Origin of Enhanced Water Oxidation Activity in an Iridium Single Atom Anchored on NiFe Oxyhydroxide Catalyst

Xueli Zheng, Jing Tang, Alessandro Gallo, Jose A. Garrido Torres, Xiaoyun Yu, Constantine J. Athanitis, Emily May Been, Peter Ercius, Haiyan Mao, Sirine C. Fakra, Chengyu Song, Ryan C. Davis, Jeffrey A. Reimer, John Vinson, Michal Bajdich, Yi Cui
Year of publication: 
Proceedings of the National Academy of Sciences

The efficiency of the synthesis of renewable fuels and feedstocks from electrical sources is limited, at present, by the sluggish water oxidation reaction. Single-atom catalysts (SACs) with a controllable coordination environment and exceptional atom utilization efficiency open new paradigms toward designing high-performance water oxidation catalysts. Here, using operando X-ray absorption spectroscopy measurements with calculations of spectra and electrochemical activity, we demonstrate that the origin of water oxidation activity of IrNiFe SACs is the presence of highly oxidized Ir single atom (Ir5.3+) in the NiFe oxyhydroxide under operating conditions. We show that the optimal water oxidation catalyst could be achieved by systematically increasing the oxidation state and modulating the coordination environment of the Ir active sites anchored atop the NiFe oxyhydroxide layers. Based on the proposed mechanism, we have successfully anchored Ir single-atom sites on NiFe oxyhydroxides (Ir0.1/Ni9Fe SAC) via a unique in situ cryogenic–photochemical reduction method that delivers an overpotential of 183 mV at 10 mA ⋅ cm2 and retains its performance following 100 h of operation in 1 M KOH electrolyte, outperforming the reported catalysts and the commercial IrO2 catalysts. These findings open the avenue toward an atomic-level understanding of the oxygen evolution of catalytic centers under in operando conditions. Computational data are available at https://www.catalysis-hub.org/publications/ZhengOrigin2020

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