New Insights into Urea Electro-Oxidation: Complete Mass-Balances and Proof of Concept with Real-Matrix Effluent

Abstract

In the present work, all the by-products formed during the urea electrooxidation (UEO) by Ni(III) in alkaline medium are identified and quantified for the first time (e.g., ). Complete mass balances are simultaneously established in both aqueous and gas phases for large urea conversion rates (>80%). The results provide clear evidence (with a maximal deviation of 2%) that, in addition to N-overoxidized by-products are produced during the UEO. Electrolyses conducted with human urine samples under identical operating conditions result in different distributions of N and C-by-products. In particular, the formation of formic and oxalic acids in these latter experiments suggests the presence of different and/or additional mechanistic pathways during the UEO process. The amount of generated at the cathode is also quantified, showing a reduction of 30% in energy consumption when compared with water electrolysis. Finally, this study assesses the energy efficiency of UEO with urea synthetic and real-matrix effluents, thereby contributing to the development of this sustainable process, which enables energy recovery from waste.