High-performance spiro-branched polymeric membranes for sustainability applications

Ion exchange membranes are semi-permeable thin films allowing for selective transport of either anions or cations and have wide applications in desalination, wastewater treatment and energy conversion and storage. Poly(aryl piperidinium) polymers are promising materials for a new generation of anion exchange membranes with high chemical stability, although their ionic conductivity remains to be further improved. Here we report a design of branched microporous poly(aryl piperidinium) membranes that combine ultra-high Cl⁻ conductivity (120 mS cm⁻¹ at 80 °C), excellent mechanical and chemical stability and solution processability. At the heart of our rational design is the use of stereo-contorted spirobifluorene monomers to control the topology and orientations of branched chains, achieving balanced rigidity and flexibility. The loose chain packing structure reduces the energy barrier for ion dissociation and diffusion within the polymer networks, which can be processed into large-area membranes aided by a colloidal method. When applied to redox flow batteries, our microporous membranes deliver record-breaking performance at a high current density of 400 mA cm⁻². Our work suggests a feasible strategy for the development of high-performance membranes that will find more applications critical to sustainability.