1. Electrode Studies at High Pressure 2. LSM Sintering at ......8th Annual SECA Workshop San Antonio...

Larry Pederson, Ben McCarthy, Matt Chou, Greg Coffey, Chris Coyle, Olga Marina,

Carolyn Nguyen, Ed Thomsen, and Xiao-Dong Zhou

Contract Manager: Heather Quedenfeld

8th Annual SECA WorkshopSan Antonio TX

August 7-9, 2007

1. Electrode Studies at High Pressure2. LSM Sintering at Low Temperature

28th Annual SECA Workshop

Objectives and Approach –Electrode Studies at High Pressure

Objectives and Approach Objectives and Approach ––Electrode Studies at Electrode Studies at High PressureHigh Pressure

Objectives: Determine how activation and concentration polarization losses for SOFC electrodes are affected by operation at elevated pressures –pressure effects identified as priority (Surdoval, 7th Annual SECA Workshop, 2006).Approach:

Pressure vessel: 130 bar maximum pressure, internal furnace maximum temperature ~950oC, flowing gas maintained across electrode surfaces, shielded leadsCathode compositions evaluated: LSM, LSM/ceria, LSF, AuElectrochemical evaluation: electrochemical impedance spectroscopy (EIS) and DC methods

Conclusion: All cathodes studied improved with increased pressure, ranging from a P1/4 to P1/2 dependence


Background – High Pressure Electrode StudiesBackground Background –– High Pressure Electrode StudiesHigh Pressure Electrode StudiesEfficiency advantages have been projected for an SOFC/gas turbine hybrid fueled by coal gas at elevated pressures (~3-15 bar) (e.g. Strakey, 7th Annual SECA Workshop, 2006)Elevated pressure operation leads to enhanced SOFC stack performance – more than can be accounted by Nernst effects alone (e.g. Minh, 7th

Annual SECA Workshop, 2006)Relatively few studies address electrode performance at elevatedpressures (many at P≤1 atm, however). On the cathode side:

For cathode-supported cells, Virkar et al. (DOE/FETC/C-98/7303, CONF-9704176, 1997) concluded that increased pressure led to a lowering of cathodic concentration polarization following a 1/P dependenceLSM electrodes improved with P(O2) up to ~20 bar at 750oC, with no further improvement at higher pressures (Drevet et al. Solid State Ionics 2000;136:807), using EIS to study symmetric cells at open circuit.

On the anode side:A minimum anodic concentration polarization at 5 bar in coal gas was projected by Gemmen and Trembly (J Power Sources 2006; 161:1084). The H2 concentration is expected to reach a maximum at 8 bar.Kikuchi et al. determined experimentally that concentration polarization rosewith P(H2+H2O) to at least 10 bar for a Ni/YSZ anode on YSZ (Solid State Ionics 2004;174:111). Activation polarization was independent of pressure.


Electrode parameters determined from Butler-Volmer equation:

where αa, αc are anodic and cathodic charge transfer coefficients, Io is the exchange current, and η is the polarization lossActivation polarization trends depend on ability of electrode to continue to adsorb oxygen. If surface sites are nearly or completely filled with oxygen adatams at high pressure (Pt is an example), performance should decreasewith pressure:

If αa ≈ αc, charge transfer resistance shows a +1/4 dependence.If few adsorption sites are filled (usually at low pressure), performance should increase with pressure, yielding a -1/4 PO2 dependence is expected if αa ≈ αc(-1/2 for associative model)

Langmuir adsorption isotherm (dissociative):

)(2/)(/12

caaOoCT PIR ααα +−≈≈

)(2/)(/12

cacOoCT PIR ααα +≈≈ (dissociative model)

(dissociative model)

Models for Cathodic Pressure DependenceModels for Models for CathodicCathodic Pressure DependencePressure Dependence

)/exp()/exp( RTFRTFII

cao

ηαηα −−=

2/1

2/1

2

2

1 Oad

Oad

PKPK

+=θ


Polarization Curves for Symmetric LSM-20/ YSZ/LSM-20 Cell at Various Oxygen Pressures

