Oxygen reduction in alkaline solution using mixed carbon paste/NixCo1-xO electrodes

Hydrogen peroxide is a valuable chemical used in various applications and there is a demand for small on-site production plants. An attractive way to produce hydrogen peroxide is by electrochemical reduction of oxygen, using electrocatalytic materials that favour formation of hydrogen peroxide instead of water. Oxygen reduction on carbon in alkaline solution is known to produce mainly hydrogen peroxide but the overpotential is large and electrocatalytic materials need to be used in combination with carbon. In the present study, the mechanism of oxygen reduction in alkaline solution was studied using NiO, Ni0.75Co0.25O and CoO powders in a matrix of carbon paste. The desired product is hydrogen peroxide and the rotating ring disc technique was used to measure the amount of hydrogen peroxide formed. Two separate processes are observed with a peak shaped wave at low overpotentials and a sigmoidal process at high overpotentials. The charge involved in the first process and the heterogeneous rate constant for the second process were determined and found to be higher in the presence of oxide compared to pure carbon paste. Maximum increase was found for the NiO containing electrode with five times higher charge in the low overpotential region and 25 times higher rate constant in the high overpotential region. The mechanism of oxygen reduction comprises redox mediated electron transfer reactions involving Ni(II)/Ni(III) states on the surface. In the low overpotential region, where oxygen reduction on carbon is mediated by native quinone groups, the increased activity is explained by an interplay between the quinone and Ni(OH)2/NiOOH redox couples.