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Development of organic-inorganic composite membranes for fuel cell applications

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2007
Erdener, Hülya
Hydrogen is considered to be the most promising energy carrier of the 21st century due to its high energy density and sustainability. The chemical energy of hydrogen can be directly converted into electricity by means of electrochemical devices called fuel cells. Proton exchange membrane fuel cells (PEMFC) are the most preferred type of fuel cells due to their low operating temperatures enabling fast and easy start-ups and quick responses to load changes. One of the most important components of a PEMFC is the proton conducting membrane. The current membrane technology is based on perfluorosulfonic acid membranes and the most common one being Nafion. Although these membranes have good thermal and chemical stability, mechanical strength and high proton conductivities, they tend to dehydrate very fast at high temperatures and low relative humidity leading to poor fuel cell performances. Moreover, the high manufacturing cost of these membranes limits the mass-production of PEMFC’s in near future. The aim of this study is to develop alternative PEMFC membranes that have sufficient thermal and chemical stability, mechanical strength and comparable proton conductivity and fuel cell performances with Nafion membranes at relatively low cost. In this context, organic-inorganic composite membranes and blends were developed. A relatively cheap and commercially available polymer, polyether ether ketone, (PEEK), was chosen as the membrane matrix for its high thermal and mechanical stability and improvable proton conductivity via post-sulfonation. The proton conductivity of SPEEK membrane (at DS 68%) was 0.06 S/cm at 60°C, and this conductivity was further increased to 0.13 S/cm with the introduction of zeolite beta crystals as inorganic fillers. The conductivity of a SPEEK blend (25wt% SPES-75wt% SPEEK) membrane was 0.08 S/cm at 90°C. In PEMFC performance tests, 397 mA/cm2 was obtained for SPEEK membrane (DS 56%) at 0.6V for a H2/O2 PEMFC working at 1 atm and 80°C. This result is promising when compared to the performance of Nafion 112® of 660mA/cm2 under same conditions. These results are welcomed since the target for commercially viable alternate membranes are reached.