Highly Sulfonated and Highly Cross-Linked Poly(ether Ether Ketone) Proton Exchange Membranes

Over the past decade, many efforts have been focused on the development of alternative hydrocarbon polymers to overcome the drawbacks of perfluoropolymers (e.g. Nafion)(1). Among these polymers, Sulfonated poly(ether ether ketone) (SPEEK) was shown to be of considerable promise, as it exhibits lower methanol permeability and more stability than that of Nafion. SPEEK is a typical water-assistant proton-conducting material. The proton conductivity of the membrane is highly correlated with the water uptake and strongly dependant on the degree of sulfonation. However, the mechanical properties of linear SPEEK tend to deteriorate upon acid loading via excess water uptake. Cross-linking is a simple and efficient way to suppress excess water uptake and enhance mechanical strength. Many cross-linked membranes have been prepared in different cross-linking methods[2-3]. However, the cross-linking based on the consumption of sulfonic acid groups leads to a decrease in ion content of the membrane and lower proton conductivity. In this work, we present an easily process for making high proton conductivity cross-linked proton exchange membrane. A functional SPEEK with defined structure was first offered by using the direct synthesis method. The free carboxylic group was activated with NHydroxysuccinimide and then reacted with Disulfonic acid-benzidine diamines to produce a series of crosslinked membranes (CSPEEK) (Figure1). One of our important findings is that the cross-linking density and proton conductivity can be improved simultaneously (Figure 2). At room temperature, the conductivity ranged from 0.48×10 to 0.58×10 S cm with increasing cross-linking density from 0% to 100%. At low cross-linking density (5%), the membrane shows a little decrease of conductivity, whereas at the crosslinking density higher than 5%, the CSPEEK membranes show high conductivity than non cross-linked SPEEK membrane. At the cross-linking density above 20%, the CSPEEK membranes show higher conductivity than Nafion117. By employing such a chemically cross-linked system, polymer electrolyte membranes with high conductivity and excellent mechanical strength can be achieved. Figure 3 and Figure 4 show the Small Angle X-ray Scattering (SAXS) curves of non-cross-linked SPEEK and CSPEEK-20%, respectively. The ionic clusters of SPEEK membrane increase from 3 nm to 6.3 nm with increasing temperature from 30 to 70 °C , suggesting that the membrane is not suitable for fuel cell membrane. While the ionic clusters of CSPEEK membrane only changed from 3.0 nm to 3.7 nm with temperature increasing from 30 to 90°C, suggesting that it is a promising candidate as a polymer proton exchange membrane in fuel cell applications. It can be inferred from the results that the CSEEK membranes are far robust than Nafion and can withstand higher operating temperature to prevent contaminants such as carbon monoxide from glomming onto the platinum catalyst. ACKNOWLEDGEMENTS