Sulfonated PEEK and fluorinated polymer based blends for fuel cell applications: Investigation of the effect of type and molecular weight of the fluorinated polymers on the membrane's properties

This work clearly demonstrates the effect of the type and molecular weight of the fluorinated polymer of SPEEK/Fluorinated polymer blends for low temperature (<80 degrees C) Fuel Cell Applications. Comparisons with trademarks (e.g., Nafion (R)) suggests that the membranes we have prepared in this study have good compatibility in all application respects. Membranes were prepared by solution casting method from four different fluorinated polymers; poly (vinylidene fluoride) with three different molecular weights (PVDF, M-w: 180.000, M-w: 275.000, M-w: 530.000); Poli(vinylidene fluoride-co-Hexafluoro propylen) (PVDF-HFP M-n:130.000) and sulfonated poly(ether ether ketone) (SPEEK) with sulfonation degree (SD) of 70. The sulfonation degree (SD) of SPEEK was determined by FTIR, H-1 NMR and ion exchange capacity (IEC) measurements. Thermo-oxidative stability and proton conductivity of the membranes were determined by using thermal gravimetric analysis (TGA) and BT-512 BekkTech membrane test systems, respectively. Chemical degradation of SPEEK membranes was investigated via Fenton test. The morphology of the membranes were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Water uptake and proton conductivity values decreased with the addition of fluorinated polymers (PVDF, PVDF-HFP) as expected, but proton conductivity values were still comparable to that of Nafion 117 (R) membrane. Addition of fluorinated polymers improved chemical degradation of the blend membranes in all ratios while addition of PVDF-HFP to the SPEEK70 caused phase separations in all ratios. Methanol permeability value of SPEEK70/PVDF(M-w = 275.000) blend membrane (3.13E-07 (cm(2)/s)) was much lower than Nafion 117 (R) (1.21E-06 (cm(2)/s). PVDF addition to the SPEEK polymers caused increase in elongation of the membranes. Increase in the molecular weight of the PVDF did not show any effect on the Young modulus, but resulted in high elongation values. On the other hand, increasing PVDF content of the blend membranes caused lower elongation values at break and didn't have any effect on the Young modulus. The gas permeability values of SPEEK70/PVDF type blend membranes were lower than that of the Nafion (R). Hydrogen and oxygen permeability values were one-tenth and one-fifth of the Nafion (R), respectively. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.