Optimized Electronic Configuration to Improve the Surface Absorption and Bulk Conductivity for Enhanced Oxygen Evolution Reaction.

The composition and structure are crucial for stabilizing an appropriate electronic configuration (unit eg electron for example) in high-efficiency electrocatalysts for the oxygen evolution reaction (OER). Here, an excellent platform to investigate the roles of the composition and structure in tuning the electron configuration for higher OER efficiency is provided by layered perovskite oxides with subtle variations of composition and structure (doping with 0%, 50%, and 100% cobalt in the Bi7Fe3Ti3O21). The crystal structures were analyzed by X-ray diffraction refinement, and the electronic structures were calculated based on X-ray absorption spectroscopy and magnetization vs temperature plots according to the Curie-Weiss law. The results indicate that the elongation of oxygen octahedra along the c-axis in layered perovskite could stabilize Co ions in the intermediate spin (IS) ( t2g)5( eg)1 state, resulting in dramatically enhanced electronic conductivity and absorption capacity. Subsequently, the OER efficiency of sample with 100% Co was found to be (incredibly) 100 times higher than that of the sample with 0% Co, with the current density increased from 0.13 to 43 mA/cm2 (1.8 V vs reversible hydrogen electrode); the Tafel slope was reduced from 656 to 87 mV/dec; and double-layer capacity enhanced from 174 to 4193 μF/cm2. This work reveals that both the composition and structure should be taken into account to stabilize a suitable electronic structure such as IS Co ions with moderate absorption and benign electronic conductivity for high-efficiency catalysis of the OER.