TWO-PHASE FLOW IN ANODE INTERDIGITAL FLOW BED OF A LIQUID FED DIRECT METHANOL FUEL CELL. Abstract

Transcription

1 TWO-PHASE FLOW IN ANODE INTERDIGITAL FLOW BED OF A LIQUID FED DIRECT METHANOL FUEL CELL H Guo, J L Jia, J Kong, F Ye, C F Ma College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, China Abstract ENR-04 Liquid fed direct methanol fuel cells have drawn increasing attention in recent years. In this paper, a liquid fed direct methanol fuel cell with transparent window was designed and fabricated for visual investigations of carbon dioxide bubbles behaviour and two-phase flow characteristics in anode interdigital flow bed under different operating conditions. The experimental results showed that the electrochemical reaction and two-phase flow interacted each other. The gas void fraction of carbon dioxide gas and mean size of CO bubbles increased with increase in current density, as well as cell temperature. The two-phase flow patterns in the interdigital flow bed are different from the phenomena in parallel channels flow bed and serpentine channel flow bed. There were gas columns in the inlet channels. The two-phase flow in outlet channels was bubbly flow. Slug flow was not observed in outlet channels of interdigital flow bed. The typical flow pattern in outlet manifold is slug flow. But bubbly flow and slug flow coexisted in the outlet manifold at low temperature. The effect of input flow rate of methanol solution was also studied. High flow rate was helpful for removal of carbon dioxide bubbles from flow bed. Keywords: Two-phase flow, Visualization, Direct methanol fuel cells, Interdigital flow bed 1. Introduction Direct methanol fuel cells (DMFC) have drawn increasing attention in recent years. In spite of relatively slow electrode reaction, liquid fed DMFCs have several advantages over vapour fed DMFCs. Those advantages include simplification of heat management systems, reduction in system size, decrease in weight, operating near atmospheric temperature and so on. Liquid fed DMFCs are considered as a promising choice for primary or auxiliary power devices in the next generation of mobile and portable systems. Most research efforts on DMFC focused on membrane and catalyst development. However, more and more researchers have recognized that there are many thermophysical issues in the fields of liquid fed DMFCs (Guo H. Ma C. F., et al., 003). The liquid fed DMFCs undergo following electrochemical reactions: Anode: CH + 3OH + H O CO + 6H + 6e (1) 3 + Cathode: O + 6H + 6e 3H O () 3 Overall: CH 3OH + O H O + CO (3) There are the aqueous methanol solution (reactant) and the carbon dioxide bubbles (product of electrochemical reaction) in the liquid fed DMFC anode. Carbon dioxide dissolvability in liquid water-methanol mixtures is fairly low. The anode side is a two-phase system primarily consisting of methanol solution and product CO gas. The presence of a lot of carbon dioxide bubbles reduces the flow area and resists transfer of reactants from bipolar plate to catalyst layer. Therefore, carbon

