ECS Transactions, 50 (2) 503-512 (2012)
10.1149/05002.0503ecst ©The Electrochemical Society
Effect of Channel Materials on the Behavior of Water Droplet Emerging From
GDL into PEMFC Gas Channels
Preethi Gopalan a, Satish G. Kandlikar b, a
a Microsystems Engineering, Rochester Institute of Technology, Rochester, New York
14623, USA
b Mechanical Engineering, Rochester Institute of Technology, Rochester, New York
14623, USA
Water accumulation in the gas channels of proton exchange
membrane fuel cells (PEMFCs) significantly affects the gas
diffusion and the overall performance. Hence, understanding the
growth and detachment of liquid water droplets from the gas
channel becomes critical. In this study, the behavior of liquid water
droplets within the gas channel in the presence of air flow (0.2 –
2.4 m/s) is experimentally investigated for different GDL types
(MRC-105, SGL-25BC, TGP-H-060) and channel wall materials
(stainless steel, copper, graphite composite, polycarbonate) that are
commercially used in PEMFC for automotive applications.
Experimental results were analyzed through Concus-Finn
condition using instantaneous dynamic contact angle (IDCA) to
determine the filling of the corners with liquid water. It was found
that the droplet detachment is influenced by air flow within the
channel and the contact angle hysteresis. Air flow produces
oscillations in the droplet and changes the IDCA the droplet makes
with the sidewalls and hence determines the corner filling.
Introduction
Proton exchange membrane fuel cells (PEMFCs) have shown tremendous potential in
becoming reliable energy source for a variety of power generation applications including
automotive purposes (1). Despite many advantages, one of the major issues still
preventing the commercialization of the PEMFC in automotive applications is the
inefficient water management at startup and shutdown of the system in the presence of
liquid water (2). Water management is of critical importance in PEMFCs to enable proper
diffusion of gases into the catalyst layer as both the liquid droplets and air share the same
pathway to initiate and sustain chemical reactions within the cell. Accumulation of the
liquid water on the cathode side of the PEMFC leads to flooding of the channels and
thereby hinders the reactant flow towards the reaction sites, consequently lowering the
efficiency of the system. On the other hand, lack of water in the cell leads to membrane
dehydration and reduction of proton exchange through the membrane, thus decreasing the
efficiency of the system (3, 4). Effective and balanced water management in PEMFCs is
of utmost importance to realize considerable performance and efficiency improvements
of PEMFCs prior to commercialization. Having a complete understanding of liquid
droplet behavior within the GDL surface and its interaction with the channel sidewall is
beneficial to improve water management and forms the major motivation for the current
work.
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ECS Transactions, 50 (2) 503-512 (2012)
Several researchers in the past have studied the effects of different forces acting
within the gas channels and analyzed the droplet interaction with channel walls to
evaluate the liquid water removal by conducting both experimental studies (in situ and ex
situ) as well as numerical simulations. Some of these preliminary studies showed that
surface forces are the primary cause for water accumulation and channel flooding in the
PEMFCs compared with viscous, pressure or inertial forces (5). Surface forces are mainly
dependent on the advancing and receding angles, and the hysteresis on the material
surface. In general, different GDL materials used in PEMFCs possess different surface
characteristics and thus, differ in their droplet accumulation characteristics within the gas
channels (6, 7).
A few detailed studies have further attempted to evaluate different channel
properties mainly focusing on liquid water stagnation. It was found that hydrophilicity of
channel surfaces facilitate the removal of water by wicking it into the channel corners (8).
These results contradicted the conclusions derived from more recent studies which
proposed that spreading of the liquid water near the hydrophilic walls lead to film
formation and blockage of the reactant channel pathway (9). To address the effect of
channel geometry on water accumulation, Zhu et al. conducted simulations on different
channel geometry and found that upside down trapezoid channel had the highest water
coverage ratio compared to any other channel configuration (10). It was also noticed that
the triangular channels performed more efficiently compared to the rectangular channels.
