Electrocatalytic Ammonia Oxidation Mediated by a Cu(II) Complex: A Hydrazine Pathway to Hydrogen Generation

Abstract

The electrocatalytic oxidation of ammonia is a rapidly growing topic due to its high-value application in fuel cells. Ammonia is also a flexible 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, proving that 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, the computational and experimental studies suggest that ammonia oxidation is catalysed by [Cu1] through proton-coupled electron transfer (PCET) via a hydrazine pathway, which is also evidenced by 1H NMR spectroscopy.