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Open AccessArticle

In Situ EPR Characterization of a Cobalt Oxide Water Oxidation Catalyst at Neutral pH

1
Max-Planck-Institut für Chemische Energiekonversion, Stiftstr. 34-36, 45470 Mülheim an der Ruhr, Germany
2
Research School of Chemistry, The Australian National University, Canberra, ACT 2601, Australia
*
Authors to whom correspondence should be addressed.
Present address: Department of Chemistry and Chemical Biology, TU Dortmund University, D-44227 Dortmund, Germany.
Catalysts 2019, 9(11), 926; https://doi.org/10.3390/catal9110926
Received: 7 October 2019 / Revised: 31 October 2019 / Accepted: 31 October 2019 / Published: 6 November 2019
(This article belongs to the Special Issue Spectroscopy in Catalysis)
Here we report an in situ electron paramagnetic resonance (EPR) study of a low-cost, high-stability cobalt oxide electrodeposited material (Co-Pi) that oxidizes water at neutral pH and low over-potential, representing a promising system for future large-scale water splitting applications. Using CW X-band EPR we can follow the film formation from a Co(NO3)2 solution in phosphate buffer and quantify Co uptake into the catalytic film. As deposited, the film shows predominantly a Co(II) EPR signal, which converts into a Co(IV) signal as the electrode potential is increased. A purpose-built spectroelectrochemical cell allowed us to quantify the extent of Co(II) to Co(IV) conversion as a function of potential bias under operating conditions. Consistent with its role as an intermediate, Co(IV) is formed at potentials commensurate with electrocatalytic O2 evolution (+1.2 V, vs. SHE). The EPR resonance position of the Co(IV) species shifts to higher fields as the potential is increased above 1.2 V. Such a shift of the Co(IV) signal may be assigned to changes in the local Co structure, displaying a more distorted ligand field or more ligand radical character, suggesting it is this subset of sites that represents the catalytically ‘active’ component. The described spectroelectrochemical approach provides new information on catalyst function and reaction pathways of water oxidation. View Full-Text
Keywords: water oxidation; cobalt oxides; in situ spectroscopy; EPR; spectroelectrochemistry water oxidation; cobalt oxides; in situ spectroscopy; EPR; spectroelectrochemistry
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MDPI and ACS Style

Kutin, Y.; Cox, N.; Lubitz, W.; Schnegg, A.; Rüdiger, O. In Situ EPR Characterization of a Cobalt Oxide Water Oxidation Catalyst at Neutral pH. Catalysts 2019, 9, 926.

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