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Water transport in a PEM fuel cell with slanted channel flow field plates
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 0
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Water transport in a PEM fuel cell with slanted channel flow field plates Preeyaphat Wawdee a,b, Sunun Limtrakul a,b,*, Terdthai Vatanatham a,b, Michael W. Fowler c a
Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, 50 Ngamwongwan Rd., Jatujak, Bangkok 10900, Thailand b Center of Excellence on Petrochemical and Materials Technology, Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand c Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada
article info
abstract
Article history:
Cell humidification is an important factor affecting PEM fuel cell performance. A new cell
Received 2 July 2014
was designed with slanted grooves on both cathode and anode sides to improve water
Received in revised form
management. Water transport measurements provided information on flooding, hydra-
5 October 2014
tion, and back-diffusion. The results showed less water flowing out from the cathode side
Accepted 9 January 2015
of an anode down-slanted channel than from that of a rectangular channel, because anode
Available online xxx
down-slanting induced a hydration gradient which caused water back-diffusion into the anode, leading to better performance due to improved membrane hydration and conduc-
Keywords:
tivity. At high humidification, performance decreased because of condensation and
Proton exchange membrane fuel cell
blocking of the gas diffusion layer, but replacing the rectangular channel with an anode
Flow field
down-slanted channel improved performance to match that of a rectangular cell at normal
Water management
humidification. However, the cathode down-slanted channel showed membrane dehy-
Water measurement
dration due to water draining away. Moreover, anode or cathode up-slanted channels
Slanted channel
induced flooding, leading to poor performance.
Back diffusion
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A fuel cell is a device that generates electricity by electrochemical reaction, using hydrogen and oxygen (or air) as the reactants to create electricity with no emissions other than water. Its efficiency is higher than that of a combustion engine. The proton exchange membrane (PEM) fuel cell is one
type of fuel cell that can be operated at low temperatures of 80e90 C [1], thus requiring a short start-up time [2e4]. Cell humidification is one of the most important factors that affect the PEM fuel cell performance. The proton exchange membrane must be sufficiently hydrated in order to facilitate proton transport. If the cell membrane is too dry, proton conductivity will decrease. But if the PEM fuel cell contains too much water, flooding can occur in the catalyst
* Corresponding author. Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, 50 Ngamwongwan Rd., Jatujak, Bangkok 10900, Thailand. E-mail address: [email protected] (S. Limtrakul). http://dx.doi.org/10.1016/j.ijhydene.2015.01.037 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
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and the gas diffusion layers [5]. Flooding affects the performance of the PEM fuel cell because the liquid water blocks the access of reactant to the catalyst surface, leading to concentration overpotential [6]. Thus good water management in the PEM fuel cell can improve cell performance. The PEM fuel cell generally consists of a membrane electrode assembly (MEA) sandwiched between two flow field plates. The plates are grooved to create the gas flow channels to provide inlets and outlets for reactant gases. The flow fields are usually made of graphite, which is electron conducting material. This component is used for feeding gas reactant into the MEA and removing water products from the cell. Water is the most important product produced at the cathode side from electrochemical reactions. It is necessary to prevent the pores of the porous media from flooding. Water management improvement can be executed by properly designing the flow fields. Generally, rectangular channels are designed. Improved cell performance has been demonstrated by replacing a rectangular channel with a slanted channel flow field [7]. Flow field plates with upslanted and down-slanted channels were tested on both the cathode and anode sides. The down-slanted plate placed on the anode side presents the best performance for all humidity levels [7]. Thus the improvement focus in this modified flow field should be water management in the cell. The hypothesis is that the anode down-slanted channel assists water flowing down and induces water back-diffusion, preventing membrane dehydration and improving humidity
balancing. Study of water transport in the modified cell is therefore important. Water measurement can provide information on water production, diffusion, and backdiffusion. This work studies the water measurements of fuel cells with an angle of 35 in slanted channels, and explains water transport behavior in terms of hydration, flooding, and back-diffusion. This work focuses on measurement of water transport in flow channels inside a cell. A fuel cell of 150 cm2 active area with assisted water flow channels was used for measurement of water flowing out from the cell. Experimental results with two different flow field designs were assembled into five configurations of single fuel cells. The cell performance was shown by current density and compared to the amount of water flowing out, from each side of the electrodes. In this way, the findings of Bunmark and coworkers (2007) can be clearly visualized, confirmed, or contrasted.
Materials and methods Fuel cell components The catalyst powder (40% Pt on carbon black, Alfa Aesar) was mixed in methanol (analytical grade, Merck KGaA). Subsequently, ionomer solution (5% Nafion, Ion Power) was added into the mixture and mixed well. The prepared catalyst ink
173.4 mm
86.5 mm
(a) 1 mm 1.5 mm 1mm 35 °
(b)
(c)
Fig. 1 e Slanted channel flow field design (a) Flow Field Plate, (b) Slanted Channels, and (c) Dimension of slanted channel.
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 0
was coated on both sides of a membrane (Nafion 112, Dupont) treated with a 0.5 M H2SO4 solution, which was then dried and sandwiched between two porous gas diffusion layers, GDL (10 BA, SGL CARBON Group) with 0.4 mm thickness, to make MEA of 0.4 mg/cm2 Pt loading with 150 cm2 active area. It was then compressed on both sides between flow fields, current collector plates, and endplates. The main objective of this work involves studying the effects of flow field channel geometries on water occurrences and flow. The flow channel plate, with a length of 173.4 mm and a width of 86.5 mm, consists of seven parallel serpentine structures with five primary grooved channels. Rectangular cross-sectional channels of 1.5 mm in width and 1 mm in depth were grooved on the conventional graphite flow field. In the modified assisted water flow field, every channel was slanted at an angle of 35 . A drawing of the modified flow field is shown in Fig. 1.
Test station The fuel cell was connected to the testing system as shown in Fig. 2. The testing system comprises a computer, test station, load box (Scribner 890 CL), feed gas lines, humidifiers, and controllers. Hydrogen gas and air were fed to the mass flow controllers before entering the column humidifiers and the fuel cell. The hydrogen and air flow rate stoichiometry was 1.7 and 2.2 on the anode and cathode, respectively. The cell temperature was measured by thermocouples installed within the endplates and controlled by a controller. Fuel cell performance and water management were evaluated at different humidifier temperatures to test the effects of humidity on water occurrences. All cases were operated at a cell temperature of 80 C. The temperatures of the saturated feed gases were 80 C (saturated condition) and 95 C (extremely wet condition). Before each measurement, the fuel cell conditioning was carried out to obtain a stable operation.
Water flow rate measurement The exhaust gases of both excess fuel and air were fed into graduated cylinders to measure the water flow rates. Water
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measurements were carried out at the flow outlets of both the anode and cathode sides. At a constant voltage, the amount of water flowing out was measured every 10 min for a total of 10 times, and the results were averaged. The cell operations were repeated three times for each condition.
