This is the submitted version of the article: Zhang X., Luo J., Wan K., Plessers D., Sels B., Song J., Chen L., Zhang T., Tang P., Morante J.R., Arbiol J., Fransaer J.. From rational design of a new bimetallic MOF family with tunable linkers to OER catalysts. Journal of Materials Chemistry A, (2019). 7. : 1616 - . 10.1039/c8ta08508k. Available at: https://dx.doi.org/10.1039/c8ta08508k Please do not adjust margins Journal of Materials Chemistry A ARTICLE From rational design of a new bimetallic MOF family with tunable linkers to an application case for OER catalyst Received 00th January 20xx, Accepted 00th January 20xx Xuan Zhanga, Jiangshui Luoa*, Kai Wana, Dieter Plessersb, Bert Selsb, Jianxun Songc, Liugang Chena,d, Ting Zhange, Pengyi Tange,f, Joan Ramon Morantef, Jordi Arbiole,g*, , Jan Fransaera* DOI: 10.1039/x0xx00000x www.rsc.org/ Innovative bimetallic MOFs offer more possibilities to further tailor the properties of MOFs, which have been attracted great attention for the wide applications. However, it is still a great challenge to rationally design bimetallic MOFs due to the lack of tunable and reasonable hybrid structure architecture. Herein, a new series of bimetallic metal-organic frameworks (MOFs) with tunable pillar linkers were prepared by a one-step synthesis method. These bimetallic MOFs retain the same crystal structure when the mole fraction (based on metal) of the two metals changes from 0 to 1 and both metal ions occupy uniform random nodal positions. The incorporation of a second metal cation has a large influence on the intrinsic properties (e.g. thermal stabilities and band gap) of the MOFs. Furthermore, these bimetallic MOFs were used as a self-sacrificial template to prepare bimetal oxide catalysts for oxygen evolution reaction (OER). After pyrolysis, a porous and hierarchical honeycomb-like structure with a carbon network covered (bi)metal oxides is formed. Among all the bimetallic MOF-derived catalysts, CoNi1@C showed the best performance for OER with the lowest Tafel slopes (55.6 mV dec−1) and overpotential (335 mV on glassy carbon electrode and 276 mV on Ni foam) at a current density of 10 mA cm−2, which is higher than state-of-the-art Co-Ni mixed oxide catalysts derived from MOFs for OER. Our results indicate that the incorporation of a second metal ion is a promising strategy to tailor the properties of MOFs. More importantly, this new bimetallic MOF family with tunable linkers is expected to serve as a flexible assembly platform to offer broad possibilities for the practical applications of MOFs. incorporation of a second metal into the framework (i.e. 1. Introduction bimetallic MOFs) is being explored recently.5 The presence of a second metal in the framework changes the intrinsic Metal-organic frameworks (MOFs), constructed by properties of the MOFs significantly. For example, by coordination bonds between metal ions (or clusters) and incorporation of different metal ions into the structure of various organic linkers, show tunable pore sizes and MOF-74, the resulting bimetallic MOF-74 showed very components, highly ordered porous crystalline structures, and different gas adsorption and catalytic activity compared with high surface areas. These characteristics make MOFs promising the monometallic MOF-74.6-9 In addition, the electronic candidates for a wide range of applications, such as gas properties of MOFs can be tailored by the metal node separation, gas storage, sensing, catalysis, energy storage and engineering, as shown in the recent work of Dolgopolova et conversion, and versatile precursors for the preparation of al.10 The stability, luminescence and magnetic properties of nanomaterials.1-4 these bimetallic frameworks have also been investigated.11-19 On the other hand, bimetallic MOFs show promise as self- In order to further optimize the structure and properties, the sacrificial templates for preparing hybrid nanomaterials, which play an essential role in sensors, catalyst, energy storage and a. Department of Materials Engineering, KU Leuven, Leuven 3001, Belgium. E-mail: conversion applications. Huang and coworkers prepared jan.fransaer@mtm.kuleuven.be; jiangshui.luo@gmail.com. porous ZnO/ZnFe2O4/C octahedra with hollow interiors by b. Centre for Surface Chemistry and Catalysis, KU Leuven, Leuven 3001, Belgium. c. Henan Province Industrial Technology Research Institute of Resources and using FeIII modified MOF-5 as the self-sacrificial template. The Materials, Zhengzhou University, 450001 Zhengzhou, China; resulting material show good lithium-storage performance.20 d. Henan Key Laboratory of High Temperature Functional Ceramics, Zhengzhou Recently, Ni–Co-mixed-oxide nanocages (prepared by using University, Kexue Avenue 100, Zhengzhou, 450001, China e. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Prussian-blue-analog)21 and carbon-coated CoxNi1−x@CoyNi1−yO Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain. E-mail: (prepared by MOF-74)22 were successfully synthesized by using arbiol@icrea.cat f. Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, bimetallic MOFs as the self-sacrificial template. The resulting Sant Adrià del Besòs, 08930 Barcelona, Catalonia, Spain Co-Ni catalysts were used for oxygen evolution reaction (OER), g. ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain which is currently a major bottleneck for electrochemical Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x water splitting.23 To generate a current density of 10 mA cm−2, This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name Figure 1. Schematic illustration of the synthesis of different bimetallic MOFs, perspective views of the crystal structure showing the coordinative environments around the MOF metal centers (all H atoms are omitted for clarity) and view of the supramolecular framework from the b axis the overpotential for the Ni–Co-mixed-oxide nanocages and controlled from 1.4 to 9.8 nm by changing the number of carbon-coated CoxNi1−x@CoyNi1−yO were 380 mV and 520 mV, phenylene rings in dioxidoterephthalate.48 However, the respectively. Meanwhile, other types of hybrid nanomaterials further modifications are limited by the type of linkers, which were also successfully prepared using bimetallic MOFs, such as hinder the potential applications of the MOF-74 family. core-shell structures and bimetallic nanoparticles.24-30 Therefore, in order to investigate a broad range of possibilities for advanced applications of bimetallic MOFs, it would be On the other hand, there are two general methods to prepare advantageous to find an appropriate prototypical MOF that bimetallic MOFs: the one-step method and the post-treatment can serve as a flexible assembly platform. method. Although the post-treatment method can prepare bimetallic MOFs, it is difficult to rationally design the molar Herein, we choose benzene-1,3,5-tricarboxylate (BTC) linker to ratios of the two metals or to obtain a uniform random form well-defined, two-dimensional (2D) plane layers with distribution of the two metal ions. Because of the small pore metal ions. By tailoring the pillars between the 2D layers, a size and the large steric hindrance within MOFs, the exchange series of bimetallic MOFs were successfully synthesized by the of metal ions needs to overcome large kinetic barriers and one-step synthesis method with different types of linkers. The diffusion problems, leading to partial exchange in some cases. chemical component and structure of bimetallic MOFs can be Moreover, when a second metal is introduced into a pristine easily tailored by the pillar-linkers. These bimetallic MOFs keep MOF, the overall morphology of the MOF can change or even their structure when the mole fraction mole fraction collapse.31, 32 Until now, the one-step synthesis method is still (calculation based on metal only) of the two metals changes the most effective way to achieve bimetallic MOFs. from 0 to 1. Additionally, the properties of the resulting MOFs Unfortunately, bimetallic MOFs often have fragile frameworks (such as thermal stabilities and band gap) change greatly with as well as unpredictable topologies and functions after the incorporating secondary metal nodes into the frameworks. introduction of new metal nodes. Some MOFs can remain their Subsequently, different CoNi-BTC-bipy bimetallic MOFs were original structure when pristine MOFs are partially replaced by used as self-sacrificial templates to synthesize catalyst for OER. a second metal ion. For example, Botas et al. report that in the Pyrolysis led to the formation of carbon-metal oxides (or dual MOF-5 framework structure, only a limited amount of Zn can metal oxides) hybrid materials. Among all the bimetallic MOF- be substituted by Co2+ (no more than 25% of the total metal derived catalysts, CoNi1@C showed the lowest overpotential mole content).33 A similar situation happens with HKUST-1 and for OER (335 mV on a rotating disk electrode and 276 mV on Ni ZIF-8/ZIF-67.34-38 Recently, some successful cases of bimetallic foam) at a current density of 10 mA cm−2, which is higher than MOFs were reported by using Prussian-blue (PB) or Prussian- state-of-the-art Co-Ni mixed oxides catalysts derived from blue-analog (PBA) as prototypical MOFs, such as Co-Ni, Co-Fe, MOFs for OER. Co-Zn, Co-Mn, Fe-Mn and Fe-Ni.39-46 However, the poor possibility for controlling pore size and linkers in PB and PBA are their key disadvantages. Compared to other MOFs, the 2. Results and discussion most promising candidate is MOF-74. It can be easily designed Benzene-1,3,5-tricarboxylate (BTC) linker based bimetallic for different Zn−Co mole fraction based on metal (from 0 to 1) MOFs have been successfully prepared by the one-step by the one-step synthesis method.6 Similarly, CoMg-MOF-74 synthesis method. The coordination environment of the metal and NiMg-MOF-74 have been synthesized successfully as center for these two kinds of bimetallic MOFs are the same well.47 Interestingly, the pore size of MOF-74 can be precisely (Figure 1), where the nickel(II) or cobalt(II) center is bound by 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Energy & Environmental Science ARTICLE structure can be maintained even when the mole ratio of the two metals changes from 0 : 1 to 1 : 0. The metal contents (Co and Ni) in the prepared MOFs were