Transcript Document 7219160

UW
ELECTROCHEMICAL
SURFACE SCIENCE
Direct Oxidation of Hydrocarbon Fuels for
Solid Oxide Fuel Cells
V. K. Medvedev, L. M. Roen
S. B. Adler, E. M. Stuve
AIChE Annual Meeting
Cincinnati, Ohio
October 31, 2005
Types of Fuel Cells
Type
Fuels/Eff.
Applications
Advantages
Disadvantages
PEM
Nafion (H+)
80 °C
H2
CH3OH
35-40%
Transp.
Port. Elec.
Home
Fast start
Simple
system
High cost
Low efficiency
Lifetime
AFC
KOH (OH–)
90–200 °C
H2
Space
Highest
power
density
CO2 instability
PAFC
H3PO4 (H+)
200 °C
H2 (CH4)
UPS
Stationary
Multi-fuel
Most
developed
Start-up time
Low energy
density
MCFC
K2CO3
(CO32–)
650 °C
H2
CH4
Stationary
High effic.
Internally
reforming
Complex
system
SOFC
YSZ, CeO2
(O2–)
800 °C
H2
CH4
(HCs?)
60-80%
Stationary
Specialty
High effic.
Internal
reforming
Complex
system
SOFC Overview
• High temperature operation (650–1000 °C)
– High system efficiency, up to 80%
– Can be internally reforming
• Applications
– Stationary power
– Marine power
– Aircraft APUs
• Operating characteristics
–
–
–
–
Stacks tend to adiabatic operation
Large excess of oxygen/air helps cooling
Constant fuel utilization (≈ 95%)
Avoid recycle, burn excess fuel
Aircraft APUs - Ground Use
Typical Turbinepowered APU
15% Efficient
=
Jet-A
(over average operating cycle)
1 litre
75% less
fuel used
=
Future 2015
SOFC APU
Jet-A
0.25 litre
60% Efficient
(at std. sea-level conditions)
DLD05-02.ppt
David Daggett
Typical SOFC Cell Operation
(–)
H2O, CO2
Fuel
O2–
Air (30x)
Anode
Electrolyte
Load
Cathode
O2
(+)
e–
Temperature ≈ 650 °C
E-lyte conductivity
Reaction rate
Nernst potential
≈ 900 °C
Low
High
<–– Approx. uniform ––>
≈ 1.1 V
≈ 0.8–0.9 V
SOFC Materials
• Electrolytes
– YSZ (yttria-stabilized zirconia) is ionically conducting
– LaSrGaMgO (LSGM) is possible alternative
– Avoid mixed conduction (ionic & electronic conduction)
• Interconnects
– Doped La-chromite (electronically conducting)
• Cathodes [Adler, Chem. Rev. (2004)]
– LSM (LaSrMg) typical choice
– LSC (LaSrCoOx), LSF (LaSrFeOx) offer better performance by virtue
of being mixed conductors
• Anodes
– Ni/ZrO2 cermet typical choice
– Ni forms carbon during operation with HC fuels
– Seek anode to avoid carbon formation
Direct HC Oxidation
• Non-hydrogen fuels
– Desire operation with liquid fuels, e.g. diesel
– Reduce/eliminate fuel reforming
• Non-coking anode catalyst
– Gorte & Vohs: Direct oxidation of hydrocarbons on Cu/CeO2
[S. Park, J. M. Vohs, R. J. Gorte, Nature 404 (2000) 265–267]
– Ceria is catalytic for HC oxidation
– Cu is current collector; electronic conductivity of Cu has influence
• Recent review of anodes
– Atkinson, Barnett, Gorte, Irvine, McEvoy, Mogenson, Singhal, Vohs,
Nature Materials 3 (2004) 17–27.
