Effect of Operating Parameters on the Direct Methanol Fuel Cell Using

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Energy & Fuels 2008, 22, 1212–1219

Effect of Operating Parameters on the Direct Methanol Fuel Cell Using Air or Oxygen As an Oxidant Gas Sang Hern Seo† and Chang Sik Lee*,‡ Graduate School of Hanyang UniVersity, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea, and Department of Mechanical Engineering, Hanyang UniVersity, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea. ReceiVed NoVember 13, 2007. ReVised Manuscript ReceiVed December 24, 2007

The objective of this study is to analyze the performance characteristics of the direct methanol fuel cell (DMFC) using air or oxygen as the oxidant gas at various operating conditions. In order to investigate the effect of various operating parameters on DMFC performance, experiments were performed with various parameters including cell temperature, methanol concentration, flow rate, cathode humidification temperature, and cathode backpressure. The membrane electrode assembly (MEA) used Nafion 117, by loading a Pt-Ru (4 mg/cm2) catalyst at the anode and Pt-black (4 mg/cm2) catalyst at the cathode. The performance of the DMFC was analyzed with a polarization curve, which was measured with the voltage-current density and power-current density. It was revealed that the performance of the DMFC was enhanced by increasing of the cell temperature, cathode flow rate, and cathode backpressure. Also, the methanol diffusion and transfer conversion rate of the fuel cell are analyzed by using the effective mass transfer coefficient and Damkohler number under various performance parameters. The comparison of the performance with both air and oxygen showed that using the oxygen resulted in higher performance than that of air by the active electrochemical reaction.

1. Introduction It is known that natural resources are being depleted and environmental regulations are being strengthened more and more. Therefore, it is necessary to develop alternative energy to replace natural resources. Fuel cells are in the limelight as an energy conversion device and have been studied actively. There are many kinds of fuel cells including the direct methanol fuel cell (DMFC). The DMFC is a kind of polymer electrolyte membrane fuel cell (PEMFC) and has potential use for applications such as automobile and portable devices in the near future. Methanol has a higher energy density and is easier to handle and store than hydrogen. Besides, it may be more profitably used than hydrogen since its cost is lower and it can be designed easily with compact size due to the lack of the need for a reforming system. However, the practical open circuit voltage (OCV) is much lower than the theoretical OCV due to the slow kinetics of methanol oxidation at the anode, oxygen reduction at the cathode, and methanol crossover in the DMFC. The phenomenon of methanol crossover is that methanol permeates through the membrane from the anode to the cathode by diffusion and electro-osmotic drag and then oxidizes with oxygen at the cathode. So, methanol crossover not only decreases cathode potential but also consumes oxygen at the cathode. With the passage of time, the cathode catalyst is poisoned by reaction intermediates such as carbon monoxide absorbed on the catalyst surface the same as anode and, hence, the cell performance decreases more and more. On the basis of these results, methanol * Corresponding author. Tel: +82-2-2220-0427. Fax: +82-2-2281-5286. E-mail: [email protected]. † Graduate School of Hanyang University. ‡ Hanyang University.

crossover leads to low efficiency, low power density, and low fuel utilization. It is therefore necessary to clarify the electrochemical reaction in the DMFC, and it is now being actively studied. Research on the DMFC has been conducted to characterize the effect of the operating parameters on DMFC performance. Ge and Liu1 experimentally studied the parameters that affect the minimum polarization and crossover on DMFC performance by changing cell temperature, methanol concentration, and flow rate. Jung et al.2,3 investigated the characteristics of the DMFC performance using Nafion 115 and Nafion 117 and proposed an optimal membrane for the DMFC with various operating conditions. Qi and Kaufman4 reported on the open circuit voltage of the DMFC with various operation conditions and proposed the possibility of reducing crossover in the DMFC. Chu and Jiang5,6 studied the performance and efficiency with various operating conditions using a passive type of DMFC that is supplied by methanol diffusion. Kim et al.7 investigated the operational characteristics of a 50-W DMFC stack at various operating conditions including the methanol concentration, flow rate, and flow direction. Shukla et al.8 experimented with a twocell DMFC stack with a methanol concentration of 2 M under ambient conditions and suggested that the management of (1) Ge, J.; Liu, H. J. Power Sources 2005, 142, 56–69. (2) Jung, G. B.; Su, A.; Tu, C. H.; Weng, F. B. J. Fuel Cell Sci. Technol. 2005, 2, 81–85. (3) Jung, D. H.; Lee, C. H.; Kim, C. S.; Shin, D. R. J. Power Sources 1998, 71, 169–173. (4) Qi, Z.; Kaufman, A. J. Power Sources 2002, 110, 177–185. (5) Chu, D.; Jiang, R. Electrochim. Acta 2006, 51, 5829–5835. (6) Jiang, R.; Chu, D. J. Power Sources 2006, 161, 1192–1197. (7) Kim, D.; Lee, J.; Lim, T. H.; Oh, I. H.; Ha, H. Y. J. Power Sources. 2006, 155, 203–212. (8) Shukla, A. K.; Christensen, P. A.; Dickinson, A. J.; Hamnett, A. J. Power Sources 1998, 76, 54–59.