Flow field arrangements in the fuel cells The experiments were carried out in fuel cells with five different configurations of graphite flow channel plates in order to study water transport in the fuel cell. Five cases shown in Table 1 were experimentally studied. In case 1, the channels on both the cathode and anode sides were fabricated with typical rectangular channel design to establish baseline conditions. In case 2, the anode flow field was modified with down-slanted channel (ADS) while keeping rectangular design on the cathode side. In case 3, the anode flow field was modified with upslanted channel (AUS) keeping rectangular design on the cathode side. In case 4, the cathode flow field was modified with down-slanted channel (CDS) keeping rectangular design on the anode side. In case 5, the cathode flow field was modified with up-slanted channel (CUS) keeping rectangular design on the anode side.
Results and discussion The aim of this work was to study the effects of flow field configuration on PEM fuel cell performance and water transport. This study examines water management in the slanted channels on flow field plates with different orientations. The explanation of flow behavior in terms of hydration, flooding, and back-diffusion is discussed. In addition, the effects of humidity on water management were also studied. This work studied the effects of five different flow channel geometries on fuel cell performance, such as rectangular channel cell, anode down-slanted channel, cathode downslanted channel, anode up-slanted channel, and cathode upslanted channel. All cases were operated at a cell temperature of 80 C with two different humidifier temperatures of 80 and 95 C. The results for cell performance and water occurrence at constant voltage of 0.7 and 0.5 V are shown in Tables 2 and 3, respectively. In addition, the polarization curves for all cases are shown in Fig. 3.
Comparison of cell performance and amount of water flowing out with rectangular channel cell (RP) on both sides, anode down-slanted cell (ADS), and cathode down-slanted cell (CDS)
Fig. 2 e Fuel cell test station.
Fig. 4 shows the current densities and amount of water flowing out at cathode and anode sides with rectangular channel cell, anode down-slanted channel cell, and cathode downslanted channel cell at a cell temperature of 80 C and different gas humidification temperatures (80 and 95 C) for the operations with constant voltages of 0.7 and 0.5 V. In addition, it also shows schematic diagrams of water flowing inside the fuel cells.
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
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Table 1 e Flow field arrangement with rectangular and slanted channels. Run
Configurations Anode
Cathode
1
Rectangular channel plate (RP)
Rectangular channel plate (RP)
2
Down-slanted (ADS)
Rectangular channel plate (RP)
3
Up-slanted (AUS)
Rectangular channel plate (RP)
4
Rectangular channel plate (RP)
Down-slanted (CDS)
5
Rectangular channel plate (RP)
Up-slanted (CUS)
The effect of using the modified flow field channel shape is shown in Tables 2 and 3, and Fig. 3. It was found that the amount of water flowing out at the cathode side was more than that at the anode side for all cases, due to the large amount of water generated at the cathode side from the electrochemical reaction, in addition to relative humidity condensation. Water at the anode side was generated from relative humidity condensation and back-diffusion. The amount of water flowing out at a cell voltage of 0.5 V, where the current density was higher, was more than that at 0.7 V for all cases because the electrochemical reaction was higher. Thus the water occurrence was greater. In order to study the effects of slanted channels on cell performance and water transport, the cathode down-slanted channel cell and anode down-slanted channel cell were tested and compared with the rectangular channel cell. Fig. 4 shows the comparison of water occurrences and current densities for three different flow field configurations at 0.7 and 0.5 V. The amount of water flowing out at the cathode side with cathode down-slanted channel cell was the most for all humidity conditions because cathode down-slanted channel allowed the water occurrences to flow down to the bottom of the flow field channel, and water at the anode side flowed to the cathode side due to a concentration gradient. The polarization and power curves of the cells with rectangular channel cell, the anode down-slanted cell, and the cathode downslanted cell were compared as shown in Fig. 3. It was found that the anode down-slanted channel cell provides the best performance for all humidity levels. The performance of the rectangular channel cell was lower than that of the anode down-slanted channel cell but higher than that of the cathode down-slanted channel cell for both high and low current density conditions (see Fig. 4). In comparing the cathode down-slanted channel cell with the rectangular channel cell, it was found that the amount of water flowing out at the cathode side with cathode downslanted channel cell was higher for all humidity levels due to downward slanting inducing the water to flow away from the cathode GDL and collect in the bottom of the flow field channels as shown in Fig. 4(e). Thus, the amount of water flowing out at the cathode side of cathode down-slanted cell
was more than that of the rectangular channel cell. Water removal at the cathode side can lead to membrane dehydration. Thus the performance of cathode down-slanted channel cell was lower than that of the rectangular channel cell. The amount of water flowing out at the cathode side with cathode down-slanted was more than that of the rectangular channel cell. Therefore, down-slanting at the cathode side cannot improve performance due to more dehydration. The anode down-slanted cell is compared with the rectangular channel cell. It was found that the amount of water flowing out at the cathode side with anode down-slanted channel cell was less than that of rectangular channel cell due to the flowing of water product from the electrochemical reactions at the cathode side to the anode side and down to the bottom of the channel as shown in Fig 4(d). Thus the amount of water flowing out at the cathode side with anode down-slanted cell was lower than that of the rectangular channel cell. It can be confirmed that water outflow from the anode side with anode down-slanted at both temperatures is higher than that from the rectangular channel plate (see Fig. 4) for both low and high voltage cases. This high water backdiffusion from cathode side to anode side in the anode down-slanted cell can improve membrane humidity and conductivity. Therefore, the anode down-slanted cell has higher efficiency than the rectangular channel cell. Tables 2 and 3 show that fuel cell current density at a humidifier temperature of 80 C is higher than that at 95 C for all flow field designs at both voltage levels. At a higher humidifier temperature of 95 C, a larger amount of water was introduced into the cell. The excessive water induces flooding and blocks the pores of gas diffusion layers. Thus, the high humidifier temperature condition leads to mass transport losses that yield poorer cell performance. At the extremely wet condition (at a humidifier temperature of 95 C), cell performance can be improved by replacing the rectangular channel plate with an anode down slanted plate. The current density at the cell voltage of 0.5 V can improve from 484 to 523 mA/cm2 (compare RP case 2 and ADS case 4 in Table 3) leading the cell performance at this wet condition (95 C) to approach the normal condition of a rectangular plate at a humidifier temperature of 80 C in
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
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Table 2 e Amount of water flowing out and current density for 150 cm2 cell with various assembly combinations at a fuel cell temperature of 80 C with feed gas humidifier temperatures of 80 C and 95 C and the hydrogen and air flow rate stoichiometry of 1.7 and 2.2. The voltage was held constant at 0.7 V. Configuration
Amount of water at cathode side (mL/min)
Amount of water at anode side (mL/min)
Total flow of water (mL/min)
Current density at 0.7 V (mA/cm2)
Case 1 RP, TH 80 C
16.34
189
Case 2 RP, TH 95 C
26.89
177
Case 3 ADS, TH 80 C
16.44
204
Case 4 ADS, TH 95 C
27.11
182
Case 5 CDS, TH 80 C
16.22
180
Case 6 CDS, TH 95 C
26.45
170
Case 7 AUS, TH 80 C
16.55
184
Case 8 AUS, TH 95 C
26.78
174
Case 9 CUS, TH 80 C
15.78
166
Case 10 CUS, TH 95 C
25.89
157
which the current density is 549 mA/cm2. The results at a cell voltage of 0.7 V were found to be similar (see Table 2). Water measurements can confirm that the water in the cathode side of the anode down-slanted cell at 95 C decreased compared to the cell without modification (see cases 2 and 4 in Tables 2 and 3).
