Membranes from Blended Ionomer/PVDF Nanofibers: II. Interplay Between Properties and Electric Response

Fact of Fiction: Do ion-exchange membranes really require complete water saturation in order to possess desirable conductivity?

The fabrication of new proton-conducting membranes with a high conductivity at high temperature and low relative humidity conditions is a critically important research challenge for fuel cell developers [1]. Similarly, high conductivity and low crossover membranes are needed in redox flow batteries. Recently, membranes were made by nanofiber electrospinning solutions of perfluoroionomer + PVDF, followed by hot-pressing the resulting blended fiber mat into a dense membrane.  These films have unusual and highly desirable properties for use in hydrogen/air fuel cells and/or aqueous flow batteries. In particular, some blended fiber membranes exhibit unusually high proton conductivity at very low water contents. The effect of processing on the complex interplay between the chemical composition, structure and conductivity mechanism of these innovative membranes is still poorly understood. Here we present a complete analysis of the physical-chemical properties as well as the conductivity mechanism of Nafion-PVDF (90wt%/10wt%) membranes obtained from a single fiber mat.  These results are contrasted with data collected using a Nafion/PVDF blended membrane prepared by conventional solvent casting.

The membranes are extensively characterized by thermoanalytical techniques (water uptake determination, HR-TGA, MDSC, DMA), vibrational spectroscopy (FT-IR ATR) [3], and broadband electric spectroscopy (BES). A Nafion-PVDF (90wt%/10wt%) membrane obtained from an electrospun single  fiber mat shows a very low level of water (3 w%) and exhibits high conductivity. The two polymers are found to be well mixed with chains of Nafion surrounding those of PVDF. In addition, there is no evidence of bulk water domains but rather the water molecules are aligned along the polymer chains facilitating a fast proton transfer mechanism. The added benefit of the low water uptake is that there is negligible swelling of the membrane, rendering membranes of this type suitable for a variety of applications.

BES measurements are carried out from -100 to 120°C and from 0.03 to 10⁷Hz and are able to elucidate different kind of conductivity mechanisms. The electric response of the Nafion-PVDF (90/10) mat (Figure 1a) and the blend fiber membrane (Figure 1b) are very different from those of Nafion [4], while that obtained from solvent casting (Figure 1c) is similar to Nafion for T>0°C and to mat membrane at T<0°C.

Acknowledgements

This research was funded by the Strategic Project of the University of Padua “Materials for Membrane-Electrode Assemblies to Electric Energy Conversion and Storage Device (MAESTRA)” and by Veneto Nanotech S.C.p.a. (Venice).

REFERENCES

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