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The effects of channel depth on the performanceof miniature proton exchange membrane fuelcells with serpentine-type flow fields
Dyi-Huey Chang a,*, Shin-Yi Wu b
a Department of Environmental Engineering and Management, Chaoyang University of Technology, Taiwanb Department of Mechanical Engineering, National Chin-Yi University of Technology, Taiwan
a r t i c l e i n f o
Article history:
Received 12 February 2015
Received in revised form
23 April 2015
Accepted 27 April 2015
Available online 4 June 2015
Keywords:
Channel depth
Fuel cell
Metallic bipolar plate
Microelectrical discharge machining
(micro-EDM)
* Corresponding author. Tel.: þ886 42 233230E-mail address: [email protected] (D.-H
http://dx.doi.org/10.1016/j.ijhydene.2015.04.10360-3199/Copyright © 2015, Hydrogen Ener
a b s t r a c t
Flow channels are one of the key components of a fuel cell, because they perform various
essential functions that enable the system to operate correctly. In this study, three-path
serpentine and parallel-serpentine flow fields with various depths were analyzed experi-
mentally. Die-sinking microelectrical discharge machining was applied to fabricate mini-
ature SUS316L bipolar plates, of which both the rib and channel widths were 500 mm and
the channel depths varied among 200, 300, 400, and 600 mm in an active area of
20 mm � 20 mm. The clamping test was performed to examine the magnitude of mem-
brane electrode assembly deformation and the contact resistance. The pressure drops for
each cell were analyzed to determine the effects of channel depth. The results revealed
that a deep channel is required to leave sufficient space for reactant transportation and
water removal; however, too low flow velocity reduces the convective mass transport and
cell performance when the channel is too deep.
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
Introduction
Fuel cells are a promising power technology because of their
compactness, silent operation, and high power density, and
because they emit nearly no pollution. Among various types of
fuel cells, proton exchange membrane fuel cells (PEMFCs) are
the leading technology for portable electronic devices and
light-duty vehicles, because they operate at relatively low
temperatures (normally below 100 �C) and produce electrical
output to meet dynamic power requirements. A substantial
00x4530; fax: þ886 42 237. Chang).53gy Publications, LLC. Publ
knowledge base that focuses on the development of high-
efficiency PEMFCs exists [1e13]. It is generally agreed that
the geometric parameters and flow patterns of flow channels
strongly influence the performance of PEMFCs.
Bipolar plates with various flow fields can be fabricated
using various methods, namely machining, etching, and
embossing. For economic reasons, embossing is the preferred
process for mass production. However, embossing exhibits
limitations in forming channels with high depth-to-width
ratios, especially for narrow channels. In addition, plate
42365.
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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 1 1 6 5 9e1 1 6 6 711660
fractures or variances in channel dimensions might occur.
Research has been conducted on enhancing the embossing
process to form deeper narrow channels [14e16]. Therefore,
investigating the impact of channel depth on the performance
of miniature PEMFCs is worthwhile.
Various simulations have been performed to explore this
topic based on computational fluid dynamics. The results of
simulating a 200-cm2 cell indicated that that fabricating a
channel that is too deep tends to reduce the local gas pressure,
leading to variances in fuel distribution [10,17]. Modeling a
130-cm2 cell indicated that the optimal channel-to-rib width
ratio is approximately one, and a channel that is too deep
provides redundant fuel [18]. Simulating a 24.6-cm2 cell
revealed that the performance of a PEMFC with a serpentine
flow channel is insensitive to channel depth in certain oper-
ation conditions [3]. In the simulation of a 2.56-cm2 miniature
cell, the results indicated that cells with a high aspect ratio
yield higher current densities [19].
These simulation results reveal a trend that deeper chan-
nels are favored for miniature cells, whereas shallower
channels are preferred for larger cells. Several studies have
experimentally examined the impacts of channel dimension
on PEMFCs of various sizes [20e32]. Few of these studies have
focused on the impacts of channel depth on miniature
PEMFCs. A miniature PEMFC normally has narrow channel
and rib widths to provide sufficient channel length in a small
active area. Fabricating a PEMFC with fine, deep channels that
produces an efficient power density is challenging. For a
PEMFC with low power densities, the impact of channel di-
mensionsmight be less substantial because efficiency inmass
transportation is lower. This study examined the effects of
channel depth on the performance of highly efficient minia-
ture PEMFCs. Die-sinkingmicroelectrical dischargemachining
(micro-EDM) was applied to prepare micro flow channels in
metallic bipolar plates. Two serpentine-type flow fields were
created; both the rib and channel widths were 500 mm and the
channel depths varied among 200, 300, 400, and 600 mm in an
active area of 20 mm � 20 mm. The clamping test was per-
formed and pressure drops were measured to examine the
effects of channel depth.
