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

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., OCN − , CO 3 2 − , NH 4 + , N 2 , NO 2 − ). 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 2 , 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 H 2 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. Elucidation of all the by-products occurring during UEO; Complete mass balances in both liquid and gas phases for large urea conversion; Nitrogen is not the main nitrogenous by-product; Comparison of electrolysis efficiency between synthetic and real matrix urine; Energy consumption of the electrolyzers for producing H 2 from urine.