What is the current status of the development of proton exchange membrane technology for AG3 battery?

AG3 battery are electrochemical reaction devices that directly convert dye chemical energy into electrical energy. The combined heat and power efficiency can reach more than 95%. At the same time, they also have the advantages of no noise, green environmental protection, high reliability, and easy maintenance. They are considered to be the most promising new power generation technology in contemporary times. Proton exchange membrane AG3 battery (PEMFC) use proton conductive materials as electrolytes. Compared with ordinary AG3 battery, they have fast start-up speed at room temperature, no electrolyte loss, and high specific power and specific energy. Therefore, they have been widely used in decentralized power stations, mobile power sources, and special aerospace. As the core material of AG3 battery, the performance of proton exchange membrane (PEM) directly affects the stability and durability of AG3 battery.

1. Classification of proton exchange membranes

According to the fluorine content, proton exchange membranes can be divided into perfluorinated proton exchange membranes, partially fluorinated polymer proton exchange membranes, non-fluorinated polymer proton exchange membranes, and composite proton exchange membranes. Among them, since the main chain of the perfluorosulfonic acid resin molecule has a polytetrafluoroethylene (PTFE) structure, it brings excellent thermal stability, chemical stability and high mechanical strength; the polymer membrane has a long life, and because of the presence of hydrophilic sulfonic acid groups on the molecular side chains, it has excellent ion conduction properties. Non-fluorinated proton membranes require a more demanding working environment, otherwise they will be quickly degraded and destroyed, and will not have the excellent performance of perfluorosulfonic acid ion membranes.

Perfluoro proton exchange membranes were the first to be industrialized. Perfluoro proton exchange membranes include ordinary perfluorinated proton exchange membranes, enhanced perfluorinated proton exchange membranes, and high-temperature composite proton exchange membranes. The production of common perfluorinated proton exchange membranes is mainly concentrated in the United States, Japan, Canada and China. The main brands include Nafion series membranes of Dupont, Dow membranes and Xus-B204 membranes of Dow Chemical Company, 3M perfluorocarbon membranes, Alciplex of Asahi Kasei Corporation, Flemion of Asahi Glass Company, C series of Japan Chlorine Engineering Company; BAM series membranes of Ballard Company of Canada, Solvay series membranes of Solvay Company of Belgium; DF988 and DF2801 proton exchange membranes of Shandong Dongyue Group of China.

Since the early 1980s, when Ballard Company of Canada used perfluorosulfonic acid proton exchange membranes for PEMFC and achieved success, perfluorosulfonic acid membranes have become the only commercial membrane material common perfluorosulfonic proton exchange membranes for modern PEMFC. Enhanced perfluorinated proton exchange membranes mainly include PTFE/perfluorosulfonic acid composite membranes and glass fiber/perfluorosulfonic acid composite membranes. High-temperature composite proton exchange membranes mainly include heteropolyacid/perfluorosulfonic acid composite membranes and inorganic oxide/perfluorosulfonic acid composite membranes.

1. Perfluorosulfonic acid proton exchange membrane

Perfluorosulfonic acid proton exchange membranes have been commercialized and have become an important fuel cell membrane material on the market. The perfluorosulfonic acid PEMs currently on the market mainly include Nafion series PEMs (Nafion117, Nafion115, Nafion112, etc.) of Dupont in the United States, XUS-B204 membranes of Dow, Aquivion membranes of Solvay in Belgium, Alciplex of Asahi Kasei in Japan, Flemion of Asahi Glass, C series of Chlorine Engineering, BAM membranes of Baliard in Canada, etc. Fleminon membranes, Aciplex membranes and Nafion membranes are similar in that they all have long side chains; the fluorinated side chains of XUS-B204 membranes are shorter, and the conductivity is significantly improved, but at the same time, the difficulty and cost of synthesis are also greatly increased, and it has been discontinued. Solvay solved this problem by introducing a higher content of sulfonate groups to maintain the water content in the membrane. The performance of the short-chain Aquivion membrane produced by Solvay has exceeded that of Nafion112 membrane.

