Polybenzimidazole membrane (PBI membrane) is a fluorine-free type of ion exchange membrane, which contains repeating units of benzimidazole in its main chain. It is a polymer with low proton conductivity, but has attracted widespread attention due to its good mechanical properties, chemical stability, thermal stability, and low cost. After research, it was found that PBI membranes can have good proton conductivity through proton acid doping, radiation grafting modification, etc. Due to the alkalinity of the benzimidazole group in the polymer matrix, acidic groups with high proton conductivity can be added as proton donors (such as phosphate groups), making it have excellent proton conductivity in high-temperature and dry environments. And due to its main chain being aromatic compounds with good stability at high temperatures, compared to perfluorosulfonic acid proton exchange membranes, it is very suitable for high-temperature proton exchange membrane fuel cell systems, and also has certain application potential in liquid flow battery systems.

PBI membranes have outstanding cost advantages and structural stability. Their benzimidazole groups are both alkaline and acidic, making them suitable for both proton exchange membranes and anion exchange membranes [1]. The potential of PBI membrane as a new type of proton exchange membrane has aroused widespread interest, but its proton conductivity needs to be further improved. Currently, there have been many advances in research in this area. This article will summarize the research work on improving the proton exchange capacity of PBI membrane, providing more references for everyone to deepen their understanding of fluorine-free proton exchange membranes.

The mechanism of proton conduction in PBI membranes is quite complex, determined by various factors such as acid doping, relative humidity, and temperature. Protons can exist in the form of NH4+, H3O+, etc. during the conduction process in aqueous solutions, and form hydrogen bonds in the presence of nitrogen atoms. The nitrogen atom in the imidazole ring of PBI membrane has the ability to provide and receive protons, playing an important role in proton conduction, but its individual conductivity is limited. The acid doping methods currently used (such as sulfuric acid, perchloric acid, hydrochloric acid, phosphoric acid, sulfonic acid, etc.) can effectively promote the long-range transport of protons in PBI membranes, thereby improving the proton conductivity of proton exchange membranes.

The modification methods for improving the proton exchange capacity of PBI membranes can be summarized into two categories: one is to start with the benzimidazole monomer and modify it by modifying the monomer before polymerization; The second type is the polymer formed after monomer polymerization, which involves block, substitution, and grafting of the polymer. The modification of monomers can directly affect the skeleton structure of PBI membranes. Blocking can effectively improve the doping acid content in PBI membranes, while substitution and grafting can further improve PBI membranes with acidic functional groups [2]. Sulfonation is a method of doping acid, commonly used in all vanadium flow battery membranes. It mainly introduces sulfonic acid groups into PBI membranes with poor ion exchange ability through chemical means. The introduction of sulfonic acid groups can improve proton conductivity and weaken the bonding ability around the carbon skeleton, making it susceptible to vanadium ion attacks in all vanadium flow batteries. Therefore, balancing the relationship between the two through blending, hybridization, filling, etc. [3].

Zou Jing et al. prepared sulfonated polybenzimidazole (SPBI) using microwave synthesis method, and then used solution film-forming method to blend the prepared sulfonated polybenzimidazole with polyethersulfone (PES) to prepare sulfonated polybenzimidazole polyethersulfone membranes for high-temperature fuel cells (HT PEMFCs). The film has good binding ability to different phases and has better film-forming ability, thermal stability, anti free radical oxidation resistance, and dimensional stability compared to the sulfonated polybenzimidazole film alone. On this basis, phosphoric acid doping was carried out, and it was found that when the phosphoric acid (PA) doping amount was 10.15%, the proton conductivity of the sulfonated polybenzimidazole polyethersulfone membrane was 79.9 mS · cm-1 at 160 ℃ without humidification. Using unhumidified hydrogen and oxygen as fuel, a fuel cell single cell test was conducted at 150 ℃, with an open circuit voltage of 0.95V and a maximum power density of 0.191W · cm-2, demonstrating outstanding performance [4].