2 dioxide removal is critical to ensure availability of adequate surface area for methanol oxidation (Mench M. M., Wang C. Y., et al., 001). Scott, K. and Argyropoulos P. did quite lot of studies on the two-phase flow in parallel flow channels (Figure 1a), spots flow bed (Figure 1b) and metal mesh flow bed of direct methanol fuel cells (Scott, K. Taama W. M., et al., 1999; Argyropoulos P., Scott K., et al., 1999a; 1999b; Scott K., Argyropoulos P., et al., 001). Lu G. Q. and Wang C. Y. developed a 5 cm transparent DMFC and visualized bubble behaviour in parallel flow channels (Lu G. Q. and Wang C. Y., 004). Nordlund J. and Picard C., et al. observed the two-phase flow in DMFC with metal mesh flow bed (Nordlund J., Picard C., et al., 004). Yang H. and Zhao T. S., et al. performed a visual investigation of CO gas bubble behaviour inside the anode flow field consisting of a single serpentine channel (Figure 1c) (Yang H., Zhao T. S., et al., 005a). They also investigated the pressure drop of two-phase flow in the channel (Yang H., Zhao T. S., et al., 005b). The interdigital flow bed was proposed by Nguyen T. V. for hydrogen proton exchange membrane fuel cells (Nguyen T. V., 1996). When the interdigital flow bed was adopted, the reactant is forced to enter into the electrode pores and exit from them under a gradient pressure by making the inlet and outlet channels dead-ended (Figure 1d). However, the study of anode interdigital flow bed of liquid fed DMFCs has been lacking in the literature by now, because that type of flow bed will lead to high rate of methanol crossover, which is methanol permeation through the electrolyte membrane caused by the high concentration gradient of methanol from the anode to the cathode. Methanol crossover may reduce cell performance. However, it was also found that the interdigital flow bed enhanced mass transport and membrane humidification allowing to achieve high power densities of DMFCs (Aricò A. S., Cretì P., et al., 000). In recent years, the development of methanol-tolerant oxygen reduction catalysts and novel membrane with very low methanol permeability (Dillon R., Srinivasan S., et al., 004) allow us to try to use interdigital flow bed as fuel distributor of anode plate in liquid fed DMFCs for mass transfer augmentation and cell performance enhancement (Guo H., Jia J. L., et al., 005). Considering advantage of interdigital flow bed in mass transfer enhancement, visualization experiments of two-phase flow in the anode flow bed of a liquid fed direct methanol fuel cell were performed in this paper. (a) (b) (c) (d) (a) Parallel channels flow bed, (b) Spots flow bed (c) Serpentine channel flow bed, (d) Interdigital flow bed Figure 1 Schematic drawing of flow beds. Experimental.1 Experimental System The sketch of the DMFC experimental system is shown in Figure. Metering peristaltic pumps (BT04) were used to supply methanol solution. Oxygen with high purity of % as cathode

3 oxidant was supplied to fuel cell. Gas flow rates were quantified by mass flow controllers (SY 931B-EX). Two buffers in the anode inlet and cathode inlet were used to alleviate the flow and pressure pulse of reactants. Two gas-liquid separators were placed in the outlets of cathode and anode. The pressure in the anode and cathode lines was regulated by two back pressure valves. The pressure and pressure drops between the inlet and the outlet of the flow channels were measured by pressure transmitters (WQSBP) and differential pressure transmitters (1151DP) in both anode and cathode sides. A nitrogen purge system made sure to clean out the fuel and oxidant reactant left in the test system pipelines and fuel cells. An ARBIN FCTS LNR was used as electronic load in the external circuit of the fuel cell. The images of flow patterns in the anode flow channels were recorded by the high-speed video (PHOTRON, FASTCAM Super 10K) and digital camera (Sony, DSC-F505V). The images in the video recorder were converted to computer pictures with the aid of a SCSI card and the Readcam software ,6 methanol solution tank;,9,3 cut-off valve; 3 peristaltic pumps; 4, preheater; 5,1 buffer; 7,19 back pressure valve; 8,18 gas-liquid separator; 9 exhaust valve; 10, 17 differential pressure transmitter; 11,16 pressure transmitter; 1 image record system; 13 cold light source; 14 DMFC; 15 electronic load; 0 drain valve; 3 gas humidifier; 4 three-way change valve; 5 check valve; 6 mass flow controller; 7 gas filter; 8 oxygen bottle; 30, 33 pressure reducing valve; 31 nitrogen bottle Figure Schematic of liquid fed DMFC test system. DMFC with a Transparent Window A DMFC with a transparent window (Figure 3) was designed and fabricated for visualization study. The interdigital channels ( mm channel width, mm rib width and mm channel depth) were machined on stainless steel bipolar plates to form flow beds with area of 70 mm x 70 mm. The flow field pattern and dimensions in anode and cathode side are same. The anode channels were completely broken through the anode stainless steel plate with thickness of mm. Polycarbonate plate was applied to close the anode channels and to form a transparent window of anode flow bed.

