The modification of aromatic polymers, such as poly(ether ether ketone) (PEEK), by sulfonation modification, can result in fabricating polyelectrolyte membranes (PEMs) as the alternatives to Nafion for direct methanol fuel cell (DMFC) applications. Due to the effective role of nanomaterials in reducing the permeability in nanocomposites, the addition of natural or organically modified montmorillonite (OMMT) nanofillers to the sulfonated matrix, with the optimum degree of sulfonation, can reduce the methanol permeability and increase the efficiency of the fuel cell.
PEEK was sulfonated at various degrees in solution state. Based on the selectivity parameter, the optimal degree of sulfonation (DS) was introduced. In order to prepare the nanocomposite membranes, using an ultrasonic agitator, different amounts of MMT and OMMT (Cloisite 15A or chitosan-modified MMT (CMMT)) nanofillers were added to the sulfonated polymer with optimal DS, and the resulting mixtures were cast. In this study, the ion exchange capacities (IEC) of the membranes were measured. The selectivity parameter (as ratio of proton conductivity to methanol permeability) at 25°C, as well as DMFC performance at 25°C and 1M feed of methanol for different membranes were determined and the results were compared with those of Nafion 117.
The optimum DS for sulfonated poly(ether ether ketone) (SPEEK) was 62%. X-ray diffraction (XRD) patterns proved that nanoclays were exfoliated in the structure of nanocomposites at small loading weight of 1% (by wt). The proton conductivity and methanol permeability, as well as the performance test, showed that SPEEK/CMMT-based nanocomposite membranes have the highest maximum power generation density compared to other nanocomposite membranes or Nafion 117. Accordingly, SPEEK/CMMT polymer electrolyte membranes are promising candidates for direct methanol fuel cell (DMFC) applications.
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