Ultra-Small Abundant Metal-Based Clusters as Oxygen-Evolving

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Ultra-Small Abundant Metal-Based Clusters as Oxygen-Evolving Catalysts Xin-Bao Han, Xing-Yan Tang, Yue Lin, Eduardo Gracia-Espino, Sang-Gui Liu, Hai-Wei Liang, Guang-Zhi Hu, Xin-Jing Zhao, Hong-Gang Liao, Yuan-Zhi Tan, Thomas Wagberg, Su-Yuan Xie, and Lan-Sun Zheng J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 12, 2018

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Journal of the American Chemical Society

Ultra-Small Abundant Metal-Based Clusters as OxygenEvolving Catalysts Xin-Bao Hana,†, Xing-Yan Tanga,†, Yue Linb,†, Eduardo Gracia-Espinod, San-Gui Liua, Hai-Wei Liangc, Guang-zhi Hud,e, Xin-Jing Zhaoa, Hong-Gang Liaoa, Yuan-Zhi Tan*,a, Thomas Wagberg*,d, Su-Yuan Xiea, and Lan-Sun Zhenga aState

Key Laboratory for Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P.R. China. bHefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, P. R. China. cDepartment of Chemistry, University of Science and Technology of China, Jinzhai Road 96, Hefei 230026, P. R. China. dDepartment of Physics, Umeå University, Umeå 90187, Sweden eKey Laboratory of Chemistry of Plant Resources in Arid Regions, State Key Laboratory Basis of Xinjiang Indigenous Medicinal Plants Resource Utilization, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011, China †These

authors contributed equally to this work.

ABSTRACT: The oxygen evolution reaction is a crucial step in water electrolysis to develop clean and renewable energy. Although noble metal-based catalysts have demonstrated high activity for oxygen evolution reaction, their application have been limited by their high costs and low availability. Here we report the use of a moleculeto-cluster strategy to prepare ultra-small trimetallic clusters by using polyoxometalate molecule as precursors. The ultrafine (0.8 nm) transition metal clusters with controllable chemical composition are obtained. The transition metal clusters enable a high-efficient oxygen evolution through water electrolysis in alkaline media, manifested by an overpotential of 192 mV at 10 mA cm‒2, a low Tafel slope of 36 mV dec‒1 and long-term stability for 30 h electrolysis. We note however that besides the excellent performance as oxygen evolution catalyst, our molecule-to-cluster strategy provides means to achieve well-defined transition metal clusters in the sub-nanometer regime, which potentially can have impact on several other applications. INTRODUCTION Electrocatalytic water splitting is widely considered as a promising and sustainable approach for the production of clean H2 fuel and O21‒3. Water oxidation, represented by the half-reaction 2 H2O → 4 H+ + 4e- + O2, constitutes a kinetic bottleneck in water splitting4,5. Therefore, the development of highly efficient water oxidation catalysts is crucial in the renewable energy field6‒8. Noble-metal catalysts such as iridium and ruthenium oxides are the highly efficient oxygen evolution reaction (OER) electrocatalysts in acid media but suffer from high-cost and relative scarcity9 and also require a substantial overpotential to reach the desired current density of ≥10 mA cm−2

