Remarkable COx tolerance of Ni3+ active species in a Ni2O3 catalyst for sustained electrochemical urea oxidation

The electrochemical urea oxidation reaction (UOR) provides a cost-effective way of generating hydrogen owing to its low thermodynamic energy barrier. Although the UOR is an effective way to generate hydrogen, sustained activity and long-term catalyst usage are retarded by COx poisoning. In nickel oxide-based UOR electrocatalysts, enhancing Ni3+ active species (NiO(OH)) is one of the effective ways to improve the activity. In this study, we show that Ni2O3 is a promising UOR catalyst with Ni3+ ions being highly active and the UOR activity is seen to be almost six times higher than that of NiO. Ni2O3 shows retention of 70% UOR performance even after 25 hours at an average current density of 25 mA cm(-2), whereas the NiO system loses 50% of its activity within 10 h even at a low current density of 5.1 mA cm(-2). The efficient and sustained UOR activity of Ni2O3 can be correlated with the highly tolerant Ni3+ ions in the Ni2O3 system towards COx poisoning compared to the tolerance of NiO, as proved by impedance studies. The impedance spectra acquired at various applied potentials in the UOR regime show a highly significant reverse loop (real impedance is negative) for NiO indicative of impeding COx desorption as a rate-determining step in the 0.48 to 0.52 V potential window, whereas for Ni2O3, it is less influential under the same conditions indicating the better tolerance of Ni2O3 towards COx poisoning. Furthermore, this is reflected in the fast electro-oxidation kinetics of Ni2O3 with a lower Tafel slope value (21 mV dec(-1)) than NiO (104 mV dec(-1)). The higher COx tolerance of the Ni2O3 catalyst is the main reason behind the remarkable stability of the catalyst. Theoretical modeling supports the high activity of Ni2O3 through effective adsorption of reactants such as urea and hydroxide with feasible CO2 removal on the Ni2O3 surface.