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

Abstract

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 (Vs-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 Vs-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 Vs-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 Vs-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.