Engineering Ni(OH)₂ with Pd for Efficient Electrochemical Urea Oxidation

Urea-assisted water electrolysis is a promising and energy-efficient alternative to electrochemical water splitting due to its low thermodynamic potential of 0.37 V, which is 860 mV less than that needed for water splitting (1.23 V). Ni(OH)₂ has proven to be an efficient catalyst for this reaction. However, the non-spontaneous desorption of CO₂ molecules from the catalyst surface leads to active site poisoning, which significantly impacts its long-term stability. Herein, we have demonstrated that Pd incorporated NiOH₂ (Pd/Ni(OH)₂) results in a significant decrease in the overpotential by 40 mV at 10 mA cm^-2 as compared to Ni(OH)₂. The decrease in the Tafel slope and charge transfer resistance of Pd/Ni(OH)₂ indicates an improvement in the kinetics of the reaction, resulting in a maximum current density of 380 mA cm^-2 at 1.5 V, which is higher than that observed for Ni(OH)₂ (180 mA cm^-2). XAS analysis was utilized to determine the nature of the metal species in the catalyst. It revealed that while Pd predominantly exists in its metallic state within the bulk of the catalyst, the surface is enriched with the oxide phase. The presence of Pd prevents the strong adsorption of CO₂ at the active site in Pd/Ni(OH)₂, resulting in a substantial improvement of stability of up to 300 h as compared to Ni(OH)₂. DFT calculations were performed to explore the detailed reaction mechanism of urea oxidation on Ni(OH)₂ and Pd/Ni(OH)₂. These calculations provided further insight into the experimental observations and evaluated the contribution of Pd in enhancing the catalytic efficiency of Ni(OH)₂. Additionally, the operando Raman and IR spectroscopy were used to understand the formation of the active sites and the intermediates during urea electrooxidation.