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Dynamic Emergence of Nanostructure and Transport Properties in Perfluorinated Sulfonic Acid Ionomers

preprint
submitted on 14.05.2020, 16:26 and posted on 15.05.2020, 08:15 by Adlai Katzenberg, Debdyuti Mukherjee, Peter J. Dudenas, Yoshiyuki Okamoto, Ahmet Kusoglu, Miguel Modestino

Limitations in fuel cell electrode performance have motivated the development of ion-conducting binders (ionomers) with high gas permeability. Such ionomers have been achieved by copolymerization of perfluorinated sulfonic acid (PFSA) monomers with bulky and asymmetric monomers, leading to a glassy ionomer matrix with chemical and mechanical properties that differ substantially from common PFSA ionomers (e.g., Nafion™). In this study, we use perfluorodioxolane-based ionomers to provide fundamental insights into the role of the matrix chemical structure on the dynamics of structural and transport processes in ion-conducting polymers. Through in-situ water uptake measurements, we demonstrate that ionomer water sorption kinetics depend strongly on the properties and mass fraction of the matrix. As the PFSA mass fraction was increased from 0.26 to 0.57, the Fickian swelling rate constant decreased from 0.8 s-1 to 0.2 s-1, while the relaxation rate constant increased from 3.1×10-3 s-1 to 4.0×10-3. The true swelling rate, in nm s-1, was determined by the chemical nature of the matrix; all dioxolane-containing materials exhibited swelling rates ~1.5 - 2 nm s-1 compared to ~3 nm s-1 for Nafion. Likewise, Nafion underwent relaxation at twice the rate of the fastest-relaxing dioxolane ionomer. Reduced swelling and relaxation kinetics are due to limited matrix segmental mobility of the dioxolane-containing ionomers. We demonstrate that changes in conductivity are strongly tied to the polymer relaxation, revealing the decoupled roles of initial swelling and relaxation on hydration, nanostructure, and ion transport in perfluorinated ionomers.

Funding

DE-AC02-05CH11231

DE-SC0014664

DE-SC0012704

DE- SC0004993

History

Email Address of Submitting Author

modestino@nyu.edu

Institution

New York University

Country

United States

ORCID For Submitting Author

0000-0003-2100-7335

Declaration of Conflict of Interest

No conflict of interest

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