An experimental study on water transport through the membrane of a PEFC operating in the dead-end mode

An experimental study on water transport through the membrane of a PEFC operating in the dead-end mode

international journal of hydrogen energy 34 (2009) 7768–7779 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he An exp...

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international journal of hydrogen energy 34 (2009) 7768–7779

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/he

An experimental study on water transport through the membrane of a PEFC operating in the dead-end mode Yongtaek Lee, Bosung Kim, Yongchan Kim* Department of Mechanical Engineering, Korea University, 5-1 Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea

article info

abstract

Article history:

Water transport through the membrane of a polymer electrolyte fuel cell (PEFC) was

Received 6 May 2009

investigated by not only measuring the voltage variation but also visualizing the accu-

Received in revised form

mulation of water at the anode for various values of operating parameters, such as the

1 July 2009

humidity, current density, stoichiometry, location of humidification, and membrane

Accepted 3 July 2009

properties. The PEFC was operated in the dead-end mode to prevent the discharge of water

Available online 5 August 2009

from the anode. The water transport in the PEFC was characterized by the elapsed time for the voltage to reach its limit. Anode visualization showed water transport under various

Keywords:

conditions. In addition, the mass balance of water at the anode of the PEFC was considered.

Polymer electrolyte fuel cell

The variations of water diffusion and electro-osmotic drag were analyzed based on the

Water transport

experimental results.

Visualization

ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Diffusion Mass balance

1.

Introduction

Water is indispensable to polymer electrolyte fuel cells (PEFCs). It hydrates the membrane and raises the ionic conductivity of the membrane. However, excessive water causes flooding, which disturbs the transport of the reactant gas. The water content, l, which is defined as the ratio of the þ number of water molecules to the number of charged SO 3H , is a key factor that determines the ionic conductivity of the membrane. l also affects the characteristics of the electroosmotic drag and water diffusion through the membrane. Simultaneously, the water content is affected by water transport through the membrane. At the anode, hydrogen oxidizes and the protons move from the anode to the cathode, dragging along a number of water molecules. At the cathode, the amount of oxygen decreases to form water molecules. To enhance the conductivity of the membrane, water is supplied together with a reactant gas to one or both sides of the

membrane. By these processes, a water-concentration gradient is produced across the thin membrane, which becomes the driving force for water diffusion. In the last few decades, the water behavior across the membrane has been one of the major areas of study in research on PEFCs. Zawodzinski et al. [1,2] measured the variation of water uptake in the membrane with the water behavior and compared the water-diffusion coefficients for several perfluorosulfonic-acid membranes at 30  C. The drag coefficient was also measured as a function of the water uptake [3–5]. Using Nafion 117, Zawodzinski et al. [3] experimentally determined the drag coefficient of a vapor-equilibrated membrane and compared their result with those of the literature [4]. Zawodzinski et al. [5] also measured the drag coefficients of several liquid-equilibrated membranes. They reported a substantial decrease in the drag as the water content of the membrane decreased [5]. Buchi et al. [6] investigated the transverse water content profile by piling several sheets of

* Corresponding author. Tel.: þ82 2 3290 3366; fax: þ82 2 921 5439. E-mail address: [email protected] (Y. Kim). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.07.010

international journal of hydrogen energy 34 (2009) 7768–7779

Nomenclature DW,F F J j MM ndrag nW,An P RH S z

Fickian diffusion-coefficient of water, cm2/s Faraday constant (96,485), C/mol molar flux, mol/cm2-s current density, A/cm2 equivalent weight of the membrane, g/mol electro-osmotic drag coefficient number of moles of water per unit area of MEA, mol/cm2 pressure, atm relative humidity, % stoichiometry perpendicular distance to the membrane, cm

