(Invited) New Levers in Fuel Cell Ionomers: Thickness Vs. Chemistry
Authors: Kusoglu Ahmet 
Published: December 2020

Polymer electrolyte fuel cells (PEFCs) offer the prospects to revolutionize the transportation by powering not only automotive but also and heavy-duty vehicles. In PEFCs, ionomers are used to function as a proton-exchange membrane (PEM) facilitating ion transport between the catalyst layers (CLs). In addition, ionomer also function as an electrolyte binder mixed with the catalyst supports and particles to facilitate mass transport of reactants to catalytic sites. When confined to nanometer thicknesses in a heterogeneous CL structure, ionomer properties exhibit a range of properties form bulk (membrane) to thin films, and influenced by ionic and interfacial interactions.
Diverse functionalities, operating environment and performance metrics in fuel cells require optimized ionomer multi-functionalities for both PEM and CLs. While reducing the thickness of PEM is commonly considered an effective strategy for improving device performance by minimizing transport losses, this was considered purely from the perspective of geometric effects and cost constraints. However, how thickness affects intrinsic properties of an ionomer from membrane to electrode structures remains to be an open question. On one hand, efforts towards improving performance and durability of PEFCs require chemically and mechanically supported PEMs, which introduce additional interactions with the secondary support materials. An improvement in transport functionality in these ionomer, however, compromises stability and separator functionality, thereby setting a constraint to their optimization. On the other hand, improving CL ionomers require an understanding of their structure in the presence of confinement and electro-catalyst interactions. Between these duality, a new paradigm for fuel-cell ionomers, where one needs to understand and tune ionomer interactions with secondary particles across a wide spectrum of thicknesses, from bulk to nm.
This talk will present an overview of ionomers from this perspective with a focus on structure-property relationship of perfluorosulfonic acid (PFSA) ionomer, in both CL and PEM regime, which enables identification of the structural transitions relevant to fuel-cell functionality. Building upon a systematic investigation of membranes in micrometer thickness range (100 to a few µm), combined with nanometer-thick supported films in sub-micron range, a comprehensive material property map is generated for PFSAs, from mechanical properties, to water and ion transport. Our data links thickness-dependent changes in bulk membrane functionality to confinement-induced changes in catalyst-ionomer structure. Given the distinct functionality expected of ionomers in these regimes, our results are used to first expand the design space of ionomer using previously overlooked thickness as a design lever, and then to identify key transition regimes to elucidate the governing phenomena and demonstrate the combinatory roles of thickness and interactions in ionomer performance. The results will be evaluated to provide a new paradigm for ionomers in hybrid structures, from composites membrane to electrodes, and to expand the ionomer parameter space enabling optimization strategies for performance and durability in fuel cells.

Acknowledgment

AK acknowledges the ECS-Toyota Fellowship and the funding from the Fuel-Cell Performance and Durability (FC-PAD) consortium through the Fuel Cell Technologies Office, Energy Efficiency, and Renewable Energy Office of the U.S. Department of Energy (DOE), (contract no. DE-AC02-05CH11231)