Density and water uptake were measured for a series of 3M ionomer membranes with different acid groups and equivalent weight (ew). We have developed a systematic method for measuring the density of hydrated membranes and assessed its statistics. In gas pycnometry, a pressure change is measured upon inserting a sample into a calibrated volume of gas. The density of several perfluorosulfonic acid (PFSA) membranes was measured as a function of water content. We also calculate the partial molar volume of water in different polymer electrolytes from the density data. The partial molar volume of water and polymer components is reported for several membranes, including 3M's perfluoroimide acid (PFIA), ortho bis-acid and perfluorosulfonic acid (PFSA) with different equivalent weights.
The proton conductivity and water uptake were measured for a series of 3M ionomer membranes with various equivalent weights (EWs) and different acid groups. For the EW variation, the conductivity of PFSA samples with EW as low as 580 g/eq were studied. Results show that conductivity increases with decreasing EW. In particular, at lower EW conductivity dramatically increases at low RH. For 580 EW at 120 o C, the conductivity exceeds the target set by DOE for automotive fuel cells (100 mS/cm). The effect of water content is investigated through water uptake measurements that showed the clear dependence of conductivity on lambda ( ). Moreover, the nature and effects of changes in the morphology are studied with simulations and NMR relaxation and diffusion measurements probing the effect of water content on long vs. short range motions. For the different acid groups, conductivity and water uptake of polymers with sulfonic acid, sulfonyl amide and asymmetric sulfonyl imide end groups were compared.
4,5-Dicyano-1H-[1,2,3]-Triazole (DCTz) is a possible water replacement for proton transport in high temperature polymer electrolyte membranes since it exhibits a favorable proton affinity. In this study, some properties of DCTz doped with Trifluoromethanesulfonic Acid (TFMSA) and Heptadecafluorooctanesulfonic acid (C8HO3F17S-HDSA) are investigated. Thermal analysis as well as FTIR data indicated the formation of the salts. After proving to be stable up to 140°C, DCTz, DCTz_TFMSA and DCTz_HDSA salts were formed into membranes in a Polyacrylonitrile (PAN) polymeric binder. Thermogravimetric analysis (TGA) showed that adding the acid increases the stability of the membranes. Electrochemical measurements showed that the acid loading increases the conductivity of these polymeric membranes. Thus, a DCTZ_TFMSA doped PAN membrane has higher conductivity than a DCTZ doped PAN membrane over a temperature range of 20°C to 160°C at low relative humidity (RH). Similarly, increasing the weight % of DCTz_TFMSA in PAN membranes leads to an improved conductivity by an order of magnitude.
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