In order to tackle the water stagnation problem at a fundamental level, Rath and
Kandlikar analyzed the droplet interactions with the sidewall of varying angles (11) and
Concus-Finn condition was used to predict the corner filling behavior of the droplet in the
gas channel (12, 13). Gopalan and Kandlikar extended this work by incorporating the air
flow into the system to simulate realistic PEMFC working conditions (14, 15). It was
established that corner filling of the channel is more dynamic in nature and it depends
upon the instantaneous dynamic contact angle the droplet makes with the sidewall and
the GDL at a given airflow rate. This work was limited to only one particular channel
wall and GDL material, so investigating the droplet growth and removal on different
pairs of materials provides useful insights. In order to address this fundamental issue, this
work focuses on analyzing the effects of different channel wall materials (stainless steel
(SS 2205), copper (Cu 110), graphite composite, polycarbonate (Lexan)) and GDL types
(MRC-105, SGL-25BC, TGP-H-060) on the liquid water droplet dynamics in the
presence of airflow within the channel and is discussed in this paper with necessary detail.
Ex-Situ Experimental
Experimental setup - A schematic of the ex-situ setup used in the present study is
shown in Figure 1. The test setup consists of a base plate made up of polycarbonate
which holds the GDL on top of it. An inlet for the water droplet was made on the base
plate near the channel end to allow water to emerge onto the GDL and visualize the
droplet interactions with the sidewall. The channel sidewall plates that are made up of
different materials with different angles (45° and 50°) machined to it were placed on top
of the GDL layer. The top wall (above the sidewall plates) made of polycarbonate was
attached to this setup forming a complete channel of length 100 mm. In our experiments,
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ECS Transactions, 50 (2) 503-512 (2012)
stainless steel (SS 2205), copper (Cu 110) and graphite composite (materials commonly
used for the bipolar plates in the PEMFCs) were used for channel sidewalls and SGL25BC, MRC-105 with 6% PTFE and TGP-H-060 with 6% PTFE were used for GDL
materials. Furthermore, the superficial air velocities were varied in the range, 0.2 – 2.4
m/s which corresponded to a current density range of 0.1 – 1 A/cm2 in typical fuel cells
with an active area of 50 cm2 (16). The water flow rate was kept constant at 0.05 ml/min
(corresponding to the water generation rate of 3 A/cm2 for an active area of 2.9 cm2).
More detailed description of the experimental setup can be found in the paper by Gopalan
and Kandlikar (15).
Pressure Sensor
Lexan
c
channel
C
Channel
angle
a
Air Flow
Supply
GDL
Water injection at the
th
center of channel
Water Inlet
Lexan
base
Figure 1. Experimental Test Setup
For each of the materials considered in this study, the advancing and receding
contact angles were measured using the VCA Optima Surface Analysis System to
quantify the contact angle hysteresis (calculated by Δθhys= θadv – θrec) and the
corresponding data is provided in Table I. Concus-Finn condition was used for the
theoretical calculation of the transition angle from non-filling to filling for a given
material pair as shown in Table II. The Concus-Finn condition is given by eqn.1.
[1]
2α = (θB + θW) – π
where θB is the advancing contact angle for the base material (GDL), θW is the advancing
contact angle for the sidewall and 2α is the channel open angle (angle between the
sidewall and GDL). It was found from this data that all material combinations had
transition angles near 50° (48°- 53°). Since the current study involves several variables,
the experimental test matrix was simplified to an extent by only including 45° and 50°
channel open angles as these angles are closer to the transition angle.
TABLE I. Static advancing and receding contact angles of sidewall materials
Material
Advancing Contact
Receding Contact
Angle (θadv)
Angle (θrec)
MRC-105 with 6% PTFE
148
138
TGP-H-060 with 6% PTFE
145
127
SGL-25BC
148
136
Polycarbonate
85
61
Copper (Cu 110)
80
53
Graphite Composite
84
34
Stainless Steel (SS 2205)
81
60
Hysteresis
(Δθhys)
10
17
12
24
27
50
19
Experimental procedure - For each pair of materials used for the experiments, the air flow
was set to the desired value and the droplet was allowed to grow on the GDL surface.
