Recent advances in catalyst materials for proton exchange membrane fuel cells Cite as: APL Mater. 9, 040702 (2021); https://doi.org/10.1063/5.0045801 Submitted: 29 January 2021 • Accepted: 08 March 2021 • Published Online: 01 April 2021 L. Mølmen, K. Eiler, L. Fast, et al. COLLECTIONS This paper was selected as Featured This paper was selected as Scilight ARTICLES YOU MAY BE INTERESTED IN Progress towards lower-cost proton exchange membrane fuel cells Scilight 2021, 141110 (2021); https://doi.org/10.1063/10.0004268 An overview of bipolar plates in proton exchange membrane fuel cells Journal of Renewable and Sustainable Energy 13, 022701 (2021); https:// doi.org/10.1063/5.0031447 Epitaxial ferroelectric oxides on silicon with perspectives for future device applications APL Materials 9, 040701 (2021); https://doi.org/10.1063/5.0039161 APL Mater. 9, 040702 (2021); https://doi.org/10.1063/5.0045801 9, 040702 © 2021 Author(s). APL Materials RESEARCH UPDATE scitation.org/journal/apm Recent advances in catalyst materials for proton exchange membrane fuel cells Cite as: APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 Submitted: 29 January 2021 • Accepted: 8 March 2021 • Published Online: 1 April 2021 L. Mølmen,1,2 K. Eiler,3 L. Fast,1 P. Leisner,2 and E. Pellicer3,a) AFFILIATIONS 1 Department of Electrification and Reliability, RISE Research Institutes of Sweden, Borås, Sweden 2 Department of Materials and Manufacturing, Jönköping University, Jönköping, Sweden 3 Departament de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain a) Author to whom correspondence should be addressed: eva.pellicer@uab.cat ABSTRACT Research on fuel cell technology is constantly gaining importance, while global emission requirements are becoming more and more restrictive. For environmentally neutral proton exchange membrane fuel cells (PEMFCs) to become a competitive technology, sustainable infrastructures need to be established. One of the main showstoppers is the utilization of the rare and therefore costly precious metal Pt as the key element in the electrocatalysis of hydrogen and oxygen. A huge amount of research is done on immensely reducing or even replacing Pt for future PEMFC technology. In this research update, the progress on oxygen reduction reaction catalysts in acidic media over the past two years is reviewed, with special attention to their durability. © 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0045801 NOMENCLATURE MA mass activity MEA membrane electrode assembly ADT accelerated degradation test MOF metal–organic framework AEM anion exchange membrane NG nanographene AEMFC anion exchange membrane fuel cell NP nanoparticle ALD atomic layer deposition NT nanotube b Tafel slope NW nanowire CB carbon black OCV open circuit voltage CNT carbon nanotube ORR oxygen reduction reaction CNW carbon nanowire PANI polyaniline CV cyclic voltammetry PEM proton exchange membrane DFT density functional theory PEMFC proton exchange membrane fuel cell DOE U.S. Department of Energy PGM platinum group metal E1/2 half-wave potential RDE rotating disk electrode ECSA electrochemically active surface area RGO reduced graphene oxide FC fuel cell RHE reversible hydrogen electrode GDL gas diffusion layer SA specific activity HOR hydrogen oxidation reaction SAC single-atom catalyst IL ionic liquid STEM scanning transmission electron microscopy jk kinetic current density TEM transmission electron microscopy LSV linear sweep voltammetry ZIF-8 zeolitic imidazolate framework APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-1 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm I. INTRODUCTION by creating edge-rich and porous structures, making use of favor- Proton exchange membrane fuel cell (PEMFC) vehicles are able structures such as graphene or carbon nanotubes (CNTs), and emerging into the commercial market as the drive for zero-emission by increasing the catalyst’s hydrophobicity. Additionally, transition vehicles increases.1 However, the cost of the PEMFC is still high, metal dithiolenes (M–S–C structures) comprising Fe or Co have with one of the major cost components being the noble metal cat- been recently proposed by density functional theory (DFT).5 A num- alyst. The rather sluggish oxygen reduction reaction (ORR) at the ber of different compounds have likewise been determined by DFT cathode requires catalysts with a higher surface area and optimized as sufficiently stable two-dimensional materials for ORR,6 although structure in order to minimize the use of platinum group metals experimental demonstration is yet to be performed. (PGMs). In the following, the most recently investigated electrocata- Today, Pt and its alloys are the most commonly used catalysts lysts for ORR will be discussed for each strategy, being split into for PEMFCs at both the anode and the cathode. In 2017, the U.S. PGM-based catalysts on the one hand and PGM-free catalysts on Department of Energy (DOE) set the technical target of reaching the other hand. The focus is on their synthesis, performance, and a total loading of platinum group metals below 0.125 mg/cm2 by durability. Due to the vast number of publications in the past two 2020. Their target activities at 0.9 V vs reversible hydrogen electrode years, the focus lies on those works providing membrane electrode (RHE) are a PGM mass activity (MA) of 0.44 A/mgPGM for PGM cat- assembly (MEA) tests in fuel cells. Additionally, works on catalyst alysts and a current density of 44 mA/cm2 for PGM-free catalysts.2 materials that have not been tested at this level but in the view of While the Pt loading can easily be reduced below 0.05 mg/cm2 at the authors present promising performance and applicability in the the anode,3 the stability and performance of the electrocatalyst at PEMFC have also been considered. The goal of this work is to give a the cathode is critical. In the acidic and oxidizing conditions at the brief overview of the latest developments and to point out the most cathode, Pt nanoparticles (NPs) tend to agglomerate and grow, thus promising approaches for future research. losing the surface area. Although some research papers have reached While ORR studies in alkaline media are currently very pop- the 2020 goals, commercial fuel cells still operate with around ular, the focus of this work is on the research dedicated to ORR 0.35 mgPt /cm2 when used in vehicles.1 in acidic media in order to identify suitable materials and synthesis There are several strategies for improving the cathode catalyst processes for the successful development of PEMFCs. The primary in terms of efficiency and durability (Fig. 1). The size of commercial objective does not necessarily lie in outperforming commercial Pt/C, Pt NPs has already been minimized to 3–6 nm. To further reduce but in achieving high reliability and durability of the catalyst in a the cost and/or improve the catalytic activity of Pt, research groups cost-efficient synthesis process. are aiming at the shape control of Pt NPs, alloying of Pt, core–shell NP structures with Pt-rich surfaces, and the synthesis of hollow NP II. PGM-BASED CATALYSTS structures. The durability can be improved by