(Invited) Interactions in Ionomers: From Cations to Confinement

Abstract

Ion-conductive polymers (ionomers) have been widely used as the solid-electrolyte in energy conversion devices, such as polymer-electrolyte fuel cells (PEFCs) fuel cells, redox flow batteries and water-splitting devices. Ionomers, such as perfluorosulfonic-acid (PFSA), function as the separator-electrolyte in these devices due their high proton conductivity and chemical-mechanical stability.1 In PEFCs, however, ionomers play another key role in Catalyst Layer (CL) structure, where they exist as nanometer-thick electrolyte “thin film" to facilitate transport of ionic and gaseous species to the catalytic particles. As a cathode catalyst film, an ionomer is expected to transport the oxygen molecules to the catalytic sites, which differs dramatically from its gas-separator functionality.2 Once confined to these nanometer thicknesses (< 100 nm), ionomer properties change drastically from the bulk (solid-electrolyte PEM), and are significantly more influenced by its interfaces with the free surface (air) and the catalytic surfaces comprised of carbon and platinum(Pt)-alloyed particles.1,2 In particular, mass-transport limitations in CLs are believed to be related to the transport resistances at the ionomer thin-film, mitigation of which requires a fundamental understanding of ionomer interfaces.3-5 In addition, ionomers, whether as PEMs or in CLs, are exposed to various cations arising from contamination, migration of radical scavengers (such as Cerium), dissolution of catalyst particles (such as Co-alloys), all of which alter the ionomer’s interactions and impact the cell performance.3,6 Thus, throughout this thickness range, it is not only the functionality of ionomer that differs, but also its morphology and transport properties, which are controlled by various interactions. This talk will present an overview of various interactions, from cationic and catalytic particles to dynamic interfaces, and how they influence an ionomer’s intrinsic morphology and transport properties across lengthscales, with a focus on thin films. The latter is in particularly important as the interactions and confinement-driven changes in thin films create an intriguing interplay governing ionomer functionality. We will explore structure-property relationship PFSA ionomers of several chemistries and thicknesses (from micrometer to 10’s of nanometers) in the presence of various types and doping levels of cations to elucidate the underlying origins of governing interactions. Water uptake and swelling behavior as well as nano-morphology of cation-exchanged PFSA membranes and thin films are studied to identify the key factors and interactions impacting their behavior. We will also discuss the role of side-chain chemistry in these interactions, especially in nano-confined thin films. Lastly, I will summarize recent progress in ionomer thin-film research, including improved model systems for thin films that can mimic fuel-cell catalyst structure and environment, and advanced characterization techniques, such as Grazing-Incidence X-ray scattering (GIXS), and how they can be used to probe various interactions. Acknowledgements We acknowledge Adam Z. Weber and Douglas Kushner of Berkeley Lab for helpful discussions, and Michael Yandrasits and Andrew Haug of 3M for helpful discussions and supplying ionomer materials. This work made use of facilities at the Advanced Light Source (ALS), supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy, and was funded under the Fuel Cell Performance and Durability Consortium (FC-PAD), by the Fuel Cell Technologies Office, of the office of the Energy Efficiency and Renewable Energy (EERE), of the U. S. Department of Energy (under contract number DE-AC02-05CH11231). References A. Kusoglu and A. Z. Weber, Chem. Rev., 117, 987 (2017).A. Kusoglu, in Encyclopedia of Sustainability Science and Technology, R. A. Meyers Editor, p. 1, Springer New York, New York, NY (2018).A. Kongkanand and M. F. Mathias, J Phys Chem Lett, 7, 1127 (2016).A. Z. Weber and A. Kusoglu, J Mater Chem A, 2, 17207 (2014).M. Tesfaye, A. N. MacDonald, P. J. Dudenas, A. Kusoglu and A. Z. Weber, Electrochem Commun, 87, 86 (2018).A. M. Baker, R. Mukundan, D. Spernjak, E. J. Judge, S. G. Advani, A. K. Prasad and R. L. Borup, J. Electrochem. Soc., 163, F1023 (2016).