Ion exchange membranes, often called proton exchange membranes, are key materials in flow battery stacks. Their quality directly affects the battery’s coulombic efficiency, voltage efficiency, and overall energy efficiency. This article looks at these membranes from three angles—their types, how they are made, and how they are used in flow batteries. It also briefly discusses current challenges and future trends.

Flow batteries are becoming an important choice for long-duration energy storage. They have several strong points: power and capacity can be designed separately, they are inherently safe, and they can last for a very large number of charge-discharge cycles. In 2025, the global market for ion exchange membranes used in all-vanadium flow batteries was about RMB 1.547 billion, and it is expected to reach nearly RMB 3.957 billion by 2032, growing at about 13.8% per year.

As a core part of the flow battery stack, the ion exchange membrane has two main jobs. First, it keeps the active materials in the positive and negative sides from mixing and causing cross-contamination. Second, it lets ions pass through to complete the internal circuit of the battery. These functions directly determine the battery’s coulombic efficiency, voltage efficiency, and energy efficiency. In flow battery research, the terms “ion exchange membrane” and “proton exchange membrane” are often used to mean the same thing, especially in all-vanadium flow batteries where proton conduction is the main function.

Types of Ion Exchange Membranes

Ion exchange membranes for flow batteries can be grouped in different ways. Based on the electrical charge of their functional groups, they fall into three types: cation exchange membranes, anion exchange membranes, and amphoteric ion exchange membranes.

Cation exchange membranes are the most widely used type in commercial products. They have negatively charged groups (like sulfonic acid -SO₃H and carboxyl -COOH) that help transport ions. They offer good chemical stability and ion transport ability. The well-known Nafion series used in all-vanadium flow batteries belongs to this group. However, these membranes do not block vanadium ions very well—too many vanadium ions pass through, causing cross-contamination and leading to capacity loss over time.

Anion exchange membranes use positively charged groups (such as quaternary ammonium) to conduct negative ions. The positive charge inside the membrane helps push away vanadium ions (which are positive), so they are better at preventing vanadium from crossing over. But they have lower conductivity and higher internal resistance, so most of them are still being studied in laboratories.

Amphoteric ion exchange membranes contain both types of functional groups. They aim to give a good balance between ion selectivity and conductivity. However, getting the right ratio of the two groups makes production complex and costly, so they face big hurdles in moving to commercial use.

Based on how much fluorine they contain, ion exchange membranes can be divided into perfluorosulfonic acid membranes, partially fluorinated membranes, and non-fluorinated membranes.

Perfluorosulfonic acid membranes are the current market leader. The C-F bond is much stronger than the C-H bond, which gives the material excellent chemical and electrochemical stability. The Nafion series from DuPont (now Chemours) is a typical example. These membranes have good chemical stability and high proton conductivity, but they do not resist vanadium well and are very expensive—costing nearly RMB 20,000 per square meter. The top three membrane makers in the world together control more than 60% of the market, giving them a strong lead.

Non-fluorinated ion exchange membranes use resins without fluorine, which cuts costs from the start. They also offer high ion selectivity and good mechanical stability. In recent years, Chinese researchers have been very active in this area. The Dalian Institute of Chemical Physics has developed porous ion-conducting membranes with better ion selectivity and capacity retention. The University of Science and Technology of China has designed new ion-conducting membranes that try to solve the trade-off between conductivity and selectivity. Still, non-fluorinated membranes are not as good as perfluorinated ones when it comes to long-term durability and batch-to-batch consistency, and they have not yet reached large-scale commercial use.

In recent years, composite membranes and new membrane materials have become important areas of progress. A team from Changsha University of Science and Technology used water-swellable materials like expansive soil and bentonite to create a “nano-gate” ion-sieving effect. This builds self-adjusting ion channels inside the membrane. Their co-swellable composite membrane showed ion selectivity 2 to 3 times better than Nafion, and extended the cycle life of all-vanadium flow batteries by 400%. This technology has passed pilot-scale tests, and they plan to build a production line with an annual capacity of 300,000 square meters. Meanwhile, new porous materials like metal-organic frameworks and covalent organic frameworks are also being explored, which could give entirely new design ideas for the next generation of membranes.

Manufacturing Processes of Ion Exchange Membranes

The preparation process of ion exchange membranes are made directly affects their performance, cost and level of industrialization. Currently, the main methods for making perfluorosulfonic acid membranes are melt extrusion and casting.

