To ran a DMFC without methanol crossover is the aim of this study.It is done by supplying fuel no more than what the anode can consume. The first is to explore the factors that may affect the time constant of vapor feed DMFC. In order to reduce the time constant of current decline, first, we decrease store tank¡¦s space of methanol with different structure of unipolar plate. Second, we reduce the thickness of anode stack and the space above the air bleed valve. Using slide plate instead of air bleed valve can shorten the diffuse distance effectively and reduce the time constant of current rise curve. The second is to explore the impact of supply of methanol on steady-state current of system. Using air bleed valve, because of its high gas tightness, the utilization rate of methanol can exceed 94% without crossover. It was found that in the slide plant experiment, steady-state current value depends mainly on the pore size of slide plate, and resistance value has nothing to do. However, the resistance value is lower, the time required to reach steady-state current is shorter. The third is to explore if the performance decay after long time test of steady-state current. It was found that the performance of MEA will decay while the water content of membrane decreased.

A combination of high prices of fossil fuels and the increased awareness of their negative environmental impact has influenced the development of new cleaner energy sources. Among various viable technologies, fuel cells have emerged as one of the most promising sources for both portable and stationary applications. Fuel cell stacks produce DC voltage with a 2:1 variation in output voltage from no load to full load conditions. Hence, to increase the utilization efficiency and system stability, a power conditioner consisting of DC-DC and DC-AC converters is required for load interface. The design of power conditioners is driven by the application. This dissertation presents several different solutions for applications ranging from low-power portable sources for small electronics and laptop computers to megawatt-power applications for fuel cell power plants. The design and analysis for each power conditioner is presented in detail and the performance is verified using simulations and prototypes. Special consideration is given to the role of supercapacitors who act as the additional energy storage elements. It is shown that the supercapacitor connected at the terminals of a fuel cell can contribute to increased steady state stability when powering constant power loads, improved transient stability against load transients, and increased fuel efficiency (i.e. reduced hydrogen consumption).

A theoretical model and numerical simulation of a direct methanol fuel cell (DMFC) is developed to simulate the reaction mechanisms and the cell voltage under several different designing parameters and operational conditions. The results of a numerical simulation include the distributions of the proton current density, the concentration of methanol, the electrochemical reaction rates, the overpotential losses, and the pressures within proton exchange membrane layer, catalyst layer and diffusion layer. In addition, the influence of aforementioned operational conditions on methanol crossover in a direct methanol fuel cell is also investigated. Finally, the results of the model are compared to the results from the experimental work. The results show that increasing of temperature, pressure and anode catalyst loading can enhance the performance of a direct methanol fuel cell, and the concentration of methanol plays an important role in its performance. The optimal concentration of methanol for a direct methanol fuel cell is about 2M. Methanol crossover can be suppressed by decreasing methanol concentration and increasing thickness of polymer electrolyte membrane (PEM). However, under operating condition of high current density, thick PEM and low methanol concentration will cause large concentration overpotential and ohmic losses, respectively.

The research of a hydrogen proton exchange membrane fuel cell is performed under certain designing and operational conditions. The water management technique is incorperated into the experimental work. The cell voltage vs. the current densities are studied by changing the stack reactive temperatures, the gas inlet temperatures and pressures, and the relative humidities in hydrogen stream. Eventually, we hope that these experimental results can provide the information about the optimizing conditions of fuel cells so that they can be used to design a high power multiple-cell fuel cell stack. A membrane and electrode assembly (called MEA) which contains a proton exchange membrane Nafion 112, anode catalyst Pt 0.4 mg/cm2, and cathode catalyst Pt 1.0 mg/cm2 is used in this experiment. The gas flowing area is about 58% of the total area. A proper heating and humidification equipment is applied in this experimental system. The experimental results show that the cell voltage at low current density is slightly influenced by the hydrogen inlet temperature; however, the cell voltage at high current density is strongly influenced by the humidity ratio of hydrogen stream. Raising the hydrogen pressure and the oxygen pressure at the same time can increases the cell voltage, but it is no obvious effects on the cell voltage when the gas pressure increases to more than 2 atm. When air is used as a oxidizer, increasing the inlet air temperature always reduces the cell voltage. With the hydrogen stream at saturated temperature 80¢XC, the assembly torque of the stack at 4 N-m, and the stack temperature at 80¢XC, the single fuel cell stack can always generate the best cell voltages at most of the current densities. At this time, the cell voltage at current density 1 A/cm2 already can reach a value higher than 0.6 V.

Abstract The characteristics of PEMFC stacks in different designs and operational conditions are studied and manufactured in this thesis. There are many factors that affect the PEMFC performance. They include the familiar humidity, the torque, the inlet pressure, the geometries of inlet ports and the flow channels in reaction regions, the cell numbers of the stacks, the type of the oxidizer and its flow rate. To understand the performance characteristics of stacks the voltage and current density will be measured as well as the interior temperature of stacks in this research. The membrane exchange assembly (MEA) with Nafion 112, anode Pt 0.4 mg/cm² and cathode Pt 1.0 mg/cm² is used in these experimental works. The experimental results display that increasing the applied torque will reduce the contact resistance between bipolar plate and diffusion layer but increase the difficulty of gas penetrating into the reaction region beneath the bipolar rib. So proper torque is necessary to obtain the best voltage output. The voltage vs. current density also increases as the inlet pressure increases, but its effect will reduce when the inlet pressure increases over 2atm. The geometry and size of inlet port to each cell for a multi-cell stack will influence the voltage output, especially in high current density, so that special attention is needed in designing inlet port. When the air is used as an oxidizer, the fan with a high rotation speed is helpful in an open circuit design. The high air volume flow rate can avoid that the voltage output decays greatly in high current density. Increasing the cell number may cause extra internal resistance due to assembling improperly and reduce the voltage output. So special attention is also needed in assembling. Keyword: Proton Exchange Membrane Fuel Cell Stacks