10.1021/ef700677y CCC: $40.75  2008 American Chemical Society Published on Web 02/09/2008

Effect of DMFC Operating Parameters

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Table 1. Specifications of the Unit Cell items

specification

electrolyte membrane effective electrode area membrane area anode catalyst cathode catalyst gasket graphite block

Nafion 117, 183µm 5 cm2 (2.24 cm × 2.24 cm) 54.3 cm2 (7.37 cm × 7.37 cm) Pt-Ru 4.0 mg/cm2 Pt-black 4.0 mg/cm2 Teflon serpentine grooves with width of 0.7874 mm and depth of 1.016 mm

carbon dioxide is extremely important. Oedegaard and Hentschel9 studied the effect of a feeding concept for the DMFC performance with the operating parameters of air flow rate, methanol concentration, and cell temperature. Wong et al.10 and Hsieh et al.11 examined the performance of micro DMFC and PEMFC with several types of flow fields under various operating conditions. Gurau and Smotkin12 experimentally studied the effect of methanol crossover, which resulted in low performance and efficiency in the DMFC, by measuring the amount of carbon dioxide with gas chromatography. Scott et al.13,14 have reported the influence of mass transport and methanol crossover on the DMFC and limiting current density characteristics of DMFC under various operating parameters. Despite previous research on the DMFC, the effects of operating parameters on DMFC performance are not fully understood, due to the complex electrochemical reaction and the difficulties of conducting experiments inside a fuel cell. The aim of this study is to investigate the effect of operating parameters on the performance characteristics of the DMFC using air or oxygen as an oxidant gas at various cell conditions. The performance was analyzed using a polarization curve, which is found by measuring the voltage-current density and power-current density, and investigated via the output characteristics at each region of current density. 2. Experimental Apparatus and Procedure A single fuel cell with an effective active area of 5 cm2 was used in this study. The membrane electrode assembly consists of two electrodes with loading of Pt-Ru (atomic ratio 1:1) of 4.0 mg/cm2 as the anode catalyst and loading of Pt on carbon 60% of 4.0 mg/cm2 as the cathode catalyst with a Nafion 117 membrane with a thickness of 183 µm. The gas diffusion layers used were carbon cloth at the anode and ELAT (E-TEK, LT1400-W) at the cathode for the electrical connection with the catalyst and good mass transport of reactants. And, the gaskets to seal reactants and products used were Teflon with a thickness of 250 µm. The graphite blocks have a serpentine flow field with a width of 0.7874 mm and a depth of 1.016 mm at both the anode and cathode sides, and the gold-coated copper plates were used as the current collectors. The single fuel cell stack was clamped by two aluminum end plates through eight screw joints for even load distribution. The specifications of the test fuel cell are listed in Table 1. The fuel cell testing apparatus illustrated in Figure 1 was utilized to obtain the performance characteristics of the DMFC in terms of the polarization curve. It consists of a methanol supply system, air supply system, flow and temperature control system, and back(9) Oedegaard, A.; Hentschel, C. J. Power Sources 2006, 158, 177– 187. (10) Wong, C. W.; Zhao, T. S.; Ye, Q.; Liu, J. G. J. Power Sources 2006, 155, 291–296. (11) Hsieh, S. S.; Yang, S. H.; Kuo, J. K.; Huang, C. F.; Tsai, H. H. Energy ConVers. Manage. 2006, 47, 1868–1878. (12) Gurau, B.; Smotkin, E. S. J. Power Sources 2002, 112, 339–352. (13) Scott, K.; Taama, W. M.; Kramer, S.; Argyropoulos, P.; Sundmacher, K. Electrochim. Acta 1999, 45, 945–957. (14) Scott, K.; Taama, W. M.; Argyropoulos, P.; Sundmacher, K. J. Power Sources 1999, 83, 204–216.