Fig. 1 e The representation of (a) SFF and (b) PSFF and the
finished unipolar plate for (c) SFF and (d) PSFF.
Experimental and procedures
Fabricating the bipolar plates
In this study, SUS316L stainless steel was used as the plate
material because it features excellent heat and corrosion
resistance. Micro-EDM is applied to fabricate unipolar plates.
Micro-EDM can be classified into various categories based on
the tool electrodes. The process applied in this study involves
using cubic electrodes and is called the die-sinking micro-
EDM. During the process, copper tool electrodes were manu-
factured using micro-high-speed milling. A 1-mm-thick
SUS316L plate was cut into a 50 mm � 50 mm area by using a
wire machine and then served as the workpiece electrode. All
channels were processed on an area basis where electricity
was discharged in a single direction.
The serpentine-type flow field was applied as the flow
design. Previous studies [1,33] have reported that single-pass
serpentine flow fields tend to cause polarization problems
attributable to excessive channel length, and executing too
many passes tends to produce an uneven fuel distribution.
Therefore, the three-pass design was adopted. Fig. 1 shows
two types of commonly used serpentine flow field; Fig. 1(a)
illustrates the three-path serpentine flow field (SFF), and
Fig. 1(b) illustrates the three-path parallel-serpentine flow
field (PSFF). Fig. 1(c) and (d) show the finished unipolar plates
for SFF and PSFF, respectively. The plates were finished using
5 A of peak discharging current at a material removal rate of
7 mm3 min�1. The most obvious difference between SFF and
PSFF is that for PSFF there are single paths collecting and
distributing both gas andwater. The PSFF has been considered
a serpentine-type flow field in various studies [9,34]. For the
test samples, the channel and rib width are constant as
500 mm in a reaction area of 4 cm2. The rib-to-channel width
ratio was set as 100% because a previous study [18] has re-
ported that such a ratio enables high power densities to be
achieved. Channel depths are varied as 200, 300, 400 and
600 mm to test their impacts.
The clamping test
When assembling PEMFCs, a strong clamping force is usually
required to prevent the leakage of reactants and to reduce the
contact resistance. However, the clamping force causes the
membrane electrode assembly (MEA) to protrude into chan-
nels, leaving insufficient space for reactant flow and water
removal. This study explored the deformation of the MEA by
using the MEA compression test. During the test, unipolar
plates with parallel channels 200 mm in depth were cut in half
by using a wire machine and assembled to become a “half-
cell,” as shown in Fig. 2.The channel and rib widths are
500 mm, consistent to the bipolar plates in Fig. 1. Clamping
Fig. 2 e (a) The half bipolar plates, (b) half gasket, (c) testing
MEA, and (d) the clamping test.
Table 1 e General values used during simulations.
Description Value Units
Active area 400 mm2
Membrane thickness 0.1 mm
GDL thickness 0.4 mm
Catalyst layer thickness 0.01 mm
Cell temperature 333 K
Inlet flow rate 60 cm3/min
Active specific area 16000 1/m
Fig. 3 e The model geometry for PSFF with the depth of
600 mm with the cut plane for further observation.
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torques ranging from 10 to 25 kgf cm were applied to examine
the magnitude of MEA deformation. The length of the MEA
protruding into channels (lp) is approximated as follows:
lp ¼ lch � lrib2
(1)
where lch and lrib are the thicknesses of the MEA in the chan-
nels and ribs, respectively.
In addition, to determine the optimal clamping force, the
contact resistances (rc) between unipolar plates and gas
diffusion layers (GDLs) are calculated as follows:
rc ¼ VJ
(2)
where J is the applied current density and V is the measured
electric potential in the test.
The single-cell test
In this study, the cell performance of metallic bipolar plates
with various channel depths was tested using the TEI-P300-
1AB2CS surveying instrument. In addition, the pressure
drops for the testing cells were measured to examine the ef-
fects of channel depth. The pressure drop is defined as the
difference between input and output gas pressure and was
obtained using a data acquisition (DAQ) system. During the
tests, a single cell 50 � 50 � 23 mm in dimensions was
assembled. The cell was fabricated with end plates, gaskets,
bipolar plates, and an MEA. The MEA comprised a proton ex-
change membrane and two GDLs with 0.25 mg cm�2 Pt for
both the anode and cathode. Pure hydrogen and oxygen were
supplied to the anode and cathode electrodes. The anode flow
rate was 60 cc min�1, and thermal gas humidification of 30 �Cwas applied; the cathode flow rate was controlled using stoi-
chiometric numbers of 1.2. The cell was operated under 1 atm
of pressure at room temperature.