determined by means of ICP-OES analysis (Figure S15). With increasing concentration of Ni in the starting solution, the content of Ni in MOF structure increases but the increase is not linear. This is caused by the different coordinating ability of Co2+ and Ni2+ towards the ligand.49 The presence of a second metal in the MOFs was confirmed by diffuse reflectance UV-Vis spectroscopy (Figure 2). In all cases, the UV-Vis spectra are consistent with an octahedral coordination sphere for Co2+ and Ni2+. Indeed, the absorption bands at 450–550 nm and 650–670 nm, detected in the spectra of these MOFs, agree very well with the d-d transitions of Co2+ and Ni2+ in an octahedral coordination sphere, respectively.9 The intensity changes of these absorption bands correspond to changes of the Co/Ni ratio in the prepared bimetallic MOFs, matching the changing content of Co and Ni precursors in the synthesis solution. These results indicate the bimetallic nature of these MOF materials. The band gaps of these MOFs were estimated by a plot of √𝐹(𝑅)ℎ𝜈 vs. energy37, 38, 50, 51 from UV-Vis data for all samples (Figure S3). From the tangent lines in these plots, the band gaps of Co-MOF, CoNiBMMOF-1, CoNiBMMOF-2, CoNiBMMOF-3 and Ni-MOF were found to be 1.61 eV, 1.72 eV, 1.79 eV, 1.82 eV and 2.4 eV, respectively. Interestingly, the synthesized bimetallic MOFs also display evident color changes, depending on the ratio of the two metals (Figure S4). This tunable property of band gaps of bimetallic MOFs may also be promising for applications in photocatalysis. In order to prove the uniform distribution of both metals Figure 3. (a) Calculated XRD patterns of Co-MOF (A) and Ni-MOF (G), experimental XRD inside the bimetallic MOFs, the relative metal element patterns of Co-MOF (B), CoNiBMMOF-1 (C), CoNiBMMOF-2 (D), CoNiBMMOF-3 (E) and Ni-MOF (F); (b) Diffuse reflectance UV-Vis spectroscopy of the different MOFs. contributions of different bimetallic MOFs were analyzed by electron probe microanalysis (EPMA) coupled with wavelength three BTC units with one bidentate carboxylic group and two dispersive spectroscopy (WDS). All the samples showed similar monodentate carboxylic groups, which form a well-defined hexagon morphology (Figure 3). Moreover, the results of Co-Ni two-dimensional (2D) layer with honeycomb pores. The free elemental relative composition mapping suggest a fairly carbonyls in the monodentate carboxylic groups are further homogenous distribution of Co and Ni throughout the bonded to a proton via hydrogen bond interactions, forming hexagonal MOF sheets. The hexagon sheets show higher Co an overall neutral framework along the ab-plane. The polar concentration than Ni, similar concentration for both Co and positions of nickel(II) or cobalt(II) center are occupied by 4,4’- Ni and lower Co concentration than Ni for CoNiBMMOF-1, bipyridine along the c axis to form a 3D framework. In order to CoNiBMMOF-2 (c) and CoNiBMMOF-3, respectively (Figure 3b- prove that this new family of bimetallic MOFs can be further 3d). These results are in good agreement with the ICP-OES tailored by ‘‘pillaring’’ strategy, a similar series of bimetallic analysis, demonstrating that these bimetallic MOFs can form MOFs based on DMF pillars were successfully prepared by the solid-solution type crystals over the whole concentration range. same method. The details of the basic characterizations (such as XRD, SEM and EDS) are shown in the Figures S1-S2. In this Determination of thermal stabilities and decomposition work, the bimetallic MOFs based on 4,4’-bipyridine are chosen temperatures of different MOFs were carried out via thermal as model MOFs for further study. gravimetric analysis (TGA) under nitrogen atmosphere. There are two main weight loss events during pyrolysis for all The crystalline structures of these synthesized MOFs are samples (Figure S5). The first one corresponds to the release characterized by powder XRD (Figure 2a). The powder XRD of guest molecules (such as ethanol and DMF). The second patterns of all samples show identical characteristic diffraction event is likely attributed to the onset of the decomposition of peaks at 2θ ≈ 7.8°, 10.6°, 15.6° and 18.5°, which match well the framework. The decomposition temperatures are 300 °C, with the simulated patterns found in literature.49 With the 419 °C, 409 °C, 412 °C and 428 °C for Co-MOF, CoNiBMMOF-1, incorporation of a second metal in the frameworks, the crystal CoNiBMMOF-2, CoNiBMMOF-3 and Ni-MOF, respectively. This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name Figure 3. SEM and corresponding EMPA elemental mapping images for Co-MOF (a), CoNiBMMOF-1 (b), CoNiBMMOF-2 (c), CoNiBMMOF-3 (d) and Ni-MOF (e), respectively. When Ni is added to the Co-MOF, the thermal stability of the high-resolution XPS spectra of Co 2p and Ni 2p (Figure 5). All MOFs increases significantly due to the higher coordination cobalt-containing samples exhibit similar results. The binding ability of Ni2+ than Co2+.14, 49 Moreover, Co2+ can be easily oxidized to Co3+ at relatively low temperature.33 After pyrolysis, all the samples retain their hexagon morphology (Figure S6- S10). At high