Lithiation-Regulated Iron Oxide Heterostructure for Hydrogen Evolution Reaction: Optimized Degree of Crystallinity for Enhanced Electrochemical Activity

Designing catalysts with optimal crystallinity is crucial for achieving enhanced electrocatalytic activity for the hydrogen evolution reaction (HER). Herein, the electrocatalytic activity of iron oxide was modified through a change in the crystallinity, induced by lithiation-activated amorphization. The electrode with the optimized crystalline-amorphous heterostructure exhibited superior electrocatalytic properties owing to the synergistic effect of the high electrical conductivity of the crystalline region and fast kinetics of the amorphous region with numerous defect sites. The HER activity of the crystalline-amorphous heterostructure was significantly improved compared to that of crystalline iron oxide, with a low overpotential of 114 mV at 10 mA cm-2 and superior stability. The electrocatalytic activity of excessively amorphous iron oxide, caused by repeated cycling in lithium-ion batteries (LIBs), was recovered through recrystallization after annealing under an Ar atmosphere. The electrocatalytic activity of the recrystallized electrode was comparable to that of crystalline iron oxide prior to cycling in LIBs. This study provides a strategy for tailoring the crystallinity and improving the reusability and electrocatalytic activity of electrode catalysts for HER electrodes to be used in LIBs.