Freestanding and Hierarchically Structured Au-Dendrites/3D-Graphene Scaffold Supports Highly Active and Stable Ni₃S₂ Electrocatalyst toward Overall Water Splitting

A free-standing three-dimensional (3D) electrocatalyst is demonstrated as an example of fine-tuning the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities through a simple electrodeposition method. The Ni3S2 nanosheets are electrodeposited onto the highly conducting Au dendrites that are grown over 3D-graphene (3DG) framework. The highest electrochemical surface area has been achieved to enhance exposure of active sites by the integration of hierarchical Au dendrites with a sufficient amount of Ni3S2 nanosheets. The optimized electrocatalyst (3DG–Au-Ni3S2-15c) delivers the current density of 10 mA cm–2 at an overpotential of 140 mV and a lowest Tafel slope of 93 mV dec–1 for HER. The higher HER activity is supported by the smaller charge-transfer resistance of 1.09 Ω, which confirms the faster interfacial electron transfer between the electrode and electrolyte phases. The OER experiments suggest that 3DG–Au-Ni3S2 (15c) has a higher current density of 91.2 mA cm–2 at a potential of 1.6 V and a lower Tafel slope of 148 mV dec–1 in alkaline media. Furthermore, X-ray photoelectron spectroscopy reveals the conversion of Ni3S2 surface into the highly OER active hydrated nickel oxide during polarization experiment. With 3DG–Au-Ni3S2 (15c) as bifunctional electrocatalysts, an alkali electrolyzer delivers the current density of 10 mA cm–2 at a low cell voltage of 1.63 V. Furthermore, this electrolyzer gives a smooth line with a very small deviation of 4% from the initial potential value for 19 h, suggesting the excellent long-term stability of 3DG–Au-Ni3S2 (15c).