The Performance of Miniature Metallic PEM Fuel Cells
Shuo-Jen Lee, Yu-Ming Lee, Chi-Yuan Lee, Feng-Hui Kuan and Chih-Wei Chuang
Department of Mechanical Engineering, Yuan Ze University
135 FarEast Road, NeiLi, TaoYuan, Taiwan, ROC
Email: mesjl@saturn.yzu.edu.tw
ABSTRACT
A prototype of metallic PEM fuel cell with thin stainless bipolar plates was tested for their
potential applications in portable electronic products. The flow field pattern was grown from the
stainless steel plates by the electroforming process. The main flow channel has the dimensions of 300
μm (width) by 300 μm (depth). The dimensions of the micro-features were 100 μm width by 50 μm
depth and 50 μm width by 50 μm depth. The material of the electroformed flow field pattern is nickel.
A prototype of a single cell with total thickness of 2.5 mm and overall reaction area of 16 cm 2 was
assembled for this study. In order to improve its corrosion resistance, the bipolar plates were coated
with 5 μm thick of multi-layered corrosion resistant material.
In this study, indices of cell performance, with and without micro-features, of flow field
concentration distribution, water draining capability were studied by both numerical simulation and
experimental measurement. The results show that the micro-features will provide a more uniform gas
concentration environment in the bipolar plate flow field. Also, the bipolar plate with micro-features
could greatly improve cell draining capability. The results of cell performance tests show that the cell
average power density reaches 173 mW/cm2. The long-term tests verify their performance were very
stable.
Keywords: electroforming, micro-features, metallic bipolar plate, numerical simulation, water draining
capability
1. Introduction
The miniaturized fuel cells require thinner
Recently, because of the rapid development
and smaller bipolar plates. The current approach
of green technologies and 3C products, the small
in developing miniature fuel cells and its bipolar
fuel cells as portable power sources are gaining
plates
more attentions.
micro-fabrication technologies like
cells
is
based
on
adoption
of
MEMS,
CMOS and etc. [1,2].
For
example,
on the gas temperature, fuel utilization, flow
the
use
of
silicon
direction and fluid dynamics [6-9]. At lower cell
micro-machining and related thin film processes
temperature, because of the water transport, the
have been employed by several research groups
cell relative humidity was not uniform. Also, the
as attractive routes for fabrication. Lee [3], by
cell current distribution could be effected by the
using sputtering and MEMS processes to create
gas flow direction. It increases in the gas
bipolar plates with depth between 50 μm and
downstream direction [10].
200 μm on silicon wafer. The micro-fuel cell
Therefore, for a miniature fuel cell, it is
performed normally with cell efficiency of 50
not
easy
to
perform
detailed
in-situ
mA/cm2. Dry etching process, RIE, could
measurements during cell operation. Yet, such
provide at least 200 μm depth of flow channel on
detailed information is essential in improving
silicon wafer [4]. However, the conductivity of
understanding of water and species transport and,
silicon-based bipolar plate is merely 0.5 S/cm. It
hence, cell performance.
was 10,000 fold smaller than that of a metallic
bipolar plate.
2. Research procedures
On the other hand, due to smaller
In this paper, thin metallic bipolar plates
dimensions of the bipolar plate, the fuel
with functional micro-features were assembled
transport and water draining capability could be
into single cells for testing. The 2D gas
greatly influenced by the channel size and
concentration distribution was simulated by
geometry. The pressure distribution in the
commercial software, FEMLAB.
bipolar plates depends on the Reynolds number
First, given the hydrogen related coefficients,
and geometric parameters of the small flow
velocity and concentration condition at the gas
channels [5]. Their results indicated that the
inlet and outlet of the bipolar plate, a numerical
pressure drop in a fuel cell bipolar plate depends
simulation model is established to simulate the
on the plate configurations. And for a given
flow field hydrogen concentration distribution
serpentine channel configuration, it depends on
for both with and without micro-features.
geometric factors such as channel hydraulic
Secondly, using CCD camera and visualization
diameter and bend geometry.
test to demonstrate the water draining capability
In addition, the cell relative humidity
of the bipolar plate with and with micro-features.
current distributions and water transport depend
Finally, the cell performance and 300 hours
long-term test was conducted to verify its
the
steady
version
of
performance stability. The research procedure
Navier–Stokes
are shown in Figure 1.
momentum conservation:
equations
the
for
well
known
mass
and
The shape and size of the bipolar plate was
4.0 cm by 4.0 cm square with effective area of

   0
(1)
2.0 cm by 2.0 cm square. The substrate material
 


    p     g
(2)
is 316L stainless steel. The width of the main
flow channel was selected to be 300 μm. The
Variables in this study include hydrogen
depth of the channel was 250 μm taking pressure
velocity and concentration. Their parametric
drop and flow resistance into consideration.
settings and related coefficients such as gas
Micro-features are designed with depth of 50μm
density (kg/m3, ρ), gas dynamic viscosity (Ns/m2,
and width of 50 μm and 100 μm. Figure 2
η) and diffusion coefficient (m2/s) are shown in
presents the geometrical dimensions of the 2D
Table 1. The modeling dimension of 20 mm x 20
schematic plot.
mm represents the size of the bipolar plate.