Comparison of cell performance and amount of water flowing out with cathode down-slanted cell (CDS) and cathode up-slanted cell (CUS) As shown in Fig. 5(a,b), the amount of water flowing out at the cathode side with the cathode down-slanted cell was higher than that of the cathode up-slanted cell for both humidity
levels due to water in downward-slanted channels flowing down to the bottom of the cathode flow field channels, and the flow of water from the anode side to cathode side resulting from hydration gradient. On the other hand, the cathode upward slanted let some water flow through the gas diffusion layers. Thus the amount of water flowing out at the cathode side with a cathode up-slanted plate was less than that of a cathode down-slanted cell. Using the cathode down-slanted flow field plate induces water to flow out from the gas diffusion layer as explained in Fig. 5(c). This avoids excessive water from the electrochemical reaction and from the humidity levels that were used in the experiment. The excessive amount of water can cause flooding in the gas diffusion layer and also block
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
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Table 3 e Amount of water flowing out and current density for 150 cm2 cell with various assembly combinations at a fuel cell temperature of 80 C with feed gas humidifier temperatures of 80 C and 95 C and the hydrogen and air flow rate stoichiometry of 1.7 and 2.2. The voltage was held constant at 0.5 V. Configuration
Amount of water at cathode side (mL/min)
Amount of water at anode side (mL/min)
Total flow of water (mL/min)
Current density at 0.5 V (mA/cm2)
Case 1 RP, TH 80 C
22.33
549
Case 2 RP, TH 95 C
33.33
484
Case 3 ADS, TH 80 C
22.66
584
Case 4 ADS, TH 95 C
33.77
523
Case 5 CDS, TH 80 C
22.00
492
Case 6 CDS, TH 95 C
32.78
457
Case 7 AUS, TH 80 C
22.23
525
Case 8 AUS, TH 95 C
33.06
476
Case 9 CUS, TH 80 C
21.78
453
Case 10 CUS, TH 95 C
31.45
419
critical pore sites of the porous electrode. Consequently, mass transfer is reduced which decreases the cell performance. As shown in Fig. 5(d), it can be seen that the upwardslanted channel can lead to increased flooding on the cathode gas diffusion layer because water at the cathode side cannot be removed effectively. Excessive water can block the gas diffusion layer and critical pore sites of the porous electrode, leading to mass transfer limitation that affects cell performance. The cell with cathode up-slanted induces water flooding which blocks mass transport of reactant gases. Therefore, it has lower performance than the cathode downslanted channel cell.
Comparison of cell performance and amount of water flowing out with anode down-slanted cell (ADS) and anode up-slanted cell (AUS) Comparison of the anode down-slanted cell to the anode upslanted cell showed that the amount of water flowing out at the cathode side with anode up-slanted cell was more than that of anode down-slanted cell for both humidifier temperatures as shown in Fig. 6(a,b). The upward-slanted anode can prevent water at the cathode side from moving toward the anode side. On the other hand, the downward-slanted anode can induce water back-diffusion from the cathode side to the anode side due to concentration gradient. It can
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
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CDS
AUS
CUS
45 Power (W)
Voltage (V)
36
0.8
27 18
0.4 a 80 °C
9
b 95 °C
0
0.0 0
200
6000
400
200
400
600
Current density (mA/cm²)
Fig. 3 e Comparison of polarization and power curves of five graphite flow channel plate configurations with the hydrogen and air flow rate stoichiometry of 1.7 and 2.2 at different gas humidification temperature of (a) 80 C, and (b) 95 C.
Amount of water (mL/min)
be also confirmed that the amount of water in the anode side of the anode down-slanted cell is higher than that of the anode up-slanted cell. The performance of the anode down-slanted cell was higher than that of the anode up-
30
Water at cathode RP
25 20
Water at anode CDS
ADS
21.7
21.9
21.3
18.3
18.4
18.2
15
12.4
11.6
10.9
10 5
4.0
4.4
3.6
0
Amount of water (mL/min)
80 95 80 95 80 95 Humidifier temperature (°C)
24
Water at cathode RP
20 16
CDS 18.6
17.7
14.7
15.1
14.2
12
9.4
8.7
7.9
8 4
1.7
2.2
1.1
0 80 95 80 95 80 95 Humidifier temperature (°C)
Membrane
630 600 570
RP
CDS
549 523
510
492
484
480
457
450 80 95 80 95 80 95 Humidifier temperature (°C) 220 210
RP
ADS
CDS
204
200 190
189 182 177
180
180 170
170 160
80 95 80 95 80 95 Humidifier temperature (°C)
(b)
Membrane
Anode
ADS 584
540
(a)
Water at anode
ADS
18.2
slanted cell for all humidity levels due to anode downward slanted shape helping to remove excess water (see Figs. 3 and 6). Thus the fuel cell continued working satisfactorily under the high gas humidity condition. In other words, the anode up-slanted shapes led to flooding at high humidity gas feed conditions. Flooding occurred at the higher gas humidifier temperature of 95 C with a cell temperature of 80 C. The downward-slanted anode can help to overcome this problem. The flow field with anode down-slanted channel induced water to flow toward the anode side as shown in Fig. 6(c) and provided better overall membrane hydration. Using anode down-slanted channel could be a remedy for the water flooding issue on the cathode side by promoting water back-diffusion to the anode side because of the hydration gradient. In addition, water back diffusion helps to prevent membrane dehydration and improves membrane conductivity. The water in the anode side flows to the cathode side because of the up-slanted channels as shown in Fig. 6(d). Therefore, the flooding can be on both sides of the cell.
