Liquid flow batteries mainly achieve the charging and discharging of batteries through reversible changes in the valence state of chemical active substances, thereby achieving the goal of mutual conversion between chemical energy and electrical energy. In the field of energy storage technology, flow batteries are widely regarded as one of the most promising new energy storage technologies due to their environmental friendliness, long lifespan, high safety, and high energy efficiency, which will be widely promoted and applied in the future.

Ion exchange membrane is an important structural component of a flow battery device, which can separate the positive and negative electrolytes and selectively penetrate ions to construct a complete circuit in the battery structure. As mentioned in previous articles, taking all vanadium flow batteries as an example, an ideal all vanadium flow battery separator needs to have the following characteristics: (1) low vanadium ion permeability, reducing pollution caused by vanadium ion transmembrane transport; (2) Excellent chemical stability and high mechanical strength enable the film to have a long lifespan under acidic conditions, thereby increasing battery life; (3) High ion conductivity and good ion selectivity result in high battery efficiency; (4) Low water flux, during the charging and discharging process, keeps the electrolyte at the anode and cathode balanced; (5) Low processing and production costs are conducive to the widespread application of diaphragms.

At present, the proton exchange membrane widely used in the commercial field is represented by the Nafion membrane produced by DuPont Company (Kemou), which uses sulfonic acid groups as exchange groups as the standard separator for all vanadium redox flow batteries. Its stability in the electrolyte is high, but due to the high permeability and high price of vanadium ions, this also limits the further development of flow batteries to a certain extent. With the increasing call for the localization of core components in China, domestic companies, enterprises, and university institutions are accelerating their research pace to promote the localization process of proton exchange membranes for liquid flow batteries. This article will mainly focus on the localization process of Nafion membranes by domestic companies and the main progress of non fluorinated membranes, with a focus on comparing the performance and cost of domestically produced films with Nafion films.

At present, most of the proton exchange membranes on the market are perfluorosulfonic acid proton exchange membranes, represented by Nafion, which have characteristics such as high mechanical strength, high stability, and high conductivity. Partial fluorinated proton membranes are mainly used to ensure a longer service life by changing the introduction method of sulfonic acid groups, such as grafting sulfonic acid branches after main chain polymerization, copolymerization of main chain followed by sulfonation branches, and direct polymerization of sulfonic monomers. The most famous one is BAM3G (sulfonated or phosphated trifluoromethylene proton exchange membrane) produced by Ballad Company, which has a cost of 50-150 US dollars per square meter, far lower than perfluorosulfonic acid proton membrane manufacturers, but has a shorter lifespan than perfluorosulfonic acid proton exchange membranes [1]. Non fluorinated proton exchange membranes are currently in the research stage, and further development is needed in terms of conductivity, chemical stability, and service life. For example, the sulfonated styrene butadiene/styrene block copolymer membrane developed by DAIS in the United States can achieve conductivity comparable to Nafion membranes when the sulfonation degree is above 50%. Among non fluorinated proton exchange membranes, polybenzimidazole (PBI) membranes have developed the best, but they are mainly used in high-temperature environments, such as in high-temperature fuel cell systems. At present, the localization of proton exchange membranes mainly focuses on the production and technological breakthroughs of perfluorosulfonic acid proton exchange membranes, which are in the mainstream demand. Both partially fluorinated and non fluorinated proton exchange membranes are currently in the initial and research stages, and there are relatively few related reports.

Taking the perfluorosulfonic acid proton exchange membrane Nafion Ⓡ series membranes (Nafion Ⓡ 112, Nafion Ⓡ 115, Nafion Ⓡ 117, etc.) produced by DuPont in the United States as an example, the products N115 and N117 sold in China have retail prices of 13500 yuan/square meter and 15000-19000 yuan/square meter, respectively, with bulk prices ranging from 2000 to 3000 yuan/square meter. According to its public information, Nafion perfluorosulfonic acid (PFSA) membrane is a PFSA/polytetrafluoroethylene (PTFE) copolymer membrane. The membranes used for energy storage mainly include NR211 and NR212, which are produced by casting method and have high stability, with thicknesses of 25.4 microns and 50.8 microns respectively, and N115 and N117, which are produced by extrusion method and have thicknesses of 125 microns and 183 microns respectively. Their main properties are as follows. The mechanical direction (MD) tensile strength of NR211, NR212, N115, and N117, as shown in the table, are 23MPa, 32MPa, 43MPa, and 43MPa, respectively. As the thickness increases, their tensile strength also increases. According to relevant literature reports, the main conductivity parameter N117 is currently 0.035 S cm-1, and the performance of other Nafions is also around 0.05-0.07 S cm-1, gradually increasing with decreasing thickness.

