Proceedings of the Fifth International Conference on Nanochannels, Microchannels and Minichannels
ICNMM2007
Proceedings of ASME ICNMM2007
June 18-20, 2007, Puebla, Mexico
th
5 International Conference on Nanochannels, Microchannels and Minichannels
June 18-20, 2007, Puebla, Mexico
ICNMM2007-30142
ICNMM2007-30142
EFFECTS OF FLOW FIELD AND DIFFUSION LAYER PROPERTIES ON WATER
ACCUMULATION IN A PEM FUEL CELL
1
1
2
2
J. P. Owejan , T.A. Trabold , D.L. Jacobson , M. Arif and S.G. Kandlikar
3
1
General Motors Fuel Cell Activities, 10 Carriage Street, Honeoye Falls, NY 14472, [email protected]
National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, [email protected]
3
Rochester Institute of Technology, Department of Mechanical Eng., Rochester, NY 14623, [email protected]
2
ABSTRACT
Water is the main product of the electrochemical reaction
in a proton exchange membrane (PEM) fuel cell. Where the
water is produced over the active area of the cell, and how it
accumulates within the flow fields and gas diffusion layers,
strongly affects the performance of the device and influences
operational considerations such as freeze and durability. In this
work, the neutron radiography method was used to obtain twodimensional distributions of liquid water in operating 50 cm2
fuel cells. Variations were made of flow field channel and
diffusion media properties, to assess the effects on the overall
volume and spatial distribution of accumulated water. Flow
field channels with hydrophobic coating retain more water, but
the distribution of a greater number of smaller slugs in the
channel area improves fuel cell performance at high current
density. Channels with triangular geometry retain less water
than rectangular channels of the same cross-sectional area, and
the water is mostly trapped in the two corners adjacent to the
diffusion media. Also, it was found that cells constructed using
diffusion media with lower in-plane gas permeability tended to
retain less water. In some cases, large differences in fuel cell
performance were observed with very small changes in
accumulated water volume, suggesting that flooding within the
electrode layer or at the electrode-diffusion media interface is
the primary cause of the significant mass transport voltage loss.
INTRODUCTION
Hydrogen fuel cells are being developed as highly efficient
and cost effective energy conversion devices that potentially
have less environmental impact than internal combustion
engines. The polymer electrolyte membrane fuel cell (PEMFC)
is the subject of the majority of fuel cell research, as it can be
operated at low temperatures, and thus can be constructed of
relatively low cost materials. This will enable the PEMFC to
compete in automobile and stationary power generation
markets which generally have very stringent cost targets.
As PEMFC technology is further refined, it is recognized
that several major hurdles must be overcome before current
research-scale units are robust enough for commercialization.
The focus in the present work is management of the water that
is produced in the cathodic oxygen reduction reaction. Because
a PEMFC operates at temperatures below 100ºC, liquid water
can form throughout the system due to condensation in the
porous gas diffusion layers (GDLs) and gas delivery channels.
Under steady-state conditions, liquid water accumulation can
be minimized by controlling parameters such as inlet relative
humidity, temperature, and pressure.
However, these
parameters must be optimized to ensure that a sufficient amount
of water is present to maintain membrane and ionomer
hydration required for adequate proton conductivity [1]. For
automotive applications in particular, the fuel cell stack will
rarely be at a steady-state condition, and the power delivery
throughout the drive cycle will be quite dynamic.
These
constant changes in fuel cell power output can cause brief
temperature variations, thus influencing the amount of liquid
water in the system. For this reason, it is believed that mass
transport losses due to liquid water accumulation in the various
fuel cell components is an inevitable problem, regardless of the
operating conditions selected for steady-state load operation.
The liquid water handling characteristics of the membrane,
electrodes, gas diffusion layers, and flow field reactant delivery
channels must be well understood, so that each of these
components can be optimized individually and as an integrated
fuel cell system.
In this study, liquid water accumulation in the carbon fiber
gas diffusion layers and reactant distribution channels was
investigated with neutron radiography imaging of 50 cm2 active
1
Copyright © 2007 by ASME
area fuel cells. Neutron radiography is ideal for these
experiments, because it is a noninvasive diagnostic that allows
the test cell to be examined without changing the thermal,
electrical or mechanical characteristics of the typical design.
Because neutrons interact with the nucleus of an atom, and not
the electrons as X-rays do, many common materials such as
aluminum are relatively transparent to neutrons while
hydrogenous materials like water are highly attenuating.