confining the PGM Four different approaches are mainly pursued on PGM-based NPs in a carbon based structure, as well as by making use of differ- catalysts: the alloying of Pt or other PGM, the optimization of the ent support materials. Recycling of membrane electrode assemblies pure Pt catalyst, an optimization of the supporting substrate, and (MEA) containing PtCo has also shown that the global impact of the the protection of the catalyst in an organic structure. Naturally, these catalyst can be reduced by extracting the metals from the aged MEA approaches are often combined to acquire superior results. and synthesizing new catalysts.4 On the other hand, Pt-free catalysts such as single-atom cata- lysts (SAC) are the focus of investigations (Fig. 1). These are most A. PGM alloys commonly of M–N–C type, where M is a metal, most commonly PGM alloys allow for reducing the use of the PGM, and elec- Fe or Co.3 Optimization of these catalysts is currently followed up tronic properties improving the catalytic activity of the PGM can be FIG. 1. Current strategies for improv- ing the efficiency and durability of PGM- based (left) and PGM-free (right) ORR catalysts for PEMFCs. APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-2 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm obtained. The alloying elements may also contribute with their own Wang et al. investigated ternary alloys of Pt with the three tran- catalytic activity. sition metals Fe, Ni, and Co, finding that the Pt–Co–Fe alloy was the One of the most popular approaches is the alloying of Pt with most favorable among the possible combinations in terms of both transition metals such as Fe, Co, Ni, and Cu, which are usually activity and durability.43 employed in the form of NPs with a Pt-rich surface. The Pt-rich The non-platinum containing PGM alloys recently reported and electrochemically stable surface is usually formed by either a for acidic ORR are Pd-based55,56 or Ir-based.57 Pd–Mo nanosheets specific etching process or the dissolution of transition metal atoms investigated by Luo et al. showed extremely high ORR activity in during operation; the result is often referred to as a core–shell alkaline media and also have superior performance in acidic media; structure. Several of these catalysts are already being used com- however, the authors considered that the stability in acidic media mercially, such as Pt–Co, which serves in Toyota’s fuel cell car;7 was insufficient for practical applications.55 More frequently, Pd however, research on different Pt–Co NP structures is still ongo- is also alloyed with Pt in a variety of nanostructures.35,58–63 Sev- ing.8–11 Current research focuses a lot on the development of Pt–Ni eral works reported ternary alloys including both Pt and Pd.64–69 alloys,12–15 where several works report the synthesis of octahedral Nan et al. investigated different Pd–M/Pt core–shell catalysts with NPs.16–20 Ni, Co, and Fe as alloying elements. The Pd–Fe core provided the Gong et al. successfully synthesized Pt–Ni as dumbbell-shaped best ORR activity, while all alloy cores outperformed the catalyst particles, which did not show increased electrochemically active sur- containing a pure Pd core.68 A ternary alloy of exclusively noble face area (ECSA) but were able to achieve both higher ORR MA metals (Pt–Pd–Ir) was investigated by Deng et al.70 and higher retention of activity after accelerated degradation testing For most reported alloy catalysts, an improvement due to alloy- (ADT) when compared to globular Pt–Ni NPs21 (Fig. 2). ing with additional elements is reported. Deng et al. found that A neural-like network of Pt–Co NPs connected by CNTs binary Pt–Pd on carbon nanowires (CNWs) outperformed ternary (Fig. 3) developed by Wang et al. was able to show outstanding Pt–Pd–Au, however, the Au phase was mostly segregated so that the performance at both H2 /O2 and H2 /air fueled PEMFCs while being effect of alloying could not be studied.59 completely free from degradation after both ADT in an electrochem- ical half-cell and continuous operation of PEMFC.22 B. Optimization of pure Pt catalysts Several other binary alloys investigated involve Pt–Ag,23 The optimization of pure Pt focuses on shape-tuning, to opti- Pt–Bi,24 Pt–Cu,25,26 Pt–Fe,27–30 Pt–Ir,31–35 Pt–Se,36 Pt–Te,37 and mize ECSA and to expose the most active crystal planes, and on the Pt–Zn.38 In addition, several ternary39–48 and quaternary alloys49,50 tuning and control of the particle size. In terms of shape, rhombic have been investigated. A Pt-rare earth catalyst is proposed by dodecahedral NPs show the highest activity in perchloric acid, while Chu et al.;51 however, such a catalyst is questionable in terms of the cubic shape is the least favorable71 (Fig. 4). large-scale application due to its low abundance. Dong et al. studied the mechanism of ORR on Pt(100), Pt(110), Apart from alloying, a few works concentrated on the phospho- and Pt(111) facets and were able to monitor the intermediates in situ rization52,53 or nitrogenation54 of Pt. by Raman spectroscopy. On Pt(111), where ORR is improved, the Dionigi et al. and Cao et al. reported further improvement in reaction takes place via the formation of HO2 radicals, whereas OH the durability of octahedral Pt–Ni by introducing additional alloy- radicals are formed on Pt(100) as well as Pt(110).72 Chen et al. syn- ing elements in a second synthesis step, such as Mo39 or Cu,40 thesized NPs in a multipod structure showing good performance; respectively. however, degradation was still an issue.73 Cheng et al. used an Se film on the carbon support in order to seed Pt NPs smaller than 2 nm, resulting not only in superior perfor- mance but also in much improved durability.74 Another approach was pursued by Chen et al., who synthesized supramolecular Pt- containing structures with promising results for ORR.75 C. Optimization of the supporting layer The interface between the PGM catalyst and its supporting layer is crucial since poor adhesion can lead to catalyst detach- ment. In addition, the support must provide an electrical connec- tion to the catalyst and may influence the density of states in the d-band of the catalyst. Carbon black (CB) is the most commonly used support for the PEMFC catalyst. Current research includes the use of CNTs,22,76–80 CNWs,9,59,81 metal oxides,82–86 a combination of those,87 and various other compounds.88–91 Those can be used to either replace CB or act as an intermediate support between the catalyst and CB. FIG. 2. Dumbbell-shaped Pt–Ni NPs for ORR captured by TEM [(a), (c), and (e)], Doping of CB with N, as well as the use of Mn as a precur- STEM [(b) and (d)], and their 3D structure model (f). The Pt–Ni/C catalyst reached sor results in more graphitic and corrosion-resistant carbon.92,93 an ORR MA of over 1.3 A/mgPt .21 Reprinted with permission from Gong et al., As reported by Yang et al., Pt particles deposited on N–CB were Appl. Catal. B 246, 277 (2019). Copyright 2019 Elsevier. smaller and better dispersed, resulting in an increase in ECSA and APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-3 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm