Two-Phase Flow And Transport Characteristics In The Anode Of Direct Methanol Fuel Cell

Power source as an important base of increasing civil economy and living standard of people is a main factor effecting the improvement of economy. At present, coal, soil, and natural gas are used mostly. But, the combustion of these fuels, such as dust, carbon dioxide, nitrogen oxide and sulfur oxide, have been badly polluted the environment. Furthermore, they can't be reproduced in short time and the reserves are limited. With rapid increase of the word population and the gradually increased wastage of power sources, the problem of shortage of power sources becomes more and more serious. Meanwhile, severe environmental pollution has been become hot point attended in the world. If there were not new power sources, the existing power sources would be exhausted. So, we must look for new power sources. Fuel cell with high advantages of efficiency and cleanness brings hope to us.Direct methanol fuel cell using a solid polymer membrane as electrolyte has the advantages of high energy density and power density, zero emission and fast response. Furthermore, it is much more convenient for DMFC to be fueled, safety to be stored, and simpleness to be handled compared with the Proton exchange membrane fuel cell (PEMFC) fueled by hydrogen. Thus, it has been projected as the most promising power source for electric vehicles, portable electronic equipment, and other mobile application. For these reasons, the researches on the DMFC are becoming hot point in fuel cell region recently.A homemade transparent DMFC was developed to visualize experimentally the two-phase flow of aqueous methanol solution and CO₂ gas bubbles by using a high-speed video camera. The dynamic behavior of CO₂ gas bubbles including nucleation, growth, coalescence, and removal in the parallel anode channels of the operating transparent DMFC were recorded in situ, and the polarization curves were measured to provide a fundamental understanding of the relationship between the behavior of carbon dioxide bubbles and performance of the DMFC. A series of parametric studies, including aqueous methanol solution flow rate, temperature, concentration, methanol solution feed styles and cell pressure difference between the anode and the cathode were performed to evaluate the effects on CO₂ gas bubbles behavior in anode channels as well as the performance of the DMFC. Meanwhile, the effects of flow field structures on formation and distribution of the CO₂ bubbles and performance of the fuel cell was also experimented. A mass transport model on gas diffusion layer, catalyst layer and membrane in the anode was developed to analogize the factors influencing the transport of methanol. And the transport phenomenon of catalyst layer was discussed especially. The flow resistance characteristics were also researched in the anode channel and the influence of feed methanol solution concentration, methanol solution rate, fuel temperature, channel width on the pressure drop was discussed. The remarkably conclusions are drawn as follows:â‘ The transparent DMFC used in the present study was in-housed made which consists of a membrane electrode assembly with an active area of 3×3 cm², two bipolar plates with parallel channels or serpentine channel, and two transparent enclosures.â‘¡The transparent DMFC was used to visualize the carbon dioxide gas bubbles flow in the parallel channels. It is observed that carbon dioxide gas bubbles nucleate at the corner of the GDL and the ribs of the channel first and form large bubble slugs by growth and coalescence in the channels. And the pores at the corner and on the intersection of the carbon cloth fibers are favored for the emergence of carbon dioxide gas bubbles. When the slugs are removed, the covered percentage of bubbles reduces gradually and the removal rate is mostly similar. Increasing flow rate of the methanol solution accelerates the removal of the discrete CO₂ bubbles, hence improving the performance of the tested DMFC. While the performance can't improve further after increasing the methanol solution flow rate over one specific value. When hoisting the temperature of the methanol solution, the characteristics of carbon dioxide gas bubbles in the anode channels changes little. Whereas the cell performance improves at high feed temperature. High methanol concentration results in high cell performance. But the exorbitant methanol concentration may aggravate the methanol crossover to lead to lose the cell performance. More CO₂ bubbles and bigger gas slugs appear in the channels with increasing pressure difference between the anode and the cathode, but the cell performance may be improved by high oxygen concentration on the catalyst surface and lower methanol crossover due to high pressure in the cathode.â‘¢The carbon dioxide gas bubbles flowing in a serpentine channel was visualized. The volume of gas bubbles reduces with the flow rate of methanol solution and the performance improves with it. The volume of gas bubbles increases with methanol solution concentration. And the performance improves first, but reduces last. The style of feeding methanol solution from lower enhances removal of CO₂ bubbles, hence improving the performance of the tested DMFC. â‘£The effect of flow field structures on the CO₂ bubbles characteristics and the performance of the operating fuel cell was also experimented. The gas bubbles in parallel channels is more than that in serpentine channel, and the performance of the fuel cell using serpentine flow field is higher at same current density under the condition of same volume flow rate. The gas bubbles in parallel channels is more than that in serpentine channel, and the performance of the fuel cell using parallel flow field is higher at same current density under the condition of same volume flow rate of single channel. The gas bubbles in parallel channels is more than that in serpentine channel, and the performance of the fuel cell using parallel flow field is higher at low current density under the condition of same mass flow rate. But at high current density, the results reverse.⑤In the catalyst layer, the methanol solution concentration gradually reduces from diffusion layer to membrane. Large methanol solution concentration gradient appears in the catalyst layer at high feed concentration. And also the higher discharged current density is, the more rapidly methanol solution concentration reduces, and the lower methanol solution concentration appears at same place. The proton current density increases nonlinearly in the catalyst layer. The anode overpotential increases nonlinearly in the catalyst layer from L1 to L2. And the higher discharge current density is, the more it increases. In the diffusion layer, the methanol solution concentration gradually reduces from the anode channels to the gas diffusion layer. Moreover the high feed methanol solution concentration induces large methanol solution concentration gradient. The methanol solution concentration reduces gradually in the flow direction of liquid in the anode channels.â‘¥The pressure drop of the methanol solution flowing through the anode channels increases with increasing flow rate of the methanol solution. For high flow rate of the methanol solution, the methanol solution concentration in the channels is less sensitive to with the current density of the cell. The pressure drop of the methanol solution flow reduces with operate temperature of the cell and the channel width.