Insights into the Ni/C-Based Thin-Film Catalyst Layer Design for Urea Oxidation Reaction in a Three-Electrode System

With the increasing interest in urea oxidation reaction (UOR) in direct urea fuel cells and urea-assisted hydrogen generation, it is highly desirable to develop a facile and reliable method for evaluating catalyst performance. A three-electrode system is commonly used to measure electrocatalytic performance for UOR catalysts before further investigation in a full cell configuration. However, the influence of the ink composition and catalyst loading and the role of the ionomer in the catalyst layer for UOR in a three-electrode system have not been studied before. In this study, we have studied the influence of Nafion ionomer in Ni/C catalyst ink, the catalyst loading on the glassy carbon electrode surface, and KOH and urea concentrations on UOR activity systematically. A standard preparation method and a benchmark protocol were proposed to obtain reliable UOR catalyst performance using cyclic voltammetry, linear sweep voltammetry, and electrochemical impedance tests. In addition, the catalyst ink quality was assessed by particle size distribution and zeta potential using the dynamic light scattering technique. The mass activity and transport resistance were found to be the optimum condition at a catalyst loading of 0.1 mg cm–2 with a ionomer-to-catalyst (I/C) ratio of 1. Further, the urea concentration of 0.33 M in 3 M KOH also exhibits the best UOR activity. Lastly, the Butler–Volmer kinetic analysis was used for the first time to investigate the effect of the I/C ratio and catalyst loading on UOR with rotating ring-disk voltammetry results. The presence of Nafion ionomer and an optimum catalyst loading improve the water uptake in the catalyst layer, which leads to an enhanced OH– and urea transport within the catalyst layer. The findings of this study provide insights into designing and evaluating UOR catalysts for advancing the urea electrolysis technology.