[PDF] - Low cost, Durable, Contaminant-Tolerant Cathodes for SOFCs Project PDF Document

SLIDE 1

1 Understanding SOFC Electrode Surfaces

Project Num ber: FC FE0 0 2610 6 DOE 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 Engineering Center for Innovative Fuel Cell and Battery Technologies Georgia Institute of Technology, Atlanta, GA 30 332-0 245, USA

Presented to DOE-NETL SOFC Kickoff Meeting Dec 3, 2015

Low Cost a nd Dura ble SOFC Ca thod es

Outline

Project inform ation
Project objectives
Technical Approaches
Project structure
Tasks to be perform ed
Milestones and Schedule
Prelim inary Results

2

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Low Cost a nd Dura ble SOFC Ca thod es

Project inform ation

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 a nd Dura ble SOFC Ca thod es

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 perform ance goals.

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Low Cost a nd Dura ble SOFC Ca thod es

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 structurally under operating conditions (w/o contaminants)?

How are these phenomena related to the
bserved electrode kinetics, catalytic

properties, and durability?

Critical questions to be answered

Low Cost a nd Dura ble SOFC Ca thod es 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 performance evaluation 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 m odel 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 m odified LSCF cathodes in commercially available cells under ROC.

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Low Cost a nd Dura ble SOFC Ca thod es

Tasks and Schedule

Task 1: Project Management and Planning; Chemical compatibility Task 2: Charactering the electrochemical behavior under realistic conditions Task 3: Understanding the mechanism of contamination tolerance Task 4: Modeling and rational design of new materials and electrode structures Task 5: Perfecting enhanced performance in button cells

Task FY2015 FY2016 FY2017 Q4 Q1 Q2 Q3 Q4 Q1 1 2 3 4 5

Low Cost a nd Dura ble SOFC Ca thod es

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.

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Low Cost a nd Dura ble SOFC Ca thod es

Task 2 Charactering the electrode behavior under realistic conditions

 ECR (Electrical Conductivity Relaxation) measurement

Performed by changing the oxygen partial pressure while recording

the 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 a nd Dura ble SOFC Ca thod es

Evaluate electrochem ical 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

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Low Cost a nd Dura ble SOFC Ca thod es

Task 3: Understanding the m echanism of contam ination tolerance Surface 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 situ Raman: monitor the surface chemistry, e.g., interactions between LSCF and B, S and/or Cr. The reaction products are Raman-active.

Low Cost a nd Dura ble SOFC Ca thod es

Surface of Cathode contamination study

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Low Cost a nd Dura ble SOFC Ca thod es 13

OH stretching (3300cm-1) and water bending (1600cm-1)

BZCYYb

BZY

Liu et al., Nano Energy, 1, 448-455, 2012. Yang et al., Science, 326 (5949) 126, 2009.

Low Cost a nd Dura ble SOFC Ca thod es

Understand the performance characteristics - Raman spectroscopy + Surface enhancement

SOFC cathodes

SERS nano probes Surface modifications

SOFC cathodes

Surface modifications Surface enhancement treatment laser laser

colossal augmentation of Raman signal

Normal Raman Surface enhanced Raman

Combination of Raman spectroscopy with surface enhancement technique

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Low Cost a nd Dura ble SOFC Ca thod es

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 deposited SEM as deposited SEM after 450C 1hr in 4%H2 SEM 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 a nd Dura ble SOFC Ca thod es

400 500 600 700 2 4 6 8 10 2000 4000 6000 8000 Time (s) Raman shift (cm-1) Intensity (a.u.)

Sample Points SERS Peak of GDC film

400 600 800 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 SERS net

I I EF 

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Low Cost a nd Dura ble SOFC Ca thod es

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 450

C

in wet C3H8 0h Blank GDC Thin Film

Raman Shift (cm

1)

GDC

3h

1000 1200 1400 1600 1800 2000

Carbon G-band Raman Shift (cm

1)

Ag@SiO2 Blank Ni SERS

Carbon D-band

1000 2000 3000 4000 5000

SERS

Time (s)

NR

1000 2000 3000

NR

Time (s)

SERS

50nm

300 600

Air 4% H

2

Air

A dsorbed Oxygen Oxygen Vacancy CeO

2

At 450

C

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 a nd Dura ble SOFC Ca thod es

 Cr2O3 and SrCrO4

bserved on poisoned

porous LSCF surface.  Increasing the H2O concentration makes the Cr poisoning more severe.

250 500 750 1000 1250 Pristine 3% H2O+Cr 5% H2O+Cr 10% H2O+Cr

Intensity, a.u. Raman Shift, cm

1

Cr2O3 SrCrO4

SERS Analysis of Cr Poisoned Sam ples (Direct Contact)

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Low Cost a nd Dura ble SOFC Ca thod es

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 situ conditions: temperature, atmosphere and bias Liu et al., Materials today (2011) 14, 534.

