Via the viewpoint of fuel and oxidant supply in this study, we compare heterogeneous carbon fiber bipolar plates with graphite bipolar plates that apply to fuel cell. In operating condition with different gas inlet pressure and compressing pressure, we study the penetrability of reactant gases that come into the carbon cloth under the rib of a bipolar plate. Eventually the output voltage and power density are measured to prove the advantages of the new bipolar plate. The experimental results show that carbon fiber bipolar bunch in low compressing pressure 2bar already display high gas penetrability. Its dimensionless flow rate is about quadruple of graphite bipolar plates. The reactant gas can enter the carbon cloth either from the side or from the top of the penetrating carbon fiber bipolar bunch. In addition, carbon fiber bipolar plates are affected slightly by compressing pressure. Further, the total electrical resistant of carbon fiber bipolar plates with carbon cloth already decreases to 18.5mΩ*cm² in low compressing pressure 2bar. Therefore, by appling the new bipolar plate, the fuel cell in compressing pressure 2bar and inlet fuel pressure 1.15bar(absolute pressure) can developed a power rate 180mW/cm². Concerning graphite bipolar plates, we can find that compressing pressure increase from 1bar to 4bar due to the reduction in total resistance so the output voltage and power density can increase to maximum value 113mW/cm2. However, while we augment more compressing pressure, the influence in reducing total resistance is much smaller than that in reducing the porosity of carbon cloth. Therefore, the output power density decreases. Also, output voltage of carbon fiber bipolar plates at 0.5mA/cm2 is 0.38 V and is higher than that of graphite bipolar plate 0.2 V.

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 heterogeneous bipolar plates are applied to pure hydrogen PEMFC (called HFC) stacks. The experimental methods are adopted to study the performance and characteristics of the cell under certain operational conditions. In order to obtain the permeability of carbon fiber bipolar plates the pressure drops and flow rates are measured on the two sides of the carbon fiber bunch. A test device has been developed to separate the gas of the two sides so that the gas can only flow through the gaps between carbon fibers. Additionally, the gas pressures on the flow channel and the output voltage of each cell in several locations are measured to help us to understand their relationships. The flow characteristics of the gas reactants in these carbon fiber bipolar plates can also be understood from these measurements. A bipolar plate with the parallel or serpentine flow channel can be formed by properly arranging the carbon fiber bunches. However, if the oxidizer is air and only single inlet and outlet in cathode chamber are designed, the oxidizer can always not be supplied sufficiently in high power density. The experimental studies display that the output voltages of cells in the midstream or downstream are far below the output voltage in the upstream. The voltage of cells in the upstream is the highest, the next one is in the downstream, and the lowest one is located in the midstream due to accumulating of nitrogen. The insufficiency of oxidizer occurs more seriously in the parallel flow channel than that in serpentine flow channel in single inlet and outlet design. The distribution of current is not uniform especially near midstream, although this phenomenon can be improved by increasing the air inlet pressure. However, the problems are hard to solve in large MEA if we just increase the inlet pressure. Another strategy is needed to solve this problem. In order to solve the insufficient supply of air in single inlet and outlet design, multiple inlets and outlets are designed. In this design multiple entries can supply enough fresh air driven by fan, and multiple exits can exhaust inert gas by exhausting fan to avoid accumulating in a reactive chamber so that the performance of stack can be improved dramatically.

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.

The characteristics of the proton exchange membrane fuel cell (called PEMFC) stacks made with the graphite unipolar/bipolar plates are studied in this thesis. Using pure hydrogen as fuel, certain experimental work is conducted to help us to understand the factors which influence on the performance of a HFC stack. The experimental work under various operating conditions starts from single cell stacks to multi-cell stacks. The maximum power is about 200 W, which is made with two 10-cell stacks in series. For simplification, all of the flow channels in the cathode are open type in which air is directly supplied from ambient by fan. The comparison of the performance of two single cells, which are made with both a graphite unipolar plate and a new-type carbon fiber unipolar plate, is conducted. The total resistances of the two types of bipolar plates with gas diffusion layers are tested to help us to understand their strong or weak points. The experimental results display that the double inlets has better performance than the single inlet due to larger entrance space. Increasing the applied torque will reduce the contact resistance between bipolar plate and diffusion layer and also the gaps between the fibers of carbon cloth. Reducing the contact resistance is helpful in increasing the performance of the cell, but reducing the gaps between fibers will inhibit the entering of reactive gas and is unfavorable for performance; therefore, the proper torque is necessary to obtain the best voltage output. When air is used as an oxidizer and the flow channel is an open type channel, the fan in high rotating speed is helpful at high current density. The high air volume flow rate can supply sufficient oxidizer and avoid the decay of the voltage output at high current density. At the current density 1 A/cm2, the power density of the single-cell stack is about 400 mW/cm2 and the power density of the 10-cell stack is down to about 310 mW/cm2 in our experiment. The rib of the carbon fiber unipolar/bipolar plate is soft, so there is no deformation in the gas diffusion layer in stack assembly. Only slight compression is needed to assemble a stack; therefore, the reactive gas can easily flow into the most of active area. This type unipolar/bipolar plate is made with low density plastic except that the rib is made with carbon fiber bunches. Thus the new plate is weight light, cost low and volume small. So it is quite possible that the new-type of carbon fiber plate is used as substitution for the graphite bipolar plate in the future. In that case the light, low cost and high performance choice can be achieved.

In this thesis the numerical method is adopted to study the flow characteristics of reactants, when the newly developed heterogeneous composite carbon bipolar plate and the traditional hard surface bipolar plate are applied to fuel cells. The simulation in this study includes the distributions of the velocity and pressure of oxidizers flowing in a parallel or serpentine flow channel under several inlet gas flow rates and pressures. The difficulty to supply reactant to the active area under the ribs is also studied in this thesis. From these studies we can understand the strong and weak points of the newly developed bipolar plates and the traditional bipolar plates better. The simulation results display that the gaseous reactants or products can permeate through the gaps between carbon fibers into or out of active area under the rib, which is formed with carbon fiber bunches. Therefore more reactant gases can be supplied with the heterogeneous carbon fiber bipolar plates than the graphite bipolar plates. In addition, the higher efficiency of fuel cells can be obtained with the new plate, especially in high power density. The pressure distribution in making use the heterogeneous carbon fiber bipolar plates are more uniform, and the pressure drop is also less than the traditional bipolar plates. For large fuel cells the current distribution will be more uniform and the pumping power will be less. The reactant gases can flow through the gaps of carbon fibers and the porous carbon cloth into the catalyst layers by convection and diffusion. So no matter what type of the flow channels used the needed oxidizer is much less with the new bipolar plate than with traditional one. The flow fields of the two types of bipolar plates are quite similar, but the gas needed to supply is also much less with the new plate. Because of the advantages mentioned above, we believe that the heterogeneous carbon fiber bipolar plate is better than the traditional graphite bipolar plate. Keywords: fuel cell, heterogeneous carbon fiber bipolar plate,hard surface bipolar plate