Acid Doped Polybenzimidazole Membranes for High Temperature
Proton Exchange Membrane Fuel Cells
Author: Ahmet Ö / zgü / r Yurdakul
One of the most popular candidates for high temperature PEMFC&rsquo / 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.
| Identifer | oai:union.ndltd.org:METU/oai:etd.lib.metu.edu.tr:http://etd.lib.metu.edu.tr/upload/2/12608506/index.pdf |
| Date | 01 July 2007 |
| Creators | Yurdakul, Ahmet Ozgur |
| Contributors | Bac, Nurcan |
| Publisher | METU |
| Source Sets | Middle East Technical Univ. |
| Language | English |
| Detected Language | English |
| Type | M.S. Thesis |
| Format | text/pdf |
| Rights | To liberate the content for METU campus |
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