advances in computational fluid dynamics modeling

Proceedings of IECEC'01 36th Intersociety Energy Conversion Engineering Conference July 29–August 2, 2001, Savannah, Georgia

IECEC2001-ET-11

EXPERIMENTAL TECHNIQUES FOR PEM FUEL CELLS *

W. K. Lee, S. Shimpalee, and J. W. Van Zee Department of Chemical Engineering, University of South Carolina Columbia, SC 29208 H. Naseri-Neshat Department of Mechanical Engineering South Carolina State University, Orangeburg, SC 29072 *Corresponding author. Tel: 1-803-777-2285; fax: 1-803-777-8625; E-mail: [email protected]

ABSTRACT With the development of improved membranes, catalysts, and bipolar plates, proton exchange membrane (PEM) fuel cells will play an important role in the near future as a new power source. The design and control of the fuel cells may be advanced by Computational Fluid Dynamics (CFD) techniques but experimental techniques to verify these predictions are not straightforward. For the fuel cell operation, proper water management inside the cell is very important. The membrane inside fuel cell requires sufficient water content to allow for conduction of hydrogen ions from the anode to the cathode. However, excessive water leads to a blocking of the catalyst for the reactions. This optimization problem depends on the current or power drawn from the PEM fuel cell and is especially important during acceleration and deceleration. Another major problem in operating a PEM fuel cell is CO poisoning on the catalyst. Even a few parts per million of CO produce a substantial degradation in the fuel cell performance. The degradation of the cell performance is associated with CO adsorption on active Pt site at the anode thus inhibiting hydrogen oxidation. Here we summarize details of experimental techniques that have been developed to enhance the understanding of PEM fuel cells and verify the numerical models. These techniques include water balance procedures to study the water management inside the system, an airbleeding procedure to remove CO adsorbed on the catalyst, and voltage step changes to capture transient behavior of the current. This paper provides future recommendations of numerical and experimental studies needed for improving the models of PEM fuel cell.

membrane electrode assembly (MEA). The schematic of a single fuel cell assembly is shown in Figure 1.

Current collector as G

(H

)i

n

2

as G

Gas diffusion layer

Gasket

MEA

ut )o

End plate

Graphite flow-channel block

as (A ir

H2

G

G

n )i

Gasket

ut )o

( as

ir (A

Figure 1. Schematic of a single fuel cell assembly [4]. In the fuel cell operation one of the major problems is water management for high performance. Insufficient water lowers the conductivity of the membrane and yields low current density. On the other hand excess water leads to flooding of the pores in the diffusion media and a decrease in performance. A number of investigators have measured water transport through the ion exchange membrane. Verbrugge and Hill [5, 6] examined the water transport through the perfluorosulfonic acid (PSA) membrane. They determined the rate of water transport and the proton diffusion coefficients in a Nafion membrane by radiotracer techniques. Fuller and Newman [7] experimentally determined the transport number of water in Nafion 117 over a wide range of water contents. Zawodzinski et al. [8-11] also investigated water uptake and transport properties for several perfluorosulfonic acid membranes. Springer et al. [12] have measured the conductivity and their reported equations show a linear dependence on water content. In their work the water sorption characteristics, diffusion coefficient of water, electro-osmotic drag, and proton

INTRODUCTION The proton exchange membrane fuel cell is regarded as highly attractive for mobile and residential applications due to high power density at low temperature and its relatively simple handling [1-3]. The PEM Fuel cell consists of six major parts: fuel cell case, current collector, flow-field plate, gasket, gas diffusion layer, and membrane electrode assembly (MEA) [4]. The cell is a sandwich of two graphite flow channel blocks laced with tiny channels and separated by a thin

