Bifunctional CoNi/CoFeO /Ni foam electrodes for efficient overall water splitting at a high current density

It is always a challenge to develop a kind of low-cost electrode with high activity and superior durability for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in the electrolysis of water at a high current density. In this study, a novel electrocatalyst composed of earth abundant metals of Ni, Fe, and Co with a three-dimensional (3D) nanostructure and high activity was successfully fabricated on a Ni foam (NF) substrate through a facile hydrothermal synthesis method followed by electrodeposition. The electrode had three levels of porous structures, including the bottom supermacroporous NF substrate, flower-like CoFeO with a macroporous structure, and the topmost mesoporous nanosheets of CoNi(oxy)hydroxide. Since this hierarchical architecture of CoNi/CoFeO/NF was binder-free, more catalytic active sites could be exposed, leading to enhanced electron transport and providing open-channels for the effective release of gas. As a result, CoNi/CoFeO/NF showed highly efficient electrocatalytic activity toward OER with an overpotential of 360 (±5) mV to achieve a current density as high as 1000 mA cm in an alkaline medium. Meanwhile, as a cathode catalyst, it also exhibited excellent performance toward HER with overpotentials as low as 82 (±3) and 189 (±3) mV to deliver current densities of 10 and 100 mA cm, respectively. Furthermore, when it was applied as a bifunctional catalyst for overall water splitting, current densities of 10 mA cm and 100 mA cm were obtained at applied potentials of 1.57 and 1.75 V, respectively, together with excellent durability.