Effect of Solvent on Ionomer Adsorption Behavior of Polymer Electrolyte Membrane Fuel Cell Catalyst

Abstract

Polymer electrolyte membrane fuel cell (PEMFC) is a promising power source with zero local carbon emissions that may aid in realizing the development of low-carbon societies. Catalyst layers (CL) composed of catalyst composites and an ionomer are key components in PEMFC. To control the structure of the CL, especially for the localization of the ionomer, understand the adsorption behavior on carbon surface is important. The quantitative analysis of ionomer adsorption isotherm is a powerful tool for analyzing the adsorption behavior, however, existing experiments have been carried out in water, and the effects of alcohol are yet to be considered. Because the mixtures of water/alcohol are used in the actual fabrication of PEMFC, the adsorption isotherm using the binary mixtures of water/alcohol as the solvents could provide practical information to optimize the PEMFC performance.Principal project of this study is to investigate and evaluate the effects of the alcohol contents on the interaction between the ionomer (Nafion) and the surface of the carbon support. The control of the interaction between ionomers and catalyst composites is as well as crucial in controlling the PEMFC performance. As the catalyst composites, commercial Vulcan/Pt (CB/Pt) and Pt-loaded polybenzimidazole (PBI) polymer-wrapped Vulcan (CB/PBI/Pt) catalysts were compared. For the detailed Nafion adsorption experiment, using synthesized CB/PBI/Pt and commercial CB/Pt disperse in aqueous ethanol solutions (87 or 20 wt%) with different amounts of Nafion (i.e., carbon-to-ionomer weight ratio, I/C=0.1~1.4). Then the dispersions were filtered with a syringe filter, and the filtrates were measured with 19F-NMR to calculate the amount of adsorption. For 19F-NMR, filtrates were added to trifluoroacetic acid and heavy water used as internal standard and deuterium solvent, respectively. The amount of Nafion was calculated based on the integral ratio of the largest peaks of Nafion ranging from −115 to −130 ppm in reference to the trifluoroacetic acid peak at −75.5 ppm. To prepare the adsorption isotherm, the adsorption amount (Γ [mg·mg−1]) of Nafion was plotted as a function of the equilibrium concentration (C e). The maximum adsorption amount (Γ max [mg·mg−1])) and Langmuir constant (K L) were determined based on the Langmuir equation. This paper will also discuss the difference in CL structure and cell performance at I/C=0.72 which formulation in low (water-) and high ratio alcohol (alcohol-rich) in ink solvent.PBI-coated CB (CB/PBI) was prepared according to our previous reports1, and the adsorption isotherms were measured in water- and alcohol-rich solvents (Figure.1), in which the BET values of 88 and 119 m2·g-1 for CB/PBI and CB/PBI/Pt, respectively, were used for the calculation of Γ’ (mg·m−2) in place of Γ (mg·mg−1). For the first layer adsorption, K L was calculated as 82 and 55 in the water- and alcohol-rich solvents, respectively. Comparing these values with those of CB, clear increase of the K L in the alcohol-rich solvent from 32 (CB) to 55 (CB/PBI) was obtained, whereas the K L values in the water-rich solvent were almost comparable between CB (89) and CB/PBI (82). In addition, the Γ max of the first layer in the alcohol-rich solvents increased from 0.024 mg mg‒1 (CB) to 0.034 mg mg‒1 (CB/PBI), while that in water-rich solvents were comparable between CB (0.043 mg mg‒1) and CB/PBI (0.043 mg mg‒1). Both results clearly indicate the stronger interaction between PBI and Nafion compared to the interaction between CB and Nafion in alcohol-rich solvents, while strong interaction in water-rich solvents was still maintained. A similar trend was also observed for Pt-loaded CB/PBI (CB/PBI/Pt), namely, the K L (43) and Γ’max (0.218 mg·m−2) were larger than those of CB/Pt (K L = 25 and Γ’max = 0.179) in the alcohol-rich solvent, while in water-rich solvent, K L (44) and Γ’max (0.277 mg·m−2) of CB/PBI/Pt were very high similar to CB/Pt (K L = 29 and Γ’max = 0.188 mg·m−2). On the other hand, when we compare Γ’max values of CB/PBI and CB/PBI/Pt, the decrease of the values after Pt loading in the water-rich and alcohol-rich solvents were observed, suggesting the Nafion adsorption onto Pt surface is inferior for PBI systems regardless of alcohol content.Furthermore, the performances of different catalysts were evaluated and discussed to understand the effect of alcohol content on Nafion adsorption and CL structure. Figure. 2 represents a performance curve that maximum power density of MEA having CB/Pt was 580 and 750W cm-2, while that of CB/PBI/Pt was 720 and 810 W cm-2, in alcohol and water-rich solvents, respectively. The high current density range (1.2~2A cm-2) also showed higher potential in water-rich conditions that related to O2 diffusion in CL.Reference 1. S. M. Jayawickrama,T. Fujigaya.et al. Electrochim. Acta. 312, 20 (2019). Figure 1