In this thesis, the treatments of MEA and the special structures within carbon fiber bunch unipolar plates on the performance of PEMFC are studied. At first, the factors affecting on the water content within MEA will be studied. A passive HFC stack usually exposes in the ambient no matter that it works or not. However, the ambient is far from saturated. The water within MEA will vaporize continuously. Especially, if the stack is shutdown for a long period, there is no water generation in the cathode and then the membrane will be short in water. If it occurs, the conductivity of H+ will decrease greatly, and the electrode of MEA is also possible to separate from its membrane. This separation will make the performance of the stack an unrecovered decay. On the other hand, in order to improve the performance of a air-breathing HFC, the inner structure within cathode carbon fiber bunch unipolar plates is modified. The structure of the unipolar plates is modified in the following three aspects: 1. Increasing soft end height of carbon fiber bunch, 2. Increasing the number of silver-coated wires in carbon fiber bunch, 3. Cutting several serrated slots on the soft end of carbon fiber bunch. In the MEA treatment, firstly, a MEA is boiled in 80oC, 0.5M H2SO4 solution and then boiled in 80oC DI water for an hour, respectively. When the single-cell HFC operates in hydrogen inlet pressure 0.1 bar, air-breathing, and room temperature, experimental results display that the power density of this HFC with the aforementioned treatments and the special structure of unipolar plates can reach a value about 185mW/cm2. This value is about 130% higher than that of the untreated MEA and about 50% higher than that of the treatment of MEA only immersed in DI water. In addition, the comparison of the performance of HFC between with carbon fiber bunch unipolar plates and with graphite unipolar plates are also studied. The experimental result displays that the performance of HFC with the carbon fiber bunch unipolar plates is superior to that with graphite unipolar plates, especially the fuel cell operating under low gas inlet pressure.

In this thesis, a portable PEMFC stack, which can directly power or charge 3C products, will be developed. The stack is developed for portable applications, so the structure of the stack is simplified as possible as we can. The PEMFC stack is made with 32 carbon fiber bunches for current collectors and two 8-cell banded-type MEAs which are made with 8 sets of electrodes on a piece of membrane. The stack can develop a high voltage by serially connecting 8 cell or 16 cell outside of the reaction chamber. The resistance of each carbon bunch assembling with carbon cloth is measured before they are assembled into the stack. Under assembly pressure 3 bar, the total resistance is about 8.7m£[ or 11m£[¡Ecm2. The resistance is about one half of that graphite plate assembling with carbon cloth. Without being compressed greatly in diffusion layer, the fluid can easily flow through the gaps between carbon fiber and within diffusion layers, and then the reactive region will react more uniformly. In addition, the connecting wires are assembled to a wire collecting board, so that the stack is look more neat, and it easier assemble or dissemble. In this thesis, the volume of the developed 16-cell hydrogen fuel cell stack is about 9.6 cm*6.3 cm*2.2 cm. The total electrode area is 50 cm2 (16-cell¡Ñ3.15 cm2 per cell). When the stack is operating at room temperature and air-breathing, an 8-cell stack in series connection can generate 3.7V voltage. Its power at voltage 3.7V is about 3.6W. It can directly power PDAs, mobile phones or digital cameras. A 16-cell stack in series connection can generate 7.2V voltage. Its power at this voltage can offer 7W. The 16-cell stack can directly power digital single-lens reflex cameras. If two or more of this stack are connected in series, it will be able to power a notebook or other more power products.