In order to verify the difference between
2.1 Models for numerical simulation
bipolar plates with and without micro-features,
2D multi-physics models of bipolar plate
all of the boundaries are no-slips except for the
flow field for hydrogen and concentration
gas inlet and outlet, hydrogen flow conditions
distributions were established using FEMLAB
are set at the gas inlet, Vi = V0, and at the outlet,
commercial
software.
The
fuel’s
mass
Vo = V1. The hydrogen concentration conditions
transportation of an ideal fuel cell system is
are assumed to be at the gas inlet, Ci=1 and at
composed of complex process such as flow of
the outlet, Co=0
gas into the bipolar plate, diffusion into the MEA
and flew in the MEA. The focus of the numerical
2.2 Micro-fabrication of bipolar plates
simulation
is
to
analyze
the
gas
mass
The bipolar plates are fabricated by the
transportation of the bipolar plate with and
UV-LIGA and electroforming processes. The
without micro-features. Therefore, the gas
thickness of the bipolar plates is around 250 μm
diffusion of the MEA is ignored.
to 300 μm. Therefore, Conventional photo resist
The resulting transport equations governing
of the MEMS process is not suitable. After many
the flow in the channels of the bipolar plate are
trials, the SU-8 photo resist is found to meet our
research goal of obtaining a 250 to 300 μm
red color stain. The draining tests are conduct
thickness of photo resist after spin coating.
with 50 and 100 SCCM (Standard Cubic
In order to create the flow field with
Centimeter Per minute) hydrogen, respectively.
micro-features, two iterations of the complete
UV-LIGA and electroforming processes were
3. Results and discussion
conducted. The first iteration is to fabricate the
3.1. Numerical simulation results
main flow field pattern. The second iteration is
Figures 5 and 6 show the results of the flow
to create the micro-features. Figure 3 shows the
field hydrogen concentration distribution, with
results of electroforming of main flow channels
and without micro-features, at gas flow velocity
and micro-features.
of 0.001 m/s and 0.0005 m/s, respectively. The
simulated data output on the dotted line of
2.3 Cell performance and water draining tests
Figures 5 and 6 were plotted in Figures 7 and 8.
A single cell was assembled for cell
Figures 5 and 6 indicate that, regardless of gas
performance test. The single cell has an overall
flow velocity, both of the flow fields seem to
size of 4 cm by 4 cm square with 2.6 mm in
have the same trend in concentration drop.
thickness. A 5-layered MEA, from Fuel Cell
In the beginning, at near the gas inlet, the
Store, #597010, is used which is composed of
hydrogen concentration is uniform in the flow
Nafion 112, GDL of carbon cloth material and
channel.
0.5 mg of Pt loading on both anode and cathode.
micro-features
The effective reaction area is 2 cm by 2 cm. The
concentration distribution of the flow field with
weight of the single cell without the fuel
micro-features
connectors is 24g. The single cell design is
micro-features. However, for the bipolar plate
shown in Figure 4.
without
Thus,
the
are
is
functions
not
equal
micro-features,
of
obviously.
to
the
that
the
The
without
difference
in
In order to verify the water draining
concentration drop of each flow channel is
capability of the micro-featured bipolar plates,
increased from the middle of the flow field. In
the water flow visualization test was employed
addition, the differences are getting more serious
to observe the water draining situation of the
when the hydrogen flows toward the gas outlet.
bipolar plate. The bipolar plates, with and
For
without micro-features, are sealed by gasket and
micro-features at the position -0.017 of the
acrylic plate. The flow channels are filled with
Figure 5 (b), the highest concentration channel is
example,
the
flow
field
without
around 0.65. But the lowest concentration
with 50 and 100 SCCM of pure oxygen and the
channel is around 0.55. The difference reaches
flow channels are full of red color stain. The
0.1.
water draining time is 3 seconds Figures 10 and
The
problem
could
be
effectively
11 shows the results of 50 and 100 SCCM.
eliminated by micro-features. Figures 7 (a) and 8
At 50 SCCM, Figure 10 (b) shows the
(a) show there are no obvious concentration
bipolar plate without micro-features still has
difference between each channel from the gas
water flooding situation. The flooding area is
inlet to the outlet. These results indicate that the
around a quarter of the flow field. On the other
micro-features could help gas diffusion between
hand, the flooding area of the micro-feature flow
each channel. Thus, the flow field has more
field is just at the bottom of the flow field. The
uniform gas concentration and the cell could
area is less than half of the flow field without
perform better.
micro-features. Therefore, the water draining
capability
3.2 Single cell and results of water draining test
The prototype of a single cell is shown in
of
the
bipolar
plate
with
micro-features seems much better than that
without micro-features.
