Nickel-based catalysts for electrolytic decomposition of ammonia towards hydrogen production

Nickel is an attractive metal for electrochemical applications because it is abundant, cheap, chemically resilient, and catalytically active towards many reactions. Nickel-based materials have been also considered as promising electrocatalysts for ammonia oxidation. The electrolysis of ammonia aqueous solution results in evolution of gaseous hydrogen and nitrogen. Up to date studies showed that metallic Ni and Ni (hydro) oxides are not catalytically active unless they are electrochemically converted to NiOOH at ~1.3 V vs. RHE. Then, dehydration of NH3 sets off by in stages electron coupled proton transfer to NiOOH resulting in a reversible reduction of the latter to Ni(OH)2. Unlike the water electrolysis process, in which solely oxygen is obtained at the anode, in that case, apart from release of N2, many undesired oxygenated nitrogen moieties may also turn up. Some reports show that these products may appear after the complete dehydrogenation of ammonia. Although, Nads dimerization is thermodynamically favorable, this coupling may be inhibited due to strong interaction of nitrogen atoms with the electrode surface, e.g. Pt. Therefore, subsequent reactions of Nads oxidation follow at more positive potentials, where water oxidation may also proceed. Studies on NiOOH activity have been conducted for systems containing various modifiers, e.g., Pt, Pd, Cu. Excluding Pt, the catalytic activity of these metals has not yet been experimentally proven. Nowadays research is being conducted in the direction of increasing the activity and stability of NiOOH.