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.