Nanoscale Effects
in Solid Oxide Fuel Cells
Xiao-Dong Zhou and Subhash C. Singhal
Pacific Northwest National Laboratory
Types of Fuel Cells
SOFC
Electrolyte
Cathode
Anode
Phosphoric
Carbonate
Acid
Polymer
Alkaline
Membrane
Y2O3Stabilized
Li2CO3-
ZrO2 (YSZ)
Sr-doped
K2CO3
Li-doped
H3PO4
KOH
sulfonic
acid
LaMnO3
NiO
Pt on C
Pt-Au
Pt on C
Ni/YSZ
Ni
Pt on C
Pt-Pd
Pt on C
650°C
200°C
100°C
90-120°C
H2, CO
H2
H2
H2
Temperature 750-1000°C
Fuel
Molten
H2, CO
Perfluoro-
2
Solid Oxide Fuel Cells - Advantages
ƒ High electric conversion efficiency
ƒ High environmental performance
•
•
•
No SOx or NOx ; Lower CO2 emissions
Quiet
Vibrationless
•
High quality exhaust heat for heating, cooling, additional power generation
ƒ Cogeneration potential
ƒ Fuel flexibility
•
•
•
•
Liquefied natural gas
Pipeline natural gas
•
•
Modularity permits wide range of system sizes
Siting flexibility for distributed power
Coal gas
Naphtha
ƒ Size and siting flexibility
•
•
Methanol
Biogases
3
SOFC Operating Principle
external electrical load
2 eHigh Po2
environment
air
Cathode reaction:
O=
½ O2 + 2 e-
Low Po2
Environment
The open circuit voltage is
given by the Nernst equation:
fuel
(H2 + CO)
O=
800 to
1000°C
porous electronic
conducting
cathode
dense oxide
ion conducting
electrolyte
La(Sr)MnO3
Yttria-stabilized
zirconia (YSZ)
Eo =
Anode reaction:
O=
½ O2 + 2 eOxidation reactions:
½ O2 + H2
H2O + heat
½ O2 + CO
CO2 + heat
Po2(c)
RT
ln
Po2(a)
4F
at 1000°C:
RT ln 0.2 = 1.1 V
4F
10-18
porous
electronic
conducting
anode
Ni - YSZ cermet
4
Cell Component Materials
Component
Material
Cathode
Doped Lanthanum Manganite
Electrolyte
Yttria-stabilized ZrO2(YSZ)
Anode
Nickel-YSZ
Interconnection Doped LaCrO3 ; High-temperature alloys
5
SOFC Designs
Tubular
(anode- and cathode-supported; microtubular)
Flattened Tubular
(anode- and cathode-supported)
Planar
(anode-, electrolyte-, and metal-supported)
6
Tubular vs. Planar Cell Designs
Tubular Cells
Planar Cells
Specific Power (W/cm2)
Low (0.2-0.35)
High (0.6-2.0)
Volumetric Power (W/cm3)
Low
High
Manufacturing Cost ($/kW)
High
Low
High Temperature Seals
Not Necessary Required
Performance Degradation
None
1-4%/1000 hrs
7
SOFC Systems
Siemens/Westinghouse 100 kW
Hexis 1 kW
FCT/SWPC 5 kW
Mesoscopic Devices(20 W to 250 W)
Delphi 5 kW APU
8
SOFC Market Drivers
¾ Positives
• Low emissions
• High efficiency, even in small size systems
• Fuel Flexibility
¾ Negatives
• Cost
• Cost
• Cost
• Lifetime
• Performance Degradation
9
SOFC Research
on Various Length Scales
meter
meter
mm
mm
http://egweb.mines.edu/faculty/rjkee/index.html
≤≤μm
μm
Uncertainty: Elementary charge-transfer chemical reaction mechanisms,
particularly related to triple phase charge exchange.
~~nm
nm
10
Zirconia-Based Electrolyte
ƒ Very low electronic conduction
(energy band gap: >7 eV)
ƒ Very high thermodynamic stability
(decomposition PO2 at 1000ºC: <10-35 atm)
ƒ Easily doped with lower valence cations
(e.g., Ca2+, Y3+, Sc3+, etc.) to create oxygen vacancies
ƒ Doped material is highly oxide ion conductive
(conductivity: >0.1 Ω-1cm-1 at 1000ºC)
11
Oxide Ion Conductivity
Temperature (℃)
1000 900 800
0.0
700
600
500
(Bi2O3)0.75(Y2O3)0.25
LSGMC
LSGM
-1.0
-4.0
(ZrO2)0.85(CaO)0.15
log(s/cm-1)
-3.0
(CeO2)0.9(Gd2O3)0.1
(ZrO2)0.9(Sc2O3)0.1
(CeO2)0.95(Y2O3)0.05
(CeO2)0.9(CaO)0.1
(ZrO2)0.91(Y2O3)0.09
-2.0
(ThO2)0.85(Y2O3)0.15
(ThO2)0.93(CaO)0.07
-5.0
0.7
0.9
1.1
1.3
1.5
1000/T (/K-1)
(LSGMC: La0.8Sr0.2Ga0.8Mg0.115Co0.085O3 ; LSGM: La0.9Sr0.2Ga0.8Mg0.1O3)
12
Nanoionics – Space Charge Model
Different
DifferentCharge
ChargeTransport
TransportPaths
Paths
Grain Boundary
J. Maier, Prog. Solid St. Chem., 23, 171
(1995); Solid State Ionics, 175 (2004) 7;
Nature Materials, 4, 805 (2005). Space
Charge
Zone
Grain
Microcrystalline Materials Nanocrystalline Materials
(Grain size » tgb & tsc)
Grain boundary can act as:
¾ Blocking layer at low T
¾ Effect is negligible at high T
(Grain size ~/< tgb & tsc)
¾ Space charge zone can cover the
whole grain, changing mobility of
charge carriers.
