A team of researchers led by Prof. XU Tongwen and GE Xiaolin from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) achieved a breakthrough in the development of anion exchange membranes (AEMs). The team designed a novel spiro-branched polymeric membrane that incorporates highly connected sub-nanometer microporous ion channels, showing exceptional performance in flow battery applications. The study was published in Nature Sustainability. 

AEMs have broad applications in the fields including chemical separations, electrochemical ammonia synthesis, water electrolysis for hydrogen production, and various energy storage systems. The ability to efficiently and reliably conduct ions is vital for improving their performance and sustainability. Traditional methods, primarily through microphase separation, have struggled with balancing ion conductivity, selectivity, and stability. This struggle often results in trade-offs that limit the overall performance of the membranes. 

In this study, the research team developed a novel spiro-branched polymeric membrane using stereotwisted spiro scaffolds and poly (aryl piperidinium) based on an all-carbon backbone. The synthesis process involved creating a spiro-branched structure that combines the rigidity of spiro units with the flexibility of branched chains. This novel configuration was aimed at enhancing the free volume within the polymer, which is crucial for forming efficient ion transport pathways.

Furthermore, the team conducted comprehensive structural characterization, including morphology analysis using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as well as porosity measurements. These analyses revealed that the membrane forms a semi-flexible 3D loosely packed network, which significantly increases the free volume and creates highly connected sub-nanometer ion channels.

Performance evaluation showed that the spiro-branched polymeric membranes demonstrates exceptional anion conductivity, with chloride ion conductivities exceeding 60 mS cm^-1 at 30°C and reaching up to 120 mS cm^-1 at 80°C. In flow battery applications, these membranes showed superior power density and energy efficiency, enabling rapid charge and discharge cycles at a high current density of 400 mA cm^-2. In vanadium redox flow batteries, they also exhibited excellent chemical stability, indicating their potential for long-term use in energy storage systems. 

This study offers a new strategy for membrane material design, addressing various energy and environmental challenges. It not only advances the field of polymer science but also paves the way for more efficient and sustainable energy storage technologies.