Polarization Curves for Symmetric LSMPolarization Curves for Symmetric LSM--20/ 20/ YSZ/LSMYSZ/LSM--20 Cell at Various Oxygen Pressures20 Cell at Various Oxygen Pressures

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.00 0.02 0.04 0.06 0.08 0.10 0.12Total Electrode Polarization Loss, V

Cur

rent

Den

sity

, A/c

m2

0.21

96.2 82.6 69.0 55.4 41.8 28.2 21.414.6

9.2

7.1

5.1

3.0

2.0

1.0

850oC


EIS Shows Diminished LSM-20 Electrode Losses with Increased Oxygen Partial Pressure; Ohmic

Contributions are Constant (OCV)

EIS Shows Diminished LSMEIS Shows Diminished LSM--20 Electrode Losses 20 Electrode Losses with Increased Oxygen Partial Pressure; with Increased Oxygen Partial Pressure; OhmicOhmic

Contributions are Constant (OCV)Contributions are Constant (OCV)-0.25

-0.2

-0.15

-0.1

-0.05

02.1 2.2 2.3 2.4 2.5 2.6 2.7

Z'

Z"

1.002.023.045.087.129.1614.6121.4128.2141.8255.4269.0382.6396.24

pressure increasing


Half-Cell Measurements for LSM-20/YSZ Show Symmetric Cathodic and Anodic PerformanceHalfHalf--Cell Measurements for LSMCell Measurements for LSM--20/YSZ Show 20/YSZ Show Symmetric Symmetric CathodicCathodic and Anodic Performanceand Anodic Performance

850oC


Performance of LSM-20 Improves as P(O2)1/2 Over Wide Pressure Range in Oxygen

Performance of LSMPerformance of LSM--20 Improves as P(O20 Improves as P(O22))1/2 1/2 Over Over Wide Pressure Range in OxygenWide Pressure Range in Oxygen


Smaller P(O2) Dependence Found for Mixed-Conducting LSM-Ceria Composite

Smaller P(OSmaller P(O22) Dependence Found for ) Dependence Found for MixedMixed--Conducting LSMConducting LSM--Ceria CompositeCeria Composite


EIS Results for LSM-Ceria – Gas

Adsorption versus Charge Transfer

EIS Results for EIS Results for LSMLSM--Ceria Ceria –– Gas Gas

Adsorption versus Adsorption versus Charge TransferCharge Transfer

100 101 102 103 104 105

-0.2

-0.1

0

0.1

Frequency (Hz)

Z''

750oC

A B

A – Low frequency arc arising from adsorption, dissociation, ~P1/2

dependenceB – High frequency arc arising from charge transfer processes,~P1/4

to P1/3 dependence

P increasing


LSF-20 Shows Low P Dependence – Greater Charge Transfer Contribution; Au Results Consistent With Greater

Gas Adsorption Contribution

LSFLSF--20 Shows Low P Dependence 20 Shows Low P Dependence –– Greater Charge Greater Charge Transfer Contribution; Au Results Consistent With Greater Transfer Contribution; Au Results Consistent With Greater

Gas Adsorption ContributionGas Adsorption Contribution


Conclusions and Future Work-High Pressure StudiesConclusions and Future WorkConclusions and Future Work--High Pressure StudiesHigh Pressure StudiesCathode performance was found to improve with increased pressure, to ~100 bar O2 partial pressure

For LSM and Au, exchange currents followed a P1/2

dependence over broad pressure ranges, attributed to dominant gas adsorption processesFor more active LSM/ceria and LSF, a lower P dependence was found, attributed to relatively greater charge transfer reaction contributions

Future studies will address pressure effects on anode performance

Concentration polarization expected to play greater roleHigh absolute steam concentrations in coal gasRole of coal gas contaminants at high pressureComparison to/validation of anode modeling e.g. Gemmen and Trembly JPS 2006 model


LSM Sintering at Low TemperatureLSM Sintering at Low TemperatureLSM Sintering at Low TemperatureObjectives: develop an electrically conductive, chemically and physically compatible, and stable contact paste to bond the SOFC cathode to the interconnect plate.Approach: Use LSM

Use alternating air and nitrogen flows to densify LSM at temperatures far below those usually required, exploiting that material’s unusual defect chemistry.Evaluate mechanical strength of bonds formed to the cathode and to the spinel-coated ferritic steel interconnect, electrical properties, and interfacial phase stability.Work with SECA modeling tasks to establish contact paste performance criteria, implement in stack.