The problem of the performance decay and the factors affecting on the DMFC performance for long-term operation are studied in this thesis. First, the influence of the initial treatments of MEA and the exposure of MEA in the atmosphere on the water content are measured. In addition, the effects of the pressure of the MEA hot press conditions, the treatments and preservation of MEA, and the operative conditions on the performance are also examined. Eventually, we expect that the best way to increase the DMFC performance and avoid the performance can be found. These can provide for references when a portable DMFC need to be designed and manufactured in future. In order to solve the problem of methanol crossover leading to the cathode poisoned, cells are operated only under the proper methanol concentration and discharged thoroughly before finishing the whole experiment. It is also necessary to maintain MEAs in proper wetness so that the performance of stack will not decay too quickly. In the initial treatment, firstly, a MEA is immersed in 3M MeOH and then boiled with 80oC DI water for an hour, respectively. The experimental conditions of this passive single-cell DMFC are pumpless in anode chamber, air-breathing, and room temperature. The power density of this DMFC with these test conditions can reach a value about 33mW/cm2. This value is about 106% higher than that of the untreated MEA. If MEA boiled with 0.5M H2SO4 for an hour and then boiled with 80oC DI water for an hour, its power density is about 75% higher than that of the untreated MEA.

In this thesis, a long-term operation direct methanol fuel cell (DMFC) stack with no performance decay is developed and improved. In a traditional passive portable DMFC, the Methanol solution is storage at anode Reaction chamber. The performance will drop after a short period due to the concentration of Methanol solution becoming lower (about 1.5M). Although High concentration of Methanol solution could increase the operation time, but it will couse crossover to poison the cathode Pt particle, and it is unable to keep long-term operation stably either. In order to achieve long-term operation stably, to maintain the concentration of methanol solution in the anode chamber will be very important. In our fuel supply stack, there are two chambers in the stack to storage methanol and water, and we could control the supplying rate by adjusting the diffusion area to control the diffusion rate of methanol and water. And the methanol solution deliver to anode reaction chamber by cotton tube. If the anode reaction chamber is filled with 1.3¢W, 2M methanol solution and without any fuel supply, operating on the 185mA constant current (82.2mA/cm2 ). The results shows that the performance begin decay about after 15minutes. If the appropriate amount of methanol and water is supplied, the performance can be steady in a long-term operation. But if supply too much methanol solution, the concentration in the anode reaction chamber will rise up, and the high concentration will cause crossover poised the cathode catalyst, and the performance will decay. If supply rate not enough, the concentration in the anode reaction chamber will become lower, and the performance will decay after long-term operation. In this study, based on operate current and the rate of evaporation of methanol solution, to supply appropriate supplying rate and concentration of methanol solution to anode reaction chamber, could keep the performance in a steady output.

The advantage for new carbon fiber bipolar plate are as follow, low cost, light weight, low contact resistance and good chemical stability. After process automation, to further reduce costs, enhance quality stability, improve production efficiency, bipolar plates can be achieved mass production. Bipolar plate manufacturing process is divided into five parts:(1) the unfolding of carbon fiber (2) automation of gluing (3) hot-compression harden (4) cutting of carbon fiber bunch (5) Injection molding of bipolar plates. Without leakage, tightness test of the carbon fibers must reach a pressure of 0.2 kg/cm2. The contact resistance is lowest when number of carbon fiber has 160 layers, and compressed fiber bunch height of 2mm on the assembly. Anode inlet pressure is 0.1 kg/cm2. Cathode is required to install a fan. And the fan speed has to cooperate with current load. The quality of carbon fiber bunch will affect the performance of the battery, such as the wide of the rubbers, the flat of the section, Tightness, and numbers of fibers. The structure of the bipolar plate must be considered fuel transfer and number of carbon fibers bunch. Fuel supply and the contact resistance value to achieve a good balance.