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conductivity were determined for a Nafion membrane. In this paper we summarize details of experimental procedures and we review data [4, 13] that show how the performance changes with humidity in the inlet stream. Another major problem of the fuel cell is CO poisoning of the catalyst. For the application of the fuel cell in the automobile, hydrogen can be produced on board vehicle by reforming the methanol or hydrocarbon fuel [14]. Although the reformer produces hydrogen rich gas containing approximately 40% H2, 43% N2, 16% CO2, there is anywhere from 5 ppm to 1% CO in this stream. Thus while, Pt has been proven to be the most effective catalyst for the hydrogen oxidation, even few part per million of CO produces a substantial degradation of the fuel cell performance with this catalyst [15, 16]. This degradation is associated with a CO adsorption on the Pt catalyst. One treatment for this poisoning is to add small amounts of air to remove CO from the Pt catalyst [17]. In the process, the chemical reaction of CO with oxygen supplied by air bleeding occurs at the anode side: Recently, Schmidt et al. [18] suggested bubbling the gas through liquid hydrogen peroxide in the anode humidifier. Bellows et al. [19] showed that the increase in performance with peroxide was a result of increased oxygen in the anode feed. The addition of liquid hydrogen peroxide to the humidification water leads to a heterogeneous decomposition of H2O2 and formation of active oxygen. In attempts to characterize the rates of poisoning and recovery, transient studies of CO poisoning with air bleeding were conducted by Bauman et al. [20] and Murthy et al. [21]. The second section of this paper reviews the experimental techniques used to study air-bleed recovery of CO poisoned MEAs. For the application of the PEM fuel cell in the automobile, it is important to know how quickly the fuel cells will response when the operating condition changes. Although, the numerical transient response of a PEM fuel cell subjected to a variable load has been investigated by Naseri-Neshat et al. [22], experimental data characterizing these load changes have not been presented. NaseriNeshat et al. [22] showed overshoot behavior of the current for step changes in the voltage as a function of anode flow rate. They claim that this overshoot will decrease as the ratio of the anode residence time to the rate of voltage change increases. They predicted the transient responses of the cell in terms of local distributions of the current density, water vapor, velocity, and pressure. Thus the third part of this paper presents new experimental data for transient responses. Typically fuel cells are tested with a station similar to that made by Fuel Cell Technology, Inc. (Los Alamos, NM). A schematic of the flow diagram of a fuel cell test station is shown in Figure 2 [4]. The gas flow rates are controlled with mass-flow controllers. The gas temperature and the degree of humidity are adjusted by the temperature of humidifier. During the fuel cell operation, the humid gases are introduced to the fuel cell and they diffused to the MEA through the gas diffusion layer while they are circulating along the gas channel. The electrochemical reactions take place at the electrodes and non-reacted gases and water are exhausted from the fuel cell. The exhausted gases pass through the backpressure regulators and then to the vents. Typically the fuel cell is operated between atmospheric pressure and 45 psig. High purity hydrogen (99.997 %) and compressed air are used as a fuel and as a reactant in most the experiments.

Fuel Cell Test Station 3 way

Bypass

H2

Mass Flow Controller

Heater

Anode Gas In Humidity Bottle

Reformat

Thermocouple Controller Oxygen

Bypass

Cathode Gas In

Fuel Cell

Mass Flow Controller

Heater

Thermocouple Controller

Humidity Bottle Thermocouple Controller

Heater

Air

Pressure Regulator

Cathode Vent

Cathode Gas Out

Anode Vent

Anode Gas Out

Figure 2. Flow diagram of fuel cell test station [4]. stream, it has been assumed frequently that the water vapor is equilibrated at saturation pressure set by the temperature of the humidifier. However, the calibration measurements [4, 13] indicated that inlet gas stream was not saturated with water vapor. Examples of water vapor conditions in the inlet gas stream are shown in Figure 3. According to the data the relative humidity can vary with flow rate of gas: 67 to 89 % for hydrogen and 83 to 98 % for air. These results show that higher flow rates of gas can contain more water vapor and that residence time in the humidity tank may not be important. We attribute this to the sparging characteristics of the humidifier chambers. Fuel Cell Technology now supplies humidifiers with membrane spargers that initially yield higher relative humidity than that shown in Fig. 3. Nevertheless, calibration of the humidifiers is critical to closing the water balance. Anode

Cathode 100

100

98 96 90

Humidification 90 oC


80 o

Humidification 80 C

Relative humidity (%)

Relative humidity (%)

94 92


90 88 86

70 Humidification 70 oC


84 82

60

80 50

100

150

Flow rate (cm3/min)

200

200

300

400

500

Flow rate (cm3/min)

Figure 3. Anode and cathode inlet humidification characteristics for different inlet flow rates and humidification temperatures [4].

