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DEFENSE ADVANCED RESEARCH PROJECTS AGENCY
MICRO HYDROGEN-AIR FUEL CELLSYEAGER CENTER FOR ELECTROCHEMICAL SCIENCES AND ELECTRONICS DESIGN CENTER
CASE WESTERN RESERVE UNIVERSITY
A Micro Hydrogen-Air Fuel Cell
PI: Robert F. SavinellCo-I(s): Professors C. C. Liu, M. H. Litt,
and J. S. WainrightCase Western Reserve University
Ernest B. Yeager Center for Electrochemical SciencesCleveland, OH 44106-7217
Overview
• Background & Motivation– On-board power is required to enable wireless
application of MEMS sensors and actuators. Polymer electrolyte fuel cell technology can provide both the steady-state and pulse power required by these devices, and can be integrated with the MEMS fabrication.
• Project Goal– Develop a microfabricated hydrogen-air fuel cell with
integrated fuel storage for autonomous operation, capable of delivering 10 mW continuous and 100 mW(pulse) power.
Microfabricated Fuel Cell Concept
• Thick Film Fabrication• Polymer Electrolyte• On-board Fuel Storage• High Efficiency • Ambient Temperature • Ambient Humidity
H2
Membrane
CatalystLayer
CarbonFlow Field
SiliconSupport
A) Silicon masked and etched
B) Alumina laser drilled sheets
C) Polymeric Porous Filters
Related microfabricated approaches: F. Prince of Stanford, J. Meyers of Lucent, W. Smyrl of Minnesota
FABRICATION CHALLENGES
• Inks for printing electrodes and current collectors
•Casting membranes with suitable electrochemical properties
•Substrate designs and materials
0.00 0.01 0.02 0.03 0.040.00
0.25
0.50
0.75
1.00
I (Amps/cm 2 )
E (V
olts
)
H2 - AIR5% H2/N2 - AIR
H2-AIR
5%H2-AIR
FUEL CELL TEST ON FREE-STANDING NAFION 117, 45 micron thick current collector
0.00 0.01 0.02 0.03 0.040.00
0.25
0.50
0.75
1.00
I (Amps/cm2
)
E (V
olts
)
Thinner goldThicker gold50
micron
30 micron
EFFECT OF THICKNESS OF ERCON GOLD CURRENT COLLECTOR-5%H2-air
Microfabricated cells on porous substratesAll fuel cell components thick film printed
• Humidified Hydrogen
• Air at 42% RH, 75 F
• Cell Area = 0.25 cm2
• I-V curve recorded at 2 mV/s
Limiting currents of 10-30 mA/cm2
due to hydrogen permeation through porous substrate beneath anodes
MICROFABRICATED FUEL CELL PERFORMANCE
Power Output at 0.5V/cell:2 mW/cm2 – Continuous operation5 mW/cm2 – 3-5 minute duration50 mW/cm2 - 10 msec pulse
10 -4 10-3 10-2 10-10
0.25
0.50
0.75
1.00
I (Amps/cm2)
E (V
olts
)
307A-L307A-MFree-standing Cell
0 20000 40000 600000
0.0025
0.0050
Time (Sec)
I (A
mps
/cm
2 )Microfabricated cells on porous substratesAll fuel cell components thick film printed
• Humidified Hydrogen
• Air at 48% RH, 75 F
• Cell Area = 0.2 cm2
MICROFABRICATED FUEL CELL PERFORMANCE
Continuous Operation at 0.5V for 20 hours
average current = 3.5 mA/cm2 (1.75 mW/cm2)
No permanent effects after over 50,000 pulses deliveredat 12.5 – 25 mW/cm2 during pulse.
Single pulse capability: 50 mW/cm2 (V min >0.5 V)
• Cell tested on H2 / Air at 48%RH
• Cell Capacitance = 13.5 mF/cm2
• Cell Area = 0.2 cm2
• Duty Cycle: 90% at 0.5 mA/cm2
10% at higher current
• Pulse Width = 10 msec
Pulse capability depends strongly on internal resistance – which is controlled by relative humidity.
Below 40%RH pulse capability is significantly decreased. At over
80%RH pulse power capability can double.
