Electrooxidation of urea and creatinine on nickel foam-based electrocatalysts

Abstract

Electrooxidation of urea and creatinine is desirable for wastewater remediation and portable dialysis to remove the two solutes from aqueous waste streams. Urine concentrations of urea (330 mM) and creatinine (18 mM) are relevant for wastewater remediation, while portable dialysis must be able to remove blood serum concentrations of urea (10 mM) and creatinine (0.18 mM) from dialysate. Three nickel foam-based electrocatalysts (clean Ni foam, Ni hydroxide, and Ni hydroxide doped with 1 mol% iron) were examined as substrates for the simultaneous electrooxidation of urea and creatinine. Electrodes were hydrothermally fabricated and characterized by loading, scanning electron microscopy, and elemental mapping. Cyclic voltammetry and chronoamperometry were used to electrochemically evaluate the three catalysts. All electrochemical experiments were performed at room temperature and pH 14. The results indicate that creatinine in urea solutions suppresses the oxidation current relative to urea-only solutions on Ni foam and Ni hydroxide catalysts. On iron-doped Ni hydroxide, an enhanced oxidation current occurs in cyclic voltammetry of creatinine/urea solutions, with the effect greater for dialysate-relevant concentrations. In chronoamperometry measurements, a slight decrease in oxidation current is observed for urea/creatinine solutions as compared with urea alone. This behavior is attributed to concurrent reactions of urea and creatinine subject to site blocking by adsorbed creatinine fragments that becomes evident at long timescales.