Ammonia electro-oxidation on nickel hydroxide: phases, pH and poisoning

Nickel hydroxide is a leading alternative to platinum group metals for electrocatalysis of the ammonia oxidation reaction (AOR), an important process for energy conversion and environmental remediation. Nevertheless, the dependence of AOR electrocatalysis on the different crystalline phases at the electrode surface (α-Ni(OH)₂/γ-NiOOH vs. β-Ni(OH)₂/β-NiOOH) has never been investigated. Herein, the crystalline β-Ni(OH)₂ and the disordered α-Ni(OH)₂ were synthesized and characterized by XRD, HRSEM, and Raman and FTIR spectroscopies. The respective electrocatalytic activity of the two phases was analysed at a broad range of ammonia concentrations (0.01-2 M) and pH values (11-13). Both phases electrocatalyze the oxidation of NH₃ to N₂, as proven by online mass spectrometry, but the α-Ni(OH)₂/γ-NiOOH couple is more active. At high ammonia concentrations (>1 M), surface poisoning by adsorbed NH₃ prevents access to OH^-, leading to less NiOOH formation, lower AOR currents, and suppression of the OER side reaction. The poisoning is strong and irreversible on α-Ni(OH)₂, as confirmed by soaking experiments. The difference in ammonia adsorption and electrocatalytic activity between the α-Ni(OH)₂ and β-Ni(OH)₂ emphasizes the importance of understanding the phase space of nickel hydroxide electrodes when designing low-cost electrocatalysts for the nitrogen cycle.