membranes. They showed that the water content at the anode side decreased with the current density and suggested that the model of Eikerling et al. [7] was in close agreement with their experimental data. Yan et al. [8] trapped and condensed the discharged water vapor and the discharged gas at the anode and cathode. They calculated the net-drag coefficient at various humidity conditions. In addition, several watertransport models were suggested, and the water transport was investigated numerically [7,9–16]. Springer et al. [9] presented a model that employs water-diffusion coefficients, electroosmotic drag coefficients, water-sorption isotherms, and membrane conductivities, which are a function of water content. Motupally et al. [11] compared the Fickian diffusioncoefficient models of Zawodzinski et al. [1], Fuller [15], and Nguyen et al. [10]. Janssen [12] demonstrated 1-D and 2-D numerical models to account for water transport through an MEA (membrane electrode assembly) for various humidification conditions of the inlet gases and compared the predicted results with those of his experimental data [17]. By using numerical methods, Kulikovsky [13] and Um et al. [14] presented the distribution of the water content in a membrane. Extensive research has been conducted on the visualization of water buildup in a flow-field by optical photography [18–25]. Yang et al. [18] visualized a droplet from its emergence to its departure and concluded that a liquid-film drain was important for preventing flooding. Through visualization, several researchers have investigated the effects of a gas diffusion layer (GDL) on water buildup and removal [19–21]. Liu et al. [22] visualized water flooding and the two-phase flow in parallel, inter-digitated, and cascade flow-fields. They revealed that the area of flooding was small in the inter-digitated and cascade flow-fields. Liu et al. [23] visualized the effects of the cell temperature, cathode flow rate, and operating temperature on the flow characteristics of PEFCs. Lu et al. [24] visualized the two-phase flow in PEFC cathode parallel channels over wide range of air velocity and water velocity in a ex situ experimental setup. Ge et al. [25] visualized the anode side of PEFCs. They concluded that the formation of liquid on the anode was caused by the condensation of water vapor and that no water droplet was found on the surface of the GDL.

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Greek letters l water content or local ratio ðH2 O=SO 3Þ in the membrane density of the dry membrane, g/cm3 rM 4 relative humidity of the reactant gas, % Superscripts SAT saturation Subscripts An anode diff diffusion drag electro-osmotic drag i inlet o outlet s saturation W water

Recently, some visualization techniques which do not use direct optical photography were widely employed [26–29]. Tsushima et al. [26] used NMR imaging to measure the spatial distribution of water in a 340 mm thick Nafion membrane of an operating PEM fuel cell. Owejan et al. [27] employed neutron imaging to visualize the steady-state water distributions in automotive fuel cells operating under a wide range of ambient temperature conditions. Park et al. [28] investigated the amount and distribution of liquid water in an operating PEFC under dynamic loading using neutron imaging technique. Nam et al. [29] employed an environmental scanning electron micrograph (ESEM), when a diffusion medium is exposed to a water saturated atmosphere (low temperature and small water vapor pressure). In this study, the characteristics of water transport through the membrane were investigated for various values of

Fig. 1 – Schematic diagram of the water-transport mechanism in the PEFC.

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0.8

Gas diffusion layer Current collector (with penetrated flow field)

0.7

Cell pote ntial (V)

Current collector

2

Gore's MEA, j=0.4 A/cm

Transparent window (Polycarbonate )

0.6 0.5 Relative humidity

0.4

40 % 60 % 80 % 100%

0.3

CCD Camera

0.2

0

10

20

30

40

50

60

T ime (min) Fig. 3 – Variation of the voltage over time for various external humidities of both gases.

End plate

MEA

Flow field plate (Graphite)

Computer

Fig. 2 – Schematic diagram of the PEFC with a transparent anode.

operating parameters, such as the relative humidity, stoichiometry of air, current density, location of humidification, and membrane thickness. Since we applied the dead-end mode in the PEFC system, we were able to evaluate the water-transport characteristics by observing the performance degradation of the PEFC and by visualizing the accumulation of water. Generally speaking, in the most previous studies, the visualization was conducted at the cathode because the flooding was easily observed at the cathode. However, it is difficult to distinguish the water that is transported through a membrane from the generated water. In this study, to observe in-situ genuine water transport, the visualization was conducted at the anode and the variation of water accumulation with time was investigated. In addition, mass-balance analysis at the anode was carried out to estimate the change in each factor that influences water transport.

