Urea Electro-Oxidation Byproducts Impact on NiO/NiOOH Anode Performance Studied by Operando Electrochemical Impedance Spectroscopy

With significant quantities of urea (CO(NH2)2) being worldwide produced and its presence at high concentration in urine, simultaneous treatment of urea effluents and hydrogen production by paired electrolysis represents a very attractive approach in circular economy. Urea electro-oxidation on Ni-based catalysts in alkaline solution represents a promising approach, but poor stability of the Ni-based catalytic performance has been reported. In this work, the performance and stability of a nanostructured NiO/NiOOH anode for urea electro-oxidation has been evaluated by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and chronoamperometry (CA). Firstly, the thickness of the NiOOH active film formed during the electrochemical activation process on NiO NPs (< 50 nm) is estimated from EIS data at about 4 nm. Secondly, the impact on the catalyst performance of all previously identified urea electro-oxidation by-products (ammonia (NH3), carbonate (CO32−), cyanate (−OCN) and nitrite (NO2−)) has been individually addressed by operando EIS, and none of them are responsible for the NiO/NiOOH catalytic deactivation. In addition to this, more insight into the competitive adsorption of CO(NH2)2 and OH− on the catalyst surface during electrode aging was gained by the response of EIS in different potential regions.