4 The channels that linked to inlet manifold are called inlet channels, the channels, which connected to outlet manifold, are called outlet channels here. In all experiments of this paper, all channels were vertical, the anode inlet manifold was in the bottom of flow bed, and the outlet manifold was on the top (Figure 1d). That arrangement smoothed the way for CO bubbles removal. In the cathode side, the inlet manifold was in top and the outlet manifold was in bottom. The membrane electrode assembly (MEA) with active area of 50 cm was produced by BCS Fuel Cells Inc. The MEA consisted of a Nafion 117 electrolyte membrane and two pieces of carbon cloth. The thickness of each carbon cloth was 0.34mm. The 4mg/cm PtRu and 4mg/cm Pt black were adopted as anode and cathode catalysts, respectively. The MEA was sandwiched between two gold plated stainless steel bipolar plates with a gasket onto either side of the MEA. An electrical heating rod was Figure 3 Transparent DMFC inserted into the cathode steel plate to heat the fuel cell. A temperature controller and temperature sensor (PT100) was used to control the temperature of fuel cell. 3. Results and Discussion 3.1 Effect of current density Polarization and power density curves of above-mentioned DMFC are shown in Figure 4. The fuel cell was operated at 70 degree Celsius. Methanol solution with molarity of 1M was supplied to anode flow bed at constant flow rate of 0 ml/min.. The inlet flow rate of pure oxygen was 800 ml/min.. The pressures in anode and cathode outlet were 0 kpa gauge. The images of two-phase flow at selected current densities, corresponding to marks of a to d in Figure 4, are shown in Figure 5(a) to Figure 5(d), respectively Voltage / V a a b b c c d d Power density / mw cm Current density / ma cm- Figure 4 Performance of liquid fed DMFC with transparent window The experimental results showed that the quantity of CO bubbles in the flow bed increased with the current density. When the current density was maintained at 0 ma/cm, there were very small bubbles in channels (Figure 5(a)). In the beginning, the bubbles accreted on the wall of channels. Then, with the electrochemical reactions, the bubbles grew slowly, and finally got away from wall

5 and enter into the fluid stream in channels. In outlet channels, the free bubbles, which were still small and scattered, moved with mainstream of methanol solution to the outlet manifold. Bubbles in inlet channels accumulated in top of channel and formed gas column because of dead-end of the channel. With increasing current density, the quantity and the size of bubbles increased gradually (Figure 5(b)). Many small bubbles were observed near the exit of outlet channels. Those small bubbles coalesced with each other and formed a large bubble. In inlet channels, the length of gas column increased. The gas column even occupied whole channel in individual inlet channels. When the current density reached 80 ma/cm, the gas / liquid ratio observably increased and the size of carbon dioxide bubble expanded remarkably. Gas slug was observed in some inlet channels (Figure 5(c)). However, gas slug was not found in outlet channels even at high current density of 10 ma/cm (Figure 5(d)). At current density of 10 ma/cm, the carbon dioxide gas column occupied quite lot of interface area between inlet channels and porous media electrode. It blocked supply of methanol solution from channels to anode catalyst layer and led to fall in cell performance (Figure 4). outlet manifold outlet channels phase interface gas slug gas column inlet channels inlet manifold carbon dioxide bubble liquid column (a) 0mA/cm (b) 40mA/cm (c) 80mA/cm (d) 10mA/cm Figure 5 Two-phase flow in anode channels at different current densities

6 At all current densities, the distribution of bubble generation sites was not uniform. Bubbles continuously generated form some sites on the porous media electrode. But in the other sites, no bubble could be observed in all experiments. The reason is that the structure and micro passage distribution in porous media electrode are not homogeneous. The two-phase flow in outlet channels was typical bubbly flow at all different current density. Slug flow was not observed in outlet channels of interdigital flow bed. This is different from the phenomena in parallel channels flow bed and serpentine channel flow bed reported in literatures (Scott, K. Taama W. M., et al., 1999; Argyropoulos P., Scott K., et al., 1999a; 1999b; Scott K., Argyropoulos P., et al., 001; Yang H., Zhao T. S., et al., 005a; 005b). In addition, outlet manifold was predominated by rather long gas slug, even at low current density of 40mA/cm. Gas columns in inlet channels increased with the current density. 3. Effect of flow rate of methanol solution Visualization images of two-phase flow in anode interdigital flow bed of the DMFC at different input flow rate of methanol solution are shown in Figure 6. (a) 5ml/min. (b) 0ml/min. (c) 40ml/min. (b) 80ml/min. Figure 6 Two-phase flow in anode channels at different flow rates