Current density (mA/cm²)
ADS
Current density (mA/cm²)
RP
1.2
Membrane
Anode
Anode
Cathode
Cathode
Cathode
Water direction
Water direction
Water direction
Baseline Condition
Water Back Diffusion
Membrane Dehydration
(c) RP
(d) ADS
(e) CDS
Fig. 4 e Current densities and amount of water flowing out at cathode and anode sides with rectangular channel cell on both sides, anode down-slanted channel cell, and cathode down-slanted channel cell at the cell temperatures of 80 C and different gas humidification temperatures (80 and 95 C) with the hydrogen and air flow rate stoichiometry of 1.7 and 2.2, at the cell voltage of (a) 0.5 V, (b) 0.7 V, with schematic diagram of water flowing inside fuel cells; (c) RP, (d) ADS, and (e) CDS. Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
8
20
Water at anode 21.9
18.4
19.8 17.7
15 10.9
11.7
10 5
3.6
4.1
CDS 80°C
CUS 80°C
0
Water at cathode
25
15.1
7.9
5 1.1
1.8
CDS 80°C
CUS 80°C
17.1
8.8
0 CDS 95°C
490
492
470 450 430
419
410 CUS
CDS
80°C 80°C
CUS
95°C 95°C
190 180
180
170
170
166
160
157
150 140 CDS
CUS 95°C
457
453
(a)
14.0
10
510
CDS
CUS 95°C
Water at anode 18.6
20 15
CDS 95°C
Current density (mA/cm²)
Water at cathode
25
Current density (mA/cm²)
Amount of water (mL/min)
Amount of water (mL/min)
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(b)
Membrane
CUS
CDS
80°C 80°C
CUS
95°C 95°C
Membrane
Anode
Anode
Cathode
Cathode
Water direction
Water direction
Water Draining
Cathode Flooding & Pore Blocking
(c) CDS
(d) CUS
Fig. 5 e Current densities and amount of water flowing out at cathode and anode sides with cathode down-slanted channel cell and cathode up-slanted channel cell at a cell temperatures of 80 C and different gas humidification temperatures (80 and 95 C) with the hydrogen and air flow rate stoichiometry of 1.7 and 2.2, at the cell voltage of (a) 0.5 V, (b) 0.7 V, with schematic diagram of water flowing inside fuel cells; (c) CDS, and (d) CUS.
Flooding on the cathode side can occur more than on the anode side due to the water production from electrochemical reaction on the cathode. Thus cell performance of the anode up-slanted cell was lower than that of the anode downslanted cell due to flooding. The amount of water flowing out at the anode side of an anode down-slanted channel cell was more than that of an anode up-slanted channel cell (see Fig. 6). It can be confirmed that the cell with anode down-slanted induces water backdiffusion to the anode side. This water back-diffusion can improve membrane hydration and conductivity. Therefore, the anode down-slanted has higher efficiency.
A practical implication of ADS A practical implication is that it can improve and stabilize cell performance through water management in a fuel cell. It is less susceptible to performance degradation in a wide
variation of air humidity conditions. The manufacturing cost of slanted channel is practically the same as that of rectangular channel plate in that both can be carried out by a conventional milling shop, without the need of special machinery.
Conclusions Water transport and current density measurements were carried out in five orientations of rectangular and slanted flow channel plates of 150 cm2. It was found that the amount of water flowing out at the cathode side of an anode downslanted channel cell was less than that of a rectangular channel cell because anode downward slanting induced water at the cathode side to highly back-diffuse to the anode side due to a hydration gradient, leading to improved membrane hydration and conductivity, thus better cell performance than
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
9
20
Water at cathode
Water at anode 22.0
21.3 18.2
18.6
15
12.4
11.1
10 5
4.4
3.7
ADS 80°C
AUS 80°C
0
25
Water at cathode
20 15
ADS 95°C
10 5
18.7
15.2 9.4
2.2
1.3
ADS 80°C
AUS 80°C
8.1
0 ADS 95°C
610 584
578 546
AUS 95°C
525
523
514 476
482 450 ADS AUS 80°C 80°C
(a)
Water at anode
17.7 14.2
AUS 95°C
Current density (mA/cm²)
25
Current density (mA/cm²)
Amount of water (mL/min)
Amount of water (mL/min)
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 0
210
ADS AUS 95°C 95°C
204
200 190
184
182
180
174
170 160
(b)
Membrane
ADS AUS 80°C 80°C
ADS AUS 95°C 95°C
Membrane
Anode
Anode
Cathode Water direction
Cathode Water direction
Water Back Diffusion
Anode Flooding
(c) ADS
(d) AUS
Fig. 6 e Current densities and amount of water flowing out at cathode and anode sides with anode down-slanted channel cell and anode up-slanted channel cell at a cell temperature of 80 C and different gas humidification temperatures (80 and 95 C) with the hydrogen and air flow rate stoichiometry of 1.7 and 2.2, at the cell voltage of (a) 0.5 V, (b) 0.7 V, with schematic diagram of water flowing inside fuel cells; (c) ADS and (d) AUS.
that of a rectangular channel cell. At a higher humidifier temperature of 95 C with the fuel cell at 80 C, cell performance decreased because of condensation and blocking of the pores of the gas diffusion layer. At this wet condition, the cell performance can be improved by replacing the rectangular channel plate with an anode down slanted plate. The current density can be improved, leading the cell performance at this wet condition to approach that of a rectangular plate under normal gas humidification conditions. The water measurement shows that the water in the cathode side of the anode down-slanted cell at 95 C is decreased, compared to the cell without modification. It can help improve cell performance at extremely wet conditions to be on par with that of a rectangular cell under normal conditions. However, in a cathode down-slanted channel cell water drains away from the cell to the cathode side leading to membrane dehydration. In addition, anode and cathode up slanted channel cells induce flooding, leading to poor performance.
Acknowledgments Financial supports from Kasetsart University Research and Development Institute (KURDI), National Research Council of Thailand (NRCT), and the Center of Excellence on Petrochemicals and Materials Technology (PETROMAT) are gratefully acknowledged.
references
[1] Mehta V, Cooper JS. Review and analysis of PEM fuel cell design and manufacturing. J Power Sources 2003;114:32e53. [2] Ma HP, Zhang HM, Hu J, Cai YH, Yi BL. Diagnostic tool to detect liquid water removal in the cathode channels of proton
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
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exchange membrane fuel cells. J Power Sources 2006;162:469e73. [3] Hu M, Sui S, Zhu X, Yu Q, Cao G, Hong X, et al. A 10 kW class PEM fuel cell stack based on the catalyst-coated membrane (CCM) method. Int J Hydrogen Energy 2006;31:1010e8. [4] Hental PL, Lakeman JB, Mepsted GO, Adcock PL, Moore JM. New materials for polymer electrolyte membrane fuel cell current collectors. J Power Sources 1999;80:235e41.
