1
Understanding SOFC Electrode Surfaces
Project Number: FC FE0026106DOE Project Manager: Dr. Briggs White
Meilin Liu, Yu Chen, Ryan Murphy
Low cost, Durable, Contaminant-Tolerant Cathodes for SOFCs
DOE-NETL SECA-CTP
School of Materials Science and EngineeringCenter for Innovative Fuel Cell and Battery Technologies
Georgia Institute of Technology, Atlanta, GA 30332-0245, USA
Presented toDOE-NETL SOFC Kickoff Meeting
Dec 3, 2015
Low Cost and Durable SOFC Cathodes
Outline
Project information
Project objectives
Technical Approaches
Project structure
oTasks to be performed oMilestones and Schedule
Preliminary Results
2
2
Low Cost and Durable SOFC Cathodes
Project information
Team members─ Georgia Tech (and an Industry partner for Phase II)
Project description─ Modify LSCF cathodes for long-term stability under realistic
conditions to enhance activity and stability─ Enhance stability against B, S, and combined effect of contaminants;
What do we expect?
─ Unravel LSCF cathode degradation mechanism when exposed to Cr, B, S and formulate strategies to mitigate degradation against contaminants (B, S, Cr, and combined effect);
─ Develop robust and electro-active catalysts against contaminants
─ Enhance the performance and durability of LSCF-based cathodes by application of a thin-film coating of robust electro-catalysts.
Low Cost and Durable SOFC Cathodes
Motivation
Cathode durability is critical to long-term reliable SOFC performance for
commercial deployment.
Current state-of-the-art SOFC cathode materials are susceptible to
degradation due to contaminants under realistic operating conditions
(ROC).
Mitigating the stability issues by design of new materials or electrode
structures will reduce the cost of SOFCs and help to meet DOE cost
and performance goals.
3
Low Cost and Durable SOFC Cathodes
• How does the electrode surface differ from the bulk chemically and structurally when exposed to air with contaminants (S, B, Cr, etc.) under operating conditions?
• How do specific elements on electrode surface change chemically and structurallyunder operating conditions (w/o contaminants)?
• How are these phenomena related to the observed electrode kinetics, catalyticproperties, and durability?
Critical questions to be answered
Low Cost and Durable SOFC Cathodes
6
Project Objectives
To identify/develop new catalysts that are compatible chemically with the state-of-the-art cathode materials at high temperatures required for fabrication and with contaminates commonly encountered under operating conditions (Cr, S, B, and combined effect);
To evaluate the electro-catalytic activity toward ORR of the chemically-stable materials when exposed to different types of contaminants using electrical conductivity relaxation measurements on bar samples and performanceevaluation of catalyst-infiltrated cathodes;
To unravel the contamination-tolerant mechanisms of the new catalyst coatings under realistic environmental conditions (with different types of contaminants) using powerful in situ and in operando characterization techniques performed on model cells with thin-film/pattern electrodes, as guided by modeling and simulation;
To establish scientific basis for rational design of new catalysts of high tolerance to contaminants;
To validate the long term stability of modified LSCF cathodes in commercially available cells under ROC.
4
Low Cost and Durable SOFC Cathodes
Tasks and ScheduleTask 1: Project Management and Planning; Chemical compatibilityTask 2: Charactering the electrochemical behavior under realistic conditions Task 3: Understanding the mechanism of contamination toleranceTask 4: Modeling and rational design of new materials and electrode structures Task 5: Perfecting enhanced performance in button cells
Task
FY2015 FY2016FY2017
Q4 Q1 Q2 Q3 Q4 Q1
1
2
3
4
5
Low Cost and Durable SOFC Cathodes
Task 1: PMP and Chemical compatibility
Finalize Project Management Plan (PMP) in order to meet all technical, schedule, and budget objectives of the project;
Coordinate activities in order to effectively complete all tasks;
Ensure that project plans, results, and decisions are appropriately documented and project reporting and briefing requirements are satisfied.
