Theory-driven design of high-valence metal sites for water oxidation confirmed using in situ soft X-ray absorption

Abstract

The efficiency with which renewable fuels and feedstocks are synthesized from electrical sources is limited at present by the sluggish oxygen evolution reaction (OER) in pH-neutral media. We took the view that generating transition-metal sites with high valence at low applied bias should improve the activity of neutral OER catalysts. Here, using density functional theory, we find that the formation energy of desired Ni4+ sites is systematically modulated by incorporating judicious combinations of Co, Fe and non-metal P. We therefore synthesized NiCoFeP oxyhydroxides and probed their oxidation kinetics with in situ soft X-ray absorption spectroscopy (sXAS). In situ sXAS studies of neutral-pH OER catalysts indicate ready promotion of Ni4+ under low overpotential conditions. The NiCoFeP catalyst outperforms IrO2 and retains its performance following 100 h of operation. We showcase NiCoFeP in a membrane-free CO2 electroreduction system that achieves a 1.99 V cell voltage at 10 mA cm–2, reducing CO2 into CO and oxidizing H2O to O2 with a 64% electricity-to-chemical-fuel efficiency.

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Figure 1: Change in Gibbs free energies for Ni2+→Ni3+→Ni4+.
Figure 2: An in situ electrochemical flow cell, enabling in situ sXAS.
Figure 3: Performance of NiCoFeP oxyhydroxides catalysts and controls in a three-electrode configuration in CO2-saturated 0.5 M KHCO3 aqueous electrolyte.

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Acknowledgements

This work was supported by the Ontario Research Fund Research Excellence Program, NSERC and the CIFAR Bio-Inspired Solar Energy programme. X.Z. acknowledges a scholarship from the China Scholarship Council (CSC) (20140625004) and the National Basic Research Program of China (2014CB931703). B.Z. acknowledges funding from STCSM (16JC1400702 and 14ZR14110200), NSFC (21503079) and the China Scholarship Council/University of Toronto Joint Funding Program (201406745001). This work has also benefited from SGM beamlines at the Canadian Light Source (CLS) and 4B9B and 4B7A beamlines at Beijing Synchrotron Radiation Facility. The CLS is supported by the Natural Sciences and Engineering Research Council of Canada, the National Research Council Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversification Canada and the University of Saskatchewan. The authors thank J. Guo and L. Zhang from Advanced Light Source for soft X-ray absorption measurements. The TEM study in this work was supported by the Center for Functional Nanomaterials, which is a US DOE Office of Science Facility at Brookhaven National Laboratory, under contract no. DE-SC0012704. First-principles simulations of X-ray absorption spectroscopy and associated interpretation and consultation by Y.L. and D.P. are provided through a user project at The Molecular Foundry (TMF), including use of its computer cluster (vulcan), managed by the High Performance Computing Services Group, at Lawrence Berkeley National Laboratory (LBNL), and associated use of TMF computing resources at the National Energy Research Scientific Computing Center (NERSC), LBNL. TMF and NERSC are US DOE User Facilities, both supported by the Office of Science of the US DOE under contract no. DE-AC02-05CH11231. DFT computations were performed using the IBM BlueGene/Q supercomputer at the SciNet HPC Consortium provided through the Southern Ontario Smart Computing Innovation Platform (SOSCIP).

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E.H.S., D.P. and B.Z. supervised the project. X.Z. and B.Z. designed and carried out the experiments. X.Z., B.Z., T.R., J.D. and R.C. performed the soft X-ray measurements. P.D.L., Y.L., D.P. and O.V. carried out simulations. L.H., H.L.X. and X.D. performed TEM measurements. X.Z., B.Z., F.P.C.d.A., M.L., C.T.D., S.H. and H.P. performed electrochemical measurements. All authors discussed the results and assisted during manuscript preparation.

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Correspondence to Bo Zhang or Edward H. Sargent.

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Zheng, X., Zhang, B., De Luna, P. et al. Theory-driven design of high-valence metal sites for water oxidation confirmed using in situ soft X-ray absorption. Nature Chem 10, 149–154 (2018). https://doi.org/10.1038/nchem.2886

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