Impedance Characteristics of the Direct Methanol Fuel Cell under

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Energy & Fuels 2008, 22, 1204–1211

Impedance Characteristics of the Direct Methanol Fuel Cell under Various Operating Conditions Sang Hern Seo† and Chang Sik Lee*,‡ Graduate School and Department of Mechanical Engineering, Hanyang UniVersity, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea ReceiVed October 10, 2007. ReVised Manuscript ReceiVed December 22, 2007

In this paper we investigate the electrochemical impedance characteristics of the direct methanol fuel cell (DMFC) under various operating conditions such as cell temperature, methanol flow rate, cathode flow rate, methanol concentration, cathode backpressure, and type of oxidant gas. The impedance characteristics of the DMFC were measured by using an alternating current impedance measurement system and simulated by an equivalent circuit at various frequencies. Nyquist diagram, impedance, and phase angle were expressed by real and imaginary components of impedance to investigate the ohmic and activation losses. The results revealed that the activation loss was decreased by increasing the cell temperature, cathode flow rate, and cathode backpressure. However, the impedance and phase angle were increased by increasing the methanol flow rate and the methanol concentration. When oxygen was used as the oxidant gas, it was found that the impedance and the phase angle decreased more rapidly than when air was used because of the active electrochemical reaction caused by the higher oxygen concentration.

1. Introduction The direct methanol fuel cell (DMFC) generally has many advantages over the polymer electrolyte membrane fuel cell because methanol is a low-priced liquid fuel which is easy for handling, storage, and refueling and has a higher energy density similar to that of gasoline. Also, the DMFC can be designed simply because of the absence of a reforming system. However, the DMFC has some drawbacks. It produces carbon dioxide as a waste product and has low performance due to the slow and complex electrochemical reaction. The voltage-current density curve of DMFC shows that the open circuit voltage is much lower than its theoretical value because of the slow kinetics at the anode and cathode as well as the methanol crossover, and there are various performance losses such as activation loss, ohmic loss, and concentration loss according to the increase of current density. It is difficult to accurately analyze performance loss with a voltage-current density graph, but it can investigate more precisely the cause of performance losses with alternating current (ac) electrochemical impedance spectroscopy.1 Many studies have been conducted both experimentally and numerically to determine the impedance characteristics of fuel cells. Mueller and Urban2 measured anode and cathode impedances of three regions separated into high, medium, and low frequency. It was shown that each of the anode and cathode impedances was obtained by the hydrogen electrode method using hydrogen at the cathode. Ciureanu et al.3 experimentally investigated the kinetic analysis of the polymer electrolyte * To whom correspondence should be addressed. Telephone: +82-22220-0427. Fax: +82-2-2281-5286. E-mail: [email protected]. † Graduate School. ‡ Department of Mechanical Engineering. (1) Yuan, X.; Wang, W.; Sun, J. C.; Zhang, J. Int. J. Hydrogen Energy 2006, 32, 4365–4380. (2) Mueller, J. T.; Urban, P. M. J. Power Sources 1998, 75, 139–143. (3) Ciureanu, M.; Mikhailenko, S. D.; Kaliaguine, S. Catal. Today 2003, 82, 195–206.

