(334b) Mixed Matrix Speek Based Membranes for Direct Methanol Fuel Cell

Fuel cells are able to deliver electrical power with high efficiency, with low emissions and at low noise levels. The main interest in South Africa is for stationary and distributed applications. In the longer term, there is no suitable alternative to hydrogen so far. However, there are still many technical drawbacks, which should be solved before hydrogen as a fuel might replace the current hydrocarbon based fuels (high cost, storage and leak-proof handling problems). The methanol is produced from natural gas, coal or biomass and it is suitable alternative for hydrogen. A Direct Methanol Fuel Cell (DFMC) utilizes methanol as fuel, thus eliminating the need for expensive reforming operations. The cell design is simple and it is possible to deliver working units to consumers for application without the need for extensive professional back-up. An important factor that is required for fuel cells to become economically viable is the development of low cost proton conducting materials that will not change their properties during long term (> 1000 h) operation. In this presentation, the development of membranes and membrane-electrode assemblies (MEAs) which can withstand DMFC operating conditions is described. Proton conducting pure-polymer membranes, mixed matrix membranes containing track etched polymer templates filled with proton conducting polymers and mixed matrix inorganic/polymer membranes have been assessed as possible candidates for DMFC applications at temperatures 20-80 C and pressures 1-2 bars, for both single cell and stack applications. Polymers such as sulfonated polyetheretherketone (SPEEK) and sulfonated poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phenylene- oxy-phenylene) (SPEEK-WC) with various degrees of sulphonation were used as proton conducting phases. The degree of membrane sulfonation was calculated by means of infrared spectroscopy. The water uptake was measured at different temperatures as a function of the degree of sulfonation. As was expected, the water uptake and proton conductivity increased with an increase in the degree of sulfonation. At the same time higher methanol cross-over was observed. In order to preserve high levels of proton conductivity and reduce the methanol cross-over, inorganic phases were added to membrane matrices. Zirconium phosphate (ZP) and heteropolyacids (HP) were used as inorganic phases in mixed matrix membranes. The following membranes were prepared by casting: 1) cross-linked SPEEK, 2) cross-linked SPEEK-WC, 3) SPEEK/ZP composite, 4) SPEEK-WC/ZP composite 5) SPEEK-WC/HP composite. They were studied using thermal, infrared and electron microscopy methods. For the composite polymer/ZP formation following synthesis procedures were used: 1) impregnation with zirconyl chloride and subsequent phosphorization; 2) adding ZrO2 nanoparticles; 3) adding ZP nanoparticles. Initially, for evaluation of membrane suitability for DMFC use the following parameters were compared: i) proton conductivity; ii) methanol crossover; iii) mechanical intergrity and iv) manufacturing reproducibility. Impedance measurements of each membrane were conducted using an Autolab potentiostat/galvanostat PGSTAT30 in combination with the computer controlled frequency response analyzer over a frequency range between 0.1 Hz and 100 kHz. The membrane to be characterized was immersed in a water, 3% methanol solution or pure methanol. Among the most important results of those tests are high conductivity of up to 0.05 S·cm-1 achieved by adding HP to the SPEEK-WC polymer membrane matrices and 2 to 30 times reduction of methanol cross-over in comparison with Nafion for all membranes studied. SPEEK-WC showed higher conductivity and stability and lower methanol crossover compared to SPEEK. The combination of the physical-chemical parameters described above and results of long-term stability tests in methanol solutions of various concentrations was used for selection of membranes for further processing into Membrane Electrode Assemblies (MEAs). High stability membranes which did not loose any of their properties after exposure to concentrated methanol solution at 80°C and 2 bar air pressure for >72h justified their further testing in DMFC. Membrane- specific procedures for MEA preparation were developed. It was found that minimal thickness of newly developed membranes required for successful MEA performance was much lower than that of Nafion membranes due to their low methanol crossover. MEAs were tested in a fuel cell setup, which consisted of an Autolab Potentiostat, Methanol Fuel Cell testing station and experimental cell. 72-500 h long DMFC tests were performed at 80oC and 2 bar air pressure. Long term DMFC operation tests were successful for a number of SPEEK-WC mixed matrix membranes.