This is the accepted version of the journal article: Horn, Michael R.; Singh, Amandeep; Alomari, Suaad; [et al.]. «Polyoxometalates (POMs) : from electroactive clusters to energy materials». Energy & environmen- tal science, Vol. 14, issue 4 (April 2021), p. 1652-1700. DOI 10.1039/d0ee03407j This version is available at https://ddd.uab.cat/record/271939 under the terms of the license Page 1 of 124 Energy & Environmental Science Polyoxometalates (POMs): from Electroactive Clusters to Energy Materials View Article Online DOI: 10.1039/D0EE03407J Michael Horn1, Amandeep Singh1, Suaad Alomari1, Sara Goberna-Ferrón2, Raúl Benages-Vilau2, Energy & Environmental Science Accepted Manuscript Nilesh Chodankar3, Nunzio Motta1, 4, Ken (Kostya) Ostrikov1, 4, Jennifer MacLeod1, 4, Prashant Sonar1, 4, Pedro Gomez-Romero2*, Deepak Dubal1,4* 1School of Chemistry and Physics, Queensland University of Technology (QUT), 2 George Street, Brisbane, 4000 Queensland, Australia. 2Catalan Institute of Nanoscience and Nanotechnology, ICN2 (CSIC-BIST) Campus UAB, E-08193 Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Bellaterra (Barcelona), Spain 3School of Chemical Engineering, Chonnam National University, Gwangju 500–757, South Korea 4Centre for Materials Science, Queensland University of Technology (QUT), 2 George Street, Brisbane, 4000 Queensland, Australia. 1 Energy & Environmental Science Page 2 of 124 Abstract View Article Online DOI: 10.1039/D0EE03407J Polyoxometalates (POMs) represent a class of nanomaterials, which hold enormous promise Energy & Environmental Science Accepted Manuscript for a range of energy-related applications. Their promise is owing to their “special” structure that gives POMs a truly unique ability to control redox reactions in energy conversion and storage. One such amazing capability is their large number of redox active sites that arises from the complex three- dimensional cluster of metal-oxide ions linked together by oxygen atoms. Here, a critical review on how POMs emerged from being molecular clusters for fundamental studies, to next-generation materials for energy applications is provided. We highlight how exploiting the versatility and activity Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. of these molecules can lead to improved performance in energy devices such as supercapacitors and batteries, and in energy catalyst applications. The potential of POMs across numerous fields is systematically outlined by investigating structure-property-performance relationships and the determinant factors for energy systems. Finally, the challenges and opportunities for this class of materials with respect to addressing our pressing energy-related concerns are identified. 2 Page 3 of 124 Energy & Environmental Science Broader context View Article Online DOI: 10.1039/D0EE03407J Unprecedented global energy demand and climate change have spawned broad based efforts Energy & Environmental Science Accepted Manuscript to develop new sustainable clean energy conversion and storage technologies. A range of electrochemical energy systems have the potential to address critical energy demands, however, their performance is far from the expectations. To this end, particular emphasis has been given to the design and engineering of new functional materials. Polyoxometalates (POMs), a class of metal oxide cluster anions with the ability to reversibly transfer multi-electrons have recently broken through as functional materials. POMs exhibit unique chemical properties unlike those of general metal Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. complexes with various ligands. The open frameworks with independent structural units and rich- redox chemistry makes them ideal materials for cutting-edge energy-related technologies. Here, we provide critical analysis of the current knowledge on the impact of structure and properties of POMs for improved performance in energy devices. We believe that this review will not only provide the fundamental knowledge in the field of POMs-for-Energy but will also help to address the current challenges in electrification to decarbonize industry and society. 3 Energy & Environmental Science Page 4 of 124 1. Introduction View Article Online DOI: 10.1039/D0EE03407J Many countries around the world have set targets to limit anthropogenic climate change and Energy & Environmental Science Accepted Manuscript with this comes an effort to reduce emissions that are the product of fossil fuel combustion.1 This is driving a shift towards electrified transport systems, with urgent demand for improved energy storage; but these systems are only as clean as the source of the electricity they consume. Therefore, the transition towards energy production via renewable sources as well as efficient storage, are two inseparable parts of the net equation. Another trend seen in recent years is the rapid increase in portable electronics and electric devices for uses including communications, computing, sensing, Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. entertainment, construction, and general maintenance.1-4 A link between this trend climate change mitigation, is a very high demand for research and development of innovative electrical energy conversion and storage. A wide range of energy conversion and storage technologies are being researched intensely. Various battery chemistries include lithium and sodium, and redox-flow.2-4 Other modes of conversion and storage include supercapacitors, hybrid battery-supercapacitors, fuel cells, hydrogen- based systems and a wide range of photovoltaics.5-10 The development of the next generation of all these modes requires materials that can be understood and controlled at the nanoscale, but also applied on the scale of industrial manufacturing processes. Polyoxometalates (POMs) are an exceptional group of inorganic redox active materials that consist of multiple metal-oxide ions linked together by oxygen atoms to form nanoclusters within an ordered three-dimensional framework. Their size allows them to be viewed as a type of oxide quantum dots,11,12 possessing a very high surface to bulk ratio. Their rapid and highly reversible redox activity endows them with excellent electrochemical properties. This underpins the motivation for an increasing amount of research into POMs for energy materials and other related applications. POMs have a history dating back to the late 1700s when heteropoly acids were first discovered. From the early 1800s their complex structure was the subject of theorizing.13 It was not until 1934 when J. F. Keggin used x-ray diffraction to interpret the structure of 12-phosphotungstic 4 Page 5 of 124 Energy & Environmental Science acid,13 that some level of agreement about structure started to be reached. The general chemical form View Article Online DOI: 10.1039/D0EE03407J of the class of POMs to which 12-phosphotungstic acid belongs can be written XM12O40, where X Energy & Environmental Science Accepted Manuscript denotes a heteroatom and M a transition metal addenda atom. This form is named after its discoverer and is referred to as the Keggin structure. It remains one of the most widely investigated and understood structural forms of POMs. Part of the attraction to these metal oxide clusters relates to their broad range of possible redox states and valences, while the molecular cluster itself may experience very little change in its physical structure. For example, stable crystal forms of ionic POM compounds can be synthesized that possess Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. the same stoichiometry in the POM anion, but the anion may carry a different charge. The salt K6[CoIIW12O40].nH2O forms green prismatic crystals. The Keggin POM anion in the compound carries a molecular charge of -6e (that is [CoIIW12O40]-6). Under careful synthesis conditions, Casañ- Pastor et al. produced a novel potassium salt of the POM anion [CoIIW12O40]-8.14 A solid crystalline product of the salt possessed a deep blue color. The authors describe how the extra two “blue electrons” hop between the (tungsten) metal atoms at the center of each tetrahedra that make up the cluster (see Figure 1 for a visualization of the tetrahedra forming a POM cluster). However, they also emphasize that molecular orbitals involving oxygen atoms play an important role in electron delocalization. Yet, despite the changes in valency, the bond lengths between the addenda atoms and other metal-oxygen bonds in the POM cluster change only on the order of a few percent of an Angstrom, and the cluster maintains a specific α type of Keggin structure (see Figure 1), that actually becomes more rigid. These features hint at the capability of POMs to act as stable and versatile molecular reservoirs for charge carriers. The Keggin structure was the first stable complex form of POM to be characterized, but many other framework forms have been discovered since.5 The structural motifs can be broadly classified into two types, isopolyoxometalates (isopolyanions) and heteropolyoxometalates (heteropolyanions). The former case possesses all the metal atoms are of the same element, where the addenda metal atoms are accompanied by heteroatoms, which may be either metal or non-metal. Common 5 Energy & Environmental Science Page 6 of 124 heteroatoms include W, Mo, V, C or Si, P, and As, whereas common addenda metals include View MoArticle andOnline DOI: 10.1039/D0EE03407J W.15 The general formation reaction of POMs can be written as Energy & Environmental Science Accepted Manuscript 𝑥𝑀𝑂24 ― + 2𝑦𝐻 + ∆⇔ 𝑀𝑥𝑂(2𝑥 ― 𝑚) ― 4𝑥 ― 𝑦 + 𝑦𝐻2𝑂 where 𝑀𝑂24 ― is an oxyanion, and 𝑀𝑥𝑂(2𝑥 ― 𝑚) ― 4𝑥 ― 𝑦 is an isopolyanion. In contrast to the formation of isopolyanions, if the condensation reaction of several (x) oxyanions occurs around (s) central heteroatoms the reaction leads to the formation of heteropolyanions with the general formula𝑋𝑠 𝑀𝑥 𝑂𝑧𝑦 ― . Various kinetically driven structures can be formed with the number of metal atoms and heteroatoms having different X/M ratios. Knowledge of isopolyoxometalates is mostly derived from Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. the Lindqvist ([Mo6O19]2-), heptamolybdate ([Mo7O24]6-) and decamolybdate anions.15 A much greater number of heteropolyoxometalates have been characterized. Based on their X/M ratio two 1 2 fundamental types of hetero-POM structures are the Keggin (X/M =12 ) and Dawson (X/M =18 ) structures,[1]16 as shown in Figure 1. However many more have been identified such as the Dexter- Silverton structure ([XM12O42]n-),17 and the much larger Keplerate cluster (Mo132-based).18 Furthermore, isomeric variations to the basic structures exist; for example, the Keggin structure has five recognized isomeric variations (denoted α through to ε – see Figure 1). The wide range of structural forms, with various properties and tunability has prompted researchers to present libraries of known POMs as building blocks for nanomaterials.19,20 In addition to the synthesis and structural studies of larger and more intricate forms of these interesting nanoparticles (NPs), facile synthesis methods are sought which avoid the need for separation and purification of intermediate products.18 Thus, extensive work has gone into the optimization of POM synthesis routes, to improve the yield of specific moieties as well as minimizing time and cost.21 For a comprehensive and recent review of POM structures and synthesis, the reader is referred to the work of Gumerova et al.15 Herein, we have presented a brief history and discussion around the classification and properties of POM materials with primary focus on the energy-related applications (Figure 2). Specifically, we focus on the properties of heteropolyoxometalates and recent progress towards their 6 Page 7 of 124 Energy & Environmental Science applications in supercapacitors (SCs), batteries and various catalytic applications. Firstly, theView review Article Online DOI: 10.1039/D0EE03407J highlights (section 2), the key advances in the enhancements of electrochemical supercapacitive Energy & Environmental Science Accepted Manuscript properties by introducing POMs as redox active moieties that offers large numbers of electrons during the oxidation/reduction processes. The challenges of anchoring POMs to other hybridizing materials are presented with other interesting properties and theoretical insights to be used in energy-devices. Section 3 carries this subject matter forward into battery applications where POMs have been much more widely explored. Several battery chemistries such as lithium ion, sodium ion and redox flow systems are briefly discussed. The lithium ion chemistry is given greatest attention based on its Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. popularity for research and the resulting depth of literature. This allows the article to present the major breakthroughs in stable attachment of POMs to conductive or chemically active electrode components, the use of POMs as precursors to other high surface area active materials, and POM hybrid materials for batteries. The following section goes into discussion of the science and engineering of POMs for catalysis applications (section 4). Research on POM-based catalysts has greatly increased in the last few years. Among the research fields, chemical oxygen reactions, such as for oxygen evolution (in electrolyzers) and for oxygen reduction (in fuel cells) represent a bottleneck for the development of energy conversion technologies. The section also includes a discussion on the progress of POM-based materials in different catalytic reactions such as carbon dioxide (CO2) reduction, nitrogen (N2) reduction and biomass conversion. Each section presents perspectives on the different roles for POMs in energy, including those which are expected to stimulate new research directions. The final section summarizes key points and conclusions as well as a short commentary on future challenges and the outlook for POMs. 2. POM-based Materials for Supercapacitors Traditional supercapacitors (SCs) store charge via the electric double layer effect (EDL). In these devices, high surface area materials are exploited to store charge via the electrophysical separation over very small distances at an electrode-electrolyte interface. Relative to current battery 7 Energy & Environmental Science Page 8 of 124 chemistries, these devices are able to deliver very high power densities but they suffer from low View Article Online DOI: 10.1039/D0EE03407J energy density.22 Over the last decade, a widely investigated challenge has been to increase the Energy & Environmental Science Accepted Manuscript amount of energy that can be stored in supercapacitors, while not trading off too heavily against their high power capability and exceptionally long cycle life. The theoretical capacitance of graphene sets somewhat of a benchmark for this work, at 550 F.g-1. The basis for this as a benchmark is rationalized by virtue of it representing a 100% exploited atomically thin surface, for the formation of an EDL. In practice, research on purely EDL capacitive materials has achieved around as high as 200- 300 F.g-1.23 However, aside from pursuing of the goal of maximal surface usage, a popular means of Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. improving energy density is via pseudocapacitance (Note that the term pseudocapacitance represents the extrinsic pseudocapacitive behavior), where chemically active species undergo fast reversible redox reactions close to the surface of electrode materials.24 POMs are great contender materials for such purpose, given their high surface-to-bulk ratio, and the numerous available redox states of the addenda metal species, which enable multi-electron charge transfer per molecule. However, there are major issues that remain to be addressed and comparison can be difficult given the wide variable space and range of metrics available for expressing results. Two of the greatest challenges for POMs are securely anchoring POMs to a host material and achieving the very long-term cycling stability. This section will discuss some advances that have been made towards all the above challenges, in applying POMs in SCs. The idea of exploiting the multiple redox states of POMs for enhancing the energy density of SCs with near-surface confined redox reactions has been known for more than two decades. As far back as 1997, POMs were being combined with conducting polymers such as polypyrrole (PPY),25 polyaniline (PANI) 26,27 and poly(3,4-ethylenedioxythiophene) (PEDOT).28 These materials were presented as organic-inorganic “hybrids”, and the focus was on their use in SCs. This work has continued in recent years, suggesting that there is much that remains to be explored and to be better understood.29 In 2012, Gomez-Romero’s group conducted one of the earlier reported studies where a PMo12-type POM was anchored to a simple activated carbon (AC) to create a hybrid material. The 8 Page 9 of 124 Energy & Environmental Science hybrid exhibited significantly enhanced performance over bare AC as large current under View theArticle CVOnline DOI: 10.1039/D0EE03407J curves can be seen in Figure 3 (a).28 Phosphomolybdic acid (H3PMo12O40) was used to decorate the Energy & Environmental Science Accepted Manuscript AC, increasing its mass by 54%. The cell capacitance of the hybrid was improved by an average 27% over a range of current densities up to 3 A.g-1 and it displayed strong cycling stability with 91% retention over 8,000 cycles. Although the cell was symmetric, with the same materials and mass loading for each electrode, the capacitance for positive and negative were different (160 F.g-1 and 183 F.g-1, respectively), suggesting that even greater performance can be achievable by refined cell construction and balancing (see Figure 3b). The authors compare their achievements to other work Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. on POM-carbon nanotube-based hybrids,30-32 citing improvements in every respect. Since then more work has reinforced that the energy density of microporous carbons can be improved by the hybridization with POM. The later work has added new knowledge about the connection to microporosity 33 and the ability of POM-based hybrids to enable high-voltage aqueous systems.34 Straightforward approaches to applying POMs have been shown to yield good results. Vanadate POMs have been used as a positive electrode with AC as the negative electrode in the construction of an asymmetric device.35 They reported 354 F.g-1 with 73 Wh.kg-1 at 0.1 A.g-1 and associated power of 312 W.kg-1. As can be seen from the Ragone plot in Figure 3 (c), power could be increased to 6238 W.kg-1 but this resulted in a reduction of energy density to 22 Wh.kg-1. The capacitance retention was 70% after 1000 cycles. In this above report, the POM based electrode was formed by simple mechanical hand mixing of the materials with a binder and carbon black. In battery research, mechanical methods of mixing POMs, such as this hand mixing, or ball milling and other techniques, are frequently reported, and described in section 3. More commonly in SCs, various solution-based chemistry approaches are applied to anchor the POMs on conducting scaffold to use as an electrode. Exotic emerging materials have also been used for anchoring POMs. For example, metallacalixarenes that are ordered crystalline materials similar to 2D metal organic frameworks (MOFs) with repeating nano-sized cup-shaped pockets in their surface (see Figure 3d).36 9 Energy & Environmental Science Page 10 of 124 Silicomolybdate-based POMs have been used to form (POM)/metallacalixarene/graphene-based View Article Online DOI: 10.1039/D0EE03407J electrode materials. Two compounds were tested in a recent study, [Ag5(C2H2N3)6][H5 ⊂ SiMo12O40] Energy & Environmental Science Accepted Manuscript – compound 1, and [Ag5(C2H2N3)6][H5SiW12O40] – compound 2, where (C2H2N3 = 1H-1,2,4- triazole). Compound 1 performed best, with a reported capacitance of 230 F.g-1 at 0.5 A.g-1. The materials both retained around 78.5% of their capacitance after 1000 cycles at 10 A.g-1. While it is difficult to account for all the variables, in a very casual comparison between the last two cited reports,35,36 the metallacalixarene provided some benefit towards holding onto the POMs for longer cycle life of the as-formed electrode. Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. The above comparison highlights the most critical challenge faced in work where POMs are applied to SCs. That is, long term stability is highly linked to the ability to anchor POMs to a conductive framework that will exploit their redox capacity but maintain cyclability.25 Progress is being made, as the metallacalixarene examples demonstrate by comparison to the AC-POM hybrid. However, when compared to commercial (AC based) supercapacitor products, where it is reasonable to see cycle ratings of over 1 million cycles with the same or greater than 75% capacitive retention and no power fade,22 it can be seen that there is still significant improvement required. The following section discusses materials that are suitable for SC electrodes, which have been synthesized with POMs, to form POM-functionalized hybrids. 2.1 POM-Functionalized Hybrids The key opportunity that POMs offer to enhance energy storage devices lies with their ultra- dispersed nature (all or most active centers are surface atoms) and with their ability to reversibly exchange large numbers of electrons. Up to +/- 24 e per molecule has been claimed in some cases.37 However, such a degree of reduction is debated and is in conflict with other knowledge on POMs, which sets a more conservative limit for the number of electrons that can be exchanged reversibly.38 As a pure material, POMs are generally electrically insulating. Therefore, exploiting the desirable qualities of POM complexes requires them to be supported on conductive pathways. In the context of SC electrodes, it is of particular importance to maintain a high surface area for double layer 10 Page 11 of 124 Energy & Environmental Science capacitance and high power handling, as well as chemical stability for long cycle life.39 TheView aim of Article Online DOI: 10.1039/D0EE03407J combining high surface area, electrical conductivity and chemical stability has stimulated the creation Energy & Environmental Science Accepted Manuscript of a wide range of hybrid materials. The definition of a hybrid material herein means a POM-based material that through a given synthesis procedure has been combined at a molecular level with some other material/s to achieve synergistic properties. This section describes the hybridization of POMs with conducting polymers and various forms of nanocarbons. The motivation for this type of best-of-both-worlds chemical hybridization is enhancing EDL performance with the redox contribution of POMs. This contribution can be quite significant. For Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. example, the use of a polymerized imidazolium linker (PIL) cation to secure GeMo12 and SiMo12 POMs to multi-walled carbon nanotubes (MWCNTs) has been shown to achieve an increase of more than nine times the capacitive performance relative to bare MWCNTs – 191 F.cm-3.40 In this study, a layer-by-layer (LbL) method of constructing the conducting polymer-carbon-POM composite was used and was compared to using a more conventional linker, poly(dimethyl diallyl ammonium chloride) (PDDA). The performance of the PIL is significant, because PDDA as a linking polymer has itself been shown to offer reasonable performance at high charge-discharge current densities, which is synonymous with high CV scan rates. For example, PDDA-functionalized reduced graphene oxide (PDDA–rGO) was used as the conductive matrix to support PMo12-xWxO403- (PMoW),41 and a capacitance of 140 F.g-1 at 10 A.g-1 was reported. Polypyrrole nanopipes (Ppy-NPipes) are another type of purely polymeric conductive framework that have been enhanced for pseudocapacitance by doping with Keggin PW12 and PMo12 type POMs.42 The two types of POM doped nanopipes showed elevated capacitive performance of almost 1.5 times that of the bare Ppy-NPipes with 294.1 F.g-1 and 341 F.g-1 for the molybdenum and tungsten POMs, respectively. By comparison, the bare Ppy-NPipe exhibited 204.5 F.g-1. Symmetric devices using each type of POM-hybrid material exhibited high performance metrics; 6.3 and 6.8 F/cm3 and 1.5 and 2.2 mWh/cm3 for the molybdenum and tungsten variants, respectively. 