Sulfur-doping assisted defective CuCo₂O₄ as a bifunctional electrocatalyst for efficient water splitting

Electrocatalytic water splitting for hydrogen production is recognized as one of the most advanced sustainable new energy resources. The design of bifunctional electrocatalysts for water splitting at the two electrodes, which involved in anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) is crucial for the development and utilization of water splitting. Herein, the sulfur (S)-doping defective CuCo₂O₄ (S-r-CCO) electrocatalyst was synthesized by doping S ions into oxygen-defect-riched CuCo₂O₄ as bifunctional electrocatalysts for both OER and HER in this work. Due to the synergistic effect by S-doping and oxygen vacancy strategy, the S-r-CCO electrocatalyst presented an enhanced electrocatalytic performance. S-r-CCO electrocatalyst achieved a low overpotential of 248 mV at a current density of 10 mA cm^-2 with Tafel slopes of 37.7 mV dec^-1 for OER and an overpotential of 113 mV at a current density of 10 mA cm^-2 with Tafel slopes of 76.5 mV dec^-1 for HER. In addition, the assembled S-r-CCO||S-r-CCO electrodes only required a cell voltage of 1.78 V to achieve a current density of 10 mA cm^-2, which is superior to that of the commercial IrO₂||Pt/Ti electrolyzer (1.93 V) for anion exchange membrane (AEM) electrolyzer. The density functional theory (DFT) result illustrated that the synthesized massive oxygen vacancies, which provided more sites for trapping S atoms to form Co(Cu)-S coordination sites, efficiently regulated the electronic structure of S-r-CCO and improved its electrical conductivity. In addition, the introduction of S could stabilize the catalytic activity for oxygen vacancies, which greatly improved the catalytic stability of S-r-CCO electrocatalyst. Therefore, this study provides a new path for the design of effective non-noble metal bifunctional electrocatalysts for electrocatalytic water splitting.