Use phase equilibria databases to guide the selection of highly-active and robust catalysts.
Evaluate the chemical compatibility of each catalyst with these contaminants using XRD and Raman spectroscopy.
5
Low Cost and Durable SOFC Cathodes
Task 2 Charactering the electrode behavior under realistic conditions
ECR (Electrical Conductivity Relaxation) measurement
• Performed by changing the oxygen partial pressure while recordingthe electrical relaxation curves of dense bar samples (w/o catalyst);
• Oxygen surface exchange rates of the cathode materials will be calculated from fitting the relaxation curves.
Low Cost and Durable SOFC Cathodes
Evaluate electrochemical stability of catalyst-coated cathodes
Two types of cells:
• Symmetrical cells of porous LSCF cathode with 3-electrode configuration;
Objective: To determine the sensitivity of cathode performance to the type and concentration of contaminants (S, B and Cr) under various testing conditions
• Thin-film dense LSCF electrode or patterned electrode with an asymmetrical electrode configuration;
Objective: To facilitate the interface analysis and correlate the degradation mechanism with the geometric factors, revealing the major path of surface reaction on the cathodes
6
Low Cost and Durable SOFC Cathodes
Task 3: Understanding the mechanism of contamination toleranceSurface Characterization
Changes in surface chemistry, structure, and morphology of LSCF cathodes, with or without exposure to various contaminants, will be characterized using SEM, AFM, EDX, XRD, Auger, XPS, Raman (SERS), synchrotron-based X-ray analyses under in situ or ex situ conditions.
in situ and ex situRaman: monitor the surface chemistry, e.g., interactions between LSCF and B, S and/or Cr. The reaction products are Raman-active.
Low Cost and Durable SOFC Cathodes
Surface of Cathode contamination study
7
Low Cost and Durable SOFC Cathodes 13
OH stretching (3300cm-1) and water bending (1600cm-1)
BZCYYb
BZYLiu et al., Nano Energy, 1, 448-455, 2012.
Yang et al., Science, 326 (5949) 126, 2009.
Low Cost and Durable SOFC Cathodes
Understand the performance characteristics -Raman spectroscopy + Surface enhancement
SOFC cathodes
SERS nano probesSurface modifications
SOFC cathodes
Surface modifications
Surface enhancement treatment
laserlaser
colossal augmentation of Raman signal
Normal Raman
Surface enhanced Raman
Combination of Raman spectroscopy with surface enhancement technique
8
Low Cost and Durable SOFC Cathodes
TEM images showing core-shell nanoparticles. Size of the silver NPs: 50nm Thickness of the SiO2: 5nm
SEM images . High temperature treatment did not change the shape and distribution.
TEM
Ag
SiO2
TEM
SEM as depositedSEM as deposited SEM after 450C 1hr in 4%H2SEM after 450C 1hr in 4%H2
Electrode
Au/Ag SERS patterns with robust coating
Gas
Heating
in situ SERS with Ag@SiO2 Particles
Low Cost and Durable SOFC Cathodes
400500
600700
0
2
4
6
8
10
0
2000
4000
6000
8000
Time (s)
Raman shift (cm-1)
Inte
ns
ity
(a.u
.)
Sample Points
SERS Peak of GDC film
400 600 800
0
1000
2000
3000
4000
5000
Wavenumbers (cm-1)
GDC blank Ag sputtered
F2g
SERS with Ag Nanoparticles (NPs)
80nm thick GDC thin film
Enhancement factor of F2g
mode is about 50
Intensity variation: 3%
Reliable for semi-quantitative analysis
Blank
SERSnet I
IEF
9
Low Cost and Durable SOFC Cathodes
•Developed thermally robust & chemically inert Ag@SiO2 core-shell nanoparticles for in situ SERS at 450C.• Detected incipient stage carbon deposition on nickel.• Detected surface defects on CeO2 powders.