FIG. 3. Scheme of hollow Pt–Co NPs in an organic network connected by CNTs inspired by the neural network (top). SEM micrographs of the intermediate products during synthesis [(a)–(c)] and the products after pyrolysis [(d) and (e)]. TEM micrographs of the obtained Pt–Co/CNT/C catalyst [(f)–(i)], which reached a peak power density of over 1 W/cm2 in a H2 /O2 PEMFC at a Pt loading as low as 60 μg/cm2 at the cathode.22 Republished with permission from Wang et al., J. Mater. Chem. A 7, 19786 (2019). Copyright 2019 Royal Society of Chemistry. PEMFC performance.92 Increased graphitization is also achieved by carbon-based matrix, where the PGM catalyst is not in direct contact N-doping along with increased porosity of the carbon support.93,94 with the polymer membrane.96,97 Zhao et al. successfully “confined” N-doping is proposed to also give a more homogeneous ionomer Pt3 Co NPs in mesoporous carbon derived from the zeolitic imida- distribution.94 Coalescence of Pt and Pt–Co particles was sup- zolate framework (ZIF-8). With the NPs trapped in the mesoporous pressed by utilizing a porous carbon support, resulting in improved structure, both detachment and agglomeration of NPs could be performance and durability.95 largely avoided, while the catalytic activity was not compromised.98 Li et al. used an ionic liquid (IL) film to cover Pt/C NPs. In addi- D. Protection of the PGM catalyst in an organic matrix tion to the protective effect of those films, their electronic prop- The most promising approach in order to limit degradation erties also improved ORR activity with respect to the bare Pt/C.99 and increase the durability of PGM catalysts is their protection in a Zhou et al. created a carbon-based shell for Pt derived from Nafion, APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-4 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm FIG. 4. Shape dependency of ORR activity for pure Pt NPs in perchloric acid. FIG. 6. SEM micrographs of Pt–Co mesoporous NTs synthesized using Te NWs and F127 block copolymer as templates. The inset in (b) shows the cross section improving the PEMFC performance at a low Pt loading of 0.07 of a NT. Interestingly, the Pt–Co NTs outperformed Pt–Ni NTs synthesized through mg/cm2 ; however, the durability was not studied.96 the same route.106 Reprinted with permission from Yin et al., ACS Sustainable Choi et al. synthesized Pt–Fe NPs in a block copolymer matrix Chem. Eng. 7, 7960 (2019). Copyright 2019 American Chemical Society. and achieved the formation of a carbon shell around the NPs. This catalyst achieved extremely high Pt mass activity in a half cell of 9 A/mg, and in PEMFC, an extremely low Pt loading of In the solvothermal reduction method, metal precursors (usu- 0.01 mg/cm2 was used to achieve a performance comparable to ally acetyl acetates) are dissolved and mixed with a reducing agent commercial Pt/C.30 and other additives. Carbon is often mixed in before the reduction Xiao et al. investigated the ORR of the noble metals Ir and Ru in process, which takes place in an autoclave at elevated temperatures. a PGM–N–C configuration as SACs, reporting extraordinarily high The product is then dried. For application in an MEA, the catalyst is ORR MAs including promising results at the PEMFC level.100,101 usually dispersed in a solvent and sprayed onto the electrode. Liu et al. determined that Ir and Rh showed favorable performance Zhao et al. studied the size dependence of icosahedral Pt NPs when applied as SAC, in contrast to Pt and Pd.102 synthesized using tetraethylene glycol as both the solvent and reduc- tant and were able to synthesize the catalyst within 20 min. Inter- E. Synthetic strategies estingly, the highest kinetic current was reached for the largest NPs (14 nm), which also exhibited the best durability.103 The most common approach for the synthesis of PGM- Lei et al. demonstrated the synthesis of Pt–Co, Pt–Fe, and based catalysts is the solvothermal reduction method in autoclave. Pt–Ni NPs through the same route, allowing a direct comparison Meanwhile, alternative synthesis routes include pyrolysis, galvanic between their ORR activities, finding that Pt–Co was the most active replacement, selective etching, and electrodeposition (Fig. 5). More- at the ORR between them.104 A similar observation was made by over, the synthesis may comprise those multiple synthesis steps. Wang et al.105 Yin et al. reported a room temperature synthesis for meso- porous Pt–Co and Pt–Ni nanotubes (NTs).106 Here, the reduction of Pt–M (M = Co, Ni) takes place using ascorbic acid as a reduc- ing agent and by galvanic replacement of the previously synthesized Te nanowires (NWs). In addition, a polymeric surfactant introduces mesoporosity in the material (Fig. 6). The same approach is used by the group to prepare mesoporous Pt–Te NTs.37 Yang et al. proposed a silica-assisted pyrolysis method for the reduction of Fe–Pt NPs.27 After pyrolysis, the silica is removed and the Pt–Fe NPs are etched to obtain a hollow structure (Fig. 7). Alternative synthesis routes for Pt–Fe NPs are the FIG. 7. Scheme for the formation of hollow Pt–Fe NPs from Pt and Fe3 O4 precur- sors by silica-assisted pyrolysis. SiO2 prevents the agglomeration of NPs during the heat treatment at 900 ○ C at which formation of Pt–Fe NPs takes place. Subse- quent etching in HF removes SiO2 and dealloys the NPs to obtain an electrochem- ically stable composition of Pt–Fe. The Pt–Fe/C catalyst reached a MA of over 1 A/mgPt and 91% retention of MA after ADT.27 Reprinted with permission from Yang et al., Chem. Eur. J. 26, 4090 (2020). Copyright 2019 Wiley-VCH Verlag FIG. 5. Main strategies for the synthesis of PGM electrocatalysts. GmbH & Co. KGaA, Weinheim. APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-5 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm ECSA and MA.17,37,65,67,69,70,106 However, their applicability in a PEMFC is questionable, as its performance depends on the number of triple points, which do not necessarily increase due to mesoporos- ity of the catalyst. In these cases, high ORR activities at the rotating disk electrode (RDE) level need to be backed up by PEMFC testing to prove the effect of a mesoporous structure on the FC. Otherwise, porous structures can still lead to improved MAs due to a decrease in the amount of non-surface material and can be ultimately optimized by the synthesis of hollow structures, as long as the stability is not compromised.12,27,69 A more promising approach is the combination of the cata- lyst with a mesoporous support layer, where the catalyst is confined within the mesopores.94,98,107 An alternative for the synthesis of PGM NPs is through pulse electrodeposition onto the carbon support.11,108 Wang et al. success- fully deposited Pt NPs within a desirable particle size of 3–10 nm after determining the optimum deposition parameters. The elec- trodeposited Pt/C showed higher half-wave potential E1/2 , kinetic current density jk , and stability with substantially lower Pt loading in comparison to commercial Pt/C.108 Hussain et al. produced an Nb–Ti oxide substrate by atomic layer deposition (ALD), followed by the deposition