11 Energy & Environmental Science Page 12 of 124 Wide ranges of synthesis methods have been explored to produce POM hybrid electrode View Article Online DOI: 10.1039/D0EE03407J materials, and variations to the methods can produce interesting results. Polyaniline (PANI), one of Energy & Environmental Science Accepted Manuscript the first discovered conducting polymers, has been combined with phosphorous, tungsten, and silicon based Keggin POMs to form molecular hybrids for supercapacitor applications.26,27 In one of these studies, PMo12 was combined with PANI and directly deposited onto current collecting foils using electrochemical deposition techniques.27 The materials were deposited by two methods. An electrochemical-only method of applying a potential sweep in H2SO4 electrolyte (ECh) was compared to a chemical-electrochemical method (Ch-ECh). In the latter, larger amounts of precursor Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. phosphomolybdic acid and aniline were added to the H2SO4 solution before the same electrochemical cycling as the former. When tested using a solid-state H3PO4 based electrolyte membrane in a two- cell configuration, the results showed that the ECh method outperformed the Ch-ECh approach. SEM further supported this, where, surprisingly, a much higher density of POM-PANI clusters is visible on the ECh produced electrode. In addition to affecting the yield of materials, variation to the reaction of precursors in POM production is known to result in different stoichiometries.21 The Wells-Dawson 18-molybdo-2- phosphate POM, K6P2Mo18O62.12H2O (P2Mo18), has been combined with Ru(bpy)3Cl2.6H2O (Ru(bpy)3) to form a molecular hybrid.43 The synthesis was performed both with and without the presence of potassium iodide (KI), and this resulted in different stoichiometry for the products. The different ratio of Ru(bpy)3 to the linked P2Mo18 molecules in turn affected the capacitance for the materials when studied for their use in neutral bio-compatible electrolyte systems. For the product formed in the presence of KI, 125 F g-1 was reported, versus 68 F g-1 for the hybrid formed without KI present. The excess of Ru(bpy)3 in the KI-derived material suggests that the interesting electronic properties of this ligand play an important role in the transport of charges associated with the reduction and oxidation of the POMs.44 While the use of polymeric linkers and other ligands typically requires synthesis via carefully measured chemical reactions, simple mixing and solvo-thermal methods are facile, and therefore can 12 Page 13 of 124 Energy & Environmental Science be easily adopted for other hybrid types. For example, POMs have been immobilized in Cu-based View Article Online DOI: 10.1039/D0EE03407J MOFs by multiple research groups, to create polyoxometalate/metal organic frameworks Energy & Environmental Science Accepted Manuscript (POMOFs).45,46 See section 3.1.1.4 for more on POMOFs in lithium batteries. Simple one-pot mixing is also a widely applied route to synthesizing POM-nanocarbon based hybrids. Dong et al. used a Mo132-based Kepplerate POM combined with reduced graphene oxide (rGO) and tested its performance in an asymmetric cell using modified AC as the opposite electrode.47 However, for carbon-based materials, the interaction with POMs is relatively weak.48 Therefore, mixing methods commonly still seek some sort functionalization of either the POM or the carbon scaffold to improve Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. the interaction strength, albeit in reactions less sensitive to the stoichiometry. In other recent work on Keplerate POMs, Pakulski et al.48 synthesized hybrid supercapacitor electrode material by mixing electrochemically exfoliated graphene (EEG) nanosheets with Mo132 POM functionalized with dodecyltrimethylammonium bromide (DTAB), resulting in the formation of porous 3D superstructures. Performance in H2SO4 electrolyte revealed 65 F g-1, 93 mFcm-2 and 93 Fcm-3 respectively for the Mo132-DTAB-EEG hybrid. Cuentas-Gallegos et al.31 fabricated novel hybrid materials using Keggin type POMs and functionalized CNTs. The importance of functionalizing the CNTs is described in some detail. It enables the POMs to more securely anchor to the carbon framework during the synthesis procedure of the electrodes. The report emphasizes again the importance of ensuring strong POM-to-scaffold interactions or linking for ensuring long-term cycle stability in such hybrid materials. In another work by the same author, -NH2 and -OH functional groups were grafted onto vulcan carbon (VC) then the material was hybridized with phosphomolybdic acid (H3PMo12O40.12H2O) to investigate its effects on capacitive performance. Functionalization and hybridization resulted in successive losses of the VC surface area. The authors propose that this is due to micropore blocking. However the -OH functionalized VC was found to improve the POM retention onto the carbon material and therefore showed better performance than other combinations studied.49 13 Energy & Environmental Science Page 14 of 124 The graphene-based material rGO is an appealing nanocarbon for hybrid electrode materials. 50 View Article Online DOI: 10.1039/D0EE03407J The appeal stems from its hydrophilic nature and well-established commercial-scale production Energy & Environmental Science Accepted Manuscript methods for its precursor, graphene oxide.51,52 As well, intrinsic defects in the material after reduction offer functional sites for the attachment of polar molecules. While the functional groups of rGO have been well-exploited for attaching metal-oxides for enhanced pseudocapacitance,53,54 research suggests that their ability to anchor POMs is less certain. In a comparative study exploring this point, phosphomolybdic acid (H3PMo12O40) was coupled with rGO and compared with another hybrid where the grafting of H3PMo12O40 onto rGO sheets is aided by poly(1‐vinyl‐3‐ethylimidazolium Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. bromide) ([VEIM][Br]).55 TEM of the polymer ionic liquid (IL) aided hybrid (POM/PIL/rGO) reveals individually-anchored POMs with no clumping. A maximum capacitance value of 408 F g-1 was recorded for the POM/PIL/rGO material, at a constant current density of 0.5 A.g−1 in H2SO4 electrolyte. This was more than two‐fold higher than that of the POM/rGO material with no polymeric linker, which displayed only 162 F.g−1. The POM/PIL/rGO retained 92% of its initial capacitance while the current density was increased up to 10 A.g−1. In contrast, the capacitance of the POM/rGO decreased more significantly as the constant current density was increased up to the same limit, down to a value of 110 F g−1. In addition, the POM/PIL/rGO material retained 98% of its capacity after 2000 cycles at 10 A g-1. In other recent and notable work utilizing POM-rGO hybrids with polymeric linking, Ensafi et al. made a sponge formulated as [(n-C4H9)4N]5[PW11CoO39]@IL-rGO. It was tested in seawater and in H2SO4 electrolyte. They report an impressive 834.3 F g-1 at a current density of 0.9 A.g-1 in seawater.56 This result is promising with respect to both energy storage applications as well as capacitive deionization for water desalination. As seen in a number of the above reports, the use of H2SO4 aqueous electrolyte is a popular choice for research level testing of materials and devices. In a comparative study centered on its use, PW12O40 was used to functionalize rGO, and the resulting phosphotungstate product was applied in symmetric devices. The devices were tested using 1M H2SO4 electrolyte and examined against the same 1M H2SO4 but with hydroquinone added for additional redox activity. The devices containing 14 Page 15 of 124 Energy & Environmental Science the doped electrolyte exhibited an energy density of 2.38 Wh cm-3 as compared to the bare View sulfuric Article Online DOI: 10.1039/D0EE03407J acid system which achieved 1.05 Wh cm-3.57 This work and other similar works demonstrate the Energy & Environmental Science Accepted Manuscript ability to boost the energy density by hybridizing the electrolyte system as well as the electrode materials.39,57,58 The same authors as 57 use PW12 and PMo12 again to dope rGO.59 The materials are tested individually in symmetric cells, and then in an asymmetric device where the rGO-PW12 hybrid is applied as the positive electrode and the rGO-MoW12 is used for the negative electrode. Careful study of the redox potentials for each hybrid via CV method in two–electrode configuration prior to the assembly of the asymmetric device enabled the use of 1M H2SO4 electrolyte in a potential window Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. of 1.6 V, an extension of 0.3 V over the typical limit of aqueous systems. Voltage windows have been extended even further using poly-vinyl-alcohol (PVA)/H2SO4- based gel electrolyte. A cobalt-containing Anderson-type POM was decorated on the surface of a reduced rGO aerogel via one-pot hydrothermal synthesis.60 An asymmetric solid-state supercapacitor denoted (Co-POM/rGO//H2SO4/PVA//rGO) was constructed using the gel electrolyte. The device exhibited an energy density of 37.6 Wh kg−1 at a power density of 500 W.kg−1, with strong cycling stability, retaining 95.2% capacity after 5000 charge/discharge cycles, at a current density of 2 A g−1 in a 2 V window.60 Given that the energy density of SC devices is proportional to the square of the voltage window, demonstrating good retention of capacity after thousands of cycles at 2 V represents significant advancement for devices using aqueous electrolytes. 2.2 POMs as Precursor/Template for Metal Oxides and Nitrides Several metal oxides and metal nitrides are known as electroactive materials offering fast reversible redox activity for enhancement of electrochemical performance of electrode materials. Generally speaking, metal-oxides are limited in their conductivity,61 while metal-nitrides have limited stable voltage windows.62 POMs offer unique avenues to address these concerns. For instance, Dubal et al. used tungsten nitride NPs derived from PW12-POM to create electrodes for asymmetric SC devices.62 The W2N NPs produced from the POM precursor are used to decorate a carbon fabric (CF) that is further enhanced by phosphorous doping, where the phosphorous source is also the PW12 POM 15 Energy & Environmental Science Page 16 of 124 precursor. An added benefit was that the negative potential window of the W2N@P-CF is larger than View Article Online DOI: 10.1039/D0EE03407J that of tungsten nitrides previously applied to SCs (1.2 V compared to ~0.8 V).63,64 When the Energy & Environmental Science Accepted Manuscript W2N@P-CF negative electrode was combined with PPy@CF for a positive electrode, the resulting devices were able to operate within a 1.6 V window. The process of producing the W2N@P-CF electrode material is represented schematically in Figure 4 (c). Firstly, the Keggin-type PW12 was anchored to the CF and subsequently carbonized in NH3 gas to form W2N NPs. During this process, the CF is simultaneously doped with phosphorous. The extended voltage window compared to other tungsten nitrides is also proposed to be a result of the unique synthesis. The authors state it as being Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. due to either, or both, the effect of phosphorous doping and the ultra-small W2N particles that are produced when using the PW12 POM as a precursor reagent. The same author65 uses PMo12 to generate molybdenum nitride nanocrystals while simultaneously doping a CF substrate with phosphorous to generate material denoted MoN@P-CF.65 Hexagonal molybdenum nitride MXenes, a class of 2D inorganic layered materials, anchored to boron and nitrogen co-doped CNTs have also been produced using molybdenum POMs, but in this case a Lindqvist type (Mo7O24) precursor was employed.66 These last three reports 62,65,66 all use high temperature thermal treatment in ammonia atmosphere to achieve the nitridation of the POM-based Mo source, showing that this is an effective route to producing nanoparticle metal nitrides. A number of Keggin-type POMs have been utilized in the synthesis of metal oxides for asymmetric SCs.61 In a single-step procedure, KMnO4 was combined with PMo12 and treated hydrothermally to produce α-MnO2 “nanoflower” type product, which can be seen in Figure 5 (a-c). The hydrothermal treatment was carried out at various temperatures and for various times. PW12 and SiW12 POMs were also explored and resulted in the effective production of MnO2 that possessed nanoparticle properties. This is an important contrast to typical metal-oxide powders that may be used for pseudocapacitive enhancement, where much larger particle sizes may be expected. In a three- electrode configuration, the PMo12 based MnO2 produced at 200 °C over 24 h outperformed the other preparations, posting 235 F g-1 at a current density of 0.5 A g-1. When the same material was tested 16 Page 17 of 124 Energy & Environmental Science in an asymmetric cell using an AC counter-electrode and neutral NaSO4 electrolyte the cell was able View Article Online DOI: 10.1039/D0EE03407J to be operated in a 2 V window and retained 91% of its 45 F g-1 initial (device) capacity after 5000 Energy & Environmental Science Accepted Manuscript cycles at 3 A g-1. The above results demonstrate that nano-sized POM particles can serve as excellent precursors to nitrides and oxides for energy storage because their high surface-to-bulk ratio enables facile and complete reactive pathways towards relevant products, and because the product materials are able to retain this property. Retaining the high surface-to-bulk means that metal oxides and nitrides produced via such routes can overcome some of the perceived limitations of conductivity and Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. electrochemical stability, since these limitations are generally associated with the bulk properties of such materials. For example, smaller particles allow intimate contact with conductive scaffold materials. It is also proposed that smaller particles promote higher voltage windows. Therefore, producing high surface-to-bulk oxides and nitrides from POMs can effectively side-step the limitations of powdered versions of such compounds to enable end-products with desirable properties. 2.3 Roadmap to POM-based Electrolytes The use of POMs as a component of conventional liquid electrolyte in SCs are have been demonstrated but not widely explored. Skunik-Nuckowska et al.58 used a phosphotungstate-based POM together with hydroquinone in aqueous solution as an electrolyte in a SC device where the electrodes consisted of a high performance carbon material. POMs have also been applied to other applications such as proton conducting membranes for fuel cells.67-69 These grossly different contexts for application as electrolytes show how this class of materials may have a wide range of prospects owing to its interesting physical properties. Wu et al.70 synthesized a phosphotungsten vanandate-based POM and investigated its properties with the view towards application as an electrolyte. The IL studied consisted of 1-(3- sulfonic group)propyl-3-methyl imidazolium cations (MIMPS) with 4PW11VO40 Keggin-based anions; [MIMPS]4PW11VO40. Their product was a quasi-solid-state IL possessing a solid layered structure below 80 °C while transitioning to a liquid above this temperature. The electrical 17 Energy & Environmental Science Page 18 of 124 conductivity of the material was measured, and the authors found a significant increaseView inArticle thisOnline DOI: 10.1039/D0EE03407J property above the liquid transition temperature. They report values of about 10 × 10−3 S.cm−1 at Energy & Environmental Science Accepted Manuscript room temperature, and 8.60 × 10−2 S.cm−1 at 83 °C. It was concluded that this result was due to the improved diffusion of the ions above the liquid transition temperature.70 In a later study by Tong et al.71 results are found for two other structurally similar POM-derived ILs, [MIMPS]4PMo11VO40 and [MIMPS]3PW11MoO40. The same-layered structure is confirmed by XRD results, and the authors show a transition from a solid to a liquid state above around 90 °C. The conductivity of these liquids was higher than that reported by Wu et al.70 where they measured it to be 0.1 S.cm-1 for both Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. [MIMPS]4PMo11VO40 and [MIMPS]3PW11MoO40, with electrochemical stability windows of 2 V and 4 V, respectively. The above cases demonstrate the interesting characteristics of these novel types of electrolytes. However, the temperature ranges in which high ionic conductivity occurs make them more appropriate for application such as fuel cells etc. Charge storage devices like SCs generally require peak performance of the materials to fall in a window a few tens of degrees either side of ambient temperatures, making these gel-liquids unsuitable. Wu et al.72 investigated two types of IL electrolytes consisting of POM polyanions combined with tetraalkylphosphonium (TBTP) cations. They conclude that the Dawson-type POM (P2W16V2O62) possesses better thermal and charge conductivity properties than the Keggin-type (PW10V2O40) to which it was compared. These conclusions are based upon the calculated energies associated with the lattice potential and the ionic migration potential, where in both cases the Dawson-type structure shows a lower energy requirement. The [TBTP]8 P2W16V2O62 IL shows a phase transition from solid-gel to liquid at around 50 °C which is greatly reduced against that seen in previous studies as well as against the [TBTP]5 PW10V2O40 Keggin-based IL against which it is compared (61°C), and almost reaching the ranges required for application in typical energy storage devices. Even though the above results lay just outside the range of practical use for energy storage, it is worth noting that the authors show a strong link between the improved properties of the Dawson type structure and its larger POM anion size. 18 Page 19 of 124 Energy & Environmental Science This offers predictive insight for the rational design and selection of POM based electrolyte systems, View Article Online DOI: 10.1039/D0EE03407J which justifies further research into these ionic gel-liquid systems. Energy & Environmental Science Accepted Manuscript 2.4 Theoretical Studies to Underpin Charge-storage in POM-Hybrids The importance of anchoring of POMs onto a supporting scaffold has been established in previous sections. Theoretical simulations provide a powerful tool to provide insight to the binding between POMs and an associated substrate or scaffold material. The mechanisms of bonding between carbon and POMs are not fully understood, and so to explore this further Muniz et al.73 performed density functional theory (DFT) investigations into the Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. electronic structure of a range of Keggin POMs (PdMo12, RuNb12, SiMo12, PMo12 and SiW12) adsorbed onto a carbon surface. These five Keggin structures were investigated for their binding lengths and electron sharing under various orientations on a carbon substrate, and neutralizing counter-ions (cations) were included to balance the charge of the Keggin anions. For PdMo12 and RuNb12 the calculation was fully predictive, as these systems have not, to date, been synthesized. For the SiW12 and RuNb12 systems, the results indicate that functionalization of the carbon surface may not be required for sufficient anchoring and feasible use as hybrid (electrode) materials. In 2019, Dubal et al.65 simulated the adsorption of a number of combinations of the materials used in their study of molybdenum nitride nanocrystals. Again, the simulations were performed using DFT, in this case in the Vienna Ab-initio Simulation Package (VASP). As described in detail in section 2.1, the experimental approach in this work used CF coated with PMo12 as a substrate. The material was then converted to MoN@P-CF, where MoN nanocrystals were formed and the CF became phosphorous doped. Simulations reveal that the phosphorous doping of the CF enhances the energy of adsorption for protons (simulated as hydronium) over the bare carbon surface. However, the MoN@P-CF material has an adsorption energy over eight times greater than that of the phosphorous-doped CF. This theoretical element of the work strongly supports the experimental findings. Likewise, Yang and co-workers45 employed DFT calculations to explain the origin of the enhanced redox reactions in POM/PIL/RGO nanohybrids. The van der Waals density functional 19 Energy & Environmental Science Page 20 of 124 (vdW-DF) for the non-local correlation part was employed in the DFT calculations. Figure 5 (d) View Article Online DOI: 10.1039/D0EE03407J shows the most stable POM/PIL/graphene structure, where two POMs and one IL dimer (i.e., Energy & Environmental Science Accepted Manuscript 2[H2PMo12O40][C14H24N4]) are included in a periodic unit cell. The role of PIL and the neighbouring POM in the proton transfer process was examined by calculating adsorptions energies of a single H atom on eight different Ob and Oc sites of POM (Figure 5e). These DFT results suggest that the H adsorption energetics on a POM in the POM/PIL nanohybrids is significantly affected by adjacent PILs and POMs. From an electrostatic standpoint, the PIL cations and the POM anions suppress and facilitate the proton adsorption reaction, respectively, on the adjacent POM. Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Gullen et al.74 performed in-silico simulations of the CNT functionalization that was experimentally explored by Gallegos et al.31 Specifically, they used DFT to investigate the bond formation between Keggin-type SiW12 and organic functional groups attached to a CNT. The functional groups are denoted φ-X, where φ denotes a phenyl group and X is one of -NH2, -OH, - COH, or -COOH. The bond lengths are similar for the different functional pairs, however SiW12@φ – COOH/CNT shows enhanced attraction over the others. Whereas the first three systems explored (-NH2, -OH, -COH ) exhibit van der Waals type interactions, the -COOH system exhibits a covalent- like binding energy, making its combinations with SiW12 a strong potential candidate for a stable hybrid system (Figure 5 f, g).74 The significance of the previous study is emphasized in closing this chapter – it is well known that stabilizing POMs on a graphene surface remains challenging.75 Therefore, predicting functional groups that are more likely to result in the successful anchoring of materials like POMs, but also other classes as well, is of great importance to rationally guide research efforts. The covalent type anchoring of POMs via carboxylate groups represents a great example of how simulation can guide novel syntheses. Therefore, combining the efforts of both theoretical work and applied materials research is an effective approach to improving devices. 20 Page 21 of 124 Energy & Environmental Science 3. Development of POMs for Battery Applications View Article Online DOI: 10.1039/D0EE03407J The development of high-performance rechargeable batteries, including lithium-ion batteries Energy & Environmental Science Accepted Manuscript (LIBs) and sodium ion batteries (SIBs), is one of the most important challenges in energy storage, and arguably stands to contribute as much as any other field toward solving increasing environmental and energy issues. The charge storage in rechargeable batteries results from reversible redox reactions in the electrode active materials.76 The stored energy in batteries can be improved by either increasing the capacity of the electrode materials or exploring new cathode materials with higher voltages.77 The following sections explore how POMs have been able to contribute to this development. Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. 3.1 Lithium-ion Batteries LIBs are the preferred energy storage system for many applications due to their high voltage of more than 3.5 V per cell, high energy density of up to 200 Wh.kg-1 and relatively long cycle life. The most common cathode materials used in LIBs are lithium transition metal oxides including layered structures such as LiCoO2, spinel structures such as LiMn2O4, or olivine structures such as LiFePO4. However, the changes in crystal structure of these materials during the intercalation (and de-intercalation) of lithium ions result in capacity loss during cycling. Therefore, their specific capacity is usually less than 200 mAh.g-1.76,78-80 For anodes, graphite is the most commonly used material in commercial LIBs. However, the theoretical capacity of graphite is low (372 mAh.g-1), which limits its applications in large-scale systems.77,80 Therefore, great efforts have been dedicated to developing new materials for LIBs. Recently, POMs have received much interest as electrode materials in LIBs (Table 1). The characteristic capacity and cycling stability of POMs are independent of the material’s crystalline stability because the POM molecular clusters show multi-electron redox as an individual molecule. Although POMs have low electrical conductivity, using nanosized POMs and/or mixing with conductive additives can improve their electrochemical properties.81-86 Their application to LIB research can be categorized by the POM type being used, such as by the addenda atom type, or by specific groupings of hybridized materials, such as the nanocarbon or MOF hybrids described in the following sections. 