Polymers of Intrinsic Microporosity for aqueous organic Redox flow batteries

Abstract

Redox flow batteries (RFBs) based on aqueous organic electrolytes are promising technologies for large-scale, safe and cost-effective electrical energy storage. The membrane separator is the key component in a RFB system to keep redox-active species separate in two half cells while allow free transport of charge-balancing ions. Delivering fast and selective ion transport is critical for battery performance yet remains a challenge for current RFB separators, owing to the difficulty in generating membranes with well-defined sub-nanometre channels to serve as efficient molecular sieves. In this thesis, a new family of ion-sieving membranes, based on Polymers of Intrinsic Microporosity (PIMs) with different backbones and integrating varied ion-conductive groups, are developed and function as efficient and stable separators in aqueous organic RFBs. PIMs are platform materials with distinct combination of properties including high microporosity, exceptional chain rigidity and good solution processibility.

A range of dibenzodioxin-based PIMs with varied spirocyclic and bridged bicyclic structural units are synthesised and provide a versatile post-functionalisation to introduce ionisable amidoxime groups. These membranes demonstrate the feasibility of PIM-based membranes serving as ion-conductive and low-resistant separators in alkaline aqueous RFB systems. Further, negative-charged sulfonate groups are introduced into spirobifluorene-based PIMs which exhibit exceptional efficiency and stability for battery operation at near neutral aqueous electrolytes, outperforming identical RFB cells that use commercial benchmark Nafion® membranes. PIMs containing Tröger’s base building blocks enable the introduction of quaternary ammonium groups together with simultaneous crosslinking network, or zwitterionic groups on amine sites. The combination of polymer rigidity, high microporosity and ion-conductive functionalities of PIMs facilitates the formation of well-confined and sub-nanometre pathway, allowing fast transport of charge-balancing ions whilst unprecedented blocking redox-active species through membranes, fulfilling the function as efficient RFB separators hence achieving impressive performance in RFBs. These proof-of-concept demos using functionalised PIM membranes as efficient molecular sieves can guide the development of new generation membrane separators for a range of clean energy technology applications.