Download Design, fabrication and performance analysis of a 200W PEM fuel

Design, fabrication and performance analysis of a 200W PEM fuel cell short stack
Fang-Bor Weng*, Bo-Shian Jou, Ay Su and Shih Hung Chan
Fuel Cells Research Center, Yuan Ze University,
135 Yuan-Tung Rd., Chung-Li, Tao Yuan, 320 Taiwan, R.O.C.,
*E-mail: [email protected]
ABSTRACT
A 200W PEM fuel cell short stack is designed and fabricated in-house. The status of the unit cell
performance is 0.55W/cm2, which using Gore 5600 series MEA, 100cm2, at standard operating
condition. Based on the unit cell technology, a short stack has been developed. The proper designs of
uniform flow distribution, cooling plate and compressed end plate are important to achieve the best
performance of the short stack. The performance of four cells stack is analyzed in static and dynamic
modes. In the static mode of polarization curve, the stack has peak power density of 0.55W/cm2
(220W) at 0.5V per cell, when the voltage is scanning form low to high voltages (1.5V~3.5V), and
resulting in minimum water flooding inside the stack. In this paper, we made a series of
dynamic-loading test to simulate the vehicle acceleration. The fuel cell performance response to
dynamic-loading influenced by the hydrogen/air stoichiometric, back pressure, and dynamic-loading
times. It needs high hydrogen stoichiometric, back pressure to maintain high dynamic performance. In
the long-time stable power testing, due to the water flooding at high output power, the stack is difficult
to maintain the high performance. A back pressure water purge method is proposed to prevent the
water flooding at flow channels and maintain the steady output power at 170W ( 0.42W/cm 2).
Keywords: Fuel cell, Stack design, Dynamic performance, Hysteresis-effect
1. Introduction
Owing to the issues of continuant development, the concerns of environmental protection and the
crisis of petroleum supply as well, fuel cell therefore develops speedily among which PEMFC is one of
various significant developments. It is believed that PEMFC plays an essential part in the advancement
of fuel cell and conducted aggressively in the worldwide research units. At the same design and
operating condition of PEMFC a single cell performs better than stack. Because of many working
conditions are not the same, especially the stack of the abounded cell number, gas distribution, water
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blocking, pressure drop, thermal distribution, ohmic resistance etc. Consequently, the approaches to
and related tests of stack research are remarkable referential information.
PEMFC performance has significant effects by the operating condition such as temperature,
reactant gases humidity, flow rate and pressure. In order to achieve a better performance, several
parameters in a fuel cell must be optimized. For example, (1) Electronic conduction from the anode and
cathode catalyst sites to the current collectors, (2) Ionic conduction from the anode catalyst sites to the
cathode catalyst sites, (3) Reactant gas access to the catalyst sites from the channels for both the anode.
Therefore moisture in the membrane electrolyte and the distribution of steam and liquid water in the
channels and porous layers has a significant influence on the cell power parameters and their long-term
stability.
GDL (gas diffusion layer) have a great influence on fuel cell, and the compression was one of the
reasons that affect the performance of the Fuel cell. Weng et al. [1] reported the reasons for the
compression of the Fuel cell hardware are to minimize the contact resistance, to avoid gas
leakage/crossover, and to optimize gas permeability. There was a plastic membrane first placed in the
testing cell to examine the gas leakage and crossover of the MEA and also the metal foil was placed to
measure the contact resistance of the testing cell. They find a way to test the GDL resistance and gas
permeability before fabricate the real MEA and it can prevent the damage of catalyst layer and
diffusion layer.
Barbir et al. [2] reported a relationship between water flooding, ohmic resistance and pressure
drop. An increase in pressure drop, particularly on the cathode side, is found to be a reliable indicator
of PEMFC flooding, while an increase in cell resistance is a reliable indicator of fuel cell drying. Both
flooding and drying have a detrimental effect on cell potential. However, drying typically causes
monotonous voltage decay, flooding causes erratic cell voltage behaviour. In addition to, the voltage
drops suddenly and then resistance increased are due to the liquid water accumulation and expulsion
inside the cell passages.