Conclusions:LSM can be densified by alternating air/nitrogen cycles.The effect decreases with increased Sr content.Bond strengths to spinel-coated ferritic steel of ~2 MPa or greater achieved in a few hours – process not yet optimized.


Background – LSM Sintering at Low Temperature

Background Background –– LSM Sintering at Low LSM Sintering at Low TemperatureTemperature

LSM exhibits “excess” lattice oxygen in air, expressed as metal vacancies (no oxygen interstitials in perovskite).Thermal cycling previously shown to enhance LSM densification:

Series of recent publications by Mori, ascribing mechanism to phase changes associated with oxygen gain/loss1997 patent by Kuo et al. on suppression of effect by aliovalent cationsubstitution on both A-and B-sites.

Our work focuses on PO2cycling (air/nitrogen), expected to affect lattice oxygen concentrations more strongly than thermal cycling.

Kuo et al. J Solid State Chemistry, 1989; 83(1): 52.

Oxygen Non-Stoichiometry in LSM-10

Nitrogen (10 ppm O2) Air


Enhanced Shrinkage of LSM-10 Occurs When Thermally Cycled in Air, Minimal at Constant

Temperature or When Cycled in Nitrogen

Enhanced Shrinkage of LSM-10 Occurs When Thermally Cycled in Air, Minimal at Constant

Temperature or When Cycled in Nitrogen

0.0E+00

2.0E-03

4.0E-03

6.0E-03

8.0E-03

1.0E-02

1.2E-02

1.4E-02

1.6E-02

0 20000 40000 60000 80000 100000 120000 140000Time, s

Dim

ensi

onal

Cha

nge,

mm

/mm

0

200

400

600

800

1000

Tem

pera

ture

, o C

10 ppm O2

Air

rate=2.10-9 mm/mm-s

rate=3.10-6 mm/mm-srate=3.10-7 mm/mm-s

Initial sintered density = 55 percent


Air/Nitrogen Cycles Also Results in LSM-10 Shrinkage, Effect Visible as Low as 700oC

Air/Nitrogen Cycles Also Results in LSMAir/Nitrogen Cycles Also Results in LSM--10 10 Shrinkage, Effect Visible as Low as 700Shrinkage, Effect Visible as Low as 700ooCC

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01

0 100 200 300 400 500 600 700 800 900Time (minutes)

Expa

nsio

n (m

m/m

m)

700oC y = -7.9E-07x800oC y = -6.9E-06x

900oC y = -1.1E-05x

1000oC y = -6.1E-06x

1100oC y = -7.5E-06x

1200oC y = -2.7E-05x

Initial, 55% DenseInitial, 55% Dense

250 2250 2--Hour Cycles at 900Hour Cycles at 900°°CC


1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

600 800 1000 1200 1400Temperature, oC

dY/d

t

55% ID, PO2 cycled62% ID, PO2 cycled68% ID, PO2 cycled55% ID, air62% ID, air68% ID, airMakipirtti-Meng model+ 95% confidence- 95% confidence

Comparison of Enhanced Sintering Rates due to PO2 Cycling to Expected Rates for LSM-10

Comparison of Enhanced Sintering Rates due to Comparison of Enhanced Sintering Rates due to POPO22 Cycling to Expected Rates for LSMCycling to Expected Rates for LSM--1010

nYYYYTnkdtdY /1]/)1)[(1()( −−=

Makipirtti-Meng model:

33

33

fo

to

fo

to

LLLL

VVVV

Y−−

=−−

=V=volumeL=length


PO2-Cycling Shrinkage Suppressed by Sr Substitution

POPO22--Cycling Shrinkage Suppressed Cycling Shrinkage Suppressed by by SrSr SubstitutionSubstitution

