Synergistic effect of sulfur atoms and ordered oxygen vacancies to enhance Fe 2 O 3 bifunctional electrocatalytic water splitting activity

Iron-based oxides are promising bifunctional electrocatalysts. The energy conversion efficiency for water splitting is limited by scarce active sites and sluggish surface reaction in Fe₂O₃. Therefore, we prepared one-dimensional Fe₂O₃ nanobelts arrays (HNBs-V_O(LRO)-S) with ordered oxygen vacancy structure by Pd-catalyzed oxygen reduction and sulfide thermal treatment. While preserving the ordered oxygen vacancy structure, making S atoms selectively fill the trap state oxygen vacancies to improve the bifunctional electrocatalytic activity and stability of Fe₂O₃. Fe₂O₃ nanobelts arrays with synergistic interaction of S-atoms and ordered oxygen vacancies have low overpotentials for anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER). Under the condition of 1 M KOH, the HNBs-V_O(LRO)-S exhibits extraordinary electrocatalytic performances for both HER (226 mV@100 mA cm^-2) and OER (262 mV@10 mA cm^-2，306 mV@100 mA cm^-2). In addition, HNBs-V_O(LRO)-S bifunctional catalyst only requires the low cell voltages of 1.92 V to deliver the current density of 100 mA cm^-2 and exhibits excellent long-term durability over 100 h. The long-range ordered oxygen vacancies serve both as a fast channel for electron transfer and as an active site for the catalytic reaction. The S atoms only fill the trap-state oxygen vacancies (TS-V_O) in the Fe₂O₃ nanobelts, which eliminates the negative effect of TS-V_O in reaction. Meanwhile, formed Fe-S coordination structure both stabilizes the ordered oxygen vacancy structure of HNBs-V_O(LRO)-S and provides more reactive active sites for the electrocatalytic reaction. Theoretical calculations show that the S atoms filling lowers the free energy barrier that the formation of OOH^* from O*, optimizes the ∆G_H* of Fe₂O₃ surface. This ingenious synergistic mechanism of vacancies filling provides new insights into the defective design of catalysts.