Hierarchically Assembling Cobalt/Nickel Carbonate Hydroxide on Copper Nitride Nanowires for Highly Efficient Water Splitting

Abstract

Electrochemical water splitting comprised of hydrogen evolution reaction (HER) at cathode and oxygen evolution reaction (OER) at anode has been intensively focused worldwide due to its pollution-free and product-unitary merits.[1] However, its sluggish kinetics renders an undesirably large potential relative to the theoretical one (1.23V), which inevitablly causes an increased overpotential (η) to proceed the overall water splitting. Thus, developing highly active and stable electrocatalysts capable of promoting both HER and OER in the integrated water splitting system is a top priority. Although noble-metal electrocatalysts (e.g., Pt and Ru/IrO2) have been demonstrated to exhibit impressive electrocatalytic activity toward water splitting, the scarcity and high cost hinder their large-scale applications. To this end, transition-metal based electrocatalysts, including oxides,[2] nitrides,[3] sulfides,[4] phosphides,[5] carbides,[6] and hydroxides,[7] have drawn increasing attentions, especially the transition metal nitrides (TMNs) due to their superior electrical conductivity and structural stability, but the low OER activity prohibits their further utilization in water splitting. Although some significant progress has been made to improve the OER activity over TMNs via heteroatom doping, heterostructure engineering, and composition regulation, the enhanced but still inadequate performance remains challenging. The heterostructure engineering strategy, in particular, appears to be a promising approach since its concomitant interface effect can contribute to the improvement of electon transfer, the optimized adsorption of key intermediates, and the accelerated kinetics caused by the modified electronic structure. Inspired by the above promising merits, the heterostructures were constructed in this work with two or more components to address the abovementioned issues and consequently acquire a desirable bifunctionality toward both HER and OER. Concurrently, directly growing active components in situ on the conductive 3-dimensional (3D) substrate could easily achieve the surface hydrophilic engineering, which could facilitate the detachment of gas products, promote the contact between electrolyte and electrode and expedite the charge and ion transfer. Among all the attractive materials, metal carbonate hydroxides (MCHs) with a layered structure have shown great potential for water splitting due to their rich redox properties and high accessibility to electrolyte, but have rarely been studied, not to mention rationally fabricating TMNs with MCHs over a conductive 3D substrate. In this work, a 3D hierarchical Cu3N@CoNiCHs on copper foam (i.e., Cu3N@CoNiCHs@CF) was synthesized as a bifunctional electrocatalyst for both OER and HER. The as-prepared Cu3N@CoNiCHs@CF exhibites an excellent catalytic activity and good stabilities, and achieves very low overpotentials (η10 ) of 155 mV for OER and 182 mV for HER to afford j10 , outperforming most of previously published benchmarking materials. More importaly, it only requires a remarkably low potential of 1.58 V to deliver a j 10 when fabricating Cu3N@CoNiCHs@CF as anode and cathode for overall water splitting in 1.0 M KOH, 30 mV lower than the IrO2/CF(+)-Pt/C/CF(-) couple (1.61 V). The superior activity credited for the Cu3N@CoNiCHs@CF could be attributed to the abundant active sites of the superhydrophilic electrode, optimized electronic redistribution and improved electron transfer caused by the strong interface effect between Cu3N and CoNiCHs, which consequently leads to optimal adsorption of the reaction intermediates.