of Pt by mag- netron sputtering, and high ORR activity at a very low Pt load- FIG. 8. TEM micrographs [(a) and (c)–(e)] and 3D model (b) of hexapod Pt–Pd–Cu ing of 9 μg/cm2 was reached.86 A sputter deposition approach NPs, which reached an ORR MA of 1.17 A/mgPt .64 Reprinted with permission was also followed by Ergul-Yilmaz et al. for the deposition from Chen et al., ACS Sustainable Chem. Eng. 8, 1520 (2020). Copyright 2020 of Pt.109 American Chemical Society. F. Durability issues Many PGM-based catalysts suffer from constant performance impregnation–reduction method28 and pyrolysis from a single pre- degradation.12 For pure Pt, this generally refers to the dissolution cursor.29 Higher process temperatures lead to the formation of an and redeposition of Pt, leading to particle growth and thus loss of ordered structure, more favorable for ORR activity.28,29 the total catalyst area.110 For alloys, leaching of non-noble metals can Chen et al. achieved the synthesis of ternary Pt–Pd–Cu alloy cause compositional and structural changes. A detailed in situ exam- NPs in a hexapod shape (Fig. 8). The formation of this unique shape ination of the degradation of octahedral Pt–Ni was performed by was triggered by a combination of etching and selective growth.64 Beermann et al., identifying coalescence and particle motion as the Mesoporous structures are reported in many cases, and their main degradation mechanisms.111 Another considerable problem of catalytic activity is certainly promising due to their generally higher pure Pt is its methanol-dependent performance. FIG. 9. Comparison of linear sweep voltammograms (LSVs) of C-supported Pt–Ni bunched nanocages (PtNi–BNCs/C), Pt–Ni bunched nanospheres (PtNi-BNSs/C), Pt NWs, and commercial Pt/C, where the Pt–Ni nanocages demonstrate an improvement of 63 mV in E1/2 with respect to commercial Pt/C (a). LSVs of C-supported Pt–Ni nanocages including cyclic voltammograms (inset) initially and after 20 000 and 50 000 cycles of ADT, showing that neither ORR nor ECSA is significantly affected by the ADT (b).12 Reprinted with permission from Tian et al., Science 366, 850 (2019). Copyright 2019 AAAS. APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-6 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm TABLE I. Characteristics of ORR for different PGM-based electrocatalysts in 0.1 M HClO4 . jk , MA, and SA are given at 0.9 V vs RHE. ADT refers to 10 000 potential cycles between 0.6 and 1.0 V unless stated otherwise. MA and ECSA usually refer to Pt mass only. All potentials refer to RHE. Kinetic current Half-wave Tafel Mass Specific Electrochemical Retained Loss in density potential slope activity activity surface area mass activity E1/2 jk E1/2 b MA SA ECSA after ADT after ADT Catalyst (mA/cm2 ) (V) (mV/dec) (mA/mg) (mA/cm2 ) (m2 /g) (%) (mV) References Ir–N–C 9.8 0.86 41 12 200a n/ab 100 c Ir–N–C 0.83 48 97 102 Na–Pt-R/C 0.71 118 35d 75 Pd–Mo/C 660a 0.47 139 55 Pt/C 0.95 2 070 3.1 70 97 114 Pt/C 870 1.49 119 Pt/C 370 0.55 69 58 73 Pt/C 860 0.99 87 83e 115 Pt/C 3.4 0.91 108 Pt/C 0.88 44 27 81 Pt/C 260 0.43 69 96 Pt/C 260 0.65 40 74f 109 Pt(N)/C 0.93 102 69 96g 54 Pt/C–N 3.13 0.87 98 5h 120 Pt/C/PANI 63 0.09 121 Pt/Co–N/C 0.89 227 48 85h 4h 10 Pt/Ir/C 2.9 0.86 373 1.30 84b 33 Pt/Mn/C 0.88 300 0.57 67 31h 93 Pt/Se/C 0.96 750 0.32 231 93 74 Pt/IL/C 800 1.32 99 Pt/Nb–O/C 0.92 64 560 0.85 66 100 84 Pt/Nb–O/C 429 0.52 83 80i 83 Pt/Nb–O/CNTj 1.1 0.86 59 57 82 93h 6h 87 Pt/Nb–Ti–O/Cj 0.77 52 56 0.32 20 86 Pt/Pd/C 750 1.01 60 62 Pt/Pd–Co/C 160a 0.58 130 68 Pt/Pd–Fe/C 260a 0.88 129 89 68 Pt/Pd–Ni/C 110a 0.69 125 68 Pt/Ti–C 0.82 80 0.13 88 1 88 Pt/Ti–N 0.85 140 0.27 79 7 88 Pt/Ti–Nb–O 150 0.35 43 82 Pt/Ti(N,C)O 115 41 n/a 85 Pt/Ti–C–O(F)/CNT 0.90 210 0.32 64 79 Pt/Ti–C–O(F)/CNT 0.88 163 0.26 63 19k 80 Pt/Pd/W–Ni/C 0.90 370a n/a h 1h 90 Pt–Ag/C 2.17 0.88 316 3.46 91 n/a 23 Pt–Au–Cu/C 0.92 78 750a 0.49 153 n/a 48 Pt–Bi/C 0.84 1 040 87c 24 Pt–Co/C 0.97 2 260 8.6 27 94l 8 Pt–Co/C 151 128 30 78d 26 Pt–Co/C 0.89 640 1.29 49 86 104 Pt–Co/C 740 1.74 91 5 105 Pt–Co/C 0.91 62 950 0.99 50 106 Pt–Co/Nb–O/C 449 0.63 71 n/a 83 Pt3 Co/C 0.93 62 830 1.44 58 87b 5b 98 Pt–Co/CNT 0.94 852 1.38 62 103 n/a 22 Pt–Co–Fe/C 0.90 650 26 84 6 43 Pt–Co–Ni–Mo/C 450 49 78 49 Pt–Cu/C 2 570 2.82 91 25 APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-7 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm TABLE I. (Continued.) Kinetic current Half-wave Tafel Mass Specific Electrochemical Retained Loss in density potential slope activity activity surface area mass activity E1/2 jk E1/2 b MA SA ECSA after ADT after ADT Catalyst (mA/cm2 ) (V) (mV/dec) (mA/mg) (mA/cm2 ) (m2 /g) (%) (mV) References Pt–Cu–Ni/C 1 290 1.7 73 93m 10m 46 Pt–Cu–Ni–W/C 2 020 126 66g 50 Pt–Cu–W/C 0.93 56 750 1.43 53 96h 42 Pt–Fe/C 0.92 58 680 2.23 30 73 28 Pt–Fe/C 300 54 29 Pt–Fe/C 48 1 020 2.73 37 91 27 Pt–Fe/C 0.88 470 0.92 51 74 104 Pt–Fe/C 50 92 Pt–Fe/C 9 000 69 30 Pt–Fe/TiCrN 674 1.28 90b 6b 107 Pt–Fe–Mo/C 0.92 780 0.56 133 67m 26m 47 Pt–Ir/C 0.87 1.18 32 23 32 Pt–Ir/C 0.91 65 522a 1.56 33 30b 35 Pt–Ir@Pd/C 1 880 1.27 122n 84 31 Pt–Ni/C 1 640 38 63 39 Pt–Ni/C 0.94 1 676 5.84 29 16 Pt–Ni/C 0.93 1 330 27 73 21 Pt–Ni/C 914 2.45 37 75o 14o 17 Pt–Ni/C 58 1 900 7.7 25 91b 5b 18 Pt–Ni/C 790 30 58p 19 Pt–Ni/C 54 3 520 5.16 68 99e 12 Pt–Ni/C 2.84 0.87 26 268 14.3 19 13 Pt–Ni/C 0.87 400 0.77 52 70 104 Pt–Ni/C 0.90 61 900 0.89 49 106 Pt–Ni/C 45 92 Pt–Ni/C 14 29 14 Pt3 Ni(Au)/C 3 080 5.74 35 95g 113 Pt–Ni/Nb–O/C 539 0.50 107 n/a 83 Pt–Ni–Cu/C 3 700 6.2 60 82m 40 Pt–Ni–In/C 760 1.96 39 98q 41 Pt–Ni–Ir/C 409 85 86b 45 Pt–Ni–Mo/C 3 430 38 72 39 Pt–P/C 0.93 915 1.86 52 93 52 Pt–Pd/C 70 92 Pt–Pd/C 920r 1.04 72r 91p 9p 58 Pt–Pd/C 0.87 411 0.43 93 88b 59 Pt–Pd/C 0.92 850a 1.33 54 73 14 60 Pt–Pd/C 480a 1.93 45 54m 30m 61 Pt–Pd/C 0.90 65 410a 0.91 52 20b 35 Pt–Ir/C 31a 57 34 Pt–Pd–Ag/C 0.90 59 610 1.11 55 11b 65 Pt–Pd–Cu/C 0.93 1 170 2.20 53 63 12 64 Pt–Pd–Ir/C 0.95 63 1 030 2.08 48 78 7 70 Pt–Pd–Ni/C 0.94 68 1 140 1.52 56 n/a 69 Pt–Pd–Ni/C 77 1 170 3.8 32 84 66 Pt–Pd–Ni–P/C 0.93 61 450 0.89 51 84 6b 67 Pt–Te/C 0.92 65 400 0.9 45 92s 37 Pt–Y–O/C 108 0.20 53 83o 51 Pt–Zn/C 0.94 1 020 1.68 61 n/a 38 Rh–N–C 0.68 51 102 APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-8 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm TABLE I. (Continued.) Kinetic current Half-wave Tafel Mass Specific Electrochemical Retained Loss in density potential slope activity activity surface area mass activity E1/2 jk E1/2 b MA SA ECSA after ADT after ADT Catalyst (mA/cm2 ) (V) (mV/dec) (mA/mg) (mA/cm2 ) (m2 /g) (%) (mV) References a g Ru–N–C 0.82 54 4 780 11.95 17 101 Se/Pt/C 0.82t 58 1.7 36 a Normalised by total PGM mass. b After 5000 potential cycles of ADT. c After 10 000 seconds of chronoamperometry. d After 1000 potential cycles of ADT. e After 50000 potential cycles of ADT. f After 3000 potential cycles of ADT. g After 20000 potential cycles of ADT. h After 30000 potential cycles of ADT. i After 25000 potential cycles of ADT. j In H2 SO4 . k After 2000 potential cycles of ADT. l At 60 ○ C. m After 15000 potential cycles of ADT. n Normalized by mass of Pt and Ir. o After 6000 potential cycles of ADT. p After 8000 