In situ SERS for Identification of Surface Species
C3H8
Coking
450°C
SOFC Anode
Ag@SiO2 NPs
In‐situ SERS with core‐shell nano probes
400 600
5h
SERS 450oCin wet C
3H
8 0h
Blank GDC Thin Film
Raman Shift (cm-1)
GDC
3h
1000 1200 1400 1600 1800 2000
CarbonG-band
Raman Shift (cm-1)
Ag@SiO2Blank Ni
SERSCarbonD-band
0 1000 2000 3000 4000 5000
SERS
Time (s)
NR
0 1000 2000 3000
NR
Time (s)
SERS
50nm
300 600
Air
4% H2
Air
AdsorbedOxygen
OxygenVacancy
CeO2
At 450oC
800 1000
Raman Shift (Δcm‐1)
Detection of Oxygen Vacancy on CeO2
Detection of Coking on nickel surface
Detection of Surface defects on CeO2 powders
SERS probes showed thermal integrity, after heat treatment.
Coking
Regeneration
Low Cost and Durable SOFC Cathodes
Cr2O3 and SrCrO4observed on poisoned porous LSCF surface.
Increasing the H2O concentration makes the Cr poisoning more severe.
250 500 750 1000 1250
Pristine 3% H
2O+Cr
5% H2O+Cr
10% H2O+Cr
Inte
nsity
, a.u
.
Raman Shift, cm-1
Cr2O
3
SrCrO4
SERS Analysis of Cr Poisoned Samples (Direct Contact)
10
Low Cost and Durable SOFC Cathodes
Synchrotron-Enabled XRD, XAS, & XPS
Provides unique ability to study bulk and surface structures simultaneously via fluorescent X-ray absorption spectroscopy (XAS), Auger electron yield, and X-ray diffraction (XRD)
Probe near-surface of electrode and identify surface composition, structure and chemical environment of specified element under in situ conditions: temperature, atmosphere, and bias
Examine interface reactions between electrode and electrolyte under in situconditions: temperature, atmosphere and bias
Liu et al., Materials today (2011) 14, 534.
Low Cost and Durable SOFC Cathodes
c
Surface
Bulk
0.1o
0.2o
0.4o
0.75o
1.0o
Inte
nsity
(a.
u.)
7 8 9 10
(002)
(002)
(101)(200)
d 0.1o
5 10 15 20 25 30 35
Two theta (o)
K0.51Mn0.93O2
Mn3O4
K0.6MnO2
Mn5O8
α-Mn2O3
K0.51Mn0.93O2
Mn3O4
K0.6MnO2
Mn5O8
α-Mn2O3 A gradient in oxidation state of cation along the thickness direction.
Glancing-Angle XRD
“Surface” is structurally quite different from that of “bulk”
Nano Lett., 2012, 12 (7) 3483; dx.doi.org/10.1021/nl300984y
11
Low Cost and Durable SOFC Cathodes21
Effect of Electrical Polarization
6540 6550 6560 6570
0.0
0.5
1.0
1.5
-1.0 -0.5 0.0 0.5 1.0
3.0
3.2
3.4
3.6
1st charging 1st discharging
Mn
vala
nce
Cell voltage ( V )
0.78V 0.24V -0.02V -0.18V -0.28V -0.45V -0.61V -0.76V
Nor
mal
ized
inte
nsity
( a
.u.
)
Energy ( eV )
(a) as‐prepared
A
B
C
6540 6550 6560 6570
0.0
0.5
1.0
1.5
-1.0 -0.5 0.0 0.5 1.0
3.0
3.2
3.4
3.6
1st charging 1st discharging
Mn
vala
nce
Cell voltage ( V )
0.78V 0.24V -0.02V -0.18V -0.28V -0.45V -0.61V -0.76V
Nor
mal
ized
inte
nsity
( a
.u.