Abounded amount of water causes flooding and then blocking the gas flow through the porous
passages of the GDL and simultaneously reduce the reactant rate of catalyst. Knobbe et al. [3] used
solenoid valve to control the inlet and outlet of each cell individually. In that study, the use of the
active gas management system developed and achieved a 30% increase in power output of a PEMFC
stack. Trung et al. [4] used stators to exhaust O2 or H2 and two sequential exhausting devices were
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tested, one based on a rotating device and another using electromechanical valves. Both showed
dynamic improvement in performance associated with equalization of flow rate.
For a fuel cell stack degradation, Giddey et al. [5] measure a long-term of 1kW PEMFC stack, the
stack showed a small degradation in its performance after undergoing more than 40 cold start/shut
down thermal cycles and ~300 hr accumulated operation over a period exceeding one year as indicated
by an increase in the average ohmic resistance of cells from 0.328 to 0.428 ohm cm2. Possible causes
for increase in the ohmic resistance and degradation in the electrical output of the stack are: membrane
degradation, increase in the contact resistance between various stack components, corrosion of the
current collection plates, deterioration of the catalyst, carbon paper and the diffusion layer. Zhigang et
al. [6] also find a new designation of flow-field plate can working under dry hydrogen and air. They
used a double-path-type counter-current flow-field design not only better utilizes the water produced by
the fuel cell to hydrate dry reactants, but also achieves a more even distribution of reactants over the
entire active area. This stack can run stable at a current density up to 0.33 A/cm2 using dry hydrogen
and dry air.
It is important to show a long-term steady and dynamic performance under different condition on
fuel cell stack, because of the genuine the application in business and the valuable academic reference.
In this study, we will discuss not only the design and experimental procedure but also the short stack’s
performance and stability of the PEM fuel cells. For the dynamic load response of fuel cell stack, a
series of experiments was developed to simulate the motor accelerate situation. The power and current
variation will accompany with loading value and loading time. In the special fuel cell operation needs
different operating condition.
2. Experimental
2.1 Design of the short stack
A self-developed short stack was designed and fabricated for this study, as schematically shown in
Fig.1a and Fig.1b, The specification of short stack was shown in Table 1. The end plates were made of
gold-coated brass, it also have a function to conduct the electron. The stack was built from graphite
composite bipolar plates, 16 serpentines type flow fields were used for gas distribution on anode and
cathode sides. There is a design of converged inlet channel to maintain the flow rate and pressure in the
whole flow field at one side, as shown in Fig. 2(a). The ditch of wide is 1.25mm, depth is 1mm and the
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wide of rib is 0.83 mm of the flow field channels and also the bipolar plate’s size is 130 mm x 130 mm.
The Gas flow field of bipolar plates are shown in Fig.2(a). In every manifold of each cell, a special
design is maintained where as the gas flow occurs through the back side than turn around to the front
side. The special design of back-sealing can improve the efficiency of the gas flowing and reduce the
route through the back side to the front side, as shown in Fig2(c). The manifold inlet-type of gas is the
back-side inlet, and there is a water cooling plate between two cells in each bipolar plate in the back
side, as seen in Fig. 2(b). This assembly, including the flow field plates, gasket layer, carbon cloth and
the MEA, was clamped between the two enclosure plates by eight M8 screw joints and each having a
torque of about 35 kgf-cm.