magnification, a porous honeycomb-structure can be clearly observed, caused by the decomposition of the organic linkers. EDS confirmed that the different MOF derived catalysts contain C (originating from the organic linkers), O (from the formed metal oxide) and M (Co or Ni). The ratios of Co vs. Ni in the pyrolized MOFs are similar to the pristine MOFs. All samples were examined by XRD to confirm the crystalline phase (Figure 4b). The identified diffraction peaks of Ni@C and Co@C can be well indexed to NiO (JCPDS 65-2901) and Co3O4 (JCPDS 43-1003) with cubic structures, respectively. It is worth mentioning that for the bimetallic samples, the presence of spinel structure of NiCo2O4 (JCPDS 20-0781) could also be possible. For both the high Co or Ni ratio samples (i.e. CoNi1@C or CoNi3@C), the spinel phase dominates the crystalline phase. In addition, the coexistence of diffraction peaks of NiO and Co3O4 for CoNi2@C implies the formation of NiO and Co3O4 composites. Furthermore, the chemical composition and valence state of Co and Ni are investigated by XPS measurements. The full scan survey XPS spectra of different catalysts in the region of 0−1000 eV (Figure S11) show evident C, O, Ni and Co peaks. Figure 4. (a) Decomposition temperature of Co-MOF, CoNiBMMOF-1, CoNiBMMOF-2, The detailed chemical compositions based on full scan survey CoNiBMMOF-3, and Ni-MOF obtained from TGA. (b) XRD pattern of NiO-JCPDS 65-2901 XPS spectra are summarised in Table S1. Additionally, the (A), Ni@C (B), CoNi3@C (C), CoNi2@C (D), CoNi1@C (E), Co@C (F) and Co3O4-JCPDS 43- chemical states of Ni and Co in the catalysts were explored by 1003 (G). 4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Energy & Environmental Science ARTICLE Figure 5 High-resolution XPS spectra of Co 2p and Ni 2p for Co@C, CoNi1@C, CoNi2@C, CoNi3@C and Ni@C catalyst powders. energies of Co 2p3/2 and Co 2p1/2 (Figure 5) show two spin- a smaller particle size (13 nm−40 nm). The corresponding NiO orbit doublets (corresponding to Co2+ and Co3+) at around lattice fringe distances were measured to be 0.209 nm, 0.242 780.1 eV and 795.4 eV, respectively, with a spin energy nm and 0.238 nm, with crystal plane angles of 55.77 and difference of around 15.3 eV, indicating the presence of mainly 126.90º which could be interpreted as the cubic NiO phase, Co3O4 phase. The weak shake-up satellite peaks for Co 2p visualized along its [101 ̅] zone axis. For the bimetal oxide spectra further reveal that the dominant phase is Co3O4.52 In catalyst of CoNi1@C (Figure 6b), a small amount of Ni doping the spectra of Ni 2p, two pairs of spin-orbit doublets and two decreases the particle size compared to Co@C and leads to shake-up satellites can be clearly observed. More specifically, particle sizes similar to Ni@C. The corresponding power the peaks at 874.3 and 856.3 eV correspond to Ni3+, while the spectrum reveals that the selected nanocrystal structure peaks at 872.5 and 854.6 eV can be attributed to Ni2+. For CoNi2@C, CoNi3@C and Ni@C, NiO is the dominant phase. Interestingly, CoNi1@C shows the opposite results with Ni3+ as the dominant phase, which is probably because Co3O4 can induce the formation of Ni3+ because in the presence of a high amount of cobalt, the Ni3+ valence state can be stabilized.53 It also implies the likely formation of a Ni3O4−Co3O4 solid solution at the calcination temperature of 500 °C.53 General HAADF STEM, TEM and HRTEM micrographs and EELS chemical composition maps of all the samples are shown in Figure 6-7 and Figure S12-S13. The Co3O4 nanocrystals derived from the Co-MOFs (Co@C) are present as hexagonal-shaped nanostructures with a size around 20 nm−40 nm (Figure 6a). In addition, according to the HRTEM images, the Co3O4 nanocrystals derived from Co-MOFs show good crystallinity. A magnified detail of the orange squared region and its corresponding power spectrum reveal that the selected nanocrystal structure is in agreement with the cubic Co3O4 phase (space group Fd3mz) with a = b = c = 0.806 nm. Based on the crystalline domain shown in Figure 6a, the Co3O4 lattice fringe distances were measured to be 0.249 nm, 0.204 nm and 0.244 nm, respectively, with crystal plane angles of either 26.83º or 51.92º, which could be interpreted as the cubic ̅1] zone axis. These Co3O4 phase that is visualized along its [01 results agree with XRD data. Similarly, the pure Ni-MOF derived catalyst (Ni@C) is in line with the NiO cubic phase Figure 6 HRTEM micrographs of the different MOF derived catalysts with (space group Fm3m) with a = b = c = 0.420 nm (Figure 6c) with corresponding power spectrum of (a) Co@C (b) CoNi1@C and (c) Ni@C. This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name corresponds to the NiCo2O4 cubic phase (space group Fd3mz) results are in excellent agreement with the XRD data. The EELS with a=b=c=0.81140 nm, which further confirms the formation maps (Figure 7 and Figures S12-S13) further reveal the of a Ni3O4−Co3O4 solid solution. Based on the crystalline chemical composition of these nanoparticles. Firstly, the domain in Figure 6b, the NiCo2O4 lattice fringe distances were mapping images display the existence