Enhancement of the electrochemical oxygen reduction performance by surface oxygen vacancies on hematite nanosheets

The surface atomic arrangement and defective structures of electrocatalysts play a crucial role in determining their catalytic activity and selectivity. Hematite (α-Fe₂O₃) nanostructures with oxygen vacancies are promising electrocatalysts for the oxygen reduction reaction (ORR) due to their low-cost and environmental friendliness. However, a systematic study of their ORR performance, especially selectivity at high oxygen vacancy concentrations, is still lacking. In this study, we synthesized α-Fe₂O₃ nanosheets with surface oxygen vacancies using a simple solvothermal reaction followed by a liquid phase NaBH₄ reduction method. The oxygen vacancy amount was adjusted by varying the concentrations of NaBH₄ solution, and it was found that increasing the oxygen vacancy concentration from 11.4% to 43.4% improved the ORR activity, but further increasing it to 77.3% deteriorated the crystalline quality and thus affected the ORR performance. The optimized sample (α-Fe₂O₃-1 M), treated with a 1 M NaBH₄ solution, showed a high limiting current density of 5.75 mA cm⁻² at 0.4 V vs. the reversible hydrogen electrode (RHE). The observed enhancement in ORR activity can be attributed to the optimal surface oxygen vacancies, which improve catalytic kinetics and increase the exposure of active sites.