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for OER in alkaline media10. In alkaline media, on the other hand, a number of research groups have demonstrated the use of earth-abundant transition metal (TM)-based catalysts, including oxides11,12, oxyhydroxides13, borides14, phosphides15,16 and molecular complexes17,18, but there are still room for significant improvement in OER systems to obtain good energy efficiency and cost effectiveness in the alkaline electrolysis systems. More importantly, these studies suggested that catalyst size and composition play crucial roles in governing their OER activities. Small-sized metal clusters, with controlled composition and size have however proved to be particularly difficult to be synthesized and stabilized. Herein we report a novel and efficient synthesis of transition metal clusters (TMCs) from welldefined polyoxometalate (POMs) molecule precursors via a molecule-to-cluster strategy. The prepared TMCs are characterized by their ultrafine size (0.8 ± 0.2 nm) and tunable chemical composition, all of which were inherited from their parent POMs. Our experimental and theoretical results demonstrate that well-defined Fe-doping of POMs allow to precisely modulate the chemical compositions of the corresponding TMCs, resulting in significant enhancement of their OER activities. The best TMC catalyst exhibited an overpotential of 192 mV at a current density of 10 mA cm‒2, had a low Tafel slope of 36 mV dec‒1, and was shown to remain stable for 6,000 cyclic voltammetry (CV) cycles and 30 h electrolysis. The molecule-to-cluster approach applied here could serve as a general and effective strategy to establish ultra-small TMclusters where practically all atoms reside at the surface and thereby are tunable for high-efficiency oxygen evolution and other important electrocatalytic reactions. RESULTS AND DISCUSSION Synthesis of TMCs. A schematic view of catalyst preparation is presented in Figure 1a-d. The self-assembly of [{Co4(OH)3PO4}4(SiW9O34)4]32− (POM 1) (Figure S1a, S1b, and Table S1, Supporting Information [SI]) and ethylenediamine-grafted C60 (EDAC60), driven by electrostatic interaction, resulted in the formation of POM 1/EDA-C60 hybrid composites. Images obtained by in-situ liquid cell transmission electron microscopy (TEM)19,20 showed that the composites contained discrete POMs (Figure 2a), suggesting that a satisfactory encapsulation of POMs by EDA-C60 is achieved, ultimately preventing self-aggregation. Similarly, TEM imaging of freeze-dried hybrid composites provided additional visual evidence for the embedment of discrete POMs in the EDA-C60 matrix (Figure S2, SI).


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Figure 1. Schematic illustration of synthesizing the TMCs. a, Ball representation of the structure of POMs 1−3. b, Self-assembly of POMs and EDA-C60 by electrostatic interaction. c, Schematic structure of the POMs/EDA-C60 composites after freezedrying. d, TMCs catalysts on carbonic support. O, gray; Co/Fe, violet; W, red; P, yellow. We speculate that the spatial confinement and single-molecule dispersion of POMs in the EDA-C60 matrix favor the formation of TMCs during thermal annealing. Indeed, annealing of the hybrid composite under 5% H2/Ar atmosphere for 3 h at 900 °C led to the transformation of the embedded POMs into TMCs and the formation of a carbonic support from EDA-C60 (Figure 1d). The resulting material was labeled 1-CoW. The morphology of 1-CoW was studied by scanning electron microscopy (SEM) and TEM. SEM images of 1-CoW showed curved sheets with few visible nanoparticles on the surfaces (Figure 2b). In contrast, abundant clusters were observed spreading over the carbonic supports in images generated by TEM (Figure 2c, 2d) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Figures 2e, 2f and Figure S3 in SI). The size distribution of the clusters was in the range of 0.6 to 3.0 nm, with a mean diameter of 0.8 nm (Figure 2f inset). Moreover, HAADF-STEM imaging of the clusters demonstrated a clear speckle pattern (Figures 2e and 2f). The speckle pattern is created due to the positive correlation of the spot brightness in HAADF-STEM images and the atomic number of the visualized atoms21,22, indicating a well-mixed W-Co system seen as contiguous brighter and dimmer spots, respectively. These results are also supported by our theoretical simulations where W-Co clusters in the range of 0.7 – 0.9 nm exhibit a chemical order parameter close to zero, meaning that no significant segregation is observed23. Furthermore, STEM-coupled energy-dispersive X-ray spectroscopy (STEM-EDXS) elemental mapping of 1-CoW (Figure 2g) also showed that Co and W were homogeneously distributed on the carbonic support. It is noteworthy to indicate that



such small TMCs (0.8 nm) have ~90 % of the atoms located at the surface and are significantly under-coordinated (Tables S2 and S3), indicating that they are feasible to be surface-passivated by O/OH as suggested by DFT, discussed later. We therefore suggest that the catalytically active sites of TMCs are closely related to metal oxyhydroxides in agreement with previous studies24‒26, which is also confirmed by the X-ray photoelectron spectroscopy (XPS) analysis of the chemical valence of Co, Fe, W and O atoms in TMCs (Figures S4‒S7, SI).

