Understanding Polymer/Particle Interactions in Fuel-Cell Inks Using Model Systems

Abstract

The formation process of hydrogen fuel-cell catalyst layers (CLs) is poorly understood, and represents a major area of effort with regard to increasing overall cell efficiency and durability. CLs are made from precursor inks that are dispersions of catalyst nanoparticles, ion-conducting polymer, and solvent (typically water and propanol mixtures). The polymer is generally a perfluorinated sulfonic-acid polymer, the prototypical example of which is Nafion. The duality between the strongly acidic sidechains and hydrophobic backbone of the polymer cause it to adopt unique conformations in solution, which are a strong function of the amount of water present in the ink. 1 Indeed, tuning the ratio of water to propanol has recently been demonstrated to change the effective pH of these polymer solutions, possibly by changing the amount of exposed sidechains. 2 The complex interplay between particle/solvent/polymer interactions govern CL ink and layer properties. 3-4 However, much remains unknown about the fundamental forces controlling these interactions. In this study, we explore the polymer/particle interaction, and how it is affected by solvent quality. To do so, we use quartz crystal microbalance with dissipation (QCM-D) with model functionalized substrates that probe specific relevant forces (electrostatic, hydrophobic, etc.) in order to elucidate interactions present on the comparably more-complex carbon-platinum particle surface. Different ionomer dispersions in a variety of solvents are studied to understand ionomer adsorption and desorption behavior. QCM-D data is coupled with calorimetry measurements to extract thermodynamic binding information. Results reveal that hydrophobic interactions are a major driving force for ionomer adsorption. Acknowledgements This study was mainly funded under the Fuel Cell Performance and Durability Consortium (FC-PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U.S. Department of Energy under contract number DE-AC02- 05CH11231. S.B. also acknowledges support from the National Science Foundation Graduate Research Fellowship under grant number DGE 1752814. References Welch, C.; Labouriau, A.; Hjelm, R.; Orler, B.; Johnston, C.; Kim, Y. S., ACS Macro. Lett. 2012, 1 (12), 1403-1407. Berlinger, S. A.; McCloskey, B. D.; Weber, A. Z., J. Phys. Chem. B. 2018, 122 (31), 7790-7796. Dixit, M. B.; Harkey, B. A.; Shen, F.; Hatzell, K. B., J. Electrochem. Soc. 2018, 165 (5), F264-F271. Hatzell, K. B.; Dixit, M. B.; Berlinger, S. A.; Weber, A. Z., J. Mater. Chem. A 2017, 5 (39), 20527-20533.