The mathematical model
In this work, a three-dimensional model for a high tempera-
ture PEMFC has been approached and implemented for its
solution in COMSOL Multiphysics 4.4. The model is similar to
those already existing in scientific literature [9,25], in which it
has been applied to study flow channel geometry influence
amongst other variables. The values of most simulation pa-
rameters are identical to [9]. Specific values consistent with
our experiment are listed in Table 1. Fig. 3 illustrates the
model geometry for PSFF with the channel depth of 600 mm.
Considering the MEA deformation, the channel depth in the
geometry is reduced by 40 mm for all simulation cases. Several
simplifications assumptions are introduced in this model: (1)
all processes are under steady-state conditions; (2) the GDLs,
the catalyst layers and the membrane are isotropic and ho-
mogeneous porous medium; (3) the gas-phase follows ideal
gas law; (4) the flows in the channels and porous layers are
assumed to be laminar and incompressible; (5) the trans-
portations of electron and proton are respectively controlled
by solid-phase electronic potential and membrane ionic
potential.
Governing equations of the model are derived as follows
(Table 2 for nomenclature):
(1) The convection and diffusion for the gaseous species (in
channels, GDLs, catalyst layers):
V$��DiVCi þ ujCi
� ¼ Si (1.1)
The diffusion coefficient of gaseous species in the diffusion
layer can be expressed as:
D ¼ Deff$ðεÞ1:5 (1.2)
Table 2 e Nomenclature used in equations of the PEMFCmodel.
C Molar concentration ðmol m�3ÞD Diffusivity ðm2 S�1ÞF Faraday's constant ðC mol�1Þi Current density ðA m�2Þkr Relative permeability
K Permeability
N Molar fluxðmol m�2 s�1ÞP Pressure (Pa)
R Gas constantðmol m�3 s�1ÞH Mass source term
u Velocity ðm s�1ÞVcell Cell voltage (V)
S Specific surface area ðm�1ÞEcell Thermodynamic equilibrium
potential (V)
Greek alphabet
h Over potential
a Charge transfer coefficient
d Conductivity ðS m�1Þ∅ Cell potential (V)
r Density ðkg m�3Þm Dynamic viscosity ðN s m�2Þs Electronic conductivity ðSm�1ÞSubscripts
a Anode
c Cathode
i Gaseous species
j Gas or liquid
ref Reference value
eff Effective value
s Electronic value
l Free electrolyte value
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(2) Momentum equation for the gaseous species (in chan-
nels, GDLs, catalyst layers):
�uj$V
�uj ¼ F� 1
rj
Vpj þmj
rj
V2uj (2.1)
Fig. 4 e The thickness of MEA (mm).
V$u ¼ 0 (2.2)
The average velocity of phase j is calculated by continuous
function based on a Darcy's law:
V$
rj
� Kkrj
mj
Vpj
!!¼ Hj (2.3)
uj ¼ �k
hVpj (2.4)
(3) The transportations of electron (in bipolar plates, GDLs,
catalyst layers) and proton (in PEM):
The source term of electronic current is set to be zero
because of no electrochemical reaction in GDLs.
�Vð�ssV∅sÞ ¼ 0 (3.1)
Similarly, the source term for the ionic current in PEM is
zero.
�Vð�slV∅lÞ ¼ 0 (3.2)
The catalyst layer is the region where reaction happens
and the ionic balance and electron balance are described as
follows:
�V�� sl;effV∅l
� ¼ �S$iloc (3.3)
�V�� ss;effV∅s
� ¼ S$iloc (3.4)
The local current density (iloc) generated by the porous
electrode reaction in the catalyst layer can be described by
Bulter-Volmer equation:
iloc ¼ io�exp
�aaF
h
RT
�� exp
��acF
h
RT
��(3.5)
iloc ¼ io�exp
�aaF
h
RT
�� exp
��acF
h
RT
��(3.6)
h ¼ ∅s �∅l � Ecell (3.7)
Results and discussion
The clamping test
Fig. 4 shows the deformation of the MEA at various clamping
forces. The values of lp and contact resistances are listed in
Table 3. The clamping force of 20 kgf,cm was adopted in the
single-cell test because the contact resistance was low. The
contact resistance did not decrease substantially when
further clamping force was applied. However, when a
clamping force of 20 kgf,cm was applied, approximately
92 mm of the MEA protruded into the channel. Obviously, a
considerable portion of channel space is required for shallow
channels.
