Nickel surface anodic oxidation and electrocatalysis of oxygen evolution

Abstract

The processes of nickel surface anodic oxidation taking place within the range of potentials preceding oxygen evolution reaction (OER) in the solutions of 1 M KOH, 0.5 M K₂SO₄, and 0.5 M H₂SO₄ have been analyzed in the present paper. Metallic nickel, thermally oxidized nickel, and black nickel coating were used as Ni electrodes. The methods of cyclic voltammetry and X-ray photoelectron spectroscopy were employed. The study was undertaken with a view to find the evidence of peroxide-type nickel surface compounds formation in the course of OER on the Ni electrode surface. On the basis of experimental results and literature data, it has been suggested that in alkaline solution at E ≈ 1.5 V (RHE) reversible electrochemical formation of Ni(IV) peroxide takes place according to the reaction as follows: ${\text{NiO}}\left( {{\text{OH}}} \right)_2 + 2{\text{OH}}^ - \Leftrightarrow {\text{NiOO}}_2 + 2{\text{H}}_2 {\text{O + 2e}}^ - .$ This reaction accounts for both the underpotential (with respect to $E_{{{{\text{H}}_{\text{2}} {\text{O}}_{\text{2}} } \mathord{\left/ {\vphantom {{{\text{H}}_{\text{2}} {\text{O}}_{\text{2}} } {{\text{H}}_{\text{2}} {\text{O}}}}} \right. \kern-\nulldelimiterspace} {{\text{H}}_{\text{2}} {\text{O}}}}}^0 = 1.77\;{\text{V}}$) formation of O₂ from NiOO₂ peroxide and also small experimental values of dE/dlgi slope (<60 mV) at low anodic current densities, which are characteristic for the two-electron transfer process. It has been inferred that the composition of the γ-NiOOH phase, indicated in the Bode and revised Pourbaix diagrams, should be ∼5/6 NiOOH + ∼1/6 NiOO₂. The schemes demonstrating potential-dependent transitions between Ni surface oxygen compounds are presented, and the electrocatalytic mechanisms of OER in alkaline, acid, and neutral medium have been proposed.