Figure 2. Morphology of TMCs on carbonic support. (a) In-situ liquid cell TEM image of the solution in which POM 1 self-assembles with EDA-C60. (b) SEM image of 1CoW. (c) and (d), TEM images of 1-CoW. (e) and (f), HAADF-STEM images of 1CoW showing nanoparticle speckling of TMCs. The inset image in (f) illustrates the corresponding size distribution of the TMCs in 1-CoW. g, HAADF-STEM image and STEM-EDXS element mapping of 1-CoW. OER performance of 1-CoW. The OER performance of the carbon-supported TMCs was compared with that of IrO2 by using a typical three-electrode system in 1 M KOH solution at a scan rate of 5 mV s‒1. Linear sweep voltammetry (LSV) was employed to obtain the polarization curves of the electrode. All potentials were calibrated in reference to the reversible hydrogen electrode (RHE). Before measurements, the as-prepared electrodes were repeatedly swept in the electrolyte until


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a steady voltammogram curve was obtained and in this process Fe and Co of the TMCs would be oxidized. Each catalyst was coated onto carbon cloth at a density of 0.64 mg cm‒2 and the generated OER currents were measured. As shown in Figure 3a, 1-CoW produced a small overpotential of 240 mV at a current density of 10 mA cm‒2 (Table 1, all current densities based on projected geometric area), which was much smaller than that of IrO2 (305 mV) under the same conditions. Table 1. Comparison of electrocatalytic activity at a current density of 10 mA cm‒2. On carbon cloth

On gold foam

Overpotential / mV

Overpotential / mV

1-CoW

240 (±5)

222 (±3)

2-CoFeW

224 (±3)

205 (±2)

3-CoFeW

205 (±3)

192 (±2)

IrO2

305 (±4)

-

Samples

Figure 3. OER polarization curves of catalysts loaded on (a) carbon cloth or (c) gold foam with a scan rate of 5 mV s−1, without iR correction. Tafel plots for different catalysts loaded on (b) carbon cloth or (d) gold foam in 1 M KOH solution. Enhancing OER performance by Fe-doping. Doping additional elements into metallic nanoparticles has proven to be an effective strategy for enhancing the performance of metallic catalysts24,27,28. The molecule-to-cluster protocol enabled convenient chemical doping of TMCs by modifying the structure of the POM precursors. By this strategy, we synthesized POM 2 24− 28− [{Fe2Co2(OH)3PO4}4(SiW9O34)4] and POM 3 [{FeCo3(OH)3PO4}4(SiW9O34)4] (Figure S1, SI) by doping POM 1 in situ with different amounts of Fe. POM 2 and POM



