Crosslinked Sulfonated Polystyrene-block-poly（ethylene-ran-butylene）-block-polystyrene as Proton Exchange Membrane
Date Issued
2011
Date
2011
Author(s)
Wang, Chih-Ting
Abstract
The major purposes of this thesis are to prepare crosslinked sulfonated poly(styrene-block-(ethylene-ran-butylene)-block-styrene) (sSEBS) membranes as the proton conducting membranes for proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) as well as to investigate the properties and the microstructure of the crosslinked membranes. Two parts of studies are presented using non-sulfonated (Part I) or sulfonated (Part II) multi-functional alcohols as crosslinkers. Crosslinking occurs within the sulfonated polystyrene (sPS) domains through the formation of sulfonate esters between the hydroxyl groups of the crosslinkers and the sulfonic acids of sPS via thermal curing. In each part, the dependence of transport properties, thermal stability, mechanical strength, chemical stability and morphology of the membrane on the chemical structure and the loading amount of the crosslinkers are systematically studied.
In the first part, the crosslinkers with different number of hydroxyl groups, including glycerol, D-mannitol and poly(vinyl alcohol) ( PVA ). The first two alcohols are classified as low molecular weight crosslinkers and PVA is classified as polymeric crosslinker.The introduction of these crosslinkers is demonstrated to improve the thermal stability, mechanical strengths and chemical stability and the enhancements are found to relate to the chemical structure and the loading amount of the crosslinkers. The microstructure of the crosslinked membranes are similar to that of the prisitine sSEBS membrane when glycerol and mannitol are used regardless the loading amount while the addition of PVA with high loading amount alters the microstructure. Ion exchange capacity and proton conductivity decrease with increasing loading of crosslinkers due to the loss of sulfonic acids to form crosslinking and the use of PVA leads to the most significant decrease comparing with pristine sSEBS. The use of glycerol and PVA provide the crosslinked membranes suppressed methanol crossover and improved selectivity. The crosslinked membranes also show much improved proton conductivity at elevated temperature and low relative humidity, probably owing to the better water retention within the membrane attributed to the presence of crosslinks and the hydroxyl groups of the crosslinkers. All the transport properties show complicated trends against the chemical structure and the loading amount of the crosslinkers.
In the second part, a novel bifunctinoal alcohol containing two sulfonic acids is designed and synthesized as the crosslinker. Interestingly, IEC and proton conductivity of the corresponding crosslinked membrane are almost constants regardless the loading amount of the crosslinkers, indicating the use of this novel crosslinker could compensate the loss of sulfonic acids originating from the crosslinking. The resulted membranes also exhibit lower methanol permeability and enhanced selectivity. In addition, these membranes also show improved chemical resistance comparing with pristine sSEBS.
Subjects
fuel cell
proton exchange membrane
crosslink
block copolymer
Type
thesis
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