Figure 1. Schematics of the experimental apparatus. Table 2. Experimental Conditions cell temperature methanol concentration anode flow rate cathode flow rate cathode humidification temperature cathode backpressure

40, 50, 60, 70, 80 °C 0.5, 1, 2, 3, 4 M 1, 3, 5, 7, 9 mL/min 100, 200, 300, 400, 500 cm3/min 30, 45, 65, 85 °C 100, 200, 300 kPa

pressure regulation system. The voltage and current were obtained by a direct current (dc) electronic load controller (AMREL, FEL 60-10-10) through potentiostatic mode which has a 0.003 V resolution and (0.2% accuracy. Methanol fed into the fuel cell through the peristaltic pump and the flow rate was changed by the control program. Air or oxygen was used as the oxidant gas, and the flow rate was controlled by a digital mass flow controller (MKS Instruments, 1179). The backpressure was controlled by a pressure regulator (Marklyn Controls, 44-2362-24) at the exit of the anode and cathode. In addition, air and oxygen were humidified by a humidity bottle assembly, and the cell temperature was measured by a type T thermocouple at the end plate of the cathode. The experiments were performed with both air and oxygen as the oxidant gas to compare the performance characteristics of the DMFC. In order to investigate the performance characteristics of the DMFC at various operating conditions, the basic parameters other than the single changing parameter were kept constant in all experiments. The standard conditions were a cell temperature of 80 °C, methanol concentration of 1 M, methanol flow rate of 3 mL/min, air or oxygen flow rate of 200 cm3/min, cathode humidification temperature of 70 °C, and cathode backpressure of 100 kPa. The details of the conditions for the experiment are listed in Table 2. In this experiment, the cell temperature was changed from 40 to 80 °C by 10 °C intervals, and the concentration of methanol solution was controlled from 0.5 to 4 M. The flow rate used was from 1 to 9 mL/min at the anode, and from 100 to 500 cm3/min at the cathode. In case of cathode backpressure, it was varied from 100 to 300 kPa at the cathode. The voltage and current were obtained with a constant voltage mode of 0.02 V drop in all experiments. The open circuit voltage was measured while there was zero current in all experiments. Also, stabilized operation, which took approximately 30 min after each experiment, was required to acquire exact voltage and power density due to the current interrupt. To investigate the influence of methanol diffusion and transfer conversion rate in the fuel cell, the effective mass transfer coefficient and Damkohler number suggested by Scott et al.13,14 were used in this study. The effective mass transfer coefficient, keff, was determined from keff )

j nFCMeOH

(1)

And, the ratio of the methanol reaction rate and methanol supply rate to the cell was demonstrated by the Damkohler number, Da, defined as

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Figure 3. Comparison of the OCV between air and oxygen according to the cell temperature.

Figure 2. Effect of the cell temperature on the DMFC performance.

jAcell 6F Da ) QaCMeOH

(2)

3. Results and Discussion 3.1. Effect of Cell Temperature. Figure 2 shows the effect of cell temperature on the DMFC performance using both air and oxygen as the oxidant gas. The performance was investigated at cell temperatures from 40 to 80 °C with an increment of 10 °C as listed Table 2. It can be seen in this figure that the DMFC performance increased for each oxidant gas as the cell temperature increased. In case of air, the maximum power density was 0.159 W/cm2 at 80 °C and was doubled compared to 0.076 W/cm2 at 40 °C, and in the case of oxygen, the maximum power density was 0.231 W/cm2 at 80 °C and was trippled compared to 0.089 W/cm2 at 40 °C. Also, it is shown that the range of increment in the oxygen case was superior to that of air due to the reduction of activation loss and concentration loss by the higher oxygen concentration. The oxygen improved the performance by about 145% compared to air and oxygen at 80 °C, which indicates the maximum power density of the DMFC. The performance improvement, corresponding to the increase in cell temperature, seemed to decrease with the ohmic loss and activation loss of the fuel cell due to the activation of the catalytic and electrochemical reactions of the oxidation–reduction kinetics at the anode and cathode. The effect of the cell temperature on the OCV was compared for both air and oxygen as the oxidant gas as shown in Figure