21 Energy & Environmental Science Page 22 of 124 3.1.1 Molybdenum-based POMs View Article Online DOI: 10.1039/D0EE03407J Phosphomolybdic acid is perhaps the most widely explored precursor to develop the electrode Energy & Environmental Science Accepted Manuscript materials for energy storage applications. This work was pioneered in the late 90s by the group of Gomez-Romero who showed the potential of POMs in LIBs.25 The group was able to demonstrate anchoring of POMs to the conducting polymer polyaniline, which allowed insertion and de-insertion of Li cations without simultaneously removing the POM anion.87 This work was followed by the demonstration of the hybrid material as an electrode in LIBs.88 Much more recently, Sonoyama and coworkers 82,89 examined the electrochemical properties Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. of a potassium-based variant of the Keggin-type polyoxomolybdate, K3[PMo12O40 ] (KPM), applying it as a cathode material for LIBs. The KPM showed a high initial discharge capacity over 200 mAh.g-1 in the potential range of 1.5 V - 4.2 V. Although a high capacity was achieved, KPM showed poor cycle stability, as only 37.5% of its initial capacity was maintained after 50 cycles, which resulted from the structural instability of the reduced [PMO12O40]3- cluster ion in the organic solvent. Furthermore, the calculated energy density of KPM was only 480 mW.h.g-1, which is not far from that of conventional cathode materials (LiCoO2~500 mWh.g-1, LiMn2O4~400 mW.h.g-1). This is a consequence of the lower discharge voltage of KPM (2.5 - 1.5 V) compared to the usual LIB cathode materials. Another Keggin-type POM, namely TBA3[PMo12O40] (TBA; [N(CH2CH2CH2CH3)4]+) was studied by Wang et al.90 as a cathode for LIBs. The charge storage mechanism in super-reduced [PMO12O40]27- was studied by X-ray absorption near-edge structure analysis (XANES) (Figure 6 (a)). The reversible redox reaction of the 24-electrons in [PMO12O40]3- during charging/discharging process resulted in a high capacity of approximately 260 mAh.g-1 at 1 mA in a voltage range of 1.5- 4.2 V. Moreover, in operando extended X-ray absorption fine structure (EXAFS) measurements indicated that POMs behave like an electron sponge as the molecular structure size of [PMO12O40]27- shrinks when compared to the original [PMO12O40]3- (Figure 6 (b)). The same group investigated Anderson-type polyoxomolybdate, Na3[AlMo6O24H6] (NAM),) as LIB-cathode.91 In order to improve the low conductivity of the POMs, NAM was mixed with two 22 Page 23 of 124 Energy & Environmental Science types of conductive additives, namely ketjen black (KB) and acetylene black (AB) using two different View Article Online DOI: 10.1039/D0EE03407J methods such as hand grinding and ball milling. Both methods and conductive additives affect the Energy & Environmental Science Accepted Manuscript capacity and cycling stability of NAM. The maximum discharge capacity of 437 mAh.g-1 was achieved, which was maintained to 399 mAh.g-1 after 50 cycles for ball-milled NAM/KB sample. The above results can, at least in part be explained by: (1) the improved electric conductivity, resulting from the high electrical conductivity of the KB and producing larger contact surface area between KB and NAM through ball milling, (2) the stable structure of [AlMo6O24H6]3- during redox reaction (Figure 6c). However, the amount of NAM in the cathode was low (32 wt %), which can restrict the Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. energy density of the cell. To further increase the discharge capacity and cycling stability, the nanosized POMs were used along with conductive additives.84 The POM (NH4)6[NiMo9O32] (ANM) was investigated using either KB or AB. In this work, four different samples were tested as LIB cathode materials; ANM microparticles hand-ground with AB (m/AB), ANM NPs hand-ground with AB (n/AB), ANM NPs hand-ground with KB (n/KB), and ANM in situ crystallized in KB (nanocomposite ANM/KB). Better discharge capacities and cycling performance were obtained with the n/KB and nanocomposite ANM/KB. The n/KB cathode exhibited a high initial discharge capacity of 490 mAh.g-1 and a very stable cycling performance, with capacity retention of 87.6% after 50 cycles. Although the nanocomposite ANM/KB cathode showed high initial capacity at low current density, it showed a poor cycling stability as compared to n/KB (61.6% after 50 cycles). This might be due to the increase in the catalytic activity of the nanocomposite ANM/KB, which leads to the formation of surface film and eventually increases its interfacial impedance. However, at high current densities (from 83 to 500 mA.g-1), the nanocomposite ANM/KB showed relatively higher discharge capacity and rate-capability than that of n/KB. Ex situ XRD analysis indicated that the crystal structure of ANM remains amorphous during insertion and extraction of Li ions (charge/discharge processes), which enables high reversible capacity with deep discharge-charge cycles. 23 Energy & Environmental Science Page 24 of 124 3.1.2 Vanadium-based POMs View Article Online DOI: 10.1039/D0EE03407J Vanadium-based POMs have proven to be practical cathode materials due to their low Energy & Environmental Science Accepted Manuscript molecular weights leading to high specific capacities and fast charging/discharging kinetics, which resulted from their stable ionic cluster.92 The electrochemical properties of V-based POMs; bi-capped Keggin-type (KH)9[PV14O42] (KPV),89 K7[NiV13O38] (KNiV),81 and K7[MnV13O38] (KMV)83, have been recently reported as LIB-cathode materials. These V-POMs displayed comparable discharge capacities to those of Mo-based POMs, but with much higher cycling performance. This indicates that the electrochemical stability of V-POM is not dependent on the stability of crystal structure, but Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. rather on the stability of their cluster ions; [PV14O42]9-, [NiV13O38]7- and [MnV13O38]7-. KPV exhibited a high initial discharge capacity of 370 mAh.g-1 with cycle stability of 95% over 50 cycles, which is very high compared to that reported from KPM (~37.5%). KNiV NPs were prepared by a re- crystallization method to improve the capacity and cycle performance of V-based materials. Re- crystalized KNiV delivered discharge capacities of 218.2 and 121.3 mAh.g-1 at current densities of 17 and 167 mA.g-1, respectively. It also showed good cycling stability with a reversible capacity of about 200 mAh.g-1 after 24 cycles at 17 mA.g-1. The observed high capacity and rate capability most likely result from the use of nanosized KNiV, which offers improved specific surface area and reduced diffusion lengths. Concerning KMV, micro-sized KMV (m-KMV) showed a discharge capacity of 187 mAh.g-1 at 17 mA.g-1. When the particle size was decreased to nanoscale, discharge capacities were increased to 308 mAh.g-1, for the same respective current density. However, at voltages higher than 3.8 V, nanosized KMV (n-KMV) showed overcharge capacities of 37 and 15 mAh.g-1, respectively. This may result in electrolyte decomposition, which in turn leads to the formation of a thick passivating layer on the cathode. The passivation layer decreases the ionic and electronic conduction at the cathode/electrolyte interface. This is why the authors proposed to use the mixture of nano and micro-sized KMV particles (n/m-KMV; 1:1 weight ratio) (n/m-KMV). Unsurprisingly, the n/m KMV showed improved cycle performance where a high capacity retention of ~ 97% after 50 cycles at both high and low rates was achieved. 24 Page 25 of 124 Energy & Environmental Science In other work, Li et al. 93 replaced the counter cations K+ in K7[MnV13O38] (KMV)View (which Article Online DOI: 10.1039/D0EE03407J + has large equivalent mass) by NH4 with a smaller equivalent mass to fabricate (NH4)7[MnV13O38] Energy & Environmental Science Accepted Manuscript (AMV). The modified AMV nanoparticles showed a reversible capacity of 234 mAh.g-1 at 0.5 C, which was retained at 234 mAh.g-1 after 100 cycles. Moreover, AMV displayed rate capability of 53% when the C-rate was increased from 1 to 5 C. An energy density of 553 mWh.g-1 was estimated over 100 cycles, which was two-fold higher than that of the layered Li-transition metal oxide cathodes. To prevent the electrolyte decomposition and increase the electron transport in POMs, authors 94 coated the surface of polyoxovanadate, K9[PV14O32] (KPV) NPs with a conducting Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. polymer, polypyrrole (PP) (5 nm thick). In this hybrid, a thin conducting layer provides high conductivity and avoids the irreversible reactions. The capacities of 410, 415 and 300 mAh.g-1 were recorded for PP-KPV at current densities of 167, 500 and 1000 mA.g-1, respectively. Interestingly, PP-KPV showed a cycling stability of 90% (370 mAh.g-1) and 93% (280 mAh.g-1) over 50 cycles at 167 and 1000 mA.g-1, respectively. This confirmed that coating PP on KPV NPs suppresses electrolyte decomposition, improves rate capability and cycling stability. Along this line, Dong and coworkers developed a symmetrical LIBs by employing Li7[V15O36(CO3)] as both cathode and anode material as shown in Figure 7 (a).95 The reason to apply Li7[V15O36(CO3)] materials as both the cathode and anode might be that the vanadium centers (VIV and VV) in the Li7[V15O36(CO3)] polyanion exhibits different redox potentials (Figure 7 (b)). The symmetric cell showed a high discharge capacity of 120 mAh.g-1 at 100 mA.g-1 and long cycle life with a capacity retention of 95% after 500 cycles at 1 A.g-1 (Figure 7c). High energy and power densities of 125 Wh.kg-1 and 51.5 kW.kg-1, respectively, were obtained. Similarly, several POM-based materials were also employed as anode materials in LIBs. For instance, Na7H2[PV14O42] (NPV) is an example of the V-based POM, which was illustrated by Huang et al.96 In operando characterizations confirmed the continuous reduction and oxidation of vanadium during discharging/charging process, and the electron/ion sponge behavior of [PV14O42]9- where at least 30 electrons can be reversibly stored by one NPV molecule (Figure 7d). Furthermore, it was 25 Energy & Environmental Science Page 26 of 124 found that NPV is an insertion-type anode rather than conversion-type anode, indicating the View Article Online DOI: 10.1039/D0EE03407J independence of the capacity and cycling stability from the stability and recoverability of the NPV Energy & Environmental Science Accepted Manuscript crystal structure. As a result, a high reversible capacity of 687 mAh.g-1 at 100 mA.g-1 was achieved with 80% cycling stability after 150 cycles. These results confirm the ability of V-based POMs to provide better capacity and cycle stability compared to other materials, however, further studies are required to achieve practical realization. 3.1.3 POM/Carbon hybrid for LIBs As discussed earlier, the direct utilization of pristine POMs as electrode materials in energy Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. storage devices is limited by their poor electrical conductivity and high solubility in water and many organic solvents. Thus, it is useful to consider the use of highly conductive organic materials as substrates such as conducting polymers or carbon materials for the fabrication of POM energy storage electrodes.97 The hybridization between POMs and nanocarbons, including carbon nanotubes (CNTs) and graphene, has been proven to increase the energy storage capability of LIBs due to the combination of the redox reaction of POMs and capacitive effect of the nanocarbons.86,98-101 Different approaches have been developed to connect the POMs onto nanocarbon materials including covalent functionalization, in which POMs are covalently grafted onto organic moieties, and non-covalent functionalization using intermolecular interactions such as electrostatic interactions and the π- π interaction between POMs and organic materials.102 For instance, Kawasaki et al.85 developed a nanohybrid POMs/SWNT system as a cathode material. They expected that the nanohybridization between the POM and SWNTs would increase the lithium ion diffusion and electron transfer through the SWNTs (Figure 8 (a)). In their study, a Keggin-type POM, TBA3[PMo12O40] was deposited onto the surfaces of the SWNTs by electrostatic interactions. The nano-hybrid POM/SWNT delivered a capacity of 320 mAh.g-1 with a good cycle performance. The high capacity of the POM/SWNT system can be ascribed to the high surface area and conducting super-highway of CNTs. 26 Page 27 of 124 Energy & Environmental Science Several POM/CNT composites have been developed through covalent and noncovalent View Article Online DOI: 10.1039/D0EE03407J bonding to improve the affinity of POMs to CNTs. For example, Song et al., 98 developed non- Energy & Environmental Science Accepted Manuscript covalent functionalization of SWNTs with a covalently functionalized pyrene-POM hybrid (Py- SiW11) and served as LIB anode. The pyrene moieties were covalently linked to the SiW11 clusters to form Py-SiW11 hybrids, which were then attached to SWNTs through both π-π stacking and electrostatic interactions. The SWNTs/Py-SiW11 nanocomposite exhibited a capacity of 1569.8 mAh.g-1 at 0.5 mA.cm-2 and capacity retention of 37 % after 100 cycles. This work was extended by covalently grafting SiW11-NH2 onto CNTs by amide bonds to prepare CNTs/SiW11 Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. nanocomposites.86 The covalent bonding introduces a stable link between the POMs and the CNTs and creates more sidewall defects in the CNTs, which results in more Li ion diffusion pathways. As an anode, CNTs/SiW11 nanocomposite showed a capacity of 1189 mAh.g-1 and capacity retention of 54.6% (650 mAh.g-1) after 100 cycles. They also prepared a ternary PMO12/PANI/MWNTs nanocomposite through noncovalent functionalization to further improve the connection between POMs and CNTs.99 This was achieved by using conductive polymers, which have strong π-π interaction with CNTs and strong electrostatic interaction with POMs. The electrochemical investigation of the PMO12/PANI/MWNTs nanocomposite as a LIB anode resulted in a capacity of 1572 mAh.g-1, which was retained at 1000 mA.h.g-1 (63.6%) after 100 cycles at 0.5 mA.cm-2. The capacitive effect of POM/nanocarbon composites can be further improved by using graphene, which has higher available specific surface area than CNTs. Hence, several studies regarding POM/graphene nanocomposites have been reported for capacitive-type devices.39,50,57 These types of composites are also appealing for battery applications, for example, Wang et al.100 reported a Keggin-type SiW12/rGO nanocomposite as a cathode material for LIBs. A capacity of 275 mAh.g-1 was achieved at a current density of 50 mA.g-1, which was ~1.7 times higher than that of the pure SiW12 (160 mAh.g-1). Kume et al.103 developed TBA3[PMo12O40]/rGO hybrid as LIB cathode material by grafting POM molecules onto the surface of rGO (Figure 8 b, c), demonstrating a capacity of ~140 mAh.g-1 with a stable capacity over 10 cycles. Likewise, Xie et al.101 prepared 27 Energy & Environmental Science Page 28 of 124 ethylenediamine-functionalized rGO (EDAG)/Anderson-type Na3[AlMo6O24H6] (NAM) composite View Article Online DOI: 10.1039/D0EE03407J and tested as anode in LIBs. The material showed a discharge capacity of 1835 mAh.g-1 at 100 mA.g-1. Energy & Environmental Science Accepted Manuscript Further work to improve the capacity and cycling stability of POM/graphene nanocomposites was performed by Ni et al.104 A 3D graphene/polyaniline/POM hybrid (rGO@PANI/PW12) was developed as LIB-cathode, which showed a specific capacity of 285 mAh.g-1 at 50 mA.g-1 with good rate capability (140 mAh.g-1 at 2 A.g-1) and excellent cycling stability (only 0.028% loss per cycle capacity) over 1,000 cycles. The high capacity and good cycle stability of the reported (rGO@PANI/PW12) cathode can be attributed to the fast electron transfer from reduced PANI Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. polycation to PW12 polyanion, as well as the high redox properties of the PW12. Nevertheless, deeper insight and understanding into the nature of the charge transfer between the POMs and the conducting supports is required in order to realize their practical application in LIBs. 3.1.4 POMs/Metal-Organic Frameworks (POMOFs) Metal-Organic Frameworks (MOFs) is a new class of porous materials, which has attracted tremendous attention due to their unique surface and structural properties. Recently, MOFs are also investigated as host to immobilize POMs to prevent their solubility in organic electrolytes.105-111 The combination of POM and MOFs is hereafter referred as POMOFs. The unique combination of features of both POMs and MOFs such as open porous structures and redox-rich chemistry makes them potential materials for LIBs. Recently, Yue et al.110 designed a POMOF, which consists of (Ni6PW6) structural building units and rigid carboxylate linkers. When applied as anode in LIBs, this POMOF showed a discharge capacity of 1421 mAh.g-1 at 1.25 C with a reversible capacity of about 350 mAh.g-1 after 500 cycles. At low C-rate of 0.25 C, a high discharge capacity of 1525 mAh.g-1 was measured, which was retained to 540 mAh.g-1 after 360 cycles. Huang et al.105 synthesized crystalline POMOF with an ultra-stable structure by connecting Zn-ε-keggin, [ε- PMo8vMo4VIO37(OH)3Zn4] with TPT ligands, (TPT=tris-(4-pyridyl)triazine) to fabricate [PMo8vMo4VIO37(OH)3Zn4][TPT]5·2TPT·2H2O (NNU-11) with π-π stacking interactions. The material was further tested as anode material in LIB, which showed a capacity of 1322.3 mAh.g-1 28 Page 29 of 124 Energy & Environmental Science with a stable reversible capacity of 750 mAh.g-1 over 200 cycles. The authors also synthesized two View Article Online DOI: 10.1039/D0EE03407J thermally-stable POMOFs with diamond topology, (TBA)3[PMo8VMo4VI O38(OH)2Zn4(IN)2] Energy & Environmental Science Accepted Manuscript (NENU-506) and (TBA)3[PMo8VMo4VI O38(OH)2Zn4(PBA)2]·H2O (NENU-507) (where IN = isonicotinic acid, TBA = tetrabutylammonium ion, H PBA = 4-(pyridin-4-yl) benzoic acid).107 When these two POMOFs were used as anode materials, the better cycling stability was obtained for NENU- 507, which delivered a reversible capacity of 640 mAh.g-1 at 100 mA.g-1. Several studies have been conducted by Lan’s group in order to improve the electronic conductivity of POMOFs. For instance, they reported on POMOF-based PMo10V2/rGO Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. nanocomposites as LIB-anode. The composite exhibited a reversible capacity of 1075 mAh.g-1 at 50 mA.g-1 and cycling stability of nearly 100% at both 2000 and 3000 mA.g-1 current densities after 400 cycles.108 They also used ILs to increase the conductivity of POMOFs by encapsulating ILs into POMOFs and fabricate a series of POMs-ILs@MOFs crystals.111 Among those, PMo10 V2- ILs@MIL-100 crystals showed an excellent cycling stability with a high capacity of 1248 mAh.g-1 at 0.1 A.g-1 after 100 cycles as LIB-anode material. Likewise, Li et al.106 reported POMOF (CuPW)/SWNTs composites, (CuPW=([Cu18(trz)12 Cl3(H2O)2][PW12O40]) as anode materials where SWNTs were utilized as a conductive scaffold. The material showed a reversible capacity of 885 mAh.g-1 after 170 cycles at 100 mA.g-1. The high capacity and reasonable rate performance can be ascribed to the high specific surface area and the improved electric conductivity due to SWNTs. From the above results, it is apparent that the POMOF/conductive composites have great potential to achieve high capacity and good cycle stability for LIBs applications. However, the complicated synthesis methods and the difficulty of anchoring the charge-balanced POMOFs on the negatively charged carbon supports limit their practical applications. Therefore, finding new simple synthesis methods to improve the conductivity of POMFs is highly demanded. 3.2 Sodium-ion Batteries Sodium-ion batteries (SIBs) are a promising technology for large-scale energy storage applications due to the large abundance of sodium in the Earth-crust (23 × 103 ppm) and it also permits 29 Energy & Environmental Science Page 30 of 124 the use of low-cost aluminum as a current collector for both anode and cathode materials. The latter View Article Online DOI: 10.1039/D0EE03407J is possible because sodium does not alloy with aluminum, unlike lithium.112-114 However, their energy Energy & Environmental Science Accepted Manuscript density is low compared to LIBs because of the large radius of the Na+ ions (Na+: 1.02 Å vs Li+: 0.67 Å), large atomic weight (Na: 23 vs Li: 7) and low operating potential (Na: 2.71 V vs Li: 3.04 V).114 Therefore, many studies have aimed to overcome these disadvantages by exploring and developing new cathode materials. Although POMs have been widely used in both cathodes and anodes in LIBs with good capacity and cycling stability, few studies have been conducted on POM-based SIBs. For instance, Liu et al.113 reported on Na2H8[MnV13O38] (NMV) as a SIB cathode material by coating Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. NMV nanoplates on graphene sheets to fabricate a robust NMV/G composite. Ex-situ X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FT-IR) spectroscopy analyses showed that the NMV cathode acts as an electron/Na+ ion sponge, in which [MnV13O38]9- is reduced to [MnV13O38]20- by storing 11 electrons with an expansion in size of ~7.5% (Figure 9). The NMV/G composite demonstrated a high reversible capacity of ~190 mAh.g-1 at 0.1 C with good rate capability (130 m.h.g-1 at 1 C), and capacity retention of 81% (140 mAh.g-1) after 100 cycles. Similarly, Chen et al.95 used Li7[V15 O36 (CO3)] as a high capacity cathode material for SIBs. According to electrochemical investigations and DFT, (V15 O36 (CO3)) can transport and store large numbers of large cations such as Na+. A reversible specific capacity of 240 mAh.g-1 was obtained at a current density of 50 mA.g-1 and a stable reversible capacity of 190 mAh.g-1 was retained after 30 cycles at 100 mA.g-1. The vanadium-based POM, Na6[V10O28]·16H2O, has been reported by Hartung et al. as an anode material for SIBs.112 Na+ is (de)inserted (from) into the space between the individual [V10O28]6- clusters instead of intercalating into their crystal structures, which resulted in the capacity of ~276 mAh.g-1 at 20 mA.g-1. Xie and coworkers101 employed NAM-EDAG (NAM: Anderson-type Na3[AlMo6O24H6], EDAG: ethylenediamine-functionalized rGO) as anode for SIB and found a reversible Na storage capacities of ~150 and ~50 mAh.g-1 after 150 and 200 cycles, respectively. However, these values are much lower than those previously reported for LIBs (> 1000 mAh.g-1 after 30 Page 31 of 124 Energy & Environmental Science 100 cycles). These low capacities can be related to sluggish electrode kinetics due to the formation View Article Online DOI: 10.1039/D0EE03407J of a Na2O passivation layer during the first charging/discharging cycles; however, future research is Energy & Environmental Science Accepted Manuscript needed to validate this conclusion. Another SIB anode material based on POMs, Na7[H2PV14 O42]·nH2O (NPV) (n=15-24), was reported by Lin et al.115 This anode showed a high reversible capacity of 322 mAh.g-1 at 25 mA.g-1 and high cycling stability 87% after 120 cycles. 