Thiol-ene Photo Crosslinking Polymerization as an Alternative Approach to Developing Hydroxide Exchange

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University of Delaware
An alternative and new approach to making hydroxide exchange membranes (HEM’s) for hydroxide exchange membrane fuel cells (HEMFC) via thiol-ene photocrosslinking polymerization is demonstrated. Specifically a novel difunctional vinyl monomer, quaternary phosphonium bisphenyl allyl ether chloride (QPBPAE-Cl), was successfully synthesized in a simple and cost effective three-step process. This synthesis protocol enables the interchangeability of the cationic functional group in the last synthesis step to readily assess hydroxide conductivity differences. This novel charged monomer was successfully photo-crosslinked in a ternary system consisting of 1,6-hexanedithiol and 1,3,5-Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO, a trifunctional ‘ene’ for crosslinking) to form the HEM. While the formed HEMs exhibit low hydroxide conductivity, they possess robust mechanical properties and excellent alkaline stability in 1M KOH for 48 hrs. The hydroxide conductivity for 200 – 290 μm thick specimens was 1.4 and 2.6 mScm-1 for a QPBPAE-Cl to TATATO ratio of 1:1.5 and 1:2, respectively, and a stoichiometric balance of thiol monomer. The swelling ratio of the HEM was 8.25% and 8.89% in water and the measured tensile strength was 57.73 and 45.28 MPa for a QPBPAE-Cl to TATATO ratio of 1:1.5 and 1:2, respectively.