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Integrated Micropower Generator
MicroSOFC
+
Swiss Roll
Combustor
High Efficiency
Thermal Management
Scott Barnett, Northwestern University
Northwestern University Role
Anode Material Development
• Develop anodes to partially oxidize high energy density
liquid hydrocarbon fuels at low temperature
• Anodes must also electrochemically oxidize resulting H2
and CO at low temperature
Approach
• Product gas analysis using differentially pumped mass
spectrometry
• Cell testing and impedance spectroscopy measurements
• Open-circuit potential measurements compared with
thermodynamic calculations
Integrated MicroPower Generator
Review, June 24, 2002
Outline
• Introduction
• Thermodynamic equilibrium calculations
– Non-coking conditions
• Mass spectrometer measurements
• Single chamber cell tests
• Dual chamber cell tests
– Thick GDC electrolyte cells
– Anode supported cells
– Open circuit voltage
• Conclusions
Integrated MicroPower Generator
Review, June 24, 2002
Thermodynamic Calculation
• Determine equilibrium gas composition and
whether coking is expected
– Used to guide choices of inlet gas composition
• Assumes 10 sccm fuel gas flow
– Propane (humidifed)
– 5% fuel utilization
• Oxygen added directly to fuel stream and/or
via fuel cell operation
• OCV calculation based on effective oxygen
partial pressure of equilibrium fuel mixture
Integrated MicroPower Generator
Review, June 24, 2002
Equilibrium Calculation: Propane, 800C
O2/C3H8 Ratio
0
1
2
3
4
1.8x10
o
800 C
CO
H2
O2
-6
1.5x10
CO2
H2O
Carbon
100
80
-6
1.2x10
60
-7
9.0x10
40
-7
6.0x10
20
-7
3.0x10
Deposition percentage (%)
Flow/deposition rate (mol/s)
-6
• Carbon deposition up to
ratio of 1.7
• Main gaseous products:
CO and H2
• CO2 and H2O gradually
increase with increasing
oxygen
0
0.0
0
200
400
600
800
1000 1200
2
Current density (mA/cm )
Integrated MicroPower Generator
Review, June 24, 2002
Equilibrium Calculation: Propane, 400C
-6
0
1
2
CO
H2
O2
o
400 C
-6
1.5x10
4
5
CO2
H2O
Carbon
100
80
-6
1.2x10
60
-7
9.0x10
40
-7
6.0x10
20
-7
3.0x10
0
0.0
0
400
800
1200
Deposition percentage (%)
Flow/deposition rate (mol/s)
1.8x10
3
• Carbon deposition up to
ratio of 4.7
• Main gaseous products:
H2, H2O, and CO2
• More oxygen required to
prevent coking than at
800C
– Due to greater amounts
of oxygen in equilibrium
products
1600
2
Current density (mA/cm )
Integrated MicroPower Generator
Review, June 24, 2002
Equilibrium Calculation: Propane
5
• Minimum O2/C3H8 ratio
required to avoid coking
• Limit at high T is partial
oxidation stoichiometry
• Limit at low T is complete
oxidation stoichiometry
Critical ratio
No carbon deposition
4
3
2
1
400
Carbon deposition
500
600
700
800
o
Temperature ( C)
Integrated MicroPower Generator
Review, June 24, 2002
Equilibrium Calculation Results
• Carbon deposition can be avoided by adding
sufficient oxygen
– Electrochemical or gas-phase oxygen source
• More oxygen required at lower temperatures
– Results from higher oxygen content of equilibrium
products
• Kinetic considerations may be completely
different
Integrated MicroPower Generator
Review, June 24, 2002
Cell Test / Mass Spectrometer
Alumina
tube
Current
lead
NiO-GDC
C3H8
+O2
+Ar
CO+
CO2
+H2
GDC
La0.5Sr0.5CoO
3
Voltage
lead
Furnace
-8
4.0x10
Flowmeter: 15.97% C3H8 - 16.81% O2 - 67.22% Ar
Mass Spec: 15.94% C3H8 - 14.35% O2 - 69.71% Ar
-8
Intensity (amps)
3.0x10
-8
2.0x10
-8
1.0x10
0.0
16
20
24
28
32
36
40
44
48
Mass/Charge
Integrated MicroPower Generator
Review, June 24, 2002
Partial Oxidation Reaction
Ar
O2
H2O
H2
CO
CO2
70
Gas content (%)
60
50
40
30
20
10
0
550
600
650
700
o
Temperature ( C)
Integrated MicroPower Generator
750
C3H8
• Mass spec measurement versus
cell temperature (no current)
• Ni-YSZ anode support
• Inlet mixture: 15.9% propaneoxygen-Ar
• Reforming products vary with T
– CO is main product (Hydrogen
sensitivity low: should be
larger than CO)
– C3H8 and O2 decrease, but not
completely consumed
– H2O, CO2 decrease w/ incr T
– Basic agreement with
calculations
Review, June 24, 2002