Water Management Water balance data requires the measurement of inlet and outlet humidity conditions. For the water vapor condition in the inlet gas

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conditions as reported by Lee et al. [13]. In the experiments RIMEA Series 5510 MEAs (0.3mg/cm2 Pt loading, 20 µm nominal membrane thickness, W. L. Gore & Associates, Inc. Elkton, Maryland, USA) were used. The catalyst loading contributes another 25-30 µm of thickness to the MEA. The diffusion medium and gasket used in the experiment was Double-Sided ELAT (E-TEK, Inc. Natic, MA, USA) and a compressible gasket of 10 mils (0.25 x 10-3 m) in thickness. The cell was tightened with four bolts clamped to 50 in-lbf of torque and the internal pressure of on the gas diffusion media was measured to be 802 psi. The experiments consisted of two parts. First, the polarization behavior of the MEA was measured at fixed stoichiometric flows for various humidity conditions, then the current was measured at various degrees of humidity while the cell voltage and cell temperature were fixed. In all of the experiments we allowed about 10 hours for steady state to be established at a given flow rate and temperature. Before the experiments were begun, the fuel cells was run at 0.6 V for about 20 hours to obtain a steady state conditioning of the MEA. We used a flow rate corresponding to a low stoichiometry of 1.2 and 2.0 times that required for hydrogen and air respectively. Once the cell reached a steady state, a polarization curve was obtained. For the polarization curve the cell voltage was changed from 0.3 V to the open circuit voltage and the corresponding currents were recorded. The flow rates were adjusted manually in an iterative manner to maintain the fixed stoichiometry at each cell voltage for the polarization data. After finishing the polarization curves, the current was measured at fixed voltage for various humidity conditions. The humidifier temperature was increased from 65 oC for anode gases and 55 oC for cathode to 95 o C for anode and 85 oC for cathode by 10 oC. At each humidifier temperature, the current was measured and recorded. All the tests were performed for about 10 to 20 hours at a fixed cell voltage (0.5 V). In the experiments, cell temperature was fixed to 50 oC. Figure 6 shows an example of the effect of inlet stream humidity on the performance of a PEM fuel cell at a cell voltage of 0.5 V and a cell temperature of 50 oC. There were four different humidity conditions examined in the experiment. For all cases the currents are stable and no oscillatory behavior was observed. In other experiments, the current can oscillate if the membrane is not completely hydrated [4, 13]. For the data of Figure 6, the membrane seems to be completely hydrated and this is probably because the cell temperature is low relative to the dew points of the entering gases. Note that there is some performance degradation observed by the long-term decrease in the current at these operating conditions. We attribute the degradation to water accumulation at the cathode side of the MEA. That is at the cathode side, water molecules are not only transported from the anode side but also generated by the cathodic reaction. If the water is not removed at the same rate at which it is generated, the pores in the gas diffusion layer may be blocked (i.e., flooded). In Fig. 7, polarization curves show the effects of different inlet humidity conditions as a function of cell voltage at a cell temperature of 50 oC. The flow rates were changed at each voltage to maintain a constant stoichiometry as discussed above. The performance is decreased as the humidification temperature is increased. This confirms our hypothesis of flooding. That is, as the cell voltage decreases and the rate of production of water increases, more active area is blocked due to water accumulation at the higher humidity condition. During the performance test the amounts of water in the exhausted gases were measured by condensing the gases. The net water transported across the membrane was determined by making a water balance with the water data and the current data. The results of

In addition to the inlet humidity, one must measure the exit humidity during operation and this can be accomplished by condensing the exhausted gases as shown in Figure 4. In the system, the temperature of gas was measured after it passed through the ice bath. We then assumed that the vented gas was fully saturated with water vapor at the measured temperature after the ice bath. The condensed water was collected and weighed every 5 minutes. An example of collected water data is shown in Figure 5. We assume steady state is obtained after the slope of the mass collected versus time was constant.