The most widely used PEM in the market is Dupont's Nafion membrane. Compared with other proton exchange membranes, Nafion membranes have higher chemical stability and mechanical strength, and can maintain high conductivity in a high-humidity working environment. Currently, almost all commercial perfluorosulfonic acid PEMs are based on the Nafion structure. However, the membrane material has high requirements for temperature and water content (the proton conductivity performance decreases seriously at medium and high temperatures). When used in direct methanol AG3 battery, the methanol permeability is high and the preparation process is difficult. Beijing University of Chemical Technology has prepared Nafion nanofiber membranes with a conductivity 5 to 6 times that of Nafion membranes, which has improved the properties of Nafion membranes.

2. Partially fluorinated proton exchange membranes

General Electric (GE) of the United States used PEM AG3 battery with sulfonated polystyrene proton membranes on spacecraft in the 1960s. In order to improve the performance of sulfonated polystyrene proton PEM, Ballard Company of Canada developed the BAM series PEM. This is a typical partially fluorinated polystyrene PEM. Its thermal stability, chemical stability and water content have been greatly improved, exceeding the performance of Nafion117 and Dow membranes. At the same time, its price is lower than that of perfluorinated membranes, and in some cases it can replace perfluorosulfonic acid membranes. However, due to the small molecular weight and insufficient mechanical strength of polystyrene PEM, its wide application is limited to a certain extent.

3. Fluorine-free proton exchange membrane

In order to meet the requirements of PEM in terms of chemical stability and mechanical strength at the same time, fluorine-free PEM is generally prepared using aromatic polymers containing benzene ring structures on the main chain. Sulfonated aromatic polymers mainly include sulfonated polyaryletherketone, sulfonated polysulfide sulfone, sulfonated polyetheretherketone, sulfonated naphthalene polyethersulfoneketone, sulfonated polyimide, sulfonated polybenzimidazole, etc. The water absorption and alcohol resistance of PEM prepared in this way are significantly higher than those of Nafion membrane. DAIS, a US company, uses sulfonated block ionic copolymers as PEM raw materials to develop sulfonated styrene-butadiene/styrene block copolymer membranes. When the sulfonation degree of the PEM is controlled between 50% and 60%, its conductivity can reach the level of Nafion membrane; when the sulfonation degree is greater than 60%, it can simultaneously obtain higher electrochemical performance and mechanical strength, achieving a balance between the two; the battery life reaches 2500h at 60°C and 4000h at room temperature, and is expected to be used in low-temperature AG3 battery.

2. Modification of proton exchange membranes

1. Composite proton exchange membranes

In order to solve the problems of high difficulty in synthesizing raw materials, complex preparation processes and high costs of perfluorosulfonic acid proton exchange membranes, researchers have used composite membrane materials to develop new proton membranes. Composite proton exchange membranes mainly include mechanically enhanced proton exchange membranes, high-temperature proton exchange membranes and self-humidifying proton exchange membranes.

(1) Mechanically enhanced proton exchange membranes

The proton conductor is combined with the reinforcing component to achieve a mechanically enhanced proton exchange membrane. Among them, proton conductors can form continuous proton transport channels and improve the conductivity of protons, such as the modification and application of Nafion membranes. Mechanical reinforcement components can effectively improve the mechanical strength of membrane materials, such as the modification and application of PTFE porous membranes. The enhanced composite PEM obtained by modifying PTFE porous membranes has increased mechanical strength and stability, while the membrane thickness has been greatly reduced. As the polymer content decreases, the production cost is also reduced; the improvement of the water content and transfer in the membrane by the modification operation can further reduce the resistance of the material and improve the overall performance of the fuel cell.

(2) High-temperature proton exchange membrane

On the one hand, at high temperatures, the water content of Nafion membranes will drop sharply, resulting in a significant decrease in conductivity; on the other hand, the chemical stability of Nafion membranes is not enough, and the occurrence of chemical degradation and structural changes also cause the mechanical strength of the membrane to decrease, thus limiting the inability to increase the electrode reaction rate and overcome catalyst poisoning by increasing the operating temperature to improve the properties of the membrane. Therefore, the research on high-temperature PEM has also become a hot topic.

