Water oxidation by amorphous cobalt-based oxides: in situ tracking of redox transitions and mode of catalysis

Abstract

Water oxidation by amorphous oxides is of high interest in artificial photosynthesis and other routes towards non-fossil fuels, but the mode of catalysis in these materials is insufficiently understood. We tracked mechanistically relevant oxidation-state and structural changes of an amorphous Co-based catalyst film by in situ experiments combining directly synchrotron-based X-ray absorption spectroscopy (XAS) with electrocatalysis. Unlike a classical solid-state material, the bulk material is found to undergo chemical changes. Two redox transitions at midpoint potentials of about 1.0 V (Co^II_0.4Co^III_0.6 ↔ all-Co^III) and 1.2 V (all-Co^III ↔ Co^III_0.8Co^IV_0.2) vs. NHE at pH 7 are coupled to structural changes. These redox transitions can be induced by variation of either electric potential or pH; they are broader than predicted by a simple Nernstian model, suggesting interacting bridged cobalt ions. Tracking reaction kinetics by UV-Vis-absorption and time-resolved mass spectroscopy reveals that accumulated oxidizing equivalents facilitate dioxygen formation. On these grounds, a new framework model of catalysis in an amorphous, hydrated and volume-active oxide is proposed: Within the oxide film, cobalt ions at the margins of Co-oxo fragments undergo Co^II ↔ Co^III ↔ Co^IV oxidation-state changes coupled to structural modification and deprotonation of Co-oxo bridges. By the encounter of two (or more) Co^IV ions, an active site is formed at which the O–O bond-formation step can take place. The Tafel slope is determined by both the interaction between cobalt ions (width of the redox transition) and their encounter probability. Our results represent a first step toward the development of new concepts that address the solid-molecular Janus nature of the amorphous oxide. Insights and concepts described herein for the Co-based catalyst film may be of general relevance also for other amorphous oxides with water-oxidation activity.