Therefore, neutron radiography is well suited for imaging water
within the metallic or carbon-based structure of a PEMFC [2][9]. The neutron images obtained in this study have been used
to qualitatively examine the regions of liquid water
accumulation, and to quantify the volume of water present
based on a system calibration relating attenuated neutron beam
intensity to water thickness. The focus of these experiments
was quantifying the impact of GDL and flow field channel
properties on water accumulation, and the attendant effect on
cell performance. These two components are known to have a
strong effect on mass transport losses in PEMFCs (e.g., [10],
[11]), but little experimental evidence exists which
demonstrates the localized impact of GDL and flow field
channel properties. Attempts have been made to acquire such
data using more standard imaging methods, but it is necessary
to alter the thermal properties of the flow field for optical
access of such visualization systems [12]-[14].
large hydraulic diameter and a minimal number of turns, this
flow field pattern yielded a low pressure drop between the inlet
and the outlet. Two different cross-sectional geometries were
tested using the serpentine channel pattern: rectangular with
1.37 mm width and 0.38 mm depth; and isosceles triangular
with 1.37 mm width and 0.76 mm depth (Figure 2). The crosssectional area was kept constant between these channel designs
to maintain a consistent mean velocity in both channel
geometries. The hydraulic diameters of the rectangular and
triangular cross-sections were 0.68 mm and 0.71 mm,
respectively, so a small difference in frictional pressure drop
existed between the two flow field designs.
EXPERIMENTAL
The hardware for the fuel cell test section was specifically
designed to optimize the quality of neutron images, and to
facilitate post-process data analysis. A commercial test stand,
as described elsewhere ([15]-[17]) was used to control
operation of the fuel cell while acquiring neutron image data
with an independent data acquisition system. The test cells
were constructed in a consistent manner with particular
consideration of compression, material integrity, and alignment.
The test conditions were chosen such that liquid water was
known to be present, although they were representative of
conditions that can exist during an automotive drive cycle.
Fuel Cell Hardware and Flow Field Design
The test hardware design was critical for obtaining the
highest resolution neutron images in the active area of the fuel
cell. The compression end plates were slightly modified from
the most common configuration used for single cell 50 cm2
testing to be more compatible with neutron imaging. The
heater rods were moved to the outer edges of the cell, beyond
the electrochemically active area, and the temperature control
thermocouple was also moved outside the active area. The bolt
pattern and dimensions were kept the same as the standard
hardware to avoid any variability in the compression
distribution. The cross-sectional thickness of hardware and
material also remained constant. The gas inlet and outlet port
locations were repositioned from the standard hardware to
accommodate space constraints within the radiation enclosure
and the test stand position relative to the neutron source.
Both the anode and cathode flow field plates used in this
study had serpentine designs as illustrated in Figure 1. The five
channel, five pass pattern had long enough flow paths to
elucidate water management phenomena similar to those
present in full-scale fuel cell bipolar plate hardware. With a
Figure 1 – Serpentine Flow Field Pattern
Figure 2 – Cross-sectional flow channel geometries
Previous work by our research group using neutron
radiography to study PEM fuel cells ([15], [16]) established a
need to unambiguously distinguish the anode flow field from
the cathode flow field. It was thus determined that arranging the
flow fields orthogonally would make it possible to discriminate
water in the anode from water in the cathode when viewing
2
Copyright © 2007 by ASME
two-dimensional neutron radiographs. This approach was first
demonstrated in the thesis by Owejan [4]. Figure 3 is a
schematic of the flow channel orientation, with the observer
looking through the anode toward the cathode. Reference
channels outside of the active area are also incorporated into
the design. These reference channels were used to verify the
calibration method applied to quantify liquid water volume
within the active area of the running cell. Because the
reference channels were not covered by GDL, the thickness of a
retained water slug was precisely known and could be used in
water quantification for verification of measurement precision.
The pattern in Figure 3 is in the same orientation as all neutron
images that were taken of the running fuel cell. Hence, a water
slug in an anode channel (black) will be easily distinguished
from a water slug in a cathode channel (red).
manufactured by SGL Carbon (Wiesbaden, Germany), each
with a PTFE treatment and microporous layer (MPL) applied to
the substrate. The in-plane gas permeability values were
obtained by forcing a controlled air flow through a hole in the
center of a disk-shaped sample of GDL that was sealed between
two plates, while measuring the upstream and exit pressures.
Porosity was calculated based on fiber size, binder volume
fraction, and manufacturing process, as disclosed by the
manufacturers. It is shown in Table 1 that the Toray and SGL
materials have large differences in through-plane thermal
resistance, but that the two SGL materials differ significantly
only in their values of in-plane gas permeability. Although
there are a variety of other material properties that may affect
fuel cell performance, it was presumed at the outset that these
two properties would have the most appreciable influence on
the accumulated water volume, as they control the amount of
convective flow through the GDL, and the effective
temperature gradient between the flow field plates and MEA.
For each fuel cell build, the same GDL material was used
on both the anode and cathode sides. The membrane-electrode
assemblies (MEAs) used in all tests were supplied by W.L.
Gore & Associates (Newark, Delaware, USA), and fabricated
from 25 Pm thick Nafion® membranes with 0.4/0.4 mg/cm2
loading of carbon supported platinum in ionomer, hot-pressed
on both the anode and cathode sides.
Table 1 – Physical Properties of Gas Diffusion Layers
Material
Thickness
MPL
P
Porosity
(%)
Substrate
PTFE
(mass %)
190
No
78
7
260
Yes
79
5
260
Yes
76
5
Figure 3 – Assembled channel orientation
(black = anode; red = cathode).
