Synthesis of anion M-doped NiCoP (M = S, Se and N) as bifunctional catalysts for alkaline seawater and urea splitting

Abstract

The current method of hydrogen generation by electrochemical water splitting is widely accepted, but the oxygen evolution reaction (OER) at the anode which involves 4e⁻ transfers makes the kinetics slow and reduces the reaction efficiency. Two promising solutions are known, one is the exploration of robust and relatively low toxicity water splitting catalysts and the other is the use of small molecules that can be easily oxidized to replace the OER, which results in a lower theoretical voltage at the anode and lower energy consumption. In this study, anion M-doped NiCoP (M = S, Se and N) catalyst nanosheet arrays were in situ generated on nickel foam by hydrothermal and calcination methods, which showed not only superior hydrogen evolution reaction (HER) activity but also excellent urea oxidation reaction (UOR) performance in 1 M KOH solution containing urea. Specifically, to drive a current density of 100 mA cm⁻², an overpotential of only 170 mV is required for HER, while only a potential of 1.451 V is required for UOR; NiCoP-S also exhibits excellent HER performance in a 1 M KOH + seawater alkaline medium, with an overpotential of only 139 mV required to obtain a current density of 100 mA cm⁻². Based on the excellent bifunctionality of NiCoP-S in urea, a urea electrolyser with NiCoP-S as the anode and cathode requires only a potential of 1.456 V to drive 10 mA cm⁻². Density functional theory calculations confirmed that the doping of elements S, Se and N all altered the electron cloud distribution of NiCoP, enhanced the hydrogen adsorption ability and electrical conductivity of the electrode, and resulted in enhancement of the performance of the catalyst, with NiCoP-S having the best ΔG_H* and metallicity. The work proposes a useful strategy for the preparation of novel bifunctional catalysts with superior performance.