)
Energy ( eV )
(a) as‐prepared
A
B
C
6540 6550 6560 6570
0.0
0.5
1.0
1.5
-1.0 -0.5 0.0 0.5 1.0
3.0
3.2
3.4
3.6
1st charging 1st discharging
Mn
vala
nce
Cell voltage ( V )
0.78V 0.24V -0.02V -0.18V -0.28V -0.45V -0.61V -0.76V
Nor
mal
ized
inte
nsity
( a
.u.
)
Energy ( eV )
(a) as‐prepared
A
B
C
6540 6550 6560 6570
0.0
0.5
1.0
1.5
-1.0 -0.5 0.0 0.5 1.0
3.0
3.2
3.4
3.6
1st charging 1st discharging
Mn
vala
nce
Cell voltage ( V )
0.78V 0.24V -0.02V -0.18V -0.28V -0.45V -0.61V -0.76V
Nor
mal
ized
inte
nsity
( a
.u.
)
Energy ( eV )
(a) as‐prepared
A
B
C
In-situ XANES during discharge
(a) In-situ XANES spectra showed an entire edge shift towards lower energy in a continuous manner, suggesting that the charge storage is mostly associated with the Mn3+/Mn4+ redox reactions as conventionally believed. (b) The behavior of the nano-porous MnOx is different.
Amorphous MnO2
6540 6550 6560 6570
0.0
0.5
1.0
1.5
-1.0 -0.5 0.0 0.5 1.0
2.6
2.8
3.0
3.2
1st charging 1st discharging
Mn
va
lan
ce
Cell voltage ( V )
0.96V 0.65V 0.44V 0.25V 0.03V -0.27V -0.79V -0.95V
Energy ( eV )
(b) optimized
A
BC
6540 6550 6560 6570
0.0
0.5
1.0
1.5
-1.0 -0.5 0.0 0.5 1.0
2.6
2.8
3.0
3.2
1st charging 1st discharging
Mn
va
lan
ce
Cell voltage ( V )
0.96V 0.65V 0.44V 0.25V 0.03V -0.27V -0.79V -0.95V
Energy ( eV )
(b) optimized
A
BC
b
Nor
mal
ize
d in
tens
ity (
a.u
. )N
orm
aliz
ed
inte
nsity
( a
.u. )
Nano-porous MnOx
Nano Lett., 2012, 12 (7) 3483; dx.doi.org/10.1021/nl300984y
Low Cost and Durable SOFC Cathodes
Synchrotron-Enabled XRD, XAS, & XPS
Liu, Alamgir et al., Materials today (2011) 14, 534.
Reversible changes in oxidation state
Mn is reduced at High Temp.Peak splitting and shifting at 2.8 Å represent slight structural deformation.
The peak growth and new features indicate ordering of the Mn local structure.
12
Low Cost and Durable SOFC Cathodes
Task 4: Modeling/rational design of new materials/electrode structures
Modeling, simulation as well as prediction tools will be used to help in formulating an effective strategy to mitigate the stability issues and predict new catalyst materials that can enhance the stability of LSCF.
Low Cost and Durable SOFC Cathodes
Design of new materials
The combination of Theoretical/continuum models and the well-controlled experiments will lead to new materials and novel structures for cathode of low polarization resistance and high durability.
Macro-prediction
Validation
Micro-prediction
Theoretical Analysisto predict certain chemical, catalytic, and transport properties of new materials with different morphologies
Continuum Modelingto predict the performance of the new materials
Electrochemical measurementsto validate predictions in a most direct way
13
Low Cost and Durable SOFC Cathodes
Surface modification
• Develop catalysts of high activity and durability
• Infiltrate catalysts into porous cathode backbones to mitigate the effect of contaminants
Catalysts Solution Infiltration
Surface Modified Cathode
Low Cost and Durable SOFC Cathodes
Task 5: Perfecting enhanced performance in button cells
• Proper fabrication processes will then be developed for implementation of the new catalysts/structure in actual cells.