Figure 4. Comparison of the Damkohler number and effective mass transfer coefficient between air and oxygen according to the cell temperature.

3. As can be seen in the figure, the OCV of oxygen was higher than that of air in the experimental results because the oxygen reacted with protons more actively as the cell temperature increased. From the results of little difference between two oxidant gases at the cell temperature of 40 °C, it was estimated that the kinds of oxidant gas do not do not effect OCV at the ambient temperature of 25 °C. Figure 4 illustrates the comparison of the effective mass transfer coefficient and Damkohler number obtained at the limiting current density between air and oxygen according to the cell temperature. The variation of keff with different temperatures increases at both oxidant gases due to the increase of methanol diffusion with temperature. It can be guessed that the high cell temperature enhanced methanol mass transport kinetics under the same experimental conditions. Also, the Damkohler number showed similar trends with an increase of cell temperature. Analyzing the distribution of the Damkohler number, it can be said that the methanol reaction rate of high cell temperature and oxygen is superior to the low cell temperature and air in fuel cell performance characteristics. On the basis of these results, the increase of cell temperature has a beneficial effect on DMFC performance due to the higher reaction rate, lower cell resistance, and enhanced mass transfer. Also, these results suggested that the supply of oxygen to the cathode reaction site was important in order to reduce the activation loss and concentration loss. However, it is possible to decrease the cell performance due to the concentration loss

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Figure 6. Comparison of the OCV between air and oxygen according to MeOH concentration.

Figure 5. Effect of MeOH concentration on DMFC performance.

caused by increasing the vapor partial pressure and water transfer from the anode to the cathode at over a 100 °C cell temperature. 3.2. Effect of the MeOH Concentration. The effect of methanol concentration on the DMFC performance using both air and oxygen as an oxidant gas is shown in Figure 5. In case of air, it was seen that the maximum performance was found for a methanol concentration of 0.5 M and the performance of the DMFC decreased sharply with the increase of methanol concentration from 0.5 to 4 M. This trend indicates that the catalyst reaction of the anode was restricted to high methanol concentration and the nonreacted methanol decreased the performance due to the methanol crossover phenomenon permeating through the membrane from the anode to the cathode as the methanol concentration increases. On the other hand, in the case of oxygen, the maximum performance is found with a methanol concentration of 1 M. The performance decreased as the methanol concentration increased, but the decrement is much less than that occurring when using air. On the basis of the above, the main reason for obtaining high performance compared to the case of air as the oxidant gas is that the high density of oxygen is large enough to react with a high methanol concentration and then reduce the concentration loss. Figure 6 shows the effect of methanol concentration on the OCV of the DMFC. It reveals that the measured results of the OCV decreased with an increase of the methanol concentration. However, the slope of the oxygen curve decreased gently, whereas in case of air, the slope decreased steeply at 3 and 4 M. It can be said that the high oxygen concentration oxidized the nonreacted methanol quickly through the membrane as the methanol concentration increased.

Figure 7. Comparison of the Damkohler number and effective mass transfer coefficient between air and oxygen according to the MeOH concentration.