Fuel cell power generation systems provide a clean alternative to the conventional fossil fuel based systems. Fuel cell systems have a high efficiency and use easily available hydrocarbons like methane. Moreover, since the by-product is water, they have a very low environmental impact. The fuel cell system consists of several subsystems requiring a lot of effort from engineers in diverse areas. Fuel cell simulators can provide a convenient and economic alternative for testing the electrical subsystems such as converters and inverters. This thesis proposes a low-cost and an easy-to-use fuel cell simulator using a programmable DC supply along with a control module written in LabVIEW. This simulator reproduces the electrical characteristics of a 5kW solid oxide fuel cell (SOFC) stack under various operating conditions. The experimental results indicate that the proposed simulator closely matches the voltage-current characteristic of the SOFC system under varying load conditions. Effects of non-electrical parameters like hydrogen flow rate are also modeled and these parameters are taken as dynamic inputs from the user. The simulator is customizable through a graphical user interface and allows the user to model other types of fuel cells with the respective voltage-current data. The simulator provides an inexpensive and accurate representation of a solid oxide fuel cell under steady state and transient conditions and can replace an actual fuel cell during testing of power conditioning equipment.

An investigation was conducted to determine the feasibility of using the technique of ultrasonic nebulization of water into the anode gas stream for evaporative cooling of a Proton Exchange Membrane (PEM) fuel cell. The basic concept of this form of internal evaporative cooling of the PEM fuel cell is to introduce finely atomized liquid water into the anode gas stream, so that the finely atomized liquid water adsorbs onto the anode and then moves to the cathode via electro-osmotic drag, where this water then evaporates into the relatively dry cathode gas stream, carrying with it the waste thermal energy generated within the fuel cell. The thermal and electrical performance of a 50 cm2 PEM fuel cell utilizing this technique was compared to the performance obtained with conventional water management. Both techniques were compared over a range of humidification chamber temperatures for both the anode and cathode gas streams so as to determine the robustness of the proposed method. The proposed method produced only meager levels of evaporative cooling (at best 2 watts, for which a minimum of 30 watts was required for adequate cooling), but the average cell voltage increased considerably (as much as a 10% gain), and the technique increased the fault tolerance of the fuel cell (the Nafion™ membrane did not dry out even if cell temperature went well in excess of 70° C despite both anode and cathode humidification temperatures of 55° C). An interesting phenomena was also observed wherein the fuel cell voltage oscillated regularly with a period of tens of seconds, and that the amplitude of this oscillation corresponded inversely with the level of humidification received by the fuel cell.

Proton exchange membrane (PEM) fuel cell possesses the characteristics of microminiaturization and low temperature operation. For this reason, the proton exchange membrane fuel cell is very suitable to serve as power source of portable electronic products. In this paper, a three-dimensional numerical model to evaluate the voltage and the total current density of a PEM fuel cell stack was developed. The polarization curves of the PEM fuel cell stack under three different operating temperatures were investigated. In this study, the micro PEM fuel cell stack contains two single cells. Pure H2 gas stream was supplied as the anode inlet flow and air as the cathode inlet flow under constant pressure at 97 kPa and constant cell temperate (298K¡B308K¡B323K) conditions. Because the cell temperature may affect the chemical reaction rate on the cathode side, we discussed the influences of different temperatures on the cell performance. Solutions were compared with the experimental data. Both the value of power density and the tendency of polarization curve are in good agreement with the experimental data.

In this experiment, we are going to discuss the possibility of zero methanol crossover to the cathode target within the capacity of DMFC electrode and with proper methanol supply. After various trials, it is found that electrospray can be used to reduce fuel demand. The methanol will be consumed immediately within the electrode capacity. The methanol solution is volatile. As a result, the actual amount of electricity generated will never accord with the input. If we supply the electrode with methanol by direct contact using infusion pump, the volatility will be reduced. The total power generated then accords with the amount of methanol input. Although only low methanol concentration is supported currently, it¡¦s hoped that the crossover problem can be solved completely. In the electrode design, we try to take away the carbon cloth from the anode and leave the catalyst layer. By this way, the methanol is in touch with the catalyst. Such change is good for this experiment. In our study, following difficulties are found: (1) Methanol input (2) The impact of volatility in electrospray (3) When supplying fuels to the surface of electrode, the reaction size is too small. More attentions should be paid in the future cell design.

In this study, copper metals were used to fabricate five different flow field plates with various open ratios using MEMS technology. Five samples were prepared for experiments with rib width varying as 150, 200, 300, 450, and 600 £gm at a fixed channel width (300 £gm). The open ratio of flow field plates was varied from 60.0% to 37.9%. Experiments with different operating parameters of anode/cathode pressure drop, cell operating temperature, and gas backpressure were conducted. Furthermore, a simple lumped capacitance model was used to predict the temperature evolution of the fuel cell system. Then, the optimum flow field design and cell operating parameters were finally found. Based on the aforementioned experiments an optimal open ratio ofunity was found like 49.2%. Further, an optimal open ratio in terms of the net power gain factor (= power gain/power consumption) of 38.7% can be obtained for the cases under study. Durability and reliability for copper bipolar plate were examined for long range tests (each run with at least 5 hours duration for consecutive two months). This strongly suggests that copper sheets can be considered as one of possible candidates for flow field material.