Nanophase Evolution, Local Water Content Distributions and Protonation Levels in Nafion—A Vibrational Spectroscopic and Molecular Dynamics Approach

Nafion has been a dominant ionomer membrane for low-temperature (~80 °C) fuel cells and electrolyzers for nearly 50 years, because of its superior chemical-mechanical stability and high protonic conductivity. Nafion morphology comprises hydrophobic (semicrystalline), interphasial (interphasial), and water-rich domains. The hydration-level-dependent size, shape, and interconnectivity of domains remains elusive, and has limited ionomer development for high-temperature operations.

For any hydrated Nafion membrane, characterized by a bulk H₂O/SO₃(H) ratio (λ), our classical molecular dynamics (CMD) simulations show a wide distribution of local λ values (λ_loc). We used density functional theory (DFT) based vibrational normal mode analysis to generate unique λ_loc = 0-15 spectra, each of which contributes to overall membrane FTIR transmission spectra. For λ_loc < 3, the SO₃H proton remains covalently bound (i.e., C₁ local symmetry), and yields normal modes that correspond to IR bands ~1414 and ~910 cm^-1 (C₁ bands). For λ_loc ≥ 3, the proton dissociates to form SO₃⁻ (C_3V local symmetry), which yields normal modes corresponding to C_3V bands ~1060 and ~970 cm^-1. We now correlate the coexistence of C₁ and C_3V bands during membrane hydration/dehydration to SO₃⁻/SO₃H ratios (i.e., protonation levels) generated by reactive force field (ReaxFF) MD simulations.

CMD simulations, governed by classical equations of motion, fail to model the dynamic exchange of protons between SO₃(H) groups (i.e., exchange sites) and water/hydronium. We used ReaxFF in Nafion MD simulations to model this exchange, and to derive λ-dependent protonation levels. This is the first such use of ReaxFF, to the best of our knowledge. Our MD systems comprised 320 exchange sites, and were hydrated as follows: λ = 0, 1, 2, 3, 5, 7, 10, 15, 20. We relate the ReaxFF generated protonation levels to C₁ and C_3V transmission-IR band intensity changes, resulting from sub-λ aliquots of water vapor injected into an initially dehydrated Nafion membrane. Illustrations of ReaxFF MD structures reveal nano-phase segregation and heterogeneity in water environments. Inner-sphere waters contribute to λ_loc values and outer-sphere waters do not. Inner sphere waters either bridge multiple exchange sites (bridged) or hydrate single SO₃(H) groups (non-bridged). Outer-sphere waters are either isolated or bulk-like. The applicability of our coordinated experimental/theoretical approach to hydrocarbon-based ionomers will be described.