Direct growth of porous crystalline NiCo2O4 nanowire arrays on a conductive electrode for high-performance electrocatalytic water oxidation

Herein we report a facile and direct synthesis of porous NiCo2O4 nanowire arrays (NWAs) with robust mechanical adhesion to conductive electrodes by a simple two-step method. Upon complete pyrolysis of the cobalt-nickel-hydroxide precursor, high-quality crystalline NiCo2O4 is achieved. The porous NiCo2O4 nanowires were found to be highly active for catalytic water oxidation when serving as the working electrodes without any external materials (binder and/or carbon black), as evidenced by exhibiting higher catalytic current density for water oxidation compared to precious metal oxide catalysts such as iridium oxide (IrO2) under the same conditions and appreciable catalytic wave at similar to 1.52 V (vs. RHE). The optimal performance of the as-synthesized NiCo2O4 nanowires showed a current density of 10 mA cm(-2) under an overpotential of only 0.46 V and 20 mA cm(-2) under an overpotential of 0.72 V, corresponding to a Faradaic efficiency of nearly 100%. The atomic-scale analysis of the NiCo2O4 nanowires was further conducted by spherical-aberration-corrected transmission electron microscopy (TEM). The highly exposed high-index facets and one-dimensional (1D) configuration of the as-synthesized porous NiCo2O4 nanowires may be responsible for the high catalytic performance of water oxidation, which exhibit excellent activity and unique advantages for catalytic water splitting.