Figure 9. The optical microscopy, micrometer
At 100 SCCM, the oxygen flow rate is
and laser displacement profiler were employed
increased. The water flooding situation is less
to exam the surface profile and height of the
damaging than 50 SCCM. Figure 11 (b) shows
flow channel. The results showed that the overall
the flooding situation of the flow filed without
size of the bipolar plate is around 4 cm by 4 cm
micro-features. It is still serious than that with
square. The height of the flow field channel is
micro-featured flow field.
around 250 to 300 μm. However, the shape is not
From these results, the water draining
very sharp which is caused by the chemical
capability of the micro-feature bipolar plate is
residues left on the pattern before electroforming.
better than that without micro-features.
There is around 20 μm difference in height over
the whole bipolar plates. This might be caused
3.3 Cell performance tests
by the initial flatness error of the SS304 plate. It
Cell performance tests were conducted
could also be resulted from the non-uniformity
with 50, 100, 150, 200, 250 and 300 SCCM
of the photo resist.
(Standard
The water draining tests were conducted
Cubic
Centimeter
Per
minute),
respectively. Pure dry oxygen and hydrogen and
the cell temperature is controlled at 49℃. I-V
results indicated that, all the tests, of without
functional micro-features, the internal flooding
curves of these cell performance tests were
may be very serious. However, the cell with
plotted in Figure 12. At the initial test of 50
functional micro-features could improve the
SCCM, the internal flooding is not obvious. The
draining ability. The cell performance could thus
cell performance decreases toward the flow rate
be higher.
of 100 SCCM. The cell performance stabilized
In
order
to
study
the
stability
in
when the flow rate exceeds 150 SCCM. For flow
performance, the single cell with functional
rates of 150 SCCM and 300 SCCM, the current
micro-features
went
through
continuous
densities at 0.6 V are 190 mA cm-2 and 250 mA
operation of 300 hours. The cell was operated
cm-2, respectively.
with a constant load of 0.4 V. Other settings
The leveling of cell performance could be
were the same with those of cell performance
caused by blockage of micro-features by the
tests. Figure 14 showed long-term test results.
GDL. Thickness of the GDL, 0.42 mm, is much
During the duration, the cell power was
larger than that of the micro-features, 0.05 mm.
maintained
between
0.4
W
and
0.55W
The optimal current density is 250 mA cm-2 at
(75mW/cm2 and 110mW/cm2) and temperature
0.6 V for flow rate of 300 SCCM. The total
current output is 1 A and power output is 0.78 W.
The average power density is 195 mW cm-2
was between 49 ℃ and 52 ℃. The cell power
fluctuated were just slightly. These results
Figure 13 shows the results of performance
indicated that the functional micro-features has
tests for both with and without micro-features.
greatly improve the cell draining capability.
At the initial test, regardless of functional
Thus, the effect of cell internal flooding was not
micro-features, the cell performance seems to be
obvious. Therefore, the cell performance was
the same. At 400 mA/cm2, nearly the end of I-V
very stable.
curves,
the
cell
voltage
with
functional
micro-features could maintain upper 0.4 V.
4. Conclusions
However, without the functional micro-features,
1. Through the UV-LIGA and electroforming
the cell voltage decreases toward 0.35 V. Also,
process, the high mechanical strength and
from the maximum power density point of view,
good
the cell with functional micro-features is
bipolar plate and miniature fuel cell had
173mW/cm2 much higher than without. These
been developed. The reaction area of the
electrically
conductivie
metallic
prototype of single cell is 4 cm2. The overall
size is 16 cm2 with thickness of 2.6 mm.
2. Through numerical
simulation,
it
[4] J. Yu, P. Cheng, Z. Ma and B. Yi, J. Power
Sources, 124 (2003) 40–46.
is
[5] S.
Maharudrayya,
S.
Jayanti,
A.
P.
concluded that the micro-features will
Deshpande, J. Power Sources, 138 (2004)
provide a more uniform gas concentration
1-13.
environment in the bipolar plate flow field.