Wang Yingzi et al. prepared a series of acid-base composite proton exchange membranes using sulfonated polybenzimidazole (S-PBI) and high degree of sulfonation polyether sulfone (ABPS) as raw materials and solution blending method. Its research shows that with the increase of sulfonated polybenzimidazole content, the size stability of the prepared membrane is significantly improved. The proton conductivity of sulfonated polybenzimidazole/high sulfonation degree polyethersulfone composite membrane decreases, but with the increase of temperature, the proton conductivity gradually increases and exhibits high conductivity [5].

Zhang Qi et al. successfully prepared graphene oxide (GO)/sulfonated polybenzimidazole (SPBI) proton exchange composite membranes through blending and in-situ polymerization methods. The experimental results show that the mechanical properties of the composite film are significantly improved after adding graphene oxide, and the tensile strength is 2.5 times higher than that of the Nafion117 film (26.65MPa). And it was found that the membrane prepared by in-situ polymerization method has good water retention ability, higher acid doping rate, and lower acid swelling, which improves the size stability of the membrane. The Y-GO/SPBI-1% proton exchange composite membrane has the highest proton conductivity of 0.113S/cm at a relative humidity of 40% and 160 ℃. The oxygen-containing functional groups on oxidized graphene contribute to the proton hopping in the composite membrane. In situ polymerization enables oxidized graphene to be more uniformly dispersed in the sulfonated polybenzimidazole matrix, which plays a key role in improving the proton conductivity of the composite membrane [6]. In addition, it also synthesized phosphonic acid modified graphene oxide grafted with phosphonic acid and sulfonic acid groups through amine phosphonation method, and doped phosphonic acid modified graphene oxide into sulfonated polybenzimidazole through in-situ polymerization method, successfully preparing SPBI/MGO proton exchange composite film. The results showed that the acid doping rate of SPBI/MGO-1% composite film reached the highest of 248.8%, and the doping of MGO improved the thermal stability of the composite film. The dry film tensile strength of the composite film increased by 36% compared to Nafion117 film (26.65MPa), and the wet film of SPBI/MGO-1% achieved a tensile strength of 69.46MPa, which increased by 41.2% compared to SPBI film. The composite film has high mechanical properties. The proton conductivity of SPBI/MGO composite membrane gradually increases with the increase of MGO content. The proton conductivity of SPBI/MGO-1% composite membrane reaches 0.193S/cm under 10% RH and 160 ℃ conditions, which has high application prospects in high temperature and low humidity proton exchange membrane fuel cells [10].

Kangsen et al. synthesized 3,3 '- sodium disulfonate 4,4' - dimethyl formate biphenyl and di (4-methylformate phenyl) phenylphosphine oxide monomers through esterification reaction, and co condensed them with 3,3 '- diaminobenzidine to prepare soluble sulfonated polybenzimidazole containing triphenylphosphine oxide groups. The presence of the benzene side group in the triphenylphosphine oxide group leads to a relatively loose arrangement of its polymer chains, resulting in an improved solubility of the membrane product; The oxygen phosphine group increases the water absorption of the product and significantly enhances its conductivity [7]. Xiao Dan et al. also used direct condensation method to polymerize 2,2 '- sodium disulfonate -4,4' - oxobenzoic acid and dicarboxylic diphenyl ether with 3,3 '- diaminobenzenediamine to prepare corresponding diphenyl ether sulfonated polybenzimidazole. By introducing flexible ether groups into the sulfonated polybenzimidazole macromolecular chain structure through this method, the flexibility of the product can be improved and the solubility of the product can be increased. Its research shows that diphenyl ether sulfonated polybenzimidazole has an amorphous structure, good solubility, thermal stability, and low swelling, which has good potential application prospects [8]. Huang Wei et al. used different ratios of sodium 5-sulfonate isophthalic acid, 4,4 '- dicarboxylic diphenyl ether, and 3,3' - diaminobenzidine in polyphosphate for co condensation reaction to produce a series of sulfonated polybenzimidazole with controllable sulfonation degree and good solubility. The results showed that the sulfonated polybenzimidazole produced had excellent thermal stability, with an initial decomposition temperature, maximum decomposition rate temperature, and 5% and 10% thermal weight loss temperature higher than 513, 586, 573, and 597 ℃, respectively [9].