[5] Guvelioglu GH, Stenger HG. Flow rate and humidification effects on a PEM fuel cell performance and operation. J Power Sources 2007;163:882e91. [6] Baschuck JJ, Li X. Modelling of polymer electrolyte membrane fuel cells with variable degrees of water flooding. J Power Sources 2006;86:181e96. [7] Bunmark N, Limtrakul S, Fowler MW, Vatanatham T, Gostick J. Assisted water management in a PEMFC with a modified flow field and its effect on performance. Int J Hydrogen Energy 2010;35:6887e96.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Water transport in a PEM fuel cell with slanted channel flow field plates Preeyaphat Wawdee a,b, Sunun Limtrakul a,b,*, Terdthai Vatanatham a,b, Michael W. Fowler c a
Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, 50 Ngamwongwan Rd., Jatujak, Bangkok 10900, Thailand b Center of Excellence on Petrochemical and Materials Technology, Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand c Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada
article info
abstract
Article history:
Cell humidification is an important factor affecting PEM fuel cell performance. A new cell
Received 2 July 2014
was designed with slanted grooves on both cathode and anode sides to improve water
Received in revised form
management. Water transport measurements provided information on flooding, hydra-
5 October 2014
tion, and back-diffusion. The results showed less water flowing out from the cathode side
Accepted 9 January 2015
of an anode down-slanted channel than from that of a rectangular channel, because anode
Available online xxx
down-slanting induced a hydration gradient which caused water back-diffusion into the anode, leading to better performance due to improved membrane hydration and conduc-
Keywords:
tivity. At high humidification, performance decreased because of condensation and
Proton exchange membrane fuel cell
blocking of the gas diffusion layer, but replacing the rectangular channel with an anode
Flow field
down-slanted channel improved performance to match that of a rectangular cell at normal
Water management
humidification. However, the cathode down-slanted channel showed membrane dehy-
Water measurement
dration due to water draining away. Moreover, anode or cathode up-slanted channels
Slanted channel
induced flooding, leading to poor performance.
Back diffusion
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A fuel cell is a device that generates electricity by electrochemical reaction, using hydrogen and oxygen (or air) as the reactants to create electricity with no emissions other than water. Its efficiency is higher than that of a combustion engine. The proton exchange membrane (PEM) fuel cell is one
type of fuel cell that can be operated at low temperatures of 80e90 C [1], thus requiring a short start-up time [2e4]. Cell humidification is one of the most important factors that affect the PEM fuel cell performance. The proton exchange membrane must be sufficiently hydrated in order to facilitate proton transport. If the cell membrane is too dry, proton conductivity will decrease. But if the PEM fuel cell contains too much water, flooding can occur in the catalyst
* Corresponding author. Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, 50 Ngamwongwan Rd., Jatujak, Bangkok 10900, Thailand. E-mail address: [email protected] (S. Limtrakul). http://dx.doi.org/10.1016/j.ijhydene.2015.01.037 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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and the gas diffusion layers [5]. Flooding affects the performance of the PEM fuel cell because the liquid water blocks the access of reactant to the catalyst surface, leading to concentration overpotential [6]. Thus good water management in the PEM fuel cell can improve cell performance. The PEM fuel cell generally consists of a membrane electrode assembly (MEA) sandwiched between two flow field plates. The plates are grooved to create the gas flow channels to provide inlets and outlets for reactant gases. The flow fields are usually made of graphite, which is electron conducting material. This component is used for feeding gas reactant into the MEA and removing water products from the cell. Water is the most important product produced at the cathode side from electrochemical reactions. It is necessary to prevent the pores of the porous media from flooding. Water management improvement can be executed by properly designing the flow fields. Generally, rectangular channels are designed. Improved cell performance has been demonstrated by replacing a rectangular channel with a slanted channel flow field [7]. Flow field plates with upslanted and down-slanted channels were tested on both the cathode and anode sides. The down-slanted plate placed on the anode side presents the best performance for all humidity levels [7]. Thus the improvement focus in this modified flow field should be water management in the cell. The hypothesis is that the anode down-slanted channel assists water flowing down and induces water back-diffusion, preventing membrane dehydration and improving humidity
balancing. Study of water transport in the modified cell is therefore important. Water measurement can provide information on water production, diffusion, and backdiffusion. This work studies the water measurements of fuel cells with an angle of 35 in slanted channels, and explains water transport behavior in terms of hydration, flooding, and back-diffusion. This work focuses on measurement of water transport in flow channels inside a cell. A fuel cell of 150 cm2 active area with assisted water flow channels was used for measurement of water flowing out from the cell. Experimental results with two different flow field designs were assembled into five configurations of single fuel cells. The cell performance was shown by current density and compared to the amount of water flowing out, from each side of the electrodes. In this way, the findings of Bunmark and coworkers (2007) can be clearly visualized, confirmed, or contrasted.
Materials and methods Fuel cell components The catalyst powder (40% Pt on carbon black, Alfa Aesar) was mixed in methanol (analytical grade, Merck KGaA). Subsequently, ionomer solution (5% Nafion, Ion Power) was added into the mixture and mixed well. The prepared catalyst ink
173.4 mm
86.5 mm
(a) 1 mm 1.5 mm 1mm 35 °
(b)
(c)
Fig. 1 e Slanted channel flow field design (a) Flow Field Plate, (b) Slanted Channels, and (c) Dimension of slanted channel.
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 0
was coated on both sides of a membrane (Nafion 112, Dupont) treated with a 0.5 M H2SO4 solution, which was then dried and sandwiched between two porous gas diffusion layers, GDL (10 BA, SGL CARBON Group) with 0.4 mm thickness, to make MEA of 0.4 mg/cm2 Pt loading with 150 cm2 active area. It was then compressed on both sides between flow fields, current collector plates, and endplates. The main objective of this work involves studying the effects of flow field channel geometries on water occurrences and flow. The flow channel plate, with a length of 173.4 mm and a width of 86.5 mm, consists of seven parallel serpentine structures with five primary grooved channels. Rectangular cross-sectional channels of 1.5 mm in width and 1 mm in depth were grooved on the conventional graphite flow field. In the modified assisted water flow field, every channel was slanted at an angle of 35 . A drawing of the modified flow field is shown in Fig. 1.
Test station The fuel cell was connected to the testing system as shown in Fig. 2. The testing system comprises a computer, test station, load box (Scribner 890 CL), feed gas lines, humidifiers, and controllers. Hydrogen gas and air were fed to the mass flow controllers before entering the column humidifiers and the fuel cell. The hydrogen and air flow rate stoichiometry was 1.7 and 2.2 on the anode and cathode, respectively. The cell temperature was measured by thermocouples installed within the endplates and controlled by a controller. Fuel cell performance and water management were evaluated at different humidifier temperatures to test the effects of humidity on water occurrences. All cases were operated at a cell temperature of 80 C. The temperatures of the saturated feed gases were 80 C (saturated condition) and 95 C (extremely wet condition). Before each measurement, the fuel cell conditioning was carried out to obtain a stable operation.
Water flow rate measurement The exhaust gases of both excess fuel and air were fed into graduated cylinders to measure the water flow rates. Water
3
measurements were carried out at the flow outlets of both the anode and cathode sides. At a constant voltage, the amount of water flowing out was measured every 10 min for a total of 10 times, and the results were averaged. The cell operations were repeated three times for each condition.