Humidifier

Fuel Cell Tester

Thermocouple Thermocouple Vent

Beaker

Ven t

Fuel Cell

Beaker

Ice bath Balance

Balance

Figure 4. Schematic of water collection set up [4].

Figure 5. Water collection data for anode side o o (Tcell = 50 C, T(A/C) = 75/65 C). We discuss next a study that illustrates the how water balance data can be obtained. In that study we were interested in quantifying the water transport across the membrane. We focused on the performance of a PEM fuel cell with various inlet stream humidity

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an overall water balance for various humidification temperatures at cell voltage 0.5 V are shown in the Table 1.

Air Bleeding In this section we discuss the experimental techniques associated with recovering from CO poisoning by using an air bleed technique. A schematic of air bleeding system is illustrated in Figure 8. As it is shown in the figure, a flow meter controls the flow rate of air. The air passes through a filter and a check valve to the anode side. One can be interested in the steady-state performance as well as the response of the MEA to transient spikes in the concentration. Details of the performance can be found in [13]. The MEAs used were PRIMEA Series 5561 MEAs, consisting of GORE-SELECT membranes (25 µm nominal membrane thickness) and catalyst loadings of 0.45 mg/cm2 Pt alloys on the anode and 0.4 mg/cm2 Pt on the cathode. The GDM used in the experiment were CARBELä CL GDM (16 mils = 0.406 x10-3 m) and SingleSided ELAT (0.406 x10-3 m) GDM. The same type of GDM was used for both the anode and the cathode sides of the MEA. The gasket used in the experiment was a compressible gasket (silicone coated fiberglass cloth, 10 mil of thickness). Another thin gasket (1.2 mils = 0.305 x 10-5 m) referred to as the sub-gasket, was placed between the MEA, the gas diffusion media, and the gasket to allow for better of the MEA during cell assembly. The cell was tightened with four bolts clamped to 50 in-lbf of torque.

1.6

1.4

Current Density (A/cm2)

1.2

T(A/C)=95/85 oC

1.0

0.8

0.6

T(A/C)=65/45 oC

T(A/C)=75/65oC T(A/C)=85/75 oC

0.4

0.2

0.0 40

50

60

70

80

90

100

110

120

Time (hours)

Figure 6. Humidity effects on PEM fuel cell performance at o 50 C cell temperature, P(A/C): 1/1 atm [13].

Flow meter 0.9

0.8

Cell voltage (V)

H2 /CO/H2O (in)

Air

O2 /H2O (in)

Filter

0.7

Check valve T(A/C)=65/55 oC

0.6

T(A/C)=95/85 oC

0.5

T(A/C)=75/65 oC

T(A/C)=85/75 oC

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1.1

H2 /CO2 /H2O (out)

Current density (A/cm2)

O2 /H2O (out)

o

Figure 7. Polarization curve for PEM fuel cell at 50 C cell temperature, P(A/C): 1/1 atm [13].

Figure 8. Schematic of air bleeding system [13].

In the table the column labeled “ maximum water out at cell temp.” indicates the amounts of water vapor that the exhausted gas could hold. All the cases the measured water out is greater than the amount of water that the exhausted gas could hold. It indicates that super saturation occurs at the anode and cathode side. According to the result water accumulation at the cathode side is increased as the humidification temperature is increased. However, negative accumulations at the anode are shown in all cases. It indicates that the amount of water out is greater than the water coming in at anode side, and the water should be transported from the cathode. So, no more water seems to be accumulated at the anode side. Also, this result shows that the water in the fuel cell is accumulated over all even though the negative accumulation occurs at the anode side.

