Introduction of surface defects in NiO with effective removal of adsorbed catalyst poisons for improved electrochemical urea oxidation

Abstract

The electrochemical urea oxidation reaction (UOR) is a promising low cost method to replace water oxidation reaction for low energy electrochemical hydrogen production. The important factors to be considered in order to attain better UOR efficiency are electrocatalyst design by tuning defects and surface area and effective removal of CO2 poisoning on the catalyst surface. In this study, we have prepared surface oxidised NiO (e-NiO) with surface defects by electron beam bombardment and shown it to oxidize urea more effectively than pristine NiO with an onset potential of 0.33 V vs Ag/AgCl(sat.KCl) and Tafel slope of 34.69 mV/dec for 0.4 M urea in 1 M KOH electrolyte. The higher UOR current density exhibited by e-NiO is associated with the higher number of Ni3+ ions on the surface as revealed by X-ray photoelectron spectroscopy, leading to enhanced formation of NiO(OH) active species. A combination of voltammetry and impedance studies are presented to understand the active species formation and surface poisoning by released CO2 species. The dominant surface poisoning on e-NiO resulting from enhanced defects can be removed by increasing KOH concentration and the impedance spectra discloses effective removal of CO2 in 6 M KOH and 0.4 M urea. This is reflected in the UOR performance of e-NiO as drastic reduction in Tafel slope (22.29 mV/dec), lower onset potential (0.24 V vs Ag/AgCl(sat.KCl)) and high UOR current maxima of 20 mA/cm2 at the applied potential of 0.35 V vs Ag/AgCl(sat.KCl). The correlation of indirect and direct electrochemical urea oxidation reactions in pristine and e-NiO catalysts can be explained in terms of the strength of urea adsorption on surface sites derived from voltammograms and fitted impedance spectra.