2.2 Fabrication and measurement
Before testing the stack performance, the ohmic resistance and gas permeability were measured to
obtain optimal compressed condition of stack assembly, as shown in Fig.3. In the part of gas diffusion
layers maintained the compress value is just around 40%. The more detailed procedure can refer to
Weng et al. [1]. The MEA was made by Gore fuel cell Tech., which using Gore 5600 series, catalyst
reaction area 100 cm2. Before the testing performance experiment, the MEA was conditioned as
suggested by Gore Fuel Cell Tech., 0.6 volts for 1800 sec, then 0.4 volts for 1800 sec and last return to
open circuit voltage for 60 sec. The MEA conditioning was repeated about five ~ six times or more
until the performances achieved a relatively stable state, as shown in Table.2. Experiments could be
started after MEA conditioning. Waste heat was used to increase the cell temperature and cooling water
used to decrease the cell temperature. The gas is humidified in a bubbler. The humidified gas is mixed
with dry gas to set the desired dew point temperature. A Scribner 890 B Fuel Cell Test Loads is
available for a series type of experiments, such as scan voltage, constant voltage, constant current, and
dynamic load response. The cell is tested under constant voltage and constant current mode to generate
a polarization curve and stabilize the stack performance respectively. The dynamic load is operated for
developing the responses of the stack.
3. Results and Discussion
3.1 Performance of polarization curve
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Fig.4 shows the performance result of 100 cm2 single cell under different temperature conditions.
The cell is operated in 75℃ for gas humidified temperature, 1.2X for H 2 stoichiometric, 2.0X for AIR
stoichiometric. Moreover, this Fig.4 shows that the performance of the cell will be superior with the
increasing temperature. Fig.5 shows the I-V polarized curve and power density of per cell from
respective single cell, 2 cells, and four cells. The utmost performances of the single cell achieved at the
operating condition of 0.5V with power density of 0.55 W/cm2 and provide the power is 55 W.
However, the maximum performance of 2 cells will be at 0.98 V with power density of 0.48 W/cm2
and the power roar to 91.6 W. On the other hand, the fulfilment of four cells will be accomplished most
at 2V with the power density of 0.38 W/cm2, and the power reaching up to 156W.
Accordingly, it is evident within this operating condition single cell provides the better polarized
curve and power density among the performance of other cell. The observation illustrates that the
performance decay not only for the increase of ohmic resistance but also for the worsening of the water
flooding. In addition, the aggravating uneven distribution of fuel resulting form the increasing number
of cells or flow-field design and gas flow control may leads to the decay of the whole performance. On
that reason, the eventual part of the polarized curve in four cells abruptly drops due to the insufficient
supply of the fuel. On the other hand, it can evident here that the ultimate portion of the polarization
curve of the single cell and the two cells doesn’t decrease owing to the satisfactory amount of fuel
supply and less water flooding.
Fig.6 shows the hysteresis effect in the polarization curve of four cell stack, and the cathode with
different initial flow (IF), 5, 8 and 10 L. The results can be divided into 3.6 V step down voltage, and
another for 1.6 V step up voltage respectively. The dash-line shows the traditional method to measure
the IV curve; the performance of step-down is obviously providing the difference from the step-up.
Initially in the IV curves at the high voltage region shows the lower current density with
step-down (dash-line) due to the lower flow of the fuel at the anode and the cathode and water also
gradually blocks the gas channel with step-down the voltage and then decrease the performance.
However flooding can be minimize by the operating condition from lower voltage to the higher
voltage. On that occasion, hysteresis is not observed on the polarization curve and cell performance
increased significantly. Then, with the step up voltage of load, the lower current accompanied with
content of the lower amount of water. The water content was not made the low performance at the low
current output.
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Fuel flow in the cathode side at the different stoichiometric rates included with the 2.5, 3.0, 3.5
and 4.0 are shown respectively in the Fig.7. The stack with a high current output needs a high flow rate
of fuel to prevent the water flooding. At the area of ohmic resistance polarization losses is very
different from the stoichiometric of the fuel flow for 4.0 and 2.5.
Higher fuel flow rate is convenient
to remove water flooding and provide the better performance.
3.2 Dynamic load response of the short stack
Figure 8(a) shows the dynamic load response with the triangle wave of step-constant current
change. This experiment can simulate the vehicle acceleration situation. In order to simulate the
acceleration, constant different current shows the different power and the flow rate under different
stoichiometric. Each step was fixed with the current correspondingly at 70, 90, 110 and 130 A for 60
sec. This figure 8(a) shows that the current can responses immediately at this operating condition.