-5.E-07

-4.E-07

-3.E-07

-2.E-07

-1.E-07

0.E+00600 700 800 900 1000 1100 1200 1300

Temperature, °C

Shrin

kage

mm

/mm

/min

LSM-0LSM-5LSM-10LSM-15LSM-20


Transient Coexistence of Oxygen and Metal Vacancies Leads to Enhanced Mobility

Transient Coexistence of Oxygen and Metal Transient Coexistence of Oxygen and Metal Vacancies Leads to Enhanced MobilityVacancies Leads to Enhanced Mobility

La(Sr)MnO3+δ+VM’’’ La(Sr)MnO3

(10 ppmO2)

O2evolved

mequilibriuVV OM >••,'''

La(Sr)MnO3

(air)O2uptake

La(Sr)MnO3+δ+ VM’’’

mequilibriuVM <'''

stoichiometric

oxygen-excess

pore

Air → Nitrogen:•Excess metal and oxygen vacancies present•Shrinkage occurs

Nitrogen → Air:•metal vacancies less than equilibrium•Minimal shrinkage


LSM Contact Paste Sintering at 900oC:Air/Nitrogen Cycles Results in BondingLSM Contact Paste Sintering at 900LSM Contact Paste Sintering at 900ooC:C:Air/Nitrogen Cycles Results in BondingAir/Nitrogen Cycles Results in Bonding

0

1

2

3

4

5

0 2 4 6 8 10 12Time, hours

Frac

ture

Stre

ngth

, MP

a

10 min Air/NitrogenCyclesAir

LSM-10

Spinel Coating

Crofer Plate

•Five specimens measured for each condition•Coverage was incomplete – ink not optimized•LSM bond strength to spinel coating estimated at >8 MPa (fractured at epoxy bond)


Conclusions – LSM Sintering at Low Temperature

Conclusions Conclusions –– LSM Sintering at Low LSM Sintering at Low TemperatureTemperature

Alternating air/nitrogen cycles is an effective way to induce LSM sintering at low temperatures

Orders of magnitude higher rates than expected for air sintering for T<1000oCFor LSM-10, maximum sintering rate at 900oC due to air/nitrogen cyclingShrinkage rates decrease with increased Sr content – LSM-20 shows no enhanced shrinkage

The mechanism of sintering is defect chemistry-driven: coexistence of transient oxygen vacancies with metal vacancies leads to enhanced mobility; no effect for stoichiometric or oxygen-deficient compositions such as LSFBond strengths between spinel-coated ferritic steel coupons of ~2 MPa or greater achieved in a few hours with un-optimized inks, incomplete coverage; estimated LSM-10 to spinel bond strength >~8 MPa (failed at expoxy bond)


Future Work – Low Temperature Sintering of LSM-Based Contact Pastes

Future Work Future Work –– Low Temperature Sintering Low Temperature Sintering of LSMof LSM--Based Contact PastesBased Contact Pastes

Improve ink formulations, burn-out/sintering procedures to achieve more uniform coverage and higher strengthEvaluate mechanical bonding strength to coated interconnect coupons and to porous cathodesEvaluate long-term electrical properties using fixture developed by Yang et al. (two paste/interconnect interfaces, two paste/porous cathode interfaces); evaluate long-term interfacial stabilityWork with SECA modeling tasks to establish contact paste performance criteria, optimize contact paste placement and coverage in a stack, evaluate critical stressesImplement contact paste compositions and processing methods in SECA stack tests


AcknowedgementsAcknowedgementsAcknowedgementsWork was supported by the US Department of Energy-Office of Fossil Energy’s SECA ProgramWe are indebted to the NETL management team for helpful discussions – especially to Wayne Surdoval and Lane Wilson for suggestions on applying LSM low temperature sintering concepts to contact paste processingPacific Northwest National Laboratory is operated for the US Department of Energy by Battelle

1. Electrode Studies at High Pressure 2. LSM Sintering at ......8th Annual SECA Workshop San Antonio...

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