Synergistic effect of sulfur atoms and ordered oxygen vacancies to enhance Fe₂O₃ bifunctional electrocatalytic water splitting activity

Iron-based oxides are promising bifunctional electrocatalysts. The energy conversion efficiency of water splitting is limited by the scarcity of active sites and sluggish surface reactions in Fe₂O₃. Therefore, we prepared one-dimensional Fe₂O₃ nanobelt arrays (HNBs-V_O(LRO)-S) with ordered oxygen vacancy (V_O) structures via Pd-catalyzed oxygen reduction and sulfide thermal treatment. While preserving the ordered oxygen vacancy structure, sulfur (S) atoms selectively fill the trap-state oxygen vacancies to improve the bifunctional electrocatalytic activity and stability of Fe₂O₃. Fe₂O₃ nanobelt arrays with synergistic interactions between S atoms and ordered oxygen vacancies have low overpotentials for the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER). Under 1 M potassium hydroxide (KOH), HNBs-V_O(LRO)-S exhibited excellent electrocatalytic performance for both the HER (226 mV@100 mA·cm⁻²) and the OER (262 and 306 mV@100 mA·cm⁻²). In addition, the HNBs-V_O(LRO)-S bifunctional catalyst only requires a low cell voltage of 1.92 V to deliver a current density of 100 mA·cm⁻² and exhibits excellent long-term durability for over 100 h. The long-range ordered oxygen vacancies serve as fast channels for electron transfer and as active sites 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 the reaction. Moreover, the 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 filling of S atoms lowers the free energy barrier such that the formation of OOH^* from O^* optimizes the free energy of uptake of the hydrogen intermediate H* (∆G_H*) of the Fe₂O₃ surface. This ingenious synergistic mechanism of vacancy filling provides new insights into the defective design of catalysts.