Recently, Toyota Motor Company launched the second-generation Mirai hydrogen fuel cell vehicle. Compared with the world's first commercial hydrogen fuel cell vehicle launched in 2014, the five seater Mirai equipped with three hydrogen fuel tanks can travel up to 850 kilometers. Its core component, the proton exchange membrane, uses Gore Select's second-generation Gore Select expanded polytetrafluoroethylene reinforced perfluorosulfonic acid resin membrane, which belongs to the low-temperature proton exchange membrane fuel cell system.

At present, most of the perfluorosulfonic acid membranes used are only suitable for use at low temperatures (<100 ℃), mainly because the conductivity of proton exchange membranes depends very much on the water content inside the membrane, and therefore requires a high ambient temperature for use (the optimal working temperature is 80 ℃). If the temperature is too high, it will lead to a decrease in the water content inside the membrane and a rapid decrease in the proton conductivity. Therefore, the most commonly used perfluorosulfonic acid proton exchange membrane is suitable for low-temperature proton exchange membrane fuel cell systems (LT-PEMFC), and the performance of common proton exchange membranes (such as Nafion) will seriously deteriorate at high temperatures. Compared to low-temperature proton exchange membrane fuel cells, high-temperature proton exchange membrane fuel cells have higher CO tolerance and better battery performance, which can reduce catalyst poisoning caused by CO and simplify hydrothermal management in the battery. The main limitations of high-temperature proton exchange membrane fuel cells currently lie in the phenomenon of bipolar plate corrosion and the maintenance of high operating temperatures, but their attention in the field of fuel cells is still increasing. For high-temperature proton exchange fuel cells (HT-PEMFC), polybenzimidazole (PBI) membranes have unique applicability and are the preferred choice for this system.

Comparison between HT-PEMFC and HT-PEMFC Source: 4G Optical Element Communication Energy Agency

The reason why polybenzimidazole membrane is suitable for high-temperature proton exchange membrane fuel cell systems is mainly due to its main chain being aromatic compounds with good stability at high temperatures. And its polymer matrix can be doped with acidic groups with high proton conductivity as proton donors (such as phosphate groups), making it have excellent proton conductivity in high-temperature and dry environments. The following figure shows common types of PBI, among which poly (2,2 '- meta phenylene-5,5' - dibenzimidazole) and poly (2,5-benzimidazole) (ABPBI) are the most commonly used PBI for HTPEM.

Common PBI types of HT-PEMFC [1]

PBI membranes such as Celazole that have been successfully commercialized so far ® PBI and Celtec ®， Compared to the common Nafion film with a glass transition temperature of 120 ℃ -140 ℃, the glass transition temperature of PBI can reach 427 ℃, which also means its excellent thermal stability, gradually applied in fuel cells and flow cells under high temperature and high pressure conditions. Celazole ® PBI is a product of PBI Corporation in the United States. It was the first commercial application of PBI membranes in 2000 and is currently the only company in the world that produces poly (2,2 '- meta phenylene-5,5' - dibenzimidazole). Its main product is Celazole ® U series and T series polymers.

In addition, BASF in Germany also commercialized PBI membranes as early as 2003, producing Celtec ® The PBI film is mainly aimed at high-temperature fuel cells and has achieved Celtec based on PBI film ®- The commercial production of P's membrane electrode, and the main process parameters of its PBI membrane product are shown in the following figure. Currently, only in 2019 has Kunai New Materials announced its successful development of high-temperature polybenzimidazole proton exchange membranes in China. Compared with existing proton exchange membranes, the molecular weight of its PBI membrane has been significantly improved, and its mechanical strength, proton channel rate, and operating life have obvious advantages. This will also initiate the localization process of PBI membranes for high-temperature fuel cells, gradually realizing the promotion and application of high-temperature proton exchange membrane electrode technology for hydrogen fuel cells in the Asia Pacific region.

     At present, the use of high-temperature proton exchange membranes has a low dependence on the preparation and storage of high-purity hydrogen. Typically, the hydrogen used in low-temperature proton exchange membrane fuel cell systems needs to reach 99.9% or more, while high-temperature proton exchange membrane fuel cell systems only require hydrogen purity of 50% or more. Impurity containing hydrogen produced by methanol reforming can be directly used, which can greatly simplify fuel cell systems and improve the failure free operation rate of fuel cells. High temperature proton exchange membrane fuel cells have great potential in the heavy-duty transportation industry and can greatly reduce the weight of heavy-duty transportation vehicles. A considerable number of people are very optimistic about the prospects of high-temperature proton exchange membrane fuel cells, and believe that they have great advantages and potential in backup power sources, auxiliary power devices, and heavy transportation. According to relevant reports, Shanghai has listed "developing high-temperature resistant proton exchange membrane technology" as the top priority for core technology localization in the 2023 fuel cell industry plan. We also believe that with the continuous development of research and application, high-temperature proton exchange membrane fuel cells will occupy a place in the future.

[1] Sun Peng, Li Zhongfang, Wang Chuangang, Wang Yan, Cui Weihui, Pei Hongchang, Yin Xiaoyan. Research progress of high-temperature proton exchange membranes for fuel cells [J]. Materials Engineering, 2021,49 (01): 23-34

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