High speed videos were recorded, using Keyence VW-6000 at 60fps, as the droplet
appeared on the GDL until it was fully removed from the channel. Initially, the droplet
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ECS Transactions, 50 (2) 503-512 (2012)
would grow to its full size before contacting the sidewall. Once the droplet touched the
sidewall it would either fill the channel corner or remain pinned on the sidewall
depending on the channel open angle until the pressure in the channel interior built up
leading to removal of the droplet. The high speed video of droplet growth and its
interaction with the channel wall was post processed using Keyence motion analyzer and
the instantaneous dynamic contact angle (IDCA) the droplet made with the channel wall
and the GDL was measured on each frame. The accuracy of the contact angle
measurement was approximately in the range of ±2°.
TABLE II. Transition Angles from Non-Filling to Filling for a Given Material Pair
Material Pair
Transition Angles
MRC-105 GDL- Polycarbonate
53°
MRC-105 GDL – Copper (Cu 110)
48°
MRC-105 GDL – Graphite
52°
MRC-105 GDL – Stainless Steel (SS 2205)
49°
TGP-H-060 - Polycarbonate
50°
SGL-25BC- Polycarbonate
51°
Results
Concus-Finn condition- This condition was used to predict the liquid droplet
behavior at the intersection of two surfaces. The different contact angles the droplet
makes with the different surfaces are shown in Figure 2(a) and these angles are plotted on
a 2D graph as shown in Figure 2(b). It can be inferred from this graph that if the contact
angle points lie in the shaded region satisfying the Concus-Finn limit condition, then the
corner-filling of the droplet will take place. Otherwise, the droplet will be pinned on the
channel surface without filling the channel corners (13). In PEMFC, once the droplet fills
the channel corner, it acts as a viable pinning site for other successive droplets emerging
onto the channel and will eventually lead to droplet accumulation and channel flooding
on the GDL surface. Therefore, it is essential to avoid corner filling in the channel to
prevent such degenerative effects and for the longevity of PEMFC components.
Gas Channel
Sidewall
Droplet
Filling
Region
θW
Droplet
et
θW
2α
GDL Base
Non
Filling
Region
GDL Base
Pore
θB
Non
Filling
Region
θB
Pore
(a)
(b)
Figure 2. Concus-Finn Condition a) Different contact angles made by the droplet with
base and sidewall b) Plot representing the Concus Finn condition when the droplet
contacts the base and the sidewall.
In our prior work, it was demonstrated that the material roughness also has drastic
effects on the contact angle measurements. To avoid such discrepancies in our
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ECS Transactions, 50 (2) 503-512 (2012)
experiments, the channel walls were smoothed down using emery cloth. The surface
roughness of the sidewall was measured using Confocal Laser Scanning Microscope
(CLSM) and the corresponding images of different channel walls and GDL samples are
shown in Figure 3. The surface roughness factor for different sidewall materials was
confirmed to be less than 900 µm from the image slices and is summarized in Table III.
200 μm
200 μm
(a)
(b)
200 μm
(d)
200 μm
(c)
200 μm
200 μm
(e)
(f)
Figure 3. Confocal Laser Microscope Images of different GDL and channel wall material.
(a) MRC-105 (b) SGL-25BC (c) TGP-H-060 (d) Cu 110 (e) Graphite Composite (f) SS
2205MRC-105
TABLE III. Transition Angles from Non-Filling to Filling for a Given Material Pair
Material
Copper (Cu 110)
Graphite Composite
Stainless Steel (SS 2205)
Polycarbonate (LexanTM)
Surface Roughness (Ra)
554 µm
873 µm
558 µm
666 µm
Effect of Channel Wall Material:
The base GDL material, MRC-105, was used for all experiments whereas the
sidewall material was changed along with the channel open angle.
