(326a) Ionomer/Particle Interactions in Inks Affect Fuel-Cell Performance

Fuel-cell catalyst layers (CLs) are heterogeneous structures comprised of agglomerates of platinum-supported-on-carbon catalyst particles bound together with ion-conducting polymer (ionomer). They are fabricated via solution-processing slurry techniques in which the active materials are mixed with a solvent in an ink, and then deposited and dried to form the CL. Interactions between the components within the ink control the eventual CL microstructure and ionomer/particle interface.¹ Unfortunately, within the CL, the ionomer often has uneven coverage on the catalyst particles, resulting in underutilized catalyst. Understanding the ionomer/catalyst particle interaction will provide insight into improving catalyst utilization, mass-transport resistances, and overall performance. Complicating our understanding are the multitude of different catalyst particles that may be used. Here, we focus on the most commonly employed: platinum-on-carbon with varying primary particle loadings (PPL, weight fraction of platinum of the carbon-platinum catalyst particles).

We first investigate the intrinsic binding behavior of ionomer to platinum and carbon surfaces in an ink using isothermal titration calorimetry and quartz-crystal microbalance. Using model inks, we reveal that carbon/ionomer interactions are stronger than platinum/ionomer interactions, adsorption occurs spontaneously, and is entropically driven.² We then extend this fundamental work to probe real-ink systems. By examining CL gas-transport resistance and zeta (ζ) potentials of corresponding inks as a function of PPL, a direct correlation between CL high-current-density performance and ink ζ-potential is observed.³ This correlation stems from changes in ionomer distributions and catalyst-particle agglomeration as a function of PPL as revealed by pH, ζ-potential, and impedance measurements. These findings are critical to unraveling ionomer distribution heterogeneity in ink-based CLs and enabling enhanced Pt utilization and improved device performance for fuel cells and related electrochemical devices.

References

(1) Berlinger, S. A.; Garg, S.; Weber, A. Z. Multicomponent, Multiphase Interactions in Fuel-Cell Inks. Curr. Opin. Electrochem. 2021, 29, 100744. https://doi.org/10.1016/j.coelec.2021.100744.

(2) Berlinger, S. A.; McCloskey, B. D.; Weber, A. Z. Probing Ionomer Interactions with Electrocatalyst Particles in Solution. ACS Energy Lett. 2021, 6 (6), 2275–2282. https://doi.org/10.1021/acsenergylett.1c00866.

(3) Berlinger, S. A.; Chowdhury, A.; Van Cleve, T.; He, A.; Dagan, N.; Neyerlin, K. C.; McCloskey, B. D.; Radke, C. J.; Weber, A. Z. Impact of Platinum Primary Particle Loading on Fuel Cell Performance: Insights from Catalyst/Ionomer Ink Interactions. ACS Appl. Mater. Interfaces 2022, 14 (32), 36731–36740. https://doi.org/10.1021/acsami.2c10499.