Cross-Linked Ionomer Membranes for High Performance Hydroxide Exchange Membrane Fuel Cells

Cristiani, Thomas
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University of Delaware
A photopolymerizable thiol-ene ionomer membrane with a variety of potential applications, most notably use in hydroxide exchange membrane fuel cells (HEMFCs), has been developed. Multiple synthesis pathways were explored, and a novel, di-functional, allyl monomer has been reproducibly synthesized using a simple, three-reaction method.[13,14] The monomer, 2,2-bis(4-allyloxy-3- phosphoniumylmethyl)phenyl-propane chloride (BAPPQP-Cl) can be subsequently co-polymerized with 1,6-hexanedithiol (HDT) and 1,3,5-Triallyl-1,3,5-triazine- 2,4,6(1H,3H,5H)-trione (TATATO) in dimethylsulfoxide (DMSO) to form a crosslinked ionomer network. Fabrication of this network has been guided by studying the resulting mechanical properties and conversion using tensile testing and FTIR spectroscopy. Tensile testing reveals that all networks are mechanically strong (for elastomers) with moduli in the hundreds of MPa range and durable with elongations at break greater than 200% (and as high as 900%) for all samples. It is shown however that mechanical properties show an inverse relationship with crosslink density in the membranes. This is most likely caused by non-covalent, ionic crosslinks between the phosphonium pendant groups and potential crystallization in the polymer networks.[15,16,17,18] Crystallization can also explain some features seen in the tensile testing results. FTIR spectroscopy also confirms that the membranes have high chemical conversions. Further experimentation is proposed to help fully understand the mechanism by which this network polymerizes as well as the membranes’ potential performance in a fuel cell.