The single-cell test
Fig. 5 shows the curves of electric potential versus power
density (VeP curves) for both SFF and PSFF. In the following
sections, the cell performances are evaluated based on the
peak power densities, as shown in Table 4 for various channel
depths and flow fields. In general, the peak power densities
Table 3 e The length of MEA protruded to channels (lp)and contact resistance (rc) in various clamping forces.
Clamping forces (kgf cm) 10 15 20 25
lp (mm) 80 86 92 96
rc (mU cm2) 72 68 64 63
Table 4 e The peak power densities (mW,cm¡2) forvarious channel depths and flow fields.
Channel depths TPS TSS
200 mm 145 144
300 mm 500 395
400 mm 562 532
600 mm 531 215
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occur around the average potential of 0.4e0.5 V. For lower
average potential, the cell performance decreases because of
the appearance of large amount of liquidwater in the cathode.
The cell performance of SFF was generally more favorable
than that of PSFF, except when the depth was 200 mm, at
which both flow fields exhibited poor performance. However,
the difference decreased as the channels deepened, because
the single path in PSFF has sufficient space. For both flow
fields, the cell performance initially increased as the channel
depth increased, and then decreased when the channel was
too deep (600 mm). The SFF with a channel depth of 400 mm
produced the highest power density of 562 mW cm�2. Fig. 5(a)
and (b) show the pressure drops for the anode and cathode
channels, respectively. The pressure drop was higher for the
PSFF than for the SFF because of the single paths. Similar to
the VeP curves, the difference decreased as the channel depth
increased.
Here, the impact of channel depth based on a cross in-
spection of Fig. 5 and Fig. 6 is discussed. At the channel depth
of 200 mm, both SFF and PSFF exhibited poor performance. The
pressure drop was high because of MEA deformation. The
cathode channel was completely clogged at a current density
of approximately 460 mA cm�2..At a channel depth of 300 mm, the performance of SFF was
muchmore favorable than that of PSFF. The cathode pressure
drop for PSFF was high and increased as the current density
increased, revealing that inefficient water removal occurs in
the single paths of PSFF flow fields because the channel space
is insufficient. It is also shown in Fig. 6 that the pressure drop
difference between SFF and PSFF at anode and cathode is
highest at 300 mm depth. This is because such depth provides
Fig. 5 e The VeP curves for the SFF and PSFF.
smaller channel cross-sectional area, comparing to deeper
channel. The single path in PSFF thus requires larger pressure
gradient to receive and distribute fluids. This is evidential for
cathode during lower cell potential, where more liquid water
is produced.
At a channel depth of 400 mm, both SFF and PSFF produced
their highest power densities compared with those at other
depths. The peak power density of SFF was the highest of all
flow fields; the pressure gradients were approximated to be
0.13 psi mm�1 and 0.83 psi mm�1 for anode and cathode
channels, respectively.
At a channel depth of 600 mm, the performance of both SFF
and PSFF was similar, indicating that deeper channels provide
sufficient space for the single path in PSFF to receive and
distributewater in the cathode. The pressure gradient for both
Fig. 6 e The pressure drops for (a) anode and (b) cathode
channels.
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flowfieldswas approximated to 0.10 psimm�1. The anode and
cathode pressure gradients were nearly identical, indicating
that the cathode water flows in the bottom of the channel,
leaving free space for gas flow. Two-phase flow considerably
complicates oxygen distribution. Therefore, the channel
depth cannot be increased excessively to improve the cell
performance.
A significant amount of researches have investigated the
effects of different channel geometrics on cell performance
based on experiments or simulations. The results of Scholta
et al. [18] suggest that the optimal channel-to-rib width ratio is
approximately one, as in Fig. 1. They also show that the
relationship between rib width and other channel geometrics
is less significant. This is because the rib width, although has
strong influences on the path length of electron transport, has
less or none effects on the gaseous flow. Therefore, the effects
of channel depth for various rib widths are theoretically
similar to the results described above. On the other hand,
Manso et al. [19] numerically investigated the effects channel
depth-to-width ratio using ten different channel geometrics.
Their results reveal that the cell performance increases for
higher channel depth-to-width ratio if the ratio value is less
than or close to one. Therefore, for different channel widths,
the overall cell performance might change while the effects of
channel depth remain the same.
