Researchers have developed a new class of ion exchange membranes, designed to enhance the efficiency and durability of redox flow batteries (RFBs).

This research marks an exciting advancement for sustainable energy storage with the findings, recently published in Joule, representing a significant step forward in the development of reliable, scalable, and cost-effective energy storage technologies.

The work is led by Dr Qilei Song from Imperial's Department of Chemical Engineering, involving several research groups across the UK, industrial partners (BP), and international collaboration with Dalian Institute of Chemical Physics (DICP) in China. 

Redox flow batteries are a promising technology for large-scale, long-duration energy storage, essential for balancing supply and demand in renewable energy systems like solar and wind. All-vanadium flow batteries have been demonstrated at 100 MW/400 MWh scale by researchers at DICP. However, the vanadium electrolytes in these flow batteries are expensive and toxic.

Next-generation systems 

Recent advancements in redox flow battery technology have focused on developing low-cost, high-performance systems such as aqueous organic redox flow batteries and alkaline zinc-iron flow batteries.

Aqueous organic redox flow batteries utilize organic molecules as active materials, offering the advantages of tunability, low toxicity, and abundant resources. Meanwhile, alkaline zinc-iron flow batteries leverage inexpensive and widely available materials, providing high energy density and long cycle life. These next-generation systems aim to address the economic and environmental limitations of traditional vanadium-based RFBs, paving the way for broader deployment in renewable energy integration. 

Membranes are critical in these systems, enabling rapid ion exchange while minimizing crossover of active electrolyte species. Traditional membranes such as perfluorosulfonic acid membranes, known as Nafion, are expensive and can constitute up to 40% of stack costs. The manufacturing of Nafion also involves per- and polyfluoroalkyl substances (PFAS), which are being increasingly regulated by EU and US environmental agencies. Hydrocarbon membranes such as sulfonated poly(ether ether ketone) (sPEEK) are low-cost and scalable alternatives with better environmental profiles than perfluorinated options.

In 2022, an international collaboration led by Prof. Xianfeng Li at DICP demonstrated the roll-to-roll manufacturing of sPEEK membranes and their integration in kW-scale flow batteries, with the findings published in Joule. However, a key challenge of sPEEK membranes is their performance, which is limited by a trade-off between ionic conductivity and selectivity. 

In this new study, the international collaboration team addressed these challenges by engineering sulfonated PEEK membranes with intrinsic microporosity, a novel material that combines superior ionic conductivity with good mechanical and chemical stability. A three-dimensional contorted monomer, known as triptycene, is incorporated into the backbone of these sulfonated PEEK membranes. These new membranes feature highly interconnected water channels that allow fast and selective transport of both cations and hydroxide ions, enabling high efficiency and reduced energy loss during battery operation. 

By tailoring the sulfonation process and incorporating microporous architecture, the membranes achieve remarkable performance metrics. Dr Toby Wong Lead author, Department of Chemical Engineering

Dr. Toby Wong, lead author of the paper, has been working on this membrane development through his PhD and postdoctoral research. “By tailoring the sulfonation process and incorporating microporous architecture, the membranes achieve remarkable performance metrics, particularly enhanced ion conductivity that overcomes the trade-off between ion conductivity and selectivity,” said Dr. Wong. 

The membranes also exhibited high chemical stability and resistance to degradation in the harsh alkaline environments of typical RFBs. Combining the contorted monomers with commercially viable polymer chemistries could also reduce manufacturing costs, making them promising candidates to replace commercial Nafion membranes. 

To understand the structures on a molecular level, the team collaborated with Professor Kim Jelfs in the Department of Chemistry at Imperial. PhD student Yijie Yang carried out molecular simulations to study the membrane structure at the atomic level, providing detailed insights into the mechanisms underlying ion transport. The molecular modelling suggested that the new sulfonated membranes form more interconnected water channels. These nanometer-sized channels facilitate the transport of both cations and hydroxide anions, which are critical for high-efficiency energy storage.

This innovation brings us one step closer to realising a sustainable energy future. Prof Qilei Song Research lead, Department of Chemical Engineering

The team tested the newly developed membranes in a wide range of redox flow battery systems, including aqueous organic redox flow batteries and alkaline zinc-iron flow batteries. The battery can be charged at high current densities of up to 500 mA/cm² with high energy efficiency, outperforming most membranes reported in the literature. This capability significantly enhances the practicality and scalability of redox flow battery systems for real-world applications. 

“Our work demonstrates the transformative potential of new ion exchange membranes in advancing the performance and scalability of redox flow batteries,” said Dr Qilei Song. “This innovation brings us one step closer to realising a sustainable energy future.” 

Future Work 

The team aims to further refine membrane designs by eliminating ether bonds, enhancing chemical stability under harsh alkaline conditions. The study also highlights the versatility of these membranes for other electrochemical applications, including fuel cells and water treatment. Future studies will focus on scaling production and exploring broader applications in sustainable processes.  

The team can manufacture these membranes with sizes up to A4 paper size.  The team are collaborating with DICP to scale up the manufacturing of the membranes using their cutting-edge roll-to-roll membrane casting facility, which is the bottleneck for upscaling and currently not available in the UK. Through this international collaboration, the Imperial team are planning to build similar facility for manufacturing of ion exchange membranes in the UK.

Collaborations  

Imperial College London invites collaborations with industry partners to accelerate the commercialization of this breakthrough membrane technology. Contact Prof Qilei Song. 

Paper information

Toby Wong, Yijie Yang, Rui Tan, Anqi Wang, Zhou Zhou, Zhizhang Yuan, Jiaxi Li, Dezhi Liu, Alberto Alvarez-Fernandez, Chunchun Ye, Mark Sankey, David Ainsworth, Stefan Guldin, Fabrizia Foglia, Neil B McKeown, Kim E Jelfs, Xianfeng Li, Qilei Song. Sulfonated poly(ether-ether-ketone) membranes with intrinsic microporosity enable efficient redox flow batteries for energy storage. Joule. 2025, in press. https://doi.org/10.1016/j.joule.2024.11.012 

Funding 

The project received funding by the European Research Council, EPSRC Programme Grant SynHiSel, and funding support from bp-ICAM and CDT in Advanced Characterization of Materials.