Atomistic characterization of hydration-dependent fuel cell ionomer nanostructure: validation by vibrational spectroscopy

DJ Donnelly and MY Yang and N Dimakis and SS Jang and WA III Goddard and ES Smotkin, JOURNAL OF MATERIALS CHEMISTRY A, 13 (2025).

DOI: 10.1039/d5ta03973h

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 (lambda), 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 lambda-dependent equilibrium proportions of protonated and deprotonated sites. We find that protonated sites persist across all lambda from 0 to 20, challenging the widely accepted notion that all sites are ionized above a threshold lambda value (e.g., 3 or 4). Our simulations generate hundreds of realistic exchange site environments, the characterization of which are based on 6 & Aring; 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 (Lambda). Our simulations produce broad lambda-dependent distributions of Lambda 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 lambda. These distributions underpin the IR spectra of hydrated membranes, with each exchange site contributing an IR spectrum characteristic of its protonation state and Lambda. We expect that these nanostructural characterizations of Nafion exchange sites will contribute to the development of new ionomers.

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