Eventual loss of phosphate and compensated passivation observed in CoPi thin films for efficient water oxidation in alkaline solutions

Abstract Electrodeposited cobalt phosphate (Co-Pi) films are an excellent choice for oxygen evolution reactions. Despite its self-repairing mechanism in neutral electrolytes, Co-Pi is mostly used as a co-catalyst for the fabrication of composite electrodes as charge collectors. In this work, the limits of Co-Pi were assessed under various operating conditions, such as those of the cationic concentration, phosphate buffering pH, concentration, and duration of the deposition. Contrasting results were observed for Co-Pi films with varied thicknesses based on the available phosphate moieties on the surfaces of the electrodes. Likewise, a highly alkaline electrolyte medium was found to provide better charge densities, at the expense of the phosphate (PO43−) species on the surface. Mechanistic details are proposed, based on the observed elemental oxidative domains, to explain both the decline in PO43− content and the formation of an active hydrous oxide passivated layer, as an alternative mechanism in non-neutral environments.