| 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 is considered as promising substitution to the conventional power sources for portable devices because of its simple system, high energy density, environmental benignancy, fast refueling and low operating temperature, has been a research hotspot in the field of electrochemistry and energy science.Based on the literature survey, it is easy to find that many researches are focused on DMFC at the higher temperatures (>60℃). However, mobile phones, laptops and other portable electronic products work at ambient temperature of25℃. Therefore, studies on DMFC at room temperature have very great significance.The work of this paper is divided into two parts:the liquid-feed DMFC experimental and the experimental study of the air-breathing DMFC. The air-breathing DMFC is one category of DMFC without external pumps or other ancillary devices for fuel and oxidant supply. Both of them are likely to substitue the battery of portable electronic devices as the power.Firstly, a transparent DMFC was developed to visualize the two-phase flow and transport in the anode and cathode flow field. The main contents are drawn as follows:Two-phase flow and characterization of flow resistance in the anode flow field; Visualization of water flooding in the cathode flow field; Effect of the anode and cathode flow fields on the cell performance.(1) Investigation on two-phase flow and transport characteristics in the anode flow field were conducted. In the serpentine flow field, the quantity of CO2bubbles increased with the increase of current densities. At low current densities, bubble flow appeared in the anode flow field; at moderate current densities, a number of gas slugs formed, and then bubble flow changed into slug flow; at high current densities, plug flow appeared. As the methanol flow rates increased, the amount of the CO2 bubbles decreased, enhancing the mass transport process of methanol. However, higher methanol flow rate led to an increase of methanol crossover and to take away more heat. This eventually, resulted in a deterioration of cell performance. Therefore, it is significant to use an appropriate methanol flow rate for the portable DMFCs operating at ambient temperature. Channel-blocking phenomenon caused by CO? gas was found in the parallel flow field and interdigitated flow field, but it was not found in the serpentine flow field. The channel-blocking phenomenon was found in the interface of the flow channel and outlet pipes of the parallel flow field and it was found in the inlet flow channel of the interdigitated flow field.(2) Characterization of flow resistance in the anode flow field was experimentally studied. At low methanol flow rates, the pressure drop enhanced at first, and then decreased and eventually to be stable with increasing current density. Methanol concentration had a significant influence on the cell performance and less effect on the pressure drop. The pressure drop in serpentine flow field was larger than that in parallel flow field and interdigitated flow field under the same operating condition. With the increase in the methanol solution flow rate, pressure drop in serpentine and interdigitated flow field increased, which was lager than that in parallel flow field.(3) Two-phase flow and transport characteristics in the anode flow field were experimentally investigated. The water fog appeared in the middle and lower reaches of the anode flow field in the serpentine flow field at the beginning of discharge test; then the water droplets emerged around the corner of the channel ribs; afterwards, the distribution of liquid water formed a cyclic process (water droplet-water slug-water membrane). For the constant current discharge test, at the same discharge time, the amount of water droplets increased with the increase of current density. At low and moderate current densities, the cell voltage decreased with discharge time; while, at high current densities, the cell voltage increased at first and then decreased. With increasing oxygen flow rate, the distribution of water in the flow field became smaller, thus enhancing the mass transfer of the oxygen and improving the cell performance. At high current densities, channel-blocking phenomenon caused by liquid water was found in the parallel flow field. With a higher oxygen flow rate, this phenomenon could be mitigated. For the interdigitated flow field, some water slugs appeared in part of the inlet channel only at low oxygen flow rate, resulting in a small effect on the cell performance.(4) Effect of the anode and cathode flow fields on the performance of DMFC was experimentally investigated. The serpentine flow field (SFF) as anode flow field had a positive effect on cell voltage and power for the better removal of COt bubbles. It was also found that CO2bubbles blocked the flow channels in the parallel flow field (PFF) and interdigitated flow field (IFF) at high current densities, leading to a worse cell performance. For the cathode flow field, it was found that water drops blocked the flow channels in the PFF, but this channel-blocking phenomenon was never found in the SFF and IFF. The DMFC equipped with the SFF and IFF yielded much better performance than that with the PFF. The IFF enhanced oxygen transport and the cell gained a better performance than that with the SFF at high current densities. It could be deduced that the SFF as the anode flow field and the IFF as the cathode flow field would be one of the best choice for the DMFC with a maximum power density of45mW·cm-2being achieved at25℃Secondly, a transparent air-breathing DMFC was developed to visualize the two-phase flow and transport process in the anode and cathode flow field. The main study contents include the transient voltage and temperature characteristics of the DMFC in the test and the visualization of the CO2bubble behavior in the anode current-collector and water droplets accumulation in the cathode current-collector. The effect of the anode and cathode current-collectors on the cell performance was also investigated. The main results are summarized as follow:(1) The transient voltage and temperature of the passive air-breathing DMFCs were investigated experimentally at different discharging current densities and methanol solution quantities. The cell performance became better with the longer waiting time, but became stable when the parameters (open circuit voltage, anode temperature difference, cathode temperature difference and methanol solution temperature difference) approch to steady. When the waiting time was longer than 30min, all of the parameters of cell were basically stable, and then the polarization data could be collected with different concentration of methanol solution. In order to get a more accurate measurement results, the waiting time could be properly extended to60min. The temperature difference increased with an increase in the discharging current density. At the low current density (20mA·cm-2), the temperature difference increased at first and decreased subsequently. At the high current density (50mA·cm-2), the temperature difference did not fall down because of the short discharging time. At the same concentration of methanol solution, with a smaller methanol solution quantity, the cell performance became better due to the higher temperature difference.(2) Visualization of the CO2bubble behavior in the anode current-collector was investigated. In the perforated current collector, with the increase of current density, the quantity of CO2gas bubbles increased progressively, most of them were presented at the upper portion of the breathing holes. Since the CO2gas bubbles could depart from the parallel current-collector more easily than that from the perforated current collector, the cell performance with parallel current-collector was better than that with perforated current collector. The parallel current-collector placed vertically could eliminate CO2bubbles more efficiently than that placed horizontally. Therefore, it gained a better performance.(3) The water droplets accumulation in the cathode current-collector was visualized during the constant current discharge test. At the beginning of discharge test, the generation rate of water droplets was slowly. However, at the end of discharge test, the generation rate of water droplets was fast. The distribution of the water droplets was not uniform. Water droplets always emerged at some preferential locations. For the constant current discharge test, at the same discharge time, with the increase of current density, the amount of water droplets increases. At the high current density (>50mA·cm-2), parallel current-collector was more efficient to remove water than the perforated current collector.(4) The effect of the current-collector structure on the performance of an air-breathing DMFC was investigated. Parallel current-collector (PACC) as anode current-collector had a positive effect on cell voltage and power density. For the cathode current-collector structure, the performance of DMFC increased at the methanol concentration of2mol·L-1for perforated current collector (PECC-2). But the methanol concentration of4mol·L-1led to an enhancement of cell performance that adopting PACC as cathode current-collector. |
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