3 were shown by single-crystal X-ray crystallography to share almost-identical structures as POM 1, albeit with different extents of Co displacement with Fe. We then generated two differently Fe-doped TMCs on carbon support, designated as 2-CoFeW and 3-CoFeW, from POM 2 and POM 3 by the same synthetic route as that of 1-CoW, respectively. As expected, both 2-CoFeW and 3-CoFeW were shown by X-ray diffraction (XRD) patterns, SEM, TEM and STEM-HAADF characterizations (Figures S8‒S11, SI) to share similar morphologies as 1-CoW and were confirmed by STEMEDXS elemental mapping to be uniformly doped with Fe (Figure S12, SI). Both 2-CoFeW and 3-CoFeW showed that the OER overpotentials reduced by 16 and 35 mV, respectively, at a current density of 10 mA cm‒2 when compared with 1-CoW. This demonstrated that a well-balanced Fe-doping could improve the catalytic performance of the corresponding TMCs (Figure 3a and Table 1), but it is noteworthy that the Fe-ratio in 2-CoFeW already is “on the down slope” of OER activity, manifested by an overpotential of 224 mV at 10 mA cm−2 (19 mV higher than that of 3-CoFeW). This behavior matches the observations in our DFT studies where a too high Fe-content leads to a too high adsorption energy of O-intermediates, and thereby an increase in the overall OER overpotential. In addition, insight in the kinetic behaviors of all TMC catalysts in OERs were obtained by analyzing their Tafel slope. As such, the superior OER performance of 3-CoFeW was supported by its smaller Tafel slope (38 mV dec‒1, Figure 3b) compared with 1-CoW (53 mV dec‒1) and 2-CoFeW (43 mV dec‒1). Turnover frequency (TOF) was also calculated at an overpotential of 250 mV in 1 M KOH solution, assuming all the Co and Fe atoms account for the catalytic activity for the OER (Table S4, SI). 3-CoFeW shows larger TOF of 0.377 s‒1 compared with 0.114 s‒1 and 0.259 s‒1 for 1-CoW and 2-CoFeW (Table S5, SI), respectively. Taken together, these data lent credence to the critical importance of finetuning the chemical composition of the TMC for improving its catalytic performance in OERs. Substrate influence on OER performance. To assess the effect of the substrate on the catalytic efficiency of the electrode, we loaded the carbon-supported TMC catalysts at a density of 0.64 mg cm‒2 on gold foam (a nickel foam that was covered with gold to avoid any spurious effects arising from interaction of the catalyst with Ni), which has a greater conductivity than carbon cloth. This switch of substrate significantly boosted the catalytic activities of all three TMCs without affecting the beneficial impact of Fe doping (Figure 3c). Specifically, 3-CoFeW showed a remarkably low overpotential of 192 mV at 10 mA cm‒2 on the gold foam (Table 1). The enhanced conductivity of the substrate also led to a substantial improvement of the OER kinetics, evidenced by the decrease of the Tafel slope for all three catalysts (Figure 3d). In addition, electrochemical impedance spectroscopy (EIS) of different samples were performed to provide further insight into the electrode kinetics under OER process. As shown in Figure S13 and Table S6 in SI, the solution resistance (Rs) of 1-CoW, 2CoFeW and 3-CoFeW coated on gold foam has a similar value, while the chargetransfer resistance (Rct) of 3-CoFeW coated on gold foam fitted according to the high frequency semicircle was only 0.93 Ω, which is smaller than 1-CoW (2.2 Ω) and 2CoFeW (1.4 Ω). Such a low Rct value of 3-CoFeW suggested faster charge transfer


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kinetics than 1-CoW and 2-CoFeW. When 3-CoFeW was coated on carbon cloth, the Rs and Rct skyrocketed form 2.4 Ω to 5.2 Ω and 0.93 Ω to 9.9 Ω, respectively, which was consistent with the disparity of catalytic activity on different substrates. Stability. We next evaluated the electrochemical stability of the carbon-supported TMC catalysts by long-term cycling. We firstly ran water oxidation on the catalyst deposited on carbon cloth (Figure 4a) and gold foam (Figure 4b) under a long-term cycling test in 1 M KOH solution. Gratifyingly, the OER polarization curve of 3CoFeW remained almost unchanged throughout the experiment and exhibited a

Figure 4. Polarization curves of 3-CoFeW loaded on (a) carbon cloth and (b) gold foam before and after 6,000 potential cycles in 1 M KOH solution. Chronoamperometric responses (i-t) of 3-CoFeW coated on (c) carbon cloth and (d) gold foam for 30 h at a constant applied potential of 1.46V versus RHE for OER. negligible potential change at 10 mA cm‒2, after 6,000 CV cycles, despite the relatively harsh experimental conditions. Chronoamperometric responses (i-t) of 3-CoFeW coated on carbon cloth (Figure 4c) and gold foam (Figure 4d) kept the activity for 30 h long term electrolysis. The STEM-HAADF and high-resolution transmission electron microscope (HRTEM) images of 3-CoFeW after the OER were found to be largely identical to those of the freshly synthesized catalyst (Figures S14 and S15, SI). In addition, XRD (Figure S16) measurements of catalysts showed almost no change before and after the long-term OER test. At last, we performed the inductively coupled plasma mass spectrometry (ICP-MS) measurements of the three catalysts to detect the amount of Co, Fe or W of the reaction solution after a long-term cycling test. The data of ICP-MS indicated that there is less than 0.56 % of W, 2.0 % of Fe and 1.3 % of Co may leach from the catalyst (Table S7, SI). The enhanced operational stability of such small clusters can be attributed to the large cohesive energy of tungsten, as well as the strength of W-C bond probably found at the cluster-carbonic substrate interface, similar results have been observed on PdW/C systems29. These results revealed the good electrochemical stability of the TMC catalysts. Although OER catalysts supported on