2. Analysis of the mass balance for water at the anode Fig. 1 illustrates the water behavior in an operating PEFC. Water enters the fuel cell along with the input gases and exits the fuel cell along with the discharged gases. The water in the

membrane is dragged by protons from the anode to the cathode of the PEFC and diffuses from one side of the membrane to the other side of the membrane. Water is also generated at the cathode by the oxygen reduction reaction (ORR). The watermass balance is considered by setting a control volume at the anode, as shown in Fig. 1. Water generation is not considered because the control volume is fixed at the anode. For a single PEFC, which includes one MEA, the inlet molar flux of hydrogen per unit area of the MEA is given by: JH2 ;i ¼ SH2

j 2F

(1)

Water is supplied along with the reactant gases through external humidification. Provided that the hydrogen is humidified externally at a relative humidity of 4, the molar flux of the water that enters the anode is given by: JW; An; i ¼ SH2

fAn PsðTAn;i Þ j 2F PAn  fAn PsðTAn;i Þ

(2)

The amount of water that exits along with the discharged gases can be calculated analytically in the same way as for externally humidified water, if the exiting water is in a vapor state and the relative humidity at the exit is known. However, in practice, there is some water in the liquid state that is mixed with the discharged hydrogen. Thus, it is very difficult to evaluate the amount of exiting water by solving Eq. (2) with appropriate modifications. Through the conduction of protons, some water molecules move from the anode to the cathode by electro-osmotic drag. The molar flux of the water that is dragged to the cathode in this way can be expressed as:

Table 1 – Properties of the MEA and GDL given by manufacturers. MEA

Thickness Loading [Ca/An] Component

GDL 

Gore 57 series

Nafion 112

18 mm (membrane) 0.4/0.4 mg/cm2 –

50 mm (membrane) 0.3/0.3 mg/cm2 –



Sigracet GDL 35BC

325 mm – Dual layers (microþmacro)

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JW;drag ¼ ndrag

j F

(3)

Using the results of Refs. [5,9] for the Nafion membrane, the dragged molar flux can be expressed as a function of the water content, l. JW;drag ¼ nSAT drag

l j 22 F

for 0  l  22

(4)

Along with the electro-osmotic drag, water diffusion also occurs across the membrane due to the water-concentration gradient between both sides of the membrane. The water diffusion can be expressed by Fick’s laws [11]. r dl JW;diff ¼  M DW;F MM dz

(5)

MM is the equivalent weight [30,31] of the membrane and z is an axis that is perpendicular to the membrane and proceeds from the cathode to the anode. If the water content at the cathode is higher than that at the anode, dl/dz is negative, which implies that the diffusive molar flux of water is positive. Actually, the water content through the thin membrane shows monotonic variation. It does not show either a local maximum or minimum inside the membrane because the generation and supply of water occur on the surface of the membrane. If we assume that the water-content gradient varies linearly through the membrane, Eq. (5) can be reexpressed as Eq. (6) through the water-content difference between the two sides of the membrane.

Molar flux of water by drag or diffusion (x10-5 mol/cm2-s)

international journal of hydrogen energy 34 (2009) 7768–7779

3.5 Electro-osmotic drag Diffusion

3.0 2.5 2.0 1.5 1.0 0.5 0.0

0

5

10

15

20

λ or dλ Fig. 5 – Variation of molar flux of water by electro-osmotic drag and diffusion according to l and dl.

However, if a PEFC operates in the dead-end mode, the system is no longer in a steady-state and there is no need to consider the water that flows out from the anode. The water at the anode accumulates (or is removed) with time. As much hydrogen is supplied in the dead-end mode as it is consumed, which leads to a stoichiometric ratio of 1 for S in Eq. (2). Thus, Eq. (7) can be rearranged as: DnW;An ¼ JW; An; i  JW;drag þ JW;diff Dt

(8)

DW,F is the Fickian diffusion-coefficient of water in the membrane, which is highly dependent on l. The relationships between the diffusion coefficients and the water content are suggested in Refs. [9,10,15]. Using the above parameters, the molar-conservation equation of water at the anode per unit area of the MEA can be expressed as:

fAn PsðTAn;i Þ DnW;An j l j r Dl ¼  nSAT  M DW;F drag Dt 2F PAn  fAn PsðTAn;i Þ 22 F MM Dz

(9)

DnW;An ¼ JW; An; i  JW; An; o  JW;drag þ JW;diff Dt

The current density, humidity, pressure, and temperature, which determine the absolute amount of the water vapor that is included in the supplied gas, are controllable parameters. By observing the accumulation of water at the anode, we can evaluate the qualitative variation of the net water transport.

r Dl JW;diff ¼  M DW;F MM Dz

(6)

(7)

If a PEFC operates in a steady-state, the time-dependent term (on the left-hand-side [LHS] of Eq. (7)) will be eliminated.