Depending on the expected tolerance of the MEA, different levels of CO can be used. In Ref [21] 500 and 3000 ppm CO in hydrogen was used. In that experiment the temperature of fuel cell was 70 oC. The humidification temperatures were 75/65 oC for Single-Sided ELAT (henceforth referred to as SSE) gas diffusion media and 85/75°C for CARBELä CL GDM. The estimated dew points for the SSE and CARBELä CL GDM were 70/60°C and 80/70°C, respectively, from the humidity calibration data. These humidification temperatures produced optimum polarization curves during preliminary experiments in neat hydrogen. To obtain the conditions for this optimum performance, these preliminary experiments consisted of fixing the

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Table 1. Water balance in PEM fuel Cell o (Cell voltage 0.5 V, Cell Temperature 50 C) [13]. Hum. Temp. (oC)


65/55 75/65 85/75 95/85

0.89 0.86 0.78 0.75

Anode Water Balance (g/min) Water in

Water out

0.0124 0.0227 0.0472 0.1198

0.0208 0.0324 0.0573 0.1674

Accum. at Anode -0.0084 -0.0097 -0.0101 -0.0475

Cathode Water Balance (g/min) Maximum water out at Cell Temp. 0.0017 0.0017 0.0017 0.0015

Water in

Gen.

Water out

0.0379 0.0682 0.1152 0.2474

0.0499 0.0483 0.0439 0.0419

0.0773 0.0986 0.1402 0.2412

Accum. at Cathode 0.0106 0.0179 0.0189 0.0481

Overall Accum. (g/min)

Maximum water out at Cell Temp. 0.0324 0.0314 0.0294 0.0275

0.0022 0.0081 0.0088 0.0005

GDM, respectively. The fact that the current is changing over this limiting plateau may indicate that over the 20 cm2 area some local over-potentials are large enough to oxidize CO.

anode and cathode humidifier temperatures and performing a polarization curve at the predetermined stoichiometric flow. The humidity temperatures were then changed and new polarization curves were obtained. Also, the pressures of the anode and cathode side were both 0 psig. Each cell was held at 0.6 V for 50 hours before the polarization data were obtained. At the beginning the current-voltage (polarization) behavior was measured in the presence of neat hydrogen at a stoichiometry of 1.2 for the anode and 2.0 for the cathode [13]. After recording a polarization curve with neat H2, polarizations in CO/H2 mixtures with and without air-bleed were obtained. In these polarization experiments, the gases used were 500 and 3000 ppm CO in H2. Two levels of air-bleed were used, 5 % and 15 % air for the 500 and 3000 ppm CO, respectively. For a polarization curve, the cell voltage was randomly set from 0.45 V to open circuit voltage and the resulting steady state currents were measured. The randomization gave reproducible results and accounted for any hysteresis in the measured current densities. While recording the polarization curve, the flow rates were adjusted manually in an iterative manner to maintain the desired stoichiometry. Figure 9 shows the polarization behavior for both GDM with neat hydrogen and with 500 ppm CO in the feed. Also, the effect of airbleed is shown in this figure. When the 500 ppm CO/H2 mixture gas was introduced in the fuel cell, the performance is substantially decreased. However, it is almost completely recovered in cell performance with 5% air-bleed. On the other hand, even 15% airbleed is ineffective with 3000 ppm CO [21]. It may be noted that there are performance differences between SSE and CARBELä CL GDM with air-bleed (especially at low cell voltages). This difference may be attributable to either differences in the permeability of the GDM or the humidity in the feed stream. Again, the different dew points for these two gas diffusion media were optimum for neat hydrogen rather than for the CO/H2 mixture as mentioned earlier. Evidence for the electrochemical oxidation of CO may be shown with a graph of anode polarization. It is difficult to compare the CO tolerance between the two GDM since the baseline performances with neat hydrogen for SSE and CARBELä CL GDM are different. This comparison is aided by using the anode over-potential graph shown in Figure 10. These over-potentials are calculated from the difference between the cell potential with neat hydrogen and the cell potential with CO/H2 at the same current density. It is assumed that the hydrogen over-potential with neat hydrogen is negligible, and that the ohmic contribution to the cell voltage and the cathodic over- potential depend only on the current density. In Figure 10, a limiting current density occurs at 0.25 A/cm2 for SSE and 0.30 A/cm2 for CARBELä CL GDM for 500 ppm without air-bleed. Without air-bleed, the active site limited current density for 3000 ppm CO is observed approximately 0.10 and 0.08 A/cm2 for SSE and CARBELä CL