3.3 POM-based Redox Flow Batteries Redox flow batteries (RFBs) are an emerging technology for stationary energy storage. Unlike conventional secondary batteries they utilize liquid electrodes.116,117 RFBs use two redox-active Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. solutions with different redox potentials, known as an anolyte and catholyte. The two solutions are stored in separate containers and are continuously pumped through battery stack compartments where reversible redox reactions can occur at the interface of each inert electrode.118 The anolyte and catholyte solutions are separated by an ionic conducting membrane or separator, which allows the transport of charge carrying ions. A schematic diagram of a RFB is shown in Figure 10. The unique design and the working mechanism of the RFB offers several promising features. As an example, their flexible modular design allows RFBs to be sized, as required, to cover a wide range of energy and power requirements of the consumer market. The energy and power characteristics of RFBs are not as explicitly coupled with each other when compared to other classes of batteries. This is due to the electrolyte and electrode being stored in separate containers. The de-coupling of these characteristics makes them unique for commercial application.117 In case of RFBs, the energy capacity is related to the volume of the electrolyte reservoirs, while the power capability depends upon the size of the battery stack. These highly customizable features of RFBs make them one of the most promising technologies for stationary energy storage. In 1970, the National Aeronautics and Space Administration (NASA) proposed the concept of RFBs for the first time, using the ferric/ferrous and chromic/chromous as an electrolyte solution. However, the limited cycling stability and the excessive hydrogen generation at the negative electrode limited the use of ferric/ferrous and chromic/chromous as practical electrolyte solutions for RFBs.119 31 Energy & Environmental Science Page 32 of 124 To enhance the energy density and achieve good cycling stability, various redox active species have View Article Online DOI: 10.1039/D0EE03407J been investigated.120 On the basis of classification by active redox species, these RFBs can be divided Energy & Environmental Science Accepted Manuscript as follows: all-vanadium, polysulphide/bromine, zinc/bromine, vanadium/cerium.121,122 Each flow battery system has certain advantages and disadvantages. Recently, POMs have been also studied in RFBs to enhance their electrochemical parameters. Pratt et al.123 developed RFBs by using the phosphorus-based POMs (A-α-PV3W9O406-, B-α-PV3W9O406-, and P2V3W15O629−). The electrochemical measurements show that the vanadium centers of these compounds could be used as the positive electrode (PVIV3WVI9O409−/PVV3WVI9O406−) and that the tungsten centers could be used Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. as the negative electrode (PVIV3WVI9O409−/PVIV3WV3WVI6O4012−). Similarly, [SiVV3WVI9O40]7- /[SiVIV3WVI9O40]10- could be used at the positive electrode and [SiVIV3WVI9O40]10- /[SiVIV3WV3WVI6O40]13- at the negative electrode, [(SiFe3W9(OH)3O34)2(OH)3]11−/[(SiFe3W9(OH)3O34)2(OH)3]14− at the positive electrode and [(SiFe3W9(OH)3O34)2(OH)3]17−/[(SiFe3W9(OH)3O34)2(OH)3]14− at the negative electrode for the RFBs.124,125 However, the low coulombic efficiency and the poor cycling stability of the POM-based anolyte and catholyte solutions are challenges that still need to be addressed to make them suitable as candidates for RFB applications. Not surprisingly, the hybridization of POMs with another conducting material has recently been proposed to enhance the electrochemical performance of POM- based RFBs.126,127 Dubal et al. proposed the hybridization of graphene and POMs (phosphomolybdate and phosphotungstate) for flow energy storage application.127 The distinctive feature of this work was the use of the customized hybrid electrolyte in a proprietary miniature flow reactor to study the electrochemical features. Remarkably, rGO/phosphomolybdate based flow cells showed encouraging electrochemical features including specific capacitance of 305 F.g-1 with cycling stability (~95%) as well as aa strong Coulombic efficiency (~77–79%) after 2000 cycles. The electrochemical results obtained for these POM based RFBs are still far from the vanadium RFBs.128 Therefore, more efforts and new approaches are necessary to utilize POMs as liquid electrodes in RFBs. 32 Page 33 of 124 Energy & Environmental Science 4. Engineering of POM-based Materials for Catalysis View Article Online DOI: 10.1039/D0EE03407J The Keggin types of POM are best suited for catalysis application as they are uniquely stable Energy & Environmental Science Accepted Manuscript amongst other structural motifs. As a result, they have already been applied in various industrial processes as a catalyst.129 They owe their extraordinary catalytic properties to the presence of numerous active sites per molecule. Their proton binding sites promote acid-catalyzed reactions and their electron-rich oxygen sites promote base-catalyzed reactions. However, the most important active sites in POM/heteropolyacid (HPA) compounds are at metal atoms, which take part in all oxidative reactions, some acid-catalyzed reactions, and other reactions. The precise active sites of POMs Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. depend upon the type of reaction. As a catalyst used in oxidative reactions, POM/HPA (mostly W/Mo-based) compounds are well explored. In the case of substituted W/Mo, the active sites depend upon the type of metal ion substituted. Considering the variety of the metals that can be introduced into POMs, these active sites can be engineered to tailor their properties for specific applications. In recent years, research on materials for clean and sustainable energy sources has grown substantially. Splitting water to produce hydrogen and oxygen is one convenient way for energy conversion.130 Due to their stability and extensive tunability, POMs are attractive as water reduction catalysts for the hydrogen evolution reaction (HER), and most expansively, water oxidation catalysts (WOCs) for the oxygen evolution reaction (OER).131-133 Indeed, POMs have been successfully implemented in the fabrication of overall water splitting systems.134 On the other hand, electrochemical oxygen redox reactions are considered to be the key for energy conversion and storage including fuel cells and metal–air batteries.135 In general, fuel cells devices are based on the electrocatalytic oxidation of a fuel (e.g. hydrogen and methanol) at the anode and the oxygen reduction reaction (ORR) at the cathode. Notably, OER (section 4.1) and ORR (section 4.2) are the two fundamental reactions in fuel cells and energy storage devices. In ORR, strong double bonds have to be broken for the formation of hydroxyl or peroxide ions from oxygen molecule and vice versa is true for OER, both reactions involving four electrons. Hence, they are often described as sluggish compared to their anodic counterparts, namely hydrogen evolution and oxidation which are 33 Energy & Environmental Science Page 34 of 124 two electron processes.136 Therefore, it is necessary that the catalysts used for ORR and OER are View Article Online DOI: 10.1039/D0EE03407J highly active for achieving better performance parameters of the devices, which has stimulated the Energy & Environmental Science Accepted Manuscript development of novel water oxidation and oxygen reduction catalysts over the past years. Furthermore, the fascinating properties of POMs, including tunable acidity and redox properties, have stimulated the research in a wide range of POM-catalyzed applications137 including other catalytic reactions such as CO2 reduction, N2 reduction and biomass conversion (section 4.3). 4.1 Water Oxidation Water oxidation involves the dissociation of protons and electrons from stable water Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. molecules, as well as the formation of an O−O bond from formally separated water molecules. The overall OER process typically occurs via four elementary steps involving different reaction intermediates and the formation of an O–O bond (eventually released as O2). This latter step has been proposed to occur through two primary pathways, the water nucleophilic attack (WNA) and the interaction of two metal-oxo entities (I2M).138 As shown in Figure 11, both reaction paths begin with two proton electron transfer (PET) events, giving rise to a metal-oxo intermediate. The WNA mechanism involves the nucleophilic attack of a solvent water molecule together with a PET resulting in a hydroper-oxo intermediate, which finally evolves molecular oxygen via another PET. On the other hand, the O–O bond formation in the I2M mechanism involves the coupling of two separate metal-oxo moieties, leading to a peroxo species that eventually produces O2 and regenerates the catalyst. In addition to the intrinsic thermodynamic demands of water splitting, kinetic obstacles impose significant activation energies (overpotentials) and require the development of efficient catalysts.129 WOCs composed of all-inorganic POMs have advanced the field of water splitting over the past decade.131 Lacunary POMs, “defect” structures made by removing M=O entities, are good inorganic multidentate ligands to stabilize multi−metal oxide clusters. Thus, the robust structure of lacunary POM ligands have been exploited for the assembly of catalytically active transition metal- oxo clusters. Herein, in the following section we discuss the progress on POM WOCs organized in terms of the transition metal providing the active sites in the catalyst. Comparison of the catalytic 34 Page 35 of 124 Energy & Environmental Science activity of different systems is only possible in the case of uniform experimental conditionsViewfor all Article Online DOI: 10.1039/D0EE03407J water oxidation catalytic species. However, since not all systems can perform under the same Energy & Environmental Science Accepted Manuscript combination of conditions, comparison of different catalysts should be done with caution. As discussed by Hill et al,133,139 there have not been any established agreements on how to compare catalyst’s activity of different systems in the literature. Nevertheless, in the following section catalysts will be often compared in terms of turnover numbers (TON: ratio of product formed per catalyst molecule) and turnover frequencies (TOF: d(TON)/dt) under analogous experimental conditions. 4.1.1 Ruthenium-based POM WOCs Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Ru-based catalysts are interesting because of their ability to act as a powerful oxidant in near- neutral pH conditions. In 2008, Bonchio 140 and Hill 141 groups simultaneously reported [Ru4O4(OH)2(H2O)4(SiW10O36)2]10- (Ru4Si2) as a highly-active WOC using strong oxidants. The structure of Ru4Si2 consists of a tetra-nuclear Ru(IV) aqua-ion, [Ru4(O)4(OH)2(H2O)4]6+, entrapped by the assembly of two kegging-type lacunary [SiW10O36]8- POM units (Figure 12a). The active site of the compound is the [Ru4O4] core with adjacently coordinated aqua ligands, which undergo four consecutive oxidation steps, from RuIV4 to RuV4, followed by nucleophilic attack and water oxidation.142-145 In 2009, Geletii et al. studied Ru4Si2 as an efficient homogeneous WOC at neutral pH for visible-light-driven water oxidation.146 This well-established technique is based on the facile generation of the oxidant Ru(bpy)33+ by photoinduced electron transfer between the photosensitizer (PS) (e.g., Ru(bpy)32+) and an appropriate sacrificial electron acceptor (SA) (e.g., S2O82-). Under the experimental conditions described in Figure 12 b, the Ru4Si2 catalyst showed high TON of up to 180, as well as initial TOF of up to 0.08 s−1 and the overall quantum yield (Φ: moles of generated product/moles of absorbed photons) as high as 4.5%. Later, the congeneric POM catalyst [Ru4(O)5(OH)(H2O)4(PW10O36)2]9− Ru4P2 was synthesized and shown to be a WOC, although it was somewhat less efficient (lower O2 yields under similar photochemical conditions).147 35 Energy & Environmental Science Page 36 of 124 Hole scavenging ability is a key parameter in the electron-transfer (ET) rate between the View Article Online DOI: 10.1039/D0EE03407J 3+ photogenerated oxidant Ru(bpy)3 and the catalyst Ru4Si2, leading to O2 evolution. The catalyst must Energy & Environmental Science Accepted Manuscript be able to scavenge the holes in Ru(bpy)33+ in competition with charge recombination. In an effort to shed light on hole scavenging, Orlandi et al. performed a nanosecond laser flash photolysis investigation on the hole transfer reaction from photogenerated oxidant Ru(bpy)33+ complexes to catalyst Ru4Si2.148 The oxidant species generated by oxidative quenching with a SA in homogeneous solution showed very fast hole scavenging from photogenerated RuIII species to Ru4Si2. However, at high catalyst concentrations, the electrostatic interaction between the PS, Ru(bpy)32+, and Ru4Si2 Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. leads to efficient static quenching of the Ru(bpy)32+ excited state.149 This energy wasting process is detrimental to photocatalytic activity, as it competes appreciably with the primary photoreaction of the excited PS with the SA, even in high ionic strength media. In this context, keeping the catalyst concentration as low as possible can prevent quenching of excited Ru(bpy)32+, however, at the cost of slowing the hole-transfer process. Another strategy to overcome this issue is the use of PSs that favor the productive, reductive quenching of the excited PS by Ru4Si2 rather than the unproductive oxidative quenching. This implies the use of PSs being strong electron acceptors in the excited state, such as a tetranuclear dendrimeric RuII polypyridine complex 150,151 or multi-perylene-bisimide chromophores.152 In the common oxidative quenching pathway in the presence of an SA, oxidized PS is irreversibly produced so that, in principle, fast hole transfer is not a strict requirement. In practice, however, the PSs, in their oxidized form, are often unstable under the reaction conditions used and fast hole scavenging is pivotal to minimize their decomposition (usually the main limiting factor in terms of turnover performance). As electron transfer from catalyst to PS is primarily diffusion controlled, PS degradation can be mitigated on a photoelectrode, where a close contact exists between the two components and transfer of electrons from the catalyst occurs rapidly after oxidation of the PS.148 Several triadic photoanodes composed of nanoporous MO2 (M = Ti, Zr and Sn) films sensitized with different cationic Ru PSs have been reported.153-155 The surface-bound cation electrostatically 36 Page 37 of 124 Energy & Environmental Science interacts with negatively charged Ru4Si2. In the presence of Ru4Si2, the electron transfer fromViewthe PS Article Online DOI: 10.1039/D0EE03407J to MO2 was found to be the main pathway, with ultrafast (nanosecond) electron transfer from Ru4Si2 Energy & Environmental Science Accepted Manuscript to the oxidized PS. The oxidized catalyst is long-lived and thus, it energetically reacts with H2O to produce O2. When using 3-aminoproyltriethoxysilane (APS) in particular, amine groups can be readily displayed on photoelectrode surfaces by its hydrolytic condensation and used to immobilize Ru4Si2 electrostatically, the very high charge of Ru4Si2 (10−) render these catalyst−surface linkages very robust. Hill's group reported that Ru4Si2 and Ru4P2 can be electrostatically adsorbed on TiO2 156 or Fe2O3 157 photoanodes after modification with APS. Under ultraviolet (UV) illumination, Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. photoelectrochemical measurements indicated an enhanced photocurrent compared to un- functionalized photoanodes. Moreover, atomic layer deposition was successfully used to protect the attachment of POM WOCs on the surface of visible-light-absorbing hematite (Fe2O3) photoanodes.157 Instead of immobilization within a host material with a well-defined porous structure, it is also possible to immobilize catalysts by in situ hybridization with polymeric materials on the surface of photoelectrodes. Very recently, Ru4Si2 was co-deposited on WO3 photoanodes with various electropolymerized conducting polymers.158 The photoelectrochemical performance for visible-light- driven water oxidation of WO3 photoanodes can be remarkably improved only when co-depositing polypyrrole (PPy) and Ru4Si2 due to the desired energy level alignment for efficient charge transfer. Electrochemical water oxidation has been demonstrated using Ru4Si2. In 2010, Toma et al. reported the effective anchoring of Ru4Si2 onto a conductive bed of MWCNTs as the conductive substrate to form an active water oxidation electrocatalyst (Figure 13).159 Compared to amorphous carbon functionalized with Ru4Si2, the electrocatalytic activity of Ru4Si2 on the derivatized MWCNTs was significantly higher, presumably due to the enhanced electron transfer events in the MWCNTs. The TOF of 300 h-1 at an overpotential of (η) 0.60 V, exceeds the values reported previously for Co- oxide and Mn-cluster systems, which were in the range 0.7 – 20 h-1 at η ≥ 0.40 V.160,161 Similarly, covalent and noncovalent approaches to functionalize CNTs with cations have been explored.162 37 Energy & Environmental Science Page 38 of 124 An improvement of the electrocatalytic performance of Ru4Si2 is conferred by using graphene View Article Online DOI: 10.1039/D0EE03407J covalently functionalized with organic hydrogen-bonding cations.163 This hybrid material catalyzes Energy & Environmental Science Accepted Manuscript oxygen evolution at a η as low as 0.3 V at neutral pH with negligible loss of performance after 4 h of testing, outperforming the catalytic efficiency of the isolated Ru4Si2 by one order of magnitude, with a two-fold enhancement compared to the nanotube analogs. Based on the observation that graphene without a modified surface can adsorb Ru4Si2 strongly, Ru4Si2 was immobilized electrostatically onto graphene electrochemically deposited on a glassy carbon or indium tin oxide (ITO) electrode.164 The composite material showed excellent catalytic activity and high stability under neutral pH conditions Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. with a moderate η of 0.35 V. Moreover, Ru4Si2 on graphene displayed higher catalytic activity compared to RuO2 on graphene, where the amount of RuO2 would be ~200 times more than the amount possible from complete decomposition of Ru4Si2. Another surface entrapment technique to fabricate catalytic electrodes consists of the employment of a conducting polypyrrole matrix and the layer-by-layer (LbL) technique. Ru4Si2 was successfully immobilized onto polypyrrole-modified glassy carbon and ITO surfaces by alternate deposition of anionic Ru4Si2 and a Ru(II)-metallodendrimer as the cationic moiety.165 The Ru4Si2- based LbL assemblies exhibited enhanced electrocatalytic abilities in pH-7, in comparison to both Ru4Si2 paste electrodes and LbL assemblies composed of the Ru-free parent POM moiety. Apart from Ru4Si2 and its analogous Ru4P2, other Ru-based POM WOCs have also been reported. In 2011, Murakami et al. synthesized and characterized two mononuclear complexes [RuIII(H2O)SiW11O39]5− and [RuIII(H2O)GeW11O39]5− for water oxidation with the strong oxidant (NH4)2[CeIV(NO3)6].166 Maximum TONs of up to 20 and 50 were reported, respectively. These catalysts turned to be less robust than Ru4Si2, despite the exposed and accessible metal center. DFT calculations indicate that the RuIII–H2O center can accommodate a stepwise multi-electron redox processes at the high reduction potential condition of CeIV/CeIII to achieve the active group [RuV=O], and the high valent oxo-RuV species are responsible for the O–O bond formation event.167 The calculated results support that the bridging O atom of the POM ligand will act as the most favorable proton acceptor in the O– 38 Page 39 of 124 Energy & Environmental Science O bond formation, with an energy barrier of 28.43 kcal.mol−1. Car et al. synthesized a triruthenium- View Article Online DOI: 10.1039/D0EE03407J substituted POM, [(Ru3O3(H2O)Cl2)(SiW9O34 )]7-, that features a capping (Ru3) unit connected to a Energy & Environmental Science Accepted Manuscript lacunary [SiW9O34]10− POM shell via Ru–O.168 The maximum TON (23) under photocatalytic conditions was reached after 120 min, with higher TOF (0.7 s−1) than the previous results for Ru– POMs. The WOC activity was derived from the formation of a recyclable POM–photosensitizer complex in the initial stages of the reaction that hinders POM decomposition into RuOx catalysts. 4.1.2 Cobalt-based POM WOCs The current approach for the development of more sustainable catalysis is the use of metals Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. with high crustal abundance. In this context, the discovery of the first Co-based POM WOC represented a great breakthrough in the field given its higher abundance than Ru. For example, reported in 2010 by Yin et al., [Co4(H2O)2(PW9O34)2]10- (Co4P2), a tetra-cobalt complex protected by two [PW9O34]9- ligands, showed water oxidation activity with TONs reaching ~ 103 in under 3 min in the presence of the chemical oxidant [Ru(bpy)3]3+ at pH 8.169 In later studies, Co4P2 activity was noted and studied for light-driven water oxidation using Ru(bpy)32+as the PS and S2O82- as the SA.170 Co4P2 showed remarkably superior photocatalytic performance to that of the Ru4Si2 catalyst,146 with a TOF of ≈5s-1, Φ of 30% and a TON > 220. Co4P2 enwrapped into ordered mesoporous carbon nitride (MCN) exhibited noticeable electrocatalytic activity in water oxidation.171 At 1.4 V vs Ag/AgCl, the TOF reaches up to 0.3 s−1 at pH 7, which is nearly 6 times higher than that with Ru4Si2@MWCNTs (0.055 s−1 at 1.4 V).159 The activity of Co4P2 as a homogeneous WOC is subject to some debate. For instance, Finke’s group first demonstrated that under electrocatalytic conditions, CoII leaching from Co4P2 can deposit onto the anode as cobalt oxide (CoOx) film that rapidly becomes the major catalyst.172 Since then, the integrity of Co4P2 has been carefully examined through a plethora of studies,173-177 leading to the conclusion that the specific conditions such as buffer, pH, and catalyst concentration matter greatly in determining the true WOC. Ultimately, Hill’s group demonstrated that Co4P2 is indeed the main WOC under the homogeneous conditions reported originally.178 The authors described a new protocol 39 Energy & Environmental Science Page 40 of 124 for the extraction of highly negatively charged POM WOCs using heptylammoniumViewnitrate Article Online DOI: 10.1039/D0EE03407J (THpANO3) dissolved in toluene to identify the active species for water oxidation. In the end, Co4P2 Energy & Environmental Science Accepted Manuscript has become a prime example for stability analysis of POM WOC systems and studies aiming to identify the main active species. Indeed, the atomic structures of POMs are highly amendable under different conditions of pH, applied potential and chemical environments, leading to ambiguity when interpreting the catalytic behaviours of OER studies. 179170, 169 Very recently, Folkman et al. reported phosphorous-31 nuclear magnetic resonance (NMR) and cathodic stripping analysis to determine the amount of detectable CoII(aq) in solutions of a series of six exemplary Co-POMs (including Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Co4P2180). Higher pH values and phosphate buffer in general led to higher levels of CoII(aq). However, under pH 5.8 the Co-POMs exhibited higher stability, where the amount of CoII(aq) detected cannot account for the observed electrochemical water oxidation activity. It seems that the specific conditions are important in determining the identity of the dominant WOC. This study exemplifies a valuable methodology for distinguishing between homogeneous and heterogeneous WOCs derived from Co-POMs, even when metal-leaching is present at just micromolar levels. Furthermore, under the umbrella of WOC optimization, advanced in situ/operando analytical techniques (such as X-ray absorption spectroscopy, XPS, X-ray scattering techniques among others) are being developed as increasingly powerful tools to elucidate the controversial discussions about the molecular or nanoscale nature of many POM WOCs. More and more of these approaches also enable the monitoring of possible key intermediates as an essential prerequisite for proposing catalytic mechanisms. An overview of emerging in situ/operando approaches as new strategies for monitoring WOC transformations has been published recently.181 Recent advances in heterogenizing POMs on electrode surfaces provide protocols and criteria to assess the stability, rates, and molecular nature of immobilized WOCs. For example, MOFs have been used to encapsulate Co4P2 displaying enhanced photocatalytic and electrocatalytic water oxidation activity as well as improved stability facilitated by the electrostatic interaction with the MOF.182 Recently, Co4P2 has been immobilized in the hexagonal channels of a porphyrinic MOF 40 Page 41 of 124 Energy & Environmental Science (MOF-545) hybrid framework (Figure 14a),183 with the catalyst and the PS (porphyrin linker)Viewwithin Article Online DOI: 10.1039/D0EE03407J the same porous solid material. As shown in Figure 14b, O2 was formed immediately upon exposure Energy & Environmental Science Accepted Manuscript to light and increased linearly with time, exhibiting high photocatalytic activity (TOF = 40 × 10−3 s−1 for the first 15 min and TON of 70), good stability and easy recyclability. It is also possible to immobilize Co4P2 by in situ hybridization with polymeric materials on the surface of photoelectrodes. Recently, a synthetic melanin thin film incorporating Co4P2 was readily deposited on the surface of various n-type inorganic semiconductors (e.g., Fe2O3, BiVO4, and TiO2) by electropolymerization.184 The formation of a melanin-based p–n heterojunction structure Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. results in an increased photocurrent density and decreased onset potential for water oxidation when compared to the bare semiconductor photoanodes. In a different approach, electrostatic, non-covalent immobilization strategies have been employed for the immobilization of Co4P2. Photoanodes for solar water oxidation have been fabricated by depositing a thin film of diverse cationic polyelectrolytes and anionic Co4P2 on the surface of various photoelectrode materials using the layer-by-layer (LbL) assembly technique.185 It was found that the performance of photoanodes was significantly improved after the deposition in terms of stability as well as photocatalytic properties, regardless of types of photoelectrodes and polyelectrolytes employed. Very recently, the Ag+ salt of Co4P2 was exploited as a hole extraction layer on the surface of BiVO4 semiconductor photoanode for boosting photoelectrochemical activity by promoting hole extraction and transfer.186 The BiVO4/Co4P2 composite exhibited a pronounced water oxidation photocurrent density of 3.06 mA.cm-2 at 1.23 V vs. RHE under (compared to the pure BiVO4 0.93 mA.cm-2), achieving a 3.3-fold improvement in photocurrent density. Furthermore, after the BiVO4/Co4P2 photoanode was further treated by UV light irradiation, a photocurrent enhancement was achieved, bringing the photocurrent density to 4.03 mA.cm-2, showing great potential for application in photo-electrocatalytic water oxidation. For P- and Si- heteroatoms, Ru4Si2 displayed higher water oxidation activity than Ru4P2 under photochemical conditions. For that reason, Zhu et al. attempted to synthesize the Si-centered analog of Co4P2, but unexpectedly obtained the tetra-Co-sandwiched POM [Co4(OH)(H2O)3](Si2W19O70)]11− 41 Energy & Environmental Science Page 42 of 124 as a 1:1 mixture of two isomers (Co4Si2).187 Although the Co4Si2 isomers are not analogous toView CoArticle 4P2Online , DOI: 10.1039/D0EE03407J a comparative study under the conditions used in Co4P2 studies 170 showed that Co4Si2 is less efficient Energy & Environmental Science Accepted Manuscript (lower maximum TON) and undergoes slow hydrolysis to more inactive forms during water oxidation. Following this work, Car et al. reported the analog compound to Co4P2, [Co4(H2O)2(SiW9O34)2]12−2, and investigated its performance as visible-light-driven WOC at pH 5.8.168 The Si-analog is an active WOC, although much less efficient than the original Co4P2. In 2014, Lv et al. synthesized a new vanadate (V)-centered POM [Co4(H2O)2(VW9O34)2]10− (Co4V2), which is isostructural