Pulse Ht. time to 0.5V cut-off
25 mA/cm2 90 min
50 mA/cm2 10 min
FUEL CELL PERFORMANCE – PULSE POWER
0
0.1
0.2
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0 0.05 0.1 0.15 0.2 0.25
Time / seconds
Cel
l Vo
ltag
e
0
5
10
15
Cu
rren
t / m
A
Wireless Sensors –A Potential Application?
Wireless Sensor
Power, Sensor, Radio and Electronics in one package
Operational life 2-3 wksContinuous power 100 ?W
Xmit power 50 mW
10 ms pulse every hourtotal energy ? 50 mWh
FuelReservoir
Micro –machinedFlow Field
Air Air BreathingCathode Controller
Unit
Sensor
_+
Sensor
• These targets have been exceeded with a single cell of 1 cm2 area.
• The weight of fuel required is <0.2g of 30% NaBH4 solution.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0 0.2 0.4 0.6 0.8 1
t*sigma/siemens
L/c
m
10 mA/cm2
20 mA/cm2
50 mA/cm2
100 mA/cm2
560 A 50%por. Au
6 micron 50%por. graphite
100 mV conductor IR-Loss
L={2x? V(t*sigma)/iavg}1/2
iavg
i
L? V
t
Sigma=conductivity
LIMIT ON ELECTRODE LENGTH FOR CO-PLANAR EDGE COLLECTION
•Electrolyte Issues:
•High Conductivity, Low Permeability over Range of Ambient Conditions
• >1 mS/cm (125 mV IR at 50mA/cm2, 25 micron)
•15%<RH<100%
•Low Crossover
•Permeable for electrode access
•Favorable environment for reaction kinetics
•Pinhole-free, Uniform Thickness Films ~25-50 microns
•Adherent to Substrate to Seal Around Anode Periphery
•Patterned to Allow for Cell to Cell Interconnections
H2H3O+ O2
H2O
e- e-
H2 H+
e-
0
1
2
3
4
5
6
0 5 10 15 20 25 30
MEMBRANE THICKNESS, MICRONS
HY
DR
OG
EN
CR
OS
S O
VE
R C
UR
RE
NT
, m
A/c
m2
0.82
0.83
0.84
0.85
0.86
0.87
0.88
0.89
0.9
0.91
OP
EN
CIR
CU
IT C
EL
L P
OT
EN
TIA
L,
VO
LT
S
NAFION 1100
Estimating Hydrogen Cross-over and Effect on Open Circuit Potential
H2H2
O2
H2O
NAFION ELECTROLYTES FOR MICROFABRICATED FUEL CELLS
• DIFFICULTIES WITH NAFION– LOW SOLUTION VISCOSITY - THIN,
POORLY DEFINED FILMS WHEN PRINTED
– LOW CONDUCTIVITY AT LOW RELATIVE HUMIDITY
– EXCESSIVE SWELLING AT 100% RH
• APPROACH - CASTING FROM ALTERNATIVE SOLVENTS
– HIGHER SOLVENT VISCOSITY– HIGHER SOLUTION
CONCENTRATION - UP TO 35%– HIGHER CONDUCTIVITY AT LOW RH
0.0 0.2 0.4 0.6 0.8 1.01E-5
1E-4
1E-3
0.01
0.1
1
Nafion / Isopropanol Nafion / DMSO Nafion / EGC
ondu
ctiv
ity
/ S/c
mRelative Humidity
• Casting from EG solutions provides acceptable conductivity, printability
• First complete fuel cells fabricated with Nafion/EG electrolyte
• Swelling at 100%RH is an issue
-{-(CF-CF3) – (CF2-CF2)X-}-?(OCFCF3)YO(CF2)2SO3H
Special Requirements for MicroFuel Cells
• High proton conductivity with low sensitivity to relative humidity.
• The conductivity should be >1 mS/cm at 15% RH. • Fuels and reactants such as hydrogen and oxygen should have essentially no permeation through the PEMs. • Can be directly cast on electrode as PEM in MEA processing. • High chemical, dimensional, and mechanical stability during the preparation of and under the working conditions of the micro-fuel cell.
