4. Conclusions

The 50-W bipolar fuel cell stack was extensively evaluated under various
constant discharge currents at different environmental temperatures. The
effects of environmental temperatures on the maximum stable power
outputs are summarized in table 1. A 62-W long-term performance was
obtained at low environmental temperature (-5 °C). As environmental
temperature was increased, the stack surface temperature increased
proportionally, causing heat to dissipate from the stack. When the stack
surface temperature reached 43°C, the stack voltage dropped signifi-
cantly within a short time. The effect of environmental temperatures on
the minimum discharge current—causing the stack voltage to drop to
zero as the stack temperature rises—is summarized in table 2. At 30°C,
stack performance was unstable at a current of 1.0 A. However, at-5 °C,
stable performance was observed until a current of 3.0 A was reached.
Improving heat dissipation efficiency will enhance the stack performance

significantly.
Table 1. Effect of Environmental Maximum stable Discharge Stack surface
environmental te
femperature stack power current temperature

temperature on
maximum stable stack eC) Ww) (A) (°C)
power output. 5 62 25 31

10 55 2.0 37

20 44 15 39

25 33 1.0 39

30 16 0.5 39
Table 2. Effect of Environmental Minimum Stack surface
temperature on temperature discharge current temperature
minimum discharge (°C) (A) (°C)
current: causing stack
voltage to drop to 35 3.0 38
zero. 10 25 43

20 2.0 44

25 15 48

30 1.0 43

15



Figure 8. Constant f@ *
current discharge
performance of 50-W 40
fuel cell stack
(CECOM 764) at 10°C: gg,
(a) voltage vs time and
(b) power vs time. H 30 Ep
inlet pressure=3 psi;
air inlet pressure = 2
5 psi; H an © 25
psi; H, purging: S
10 s/5 min; cooling by
fan; dry H, and dry air % 20
fill. Fy
15
i 1.0A(V)
10 me 154A) 4
5 —o- 20A(V) |_|
—@ 25A(V)
0 +
t) 20 40 60 80 100 120 140
Time (min)
o) *

Stack power (W)

20:

10: —1.0A(W) —i 1.5A(W)

—O-2.0A(W) —@ 25A(W)
T I

0 T T
0 20 40 60 80 100 120 140

Time (min)

However, stack performance dropped at 2.5 A discharge current. The data
from figure 8(a) are also plotted for power versus time in figure 8(b).
Approximately 55 W constant performance was obtained at a current of
2.0 A. However, when the discharge current was increased to 2.5 A, the
stack power dropped off within 45 min. Based on the results shown in
figures 6 to 8, we expected the stack performance to improve at lower



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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED
February 2000 Progress, 10/98-9/99
‘atm anpsuatme Performance and Evaluation of Bipolar Fuel Cell 5. FUNDING NUMBERS
Stacks DA PR:—
PE: 62120A

s.auttors) Deryn Chu, Rongzhong Jiang, Charles Walker (ARL),
Richard Jacobs, Krist Gardner, and Jim Stephens (Power Sources
Division, U.S. Army CECOM)

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8, PERFORMING ORGANIZATION
U.S. Army Research Laboratory REPORT NUMBER
Attn: AMSRL-SE-DC email: dchu@arl.mil ARL-TR-2064

2800 Powder Mill Road
Adelphi, MD 20783-1197

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING
U.S. Army Research Laboratory AGENCY REPORT NUMBER

2800 Powder Mill Road
Adelphi, MD 20783-1197

11. SUPPLEMENTARY NOTES

ARL PR: JNV4VV
AMS code: 622120.H16

Yaa, DISTRIBUTIOWAVAILABILITY STATEMENT Approved for public release; 72b, DISTRIBUTION CODE
distribution unlimited.

