A comparative study of the oxygen evolution reaction on oxidized nickel, cobalt and iron electrodes in base

Abstract

Despite the recent renewal in interest in the oxygen evolution reaction (OER) at transition metal oxide based electrodes in alkaline solution, the details of the mechanism remain controversial. While most studies focus on a particular oxide in isolation, a consistent experimental examination of the oxides of adjacent elements is likely to be fruitful with respect to mechanistic elucidation. In the present comprehensive work, the kinetics of the OER proceeding on the anodic passive oxides of iron, cobalt and nickel are probed using steady state polarisation to ascertain values of the Tafel slope, b, and the OH? ion reaction order, mOH-. The critically important matter of the interplay between the observed OER kinetic parameters and the electrochemistry and structure of the underlying oxide, is explored using cyclic voltammetry. Tafel slopes of b ? 46 mV dec?1 observed for pre-reduced Fe and Co anodes, are rationalised on the basis of a ?barrier oxide? associated with the inner anhydrous region of the passive film. A reaction path involving the rate determining formation of a superoxy (OOH) intermediate is proposed. A meaningful comparison of the catalytic performances of the oxides is facilitated by the estimation of active surface areas, using a transient decay measurement technique.