potential cycles of ADT. q After 4000 potential cycles of ADT. r Normalized by total catalyst mass. s After 5 h of chronoamperometry. t Recalculated from Ag/AgCl assuming a pH of 1. Mom et al. investigated the oxidation behavior of Pt under Tu et al. found that the addition of W to Pt–Cu led to the PEMFC conditions and found that oxidation of Pt was favored outstanding stability of the catalyst after 30 000 potential cycles, under wet conditions at oxidizing potentials, especially on Pt NPs. even though the performance has been notably decreased after 5000 Furthermore, the authors showed that the use of a Br-containing cycles.42 This example shows that sufficiently long durability tests membrane instead of a regular Nafion membrane could suppress are indispensable for the evaluation of an ORR catalyst and 10 000 oxide formation.112 potential cycles, as suggested by the DOE, should be the absolute Outstanding durability has been reported by Tian et al. on minimum.2 bunched Pt–Ni nanocages, with only marginal decrease in MA after The fact that MA of the PGM-based catalysts is often reported 50 000 cycles of ADT (Fig. 9). This performance has been related to with respect to Pt mass leads to the questionable assumption that the protective Pt skin structure of the catalyst.12 Wu et al. improved ORR solely takes place on Pt atoms. Especially, if other PGM- the durability of Pt3 Ni NWs by Au doping, while the ORR activ- metals such as Pd or Ir are involved, but also in the case of alloy- ity was unaffected by the addition of Au.113 Corroborated by DFT ing of Pt with non-noble elements, their contribution to ORR studies, the authors suggest that Au occupies surface defects of the activity is neglected and unrealistically high MA are reported. Pt3 Ni matrix and immobilizes Ni atoms. A similar observation was Although the Pt MA is therefore little meaningful from a sci- made by Shen et al. using In in order to limit the diffusion of Ni.41 entific viewpoint, it can give a good estimation on the required Although the initial performance was inferior to that of a Pt–Ni amount of Pt and therefore the cost efficiency. From a purely alloy, an improved durability of Pt–Ni–In was found, and the opti- electrochemical view, the ECSA-normalized specific activity (SA) mum In content is the result of a trade-off between activity and should provide better comparability (Table I). A sound guidance durability. for measuring and reporting the catalytic activity was produced by Feng et al. produced Pt–Te nanosheets, which were then Wei et al.117 submitted to electrochemical dissolution of Te. The so-obtained Zalitis et al. targeted the issue that although Pt alloys often show porous Pt nanosheets showed very high MA and durability even higher activity around the half-wave potential, they often do not after 30 000 potential cycles.114 Similar observations were made by exhibit as high current densities at higher ORR overpotential, close Kong et al. after dealloying Pt–Ni NWs115 and referred to the to the working potential of the PEMFC.118 It is therefore impor- higher defect density of dealloyed Pt, leading to improved ORR tant to document the MA at a higher potential such as 0.9 V vs performance.116 RHE. In a different approach, a significant improvement was made by Ir-based catalysts57 and Pt–Pd alloys35 show very good CO Guo et al. to Pt/C by applying a phosphorization treatment, leading tolerance in contrast to Pt. Bak et al. further found that carbon to the formation of Pt2 P.52 corrosion was suppressed when Ir was present.34 APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-9 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm III. PGM-FREE CATALYSTS doped with elements such as nitrogen,125,126 sulfur, phosphorous, and their combinations127–129 has proven to work as ORR cata- Research on PGM-free catalysts for the ORR in acidic media lysts in both half-cell and PEMFC experiments. However, the cur- includes ceramic catalysts such as CoS2 ,122 NiN123 and Fe3 C;124 rent densities of these catalysts are not comparable to their PGM however, the majority of the PGM-free catalysts are carbon-based. counterparts yet. Therefore, co-doping carbon with nitrogen and Carbon-based electrocatalysts profit from the huge abundance of a transition metal to form SAC M–N–C structures where M=(Fe, carbon as well as from the highly advanced existing technologies Co, Cr, Ni, Mn, Ti, Zn)130–135 is the focus of current PGM-free able to tune carbon to the desired electronic properties. Carbon research. FIG. 10. X-ray photoelectron spectra of single-atom Fe anchored to nanographene (NG) and reduced graphene oxide [(RGO, (a) and (b)] and schematic of the Fe–N4 , Fe–N5 , Fe–N3 , and Fe–C4 active sites where Fe is indicated in gold, N is indicated in gray, and C is indicated in dark orange (c). The free-energy diagram of these sites also at the Pt(111) facet in acidic media from DFT calculations (d) and a volcano plot of the ORR activity by plotting the overpotential as a function of the binding energy of OH of these sites, Pt(111), nitrogen-doped graphene, and graphene (e).146 Reprinted with permission from Chen et al., Nano Energy 66, 104164 (2019). Copyright 2019 Elsevier. APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-10 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm FIG. 11. Scheme for the synthesis of SAC electrocatalysts. These SACs utilize abundant materials and thus have the poten- etching to remove the support, surface oxides, and unstable metal tial to significantly reduce the cost of PEMFCs. The electrocatalytic clusters138,147,148 (Fig. 12). An additional pyrolysis step ensures car- activity of these materials is highly dependent on the type of metal bonization and yields a more uniform morphology.147,148 Several and is reported to follow the order Fe > Co > Mn > Cu > Ni.136 groups have added excess Fe to form NPs of iron carbides or nitrides Fe has therefore been studied extensively.130,137–144 To facilitate the within the Fe–N–C.141,165,166 optimization of SACs, studies to understand which are the active Gao et al. combined the pyrolysis and etching step by adding sites have been performed. Zhang et al. found pyrrole-type FeN4 H2 to atmosphere during pyrolysis of a MOF-based catalyst. Tun- to be most active through both DFT calculations and from high able hierarchical porosity was obtained.131 Li et al. added addi- open circuit voltage (OCV) and peak power density from tests in tional metal ions to the pyrolysis. The metal ions were oxidized PEMFC. Chen et al. reached the same conclusion145 (Fig. 10). Mun during heat treatment and ensured increased porosity while being et al. investigated the effect of the carbon support on the activ- easily removed during subsequent acid leaching.154 A hierarchi- ity of FeN4 and found that the activity can be tuned by adding cally open-porous structure has been obtained by the addition electron-withdrawing/donating groups.146 of graphitic carbon nitride to ZIF-8 and iron precursor prior to pyrolysis.139,167,168 A. Synthetic strategies Similarly, ammonium chloride or sodium chloride has been The synthesis of M–N–C catalysts is most commonly