Stainless Steel (SS 2205) sidewall: For the channel sidewall of SS 2205, the
transition angle was found to be 49° according to the theoretical prediction using ConcusFinn condition. The corner filling did not occur for the 45° channel in the experiments
agreeing with the theoretical predictions. A recorded image sequence of the droplet
growth and its interaction with the SS 2205 sidewall for an open angle of 45° is shown in
Figure 4. The IDCA for the SS 2205 sidewall and the MRC-105 base measured from
each frame of the video sequence was plotted against the Concus-Finn Limit (CFL) as
shown in Figure 5(a). The observed behavior can also be compared to the plot of the
IDCA points that lie above the Concus-Finn Limit (CFL) line. Hence, the findings from
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ECS Transactions, 50 (2) 503-512 (2012)
our experiments regarding droplet filling agreed with the corresponding theoretical
predictions.
1 mm
1 mm
1 mm
Droplet emergence
Touches sidewall
1 mm
1 mm
Touches top wall
Touches third wall
1 mm
Droplet removal
from channel
Corner not filled
Figure 4. Sequence image of the droplet interaction with the Stainless Steel (SS 2205)
sidewall and MRC-105 base in 45° open angle channel with 1.6 m/s air velocity
introduced into the channel. The red circle in the image shows that the droplet does not
fill the corner of the channel.
180
180
45 deg_Limit
0.2 m/s_Non Filling
0.2 m/s_Filling
1.6 m/s_Non Filling
1.6 m/s_Filling
160
140
50 deg_Limit
0.2 m/s_Filling
0.2 m/s_Non Filling
1.6 m/s_Filling
1.6 m/s_Non Filling
160
140
2α=45˚˚
100
2α=50˚
ΘW
120
100
ΘW
120
80
80
ΔΘW, Stainless Steel
60
ΔΘW,Stainless Steel
60
ΔΘB, Baseline GDL
40
ΔΘB, Baseline GDL
40
20
20
0
0
0
20
40
60
80
100
120
140
160
180
0
20
40
60
80
100
120
140
160
180
ΘB
ΘB
(a)
(b)
Figure 5. Plot of dynamic contact angle made by the droplet with the stainless steel
sidewall and the GDL base for (a) 45° open angle channel and (b) 50° open angle channel
at different air velocities.
However for a 50° channel, contradicting results were obtained between the
experimental observation and the theoretical prediction. In the former case, corner filling
was observed only for the lower air velocities (0.2 - 0.6 m/s) compared to the higher air
velocities (0.7 – 2.4 m/s). Image sequences of the droplet interacting with the 50° channel
walls for an air velocity of 1.6 m/s are shown in Figure 6. It was also observed that the
IDCA for lower air velocities fell below the CFL line satisfying the corner filling criteria
(Figure 5(b)). In contrast, the IDCA for higher air velocities fell above the CFL line
leading to non-corner filling behavior. Therefore, from the experiments, the effect of air
velocities were pronounced and caused an “imbalance” in the droplet at higher velocities
causing noticeable oscillations. This “imbalance” actually marked the transition between
channel filling and non-filling behavior in such cases (disagreement with traditional CF
condition). It emphasized the use of dynamic contact angle information in order to predict
the droplet behavior and thus, the overall water coverage ratio on the GDL more
accurately.
Copper (Cu 110) sidewall – A similar set of experiment was conducted with the
Cu 110 sidewall. According to the theoretical prediction, the transition angle for Cu 110
should be 48°. The experimental results were similar for Cu 110 for both the 45° and 50°
channels as observed for SS 2205 sidewalls.
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ECS Transactions, 50 (2) 503-512 (2012)
1 mm
1 mm
Droplet emergence
Touches side wall
1 mm
1 mm
1 mm
Touches top wall
Touches third wall
1 mm
Droplet removal
from channel
Corner not filled
Figure 6. Sequence image of the droplet interaction with the Stainless Steel (SS 2205)
sidewall and MRC-105 base in 50° open angle channel with 1.6 m/s air velocity
introduced into the channel. The red circle in the image shows that the droplet filled the
corner of the channel.