Abstract The improvement of performance and the maintenance of stability of a portable air-breathing DMFC are studied in this thesis. The effect of the improvement of the internal structural of carbon fiber bunches on the cell performance is studied firstly. The small channels in the soft end of the carbon fiber bunches can be formed by changing the thickness of the copper plates burry within the gluing zone of the fiber bunches. Then one or two transverse grooves are form in proper location by cutting part of the carbon fibers at the soft end to shorten the airflow path to the area of electrode which is covered by the carbon fiber bunches so that the reaction area can obtained enough oxygen or fuel. Experimental results show that the maximum power density is about 20 mW/cm2 with no structure but it raised to about 24 mW/cm2 with the burry a 0.5mm thick copper plate and the two transverse grooves. It improves about 20% power density. The experiments prove that the improvement of the internal structure of the carbon fiber bunches is helpful in stack performance. In order to reduce the unneeded depletion of fuel, the bare nafion membrane pastes another special membrane to block methanol and water leakage. The strategy to block the leakage improved the rate of fuel utilization about 24%. In order to make the direct methanol fuel cell operating stably, a fuel supplying system by gravitation and diffusion forces is delivering the consumed fuel to maintain the concentration of methanol solution in anode reaction, by adjusting a sliding gate to control the diffusion area and utilizing three cotton threads and hoses to distribute the fuel to proper location. The multi-point type of fuel supplementary system allows the methanol solution to be distributed uniformly, so that the stack can maintain stable operation for a long period. In order to make the stack size to a minimization, the volume of the anode reaction chamber will be minimized as possible; however, the reduced chamber is still able to supply sufficient fuel maintaining operating stably in the high-current condition. The transient phenomena of output voltage under the various volume of the reaction chamber are also studied in this research. Finally, we hope to be able to identify the most appropriate space to meet demand. The above optimization results are able to provide a reference in the future design and production of portable DMFCs.

The effects of the structure of new carbon fiber bunch heterogeneous unipolar plates on the performance of PEMFC are studied in this thesis. Internal structure of carbon fiber bunches can be modified by embedding different thickness or number of copper plates in the glue bonding area to increase the air permeability of carbon fiber bunches in its soft end. We can add different thickness or amount of coppers at the middle of bonding area, making the carbon fiber bunches soft side to form parallel to the longitudinal fiber bunch with a small flow channel. We can also make a trench at the appropriate place of the soft side of the carbon fiber bunches to form an extra air passage. In order to make the above flow channel, a new process for making the carbon fiber bunches is developed also. This process will be easier to produce a variety of different structures of carbon fiber bunch. Finally, several different experiments are performed to help us to understand the effect of the carbon fiber bunch structure on the performance and find out the best structure of the carbon fiber bunches. The carbon fiber bunch structures of the test cells on the anode side are all the same, but the carbon fiber bunch structures of on cathode side are all different. Experiments show that there are two structures among all test structures displayed better gas permeability. The first one is two 0.2 mm copper plates embedded within both sides of the glue ends of a cathode carbon fiber bunch, so that a small longitudinal flow channel are formed in soft end of the cathode carbon fiber bunch. When the HFC operates at room temperature and by air-breathing, the highest performance of the HFC can reach a value of 185 mW/cm2. The second one is a 0.2 mm copper plate embedded in the center of the glue end of a carbon fiber bunch, and then three 2 mm wide serrated slots are cut on the soft end of the carbon fiber bunch. The highest performance of the HFC can reach a value of 190 mW/cm2. The highest performance of the HFC with no copper plate and no slot structure can only reach a value 160 mW/cm2. The second design can increase the no structure cell performance 18.8%. Therefore, the internal structures of carbon fiber bunches are significant to affect on the fuel cell performance, and its internal design must be considered.

