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

Abstract

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 CO_x 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 CO_x 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 CO_x desorption as a rate-determining step in the 0.48 to 0.52 V potential window, whereas for Ni₂O_3, it is less influential under the same conditions indicating the better tolerance of Ni₂O₃ towards CO_x 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 CO_x 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.