Design Of Structure And Investigation On Performance For Passive Direct Methanol Fuel Cell

The passive direct methanol fuel cell (DMFC) is a kind of fuel cell using methanol solution or gas as the anode, oxygen or air as the oxidants. Passive DMFCs have some advantages such as higher reliability, low cost, higher fuel utilization and higher energy density due to the absence of external devices such as a pump or fan. Therefore, passive DMFCs are regarded as one of the most promising candidate for lithium ion batteries as the power for portable devices. This thesis is aimed to improve the water flood in the cathode and methanol crossover in the anode. Cathode water flood is improved by optimizing the operating parameter, studying the long-time stability of different cell orientations and designing novel flow field. The influencing factors of methanol crossover are investigated by changing methanol concentration, the total amount of methanol and the opening area of anode flow field.The factors influencing the performance of passive DMFCs are investigated in detail. The operating conditions are optimized. The best performance of passive DMFCs could be obtained when the membrane electrode assembly (MEA) is activated at a constant voltage of 0.25 V for 36 h. The maximum power density at 30℃is 13 mW·cm^-2 with the optimal methanol concentration of 1.5^-2.0 mol·L-1. The cathode oxygen transportation is the key factor influencing the cell performance, which could be improved by increasing the temperature. The cell temperature could be raised by increasing the cathode catalyst load and thus increasing the electrochemical reaction rate and the diffusion rate. The optimal value of catalyst load for the anode and cathode are both 4.0 mg·cm^-2. The humidity of air has small effect on the power density of the cell but strongly affects the long-term performance. It is found that better performance could be obtained by using perforated flow field for the cathode but parallel one for the anode.The effects of water flood on the long-term performance are studied. Better performance could be obtained with anode facing upward which is good for water removal in cathode with the help of gravity. The shortage of the cell with anode facing downward is that CO2 bubbles produced in the anode block methanol pathway. On the other hand, the key factor influencing the long-term performance of the cell with vertical orientation is the water flood in the cathode. A novel flow field is designed to improve the cathode flood. The potential loss is 9.9 % for the novel flow field while 15.3 % for the traditional one at a constant current density of 22.2 mA·cm^-2 for 10 h. The cell with novel flow field is not sensitive to the environmental humidity. When the relative humidity increases from 10 % to 90 %, the potential loss increases by 4.4 % after discharging at 33.3 mA·cm^-2 for 5 h.The effects of methanol crossover are discussed. Methanol crossover rate decreases when reducing the opening area of anode flow field and improve the cell performance with high concentration of methanol. By using high concentration of methanol, better performance at higher current density could be obtained owing to the fast diffusion rate of methanol. The Faradic efficiency decreases when using low concentration of methanol due to the serious crossover effect. The total amount of methanol will not influence the working time and the Faradic efficiency.The passive DMFC stacks are fabricated. Long-term performance is influenced by the corrosion of gold-plated PCB flow field. The performance of each single cell is uniform. The maximum power density reaches 480 mW. The stack operate stably at both constant or varied current densities. The Faradic efficiency is 44.6 % when the stack is operating at 200 mA. The stack displays good continuous operating performance in 5 cycles. The Faradic efficiency is 43.5 % when the stack is under intermittent discharge mode.