Secondary sulfidation-engineered Co/N–graphene catalyst for boosted ORR/OER bifunctionality in zinc–air batteries

The development of highly efficient and stable bifunctional catalysts is of crucial importance for advancing the large-scale application of zinc-air batteries (ZABs). This present study introduces a composite catalyst composed of cobalt-based sulfides supported on nitrogen-doped graphene (Co@NG-S2), synthesized through a two-step sulfurization strategy. The first sulfurization stage was successful in achieving co-doping of sulfur (S) and nitrogen (N) while enhancing the conductivity and activity of the graphene support. The second sulfurization step promoted the uniform distribution of cobalt-based sulfides and strengthened their interfacial coupling with graphene, thereby significantly improving their bifunctional catalytic performance. Electrochemical characterization revealed that Co@NG-S2 exhibited an oxygen reduction reaction (ORR) onset potential of 0.868 V and a half-wave potential (E_1/2) of 0.798 V (vs. RHE). For the oxygen evolution reaction (OER), Co@NG-S2 demonstrated a low overpotential of merely 430 mV at a current density of 10 mA cm^-2, thereby demonstrating exceptional ORR/OER bifunctional activity. In addition, a liquid ZAB assembled with this catalyst achieved a peak power density of 110 mW cm^-2 and exhibited cycling stability exceeding 200 h. This study proposes a novel strategy for the synthesis of high-performance transition metal sulfide-carbon-based catalysts, demonstrating considerable promise for future applications.