At present, the main transport carriers of high-temperature proton exchange membranes include high-boiling-point inorganic acids or heteropoly acids, such as phosphoric acid, silicotungstic acid, phosphotungstic acid, etc. The NASTA series of heteropoly acid blend membranes and NASTATH series of heteropoly acid blend membranes launched by Ecole Polytechnique of Canada have improved proton conductivity and water absorption compared with Nfion membranes. The performance of AG3 battery assembled using them is also better than that of AG3 battery made of Nafion membranes. Among them, the NASTA series of heteropoly acid blend membranes are prepared by adding silicotungstic acid to Nafion solution and using the injection method. The NASTATH series of heteropoly acid blend membranes are prepared by mixing silicotungstic acid, plasticizer liquid thiophene and Nafion solution.

(3) Alcohol-repellent proton exchange membranes

Direct methanol AG3 battery have the advantages of high low-temperature start-up speed, green environmental protection and simple battery structure, and have great application potential in the field of mobile power sources. However, the alcohol-repellent performance of perfluorosulfonic acid PEM is poor, and direct methanol AG3 battery cannot be prepared. At present, the modification of Nafion membrane is usually used to improve the alcohol resistance of membrane materials. Tianjin University prepared two kinds of blended PEMs, PVDF-PSSA and PVDF-Nafion, by blending Nafion with proton conductivity, polystyrene sulfonic acid solution and polyvinylidene fluoride with high alcohol resistance. Compared with Nafion117 membrane, these two membranes have obvious advantages in alcohol resistance. When the mass fraction of Nafion is 25%, the conductivity of PVDF-Nafion membrane decreases by 100 times, but the methanol permeability decreases by nearly 1000 times.

(4) Self-humidifying proton exchange membrane

In order to maintain good proton conductivity, PEM needs to maintain sufficient water. AG3 battery made of self-humidifying PEM have a simpler structure. At the same time, due to the existence of self-humidifying PEM, water vapor will not liquefy and condense during the battery reaction. Therefore, self-humidifying PEM also has a wide range of application potential.

At present, there are two main types of self-humidifying PEM: hydrophilic oxide-doped self-humidifying PEM and H2-O2 self-humidifying composite PEM.

Hydrophilic oxide-doped self-humidifying composite membranes generally use hydrophilic oxide particles such as SiO2 and titanium dioxide (TiO2) to dope the membrane material. Due to the presence of these hydrophilic ions, PEM can absorb water generated during the battery reaction, thereby keeping the proton membrane moist. The humidification properties of the membrane can be adjusted by factors such as the content, diameter, and crystal type of the hydrophilic oxide. Honamai et al. combined siloxane and polymer electrolyte membranes to produce a nanosiloxane skeleton, which significantly increased the moisture content of PEM. They further introduced dispersed SiO2 and TiO2 particles into Nafion112 membranes, and also obtained good humidification effects.

The working principle of the H2-O2 self-humidifying composite membrane is to add a certain amount of Pt as a catalyst into the PEM to allow the hydrogen and oxygen diffused into the PEM to react to generate water. This method can not only achieve real-time humidification of PEM, but also prevent hydrogen (H2) from generating mixed potential at the oxygen electrode, thereby improving current efficiency and increasing battery safety. However, self-humidifying proton membranes also have certain defects. Mainly including: since the Pt particles in the PEM cannot be fixed, the Pt particles are easy to gather into clusters and form conductive paths; furthermore, these inorganic particles are incompatible with Nafion, which can easily cause the local pressure of spherical particles to increase in the water concentration gradient environment, resulting in reduced mechanical properties of the composite PEM and aggravated diffusion of the reaction gas in the membrane.

III. Conclusion

Proton exchange membrane is the core material of AG3 battery. The performance of proton exchange membrane will directly affect the industrialization process of AG3 battery and one of the key factors for large-scale application. In order to realize the practical application and industrialization of AG3 battery, people have conducted a lot of research on the manufacturing process and material modification of PEM. At present, further improving the durability, life and working performance of PEM is still the main task facing the industrialization of PEM AG3 battery. The fuel cell PEM market is still an emerging market, and it has not formed a large scale at home and abroad. Driven by the huge market demand for AG3 battery, PEM will surely achieve further development. It is believed that PEM products with higher performance and lower cost will be launched soon, which will greatly promote the development of fuel cell technology and its industrial application.

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