In this paper, a 16-cell direct methanol fuel cell (called DMFC) stack was developed to power or charge a mobile phone without any voltage transformer. The various types of the performance decay of DMFCs are studied before a 16-cell DMFC stack is made. The decays due to improper storage are found and avoided. The influences of the MEA treatments on the performance are also studied. Eventually, we try to find the best storage and treatment methods to keep stacks in a good condition all the way. In order to solve the problem of methanol crossover lead to the cathode poisoned, it is necessary to operate under the proper methanol concentration and to discharge before finishing the whole experiment. It is also necessary to maintain MEAs in proper wetness so that the performance will not decline during storage. Additionally, the catalyst in the cathode will use Pt/Ru to replace Pt. This 16-cell DMFC stack is composed of two 8-banded MEAs and 16 carbon fiber bunches. Each MEA is made with 8 sets of electrodes on a piece of membrane. The stack with 16 cells will be connected in series outside of the reaction chamber. The weight and volume of this 16-cell DMFC stack are 55 g (not including 20 c.c. methanol solution) and 99 cm3. The total electrode is 50 cm2 (16-cell¡Ñ3.15 cm2 per cell). The power at voltage 4V is 1680mW when it is operating at room temperature and air breathing. The maximum power density can reach 33 mW/cm2. The specific power density is 22 mW/g and the volumetric power density is 16.9 mW/cm3. This stack can power or charge a mobile phone directly.

There are several disadvantages in conventional unipolar/bipolar plates such as cost expensive, weight heavy and volume large. Therefore, it is difficult in making use conventional 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, 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. A two-layer 16-Cell DMFC Stack had been designed and made by using the heterogeneous carbon fiber monopolar plates developed in our fuel cell laboratory. With this design, the methanol solution can be directly stored in the anode chamber which can store fuel 17 ml and does not need any auxiliary equipment, so it easy to apply to the portable power source. Not including fuel, total weight of stack is only 50g and the volume is 75 cm3. The 16 cell stack includes two pieces of 117 membrane, 16 anode electrodes loading Pt/Ru 5 mg/cm2 and 16 cathode Pt loading 5 mg/cm2. Each single cell electrode area is about 3.5 cm², so the total electrode area of the 16-cell stack is 56cm2. With methanol concentration 3 M, pumpless, air-breathing, and room temperature, the largest output power density of the fuel cell can reach 10.3 mW/cm², and the total power can reach 578 mW in this stage. The performance of the stack will be further improved in the next stage.

In this thesis, a long-term operation direct methanol fuel cell (DMFC) stack is developed. In order to reach this goal required in many ways, including select highly chemical stability materials, operating conditions must also be stable, and avoid changing the MEA structure when preserved, then can cause the DMFC to maintain stable operation for a long time. First of all, in order to avoid contaminating electrode, this study find out the chemical instability materials. Second, this study design a device which does not require power then can stability supply consumption fuel, and apply this device in 16-cell DMFC. Finally compare with continuous fuel supply and without fuel supply, two operating conditions performance stability. From these experiments can find out, the DMFC indeed in stable operation for a long time under the appropriate supplement. Traditional fuel supply systems typically using the pump fuel recycling, so the structure is more complex, difficult to reduce the volume, and not conducive to carry. If using a passive operation, fuel completely stored in the reaction Chamber, even though the structure is simple there will be a problem with fuel supply. In recent years, someone use vapors of methanol to supply the fuel, although can use high concentration methanol to extend operating time, but the evaporation rate is difficult to control, the fuel can¡¦t be supplied in time, especially when the large current is needed, and CROSSOVER issues would be difficult to overcome. In our 16-cell DMFC, continues to add appropriate amount of fuel consumed which according to the different current. The fuel supply device with a sliding control plate which can control methanol and water diffusion rate respectively. This device only to provide consumed by reaction and leaked fuel in anode chamber, so that the methanol concentration can maintained in the proper range at anode chamber. This device only use diffusion and gravity effects, don't use a fuel pump, so will not consume DMFC power.

Abstract A one-watt portable air-breathing direct methanol fuel cell stack (called DMFC), which can supply fuel passively and operate steadily, is developed in this thesis. A DMFC to maintain its performance stable, the most important strategy is to keep the methanol concentration in reacting chamber to be proper and stable. A fuel supplying system will be in accordance with the depletion of chemical reaction and the leakage of fuel under different circuit current to supplying fuel. To regulate the methanol and water supplying, a fuel supplying system by gravitation and diffusion forces deliver methanol and water to fill up the consumed fuel to maintain the concentration of methanol solution in anode reaction chamber, by adjusting a sliding gate to control the area of a diffusive membrane and utilizing three cotton threads and hoses to distribute the fuel to proper location. In doing so, the methanol concentration in the anode chamber can keep within an appropriate range, so that the DMFC stack can operate stably for a longer period. Yet the diffusivity of the diffusive membrane is comparatively less, the supply system is not easy to downsize. To reduce the size of portable DMFC, we make use of a fuel plug tank to combine the supply tank and reacting chamber, and thus the cell package is more portable. Between the plug tank and the reacting chamber, the three cotton threads are used to distribute the fuel to proper location. The above two design with no extra auxiliary device; therefore, no extra energy will be consumed. To reduce the fuel leakage, and make more use of fuel, four block films is pasted on the bare area of the nafion membranes in a 16-cell DMFC stack. If no fuel is fed into reaction chamber, this will prolong the cell operation time. Under the condition of 3.7 V (cell phone rated voltage) and the operating current 225 mA, our experiments display that the stacks with the two fuel supplying systems can continuously operate for more than 3 hours with no obvious change in methanol concentration within reaction chamber. The experimental results show that this simple passive fuel supplemental system can really keep the DMFC stack operating stably for a sufficient long period.