Graphite composite sidewall - The transition angle for the graphite sidewall
according to the theoretical prediction was 52°. From the experimental results, it was
learnt that for 45° and 50° open angle channel, the droplet did not fill the channel corners
for any air velocities (0.2 – 2.4 m/s) and the results agreed with the IDCA plot (Figure 7)
which showed non-filling for all air flow velocities as well.
180
140
50 deg_Limit
0.2 m/s_Non Filling
0.2 m/s_Filling
1.6 m/s_Non Filling
1.6 m/s_Filling
160
140
2α=45˚
120
2α=50˚˚
120
100
100
ΘW
ΘW
180
45 deg_Limit
0.2 m/s_Non Filling
0.2 m/s_Filling
1.6 m/s_Non Filling
1.6 m/s_Filling
160
80
60
ΔΘW, Graphite
80
ΔΘW, Graphite
60
ΔΘB, Baseline GDL
40
ΔΘB, Baseline GDL
40
20
20
0
0
0
20
40
60
80
100
120
140
160
180
0
20
40
60
80
100
120
140
160
180
ΘB
ΘB
(a)
(b)
Figure 7. Plot of dynamic contact angle made by the droplet with the graphite sidewall
and the GDL base for (a) 45° open angle channel and (b) 50° open angle channel at
different air velocities.
The three materials proposed for use in automobile applications (stainless steel,
copper, graphite) have been found to have varying effects on the droplet behavior
depending upon the contact angle hysteresis value. The graphite composite, which has
larger contact angle hysteresis, behaved differently compared to the materials having
lower contact angle hysteresis near the transition angle (50°). It can be concluded that
contact angle hysteresis affects the droplet behavior in the gas channel. Materials having
large hysteresis value can take larger oscillations of a droplet due to pinning and lead to
non-filling of the channel corner whereas materials having smaller hysteresis can only
accommodate smaller oscillations produced by the air flow without leading to corner
filling of the channel.
When using MRC-105 GDL with any of the selected channel materials, the
droplet filling of the channel corners did not take place for angles below 45° and the 50°
channel behaved as a transition angle between filling and non-filling. The Concus-Finn
condition could not accurately predict the corner filling behavior of the droplet near the
transition angle. Hence, using IDCA information would be helpful in predicting the
droplet behavior in the gas channel of PEMFC instead of static contact angle information.
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ECS Transactions, 50 (2) 503-512 (2012)
Effect of GDL Material:
There are different kinds of GDL materials that are used for the PEMFC
applications and the most commonly used are Toray, SGL and the MRC-105. To evaluate
the effect of a GDL material on droplet growth and removal, similar experiments were
performed under similar air flow rates. In these experiments the channel wall material
was kept constant (polycarbonate) while the GDL materials were changed along with the
channel open angle.
TGP-H-060 and polycarbonate channel wall – For TGP-H-060 with 6% PTFE,
the transition angle was found to be 50° according to the theoretical prediction using
Concus-Finn condition. The experimental results showed no corner filling for the 45° and
50° channels which agreed with the theoretical predictions for all air velocities. The
IDCA plot for the 45° and the 50° open angle channel is shown in Figure 8.
180
180
Limit
0.2 m/s_Non Filling
0.2 m/s_Filling
1.6 m/s_Non Filling
1.6 m/s_ Filling
160
140
140
2α=45˚
120
2α=50˚
120
100
100
ΘW (˚)
ΘW (˚)
Limit
0.2 m/s_Non Filling
0.2 m/s_Filling
1.6 m/s_Non Filling
1.6 m/s_ Filling
160
80
ΔΘW, Polycarbonate
60
80
ΔΘW, Polycarbonate
60
ΔΘB, TGP-H-060
40
ΔΘB, TGP-H-060
40
20
20
0
0
0
20
40
60
80
100
ΘB (˚)
120
140
160
180
0
20
40
60
80
100
ΘB (˚)
120
140
160
180
(a)
(b)
Figure 8. Plot of dynamic contact angle made by the droplet with the polycarbonate
sidewall and the TGP-H-060 base for (a) 45° open angle channel and (b) 50° open angle
channel at different air velocities.
