Nickel sulfide wrapped by porous cobalt molybdate nanosheet arrays grown on Ni foam for oxygen evolution reaction and supercapacitor

Highlights: • The Ni₃S₂ warped by Co-Mo-O (Co_1.2MoO_4.2·1.3H₂O) fabricated by hydrothermal method. • Co-Mo-O/Ni₃S₂/NF exhibits an excellent performance for oxygen evolution reaction and supercapacitor. • The asymmetric supercapacitor device obtains high energy density and power density. Co-Mo-O (Co_1.2MoO_4.2·1.3H₂O)/Ni₃S₂ nanosheet arrays were in-situ grown directly on Ni foam (NF) by a facile two-step hydrothermal reaction, in which the untrathin Co-Mo-O nanosheets are connected to each other into a three-dimensional (3D) porous hierarchical architecture with Ni₃S₂ entrapped on the underneath Ni support. The resulting Co-Mo-O/Ni₃S₂/NF exhibits an excellent electrocatalytic activity for oxygen evolution reaction (OER) in KOH solution, which yields a 50 mA cm⁻² of OER current density at an overpotential of 0.29 V, outperforming the state-of-the-art noble RuO₂ (η₅₀ = 0.42 V), Co-Mo-O/NF (η₅₀ = 0.33 V) and Ni₃S₂/NF (η₅₀ = 0.31 V). And it demonstrates a high specific capacity of 340 mAh g⁻¹ at 2 mA cm⁻² with good capability (82.1% of the retention at 32 mA cm⁻²) in basic solution when used as supercapacitor electrode, which is superior to those of Co-Mo-O/NF (183 mAh g⁻¹) and Ni₃S₂/NF (70 mAh g⁻¹). The outstanding electrochemical performance of Co-Mo-O/Ni₃S₂/NF is associated with the synergistic effect of the individual components as well as the nanosheets directly grown on Ni conducting support with more active sites exposed. Furthermore, Co-Mo-O/Ni₃S₂/NF displays good durability in 12 h-electrolysis experiment for OER and 10000 charge-discharge cycles. It is expected to benefit from Ni₃S₂ entrapped by the protective layer of 3D porous Co-Mo-O nanosheet arrays, which not only can provide channel for electrolyte ion to pass through, but also improve the stability of Ni₃S₂. In addition, the fabricated Co-Mo-O/Ni₃S₂/NF//Activated carbon (AC) asymmetrical supercapacitor shows an ultra-high energy density of 57.9 Wh kg⁻¹ at a power density of 1.1 kW kg⁻¹.