Experimental Study Of Dynamic Per-Formance Of Small Passive Direct Methanol Fuel Cells

Fuel cells are the devices which continually convert the chemical energy of the fuel and oxidant into electrical energy through the electrochemical reaction directly. Because of their environmental compatibility and high efficiency, fuel cells have drawn increasing attention from governments and enterprises all over the world. Direct methanol fuel cells (DMFCs) have the advantages of high energy density and convenience in refueling. Hence, they are regarded as one of the most promising candidates to replace the conventional batteries for powering portable electronic devices, such as laptops, cellular phones and personal digital assistants (PDAs). Traditional active DMFCs use the pump to supply the fuel and a gas compressor or high pressure tank to supply the oxidant. but these auxiliary devices will cause parasitic energy losses and system complexity. Therefore, passive DMFCs are concerned gradually.Up to now, some research institutions have studied the passive fuel cells. However, most of these studies were focused on how to supply fuel passively and the static performance of the passive fuel cells. Research of dynamic performance of passive DMFCs is still lacking in literatures.A miniature passive direct methanol fuel cell, which limited current density could reach 110mA/cm2 and could work at any orientation, has been designed and fabricated in this thesis. Experimental study of dynamic response characteristics of the passive fuel cell has been conducted. The experimental results indicated that the dynamic performance of passive direct methanol fuel cells is different from it of active direct methanol fuel cells. The results of this study are as follows:1. A passive direct methanol fuel cell has been designed.(1) Use graphite as the material of polar plates, which has punched flow bed with the open ratio of 42%.(2) The membrane electrode assembly (MEA), which has 25cm2 active area, consists of Nafion 117 proton exchange membrane and two pieces of carbon cloth as anode and cathode diffusion layer. The catalyst of the anode and cathode is 4mg/cm2 Pt-Ru/C and 4mg/cm2 Pt/C, respectively. (3) In order to make sure that the fuel cell can be supplied liquid fuel at any orientation continually, A passive liquid supply system, which consist of liquid storage tank, bilayer structure, locating plate and so on.2. The MEA was activated and had high and stable performance after activation.3. Dynamic performance of passive fuel cells has been investigated experimentally.(1) Tests of static performance of the fuel cell with different concentrations feeding were conducted. Finally, 9mol/L was selected as the methanol concentration for series of dynamic performance tests to get relatively high and stable cell performance. At the same time, stability performance of fuel cells is also tested. In situ visual observation of water condensation in cathode under different current densities has been done.(2) When the current decrease after the dynamic loading increase by degrees, and there are unloading period between the two loading square waves of current, the open circuit voltage of the cell drops to a low value, and then rapidly rebound again while the current suddenly loads from zero to a small value. . With the current increasing, those phenomenona becomes more and more inconspicuous.(3) The voltage is increasing when the current load is small. While in the high current, the voltage is decreasing continuously over whole loading period. The voltage tend to be a constant after loading of 60s. However, when the load time is over 120s, cell performance declines gradually.(4) The results of dynamic response of step change of current shows the capacity effect in most conditions is not obvious.(5) The impact of operating angle on cell performance was investigated. When the fuel cell was rotated in the direction, which is parallel with polar plate, it was found that the best cell performance is got when the rotation angle is 0°, and it take second place at angle of 45°, and the worst cell performance is obtained at 90°. When the fuel cell was rotated in the direction, which is perpendicular to polar plate, the best performance also appears when the working angle is 0°.(6) The instability of loading curren was studied when the methanol concentration is relative low. Influential factors of the instability were analyzed and discussed.