Flow field arrangements in the fuel cells The experiments were carried out in fuel cells with five different configurations of graphite flow channel plates in order to study water transport in the fuel cell. Five cases shown in Table 1 were experimentally studied. In case 1, the channels on both the cathode and anode sides were fabricated with typical rectangular channel design to establish baseline conditions. In case 2, the anode flow field was modified with down-slanted channel (ADS) while keeping rectangular design on the cathode side. In case 3, the anode flow field was modified with upslanted channel (AUS) keeping rectangular design on the cathode side. In case 4, the cathode flow field was modified with down-slanted channel (CDS) keeping rectangular design on the anode side. In case 5, the cathode flow field was modified with up-slanted channel (CUS) keeping rectangular design on the anode side.
Results and discussion The aim of this work was to study the effects of flow field configuration on PEM fuel cell performance and water transport. This study examines water management in the slanted channels on flow field plates with different orientations. The explanation of flow behavior in terms of hydration, flooding, and back-diffusion is discussed. In addition, the effects of humidity on water management were also studied. This work studied the effects of five different flow channel geometries on fuel cell performance, such as rectangular channel cell, anode down-slanted channel, cathode downslanted channel, anode up-slanted channel, and cathode upslanted channel. All cases were operated at a cell temperature of 80 C with two different humidifier temperatures of 80 and 95 C. The results for cell performance and water occurrence at constant voltage of 0.7 and 0.5 V are shown in Tables 2 and 3, respectively. In addition, the polarization curves for all cases are shown in Fig. 3.
Comparison of cell performance and amount of water flowing out with rectangular channel cell (RP) on both sides, anode down-slanted cell (ADS), and cathode down-slanted cell (CDS)
Fig. 2 e Fuel cell test station.
Fig. 4 shows the current densities and amount of water flowing out at cathode and anode sides with rectangular channel cell, anode down-slanted channel cell, and cathode downslanted channel cell at a cell temperature of 80 C and different gas humidification temperatures (80 and 95 C) for the operations with constant voltages of 0.7 and 0.5 V. In addition, it also shows schematic diagrams of water flowing inside the fuel cells.
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
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Table 1 e Flow field arrangement with rectangular and slanted channels. Run
Configurations Anode
Cathode
1
Rectangular channel plate (RP)
Rectangular channel plate (RP)
2
Down-slanted (ADS)
Rectangular channel plate (RP)
3
Up-slanted (AUS)
Rectangular channel plate (RP)
4
Rectangular channel plate (RP)
Down-slanted (CDS)
5
Rectangular channel plate (RP)
Up-slanted (CUS)
The effect of using the modified flow field channel shape is shown in Tables 2 and 3, and Fig. 3. It was found that the amount of water flowing out at the cathode side was more than that at the anode side for all cases, due to the large amount of water generated at the cathode side from the electrochemical reaction, in addition to relative humidity condensation. Water at the anode side was generated from relative humidity condensation and back-diffusion. The amount of water flowing out at a cell voltage of 0.5 V, where the current density was higher, was more than that at 0.7 V for all cases because the electrochemical reaction was higher. Thus the water occurrence was greater. In order to study the effects of slanted channels on cell performance and water transport, the cathode down-slanted channel cell and anode down-slanted channel cell were tested and compared with the rectangular channel cell. Fig. 4 shows the comparison of water occurrences and current densities for three different flow field configurations at 0.7 and 0.5 V. The amount of water flowing out at the cathode side with cathode down-slanted channel cell was the most for all humidity conditions because cathode down-slanted channel allowed the water occurrences to flow down to the bottom of the flow field channel, and water at the anode side flowed to the cathode side due to a concentration gradient. The polarization and power curves of the cells with rectangular channel cell, the anode down-slanted cell, and the cathode downslanted cell were compared as shown in Fig. 3. It was found that the anode down-slanted channel cell provides the best performance for all humidity levels. The performance of the rectangular channel cell was lower than that of the anode down-slanted channel cell but higher than that of the cathode down-slanted channel cell for both high and low current density conditions (see Fig. 4). In comparing the cathode down-slanted channel cell with the rectangular channel cell, it was found that the amount of water flowing out at the cathode side with cathode downslanted channel cell was higher for all humidity levels due to downward slanting inducing the water to flow away from the cathode GDL and collect in the bottom of the flow field channels as shown in Fig. 4(e). Thus, the amount of water flowing out at the cathode side of cathode down-slanted cell
was more than that of the rectangular channel cell. Water removal at the cathode side can lead to membrane dehydration. Thus the performance of cathode down-slanted channel cell was lower than that of the rectangular channel cell. The amount of water flowing out at the cathode side with cathode down-slanted was more than that of the rectangular channel cell. Therefore, down-slanting at the cathode side cannot improve performance due to more dehydration. The anode down-slanted cell is compared with the rectangular channel cell. It was found that the amount of water flowing out at the cathode side with anode down-slanted channel cell was less than that of rectangular channel cell due to the flowing of water product from the electrochemical reactions at the cathode side to the anode side and down to the bottom of the channel as shown in Fig 4(d). Thus the amount of water flowing out at the cathode side with anode down-slanted cell was lower than that of the rectangular channel cell. It can be confirmed that water outflow from the anode side with anode down-slanted at both temperatures is higher than that from the rectangular channel plate (see Fig. 4) for both low and high voltage cases. This high water backdiffusion from cathode side to anode side in the anode down-slanted cell can improve membrane humidity and conductivity. Therefore, the anode down-slanted cell has higher efficiency than the rectangular channel cell. Tables 2 and 3 show that fuel cell current density at a humidifier temperature of 80 C is higher than that at 95 C for all flow field designs at both voltage levels. At a higher humidifier temperature of 95 C, a larger amount of water was introduced into the cell. The excessive water induces flooding and blocks the pores of gas diffusion layers. Thus, the high humidifier temperature condition leads to mass transport losses that yield poorer cell performance. At the extremely wet condition (at a humidifier temperature of 95 C), cell performance can be improved by replacing the rectangular channel plate with an anode down slanted plate. The current density at the cell voltage of 0.5 V can improve from 484 to 523 mA/cm2 (compare RP case 2 and ADS case 4 in Table 3) leading the cell performance at this wet condition (95 C) to approach the normal condition of a rectangular plate at a humidifier temperature of 80 C in
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Table 2 e Amount of water flowing out and current density for 150 cm2 cell with various assembly combinations at a fuel cell temperature of 80 C with feed gas humidifier temperatures of 80 C and 95 C and the hydrogen and air flow rate stoichiometry of 1.7 and 2.2. The voltage was held constant at 0.7 V. Configuration
Amount of water at cathode side (mL/min)
Amount of water at anode side (mL/min)
Total flow of water (mL/min)
Current density at 0.7 V (mA/cm2)
Case 1 RP, TH 80 C
16.34
189
Case 2 RP, TH 95 C
26.89
177
Case 3 ADS, TH 80 C
16.44
204
Case 4 ADS, TH 95 C
27.11
182
Case 5 CDS, TH 80 C
16.22
180
Case 6 CDS, TH 95 C
26.45
170
Case 7 AUS, TH 80 C
16.55
184
Case 8 AUS, TH 95 C
26.78
174
Case 9 CUS, TH 80 C
15.78
166
Case 10 CUS, TH 95 C
25.89
157
which the current density is 549 mA/cm2. The results at a cell voltage of 0.7 V were found to be similar (see Table 2). Water measurements can confirm that the water in the cathode side of the anode down-slanted cell at 95 C decreased compared to the cell without modification (see cases 2 and 4 in Tables 2 and 3).