membrane fuel cells by impedance spectroscopy. They examined the effects of mass transport, kinetics of electrode reactions, and charge transfer on polymer electrolyte membrane fuel cell performance. Li and Pickup4 compared the impedance of the polymer electrolyte membrane fuel cell with that of the DMFC. They demonstrated the effects of the anode and cathode within the fuel cell by using a reference electrode in both fuel cells. Kurzweil and Fischle5 proposed a new monitoring method which expresses resistance and capacitance by an impedance measurement system and calculated equations. With these results, they were able to determine the optimum operating state at the highest capacitance and lowest resistance in a fuel cell system. Zhao et al.6 studied the effects of Nafion content and catalyst loading on the DMFC performance using the ac impedance method. Hakenjos et al.7 and Yuan et al.8,9 conducted the measurement of impedance spectroscopy in a polymer electrolyte membrane fuel cell stack. They found that the sum of the single cell impedance is in good agreement with whole stack impedance values. Also, Oedegaard10 examined overvoltage and impedance in a DMFC under ambient conditions of varying operating temperature and methanol concentration. Cha et al.11 reported the electrochemical impedance of a proton exchange membrane fuel cell changing dimensions of the micro-scale flow channel and the gas diffusion layer thickness. Wang et al.12 compared the electrochemical impedance between a Pt-Ru-Ni catalyst (4) Li, G.; Pickup, P. G. Electrochim. Acta 2004, 49, 4119–4126. (5) Kurzweil, P.; Fischle, H.-J. J. Power Sources 2004, 127, 331–340. (6) Zhao, X.; Fan, X.; Wang, S.; Yang, S.; Yi, B.; Xin, Q.; Sun, G. Int. J. Hydrogen Energy 2005, 30, 1003–1010. (7) Hakenjos, A.; Zobel, M.; Clausnitzer, J.; Hebling, C. J. Power Sources 2006, 154, 360–363. (8) Yuan, X.; Sun, J. C.; Blanco, M.; Wang, H.; Zhang, J.; Wilkinson, D. P. J. Power Sources 2006, 161, 920–928. (9) Yuan, X.; Sun, J. C.; Wang, H.; Zhang, J. J. Power Sources 2006, 161, 929–937. (10) Oedegaard, A. J. Power Sources 2006, 157, 244–252. (11) Cha, S. W.; O’Hayre, R.; Park, Y. I.; Printz, F. B. J. Power Sources 2006, 161, 138–142.

10.1021/ef700602k CCC: $40.75  2008 American Chemical Society Published on Web 02/12/2008

Impedance of the Direct Methanol Fuel Cell

Energy & Fuels, Vol. 22, No. 2, 2008 1205

Figure 1. Schematics of the fuel cell. Figure 2. Definition of the impedance curve. Table 1. Specifications of the Fuel Cell item

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 4.0 mg/cm2 Teflon serpentine grooves with width of 0.7874 mm and depth of 1.016 mm

Table 2. Experimental Conditions cell temperature methanol concentration anode flow rate cathode flow rate cathode backpressure

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

Figure 3. Equivalent circuit for modeling of DMFC impedance.

during current measurement.17 There are many other methods such as pressure drop diagnosis, current density mapping, inspection microscopy, and chemical analysis.18–20 The objective of this study is to investigate the impedance characteristics of the DMFC under various operating conditions such as cell temperature, methanol flow rate, cathode flow rate, methanol concentration, and cathode backpressure. In addition, the phase angle when air is used as the oxidant gas is compared to that when oxygen is used to determine the effect of oxygen concentration on the impedance of the DMFC.

and a Pt-Ru catalyst at the anode, and the results showed that the performance of the Pt-Ru-Ni catalyst is much better than that of the Pt-Ru catalyst because of the chemical action of Ni. In addition, other techniques have been developed to confirm the effect of operating parameters on fuel cell performance. First of all, fuel cell performance is characterized by its polarization curve expressed voltage and current density.13–15 From the polarization curve, not only the polarization characteristics such as activation loss, ohmic loss, and concentration loss were analyzed by increasing the current density but also the flooding or drying in the fuel cell was estimated by increasing and decreasing current density. In addition, the current interrupt method, in which the cell current was rapidly interrupted and the terminal voltage measured before and during the interruption of the current, is one of the methods to measure the resistance because of the insufficient information of the polarization curve. This method can determine the ohmic loss and nonohmic processes of the fuel cell performance.16 Cyclic voltammetry is another technique that provides information on fuel cell reaction kinetics of catalyst activity in detail. In this method, the voltage is swept backward and forward between two voltage limits