Experiments of PSFF with various flow rates are evaluated
because the impacts of channel depth are much sensitive in
such a flow field. Fig. 7 shows the comparison of cell perfor-
mances at anode flow rates of 60 and 80 c.c. min�1. The
cathode flow rate was controlled using stoichiometric
numbers of 1.2. It can be seen at a channel depth of 300 mm
that the cell performance improves for higher flow rate. It is
because patterns of two-phase flow in the cathode channel
are more likely to change from plug flow to more efficient
annular flow due to high flow velocity. At a channel depth of
400 mm, the cell performance slightly decreases at higher flow
rates. It reveals that the cell performance at the flow rate of
60 c.c.min�1 is at its optimum. Too high flow velocity shortens
the gaseous retention time and reduces the cell performance.
Fig. 7 e The cell performances of PSFF with different
channel depths at anode flow rates of 60 and 80 c.c. min¡1.
At a channel depth of 600 mm, the increase of flow rates im-
proves cell performance a little. In such a flow rate, the IeP
curves is very similar for the cell with the depth of 400 and
600 mm. Deeper channel has larger cross-sectional area,
leading to lower flow velocity. It requires larger flow rate to
increase flow velocity and convective mass transport. As a
result, it is economically inefficient to fabricate bipolar plates
with too deep channel and operate at a high flow rate.
The numerical analysis
Fig. 8 compares experimental and simulated polarization
curves. In general, calculated results agree with experimental
data. However, the finite elemental method diverges and fails
to find the solution for the average potential smaller than
0.45 V. In addition, simulation results slightly over estimates
the current densities for smaller average potential. It appears
that the proposed model has limitation in dealing with the
complicity of two-phase flow. As described in existent litera-
ture [15,16] that the process of random droplet emergence and
liquid water clogging is not clearly understood because water
clogging is highly localized to certain regions in specific
channels. Nevertheless, the observation of individual channel
velocity provides some insight about the pattern of two-phase
Fig. 8 e Model validation for (a) SFF and (b) PSFF; Solid line:
experiment, Dotted line (Simulation).
Fig. 9 e Predicted flow velocity distribution (m/s) at 0.45 V. (a) SFF, (b) PSFF; (1) 300 mm, (2) 400 mm, (3) 600 mm.
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flow. Fig. 9 shows the predicted flow velocity distribution at
0.45 V. As mentioned before, the cell with the depth of 600 mm
has lower average velocity and thus reduces the convective
mass transport and cell performance. However, the cell with
shallower depth, although provides higher average flow ve-
locity, shows highly heterogeneous in the distribution of ve-
locity. It can be observed that very low flow velocity exists in
most inner channels. In such a case, the plug flow may
dominate in the cathode channel and have more chance of
water clogging. On the other hand, deeper channel provides
sufficient cross-sectional area and may lead to annular flow.
Fig. 10 compares the vorticities in the cut plane at the six
channels near the cathode output (Fig. 3) for PSFF 300 mm and
600 mm. It can be seen in the channel depth of 300 mm that the
vorticities is significant small in the 4th to 6th channels. In
Fig. 10 e The vorticities (1/s) in the cut plane (Fig. 3
contrast, the vorticities is significant and uniform in all
channels with the depth of 600 mm. The vortices create cen-
trifugal forces to lead the liquid flow against the channel wall,
leaving channel central space for gaseous flow. Such an
annular flow removes water more efficiently and yields better
cell performance.
Conclusion
This study involved applying die-sinking micro-EDM to pre-
pare micro flow channels in metallic bipolar plates. The ef-
fects of channel depth were examined by measuring pressure
drops, observing MEA deformation, and numerical analysis.
The conclusions are listed as follows:
) for PSFF (a) 300 mm and (b) 600 mm at 0.45 V.
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1. An SFF with a channel depth of 400 mm produced the
highest power density of 562 mW cm�2.
2. The cell performance of SFF is generally more favorable
than that of PSFF. However, the cell performance becomes
similar as the channel depth increases, because the single
paths in PSFF cathode flow fields have sufficient space to
distribute water.
3. Because MEA deformation occupies portions of channel
space, a sufficient channel depth is required to enable
reactant transportation and water removal. However,
when the channel is too deep, too low flow velocity further
reduces the convective mass transport and cell perfor-
mance. The increment of flow rate slightly improves the
cell performance. However, it is economically inefficient to
fabricate bipolar plates with too deep channel and operate
at high flow rate.
Acknowledgments
This work was supported by the National Science Council of
Taiwan under grant numbers NSC 102-2221-E-324 -021 and
NSC 102-2622-E-324 -003 -CC3.
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