Study On Key Components Of An "Air-Breathing" Direct Methanol Fuel Cell

Direct methanol fuel cell (DMFC) is viewed as a promising power source for various portable devices due to its simplicity, convenience and higher energy density of the system. Water management within the cathode and methanol feeding on the anode side of a DMFC are two key factors for the improvement of performance and stability of a DMFC.To choose the suitable carbon materials used in micro porous layer (MPL) within the cathode, the performance and stability of the membrane electrode assemblies (MEAs) by using different kinds of carbon powders as MPLs are compared. By mixing different carbon powders at a given mass ratio as MPL, the pore structure of cathode MPL could be optimized, leading to a decrease in mass transfer resistance and to an improvement in the cathode water management. As a result, the water-flooding within the cathode could be avioded under "air-breathing" mode and the performance and stability of a DMFC can be improved greatly. The experimental results show that the addition of Black Pearl 2000 (BP) into the cathode diffusive layer has a significant influence on the performance of a DMFC. When the ratio of Ketjen Black (KB) and BP is ca. 9:1, the maximum power density of an air-breathing DMFC at a temperature of ca. 25℃is 20.8 mW/cm~2. Such a DMFC also exhibits a better stability. The enhanced power density and improved stability of the DMFC could be ascribed to the improvement in the porous structure within the cathode due to the addition of small amounts of BP. The addition of pore-former into the MPL and the catalystic layer of a DMFC can also improve the pore structure of cathode, thus decreasing the mass transport resistance of gas reactants.Based on the improvement in the pore structure of cathode MPL, another choice to address the water management in this work is a hydrophobic treatment of silicon based flow field plate on cathode side of a micro DMFC fabricated by MEMS technology. The hydrophobic treatment of Si plate efficiently prevents water from aggregation on the outside of cathode, thus leading to fast evaporation of water into the air and decreasing the possibility of water-flooding within the cathode. Anode reaction of a passive DMFC will consume methanol and water under operation, which will result in a change in methanol concentration, thus affecting the stability of power output. To carry out continuous and stable supplement of methanol for anode side, in this work, a kind of composite membrane with the functions of water-proofing, and gas and alcohol permeation was developed. Using this composite membrane between pure methanol tank and methanol solution chamber, methanol could penetrate through the membrane from fuel tank to mixing chamber at a certain velocity, thus enabling a stable fuel delivery for anode reaction. Such a technique ensures the addition and use of pure fuel for a DMFC system, which could be benefical for the increase in energy density of a DMFC system.