13. ABSTRACT (Maximum 200 words)
Under a joint technology planning annex (TPA) agreement, fuel cell groups at the U.S. Army

Research Laboratory (ARL) and the U.S. Army Communications-Electronics Command (CECOM)
worked together to develop Army power sources for soldier applications. Two 50-W bipolar fuel
cell stacks designed by CECOM were extensively evaluated. The performance of the stacks
depended significantly on the environmental temperature. Decreasing environmental tempera-
ture granted better heat dissipation in the stacks, resulting in improved stack performance.
Long-term performance of 62 W was obtained at low temperature (-5 °C). Higher environ-
mental temperatures caused an increase in stack surface temperature. When the stack surface
temperature reached 43 °C, the stack voltage dropped to zero within a short time. The maximum
power density for long-term operation was 97.3 W/kg, or 167 W/L. The average hydrogen
utilization efficiency was 95 percent. The water production efficiency was dependent on the
discharge currents, varying from 40 percent (at 1.0 A) to 90 percent (at 2.5 A).

14. SUBJECT TERMS 15. NUMBER OF PAGES
Fuel cell, bipolar 25

16. PRICE CODE

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19, SECURITY CLASSIFICATION 120. LIMITATION OF ABSTRACT
OF REPORT OF THIS PAGE OF ABSTRACT
Unclassified Unclassified Unclassified UL
NSN 7540-01-280-5500 ‘Standard Form 298 (Rev. 2-89)
Prescribed by ANSI Sid, 239-18
200-102

19



4.

Contents

Introduction

Experiment

Results and Discussio:
3.1 CECOM Fuel Cell Stack Tag 764 .
3.1.1 Stack Performance .........
3.1.2 Optimizing Fuel Cell Stack Performance
3.1.3 Hydrogen Utilization Efficiency .
3.1.4 Water Production Efficiency
3.2 CECOM Fuel Cell Stack Tag 762 ...
3.2.1 Stack Performance .....
3.2.2 Constant Current Discharge

CONCIUSIONS ........-ssesssesssseesseeessesessnecesnecsencesuneesasectsnessutassneecsnessaseesnee

References .........s:ssscsesssssessseeseesneesnecnecstenneenneennes

Distribution

Report Documentation Page

y

Be

12.
13.

1.
2.

FS pana

Figures
Dimensional drawing of 50-W fuel cell stack sessteesessee B
Discharge performance improving with electrolyte membrane humidification until ,
reaching maximized performance for 50-W fuel cell stack at 10 °C.... 74
5

Discharge performance of 50-W fuel cell stack at different temperatures.
Constant current discharge performance of 50-W fuel cell stack at different
environmental temperatures ....
Effect of discharge on stack surface temperature at various s environmental
temperatures for 50-W fuel cell stack
Constant current discharge performance of 50-W fuel cell stack at .
Constant current discharge performance of 50-W fuel cell stack at 20°C.
Constant current discharge performance of 50-W fuel cell stack at 10 °C.
Constant current discharge performance of 50-W fuel cell stack at -5 °C.
Effect of current on Hz flow rate and H, efficiency for 50-W fuel cell stack .
Effect of environmental temperature and discharge current on product

water efficiency for 50-W fuel cell stack ..
Discharge performance of 50-W fuel cell stack at 25 Cc.
Constant current discharge performance of 50-W fuel cell stack at 25 °C
and 64% HUMIity -..csssesesssseescececssneeteeeeeeeessnsnmeeseenesnessans

Tables

Effect of environmental temperature on maximum stable stack power output
Effect of temperature on minimum discharge Current...

iii



References

16

A. J. Apply and E. B. Yeager, Energy 11 (1986), 137.

D. Linden, Handbook of Batteries and Fuel Cells, McGraw-Hill, Inc. (1984),
pp 41-43.

S. Srinivasan, E. A. Ticianelli, C. R. Derouin, and A. Redondo, J. Power
Sources 22 (1988), 235.

4. K. Prater, J. Power Sources, 29 (1990).
5. M.S. Wilson and S. Gottesfeld, J. Appl. Electrochem. 22, 1 (1992).

Guenther Scherer, “PEFC Research at Paul Scherrer Institute,” Proceedings
of the IEA Workshop for the PEFC Annex, 14-15 September 1992, Ottawa,
Canada.

* “How to Build a Clean Machine,” Business Week, 27 May 1996.

