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.

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.