An important parameter in the present study was the
surface energy of the flow field plates. To achieve a decreased
surface energy (i.e., increased hydrophobicity) of the gold
plated aluminum surfaces, the plates were coated with an
ionically bonded polytetrafluoroethylene (PTFE), provided by
TUA Systems (Merritt Island, Florida, USA). The coating was
found to be very uniform with an average thickness of less than
2 microns, and was applied to two cathode flow fields
consisting of each of the two cross-sectional geometries
described above (rectangular and triangular).
Gas Diffusion Layer (GDL) Selection
Three commercially available GDLs were investigated in
this study, as summarized in Table 1. The first material tested
was T060 from Toray Industries (Tokyo, Japan)1, which was
treated with PTFE, but did not have a microporous layer. Two
additional materials studied were 20BC and 21BC
Material
T060
SGL 21BC
SGL 20BC
In-plane gas
permeability1,2
(Darcy)
5.32
3.20
1.11
Through-plane
thermal resistance3
(m2-K/W)
1.12E-04
5.13E-04
3.98E-04
1
1
Certain trade names and company products are mentioned in the text or
identified in illustrations in order to adequately specify the experimental
procedure and equipment used. In no case does such identification imply
recommendation or endorsement by the National Institute of Standards and
Technology, nor does it imply that the products are necessarily the best
available for the purpose.
In-plane gas permeability measurements made with GDL samples under 1.38
MPa compression.
SGL measurements of through-plane gas permeability for 21BC and 20BC are
2.35 and 0.65 cm3/(cm2·sec), respectively, using a Gurley model 4118, 300 cm3,
0.1 in2 orifice (www.sglcarbon.com; SIGRACET GDL 20 & 21 Series Gas
Diffusion Layer).
3
Thermal resistance measurements made with GDL samples under 1.64 MPa
compression.
2
3
Copyright © 2007 by ASME
Neutron Imaging System
Experiments were conducted at the neutron source
operated by the Center for Neutron Research at the National
Institute of Standards and Technology (NIST), in Gaithersburg,
Maryland, USA. Thermal neutron beam line BT-6 was utilized
with an aperture of 1 cm and a resulting L/d ratio of 400. A
complete description of the BT-6 neutron imaging facility is
provided by Hussey et al. [17].
The image exposure time was set to 1 second after
preliminary experiments were conducted to optimize the
neutron image contrast. Ideally, the exposure time must be
minimized to visualize transient behavior within the cell.
Conversely, short exposure time does not provide enough light
to expose each image with the desired contrast. Each pixel
value was saved to a .fits (formatted image transfer system) file
in 16-bit double precision format. The CCD chip was a 2048 x
2048 array of pixels, and with binning set at 2, the images were
saved as 1024 x 1024 pixel arrays. In the present study, a series
of 300 images was taken once the fuel cell operating point was
considered to be at steady state condition. These images were
later averaged to increase the signal-to-noise ratio. Images
were also analyzed individually to verify that the liquid water
profile was constant throughout he averaging period.
Liquid water content was quantified using a calibration
based on the exponential attenuation law, as described in [15].
The macroscopic neutron cross-section was determined
experimentally to be 2.958 r 0.010 cm-1.
RESULTS AND DISCUSSION
All experiments were conducted using a consistent set of
test parameters (Table 2), which were chosen specifically to
ensure that liquid water would be present during fuel cell
operation. Four cathode flow field configurations were
investigated: rectangular and triangular geometry (Figure 2),
each with and without the ionic PTFE coating which increased
the surface static contact angle. The anode flow field was
constant through all tests (rectangular channels, with no PTFE
coating), and water accumulation in the vertically traversing
anode channels should not be confused with the water
accumulation of interest in the horizontally traversing cathode
channels. It was consistently observed that the anode channels
contained large, stagnant water slugs which were present
throughout the entire measurement sequence. This level of
anode water is attributed primarily to condensation which
occurs as the hydrogen fuel is consumed in the fully humidified
gas stream, and also to liquid water that is periodically
introduced at the inlet due to condensation upstream of the fuel
cell. In either case, once water enters the anode channels, it
cannot be continuously purged by the low density hydrogen gas
flowing at relatively low velocity. However, it was observed
that fuel cell performance was minimally influenced by the
presence of water in the anode channels.
Effects of GDL Properties on Water Accumulation
In this section, results are presented to contrast the water
accumulation behavior of fuel cells run with three GDLs
havingvarying physical properties, as summarized in Table 1.
Throughout the following discussion, neutron radiographs are
presented as either gray scale images, or as RGB images in
which the top of the color scale (red) corresponds to a water
Table 2 – Fuel Cell Parameters for Neutron Radiography
Experiments
Parameter
Value
Active area
50 cm2
Membrane thickness
25 Pm
Catalyst loading
0.4 / 0.4 mg Pt/cm2
Anode fuel
Hydrogen
Cathode oxidant
Air
Cell temperature
80 C
Back pressure
200 kPa
Inlet humidification
100% / 100%
Stoichiometric ratio
2/2
Polarization curve
0, 0.1, 0.5, 1.0, 1.2, 1.5 A/cm2
Start-Up
1 hr. at 0.6 V
thickness of 0.30 mm, which is close to the rectangular channel
depth of 0.38 mm. Portions of the active area quantified by
colors in the orange to red range thus represent water
thicknesses that can only exist inside a flow field channel. A
deep green to yellow color is representative of liquid water
contained primarily in the GDLs and MEA.
