Quaternized cardo polyetherketone anion exchange membrane for direct methanol alkaline fuel cells
Introduction
Recently alkaline direct methanol fuel cells (ADMFCs) have attracted more and more research interest. Compared with the traditional proton exchange membrane fuel cells (PEMFCs), the ADMFCs have lower methanol crossover, higher catalyst efficiency and lower cost [1], [2], [3], [4]. Many investigators have developed various exchange membranes for application in ADMFCs [5], [6], [7], [8], [9], [10], [11], [12], [13].
There are many ways to prepare anion exchange membranes, such as radiation grafting of styrene onto polymer films [7], [8], dipping with an alkali metal hydroxide [13], and electrophilic substitution [5], [6]. Chloromethyl groups can be introduced onto the phenyl ring of the aromatic compounds via the Blanc chloromethylation reaction. Following this reaction, quaternary ammonium groups can be introduced into the aromatic compounds via quaternization and alkalization and hence an anion exchange membrane is produced. Many polymers, such as polysiloxane [14], polyarylether sulfone (PSF) [15], poly(phthalazinon ether sulfone ketone) (PESK) [11], cardo polyethersulfone (PES-C) [9], poly(ether-imide) [16], have been used in this manner to produce anion exchange membranes for application in ADMFCs. Chloromethylether (or bichloromethylether), paraformaldehyde (CH2O)n, hydrochloric acid (HCl) gas, chloromethyl alkyl ether, etc. are normally used for chloromethylation [11], [17], [18], [19], [20], [21], [22], [23]. Of those chloromethylation reagents, chloromethylether and bichloromethylether are toxic and carcinogenic, and chloromethyl alkyl ether needs to be produced from chloromethylether. In order to avoid using those toxic substances and to simplify the reaction route, we used paraformaldehyde and HCl gas as the chloromethylation reagents. Fang and Shen [11] prepared quaternized poly(phthalazinon ether sulfone ketone) anion exchange membrane using paraformaldehyde and HCl gas as the chloromethylation reagents.
Cardo polyetherketone (PEK-C) is an excellent resistance polymer with strong mechanical properties and good thermal stability. With the bulky cardo group on a side chain, PEK-C can dissolve in some organic solvents, such as NMP, DMF, CHCl3 to produce membranes with ease. Based on the above advantages, PEK-C membranes have potential applications as functional materials, such as for pervaporation and fuel cells [24], [25], [26]. Basile et al. [26] prepared sulfonated PEEK-WC (another name for PEK-C) membranes, and applied them in an H2/air fuel cell. The sulfonated PEEK-WC membranes exhibit comparable electrochemical performance. These studies show the ideal potential applications of PEK-C materials for fuel cells.
To our knowledge, quaternized PEK-C membranes have not been reported as having been used for alkaline direct methanol fuel cells. The objective of this work was thus to prepare QPEK-C membranes, using PEK-C as a starting material for the quarternized membranes, for application in alkaline direct methanol fuel cells.
Section snippets
Materials
Cardo polyetherketone (polymerization degree of 101) was supplied from Xuzhou Engineering Plastic Factory (China). N-methyl-2-pyrrolidone (NMP) and paraformaldehyde (CH2O)n were purchased from the Shanghai Chemical Reagent Store (China) and used as received.
Chloromethylation of PEK-C
The chloromethylation reaction of the PEK-C was carried out according to the scheme reported in the literature [11], [21], [22], [23], where the chloromethyl groups were successfully introduced onto the aromatic ring. The ketone group is
Confirmation of the reaction
The chloromethylation reaction was confirmed by 1H NMR. Fig. 1 shows the 1H NMR spectra of the PEK-C (a) and CPEK-C (b and c). The protons associated with benzene ring of PEK-C structure appear at 7–8 ppm, which can be found on both the spectra of the PEK-C and CPEK-C. Four new peaks appeared in the 1H NMR spectrum of the CPEK-C. There is a five-fold multiplet centered at 2.0 ppm, two three-fold multiplets centered at 2.4 and 3.5 ppm, and finally two singlet peaks at 2.2 and 2.8 ppm. There is
Conclusions
In this paper, a novel anion exchange membrane, namely quaternized cardo polyetherketone (QPEK-C), was prepared based on polyetherketone (PEK-C). The conductivity of the QPEK-C membrane was 5.06 × 10−3 S cm−1 at 60 °C. The methanol permeability, being less than 10−9 mol cm−2 min−1, suggests that the QPEK-C membrane has excellent methanol resistance. This performance criterion indicates that the QPEK-C membrane has a strong potential for application in alkaline methanol fuel cells.
Acknowledgements
The support of National Nature Science Foundation of China Grant No.50573063, the research fund for the Doctoral Program of Higher Education (No.2005038401) and Program for New Century Excellent Talents in University in preparation of this article is gratefully acknowledged. The authors are grateful to Prof. James R. Bolton from the Department of Civil and Environmental Engineering at the University of Alberta for his kind assistance with our English writing.
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