Characterization of amorphous and crystalline nickel-cobalt alloys as electrocatalysts for oxygen evolution in alkaline media

The electrocatalytic behaviour of amorphous and crystalline Ni-Co alloys for the oxygen evolution reaction (OER) in 1 M KOH has been assessed. Ni-Co alloys were activated electrochemically by steady-state preoxidation and dynamic potential cycling, and were characterized by Tafel parameter measurement, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and x-ray photoelectron spectroscopy (XPS).;Both dc and ac electrochemical measurements showed that the electrocatalytic activity of amorphous and crystalline Ni-Co alloys strongly depended on the electrochemical pretreatment. The preoxidized amorphous and crystalline Ni-Co alloys showed Tafel plots with a "limiting current" transition characteristic of Co oxides. The analysis of the "limiting current" showed that the phenomenon was associated with a time-dependent electrochemical impedance. This was attributed to a slow surface place-exchange between Ni and Co sites and an increase in degree of oxidation of the surface species from the low to the high Tafel regions.;After potential cycling, the electrocatalytic activity of Ni-Co alloys increased drastically and their Tafel behaviour appeared Ni-like. Ni-rich amorphous alloys showed particularly high activity and stability for the OER. XPS analysis showed that the potential cycled amorphous Ni-Co alloys had a surface oxide with relatively high oxidation states and a high degree of hydration on the film. The hydrated oxide film is more active for the OER than the anhydrous film formed by preoxidation.;The oxides formed by preoxidation on the alloys were thin and showed reversible electrochemical behaviour for the oxidation and reduction. After repeated potential cycles, a thick hydrated oxide layer was formed on the electrode surface, which demonstrated a diffusion-like behaviour for the oxidation/reduction similar to that of redox polymer thin films. A modified Randles-type equivalent circuit was proposed to interpret the EIS response and to characterize the oxide film/solution interface of these electrodes. The hydrous oxides formed on amorphous Ni-Co alloys can be considered as inorganic redox polymers in which the electroactive centres are cross linked by OH...