of M (Co or Ni), O and C measured to be 0.243 nm, 0.459 nm and 0.233 nm, elements in these nanoparticles derived from different MOF respectively, at crystal plane angles of 58.37º or 116.59º, samples. The elemental maps clearly confirm that C is mainly which could be attributed to the cubic NiCo2O4 phase, concentrated on the exterior of the nanostructures for all measured to be 0.243 nm, 0.459 nm and 0.233 nm, samples, suggesting the formation of a core-shell structure. respectively, at crystal plane angles of 58.37º or 116.59º, These metal oxide particles are connected together by a which could be attributed to the cubic NiCo2O4 phase, carbon net, which could impart a high electronic conductivity ̅11] zone axis. On the other hand, it is visualized along its [2 to the catalyst. Moreover, the mapping images display the difficult to distinguish the mixed phase for CoNi2@C and uniform distribution of Ni and Co for CoNi3@C (Figure S12). CoNi3@C due to the uniform overlap. Nevertheless, all these Surprisingly, CoNi1@C and, in particular, CoNi2@C show an uneven distribution of Ni and Co as determined from the Ni-Co elemental mapping (Figure 7 and Figure S13), which could be caused by the different diffusion coefficients of Co and Ni in the Ni-Co system during pyrolysis. A similar phenomenon has been discussed in detail by Han et al.54 To explore the porous structure of the resulting catalyst, the nitrogen adsorption and desorption isotherms and pore size distributions (based on density functional theory method with slit pore model) of the different catalysts are measured (Figures S14). All the adsorption-desorption isotherms exhibit a typical type II curve with a small H3 hysteresis loop in the P/P0 range of 0.8–1.0, indicating the co-existence of large mesopores and small macropores. The BET specific surface areas of Co@C, CoNi1@C, CoNi2@C, CoNi3@C and Ni@C samples are found to be 22.5, 33.7, 29.0, 28.9 and 36.6 m2 g−1, respectively. With a small amount of Ni doping, the specific surface areas of CoNi1@C increases considerably compared to pure Co-MOF derived materials, which is attributed to the decrease of particle size as proved by TEM results. The pore- size distribution of CoNi2@C, CoNi3@C and Ni@C ranges from 40 nm to more than 100 nm, which is dominated by small macropores. Co@C and CoNi1@C display a broader pore-size distribution with smaller pore sizes and more mesopores, which range from 20 to more than 100 nm. These hierarchical porous structures would be beneficial for the transport of electrolyte ions onto the surface of the active material and facilitate gas bubble dissipation ability.55-59 Electrochemical water splitting is a promising strategy for renewable energy conversion and storage, which combines OER and hydrogen evolution reaction (HER). However, the sluggish nature of the OER greatly impedes large-scale applications of electrochemical water splitting. Therefore, the catalytic performance of the as-prepared catalysts for OER was explored by linear sweep voltammetry (LSV) (Figure 8). The precious metal catalyst IrO2 was also tested as a comparison under the same conditions. Compared to bare glassy carbon electrode (GCE), all catalysts show obvious catalytic activity for Figure 7 EELS chemical composition maps of the different MOF derived catalysts OER. The on-set overpotentials of different catalysts were obtained from the red squared area of the STEM micrograph of (a) Co@C (b) CoNi1@C 0.309 V, 0.263 V, 0.283 V, 0.293 V, 0.298 V and 0.270 V for and (c) Ni@C, showing individual Ni L2,3-edges at 855 eV, Co L2,3-edges at 779 eV, O K- Co@C, CoNi1@C, CoNi2@C, CoNi3@C, Ni@C and IrO2 catalysts, edge at 532 eV and C K-edge at 284 eV as well as the composite (Ni-Co, O-C, Co-O-C respectively (Figure 8). Among these catalysts, CoNi1@C and and Ni-O-C) elemental mapping of these nanostructures. IrO2 have the lowest on-set overpotentials. The 6 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Energy & Environmental Science ARTICLE parameter to evaluate the activity of OER catalysts because this current density matches that of a solar water-splitting device with 12.3% efficiency.60 Even though IrO2 has a low on- set overpotential, it has the highest overpotential (0.401 V) at a current density of 10 mA cm−2 because of the high Tafel slope. Among all the MOF-derived catalysts, CoNi1@C shows the lowest overpotential (0.335 V) at a current density of 10 mA cm−2, which is attributed to the combination of low on-set overpotential and low Tafel slope. The electrochemical impedance spectroscopy (EIS) data are recorded in the frequency range from 100 kHz to 0.1 Hz at ƞ = 380 mV with an amplitude of 5 mV, where Z' and Z'' are the real and imaginary parts of the impedance, respectively. The data was fitted adequately by a modified Randles’ equivalent circuit shown in the inset of Figure S15. The intersection of the plots on the real axis in the higher frequency region represents the equivalent series resistance (Rs), which includes the electrolyte resistance, the contact resistance and the intrinsic resistance of the electrode. The diameter of the semi-circle represents the charge transfer