Role of Cu and Ni in dual sites layered electrocatalysts for ammonia electrooxidation

Abstract

Single metal layered α-Ni(OH)₂, β-Ni(OH)₂ or dual metal layered NiCu hydroxides were hydrothermally deposited on nickel foam (NF) and investigated for electrochemical oxidation of ammonia. Copper induced conversion of hydrotalcite (α-Ni(OH)₂) phase into the mixed hydrotalcite/brucite (α/β-NiCu) structure, which is distinguished by smaller stacks and larger interplanar distances, demonstrated the highest catalytic performance. Copper inclusion resulted in two-fold increase in the current density (to 55.2 mA/cm² at 1.55 V vs RHE for α/β-NiCu_(1/1)) as well as significant increase in the electrochemical active surface area (387.5 cm²) and turnover frequency (0.56 1/cm²). The presence of Cu does not affect the onset potential of the reaction but elevated the overpotential of water oxidation, thereby increasing N₂ selectivity. We proposed formation of temporary vacancies on the copper sites during AOR, which allowed the adsorption of OH followed by their coupling with hydrogen atoms derived from NH_x molecules bound to nickel sites (dual-sites mechanism).