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Operando X-Ray Absorption Spectroscopy (XAS) Observation of Photoinduced Oxidation in FeNi (Oxy)hydroxide Overlayers on Hematite (α-Fe2O3) Photoanodes for Solar Water Splitting

preprint
submitted on 18.08.2020 and posted on 19.08.2020 by Anton Tsyganok, Paolo Ghigna, Alessandro Minguzzi, Alberto Naldoni, Vadim Murzin, Wolfgang Caliebe, Avner Rothschild, David Ellis
A FeNi (oxy)hydroxide co-catalyst overlayer was photoelectrochemically deposited on a thin film hematite (α-Fe2O3) photoanode, leading to a cathodic shift of ~100 mV in the photocurrent onset potential. Operando X-ray absorption spectroscopy (XAS) at the Fe and Ni K-edges was used to study the changes in the overlayer with potential, in dark and under illumination conditions. Potential or illumination only had a minor effect on the Fe oxidation state, suggesting that Fe atoms do not accumulate significant amount of charge over the whole potential range. In contrast, the Ni K-edge spectra showed pronounced dependence on potential in dark and under illumination. The effect of illumination is to shift the onset for the Ni oxidation because of the generated photovoltage, and suggests that holes which are photogenerated in hematite are transferred mainly to the Ni atoms in the overlayer. The increase in the oxidation state of Ni proceeds at potentials corresponding to the redox wave of Ni, which occurs immediately prior to the onset of the oxygen evolution reaction (OER). Linear fitting analysis of the obtained spectra suggests that the overlayer does not have to be fully oxidized to promote oxygen evolution. Cathodic discharge measurements show that the photogenerated charge is stored almost exclusively in the Ni atoms within the volume of the overlayer.

Funding

CALIPSOplus Grant Agreement 730872 EU Framework Programme for Research and Innovation HORIZON 2020

European Research Council under the European Union’s Seventh Framework programme (FP/2007-2013)/ERC (grant agreement no. 617516)

PAT Center of Research Excellence supported by the Israel Science Foundation (grant no. 1867/17)

Helmholtz Association (HGF)

Israel Ministry of Energy as a part of the scholarship program in the field of energy (219-01-044)

Operational Programme Research, Development and Education - European Regional Development Fund, project no. CZ.02.1.01/0.0/0.0/15_003/0000416 of the Ministry of Education, Youth and Sports of the Czech Republic

BMBF project 05K19PXA

History

Email Address of Submitting Author

ellis@technion.ac.il

Institution

Technion Israel Institute of Technology

Country

Israel

ORCID For Submitting Author

0000-0002-7639-3264

Declaration of Conflict of Interest

none

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