Nickel-doped lithium-vacant layered LiyCr1−xNixO2: a potentially active electrocatalyst for the oxygen evolution reaction

Abstract

Enormous research efforts to use lithium transition metal-containing layered oxides (LiMO₂) as superior alternatives for noble metal oxide catalysts in OER electrocatalysis are currently underway. The mixing of the O(2p) band in M(3d) levels by tuning the oxidation state of transition metal ions by varying the Li content and metal substituents in the crystal structure can greatly enhance the OER activity of the catalyst. Herein, we systematically investigated the OER activity of layered Li_yCr_1−xNi_xO₂ (y ≤ 1 and 0 ≤ x ≤ 0.2) with different Ni doping amounts synthesized via a solid-state reaction and solution combustion method. The strategy of more electronegative Ni³⁺-ion substitution in layered LiCrO₂ can significantly increase the ionicity of the counterpart Cr–O bond through an inductive effect. Higher ionicity leads to a positive shift in the redox potential, resulting in a greater overlap between the Cr(3d) and O(2p) orbital and thus leading to superior OER activity. We also demonstrated that Ni substitution can considerably induce the oxidation of Cr³⁺ into the Cr⁶⁺ state through the inductive effect, resulting in partial Li-vacancy in the structure, and part of these vacancies is occupied by some of the Cr⁶⁺ ions, resulting in the formation of a dumbbell defect in the layered Li_yCr_1−xNi_xO₂ lattice and contribute to the superior OER activity of the catalyst. Our results reveal that solution combustion synthesized LCNO-10(c) with the nominal composition of LiCr_0.9Ni_0.1O₂ (Tafel slope of 63 mV dec⁻¹ and an overpotential of 277 mV at a current density of 10 mA cm⁻²) showed superior OER activity among all the synthesized catalysts and remarkably competes with the performance of benchmark RuO₂ electrocatalysts (336 mV at 10 mA cm⁻²). These findings validate that the presence of partial Li vacancy and cation mixing coupled with the inductive effect to enhance the ionicity of neighboring bonds in layered LiCrO₂via Ni doping (Li_yCr_1−xNi_xO₂) can be an effective technique to develop efficient electrocatalysts with superior OER activity.