Oral: Water Volume Fraction Governs the Thermodynamics of Water Adsorption of Anion Exchange Membranes

Water absorption in polymeric anion exchange membranes influences their performance as environment-friendly solid electrolytes in hydrogen electrochemical devices for clean energy conversion, as it affects important properties such as ionic conductivity and mechanical strength. However, computational approaches that address the relationship between the polymer design and the absorption process are scarce. In this work, we developed a simple mean-field model to predict water absorption isotherms of polyelectrolyte membranes in contact with a water vapor reservoir that smoothly considers the specific chemical design of the polymers. The model accurately predicts the water content and the macrostructural properties of polynorbornene membranes versus the water activity and successfully captures the effect of various polymer design parameters. The predicted energy of pairwise attractive interactions provided a way to interpret the absorption process at the molecular level. Finally, we dissected the thermodynamic process in favorable and unfavorable free energy contributions and found that their balance is governed by the water volume fraction, independent of the polymer design. This universal behavior leads to important implications in the search for better ion exchange membranes.