to Co4P2.188 Under photo-induced water oxidation, Co4V2 performs exceptionally fast Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. (up to ≈103 s-1 TOF) with final O2 yields of ~68%, twice as high as that of Co4P2. The stability of Co4V2 has also been subject to debate.189,19051,52 Recent vanadium NMR solution studies on the NMR/CoIIaq/CoOx system describe a complex speciation behavior in solution, making the identification of the various equilibrating aqueous active catalysts in the system is a challenging task.191 Despite several advances, the large catalytic difference between Co4V2 and Co4P2 called for examining the significance of substituting phosphorus centers in Co4P2 with vanadium. A thorough DFT study on the OER mechanism for Co4P2 suggested that the O–O bond formation takes place from a Co3+-oxyl radical through the WNA attack pathway.192 The computation of the reaction mechanism with Co4V2 showed a higher oxidation potential for the first proton coupled electron transfer (PCET) event and a reduction in the activation energy for the WNA pathway. The authors ascribed both effects to the decrease in the molecular orbital energies due to the orbital coupling between vanadium and cobalt atoms. In 2018, evidence for the significance of the vanadium centers to catalysis, predicted by theory, was found from soft X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS).193 The results revealed that Co4V2 possesses a smaller ligand field within the tetra-cobalt core and a cobalt-to-vanadium charge transfer band, supporting previous theoretical predictions. Interestingly, Soriano-López et al. recently reported that it is possible to dope molybdenum into the Co4P2 structure synthesizing a mixed Mo and W (Co4MoxWy) analog.194 Doping the W framework of Co4P2 with Mo leads to the electrochemical oxidation of water at a low 42 Page 43 of 124 Energy & Environmental Science onset potential, and with no catalyst degradation while reducing the overpotential of the View oxygen Article Online DOI: 10.1039/D0EE03407J evolution (ηO2) reaction by 188 mV compared with the pure tungsten framework. Energy & Environmental Science Accepted Manuscript Goberna-Ferrón et al. discovered the catalytic activity of the high-nuclearity Co-based POM [Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]16− (Co9).195 Its molecular structure (Inset of Figure 15 (a)) contains three triads of edge-sharing CoIIO6 octahedra supported by three tri-vacant Keggin-type [PW9O34]9− units. The triads are interconnected through three OH− and two HPO42− bridges. Six of the Co ions complete their coordination sphere with terminal water molecules. Co9 is as an effective WOC using NaClO as the oxidant; when the ratio of catalyst to NaClO is 1:100, the reaction rate with Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Co9 is at least 8 times faster than that with Co4P2 at pH 8. The catalytic process can be maintained for several days without fatigue or decomposition. The homogeneous character of Co9 during the oxygen evolution reaction has been well established from postreaction characterization. In addition, small- angle X-ray scattering experiments were carried out to investigate, in detail, the speciation under catalytic working conditions, confirming the remarkable stability of Co9 in water oxidation conditions.196 As an electrocatalyst in homogeneous conditions, decomposition reactions that yield a thin CoOx film on the anode can be easily avoided by the addition of chelating agents. After the reaction cycles, Co9 can be recovered from solution by recrystallization with additional alkali-metal cations or by slow evaporation of the reaction medium. The Cs+ insoluble salt of Co9 can be blended into a carbon paste electrode to yield a robust catalytic process in the pH range from 1 − 8.197 Co9- modified electrodes exhibited exceptional stability during bulk water electrolysis, where the current remains constant for at least 8 h. Moreover, Cs-Co9 electrodes are more active than Co3O4 electrodes under the same conditions, yielding 2 orders of magnitude larger current densities for equimolar amounts of Co content (Figure 15 a). The most remarkable feature of Cs-Co9 is its stability at a low pH values (<1.5), where CoOx is unstable. This is important considering that acidic solutions appear to be preferred as media to carry out water electrolysis since hydrogen production is much easier at lower pH, where the proton (as reagent) concentration is higher. In this context, the Ba+ salt of Co9 showed a significantly better electrocatalytic performance in 1M H2SO4 than the Cs+ salt, with onset 43 Energy & Environmental Science Page 44 of 124 potentials as low as η = 88 mV (vs. 196 mV) and higher current densities due to better electron View transfer Article Online DOI: 10.1039/D0EE03407J kinetics.198 Moreover, the Ba-Co9 outperforms the state-of-the-art IrO2 catalyst even at pH < 1, with Energy & Environmental Science Accepted Manuscript an ηO2 of 189 mV at 1 mA.cm−2. Notably, the authors observed an enhancement of the catalytic stability in acidic medium by using a carbon-paste conducting support with a hydrocarbon binder, which provides a hydrophobic environment. Very recently, the same authors reported a complete series of catalytic studies with multiple Co-based POMs, including Co9, following the same working electrode strategy.199 Composition-structure-activity relationships indicated that i) only those species containing P(V) as heteroatoms appeared to be stable; ii) the Keggin-derivatives exhibit faster Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. performance than their Dawson-Wells counterparts; and iii) among them, catalytic activity is favored by high Co nuclearity and overall total charge. Since carbon paste is limited by its poor mechanical and redox stability, the authors recently turned their interest in conducting organic polymers, incorporating the Co9 into a polypyrrole matrix to obtain organic-inorganic hybrid anodes.200 These electrodes sustain robust electrocatalytic water oxidation in neutral media, offering very good processability. It is remarkable that less than 6% POM content in a polypyrrole film multiplies OER currents by one order of magnitude, reducing the required overpotentials by approximately 200 mV. During these experiments, however, roughly 65% efficiency towards O2 evolution was found, suggesting the occurrence of competing oxidations. It is also noteworthy that a Ba-Co9/lead composite has been investigated recently as anode for OER during zinc electrowinning.201 The OER is the most common anode or counter reaction in the electrowinning of metals from acidic electrolytes. Zn electrowinning consumes a lot of electrical energy due to the high ηO2, so even a modest decrease in the ηO2 would be beneficial for industry to save energy. The ηO2 of the Ba- Co9/lead composite decreased by about 137 and 40 mV compared to pure Pb foil and Pb-0.75Ag alloy (commonly used in the hydrometallurgical industry), respectively. Therefore, the Ba-Co9/Pb composite could be a suitable substitute additive for Ag, with a strong potential industrial application. Recently, the Cs+ salt of Co9 was tested as heterogeneous oxygen evolution catalyst in light-induced experiments, using Ru(bpy)32+as the PS and S2O82- as the SA in neutral pH.202 Post-catalytic assays 44 Page 45 of 124 Energy & Environmental Science indicated that the Cs-Co9 is transformed into the corresponding [Ru(bpy)3]2+ salt upon cesium loss. View Article Online DOI: 10.1039/D0EE03407J Remarkably, analogous photoactivated oxygen evolution experiments starting with a [Ru(bpy)3]2+ Energy & Environmental Science Accepted Manuscript salt of Co9 demonstrated much higher efficiency and kinetics, with maximum TON and TOF of 27.3 and 19.1 h−1 respectively (14.2 and 10.8 h−1 for Cs-Co9), affording chemical yields close to 50% (10% for Cs-Co9). The origin of this improved performance is the closer cation-anion (PS/catalyst) interaction in the solid state. This is beneficial for the electron transfer event, and for the long-term stability of the PS. Han et al. reported a series of high-nuclearity Co-based POM WOCs, ([Co4(OH)3(PO4)]4(XW9O34)4)n− (X = Si, Ge, P, As; n= 32, 28) (Co16X4).203 These Co-POMs contain Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. a high-nuclearity Co–Pi cluster (Co16(PO4)4(OH)12) encapsulated by four [XW9O34]n− units. The (Co4(PO4)4(OH)12) core contains a Co4O4 cubane structurally analogous to the [Mn3CaO4] core of the oxygen-evolving complex (OEC) in photosystem II (PSII). The trend in photocatalytic activity was Co16Ge4 > Co16As4 > Co16Si4 ≥ Co16P4. The values for Co16Ge4 were 31% O2 yield, TON = 38.75 and initial TOF in the first 300 s of 0.105 s−1. Furthermore, the structural integrity of the POMs under photocatalytic conditions was confirmed by recrystallization, laser flash photolysis, dynamic light scattering (DLS), phosphorous-31 NMR, UV−vis, POM extraction, and inductively coupled plasma mass spectrometry (ICP-MS). Recently, the photocatalytic activity towards water oxidation of Co16P4 has been compared to that of Co9 and another Co-based POM, [Co6(H2O)30(Co9Cl2(OH)3 (H2O)9(SiW8O31)3)]5− (Co15).204 The WOC performance followed the trend Co15 ∼ Co16 > Co9, in agreement with flash photolysis results, indicating that fast multi-hole accumulation is a prelude to fast oxygen evolution under the conditions explored. The resident state of the Co-POMs after O2 evolution displays XANES spectra that are consistent with a photo-induced oxidation of the cobalt centers while retaining the POM features, thus ruling out structural leaching of CoII ions and evolution to CoOx colloids. Another high-nuclearity Co-based WOC that resembles the OEC in PSII is the double CoII4O3 core POM [(A-α-SiW9O34)2Co8(OH)6(H2O)2(CO3)3]16-.205 This octo-nuclear cobalt complex contains two cubical CoII4O3 cores, with two of these Co atoms coordinated to terminal aquo ligands. The two 45 Energy & Environmental Science Page 46 of 124 CoII4O3 cores are located in two trivacant [SiW9O34]10- ligands and are connected by three CO32-Online View Article DOI: 10.1039/D0EE03407J anions. This catalyst exhibited excellent photochemical water oxidation activity, reaching a TON of Energy & Environmental Science Accepted Manuscript 1436, currently the highest amongst bioinspired catalysts with a cubical core, and a high initial TOF of 10 s−1. Zhang et al. combined the well-established bpy ligand and Co metal ion with stable Keggin- type Mo-POMs to design a robust organic-inorganic hybrid molecular WOC, ([(CoIIbpy)2(PMo8VIMo4VO40)]3[(CoIIbpy)(PMo8VIMo4VO40)])8-.206 This compound was shown to be an efficient visible light-driven catalyst for water oxidation. Its stability was demonstrated by DLS, POM extraction, UV-vis and FT-IR. Moreover, control experiments demonstrated that POM anions Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. can enhance the WOC activity of bipyridine-substituted cobalt complexes. Chen et al. synthetized the first POM with a cobalt–arsenic core, [(CoII7AsIII6O9(OH)6)(A-α-SiW9O34)2] 12-, which contains two [SiW9O34]10- fragments encapsulating a hepta-nuclear cobalt-arsenic (CoII7AsIII6O9(OH)6) core that show a ‘‘fused’’ double-quasi-cubane-type topology.207 The unique CoII7O6 core provides another example of bio-mimetic structural model that catalyzes water oxidation. Very recently, Gong et al. reported a new sandwich tungstobismuthate WOC, (Hdamp)2[(Na4(H2O)9Co2(H2O)6WO3)2(BiW8O30)2] (dmap = N-(4-pyridyl)dimethylamine), which is an efficient catalyst for water oxidation under visible light irradiation at pH 9 (TON of 205.5 and O2 yield of 32.88%).208 The basic structural unit can be described as two (BiW8O30)9- POM units sandwiched by an unusual 14-nucleus mixed-metal cluster (Na4(H2O)9Co2(H2O)6WO3)2. Interestingly, the sandwich units themselves are directly interconnected to form independent chains and layers, forming extended 2D layers linked to each other by H-bonds and supramolecular interactions between the 4-dimethylaminopyridine (dmap) groups and surface oxygen atoms of the POM units. The stability of this catalyst was confirmed through UV−vis, DLS, catalyst aging, and POM toluene extraction experiments. It is important to note that high cobalt nuclearity is not a requirement for POM-WOC design. In 2012, Tanaka et al. reported two POMs containing mono- and di-CoIII catalyst cores, [CoMo6O24H6]3- (Co-Mo) and [Co2Mo10O38H4]6-(Co2Mo), as the first examples of low nuclearity Co- 46 Page 47 of 124 Energy & Environmental Science based POM WOCs.209 Photo-induced water oxidation at pH=8 revealed that both compoundsView possess Article Online DOI: 10.1039/D0EE03407J higher activity than Co4P2 under the same experimental conditions, thus demonstrating that neither a Energy & Environmental Science Accepted Manuscript critical POM size nor multi-site metal cores are mandatory for catalytic activity. The POM charges, normalized to the number of Co centers, are similar to each other (−3, −3, and −2.5 for Co-Mo, Co2- Mo and Co4P2, respectively). Such an electronic factor may affect the O2 evolving activity of cobalt centers. Moreover, the authors demonstrate that Mo-based POMs instead of W-based POMs can also serve as catalysts for O2 evolution. Evangelisti et al. investigated sandwich-type [(Co(H2O)3)2(CoBi2W19O66(OH)4)]10−, with two identical lacunary (BiW9O33) units connected via Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. two disordered (WO2|CoO2) internal groups and two terminal (Co(H2O)3) moieties forming a tetranuclear metal core.210 This POM exhibited high photocatalytic activity with maximum TON of 21 and 97% oxygen yield. However, the isostructural [Co2.5(H2O)6(Bi2W19.5O66(OH)4)]8−, that displays the same disorder pattern on the internal sites together with a 3:1 Co:W ratio on the terminal core positions, turned out to be inactive for water oxidation. The subtle difference in the Co occupancy of the terminal positions had a decisive effect on WOC activity, suggesting that only terminal core sites are essential for WOC activity and, therefore, that high nuclearity core clusters are not essential for the catalytic activity. A mono-nuclear CoII-substituted POM, the Keggin [CoIIICoII(H2O)W11O39]7- , was investigated for visible-light driven water oxidation.211 The authors proposed that the CoIIIO4 tetrahedra, located in the center of the Keggin polyanion, plays an important role in maintaining the integrity of the structure, while the peripheral CoIIO6 octahedral cluster is the active site for water oxidation. Notably, this Keggin POM catalyst was later applied to efficient hydrogen evolution under visible light irradiation.212 Another mono-nuclear CoII-substituted POM, [Co(H2O)2(γ-SiW10O35)2]10-, was investigated as a light-driven WOC acquiring high TON of 313, a TOF of 3.2 s-1, and Φ of 27% for O2 evolution at pH 9.0.213 Structural analogs incorporating Ni and Mn resulted in little water oxidation activity, suggesting that the Co center is the active site. In addition, the catalytic activity was better than that of the CoIIICoII Keggin POM discussed previously and comparable to that of Co4P2 under the same conditions. The hydrolytic stability of this mono-nuclear Co-POM was 47 Energy & Environmental Science Page 48 of 124 confirmed by UV/Vis, linear sweep voltammetry measurements of the catalyst and by cathodic View Article Online DOI: 10.1039/D0EE03407J adsorption stripping analysis. Energy & Environmental Science Accepted Manuscript 4.1.3 Manganese-based POM WOCs In natural photosynthesis, the WOC is a tetranuclear manganese cluster (Mn3CaO4) stabilized by amino acids, water molecules and counter ions. However, there are only a few examples of Mn- POMs reported. This lack Mn examples is not exclusive to POMs,214 and even if one considers heterogeneous catalysts, Mn oxide is much less active than the corresponding Co or Ni analogs.215 Still, the use of Mn is attractive because it is earth-abundant and relatively nontoxic when compared Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. with the other more active metal-based catalysts. Al-Oweini et al. reported the first manganese tungstate WOC, [MnIII3MnIVO3(CH3COO)3(SiW9O34)]6, a mixed-valent (Mn4) core, which is stabilized by a hybrid set of ligands, including an inorganic POM platform and carboxylate bridges.216 The interplay of organic and inorganic residues provides a coordination environment with both stability and flexibility to assist stepwise one-electron oxidation of the catalytic core and to access high-valent Mn states that are responsible for water oxidation. Photo-induced water oxidation at pH 5 exhibited a TOF per Mn center of 0.71 × 10−3 s−1, TON up to 5.2, and a Φ of 1.7%. Schwarz et al. designed a Mn-based POM, [Mn4V4O17(OAc)3]3-, based on a [Mn4O4]6+ cubane core stabilized by a [V4O13]6- ligand, that interacts with three Mn centers of the cubane, and three acetate ligands.217 Under visible-light irradiation, the cluster shows high WOC activity (TON ˜ 1150, TOF ˜ 1.75 s-1). Furthermore, the cluster stability and reactivity were checked by electrochemical, mass spectrometric and spectroscopic studies. Recently, the photochemical and electrochemical water oxidation activity of the sandwiched dimer [Mn3(H2O)3(SbW9O33)2]12− was reported.218 The authors demonstrated that heteroatoms in the POM play important role for the water oxidation; compared to its [PW9O34]9− and [AsW9O33]9− analogs, the Mn3-POM based on a Sb heteroatom has the best water oxidation activity. Multiple experiments including UV−vis, catalyst aging, POM extraction, electrochemical and capillary electrophoretic experiments confirmed that the catalyst is a homogeneous molecular catalyst. The 48 Page 49 of 124 Energy & Environmental Science search for Mn-POMs which could exhibit WOC activity called for examining the Mn-analog of the View Article Online DOI: 10.1039/D0EE03407J well-studied Co4P2, Mn4(H2O)2(PW9O34)2 ]10−.219 Electrochemical water oxidation experiments Energy & Environmental Science Accepted Manuscript showed initial activities comparable to those of the Co counterpart, although current density decreased very rapidly to become negligible just after 30 min, with the appearance of an inactive MnOx layer on the electrode. Another high nuclearity Mn-POM, the mixed-valent [MnIII10MnII6O6(OH)6(PO4)4(A-a-SiW9O34)4]28- polyanion supported on graphene (Mn16@graphene),220 was tested by using rotating ring-disk voltammetry (Figure 15 b) showing comparable TOF values to the Ru4Si2-graphene system164 at neutral pH. As shown in the inset of Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 15 (b), the structure of this Mn-POM polyanion comprises a central (MnII4O4) cubane, capped by 4(Mn3(SiW9)) Keggin units. The overall assembly is connected to each other in a tetrahedral fashion by four phosphate (PO4) linkers. This Mn-POM has been deposited on an ITO-coated glass electrode and on a glass carbon electrode by a layer-by-layer assembly method for electrocatalytic water oxidation.221 Very recently, Teillout et al. synthetized and characterized the Mn-POM [(Mn(H2O)3)2(H2W12O42)]6−, which is an extended 2-D network consisting of [H2W12O42]10− moieties linked together by Mn2+, K+ and Na+ ions.222 This complex undergoes multiple electron-transfer processes that lead to the electro-generation of manganese high-oxidation-state species that catalyze water oxidation. 4.1.4 Nickel-based POM WOCs The first Ni-containing POM WOC, [Ni5(OH)6(OH2)3(SiW9O33)2]12−(Ni5Si2) was reported by Zhu et al. in 2012.223 This Ni-POM was constructed from an open Wells–Dawson [Si2W18O66]16− fragment (built up from two [SiW9O34]10- keggin-type subunits) embedded by a large [Ni5(OH)6(OH2)3]4+ unit in the central cavity. Ni5Si2 exhibited efficient chemical and photochemical water oxidation activity. Moreover, light-driven water oxidation revealed the formation of a Ni5Si2– PS complex which exists in the form of NPs that remain molecular and do not decompose to nickel hydroxide particles, and which are more active for water oxidation than freely diffusing Ni5Si2. Three polynuclear nickel clusters, [Ni12(OH)9(CO3)3(PO4)(SiW9O34)3]24−, 49 Energy & Environmental Science Page 50 of 124 [Ni13(H2O)3(OH)9(PO4)4(SiW9O34)3]25−; and [Ni25(H2O)2OH)18(CO3)2(PO4)6(SiW9O34)6]50−View Article wereOnline DOI: 10.1039/D0EE03407J reported as structurally stable WOCs under photochemical conditions (Figure 16).224 All three Energy & Environmental Science Accepted Manuscript compounds contain (Ni3O3) quasi-cubane or (Ni4O4) cubane units, which are similar to the [Mn3CaO4] core of the OEC in PSII. The trend in efficiency was Ni12 < Ni13 < Ni25 (TON = 204.5), suggesting activity could be related to the Ni nuclearity. Following this work, Yu et al. reported the photocatalytic activity towards water oxidation of a tetra-Ni-based POM, [(SiNi2W10O36(OH)2(H2O))4]24−.225 This Ni-POM can be seen as two lacunary [SiW10O37]10− Keggin units where each unit accommodates two Ni2+ ions linked together by two Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. bridging -OH groups. The cluster shows higher WOC activity than the previously reported Ni-POMs, according to the O2 yield (27.2%), high TON (335) and TOF of 1.7 s−1. Singh et al. recently reported a hybrid POM-supported NiII coordination complex, [(NiII(bpy)2(H2O))(HCoIIWVI12O40)]23-, containing the Keggin anion (H(CoIIW12O40)5− as the inorganic unit and 2,2′-bpy as the organic unit linked together with a NiII metal ion that acts as the catalytic center.226 This catalyst was found to be highly stable and robust under electrochemical water oxidation conditions at pH 7, exhibiting a moderate η value of 475.6 mV with a high TOF of 18.49 s−1. However, the crystal structure of the compound was not investigated completely and hence more studies need to be conducted in order to tune the structure for real world applications. 4.1.5 Other POM WOCs Several other POMs WOC have also been investigated. For instance, Cao et al. reported the first Ir-POM WOC, [(IrCl4)KP2W20O72]14−.227 Its water oxidation activity was determined at pH 7.2 using [Ru(bpy)3]3+ via electrochemical methods. Stability studies of the catalyst indicated a dissociation in aqueous solution into the [IrCl4(H2O)2]− unit and the [KP2W20O72]13− ligand (based on 2 PW9O34]9- linked by two [WVIO2]2+) However, the oxidation of water proceeds two orders of magnitude faster than the hydrolytic decomposition. Further water oxidation studies of the dissociation products indicated that both [IrCl4(H2O)2]− and IrCl3 show similar activities, while the ligand [KP2W20O72]13− is inactive to water oxidation, implying that the intact catalyst could well 50 Page 51 of 124 Energy & Environmental Science catalyze water oxidation. However, this study was inconclusive regarding the formation of View Article Online DOI: 10.1039/D0EE03407J catalytically active IrO2 NPs. Interestingly, a heteropolyoxoniobate compound, Energy & Environmental Science Accepted Manuscript [(Nb2(O)2(H2O)2)(SiNb12O40)]10−, was reported as a water oxidation photocatalyst.228 The structure is composed of Nb-Keggin clusters linked into infinite 1D chains by dinuclear (Nb2(O)2(H2O)2) moieties that contain a water molecule directly coordinated to each of the two bridging Nb active sites. Initial studies also demonstrated that this Nb−POM can be combined with a hydrogen-evolving catalyst (NiO), forming a heterogeneous system for the splitting of water into oxygen and hydrogen, albeit at relatively low TON. Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Three new Ru dye-sensitized Mo-POM–[Ru(phen)3] (phen = tris(1,10- phenanthroline)) hybrids were reported as active WOCs under photocatalytic conditions. [Ru(phen)3CH3OH][Mo6O19], [Ru(phen)3][(C2H8N2)2(C3H7NO)][Mo5S2O23], and [Ru(phen)3CH3CN]2[Mo8O26] were made of anionic Mo-POM clusters, charge-balancing [Ru(phen)3]2+ PS unit and solvent molecules.229 The trend in catalytic efficiency was determined to increase with increasing Mo active sites. Impressively, the AgI-substituted Keggin POM [H3AgIPW11O39]3- can be oxidized by S2O82− in aqueous solutions to generate [H3AgII(H2O)PW11O39]2- and a small amount of [H3AgIIIOPW11O39]3−, simultaneously evolving O2 with higher efficiency than simple Ag salts.230 This work illustrated that the [PW11O39]7− ligand plays important roles in the transmission of both electrons and protons, and in the improvement of the redox performance of silver ions. In 2019, another Ag-based POM was reported by Chaojie Ciu et al.,231 H17[Ag9(AgH8W11O44)2(bpy)9], where [AgH8W11O44]13- polyoxoanions are connected by 1D chains formed by Ag(I) cations ligated by bpy ligands to produce a 3D Ag(I)-containing POM-based MOF. This hybrid material exhibited photocatalytic O2 production with an average rate of 964 µmol.g-1 in the first 10 minutes. A POM WOC containing only vanadium atoms as metal centers, [(VIV5VV1)O7(OCH3)12]−, was reported in 2014.232 The mixed-valence cluster is formally derived from the Lindqvist structure [M9O19]n− substituting the 12 μ-bridged oxo ligands by methoxo ligands. Under photoinduced 51 Energy & Environmental Science Page 52 of 124 conditions, the Φ of 40% was comparable with the values obtained using one of the most View studied Article Online DOI: 10.1039/D0EE03407J CoIII) cubane-type species 233 in similar conditions. The first Fe-POM WOC reported, Energy & Environmental Science Accepted Manuscript [Fe11(H2O)14(OH)2(W3O10)2(SbW9O33)6]27- (Fe11) 234,235 features six tri-lacunary Keggin [SbW9O33]9- ligands gathered together by a (Fe11(H2O)14(OH)2(W3O10)2)27+ core. Under photochemical water oxidation, the TON reached 1815 with an initial TOF of 6.3 s-1. Recently, Hu et al. implemented this catalyst into a three-component photocatalytic water oxidation system with BiVO4 as light-harvesting material and NaIO3 as SA. The system produced an O2 yield of 19.1% sustaining initial TOF of 173 h-1 with a Φ of 25.3% in acidic medium.236 More recently, a well-known Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. giant Mo72Fe30 POM cluster, Mo72Fe30O252(CH3COO)12[Mo2O7(H2O)]2[H2Mo2O8(H2O)](H2O)91, has been explored as an efficient and stable WOC in the presence of light at pH 9 exhibiting TON of 13.99 with TOF of 3 min-1.237 In a different strategy, Fe(III)-substituted anions, [PW11O39Fe]4––O–, were reported as oxo-donor ligands for hematite (α-Fe2O3) cores (Figure 17 (a)), giving stable, water- soluble catalysts for water oxidation using visible light. The authors identified a new mechanism of freely diffusing fragments of solid-state metal oxide taking part in multi-electron oxidation of water.238,239 Yu et al. reported a Cu-POM WOC, the sandwich-type [Cu5(OH)4(H2O)2(SiW9O33)2]10-, which was able to catalyze photochemical water oxidation with an O2 yield of 12.3% and a TON of 61.240 Very recently, another sandwich-type Cu-POM cluster, [Cu3(H2O)3(SbW9O33)2]12- was shown to catalyze water oxidation at a neutral pH without decomposition under homogeneous electrochemical conditions.241 Interestingly, [Cu5(OH)4(H2O)2(SiW9O33)2]10- showed negligible activity under the same conditions. The molecular nature of POMs represent a unique opportunity for the development of WOCs. Furthermore, investigations have shown that the stability of the catalysts must be taken into consideration, studying the speciation and the mechanism of catalysis with POMs is essential for garnering fundamentally useful insights into catalysis by POM-based systems. 