Acid doped polybenzimidazole membranes for high temperature proton exchange membrane fuel cells

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2007

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Yurdakul, Ahmet Özgür

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Acid Doped Polybenzimidazole Membranes for High Temperature Proton Exchange Membrane Fuel Cells Author: Ahmet Özgür Yurdakul One of the most popular candidates for high temperature PEMFC’s is phosphoric acid doped polybenzimidazole (PBI) membrane due to its thermal and mechanical stability. In this study, high molecular weight PBI was synthesized by using PPA polymerization. The stirring rate of reaction solution was optimized to obtain high molecular weight. The inherent viscosity of polymer was measured at four points in 96 percent sulphuric acid solution at 30 degree centigrade by using an Ubbelohde viscometer. The highest average molecular weight was found as approximately 120,000 using the Mark-Houwink equation. The polymer was dissolved in N,N-dimethylacetamide at 70 degree centigrade with an ultrasonic stirrer. The membranes cast from this solution were doped with phosphoric acid solutions at different concentrations. The doping levels of the membranes were 6, 8, 10 and 11 moles phosphoric acid/PBI repeat unit. The mechanical strength of the acid doped membranes measured by tensile tests were found as 23, 16, 12 and 11 MPa, respectively. Conductivity measurements were made using the four probe technique. The membranes were placed in a conductivity cell and measurements were taken in humidity chamber with temperature and pressure control. The conductivity of membranes was measured at 110, 130 and 150 degree centigrade in both dry air and water vapor. The highest conductivity was 0.12 S/cm at 150 degree centigrade and 33 percent relative humidity for the membrane doped with 11 moles of H3PO4. The measurements showed that conductivity increased with increasing doping and humidity. Moreover, membranes had acceptable conductivity levels in dry air.

Subject Keywords

Chemical engineering.

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http://etd.lib.metu.edu.tr/upload/2/12608506/index.pdf
https://hdl.handle.net/11511/16926

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Graduate School of Natural and Applied Sciences, Thesis

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Citation Formats

A. Ö. Yurdakul, “Acid doped polybenzimidazole membranes for high temperature proton exchange membrane fuel cells,” M.S. - Master of Science, Middle East Technical University, 2007.