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 CoOx 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.

In situ experiments combining X-ray absorption spectroscopy (XAS) and electrochemistry have now become an indispensable tool for understanding the mechanisms of operation, structure, and the modes of degradation of electrocatalysts under operational conditions. Herein, the design of a gas- and liquid-tight spectroelectrochemical cell (SEC) and an experimental protocol for the simultaneous collection of high-quality XAS and electrochemical data are introduced. The effects of the working electrode, loading of active material, and X-ray damage are demonstrated and interpreted by an example of a well-known heterogenite-like cobalt oxide water oxidation catalyst. The SEC permitted reproducible X-ray absorption near edge structure to be collected with a resolution of at least 0.05 eV (equivalent to approximately 0.02 unit oxidation state sensitivity) and allowed X-ray-mediated photoeffects to be examined in detail. Furthermore, tracking of the potential-dependent changes in the oxidation state of a cobalt oxide catalyst with high precision and reproducibility is demonstrated. These in situ XAS data are correlated with a previous detailed electrokinetic analysis to identify the nature of the active state of the heterogenite-like water oxidation catalyst and conclude that metal oxidation states higher than IV are not involved in the catalytic mechanism. Finally, the implications of the significantly different timescales of the probed electron transfer events and the XAS analysis on the interpretation of the in situ spectroelectrochemical data are critically discussed, focusing on the mechanism of the water oxidation reaction.

We describe the synthesis and characterization of a novel 2D-MnO x material using a combination of HR-TEM, XAS, XRD, and reactivity measurements. The ease with which the 2D material can be made and the conditions under which it can be made implies that water oxidation catalysts previously described as "birnessite-like" (3D) may be better thought of as 2D materials with very limited layer stacking. The distinction between the materials as being "birnessite-like" and "2D" is important because it impacts on our understanding of the function of these materials in the environment and as catalysts. The 2D-MnO x material is noted to be a substantially stronger chemical oxidant than previously noted for other birnessite-like manganese oxides. The material is shown to both "directly" and "catalytically" oxidize water in the presence of Ce 4+ , and to directly oxidize H 2 O 2 in the absence of any other oxidant.