Synergistic effect of sulfur atoms and ordered oxygen vacancies to enhance Fe <sub>2</sub> O <sub>3</sub> bifunctional electrocatalytic water splitting activity
Kaining Shi, Shihao Ding, Xueli Zhang, Hengrui Jian, Chenlong Dong, Qianqian Shen, Luhao Yang, Jinbo Xue
Abstract
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 Fe<sub>2</sub>O<sub>3</sub>. Therefore, we prepared one-dimensional Fe<sub>2</sub>O<sub>3</sub> nanobelts arrays (HNBs-V<sub>O</sub>(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 Fe<sub>2</sub>O<sub>3</sub>. Fe<sub>2</sub>O<sub>3</sub> 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<sub>O</sub>(LRO)-S exhibits extraordinary electrocatalytic performances for both HER (226 mV@100 mA cm<sup>-2</sup>) and OER (262 mV@10 mA cm<sup>-2</sup>，306 mV@100 mA cm<sup>-2</sup>). In addition, HNBs-V<sub>O</sub>(LRO)-S bifunctional catalyst only requires the low cell voltages of 1.92 V to deliver the current density of 100 mA cm<sup>-2</sup> 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<sub>O</sub>) in the Fe<sub>2</sub>O<sub>3</sub> nanobelts, which eliminates the negative effect of TS-V<sub>O</sub> in reaction. Meanwhile, formed Fe-S coordination structure both stabilizes the ordered oxygen vacancy structure of HNBs-V<sub>O</sub>(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<sup>*</sup> from O*, optimizes the ∆G<sub>H*</sub> of Fe<sub>2</sub>O<sub>3</sub> surface. This ingenious synergistic mechanism of vacancies filling provides new insights into the defective design of catalysts.