If the properties of the membrane such as rM and MM are unchanged, Eq. (9) can be represented as:   DnW;An Dl ¼ f j; fAn ; PAn ; TAn; i ; l; Dt Dz

(10)

Molar flux of water by external humidification (x10-7 mol/cm2-s)

10

8

Table 2 – Values of parameters for evaluating the values of each term in Eq. (9).

6

Parameter

4

2

0 0.0

0.2

0.4

0.6

0.8

Relative humdiity Fig. 4 – Variation of molar flux of water by external humidification according to the relative humidity.

1.0

Value

j F PAn Tsat nSAT drag MM

0.4 A/cm2 96,485 C/mol 1 atm 70  C 2.5 1100 g/mol (Gore) 1000 g/mol (Nafion)

rdry

2000 kg/m3 (Gore) 1970 kg/m3 (Nafion) 1.5  106 cm2/s (Gore) 3.0  106 cm2/s (Nafion)

DW,F

Reference Experimental condition – Experimental condition Experimental condition Zawodzinski et al. [5] Ju et al. [31] O’ Hayre et al. [30] Ju et al. [31] O’ Hayre et al. [30] Ju et al. [31] Springer et al. [9]

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Therefore, in this study, we measured and analyzed the effects of the current density, humidity, humidification conditions, stoichiometry, and membrane thickness on water accumulation at the anode.

3.

Experimental details

3.1.

Experimental setup

The experiment was carried out using a test station. The rate of flow of the reactant gas was controlled by a mass-flow controller (MFC). The humidity was controlled by adjusting the temperature of the sparging-type humidifier. A band-type heater was wrapped on the gas line between the humidifier and the cell to control the temperature of the gas that enters

the cell. The temperature of the heater was maintained at 10  C higher than that of the humidifier to prevent condensation of water in the gas line. Back-pressure regulators were attached at the exits of the anode and cathode lines to maintain the system pressure at the design value. A two-way valve was employed at the exit of the anode to switch the mode of flow between flow-through and dead-end. A unit cell with an active area of 25 cm2 was employed in this experiment, as shown in Fig. 2. A four-channel serpentine-type flow-field was applied. A cathode flow-field was machined on the current collector that was made of graphite. A gold-coated stainless steel plate of 1 mm thickness was used as the current collector at the anode. The anode flow-field was penetrated on the plate to allow hydrogen flow along the channel. The water accumulation at the anode was visualized through a transparent, polycarbonate window. A CCD camera

Fig. 6 – Photographs of water buildup at the anode with 100% RH: (a) just before the closure of the exit valve; (b) just before the voltage reaches the limit; and (c) an enlargement of parts of the second photograph.

international journal of hydrogen energy 34 (2009) 7768–7779

was installed to take pictures of the anode every 30 s. The operating temperature of the fuel cell was controlled by a heater that was inserted in the cathode-side endplate and an air-cooling fan that was attached to the surface of the endplate. Two types of MEA, Gore’s 57 series and Nafion 112, were used in this experiment. The specifications of MEAs and the GDL are listed in Table 1.

3.2.

Test conditions

Throughout the experiment, the operating temperature of the cell was maintained at 70  C. The cell temperature was measured by a T-type thermocouple that was located at the center of the cathode-side flow-field plate. The relative humidities of hydrogen and air were maintained at 100% RH, unless otherwise mentioned. The stoichiometries were maintained at 2.5 and 1.5 for air and hydrogen, respectively, in the flow-through mode. However, after the exit valve of the anode

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was closed, the flow rate of hydrogen was determined by the current instead of the flow controller, which was fully opened. The experiment was carried out in the dead-end mode. Therefore, water accumulation can occur at the anode and affect the performance of the system. The recording of data started simultaneously with the shut-down of the two-way valve at the exit of the anode. Before shut-down of the valve, the cell was operating at flow-through mode. The flowthrough operation took enough time to the parameters in the cell such as water content of membrane reached the steadystate. During the flow-through operation, the anodic pressure was maintained at the atmospheric pressure. However, as soon as the valve was closed, the pressure started to increase and reached the set value of the pressure regulator. The pressure regulator that was installed in the hydrogen bottle was preset to keep the anodic pressure of the PEFC in the dead-end mode to 20 kPa after the pressure reached the steady-state value. The cathodic pressure was also controlled

Fig. 7 – Photographs of water buildup at the anode with 40% RH: (a) just before the closure of the exit valve; (b) after the same period at which Fig. 6(b) was taken; and (c) an enlargement of parts of the second photograph.

international journal of hydrogen energy 34 (2009) 7768–7779

4.