done added to the pyrolysis to achieve an edge rich structure with more through pyrolysis, where nitrogen containing the organic precursor accessible active sites.149,169 Wang et al. also achieved an edge rich and metal precursor are mixed and subsequently heated in an structure by using iron chloride as a precursor, and both experi- inert atmosphere.138,147,148 Some research groups add sulfur to the ments and DFT calculations showed higher catalytic activity of the M–N–C to improve catalyst activity.149–152 To ensure porosity, Fe-Nx sites at edges compared to those at the center of the carbon a sacrificial support may be added147,153–157 or a metal–organic matrix.138 Another strategy is to increase the density of active sites framework (MOF) such as ZIF-8 can be used as the organic as done by Wan et al. by making concave structures.142 precursor.148,158–163 In principle, several different precursors may Wang et al. produced iron oxide NPs in a mesoporous N-doped be combined (Fig. 11). Zhu et al. combined the use of a sac- carbon, which after pyrolysis formed a core–shell structure with iron rificial polystyrene template and ZIF-8 to obtain a hierarchi- oxide encapsulated in graphitic carbon, and x-ray photoelectron cally porous structure.164 The first pyrolysis step is followed by spectroscopy analysis showed Fe–N active sites.170 FIG. 12. Formation of Fe–N–C by pyrolysis using L-aspartic acid, dicyandiamide (DCD), and FeCl3 as precursors.138 Reprinted with permission from Wang et al., Adv. Mater. 32, 2000966 (2020). Copyright 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-11 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm As an alternative to the pyrolysis of MOF and sacrificial sup- fabricated carbon nanospheres connected with CNTs to improve port, Wang and Berthon–Fabry synthesized Fe–N–C using the the electrical conductivity of the catalyst.173 Liu et al. also anchored sol–gel method to produce an aerogel.171 Fe single atoms on CNTs174 but further improved the perfor- Highest performance is, however, found for Fe–N–C cata- mance by exchanging CNTs for graphene175 (Fig. 13). The high- lysts using CNTs and/or graphene as the carbon precursor, with est performance and durability of a PGM-free catalyst were the advantage of not needing a sacrificial support.172 Li et al. achieved by Chen et al. through supporting NG doped with FIG. 13. Scheme of Fe–N-graphene synthesis (a), TEM image of the intermediate Fe/ZIF-8/graphene oxide (b), TEM and STEM images of the Fe–N–graphene [(c)–(e), and (g)], and elemental mapping of the final product (f).175 Reprinted with permission from Liu et al., Small Methods 4, 1900827 (2020). Copyright 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-12 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm TABLE II. Characteristics of the ORR for different PGM-free electrocatalysts. ORR was performed in 0.1 M HClO4 unless stated otherwise. MA and SA are given at 0.8 V. All ECSA were determined by the Hupd method. ADT refers to 10 000 cycles between 0.6 and 1.0 V unless stated otherwise. All potentials refer to RHE. Kinetic Half-wave Tafel Mass Specific Electrochemical Retained Loss in current potential slope activity activity surface area current density E1/2 density jk E1/2 b MA SA ECSA after after ADT Catalyst (mA/cm2 ) (V) (mV/dec) (mA/mg) (mA/cm2 ) (m2 /g) ADT (%) (mV) References C(N,S) 0.74 60 78a 128 C(P,N)b 0.96 at 0.5 V 127 N–C 0.76 91c 125 N–C 0.61 108 87d 126 N–P–S–C 0.64 126 84a 129 Co–In–C–Nb 0.7 79a 180 Co–S 62 122 Co–N–Cb 0.63 15e 177 Co–N–Cb 0.84 40f 176 Co–N–Cb 0.69 48 99g 178 Co–S–N–C 5.53 at 0.5 V 0.68 91.7a 152 (Co,Ni)–C 0.34 104 87c 181 (Co,Zn)–N–CNW 0.80 82 12 182 (Co,Zn)–N–S–Cb 0.70 85d 150 (Co,Zn)–N–C 0.80 n/a 183 Cr–N–C 24 at 0.75 V 0.77 37 15h 132 Fe–N–C 5.8 at 0.3 V 0.76 137 Fe–N–C 0.82 142 Fe–N–Cb 0.75 82d 139 Fe–N–C 0.78 65 67d 167 Fe–N–C 0.78 90i 130 Fe–N–C 0.78 10.5 0.0135 75j 140 Fe–Fe3 C/C 0.73 38 27f 141 e Fe–N–C 37 at 0.75 V 0.8 68 n/a 138 Fe–N–Cb 0.8 72 6.89k 26 143 Fe–N–C 0.81b 6.50 45 53 144 Fe–N–C 6.12 at 0.8 V 0.81 63 18 131 Fe–N–C 0.73 503 19l 145 Fe–N–C 0.74 59 38 162 Fe–N–C 0.81 16 163 Fe–N–C 6.14 at 0.8 V 0.80 n/a 157 Fe–N–Cb 3.82 at 0.8 V 0.80 65 169 Fe–N–Cb 0.84 31f 158 Fe–N–C 4.6 at 0.8 V 0.80 66 9 172 Fe–N–Cb 0.925 at 0.8 V 0.81 75 17 165 Fe–N–C 0.84 73 n/aa 175 Fe–N–C 0.79 90.8h 168 Fe–N–C 0.76 10f 156 Fe–N–C 0.79 72 91a 159 Fe–N–Cb 0.82 92 1 160 Fe–N–CNW 8.0 at 0.8 V 0.82 63 16 154 Fe–N–C/CNT 13.1 at 0.8 V 0.84 67 565 15e 173 Fe–N-CNT 0.73 28 174 Fe–Fe3 C/Fe–N–Cb 0.56 at 0.85 V 0.79 166 Fe2 –N–Cb 16.4 at 0.75 V 0.78 83 20h 184 Fe2 N6 –Cb 0.84 82 8.48 26.2 24 185 SiO2 –Fe–N–C 5.7 at 0.2 V 0.8 153 Fe–N-NG/RGO 0.84 63 5m 146 APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-13 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm TABLE II. (Continued.) Kinetic Half-wave Tafel Mass Specific Electrochemical Retained Loss in current potential slope activity activity surface area current density E1/2 density jk E1/2 b MA SA ECSA after after ADT Catalyst (mA/cm2 ) (V) (mV/dec) (mA/mg) (mA/cm2 ) (m2 /g) ADT (%) (mV) References Fe–N–S–CNNb 0.78 151 (Fe,Ni)–N–C 12e 179 (Fe,Ni)–N–C 0.84 60 92a 164 Fe–O/C–Nb 3.0 at 0.8 V 0.81 64 170 b Mn–N–C 0.78 20 133 NiN 0.49 123 Ti-MOF 0.75 18 134 Zn–N–C 0.75 135 a After 30000 s of chronoamperometry. b In 0.5 M H2 SO4 c After 10000 s of chronoamperometry. d After 40000 s of chronoamperometry. e After 5000 potential cycles of ADT. f After 30000 potential cycles of ADT. g After 50000 s of chronoamperometry. h After 20000 potential cycles of ADT. i After 20000 s of chronoamperometry. j After 24 h of chronoamperometry. k With respect to BET surface area. l After 3000 potential cycles of ADT. m After 15000 potential cycles of ADT. Fe and N on RGO with minimal loss after 15 000 potential B. Durability issues cycles.146 He et al. observed an initial high rate of degradation before Co–N–C catalysts have also been researched.152,161,176–178 an apparent stabilization of the Co–N–C. This is similar to what is Although Co–N–C so far has shown lower catalytic activity than found in PGM based catalysts and is due to an initial loss of unstable Fe–N–C (Table II), Co ions are less critical when released into both catalytic sites and oxidation of carbon.176 the membrane and ionomer since they are not Fenton active, and Kumar et al. found that carbon corrosion is occurring also in hence, a better durability is expected.161 However, the Co–N–C Fe–N–C catalysts. The deactivation of the catalyst is increased by needs advanced engineering to reach potentials comparable to the H2 O2 byproduct from the ORR on the Fe–N–C, as there is sig- Fe–N–C and Pt. Although half-wave potentials of up to 0.84 V vs nificant increase in corrosion when cycled in O2 compared to Ar.144 RHE have been reached by a surfactant-covered Co–N–C struc- Bae et al. studied the effect of H2 O2 with varying pH. With the ture,176 they need to be further stabilized to achieve the superior decrease in pH in the presence of H2 O2 , the amount of oxygen in durability expected. the sample after the test increased. This is in accordance with the Promising results have also been obtained by adding more