resistance (Rct). A lower Rct corresponds to a faster charge transfer process.61, 62 Using the equivalent circuit as shown in Figure S15, the fitting results for EIS measurements of different catalysts are obtained (Table S2). Among these catalysts, CoNi1@C exhibited the lowest Rct resistance (3.4 Ω), implying its fast charge transfer process and thus favorable electrocatalytic activity for OER. In order to increase the surface area of the electrode, nickel foam was chosen as a substrate to load the catalysts in a three-electrode system for electrochemical polarization studies (Figure 9a). Both IrO2 and CoNi1@C exhibit a significant enhancement of OER catalytic performance compared to GC based samples. Particularly, CoNi1@C displays an overpotential of as low as 276 mV on Ni foam at a current density of 10 mA cm−2 (Figure 9a and Table S3), which is higher than state-of-the-art Co-Ni mixed oxide catalysts derived from MOFs for OER. At an overpotential of 400 mV (i.e. at a potential of 1.63V vs. RHE), the current density of CoNi1@C covered electrode (197 mA cm−2) is 3.6 times that of the IrO2 covered electrode (55 mA cm−2) (Figure 9a). In addition, a higher pyrolysis temperature has been explored for CoNi1@C. With the increase of temperature, the catalytic performance for OER slightly decreased due to the decrease of surface area (Figure S16), which could be caused by the increase of particle size with a higher pyrolysis temperature.63 These results are in agreement with previous Figure 8 (a) OER polarization curves using Co@C, CoNi1@C, CoNi2@C, CoNi3@C, Ni@C, studies for MOF-derived materials.64-66 Chronopotentiometric or IrO2 catalysts covered GCE and bare GCE at a rotating speed of 1600 rpm in 1 M KOH; (b) Corresponding Tafel plots obtained from respective polarization curves; (c) curves of CoNi1@C at different current densities (from 10 to Comparison of the on-set overpotentials, overpotentials at a current density of 10 mA 100 mA cm−2) and chronopotentiometric durability tests at a cm−2 and Tafel slopes of various catalysts. All potentials are against the RHE. constant current density of 20 mA cm −2 were recorded (Figure OER kinetics of the catalysts was estimated from Tafel plots. A 9b). CoNi1@C exhibits excellent durability at large current smaller Tafel slope indicates more favorable OER kinetics of densities (e.g. 100 mA cm−2) over a long time (e.g. 48 h), which the catalysts. The Tafel plots corresponding to Figure 8a are could be associated with the thin carbon shell of the CoNi1@C shown in Figure 8b. The Tafel slopes of Co@C, CoNi1@C, catalyst. Obviously, the high catalytic activity of CoNi1@C for CoNi2@C, CoNi3@C, Ni@C and IrO2 catalysts are 41.8, 55.6, OER is promising for electrochemical water splitting. 48.1, 57.3, 63.2 and 86.8 mV dec−1, respectively, indicating that the OER was kinetically faster on the MOF-derived catalysts, Furthermore, while Pt/C catalyst still greatly surpasses all especially for the Co@C catalyst. The overpotential at a other kinds of catalysts for HER, in this study, Pt/C catalyst and current density of 10 mA cm−2 is chosen as another key CoNi1@C catalyst were chosen as the catalysts for the cathode This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name and the anode, respectively, to establish a single electrolyzer (24.6 at.%) than on the Ni foam (32.9%), confirmed by WDS cell. At current densities of 10, 20 and 50 mA cm−2, the cell (Table S3). voltages are 1.53 V, 1.58 V and 1.66 V, respectively (Figure 9c). The catalyst performance of the CoNi1@C catalyst for OER In addition, the electrolyzer based on Pt/C catalyst and not only surpasses bimetallic MOFs of PBA21, MOF-7422 or CoNi1@C catalyst exhibits excellent stability over 48 h during partially doped ZIF-67(Co-Ni)68 derived Co-Ni mixed oxide the electrolysis tests (Figure 9d). catalysts, but also outperforms most Co-Ni based catalysts After stability test, the chemical composition and developed via other methods (Table S4). Compared with some morphology are explored via TEM and EELS mapping. state-of-the-art seamless integration electrodes with Ni foam Compared with the catalyst of CoNi1@C before test, no as the substrate,69-71 CoNi1@C exhibits a slightly higher distinct change in the morphology can be observed, which overpotential at the high current density of 100 mA cm−2, could benefit from a carbon shell network of CoNi1@C. Figure which is probably caused by the addition of binder during our S17 shows the EELS chemical composition maps obtained from coating procedure. the red squared region in the HAADF STEM micrograph. As can The high OER catalyst activity of CoNi1@C could be be seen in the maps, the region of interest contains attributed to the following reasons: (1) the synergistic effect of information about a few nanostructures. Firstly, the mapping bimetal oxides. More importantly, the stabilized high valence images display the existence of Ni, Co, O and C in the state of Ni, which is regarded as the real active site, could nanostructures present in the sample. There are no other promote the OER. In the spinel structure, the trivalent ions possible contamination elements