Results and discussion

The voltage of PEFC was measured over time at various conditions. Data were recorded immediately after the valve at the exit of the anode was closed. Generally, the voltage decreased with time. The initial voltage was maintained for a while but after a certain period, dropped dramatically. Since the anode was in the dead-end state, the water that was supplied with the reactant gas and diffused from the cathode through the membrane could not be discharged. As the water accumulated at the anode, it covered the catalyst layer and blocked the small holes in the gas diffusion layer to increase the mass-transport loss at the anode. The required time for the voltage to drop below 0.2 V varied with the operating condition. The water transport in the PEFC was characterized over time as the voltage reached the limit, and the water accumulation was visualized. Gore’s 57 series MEA was used for the effects of humidity, stoichiometry of air, current density, and location of humidification (Sections 4.1–4.4), while Nafion 112 was used for the effect of the membrane thickness (Section 4.5).

4.1.

Effect of the humidity

Fig. 3 shows the variation of the voltage with the relative humidity (RH or 4). Both the hydrogen and air were humidified externally at a specified humidity. The current density was fixed at 0.4 A/cm2. At 100% RH, the voltage dropped below 0.2 V in about 22 min. As the humidity decreased, the time to reach the voltage limit increased, possibly indicating that the rate of water accumulation increases as the humidity increases. The increase in water accumulation implies that the LHS of Eq. (9),DnW;An =Dt, which represents the rate of change of water inside the control volume (anode), increases with the humidity (4) of the reactant gas. The first term of the right-hand-side (RHS), which refers to the molar flow rate of water per unit area of the MEA that is supplied by external humidification, increases with 4. The second term, nSAT drag lj=ð22FÞ, which is linearly proportional to l, also increases with 4 because the increased humidity raises the water content of the membrane. The absolute values of each term in the RHS of Eq. (9) are plotted in Figs. 4 and 5. Fig. 4 shows the variation of molar flux of water by external humidification at the current density of 0.4 A/cm2. It increases from 0 to 8.1  107 mol/cm2-s as the relative humidity varies from 0 to 1. Fig. 5 shows the absolute values of molar flux of water by electro-osmotic drag and diffusion. The drag flux varies from 0 to 1.03  105 mol/cm2-s as the l varies from 0 to 22. The water flux by diffusion ranges from 0 to 3.33  105 mol/cm2-s at the range of dl from 0 to 22. Table 2 shows the values of the parameters used in the calculation of each term. Considering only the first two terms of the RHS, the rate of change of the water inside the anode must have a negative value. However,

0.8 Gore's MEA, j=0.4 A/cm2 0.7

Cell potential (V)

at 20 kPa by the back-pressure regulator to remove the convection of water due to the pressure difference between the anode and the cathode. The variation of the voltage at a given current density was measured every 5 s until the voltage dropped below 0.2 V.

0.6 0.5 0.4

Soichiometry of air 1.5 2.5 3.5

0.3 0.2

0

5

10

15

20

25

30

Time (min) Fig. 8 – Variation of the voltage with time for various stoichiometric ratios of air.

this is not consistent with the experimental results: the rate of change of the water at the anode increased with time once the exit valve was closed. Consequently, the third term of the RHS, rM DW;F Dl=ðMM DzÞ, should have a negative value, as a result of which Dl/Dz has a negative value. A negative value of Dl/Dz indicates that the water concentration at the cathode is higher than that at the anode because the z-axis is in the direction from the cathode to the anode, as shown in Fig. 1. The experimental results showed that the rate of water accumulation increased with the humidity, which means that the water-concentration gradient increases with the humidity. The water-concentration gradient can be expressed as the absolute value of Dl/Dz. Fig. 6 visualizes the buildup of water at the anode at 100% RH. Fig. 6(a) shows the surfaces of the GDL and the flow channel just before the outlet valve was closed. Before the valve was closed, the PEFC was maintained at the flowthrough mode for a while so that it could reach a steady-state. Some fog on the inner surface of the transparent window covered about half of the channel. However, water columns were not observed in either the channel or the water film. Ge et al. [25] also reported that the liquid that forms at the anode 0.8