1.0 0.9 0.8 CARBEL CL

Cell voltage (V)

0.7

Neat H2

0.6 0.5 0.4

Single Side ELAT

Single Side ELAT

w/o air bleed

air bleed (5 %)

0.3

CARBEL CL

Single Side ELAT

w/o air bleed

Neat H2

CARBEL CL air bleed (5 %)

0.2 0.1 0.0 0

200

400

600

800

1000

1200

1400

1600

1800

Current density (mA/cm2)

Figure 9. Performance comparison between CARBELä ä CL and Single-Sided ELAT GDM for 500 ppm CO [21]

Anode Overvoltage (EH2 - EH2/CO), V

0.5

0.4

0.3

0.2

0.1

0.0

101

2

3

4

5

6

7 8

102

2

3

4

5

6

7 8

103

2

Current density (mA/cm2)

Figure 10: Anode over-potential due to CO poisoning 3000 ppm CO/H2 (dashed lines) & 500 ppm CO/H2 (solid lines); SSE with air- bleed (■); SSE without air-bleed (□); CARBELä ä CL without air-bleed (○) [21].

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For SSE, air-bleed with 3000 ppm CO yields polarization performance similar to the performance with 500 ppm without air-bleed. The airbleed technique is effective in regenerating enough sites so that this limiting current for SSE at 500 ppm CO increases from 0.25 A/cm2 to approximately 1 A/cm2.

0.6

0.8

0.5 Cell voltage (V)

Transient Response to Load Changes Transient response tests can be performed to study the effect of changing the load. Here we present data for four different cases at a constant cell temperature of 70 oC and 1 atm operating pressure for both the anode and cathode streams (i.e. 0 psig back pressure). For the first case, a cell voltage change from 0.7 V to 0.5 V was applied. For the second case, the cell voltage was changed from 0.5 V to 0.7 V. In both the first and second cases, the flow rates of the anode and cathode gases corresponded to a stoichiometry of 1.2 and 2.0 (i.e. 20 % excess hydrogen and 100 % excess air) for 0.5 V. Thus one can consider case one to be a change from excess oxygen to a normal stoichiometry for 0.5V. Then case 2 corresponds to a move from a normal amount of oxygen to and excess amount. The flow rates were 62 and 259 standard cm3/min (0 oC and 101 KPa) for anode and cathode, respectively. For the third and fourth cases, the voltage step changes were same as case one and two. However, the flow rates were changed to correspond to 1.2/2.0 for the cell voltage of 0.7 V (i.e. flow rates (A/C) = 50/99 cm3/min). Then in case three one can consider the change from a normal oxygen flow rate to a starved condition. Also, case four would correspond to a change from a starved condition to a normal stoichiometry of 2.0. We used an oscilloscope (TDS 200Series, Tektronix) to capture the transient response of current in a short time period. In the experiments Series 5510 PRIMEA MEA (0.3 mg/cm2 Pt loading, 20 µm nominal membrane thickness, W.L. Gore &  Associates, Inc) and Double-Sided ELAT (thickness 500 µm) were used as a MEA and gas diffusion media, respectively. The cell was tightened with four bolts clamped to 150 in-lbf of torque. The active

0.6


0.7

0.4 0.5

0.4

0.3

-0.3

-0.2

-0.1

-0.0

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0.7

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Time (sec.)

