Engineering Disorder into Heterogenite-Like Cobalt Oxides by Phosphate Doping: Implications for the Design of Water-Oxidation Catalysts

Amongst the most promising materials designed to catalyse water oxidation from earth-abundant materials are the metal oxides. Despite the success of these materials, understanding the relationship of structure to function has been very challenging. It has been noted that many metal oxide water-oxidation catalysts function best in a proton-accepting electrolyte, such as a borate or phosphate buffer. However, these same electrolytes are known to significantly affect the metal oxide structures by imparting a level of “disorder” or “molecular nature” to the materials. The most well-known case is that of Nocera's Co–Pi catalyst (Pi: inorganic phosphorus). In this study, we have synthesised a series of “heterogenite-like” cobalt oxides with different levels of phosphate doping (0–9 %P). Our synthetic method enables us to make “bulk materials”, the structural properties of which (as observed by X-ray absorption spectroscopy and transmission electron microscopy) mimic those observed directly on electrode surfaces. The changes made to the bulk phases were directly correlated with the reactivity for water-oxidation catalysis and the ability of the CoO_x materials to act as sacrificial oxidants. The most disordered materials were most reactive for sacrificial oxidation but were less effective as water-oxidation catalysts. These results help us understand how disorder changes the thermodynamic stability of metal oxides and how this impacts on efficiency for water oxidation.