Highly dispersed and disordered nickel–iron layered hydroxides and sulphides: robust and high-activity water oxidation catalysts

Abstract

The present work introduces a rapid low-temperature microwave-assisted synthesis of nickel(iron) layered hydroxides and sulphides that exhibit robust catalytic activity for electrooxidation of alkaline water – the most feasible source of electrons for any renewable fuel synthesis. The procedures require not more than an hour to complete at 120–150 °C with quantitative yields of: (i) few-atomic-layers thick porous sheets of Ni_0.75Fe_0.25(OH)_2+x with surface area A_BET = 149 m² g⁻¹, and (ii) interconnected Ni_0.75Fe_0.25S_2+y particles of few nanometers in size covered with a thin oxide/hydroxide layer having A_BET = 87 m² g⁻¹. These and other morphological and structural features of the materials were inferred from XRD, XPS, Ni- and Fe-edge EXAFS/XANES, TEM/SAED, EDX mapping, SEM, N₂ adsorption–desorption, and electrochemical techniques. At lower loadings on the electrode surface (≤0.01 mg cm⁻²), the specific activity for water (1 M KOH) electrooxidation at 0.3 V overpotential is 210 A g⁻¹ for Ni_0.75Fe_0.25(OH)_2+x, and 384 A g⁻¹ for Ni_0.75Fe_0.25S_2+y, which excels the performance of the best-performing analogues. The enhanced electrocatalytic activity of sulphides over hydroxides is defined by the better electrical conductivity and different nature of the electrochemically active surface species. At higher loadings, the activity of the microwave-synthesised NiFe catalysts is found to be partially limited by agglomeration, though still high enough to enable the water oxidation rate of 10 mA cm_geom⁻² at overpotentials of only 0.270 ± 0.005 (flat support) and 0.21 V (foam support) with Ni_0.75Fe_0.25S_2+y. The developed methods offer a new facile strategy for the creation of high-performing multicomponent catalysts.