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

Perfluorinated sulfonic acid (PFSA) ionomers like Nafion have dominated as membranes for low-temperature fuel cells and electrolyzers for nearly 50 years, because of their high chemical-mechanical stability and high protonic conductivity. PFSAs are expensive and are poor conductors beyond 90 °C, however, in addition to being environmental hazards at the end of their lifetimes. PFSAs are often coated with expensive catalysts (e.g., Pt, Ru, Ir, etc.), whose recovery demands inherently toxic incineration processes. Hydrocarbon based ionomers, like sulfonated poly(ether ether ketone) (SPEEK), are synthesized via relatively green synthetic pathways, and lend themselves to catalyst recovery with less environmental impact upon incineration. The advancement of SPEEK and other sulfonated polyaromatic ionomers requires a thorough understanding of their bulk H₂O/SO₃(H) ratio (λ) dependent protonation levels (i.e., SO₃⁻/SO₃H ratios).<br/>We report the use of reactive force field (ReaxFF) molecular dynamics (MD) simulations on SPEEK at the following hydration levels: λ = 0, 1, 2, 3, 5, 7, 10, 15, 20. Each SPEEK system comprises 58 chains with 10 equally spaced sidechains terminated by SO₃(H) groups (i.e., exchange sites). SPEEK protonation levels, inner- and outer-sphere water proportions, and nanophase evolution are contrasted to those of Nafion’s across every considered λ value. Moreover, the ReaxFF generated protonation levels are correlated to changes in SPEEK’s transmission IR band intensities. We focus on the IR bands related to SO₃H and SO₃⁻ vibrations, and conduct density functional theory based vibrational assignments of these bands across different λ values, local to each exchange site (λ_loc). We show that SPEEK’s overall membrane spectra results from a distribution of λ_loc spectra.