Direct Oxidation of Liquid Fuels
Kim, Park, Vohs, Gorte,
J. Electrochem. Soc. 148 (2001) A693–695.
CuCeO2 / YSZ / LSM
700 °C
Decane, toluene, diesel
0.5 V & 0.2 A/cm2
Stable for hours
Decane
Toluene
Diesel
Influence of Carbon Formation
Increased performance in H2
attributed to carbon formation
on anode following butane
oxidation
CuCeO2 / YSZ / LSM
700 °C
H2, C4H10, H2
0.5 V & 0.2 A/cm2
McIntosh, Vohs, Gorte,
J. Electrochem. Soc. 150 (2003) A470–476.
H2
C4H10
H2
Influence of Carbon Formation
C4H10
Carbon deposits increase
electrical conductivity of anode
McIntosh, Vohs, Gorte,
J. Electrochem. Soc. 150 (2003) A470–476.
Motivation for Our Research
• Fundamental surface chemistry of electrocatalytic
hydrocarbon oxidation reactions
– Reaction pathways & kinetics in direct oxidation
– Surface intermediates & coverages (C, O, others)
– Surface electric field; influence of adsorbates
• Characterize fuel/catalyst combinations
– Role of surface/substrate oxygen in direct oxidation
– Bond breaking tendencies for C–C, C–H, and C–O
• Characterization of electrolyte & catalyst
– Influence of electrolyte preparation
– Electrochemical activation of catalysts
Catalysis & Electrocatalysis
C7H8 + 9 O2 ––> 7 CO2 + 4 H2O
Test reaction:
O2
CO2
H2O
CO2
H2O
C7H8
C7H8
Sm-CeO2
1000 K
O2
Pt anode
Solid oxide
electrolyte
Pt cathode
Catalytic combustion:
all O2 from gas phase
What is the role of oxygen from
gas phase vs. from electrolyte?
O2–
O2
Electrocatalysis: all
O from electrolyte
Oxygen Transport (at cathode)
Three-phase
boundary (TPB)
Catalyst
Electrolyte surface
J. Flieg, Annu. Rev. Mater. Res. 33 (2003) 361-82.
• Similar situation at anode
• Different reactivities of chemisorbed O
(TPB, catalyst, and electrolyte)
• Possible role of O2–?
Anode Reaction Network
CO2
H2O
CxHy
CxHy
(8)
(7)
(6)
(9)
(14)
(5)
O
2
(4)
(3)
(2)
(1)
O
CH
(11)
O O
Catalyst O2–
TPB
(10)
O2–
O2–
Solid Oxide Electrolyte
TPB
(13)
e–
O2–
O2
(12)
O2–
• Fuel adsorption, oxygen transport, and reaction at a
solid oxide FC anode.
SOFC Cell
Fuel
Oxygen
Hot Zone
Quartz Tubes
Electrolyte
Oxygen
Pt
Au
Screw / Nut
/ Washer
Spring
NiCr Wire
Alumina
UHV-SOFC System
Baratron 0.01-100 Torr
Oxygen
Viscovac
10-6 – 10-1 Torr
Baratron 0.01-100 Torr
Leak
Valve
to
UHV Chamber
with Calibrated Mass Spec
Fuel
Pumping
Oxygen
Activation by Oxide Ion Flux
O2
CO2, H2O
CO2, H2O
O2
C7H8
C7H8
C-layer?
Sm-CeO2
1000 K
Pt
O2–
O2
O2
O2– removes carbon
layer; surface reaction
proceeds much faster
CO2 production /
arb. units
O2– current
0
0
t / min
60
Surface Flux with Oxide Ions
•
O2
Now add influence of oxide ions
from electrolyte
ro  2No so (tot ) r
O2
C 7 H8
rf
WE
O2-
At high surface coverage,
so << 1, so rO2– dominates and can
 ignite
reaction
•
slow reaction
Pt
•
•
ro
Once reaction proceeds, tot
decreases and now gas supplies
reactants at rate much faster than
rO2–.
YSZ
CE
fast reaction
O2
C 7 H8
rf
ro
Pt
WE
O2-
With fast reaction ro >> rO2– giving
rise to electrochemical modification
of catalytic activity.