that can be detected. Then play an important role in stimulating the absorbed oxygen by the same results of metal elements are further confirmed by the beneficial electrostatic adsorption of OH− ions to the WDS (detection limit can reach ppm level and around 0.05 formation of MOOH and then inducing deprotonation of OOH− wt%)67 as shown in Figure S18. However, the increase of Ni can species to produce O2.72-75 (2) This unique structure of be clearly observed, compared with CoNi1@C before test, hierarchical porous honeycomb-like structure could facilitate which is probably come from anodic dissolution of Ni foam. In the transportation of electrolyte ions and enhance the stability order to prove this hypothesis, the same stability test was of the structure during the electrochemical test.76-78 carried out by the substrate of carbon paper. After stability test, CoNi1@C on the carbon paper exhibited lower Ni content Figure 9 (a) OER polarization curves of CoNi1@C, IrO2 catalysts covered Ni foam and bare Ni foam in 1 M KOH in a three-electrode system; (b) Multi-current process of CoNi1@C covered Ni foam at different constant current densities and durability test at a constant current density of 20 mA cm−2 in a three- electrode system; (c) Multi-current process in a two-electrode configuration as a single electrolyzer cell, using CoNi1@C as catalyst for OER and Pt/C for HER in 1 M KOH. Support: Ni foam; loading of catalyst: 3 mg cm−2. (d) Durability test at a constant current density of 20 mA cm−2 for the single electrolyzer cell over 48 h. Inset shows a photograph of the electrolyzer cell system during water electrolysis in 1 M KOH to generate oxygen gas (left) and hydrogen gas (right). 8 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Journal of Materials Chemistry A ARTICLE In addition, a carbon shell network could ensure the electrical powder was washed several times with DMF and ethanol continuity of bimetal oxide particles and suppress the successively. The corresponding samples from pure cobalt- agglomeration of nanoparticles during pyrolysis and MOF to pure nickel-MOF were denoted as Co-MOF, electrochemical tests. CoNiBMMOF-1, CoNiBMMOF-2, CoNiBMMOF-3, CoNiBMMOF- 4, CoNiBMMOF-5, and Ni-MOF, respectively. In order to prove that new families of bimetallic MOFs can be tailored by the 3. Conclusions pillar strategy, an identical series of bimetallic MOFs based on In summary, a new series of bimetallic MOFs were successfully DMF as the pillar were prepared by the same method at 100 °C. prepared using the one-step synthesis method. The molar The details are shown in Table S5. ratio of the metal nodes in these bimetallic MOFs can be arbitrarily varied while the nodes are randomly and uniformly 4.2 Preparation of MOF derived bimetallic catalysts distributed inside the frameworks. Moreover, the pillars in this series of bimetallic MOFs are tunable. As expected, with The Co-MOF, CoNiBMMOF-1, CoNiBMMOF-2, CoNiBMMOF- incorporation of a second metal ion, intrinsic properties (such 3 and Ni-MOF were pyrolyzed at 500 °C for 8 h in a nitrogen as thermal stabilities and band gap) of the pristine MOFs can gas atmosphere with a flow rate of 300 mL min−1. The be tailored. In order to prepare bimetal oxide catalysts for OER, temperature inside the furnace was gradually increased to these bimetallic MOFs were pyrolized and used as self- 500 °C at a heating rate of 3 °C min−1. After the pyrolysis, the sacrificial templates. The resulting materials showed a furnace was cooled down to room temperature with a cooling hierarchical porous honeycomb-like structure of carbon rate of 3 °C min−1. The resulting powder was ultrasonically covered (bi)metal oxide nanostructure. After pyrolysis, the cleaned with DI water and EtOH successively, and then dried at bimetallic MOF with a high ratio of Co leads to the highest 60 °C. After that, the as-prepared black powder was further catalyst performance for OER due to the unique structure and annealed at 350 °C for 3 h with a heating rate of 3 °C min−1 in high valence state of Ni. At a current density of 10 mA cm−2, air to oxidize the metal phase. The different kinds of MOF- the overpotentials of CoNi1@C are 335 mV and 276 mV for derived catalysts were denoted as Co@C, CoNi1@C, CoNi2@C, OER on RDE and Ni foam, respectively. These promising results CoNi3@C and Ni@C, respectively. not only prove that the properties of MOFs can be tailored by incorporation of a second metal into the MOF structure but 4.3 Material characterizations also show possibilities for wider applications of this new bimetallic MOF family. Thermogravimetric analysis (TGA) was performed on a thermogravimetric analyzer (AutoTGA 2950HR V5.4A, TA Instruments) using platinum pans at a heating rate of 5 °C 4. Experimental min−1 in N2 atmosphere. Decomposition temperatures are deduced by intersection of a straight baseline with the tangent 4.1 Preparation of bimetallic MOFs of the weight versus temperature. A Bruker AXS D8 All the chemicals were used without further purification. In diffractometer (Cu Kα radiation with λ = 0.15405 nm and Ni a typical method, different proportions of Co(NO3)2·6H2O filter) with 2θ ranging from 5° to 30° and a step of 0.02° (2 s (≥99%, Merck, Germany) and Ni(NO3)2·6H2O (≥99%, VWR per step) was used to record X-ray diffraction (XRD) patterns of International bvba, Belgium) were dissolved in 40 ml N,N- all samples. The morphologies and structures of samples were dimethylformamide (DMF, ≥99% Chem-Lab). 