Iron-doping-induced formation of Ni-Co-O nanotubes as efficient bifunctional electrodes

The rational design of earth-abundant and efficient electrocatalysts to replace precious metal-based materials is highly anticipated for overall water splitting. Herein, NiCo₂O₄ electrocatalysts with different Fe doping amounts (Fe-NCO, x = 1, 2, 3) were synthesized by a low-temperature chemical method. It was interesting to find that the doping of Fe induced the formation of NiCo₂O₄ nanotube arrays by modulating the Fe content. The Fe₃-NCO electrode with a nanotube structure and rich oxygen vacancies exhibited exceptional electrocatalytic activities for the hydrogen evolution reaction (97 mV, 10 mA cm^-2) and oxygen evolution reaction (188.4 mV, 10 mA cm^-2). DFT calculations revealed that Fe promoted the modulation of the electronic structure, which played a crucial role in optimizing the reaction intermediates and altered the energy level of the d band center, and as a result, enhanced the water dissociation ability. Additionally, a low cell voltage of 1.56 V (10 mA cm^-2) was realized for water splitting based on an as-fabricated Fe-doped NiCo₂O₄ nanotube array bifunctional electrode.