Electrochemical impedance investigation of urea oxidation in alkaline media based on electrospun nanofibers towards the technology of direct-urea fuel cells

Abstract

Electrooxidation of urea was investigated by use of electrospun nanofibers through electrochemical impedance analyses (EIS) which is the best technique to study the electron transfer in anodic reactions. The effects of the fitting method, urea concentration and polarized potential on the impedance behavior are deeply studied to understand more about the anodic process of urea. The best-fitting equivalent circuit (Rh, R1/CPE1, R2/CPE2) should have two charge transfer processes in addition to electrolyte resistance, i.e., the formation of NiOOH and urea degradation. The Nyquist (Z/, -Z//) plots showed two semicircles in the presence of urea/KOH, which can be related to the conversion of Ni(OH)2 to NiOOH and urea oxidation at high and low-frequency regions, respectively. At higher urea contents, the electrode surface was covered by more urea beside OH−, and hence the Ni-activation process was decreased which interpreted the decreasing of oxidation at higher urea concentrations. Among the investigated polarized potentials, the best impedance behavior was observed at 0.40 V. The higher impedance behavior in the case of higher potentials can be interpreted via the adsorption of intermediates at the surface of the electroactive catalyst. The mechanism of electrooxidation was suggested deeply to understand the anodic behavior of urea-fuel cells.