carbonic supports are widely used for fundamental study30‒34, it is worth noting that carbonic supports suffer from carbon corrosion for practical application under prolonged anodic bias in an alkaline electrolyzer.35,36 We are continuing our efforts to prepare TMC catalysts supported on more stable supports for practical applications. Theoretical description of the OER. The catalytic activity of CoFeW clusters was also investigated by DFT. Three different clusters sizes were prepared by a modified Basin-Hopping Monte Carlo (BHMC) method with an average diameter of 0.7, 0.8, and 0.9 nm, and a chemical composition of W13Co16, W23Co16, and W36Co16 respectively (Figures S17 and S19, SI). The resulting clusters exhibited an average coordination number of ~7.2, irrespective of the particle size. Interestingly, the addition of Fe does not have a significant effect on the average coordination number. However, the individual coordination number of Co is increased in the presence of Fe (Table S3), suggesting a Co migration to the cluster core, which in a 0.8 nm particle correspond to ~4 atoms, while Fe is more often found at the surface. In any case, the severe undercoordination results in large adsorption energies for OER intermediates (*O, *OH, and *OOH) degrading the catalytic activity, but it also indicates the feasibility to form an oxyhydroxide surface layer and confirmed by the XPS analysis of Co, Fe, W and O atoms in TMCs (Figures S4‒S7, SI). The OER activity was investigated only on clusters with an average diameter of 0.8 nm since these were the most abundant experimentally. The clusters were initially passivated with hydroxyl groups until the changes in free energy were zero. During the passivation process several water molecules were formed and removed from the system, resulting in an OH/O surface-passivation (Figure S7, SI) with a ratio of 4.2, 4.9 and 6.0 and a final composition of W23Co16O9(OH)38, W23Co12Fe4O8(OH)39 and W23Fe16O7(OH)42, respectively. The differences in OH/O ratio already shows the stronger interaction that Fe exhibit over the hydroxyl groups compared to a weaker OH interaction when only Co atoms are present. The catalytic activity was investigated using the four-electron reaction pathway37 and the free energy difference between the *O and *OH intermediate (ΔGO-ΔGOH) as universal descriptor37,38. The latter results in a Volcano-like plot as the one seen on Figure 5, where the highest OER activity is expected for those materials with a free energy difference ΔGO-ΔGOH = 1.6 eV, while those at a lower or higher energy difference will exhibit a lower OER activity. In our case, the clusters are mostly asymmetric with a large variety of active sites with diverse OER activity. As seen in Figure 5, all three clusters exhibit sites with high activity reflecting the performance seen experimentally. It can however be seen that W23Co16O9(OH)38 favours weak adsorption energies seen as ΔGO-ΔGOH > 1.6 eV with the best site having a ηOER equal to 0.45 V, while W23Fe16O7(OH)42 tends towards a higher adsorption energy with ΔGO-ΔGOH < 1.6 eV and the lowest ηOER = 0.39 V, and the tri-metallic W23Co12Fe4O8(OH)39 mostly lies around ΔGO-ΔGOH = 1.6 eV with an optimum ηOER of 0.37 V. It is noteworthy to mention that we did not observe any significant difference between Co of Fe in the mixed W23Co12Fe4O8(OH)39 cluster. However, we did notice differences when considering the coordination environment, and those Fe/Co atoms with a complete octahedral coordination exhibit the lowest ηOER irrespective of the neighbors’ type. Similar results were observed when using a different


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set of atomic models constructed using the γ-FeOOH framework instead of the BHMC method (W20Co16O30(OH)51, W20Co12Fe4O33(OH)44, and W20Fe16O30(OH)51), see Figure S18 and S20 in SI for further details. Our results are in line with previous publications where CoOOH exhibit a weak interaction with oxygenated species, whereas it is too strong on FeOOH24, therefore a combination of these two ultimately results in a material with an optimal adsorption energy.

