Fuel cells find application in transportation, and in stationary and portable power generation. Among them, Direct Methanol Fuel Cells (DMFC) are attracting considerable attention due to the fact that it can be operated using liquid methanol as a fuel, both for transportation as well as for portable power generation. DMFC, however, faces today two major technical problems. One is slow methanol anode oxidation kinetics, and the other is methanol crossover (diffusion) across the polymer electrolyte membrane. This causes a decrease in the fuel cell voltage, and loss in fuel cell efficiency. Commercially available membranes like Nafion® are very expensive; it has been found, furthermore that sometimes over 40% of the methanol is lost through crossover during fuel cell operation. [1 ] Our study aims at developing a new, cost-effective membrane with superior methanol barrier properties and reasonable proton conductive properties; these are composite membranes that are made by incorporating a layered material (hydrotalcite (HT)) into the matrix of a sulfonated polymer, namely Sulfonated Poly Ether Ether Ketone or SPEEK. These SPEEK-HT hybrid membranes show good properties and are potential candidates for replacement of Nafion®.[2, 3]

During the study, several SPEEK membranes were synthesized by sulfonation from PEEK by varying the reaction time at room temperature. SPEEK membranes were casted on Petri dishes using DMAc as the solvent and were characterized using several analytical techniques. We have also investigated the methanol permeability (P), proton conductivity (C), and selectivity (defined as the ratio of C/P.) for both the SPEEK as well as SPEEK-HT hybrid membranes. The methanol permeability of the hybrid SPEEK-HT membranes was around 4 ~ 18 times smaller than that of Nafion®. On the other hand, SPEEK and SPEEK-HT membranes showed proton conductivity which is 3 ~ 5 times smaller than that of Nafion®'s. When comparing the conductivity of SPEEK-HT membranes to that of SPEEK membranes it was found to be up to 6 times higher at maximum. The membrane properties depend strongly on sulfonation degree and the fraction of hydrotalicte in the hydrid membranes. Therefore, the selectivity (C/P) of our membranes was generally comparable to that of the commercially available Nafion® membranes, and therefore these membranes are promising candidate for proton exchange membrane systems for fuel cell applications.

References 1. V. Tricoli, “Proton and methanol transport in poly(perfluorosulfonate) membranes containing Cs+ and H+ cations”, J. Electrochem. Soc., 145(11), 3798 (1998) 2. P. Xing et al., “Synthesis and characterization of sulfonated poly(ether ether ketone) for proton exchange membranes”, J. Memb. Sci., 229 (2004) 95-106 3. B. Liu et al., “ Fluorenyl-containing sulfonated poly(aryl ether ether ketone ketone)s (SPFEEKK) for fuel cell applications”, J. Memb. Sci., 280 (2006) 54-64