Melt extrusion is the older method. In this process, the perfluorosulfonic acid resin is melted and pushed through a die to form a film, which is then treated with hydrolysis to become the final product. This method is well-established, efficient, and environmentally friendly, but the films tend to be thicker. Also, the hydrolysis step is mostly controlled by American and Japanese companies, which creates a technical bottleneck for others.

Casting involves dissolving the resin in a suitable solvent to make a casting solution, removing air bubbles, and then spreading it on a steel belt or polyester film to dry into a membrane. This method produces thinner films with better performance. Domestic Chinese companies like Dongyue Future, Korun New Materials, and Guorun Energy Storage all use the steel-belt casting method. In addition, Guizhou Zhixi Technology has introduced a wet casting process for perfluorosulfonic acid membranes. Their product has mechanical strength over 80% of imported membranes at the same thickness.

Making non-fluorinated and composite membranes involves more varied methods. Non-fluorinated membranes are often made by solution casting or phase inversion. These methods control the membrane’s microscopic structure by adjusting things like how fast the solvent evaporates and the makeup of the coagulation bath. For the co-swellable composite membrane mentioned earlier, the team had to solve the problem of nanoparticles clumping together in the polymer. They used an “acid-alkali activation and ion intercalation” process to precisely control the space between layers and the surface charge. Then they used a “suspension-thermal setting” process to make sure the nanoparticles were evenly spread throughout the polymer. This makes the production chain quite long and requires very tight control.

Looking at the industry today, the process for perfluorosulfonic acid membranes is fairly mature. But making the core resin (perfluorosulfonic acid resin) on our own is still a bottleneck for China’s membrane industry. For non-fluorinated and composite membranes, the processes are still moving from the lab to pilot-scale production. Problems like film uniformity, batch stability, and yield still need further improvement.

Applications of Ion Exchange Membranes in Flow Batteries

Different flow battery systems have different needs for ion exchange membranes, so their applications vary quite a bit.

In all-vanadium flow batteries, the main challenge is balancing vanadium resistance with chemical stability. Traditional Nafion membranes let vanadium ions pass through at a rate higher than 10⁻⁷ cm²/s, which causes the battery to lose capacity over time. To fix this, researchers have tried several approaches: adding SiO₂ nanoparticles to fill the hydrophilic areas of Nafion, which physically blocks some vanadium ions and cuts permeability by several times; building a positive-charge layer on the surface to push away high-valence vanadium ions through electrostatic repulsion; and developing non-fluorinated membranes as a complete replacement. Currently, performance targets for membranes in all-vanadium flow batteries usually include energy efficiency above 75% (at 100 mA/cm²) and less than 15% capacity loss over 2000 cycles.

In zinc-based flow batteries, the membrane not only needs to filter ions but also must stand up to strong alkaline corrosion and the risk of zinc dendrites poking through. Porous membranes are the top choice here because they cost less (below RMB 500/m²). Polysulfone/polyacrylonitrile-based asymmetric membranes use size-based filtering to stop active ions from crossing over. Chitosan-modified layers help guide zinc to deposit evenly through hydroxyl complexation, which stops dendrites from growing and greatly increases cycle life.

In organic flow batteries, traditional ion exchange membranes allow too much permeability (over 10⁻⁸ cm²/s) for active molecules like quinoline derivatives. This is a major problem. New materials like polymers of intrinsic microporosity and covalent organic framework membranes have pores smaller than an angstrom, which lets them filter molecules very precisely. Quinoline permeability can be as low as 10⁻¹¹ cm²/s, which offers key support for making organic flow batteries practical.

On the industry side, replacing imported membranes with domestic ones is speeding up. Since 2024, several Chinese group standards and industry rules for ion exchange membranes in flow batteries have been released. These set clear requirements for things like thickness consistency, area resistance, and ion selectivity. Working together on custom development between membrane makers and stack manufacturers has become common. Membrane costs are coming down by 8% to 12% each year. That said, we still do not have enough long-term performance data from large energy storage stations, and non-fluorinated membranes still need more testing to prove they can last for tens of thousands of hours.

Ion exchange membranes for flow batteries are moving from a time when perfluorosulfonic acid membranes dominated alone to one where many different approaches are competing and making progress together. Perfluorinated membranes keep the high-end market because of their performance. Non-fluorinated and composite membranes are pushing hard to break through by focusing on cost. Over the next five years, as mass production of non-fluorinated membranes improves, composite membrane technologies mature, and standards keep getting better, this field is likely to see a major reshaping of its competitive landscape. At the same time, making membranes work better together with other stack parts like electrodes and bipolar plates, and developing special membranes for different types of flow batteries, will be important forces in driving large-scale commercial use of flow batteries.