Intercalation of Hydrogen in Perovskite Oxide for Pseudocapacitive Energy Storage

Perovskite-structured oxides have been commonly used as electrode materials in pseudocapacitive energy storage. The prevailing charge storage model in perovskite oxides implies a variation of oxygen vacancies and electrons in the bulk of oxides. Thus, the conventional wisdom lies in the energy being stored through an anion intercalation mechanism involving oxygen vacancy transport. However, as the chemical bulk diffusion of oxygen is extremely sluggish at room temperature, altering the oxygen content in oxides is thus kinetically unfavorable. Such a sluggish energy storage mode is thus inconsistent with the fast kinetics that have been generally observed for perovskite oxide electrodes in pseudocapacitors. Herein, we report that the pseudocapacitances of perovskite oxides in alkaline electrolytes can stem from hydrogen intercalation, realized by the dissociation of protons and electrons in oxides. We combine the electrochemical studies of a series of perovskite oxides in KOH electrolytes with detailed structural and chemical characterizations, employing surface-sensitive X-ray spectroscopy and depth profiling X-ray photoelectron spectroscopy. It is found that the origin of the pseudocapacitance arises from subsurface hydrogen intercalation rather than classic bulk anion intercalation. The finding of a new charge storage mechanism in perovskite oxides provides a better understanding of redox behaviors of functional oxides in alkaline electrolytes, highlighting the importance of reconsidering surface hydrogen intercalation for the diagnosis and design of electrode materials toward pseudocapacitive applications.