8. “Detroit’s Impossible Dream?” Business Week, 2 March 1998.

10.

11.
12.
13.
14.

D. Chu, S. Gilman, and L. Jarvis, Electrochemical Society Meeting, Fall
1997.

D. Chu, R. Jiang, and Charles Walker, accepted for publication by J. Appl.
Electrochem.

D. Chu and R. Jiang, J. Power Sources 80 (1999), 226-234.

D. Chu and R. Jiang, accepted for publication in J. Power Sources.
R. Jiang and D. Chu, 6th Grove Fuel Cell Seminar, September 1999.
D. Chu and R. Jiang, Proc. J. Electrochem. Soc. PV 98-27, 470.



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3.2.1

3.2.2

Figure 12. Discharge.
performance of 50-W
fuel cell stack
(CECOM 762) at 25 °C.
Dry air and dry H,
fill; H, pressure

=3 psi; air pressure
= 5 psi; H, purging:
10 s/5 min; at 2.03 A,
stack temperature
increased to 52 °C.

CECOM Fuel Cell Stack Tag 762

This stack leaked during evaluation, so only limited data were obtained.

Stack Performance

Figure 12 shows a polarization curve for the fuel cell stack at 25°C. The
open circuit voltage was approximately 42 V. Because of poor heat
dissipation, which caused the electrolyte membrane to dehydrate, the
voltage-current curve decreased significantly at currents greater than
1.0 A. The maximum power output was approximately 39 W.

Constant Current Discharge

Figure 13(a) shows voltage versus time plots for three different discharge
currents for the fuel cell stack at 25°C. At 1.0 A discharge current, the
stack voltage was very stable. When the discharge current increased to
1.25 A, the voltage decreased a little with time. At 1.5 A discharge current,
the stack voltage dropped very fast within 20 min. Figure 13(b) shows the
corresponding stack power versus time plots at the same three discharge
currents: at 1.0 A and 25 °C, a stable power output of approximately 33 W
was obtained.

45 45
40 40
35 35
30 30
= SI
& 25 2 >
8 » 2
% 20 3
a x
15 16
10 7 10
—- Voltage (V)
54 H 5
+ Power (W) :
ot +—_+—_+—_4 °
00 02 04 06 08 10 12 14 16 18 20 22 24

Current (A)

13



Figure 3. Discharge
performance of 50-W
fuel cell stack
(CECOM 764) at
different
temperatures. Dry air
and dry H, fill; H,
pressure = 3 psi; air
pressure = 5 psi; H,
purging: 10 s/5 min.

3.1.2

40 _
35
+60
= #
© Te& =
8 25 re
$ tao &
x
g 70 s
° doo &
15
10 +2
51 -o- 20°C(vy) = 10°C(V) | 4 10
— 20°C (W) 9 -@ 10°C (W)
° : 0
0.0 05 1.0 15 2.0 25 3.0 3.5

Current (A)

Optimizing Fuel Cell Stack Performance

Based on the stack performance results given in figures 2 and 3, various
constant discharge currents at different environmental temperatures
(from —5 to 30 °C) were performed for longer (2-hr) evaluation periods.

Effect of temperature during constant discharge current.—Figure 4 shows the
stack performance under constant discharge current (I= 1.0 A) at different
temperatures. At lower temperature (10 °C), the stack has a lower initial
voltage; however, the long-term performance of the stack at 10 and 20°C
is almost identical. Approximately 32 W output was obtained at 1.0 A
constant current discharge. At 30 °C, the stack has good performance
during the first ~20 min, after which the voltage rapidly drops to zero
within 40 min. In order to understand the reason for the poor perform-
ance at 30 °C, we monitored the stack surface temperature during
operation. Figure 5 shows the stack surface temperatures at different
currents and environmental temperatures; the stack surface temperatures
increase significantly with increasing discharge current and environmen-
tal temperature. At 1.0 A and 30°C environmental temperatures, the stack
surface temperature is as high as 43°C. Undoubtedly, the temperature
inside the stack would be much higher than 43 °C, causing the electrolyte
membrane to dehydrate, which would result in a large stack voltage
drop.



