Influence of Sidechain Length in Perfluoro-Sulfonic Acid Ionomer on Electrode Slurry Interactions and Finalized Microstructure in the Polymer Electrolyte Fuel Cells

Abstract

Polymer electrolyte fuel cells (PEFCs) have emerged as one of the most promising next-generation energy devices for both passenger-owned and heavy-duty vehicles due to their high energy efficiency, low emissions, and quiet operation. However, the performance of the fuel cell electrode is often limited by the strong specific adsorption of long sidechain (LSC) perfluoro-sulfonic acid (PFSA) ionomer onto the platinum (Pt) catalyst2, which results in a decrease in the electrocatalytic activity1 as well as increase oxygen mass transport resistenace at the MEA level. To address this issue, a short sidechain (SSC) ionomer has been developed, which has been found to exhibit excellent efficiency over LSC ionomer3 The reduced specific adsorption of SSC ionomer onto the Pt catalyst results in a higher available surface area for the electrochemical reaction, which improves the overall fuel cell performance. The sidechain length of ionomers plays a crucial role in determining the overall cell performance4 however, due to the differences between them, the interactions among slurry components are not yet fully understood.Unlike the previous literatures, we investigates the details of ionomer sidechain length and its impact on the interaction among slurry components and resulting electrode microstructure, through a comparative analysis of LSC and SSC ionomers.5 Our hypothesis propose that the physical interactions between components are considerably impacted by the sidechains with a specific emphasis on the mobility and ion-pair association in the slurry. To probe the influence of ionomer sidechains, we conducted a thorough investigation wherein incrementally varied its concentration from 0 to 1.5 mmolSO3-gC -1, at intervals of 0.3 mmolSO3-gC -1. The main purpose of this evaluation was to observe the evolution of particle aggregation according to the correlation between adsorbed ionomer and free ionomer in electrode slurry. Drawing from the U-shaped viscosity profile obtained through rheological measurements, we propose an optimized ionomer concentration of 0.6 mmolSO3-gC -1 for the SSC ionomer-based slurry and 0.9 mmolSO3-gC -1 for the LSC ionomer-based slurry. To characterize the microstructure of the as-prepared electrode, we employed scanning electron microscopy (SEM) and confocal microscopy, with a focus on particle agglomeration and the homogeneous ionomer distribution. Finally, the electrochemical analysis was performed on the fabricated membrane electrode assembly (MEA) in a 25 cm² single cell for a cathode Pt loading of 0.1 mgPtcm⁻². As a result, the optimized SSC ionomer-based electrode exhibited low local O₂ transport resistance and high Pt utilization, leading to over a 40 % enhancement in fuel cell performance at 0.6 V with a high proton accessibility value above 0.85 when compared to the optimized LSC ionomer-based electrode. This study highlights the crucial importance of understanding the effects of ionomer sidechain length on electrode slurry interactions and microstructure and establishes a significant correlation between them. Our findings contribute to the ongoing development of high-performance PEFCs, offering valuable insights into optimizing electrode composition and structure for enhanced fuel cell performance.(1) Ahn, C.-Y.; Park, J. E.; Kim, S.; Kim, O.-H.; Hwang, W.; Her, M.; Kang, S. Y.; Park, S.; Kwon, O. J.; Park, H. S. Differences in the electrochemical performance of Pt-based catalysts used for polymer electrolyte membrane fuel cells in liquid half-and full-cells. Chemical Reviews 2021, 121 (24), 15075-15140.(2)Kodama, K.; Motobayashi, K.; Shinohara, A.; Hasegawa, N.; Kudo, K.; Jinnouchi, R.; Osawa, M.; Morimoto, Y. Effect of the side-chain structure of perfluoro-sulfonic acid ionomers on the oxygen reduction reaction on the surface of Pt. ACS Catalysis 2018, 8 (1), 694-700. (2) Garsany, Y.; Atkinson, R. W.; Sassin, M. B.; Hjelm, R. M.; (3) Gould, B. D.; Swider-Lyons, K. E. Improving PEMFC Performance Using Short-Side-Chain Low-Equivalent-Weight PFSA Ionomer in the Cathode Catalyst Layer. Journal of The Electrochemical Society 2018, 165 (5), F381-F391.(4) Ramaswamy, N.; Kumaraguru, S.; Koestner, R.; Fuller, T.; Gu, W.; Kariuki, N.; Myers, D.; Dudenas, P. J.; Kusoglu, A. Editors’ Choice—Ionomer Side Chain Length and Equivalent Weight Impact on High Current Density Transport Resistances in PEMFC Cathodes. Journal of The Electrochemical Society 2021, 168 (2), 024518.(5) Mauritz, K. A.; Moore, R. B. State of Understanding of Nafion. Chemical Reviews 2004, 104 (10), 4535-4586. DOI: 10.1021/cr0207123.