However, the power output is not getting high enough with higher current output. Therefore,
higher current outputs generates in the lower voltage and then resemble the power output is uncertain.
Understanding the better performance of the simulated vehicle acceleration we can change the loading
time from 60 sec to 5 sec as represents in the Fig. 8 (b). This study consider the three series of
condition, (1) Anode stoichiometric 1.2, Cathode stoichiometric 3.0, backpressure 0 psi. (2) Anode
stoichiometric 1.5, Cathode stoichiometric 4.0, backpressure 0 psi. (3) Anode stoichiometric 1.2,
Cathode stoichiometric 3.0, backpressure 15 psi. Compare with (1) and (2) condition, only condition (2)
can follow the current setting in first cycle, even create the flow rate to 1.5 of stoichiometric in anode
and 4.0 of stoichiometric in cathode. The loading time in 5 sec is not enough for obtaining the fuel cell
higher range of the current.
In the lower stoichiometric and the shorter response time cannot generated the higher range of the
current in the fuel cell. Therefore, the current cannot generate completely in time and the current setting
frequently moving onward.
Furthermore, the principle of fuel cell power equals the voltage plus current. In the higher current
region provides the voltage as much as inferior than the lower current region. On that basis, total power
is not so high enough in the higher current region.
As mentioned above, the flow rate can not satisfy the fuel cell to generate enough current in the
very short time of response.
In order to solve this problem, the back-pressure in fuel cell operated at
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the 15 psi, as condition (3). It is easily to observe condition (3) is the only one which can generate the
corresponsive current without operating in high flow rate of fuel. On that regards, fuel cell immediately
generate extremely high current for the acceleration of the motor and back pressure is effective to
handle out the above situation.
To compare the effect of loading time on power generating is represents in Fig.8(c). The three
conditions are 60 sec, 20 sec and 5 sec of constant current loading time. The flow rate in the high
stoichiometric, thus the fuel cell can generate the higher performance of the total power around 200 W
at 60 sec of loading time.
The condition of 20 sec can not maintain the high current and following
condition of 5 sec is unpredictable for sustain the higher current.
Given above, the shorter time of power generation in the fuel cell is more difficult for the current
response. Consequently, high current output at very short time will results the total power output is
unstable and decreased.
3.3 Stability of the short stack
Long-term operating shows the stability of fuel cell, and diagnoses the design of flow-field and
figure out the solution. Fig.9 shows the stability of the constant 1.9 voltage, in this figure also shows
the measurement of high frequency cell resistance. The meaning of resistance is the proton conduct
activity; the lower resistance means the better conduct activity. At the first curve is A/C stoichiometric:
1.2/3.0, at the second curve is A/C stoichiometric: 1.5/3.5. In the long-term operating of the fuel cell at
constant voltage, the power decreases slightly and consequently increases the resistance. After
increasing the fuel flow rate as like as the A/C stoichiometric:1.5/3.5 then the power increases
enormously and subsequently the resistance immediately decreased in a greater value. The reason is
that the water block the porous gas diffusion layer and catalyst layer during the long-term operation,
and finally causes the decay of proton conductivity and increased the resistance.
Especially in stack, the abundant amount of water generated frequently with the current output.
Water removing from the fuel cell operation is a challenge for the flow field system. On that occasion,
another long-term fuel cell performance experiment is performed is in the constant current. For fuel cell
long-term experiment, the constant current is generated the constant water and shown in the Fig.10.
The first and second curve is represented the constant 90, 70 A respectively. In this period, the water
flooding in high current output results the decay of fuel cell performance.
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Initially performance of the
fuel cell power output gradually decreased and to overcome this performance loss, the cathode outlet of
the stack is opened momentarily to apply a back-pressure release impulse to the cathode gas stream.
This removes the liquid water from the gas channels and then the cell voltage jump to higher
values for the impulse of back-pressure release and consequently maintain the trend to avoid decreasing
the power output.