The single fuel cell tested in this study had an active area of 5 cm2 and a serpentine flow type with a width of 0.7874 mm and a depth of 1.016 mm. The membrane electrolyte assembly consisted of Nafion 117 loading Pt-Ru of 4 mg/cm2 at the anode and Pt black of 4 mg/cm2 at the cathode. Figure 1 shows the experimental apparatus used in this study. The voltage and current were controlled by a direct current (dc) electronic load (AMREL, FEL 60-10-10) through potentiostatic techniques. The fuel used at the anode was mixed methyl alcohol and purified water, and the methanol solution was supplied to the fuel cell via a peristaltic pump (Gilson, minipuls3 peristaltic pump). The cathode gas humidified by a humidification system used air or oxygen as the oxidant gas, and the flow rate was controlled by a mass flow controller (MKS Instruments, 1179). The backpressure of the cathode gas was managed by a backpressure regulator (Marklyn Controls, 44-2362-

(12) Wang, Z. B.; Yin, G. P.; Shao, Y. Y.; Yang, B. Q.; Shi, P. F.; Feng, P. X. J. Power Sources 2007, 165, 9–15. (13) Ge, J.; Liu, H. J. Power Sources 2005, 142, 56–69. (14) Jung, G. B.; Su, A.; Tu, C. H.; Weng, F. B. J. Fuel Cell Sci. Technol. 2005, 2, 81–85. (15) Hsieh, S. S.; Yang, S. H.; Kuo, J. K.; Huang, C. F.; Tsai, H. H. Energy ConVers. Manage. 2006, 47, 1868–1878. (16) Cooper, K. R.; Smith, M. J. Power Sources 2006, 160, 1088–1095.

(17) Chakraborty, D.; Chorkendorff, I.; Johannessen, T. J. Power Sources 2006, 162, 1010–1022. (18) Barbir, F.; Gorgun, H.; Wang, X. J. Power Sources 2004, 141, 96–101. (19) Cleghorn, S.; Derouin, C. R.; Wilson, M. S.; Gottesfeld, S. J. Appl. Electrochem. 1998, 28, 663–672. (20) Jeon, M. K.; Won, J. Y.; Oh, K. S.; Lee, K. R.; Woo, S. I. Electrochim. Acta 2007, 53, 447–452.

2. Experimental Apparatus and Procedure

1206 Energy & Fuels, Vol. 22, No. 2, 2008

Seo and Lee

Figure 5. Effect of the cell temperature on the DMFC impedance and phase angle. Figure 4. Effect of the cell temperature on the DMFC Nyquist diagram.

24) at the cathode vent. The operating temperature and humidification temperature were measured by type T thermocouples and controlled by a software program. The ac impedance measuring system was connected with an electronic load and measured with the impedance system data acquisition card. The Nyquist curve was plotted with real and imaginary components of impedance according to the frequency range. The detailed specifications of the fuel cell system are listed in Table 1. Experiments were conducted with both air and oxygen as the oxidant gas to compare the impedance characteristics under various operating conditions. The various operating conditions used in this study are summarized in Table 2. The operation cell temperature was varied from 40 to 80 °C at intervals of 10 °C. The methanol flow rate ranged from 3 to 9 mL/min at intervals of 2 mL/min, and the concentration of methanol solution was varied from 1 to 4 M. The air and oxygen supplied to the cathode were varied from 100 to 400 cm3/min at intervals of 100 cm3/min, and the backpressure was regulated from 100 to 300 kPa at intervals of 100 kPa. The impedance measuring system modulated the cell current by sending a sinusoidal voltage of 5 mV to the dc electronic load, and the response of the voltage and current was measured with the impedance system data acquisition card at the operating voltage of 0.3 V. The sine correlation method was used to calculate the cell complex impedance from the waveforms measured at frequencies between 0.1 and 10 000 Hz with six points per decade, as shown (21) Harrington, D. A.; Conway, B. E. Electrochim. Acta 1987, 32, 1703–1712.

