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

0.3

0.4

0.5

0.6

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

0.25

0.50

0.75

1.00

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

0.2

0.4

0.6

0.8

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