Exploring the Limits of Self-Repair in Cobalt Oxide Films for Electrocatalytic Water Oxidation

We analyze the stability of anodically electrodeposited cobalt hydroxide films during operation as electrocatalysts for water oxidation and show that the stability considerations of these films are a complex issue concerning multiple distinct phenomena. We combine the electrochemical analysis with the quantitative analysis of cobalt content using X-ray fluorescence spectroscopy, transient behavior analysis using in operando XANES electrochemical analysis, and structural analysis using cryogenic X-ray absorption spectroscopy to show that three distinct behavioral regimes exist. Each behavioral regime suffers from a degree of corrosion, but both self-healing and self-repair mechanisms are capable of partially stabilizing the cobalt content within the catalyst film. However, even if the absolute cobalt content of the film is fully stabilized, the catalytic behavior of the film degrades over time. This behavior is attributed to observed structural evolution of the material, where mid- and long-range structural order increases during operation. The increased structural order appears independent of whether self-healing or self-repair mechanisms are activated and results in fundamental changes in the catalytic behavior, observable as decreased turnover frequency and increased Tafel slopes. These results reveal that the cobalt content or electrocatalytic performance of the catalyst films can be stabilized by controlling the applied voltage or solution composition, but simultaneous stabilization of both is not currently possible.