Unique hierarchical NiFe-LDH/Ni/NiCo2S4 heterostructure arrays on nickel foam for the improvement of overall water splitting activity

Abstract

The development of environmentally friendly, high-efficiency, stable, earth-abundant and non-precious metal-based electrocatalysts with fast kinetics and low overpotential for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of exceeding significance but still challenging. Herein, a bifunctional electrode of unique hierarchical NiFe-LDH/Ni/NiCo₂S₄/NF (NiFe-LDH = nickel–iron layered double hydroxide and NF = nickel foam) electrocatalytic architecture, which is built up from NiFe-LDH nanosheets, Ni nanoparticles and NiCo₂S₄ nanoneedles sequentially arrayed on a porous NF substrate, has been prepared by a facile hydrothermal and electrodeposition method. This electrocatalytic architecture is binder-free and its outer NiFe-LDH nanosheets can effectively prevent the oxidation of inner Ni nanoparticles and corrosion of NiCo₂S₄ nanoneedles during water electrolysis. The integration of effective HER Ni nanoparticles into OER NiFe-LDH and NiCo₂S₄ electrocatalysts can not only overcome the disadvantage of their poor electrical conductivity, but also greatly improve their HER and OER activities, owing to the unique hierarchical heterostructure and multicomponent synergies among Ni, NiFe-LDH, and NiCo₂S₄. Consequently, this electrode shows better OER performance with ultra-low overpotential of 192 mV at 10 mA cm⁻², a small Tafel slope of only 37 mV dec⁻¹ and high stability in comparison with commercial RuO₂, and also exhibits excellent HER activity with a low overpotential of 83 mV at 10 mA cm⁻². More importantly, the NiFe-LDH/Ni/NiCo₂S₄/NF composite serves as anode and cathode for overall water splitting, and it requires a low cell potential of only 1.53 V to reach a current density of 10 mA cm⁻², which evidently exceeds that of the standard Pt–C/NF//RuO₂/NF cell of 1.61 V. This work provides a feasible strategy to prepare high-efficiency, low-cost and non-precious metal-based electrocatalysts for overall water splitting.