Preparation And Properties Of Proton Conductive Polymers Containing Tetrazole Moieties For High-temperature Fuel Cells

Proton exchange membranes fuel cells （PEMFCs） have been attracting more and more attentionat present for its fast staring and no pollution. The proton exchange membranes （PEM） is the keycomponent in PEMFCs. Perfluorinated sulfonic acid membranes used at low temperature such hvebeen commercialized. However, these membranes excessively depend on water supply which limitsthe operation temperature below100℃. At low temperature, CO from the fuel gas can poison thecatalyst and reduce the efficiency of the fuel cells. So the high-temperature PEMFC is highlyrecommanded. N-containing heterocyclic polymers are thermally stable and the heterocycle rings areboth proton carriers and conductors in the proton transfer process. Thus, they become the focus inhigh-temperature PEM research. The tetrazole polymers are more advantageous for proton conductionbecause they have four N atoms and several tautomers. However, because of the low ionization degree,the pristine tetrazole polymers have very low conductivity. So in order to overcome this shortcoming,the acid groups with high ionization degree（e.g sulfonic acid） needed to be introduced. In thiscontribution, poly（5-vinyl tetrazole）s are modified by blending with sulfonated polymers andsulfonation to achieve higher conductivity at high-temperature. The specific process was as follows:（1）The polyacrylonitile （PAN） with high molecular weight was prepared from acrylonitrile as theraw material by emulsion polymerization and the poly5-vinyl tetrazole （PVTra） with a tetrazoliumconversion rate up to85%, was prepared by Click[3+2] principles of cyclization. The H in thetetrazole in highly active and can be replaced easily. Poly2-（3-sulfonic acid propyl）-5-vinyl tetrazole（SPVTra） with different sulfonation degrees were thus prepared from poly（5-vinyl tetrazole） and1,3-propane sultone. The structures of PAN、PVTra and SPVTra were characterized by FTIR and1HNMR, respectively.（2）The sulfonation degree of72%sulfonated polystyrene （SPS） was prepared from styrene as theraw material, then blended with PVTra by solution casting method. The proton exchange membranesof PVTra（SPS）0.5、PVTra（SPS） and PVTra（SPS）₂were obtained. Their performance used forhigh-temperature PEMs were preliminarily evaluated. The results showed that the blend membraneshave improved ion exchange capacity （IEC）, mechanical properties and conductivity at hightemperature. But blending can cause phase separation and the water uptake of the blend membranesare high. However, it is worth noting that the conductivity of PVTra（SPS）₂can reach to1.23×10^-2 S/cm at100℃and4.40×10^-2S/cm at120℃without extra humidity supply respectively, which arecomparable to the conductivity of Nafion at low temperature with high moisture.（3）The proton exchange membranes with three different sulfonation degrees, SPVTra-20、SPVTra-40and SPVTra-55, were prepared by solution casting method. Then the performance usedfor high-temperature PEMFC was evaluated. The results showed that the introduction of propylsulfonic acid improved the ion exchange capacity of the membranes （IEC）, but the mechanicalproperties decreased. The thermal decomposition temperature and glass transition temperatures arearound200℃. The conductivity for the SPVTra-55at120℃without extra humidity supply reachesto3.74×10^-3S/cm.