Abstract Traditional unipolar/bipolar plates, such as the metal and the graphite unipolar/bipolar plates, are expensive, weight heavy and volume large, so that it is hard to be used in the portable application. A high efficiency, low cost and lightweight portable proton exchange membrane fuel cell (called PEMFC or called HFC when using pure hydrogen fuel), which is made with a new heterogeneous composite carbon fiber bipolar plate and a MEA, is developed in our lab. There are many advantages of the new carbon fiber unipolar/bipolar plates, such as low contact resistance, low cost, lightweight and small volume. We hope that the new unipolar/bipolar plate will be able to replace the conventional metal and graphite unipolar/bipolar plates in the future. The characteristics of a portable PEMFC in different operational conditions are studied in this research. From our experimental result, we find that the factors which affect the HFC performance include the gas temperature, humidity ratio, inlet gas pressure in anode, the geometry of inlet ports, the flow channels within cell, and the oxidant flow rate etc. In addition, the contact resistances between different materials within each cell all strongly influence HFC performance. The ribs of the carbon fiber unipolar/bipolar plates is pored structure, and the gas diffusion layer is no deformation because of only slight compression in stack assembly; therefore, the reactive gas can easily flow into the most of active area. In addition, the contact resistance between the carbon fiber unipolar plate and the gas diffusion layer is lower than that between the traditional unipolar plate and the gas diffusion layer, so that the electrons in active layer is easily to exit or enter this region. The experimental result at 1.15 atm and 40 oC displays that the current density with the new unipolar plate is about twice higher than that with the graphite unipolar plate at overpotential 0.6 V. With air as an oxidizer, we find that increasing the fan rotation speed can avoid output-voltage decay in high current density, but the design with fan is unfavorable for portable application. So a front open unipolar plate and air-breathing design is adopted on the cathode. The power density of this design is slightly lower than that with fan, but it still can reach a value 160 mW/cm2 without any heating and humidification in the anode. Because this design needs little supplement device, the application in portable fuel cells of the new design will be wider than that of a traditional design.

Several disadvantages in general unipolar/bipolar plates are that cost is expensive, weight is heavy and the volume is large. The high compressing pressure is also necessary to reduce the contact resistance in making up a fuel cell stack. Therefore, it is difficult in making use general unipolar/bipolar plates to portable fuel cells. With a new heterogeneous carbon fiber bipolar plate, pumpless and air-breathing design and in cooperating with a special MEA, a portable fuel cell stacks developed in our lab have made portable applications to be possible. The structure of the DMFC stack made with the new carbon fiber bipolar plate is much more simple and weight-light than the other designs. The three portable DMFC stacks flat type, cylinder type (I), and cylinder type (II) are developed in series in our lab. The methanol solution can be stored directly in the flow channel of the anode, and does not need the extra auxiliary equipment, so it easy to apply to the portable fuel cell. The developed portable DMFC of cylinder type (II), weight is only 20g, volume is 30cm3, and the fuel stored capacity is 7.5ml. In the flat type DMFC, In anode Pt-Ru loading 3 mg/cm2, and cathode Pt loading 1 mg/cm2, methanol concentration 3 M, pumpless, air-breathing, and room temperature, the largest of output power density of the fuel cell can reach 5.27 mW/cm2, and the total power can reach 71 mW. The weight of DMFC of cylinder type (II) is far lower than DMFC of flat type in addition, so its power density 1.33mW/g is about 1.68 times of flat type 0.79 mW/g.

In this thesis the experimental method is used to study the characteristics of a DMFC when a heterogeneous carbon fiber bipolar plate is applied to it. The first main study is about the effect of the different structures of the carbon fiber bunch on the fuel cell performance. Additionally, a high temperature hot-pressing process is performed to change the inner molecular structure so that the hydrogen ion can be blocked to avoid the lateral migration between two adjacent cells. Finally, the two techniques are applied to make our new portable DMFC stack. The bipolar plates with the sawtooth or non-sawtooth carbon fiber bunches have been used in making our DMFC stack. The experimental results display that the performances of the two structures both are better than the traditional graphite bipolar plate. However, the performance of DMFC with the sawtooth bipolar plate is much better than that without sawtooth, especially in high current density. When carbon fiber bunches with sawtooth use at anode and cathode of bipolar plates, the performance can be enhanced and its power density 27.6% higher than that without sawtooth. During our study we also found that part of hydrogen ions can laterally migrate to its adjacent cathode and do not directly cross to its opposite cathode, when the banded type MEA are used to multiple cell stack. Therefore, the performance cannot be performed well due to this type ion transfer. In order to block the lateral migration, the narrow area of the membrane between two adjacent electrodes is pressed with a high temperature hot-pressing device. After a short time hot-press between two adjacent electrodes, the hydrogenion migration phenomenon reduced, and the performance had been improved about 10% higher than that without hot-press. Finally, a double layer 2x6-cell flat type DMFC is made. This 12-cell stack is composed of each electrode area 0.5x5cm2, two sheets of membrane for 6-cell using Nafion 117, the anode catalyst Pt-Ru loading 4mg/cm2, and cathode catalyst Pt loading 4mg/cm2, the methanol concentration 3M, air-breathing, and operating in room temperature. The output power of the cell can reach an average power density 8.0mW/cm2 and total power 240mW with our handmade stack. If the performance of each fuel cell is more uniform, we expect that total power can reach 480 mW. The power level should be satisfied for any kind mobile phone.