• Button cells with a diameter of about 1” (~2 cm2 active electrode area, for quick check)
• A single cell with dimensions of 4”x4” (~100 cm2 active electrode area) with the help of an industry partner
• Once enhanced tolerance to impurities is demonstrated, the detailed microstructure, morphology, and composition will be carefully characterized using various in-situ and ex-situ measurements.
• New catalysts or structures will be first examined in symmetric cells to characterize the electrochemical behavior of the modified LSCF cathode under ROC with different concentrations of S, B and/or Cr.
14
Low Cost and Durable SOFC Cathodes
Validation in actual fuel cells
• Fabrication of anode-supported cells of high performance;
• Demonstration of enhanced durability while maintaining highperformance by infiltrating newly developed catalysts into porous LSCFcathode;
• Demonstration of enhanced durability in commercially available cells;
• Post-analysis of tested cells
15
Low Cost and Durable SOFC Cathodes
Preliminary Results
Chemical compatibility of catalysts with contaminant (Cr, B, S), using XRD
ECR measurement for blank LSCF
Preparation of LSCF thin films and patterned electrodes
Understanding SOFC Electrode Surfaces
Screening of Catalysts using Raman Spectroscopy
200 400 600 800 1000
LSM
LCNF
PNM
PCM
Inte
nsi
ty (
a.u
)
Raman shift (cm-1)
XCrO4
PCF
BaO
LCF
Blank
Conditions: Crofer 22 APU, 750 oC for 75 h, with air containing 3 % H2O
16
Understanding SOFC Electrode Surfaces
Catalyst coating enhances Cr tolerance
0 20 40 60 80 100
-0.114
-0.112
-0.110
-0.108
-0.106
-0.104
-0.102
PNM infiltrated LSCF Blank LSCF
C
ath
odic
ove
rpo
ten
tial /
V
Time / h
3% H2O+Cr (Direct Contact)@750 oC
Cathodic overpotentials of a catalyst ‐infiltrated LSCF and blank LSCF cathode in contact with Cr materials at 3% H2O+1% CO2, measured at 750oC at a constant voltage of 0.40 V and 0.25 V at 750oC, respectively.
Low Cost and Durable SOFC Cathodes
Experimental conditions
Electrical conductivity Relaxation
4‐probe DC method Standard gas mixtures of O2 and Ar Flow rate: 300mL/min Temperature: 550‐800oC
Digital multimeter
17
Low Cost and Durable SOFC Cathodes
In progress
)(
)/exp(21
)0()(
)0()()(
221
21
221
2
1 CC
ltDCttg
mm
chemm
m
Dchem and kchem were extracted with fitting by a least square method to an analytical solution of Eq. g(t)
Low Cost and Durable SOFC Cathodes
In progress: ECR test for Blank LSCF
PBSCFk (cm/s) D (cm2/s)
Temp.
600oC 1.00±0.0188 x 10-7 4.87±0.0188 x 10-11
650oC 3.22±0.0188 x 10-7 2.28±0.0188 x 10-10
700oC 4.26±0.0188 x 10-6 1.73±0.0188 x 10-8
Relative conductivity
Experiment condition• Temperature range: 600,650,700oC• pO2 range: 1 atm to 0.01 atm• Flow rate: 300mL/min• O2 and Ar mixture gas• Current: 10mA
18
Low Cost and Durable SOFC Cathodes
200nm 500nm
200nm 500nm
SEM Images of LSCF Films
SubstrateCross
SectionSurface
PolycrystallineGDC
Single CrystalSi
The sputtered LSCF film with 1:1 A/B ratio is annealed at 800ºC for 1hr, and SEM characterization is performed to identify the sputtering rate (~30nm/hr) and surface morphology.
Low Cost and Durable SOFC Cathodes 36
SEM Images of the Patterned Electrodes
~40m
71m
111m
145m
100m
1m
Top: high magnification, at the middle of an electrode
Right: low magnification, various electrode sizes
19
Low Cost and Durable SOFC Cathodes
DOE-SECA core technology programGrant No. FE0026106
Acknowledgement
Discussions with Dr. Briggs White