52 Page 53 of 124 Energy & Environmental Science 4.2 Oxygen Reduction View Article Online DOI: 10.1039/D0EE03407J The reduction of molecular oxygen occurs inside fuel cells and metal-air batteries. The ORR Energy & Environmental Science Accepted Manuscript follows very slow kinetics and has a very high overpotential. Therefore, significant research effort is devoted to improving this reaction. The reduction of molecular oxygen can occur through two pathways. First is the four-electron pathway, leading to the formation of water and second is the two- electron pathway in which the product is hydrogen peroxide (H2O2). POMs are normally used as active centers for a reduction supported over a conducting substrate to transport the electrons and accelerate the reaction kinetics. Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. 4.2.1 POMs as Catalysts for ORR In recent years, many POM compounds incorporating transition metal clusters have been shown to serve as robust catalysts, especially in the water oxidation field (section 4.1). However, relatively few reports have been devoted to the ORR catalytic activity of transition metal-substituted POMs. In 2015, Rousseau et al. reported a nanocomposite based on the heterometallic complex [((PW9O34)Co3(OH)(H2O)2[O3PC(O)(C9H13N2[Zn(H2O)3])PO3])2Co]12− (Co7(AlePyZn)2) (Inset of Figure 17 (b)) and cost-effective VC XC-72 as ORR electrocatalyst.242 This heterometallic POM complex contains a heptanuclear CoII core sandwiched by two [PW9O34]9− units and connected to two [O3PC(O)(C3H6NHCH2(C5NH4))PO3]5− (AlePy) ligands linked to two ZnII cations. Both ZnII ions complete their coordination spheres with one oxygen atom of the POM and three terminal water molecules. The Co7(AlePyZn)/C composite showed high activity for the reduction of O2 (Figure 17 (b)), better tolerance to methanol crossover, which is important for practical applications of direct methanol fuel cells (DMFCs), and superior durability compared to commercial Pt/C. Recently, the same group reported a series of POM-based composites as efficient cathode materials for the ORR, in neutral or basic media, via a four-electron mechanism with high durability.243 Similarly to Co7(AlePyZn)2, the reported POM structures consist of a M7 (M = Co or Ni) core encapsulated between two [PW9O34]9− units and structured by two alendronate (Ale, (H2O3PC(OH)(C3H6NH2)PO3)2-) ligands. The activity of this POM systems was optimized by the 53 Energy & Environmental Science Page 54 of 124 judicious choice of 3d metals incorporated in the ((PW9)2M7) (M = Co, Ni) POM core, the POM View Article Online DOI: 10.1039/D0EE03407J counter ions and the support (thermalized triazine-based frameworks (TTFs), fluorine-doped TTF Energy & Environmental Science Accepted Manuscript (TTF-F), rGO, or carbon VC XC-72). In particular, the overpotential required to drive the ORR for the composite associating the ((PW9)2Ni7) POM, [Cu(ethylenediamine)2]2+ counter-cations, and the highly conductive and ORR active TTF-F carbonaceous material is very close to that of commercial Pt/C. The good ORR electrocatalytic activity can be attributed to two synergistic effects due to the binary combination of the Cu and Ni centers and the strong interaction between the POM molecules and the porous and highly conducting TTF-F framework. However, the mechanisms behind these Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. synergies are still unclear and require further studies. The well-known WOC [Co4(H2O)2(α-PW9O34)2]10- (Co4P2) was tested as an ORR electrocatalyst in 2018.244 Co4P2 was immobilized onto different carbon nanomaterials (SWCNT, graphene flakes (GF), N-doped CNTs, and N-doped few layer graphene (N-FLG)). In alkaline media, all nanocomposites exhibited superior overall electrocatalytic activity when compared to the corresponding carbon materials with onset potentials (0.77−0.90 V vs RHE) comparable to commercial Pt/C (0.91 V vs RHE). In particular, Co4P2@N-CNT presented greater diffusion-limiting current density (1.8 times) than Pt/C. Moreover, these nanocomposites showed excellent tolerance to methanol with no significant changes in current density, in contrast to Pt/C (decrease of ≈59% after methanol addition), and good long-term electrochemical stability with current retentions between 75 and 84%. The integrity of the Co4P2 POM structure after the long-term stability tests was confirmed by XPS analysis. Simple bare POM structures have also been reported as ORR electrocatalysts when combined with carbon-based materials. In 2014, a negatively charged Pressler-type POM, [NaP5W30O110]14-, GO and poly(ethyleneimine) (PEI) were constructed into (PEI/POM−rGO)n nanocomposite multilayer with protonated PEI via LbL self-assembly route combined with post-photoreduction method.245 The results obtained proved that the POM played a main role in ORR electrocatalysis while rGO promoted the advance of the onset potential, Eonset, for ORR to ∼80 mV. Nunes et al. 54 Page 55 of 124 Energy & Environmental Science prepared various nanocomposites based on vanadium-substituted phosphomolybdate, [PMo11View 4− VOArticle 40] Online DOI: 10.1039/D0EE03407J (PMo11V), incorporated into different carbon-based materials.246 All nanocomposites showed ORR Energy & Environmental Science Accepted Manuscript electrocatalytic activity in acidic medium, with a strong dependency between the electrocatalytic ORR performance and the carbon material used as support. The Eonset shifted to less negative values in the order PMo11V@carbon-black < PMo11V@SWCNT < PMo11V@graphene; for PMo11V@graphene, the most promising composite, it was measured at Eonset = 0.18 V vs. RHE. Recently, a lacunary Keggin POM, [PW11O39]7-, was covalently linked with 3D ordered macroporous graphitic carbon nitride (3DOM g-C3N4) through an organic linker strategy.247 This Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. covalent combination selectively promoted the two-electron O2 reduction due to the enhanced O2 adsorption of 3DOM g-C3N4-POM compared to its counterparts. The performance of the described POM ORR catalysts is summarized in Table 2. 4.2.2 POMs as Co-catalysts for ORR POMs have also been combined with other materials to develop POM-based composites with improved catalytic properties for ORR as result of the synergetic effects between all components. Typically, Pt-based materials are state-of-the-art catalysts for ORR despite their high cost. However, due to the scarcity and high cost of noble metals, their integration with other sustainable and available materials such as POMs is an attractive way to optimize their properties and minimize their consumption. Additionally, using M0 nanostructures (i.e. zero-dimensional) increases the overall surface-to-volume ratio and hence reduces the metal loading. This is why great efforts have been focused on decreasing the Pt loading or replacing the electrocatalysts with non-Pt based materials. Hsu-Yao et al. synthetized 3-4 nm sized POM-stabilized Pt0 NPs using electrochemically reduced POM anions, SiW12O404- and H2W12O406-, as reductant and protecting/stabilizing ligands.248 Compared to the commercial Pt black, both the POM-stabilized Pt0 NPs showed higher ORR, with Pt0@W12O40 NPs having better performance (E1/2 showing a 30 mV positive shift while that of the Pt0@Si12O40 being 17 mV as compared to that of the Pt black). The observed activity enhancements may be related to the generally weakened reactant–metal surface binding as implied by cyclic 55 Energy & Environmental Science Page 56 of 124 voltammetry (CV) analysis. In a different strategy, Renzi et al. recently evaluated the ORR catalytic View Article Online DOI: 10.1039/D0EE03407J activity of Pt0 NPs mixed with insoluble Cs3HPMo11VO40 249,250 and Cs3H2PMo10V2O40251 POMs. The Energy & Environmental Science Accepted Manuscript Pt amount was reduced through the use of POMs as co-catalytic supports that are able to: i) enhance catalyst dispersion and limit the aggregation during fuel cell operation, induced by the mesoporous features of POM that allow the embedding of Pt inside the micro-mesopores; and ii) enhance the kinetics of the ORR redox process, owing to the high surface acidity of POMs and consequent high proton mobility. This strategy allowed decreased Pt loading while maintaining the electrocatalytic performance towards ORR. Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Xin et al. fabricated stable Au0 NPs/POM modified glassy carbon electrodes (GCE) using LbL assembly. PPDA-stabilized Au0 NPs were synthetized first using NaBH4 as reducing agent.252 Next, GC/(POM/Au-NP)n LbL hybrid films were prepared by the sequential deposition PDDA- stabilized Au0 NPs and POMs on GCE via the electrostatic forces between the negatively charged POM and positively charged PDDA. The authors compared the electrocatalytic behavior in terms of half-wave potential for the ORR (E1/2) of LbL monolayers built using W-, Ru-, and Co-substituted Keggin POMs. The trend in catalytic activity was PCoW11O395- > PRuW11O395- > PW12O403-, where POM co-catalysts containing more labile M–O bonds (e.g. Co-substituted) were better ORR co- catalysts. In the same year, Bao et al. chose a wheel-shaped VV–VIV mixed-valence tungstovanadate, [P8W48O184(V4V V2IVO12(H2O)2)2]32- (V12), that acts as both reducing (VIV reduced centers in V12) and capping ligand to synthetize 30 nm POM-stabilized Au0 spherical NPs (Au0@V12 NPs) in a one-pot procedure.253 The negatively-charged surface of the Au0@V12 NPs allowed the easy fabrication of multilayer films on ITO-coated electrodes based on the electrostatic attraction with the positively charged PEI using a LbL assembly method (Figure 17 (c)). CV experiments indicated the Au0@V12 NPs based composite films possessed good electrocatalytic activities in ORR. Reduced POMs (rPOM) are extremely stable in alkaline solutions and can participate in electrocatalytic cycles. In 2015, a rPOM was used as assistant catalyst substrate for Pd0 NPs.254 In the synthetic procedure, (NH4)PMoVI12O40 POM was reduced (in situ) using NaBH4 at the same time as 56 Page 57 of 124 Energy & Environmental Science the Pd2+ salt was reduced to form a rPOM@Pd0 hybrid with the Pd0 NPs sitting on the surface of the View Article Online DOI: 10.1039/D0EE03407J rPOM. The use of rPOM as a support for Pd0 NPs not only enhances the stability of the Pd0 NPs via Energy & Environmental Science Accepted Manuscript powerful support interactions minimizing the agglomeration but also efficiently enhances the ORR activity of the Pd0 NPs via electron transfer between Pd0 and rPOM. This rPOM@Pd0 hybrid showed better electrocatalytic performance for ORR in alkaline media than the commercial Pt/C EC: Eonset = 1.078 V vs. RHE (0.051 V more positive compared to Pt/C EC) and almost no declining of catalytic activity during 1500 cycles. The performance enhancement is a result of the changes in the electronic structure of Pd0 induced by synergistic interaction with the rPOM support, which weakens the Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. interaction between Pd0 and non-reactive oxygenated species (e.g. H2O2), providing more active sites for O2 adsorption and activation. Taking advantage of the high surface area and electrical conductivity of nanocarbons and their ability to be hybridized with POMs, several POM and carbon-based composites have been tested as ORR electrocatalysts. For instance, Wang et al. introduced a novel Pd0/POM-PDDA-MWCNTs composite with high electrocatalytic activity for the ORR of fuel cells (Inset of Figure 18 (a)).255 The construction implied the immobilization of Keggin-type tungstophosphoric acid (H3PW12O40, PW12) on the outer walls of MWCNTs functionalized by PDDA via the electrostatic forces between the negatively-charged PW12 and positively charged PDDA, followed by reductive deposition of Pd. The highly dispersed Pd0/PW12 NPs supported on PDDA-MWCNTs showed a much higher ORR activity (in terms of kinetic current density (Ik), defined as the current in the absence of mass-transfer limitations) than acid-treated MWCNT-supported Pd catalysts and comparable activity with that of commercial Pt/C catalysts as seen in Figure 18 (a). The authors suggest that PW12 could act as both the effective mediator and as a source of mobile protons at the electrocatalytic interface. Moreover, XPS data indicate that the composite material possesses a significantly higher amount of metallic Pd compared with Pd/acid treated-MWNTs, and it would facilitate desorption of adsorbed oxygen. This is a critical parameter to maximize electrode performance, as limited oxygen desorption and bubble formation on the electrode can significantly increase the overpotential. 57 Energy & Environmental Science Page 58 of 124 More recently, a novel Pd0/POM/rGO nanocomposite was prepared by the assistance of View Article Online DOI: 10.1039/D0EE03407J H3PMo12O40 (PMo12) and hydrazine hydrate as reducing agent.256 The electrostatic repulsive Energy & Environmental Science Accepted Manuscript interactions derived by PMo12 anions effectively prevented the agglomeration of Pd0 NPs, which were uniformly distributed over the surfaces of rGO. The composite revealed enhanced catalytic activity compared to its counterparts, as indicated by the higher current density on the linear sweep voltammograms (LSVs). Moreover, when methanol is added the LSV remains the same, demonstrating that the Pd0/PMo12/rGO catalyst has an excellent tolerance to the methanol cross-over effect. Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Initial steps have been taken to replace the traditional noble-metal particles for the ORR by cheaper metals. POM-stabilized Ag0 NPs on CNTs 257 or rGO 258 were prepared by a facile one-pot synthetic procedure using in situ photo-reduced PW12 Keggin anions as reducing agent and capping ligands. These nanohybrid tri-component systems were shown to be competitive to the commercial Pt/C for ORR. Using an alternative synthetic method, 2D Ag0 nanonets (NN)@POM–graphene nanosheets (GNSs) were reported.259 The composite was prepared using a MoV/MoVI mixed-valence POM, H7[β-PMo4VMo8VIO40], as the reducing, encapsulating and bridging agent to deposit an Ag0 nanostructure on graphene. The as-prepared ternary Ag0NN@POM–GNS composites showed high electrocatalytic ORR activity, with more positive onset (Eonset) and cathdoic peak (Epc) potentials and higher cathodic peak current density (ipc) compared to the binary Ag0NN@POM composites. The authors attribute the excellent performance to the high catalytic activity of the Ag0NN and the excellent electron transfer properties of GNSs, and their synergistic effect reinforced by the POM presence. Liu and co-workers reported the electrochemical-reduction-assisted assembly of ternary Ag0NPs@PW12–rGO,260 where the POM worked as both electrocatalyst and capping ligand as displayed in Figure 18 (c). This composite showed much better electrocatalytic activities towards the ORR than binary Ag0NPs@ PW12 and PW12/rGO nanohybrids, with higher ipc and a more positive Eonset and Epc values. Moreover, these results proved that the hybrids were sufficiently competitive 58 Page 59 of 124 Energy & Environmental Science with commercial Pt/C. More recently, a PW12-modified Ag@Pt/MWCNT electrocatalyst was View Article Online DOI: 10.1039/D0EE03407J prepared by electrostatic interaction with positively charged chitosan-functionalized core-shell Energy & Environmental Science Accepted Manuscript Ag@Pt NPs on MWCNTs.261 The results showed that PW12 improves the electrocatalytic activity of Ag@Pt NPs; with a E1/2 (half-wave electrode potential) for the ORR which positively shifted 44 mV from that of Ag@Pt NPs/MWCNTs, and a mass activity (Im) and specific area activity (Is) about 2 times higher than those of Ag@Pt NPs/MWCNTs. The enhanced activity can be explained by the protective effect of the PW12 layer on the catalyst surface. The negative surface potential established over the catalyst surface protects complex metal anions from being subjected to electrostatic repulsion Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. thereby conserving the active metal species on the catalyst surface. The performance of Nano- Heterostructure POM is summarized in Table 3. In the search for Pt-free catalysts, considerable efforts have also been devoted to investigating metalloporphyrin catalysts for ORR, particularly Fe or Co porphyrin complexes.262 However, little attention has been given to the combination of cationic metalloporphyrins and anionic POMs. In 2012, a PW12/porphyrin composite based on a mixture of cationic [5,10,15,20-tetrakis(1-methyl-4- pyridinio)porphyrin]cobalt(II) (CoPP) and anionic PW12 supported on carbon black was reported as an active ORR electrocatalyst.263 The results showed that cobalt ion was vital for the formation of the active ORR species, but might be attributed to the interaction of Co with POM that significantly improved the ORR activity. Hybrid ORR electrocatalysts have been prepared by ionic self-assembly of positively charged Fe(III) meso-tetra(N-methyl-4-pyridyl) porphyrin (FeP) with negatively- charged PMo12 in the presence of suspended MWCNTs.264 Figure 18 (b) shows the schematic illustration of steps involved in the synthesis of hybrid ORR catalyst. Compared with individual PMo12 and FeP, the hybrid FeP/PMo12-MWCNT electrocatalyst demonstrated a much higher ORR activity in terms of Eonset and E1/2 in acidic media due to the high proton conductivity of PMo12 together with the large surface area and high electrical conductivity from the MWCNTs. Interestingly, Deng’s group has developed a liquid-catalyst-mediated fuel cell (LCFC) employing only Mo–V–P POM solutions as electrocatalysts and constructed using a Nafion 115 59 Energy & Environmental Science Page 60 of 124 membrane sandwiched between two 3D graphite electrodes with no metal loading.265-267 In these View Article Online DOI: 10.1039/D0EE03407J cells, a Keggin-type H3PW11MoO40 solution oxidizes the biomass under sunlight or heating (at the Energy & Environmental Science Accepted Manuscript anode), while a modified (non-Keggin) H12P3Mo18V7O85 aqueous solution leads to the reduction of oxygen (in the cathode). The two POM solutions have different redox potentials and, unlike noble- metal catalysts, have the advantage of being tolerant to catalyst-poisoning organic and inorganic contaminants. Therefore, raw biomasses, such as cellulose, starch, and even grass or wood powders can be used directly to produce electricity without prior purification. It is noteworthy that, contrary to conventional fuel cells, in which small molecules such as hydrogen and methanol are the best fuels, Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. biomass fuels are more favorable for this novel LCFC with respect to their power densities. In another study, Mo–V–P POM solutions, H3+xPVxMo12−xO40 (x = 0, 2, and 3), were applied as redox mediators for the ORR in polymer electrolyte fuel cells.268 In this electrochemical system, the POM is reduced over the carbon cathode, and then the resultant rPOM is re-oxidized by exposure to the dissolved oxygen in the regenerator. The substitution quantity of V in the POM significantly affected the onset reduction potential as well as the current density achieved; H6PV3Mo9O40 was the most suitable mediator in terms of the electrochemical reduction property. The principle of using POM solutions and carbon as the cathode ORR catalyst can provide new methods for the improvement of other fuel cell systems. 4.3 Other Catalytic applications 4.3.1 Carbon dioxide reduction reaction The increasing concentration of atmospheric CO2 due to the industrial era is alarming and has now crossed the threshold of 400 ppm.269 Although massive emission of CO2 has resulted in serious environmental problems, it still is an important non-toxic, highly abundant and cheap carbon feedstock. Photo/electrocatalytic carbon dioxide reduction reaction (CO2RR) represents a favorable means that can reduce CO2 to highly valued hydrocarbon fuels or chemicals by using renewable solar energy or electricity as driving force. For CO2RR, the most important and ambitious goal is to realize 60 Page 61 of 124 Energy & Environmental Science the efficient, selective and durable transformation of CO2 into specific product with a View view to Article Online DOI: 10.1039/D0EE03407J increasing the possibility of large-scale utilization. The major challenge is to develop efficient Energy & Environmental Science Accepted Manuscript catalysts for CO2RR. The desirable photocatalyst for CO2RR should at least have following features: i) a suitable band gap structure for absorbing photons; ii) an efficient separation and transport of photo-generated electron-hole pairs; iii) large number of active adsorption and reaction sites for promoting photocatalytic reaction.278-280 The research in photocatalytic CO2RR is focused on finding catalysts that can work in visible light (>380nm) and are able to sustain the multi-electron transfer process. The final products of such Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. a process are carbon monoxide (CO), formic acid (HCOOH), formaldehyde (HCHO), methanol (CH3OH), and methane (CH4).270,271 It is noteworthy that some of the end products of this reaction are useful in their own right. For example, formic acid is a very promising hydrogen carrier and an important reagent in organic synthesis processes. POMs as one kind of molecular photocatalysts that display quasi-semiconductor photochemical properties also have many advantages in performing CO2RR, such as superior solution, thermal stability and adjustable redox potential and reversible redox activity, which have also been reflected in photocatalytic HER and OER. Their good solution stability can enable them to carry out photocatalytic CO2RR in water or other solvents, and their own redox properties can assist the multi-electron and multi-proton transfer process required for the conversion of CO2 to products. Of course, the research on CO2RR based on POM-based photocatalysts is still in its infancy, some meaningful conclusions need to be obtained based on more investigations. Neumann and co-workers283 demonstrated the first practical utilization of RuIII substituted Keggin structure, [(C6H13)4N5][RuIII-(H2O)SiW11O39] for the photoreduction of CO2 to CO with Et3N as reducing agent. DFT calculations suggests that CO2 is preferably coordinated in a side on manner to RuIII in the POM through formation of a RuO bond, further stabilized by the interaction of the electrophilic C atom of CO2 to an O atom of the POM. The interaction of the nucleophilic O atom of CO2 to Ru atom and the formation of O2C-NMe3 zwitterion stabilize both Ru- O and C-N interactions and determine the promotional effect of an amine on the activation of CO2 by 61 Energy & Environmental Science Page 62 of 124 POM molecule. Besides, several approaches are employed to construct heterogeneous crystalline View Article Online DOI: 10.1039/D0EE03407J POM-based photocatalysts for CO2RR. Recently, Lan et al.288 prepared several efficient Energy & Environmental Science Accepted Manuscript heterogeneous POM-based photocatalyst systems and demonstrated an effective reduction of CO2 to CH4 with high selectivity and activity. They first synthesized two pure inorganic POM-based frameworks constructed with strong reductive [P4Mo6V] units, NENU-605 and NENU-606, both of which can be used as heterogeneous catalysts for photocatalytic CO2RR. It should be noted that this is the first POM-based crystalline photocatalysts system to reduce CO2 to CH4 in water with a high selectivity of 85.5%. In