13
Enhanced Oxygen Diffusivity
in Interfaces of Nano YSZ Disk
~ 60 nm
> 1 mm
‰ 18O Diffusion profile analysis
‰ Bulk and interface diffusion 3
order faster in nano than in micro YSZ
‰ Similar magnitude of oxygen
exchange coefficient for nano
and micro YSZ
G. Knoner, K. Reimann, R. Rower, U. Sodervall, and H. E Schaefer, PNAS, 100, 3870 (2003).
14
Ionic Conduction in Nanocrystalline YSZ Films
YSZ
MgO
~ 15 μm
12 and 25 nm
I. Kosacki et al., Solid State Ionics, 176, 1319 (2005).
X. Guo, Acta Mater., 53, 5161 (2005).
15
Oxygen Content (%)
Oxygen Content in YSZ Films
Determined by RBS and NRA
17
17nm
nmthick
thickfilms
films
•• More
Moreexcess
excessoxygen
oxygenininas
asdeposited
depositedfilms;
films;
•• Increasing
Increasingoxygen
oxygencontent
contentupon
uponannealing.
annealing.
427
427nm
nmthick
thickfilms
films
•• More
Moreoxygen
oxygenvacancies
vacanciesininas-deposited
as-depositedfilms
films
•• Mechanisms:
Mechanisms:dopant
dopantsegregation;
segregation;surface
surface
oxygen
oxygenvacancies
vacancies
Annealing Time (hr)
16
Nanosize Effects on Electrolyte
Properties
• Additives which contribute to ion blocking at grain boundaries are
diluted in nanocrystalline oxides giving rise to substantial reductions in
specific grain boundary resistivities. This leads, in some cases, to an
overall decrease in grain boundary resistance.
• The case for enhanced ionic conduction in nominally undoped
nanocrystalline oxides remains unresolved. In thin films,
enhancements of several orders of magnitude are reported. It remains
to be seen if this discrepancy is related to differences in the manner in
which the dopants are distributed between grain and grain boundary
during processing, or, in the case of the films, are due to spurious
effects such as humidity or film substrate interactions.
H. L. Tuller, Solid State Ionics, 131, 143 (2000).
E. D. Wachsman, MRS Spring Meeting, (2007).
17
Reaction and Length Scale
in SOFC Anode
H2 on Ni: H 2 ↔ 2H
Desorption Rate:
rd ~ n o υ d exp(− E d RT )θ 2H
Mean lifetime of
chemisorbed H ~ 700oC
exp(E d /RT)
τH =
≈ 12 ns
υdθ H
Diffusion of H on Ni: D H ≈ 0.0025 exp(− 1762 T) cm 2 /s
Diffusion length scale at ~ 700oC: t d ≈ D H τ H ≈ 20 nm
R. J. Kee, H. Zhu, D. G. Goodwin, Proc. Comb. Institute, 30, 2379 (2005).
18
Improved Anode Performance
YSZ
Kim et al, J. Power Sources, 163, 392 (2006).
19
Ni Particle Size vs CH4 Reforming
CH4 conversion
0.8
Methane Steam Reforming
o
(S/C/H=3/1/1) over Ni/YSZ(baker)_cal1375 C
0.7
0.6
0.5
o
700 C/1h
o
750 C/3%H2O
159h
0.4
0.3
o
1000 C/4h
0.2
0.1
0.0
0
10
20
30
40
50
60
70
Time / h
Dave King et al. 2006 Office of Fossil Energy Fuel Cell Program Annual Report. 20
Cathode for Oxygen Reduction
Length scale in SOFC cathode
21
Cathodic Resistance vs.
Triple Phase Boundary Length
lTPB ∝ 1/d
(d: grain size)
R. Radhakrishnan, A. V. Virkar, and S. C. Singhal, J. Electrochem. Soc., 152, A927 (2005).
R. Radhakrishnan, A. V. Virkar, and S. C. Singhal, J. Electrochem. Soc., 152, A210 (2005).
22
Infiltration of Nanoparticles into Cathode
No Infiltration
Infiltration w/ Co3O4
S. Visco, SECA core technology peer review workshop, (2005).
23
Linear Expansion
Instability of the Cathode
Instability
Instabilityof
ofdimension
dimensionand
and
mass
massisisbecause
becauseof
ofloss
lossof
of
lattice
latticeoxygen,
oxygen,resulting
resultinginin
more
morelow-valence
low-valencestate
state
transition
transitionmetal
metalions.
ions.
For
Forhighly
highlyactive
activenanograin
nanograin
cathodes
cathodesto
tohave
havestable
stable
performance
performancefor
for~~40,000
40,000hrs,
hrs,
temperature
temperaturemust
mustbe
bereduced
reduced
o
to
tobelow
below600
600oC.
C.
J. Stevenson, PNNL, Unpublished.
24
Improving SOFC Seals
Ron Loehman, www.osti.gov/bridge/servlets/purl/839246-r8KjrS/native/839246.pdf.
25
Summary
¾ Major impact of nanotechnology in SOFCs
• Enhancing ionic conduction in the electrolyte
• Decreasing grain size of the electrodes (increasing
surface area) to improve electrocatalysis
• Optimizing electronic/ionic conduction paths in electrodes
• Optimizing sinterability of the seals
¾ Major Challenges
• Stability of the cathode at high operation temperatures
• Stability of the anode during cell fabrication
• Lowering operation temperature to below 600oC to take
advantage of beneficial effects of nanotechnology
26