Organic–inorganic hybrid proton exchange membrane based on polyhedral oligomeric silsesquioxanes and sulfonated polyimides containing benzimidazole
Graphical abstract
Introduction
Energy crisis and environmental pollution are the most severe challenges faced by the current generation. Fuel cells (FCs) have attracted considerable research interest because of their high efficiency and environment friendliness [1], [2], [3]. As one of the key components of the proton exchange membrane FCs (PEMFCs), proton exchange membranes (PEMs) serve two functions: to transfer protons and separate the fuel and comburent. Preferred PEMs have high proton conductivity and low methanol permeability along with high chemical, thermal, and mechanical stability [4]. In the past few decades, several types of PEMs have been synthesized and investigated for application in FCs. Perfluorosulfonic acid membranes, such as Nafion®, are predominantly used in practical and scientific systems. However, these membranes are expensive, have poor dimensional stability, and low mechanical property at high temperature and high humidity [5], [6], [7]. As alternatives to the perfluorinated polymers, various inexpensive sulfonated aromatic hydrocarbon polymers with excellent comprehensive properties have been developed [8], [9], [10], [11]. Among these, sulfonated polyimides (SPIs) have attracted much attention for their potential use as the PEM of FCs [12], [13], [14], [15].
Aromatic polyimides are a class of high performance engineering plastics with high tensile strength, unique thermal stability, excellent chemical resistance, and so on [16], [17]. However, SPIs used in FCs have poor hydrolytic stability due to decomposition of the susceptible imide (–OC–N–CO–) groups in the polymer backbone. Although better than five-membered ring SPIs, six-membered ring SPIs also suffer from low hydrolytic stability [18], [19], [20]. Polybenzimidazole (PBI), with high heat resistance, excellent mechanical property, and anti-free-radical oxidative stability, shows great potential for use in the PEMFCs under high temperature and low humidity conditions. However, practical applications of PBI are restricted due to limitations. First, the solubility of PBI in most solvents at low temperature is poor, and second, the proton conductivity of PBI (sulfonated PBI or PBI doped with acid) is low [21], [22], [23], [24]. Therefore, we introduced benzimidazole groups into the main chain of SPI in order to combine the advantageous properties and overcome the drawbacks of both SPI and PBI. One effective method to enhance the hydrolytic stability of SPI is cross-linking. Polyhedral oligomeric silsesquioxanes (POSS) have well-defined nanosized Si–O cage structures which make them a versatile additive for imparting enhanced mechanical property, thermal stability, oxidative resistance, and abrasion resistance [25], [26], [27]. POSS-based PEMs can improve the dimensional and hydrolytic stabilities as well as water retaining property [28]. Therefore, we selected the glycidyl ether of POSS (G-POSS) as the cross-linker to prepare cross-linked organic–inorganic hybrid PEMs. G-POSS has eight epoxy groups that can react with benzimidazoles and enhance cross-linking density.
In this study, we report the synthesis of SPIs containing benzimidazole groups (SPIBIs) by direct copolymerization. By changing the feed ratio of sulfonated and non-sulfonated diamines, a series of SPIBIs with different levels of sulfonation were synthesized successfully. A new type of SPIBI/POSS-based PEM was prepared by the solution casting method and in-situ cross-linking. Benzimidazoles in SPIBI react with the epoxy groups in G-POSS and form a polymer network within the membranes. The mechanical property, proton conductivity, and oxidative and hydrolytic stability of these membranes were also investigated.
Section snippets
Materials
G-POSS was purchased from Hybrid Plastic. 4,4′-diaminodiphenyl ether (ODA) was purchased from Sinopharm Chemical Reagent Co. and recrystallized from ethanol. 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) was purchased from Alfa Aesar Co. 5-Amino-2-(4-aminophenyl) benzimidazole (APBIA) was purchased from Zhejiang Dragon Chemical Group Co. Triethylamine(Et3N), methanol (AR grade), dimethyl sulfoxide (DMSO, analytical reagent grade), benzoic acid (CR grade), and acetone were supplied by
Synthesis of the benzimidazole-containing sulfonated polyimide
SPIBIs are synthesized by the co-polymerization of ODADS, APBIA, and NTDA monomers (Fig. 1). By changing the ODADS-to-APBIA molar ratio, a series of SPIBIs with different degrees of sulfonation (DS) are obtained. The SPIBI is designated as SPIBI-X, where X (in %) represents the expected DS. DS is calculated from the ODADS-to-APBIA feed ratios, which are listed in Table 1. The intrinsic viscosity ([η]) of SPIBI varies between 0.44 dL g−1 and 1.99 dL g−1 and increases with the increasing DS (
Conclusions
In this study, SPIBIs are synthesized using NTDA, APBIA and ODADS as the monomers. By changing ODADS-to-APBIA molar ratio, a series of SPIBIs with different DS are obtained. During the membrane preparation, benzimidazoles in the obtained SPIBI react with the epoxy group of G-POSS to afford cross-linked organic–inorganic membranes. Compared to pure SPIBI, the cross-linked membranes exhibit highly improved dimensional stability, mechanical property, anti-free-oxidative and hydrolytic stability.
Acknowledgments
The project is sponsored by National Science Foundation of China (51303134, 51203119), Fundamental Research Funds for the Central Universities (0500219149), Research Fund for the Doctoral Program of Higher Education (20120072110062), China High-Tech Development 863 Program (SS2012AA110501).
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