3D urchin-like NiCo₂O₄ coated with carbon nanospheres prepared on flexible graphite felt for efficient bifunctional electrocatalytic water splitting

Bifunctional non-precious electrocatalysts with high efficiency are desirable for electrolysis under alkaline conditions. Herein, 3D NiCo₂O₄ nanocones are coated with carbon nanospheres with four different sizes and prepared on flexible graphite felt by a two-step hydrothermal technique. Strong interactions occur between carbon nanospheres and urchin-like NiCo₂O₄, leading to the superior performance toward HER and OER. As the size of the nanospheres increases, the catalytic performance of the electrode increases initially and then decreases. The optimized carbon-coated NiCo₂O₄ electrode exhibits excellent catalytic reactivity for OER (10 mA cm⁻² at an overpotential of 196 mV and Tafel slope of 45.3 mV dec⁻¹), as well as a low potential of −69 mV versus RHE at 10 mA cm⁻² and Tafel slope of 66.3 mV dec⁻¹ for HER. In the two-electrode water-splitting equipment, the electrocatalyst only needs a low cell voltage of 1.57 V to drive a 10 mA cm⁻² current density and exhibits excellent durability in 1 M KOH. This study improves the electrode interface for optimizing its catalytic performance.