Controlled synthesis of 3D porous structured cobalt-iron based nanosheets by electrodeposition as asymmetric electrodes for ultra-efficient water splitting

Exploring more active, low-cost and stable electrocatalysts to substitute precious noble metals has always been a research focus on the application of water splitting. In this study, firstly, we prepared CoFe-layered double hydroxide (CoFe-LDH) via one-step electrodeposition. The CoFe-LDH nanosheets grew on the nickel foam and formed a 3D hierarchical structure, making the CoFe-LDH exhibit excellent activity for oxygen evolution reaction (OER). The overpotential was 250 mV and the Tafel slope is 35 mV dec(-1) at 10 mA cm(-2) in alkaline condition. With such a 3D porous structure as a framework, we fabricated CoFe phosphides using a solid-phase reaction. After phosphorization, the nanosheets retained the original 3D porous structure and electrochemical performance for hydrogen evolution reaction had been greatly enhanced. The overpotential in alkaline condition was 58 mV and the Tafel slope is 46 mV dec(-1) at 10 mA cm(-2). The CoFe-LDH and CoFe-P with Ni foams were used as asymmetric electrodes for overall water splitting. At the potential of 1.51 V, this electrolysis apparatus can reach a current density of 10 mA cm(-2) in 1 M KOH, which is superior to all of the reported non-precious metal based ones.