N
SO3H
SO3H
O
O
O
O
N N ArN
O
OO
O
xz
(a). Homopolymer or copolymer with linear, small comonomer
(b).Copolymers with linear,
bulky comonomer
(c). Copolymers with angled, and/or rigid comonomers
Molecular DesignThe rigid-rod liquid crystalline feature enables a few bulky or angled co-monomer units to separate the polymer chains over their whole length. This creates permanent pores lined with SO3H groups. Water is held very strongly
SO3H
SO3H
SO3H
SO3HH2OH2O
H2OH2O H2O
SO3H
SO3H
SO3H
SO3HSO3H
SO3H
SO3H
SO3H
H2OH2O
SO3H
SO3H
SO3H SO3H
SO3H SO3H
SO3H
SO3H
H2OH2OH2OH2O
H2O
H2O
Novel Polymer Electrolyte Synthesis
SULFONATED POLYIMIDE SYNTHESIS
SO3H
SO3H
*N
O
O
N
O
O
N
O
O
N
O
O
* X
Y
Z
R(4P)T5 X=0.05, Y=0.95
NN
O
O
O
O
NN
O
O
O
O
SO3H
SO3H
* *Y
X
RF5 X=0.05, Y=0.95
Relative Humidity (%)
0 20 40 60 80 100
Con
duct
ivity
(s/c
m)
1e-5
1e-4
1e-3
1e-2
1e-1
1e+0
Homopolymer RF5 RO5 R(4P)T5 RD5 Nafion
15%RH, 1st Generation Conductivities ? 1 mS/cm.
Conductivity Of 1st Generation PolyimideElectrolytes
PERMEABILITY OF POLYIMIDE ELECTROLYTES
H2 Permeability of Polyimides is ? 20X lower than Nafion
2.6 E-13R(4P)T5
2.7 E-13RF5
5.8 E-12Nafion (this work)
4.7 E-12Nafion (literature)
D*C / mole /cm/sSample
Rotating Disk Measurement of Limiting Current
Polymer Coated Pt Disk
0.1 M H2SO4 saturated w/ H2
Permeability Evaluated fromKoutecki-Levich Plot
NN
O
O O
O SO3H
HO3Sx
SO3H
SO3H
SO3H
SO3HH2O
[ ]x
[ ]x
H2O
H2O
2nd GenerationPolyimide Electrolytes
MONOMERS
• Fluorinated monomer added to control solubility in water
2nd Generation POLYMER
.
O O
O O
OO
SO3H
HO3 S
NH2H2N
CF3
F3C
NH2H2N
NH2
NH2
(m-PDA) (TFMB)
(NTDA) (DAPS)
.
.
.
n7N N
O O
OO
SO3H
HO3S
NO
O
NO
O
N N
CF3
F3C
O
O
O
O31
• Addition of fluorinated monomer alone lowers conductivity
• Co-addition of m-PDA balances conductivity and stability
• However, conductivity at 15%RH is below target (1 mS/cm)
0.114.2 e-31.2 e-41.8 e-4
with 10% m-PDA and 30% TFMB
0.062.8 e-31.3 e-5with 33% TFMB alone
0.2110.0 e-35.1 e-4with 10% m-PDA alone
0.23S/cm
9.3 e-3S/cm
2.9 e-4 S/cm
homopolymer
100%RH50%RH15% RHPolymer
• 4 probe conductivity measurement
• room temperature
• films cast in acid form
Conductivity of 2nd Generation Polyimides
• Humidified Hydrogen
• Air at 42% RH, 75 F
• Cell Area = 0.25 cm2
• I-V curve recorded at 2 mV/s
• Limiting currents of ?10 mA/cm2
due to hydrogen permeation through substrate beneath anodes
• Use of the water-insoluble polymer will provide for improved fabrication routes
Microfabricated Fuel Cell Performance
Power Output at 0.5V/cell:
1.5 mW/cm2 – Continuous operation
Using 2nd Generation PolyimideElectrolyteAll fuel cell components thick film printed
081301 series
10-5 10-4 10-3 10-2 10- 10
0.2
0.4
0.6
0.8
1.0
I (Amps/cm2)
E (V
olts
)
b-l-fc-swpx-cut.corb-u-fc-swpx-cut.corc-l-fc-swpx-cut.corc-u-fc-swpx-cut.cord-l-fc-swpx-cut.cor
0.0
0.1
0.2
0.3
155016001650170017501800
wavenumber (cm-1)
abso
rban
ce
40 days20 days0 day
1786
1714
1676
1582
2nd Generation Polyimide Electrolytes
ISSUE – ACID CATALYZED HYDROLYSIS
• Polyimide films were exposed to 90?C water for 0, 20 and 40 days
• FT-IR was used to track the composition of the polymer
• The imide peaks at 1714 and 1676 cm-1 decreased with time.