At the outset, it is important to note that under the fully
humidified conditions investigated in this study, the Toray
diffusion media experienced significant mass transport loss and
could not be run beyond 1.0 A/cm2. In Figure 4, it is shown that
the cell voltage with Toray 060 was below 0.4 V at this current
density, while both SGL materials performed well out to 1.5
A/cm2 with relatively little mass transport voltage loss. This is
due to the use of a microporous layer that is known to improve
water transport from the MEA into the diffusion medium [18].
In Figure 5, a direct comparison is made among the water
distributions for Toray 060, SGL 20BC and SGL 21BC at 0.1
and 1.0 A/cm2. Several trends are apparent in this figure. First,
the large stagnant water slugs present in the anode channels of
all cells at 0.1 A/cm2 are largely eliminated with a 10-fold
increase in gas flows at 1.0 A/cm2. Secondly, it appears that the
quantity of water within the softgoods (i.e., GDL and MEA)
increases with increasing current density, as indicated by
visually comparing the relative gray scales of non-channel
portions of each image pair. It is also important to note that the
GDL/MEA immediately adjacent to the reactant inlets (upper
right hand corner of each image) are relatively dry in
comparison to the rest of the active area. The land areas
between the first pass of the anode and cathode channels show
appreciable gradients in water content, as if water is removed
from the GDL where the gas velocity is greatest (i.e., in the first
pass of the serpentine flow field). The overall dry trend near
the anode inlet, which is most pronounced at 1.0 A/cm2, is
attributed to lack of membrane hydration in this location.
Because the conditions in this area are less accommodating to
facilitate proton conduction, the corresponding cathode reaction
on the opposite side of the MEA is suppressed. Without an
efficient cathode reaction, the local current density is
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Copyright © 2007 by ASME
Toray
SGL 20BC
SGL 21BC
Performance Comparison
0.95
0.1 A/cm2
Voltage (V)
0.85
0.75
0.65
0.55
0.45
0.5 A/cm2
0.35
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Current Density (A/cm2)
Figure 4 – Performance comparison among GDL materials
0.1 A/cm2
1.0 A/cm2
1.0 A/cm2
Toray 060
1.2 A/cm2
SGL 21BC
1.5 A/cm2
Figure 6 – Water distributions and computed masses for
SGL 20BC
SGL 20BC
Figure 5 – Effect of GDL type on water distributions at 0.1
and 1.0 A/cm2
accordingly lower, and hence less liquid water is produced in
this region over an averaged period of operation. The observed
gradient in water content over the land regions of the flow field
is a function of gas permeability and in-plane pressure drop;
this observation will be discussed further later in this section.
In Figure 6, neutron radiographs are presented for SGL
20BC operated at all 5 current densities. For each condition,
the water distribution is first shown as a time-averaged gray
scale image, and then as a colorized RGB scale image with the
computed water mass, based on the macroscopic neutron crosssection determined via calibration as described above. This
computation considers only water present within the square 50
cm2 active area, and does not includes the reference channels in
the upper right-hand corner, nor the outlet ports in the lower
left-hand corner. From this series of images, one can extract a
clear qualitative indication that with increasing current density,
the amount of channel-level water decreases while the water
content within the softgoods increases. It is also notable, from
the data presented in both Figures 5 and 6, that the effect of the
microporous layer (MPL) on the SGL gas diffusion media is to
produce a more even water distribution than the Toray material
which does not have an MPL. By summing the individual pixel
water thickness values and multiplying by the active area, the
total average water mass was determined and plotted for all
three GDL materials (Figure 7). The cell constructed with
Toray material shows a trend of a slightly increasing then
decreasing water mass with a change in current density from
0.1 to 1.0 A/cm2. Conversely, the SGL materials display the
opposite trend: decreasing water mass from 0.1 to 0.5 A/cm2,
with a generally increasing water mass from 0.5 to 1.5 A/cm2.
This figure does not show a trend of monotonically increasing
water mass with current density because the values plotted also
include the water slugs in the channels for d0.5 A/cm2. To
5
Copyright © 2007 by ASME
Average Water Mass Comparison
Toray
SGL 20BC
SGL 21BC
0.58
0.56
Water Mass (g)
0.54
0.52
0.5
0.48
0.46
0.44
0.42
0.4
0.38
0
0.2
0.4
0.6
0.8
1
Current Density (A/cm 2)
1.2
1.4
Figure 9 – Discrimination of water in softgoods and
channels
1.6
Figure 7 – Total water mass variation with current density
Toray
SGL 20BC
SGL 21BC
Average Water Mass in DM Only
0.44
Anode/Cathode Channel
Channels
0.42
Water Mass (g)
0.4
0.38
0.36
0.34
0.32
0.3
0.28
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
I (A/cm2)
Figure 10 – Water mass in softgoods only, resulting from
water separation procedure
Only GDM+MEA
Figure 8 – Discrimination of water in softgoods and
channels
Toray
SGL 20 BC
SGL 21 BC
0.2
Water Mass (g)
develop a better understanding of how accumulated water mass
varies with fuel cell operating conditions, an image processing
procedure was applied to enable separate analysis of water in
the channels from that in the softgoods.
