Constructing hierarchical mushroom-like bifunctional NiCo/NiCo2S4@NiCo/Ni foam electrocatalysts for efficient overall water splitting in alkaline media

Abstract Electrochemical water splitting into hydrogen energy offers a promising pathway for renewable and clean energy conversion and storage, but high overpotentials are needed to drive the process owing to the sluggish kinetics of the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER). Herein, three-dimensional (3D) mushroom-like NiCo/NiCo 2 S 4 @NiCo arrays grown directly on Ni foam (NF) have been constructed via hydrothermal and electrodeposition methods. Owing to the unique mushroom-like structure and the beneficial effect of the composite catalyst, the NiCo/NiCo 2 S 4 @NiCo/NF electrode exhibits excellent activity and stability for both OER and HER. When serving as an anode electrocatalyst, the composite catalyst only requires a overpotential of 294 mV to achieve a high current density of 100 mA/cm 2 for the OER in 1 M KOH. Moreover, in chronopotentiometric and chronoamperometric measurements, the NiCo/NiCo 2 S 4 @NiCo/NF electrode displays distinguished electrochemical and structural stability for at least 100,000 s. Similarly, the NiCo/NiCo 2 S 4 @NiCo/NF catalyst delivers a hydrogen production current density of 10 mA/cm 2 at an overpotential of 132 mV and shows no obvious attenuation of performance after stability testing for 100,000 s in an extreme alkaline environment. Accordingly, the remarkable and stable bifunctional abilities enable the application of the NiCo/NiCo 2 S 4 @NiCo/NF catalyst in two-electrode water-alkali electrolyzers with a low cell voltage of 1.55 V to deliver a current density of 10 mA/cm 2 and excellent stability for over 27 h.