Figure 7 shows the comparison of the effective mass transfer coefficient and Damkohler number for both oxidant gases according to the various MeOH concentrations. The mass transfer coefficient in both oxidant gases gradually decreased accordingly with the methanol concentration from 0.5 to 4 M due to the decrease of the limiting current density. In the same way, the Damkohler number decreased significantly with the increase of methanol concentration. It can be presumed that DMFC performance was enfeebled by the increase of methanol crossover due to the inactive methanol reaction rate as the methanol concentration increased. 3.3. Effect of the Anode Flow Rate. Figure 8 shows the results of the effect of the anode flow rate on DMFC performance using air and oxygen as the oxidant gas. Both oxidant gases provide maximum performance with an anode flow rate of 3 mL/min, and the performance decreases slightly due to the increase of methanol crossover at an anode flow rate above 3 mL/min. In particular, the anode flow rate of 1 mL/ min appeared to be at an unstable state at 600 mA/cm2 for both oxidant gases due to the concentration loss caused by an insufficient supply of methanol to react with the catalyst in the region of high current density. The performance of the DMFC is not affected much by the anode flow rate except at 1 mL/min, as shown in Figure 8, but it should be noted that the anode flow rate changes the OCV, as shown in Figure 9. As the anode flow rate increases, the OCV decreases with both oxidant gases. This means that the

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Figure 10. Comparison of the Damkohler number and effective mass transfer coefficient between air and oxygen according to the anode flow rate.

Figure 8. Effect of the anode flow rate on DMFC performance.

Figure 9. Comparison of the OCV between air and oxygen according to the anode flow rate.

methanol crossover by permeation through membrane is increased as the methanol flow rate is increased, in excess of the reaction rate of the catalyst. For the case of oxygen, the permeated methanol is not affected much by a faster oxidation velocity than the air at the cathode. The anode flow rate of 1 mL/min, which has a high open circuit voltage, indicates the smallest phenomenon of crossover, whereas it shows the unstable state at the region of high current density, so that it is not the optimal flow rate with respect to the performance aspect. To investigate of the methanol mass transfer and reaction rate with the increase of methanol flow rate, the effective mass transfer coefficient and Damkohler number obtained at the limiting current density between air and oxygen is illustrated

in Figure 10. The mass transfer coefficient in both oxidant gases increased gradually from 0.5 to 3 mL/min, but it was almost constant above the anode flow rate of 3 mL/min. This means that the methanol flow rate of 3 mL/min has the maximum diffusion rate in this study. However, the Damkohler number decreased rapidly in both oxidant gases with the increase in the methanol flow rate. It expected that the catalyst reaction reached the limit as the methanol flow rate increased. Judging from this result, it can be conjectured that methanol crossover severely increased because methanol was not reacted by the low reaction rate according to the increase of the methanol flow rate. 3.4. Effect of the Cathode Flow Rate. The effect of the cathode flow rate on the DMFC performance is shown in Figure 11. For the case of air as the oxidant gas, cell performance increases with the increase of cathode flow rate as illustrated in Figure 11a. At a cathode flow rate of 100 cm3/min, the concentration of oxygen was rapidly decreased by the insufficient oxygen oxidized the hydrogen proton through the membrane as the current density increased. Then, the electrode and gas diffusion layer dries out, due to the shortage of water that is a reaction product, so that the concentration loss is increased by the changes of concentration at the surface of the electrode in the region of high current density. As the air flow rate increases to the cathode flow rate of 300 cm3/min, the cell performance is improved by the active reduction reaction due to the sufficient supply of oxygen in the electrode catalyst and the increment of performance decreases above a cathode flow rate of 400 cm3/min. On the other hand, in the case of oxygen in Figure 11b, the cell performance decreases slightly by the increase of the cathode flow rate. The maximum power density appeared at a cathode flow rate of 200 cm3/min, and the power density decreases with the increase of the oxygen flow rate in the region of high current density. This is caused by the flooding phenomenon, which blocks the porosity of the electrode and the gas diffusion layer with the water as the reaction product. Therefore, the excessive production of water in the fuel cell interferes with the oxygen movement due to the closed cathode flow channel, so that an optimum cathode flow rate is necessary to minimize the voltage loss at the cathode. Figure 12 shows that the effect of the cathode flow rate on the OCV was investigated at the same experimental conditions. As shown in the figure, the OCV decreased slightly with an increase of the cathode flow rate which is seen for both oxidant gases. It was presumed that the OCV values are decreased by

Effect of DMFC Operating Parameters

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Figure 13. Comparison of the Damkohler number and effective mass transfer coefficient between air and oxygen according to the cathode flow rate.

Figure 11. Effect of the cathode flow rate on the DMFC performance.

