Electrochemical investigation of urea oxidation reaction on β Ni(OH)₂ and Ni/Ni(OH)₂

Urea oxidation is the key limiting reaction in energy conversion devices based on this molecule. Ni-based catalysts are widely used to catalyze this reaction via the intermediate formation of reactive NiOOH from nickel hydroxide. In this study, β Ni(OH)₂ urea oxidation activity is compared to that of Ni/Ni(OH)₂. Electrochemical active surface area, exchange current density, rate constant, and capacitance are estimated for these catalysts to mechanistically probe the reaction. A quantitative electrochemical analysis of urea oxidation on these catalyst surfaces yields important reaction parameters. The reaction orders of β Ni(OH)₂ with respect to KOH and (NH₂)₂CO are 1.22 and 0.26, respectively, at a kinetically-controlled potential of 1.43 V vs. RHE. The reaction order with respect to KOH decreases gradually with potential and it is almost constant with urea. The similar trends in reaction order are observed with Ni/Ni(OH)₂. Electrochemical impedance measurements displayed lower charge-transfer resistance of β Ni(OH)₂ indicative of faster urea oxidation kinetics. It is observed that at the potential of 1.43 V, the charge transfer resistance of β Ni(OH)₂ (87.3 Ω cm_ECSA ²) lowered by a factor of ∼1.23 compared to Ni/Ni(OH)₂ (107.6 Ω cm_ECSA ²). The electrochemical surface area normalized heterogeneous rate constant of β Ni(OH)₂ is ∼2 times higher than that of Ni/Ni(OH)₂, in line their high intrinsic urea oxidation activity, capacitance and higher electrochemical phase stability. Moreover, the electrochemical chemical mechanism is observed on both catalysts in support with earlier report [1].