Effect of Ni-doping in cobaltite oxides on alkaline water electrocatalysis

The Ni-doped oxide electrodes were prepared by the span-60 sol-gel route for the electrochemical formation of oxygen in an alkaline medium. The prepared oxides were characterized physicochemically by the FTIR, P-XRD, and SEM techniques to study their formation, structure, and morphology. The prepared oxide electrodes were tested for their electrochemical performance for oxygen evolution reaction by the cyclic voltammetry and the Tafel polarization techniques. The voltammograms of each oxide electrode showed two redox peaks, one cathodic peak (E_pa = 170-245 mV) and another anodic peak (E_pa = 541-603 mV). The electrocatalytic performance of oxide electrodes in 1 M KOH at 25°C was investigated using the Tafel polarization method. Doping nickel in the oxide matrix greatly enhanced the electrocatalytic activity for the oxygen evolution reaction (OER). The most active electrode in the current study was the 0.8-mol nickel substituted oxide electrode, which demonstrated a Tafel slope (b) of 88 mVdec^-1 and a current density (j) of 50 mA cm^-2 at 331 mV oxygen over potential. It followed a first-order reaction mechanism regarding the change in [OH^-] concentration. The temperature-dependent kinetics of the oxide electrode were also investigated at various temperatures, revealing thermodynamic characteristics including the standard entropy of reaction ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Δ</mml:mi> <mml:msubsup><mml:mi>S</mml:mi> <mml:mrow><mml:mi>el</mml:mi></mml:mrow> <mml:mrow><mml:mn>0</mml:mn> <mml:mo>≠</mml:mo></mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ) for the OER ranging from 232 to 303 J deg^-1 mol^-1 and the standard electrochemical activation energy (Ea) ranging from 10 to 30 kJ mol^-1. A high negative reaction entropy value indicates that the adsorption of reaction intermediate species at the surface electrode is the mechanism by which OER takes place.