An interpolation routine was derived to exclude the water
in the channels for each thickness value in the original
thickness matrix. Several methods were considered, but the
most sophisticated would have to preserve the thickness
gradient in the MEA and GDM over the channel area. The
image was examined manually at all areas consisting of only
GDM and MEA (i.e., aligned with flow field lands), and a
maximum thickness was determined from the grid of
intersecting land areas (see Figure 8). Then an algorithm was
developed to replace areas with water slugs with a thickness
value equal to the maximum thickness value in the GDM and
MEA in the grid. The assumption made with this interpolation
was that the portion of the softgoods adjacent to a channel
water slug would contain the same amount of liquid as that in
the neighboring land area. After applying this algorithm, line
plots were generated from horizontal slices of the thickness
matrix (1 x 1024). By observing the magnitude of thickness
values before and after the interpolation it could be determined
if the interpolation was sacrificing the integrity of the thickness
values across the flow field lands. These areas were to remain
unchanged through the interpolation, and if “clipping” of the
Average Water Mass in Channels Only
0.25
0.15
0.1
0.05
0
0
0.2
0.4
0.6
0.8
I (A/cm 2)
1
1.2
1.4
1.6
Figure 11 – Water mass in channels only, resulting from
water separation procedure
thickness signal was observed in these land areas, the
maximum thickness in only GDM and MEA was re-evaluated.
Figure 9 illustrates the effect of the interpolation routine for a
particular test point plot; note that the color values were
rescaled to a broader range of the colorbar spectrum in the
interpolated plot. Horizontal line plots of the top, middle, and
bottom of the active area were also generated for each
interpolated data array. The line plots were used to ensure that
the interpolation only “clipped” the data array in areas
consisting of a channel with a water slug inside.
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Copyright © 2007 by ASME
Average Liquid Water Thickness Gradient Across Flow Field Land at 1.0 A/cm
2
Toray 1.0 A/cm2
0.0125
SGL 20BC 1.0 A/cm2
SGL 21BC 1.0 A/cm2
0.0115
Water Thickness (cm)
0.0105
0.0095
0.0085
0.0075
0.0065
0.0055
0.0045
0.0035
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
Land Width (mm)
Figure 12 – Water thickness profiles across cathode lands
4.5
T060
4.0
SGL 20BC
SGL 21BC
3.5
Tmembrane - Tplate (C)
Interpolating such that the average water mass values did
not include the water slugs in the channels enabled conversion
of the data in Figure 7 into the plots shown in Figures 10 and
11. By excluding the volume of water slugs in the channels, a
trend of increasing water accumulation with load was observed
for all three GDL materials. Furthermore, by subtracting the
water mass values in Figure 10 from the total water mass in
Figure 7, a trend of decreasing water in the channels with
increased load can be observed.
The water mass profiles for the three GDL types can be
compared to the performance comparison illustrated in Figure
4. An obvious correlation between accumulated water mass
and cell voltage is observed, where the Toray GDL
demonstrated largest mass transport loss. The performance of
the two SGL GDM samples were similar as the average amount
of liquid water accumulated at each test point was comparable,
and consistently lower than for Toray. The microporous layer
on the SGL gas diffusion media samples likely plays in a key
role in optimizing the fuel cell water management. This is
accomplished by facilitating transport of product water away
from the MEA, and distributing the water produced in the
electrode layer more evenly over the active area.
Aside from the well-documented benefit of a microporous
layer on water management (e.g., [19]), it is believed that two
properties of the GDL substrate play a key role in the water
accumulation within fuel cells operating at the same nominal
conditions: in-plane gas permeability which influences the
amount of convective flow through the GDL, and throughplane thermal conductivity which affects the temperature
gradient from the MEA to the flow field plate, and therefore the
local relative humidity of the reactant gases.
The in-plane gas permeabilities of the three GDLs (Table
1) had a strong effect that was observed in each radiograph in
the form of accumulated water gradients across the land areas
of the flow field. The Toray GDL had the highest permeability,
and hence the least resistance to gas transport over the lands.
SGL 21BC and 20BC had lower permeability, with that of the
20BC being the lowest. In Figure 12 a plot is presented of the
water thickness gradient over a cathode land near the reactant
inlets (upper right-hand corner of the radiographs) at a current
density of 1.0 A/cm2. This specific location and test condition
demonstrated the most pronounced effect, but the general trend
was observed throughout the analysis. Figure 12 clearly shows
that a higher in-plane permeability in the GDL yields more
effective gas transport over the land area. This effect may be
associated with anode channel water slugs observed at lower
current densities in the Toray test cell, where at the same
condition, SGL test cells did not retain water in the anode
channels. The significant increase in the in-plane pressure drop
for the SGL GDLs is a result of the properties of the paper
composition and manufacturing processes. Because of the
much higher flow resistance in these materials, more of the gas
is forced along the channels themselves, thereby enhancing the
convective removal of liquid water. The use of low in-plane
permeability gas diffusion layers to augment water transport in
the bipolar plate has been recommended in a published patent
application [19].
