Effect of Urine Compounds on the Electrochemical Oxidation of Urea Using a Nickel Cobaltite Catalyst: An Electroanalytical and Spectroscopic Investigation

Cyclic voltammetry (CV) and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy were used to investigate the effect of major urine compounds on the electro-oxidation activity of urea using a nickel cobaltite (NiCo2O4 ) catalyst. As a substrate, carbon paper exhibited better benchmark potential and current values compared with stainless steel and fluorine-doped tin oxide glass, which was attributed to its greater active surface area per electrode geometric area. CV analysis of synthetic urine showed that phosphate, creatinine, and gelatin (i.e., proteins) had the greatest negative effect on the electro-oxidation activity of urea, with decreases in peak current up to 80% compared to that of a urea-only solution. Further investigation of the binding mechanisms of the deleterious compounds using in situ ATR-FTIR spectroscopy revealed that urea and phosphate weakly bind to NiCo2O4 through hydrogen bonding or long-range forces, whereas creatinine interacts strongly, forming deactivating inner-sphere complexes. Phosphate is presumed to disrupt the interaction between urea and NiCo2O4 by serving as a hydrogen-bond acceptor in place of catalyst sites. The weak binding of urea supports the hypothesis that it is oxidized through an indirect electron transfer. Outcomes of this study contribute to the development of electrolytic systems for treating source-separated urine.