Studies On Methanol-resistant Membrane Electrode Assembly For Direct Methanol Fuel Cell

Direct Methanol Fuel Cell (DMFC) has promising applications as portable power sources due to its high energy density, simplicity in the system structure and easiness in fuel storage and supply. Membrane & Electrode Assembly (MEA) is the core component of DMFC. And methanol crossover and cathode flooding are the main problems in the MEA that affect not only the cell performance but also stability and lifetime of DMFC.In this dissertation, many efforts of preparing the orgnic/inorgnic composite membrane and MEA with water backflow structure have been devoted to improve the performance and stability of DMFC. Besides, investigations on the thermal stability of MEA and methods of fabricating MEA in high volume process were also carried out. The main conclusions are as below:1. The Pd/Nafion composite membrane was prepared by an electroless plating method. The Pd/Nafion membrane was characterized by macroscopic and microscopic composite structures, and Pd layer attached closely to Nafion membrane. By employing the novel composite membrane in MEA, the limiting current density of methanol crossover through the MEA decreased 36% and the peak power density of single cell increased more than two times.2. Toray 060 carbon paper was used as backing layer to prepare gas diffusion layer (GDL). The pore structure and hydrophobicity of the GDL were optimized by modifying the loading of PTFE and XC-72 carbon black, and so the transport properties of air and water in GDL were improved obviously. The cell voltageat 100 mA/cm~2 increased from 0.31V to over 0.40V when the cell was operated with ambient air at a stoichiometry of three. MEA with water backflow structure was prepared successfully. In this MEA, part of water returned from the cathode to the anode directly and flooding in the cathode was reduced, the methanol crossover could be also suppressed partially by the process.3. Investigation on thermal stability of MEA showed that during the thermal cycles between 60°C and -20°C, the electrochemical active surface area decreased, the membrane and the catalyst layer was delaminated, the structure of GDL was deformed partially, and hence the cell performance decreased continuously. The performance degradation rate was partially reduced by gas purging before the cell was frozen.4. The method of fabricating MEA in high volume process was investigated. Ethylene glycol was used as solvent to improve the viscosity and fluidity of catalyst slurry, and a uniform catalyst layer with good reproducibility was prepared successfully by screen printing.