in Figure 2. The impedance is expressed by the real and imaginary components, and the equations of the impedance can be shown as follows: Z)

V0e jwt V0 cos(wt) ) (jwt-jφ) ) Z0e jφ i0 cos(wt - φ) i e 0

) Z0(cos φ + j sin φ) ) Re Z + j Im Z

(1)

Also, both the magnitude and the phase angle can be expressed by these two parts of the impedance as follows: |Z| ) [(Re Z)2 + (Im Z)2]1/2 Im Z φ ) tan-1 Re Z

( )

(2) (3)

3. Results and Discussion To compare with the experimental results, the impedance was analyzed with an equivalent circuit for modeling as shown in Figure 3. The impedance behavior of methanol oxidation on the anode proposed by Harrington and Conway21 is based on the concept of relaxation impedance, which is associated with the surface relaxation of the interface by the carbon monoxide poisoning and pseudoinductive behavior in the low frequency range. Also, the electrode resistance represents the charge transfer resistance of a reaction and the capacitance attributes to double-layer capacitance across the interface at the anode and cathode. In the equivalent circuit, Rm denotes the resistance of the membrane itself; Rct,A and Cdl,A define the charge transfer resistance and the double layer capacity of the



Figure 7. Effect of MeOH concentration on the DMFC impedance and phase angle. Figure 6. Effect of MeOH concentration on the DMFC Nyquist diagram.

anode, respectively; Rct,C and Cdl,C define the charge transfer resistance and the double layer capacity of the cathode, respectively; and L and Rk express the pseudoinductance and the relaxation resistance, respectively. 3.1. Effect of Cell Temperature on the DMFC Impedance. Figure 4 depicts the effect of the cell temperature on the DMFC impedance.The simulated Nyquist plot by the equivalent circuit is also shown with the experimental results. The cell temperature was changed from 40 to 80 °C in intervals of 10 °C, and the impedance was measured when air and oxygen were used as the oxidant gas. From the impedance results, it was observed that the ohmic and activation losses decreased as the cell temperature was increased, and the simulated results by the equivalent circuit properly predict the tendency in accordance with experimental results although it overestimates the capacitance at the high frequency region and underestimates the resistance and capacitance at the low frequency region. In the case of air, the ohmic loss made little difference, about 0.05 Ω cm2, but the activation loss at 80 °C was 66% lower than that activation loss at 40 °C. The activation loss decreased rapidly because of the increased velocity of the electrochemical reaction and the increased activity of the catalyst with increase of the cell temperature. When oxygen was used as the oxidant gas, the impedance was decreased by decreasing ohmic and activation losses with increase of the cell temperature in the same way as using air as an oxidant gas. The ohmic loss

of using oxygen was almost equal to that of using air, but on the other hand, the activation loss decreased sharply because of the higher concentration of oxygen than air. At the cell temperature of 40 °C, the diameter of the semicircle was about 60% less for oxygen (0.84 Ω cm2) than for air (2.08 Ω cm2). On the basis of these results, the impedance can be reduced to a minimum by increasing the cell temperature and using oxygen as the oxidant gas. Figure 5 shows the impedance and phase angle for various frequencies from 0.01 to 10 000 Hz when air is used as the oxidant gas in both experiment and simulation. The real and imaginary components of impedance increased in the region of low frequency from 0.1 to 10 Hz, but cell temperature had little influence on the impedance in the region of frequency above 100 Hz. The maximum impedance at 80 °C was about 63% lower than that at 40 °C. The phase angle increased from the region of low frequency to the maximum near the frequency of 10 Hz, and after that it decreased to the region of high frequency. Also, the phase angle at 80 °C was about 7° lower than at 40 °C. In the region of low frequency, the phase angle reversed with increase of cell temperature because the real component of impedance decreased and the imaginary component of impedance had a negative value. When oxygen was used as the oxidant gas, the results showed similar tendencies to those shown by the impedance and phase angle when air was used. The maximum impedance decreased by about 55% and the phase angle decreased about 7° compared to when air was used. From the equality of the phase angle in the frequency region greater than 100 Hz, it is supposed that the variation of the real


