Electrochemical Investigation of Urea Electro-oxidation on Ni, α and β Phases of Ni(OH)₂: A Comparative Study

In the work, the electro-activity of carbon-supported Ni, α and β phases of Ni(OH)₂ is investigated for urea oxidation reaction. A comparative analysis is carried out to understand the mechanism of urea oxidation on different surfaces of Ni(OH)₂. The α and β phases of Ni(OH)₂ is characterized by X-ray diffraction (XRD), Raman spectroscopy and inductively coupled plasma-optical emission spectrometer (ICP-OES). These catalysts are electrochemically studied by cyclic voltammetry (CV), chronoamperometry, and electrochemical impedance spectroscopy (EIS) in standard three electrode configuration. Highest activity towards urea oxidation is observed with Ni/C with an onset potential of 0.35 V vs. Ag/AgCl. Effect of alkali and urea concentration is systematically varied on Ni, α and β-Ni(OH)₂/C and the order of reaction with respect to alkali and KOH is deduced. The chronoamperometric plots suggest catalysts are stable in 1 hour in oxidizing urea in 1M KOH. The scan rate dependent analysis carried out to understand the urea oxidation mechanism. Impedance analysis suggest the sluggish kinetics of Ni(OH)₂ towards urea oxidation reaction is caused by high charge transfer resistance and it is in the order of β-Ni(OH)₂/C>α-Ni(OH)₂>Ni/C. A mechanistic study on these catalysts suggests that the urea oxidation is on Ni-based catalysts is governed by NiOOH hydroxide intermediate in an electrochemical-chemical (EC) mechanism in support with previous reports.

Key Words: Urea oxidation, α and β Ni(OH)₂, electrochemical impedance spectroscopy, direct urea fuel cells