Mo-induced in-situ architecture of NixCoyP/Co2P heterostructure nano-networks on nickel foam as bifunctional electrocatalysts for overall water splitting

Abstract

Transition metal phosphides (TMPs) exhibit excellent competitiveness for electrochemical splitting water due to their high electrical conductivity and remarkable physicochemical stability. However, pure single-phase TMPs still possess some disadvantages, such as the limited number of active sites and difficulty in greatly accelerating mass/charge transfer. Elemental doping is a practical way to manipulate the intrinsic electronic structures of TMPs and optimize the adsorption and desorption process of intermediates during the water splitting reaction. Rational construction of heterostructure electrocatalysts with high catalytic activity and excellent durability shows great potential for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, we developed a nickel foam (NF)-supported NixCoyP/Co2P heterostructure anchoring on N-doped carbon (NixCoyP/Co2P@NF) via the phosphating of the organic frameworks (ZIF-67) with Mo species added. The as-synthesized NixCoyP/Co2P@NF catalyst shows a unique three-dimensional (3D) nano-network morphology with a large specific surface area and abundant channels for mass exchange. Mo doping alters the valence electronic states of the constituent elements in the initial phase (NixCoyP), thus optimizing the proton transfer from P sites to metal sites, while Co species in the secondary phase (Co2P) is conducive to the water decomposition. Moreover, the heterogeneous interface between initial and secondary phases accelerates electron conduction and further enhances the electrocatalytic efficiency. The NixCoyP/Co2P@NF catalyst shows overpotentials of 170 mV for HER and 352 mV for OER at a current density of 100 mA cm−2 in 1.0 M KOH. The current density can reach 100 mA cm−2 by applying a voltage of 1.75 V for overall water splitting in an alkaline water electrolyzer with NixCoyP/Co2P@NF acting as the anodic and cathodic electrodes. This work has achieved a heterostructure catalyst via the heteroatom-dopant induced phase transition and offered an interfacial engineering strategy for further improving the activity of bifunctional TMP catalysts.