NiCo₂S₄ spheres grown on N,S co-doped rGO with high sulfur vacancies as superior oxygen bifunctional electrocatalysts

Rational design of stable and active bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is desirable but it remains challenging. In this work, NiCo₂S₄ spheres grown on N,S co-doped reduced graphene oxide sheets with sulfur vacancies (V_s-NiCo₂S₄/N,S-rGO) have been successfully fabricated through a facile solution method with post heat-treatment. As ORR catalyst, it presents more positive half-wave potential (0.84 V), smaller Tafel slope (43.8 mV dec⁻¹) than that of pure NiCo₂S₄ nanoparticles (0.59 V and 75.6 mV dec⁻¹) and N,S-rGO (0.54 V and 113.7 mV dec⁻¹) in 0.1 M KOH. As OER catalyst, the V_s-NiCo₂S₄/N,S-rGO only requires an overpotential of 340 mV to deliver a current density of 10 mA cm⁻² in 0.1 M KOH. Significantly, as oxygen bifunctional electrocatalyst, the V_s-NiCo₂S₄/N,S-rGO provides a smaller potential difference (0.73 V) than that of pristine NiCo₂S₄ nanoparticles (1.11 V), N,S-rGO (1.36 V) and most non-precious electrocatalysts. The much improved electrochemical performance of the V_s-NiCo₂S₄/N,S-rGO for both OER and ORR in 0.1 M KOH could be ascribed to considerable high-valence Ni³⁺ and Co³⁺ in spinel-type material and sulfur vacancies, which can improve electrical conductivity and increase the exposure of metal active sites, respectively, and thereby facilitate catalytic process. This work provides a feasible strategy to develop high-efficient bifunctional electrocatalysts with promising applications in rechargeable metal-air batteries and fuel cells.