Seo and Lee

Figure 9. Effect of the anode flow rate on the DMFC impedance and phase angle. Figure 8. Effect of the anode flow rate on the DMFC Nyquist diagram.

and the imaginary components of impedance was almost equal. Judging from the higher impedance and phase angle at 80 °C when using air than that at 40 °C when using oxygen, it can be said that using oxygen at the lower temperature has the effect of using air at the higher temperature. On the other hand, if the cell temperature is too high, there are several negative effects on fuel cell performance such as the decrease of the oxygen partial pressure as a result of the increase of vapor partial pressure and increase of methanol crossover and water transfer from the anode to the cathode.13 Therefore, the diameter of the semicircle was increased by increase of the mass transfer resistance as the cell temperature increases.22 3.2. Effect of the Methanol Concentration on the DMFC Impedance. The effect of the methanol concentration on the DMFC impedance is shown in Figure 6. As shown in this figure, when air is used as the oxidant gas, the impedance rapidly increased as a result of increasing activation loss as the methanol concentration was increased in both the experimental and the simulation results. The diameter of the semicircle at the methanol concentration of 4 M was about 33% greater than that at 0.5 M. This is why the remaining carbon dioxide generated from the oxidation reaction can decrease the effective mass transfer as the methanol concentration increases, and the methanol crossover occurs through the membrane from anode (22) Yan, X.; Hou, M.; Sun, L.; Liang, D.; Shen, Q.; Xu, H.; Ming, P.; Yi, B. Int. J. Hydrogen Energy 2007, 32, 4358–4364.

to cathode. Also, the catalyst was poisoned by carbon monoxide at high methanol solution concentration. However, the impedance at high methanol concentration appeared to be an unstable condition in the region of low frequency. This is likely due to the fact that the oxidation reaction of methanol is slow because of the low oxygen concentration of air in this region. When oxygen is used as the oxidant gas, the impedance sharply decreased as a result of the decrease of activation loss and increased with the increase of the methanol concentration, which was similar to what was observed when air was used as the oxidant gas. The figure shows that at the concentration of 1 M the diameter of the semicircle with oxygen as the oxidant gas (0.30 Ω cm2) was lower than the loss when air was the oxidant gas (0.850 Ω cm2). This indicates that when the oxygen was used at the cathode the permeated methanol may be oxidized quicker than when using air.13 Therefore, maximum performance in the DMFC was achieved by reducing the impedance at a methanol concentration of 1 M with oxygen as the oxidant gas. The impedance and phase angle when using air as the oxidant gas were investigated at various methanol concentrations as illustrated in Figure 7. The impedance has a maximum value at the methanol concentration of 4 M in the region of low frequency and then has the convergence with increase of the frequency. The phase angle at the methanol concentration of 4 M was 5° higher than the angle at 1 M as shown in Figure 10. At the same methanol concentrations, the impedance with oxygen as the oxidant gas was about 50% less than when air was used as the oxidant gas, and the tendency was also increased with increase of the methanol concentration. The difference in



Figure 11. Effect of the cathode flow rate on the DMFC impedance and phase angle. Figure 10. Effect of the cathode flow rate on the DMFC Nyquist diagram.

