Temperature Characteristics And Visualization Study Of A Passive Air-breathing Direct Methanol Fuel Cell

With the rapid growth of world population and continued increase of per capita energy consumption, the problem of global energy shortage is getting increasingly pronounced. Meanwhile, the increasingly serious problem of environmental pollution has become the focus of attention. If there were no new power sources, the existing power sources would be exhausted. Therefore it is urgent to develop clean and efficient energy technology. It is Fuel cell that brings the dawn for people because its efficiency and cleanliness. Direct methanol fuel cell (DMFC) is one of the high technologies with broad market prospect and now becoming a research hotspot in electrochemical science and energy field due to its high energy density, long-life and no need for charging. The passive air-breathing DMFC operates without the help of external devices for pumping methanol and blowing air into the cell, and thus oxygen diffuses into the cathode from the ambient air, and methanol diffuses into the anode from a built-in feed reservoir driven by concentration gradient between the anode and the reservoir. The passive air-breathing DMFC becomes the preferred power for mobile devices because its high reliability, low cost, simple structure, high fuel efficiency and high energy density.In the present study, a passive air-breathing direct methanol fuel cell with an active area of 4.41 cm2 and a built-in methanol solution reservoir of 6.7 ml was used to investigate the cell performance, visualization study of cathodic flooding and temperature characteristics. The effects of catalyzer, Nafion membrane, gas diffusion layer material, methanol concentration, relative humidity (RH) and temperature of ambient atmosphere on cell performance were discussed. The distribution of water droplets at different discharging current densities, relative humidities and temperatures were visualized and discussed. The effects of methanol concentration and discharging current density on the cell temperature characteristics were also discussed. Some remarkable conclusions are drawn as follows:①A series of experiments were carried out to optimize the cell performance. It was found that in the production of MEA catalyst layer, a better cell performance was gained when the amount of carbon was 15 wt%. The performance becomes better with higher concentrations, which can be attributed to the higher temperature caused by the exothermic reaction between the permeated methanol and oxygen on the cathode. However, when the concentration was too high, methanol crossover would cause excessive mixed potential and result in the reduction of cell performance. A maximum power density of 12.5 mW/cm2 was obtained with 4.0 M methanol solution. As to different GDL material, carbon cloth got a better cell performance because the pores distribution of carbon cloth was more uniform, resulting in more evenly distributed catalytic layer and smaller mass transfer resistance. Methanol crossover was more serious when the thin membrane was used, which caused a sharp decline in cell performance. So a thicker membrane was recommended for passive DMFC that operate with high methanol concentrations.②It was found that the effects of RH on cell performance were dependent upon temperature. When the temperature is below 10 oC, RH shows no effect on the cell performance. At the temperature of 20 oC and 35 oC, RH also shows little influence on the cell performance at low to medium current densities, and the cell performance decreases with the increase of RH at high current densities. In the case of 50 oC, the increasing RH has a greater effect on the cell performance than those at lower temperature: the cell performance is improved at low to medium current densities, whereas it is depraved at high current densities.③It was found that the cell performance improved with the increase of ambient temperature.④Some dispersed liquid droplets appeared at the beginning of constant current discharge, and then grew big enough to coalesce. The liquid film formed and covered the whole air-breathing hole at the end of discharge. The cell voltage dropped evidently when the serious water flooding happened in the cathode. The cover ratio of liquid in the cathode increased with the increase of discharging current density, and it increased sharply at the beginning of discharge. A higher relative humidity and lower ambient temperature led to more serious water flooding in the cathode.⑤The open circuit voltage rise rapidly after the fuel injection, then declined slowly, and a slight rebound in the final, while the cell temperature increased because of methanol crossover. After discharge started the cell voltage dump firstly, and then declined slowly with the discharge. The cell temperature increased at first and then decreased continuously, and it dropped evidently after constant current discharge. The cell temperature increased with an increase in discharging current density. A higher methanol concentration led to a higher temperature due to much more permeated methanol.⑥The cell energy efficiency improved significantly with the increase of ambient temperature, and reduced with the increase of relative humidity, methanol concentration and discharge current density.