Physics & Astronomy Faculty Publications and Presentations
Document Type
Article
Publication Date
6-3-2025
Abstract
The development of Nafion alternatives for fuel cells and electrolyzers requires a fundamental understanding of hydration-dependent ion-exchange site acid/base chemistry. We present here reactive force field (ReaxFF) molecular dynamics (MD) simulations of Nafion at varying molar water/ion-exchange-site ratios (λ), which we correlate to our experimental and density functional theory-based vibrational spectra. ReaxFF describes the formation and breaking of covalent bonds, enabling simulations of proton exchange between sulfonic acid/sulfonate groups and water/hydronium. Our MD simulations determine the λ-dependent equilibrium proportions of protonated and deprotonated sites. We find that protonated sites persist across all λ from 0 to 20, challenging the widely accepted notion that all sites are ionized above a threshold λ value (e.g., 3 or 4). Our simulations generate hundreds of realistic exchange site environments, the characterization of which are based on 6 Å radii sulfur-centered ‘inner-spheres’. These inner- and outer-sphere regions elucidate an interplay of stereoelectronic factors that influence protonation states, including the number of inner-sphere waters (Λ). Our simulations produce broad λ-dependent distributions of Λ values, representing non-uniform exchange site hydration. Moreover, we demonstrate that these distributions are specific to both protonated and deprotonated sites, with significant overlap between the two distributions for all non-zero λ. These distributions underpin the IR spectra of hydrated membranes, with each exchange site contributing an IR spectrum characteristic of its protonation state and Λ. We expect that these nanostructural characterizations of Nafion exchange sites will contribute to the development of new ionomers.
Recommended Citation
Donnelly, Dan J., Moon Young Yang, Nicholas Dimakis, Seung Soon Jang, William A. Goddard, and Eugene S. Smotkin. "Atomistic characterization of hydration-dependent fuel cell ionomer nanostructure: validation by vibrational spectroscopy." Journal of Materials Chemistry A (2025). https://doi.org/10.1039/d5ta03973h
Creative Commons License
This work is licensed under a Creative Commons Attribution 3.0 License.
Publication Title
Journal of Materials Chemistry A
DOI
10.1039/d5ta03973h
Comments
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.