the phase angle between 4 and 1 M was about 7°, and the difference between the phase angles with oxygen and air was about 15° at the same methanol concentrations. For this reason, it can be said that the activation loss was reduced by decreasing the methanol concentration and using oxygen as the oxidant gas. However, it may not be suitable to use a methanol concentration greater than 1 M because of the possibility of an unstable condition. 3.3. Effect of the Anode Flow Rate on the DMFC Impedance. Figure 8 illustrates the effect of the methanol flow rate on the DMFC impedance using air as the oxidant gas in both the experimental and simulation results. According to the increase of the methanol flow rate, the ohmic loss was almost equal but the impedance increased because of the increasing diameter of the semicircle. The minimum diameter of the semicircle occurs at a flow rate of 3 mL/min which is about 10% lower than the loss at a flow rate of 9 mL/min. The main reason for this may be the increasing methanol crossover and heat removal from the catalyst layer with the increase of methanol flow rate.13,23 In the case of using oxygen, the minimum impedance occurs at a methanol flow rate of 3 mL/ min, and the activation loss increased as a result of increasing the methanol flow rate. Compared with air, the impedance with (23) Scott, K.; Taama, W. M.; Kramer, S.; Argyropoulos, P.; Sundmacher, K. Electrochim. Acta 1999, 45, 945–957.

oxygen as the oxidant gas was decreased by about 60% at the same methanol flow rate. Figure 9 shows the impedance and phase angle of the impedance at various methanol flow rates. With air as the oxidant gas, the minimum impedance and phase angle occur at the methanol flow rate of 3 mL/min, and the impedance was almost the same for all regions above 100 Hz. With oxygen as the oxidant gas, the maximum impedance and phase angle values occur at a flow rate of 9 mL/min; however, the values between 9 and 3 mL/min were not widely different. Compared with air, the impedance decreased by about 60%, and the phase angle decreased by about 50% from 32° to 17° at the same methanol flow rate. Therefore, on the basis of the experimental results, it can be said that the methanol flow rate should be supplied as low as possible to reduce the methanol crossover so as not to affect performance. 3.4. Effect of the Cathode Flow Rate on the DMFC Impedance. Figure 10 shows the effect of the cathode flow rate on impedance of the DMFC in both the experimental and the simulation results. With air as the oxidant gas, the ohmic loss is almost the same at all flow rates, but the activation loss rapidly decreased by reducing the real and imaginary components. That is why the electrochemical reaction is more active as a result of the increase of oxygen concentration at the cathode and the decrease of the mass transport limitation by removing liquid water in the diffusion layer and the catalyst layer.13,22 The activation loss at the cathode flow rate of 400 cm3/min was about 50% lower than the loss of 100 cm3/min. According to the increase of the cathode flow rate, the activation loss was


Seo and Lee

Figure 13. Effect of the cathode backpressure on the DMFC impedance and phase angle. Figure 12. Effect of the cathode backpressure on the DMFC Nyquist diagram.

reduced, but the decrement was decreased. With oxygen as the oxidant gas, the impedance was decreased by reducing the real and imaginary components in the same way. Compared with air, the activation loss for when oxygen is the oxidant gas decreased about 70% at the same cathode flow rate. The impedance and phase angle were plotted at various frequencies in Figure 11. In the case of air as the oxidant gas, there is a large difference among the impedance values at the lower frequencies but there is little difference over the frequency of 10 Hz. In addition, the phase angle decreased about 7° from 37° at the cathode flow rate of 100 cm3/min to 30° at the cathode flow rate of 400 cm3/min. With oxygen as the oxidant gas, the impedance and phase angle decreased as a result of increasing cathode flow rate, which is similar to what was observed with air as the oxidant gas. Compared with air, the impedance decreased by about 60% and the phase angle was decreased about 15° when oxygen was used as the oxidant gas. The point of inflection for the phase angle occurs at a frequency of 10 Hz for the very simple reason that the real and imaginary components of the impedance fluctuated repeatedly. The results show that using oxygen at a flow rate of 100 cm3/min improves the performance of the DMFC because of the decreases in impedance resulting from the reduction of activation loss in comparison to the use of air at a flow rate of 400 cm3/min. 3.5. Effect of the Cathode Backpressure on the DMFC Impedance. Figure 12 shows the effect of the cathode back-

