Bifunctional CoNi/CoFe2O4/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 CoFe2O4 with a macroporous structure, and the topmost mesoporous nanosheets of CoNi(oxy)hydroxide. Since this hierarchical architecture of CoNi/CoFe2O4/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/CoFe2O4/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(-2) 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(-2), respectively. Furthermore, when it was applied as a bifunctional catalyst for overall water splitting, current densities of 10 mA cm(-2) and 100 mA cm(-2) were obtained at applied potentials of 1.57 and 1.75 V, respectively, together with excellent durability.