Figure 5. Theoretical catalytic activity for metal oxyhydroxide clusters. OER activity volcano plot for W23CowFexOy(OH)z clusters. CONCLUSIONS In summary, a series of carbon supported TMCs have been successfully prepared by a molecule-to-cluster strategy and employed as electrocatalysts for OER in 1 M KOH. These catalysts exhibited high electrocatalytic activity and long-term catalytic stability. Particularly, 3-CoFeW generated a current density of 10 mA cm‒2 at an overpotential of only 192 mV when loaded on gold-plated nickel foam as working electrode, making it one of the most active OER catalysts reported to date. Even more importantly, our molecule-to-cluster strategy presented here provides a unique approach to fine-tune the catalytic properties of ultrafine clusters by selective and controlled surface doping to address adsorption properties of reaction intermediates at the surface of the TM-clusters. We note that our approach has implications far beyond oxygen evolution reactions. EXPERIMENTAL SECTION Materials and Methods. KOH was purchased from Aldrich Chemical Inc. Other reagents and solvents were used of commercial grade, without further purification. The SEM images were performed on a Hitachi S-4800 scanning electron microscope at an accelerating voltage of 15 kV.



TEM and HAADF-STEM images were collected on a JEOL ARM-200F field-emission transmission electron microscope operating at 200 kV accelerating voltage. STEM-EDXS images were collected on a Titan Themis transmission electron microscope (FEI). The in-situ TEM observation was carried out using a FEI Tecnai F20 transmission electron microscopy with a CMOS based camera TemCam-XF416 (TVIPS GmbH, Germany). Synthesis of POM 1. POM 1 was synthesized according to a previously described method39. Briefly, CoCl2·6H2O (0.78 g, 3.26 mmol) was dissolved in 40 mL of distilled water, followed by the addition of Na10[α-SiW9O34]·18H2O40 (1.18 g, 0.40 mmol). The reaction mixture was stirred until a clear, purple solution was obtained. Na3PO4·12H2O (0.60 g, 1.58 mmol) was then added and the pH was maintained in the range of 8.5−9.0 with 1.0 M KOH (aq). The resultant turbid solution was stirred for 3 h at room temperature and the purple precipitate was removed by filtration. The obtained filtrate was subsequently mixed with 5 mL of 1.0 M KCl solution, stirred for another 30 min and filtered into a 50 mL beaker to allow slow evaporation at room temperature for one week, resulting in the precipitation of 1 as dark purple crystals. The crystalline product was collected by filtration, washed with cold water, and air-dried. Synthesis of POM 2. CoCl2·6H2O (0.18 g, 0.75 mmol) and Fe(NO3)3·9H2O (0.34 g, 0.84 mmol) were dissolved in 50 mL of distilled water. Na10[α-SiW9O34]·18H2O (1.18 g, 0.40 mmol) was then added and the mixture was stirred for 15 min. Next, Na3PO4·12H2O (0.60 g, 1.58 mmol) was added and the pH was maintained in the range of 8.0−8.5 with 1.0 M NaOH (aq.). The resultant turbid solution was stirred for 2 h at 80 °C, and the precipitate was removed by filtration. The obtained filtrate was subsequently mixed with 5 mL of 1.0 M NaCl solution, stirred for another 30 min and transferred to a 100 mL beaker to allow slow evaporation at room temperature over one or two weeks. Eventually, the product 2 precipitated as light brown crystals, which were washed with cold water and air-dried. Overall, 239 mg of 2 was obtained and the yield was calculated to be 19.35%. Anal. Calcd (%): Na, 4.68; Fe, 3.80; Co, 4.00; W, 56.15; Found: Na, 4.76; Fe,3.73; Co, 3.92; W, 56.41. Synthesis of POM 3. CoCl2·6H2O (0.36 g, 1.51 mmol) and Fe(NO3)3·9H2O (0.23 g, 0.57 mmol) were dissolved in 50 mL of distilled water. Na10[α-SiW9O34]·18H2O (1.18 g, 0.40 mmol) was then added and the mixture was stirred for 15 min. Next, Na3PO4·12H2O (0.60 g, 1.58 mmol) was added and the pH was maintained in the range of 8.0−8.5 with 1.0 M NaOH (aq.). The resultant turbid solution was stirred for 2 h at 80 °C and the precipitate was removed by filtration. The obtained filtrate was mixed with 5 mL of 1.0 M NaCl solution, stirred for another 30 min and subsequently transferred to a 100 mL beaker to allow slow evaporation at room temperature over one or two weeks. The product 3 precipitated as reddish-brown crystals, which were washed with cold water and air-dried. Overall, 201 mg of 3 was obtained and the yield was calculated to be 16.27%. Anal. Calcd (%): Na, 5.38; Fe, 1.87; Co, 5.91; W, 55.24; Found: Na, 5.45; Fe,1.82; Co, 5.78; W, 55.49. Synthesis of EDA-C60. EDA-C60 was synthesized by a modified method.41,42 Typically, C60 (500 mg) and freshly distilled EDA (500 mL) were added to a three-necked flask and stirred in an oil bath for 4 days at 80 °C under an Ar atmosphere. The excess EDA was removed from the crude reaction mixture by rotary evaporation, followed by the addition of 10 mL of deionized H2O to dissolve the residuals. The target product was precipitated by adding 500 mL of acetone, washed several times by acetone, vacuum-filtered and then dried at 60 °C overnight.


