Sulfur-modified Co₃O₄ as a bifunctional oxygen catalyst for zinc-air batteries

The development of highly active non-precious metal bifunctional oxygen catalysts is of great significance in improving the efficiency of zinc-air batteries (ZABs). In this work, Co₃O₄ is modified by a simple sulfur-doping strategy, which   achieves a sulfur-doped Co₃O₄ (S-Co₃O₄) with significantly-enhanced  oxygen catalytic activity. The doping of sulfur promotes the exposure of cobalt active sites and brings abundant oxygen vacancies, which provide additional active sites and enhance the conductivity of Co₃O₄. Consequently, S-Co₃O₄ exhibits a low overpotential of 318 mV in the oxygen evolution reaction (OER) and an improved half-wave potential (0.788 V) for oxygen reduction reaction (ORR). In addition, the sulfur doping increases the ORR kinetic current density of pristine Co₃O₄ by a factor of 49. The assembled ZABs based on S-Co₃O₄ cathodes show a higher peak power density (98.90 mW cm⁻²) than the commercial Pt/C + RuO₂-based ZABs. This work reveals that the intrinsic oxygen catalytic activity of Co₃O₄ can be significantly strengthened by sulfur doping, which can  modify its structural characteristics and introduce structural defects.