Thus, the cell performance could be
[6] P. Quan, B. Zhou, A. Sobiesiak, Z. S. Liu, J.
Power Sources, 152 (2005) 131-145.
[7] P. T. Hguyen, T. Berning, N. Djilali, J.
improved.
3. From results of water draining tests,
Power Sources, 130 (2004) 149-157.
regardless of gas flow rate, the bipolar plate
[8] W. Ying, Y. J. Sohn, W. Y. Lee, J. Ke, C. S.
with micro-features could greatly improve
Kim, J. Power Sources, 145 (2005) 563-571.
[9] S. Um, C. Y. Wang, J. Power Sources, 125
cell draining capability.
4. The results of cell performance tests show
that the cell average power density has
reached 173 mW cm-2. The long-term tests
5. The proposed manufacturing procedures
prove to be a promising technology in
producing metallic bipolar plates with
micro-features.
References
[1] S. Aravamudhan, A. R. A. Rahman, S.
Sensors
and
Actuators
A,
123–124 (2005) 497–504.
[2] P. Tristan, G. M. Bernard, H. Daniel, Fuel
Cells Bulletin, 2004 (2004) 11-14.
[3] S. J. Lee, S. W. Cha, Y. Liu, R. O'Hayre, F.
B.
Prinz,
Electrochemical
Proceedings, Spring (2000).
[10] H. Nishikawa, R. Kurihara, S. Sukemori, T.
Sugawara, H. Kobayasi, S. Abe, T. Aoki, Y.
Ogami, A. Matsunaga, J. Power Sources,
also verify their stable performance.
Bhansali,
(2004) 40-51.
Society
155 (2006) 213-218.
The Performance of Miniature Metallic PEM Fuel Cells
Hydrogen flow velocity
Hydrogen density,
dynamic viscosity
Hydrogen concentration
Bipolar plate
modeling
Diffusion coefficient
Bipolar plate creating by UV-LIGA
EMM experiments
Water draining test
Cell performance test
Measurement
Long term test
Fig. 1. Research procedures
micro-feature width 100μm
and 50μm
Fig. 2. Schematic picture of the geometry model (mm)
Fig. 3. Main flow channels and micro-features
Fig. 4. Single cell design
(a) with micro-features
(b) without micro-features
Fig. 5. Hydrogen concentration distribution with v = 0.001 m/s
(a) with micro-features
(b) without micro-features
Fig. 6. Hydrogen concentration distribution with v =0.005 m/s
Without micro-features
With micro-features
Concentrationn (x 100%) .
v = 0.001m/s
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.01335
Outlet
0.01336
-0.02
0.01337
-0.017
0.01338
-0.01
0.01339
-0.09
X measurement position
0.0134
-0.095
0.013410 Gas0.01342
inlet
Flow field vertical distance
(m)
Fig. 7. Values of hydrogen concentration with v = 0.001 m/s
Without micro-features
With micro-features
Concentration (x100%) .
v = 0.005 m/s
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Outlet
0.01302
-0.018
0.01303
-0.013
0.01304
-0.01
0.01305
x
-0.09
0.01306
0
Gas inlet
0.01307
0.01308
Flow field vertical distance
(m)
Fig. 8. Values of hydrogen concentration with v = 0.0005 m/s
Fig. 9. Prototype of a single cell
Inlet
Outlet
(a) with micro-features (b) without micro-features
Fig. 10. Water draining test at flow rate of 50 SCCM
Inlet
Outlet
(a) with micro-features (b) without micro-features
Fig. 11. Water draining test at flow rate of 100 SCCM
Compensated cell potential (V)
I-V Curve
1
50c.c.
100c.c.
150c.c.
200c.c.
250c.c.
0.8
0.6
0.4
0.2
300c.c.
0
0
100
200
300
400
500
600
Current density(mA cm-2)
Fig. 12. I-V curves of the cell performance tests
I-V curves of the cell with and without micro-features
V (voltage)
130
90
With micro-features
Without micro-features
With micro-features
Without micro-features
I (mA/cm2)
Fig. 13. Cell performance tests
60
20
Power (mW/cm2)
170
Fig. 14. 300 hours long-term test with micro-features of single cell
Table 1. Variables and their parametric settings
Parameters
Setting values
Variables
Hydrogen velocity (m/s)
0.001
0.0005
Hydrogen concentration
1 (inlet)
0 (outlet)
Coefficients
Parameters
Setting values
Density
(kg/m3, ρ)
0.09
Dynamic viscosity
(×10-5 Ns/m2, η)
0.9
Diffusion coefficient
(×10-4 m2/s)
0.7