fact that Fenton’s reaction is pH dependent and results in reducing than one metal to the M–N–C such as Fe–Ni,164,179 Co–In,180 the turnover frequency of nearby active sites.186 Co–Ni,181 and Co–Zn.150,182,183 Zang et al. reported increased activ- Doping graphene, CNTs, or CNWs with a transition metal and ity, both experimentally and from DFT calculations, due to Co–Ni nitrogen has been proven by several groups to increase the durabil- dual active sites,182 similar to the results produced by Ye and co- ity,146,154,178 with a further advantage of ensuring good conductivity workers using Fe–Fe dual sites.184 Fe–Fe was also studied by Zhang in the catalyst layer182 et al. by thermal migration of adjacent FeN4 sites to form Fe2 N6 A major advantage of the SACs is that they are superior to dual sites. These dual sites show high selectivity toward the four Pt/C when it comes to methanol tolerance.148,151 This would lower electron ORR due to the ability to adsorb two O atoms simulta- demands on gas purity for the PEMFC, which further reduces the neously.185 On the other hand, Zhu et al. found that Fe and Ni operating costs. formed single atom sites.164 This shows the immense number of variations that can be found for SACs, while only a handful has IV. PEM FUEL CELL TESTING yet been explored, and that further research is needed to find a durable catalyst with a high catalytic efficiency and low production While the electrochemical activity of a catalyst can be tested in cost. a three electrode setup, PEMFC testing is required to evaluate the APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-14 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm TABLE III. Characteristics of PEMFC tests in H2 /O2 for different electrocatalysts at 80 ○ C and 100% relative humidity. ADT refers to cycling between 0.6 V and 1.0 V unless stated otherwise. Peak power Open Specific Mass Tafel Current Retention density Cathode circuit activity activity slope retention of ECSA Pmax loading voltage SA MA b after after Catalyst (W/cm2 ) (mg/cm2 ) OCV (V) (A/cm2 ) (A/g) (mV/dec) ADT (%) ADT (%) References C(N,S) 0.28 2.5 0.78 128 Co–N–C 0.87 4 0.92 176 Co–N–C 0.92 4 161 (Co,Zn)–N–CNW 0.60 4 0.88 182 Fe–N–C 1.18 2.7 142 Fe–N–C 0.48 4 139 Fe–N–C 0.78 1 130 Fe–N–C 0.63 4 49a 140 Fe–N–C 0.68 3 0.29 157 Fe–N–C 0.73 3.5 0.98 158 Fe–N–C 0.75 2 172 Fe–N–C 0.7 4 1.01 143 Fe–N–C 0.86 4 169 Fe–N–C 0.65 4 165 Fe–N–C 0.6 4.0 160 Fe–N–S–C 0.53 4 149 Fe2 N6 –C 0.845 4 0.98 185 Fe–Fe3 C/C 0.24 2 0.77 141 Fe–Fe3 C/Fe–N–C 0.76 4 166 Fe–O/C–N 1.05 3.0 2 070 170 (Fe,Ni)–N–C 0.22 4 179 (Fe,Ni)–N–C 0.58 4 0.96 164 SiO2 –Fe–N–C 0.32 3 153 (Zn,Co)–N–C 0.71 183 C(P,N) 0.14 3.5 0.86 127 Ir–N–C 0.93 4 (0.008b ) 0.955 100 Ir–N–C 0.87 3 (0.135b ) 102 Pt/C 1.39 0.11c 400 90 94 Pt/C–N 0.33 0.5c 0.93 120 Pt/Co–N/C 1.25 0.175c 10 Pt/Nb–O/CNT 0.77 0.15 96d 87 Pt/Ti–Nb–O 0.2 77e 82 f Pt/Ti(N,C)O 0.74 94 85 Pt–Co/C 1.16 0.1c 720 61 15 Pt–Ni/C 1.21 0.1c 660 65 15 Pt–Co/CNT/C 1.02 0.06c 22 Pt–Cu/C 0.4 0.39 189 Pt–Cu–Ni/C 0.46 0.2c 46 Pt–Fe/C 0.4 0.45 189 Pt–Fe/C 1.08 0.3 92 Pt–Fe/C 0.96 0.01c 1 620 50g 30 Pt–Ni/C 0.4 0.47 189 Pt–Ni/C 0.82 0.3 92 Pt–Ni/C 0.88 0.31 3.26 70 38 72h 40h 14 Pt–Ni–Mo/C 0.7i 0.1c 450j 39 Pt–Pd/C 1.14 0.3 92 Pt–Pd/C-CNT 0.31k 0.5c 96h 63 APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-15 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm TABLE III. (Continued.) Peak power Open Specific Mass Tafel Current Retention density Cathode circuit activity activity slope retention of ECSA Pmax loading voltage SA MA b after after Catalyst (W/cm2 ) (mg/cm2 ) OCV (V) (A/cm2 ) (A/g) (mV/dec) ADT (%) ADT (%) References g Pt–Zn/C 2.00 520 83 38 Ru–N–C 0.64 4 0.93 76l 101 a After 8 h at constant voltage. b PGM loading. c Pt loading. d After 96 h at maximum current. e After 500 potential cycles from 1.0 V to 1.5 V. f After 5000 potential cycles from 1.0 V to 1.5 V. g After 30000 potential cycles of ADT. h After 3000 potential cycles of ADT. i Value determined under H2 /air supply. j With respect to Pt mass. k At 60 ○ C. l After 100 h of constant operation. performance of the catalyst when the electrolyte is changed from liq- metal catalyst needs to be solved. It should also be noted that uid to solid. However, comparison of the catalyst performance in a although 4 mg/cm2 is a common loading for SACs, some of the PEMFC is more complex. In Tables III and IV, test conditions and best performances come from catalysts with half the loading.142 reported performance are shown for PEMFC tests using H2 /O2 and In a recent review by Satjaritanun and Zenyuk, pooling of water H2 /air, respectively. Many parameters of PEMFC testing affect the at interfaces between the catalyst layer and the microporous layer results, although they are reported in varying degrees. The ionomer or the membrane as a result of non-homogeneous fabrication was content in the catalyst layer and coating technique on membrane found as an obstacle for these catalysts.190 Liu et al. concluded that affect the proton conduction from the catalyst layer. The type of sup- the hydrophobicity of the catalyst layer was the reason for water port, choice of gas diffusion layer (GDL), and compression of the cell accumulation in a study where both the catalyst coated membrane affect the electrical conductivity. Finally, the porosity in GDL and and gas diffusion electrode assemblies were analyzed.191 Yang et al. catalyst layer and the flow field design and size of the cell affect how kept the SiO2 support in the Fe–N–C catalyst, which increased the homogeneously the reactant gases are distributed across the catalyst hydrophobicity of the catalyst layer and thus improved the PEMFC layer. performance.153 Half of the reported tests use air at the cathode, while the Furthermore, it was concluded that although flooding of micro- rest use O2 . Several groups, testing both PGM and PGM-free cat- pores is often cited as a major degradation mechanism and it can alysts, have tested their catalyst in both and found that the peak block O2 transport to the active site, it is not the most impor- power density is almost twice as high when using O2 compared to tant. In the review by Satjaritanun and Zenyuk, Fe–N4 demetal- air.22,130,169,172 Using pure oxygen mitigates some of the mass trans- ization was instead pointed out as the major degradation mecha- port limitations and is more comparable to using air. Contaminants nism.190 Electrochemical impedance spectroscopy of aged PEMFCs in air are rarely specified, but it is known that CO and SO2 both with Fe–N–C catalysts has confirmed that there is a loss of active poison the catalyst.187,188 sites, which further leads to Fe-ion poisoning of the ionomer and For PGM containing catalysts, the catalyst layer has a thick- membrane.192,193 ness of 10 μm and the catalyst loading varies between 0.06 and It should be noted that in alkaline environment, many of the 0.4 mg/cm2 . The focus of the current research is on improving the PGM-free catalysts have higher performance than the Pt-based cat- corrosion resistance of the carbon support.92–95 Daş et al. tested Pt- alyst. Therefore, these catalysts would have an advantage if the PEM M alloys