low cost, durable, contaminant-tolerant cathodes for sofcs · 2018. 11. 6. · ─ develop robust...

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


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.


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


• 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


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


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


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


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.


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


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.


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.


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


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


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


•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


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


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.


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


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


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


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


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


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


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


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.


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


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


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


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


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


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


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.


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


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)


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


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


DOE-SECA core technology programGrant No. FE0026106

Acknowledgement

Discussions with Dr. Briggs White

low cost, durable, contaminant-tolerant cathodes for sofcs · 2018. 11. 6. · ─ develop robust...

Documents