This proves the long-term operation in stack required several times of water
purging to maintain the stability of higher output power at 170 W ( 0.42 W/cm2) .
4. Conclusion
The ohmic resistance and gas permeability test can ensure MEA is newly set and avoid the factor
that fabrication too many times and induce the increase of variable factors. The intricate design of stack
used in this experiment in the flow field has the capability to improve the smoothness of gas flowing
into the fuel cell. The cell devise in this study of 1, 2 and 4 cells has provide the best performance at
70℃ of cell temperature, and 75℃ of humidified temperature with 0.55, 0.47 and 0.39 W/cm2 of peak
power density respectively. The hysteretic effect in the polarization curve experiment has different
performance; step-down and step-up of voltages and has the different experimental results. Step-up of
the scanning voltage will have the better performance because of the water flooding effect is smaller.
However flooding can be minimize by the operating condition from lower voltage to the higher voltage.
On that occasion, hysteresis is not observed on the polarization curve and cell performance increased
significantly.
Dynamic load-response provides the information of stack operation under strict condition. High
power output in the short loading time and simultaneously to generate the high current output of the
stack is difficult to obtain, unless operating under high H 2 flow rate or raising the back pressure.
In general fuel cell experiment, the IV curve, constant voltage, constant current are operating
under the long loading time. In the long term experiments, the stack operation’s skill is so important to
maintain the fuel cell performance and stability. There needs some operating skill to maintain the
performance in the stack operation, such us purged water is a good way to maintain the stability.
Acknowledgments
We would like to acknowledge gratefully for the financial support by the Technology Development
Program to Academia, DIT Department, the Ministry of Economic Affairs, Taiwan, R.O.C. under the grant
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No.93-EC-17-A-05-S1-0012. Also, the Aim for the International Top University Program under the grant
No.0950026846, and the Research Team members in Yuan Ze Fuel Cell Center.
References
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Captions
Fig.1. (a) Schematics of fabricated PEM fuel cell short stack; (b) Picture of short stack under operation
Fig.2. Special design of bipolar plate, (a) Front side of bipolar plate. (b) Back side of bipolar plate. (c) Manifold
sealing for gas inlet
Fig.3. Design, fabrication and experimental procedure
Fig.4. Effect of cell temperature on unit cell performance. Anode/Cathode:1.2/2.5 stoich, Cell temp. 70℃. Humid.
temp.75℃ , 0psig
Fig.5. Polarization and power density curve/per cell, Cell temp.70℃, H2 hum.temp.75℃, Air-hum.temp.75℃, H2
stoich. ratio 1.2, Air sto.ratio 2.5
Fig.6. The hysteresis effect of the stack performance. Real line: The voltage is scanning from low to high voltages.
Dash ling: The voltage is scanning from high to low voltages. Anode/Cathode:1.2/2.5 stoich, Cell temp. 70℃.
Humid. temp.75℃ , 0psig
Fig.7. Effect of cathode stoich. rates on the stack performance, cell temp.60℃, humid. temp. 60℃,
backpresure:15psi
Fig.8(a). Dynamic load response with step-triangle wave const. current, 60 sec. of each step current, cell
temp.60℃, humid. temp. 60℃, 15psi
Fig.8(b). Dynamic load response with high freq. step-triangle wave const. current, 5 sec. of each step current, cell
temp.60℃, humid. temp. 60℃, 0psi
Fig.8(c). Dynamic load response with variable freq. step-triangle wave const. current, 60 sec., 20 sec. and 5 sec. of
each step current (T/P=Time/Point), cell temp.60℃, humid. temp. 60℃, 0psi, Anode/Cathode Stoich.1.5/4.0
Fig.9. Stability of constant voltage, 1.9V, without water purge procedure, cell temp.60℃, humid. temp. 60℃, 15psi
Fig.10. Stability of const. current, 90A, 70A, with water purge procedure, cell temp.60℃, humid. temp. 60℃,
15psi, H2 stoic.2.0, Air stoic.3.5
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