this work, the strong reductive [P4Mo6V] units serving as multielectron donor Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. played a crucial role in promoting the 8 electrons and 8 protons transfer process required for CO2-to- CH4 conversion. These works indicate that developing strong reducing POM into photocatalyst architecture can increase the possibility of reducing CO2 to CH4 or other high-value carbon-based products accompanied with multiple processes. Table 4 shows the examples of best POM based photocatalysts for CO2RR. Reducing CO2 into value-added fuels (such as C2H4, CH4, CO) or chemicals (such as HCOOH, CH3OH, C2H5OH) is one of the promising paths to address environmental issues and affords a convenient solution for energy conversion. Compared with the photocatalytic CO2 reduction, electrical energy-driven CO2 reduction is also an effective technology to realize this reaction process and commonly shows higher conversion efficiency. The brief list with their standard reduction potential and chemical reaction can be found in a paper by Qiao et al.272 Nevertheless, HER as competitive reactions in aqueous electrolytes reduces the conversion efficiency of CO2 during the electrochemical process due to their comparable thermodynamic potential. Therefore, the search of superior electrocatalysts with excellent electrochemical activity and suppressive HER is of great importance to develop CO2RR. An ideal electrocatalysts for CO2RR are expected to have following characteristics: i) flexible redox ability for the multielectron transfer, ii) high Faradaic efficiency and selectivity for the CO2RR, iii) high structural and performance stability during a long-term electrochemical process, iv) high catalytic activity with a low electrochemical overpotential, and v) 62 Page 63 of 124 Energy & Environmental Science scalable synthesis from low cost raw materials. Recent focus is on the development of heterogeneous View Article Online DOI: 10.1039/D0EE03407J elelctrocatalysts in nanostructured form, while the contrary homogeneous elelctrocatalysts mainly Energy & Environmental Science Accepted Manuscript involve molecular metal complexes and macrocyclic organics. Owing to the capability of POMs to undergo reversible multi-electron redox reactions without decomposition, they can be used along with a co-catalyst that brings down the activation energy for CO2 reduction. Moreover, POMs as weak bases could promote the formation of hydrogen-bond networks near the CO2 coordination center to favor proton-coupled electron transfer that occurs when the proton source is close to the catalytically active center. Electrocatalytic reduction of CO2 with Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. POM was first reported by Kozik et al.278. It was observed that α-[SiW11O39Co]6− showed the electrocatalytic activities for CO2 reduction, however, the final CO2 reduction product was unstated. More than decade later, Proust et al. reinvestigated this reaction in more detail by using [α- SiW11O39Co]6− in the electro-assisted reduction of CO2.290 The [α-SiW11O39Co]6− contained a CoII in place of a WVI. The square-pyramidal CoO5 with a vacant site was generated by losing a coordinated water molecule from CoO5(H2O) octahedral when the POM was extracted from aqueous to organic media. Except CO and HCHO, neither H2 nor other CO2 reduction products were detected. This indicated the unique selectivity of [α-SiW11O39Co]6− POM catalyst in the electroreduction of CO2. The turnover of CO2 to CO reached to 3.7 with the faradic efficiency of 13%. HCHO was the other detectable product with amount varied from 2.1 × 10−7 to 2.2 × 10−6 mol with the faradic yield varying from 25% (high HCHO content with low electrolysis charge) to 0.8% (low HCHO content with high electrolysis charge). Likewise, Wang et al has investigated MOFs as co-catalysts and combined them with POMs.273 The bonds between Zn2+ ions on POM and carboxylates porphyrin were formed under hydrothermal conditions using M-TCPP = tetrakis[4-carboxyphenyl]-porphyrinato-M, as ligands. Here M is for metal Co, Fe, Ni and Zn. The electrochemical performance of the reduction reaction was evaluated in an airtight three-electrode electrochemical H-type cell, where a proton exchange membrane (Nafion® 212) separated the compartments. The performance of different metal POM- MOF are summarized in the Table 5. The mechanism of CO2 reduction in presence of POM and effect 63 Energy & Environmental Science Page 64 of 124 of temperature on reduction reaction initiated by different driving forces needs to be investigated View Article Online DOI: 10.1039/D0EE03407J further to develop this technology for commercial application. Energy & Environmental Science Accepted Manuscript 4.3.2 Nitrogen reduction reaction Nitrogen (N2) makes up 78% of the total gaseous composition of the atmosphere. However, fixing nitrogen in soil is still difficult but important reaction needed to maintain the global ecosystem. To supply the necessary nitrogen to plants, ammonia is produced using the Haber–Bosch approach, which is energy intensive and carried out under harsh conditions. Hence, finding alternative ways or Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. catalysts for the nitrogen reduction reaction (NRR) is crucial. Recently, Li et al used a typical MOF material, zeolitic imidazolate framework-67 (ZIF-67) with network tetrahedral topologies to adsorb N2 on its porous structure.274 The ZIF-67 was combined with various transition-metal-substituted POMs to supply multiple electrons to efficiently carry out NRR. Their performance towards NRR was evaluated in a sealed reactor with 300 mg of photocatalyst, 80 mL of distilled water and 20 mL of ethyl alcohol. The reaction was carried out at room temperature and N2 was introduced in the reaction vessel at 1 L/minute, for an hour. As seen from Figure 19 (a), the ZIF-67/POM composite nanomaterials showed higher catalytic performance than that of ZIF and POMs alone with highest N2 fixation efficiency reaching 149.0 mol/L at 1 h, yielding a solar to ammonia (STA) efficiency of 0.032 % for ZIF-67@PMo4V8. The energy levels and electron-transfer mechanism for ZIF-67 and POMs composite photocatalyst is shown in Figure 19b. Accordingly, the porous ZIF-67 adsorbs a large amount of N2 while the POMs are reduced under light conditions, and the reduced POMs more easily stimulate electrons to participate in the catalytic process. After the reaction, the reduced POMs react with the oxidant (e.g., O2) to turn into their oxidized state and forms a complete self-healing and circulatory system (Figure 19b). Another important example of POM-based catalyst for NRR is demonstrated by Wang et al.275 They used PMo12@MOF-100(Fe) POM with PVP (polyvinylpyrrolidone) as a precursor to synthesize two FeMo-based electrocatalysts, Fe1.89Mo4.11O7/FeS2@C and FeMoO4/FeS2@C by hydrothermal methods. The 64 Page 65 of 124 Energy & Environmental Science Fe1.89Mo4.11O7/FeS2@C showed outstanding catalytic performance with a high NH3 yield rateView(105.3 Article Online DOI: 10.1039/D0EE03407J μg h-1 mgcat.-1) and Faradaic efficiency (54.7%) in acidic electrolytes at -0.4 V vs. RHE, which is Energy & Environmental Science Accepted Manuscript higher than FeMoO4/FeS2@C, and most reported electrocatalysts for NRR. Further electrochemical experiments in alkaline electrolytes reveal the overall NRR catalytic activity of Fe1.89Mo4.11O7/FeS2@C (NH3 yield of 86.3 μg h-1 mgcat.-1, FE of 53.6% at -0.4 V vs. RHE). The NRR performances of POM based catalysts is summarized in Table 6. 4.3.3 Biomass conversion using POMs Due to the oxygen rich characteristics of biomass, POMs are now being investigated to Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. catalyze biomass conversion reactions for obtaining valuable oxygen-containing chemicals. Along these lines, Voß et al investigated the conversion pathway of glucose using either an oxygen or a nitrogen atmosphere.276 The reaction was catalyzed by Lindqvist-type POM K5[V3W3O19]. The time dependent distribution of products of the above catalyzed reaction is shown in Figure 20. Further, the Keggin type POM catalyst H6[PV3Mo9O40] and a substrate variation (glucose and glyceraldehyde) was also tested under aerobic (oxygen) and anaerobic (nitrogen) environment (see Table 7). It was found that the substrate and the reaction conditions (whether it is aerobic or anaerobic) determine the final product (folic acid (FA), or lactic acid (LA)). Hence, it is shown that a suitable catalyst can be chosen, which can be combined with a given set of parameters to convert biomass into the desired chemical/s. Liu et al used various kinds of biomass (cellulose, lignin and even wood and grass powders) in a novel electrolytic reduction process at low temperature.277 The process was catalyzed by a POM solution as depicted in figure below. No current was observed initially at an applied potential of 1.1V, even in the presence of glucose. However, when a PMO12 solution was pumped through the anode along with biomass, a sharp increase in the electrolysis current was observed together with the corresponding bubbles of H2 gas at the cathode side as can be seen from Figure 21 (a, b). The use of POMs to produce H2 at low electric energy consumption from biomass is very young but promising 65 Energy & Environmental Science Page 66 of 124 technology that remains to be deeply explored. Various parameters such as temperature, active sites View Article Online DOI: 10.1039/D0EE03407J on the POM, electrolyte effect and electron transport paths need further investigation. Energy & Environmental Science Accepted Manuscript Anna Bukowski et al developed a concept called the Ionosolv method in which fractionation of lignocellulosic biomass (wood) was achieved using a low-cost IL.278 Bio-derived formic acid was produced using a POM catalyst and molecular oxygen as an oxidant. The IL alkylammonium [TEA][HSO4] was used as a solvent to break down the lignocellulosic feedstock. The best performance was achieved using vanadium substituted POM (H8[PV5Mo7O40]) with respect to folic acid yield (23%) from hardwood (beech), which corresponds to a folic acid selectivity of 54%. The Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Ionosolv method was also used with other types of wood such as softwood (pine) and grass biomass (Miscanthus). This method should be evaluated further for final product purity, catalyst recyclability and cost effectiveness of the whole process. This step will test the commercial viability of the concept. 4.3.4 Methanol oxidation using POMs Direct alcohol fuel cells (DAFCs) are systems that extract energy by selective oxidation of the alcohol. Hydrogen ions produced by alcohol oxidation are transported across the proton‐exchange membrane to the cathode, where they react with O2 to form water. The overall reaction occurring in a DAFC is 2 CnH2n+1OH+3 nO2→2 nCO2+2 (n+1)H2O. DAFCs have been also proposed as the green energy for future vehicles due to its easy fuel storage, high energy density and low emission of pollutants.279 The most widely studied ones utilize methanol and ethanol and are referred as a direct methanol fuel cell (DMFC) or a direct ethanol fuel cell (DEFC), respectively. Several efforts have been devoted in the past years to develop efficient catalysts and are outlined in the previous review articles. For instance, platinum280 based or nano-metals (noble and non-noble)281 catalysers with their potential application in portable devices282 due to high energy density and easy handling among others. In this section, we have highlighted key features and complement the abovementioned reviews with the most important investigations that have reported POMs-based catalytic reactions. Pt is commonly employed as the catalyst in acidic media in DMFC or DEFC devices. However, Pt on its own is not a highly efficient catalyst due to poisoning from the strongly adsorbed 66 Page 67 of 124 Energy & Environmental Science species such as CH(ad),CH3(ad), and CO(ad) generated during oxidation. Accordingly, many efforts have View Article Online DOI: 10.1039/D0EE03407J been dedicated to improve the poisoning tolerance of the metal. Normally researchers encounter Energy & Environmental Science Accepted Manuscript partial oxidation to aldehydes, ethers or carboxylic acids283-285 therefore the system is not extracting all the power of the system. Most of the papers reviewed here are uses commercial POMs (as seen in Table 8), but as an anti CO poisoning agent of the real catalyst (which is Pd, Pt, Ru, Au or composites) and/or morphology control rather than alcohol oxidation. For example, Shi et al279 studied the performance of Pt/Pyridine ionic liquid POM/rGO (Pt/(epy)3PMo12O40/rGO) in acidic conditions, suggesting improvement in the reaction. The reason attributed to the fact that POM promote Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. uniformity of Pt dispersion ((111) face) and exhibits anti-CO poisoning properties. This is one of the main conclusions extracted from the past research in the field as the POM improve the crystal size homogeneity and decrease the size. The second conclusion is that POMs improve the catalyst stability by preventing the surface poisoning with by-products that are strongly adsorbed.286-288 However, Barczuk et al.289 doubts the electron mediating function of the PMo12 in the electrooxidation of ethanol, despite the fact the chronoamperometric results evident an increase in current and stability of the metal catalyst (Pt, Pt-Ru, Rt-Sn/C etc.). They propose that PMo12 stabilize the oxo-groups on top of metal, which are known to induce the oxidation of the poisoning species (such as CO). Various POMs have been proposed and tested as additives for methanol oxidation including PMo12, SiW12, PW12,290,291 wheel-shaped VV –VIV mixed-valence tungstovanadate (V12),253 and their composites. Of special interest, Bommineni S et al.283 investigated the methanol oxidation with pure PMo12 and three derivative (pyridine, niobium and pyridine-niobium salt). The authors analyse the methanol oxidation under a gas stream (Ar, O2 MeOH) at 533K adjusting the conversion around 10%. It was revealed that the PMo12 and its niobium salt give mainly dimethoxymethane while the niobium pyridine derivative gives preferentially dimethyl ether. The pyridine POM salt gives mixture of the products. Likewise, Chen et al.292 studied the influence of WOx and PW12 on the PtRu/C catalyst, suggesting an improvement in the catalytic properties due to both: 1) WOx facilitates the nanoparticles distribution on carbon while 2) PW12 improve the poison resistance behaviour of the 67 Energy & Environmental Science Page 68 of 124 catalyst. Similarly, the catalytic effect of a complex POM (H3+xPMo12xVxO40 (x =0,1,2,3)) and View Article Online DOI: 10.1039/D0EE03407J surface-supported VOx/H3PMo12O40 was also investigated.293 The study was performed in isothermal Energy & Environmental Science Accepted Manuscript fixed bed reactor with a gas stream at 250ºC. The volume composition of the gaseous reactant feed was 6% CH3OH, 13% O2 and balance He, with a total flow rate of 100 mL/min. The results suggests that the introduction of the VOx sites increases the formaldehyde selectivity and decreases the dimethyl ether selectivity. The VOx units in the primary Keggin structure were slightly more active than the surface VOx species, and both VOx sites were significantly more active than the MoOx sites in the primary Keggin structure. Khadempir et al. synthetized pseudospherical palladium Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. nanoparticles protected by PMo12290 at different pH or supported on graphene nanosheets (GNS)294 in acid solution. According to the results, Pd/PMo12/GNS demonstrates better poison tolerance as compared to the reference sample due to a superior ionic and electronic conductivity, small and highly dispersed nanoparticules. We have summarized recent works in the Table 8. 5. Conclusion and Future Challenges Polyoxometallates (POMs) is a unique class of materials that have a broad range of potential forms and applications. Both their possible forms and their subsequent applications continue to be widely studied. In this review, we briefly described the history and progress of this class of materials with particular emphasis on energy-related applications. The use of POMs in supercapacitors can offer significant opportunity to enhance the energy density of these devices by contributing pseudocapacitive charge storage effects in an efficient manner. This is a consequence of the high surface-to-bulk ratio of POMs, as well as their multiple redox states. POMs were shown to be suitable for improving performance in a variety of approaches to their use. These range from very simple addition of POMs to typical AC electrode pastes, to more complex methods of chemical linking to 2D materials and other emerging nanomaterials. However, because of their generally low electrical conductivity, the linking of POMs to an appropriate 68 Page 69 of 124 Energy & Environmental Science conducting scaffold is particularly important and challenging, however progress is being made, for View Article Online DOI: 10.1039/D0EE03407J example, by using DFT simulation and prediction. Energy & Environmental Science Accepted Manuscript For similar structure-property reasons as with SCs, these small cluster molecules offer the opportunity to improve the performance of battery technologies, and relatively speaking, this is being more widely explored. The poor electrical conductivity of pristine POMs and their typical high solubility in battery electrolytes results in a low cycling stability and poor rate performance when used as electrode materials. POMs-based composites can overcome these issues. Therefore, developing new POM‐based composites is needed to expand the use of POMs in the field of Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. rechargeable batteries as well as SCs. Particular attention could be given to POMOFs/carbon nanostructures, in which MOFs can mitigate the dissolution issue, while carbon nanostructures can increase the conductivity of POMs. More work focused on developing simple low-cost methods for the synthesis of POMOFs with high conductivity and controllable homogeneity, would be expected to improve their electrochemical performance in energy storage devices. The energy storage mechanism for POMs-based electrode materials for rechargeable batteries and SCs are still not fully understood. The relatively high number of electron exchanges per molecule, when compared to other redox materials used in cathodic and anodic battery storage, complicate this. Therefore, detailed analysis is necessary to understand the reactions at the electrode/electrolyte interface. Advanced in situ/operando characterisation (microscopic and spectroscopic) and continued theoretical studies (such as by DFT) will likely be a necessary part of gaining an in‐depth understanding of the reaction mechanisms and structural changes in POM-based electrodes, in order to promote further improvement in this field. The interesting properties of POMs for energy storage are linked to the number of active sites in POM molecules, and these are also a critical property for catalysis. The application of POMs as catalysts is discussed for both water oxidation and ORR, as well as for other catalytic applications. Regarding water oxidation, the following three principles for the design of molecular WOCs recently outlined by Craig et al provide a strong point of reference for summarizing the topic:295 (1) Research 69 Energy & Environmental Science Page 70 of 124 should focus on transition metal complexes which can undergo additional electron transfer steps by View Article Online DOI: 10.1039/D0EE03407J stabilizing high oxidation states, hence reducing the overpotential. (2) existing knowledge of Energy & Environmental Science Accepted Manuscript heterogeneous catalysis, where DFT is used for material design, should be used to optimize the binding of the different reaction intermediates to the transition metal center. (3) electronic and steric effects of metal ligands should be investigated systematically. Advances in this field will allow the future implementation of ideal molecular WOCs into commercial electrolyzers with greatly enhanced selectivity, stability, and electron transport. In the case of ORR catalysts, hybrid materials combining POMs with metal NPs, carbon- Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. based materials and porphyrins have shown favorable performances. The synergies between POMs and the supports are crucial to improve the final composite stability and to achieve favorable ORR performances. On the other hand, pristine POMs with intrinsic catalytic activity towards ORR are scarce, and continuous efforts should be devoted to the discovery of cost-effective, efficient, and durable noble metal-free POM catalysts with good ORR performance. In general, the discussed examples demonstrate that the catalytic properties of POMs can be engineered finely to meet the requirements of different reactions. The molecular nature of POMs is enormously tunable and continues to present unique opportunities in the investigation of catalytic systems. Future research will focus on further advancements in POM design and immobilization, with special attention to the stability of the catalysts. The combination of POMs with metal NPs, carbon-based materials and other molecules has been shown to lead to favorable performance. In a synergistic fashion, these hybrid materials exhibit properties not accessible from the individual components. Furthermore, POMs have been shown to be useful for efficient production of hydrogen and for improved fuel cell performance. In all three of the main energy-related contexts of applying POMs, one of the critical challenges that continues to be investigated is the anchoring of the materials to suitable substrates. POMs can very much be viewed as water-soluble oxyanions, just as can be found in the molecular pairing of many acid types. However, their relatively large molecular size also makes them true NP materials that present the opportunities mentioned in this review, when the particles are well dispersed 70 Page 71 of 124 Energy & Environmental Science and fixed to a supporting scaffold. Amongst other methods, achieving this has been done View through Article Online DOI: 10.1039/D0EE03407J covalent and non-covalent approaches for carbonaceous and polymeric materials. Energy & Environmental Science Accepted Manuscript Despite this, there remains considerable room to improve knowledge in this area, and for it to be rationally guided, such as by DFT investigative work. Collaboration between theoretical modelers and experimental researchers will enable the discovery of new structures and new results in applications of POMs. Together the outputs of theoretical investigations, advanced characterization work, and the synthesis and testing of novel POMs and POM composites can be expected to yield significant Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. benefits. A deeper understanding of these materials and their structure and activity represents an accomplishment to advanced physical chemistry, given their complexity. While at the same time, their excellent activity and versatility make them exciting materials for applications such as in energy as we have discussed here. Overall, continued research into POMs for energy can be expected to make a significant contribution towards solving sustainable energy problems, at large. Author contributions D.P.D. and P.G.R. conceived the idea of the project. The energy storage part was written by M.H., S.A. and N.R.C. while A.S., S.G.F. and R.V.B. have contributed the energy conversion sections. All authors contributed equally to the preparation and reviewing manuscript. Acknowledgements D.P.D. acknowledges the Queensland University of Technology (start-up grant: 323000-0424/07) and Australian Research Council (ARC), Australia for the Future Fellowship (FT180100058). D.P.D. would also like to acknowledge the financial support from Centre for Materials Science, QUT, Australia. Sara Goberna-Ferrón has received funding from the postdoctoral fellowship programme Beatriu de Pinós, funded by the Secretary of Universities and Research (Government of Catalonia) 71 Energy & Environmental Science Page 72 of 124 and by the Horizon 2020 programme of research and innovation of the European Union under the View Article Online DOI: 10.1039/D0EE03407J Marie Skłodowska-Curie grant agreement No 801370. 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High performance of polyoxometalate/PtPd nanoparticles/carbon nanotubes electrocatalysts for the methanol electrooxidation. International journal of hydrogen energy 2013, 38 (25), 11074. (328) Medetalibeyoğlu, H.; Manap, S.; Yokuş, Ö. A.; Beytur, M.; Kardaş, F.; Akyıldırım, O.; Özkan, V.; Yüksek, H.; Yola, M. L.; Atar, N. Fabrication of Pt/Pd nanoparticles/polyoxometalate/ionic liquid nanohybrid for electrocatalytic oxidation of methanol. Journal of The Electrochemical Society 2018, 165 (5), F338. (329) Çolak, A. T.; Eren, T.; Yola, M. L.; Beşli, E.; Şahin, O.; Atar, N. 3D polyoxometalate- functionalized graphene quantum dots with mono-metallic and bi-metallic nanoparticles for application in direct methanol fuel cells. Journal of the Electrochemical Society 2016, 163 (10), F1237. 