Low Cost a nd Dura ble SOFC Ca thod es

c

Surface Bulk 0.1o 0.2o 0.4o 0.75o 1.0o

Intensity (a.u.)

7 8 9 10

(002) (002) (101) (200)

d 0.1o

5 10 15 20 25 30 35 Two theta (

)

K0.51Mn0.93O2 Mn3O4 K0.6MnO2 Mn5O8 α-Mn2O3 K0.51Mn0.93O2 Mn3O4 K0.6MnO2 Mn5O8 α-Mn2O3

A gradient in

xidation state
f 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

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Low Cost a nd Dura ble SOFC Ca thod es 21

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 valance Cell voltage ( V )

0.78V 0.24V

0.02V
0.18V
0.28V
0.45V
0.61V
0.76V

Normalized intensity ( 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 valance Cell voltage ( V )

0.78V 0.24V

0.02V
0.18V
0.28V
0.45V
0.61V
0.76V

Normalized intensity ( 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 valance Cell voltage ( V )

0.78V 0.24V

0.02V
0.18V
0.28V
0.45V
0.61V
0.76V

Normalized intensity ( 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 valance Cell voltage ( V )

0.78V 0.24V

0.02V
0.18V
0.28V
0.45V
0.61V
0.76V

Normalized intensity ( 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 valance Cell voltage ( V )

0.96V 0.65V 0.44V 0.25V 0.03V

0.27V
0.79V
0.95V

Energy ( eV ) (b) optimized

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 2.6 2.8 3.0 3.2 1st charging 1st discharging

Mn valance Cell voltage ( V )

0.96V 0.65V 0.44V 0.25V 0.03V

0.27V
0.79V
0.95V

Energy ( eV ) (b) optimized

A B C

b

Normalized intensity ( a.u. ) Normalized intensity ( a.u. )

Nano-porous MnOx

Nano Lett., 2012, 12 (7) 3483; dx.doi.org/10.1021/nl300984y

Low Cost a nd Dura ble SOFC Ca thod es

Synchrotron-Enabled XRD, XAS, & XPS

Liu, Alamgir et al., Materials today (2011) 14, 534.

Reversible changes in oxidation state

Mn is reduced at High Tem p. Peak splitting and shifting at 2.8 Å represent slight structural deformation. The peak growth and new features indicate

rdering of the Mn local structure.

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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 a nd Dura ble SOFC Ca thod es

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 Analysis to predict certain chemical, catalytic, and transport properties of new materials with different morphologies Continuum Modeling to predict the performance of the new materials

Electrochemical measurements to validate predictions in a most direct way

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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 a nd Dura ble SOFC Ca thod es

Task 5: Perfecting enhanced perform ance 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

f S, B and/or Cr.

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Validation in actual fuel cells

Fabrication of anode-supported cells of high performance;
Demonstration of enhanced durability while maintaining high

performance by infiltrating newly developed catalysts into porous LSCF cathode;

Demonstration of enhanced durability in commercially available cells;
Post-analysis of tested cells

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Low Cost a nd Dura ble SOFC Ca thod es

Prelim inary 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 Ram an Spectroscopy

200 400 600 800 1000

LSM LCNF PNM PCM

Intensity (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

SLIDE 16

16 Understanding SOFC Electrode Surfaces Catalyst coating enhances Cr tolerance

20 40 60 80 100

0.114
0.112
0.110
0.108
0.106
0.104
0.102

PNM infiltrated LSCF Blank LSCF

Cathodic overpotential / V Time / h

3% H2O+Cr (Direct Contact)@750

C

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 a nd Dura ble SOFC Ca thod es

Experim ental conditions

Electrical conductivity Relaxation

4‐probe DC method
Standard gas mixtures of O2 and Ar
Flow rate: 300mL/min
Temperature: 550‐800oC

Digital multimeter

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Low Cost a nd Dura ble SOFC Ca thod es

In progress

) ( ) / exp( 2 1 ) ( ) ( ) ( ) ( ) (

2 2 1 2 1 2 2 1 2 1

C C l t D C t t g

m m chem m m

         

 

      

Dchem and kchem were extracted with fitting by a least square method to an analytical solution of Eq. g(t)

Low Cost a nd Dura ble SOFC Ca thod es

In progress: ECR test for Blank LSCF

PBSCF k (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

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Low Cost a nd Dura ble SOFC Ca thod es

200nm 500nm 200nm 500nm

SEM Im ages of LSCF Film s

Substrate Cross Section Surface Polycrystalline GDC Single Crystal Si 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 a nd Dura ble SOFC Ca thod es

36

SEM Im ages of the Patterned Electrodes

~40m 71m 111m 145m 100m 1m

Top: high magnification, at the middle of an electrode Right: low magnification, various electrode sizes