In recent decades, special attention has been paid to the electrochemical oxidation of organic compounds, such as urea (H₂NCONH₂) from urine, in order to avoid their natural degradation into detrimental products, as well as to produce energy by using fuel cells [1]. Its presence in sewages and its natural decomposition into ammonia (NH₃) can adversely affect our environment. Urea is naturally decomposed into ammonia by the enzyme urease, therefore, the electro-oxidation of both compounds must be addressed as concomitant reactions, and requires a comparative study. Nickel-based materials have received attention for both urea and ammonia electro-oxidations due to their low cost, high abundance, and high activity in alkaline solutions. However, it has been suggested that urea follows an indirect pathway involving a chemical reaction with NiOOH (EC’ mechanism), different to ammonia, in which case it follows a direct electrochemical pathway (E mechanism) [2].

This work aims to compare the mechanism of urea and ammonia electro-oxidation reactions on Ni-based nanocatalysts (Ni and NiO) as a function of applied potential under controlled mass transport conditions. Hence, rotating disk electrode (RDE), cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and chronoamperommetry results are presented (fig.1). The effect of urea electro-oxidation byproducts such as cyanate, ammonia, nitrite, and carbonate, the impact of the supporting electrolyte and urea concentrations, as well as the effect of ions and other molecules present in samples of artificial human urine such as Cl^- and creatinine have been successfully studied herein.

Authors would like to acknowledge funding from the ANR project HYUREA devoted to the production of hydrogen from urea electro-oxidation.

References:

[1] E.T. Sayed, T. Eisa, H.O. Mohamed, M.A. Abdelkareem, A. Allagui, H. Alawadhi, K.-J. Chae. Direct urea fuel cells: Challenges and opportunities. J. Power Sources, 417. (2019) 159–175.

[2] A. Kapałka, A. Cally, S. Neodo, C. Comninellis, M. Wächter, K.M. Udert. Electrochemical behavior of ammonia at Ni/Ni(OH)₂ electrode. Electrochem. Commun, 12. (2010) 18–21.