Gore's MEA, 100% RH

0.7

Cell potential (V)

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0.6 0.5 0.4 Current density 0.2 A/cm2 0.4 A/cm2 0.6 A/cm2

0.3 0.2

0

5

10

15

20

25

30

Time (min) Fig. 9 – Variation of the voltage with time for various current densities.

35

international journal of hydrogen energy 34 (2009) 7768–7779

side is mainly from the condensation of water vapor on the cooler and on the hydrophilic surface of the channel walls; they did not find water droplets on the surface of the GDL. In the flow-through mode, the water activity at the anode was relatively low and it was not enough to form water droplets inside GDL. Fig. 6(b) and (c) shows photographs of the anode in the dead-end mode just before the voltage reached its limit of 0.2 V. The diffused water droplets merged with the condensed droplets on the surface of the channel wall to form the water columns. At some parts of the channel, the surface of the GDL was covered with a water film. At the corner of the gas channel where the path turns by a right-angle, water gathered and clogged the gas flow. In addition, water accumulation was observed mainly in the last half of the flow path. The water content at the cathode increased as the flow moved towards the exit because the generated water was swept away by the gas that flowed towards the exit. Therefore, even though the

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current distribution was high near the inlet of the gas [32–35], the water activity increased along with the flow. The increased water-concentration gradient caused further diffusion of water to the anode. Um et al. [14] represented waterconcentration profiles along flow directions and reported that there is a point of zero net water flux. Before this point, electro-osmotic drag is dominant but after this point, diffusion is dominant. There is another reason for water accumulation in the last half of the flow path. Even though the exit of the anode was closed, there was a still little flow of hydrogen. Since hydrogen was consumed continuously by oxidation, it flowed towards the closed exit as much as it was consumed. The flow dragged and pushed the water droplets and water columns. The flow rate of hydrogen reduced with the approach of the exit and water accumulated near the exit. Fig. 7 shows the anode at 40% RH. Fig. 7(a) shows the anode just before the outlet valve was closed and Fig. 7(b) shows the

Fig. 10 – Photographs of water buildup at the anode at the current densities of: (a) j [ 0.2 A/cm2; (b) j [ 0.4 A/cm2; and (c) j [ 0.6 A/cm2. All photographs were taken after the same period at which the voltage under 0.6 A/cm2 reached the limit of 0.2 V.

international journal of hydrogen energy 34 (2009) 7768–7779

anode at the same time-point when Fig. 6(b) was taken (a lapse of 22 min). A greater area of the channel is covered with water droplets in Fig. 7(b) than in Fig. 7(a). As shown in Fig. 7(c), even though large water droplets existed in the channel, a water film was not observed on the GDL. The diminished extent of back-diffusion of water due to the low humidity was observed through the visualization of the anode. Consequently, the high external humidification accelerated water accumulation at the anode, which led to a high rate of decrease in the voltage.

4.2.

Effect of the stoichiometry of air

Fig. 8 shows the variation of the voltage when the cathodic stoichiometry was varied from 1.5 to 3.5. As the stoichiometry increased from 1.5 to 3.5 at a fixed current density of 0.4 A/ cm2, the time to reach the voltage limit of 0.2 V increased from 14 to 25 min. The higher flow rate of air at the cathode dragged more water droplets in the gas channel out of the cell. It also reduced the water content of the membrane especially at the cathode side, thereby decreasing the water-concentration gradient between both sides. Since there was no variation in the humidity, temperature, and current density, the externalhumidification term in Eq. (9) was not changed. It can be inferred from Eq. (9) that the water-concentration gradient, which is the absolute value of Dl/Dz, decreases as the cathodic stoichiometry increases. Therefore, more time was required for the voltage to reach its limit at high stoichiometry than at low stoichiometry.

4.3.