• A new peak at 1786 cm-1 appeared, which is assigned to an anhydride group resulting from chain cleavage.
• It is not clear if this imbrittlement will cause failure of an operating fuel cell, however, cells cannot be fabricated with aged polymer.
Summary – novel Electrolytes
• 1st Generation Polyimides (9/97 – 9/00)• achieved conductivity target of 1 mS/cm @ 15%RH• poor mechanical properties - brittle• low hydrogen permeability• all versions water-soluble to some extent
• 2nd Generation Polymides (9/00 – 9/01)• incorporated fluorinated monomer – water insoluble• better mechanical properties• low hydrogen permeability• only 0.2 mS/cm @ 15%RH• successfully incorporated into microfuel cell fabrication
All polyimide electrolytes synthesized degrade by hydrolysis – new family of linear aromatic polymers proposed (BAA 01-09)
Fuel Considerations:
Palladium Hydride• 2288 mAh/cm3, 190 mAh/g• near 100% utilization - no anode exhaust, no fuel wasted• difficult to refuel• Hydrogen pressure controlled by equilibrium w/ hydride, temperature - passive system• low H2 delivery pressure limits power output of fuel cell
Methanol:Water• 2772 mAh/cm3, 3216 mAh/g• must exhaust CO2
• fuel utilization is lowered by crossover, losses in exhaust• readily refuelable• slower kinetics, lower thermodynamic efficiency than with hydrogen•active control required to circulate fuel
RESULTS
Silicon-bonded Pd inks
•Near 100% solids system, minimal shrinkage-readily printed
•equilibrium sorption in 0.2 sec for 100? m thick film
•energy density ? 450 mWh/cm2
HYDROGEN FROM AQUEOUS NaBH4 SOLUTIONS
NaBH4 + 2H2O ? 4H2 + NaBO2•1900 mAh/cm2, 1700 mAh/g
•Reaction rate controlled by decomposition catalyst, temperature
• Decomposition in absence of catalyst minimized by addition of NaOH
•Catalyst requirement is small: 0.4 sccm H2 produced per mg Pt at 25C using 10wt % NaOH, 20 wt% NaBH4 (0.4 sccm H2 ? 62 mA)
•Passive system may require fixed orientation and overpressure relieve valve may be required
• Fuel solution is not sensitive to air, has moderate shelf-life (months) and is non-flammable
• Waste product is non-toxic, water soluble (borax)
• Larger scale systems under commercial development
(Millennium Cell LLC, www.millenniumcell.com)
H2, water vapor
NaBH4 solutionCatalyst
Fuel Cell
Air
MICROFABRICATED FUEL CELL PERFORMANCE WITH INTEGRATED HYDROGEN GENERATOR
Microfabricatedcells on porous substratesAll fuel cell components thick film printed
0 0.005 0.010 0.015 0.0200
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I (Amps/cm2)
Pot
entia
l (V
)
H2 FROM BOROHYDRIDETANK H2
0
0.001
0.002
0.003
0.004
0.005
Pow
er (W/cm
2)
No significant difference in power output using hydrogen generated from NaBH4.