Figure 12. Comparison of the OCV between air and oxygen according to the cathode flow rate.

the flooding phenomenon as the water production increases at the cathode in the fuel cell. Figure 13 shows the effective mass transfer coefficient and Damkohler number calculated at the limiting current density using both oxidant gases. The effective mass transfer coefficient of air as an oxidant gas increased with the increase of the cathode flow rate, whereas that of oxygen was almost constant regardless of the cathode flow rate. The Damkohler number of using air increases from 0.1 to 0.17, but that of using oxygen is almost the same at about 0.2. In the case of air, the mass diffusion was increased with the increase of the cathode flow

Figure 14. Effect of the cathode humidification temperature on the DMFC performance.

rate because the air contains only oxygen of 21%. But, in the case of oxygen, the cathode flow rate was not affected by the mass diffusion rate because the oxygen concentration was enough to react with methanol. 3.5. Effect of the Cathode Humidification Temperature. Figure 14 illustrates the performance characteristic of the DMFC under various cathode humidification temperatures. The experiment was carried out maintaining a cell temperature of 80 °C, methanol (1 M) flow rate of 3 mL/min, and cathode flow rate of 200 cm3/min. It was found that DMFC performance is highest at the cathode humidification temperature of 30 or

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Figure 15. Comparison of the OCV between air and oxygen according to the cathode humidification temperature

Figure 17. Effect of the cathode backpressure on DMFC performance. Figure 16. Comparison of the Damkohler number and effective mass transfer coefficient between air and oxygen according to the cathode humidification temperature.

45 °C in both oxidant gases and decreases as the cathode humidification temperature increases to higher than 45 °C. It can be said that the increase of humidification temperature may induce a high relative humidity of the reactant gas and, therefore, the performance decreases because the oxygen partial pressure reduces and the excessive water disturbs the mass transfer on the electrode at the region of high current density. However, the low humidification temperature of the reactant gas cannot hydrate the membrane sufficiently, so the performance may decrease due to the low ionic conductivity. Therefore, it is necessary to require the optimal cathode humidification temperature for the maximum performance. The comparison of OCV between air and oxygen as the oxidant gas was investigated, and the results are shown in Figure 15. In this figure, it is illustrated that the OCV of oxygen under the same operating conditions is much higher than that of air. Then, the OCVs of both gases were increased slightly with the increase of cathode humidification temperature. From these results, it can be said that the cathode humidification temperature has a little effect on the methanol crossover caused by electroosmotic drag at the region of low current density. In order to analyze the effect of cathode humidification temperature on the mass diffusion rate and methanol reaction rate at limiting current density, the effective mass transfer coefficient and Damkohler number were investigated as shown in Figure 16. The effective mass transfer coefficients of both gases were decreased slightly because it may be conjectured that the concentration loss was increased by flooding with the

increase of cathode humidification temperature. The values of Damkohler number that are closely related with the electrochemical reaction in a DMFC are in agreement with the tendency of the effective mass transfer coefficient with both oxidant gases. Consequently, the cathode humidification temperature cannot enhance DMFC performance due to the low mass transfer by flooding at the region of high current density. 3.6. Effect of Cathode Backpressure. Figure 17 shows the effect of the cathode backpressure on DMFC performance. In order to investigate the effect of cathode backpressure, an experiment of cathode backpressure was performed at the anode backpressure of atmospheric pressure. From the results, it is revealed that the DMFC performance was increased by the increase of cathode backpressure, and this occurs at a higher rate of increase in the case of air than for oxygen. When the cathode backpressure increased up to 300 kPa, the highest power density of air was 0.178 W/cm2 and that of oxygen was 0.258 W/cm2 with 1 M methanol feed at a cell temperature of 80 °C. This value was the highest in this paper, thus cathode backpressure seems to be a significant role in improving the performance of the DMFC. From these results, it can be conjectured that the methanol crossover is decreased by a decrease of electro-osmotic drag inside the membrane and the reduction reaction at the cathode was activated by an increase of oxygen partial pressure according to the cathode backpressure. It was shown that the OCV increases rapidly as the cathode backpressure increases with both oxidant gases, as shown in Figure 18. In particular, the increment of air increases as much as the oxygen at the backpressure of 300 kPa, and the differences of OCV between both oxidant gases are decreased as the cathode

Effect of DMFC Operating Parameters

Figure 18. Comparison of the OCV between air and oxygen according to the cathode backpressure.

