Blends based on sulfonated poly[bis(benzimidazobenzisoquinolinones)] and poly(vinylidene fluoride) for polymer electrolyte membrane fuel cell
Introduction
Polymer electrolyte membranes (PEMs) constitute an important class of polymeric materials for use as ion-exchangers, electrolytes for batteries and sensors, and dopants for electronic conductors. Recently, studies on PEM materials have been strongly promoted as a result of a considerable interest in the development of high-performance polymer electrolyte membrane fuel cells (PEMFCs) for transportation, stationary and portable power applications [1], [2]. In order for a fuel cell to work effectively and to be widely adapted, the PEM must present a portfolio of properties including an acceptable cost, a high proton conductivity, good chemical and thermal stabilities, a decent mechanical strength, and a low fuel crossover [3].
Nafion (from Dupont) is a perfluorosulfonated membrane material with a hydrophobic fluorocarbon backbone and hydrophilic sulfonic pendent side-chains, and is capable of satisfying a large number of the requirements concerning polymer electrolyte membranes. Nafion has found practical use in PEMFCs [4]. However, various limitations exist for Nafion membranes, and these include a very elevated cost, a high fuel permeability, and the loss of the preferable properties at temperatures >80 °C [5], [6], [7], [8], [9].
In view of this, numerous efforts have been focused on developing more economical alternatives based on non-fluorinated or partially fluorinated polymers. Up to now, a large number of sulfonated aromatic polymers such as poly(ether ether ketone) [10], [11], [12], [13], [14], poly(ether sulfone)s [15], [16], [17], [18], poly(arylene ether)s [19], [20], polyimides [21], [22], [23], [24], [25], [26] and poly(p-phenylene)s [27], [28], [20] have been developed. Generally, such polymer membrane materials for PEM fuel cells rely on the presence of absorbed water and its interaction with acid groups to produce proton conductivity. To ensure high proton conductivity, polymers presenting significant ion-exchange capacities (IECs) have been utilized, but these materials were found to swell excessively in water (thereby losing their mechanical properties) and were moreover brittle when dry.
Previous papers [29] have reported on poly[bis(benzimidazobenzisoquinolinones)] ionomers, a class of ladder polymers, that are hydrolytically and dimensionally stable. This enhanced stability toward water has been attributed to the acid–base interactions between the pyridinone ring and sulfonic acid groups. However, the proton conductivity and oxidative stability for practical applications requires further improvement. The present study thus attempts to investigate the possibility of obtaining, through blending, a membrane demonstrating high proton conductivity as well as high oxidative stability.
Polymer blending has been known to be the most frequently used means of overcoming the shortcomings of an individual polymer and of obtaining inexpensive materials with desirable properties by combining the advantages of two or more individual polymer components. A key factor in acquiring polymer electrolyte membranes via blending relies on finding an optimum combination of thermal, chemical and mechanical properties with hydrophilicity in the employed polymers. Among various counter-polymers, poly(vinylidene fluoride) (PVDF), a semi-crystalline and chemically resistant material, is well suited for the fuel cell environment [30]. Blends of PVDF with Nafion [31], [32] and sulfonated poly(ether ether ketone) [33], [34] have been prepared for potential use in a direct methanol fuel cell. PVDF also demonstrates sufficient dimensional stability and mechanical strength. Thus, a successful combination of the conducting characteristics of SPBIBI and the physical stability of PVDF should be able to be achieved provided that it is possible to obtain a miscible blend of the two materials.
The present article reports on a blend of SPBIBI with PVDF for use as a proton exchange membrane for fuel cells. The effect of the PVDF content on water uptake, swelling ratio, oxidation stability, proton conductivity and methanol permeability of the prepared SPBIBI/PVDF membranes were investigated to evaluate their potential applications in direct methanol fuel cells (DMFCs).
Section snippets
Materials
PVDF was purchased from Aldrich having a weight average molecular weight (Mw) of 530,000. Dimethylsulfoxide (DMSO) was dried over CaH2, then distilled under reduced pressure, and stored over 4 Å molecular sieves under nitrogen in the dark. 4,4′-Binaphthyl-1,1′,8,8′-tetracarboxylic dianhydride disulfonic acid (SBTDA) was prepared according to a previously reported method [35]. All other reagents were obtained from commercial sources and used as received.
Synthesis of SPBIBI
The sulfonated
Compatibility of the blend
The optical clarity of the membranes was visually inspected as a preliminary means of identifying the compatibility between the polymers. PVDF, a semi-crystalline and chemically resistant polymer, is white and opaque. The blend membranes were denoted as SPBIBI/PVDF(χ), where χ is the weight ratio of PVDF in the feed. As shown in Fig. 1, all blend membranes were transparent, and when increasing the content of PVDF, the color turned from burgundy to light red.
The compatibility of the two polymer
Conclusions
Partially fluorinated membranes based on blends of SPBIBI and PVDF were successfully prepared with a solution casting method. The results of WXRD and DSC indicated that the blend membranes showed a decent compatibility within a broad range of compositions up to approximately 70% of PVDF. The blends could produce good ion conduction channels especially when the blend contained 10% of PVDF. This gave rise to a high proton conductivity. The dimensional stability, the elongation at break, the
Acknowledgements
We thank the National Basic Research Program of China (No. 2009CB623401), the National Science Foundation of China (Nos. 50673087 and 50825303) and the Development of Scientific and Technological Project of Jilin Province (No. 20080620) for the financial support.
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