Fluorinated SPEEK Membranes For DMFC Applications

Issues such as high methanol crossover through the PFSA (perfluorosulfonic acid) membrane and its high cost are motivating research into alternative membranes such as SPEEK (sulfonated poly-ether ether ketone).  However, although SPEEK membranes show reduced methanol crossover, their chemical stability and ionic conductivity is lower than that of PFSA membranes such as Nafion^®.  The stability of Nafion and the acidity of its sulfonic group is due to the presence of strong electron-withdrawing fluorinated derivates on its backbone. The proximity of electron-accepting substituents such as fluorine to the sulfonic group increases the acidity of the Nafion membrane. However in the case of SPEEK membranes the sulfonic groups are attached directly to the polymer backbone and the acidity of the sulfonic group is lowered by the proximity of electron-donating constituents such as oxygen to the sulfonic groups (Figure 1). 

Our objective in this study is to improve the stability and acidity of the SPEEK membrane by modifying its functional groups.  We propose to modify the chemical structure of SPEEK membranes by increasing the separation between the sulfonic acid group and the main backbone by introducing short fluorinated carbon chains. Such a modified structure, shown in Figure 2, would be morphologically similar to that of Nafion.  The addition of sultones (hydrocarbon rings incorporating the sulfonic group) to PEEK will create the desired short hydrocarbon chains.

            Separating the sulfonic group from the polymer backbone would lead to a phase separation of the hydrophilic/hydrophobic phase. Fluorinated derivatives of such short hydrocarbon chains would also increase the acidity of the membrane. We have identified the Friedel-Crafts reaction of fluorinated sultones on PEEK in the presence of Nitrobenzene solvent to produce the molecule shown in Figure 2. After developing the polymer, the membranes will be prepared by casting them on a flat plate and drying them in an oven. The mechanical and electrical performance properties of the membrane will be evaluated and compared with SPEEK and Nafion membranes.