Comparison of cell performance and amount of water flowing out with cathode down-slanted cell (CDS) and cathode up-slanted cell (CUS) As shown in Fig. 5(a,b), the amount of water flowing out at the cathode side with the cathode down-slanted cell was higher than that of the cathode up-slanted cell for both humidity
levels due to water in downward-slanted channels flowing down to the bottom of the cathode flow field channels, and the flow of water from the anode side to cathode side resulting from hydration gradient. On the other hand, the cathode upward slanted let some water flow through the gas diffusion layers. Thus the amount of water flowing out at the cathode side with a cathode up-slanted plate was less than that of a cathode down-slanted cell. Using the cathode down-slanted flow field plate induces water to flow out from the gas diffusion layer as explained in Fig. 5(c). This avoids excessive water from the electrochemical reaction and from the humidity levels that were used in the experiment. The excessive amount of water can cause flooding in the gas diffusion layer and also block
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
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Table 3 e Amount of water flowing out and current density for 150 cm2 cell with various assembly combinations at a fuel cell temperature of 80 C with feed gas humidifier temperatures of 80 C and 95 C and the hydrogen and air flow rate stoichiometry of 1.7 and 2.2. The voltage was held constant at 0.5 V. Configuration
Amount of water at cathode side (mL/min)
Amount of water at anode side (mL/min)
Total flow of water (mL/min)
Current density at 0.5 V (mA/cm2)
Case 1 RP, TH 80 C
22.33
549
Case 2 RP, TH 95 C
33.33
484
Case 3 ADS, TH 80 C
22.66
584
Case 4 ADS, TH 95 C
33.77
523
Case 5 CDS, TH 80 C
22.00
492
Case 6 CDS, TH 95 C
32.78
457
Case 7 AUS, TH 80 C
22.23
525
Case 8 AUS, TH 95 C
33.06
476
Case 9 CUS, TH 80 C
21.78
453
Case 10 CUS, TH 95 C
31.45
419
critical pore sites of the porous electrode. Consequently, mass transfer is reduced which decreases the cell performance. As shown in Fig. 5(d), it can be seen that the upwardslanted channel can lead to increased flooding on the cathode gas diffusion layer because water at the cathode side cannot be removed effectively. Excessive water can block the gas diffusion layer and critical pore sites of the porous electrode, leading to mass transfer limitation that affects cell performance. The cell with cathode up-slanted induces water flooding which blocks mass transport of reactant gases. Therefore, it has lower performance than the cathode downslanted channel cell.
Comparison of cell performance and amount of water flowing out with anode down-slanted cell (ADS) and anode up-slanted cell (AUS) Comparison of the anode down-slanted cell to the anode upslanted cell showed that the amount of water flowing out at the cathode side with anode up-slanted cell was more than that of anode down-slanted cell for both humidifier temperatures as shown in Fig. 6(a,b). The upward-slanted anode can prevent water at the cathode side from moving toward the anode side. On the other hand, the downward-slanted anode can induce water back-diffusion from the cathode side to the anode side due to concentration gradient. It can
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
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CDS
AUS
CUS
45 Power (W)
Voltage (V)
36
0.8
27 18
0.4 a 80 °C
9
b 95 °C
0
0.0 0
200
6000
400
200
400
600
Current density (mA/cm²)
Fig. 3 e Comparison of polarization and power curves of five graphite flow channel plate configurations with the hydrogen and air flow rate stoichiometry of 1.7 and 2.2 at different gas humidification temperature of (a) 80 C, and (b) 95 C.
Amount of water (mL/min)
be also confirmed that the amount of water in the anode side of the anode down-slanted cell is higher than that of the anode up-slanted cell. The performance of the anode down-slanted cell was higher than that of the anode up-
30
Water at cathode RP
25 20
Water at anode CDS
ADS
21.7
21.9
21.3
18.3
18.4
18.2
15
12.4
11.6
10.9
10 5
4.0
4.4
3.6
0
Amount of water (mL/min)
80 95 80 95 80 95 Humidifier temperature (°C)
24
Water at cathode RP
20 16
CDS 18.6
17.7
14.7
15.1
14.2
12
9.4
8.7
7.9
8 4
1.7
2.2
1.1
0 80 95 80 95 80 95 Humidifier temperature (°C)
Membrane
630 600 570
RP
CDS
549 523
510
492
484
480
457
450 80 95 80 95 80 95 Humidifier temperature (°C) 220 210
RP
ADS
CDS
204
200 190
189 182 177
180
180 170
170 160
80 95 80 95 80 95 Humidifier temperature (°C)
(b)
Membrane
Anode
ADS 584
540
(a)
Water at anode
ADS
18.2
slanted cell for all humidity levels due to anode downward slanted shape helping to remove excess water (see Figs. 3 and 6). Thus the fuel cell continued working satisfactorily under the high gas humidity condition. In other words, the anode up-slanted shapes led to flooding at high humidity gas feed conditions. Flooding occurred at the higher gas humidifier temperature of 95 C with a cell temperature of 80 C. The downward-slanted anode can help to overcome this problem. The flow field with anode down-slanted channel induced water to flow toward the anode side as shown in Fig. 6(c) and provided better overall membrane hydration. Using anode down-slanted channel could be a remedy for the water flooding issue on the cathode side by promoting water back-diffusion to the anode side because of the hydration gradient. In addition, water back diffusion helps to prevent membrane dehydration and improves membrane conductivity. The water in the anode side flows to the cathode side because of the up-slanted channels as shown in Fig. 6(d). Therefore, the flooding can be on both sides of the cell.