Future man-portable power sources systems will need high power
density and.a long operating life, and they will have to be small and
lightweight. Using a lightweight PEMFC stack as a portable power source
can reduce the physical burden on the carrier. Several key areas need to
be addressed to successfully produce the desired high-performance,
lightweight, ambient-temperature and -pressure fuel-cell system [9-14]:
(1) thermals and heat transfer management, (2) water management,

(3) environmental factors, (4) hydrogen storage conditions, (5) determina-
tion of the best optimum stoichiometry of fuel and oxidant, and (6) sys-
tem integration for high-performance PEMFCs.

Fuel cell groups from the U.S. Army Research Laboratory (ARL) and the
U.S. Army Communications-Electronics Command (CECOM) worked
together on a PEMFC project under a joint technology planning annex
(TPA) agreement (TPA SE-CE-O2-98). In this report, we give performance
evaluation results for the bipolar fuel cell stacks. Two identical 50-W
bipolar fuel cell design stacks (CECOM tags 764 and 762) were evaluated.


1. Introduction

A fuel cell is an electrochemical device that directly converts the chemical
energy of the reactants, a fuel and an oxidant, into electrical power [1,2].

A fuel cell will continue to operate as long as the externally stored
reactants are supplied. Fuel cells are more efficient than combustion
technology, partly because they avoid the Carnot cycle limitation. Since a
fuel cell generates electricity without combustion, it does not produce the
air pollutants that are byproducts of the combustion process. Similar to an
internal combustion engine, and unlike a battery, a fuel cell does not
require recharging—it will provide power indefinitely, as long as fuel is
supplied.

The basic H,/air polymer electrolyte membrane fuel cell (PEMFC)
consists of (a) an anode (carbon-supported platinum black), (b) an
electrolyte (Nafion), and (c) a cathode (carbon-supported platinum black).
Nafion is a cation-exchange polymer membrane that has a perfluorinated
polymer backbone with sulfonic acid substituents that are periodically
attached. Nafion exhibits exceptionally high electrochemical, thermal, and
chemical stability in fuel cell environmental conditions. In order to
achieve high voltage and high power, a huge number of single cells are
assembled together to act as a power source in practical applications.
Such an assembly is called a fuel cell stack.

For an H,/O, fuel cell, hydrogen would become oxidized at the anode
via the half reaction

H, ——> 2H* + 2e- Po=0V, (1)
whereas oxygen would become reduced at a cathode via the reaction

O, + 4H* + 4e° —> 2H,O F0=1.23V. (2)
The overall reaction is

2H, +O, —-> 2H,O £2 =1.23V. (3)

The net result is the generation of electricity, heat, and water, as shown in
equation (3).

PEMECs are the most desirable portable power supply device, primarily
because they are lightweight and have a high power density. A PEMFC
power source is being developed for a wide variety of applications that
now use batteries, from laptop computers to electric vehicles [3-8]. A
PEMEFC system can be used with a replaceable fuel cartridge (e.g.,

tanked /metal or chemical hydrides) for practical operations without any
environmental concern. Small fuel cell systems, if successfully developed,
can replace batteries by directly providing power.



Figure 13. Constant
current discharge
performance of 50-W
fuel cell stack
(CECOM 762) at 25 °C
and 64% humidity:

{a) voltage vs time and
(b) power vs time. H,
inlet pressure = 3 psi.
Air inlet pressure =

5 psi. H, Purging:

10 s/5 min. Cooling by
fan. Dry H, and dry
air fill.

14

(a) 45
40
35
30) ooo}
; pRraiorones?
& 254
g
$
S 20
&
ao
15
He 10A)
10 oe 125A) |_|
5 —“O- 15A(V) -4
0
0 20 40 60 80 100 120 140
Time (min)
©)

=
3
Fs
&
=
8
16
° e10A) |
1.25 A(V)
8 L|
—@15A(V)
0.
0 20 40 60 80 100 120 140

Time (min)


3. Results and Discussion

3.1

3.1.1

Figure 2. Discharge
performance
improving with
electrolyte membrane
humidification until
reaching maximized
performance for 50-W
fuel cell stack
(CECOM 764) at 10 °C.
Dry air and dry H,
fill; H, pressure

=3 psi; air pressure

= 5 psi; H, purging:
10 s/5 min.

