Cross-linked polybenzimidazole membranes for high temperature PEM fuel cells

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2018
Özdemir, Yağmur
Literature studies have shown that it is desirable to increase operation temperature of Proton Exchange Membrane Fuel Cells (PEMFCs) due to the reasons like reduced fuel impurity sensitivity, fast electrode kinetics, simple thermal and water management. However, during fuel cell operation at high temperatures, the PEM suffers from inevitable leaching out of the doped acid, which can have deteriorating effect on the membrane performance. Thus, there is always a need to minimize this problem by making ways so that the PBI based membrane can retain sufficient acid even at elevated temperatures. In this study, developing cross-linked PBI membranes for High Temperature PEMFC (HT-PEMFC) membranes with enhanced acid retention capability and better HT-PEMFC performance is the primary goal. Significant progress was made regarding the understanding of polybenzimidazole (PBI) chemistry. Extensive work has been conducted concerning preparation and characterization of covalently cross-linked PBI membranes. PBI membranes cross-linked by BADGE, DBpX, EGDE and TPA were prepared and their properties and performances were studied comperatively along with pristine PBI membrane. Membranes were characterized using acid doping, and acid leaching, proton conductivity, extraction in DMAc, SEM, TGA, and FTIR analyses in order to investigate the influences of cross-linking. The membranes prepared with TPA, BADGE and DBpX were tested in a single cell HT-PEMFC test unit. The membranes prepared with EGDE cross-linker was not studied further after the observation of nonpromising results. HT-PEMFC tests were conducted with dry air and H2 as reactants, at 1650C. The pristine PBI membrane based MEA reached to 0.085 W.cm-2 maximum power density and 0.081 A.cm-2 current density at 0.6 V. PBI-TPA-5 has shown the poorest performance among all the membranes with 0.051 W.cm-2 maximum power density and 0.048 A.cm2 current density. PBI-DBpX-3 membrane gave 0.106 W.cm-2 power density and 0.100 A.cm-2 current density. The current density for PBI-BADGE-5 was found to be 0.121 A.cm-2 and maximum power density was determined as 0.123 W.cm2. This is the highest performance obtained in this study. PBI-BADGE membranes were further investigated in order to determine the influence of cross linker content. Two additional sets of membranes with different cross linker content (2.5 and 7.5%) were prepared, characterized and tested. PBI-BADGE-7.5 showed maximum power density of 0.019 W.cm-2, while PBI-BADGE-2.5 showed maximum power density of 0.027 W.cm-2. After these results, PBI-BADGE-5 was found as the best performing membrane.

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Citation Formats
Y. Özdemir, “Cross-linked polybenzimidazole membranes for high temperature PEM fuel cells,” M.S. - Master of Science, Middle East Technical University, 2018.