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Synthesis of the carbon supported TMCs catalysts. EDA-C60 (6 ml, 5 mg·mL‒1) was added to a 150 mL beaker and further diluted by distilled water to a final concentration of to 0.5 mg·mL‒1. The solution was vigorously stirred and its pH was recorded to be 3.57. Next, POM 1 (20 mL, 0.5 mg·mL‒1) was added drop-wise. The resultant solution was vigorously stirred for 1 h at room temperature and further ultrasonicated for 30 min. The 1-CoW catalyst was obtained by freezedrying the reaction solution and then annealing under 5% H2/Ar atmosphere for 3 hours at 900 °C. The preparation of 2-CoFeW and 3-CoFeW followed a similar protocol to that of 1-CoW except that different starting materials were used. Specifically, 20 mL of 0.5 mg·mL‒1 of POM 2 and POM 3 were used in place of POM 1 for the synthesis of 2-CoFeW and 3-CoFeW, respectively. X-ray structure analysis. Single crystal datasets and unit cells were collected on an Agilent Super Nova X-ray single crystal diffractometer using Cu Kα (λ = 1.54184 Å) micro-focus X-ray sources at 100 K. The structures of POM 2 and POM 3 were solved by the direct method and refined by the full-matrix least-squares method on F2 using the SHELX-2014 crystallographic software package43,44. Ab initio calculations were performed by means of density functional theory as implemented in the Siesta code45. The generalized gradient approximation and the revised model of Perdew, Burke, and Ernzerhof46 were used to describe the exchange and correlation functional. The valence electrons are represented by a linear combination of pseudo-atomic numerical orbitals using a double-ζ polarized basis. An energy cut-off of 250 Ry was used for the charge and potential integration in real-space. The Co/Fe content of W-Co and W-Fe-Co clusters was selected to emulate the precursors POM 1 and POM 3. Additional W-Fe clusters were built to study the extreme case where all Co atoms have been replaced by Fe. The particle size was adjusted by varying the W content until an average diameter of 7, 8, and 9 Å were obtained, resulting in W13Co16, W23Co16, and W36Co16 clusters, respectively. A modified BHMC method was used to find an energetically favourable atomic configuration47. In brief, an initial structure was randomly created and geometrically optimized using the conjugate gradient method, afterwards the position of two atoms with different chemical elements are randomly swapped followed by a geometrical optimization. A total of 80 Monte Carlo steps were carried out and the lowest energy configuration was used for the OER study. A thermal energy kBT of 0.2 eV was used as criterion to accept or reject a configuration. Finally, the optimized clusters were passivated with hydroxyl groups until the change in free energy was zero, any created water molecule was removed from the system. The final OH/O ratio was 4.2, 4.9 and 6.0 for W23Co16, W23Co12Fe4, and W23Fe16, respectively. The catalytic activity was studied using the computational hydrogen-electrode model48 by setting the reference potential equal to the standard hydrogen electrode. In this way, the chemical potential of a proton–electron pair (H+ + e–) in solution is equal to half of the chemical potential of a gasphase H2 molecule. Under these conditions, the explicit treatment of solvated protons is avoided. The adsorption free energy is calculated as ΔG(i) = ΔE(i) + ΔZPE(i) – TΔS(i), where ΔE(i) is the adsorption energy of the intermediate i evaluated using H2 and H2O as reference states49. ΔZPE is the change in zero-point energies, T is the temperature (298.15 K), and ΔS is the change in entropy, values taken from Ref. 48. The oxygen adsorption energy was estimated by using the scaling relation for transition metal oxides given by Δ𝐸𝑂𝐻 = 0.55Δ𝐸𝑂 ―0.48 in eV38. The universal descriptor, (ΔGO-ΔGOH), for transition metals and OER was used to calculate the theoretical overpotential (ηtheory), where under standard conditions the overpotential is calculated as ηtheory = {max[(ΔGOΔGOH), 3.2 eV -(ΔGO-ΔGOH)]/e}-1.23 V37.