Effect of the current density

Fig. 9 shows the variation of the voltage with time for three different current densities. The PEFC was operated in the flowthrough mode at each current density until it reached the steady-state in terms of the voltage, temperature, humidity, and water buildup, at which point the exit valve was closed. As the current density increased from 0.2 to 0.6 A/cm2 under 100% RH, the time to reach the voltage limit of 0.2 V decreased from 30 min at 0.2 A/cm2 to 10 min at 0.6 A/cm2. This may have arisen because a high current density would yield a larger difference between the initial voltage and the voltage limit than a low current density. However, as shown in Fig. 9, the rate of decrease of the voltage is so steep that the effect of the difference between the initial voltage and the voltage limit is not significant. In this case, careful attention is required to correlate the rate of voltage decrease with the rate of water accumulation at the anode. Even when the same amount of water accumulates at the anode and the water blocks the same area of the GDL, the remaining area without waterblocking can be either sufficient for a low current density or insufficient for a high current density. Therefore, the voltage can be maintained under a low current density while the voltage under a high current density drops dramatically for the same amount of water accumulation. Fig. 10(a)–(c) shows the water buildup at the anode at the current densities of 0.2 A/cm2, 0.4 A/cm2, and 0.6 A/cm2, respectively. All photographs were taken after the same lapse of time that was required for the voltage to reach the limit of 0.2 V under a current density of 0.6 A/cm2. The amount of

water accumulation increased slightly with the current density but the effect of the current density on the rate of water accumulation was not significant. In Eq. (9), the external-humidification term increases with the current density as more hydrogen is supplied. As the water-concentration gradient increases with the current density [6,9,15], the variation of the water diffusion from the cathode to the anode also contributes to the increase in the water accumulation at the anode. The increased water production by the increased current density can yield the increase of l. The electroosmotic drag term is determined by l and the current density. With the increasing current density the drag term shows the largest variation because it is directly proportional to the current density. However, as shown in Fig. 5, the absolute quantity is much smaller than that of diffusion. As shown in Fig. 10, the variation of the current density yields only a small increase in the water accumulation at the anode, which implies that the increase in the water outflow from the control volume, owing to electro-osmotic drag, is no less than that of the water inflow that is caused by humidification and backdiffusion. Even though each term in Eq. (9) varies greatly with the current density, the rate of change of the amount of water at the anode increases only slightly. Consequently, the major parameter that determines the time for the performance decay is the effective area (uncovered by water accumulation) which can be sufficient or insufficient according to the current density.

4.4.

Effect of the location of humidification

The method of external humidification was varied as anodic humidification, cathodic humidification, and bilateral humidification (i.e., at both the anode and the cathode). Fig. 11 presents the variation of the voltage with the location of humidification at a current density of 0.4 A/cm2. The highest rate of voltage reduction was observed for bilateral humidification. However, for anodic humidification, there was no voltage reduction within 60 min, except for intermittent fluctuations. For cathodic humidification, the time taken for the voltage to reach the limit of 0.2 V was slightly greater than the corresponding value for bilateral humidification. Even though

0.8

Gore's MEA, j=0.4 A/cm2

0.7

Cell potential (V)

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0.6 0.5 0.4

Location of humidification

0.3

Cathodic Bilateral Anodic

0.2

0

10

20

30

40

50

Time (min) Fig. 11 – Variation of the voltage with time for various locations of humidification.

60

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there was no external humidification at the anode under cathodic humidification, the water accumulated at the anode, which caused the voltage to drop. Fig. 12(a) and (b) visualizes the water buildup at the anode for cathodic and anodic humidifications, respectively. Fig. 12(a) was taken just before the voltage reached the limit of 0.2 V. Many water droplets existed in the gas channel in the form of slug flow. However, for anodic humidification, as shown in Fig. 12(b), no water droplet was observed on the transparent window and the surface of the GDL. When only the cathode is humidified, the external-humidification term of Eq. (9) can be eliminated because hydrogen is supplied in a dry

state. The water activity at the cathode is much higher than that at the anode due to the external humidification and the water generation. The high water-concentration gradient across the membrane increases the rate of change of the water at the anode. However, when only the anode was humidified, water accumulation was not observed at the anode although this does not mean that the rate of change of the water at the anode was negative. If DnW;An =Dt is negative, the membrane will suffer dry-out, which will decrease the voltage. Thus, it can be concluded that the electro-osmotic drag and back-diffusion of water are in an equilibrium state.