Cell Tests
Types of Cells
• Thick GDC electrolyte
– Anode: 60% NiO – GDC
– Gd0.5Sr0.5CoO3 cathode (similar to SmSrCoO3)
• Anode supported cells
– Thin YSZ electrolyte
– Ni-YSZ anode
– LSM-YSZ cathode
Test Conditions
• Standard fuel mixture:
– 10-25% propane, balance Ar-O2(20%)
• Temperatures reported are measured at cell
– ~50C higher than furnace temperature
Integrated MicroPower Generator
Review, June 24, 2002
Effect of Anode Material
• Ni-GDC thin anodes showed no coking in 15.9% propane
mixture
• Ni-YSZ thick anodes showed obvious coking in 15.9%
propane mixture
– May be related to higher Ni content of thick anode, or NiGDC versus Ni-YSZ
• Both types of anodes coke-free with 10.7% propane
Integrated MicroPower Generator
Review, June 24, 2002
Single Chamber: Thick GDC
0.6
0.5
8
0.4
6
0.3
4
0.2
o
511 C
o
o
732 C
2
o
663 C
o
654 C
2
614 C
0.1
o
561 C
0
0.0
0
5
10
15
20
25
30
2
Current density (mA/cm )
Integrated MicroPower Generator
35
Power density (mW/cm )
Terminal voltage (V)
10
40
• Ni–GDC|GDC|Gd0.5Sr0.5CoO3
• 10.7% propane, balance air
• Unstable performance
between 511 and 732C
• Stable at endpoint
temperatures
• OCV ~ 0.5V
– lower than Hibino reports
• Very low current density
• No carbon deposition detected
Review, June 24, 2002
Dual Chamber: Thick GDC
0.8
300
o
250
0.6
200
0.4
150
100
0.2
50
0.0
0.0
0.3
0.6
0.9
1.2
2
Current density (A/cm )
Integrated MicroPower Generator
0
1.5
2
Terminal voltage (V)
o
790 C
Power density (mW/cm )
827 C
• Ni–GDC|GDC|Gd0.5Sr0.5CoO3
• 10.7% propane, balance air
• Low OCV
– As
expected
for
GDC
electrolyte
– But ~0.1V higher than single
chamber
• Power density similar to such
cells run on hydrogen
– Limited by thick 0.5-mm GDC
– But much higher power
density than single chamber
Review, June 24, 2002
Dual Chamber: Anode Supported
1.0
0.7
o
678 C
o
779 C
o
819 C
0.5
0.6
0.4
0.4
0.3
0.2
0.2
2
Terminal voltage (V)
0.8
0.6
o
Power density (mW/cm )
728 C
0.1
0.0
0.0
0.5
1.0
1.5
0.0
2.0
• NiO-YSZ|YSZ|LSM-YSZ
(anode supported)
• 10.7%C3H8–balance air
– Propane just below
partial oxidation
stoichiometry
• Open circuit voltage = 0.9
to 0.95V
• Power density actually
higher than with
hydrogen!
2
Current density (A/cm )
Integrated MicroPower Generator
Review, June 24, 2002
Open Circuit Voltage: Propane-Air
• 800oC, dual chamber cell
• Experiment:
1.2
– Voltage increases from
~0.9 to 1.0V with
increasing propane
OCV (V)
0.9
• Equilibrium calculation
0.6
0.3
0.0
OCV800 theoretical
OCV794 experimental
10
15
20
25
Propane content
Integrated MicroPower Generator
30
– Voltage increases rapidly
from 1.0 to 1.1V with
increasing propane to 11%
– Voltage flat for higher
propane (solid C present)
Review, June 24, 2002
1.2
2
Peak power (mW/cm )
Open Circuit Voltage (V)
OCV and Max Power: Anode Supported
0.6
1.0
0.8
0.4
0.6
0.4
0.2
C3H8
H2
0.2
0.0
600
650
700
750
800
• Dual chamber cell
• Two fuels:
– 10.7% propane –
balance air
– Humidified hydrogen
• H2 gives higher OCV
• C3H8 gives higher power
density
0.0
o
Temperature ( C)
Integrated MicroPower Generator
Review, June 24, 2002
Summary
• Thermodynamic calculation shows that more oxygen is
required to suppress coking at lower temperature
• Mass spectrometer measurements show expected
reforming behavior, agree with calculations
• Single-chamber tests show low voltage and low current
density in propane-air
• Dual chamber tests:
– High power density for anode supported cells
– No coking for propane content < 10.7% in air
– More tendency for coking on thick anodes for higher
propane content
– Measured open circuit voltages slightly less than equilibrium
calculation
Integrated MicroPower Generator
Review, June 24, 2002
Propane OCV
1.30
OCV (V)
1.25
1.20
1.15
1.10
400
500
600
700
800
o
Temperature ( C)
• Humidified propane
• Dual-chamber cell
• Relatively high OCV due
to low H2O and CO2
partial pressures
• Low T slope resembles H2
fuel operation
• High T slope resembles C
partial oxidation
Dependence of OCV on temperature for propane based fuel cells
Integrated MicroPower Generator
Review, June 24, 2002