In Table 1, it is evident that differences exists in throughplane thermal resistance between the Toray and SGL materials.
This parameter effectively controls the increase in temperature
of the MEA above that of the flow field plate, which in these
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
current density (A/cm2)
Figure 13 – Computed temperature differences between
membrane and flow field plate
experiments was where the control thermocouple was located.
Based on the known waste heat flux and geometry of the flow
field plates, a two-dimensional conduction model was used to
estimate the difference in temperature between the membrane
and flow field lands. Similar calculations over the channels are
complicated because the actual compression force, contact
resistances, etc. are not well characterized. As shown in Figure
13, there is very little difference in the values of Tmembrane – Tplate
for the three GDL materials. Although the waste heat flux for
the Toray material is generally higher that that for SGL (due to
the lower cell voltage performance at the same current density,
Figure 4), this is counteracted by a much lower thermal
conductivity. From this simple analysis, it would be concluded
that the differences in land-on-land water accumulation
observed for the three different GDL materials is primarily a
result of variations in the in-plane gas flow, and not thermal
effects.
Effects of Channel Properties on Water Accumulation
Based on the measurements described above using
different GDLs, it was decided that further experiments were
warranted to understand the effects of channel properties on
water accumulation in the cathode flow field. For this phase of
the work, Toray 060 GDL was used exclusively, as this
7
Copyright © 2007 by ASME
Uncoated
Uncoated
Rectangular
PTFE Coated
Figure 14 – Uncoated and PTFE coated rectangular
channels at 0.5 A/ cm2
Figure 16 – Uncoated rectangular and triangular channels
at 0.5 A/cm2
Rect No PTFE
Rect PTFE
Tri No PTFE
Tri PTFE
Water Mass Comparison
Water Mass (g)
Uncoated
Triangular
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Figure 17 – Enlarged view of slug formation in rectangular
(top) and triangular (bottom) channels
0
0.2
0.4
0.6
0.8
1
Current Density (A/cm2)
1.2
1.4
1.6
Figure 15 – Measured water mass profiles for channel
property study
material was observed to consistently retain more water than
either of the SGL materials with microporous layers. The
baseline gold-plated aluminum flow field surface (hereafter
referred to as “uncoated”) had an average static contact angle of
40°, as measured using a Krüss Model DSA 10 Drop Shape
Analysis System. This surface is contrasted by PTFE coated
gold (“coated”) with an average static contact angle of 95°. A
comparison of water accumulation in each flow field crosssectional geometry (rectangular and triangular) was made for
the test points summarized in Table 2. In all cases, the anode
flow field retained the baseline characteristics of rectangular
geometry with no PTFE coating.
Throughout this part of the experimental program, a
consistent trend was observed, attributed to the effect of water
slug geometry upon increasing the static contact angle of the
channel surface. The PTFE coated cathode flow channels
generally formed smaller, more distributed water slugs
throughout the channel compared to the uncoated flow fields.
It is clear that the water slugs in the uncoated cathode channels
block a large fraction of the channel cross-section in the twodimensional area captured by the radiograph (Figure 14). The
average water mass plot in Figure 15 demonstrates that the
PTFE flow field configuration retains more liquid water at a
given current density. Over an averaged period of time this is
concurrent with the behavior of a water slug, as the larger
channel blocking slugs will be periodically purged out of the
channel by the pressure drop they induce. In contrast, the
smaller, more distributed slugs will remain in the flow field the
entire period of time, because by not obstructing a large
fraction of the channel, the pressure gradient required to
remove these small water slugs will not be generated.
In general, the triangular cross-section channels retained
less water than the rectangular cross-section at the same current
density (see Figure 15). This is consistent with the distinct
differences in slug shape observed in the two channel
geometries. Generally smaller water slugs were retained in the
triangular channels at the corners adjacent to the GDL; this was
a result of the surface tension acting to force water to the
corners encompassed by smaller angles. This observation is
consistent with published results of phase distributions in airwater flows through small, non-circular channels (e.g., [20]),
where the water tends to be transported in the corners with the
air flowing in the high-velocity core. Figures 16 and 17
illustrate the contrast in water slug shape with two radiographs
taken at 0.5 A/cm2 for uncoated rectangular and triangular
channels. It appears that in the rectangular case, the water slugs
fill most of the channel cross-section and are affected by
gravity, as many of these large slugs are retained at the lower
channel edge. Between the large slugs are many smaller
“satellite” droplets that are also stationary over the course of
the neutron imaging sequence. Conversely, in the triangular
channels, the slugs are usually formed in pairs and do not seem
to be influenced by gravity to as great a degree, but are retained
in the 43° angles adjacent to the GDL (see Figure 2). A
comparison of uncoated and coated triangular channels is
illustrated in Figure 18. As observed in the rectangular channel
comparison in Figure 12, with PTFE coating the water slugs are
generally smaller although they are still formed at many
8
Copyright © 2007 by ASME
Flow Field Study Performance Data
Toray Rect No PTFE
Toray Rect PTFE
0.95
Toray Tri No PTFE
Voltage (V)
0.85
Toray Tri PTFE
0.75
0.65
0.55
0.45
0.35
0
Uncoated
PTFE Coated
locations as pairs retained at the channel angles nearest the
GDL. It is expected that other channel cross-sectional
geometries can be used to control the location of liquid water
accumulation, preferably away from the GDL to minimize its
influence on reactant mass transport [21].