rO2–
YSZ
rO2–
CE
Detection and Analysis of Coking
CH4 + 2 O2 ––> CO2 + 2 H2O
Anode
Pt/Gd0.1Ce0.9Ox
915 K
Short-circuit
Cathode
La0.8Sr0.2CoO3
92 torr O2
C removal
a
b
e
g
rCH4
c
h
pCO2
d
pO2, anode
f
4.5 torr
0.25 torr
pCH4, anode
Time
a. Large reaction of CH4 on
initially clean surface
b. Reaction slows with C
formation
c. Reaction goes through
minimum as C layer
rearranges
d. Reaction on C-covered
surface reaches steady state
e. End of CH4 reverses step c
f. O2 reaches prolonged
minimum as C-layer removed
g. Reaction of residual CH4
increases as C-layer
removed
h. Reaction ends on clean
surface
Multi-fuel Polarization Curves
Cell Potential / V
0.8
Anode: Pt/Gd0.1Ce0.9Ox
Cathode: La0.8Sr0.2CoO3
Pfuel = 4 torr
pO2, anode = 0.25 torr
pO2, cathode = 74 torr
915 K
0.6
0.4
0.2
C2H5OH
H2
CH4
0
0
1
2
3
Current density / mA cm–2
4
• Multi-fuel capability
• Cathode partially optimized;
further improvements
possible
• Oxygenated fuels (H2, CO,
CH3OH, C2H5OH) exhibit
higher open circuit voltages
• Parafins and olefins have
lower open circuit voltages
• F/C seals improved: O2
pressure ratio of ~300 across
5.0 fuel cell; further
improvements possible
Spontaneous Oscillations / C2H4
834 K
Pt/GdCeO2/Pt
Ethylene: 0.11 Torr
Oxygen: 0.29 Torr (anode)
5.5 Torr (cathode)
CH4 Oscillations – A Closer Look
Total Pressure
Current
Partial Pressures at Spike
All Reaction Rates
0.010
0.008
H2
Rate / Torr l s–1
0.006
H2O
0.004
CO
0.002
CO2
0
O2
–0.002
CH4
–0.004
–500
–400
–300
–200
–100
0
Time / s
100
200
300
400
500
Atom Balance (H,C,O)
Net O2– through
electrolyte
≈ 40 mA
0 (Balanced)
Reactions to Consider
• Combustion
CH4 + 2 O2 ––> CO2 + 2 H2O
Dn
0
• Electrocatalysis
CH4 + 4 O2– ––> CO2 + 2 H2O
2
• Reforming
CH4 + H2O ––> CO + 3 H2
2
• Water Gas Shift
CO + H2O <––> CO2 + H2
0
Analysis of Oscillation
• Initiation: increase in H2O production, perhaps
coupled with decrease in carbon layer
• Increase in direct oxidation rate
• Large increase in reforming (H2, CO)
• Increase in current => electrocatalysis
• Increase in pressure => electrocatalysis &
reforming
• Post-spike: deficit in CO2 production indicates
return of carbon layer
• Termination: completion of carbon layer?
Big Question
• What caused the increase in H2O production?
• Speculation: Change in O2– conduction mech.
Alternating Conduction Modes
CO2
H2O
CxHy
CxHy
(6)
(9)
(5)
TPB
O2
(8)
(7)
O
CH
(11)
Catalyst
Solid Oxide Electrolyte
O O
O2
O2–
e–
O2–
• Spontaneous oscillations possibly due to electronic
change between ionically conducting and electronically
conducting
O2– Transport
O2
CO2, H2O
C2 H4
n O2– ––> 2n e–
Eoc
2– e– O2– –
+
Ce4+/3+ redox
Anode
–
–
+
+
GDC
+
O
Cathode
O2
O2– transport through electrolyte
Summary
• UHV-SOFC Studies
– Catalytic oxidation of C7H8, C2H4, CO on Pt, Pt/YSZ
– Catalytic activity controlled by surface coverage; sticking
coefficients of gas phase species important
– Electrochemical catalyst activation by modulating oxide ion
flux
– Frequency response consistent with Ce4+/3+ redox!
– Direct oxidation coupled with reforming
– Spontaneous oscillations related to changing conduction
modes in the solid oxide electrolyte
Acknowledgements
• Personnel
–
–
–
–
Jamie Wilson (Adler group)
David Daggett (Boeing)
Ray Gorte
John Vohs
• Funding
– Office of Naval Research
CH4 Consumption at Spike
O2 Consumption Rate
CO2 Production at Spike
H2, CO Production Rates
H2O Production Rate
Electrochemical Catalyst Activation
Alternating Oxide Current / C2H4
Spont. Oscil. Temp. Variation
Spontaneous Oscillations: C2H4/Ce0.9Gd0.1O1.95
0.6
C2H4/Ce0.9Gd0.1O1.95
CO2 / arb. units
0.5
p / Torr
C2H4: 0.12
O2 anode: 0.28
O2 cathode: 5.5
0.4
0.3
889
839
836
834
789
0.2
0.1
K
K
K
K
K
0
0
200
400
600
800
Time / min
1000
1200
1400
1600
Frequency Dependence
Oscillations in Ethylene Oxidation
0
-1
ln(Frequency / min )
-1
-2
-3
-4
-5
-6
152.4 ± 2.8
-7
-8
0.0011
0.00115
0.0012
0.00125
0.0013
1/Temperature / K -1
0.00135
0.0014
Absolute Reaction Rates
• Comparative measurements of mass
spectrometer signal with pressure gauges
(Viscovac & Baratron)
• Measure reactor volume V
• Measure pumping speeds of all species Si
• Convert MS signal to production rates
ri  V

dp
 Si p
dt
SOFC Designs
Interconnect
Anode
Air
Fuel
Electrolyte
Cathode
Electron
path
Planar
Tubular