Lattice strain-induced electronic effects on a heteroatom-doped nickel alloy catalyst for electrochemical water splitting

Abstract

The development of a high-performance water splitting electrocatalyst is essential for boosting green hydrogen energy technologies. Lattice strain affects the electrocatalytic activity in a significant way in metal alloy catalysts. However, there is a limited understanding of the effect of nanoscale morphological changes and dopant metal–substrate interactions on the lattice strain. Herein, we demonstrate the effect of lattice strain on the overall water splitting reaction by engineering the nanoscale surface morphology of heteroatom (Mn, Fe, Co, and Cu)-doped free-standing nickel electrodes. A porous NiFe alloy catalyst with a negative enthalpy of mixing and lower Fermi energy exhibits the best performance towards the oxygen evolution reaction (OER) with an overpotential (η) of 275 mV at a current density of 10 mA cm⁻². However, the NiCu electrode with a positive enthalpy of mixing showed the highest activity towards the hydrogen evolution reaction (HER) with an overpotential of 216 mV at 10 mA cm⁻². The morphological evolution of the catalytic surface at the nanoscale level with the doping of the heteroatom is due to differences in the standard electrochemical reduction potential (ΔE°), enthalpy of mixing (ΔH_mix), surface energy change (Δγ_o), and chemical potential (Δμ) of the dopant and the substrate–metal. First-principles density functional theory (DFT) calculations have been performed to check the changes in the density of states (DOS) resulting from lattice strain due to the alloying of metals. The changes in the DOS of the fabricated electrodes affect the adsorption of the reactant species, favoring the OER on NiFe and the HER on the NiCu catalyst.