Figure 19. Comparison of the Damkohler number and effective mass transfer coefficient between air and oxygen according to the cathode backpressure.

backpressure is increased. It can be guessed that the methanol crossover decreases in the region of low current density as the increase of cathode backpressure because the high cathode backpressure would enhance the reduction reaction by increasing the partial pressure in both reactant gases. Figure 19 shows the comparison of the effective mass transfer coefficient and Damkohler number at the limiting current density in both oxidant gases as a function of cathode backpressure. As shown in this figure, the effective mass transfer coefficients of both oxidant gases grow with an increase of the cathode backpressure. Moreover, the Damkohler number increased slightly due to the active electrochemical reaction caused by increase of the reaction time, and this result can also be estimated in the performance results. It can be said that the high cathode backpressure in the DMFC may induce the active reduction and electrochemical reactions; therefore, the performance increases significantly. From these results, cathode backpressure decreases the concentration loss at the region of high current density, so it is helpful to form the high performance in DMFC. 4. Conclusions In this study, the effect of operating parameters on the DMFC performance using air as the oxidant gas was investigated experimentally in terms of the polarization curve, open circuit voltage, effective mass transfer coefficient, and Damkohler

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number, and it compared with results of using oxygen as the oxidant gas. The following conclusions can be drawn based on the experimental results. 1. The increase of cell temperature has a beneficial effect on DMFC performance due to the higher reaction rate, lower cell resistance, and enhanced mass transfer. Also, the supply of oxygen to the cathode reaction site was important in order to reduce the activation loss and concentration loss. 2. In case of air, the maximum performance of the methanol concentration is at 0.5 M, but the OCV is decreased by the methanol crossover in the region of low current density at 3 and 4 M. On the other hand, in the case of oxygen, the maximum performance of the methanol concentration is at 1 M, and the decrement of performance by concentration loss at 3 and 4 M is lower than that in air. 3. The maximum performance appears at the anode flow rate of 3 mL/min in both oxidant gases, and the performance decreases rapidly due to the shortage of methanol at an anode flow rate of 1 mL/min. Also, it can be conjectured that the methanol crossover is severely increased because the methanol was not reacted by the low reaction rate according to the increase of methanol flow rate. 4. In the case of air, the performance increase as the cathode flow rate increases, but in the case of oxygen, the performance is almost equal because the oxygen concentration was enough to react with methanol. Moreover, the open circuit voltage is decreased by the flooding of water inside the fuel cell as the cathode flow rate increased with both oxidant gases. 5. The cathode humidification temperature cannot enhance DMFC performance due to the low mass transfer by flooding at the region of high current density. Therefore, it is necessary to require the optimal cathode humidification temperature for the maximum performance. 6. The methanol crossover was decreased by the reduction of electro-osmotic drag inside the membrane, and the reduction reaction at the cathode was activated by the increase of the oxygen residence time as the cathode backpressure increased. Thus, cathode backpressure seems to be a significant role in the improvement of the performance of the DMFC. Acknowledgment. This study was supported by the CEFV (Center for Environmentally Friendly Vehicle) of the Eco-STAR project from the MOE (Ministry of Environment, Republic of Korea). Additional support for this work is from the Ministry of Education and Human Resources Development (MOE), the Ministry of Commerce, Industry and Energy (MOCIE), and the Ministry of Labor (MOLAB) through the fostering project of the Lab of Excellency. Also, this work was supported by grant no. R01-2006000-10932-0 from the Basic Research Program of the Korea Science & Engineering Foundation and the Second Brain Korea 21 Project in 2006.

Nomenclature Acell ) cross-sectional area of cell CMeOH ) methanol concentration Da ) Damkohler number F ) Faraday’s constant j ) current density keff ) effective mass transfer coefficient n ) number of electrons in methanol oxidation Pcb ) cathode backpressure Qa ) anode flow rate Qc ) cathode flow rate Tcell ) cell temperature Tch ) cathode humidification temperature EF700677Y