1,3,5- observed on a FEI/Philips XL30 FEG microscope (SEM). Metal benzenetricarboxylic acid (0.844 g, H3BTC, 98% purity, ABCR, compositional analyses of BMMOFs were determined by full Germany) and 4,4-dipyridyl (0.768 g bipy, 98% purity, Acros quantitative electron probe micro-analyzer system coupled Organics) were dissolved in 40 ml DMF as well. The two with wavelength dispersive spectroscopy (EPMA-WDS; JEOL homogeneous solutions were mixed together into a 100 mL JXA-8530F, Tokyo, Japan) with an acceleration voltage of 15 kV glass bottle. The molar ratios of Co(NO3)2·6H2O vs. at a probe current of 15 nA. Before SEM and EPMA test, Ni(NO3)2·6H2O are shown in Figure S19 while the total molar BMMOFs were sputtered with a 10 nm layer of Pt to improve concentration of metal salts is 12 mmol. After that, the glass the electronic conductivity. Scanning Transmission Electron bottle was maintained at 50 °C in a water bath (Memmert, Microscopy (STEM) and High Resolution Transmission Electron WNB 45) for 72 h and then naturally cooled down to room Microscopy (HRTEM) images were collected using an FEI temperature. In order to remove the residual reactant, the Tecnai F20 field emission gun microscope operated at 200 kV This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 9 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name with a point-to-point resolution of 0.19 nm, which is equipped where 𝑖 is the current (A) and R (Ω) is the electrolyte resistance with high angle annular dark field (HAADF) and electron energy measured by electrochemical impedance spectroscopy (EIS). loss spectroscopy (EELS) detectors. The obtained images and spectra were analyzed by Gatan Digital Micrograph software. ESCALAB250 (monochromatic Al Kα with 1486.6 eV) was used Conflicts of interest to collect X-ray photoelectron spectroscopy (XPS) data. The There are no conflicts to declare metal compositions in Co-MOF, Ni-MOF and BMMOFs (dissolved in 0.5M HCL) were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES, Varian 720-ES). Acknowledgements A micromeritics ASAP 2020 apparatus was used to investigate X. Zhang is grateful to China Scholarship Council. J. Luo the specific surface area and pore size distributions. Diffuse acknowledges the Research Foundation – Flanders (FWO) for a reflectance UV-Vis-NIR spectroscopy was carried out on a Research Project (G0B3218N), Research Grant (1529816N) and Varian Cary 5000 UV–Vis–NIR spectrophotometer at room a travel grant (V410316N) for a Visiting Professorship at the temperature. Technical University of Denmark. TZ, PYT and JA acknowledge funding from Generalitat de Catalunya 2017 SGR 327 and the 4.4 Electrochemical characterizations: Spanish MINECO project VALPEC (ENE2017-85087-C3). ICN2 acknowledges support from the Severo Ochoa Programme Preparation of working electrodes. The catalysts inks were (MINECO, Grant no. SEV-2017-0706) and is funded by the prepared by dispersing 10 mg catalyst into 1 mL solvent (800 CERCA Programme / Generalitat de Catalunya. Part of the μL ethanol and 200 μL DI water) containing 20 µL 5 wt% Nafion present work has been performed in the framework of (Alfa Aesar Germany). Before loading the catalysts on the GCE, Universitat Autònoma de Barcelona Materials Science PhD the ink was sonicated for 30 minutes. Then 10 µL as-prepared program. TZ has received funding from the CSC-UAB PhD catalyst ink was loaded on the surface of the GCE (5 mm in scholarship program. D.P. acknowledges the Research diameter) corresponding to an aerial mass loading of about Foundation – Flanders (FWO) for Ph.D. (aspirant) Fellowship. 0.50 mg cm−2. For comparison, IrO2 (P40V020, particles from We thank Iris Cuppens and Dr. Fei Wang (Department of Premetek Co.) were used in identical electrochemical tests Materials Engineering, KU Leuven) for the assistance of EPMA with the same mass loading. Electrochemical water splitting and ICP measurements. Funding from the National Natural was carried out with Pt/C catalyst (10% of platinum, Alfa Aesar Science Foundation of China (No. 21776120), “Minjiang Germany) and bimetallic MOFs derived catalyst as cathode and Scholar” Program and key project (No. JZ160480) of anode, respectively. Ni foam electrodes were prepared using Department of Education, Fujian Province, China, and Open the same procedure with catalysts aerial mass loading of Project (No. 201607) of State Key Laboratory of Physical around 3.0 mg cm−2 based on the geometrical area for both Chemistry of Solid Surfaces, Xiamen University is also Pt/C catalyst and bimetallic MOF derived catalyst. acknowledged. Catalytic performance. The OER catalytic performance of the samples was evaluated by linear sweep voltammetry (LSV) technique using an Autolab electrochemical workstation at room temperature. 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