CECOM Fuel Cell Stack Tag 764

Stack Performance

Figure 2 shows the polarization curves for the fuel cell stack at 10 °C, fed
with dry H, and dry air at different discharge currents. The open circuit
voltage was about 42 V. Initially, the stack power output increased at low
currents and plateaued at currents higher than 2 A; then it decreased at
currents higher than 2.5 A. This behavior is probably caused by incom-
plete hydration of the electrolyte membrane. The maximum power
output is approximately 50 W. As operating time and current increased,
the membrane became more hydrated and stack performance improved
significantly. Maximum power output was obtained at currents of 3.0 A
(~67 W), and plateaus at currents higher than 3 A.

Figure 3 shows the stack performance at 10 and 20 °C environmental
temperatures, under different discharge currents. Performance at 10 °C is
slightly better than that at 20 °C.

45 80
40 + 70
Foren r
35 | 60
30
= pe =
B 25 5
g t4o &
2 a0 g
j fA i
8 20 &
B45 a
to 20
|-m@— Initial measured (V)  -®~ Maximized (V)
5] L+ 10
-O- Initial measured (W) — —O— Maximized (W)
° } f f i f °
0.0 05 1.0 15 2.0 25 3.0 3.5

Current (A)



ARMY RESEARCH LABORATORY

Performance and Evaluation of
Bipolar Fuel Cell Stacks

Deryn Chu, Rongzhong Jiang, Charles Walker, Richard Jacobs,
Krist Gardner, and Jim Stephens

ARL-TR-2064 February 2000

20000321 106

Approved for public release; distribution unlimited.

a)

DTIC QUALENY tho ee



Fi;

igure 6. Constant

current discharge
performance of 50-W
fuel cell stack
(CECOM 764) at 30 °C:
(a) voltage vs time and

(b)

) power vs time. H,

inlet pressure = 3 psi;
air inlet pressure =

5 psi; H, purging:

10 s/5 min; cooling by

fa

fill.

n; dry H, and dry air

(a) 45

DOO Ir

25

20

Stack voltage (V)

15

10

0.5 A(V)

—e 10a)! |

|

T
0 20 40 60 80 100 120 140
Time (min)

°

&® 8 & 8

Stack power (W)
8

DOC eee eee

05 A (W) | |
—# 1.0A(W)

+
0 20 40 60 80 100 120 140
Time (min)

obtained at 0.5 A and 30 °C environmental temperature. Figure 7(a) shows
the voltage versus time plot for constant current discharge at 20°C.
Compared with the results at 30 °C, the performance of the stack was
much improved. At 1.0 and 1.5 A discharge currents, the stack voltage
was stable for the entire test period. However, when the discharge current
was increased to 2.0 A, the voltage dropped again. From the data shown
in figure 5, we know that the stack surface temperature had increased to
44°C at 2.0 A. At this temperature, the membrane electrolyte was



Figure 4. Constant
current discharge

(I= 1.0 A) perform-
ance of 50-W fuel cell
stack (CECOM 764) at
different environ-
mental temperatures.
H, inlet pressure

=3 psi; air inlet
pressure = 5 psi; H,
purging: 10 s/5 min;
cooling by fan; dry H,
and dry air fill.

Figure 5. Effect of
discharge on stack
surface temperature at
various environmen-
tal temperatures for
50-W fuel cell stack
(CECOM 764).