pressure on the impedance of the DMFC in both the experimental and the simulation results. As the cathode backpressure is varied from 100 to 300 kPa at increments of 100 kPa, the activation loss was remarkably reduced by decreasing the real and imaginary components of the impedance. The diameter of the semicircle was decreased by about 38% from 0.9 Ω cm2 at the backpressure of 100 kPa to 0.55 Ω cm2 at the backpressure of 300 kPa. This is why the electrochemical reaction was accelerated by increasing the diffusion of the reactants in the fuel cell by means of the partial pressure increase of the reactant gases and the membrane was hydrated by backdiffusion.15 However, the increment was decreased with the increase of cathode backpressure, and this tendency occurred similarly to when oxygen was used. When oxygen is the oxidant gas, the activation loss decreased by about 50% compared to when air was used as a result of the active reduction reaction resulting from the higher oxygen concentration at the same backpressure. Figure 13 shows the impedance and phase angle at various cathode backpressures. For air, the impedance decreased by about 40% from 1.1 Ω cm2 at 100 kPa to 0.65 Ω cm2 at 300 kPa at the frequency of 0.1 Hz, and the phase angle decreased by about 4°. The maximum phase angle was shifted to the higher frequency with the increasing cathode backpressure. For oxygen, the impedance and phase angle also decreased with the increasing cathode backpressure, and the amount of decrease was smaller than it was when air was used. Compared with when air was used, the impedance decreased by about 60%, and the phase angle decreased by about 13° at the same cathode backpressure. Therefore, it can be said that the fuel performance


was increased by increasing the cathode backpressure because of the reduction of activation loss. 4. Conclusions In this work, the impedance characteristics of the DMFC were investigated experimentally under various operating conditions. The following conclusions were obtained through this investigation. As the operation temperature increased, the impedance was decreased as a result of the reduction of the ohmic and activation losses resulting from the active electrochemical reactions in the fuel cell. The activation loss was lower when oxygen was used as the oxidant gas by reason of the higher oxygen concentration, and the maximum impedance decreased by about 55% and the phase angle decreased about 7° compared to when using air. The impedance was remarkably increased with increasing methanol concentration, and it is considered that the activation loss was increased by the limited mass transfer due to the remaining carbon dioxide, the methanol crossover through the membrane from anode to cathode, and the catalyst poisoned by the carbon monoxide. The impedance has a maximum value at the methanol concentration of 4 M in the region of low frequency, and the phase angle at the methanol concentration of 4 M was 5° higher than the angle at 1 M. It can be concluded that the methanol crossover and heat removal from the catalyst layer increase with the increase of methanol flow rate; hence, the impedance and phase angle increased slightly with the increase of methanol flow rate, but the ohmic loss was not affected by the methanol flow rate. As the cathode flow rate was increased, the impedance decreased as a result of the reduction of concentration loss resulting from the supply of sufficient oxygen for the active reduction reaction at the cathode. Compared with air, the impedance decreased by about 60% and the phase angle was decreased about 15° when oxygen was the oxidant gas at the same cathode flow rate. The impedance decreased remarkably by increasing the cathode backpressure that is caused by the partial pressure increase of the reactant


gases. In the case of oxygen as an oxidant gas, the impedance decreased by about 60% and the phase angle decreased by about 13° compared with those of air at the same cathode backpressure. Also, the maximum phase angle was shifted to the higher frequency with the increasing cathode backpressure. On the whole, the simulated results by the equivalent circuit properly predict the tendency in accordance with experimental results although it may overestimate or underestimate it somewhat. 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 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 CMeOH ) methanol concentration Im Z ) imaginary component of impedance i0 ) amplitude of the current signal j ) imaginary number (j ) √-1) Pc ) cathode backpressure Qa ) anode flow rate Qc ) cathode flow rate Re Z ) real component of impedance Tcell ) cell temperature t ) time V0 ) amplitude of the voltage signal Z ) impedance Z0 ) impedance magnitude φ ) phase angle w ) radial frequency EF700602K

Impedance Characteristics of the Direct Methanol Fuel Cell under

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