as the anode, cathode, and both and found that the highest was changed to an anion exchange membrane (AEM), ensuring performance gain was obtained when the bimetallic catalysts were a less corrosive environment in the cell. Seeberger et al. demon- applied as the anode.189 strated the feasibility of a bipolar membrane assembly for PGM- A highly desirable durability in PEMFC application was free FCs, combining the alkaline ORR with acidic hydrogen oxida- achieved by Gao et al., where the performance did not decrease tion reaction (HOR) by joining a PEM with an AEM as a double even after 50 000 potential cycles of ADT for a Pt-Co/CNW catalyst membrane.194 (Fig. 14).9 Fe–N–C has also been pointed out to be an alternative to Pt/C For PGM-free catalysts, low volumetric activity leads to ten in high temperature PEMFCs. In these cases, where the membrane is times thicker catalyst layers. Consequently, mass transport and resis- polybenzimidazol doped with phosphoric acid, Pt is poisoned by the tance problems that are minimized or already overcome for noble phosphate ions, while Fe–N–C is immune to the poisoning effect.156 APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-16 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm TABLE IV. Characteristics of PEMFC tests in H2 /air for different electrocatalysts at 80 ○ C and 100% relative humidity. ADT refers to cycling between 0.6 V and 1.0 V unless stated otherwise. Peak power Open Specific Mass Tafel Current Retention density Cathode circuit activity activity slope retention of ECSA Pmax loading voltage SA MA b after after Catalyst (W/cm2 ) (mg/cm2 ) OCV (V) (A/cm2 ) (A/g) (mV/dec) ADT (%) ADT (%) References Fe–N–C 0.42 2 142 Fe–N–C 0.46 1 68a 130 Fe–N–C 0.35 2 172 Fe–N–C 0.43 4 169 Pt/C 0.22 800 115 Pt/Cb 0.78 0.10 1.15 91c 67 188 Pt/C 0.85 0.1 81 Pt/C 1.20 0.07 96 Pt(N)/C 1.25 0.3 54 Pt/Nb–O/C 0.10 288 68c 83 b Pt/Ti–C–O/CNT 0.18 0.2 80 Pt–Co/CNW 0.71 0.09d 107e 9 Pt–Ni/Cf 0.88 0.4d 88g 16 Pt–Ni/C 0.92 0.15 >97h 12 Pt–Ni–Ir/C 0.40 0.35d 80i 45 Pt/C + Pd–Auj 0.54b 0.1d 160 79k 68c 56 Pt/Mn/C 0.12 373 66l 93 Pt–Co/C 0.11d 0.002 560 63 81c 8 Pt3 Co/C 0.98 85m 79n 98 d Pt–Co/CNT/C 0.54 0.06 22 Pt–Ir/C 0.4d 99k 34 Pt–Y–O/C 0.84 0.4 51 a Voltage retention after 10 h at constant-current operation. b Tests performed at 60 ○ C. c After 30000 potential cycles of ADT. d Pt loading e After 50000 potential cycles of ADT. f Tests performed at 80% relative humidity g After 100 h at constant voltage. h After 180 h at constant voltage. i After 1000 potential cycles of ADT. j Pd–Au NPs are coated onto the Nafion membrane, while a conventional Pt/C catalyst is used in addition. k After 15000 potential cycles of ADT. l After 5000 potential cycles from 1.0 to 1.5 V. m After 20000 potential cycles of ADT. n After 10000 potential cycles of ADT. V. RECYCLING OF PEMFC shown that Pt can be dissolved from a gas diffusion electrode by elec- trochemical dissolution in HCl, with a recovery efficiency of more Recycling of PEMFC components and particularly of PGM than 90% when producing a new catalyst from the recycled mate- electrocatalysts is of utmost importance considering the current rial.197,198 However, the gas diffusion electrode was never assembled striving toward sustainability. Yet, this is an aspect that deserves or cycled before dissolution, and the recovery efficiency may there- larger attention from the scientific community in the field. fore be lower considering Pt dissolution and migration into the Lotrič et al. performed a life cycle assessment of the PEMFC membrane and the fact that Pt particles adhered to the membrane stack and found that Pt is the component in the PEMFC with the upon disassembly. Duclos et al. studied the recovery of PtCo from largest environmental impact, and hence, recycling of the Pt can an MEA by two different hydrometallurgical routes. Both methods reduce the environmental footprint of the PEMFC.195 In general, include incineration of fluorinated polymers, causing emissions, and the global recycling of PGM has the potential to over 95% recov- the authors point out the need for recycling of the fluorinated poly- ery.196 For PGM used in PEMFC specifically, Sharma et al. have mers as well. The ion exchange resin and solvent separation methods APL Mater. 9, 040702 (2021); doi: 10.1063/5.0045801 9, 040702-17 © Author(s) 2021 APL Materials RESEARCH UPDATE scitation.org/journal/apm Another important durability issue occurring at high poten- tial cycling, the corrosion of the carbon support, can be tackled by replacing or modifying the carbon support material. Many dif- ferent approaches and materials have been studied to modify the support of PGM-based catalysts, showing desirable improvements. Considering the high amount of studies on the field in general, optimized support structures can soon be combined with opti- mized PGM catalysts in order to eliminate all durability issues and to provide reliable performance at low Pt loading. As there is a well-established industry for the production of carbon supported Pt NPs, these improvements can be implemented without needing major changes, and so, the way to commercialization is relatively short. Regarding PGM-free catalysts, sufficient durability is the key issue. As a result of this being a new technology, current work focuses on performance optimization, while works focusing on their durability are rather scarce. For the SACs, solutions have to be found to effectively prevent metal dissolution in acidic environments and increase durability during potential cycling. Furthermore, optimiza- tion of the catalyst layer in the PEMFC, achieving a more homoge- neous deposition, and upscaling of the fabrication are key to bring- ing these catalysts closer to broad commercialization. Otherwise, SACs are readily applicable in AEMFCs. AUTHORS’ CONTRIBUTIONS L.M. and K.E. contributed equally to this work. FIG. 14. Illustration of the CNW-supported Pt–Co catalyst in the PEMFC setup, which reached 8.2 W/mgPt .9 Reprinted with permission from Gao et al., ACS ACKNOWLEDGMENTS Sustainable Chem. Eng. 8, 13030 (2020). Copyright 2020 American Chemical This work received funding from the European Union’s Hori- Society. zon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No. 764977. E.P. also acknowl- edges funding from the Generalitat de Catalunya under Project No. gave Pt yield of 77% and 83%, respectively, while the ion exchange 2017-SGR-292. resin is advantageous with respect to synthesizing a new catalyst material from the product.4 DATA AVAILABILITY Data sharing is not applicable to this article as no new data were VI. SUMMARY created or analyzed in this study. Pt remains as the most important component of reliable and efficient PEMFC ORR electrocatalysts—and will continue to do so for the time being. The cost of the fuel cell stacks can be efficiently REFERENCES reduced by lowering the amount of Pt used and by increasing its 1 B. G. Pollet, S. S. Kocha, and I. Staffell, “Current status of automotive fuel cells efficiency through alloying, shape-tuning, and creating NPs with Pt- for sustainable transport,” Curr. Opin. 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