Tailoring the Proton Conductivity and Microstructure of Block Copolymers by Countercation-Selective Membrane Fabrication

Here we report a simple but effective method to control the membrane morphology and transport properties of aromatic multiblock copolymers bearing perfluorosulfonic functions, via casting with different countercations. Five monovalent cations with different sizes, polarities, and hydrophobicities, i.e., H⁺, Li⁺, K⁺, Cs⁺, and TEA⁺, and one double-valence cation, i.e., Ca²⁺, were selected for preparing block copolymer membranes. We show that the countercation has a strong impact on the superstructure long-range order by acting as either a block separator or a block compatibilizer, therefore tuning the thermodynamics of the self-assembly process. Hence, by selecting the cations, highly ordered or completely disordered phase-separated block morphologies can be created. The effect of the countercation nature on the morphology is strongly reflected in the proton conductivity of acidified membranes. At 25 °C and 10% relative humidity, the acidified TEA⁺-cast membranes are ∼22 times less conductive than the acidified Cs⁺-cast ones. By combining microscopy and neutron scattering techniques, we reveal the direct correlation between enhanced functional properties and quality of membrane microstructure directed by the nature of cations with beneficial characteristics. Our findings highlight the role and importance of cation selection to tailor the functional properties of multiblock ionomers applicable as solid electrolytes for energy conversion devices.
https://doi.org/10.1021/acs.jpcc.0c04682