Two-Phase Flow And Transport Characteristics In Air-Breathing Direct Methanol Fuel Cells

The traditional energy conversion process is usually carried out by the heat engines, which is limited by the Carnot cycle leading to low efficiency and serious energy waste. Meanwhile, severe environmental pollution problem arises from the generated deleterious species and noise during the process. However, unlike the power generation process of heat engines, fuel cells are energy conversion devices in which the chemical energy is directly converted into electricity without the intermediate of thermal energy. They are regarded as clean power generation technique with high efficiency due to less energy conversion steps, no combustion process and rotated components, and without the limitation of Carnot cycle. In recent years, with the rapid development of electric devices such as mobile phones, personal digital assistants and laptop computers, which demand much more power due to new functions, the present battery technology is unlikely to keep pace with these growing power demands. The air-breathing direct methanol fuel cell (DMFC) which are considered as an attractive alternative to the conventional power sources for portable devices because of its simple system, high energy density, environmental friendly emissions, fast refueling and low operating temperature, are becoming a research hotspot in the field of electrochemistry and energy science.Compared with the active DMFC, the electrochemical reaction rate of the air-breathing DMFC is lower resulted from the slow mass transport of reactants. The different structure from active ones causes not only the poorer performance but also the various heat and mass transport characteristics. Thus, it is necessary to investigate the two-phase flow and transport characteristics inside the air-breathing DMFC.The cell performance and mass transport characteristics of an air-breathing DMFC were investigated with a home-made membrane electrode assembly (MEA). The effect of ambient conditions on the cell performance was discussed experimentally. The water droplets accumulation in the cathode and its effect on discharging performance were studied with the aid of visualization technique. The operating temperature characteristics in the whole test including before, during, and after the discharging process were recorded in the present study. The effect of wettability of anode microporous layer (MPL) on the performance and operation duration was also tested. A mathematical model was developed to comprehensively describe the two-phase flow and mass transport and analyze the mechanism of methanol and water crossover inside an air-breathing DMFC with active fuel supply. The mass transport in the catalyst layer and the discontinuity in liquid saturation at the interface between the diffusion layer and the catalyst layer are particularly considered. The main results are summarized below:1) The performance of air-breathing DMFC was improved to four times after being activated by adding methanol solution into reservoir and then resting for 24 h. A simpler filtration method with high reproducibility to fabricate MPL was proposed, through which more flat MPL, higher cell performance and longer operation duration could be obtained.2) The effects of catalyst, diffusion layer material, methanol concentration and current collector structure on the cell performance and mass transfer characteristics were discussed. The experimental results show that the mass transfer of reactants and the cell performance are enhanced by using catalyst with high noble metal loading or carbon cloth in the MEA. The better performance can also be obtained by increasing methanol concentration properly or testing with parallel current collectors.3) The influences of ambient temperature and relative humidity on the cell performance were investigated. The results reveal that at the same relative humidity, the performance of air-breathing DMFC increases with the increasing of ambient temperature. At the same temperature, the effect of relative humidity on the cell performance is related to the value of the ambient temperature. There is no evident effect of relative humidity observed at a low temperature. However, with the increasing of temperature, the relative humidity plays a more significant role in the cell performance.4) The water droplets accumulation in the cathode was visualized with a digital camera during the constant current discharge test. It is observed that water droplets always emerge at some preferential locations and the amount of water still increases in the air-breathing holes fully covered by liquid droplets. The discharging time at a low current density is dominated by the water droplets accumulation at the cathode, while the consumption of methanol is the dominator for that at a high current density. The longer discharging time can be obtained by improving methanol concentration properly, reducing the discharging current density and the relative humidity.5) The experiments on the operating temperature characteristics during the whole test process were conducted. It is found that after injecting fuel into the cell, the temperature difference between the cell and ambient increases rapidly in a few minutes, but after a mild increase it tends to be a constant value. During the constant current discharge, the temperature difference rises at first and then goes down continuously, and it increases with the increasing of discharging current density. However, at a high discharging current density, the temperature difference goes up at all times due to short discharging time. Once the constant current discharge terminated, the temperature difference drops significantly. In addition, a higher methanol concentration leads to a higher the temperature difference due to a larger methanol crossover rate.6) Comparative studies on cell performance and operation duration of air-breathing DMFCs with different anode GDLs were performed experimentally. The results demonstrate that when the performance evaluation was conducted in the fast scan mode (a holding time of 45 s at each current density), the DMFC with a hydrophilic MPL (DMFC-L) in the anode shows superior performance to that with a hydrophobic MPL (DMFC-B). While the DMFC-B shows better performance at medium and high current densities in the slow scan mode (a holding time of 150 s), which results principally from blockage of the oxygen supply due to the water droplets accumulated during the cell performance evaluation. Although the DMFC-L can yield a better performance at the beginning of constant current discharge, the DMFC-B exhibits a higher performance and longer operation duration in the following discharge process due to a lower rate of water accumulation with the hydrophobic MPL in the anode.7) A two-dimensional two-phase mass transport model was developed to predict methanol and water crossover in an air-breathing DMFC with active fuel supply. The modeling results agree well with the experimental data of a home-assembled cell. The numerical results indicate that for a given current density, the methanol crossover flux increases with increasing methanol concentration; for a given methanol concentration, it increases with increasing current density for the methanol concentrations of 2 M and 4 M, while it increases slightly at low current densities and decreases at high current densities for 1 M. Diffusion predominates the methanol crossover at low current densities, while electro-osmosis is the dominator at high current densities. Water transport through the membrane depends on electro-osmosis and hydraulic pressure difference across the membrane at low current densities; however, electro-osmosis plays a critical role in the water crossover at high current densities. The total water flux at the cathode is originated primarily from the water generated by the oxidation reaction of the permeated methanol at low current densities, while the water crossover flux is the main source of the total water flux at high current densities.