0.8

0.6

0.7

0.5

0.6

0.4

0.5

0.3


Cell voltage (V)

Figure 12. Transient behavior for voltage change from 0.5 V 3 to 0.7 V at flow rate 62/259 cm /min (case 2).

2

area of the membrane was 10 cm and the serpentine flow field consisted of 20 equally spaced channels and bends for a total length of approximately 0.6 m. The flow channel height and width are 0.1 cm and 0.08 cm, respectively.

0.4

0.2

-0.1

0.0

0.1

0.2

0.3

0.4

Time (sec.)

Figure 11. shows the response of the current density when the cell voltage was changed from 0.7 V to 0.5 V (case 1). One may notice the maximum overshoot in the current density at 0.25 seconds. Figure 12 shows the current density decreases when the cell voltage increases from 0.5 V to 0.7 V (case 2). There is a little under shoot observed


0.5

0.8 0.8

0.6

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Cell voltage (V)

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Cell voltage (V)

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Time (sec.)

Time (sec.)



6

less than 0.05 seconds and the current density decreases exponentially within 0.7 seconds and approaches to 0.34 A/cm2. The current density responses corresponding to cell voltage change shown in Figures. 13 and 14 (case 3 & 4) are similar as Figures 11 and 12, respectively. In Figure 13, maximum overshoot in current density was observed at 0.13 seconds. Also, Figure. 14 shows that the under shoot within 0.05 seconds and current density approaches to 0.31 A/cm2.

Uptake by and Transport Through Nafion 117 Membranes”, Journal of Electrochemical Society, 140, pp. 1041-1047. [10] Zawodzinski, T.A., Springer, T.E., Davey, J., Jestel, R., Lopez, C., Valerio, J., and Gottesfeld, S., 1993, “A Comparative Study of Water Uptake By and Transport Through Ionomeric Fuel Cell Membranes”, Journal of Electrochemical Society, 140, pp. 1981-1985. [11] Zawodzinski, T.A., Davey, J., Valerio, J., and Gottesfeld, S., 1995, “The Water Content Dependence of Electro-osmotic Drag in Proton-conducting Polymer Electrolytes”, Journal of Electrochimica Acta, 40, PP. 297-302. [12] Springer, T.E., Zawodzinski, T.A., and Gottesfeld, S., 1991, “Polymer Electrolyte Fuel Cell Model”, Journal of Electrochemical Society, 138, pp. 2334-2342. [13] Lee, W. K., 2000, "The effect of clamp torque, humidity, and Co poisoning on PEM fuel cell," Ph.D. dissertation, Department of Chemical Engineering, University of South Carolina, Columbia [14] Schmidt, V.M., Brockerhoff, P., Hohlein, B., Menzer, R., and Stimming, U., 1994, “Utilization of methanol for polymer electrolyte fuel cells in mobile systems”, Journal of Power Sources, 49, pp. 299313. [15] Oetjen, H.-F., Schmidt, V.M., Stimming, U., and Trila, F., 1996, “Performance Data of a Proton Exchange Membrane Fuel Cell Using H2/CO as Fuel Gas”, Journal of Electrochemical Society, 143, pp. 3838-3842. [16] Dhar, H.P., Christner, L.G., and Kush, A.K., 1987, “Nature of CO Adsorption during H2 Oxidation in Relation to Modeling for CO Poisoning of a Fuel Cell Anode”, J. of Electrochemical Society, 134, pp. 3021-3026. [17] Gottesfeld, S., and Pafford, J., 1988, “A New Approach to the Problem of Carbon Monoxide Poisoning in Fuel Cells Operating at Low Temperatures”, J. of Electrochemical Society, 135, pp. 26512652. [18] Schmidt, V.M., Oetjen, H.-F., and Divisek, J., 1997, “Performance Improvement of a PEMFC Using Fuels with CO by Addition of Oxygen-Evolving compounds”, J. of Electrochemical Society, 144, pp. L237-238. [19] Bellows, R.J., Marucchi-Soos, E., Reynolds, R.P., 1998, “Proposed Mechanism of CO Mitigation in PEMFCs by Using Dilute H2O2 in the Anode Humidifier”, Electrochemical Society Proceedings, 98-27, pp. 121-126. [20] Bauman, J.W., Zawodzinski, T.A., and Gottesfeld, S., 1998, “An Investigation of Transient Carbon Monoxide Poisoning Effects in Polymer Electrolyte Fuel Cells”, Electrochemical Society Proceedings, 98-27, pp. 136-149. [21] Murthy, M., Esayian, M., Hobson, A., MacKenzie, S., Lee, W.K., and Van Zee, J.W., 2001, “The Performance of a PEM Fuel Cell Exposed to Transient CO Concentrations”, submitted to J. of Electrochemical Society. [22] Naseri-Neshat, H., Lee, W. K., Shimpalee, S., and Van Zee, J. W., 2001, "Numerical prediction of transient response on a PEM fuel cell performance," submitted to J. of Electrochemical Society.