The observed effect of gravity described above was
initially somewhat unexpected, because it is widely reported
that interfacial forces should dominate in two-phase flow
through small channels. The Bond number characterizes the
relative influence of gravity and capillary forces, and is defined
as:
'Ugl
V
0.4
0.6
I
Figure 18 – Uncoated and PTFE coated triangular channels
at 0.5 A/ cm2
Bo {
0.2
2
(1)
is the difference
where g is gravitational acceleration,
between liquid and gas densities, l is the characteristic length
scale and
is the surface tension. For capillary forces to
dominate, the channel size needs to be selected such that the
condition Bo<<1 is satisfied. For example, for Bo = 0.1 in an
air-water system at 80 C, this dictates that the characteristic
length scale must be less than 0.8 mm. This value is well less
than the width of the rectangular channels used in the current
study (1.37 mm) which may be the characteristic length scale,
because this is the dimension aligned with gravity. However,
the channel hydraulic diameter (0.59 mm) is less than this
“critical” channel size based on the Bond number criterion.
Therefore, at least for the rectangular geometry, it is reasonable
to expect that gravity will play some role in the morphology of
the water slug distribution. In Figures 16 and 17, it is apparent
that many of the largest water volumes reside on the lower
channel edge, and are clearly influenced by the action of
gravity.
All four flow field configurations performed similarly in
regard to their respective polarization curves (Figure 19).
Every configuration displayed significant mass transport losses
at 1.0 A/cm2 and higher loads. The flow fields were chosen to
exaggerate losses in the mass transport region; hence voltage
losses were expected at high current densities. Again, a
performance correlation is ascertained relating accumulated
water mass and performance. It is evident in Figure 19 that
smaller water slugs adjacent to the GDL, produced by altering
0.8
1
(A/cm 2)
Figure 19 – Fuel cell performance comparison for channel
variations
channel surface energy and geometry, can improve
performance. The channel water slug size and distribution are
also important considerations for reasons other than fuel cell
power performance alone. For example, in a triangular
channel, the smaller accumulated water slugs left by the
operation of a PEMFC after shut-down have more space in the
channel to expand under a freeze condition than larger slugs
accumulated in rectangular channels that could potentially
damage the brittle GDL.
CONCLUSIONS
The neutron radiography method has been applied to
operating 50 cm2 PEM fuel cells, to assess the effects of gas
diffusion layer and flow field channel properties on liquid water
accumulation. The test apparatus featured anode and cathode
flow fields which were arranged orthogonally, to enable
separate analysis of the water content on either side of the fuel
cell. The gas diffusion layers manufactured by Toray and SGL
varied most significantly in their in-plane gas permeability and
through-plane thermal conductivity, which control the
convective flow through the material and the effective
temperature gradient between the membrane and the flow field
plate. It was determined that the relatively low in-plane gas
permeability of the Toray material accounts for the greater
volume of retained water under the flow field lands. Despite the
wide differences in thermal conductivity between the Toray and
SGL samples, a simple two-dimensional thermal model
indicated that the temperature gradient from the membrane to
the flow field was within 0.5 C over the entire range of current
densities.
It was observed that channel geometry and surface
property both have appreciable effects on the volume of
accumulated water, and on the morphology of water droplets
retained in the flow field channels. For both a rectangular and
triangular channel with the same cross-sectional area, channellevel water accumulation was reduced by use of a PTFE
coating, which provided a static contact angle of 95°. For a
given flow field surface energy (either PTFE or “uncoated”
gold-plated aluminum) the triangular channels retained less
water. Moreover, the water morphology was generally
characterized by pairs of droplets captured in the channel
angles between the diffusion media and the flow field plate. In
9
Copyright © 2007 by ASME
the rectangular channels, the water droplets were larger and
dispersed individually in the direction of flow, with smaller
“satellite” droplets between. It was also apparent that
gravitational forces influenced the water accumulation profile,
at least for the rectangular channels. These results provide
strong evidence that channel geometry and surface properties
must be accounted for in the design of fuel cell systems, due the
affects on cell voltage performance, and water accumulation
which would be expected to impact freeze operation and longterm material durability.
ACKNOWLEDGMENTS
Lee Whitehead is acknowledged for his calculation of the fuel
cell temperature gradients for the various GDL materials.
REFERENCES
1. A. Z. Weber, J. Newman, Transport in polymer-electrolyte
membranes. I. Physical model, J. Electrochem. Soc. 150
(2003) A1008-A1015.
