(Invited) Highly Stable, Crosslinked Polymer Membranes for Non-Aqueous Flow Batteries

Abstract

Long-term energy storage is a crucial stepping stone for the introduction of renewable sources in the energy mix due to their intermittent nature. Non-aqueous Flow Batteries (NFBs) are expected to operate at higher potential differences in comparison to conventional aqueous systems, thus yielding an improved power density. However, among the several issues of NFBs, it is pivotal to develop ion-conducting separators stable in organic solvents.Here we present a new family of ionomers which proved promising as candidates to implement this crucial function. The ionomers are founded on the strong ionic interactions between the functionalities of a polyamide-based material (P) and a perfluorosulfonic acid macromolecule (N). The latter interactions establish crosslinks between the two starting macromolecules. Consequently, the polymeric structure of the final ionomer is reorganized, improving the mechanical properties and curtailing the swelling upon immersion in organic solvents.The membranes obtained from two of the proposed ionomers, labelled “P-N0.77” and “P-N0.89”, show a swelling < 7% after 1000 hours of immersion both in acetonitrile (ACN) and in a 1:1 vol./vol. mixture of ethylene carbonate and propylene carbonate (EC:PC). Upon exchanging the membranes with tetraethylammonium (TEA+) cations, a common benchmark mobile specie for NFBs, it is shown that: (i) P-N0.77 exhibits a conductivity of 0.4∙10-3 S∙cm-1 and 0.9∙10-3 S∙cm-1 at 25°C in 0.1 M TEA+ in ACN and 0.1 M TEA+ in EC:PC, respectively; and (ii) P-N0.89 shows a conductivity of 1.5∙10-3 S∙cm-1 and 0.5∙10-3 S∙cm-1 in the same conditions.These results suggest that the proposed materials are suitable for application in NFBs, opening the door to the long-term operation of these devices.