45 45
40 40
35 35
(SESUEUSUUUGEEREGSEOEEENEO STOR]
s foals 30
3 z
8 25 25 5
3 % a
&

% 20 2 §
@ Oo
15 15

—e 10°C
L+ 10
10 “20°C
5 ~o-30°c [75
0 f 0
0 20 40 60 80 100 120 140
Time (min)
50
45 wi
S 40 ZA
: Wai Xx VAI
3
5 a5
5
z "4
§
8
g 9
2 Envir. temp =-5 °C
B os L
a Y VA —t-Envir. temp = 10°C
Envir. temp = 20°C
20 al H
=x Envir. temp = 30 °C
15 +
0.0 05 1.0 15 2.0 25 3.0 35
Current (A)

Effect of temperature at different discharge currents.—Figure 6(a) shows the
voltage versus time plots for constant current discharge at 30°C. At 0.5A
discharge current, the voltage was very stable and lasted more than

120 min. The stack voltage decreased slightly after over the first 40 min
and then stabilized. For a discharge current of 1 A, the stack voltage
dropped very fast, reaching zero in only about 40 min. The corresponding
power versus time plot is shown in figure 6(b). A 16-W output was



2. Experiment

Figure 1. Dimensional
drawing of 50-W fuel
cell stack (42 cells,
CECOM 762 or 764).
Dry weight (before
electrolyte membrane
was humidified):

637 g (including two
electric wires and two
H, tubes, and exclud-
ing air or water tubes);
wet weight (after
membrane was
humidified): 643 g.

In this experiment, high-purity hydrogen (99.99%) was used as the fuel
and compressed air as an oxygen source. The environmental temperature
was controlled by a Tenney environmental chamber (model BTRC), which
was programmed through a computer with Linktenn II software. An
Arbin battery tester (BT-2043) was used to administer constant current
discharge tests. A Hewlett-Packard electronic load (model 6050A) and
multimeter were used to measure current and voltage when the stack
voltage was greater than 35 V. A Matheson digital flowmeter (LFE 1000H)
was used to measure hydrogen flow, and a hydrogen purger was set to
purge for 10 s every 5 min during all measurements. The inlet hydrogen
and air pressures were adjusted to 3 and 5 psi, respectively. An ac electric
fan (~10 W) was pointed toward the stack during testing to dissipate
excess heat. A thermocouple was attached to the top surface of the stack
to measure stack surface temperature.

CECOM provided two identical fuel cell stacks for evaluation (CECOM
tags 764 and 762). CECOM did not relate the nominal voltage and power
ratings. Each stack contained 42 single cells connected in series. The dry
stack weight was 637 g, and the wet stack weight (after humidification)
was 643 g. (The stacks were humidified before evaluation.) The electrode
area was about 17.6 cm? (4.0 x 4.4 cm). The dimensions of the stack are
given in figure 1.

5.6 cm

8.5cm Electrode: 4.0 x 4.4 cm2



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17



Abstract

Under a joint technology planning annex (TPA) agreement, fuel cell
groups at the U.S. Army Research Laboratory (ARL) and the U.S.
Army Communications-Electronics Command (CECOM) worked
together to develop Army power sources for soldier applications.
Two 50-W bipolar fuel cell stacks designed by CECOM were
extensively evaluated. The performance of the stacks depended
significantly on the environmental temperature. Decreasing
environmental temperature granted better heat dissipation in the
stacks, resulting in improved stack performance. Long-term

- performance of 62 W was obtained at low temperature (-5 °C). Higher

environmental temperatures caused an increase in stack surface
temperature. When the stack surface temperature reached 43 °C, the
stack voltage dropped to zero within a short time. The maximum
power density for long-term operation was 97.3 W/kg, or 167 W/L. The
average hydrogen utilization efficiency was 95 percent. The water
production efficiency was dependent on the discharge currents,
varying from 40 percent (at 1.0 A) to 90 percent (at 2.5 A).



Figure 9. Constant
current discharge
performance of 50-W
fuel cell stack
(CECOM 764) at -5 °C:
(a) voltage vs time and
(b) power vs time. H,
inlet pressure = 3 psi;
air inlet pressure =

5 psi; H, purging:

10 s/5 min; cooling by
fan; dry H, and dry air
fill.

