The Influence of Catalyst Ink on the Urea Oxidation Reaction Activity Evaluated in a Three-Electrode System

Abstract

Urea oxidation reaction (UOR) has been considered as a promising electrochemical reaction for hydrogen production since the theoretical reversible potential of urea electrolysis (0.084 V) is much lower than that of the water electrolysis (1.23 V)1. Urea is solid, non-toxic, non-flammable and stable compound, and hence easy to store and transport. In addition, urea is the largest constituent of urine, which is one of the most abundant wastes on earth. Among the extensively studied fuels in fuel cell systems, urea is the only non-flammable compound. In 2009, Botte et al. first developed the urea electrolysis technique to produce hydrogen2. Then in 2010, Irvine et al. reported a fuel cell system to generate electricity directly from urea3. With increasing interest in UOR in alkaline medium, a fast and reliable evaluation method for UOR catalysts is highly desirable. Currently, the validity of the test results is compromised due to uncertainties resulting from random selected ink composition, test protocols, operating parameters, etc. To understand and compare actual catalyst performance, it is necessary to minimize these uncertainties for further development of efficient UOR catalysts. Three-electrode system is the most commonly employed tools for evaluating UOR electrocatalysts due to its simplicity of the test setup and accuracy of the catalyst activity. The preparation and deposition technique of the catalyst ink as a thin-film catalyst layer (CL) on the glassy carbon electrode (GCE) have a significant effect on obtaining the intrinsic properties of applied materials. Although a few studies investigated the effect of catalyst ink on oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) in a thin film rotating disk electrode system (TF-RDE), there are no report yet on the study related to ink composition optimization for anodic UOR in a three-electrode system4,5. In this study, we have investigated the role of solvent ratios (isopropyl alcohol and DI H2O) and ionomer on UOR catalyst activity in a three-electrode system. A commercial Ni-based catalyst is selected as our baseline catalyst, since Ni is found to be the most stable, economical and active catalyst for alkaline UOR. It is observed that the stability of the ink and particle size change with different solvent ratios, which leads to the variation of the catalyst film homogeneity on the GCE and consequently their electrochemical behavior. The role of ionomer is also studied through ex-situ measurements and in-situ performance evaluation including cyclic voltammetry, linear sweep voltammetry, chronoamperometry, and electrochemical impedance tests. Our results suggest that catalyst evaluation using a three-electrode system requires optimization of the catalyst ink to further understand the interfacial reaction mechanism. Detail experimental results will be shared at the conference.