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.

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.