Small Direct Methanol Fuel Cell Stack

Direct methanol fuel cells(DMFCs) are regarded as one of the most promising candidates for powering portable electronic devices (laptops, cellular phones and digital cameras) because of their advantages such as high energy density, simplicity in structure, convenience in fuel storage and refueling. However, the low performance of membrane electrode assembly(MEA),the core component of a DMFC, has become one of the key obstacles to its commercial application. In particular, methanol crossover and cathode flooding are two problems in MEA that affect not only the cell performance but also the stability and lifetime of DMFCs. In this thesis, the effect of Nafion membrane modification, preparation parameters of MEA as well as the operating conditions on the cell performance has been investigated, and DMFC stack has been designed and installed to provide some experiences for the practical application of DMFCs.The performance of MEA fabricated by spray method has been compared with those by brush method. The results showed that the performance of MEA prepared by the spray method was better because of the more flat and smooth surface of the catalyst layer. The cell activation of discharging with small current density improved the three reaction zone and Nafion distribution in the electrode to enhance the electrochemical active surface area. The appropriate operating conditions for the single DMFC were: oxygen flow rate of 100mL/min, methanol concentration of 2.5mol/L, methanol flow rate of 2 mL/min. Under these conditions, the highest power density at the temperature of 25, 40, 60 and 80℃were 19.42, 37.86, 59.96 and 82.82 mW/cm~2, respectively.Nafion 117 membrane was modified by polypyrrole using pyrrole monomer as raw materials and hydrogen peroxide as oxidant. It was found that the modification of Nation membrane with polypyrrole can improve the performance of MEA to a great extent when the cell was operated at higher methanol concentrations and higher discharging current densities, which can be attributed to the lower methanol permeability of Nafion membrane modified by polypyrrole. In addition, Co was chemically reduced on the cathode side of the modified membrane to prepared Co/PPy-35 membrane, which demonstrated higher cell performance when compared to that of Ppy-35 membrance.A DMFC stack was designed and installed, and its polarization performance, efficiency as well as stability were investigated. The results show that the maximum power density and maximum efficiency of the stack were208.7mW and 23.44%, respectively.