posted on 2023-12-07, 17:03authored byMeng-Hua Lin, Ming-Yen Lu, Hung-Lung Chou, Gang Wan, Chia-Chin Chen
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.