SGL-25BC and polycarbonate channel wall - The transition angle for SGL-25BC
with polycarbonate sidewall was found to be 51°. Similar experimental results were
observed for SGL-25BC in which the droplets did not fill the channel corners for the 45°
and 50° channels for all air velocities.
However, there was a significant difference in the way the droplet interacted with
the channel walls. High speed videos of the droplet interaction with the channel walls for
a SGL-25BC base revealed that at any given air velocity within the channel, the droplet
would grow on the GDL until it would jump and make its first contact with the top wall.
In other material studies the first contact of the droplet was with the sidewalls. Once the
contact was established, the droplet would begin to move to the sidewall and eventually
block the entire channel cross section. The sequence of images of the droplet growth in
the 50° open angle channel at 1.6 m/s air velocity is shown in Figure 9.
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ECS Transactions, 50 (2) 503-512 (2012)
1 mm
Droplet Movement Along
Side wall
1 mm
1 mm
Droplet Growth
Droplet Touching Top Wall
1 mm
1 mm
Droplet Touching Third wall
Droplet Removal from Channel
1 mm
Droplet Movement Along
Top wall
1 mm
Complete Droplet Removal
from Channel
Figure 9. Sequence image of the droplet interaction with the polycarbonate sidewall and
SGL-25BC base in the 50° open angle channel with a 1.6 m/s air velocity.
This behavior of droplet jumping and clinging to the top wall is mainly attributed
to the fiber entanglement of the GDL surfaces. In general, SGL has more openly packed
fiber structure compared to the TGP-H-060 or MRC-105 as shown in Figure 3. The open
fiber structure allows the water to be pushed through the GDL easily and leads to the
vertical motion of the droplet. Secondly, SGL-25BC and TGP-H-060 have their transition
angle near 50°, but both did not fill the channel corners for lower air velocities at the 50°
channel. The MRC-105 showed corner filling at lower air velocities and non-filling at
higher air velocities. Thus, the oscillations produced by the air flow and the contact angle
hysteresis due to material surface characteristics and fiber entanglement pattern have a
significant role in capturing the droplet dynamics within a PEMFC channel.
Conclusion
In this work, ex-situ experiments were conducted to study the water droplet
dynamics in a gas channel with different channel sidewall angles, channel wall materials,
and GDL materials under controlled air flow velocities between 0.2 – 2.4 m/s. It was
revealed through the recorded high-speed videos that the droplet dynamics at the channel
corners are significantly affected by the channel open angles, air flow velocities and the
channel materials.
x
Sidewall Material: Characteristic behavior of different channel materials differs
considerably. The contact angle hysteresis played a major role in determining the
corner filling criteria for each type of the channel. Materials such as graphite,
which have larger contact angle hysteresis, showed no corner filling behavior at
any air velocities for the transition angle. These materials can take larger
oscillations before the droplet starts to move on the surface to fill the channel
corner. However, for materials having smaller hysteresis (copper, stainless steel
and polycarbonate) corner filling occurred at lower air velocities.
x
Air Flow Rate: Air flow in the channel produced oscillations in the water droplet
which determined the instantaneous contact angle the droplet made with the
sidewall and the base GDL. In this case, the instantaneous dynamic contact angles
of the droplet determined the corner filling behavior.
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ECS Transactions, 50 (2) 503-512 (2012)
x
GDL Materials: For channel open angles near transition point, SGL-25BC and
TGP-H-060 showed no corner filling activities. However, MRC-105 showed
corner filling. Contact angle hysteresis, the oscillations in the water droplet and
the GDL fiber entanglement determined the corner filling criteria for a given GDL
and wall material.
x
Concus-Finn Condition: Concus-Finn condition using static contact angle
predicted the corner filling behavior inaccurately. Instead, using instantaneous
dynamic contact angle information in Concus-Finn condition detected corner
filling more accurately, especially near transition angle.
Acknowledgments
Support for this project was provided by the US Department of Energy under award
number: DE-EE0000470.
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