Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 114, Issue 15, Pages 3855-3860Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1701816114
Keywords
water splitting; renewable energy; solar-to-fuels; electrocatalysis; oxygen evolution reaction
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Funding
- Solar Photochemistry Program of the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences of the US Department of Energy
- Solar Energy Conversion program in Chemical Sciences and Engineering Division at Argonne National Laboratory (ANL) [DE-AC02-06CH11357]
- US Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]
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The Co4O4 cubane is a representative structural model of oxidic cobalt oxygen-evolving catalysts (Co-OECs). The Co-OECs are active when residing at two oxidation levels above an all-Co(III) resting state. This doubly oxidized Co(IV)(2) state may be captured in a Co(III)(2)(IV)(2) cubane. We demonstrate that the Co(III)(2)(IV)(2) cubane may be electrochemically generated and the electronic properties of this unique high-valent state may be probed by in situ spectroscopy. Intervalence charge-transfer (IVCT) bands in the near-IR are observed for the Co(III)(2)(IV)(2) cubane, and spectroscopic analysis together with electrochemical kinetics measurements reveal a larger reorganization energy and a smaller electron transfer rate constant for the doubly versus singly oxidized cubane. Spectroelectrochemical X-ray absorption data further reveal systematic spectral changes with successive oxidations from the cubane resting state. Electronic structure calculations correlated to experimental data suggest that this state is best represented as a localized, antiferromagnetically coupled Co(IV)(2) dimer. The exchange coupling in the cofacial Co(IV)(2) site allows for parallels to be drawn between the electronic structure of the Co4O4 cubane model system and the high-valent active site of the Co-OEC, with specific emphasis on the manifestation of a doubly oxidized Co(IV)(2) center on O-O bond formation.
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