Precision vibrational spectroscopy of sulfonated ionomers

Abstract

Application of polymer electrolytes in devices (i.e., fuel cells, batteries, actuators and dynamic sensors) requires a fundamental understanding of exchange site variations with environmental changes. The categorization of sulfonated ionomer IR group modes by exchange site local symmetry enables the ionomer structure and properties to be correlated to hydration state and ion exchange. During dehydration, the fully hydrated perfluorinated sulfonated ionomer, PFSI, membranes (i.e., Aquivion, Nafion and 3M) IR spectra show the reversible diminishing of the ~970 cm-1 and ~1060 cm-1 bands concurrent with emergence of the ~920 cm-1 1000 cm-1 and ~1400 cm-1 bands. The first set of group modes is assigned to a dissociated exchange group (sulfonate) with a three-fold local symmetry (C3V). The C3V group modes are replaced by the set of group modes assigned to an associated exchange group (sulfonic acid) with no local symmetry (C1). The density functional theory normal mode analysis confirms that the sulfonic acid/sulfonate site plays a dominant role in the C1 and C3V group modes, respectively. The hydration dependent sulfonated poly (ether ether ketone), SPEEK, IR spectra show similar group modes to that of PFSIs. The SPEEK C3V bands are at 1023 cm-1 and 1061 cm-1. With dehydration, the C3V bands diminish, while C1 bands (898 cm-1 and 1362 cm-1) emerge. PFSI-M (M is Li, Na, K, Rb and Cs) retain C3V local symmetry at all hydration states. The symmetry-based assignments suggest that at dehydration, an aggregate form of exchange site exists, where each M+ provides 1/3 of a charge per sulfonate oxygen with an overall C3v symmetric motif. Membrane ensembles would consider cation size, anionic site vicinity and hydrophobic polymeric matrix rigidity for large scale aggregation. These restrictions lead to reduced aggregation related to C3V, HF band broadening by smaller cations and 3M PFSI. Upon full hydration, C3V, HF band FWHMs reduce with the reduction of λ dependent exchange-site states. At total dehydration, cations, stripped of solvation spheres, have the closest approach to exchange-sites, and C3V, HF band frequencies decrease with cation polarizability. Highly polarizing bare cations (e.g., Li+ and Na+) are less able to electron withdrawal from sulfonates compared to that of highly polarizable cations (e.g., K+, Rb+ and Cs+). The down shift with electron withdrawal from the exchange site suggest a decrease in the sulfonate dipole polarization by larger highly polarizable cations. Upon hydration, the C3V, HF band shift is not monotonic with cation n quantum number (i.e., Li, Na and K down shift, Rb no notable shift and Cs upshifts). Hydration waters alter the cation closest-approach to the exchange site and thus the electric field polarizing the exchange-site. Two events alter the exchange site dipole polarization 1) electron density withdrawal by hydrogen bond exchange site hydration waters (down shift) and 2) electron transfer from departing cations (up shift). The latter explains the curvature of the hydrated C3V, HF ν vs. ion plots. Hard ions have compact frontier orbitals and small α (i.e., highly polarizing). Soft ions have diffusely distributed frontier orbitals and large α (i.e., highly polarizable). Lithium ions are 1) highly hydrated (large down shift) and 2) very hard (very little to no up shift). The combination results in a large overall down shift. Cesium ions are 1) poorly hydrated (very little down shift) and 2) soft (up shift). The combination results in a small overall up shift. Upon saturation, an overall C3V, HF band shift can be predicted by comparison of the cation polarizability to that of sulfonate hydration water (0.39 ± 0.01 Å3). In SPEEK, metal ions also bind to the exchange site with C3V symmetry at all hydration states. The low frequency C3V diminished with dehydration for cations with Hhyd 1450 kJ/mol. The C3V band persistence with divalent cations is reconciled by charge the balancing of (1) another crosslinking exchange site or (2) a hydroxide ion. The cross-linking degree is expected to be inversely related to the C1, HF band intensity. Lower hydration enthalpy cations (Ba2+ and Sr2+) have smaller C1, HF intensity and more crosslinking. Higher enthalpy ions (Cd2+ and Cu2+) bind more weakly to the exchange site because exchanges site cannot compete with hydration waters.