2. R.J. Bellows, M.Y. Lin, M. Arif, A.K. Thompson, D.
Jacobson, Neutron imaging technique for in situ
measurement of water transport gradients within Nafion in
polymer electrolyte fuel cells, J. Electrochem. Soc.146
(1999) 1099-1103.
3. A.B. Geiger, A. Tsukada, E. Lehmann, P. Vontobel, A.
Wokaun, G.G. Scherer, In situ investigation of two-phase
flow patterns in flow fields of PEFC’s using neutron
radiography, Fuel Cells 2 (2002) 92-98.
4. J. Owejan, Neutron radiography study of water transport in
an operating fuel cell, M.S. Thesis, Rochester Institute of
Technology (2003).
5. R. Satija, D.L. Jacobson, M. Arif, S.A. Werner, In situ
neutron imaging technique for evaluation of water
management systems in operating PEM fuel cells, J. Power
Sources 129 (2004) 238-245.
6. N. Pekula, K. Heller, P.A. Chuang, A. Turhan, M.M.
Mench, J.S. Brenizer, K. Unlu, Study of water distribution
and transport in a polymer electrolyte fuel cell using
neutron imaging, Nucl. Instrum. Methods Phys. Res. A 542
(1–3) (2005) 134.
7. D. Kramer, J. Zhang, R. Shimoi, E. Lehmann, A. Wokaun,
K. Shinohara, G.G. Scherer, Electrochim. Acta 50 (13)
(2005) 2603.
8. D. Kramer, E. Lehmann, G. Frei, P.Vontobel, A.Wokaun,
G.G. Scherer, Nucl. Instrum. Meth. A 542 (1–3) (2005) 52.
9. Hickner, M.A., Siegel, N.P., Chen, K.S., McBayer, D.N.,
Hussey, D.S., Jacobson, D.L. and Arif,. M., Real-time
imaging of liquid water in an operating proton exchange
membrane fuel cell, J. Electrochem. Soc. 153 (2006)
A902-A908.
10. C.S. Kong, D.-Y. Kim, H.-K. Lee, Y.-G. Shul, T.-H. Lee,
Influence of pore-size distribution of diffusion layer on
mass-transport problems of proton exchange membrane
fuel cells, J. Power Sources 108 (2002) 185-191.
11. J. Scholta, G. Escher, W. Zhang, L. Küppers, L. Jörissen,
W. Lehnert, Investigation on the influence of channel
geometries on PEMFC performance, J. Power Sources 155
(2006) 66-71.
12. K. Tüber, D. Pócza, C. Helbig, Visualization of water
buildup in the cathode of a transparent PEM fuel cell, J.
Power Sources 124 (2003) 403-414.
13. F.Y. Zhang, X.G. Yang, C.Y. Wang, Liquid Water
Removal from a Polymer Electrolyte Fuel Cell, J.
Electrochem. Soc. 153 (2) (2006) A225-A232.
14. F.-B. Weng, A. Su, C.-Y. Hsu, C.-Y. Lee, Study of waterflooding behavior in cathode channel of a transparent
proton-exchange membrane fuel cell, J. Power Sources 157
(2006) 674-680.
15. Trabold, T.A., Owejan, J.P., Jacobson, D.L., Arif, M. and
Huffman, P.R., “In-situ investigation of water transport in
an operating PEM fuel cell using neutron radiography. Part
1 – Experimental method and serpentine flow field
results,” Int. J. Heat Mass Trans. 49 (2006) 4712-4720.
16. Owejan, J.P., Trabold, T.A., Jacobson, D.L., Baker, D.R.,
Hussey, D.S. and Arif, M., “In-situ investigation of water
transport in an operating PEM fuel cell using neutron
radiography. Part 2 – Transient water accumulation in an
interdigitated cathode flow field,” Int. J. Heat Mass Trans.
49 (2006) 4721-4731.
17. D.S. Hussey, D.L. Jacobson, M. Arif, P.R. Huffman, R.E.
Williams, J.C. Cook, New neutron imaging facility at
NIST, Nucl. Instrum. Methods Phys. Res. A 542 (2005) 915.
18. M. Mathias, J. Roth, J. Fleming, W. Lehnert, Diffusion
media for PEM fuel cells, Chapter 46 in: Handbook of Fuel
Cells – Fundamentals, Technology and Applications, W.
Vielstich et al. (Eds.), Volume 3: Fuel Cell Technology
and Applications, John Wiley & Sons, Ltd. (2003).
19. H. Haas, D.C.L. Cheng, K.K.-V. Fong, J.D. Glandt, C.-Y.
Lim and C.N. Startek, Fluid diffusion layers with in-plane
permeability characteristics for improved performance in
fuel cells, World Patent Application 2005/124908,
December 29, 2005.
20. T.S. Zhao, Q.C. Bi, Co-current air-water two-phase flow
patterns in vertical triangular microchannels, Int. J.
Multiphase Flow 27 (2001) 765-782.
21. T.A. Trabold and J.P. Owejan, Flow Field Geometries for
Improved Water Management, U.S. Patent 7,087,337,
August 8, 2006.
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