Sulfonated poly(aryl sulfide sulfone)s containing trisulfonated triphenylphosphine oxide moieties for proton exchange membrane
Graphical abstract
Introduction
Fuel cells, including polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), are widely regarded as promising clean power sources for automotive, stationary, and portable power devices because of their high efficiency, high power density, quiet operation and environmental friendliness [1], [2]. As one of the key components in fuel cells, the proton exchange membranes (PEMs) acts as a separator for the reactants and provides ionic pathways for proton transport. Currently, perfluorosulfonic acid (PFSA) polymers, such as Nafion® (DuPont), are commercialized PEMs normally utilized in PEMFC and DMFC because of their excellent chemical stability and high proton conductivity. However, their well-recognized drawbacks such as high cost, difficult synthetic procedure, constrained operating temperature (≤80 °C) and high methanol/fuel gas diffusion may limit their widely application in related areas. Hence, a variety of proton conductive materials, especially some aromatic hydrocarbon ionomers, have been proposed as alternative membranes [3], [4], [5].
The most widely reported aromatic PEMs include sulfonated derivatives of poly(arylene ether ketone)s [6], [7], poly(arylene ether sulfone)s [8], [9], [10], poly(arylene ether nitrile)s [11], polyimides [12], poly(benzimidazole)s [13] and poly(benzothiazole)s [14]. Generally, these aromatic ionomers are advantageous in terms of thermal stability and gas impermeability. However, they require larger amount of water to achieve similar level of proton conductivity to PFSA membranes because high-separate sulfonic groups in rigid polymer chains was difficult to form distinct phase-separated structures, which consequently lead to large dimensional variations and poor mechanical properties [15], [16], [17]. For these reasons, many studies have been carried out to improve the balance between dimensional stability and proton conductivity through careful structural design, including block copolymers [18], [19], [20], pendent or comb-shaped copolymers [21], [22], [23], [24], and highly sulfonated copolymers [25], [26].
In order to develop advanced PEMs, further detailed understanding of the relationship between the chemical structure and properties of the polymers is necessary. Some sulfonated aromatic polymers with functional atoms or groups were designed for polymer eletrolyte membranes. Sulfonated polymers containing sulfur atoms (-S-, -SO-, and -SO2-) are considered promising PEMs due to their good chemical oxidative ability [27], [28], [29]. The electron-donating thioether groups play an important role as a free-radical-scavenging moiety and can be oxidized to sulfoxide groups by reacting with hydroxyl radicals during fuel cell operation. This transformation is thought to be an advantage to enhance the membrane oxidative stability and improve the membrane lifetime. In addition, aromatic sulfonated polymers containing phosphine oxide moieties are also considered promising PEMs due to their good film forming and adhesive abilities [30], [31], [32]. However, these sulfonated polymers often contain one sulforic group in the repeated hydrophilic units. As a result, they only attain suitable conductivities comparable with Nafion just at high sulfonated degree (70%-100%), which lead to large dimensional variations due to high and randomly distributed sulfonic acid groups in polymers chain [31], [32].
In our previous work, we reported some poly(arylene ether sulfone)s containing densely populated sulforic acid groups in hydrophilic units [33], [34]. The materials displayed advantageous proton conductivities with relatively low water contents and swelling ratio. In this article, a series of poly(arylene sulfide sulfone) copolymers containing trisulfonated triphenylphosphine oxide moieties were prepared by direct polymerization. Considering the characteristics of sulfur and phosphine oxide groups and dense sulfonic acid moieties, these membranes are expected to possess good combantion of proton transporting properties and dimensional and oxidative stability. The resulting copolymers were characterized in detail, and their properties are investigated.
Section snippets
Materials
4,4′-Difluorodiphenyl sulfone (DFDPS, 99+%) and 4,4′-thiobis(benzenethiol) (TBT, 99+%) were purchased from TCI Chemical Company. Anhydrous potassium carbonate (99+%), N-methylpyrrolidone (NMP, 99+%) and dimethyl sufoxide (DMSO, 99+%) were purchased from Alfa aesar Chemical Company. Bis(4-fluoro-3-sulfonated phenyl)-(3-sulfonated phenyl) phosphine oxide trisodium salt (TSBPO, 99+%) was provided by Yanjin Technology Company and dried under vacuum before copolymerization. All other solvents and
Synthesis and characterization of sulfonated polymers
A series of poly(arylene sulfide sulfone) copolymers containing trisulfonated triphenylphosphine oxide moieties were prepared via a standard K2CO3-catalyzed aromatic nucleophilic substitution polycondensation using TSBPO and DFDPS with TBT, as shown in scheme 1. The copolymers were denoted as TS-PASS-xx, where the “xx” is the molar percent ratio of TSBPO. The feed ratios for monomers were given in Table 1. NMP was used as the solvent and toluene was employed for the removal of water produced
Conclusions
A series of sulfonated poly(aryl sulfide sulfone)s containing trisulfonated triphenylphosphine oxide moieties (TS-PASS-xx) were successfully prepared by the nucleophilic aromatic substitution reaction using TSBPO, DFDPS and TBT as starting materials. The IEC values (1.15–1.60 mequiv./g) of the obtained copolymers can be readily controlled by adjustiing different ratio of TSBPO and DFDPS. Their membranes exhibited moderate water uptake and low swelling ratio with the values of 25.6%-49.1% and
Acknowledgement
This work was supported by the National Natural Science Foundation of China (No. 21404016), Natural Science Foundation of the Jiangsu Higher Education Institutions (No. 13KJB430003), the Opening Project of State Key Laboratory for Modification of Chemical Fibers and Polymer Materials (No. LK1222), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions of China.
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