ASSOCIATED CONTENT Supporting Information. X-ray crystallography, supplementary figures and tables, IR spectra, and CIF files. These materials are available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author. *[email protected], [email protected] ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21771155, 21721001, 11404314), the Ministry of Science and Technology of China (2014CB845603, 2017YFA0204902) and Anhui Provincial Natural Science Foundation (1708085MA06). EGE acknowledges the Carl Tryggers foundation (CTS16-161) for the financial support. The theoretical simulations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at the High Performance Computing Center North (HPC2N). Dr. Dong-Fei Lu, Dr. Yu-Hui Luo, Prof. Xin-Long Wang, and Prof. Da-Qi Wang are gratefully acknowledged for their help during reﬁning the X-ray crystal structures. REFERENCES 1. McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. J. Am. Chem. Soc. 2015, 137, 4347– 4357. 2. Kornienko, N.; Resasco, J.; Becknell, N.; Jiang, C.-M.; Liu, Y.-S.; Nie, K.; Sun, X.; Guo, J.; Leone, S. R.; Yang, P. Operando spectroscopic analysis of an amorphous cobalt sulfide hydrogen evolution electrocatalyst. J. Am. Chem. Soc. 2015, 137, 7448−7455. 3. Gao, M. R.; Zheng, Y.-R.; Jiang, J.; Yu, S.-H. Pyrite-type nanomaterials for advanced electrocatalysis. Acc. Chem. Res. 2017, 50, 2194–2204. 4. Hunter, B. M.; Gray, H. B.; Müller, A. M. Earth-abundant heterogeneous water oxidation catalysts. Chem. Rev. 2016, 116, 1420–14136. 5. Xu, P.; Huang, T.; Huang, J.; Yan, Y.; Mallouk, T. E. Dye-sensitized photoelectrochemical water oxidation through a buried junction. Proc. Natl. Acad. Sci. U.S.A. 2018, 115, 6946–6951. 6. Symes, M. D.; Cronin, L. Decoupling hydrogen and oxygen evolution during electrolytic water splitting using an electron-coupled-proton buffer. Nat. Chem. 2013, 5, 403–409. 7. Blasco-Ahicart, M.; Soriano-López, J.; Carbó, J. J.; Poblet, J. M.; Galan-Mascaros, J. R. Polyoxometalate electrocatalysts based on earth abundant metals for efficient water oxidation in acidic media. Nat. Chem. 2018, 10, 24–30. 8. Zhang, T.; Wang, C.; Liu, S.; Wang, J.-L.; Lin, W. A biomimetic copper water oxidation catalyst with low overpotential. J. Am. Chem. Soc. 2014, 136, 273−281. 9. Lei, C.; Chen, H.; Cao, J.; Yang, J.; Qiu, M.; Xia, Y.; Yuan, C.; Yang, B.; Li, Z.; Zhang, X.; Lei, L.; Abbott, J.; Zhong, Y.; Xia, X.; Wu, G.; He, Q.; Hou, Y. Fe‒N4 sites


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Ultra-Small Abundant Metal-Based Clusters as Oxygen-Evolving

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