4.5.

Effect of the membrane properties

The effect of the membrane properties on the water-concentration gradient was investigated by employing Nafion 112. The properties of membrane that affects the water transport are equivalent weight, membrane density, diffusivity and thickness. As shown in Table 2, the equivalent weight and density show little difference. The thickness of Nafion 112 was 50 mm, which was much higher than that of Gore’s 57 series that had a thickness of 18 mm. The water diffusivity of Gore’s membrane has approximately half of the value of Nafion because the Gore’s membrane was reinforced [31]. Fig. 13 shows the variation of the voltage with the humidity. At low humidity, the voltage decreases slowly with time, which is consistent with that of the Gore’s MEA. However, the voltage required much more time than the voltage of Gore’s MEA to reach the limit of 0.2 V. The water flux of Nafion by diffusion is about 0.7 times that of Gore’s membrane because the diffusivity is twice larger and the thickness is 2.8 times larger. The reduced diffusion effect decreased the rate of water buildup and consequently increased the elapsed time for the voltage to reach the limit of 0.2 V. However, when the humidity was low, it required much longer time for performance decay. At low humidity condition, the water content of membrane was low. As the water was accumulated at the anode, some water could be used to membrane hydration at low humidity condition. Much more water transporting through membrane might be captured in the Nafion which is 2.8 times thicker than Gore’s membrane.

0.8 Nafion 112 MEA, j=0.4 A/cm2

Cell potential (V)

0.7 0.6 0.5 0.4

Relative humidity 60% 80% 100%

0.3 0.2

Fig. 12 – Photographs of water buildup at the anode for various locations of humidification: (a) cathodic humidification (at the limit of 0.2 V) and (b) anodic humidification (after 3 h).

0

10

20

30

40

50

60

Time (min) Fig. 13 – Variation of the voltage with time for Nafion 112 at various humidities.

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Fig. 14 – Photographs of water buildup at the anode with 100% RH using Nafion 112: (a) just before the closure of the exit valve and (b) after the same period at which the voltage reached the limit under Gore’s MEA [(c) and (d) are enlargements of parts of the second photograph].

Fig. 14 shows the water accumulation at the anode through Nafion 112 at 100% RH. Fig. 14(a) visualizes the anode in the flow-through mode. Fig. 14(b)–(d) shows the anode after the exit valve was closed. As shown in Fig. 14(c), some water columns were plugged in the channel. However, as shown in Fig. 14(d), there was no water droplet or water film over a fairly large area of the GDL. Compared with the photograph of Gore’s MEA (Fig. 6), the amount of water accumulation was very low not only before the close of the exit valve but also after the same period over which the voltage of Gore’s MEA reached the limit of 0.2 V.

5.

Conclusions

Water transport through the membrane of a PEFC that was operated in the dead-end mode was investigated by varying the operating parameters that affect the performance of PEFCs. The anode was also visualized to observe the variation of the net water transport. In addition, mass-balance analysis inside the anode revealed the variation of the water-

concentration gradient across the membrane for diverse operating conditions. As the humidity of the reactant gas increased, the voltage drop accelerated and the amount of water accumulation at the anode increased. The mass-balance analysis indicated that both the water-concentration gradient and the rate of back-diffusion of water increased with the humidity. As the stoichiometry of air increased, more time was required for the voltage to drop to the limit of 0.2 V. Since a high rate of airflow caused more water to discharge out of the cathode, the water content and water-concentration gradient of the membrane decreased. As the current density increased, the voltage drop accelerated. However, the rate of water accumulation at the anode increased only slightly because of the high increase in the electro-osmotic drag. When only the anode was humidified externally, no water accumulated at the anode because of the relatively low water-concentration gradient. Only cathodic humidification resulted in water accumulation at the anode. The water transport rate was decreased when thicker membrane was used. Not only the decrease of waterconcentration gradient, but also storage capacity reduced the rate of net water transport.

international journal of hydrogen energy 34 (2009) 7768–7779

Acknowledgements This work was supported by a grant (No. R01-2006-000-11014-0) from the Basic Research Program of the Korea Science & Engineering Foundation.

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