7 The fuel cell was operated at 70 degree Celsius and a constant current density of 80mA/cm. Methanol solution molarity was 1M. The inlet flow rate of pure oxygen was 800 ml/min.. The pressures in anode and cathode outlet were 0 kpa gauge. At rather low flow rate, many bubbles went up in the outlet channels. A majority of inlet channels was blocked by CO gas column (Figure 6(a)). With increasing of methanol solution flow rate, both gas void fraction and average size of bubbles in outlet channels were reduced. The length of gas column in inlet channels also grew down (Figure 6(b), (c)). As the methanol solution flow rate was increased to 80 ml/min., as shown in Figure 6(d), the number of carbon dioxide bubbles was reduced because the sweeping rate of the CO bubbles was increased with the methanol solution flow rate. Meanwhile, just very small bubbles were observed in outlet channels because it did not have enough time for the growth of bubbles at high flow rate. The long gas columns almost disappeared from the inlet flow channels. This can be explained by the increasing pressure in inlet flow channels caused by dead-ends. The experimental result indicated that increasing input flow rate of aqueous methanol solution is helpful for removal of carbon dioxide bubbles. 3.3 Effect of temperature The effect of cell temperature on the polarization characteristics and bubble behaviour are shown in Figure 7 and 8, respectively. The fuel cell was operated at a constant current density of 80mA/cm. Methanol solution with molarity of 1M was supplied to anode flow bed at constant flow rate of 10 ml/min.. The input flow rate of pure oxygen was 800 ml/min.. The pressures in anode and cathode outlet were 0 kpa gauge. 0.8 Voltage / V o C 50 o C 80 o C Current density / ma cm - Figure 7 Polarization curves of DMFC at different temperatures As seen from Figure 7, increase in the cell temperature from 30 o C to 80 o C led to the improved cell performance because of faster electrochemical reaction. Meanwhile, the quantity of the reaction product, the carbon dioxide gas, had increased (Figure 8). The results of visual investigation also shows that the average size of CO bubbles in the anode flow bed increased gradually and small bubbles tended to coalesce with each other and to form larger bubbles with increase in the cell temperature from 30 o C to 80 o C. There are two reasons associated with this phenomenon. Firstly,

8 the vaporizing rate of methanol increased with increasing temperature. Secondly, the surface tension of methanol solution reduced when the temperature became higher. The experimental results also showed that the two-phase flow in outlet channels was bubbly flow at all temperatures. At low temperature (e. g. 30 o C), bubbly flow and slug flow coexisted in the outlet manifold (Figure 8(a)). With the further increase in the temperature, gas slug was lengthened (Figure 8(b)). Finally, as seen from Figure 8(c), slug flow was present almost in entire outlet manifold at high temperature of 80 o C. (a) 30 o C (b) 50 o C (c) 80 o C Figure 8 Two-phase flow in anode channels at different temperatures 4. Conclusions (1) A liquid fed direct methanol fuel cell with transparent window was designed and fabricated. With the aid of visualization technology, the carbon dioxide bubbles behaviour and the two-phase flow characteristics in the anode interdigital flow bed were experimentally studied under different operating conditions. () The experimental results showed that the two-phase flow patterns in the interdigital flow bed are different from the phenomena in parallel channels flow bed and serpentine channel flow bed. There were gas columns in the inlet channels. The two-phase flow in outlet channels was bubbly flow. Slug flow was not observed in outlet channels of interdigital flow bed. Although bubbly flow and slug flow coexisted in the outlet manifold at low temperature, the typical flow pattern in outlet manifold is slug flow. (3) The quantity and average size of carbon dioxide bubbles increased with increase in current density, as well as temperature. Increasing input flow rate of methanol solution was helpful for removal of carbon dioxide bubbles. This work will be helpful for understanding fluid processes occurred inside anode flow beds of liquid fed direct methanol fuel cells. The results also provide reference for optimization of fuel cell design. Further studies will be reported subsequently. Acknowledgements The authors are grateful to the National Natural Science Foundation of China for the financial support (Grant Nos.: , ) and to Mr. Wang M. H. and Liu X. for the helpful discussions.

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