Uncovering the Change in Catalytic Activity during Electro-oxidation of Urea: Answering Overisolation of the Relaxation Phenomenon

Electro-oxidation of urea offers tremendous opportunity for the economical hydrogen production option owing to its lower thermodynamic potential barriers. Slower reaction rate and multiple gas desorption steps hinder the implementation of urea-based fuel cells, and thus understanding the urea oxidation process is key for the commercialization of this technology. In this report, we systematically examined the electrocatalytic urea oxidation activity of nickel-based model catalysts such as Ni12P5 and Ni(OH)2 in an alkaline medium at a fixed urea concentration of 0.33 M using electrochemical impedance spectroscopy (EIS). Ni12P5 and Ni(OH)2 require potentials of 0.51 and 0.54 V vs Hg/HgO, respectively, to achieve a current density of 10 mA cm–2. The origin of the catalytic activity difference of the urea oxidation reaction (UOR) between these two Ni catalysts is briefly analyzed using nondestructive in situ EIS. We measured the electrochemical impedance spectra of both the catalysts at their faradic potentials, and the obtained Nyquist plots suggest that the depressed arcs possess more than one time constant. We used the impedance spectroscopy genetic programing (ISGP) method to obtain the distribution function of relaxation times (DFRTs). DFRTs show the existence of three relaxation processes that occur during the anodic UOR, which are the effect of pores or active material (P1), charge transfer resistance related to oxidation of Ni (P2), and charge transfer resistance associated with oxidation of urea (P3). The location of P2 and P3 at higher frequencies for Ni12P5 compared to its counterpart Ni(OH)2 confirms that Ni12P5 can be a better catalyst.