10

temperatures. Figure 9(a) shows the voltage versus time plots for constant
current discharge at —5 °C. Stack performance was very stable up toa
current of 2.5 A, but failed at a discharge current of 3 A. Figure 9(b) shows
the corresponding power versus time plots. An approximately 62-W

(a) 45

40

35

30

25

Stack voltage (V)

He 25A(V)

: “1 20A(V)
:
5

—O- 3.0 A(V)

0 20 40 60 80 100 120 140
Time (min)

Stack power (W)
é
OFO-0-0-O-070D-00)

20a) — 25AW)
10 -O- 3.0 A(W)

0 20 40 60 80 100 120 140
Time (min)



Figure 11. Effect of (a) 100
{a) environmental
temperature (at 2.0 A
discharge) and 80
(b) discharge current —_—*
(at 10 °C environmen- —— a |
tal temperature) on = |
product water = 60
efficiency for 50-W 2
fuel cell stack 2
(CECOM 764). 2
2 40.
Ss
20.
0.
10 5 5 10 15 20 25
Temperature (°C)
& ©
a
80. ZO
~~
fi
z 60:
3
3
3 40:
s
20
0
0.0 05 1.0 16 2.0 25 3.0
Current (A)

12


Army Research Laboratory

Adelphi, MD 20783-1197

ARL-TR-2064 February 2000

Performance and Evaluation of
Bipolar Fuel Cell Stacks

Deryn Chu, Rongzhong Jiang, Charles Walker

Sensors and Electron Devices Directorate, ARL

Richard Jacobs, Krist Gardner, and Jim Stephens
Power Sources Division, U.S. Army CECOM

Approved for public release; distribution unlimited.



The findings in this report are not to be construed as an
official Department of the Army position unless so
designated by other authorized documents.

Citation of manufacturer's or trade names does not
constitute an official endorsement or approval of the use
thereof.

Destroy this report when it is no longer needed. Do not
return it to the originator.



Figure 7. Constant
current discharge
performance of 50-W
fuel cell stack
(CECOM 764) at 20 °C:
(a) voltage vs time and
(b) power vs time. H,
inlet pressure = 3 psi;
air inlet pressure =

5 psi; H, purging:

10 s/5 min; cooling by
fan; dry H, and dry air
fill.

(a) 45

40

35

Toor

TOO ooo

Stack voltage (V)

+ 10a)
a 15A(V)

—o- 2.0A (vy) ]

0 20 40 60 80 100

Time (min)

120 140

() 80

70

ainsi

g
3

Stack power (W)
>
&

COC TS er

ae

+ 1.0A(W)

10

—O- 2.0 A (W)

He 154)

60 80 100

Time (min)

120 140

dehydrated. Figure 7(b) shows the corresponding power versus time
plots. The stable stack power outputs were 44 and 33 W for 1.5 and 1.0 A,
respectively. Figure 8(a) shows the stack voltage versus time plots for
different constant current discharges. Even when the discharge current
increases to 2.0 A, the stack could maintain stable discharge performance.



3.1.3

3.14

Figure 10. Effect of
current on H, flow
rate and H, efficiency
for 50-W fuel cell
stack (CECOM 764,
containing 42 single
cells connected in
series).

constant output was obtained at 2.5 A during the entire 120-min
experiment. When discharge current was increased to 3.0 A, the stack

power dropped to zero within 20 min. Many problems (for example, with

heat dissipation, water management, and supplies of oxygen and
hydrogen) may occur as a consequence of high discharge current (3 A).

Hydrogen Utilization Efficiency

Figure 10 shows that the effect of discharge current on hydrogen flow
rates is linear. It also shows the efficiency of hydrogen utilization: the
hydrogen utilization efficiency was approximately 95 percent for
discharge currents of 1.0 A and above.

Water Production Efficiency

Figure 11(a) shows the effect of environmental temperature on water
production efficiency, which increased only slightly from -5 to 20°C.
Figure 11(b) shows that an increase in discharge current increased water
production efficiency.

1000 F120
900
+100
£. -2
800 Pe) oo
a
= 700
E ¢ + 80
=
= 600
g
3 500 60
> YY
400
4 40
300
200 1—
yf © H, flow (mVmin) 4 20

100 WA © Hy efficiency (%)
0 f | 0

0.0 05 1.0 15 2.0 25 3.0 3.5
Current (A)

H, efficiency (%)

11