91 Energy & Environmental Science Page 92 of 124 View Article Online DOI: 10.1039/D0EE03407J TOC: Polyoxometalates as anionic molecular metal oxides clusters with open frameworks and rich redox chemistry have outstanding versatility in energy conversion and storage research. Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. 92 Page 93 of 124 Energy & Environmental Science View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 1 Keggin and Dawson POM structures including the acknowledged Keggin isomeric variations. Adapted with permission from Ref. 15, Copyright 2018, Macmillan Publishers Limited, part of Springer Nature. 93 Energy & Environmental Science Page 94 of 124 View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 2. Polyoxometalates molecules and their range of energy related applications for a cleaner environment. 94 Page 95 of 124 Energy & Environmental Science View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 3. (a, b) Cyclic voltammetry and charge/discharge curves for AC and AC/PMo12 hybrid materials. Reprinted with the permission from Ref.18, copyright 2012, Elsevier. (c) Ragone plot demonstrating the high energy density of vanadium based POMs. Reproduced with the permission from Ref. 36, copyright 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. (d) The metallacalixarenes are able to anchor POM clusters inside the pockets of the material. Reprinted with the permission from Ref. 36, copyright 2019, American Chemical Society. 95 Energy & Environmental Science Page 96 of 124 View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 4. (a, b) CV curves of PDDA, PMoW, RGO and the PMoW-PDDA-RGO composite at 10 mV s-1. (d) Rate performances of RGO and the PMoW-PDDA-RGO composite determined from discharge curves. Reprinted with the permission from Ref.33, copyright 2016, The Royal Society of Chemistry. (c) Schematic illustration of synthesis of W2N@P doped CF using PW12 as precursor with corresponding TEM and electrochemical analysis. Reprinted with the permission from Ref. 62, Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. 96 Page 97 of 124 Energy & Environmental Science View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 5 (a-c) SEM images of MnO2 coated with different POMs such as PMo12, PW12 and SiW12, respectively. Reprinted with permission from 61, Copyright 2018, American Chemical Society. (d) POM/PIL/graphene structure, where two POMs and one IL dimer (2[H2PMo12O40][C14H24N4]), are included in a periodic unit cell. (e) The calculated adsorption energies compared with those obtained for the H adsorptions on the same O sites of an isolated POM (H2PMo12O40) on graphene. Reprinted with the permission from Ref.45, copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (f, g) Representation of anchoring of SiW12@CNT where X denotes the functional groups -NH2, -OH, -COH and –COOH, (f) Top(T), Bridge(B) and Hollow(H) points on the carbon network where the central axis of the SiW12 cluster is projected. Reprinted with permission from 64, Copyright 2018, Elsevier. 97 Energy & Environmental Science Page 98 of 124 View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 6 The ability of PMOs to discharge multiple electrons per molecule determines their outstanding ability to control redox processes in energy storage devices. (a) The reduction of all 12 Mo6+ to Mo4+ in [PMO12O40]3- to form [PMO12O40]27-, (b) Mo K-edge XANES spectra for PMo12 MCBs in the first discharging and second charging. (c) Schematic illustration of reaction mechanism for NAM during the discharge-charge process. Reproduced with the permission from Ref. 90, Copyright 2012 American Chemical Society and Ref. 91, Copyright 2014, Elsevier. 98 Page 99 of 124 Energy & Environmental Science View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 7 (a) Scheme showing the charging process of [V15O36(CO3)]-based symmetrical LIBs (VV centers; yellow, VIV centers; teal), (b) Galvanostatic charge and discharge curves of Li7[V15O36(CO3)] half-cell in within different voltage ranges at 100 mA.g-1 (c) Cyclic stability of symmetric Li7[V15O36(CO3)] device (d) the concept of electron/ion sponge in NPV anode. Reprinted with permission from95, Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim and from ref. 96 , Copyright 2019 Elsevier. 99 Energy & Environmental Science Page 100 of 124 View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 8 (a) A possible interaction of lithium with POM/SWNT hybrid and TEM image of the POM/SWNT hybrid (b) schematic demonstration and TEM image of TBA3[PMo12O40]/rGO hybrid, (c) electrochemical performance of TBA3[PMo12O40]/rGO in Li-ion battery. Reproduced with the permission from ref. 85, Copyright 2011, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim and from ref. 103, Copyright 2014, Royal Society of Chemistry. 100 Page 101 of 124 Energy & Environmental Science View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 9 Charge storage mechanism in NMV cathode of (a) the molecular structure of [MnV13O38]8- and [MnV13O38]20- upon the insertion of Na+, (b) charge redistribution of [MnV13O38]20- analyzed by the charge difference, and (c) electron/Na+ ion sponge behavior. Reprinted with permission from 113, Copyright, 2017, American Chemical Society. 101 Energy & Environmental Science Page 102 of 124 View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 10 Schematic of the structure of a redox flow battery. Reprinted with permission from 128, Copyright 2011, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. 102 Page 103 of 124 Energy & Environmental Science View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 11 Catalytic cycles for the two primary reaction pathways proposed for the OER. Reproduced with permission from 138. 103 Energy & Environmental Science Page 104 of 124 View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Figure 12 a) Structure of Ru4Si2, highlighting the central [Ru4(O)4(OH)2(H2O)4]6+core (ball-and-stick representation, Ru: blue, O: red, O(H2): orange. Hydrogen atoms omitted for clarity). The POM fragments are shown as gray polyhedra. Reproduced with permission from 141, Copyright 2008, Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. b) Kinetics of O2 formation (blue) and SA consumption (red) during light-induced catalytic water oxidation by Ru4Si2 5.0 μM using Ru(bpy)32+ 1 mM as the PS and S2O82-5 mM as SA in sodium phosphate buffer 20 mM (initial pH 7.2). Reproduced with permission from 146 104 Page 105 of 124 Energy & Environmental Science View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Figure 13 (a) Electrostatic capture of polyanion Ru4Si2 (red) by polycationic dendrons on the MWCNT surface (blue) and polyhedric structure showing the side and front view of Ru4Si2 with the [Ru4O4] core in yellow. (b) General scheme for a water-slitting electrocatalytic cell with the integrated Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. nanostructured O2-evolving anode. Reprinted with permission from 159, Copyright 2010 Macmillan Publishers Limited. 105 Energy & Environmental Science Page 106 of 124 View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Figure 14 (a) Co4P2@MOF-545 components: Co4P2 ([Co4(H2O)2(PW9O34)2]10- ); tetrakis(4- carboxyphenyl)porphyrin linker (TCCP-H2); and Zr-based building blocks of the MOF ([Zr6(μ3- Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. O)4(μ3-OH)4(OH)2(H2O)6]10+). The position of Co4P2 in the MOF channel was obtained from computations. WO6: green polyhedral, ZrO8: blue polyhedra or spheres, Co: cyan spheres, O: red spheres, C: gray, H: light gray, N: dark blue. (b) Kinetics of photocatalytic O2 production over 0.5 mg of Co4P2@MOF-545 (blue), Co4P2@MOF-545 recycled once (red), twice (pink), and a solution containing PS porphyrin linker 131 μM and Co4P2 13 μM (green). Reaction conditions: S2O82- 5 mM as SA in borate buffer 80 mM solution, pH 8, visible light (λ > 420 nm, 280 W). (b) and (c) reprinted with permission from183, Copyright 2018, American Chemical Society. 106 Page 107 of 124 Energy & Environmental Science View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Figure 15 (a) Current density at +1.3 V (vs NHE) with a 44% Co9-C electrode (black) and with Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Co3O4-C working electrodes at different Co contents (color lines). Labels indicate the total amount of Co in the blends. Reprinted with permission from 197. Inset: Structure of Co9, WO6 (gray), PO4 (black), Co (pink), P (black), O (red), reproduced with permission from 195. (b) Linear sweep voltammetry (LSV) of Mn16@graphene modified rotating ring-disk electrode in pH 7.5 phosphate buffer containing 1 M NaNO3. The scan potential was from 0.3 V to 1.4 V at 50 mV/s on the disk electrode (blue line) and a constant potential of -0.35 V was applied to the ring electrode (green line). The bare graphene electrode was used as a reference under the same conditions (black and red lines). Inset: Structure of Mn16, WO6 (aqua), PO4 (pink), SiO4 (blue), MnII (yellow), MnIII (brown), O (red). Reproduced with permission from 220, Copyright 2016, Royal Society of Chemistry. 107 Energy & Environmental Science Page 108 of 124 View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Figure 16. Ball-and-stick and polyhedral representation of the polyoxoanion structures of compounds (a) Ni12, (c) Ni13 and (d) Ni25 ; (b) Schematic view of the synthetic procedures. WO6, teal octahedra; Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. PO4, pink tetrahedra; SiO4, orange tetrahedra; C, black sphere; O, red sphere; Ni, lime sphere. (e) Kinetics of O2 evolution in the photocatalytic system at different concentrations of Ni25 3. The symbols used to denote catalyst concentrations are 0 μM (black ■), 1 μM (red ●), 2 μM (royal blue ▲), 3 μM (teal blue ▼), 5 μM (magenta leftpointing triangle), and 10 μM (olive right-pointing triangle). Conditions: 300 W Xe lamp equipped with a long-pass filter (420 nm cutoff); 1.0 mM [Ru(bpy)3]Cl2, 5.0 mM Na2S2O8, sodium borate buffer pH 9.0 (80 mM); total reaction volume 20 mL; vigorous stirring (1.5 × 103 rpm). Reproduced with permission from 224, Copyright 2015, American Chemical Society. 108 Page 109 of 124 Energy & Environmental Science View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 17 (a) Polyhedral presentation of the hematite surface with one Fe–OH reactive site highlighted in blue (two bound POM ligands and one [H3IVIIO6 ]2- SA drawn to scale). Reproduced with permission from 239, Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Linear sweep voltammograms of Co7(AlePyZn)2/C and Pt/C in O2-saturated 0.2 M NaH2PO4/NaOH (pH 7.4) and a scan rate of 10 mV/s. Reproduced with permission from ref 253, Inset: Structure of the polyanionic unit in Co7(AlePyZn)2, WO6(blue), PO4 (orange), Co (pink), P (orange), C (black), N (blue), O (red), and Zn (grey). Redrawn from ref 242, Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Left: Structure of V12, V (dark yellow), O (red), WO6 (cyan), and PO4 (blue). Right: Scheme of preparation of (PEI/ V12@Au0)n LbL multilayer films on the ITO-coated electrodes (not scaled). . Reprinted with permission from 253, Copyright 2011, Elsevier. 109 Energy & Environmental Science Page 110 of 124 View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 18 (a) Kinetic current density (Ik) obtained from rotating disk electrode experiments normalized by the electrochemical surface area of the catalysts. Dashed line: Pd/POM-PDDA- MWCNT; solid line: commercial Pt/C catalysts; and dotted line: acid-treated MWCNT-supported Pd. Reproduced with permission from255. Inset: Representation of Pd0/POM-PDDA-MWCNT composite material. Reproduced with permission from 296, Copyright, 2015, Royal Society of Chemistry. (b) Schematic representation of the synthesis of FeP/PMo12-MWCNTs electrocatalysts. For simplicity, the axial ligands of FeP were omitted. Reproduced with permission from 264, Copyright 2019, World Scientific Publishing Company (c) Electrochemical-reduction-assisted assembly procedure of the representative ternary Ag0NPs@POM–rGO nanohybrids. Adapted with permission from 260, Copyright 2015, Royal Society of Chemistry. 264 255 254. 110 Page 111 of 124 Energy & Environmental Science View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Figure 19 (a) NH3 yield of various ZIF-67/POMs hybrids. (b) Energy levels and electron-transfer mechanism for ZIF-67 and POMs composite photocatalyst. 274 Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. 111 Energy & Environmental Science Page 112 of 124 View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Figure 20 Time-dependent product distribution in the K5[V3W3O19]-catalyzed glucose conversion under (a) oxygen and (b) nitrogen atmospheres. Reaction conditions: 2 mmol of glucose, 0.167 mmol of K5[V3W3O19], 10 g of water as solvent, 160 °C reaction temperature, 20 bar pressure, 1000 rpm stirrer speed, 0−5 h reaction time. With permission from 276 112 Page 113 of 124 Energy & Environmental Science View Article Online DOI: 10.1039/D0EE03407J Energy & Environmental Science Accepted Manuscript Figure 21 (a) Schematic illustration of the chemical-electric conversion (CEC) process. The anode is a simple graphite-felt electrode; the cathode is a gas-diffusion electrode loaded with the Pt black Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. catalyst (4 mg cm-2). (b) Polarization curves of PMo12 (0.3 mol l-1) and biomass solution (fed with the anode side) preheated at 100 oC for 2 hours. The cathode side is pumped with 1 mol l-1 H3PO4 solution. Figure adapted from 277 113 Energy & Environmental Science Page 114 of 124 Table 1. POMs-based materials for lithium-ion batteries. View Article Online DOI: 10.1039/D0EE03407J Initial Current Capacity discharge /Current Cycle Energy & Environmental Science Accepted Manuscript Materials Electrode retentio Ref. capacity density /Rate number n (%) (mAh.g-1) 50@100 8 K3[PMo12O40] (KPM) Cathode >200 50 µA 37.5 2,89 µA 9 TBA3[PMo12O40] Cathode 260 1 mA 80.7 10 0 Na3[AlMo6O24H6] Cathode 437 0.04 C 91.2 50 91 (NAM)/KB (NH4)6[NiMo9O32] Cathode 490 17 mA.g-1 87.6 50 84 (ANM) NPs/KB (KH)9[PV14O42] 8 Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Cathode 370 100 µA 95 50 9 (KPV) K7[NiV13O38] (KNiV) Cathode 218.2 17 mA.g-1 ~91.6 24 81 K7[MnV13O38] Cathode 300 17 mA.g-1 97.3 50 83 (KMV) (NH4)7[MnV13O38] 9 Cathode 234 0.5 C 100 100 3 (AMV)NPs K9[PV14O42]/PP Cathode 410 167 mA.g-1 90 50 94 9 Li7[V15O36(CO3)] Cathode 174 2 A.g-1 85 100 2 Na7H2[PV14O42] 9 Anode 961 100 mA.g-1 ~57.2 150 6 (NPV) TBA3[PMo12O40]/SW Cathode ~320 1mA ~96 10 85 NTs SWCNTs/ Py-SiW11 Anode 1569.8 0.5 mA.cm-2 37 100 98 8 CNTs/ SiW11-NH2 Anode 1189 0.5 mA.cm-2 54.6 100 6 PMO12/PANI/MWNT Anode 1572 0.5 mA.cm-2 63.6 100 99 s 1 SiW12/rGO Cathode 275 50 mA.g-1 ~99.5 20 00 1 TBA3[PMo12O40]/rGO Cathode 140 1 mA ~99 10 03 (EDA- rGO)/Anderson-type Anode 1835 100 mA.g-1 ~54.4 100 101 Na3[AlMo6O24H6] (NAM) 114 Page 115 of 124 Energy & Environmental Science 3D rGO@PANI/PW12 Cathode 235 200 mA.g-1 87 50 View Article Online 104 DOI: 10.1039/D0EE03407J 2 Mo6-SCN Anode 1678 50 mA.g-1 52.2 100 Energy & Environmental Science Accepted Manuscript 97 2 (Py-Anderson) -CNTs Anode 1898.5 0.5 mA.cm-2 35.1 100 98 2 Mn-Anderson /SWNT Anode 3405.4 0.5 mA.cm-2 ~27.4 100 99 3 TBA-PMO11/CNT Anode 3014.1 0.5 mA.cm-2 28.2 100 00 POMOF-based on 1 Anode 1421 1.25 C ~24.6 500 10 (Ni6PW9)SBUs 1 NNU-11 Anode 1322.3 50 mA.g-1 56.7 200 05 Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. 1 NENU-507 Anode 1008 100 mA.g-1 ~63.5 100 07 POMOFs-based 1 Anode 2367.8 50 mA.g-1 45.4 100 08 PMo10V2/rGO 1 POMs-ILs@MOFs Anode 1665.5 0.1 A.g-1 ~75 100 11 CuPW/SWNTs Anode 1231.7 100 mA.g-1 71.8 170 106 3 PMo12@FeBTC Anode 1542.6 100 mA.g-1 42.6 50 01 3 Ni-POM Anode 1325 100 mA.g-1 53.5 50 02 3 Ag NPs/POMs Anode 1760 0.1 C - - 03 POM-derived metal composites as anode for Li-ion batteries 3 Hierarchical MoS2 Anode 980 100 mA.g-1 97.6 50 04 MoO2/Mo2C/C 3 Anode 1172 100 mA.g-1 ~68.2 100 05 spheres 3D porous MoP@C 3 Anode 1439 100 mA.g-1 71.4 100 06 hybrid MoO2@C nano- 3 Anode 2936.8 100 mA.g-1 ~49 50 07 octahedrons Mo2C/N–C 3 mesoporous hetero- Anode 1206.2 0.1 A.g-1 78 50 08 NWs 3D hierarchically 3 Anode 1064.8 0.1 A.g-1 87 100 09 porous Mo2C 3 MoS2–C Anode 1375 1 A.g-1 90.9 200 10 115 Energy & Environmental Science Page 116 of 124 MoO2@MoS2 3 Anode 1259 100 mA.g-1 80.9 200 View Article 11 Online Nanoarchitectures DOI: 10.1039/D0EE03407J 3 MoC–N–C Anode 2479 100 mA.g-1 50.2 300 Energy & Environmental Science Accepted Manuscript 12 2D layered 3 mesoporous Anode 1238.4 100 mA.g-1 67 150 13 MoS1.12Se0.88/rGO Red MoSe2 3 Anode 1706.88 1 A.g-1 65.9 500 14 nanosheets Mo6-SCN =[(Bu4N)2[Mo6O18-N-Ph-(o-CH3)2-p-SCN], NNU-11= [PMo8vMo4VIO37(OH)3Zn4][TPT]5·2TPT·2H2O (TPT=tris-(4-pyridyl)triazine), NENU-507= (TBA)3[PMo8VMo4VI O38(OH)2Zn4(PBA)2]·H2O (TBA = tetrabutylammonium ion, H PBA = 4-(pyridin-4-yl) benzoic acid), CuPW= ([Cu18(trz)12 Cl3(H2O)2][PW12O40]), BTC= Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. benzene-1,3,5,-tricar- boxylate, Ni-POM = [Ni5(OH)3(trzS)3(en)(H2O)(B-α-PW9O34)], Ag NPs/POMs =Silver nanoparticles/[[Ni2,5(Hpen)4(PW9O34)]·5H2O] 116 Page 117 of 124 Energy & Environmental Science Table 2. Performance summary for POMs applied as ORR catalyst materials. View Article Online DOI: 10.1039/D0EE03407J Catalyst Medium On set for ORR Remarks Ref Energy & Environmental Science Accepted Manuscript Co7(AlePyZn)2 Neutral 0.710 V (Neutral) 4 electron 242 and basic and 0.75 V (Basic) pathways of vs RHE. reaction. NH4Co7/TTF-F (i) Neutral 0.838 V vs RHE (i) One-step 243 Cu6Ni7/TTF-F (ii) 0.854 V vs RHE (ii) reduction of oxygen Co4P2@Carbon Neutral 0.90V @N-CNT VS Out of various 244 (SWCNT, Graphene RHE carbon support, flakes, N doped CNT) the nitrogen Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. doped CNT gives best performance. (PEI/P5W30−RGO)10 Acidic 0.80V vs Ag/AgCl POM played 245 multilayers electrode. main role in electrocatalysis PMo11V@carbon suports Acidic 0.18 V vs. RHE Strong 246 (carbon-black, SWCNT PMo11V@graphene dependency on and graphene) (best) carbon support. PW11-3DOM Acidic 0.69 V vs. NHE H2O2 Formation 247 rate 2.4 μmol h-1 117 Energy & Environmental Science Page 118 of 124 Table 3. Performance summary for co-catalyst POMs applied as ORR catalyst materials. View Article Online DOI: 10.1039/D0EE03407J Catalyst Medium Onset for ORR Remarks Ref Energy & Environmental Science Accepted Manuscript Pd/PMo12/rGO Acidic 0.6 V vs. Ag/AgCl High methanol tolerance 256 Ag/PW12/CNT Basic -0.4 V vs. SHE Instability of POM in 257 alkaline medium Ag/PW12/rGO Basic 0.15 V vs. RHE 30% Ag NPs loading has 258 comparable performance as commercial Pt/C catalyst with 20% loading. Ag0NN@H7[β- Basic +0.924 V vs. RHE Green, facile and large- 259 PMo4VMo8VIO40]– scale synthesis of Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. GNS nanohybrids. Ag NPs@PW12/rGO Basic 0.86V vs RHE 20% loading of Ag NPs 260 gives best performance. Ag@Pt/MWCNTs- Acidic 0.85V vs. Ag/AgCl 25% of PW12, 10% each 261 PW12 loading of Ag and Pt NPs 118 Page 119 of 124 Energy & Environmental Science Table 4. Summary of performance of Photocatalytic reduction of CO2 View Article Online DOI: 10.1039/D0EE03407J Catalyst Final products yield Remarks Ref CO Energy & Environmental Science Accepted Manuscript RuIII(H2O)SiW11O39]5- 50µmol 20-hour irradiation, 315 Φ = 2% H5PWV2W10O40 0.9 µmol The POM act as co- 316 catalyst to a dirhenium molecular catalyst Na10[Co4(H2O)2(PW9O34)2 107 µmol g-1 h-1 43 wt% Co4 content. 317 @g-C3N4 Convenient recovery, steady reuse. Catalyst Product yield µmol g-1 h-1 Remarks Ref Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. CO CH4 H2 Au@[PTi2W10O40]7- 12.8 2.1 2.6 318 Au@[PW12O40]3- 0.5 0.15 Keggin-type POM Au/K7(PTi2W10O40) 2.1 0.35 0.29 Catalyst Final product TON Remarks Ref µmol HCOOH [Mo154]1165 116.7 778 POM microions, 319 HCHO was also obtained. [Mn6P3W24]931 40.6 270 No HCHO is produced [Mo132]1064@rGO 205 1366 HCHO was also obtained. Catalyst Final product TON Remarks Ref nmol/g CO CH4 NENU-605 52 170 135.9 TON of CH4 alone 320 104.1. TON here is per 1000 NENU-606 68 402 282.2 TON of CH4 alone 241.4. TON here is per 1000 NENU-607 47 70 25.4 TON of CH4 alone 15.2. TON here is per 1000 (NENU-605)-H[[Na2K4Mn4(PO4) (H2O)4]3[[Mo6O12(OH)3(HPO4)3(PO4)]4[Mn6(H2O)4]] (NENU-606)-H[[Na6CoMn3(PO4)(H2O)4]3[[Mo6O12(OH)3(HPO4)3(PO4)]4[Co1.5Mn4.5]] (NENU-607)-Mn [Mo6O12(OH)3(HPO4)3(PO4)]2 119 Energy & Environmental Science Page 120 of 124 Table 5 Summary of performance of different POM-MOF catalysts. View Article Online DOI: 10.1039/D0EE03407J Catalyst E Onset Current Charge TOF (h-1) Ref. Energy & Environmental Science Accepted Manuscript (V vs. SHE) Potential density transfer (V) (mA cm-2) resistance (Ω) Co-PMOF -0.8 -0.35 18.08 9.83 1656 Fe-PMOF -0.7 -0.53 0.47 10.26 17.45 291 Ni-PMOF -0.8 -0.58 0.27 10.70 8.11 Zn-PMOF -0.9 -0.60 0.02 12.17 0.005 Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Note: Charge transfer resistance was measured by electrochemical impedance spectroscopy (EIS). 120 Page 121 of 124 Energy & Environmental Science Table 6. The performance of different electro-(photo)-catalyst towards NRR where STA- View solar to Article Online DOI: 10.1039/D0EE03407J ammonia. Sample NH3 yield STA TOF Faradaic Double Ref. Energy & Environmental Science Accepted Manuscript efficiency (h-1) efficiency layer (%) (%, vs Capacitance RHE) (mF cm-2) ZIF-67 22.2 µmol/L- - - - - 1 h-1 292 PMo12 15.8 µmol/L- - - 1 h-1 ZIF-67@ PMo12 39.4 µmol/L- 0.008 24 - 1 h-1 292 PMo11V 16 µmol/L-1 - - - h-1 Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. ZIF-67@ PMo11V 70 µmol/L-1 0.015 42 - h-1 PMo10V2 23 µmol/L-1 - - - h-1 ZIF-67@ PMo10V2 74.8 µmol/L- 0.016 42 - 1 h-1 PMo9V3 27 µmol/L-1 - - - 292 h-1 ZIF-67@ PMo9V3 134.6 0.029 76 - µmol/L-1 h-1 PMo4V8 28.4 µmol/L- - - - 1 h-1 ZIF-67@ PMo4V8 149 µmol/L- 0.032 94 - 1 h-1 Fe1.89Mo4.11O7/FeS2@C 105.3 µg h-1 - - 54.7% at - 3.5 mgcat-1 0.4 V 293 FeMoO4/FeS2@C 51 µg h-1 - - 43.9% at - 3.4 mgcat-1 0.5 V 121 Energy & Environmental Science Page 122 of 124 Table 7. Yields of the main acids produced from the switchability experiments under oxygen and View Article Online DOI: 10.1039/D0EE03407J nitrogen atmosphere. Reaction conditions: 2 mmol of substrate, 0.5 mmol of (vanadium content) H6[PV3Mo9O40] or K5[V3W3O19] catalyst, 10 g of water as solvent, 160 °C reaction temperature, 20 Energy & Environmental Science Accepted Manuscript bar oxygen pressure or nitrogen pressure, 1000 rpm stirrer speed, 1 h reaction time. POM Type Substrate Environment Yield Ref. K5[V3W3O19] Glucose Oxygen YFA=20% Nitrogen YLA=17% Glyceraldehyde Oxygen YFA=16% 294 Nitrogen YLA=17% H6[PV3Mo9O40] Glucose Oxygen YFA=36% Nitrogen YLA=10% Glyceraldehyde Oxygen YFA=42% Nitrogen YLA=40% Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. 122 Page 123 of 124 Energy & Environmental Science Table 8. Literature overview of POM based materials for alcohol oxidation in direct View alcohol Article Online DOI: 10.1039/D0EE03407J fuel cells (DAFCs) Energy & Environmental Science Accepted Manuscript POM/composite Electrolyte solution Chronoamperometry Ref. Voltage Intensity Time (V) plateau/ (s) (mA.mg-1) Pt/(epy)3PMo12O40/r 0.5M CH3OH + 0.5 M H2SO4 0.7 118 3600 279 GO N2-saturated Pt/H3PMo12O40/rGO 0.5M CH3OH + 0.5 M H2SO4 0.7 24 3600 279 N2-saturated Pd0.5Ni0.5/RGO/HPW 0.5 M CH3CH2OH + 0.5 M n.a. ~180 4000 321 -3 KOH Pt/H3PMo12O40/GO/ 1 M CH3OH + 0.1 M KOH 0.7 ~50 7200 322 Nickel foam 1 Published on 04 January 2021. Downloaded on 1/7/2021 1:27:30 AM. Pt/H3PMo12O40/GO/ 1 M CH3OH + 0.1 M KOH 0.7 ~0 7200 322 Nickel foam 2 Pt/H3PMo12O40/GO/ 1 M CH3OH + 0.1 M KOH 0.7 ~0 7200 322 Nickel foam 3 Pt/H3PMo12O40/GO/ 1 M CH3OH + 0.1 M KOH 0.7 ~0 7200 322 Nickel foam 4 Au@Pd/PMo12/RGO 1 M CH3OH + 1 M NaOH -0.3 ~4 mA.cm-2 1000 323 Activated 1 M CH3OH + 1.0 M NaOH -0.2 ~50 800 294 PdNPs@PMo12 Non-Activated 1 M CH3OH + 1.0 M NaOH -0.2 ~0 800 294 PdNPs@PMo12 H5PMo10V2O40 1 M CH3OH + 0.5 M H2SO4 <0.1 1800 324 adsorbed on Pt mA.cm−2 H5PMo10V2O40 1 M CH3OH + 0.5 M H2SO4 0.3 <0.006 mA 1800 324 H3PMo12O40 Pt-Ru 1 M CH3OH + 0.5 M H2SO4 0.3 <0.04 mA 200 289 H3PMo12O40 Pt-Sn/C 1 M CH3OH + 0.5 M H2SO4 0.3 <0.4 mA 200 289 H3PMo12O40 Pt 1 M CH3OH + 0.5 M H2SO4 0.4 <0.003 mA 200 289 H3PMo12O40 Pt-Ru 1 M CH3OH + 0.5 M H2SO4 0.4 <0.154 mA 200 289 H3PMo12O40 Pt-Sn/C 1 M CH3OH + 0.5 M H2SO4 0.4 ~0.4 mA 200 289 SiW12O404− on Pt/C 1 M CH3OH + 0.5 M H2SO4 0.7 ~1 A.mg- 1200 325 catalyst 1.cm-2 PtRu Nanoparticles 1 M CH3OH + 0.5 M H2SO4 0.4 ~0.8 3600 286 with H3PMo12O 40 H3PMo12O40 1 M CH3OH + 0.5 M H2SO4 0.4 ~1 3600 286 PtRu/PDDA/MWCN T Pt black 0.5 M CH3OH + 0.5 M H2SO4 0.1 ~0.2 mA.cm-2 200 326 nanoparticles deposited onto TiO2- supported Au-PMo12 Pt black 0.5 M CH3OH + 0.5 M H2SO4 0.2 ~0.65 200 326 nanoparticles mA.cm-2 deposited onto TiO2- supported Au-PMo12 123 Energy & Environmental Science Page 124 of 124 PtIr/PW12/MWCNT 0.5 M CH3OH + 0.5 M H2SO4 0.6 200 3000 326 View Article Online PtRu/C PW12 0.5 M CH3OH + 0.5 M H2SO4 0.5 ~3 mA.cm-2 1500 292 DOI: 10.1039/D0EE03407J PMo12/Pt4Pd1/MWC 0.5 M CH3OH + 0.5 M H2SO4 0.6 200 3000 327 NT Energy & Environmental Science Accepted Manuscript PdNPs@PMo12-GCE PBS (pH= 8.5) Phosphate -4.5 -0.5mA 300 290 buffer solution PdNPs/ 0.5 M CH3OH + 0.1 M HClO4 0.6 ~0.045 20000 328 PWO12/ILs/GCE mA.cm-2 Pt/PdNPs/PWO12/ILs 0.5 M CH3OH + 0.1 M HClO4 0.6 ~0.050 20000 328 /GCE mA.cm-2 Pt- 0.5 M CH3OH + 0.1 M HClO4 0.6 ~0.045 20000 329 PdNPs/NaPWO/GQ mA.cm-2 D PtNPs/NaPWO/GQD 0.5 M CH3OH + 0.1 M HClO4 0.6 ~0.035 20000 329 mA.cm-2 Published on 04 January 2021. 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