Electrocatalytic ammonia oxidation mediated by a Cu(II) complex: a hydrazine pathway to hydrogen generation

The electrocatalytic oxidation of ammonia is a rapidly growing topic of study due to its high-value application in fuel cells. Ammonia is also a promising fuel with a high energy density that can serve as a hydrogen storage medium, specifically in direct ammonia fuel cells (DAFCs). Herein, we report the synthesis and characterization of a Cu(II) catalyst, [Cu(^tBuN₂Py₃)](PF₆)₂[Cu1] (where ^tBuN₂Py₃ = N,N'-(pyridine-2,6-diylbis(methylene))bis(2-methyl-N-(pyridin-2-ylmethyl)propan-2-amine)), capable of electrocatalytic oxidation of ammonia (NH₃) into dinitrogen (N₂) and dihydrogen (H₂) under ambient conditions. Using controlled potential electrolysis (CPE) in 0.84 M NH₃ at an applied potential of 1.0 V vs. SCE for 1 h, H₂ and N₂ (2.92 : 1 molar ratio) evolved with faradaic efficiencies of 89 and 91%, respectively. Incorporating ND₄OD, ⁴D₂ and ²⁸N₂ were detected using an online gas-mass analyser, confirming the electrooxidation of ND₃ into N₂ and D₂. Several techniques were employed to characterize the stability and homogeneity of the catalyst following the ammonia oxidation reaction (AOR). In addition, computational and experimental studies suggest that ammonia oxidation is catalysed by [Cu1] through proton-coupled electron transfer (PCET) via a hydrazine pathway, as supported by ¹H NMR spectroscopy.