CONCLUSIONS In an effort to discuss experimental techniques used to study PEM fuel cells, we have present procedures and data that can be used to study the effects of humidity, the effects of air bleed recovery from CO poisoning, and the effects of changing the load. Typically the water management techniques can be used to detect flooding of the diffusion layer was detected for various humidity conditions. The technique of monitoring humidity in the exhausted gas and collecting water was explained. Also, the overall water balance data for the system was reported. For CO poisoning, air bleeding techniques may be used to recover the performance. We showed how this recovery can change the polarization data are reported for 500 ppm and 3000 ppm CO in hydrogen. Depending on the tolerance of the MEA to CO, injection of 5% air may completely recover the performance of the MEA at 500 ppm CO for current densities less than 0.6 A/cm2. Finally, techniques to study the transient behavior of PEM fuel cell showed how the transient response of current might change when a voltage step change was applied. A maximum overshoot in the current density was observed at 0.25 second for the first case and 0.13 seconds for the third case when the voltage changed from 0.7 to 0.5 V. Also, a under shoot in current density was detected within 0.05 seconds for the case 2 and 4. ACKNOWLEDGEMENTS The author acknowledges financial support for this work from the W. L. Gore & Associates, Inc. and Department of Energy (DE-FG0291ER75666). REFERENCES [1] Lemons, R.A., 1990, “Fuel Cells for Transportation”, J. Power Sources, 29, pp. 251-264. [2] Appleby, A.J., 1994, “Fuel cell electrolytes: evolution, properties and future prospects”, J. Power Sources, 49, pp. 15-34. [3] Shoesmith, J.P., Collins, R.D., Oakley, M.J., and Stevenson, D.K., 1994, “Status of solid polymer fuel cell system development’, J. Power Sources, 49, pp. 129-142.

[4] Lee, W.K., Van Zee, J.W., Shimpalee, S., Dutta, S., 1999, “Effect of Humidity on PEM Fuel Cell Performance Part I: Experiments,” Proceedings of the ASME IMECE, 5, pp. 454460. [5] Verbrugge, M.W. and Hill, R.F., 1990, “Ion and Solvent Transport in Ion-Exchange Membrane”, Journal of Electrochemical Society, 137, pp. 893-899. [6] Verbrugge, M.W. and Hill, R.F., 1990, “Transport Phenomena in Perfluorosulfonic Acid Membranes during the Passage of Current”, Journal of Electrochemical Society, 137, pp. 1131-1138. [7] Fuller, T. F. and Newman, J., 1992, “Experimental Determination of the Transport Number of Water in Nafion 117 Membrane”, Journal of Electrochemical Society, 139, pp. 1332-1337. [8] Zawodzinski, T.A., Springer, T.E., Davey, J., Valerio, J., Gotesfeld, S., 1991, “Water Transport Properties of Fuel Cell Ionomers”, Electrochemical Society Proceedings, 91-10, pp. 187-196. [9] Zawodzinski, T.A., Derouin, C., Radzinski, S., Sherman, R.J., Smith, Van T., Springer, T.E., and Gottesfeld, S., 1993, “Water

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