At present, PBI membrane designs and modification methods vary for different application scenarios, but they are all aimed at replacing traditional Nafion membranes to achieve higher cost-effectiveness. PBI membrane, as a cutting-edge ion exchange membrane material, has great potential in fuel cells, flow cells, and hydrogen production through electrolysis of water. Despite the high cost of traditional perfluorinated membranes, the cost of core components can be reduced through technological innovation and domestic substitution, which is of great significance for the large-scale promotion and application of future technologies.

Reference materials

[1] Li Yan New type of polybenzimidazole film and its application in lithium batteries [D]. Zhejiang University, 2021 DOI:10.27461/d.cnki.gzjdx.2021.002214.

[2] Wang Di Preparation and performance study of high-temperature proton exchange membranes based on grafting/block modification of polybenzimidazole [D]. Changchun University of Technology, 2021 DOI:10.27805/d.cnki.gccgy.2021.000399.

[3] Dong Ziwei Preparation of Modified Sulfonated Polyphenylimidazole Amphoteric Films and Performance of Vanadium Batteries [D]. Dalian University of Technology, 2020 DOI:10.26991/d.cnki.gdllu.2020.003518.

[4] Zou Jing, Cui Qiujuan, Yang Yanli, Ma Shaohua. Preparation and performance study of sulfonated polybenzimidazole polyethersulfone high-temperature composite proton exchange membrane [J]. Aging and Application of Synthetic Materials, 2021,50 (06): 55-58 DOI:10.16584/j.cnki.issn1671-5381.2021.06.018.

[5] Wang Yingzi, Shang Yuming, Feng Shaoguang, Dong Wenqi, Wang Yaowu, Xie Xiaofeng, Lv Yafei. Preparation and characterization of sulfonated polybenzimidazole/sulfonated polyether sulfone acid-base composite proton exchange membranes [J]. Chemical Progress, 2010,29 (05): 843-846 DOI:10.16085/j.issn.1000-6613.2010.05.034.

[6] Zhang Qi, Pan Liyan, Xu Rong, Zhou Shouyong, Zhong Jing. Preparation and characterization of high-temperature proton exchange membranes of oxidized graphene/sulfonated polybenzimidazole [J]. Chemical Progress, 2018,37 (12): 4758-4764 DOI:10.16085/j.issn.1000-6613.2018-0430.

[7] Kang Sen, Zhang Chunjie, Xiao Guyu, Yan Deyue. Sulfonated polybenzimidazole proton exchange membranes containing triphenylphosphine oxide groups [J]. Journal of Functional Polymers, 2009,22 (02): 199-202 DOI:10.14133/j.cnki.1008-9357.2009.02.013.

[8] Xiao Dan Preparation and characterization of sulfonated polybenzimidazole by direct condensation method [D]. Shanghai Jiao Tong University, 2007

[9] Qing Shengbo, Huang Wei, Yan Deyue. Synthesis and characterization of high-temperature resistant sulfonated polybenzimidazole [J]. Journal of Chemistry of Higher Education, 2005 (11): 177-180+9

[10] Zhang Qi, Zhang Kui, Zhong Jing, Xu Rong. Preparation and properties of sulfonated polybenzimidazole/phosphonic acid modified graphene oxide proton exchange composite membranes [J]. Chemical Progress, 2020,39 (07): 2751-2757 DOI:10.16085/j.issn.1000-6613.2019-1573.

More articles: