Boosting urea electro-oxidation activity by pairing nanoporous nickel with borate anions

In this work, we elucidate the crucial role of borate anions ([B(OH)₄]^-) for the electrocatalytic urea oxidation reaction (UOR) using a nanoporous metallic nickel (NP-Ni) catalyst grown on Si substrates. The UOR activity of the NP-Ni catalyst has been studied at various boric acid (H₃BO₃) concentrations, demonstrating superior activity at a specific electrolytic composition of 0.5 M KOH, 0.33 M urea, and 50 mM of H₃BO₃. Based on a wide range of electrochemical techniques, such as, cyclic voltammetry (CV), linear sweep voltammetry (LSV), Pb-anodic deposition, and chronoamperometry (CA), we develop a potential mechanism for the [B(OH)₄]^--mediated UOR. The high double layer capacitance, surface density of Ni redox sites, and urea oxidation currents, clearly demonstrate the significant impact of [B(OH)₄]^- during electrolysis. Furthermore, we find that UOR catalyzed by the NP-Ni is controlled by diffusion both in presence and absence of [B(OH)₄]^-. Finally, a set of physical characterizations, including XPS, SEM, and TEM were performed to correlate the composition and structure of the NP-Ni to the [B(OH)₄]^--mediated increased UOR activity. The improved UOR activity attributed to the [B(OH)₄]^- mediation could open a new possibility of research for UOR driven wastewater treatment.