Redox flow batteries (RFBs) have great potential for long-duration grid-scale energy storage. Ion-conducting membranes are a crucial component in RFBs, allowing charge-carrying ions to transport while preventing the cross-mixing of redox couples. Commercial Nafion membranes are widely used in RFBs, but their unsatisfactory ionic and molecular selectivity, high costs, and environmentally damaging production limit the performance and the widespread deployment of this technology. Inexpensive ion-selective membranes are highly desired that concurrently deliver low ionic resistance and high selectivity towards redox-active species.
Here, I will discuss our recent efforts in the development of next-generation RFB membranes based on the molecular design and engineering of polymers of intrinsic microporosity (PIMs, Fig. 1). PIMs possess highly interconnected sub-nanometre pores resulting from the inefficient packing of their rigid and contorted macromolecular chains. Modular synthesis of PIMs allows a variety of cation-coordinating functionalities, including amidoxime, sulfonate, and carboxylate, to be introduced for ion conduction optimization, while pore architecture and size can be effectively tuned by varying polymer chain topology and rigidity. Membranes based on these ionic PIMs overcome the trade-off between ionic conductivity and selectivity found in all existing materials, and enable exceptional RFB performance in RFBs. Importantly, these polymers are synthesized by environmentally friendly reaction routes using low-cost starting materials, and can be solution processed to form high-quality membranes at large areas, demonstrating great potential for commercialization and providing step-changing opportunities for realizing electrochemical devices with lower costs and longer lifetimes for grid-scale energy storage.

<b><i>Figure 1. Ion-conducting polymers of intrinsic microporosity (PIMs): modular synthesis, selective ion transport and ultra-stable battery performance.</i></b><i> <b>A</b>. PIMs featuring highly rigid, contorted chain structures and diverse ion-exchange chemistries. <b>B</b>. Molecular-sized sub-nanometre cavities resulting from the inefficient packing of PIM macromolecular chains. <b>C</b>. RFB incorporating highly selective PIM membranes.</i>

Figure 1. Ion-conducting polymers of intrinsic microporosity (PIMs): modular synthesis, selective ion transport and ultra-stable battery performance. A. PIMs featuring highly rigid, contorted chain structures and diverse ion-exchange chemistries. B. Molecular-sized sub-nanometre cavities resulting from the inefficient packing of PIM macromolecular chains. C. RFB incorporating highly selective PIM membranes.

Anqi Wang

Appearances