Proton-Conducting Cross-Linked Sulfonated Aromatic Polymers for Fuel Cells Application
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Proton-Conducting Cross-Linked Sulfonated Aromatic Polymers for Fuel Cells Application B. Maranesi,1 L. Pasquini,1 M. Khadhraoui,2 P. Knauth,2 M. L. Di Vona1 1
2
University of Roma Tor Vergata, Dip. Scienze Tecnologie Chimiche, 00133 Roma, Italy Aix-Marseille Univ, Laboratoire Chimie Provence, Centre St Jérôme, 13397 Marseille, France
Abstract Thermal stability, hydration and mechanical properties of thermally cross-linked Sulfonated Aromatic Polymers (SAP) with high ionic exchange capacity (IEC) were measured and compared to untreated samples. The formation of cross-linking greatly stabilizes SAP in terms of thermal, mechanical, and hydrolytic degradation: they can resist in water even at a temperature of 145 °C with improved mechanical properties. Acid-base titration and FTIR spectra consistently indicate that SAP microstructure stabilization is related to cross-linking of the polymer chains by SO2 bridges, which is promoted by temperature. Introduction Sulfonated aromatic polymers (SAP) are promising alternative membrane materials to perfluorinated membranes operating up to 80-90°C [1]. It is desirable to operate in the range of temperature between 120 °C to 140 °C to enhance the conductivity of membranes, reduce anode poisoning and improve the fuel oxidation kinetics [2]. The most important examples of SAP are sulfonated poly(ether ether ketone) (SPEEK), sulfonated poly(phenyl sulfone) (SPPSU) and sulfonated poly (ether sulfone) SPES, with high degrees of sulfonation (DS) required for appropriate proton conductivity [5] (Figure 1). However, this lowers the mechanical and morphological properties and leads to highly swellable or even water-soluble products [6,7]. A powerful method to improve solvent resistance, dimensional stability and mechanical strength, maintaining a locally high density of functional groups such as sulfonic acid groups is the formation of covalent cross-links between the macromolecular chains. A direct cross-linking reaction performed in situ during the casting procedure is certainly an interesting and promising methodology to obtain stable and long-life membranes.
O
O
O
SO3H
O
O
S
S O
O
SPES
S O
0.83
SO3H
0.17
O
SO3H
SO3H
SPPSU
O O
O
O
O C O
C O
0.83
0.17
SPEEK
Figure 1. Repeat unit for SPPSU with DS = 2, SPES with DS = 0,83 and SPEEK with DS = 0,9.
Furthermore, the introduction of a certain number of covalent bonds between adjacent polymeric chains by thermal treatment of cast membranes is really economic. Due to its simplicity, this method is very suitable for industrial preparation of cross-linked membranes, because the procedure can be easily up-scaled. In this work, we will show that thermal treatments of sulfonated aromatic polymers membranes performed above 150 °C can significantly improve thermal stability, hydration and mechanical properties: they can resist in water even at a temperature of 145 °C. These results are of significant importance for the improvement of proton-exchange membrane fuel cell. Experimental section Sulfonated poly(ether ether k
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