Current density (mA/cm²)
ADS
Current density (mA/cm²)
RP
1.2
Membrane
Anode
Anode
Cathode
Cathode
Cathode
Water direction
Water direction
Water direction
Baseline Condition
Water Back Diffusion
Membrane Dehydration
(c) RP
(d) ADS
(e) CDS
Fig. 4 e Current densities and amount of water flowing out at cathode and anode sides with rectangular channel cell on both sides, anode down-slanted channel cell, and cathode down-slanted channel cell at the cell temperatures of 80 C and different gas humidification temperatures (80 and 95 C) with the hydrogen and air flow rate stoichiometry of 1.7 and 2.2, at the cell voltage of (a) 0.5 V, (b) 0.7 V, with schematic diagram of water flowing inside fuel cells; (c) RP, (d) ADS, and (e) CDS. Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
8
20
Water at anode 21.9
18.4
19.8 17.7
15 10.9
11.7
10 5
3.6
4.1
CDS 80°C
CUS 80°C
0
Water at cathode
25
15.1
7.9
5 1.1
1.8
CDS 80°C
CUS 80°C
17.1
8.8
0 CDS 95°C
490
492
470 450 430
419
410 CUS
CDS
80°C 80°C
CUS
95°C 95°C
190 180
180
170
170
166
160
157
150 140 CDS
CUS 95°C
457
453
(a)
14.0
10
510
CDS
CUS 95°C
Water at anode 18.6
20 15
CDS 95°C
Current density (mA/cm²)
Water at cathode
25
Current density (mA/cm²)
Amount of water (mL/min)
Amount of water (mL/min)
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 0
(b)
Membrane
CUS
CDS
80°C 80°C
CUS
95°C 95°C
Membrane
Anode
Anode
Cathode
Cathode
Water direction
Water direction
Water Draining
Cathode Flooding & Pore Blocking
(c) CDS
(d) CUS
Fig. 5 e Current densities and amount of water flowing out at cathode and anode sides with cathode down-slanted channel cell and cathode up-slanted channel cell at a cell temperatures of 80 C and different gas humidification temperatures (80 and 95 C) with the hydrogen and air flow rate stoichiometry of 1.7 and 2.2, at the cell voltage of (a) 0.5 V, (b) 0.7 V, with schematic diagram of water flowing inside fuel cells; (c) CDS, and (d) CUS.
Flooding on the cathode side can occur more than on the anode side due to the water production from electrochemical reaction on the cathode. Thus cell performance of the anode up-slanted cell was lower than that of the anode downslanted cell due to flooding. The amount of water flowing out at the anode side of an anode down-slanted channel cell was more than that of an anode up-slanted channel cell (see Fig. 6). It can be confirmed that the cell with anode down-slanted induces water backdiffusion to the anode side. This water back-diffusion can improve membrane hydration and conductivity. Therefore, the anode down-slanted has higher efficiency.
A practical implication of ADS A practical implication is that it can improve and stabilize cell performance through water management in a fuel cell. It is less susceptible to performance degradation in a wide
variation of air humidity conditions. The manufacturing cost of slanted channel is practically the same as that of rectangular channel plate in that both can be carried out by a conventional milling shop, without the need of special machinery.
Conclusions Water transport and current density measurements were carried out in five orientations of rectangular and slanted flow channel plates of 150 cm2. It was found that the amount of water flowing out at the cathode side of an anode downslanted channel cell was less than that of a rectangular channel cell because anode downward slanting induced water at the cathode side to highly back-diffuse to the anode side due to a hydration gradient, leading to improved membrane hydration and conductivity, thus better cell performance than
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
9
20
Water at cathode
Water at anode 22.0
21.3 18.2
18.6
15
12.4
11.1
10 5
4.4
3.7
ADS 80°C
AUS 80°C
0
25
Water at cathode
20 15
ADS 95°C
10 5
18.7
15.2 9.4
2.2
1.3
ADS 80°C
AUS 80°C
8.1
0 ADS 95°C
610 584
578 546
AUS 95°C
525
523
514 476
482 450 ADS AUS 80°C 80°C
(a)
Water at anode
17.7 14.2
AUS 95°C
Current density (mA/cm²)
25
Current density (mA/cm²)
Amount of water (mL/min)
Amount of water (mL/min)
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 0
210
ADS AUS 95°C 95°C
204
200 190
184
182
180
174
170 160
(b)
Membrane
ADS AUS 80°C 80°C
ADS AUS 95°C 95°C
Membrane
Anode
Anode
Cathode Water direction
Cathode Water direction
Water Back Diffusion
Anode Flooding
(c) ADS
(d) AUS
Fig. 6 e Current densities and amount of water flowing out at cathode and anode sides with anode down-slanted channel cell and anode up-slanted channel cell at a cell temperature of 80 C and different gas humidification temperatures (80 and 95 C) with the hydrogen and air flow rate stoichiometry of 1.7 and 2.2, at the cell voltage of (a) 0.5 V, (b) 0.7 V, with schematic diagram of water flowing inside fuel cells; (c) ADS and (d) AUS.
that of a rectangular channel cell. At a higher humidifier temperature of 95 C with the fuel cell at 80 C, cell performance decreased because of condensation and blocking of the pores of the gas diffusion layer. At this wet condition, the cell performance can be improved by replacing the rectangular channel plate with an anode down slanted plate. The current density can be improved, leading the cell performance at this wet condition to approach that of a rectangular plate under normal gas humidification conditions. The water measurement shows that the water in the cathode side of the anode down-slanted cell at 95 C is decreased, compared to the cell without modification. It can help improve cell performance at extremely wet conditions to be on par with that of a rectangular cell under normal conditions. However, in a cathode down-slanted channel cell water drains away from the cell to the cathode side leading to membrane dehydration. In addition, anode and cathode up slanted channel cells induce flooding, leading to poor performance.
Acknowledgments Financial supports from Kasetsart University Research and Development Institute (KURDI), National Research Council of Thailand (NRCT), and the Center of Excellence on Petrochemicals and Materials Technology (PETROMAT) are gratefully acknowledged.
references
[1] Mehta V, Cooper JS. Review and analysis of PEM fuel cell design and manufacturing. J Power Sources 2003;114:32e53. [2] Ma HP, Zhang HM, Hu J, Cai YH, Yi BL. Diagnostic tool to detect liquid water removal in the cathode channels of proton
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037
10
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 0
exchange membrane fuel cells. J Power Sources 2006;162:469e73. [3] Hu M, Sui S, Zhu X, Yu Q, Cao G, Hong X, et al. A 10 kW class PEM fuel cell stack based on the catalyst-coated membrane (CCM) method. Int J Hydrogen Energy 2006;31:1010e8. [4] Hental PL, Lakeman JB, Mepsted GO, Adcock PL, Moore JM. New materials for polymer electrolyte membrane fuel cell current collectors. J Power Sources 1999;80:235e41.
[5] Guvelioglu GH, Stenger HG. Flow rate and humidification effects on a PEM fuel cell performance and operation. J Power Sources 2007;163:882e91. [6] Baschuck JJ, Li X. Modelling of polymer electrolyte membrane fuel cells with variable degrees of water flooding. J Power Sources 2006;86:181e96. [7] Bunmark N, Limtrakul S, Fowler MW, Vatanatham T, Gostick J. Assisted water management in a PEMFC with a modified flow field and its effect on performance. Int J Hydrogen Energy 2010;35:6887e96.
Please cite this article in press as: Wawdee P, et al., Water transport in a PEM fuel cell with slanted channel flow field plates, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.01.037