Construction of a hierarchically structured, NiCo–Cu-based trifunctional electrocatalyst for efficient overall water splitting and 5-hydroxymethylfurfural oxidation

In this study, Cu nanowire arrays grown on a three-dimensional Cu foam substrate are modified by nickel–cobalt layered double hydroxide nanosheets (NiCoNSs/CuNWs), which can be used as a self-supporting trifunctional electrocatalyst for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and 5-hydroxymethylfurfural (HMF) oxidation. This hybrid material as an electrocatalyst possesses remarkable advantages including low cost, freestanding nature, high conductivity and a unique hierarchical structure with a potential bimetallic synergistic effect. The optimized NiCoNSs/CuNWs electrode shows excellent HER and OER performances in alkaline solution, making it a remarkable bifunctional electrode for efficient overall water splitting. When assembling a two-electrode electrolytic cell with NiCoNSs/CuNWs as both the anode and cathode, it requires an applied cell voltage of only 1.69 V to deliver a current density of 10 mA cm−2 in water electrolysis. Electrochemical oxidation of 5-hydroxymethylfurfural (HMF) is also carried out in 1.0 M KOH. HMF oxidation occurs upon the formation of Ni3+ species, leading to earlier catalytic onset (around 300 mV ahead) in comparison with the OER trigged by the higher-oxidation-state Ni4+ species. When HMF oxidation and H2 evolution are integrated into a two-electrode electrolyzer with the NiCoNSs/CuNWs catalyst couple, the voltage required to achieve a current density of 10 mA cm−2 is 1.44 V, approximately 250 mV lower than that of overall water splitting. The present work provides useful guidance for designing and developing efficient multifunctional electrocatalysts for energy-saving hydrogen production coupled with oxidative biomass upgrading.