     Dongyue Group's Future Hydrogen Materials Company is one of the leading domestic enterprises in the production of proton exchange membranes. Future Hydrogen Company currently has a 1.5 million square meter fuel cell proton membrane production plant, consisting of a comprehensive research and development center and an internationally advanced standardized membrane workshop. Its main products also include proton exchange membranes for liquid flow batteries. At present, it has formed a complete industrial chain from raw materials, intermediates, monomers, polymers, to film-forming technology, functionalization technology, etc., with independent core technologies and strong market competitiveness.

According to its public information, the proton exchange membrane used in the perfluorinated sulfonic acid series membrane produced by it enhances the material composite, and has the characteristics of unidirectional cation passing, thin thickness, high strength, low swelling, high dimensional stability, and good durability. The current production model is DMV850, with a thickness of 50 microns. Its tensile strength is 20MPa, elastic modulus is 300MPa, elongation at break is 200%, water content is 3-5%, and vanadium ion permeability is less than 15 × 10-7 cm2/min. Compared with the parameters related to Nafion membrane NR212, it can be seen that the tensile strength of DMV850 is lower than that of NR212, about 62.5% of that of NR212 membrane, and the elongation at break is lower than that of NR212, about 343% of that of NR212 membrane, about 58.3% of that of NR212 membrane, indicating that there is still a gap in its mechanical strength compared to Nafion membrane. And its moisture content is also lower than 5-8% of NR212, which suggests that its conductivity performance may be worse than NR212. But currently, its market share in the domestic market is not large, but it has a certain price advantage, about 1000-1500 yuan/square meter.

Main technical parameters of DMV850 flow battery membrane

     Suzhou Kerun New Materials Co., Ltd. was established in 2019 and has nearly 10 years of experience in the research and manufacturing of perfluorinated ion membranes and proton exchange membranes. Currently, Kerun New Materials has achieved mass production of perfluorinated ion membranes through steel strip casting. According to its public information, its production product Nepem-11 series vanadium battery specific low vanadium permeation ion exchange membrane is an ion exchange membrane manufactured by Korun using a new casting process. Due to the combination of casting process and doping technology, the membrane manufactured has advantages such as high tensile strength, isotropy, high conductivity, and good chemical properties. During the casting process, a new type of fluorine resin mixture with good vanadium resistance was added to the Nepem membrane. After testing, the addition of the mixture ratio reduced vanadium penetration by more than 80%, and the conductivity only decreased by 5%. Nepem-115, 1135, and 1125 use 1000 equivalents of high-quality perfluorosulfonic acid ion exchange resin as the raw material, which has stable performance and is doped with specially tuned vanadium blocking fluorine resin, ensuring high conductivity and low vanadium permeability. Comparing N-112 with a thickness of 50 microns and N-115 with a thickness of 125 microns with corresponding Nafion films (NR212 and N115), the tensile strength (greater than 28 MPa) is almost the same as that of Nafion films, and its elastic modulus (greater than 400 MPa) is much higher than that of Nafion films. However, its elongation at break (greater than 120%) is relatively small. Overall, the mechanical strength of the Korun Nepem-11 series should have a small difference from Nafion films and have its own advantages, and its moisture content index is the same as that of Nafion films. Its current reported conductivity is 0.1 S/cm, which is better than Nafion's conductivity performance of 0.035-0.07 S/cm.

     Hancheng New Energy was founded in 2017, mainly dedicated to the research and development, production, and sales of world leading special fluorine-containing new materials, key materials for hydrogen energy fuel cells, and other products. According to its public information, the Hancheng Hyproof enhanced perfluorosulfonic acid proton exchange membrane produced by it belongs to ultra-thin enhanced composite membranes, which can be used for fuel cell proton exchange membrane electrodes, water electrolysis hydrogen production separators, energy storage battery separators, and many electrochemical applications. It has good mass selectivity, high mechanical strength, high ion conductivity, low swelling deformation, strong acid resistance, and long service life. The main technical advantages claimed by it for proton exchange membranes include: (1) improving the overall physical performance of proton exchange membranes through corresponding process optimization, which is conducive to enhancing the structural stability and service life of the membranes; (2) In terms of perfluorosulfonic acid resin, we have broken through foreign patent restrictions and achieved the localization and industrialization of functional resins through independent development and production processes; (3) Having multiple national and international invention patents, and successfully achieving industrial transformation and application of patents. However, its specific process and performance parameters have not been publicly displayed.

