Study On The Water Management Of A Direct Methanol Fuel Cell With An Air-breathing Cathode

Along with the development of microelectronics, energy needs for portabledevices are rising rapidly in the past few years due to their increasingfunctionalities. Direct methanol fuel cell (DMFC) has been regarded as one of themost promising candidates to replace the current batteries for its convenience touse, high energy density, low-temperature start and environmental friendliness.However, the commercialization of DMFC technology is still hindered by sometechnological problems, such as methanol crossover, cathode water managementand stability. This paper presents a theoretical and experimental study on the watermanagement of a passive DMFC with an air-breathing cathode. The behaviors ofwater transport in the membrane electrode assembly (MEA) are carefully studiedand a novel MEA with a water retention layer in the cathode was designed andfabricated to suppress the water crossover flux.With the help of a two-dimensional two-phase mathematical model, the watertransport behaviors, electrochemical kinetics and the effect of the existence of thewater retention layer in the cathode are well examined. The simulation resultsshow that the mass transport of methanol in the anode porous area of a passiveDMFC mainly depends on the free diffusion while oxygen transport in the cathodeis highly affected by the liquid saturation. The water content distribution in theproton exchange membrane (PEM), which has a significant impact on the watercrossover flux, is dominated by the liquid saturation in the catalyst layers of bothanode and cathode. When the water retention layer is put into the MEA, a lessinner resistance of the PEM and a less water crossover flux are obtained.Using carbon nanotubes (CNTs) as the water retention layer, a novel MEA ofthe new structure was fabricated by two means: GDE and CCM. According to theexperimental results, the novel MEA made by the CCM method had a much betterperformance than the other. An Electrochemical Impedance Spectroscopy (EIS)test revealed that the inner resistance of the MEA was less than the standard MEAwhen the CNTs loading was no more than0.4mg/cm~2. The CNTs loading had afurther influence on the cell performance and water crossover: when the loadingwas too high,0.8mg/cm~2, the water and methanol crossover downgraded to theleast, but the fuel cell suffers the worst performance. When the loading was toolow,0.2mg/cm~2, almost no contribution could it make to improving the cellperformance as well as suppressing the water crossover. A cell performance of32.7mW/cm~2, about9%higher than the standard MEA, was obtained when the CNTs loading came to an appropriate value of0.4mg/cm~2. At last, theexperimental results also show that the novel MEA had advantages over thestandard one on the stability test.