Status – Metal Hydride Storage
• Successfully demonstrated thick film printable Palladium inks (01/99)• 765 mAh/cm3 (as ink), 2288 mAh/cm3 (theoretical)
• low power output due to PH2 ? 15 torr (0.02 atm)
New area of effort (9/01)• AB5 materials, modified to prevent de-activation
• LaAl0.3Ni4.7 with additional surface modification
• much lower binder content than in Pd inks• equilibrium pressure: 0.1 – 0.7 atm
• over 5 weeks in air without deactivation • over 40 sorption/desorption cycles without deactivation
• initial fuel cell tests promising
Thick Film Metal Hydride Storage
Pressure – Composition curves for thick film printed Pd and LaAl0.3Ni4.7 @ 22C
Sorption/DesorptionCurves for modified LaAl0.3Ni4.7
after 5 wks exposure to air
and after exposure and 40 S/D cycles
1
10
100
1000
0 0.2 0.4 0.6 0.8 1H/M
Pre
ssu
re, t
orr
LaAlNi
Pd
0
200
400
600
800
1000
0 0.2 0.4 0.6 0.8 1H/M
Pre
ssu
re, t
orr
5 wk aged in air.
aged plus 40 cycles
Thick Film Metal Hydride StorageConventional Fuel Cell Coupled to Hydride Ink
H2 Pressure and Cell Voltage - 20 mA• initial pressure = 748 torr• final pressure = 59 torr• achieved over 90% utilization of H2 stored in Hydride
Fuel Cell Polarization Curve with Hydride Source• over 200 mA draw without excessive Pressure drop
• H2 release rate far above micro fuel cell requirements
0
200
400
600
800
1000
0
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1
1.2
0 3600 7200 10800 14400 18000 21600 25200
Pressure
Potential
Time, Second
0
0.2
0.4
0.6
0.8
1
1.2
0 5 0 100 150 200 250
Current, mA
Future Plans• Improve porosity of substrate/current collectors – increase
power density Goal: 10-20 mW/cm2
• Develop improved methods for depositing electrolyte layerGoal: Enhance capability to manufacture
higher voltage stacks
• Develop improved metal hydride storage using higher pressure hydride materials
Goal: Increase energy density to 500 mWh/cm3
High Temperature Mesoscale Fuel Cells
• Joint Effort with Battelle – PNL• Electrode areas ? 1 cm2
• Electrodes, current collectors printed on PBI electrolyte membrane
• Cell interconnects, heaters, RTD printed on alumina• Adhesive assembly of components• Cathodes exposed to ambient air
Greek “Mesos”; middle
• this is an intermediate sized device, and the construction uses both microfabricated and conventionally fabricated components
High Temperature Mesoscale Fuel Cells
N
NH n
HN
N
H+ H2PO4-
H+ H2PO4-
H+ H2PO4-
H+ H2PO4-
H+ H2PO4-
PBI/PhosAcid membrane
High Temperature Mesoscale Fuel Cells
View of Cathode Side showing heater
Side View –showing fuel inlet/outlet, insulation, anode manifold
1 cm
Polarization curves for four, two-cell mesofabricated devices. Temperature 150?C. Fuel: H2 at 1.2 sccm (nominal). Air breathing
0 0.05 0.10 0.150.7
1.0
1.3
1.6
1.9
I (Amps)
E (V
olts
)
Meso PBI Fuel Cells Shipped to BattelleFabricated 12/14/01
ABDE
High Temperature Mesoscale Fuel Cells
THE CWRU FUEL CELL GROUP
FACULTY: M. LITT, C.C. LIU,
SENIOR RESEARCHERS: J. WAINRIGHT
RESEARCH ASSOCIATES: N. MIYAKI, A. SCHECTER, J. VENDRAMINI, L. DUDICK
GRADUATE STUDENTS: S. SAMMS, J. BAUMAN, Y. MA, Z. LIU, L. CHEN, K. YEE, Y. ZHENG, H. KIM
FORMER STUDENTS, ASSOCIATES: U. LANDAU S. ZECEVIC, Z. BAO, S. WASMUS, S. GOJKOVIC, K.TAKUMITSU, S. GUPTA, G. DU, G. SUN, W. LIN, J.T. WANG, D. WENG, H. MOEADDEL, R. AMERI, Y. WANG, K. YEE, A. DALMIA, C. LINIKEN, Z. JIN, C. KIM, Y. MORIMOTO
ACKOWLEDGEMENTS
DARPA