Zhongke Energy Materials Technology (Dalian) Co., Ltd., as a subsidiary of the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, has a close cooperative relationship with its industrial development. Zhongke Energy Materials Company is mainly engaged in the production of energy storage materials, including liquid flow batteries, fuel cell membrane materials (acidic/alkaline systems), bipolar plates, PBI polymers, and so on. According to its public information, the proton exchange membrane used in its production of flow batteries has the characteristics of high stability, high ion selectivity, and high mechanical strength. Its thickness range is 10-100 microns, with a tensile strength greater than 100 MPa and a swelling rate less than 30%. Compared to traditional Nafion proton exchange membranes, there is still a certain gap. However, it is worth mentioning that it also has certain production and research in the field of PBI polymers.

Shanxi Guorun Energy Storage Technology Co., Ltd. is also engaged in the production of high-end ion exchange membranes in liquid flow battery energy storage systems, liquid flow batteries, and hydrogen fuel cells. It claims to be the only enterprise in China that comprehensively layout equipment manufacturing and core material production. According to public information, its production of perfluorinated ion membranes adopts advanced membrane forming integration technology, achieving the regeneration and recycling of perfluorinated ion membranes. Its advantages are high conductivity, high crystallinity, high tensile strength, isotropy, and long service life. Its main focus is also on ion exchange membranes for liquid flow batteries. At present, Shanxi Guorun Energy Storage Technology Co., Ltd. has built a large-scale energy storage and high-quality ion membrane material production line for vanadium flow batteries, and has achieved full production.

There are also some domestic enterprises that can independently produce proton exchange membranes, but their product focus and solution mainly focus on fuel cells. Their rapid development undoubtedly indicates the continuous acceleration of the localization process of proton exchange membranes.

According to publicly available information from Shenzhen General Hydrogen Energy, its ePTFE reinforced PEM has higher mechanical properties and dimensional stability; Short branched perfluorosulfonic acid resin has higher ion conductivity and higher output performance under high temperature and low humidity conditions; Contains free radical scavengers for higher durability; The consistency of MD and TD direction performance and more stable output performance of internationally competitive price products are also being explored for power supply solutions.

On December 5, 2021, the 300000 square meter proton exchange membrane production line of State Power Investment Hydrogen Energy Company was officially put into operation, which is also the first fully autonomous and controllable proton exchange membrane production line in China. Each link does not rely on foreign parts and technology, achieving the localization of key components of hydrogen fuel cells and breaking the long-term monopoly of the domestic proton exchange membrane market by foreign manufacturers. Moreover, according to relevant data, its production line can produce proton exchange membranes with thicknesses ranging from 8 micrometers to 20 micrometers. Compared with similar domestic and foreign competitors, the proton exchange membranes produced are equivalent or superior to similar foreign products in terms of proton conductivity, gas permeability (H2), mechanical strength, and other aspects.

Pan Asia Microporous was founded in 1995. It is an independently innovatively developed micro permeable polymer breathable new material and device, which is applied in the automotive industry, packaging industry, and other protective breathable fields; Master the core technology of e-PTFE membranes and have the ability to fully develop the industry chain of micro permeable membranes and membrane components; According to its relevant information, Pan Asia Microfiltration is expected to invest in the construction of ePTFE functional membranes and hydrogen fuel cell engineering technology research institute, as well as a 1.5 million square meter hydrogen proton exchange membrane industrialization project.

According to the statistical data of the North Star Energy Storage Network and relevant government project announcements, the current scale of all vanadium flow battery energy storage projects in China exceeds 120MW, with an under construction scale of over 110MW. According to the current construction scale, it consumes approximately 150000 to 200000 square meters of proton exchange membranes, and its current demand still exceeds 150000 square meters. Although it can be seen from existing data that the development of domestic proton exchange membrane technology cannot surpass traditional Nafion membranes, in recent years, with continuous core technology research and development, good results have been achieved. We believe that with the further development of energy storage technology and industrial layout of all vanadium flow batteries in China, the market space for proton exchange membranes for flow batteries required for their construction has also increased. With the synchronous increase in demand for proton exchange membranes for fuel cell vehicles and fuel cell technology, it is imperative for domestic proton exchange membranes to replace imported proton exchange membranes to reduce costs and avoid bottlenecks.



Reference materials

[1] CHEN QUAN SHI, QIU BIN, XIE QI CHENG. Fuel cell electric vehicles [M] Beijing: Tsinghua University Press, 2005

[2] CITIC Securities, Hydrogen and Fuel Cell Market Outlook and Investment Research Report: Proton Exchange Membrane Market and Localization [R] two thousand and twenty-one



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