Detachment of aluminium in NiCoAl-LDH modulates the active metal species to enhance oxygen evolution reaction activity

Abstract

Transition metal-based layered double hydroxide catalysts show high catalytic activity for the oxygen evolution reaction (OER), during which Co-based catalysts will undergo electro-induced chemical reconfiguration to form a variety of high valence Co species, such as Co⁴⁺. Co⁴⁺ has a high intrinsic activity for the OER and can enhance the electrostatic attraction of negatively charged oxygenated species (such as OH⁻) in the electrolyte. Meanwhile, the strong charge density of Co⁴⁺ can reduce the thickness of the double electric layer formed with OH⁻, shorten the ion diffusion distance, accelerate the adsorption of OH⁻, and regulate the OER kinetics to bring higher activity. However, the formation of Co⁴⁺ is a thermodynamically unfavorable process. In this study, NiCoAl layered double hydroxide (LDH) was optimized by the embedding and removal of aluminum, leading to the change of morphological structure and chemical composition, especially promoting the favorable formation of Co⁴⁺. Quasi in situ X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) tests demonstrated that the dissolution of aluminum induced the formation of Co⁴⁺, and then significantly boosted the OER activity, resulting in a lower overpotential (η₁₀₀ = 303 mV), a reduced Tafel slope (34.3 mV dec⁻¹) and a smaller R_ct (3.75 Ω), while simultaneously maintaining excellent OER stability at a high current density. This work provides a reference for guiding the design of OER electrocatalysts and the study of the interface between active sites and electrolytes.