In this paper a proton exchange membrane fuel cell (called PEMFC) stack was developed to power or charge 3C products without any voltage transformer. PEMFC stacks made with traditional bipolar plates to generate a high voltage are usually by accumulating multiple single fuel cells together. The design with traditional heavy and large bipolar plates is inconvenient for 3C products to generate a high voltage in a finite volume. To solve this problem, a heterogeneous carbon fiber bunch unipolar plate is adopted to replace traditional bipolar plates, and a special membrane electrode assembly (called MEA) with multiple sets of banded electrodes is used to replace a traditional MEA that is made with only a set electrodes. With this new design, the fuel cell voltage can easily increase in a layer. The designed stack can provide multiple voltages and currents by proper series and/or parallel connections. The variable voltage 16-cell fuel cell is composed of 4-layer 4-banded type MEAs and 5 heterogeneous carbon fiber bunch bipolar plates. The 16-cell stack is divided into 4 sets. Each set of 4 series connection cell is arranged in a line in 4 different layers. The 4-cell sets can connect by series/parallel on the two ends of the stack. The total volume of the 16-cell stack is 385cm3 and its weight is 365g. The new design can power or charge certain 3C products directly. If 2 sets of 4-cell fuel cells are connected in series, the stack can provide 2A at 3.6V. With the above 2 sets of 2*4-cell connected in parallel, the stack can provide 3.5A at 3.6V. If the 4 sets of 4-cell are all connected in series, the stack can provide 1.8 A at 7.2V. These voltages and currents derived from these stacks can power or charge a mobile phone, a photo camera and a video camera directly. If a higher voltage or current are needed, two or more 16-cell stacks can be connected in series XI or parallel. Then notebooks or any other 3C products in which higher power are needed can be driven.

¡@¡@In this thesis, we develop a self-humidifying PEMFC. The humidifying effects on the stability and impedance of the fuel cell are studied. A portable and passive PEMFC stack usually exposes in the ambient no matter that it works or not. However, the ambient is far from saturated. The water within MEA will diffuse to the membrane¡¦s surface and evaporate continuously. The membrane will be short in water without water supplying. Because the conductivity of H+ of the membrane is highly dependent on water content, the dehydration of the membrane will reduce the interconnected passageway of H+ and affect the performance of fuel cell directly. And because of the different expansion rate the electrode of MEA is also possible to separate from its membrane when it operates repeatedly. This separation will make the performance of fuel cell an unrecovered decay. ¡@¡@At first, the hydration status of the dry membrane is observed. We measure the addition weight of water into membrane by using cotton thread humidifying, and estimate the water permeation distances. The maximum water supply rate of cotton thread is 4.26mg/min, and the permeation rate of water through membrane where is 2.5cm from water surface is 0.15mg/cm¡Dmin. Then we design the self-humidifying devices of PEMFC stack. The humidifying effects on performance and stability of the fuel cell are studied. ¡@¡@When the active area is 0.7¡Ñ4.5cm2 and the cotton thread is 5mm from the center of electrode the supplying water can arrive at the reaction area under the electrode through the membrane in one minute. The difference of the supplying water between the bottom and top is 7% by using 6cm cotton thread. Therefore water can hydrate the membrane and the difference of the supplying water between bottom and top is not oversize. The higher current load, the voltage efficiency is lower. The increasing heat generation rate results in the water evaporation rate would be greater than the water generation rate. So the drop of voltage under higher current is greater than lower current. By comparing with the difference of high frequency impedance the change of humidifying is smaller between 1hr operating. It indicates that humidifying by cotton thread keeps the membrane hydration.

This study has two purposes: First, the catalyst-coated membrane (CCM) method to produce high performance and high utilization of electrode, and the other is to enhance the fuel cell performance with the heterogeneous carbon fiber bunch framework of stack. First, to establish an ideal electrode structure, there has an intensive triple phase boundaries. We will describe how the procedure of reliable and practical electrode improved following the optimization of (1) the spray system, and (2) the catalyst dispersion. We will also focus (3) modification of the spray system, and (4) electrode performance analysis. In addition, investigate of the single cell performance in heterogeneous carbon fiber bunch framework. We will find that: (1) Increasing the catalyst loading and concentrated the catalyst activation reaction, can be improve the electrode performance and catalyst utilization. (2) Coating a thin conductive layer onto membrane electrode (ME), be a precise hot-pressue process in the Stack and MEA or GDL and ME, can be reduce the contact resistance. Specially, reduce the carbon fiber coverage fraction with electrode area, result the activation reaction decay and ohmic loss obviously. (3) Increasing the gas flow rate, can enhance the mass transfer performance, but increase the pressure of the reaction gas, can¡¦t significant effect on performance. Besides, when the stack is anode side up, seems favorable to the exclusion the generate water of cathode.