Electrocatalytic activity of Li_xNi_1-xO (0 ≤ x < 0.5) solid solutions in the oxygen evolution reaction

Cyclic voltammetry and steady state current-potential measurements, as well as physicochemical methods, have been used to elucidate the electrocatalytic properties of Li_xNi_1-xO electrodes in the oxygen evolution reaction in KOH and LiOH electrolytes. By varying the lithium content in Li_xNi_1-xO (x = 0 to x = 0.42) and correspondingly the mean oxidation state of nickel (2 to 3) and the crystal structure type (from disordered to ordered rock salt type), it has been shown that nickel hydroxide thin films are formed electrochemically on the oxide surface. The structures of these nickel hydroxide films are determined not only by the chemical composition and crystal structure of Li_xNi_1-xO substrates but also determined by the anodic polarization conditions and the electrolyte type. α-Type Ni · OH · H₂O is deposited on NiO, and during anodic polarization it is partially transformed to β-type Ni · OH, resulting in an enhanced electrocatalytic activity. On ordered Li_xNi_1-xO (0.3 ≤x < 0.5), oxyhydroxide films of Ni · O · OH are formed by ion exchange of Li⁺ with electrolyte protons. During anodic polarization of these samples, Ni⁴⁺ ions are stabilized, with a consequent significant decrease in their electrocatalytic activity. The highest activity towards O₂ evolution is displayed by Li_xNi_1-xO (0.05 ≤x≤0.2) with a disordered rock salt structure, on which β-type Ni · OH films, which are stable during anodic polarization, are formed. The possible insertion of electrolyte ions into the nickel hydroxide films is discussed on the basis of comparative measurements performed in KOH and LiOH electrolytes.