Ba deficient NdBaCo2O5+δ oxides for Intermediate Temperature Solid Oxide Fuel Cell Cathodes

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Ba-deficient NdBaCo

₂

O

_5+δ

oxides for intermediate

temperature SOFC

cathodes

Renato Pelosato, A. Donazzi, G. Cordaro, D. Stucchi, C. Cristiani, G. Dotelli

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FINAL REMARKS

• MAIN OUTCOMES

• WHAT IS MISSING

• WHAT TO DO NEXT

outlinene

FINAL REMARKS ELECTROCHEMICAL CHARACTERIZATI COMPATIBILITY vs ELECTROLYTES CHARACTERIZATION BACKGROUND

ELECTROCHEMICAL

CHARACTERIZATION

• SYMMETRIC CELLS

• ASRs

• EQUIVALENT CIRCUITS

• [CONDUCTIVITY]

COMPATIBILITY vs

ELECTROLYTES

• vs LSGM

• vs GDC

SYNTHESIS &

CHARACTERIZATION

• SSR SYNNTHESIS

• STRUCTURAL

• MICROSTRUCTURAL

• PHYSICO-CHEMICAL

BACKGROUND

• SOFC CATHODES

• LAYERED PEROVSKITES

• UNDERSTOICHIOMETRY

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900-1000 °C

HT-SOFC

_IT-SOFC

500-800 °C

background

e- O₂ O2- Electrolyte MIEC Cathode FINAL REMARKS ELECTROCHEMICAL CHARACTERIZATI COMPATIBILITY vs ELECTROLYTES CHARACTERIZATION BACKGROUND

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AA’B₂O_5+δ general formula with 0 < δ < 1

high electronic (metallic) conductivity at intermediate to high temperatures

Can accept high oxygen vacancy concentration

high oxygen ion transport

high oxygen surface exchange coefficients

cathode materials: layered perovskites

A. Tarancón A, Skinner SJ, et al. ‘Layered perovskites as promising cathodes for intermediate temperature solid oxide fuel cells’ Journal of Materials Chemistry 2007, 17, 3175.

Kim JH, Manthiram A, ‘LnBaCo2O5+δ Oxides as Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells’ Journal of the Electrochemical Society 2008, 155, B385.

FINAL REMARKS

ELECTROCHEMICAL CHARACTERIZATI

COMPATIBILITY vs ELECTROLYTES CHARACTERIZATION

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layered perovskites

A-O B-O A-O

ABO

_3-δ

A,A’-O B-O A,A’-O

A

_1-x

A’

_x

BO

_3-δ

A-O B-O A’-O B-O A-O

AA’B

₂

O

_5+δ

A- B-O A’-O B-O A- FINAL REMARKS ELECTROCHEMICAL CHARACTERIZATI COMPATIBILITY vs ELECTROLYTES CHARACTERIZATION BACKGROUND

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Survey on REBaCo

₂

O

_5+d

RE La Y Sm Nd Pr Gd FINAL REMARKS ELECTROCHEMICAL CHARACTERIZATI COMPATIBILITY vs ELECTROLYTES CHARACTERIZATION BACKGROUND

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A

A’

_1-x

B

₂

O

₅₊_δ

PrBa

_1-x

Co

₂

O

_5+δ Pang et al., J. Power Sources, 2012. 204: 53-59

cation deficiency in layered perovskites

Max deficiency: 15%

Cryst structure Tetragonal, unchanged Conductivity decrease monotonic ASR and Ea decrease min. at x=0.10 TEC --

Pang et al. J. Power Sources, 2013. 240: 54-59

LaBa

_1-x

Co

₂

O

_5+δ

Max deficiency: 8%

Cryst structure Orthorombic, shrinking in a,b Conductivity increase, minimum at x=0.03 ASR and Ea decrease monotonic

TEC nearly no effect

FINAL REMARKS

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A

_1-x

A’B

₂

O

₅₊_δ

Sm

_1-x

BaCo

₂

O

_5+δ Jiang at al. Int. J. Hydrogen En. 2014. 39: 10817-10823

Pr

_1+x

Ba

_1-x

Co

₂

O

_5+δ

cation deficiency in layered perovskites

A

_1+x

A’

_1-x

B

₂

O

₅₊_δ

Jiang at al. Electrochimica Acta 2014, 133: 364– 372

Max deficiency: 5%

Cryst structure Orthorombic, enlarge Conductivity decrease, monotonic ASR and Ea decrease, monotonic TEC --

Max ‘deficiency’: 30%

Cryst structure Tetragonal, shrinking Conductivity decrease, monotonic ASR and Ea U shape, min at x=0.10 TEC decrease monotonic FINAL REMARKS

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experimental work

x formula ID 0.00 NdBaCo₂O_5+δ (NBC0) 0.05 NdBa_0.95Co₂O_5+δ (NBC5) 0.10 NdBa_0.90Co₂O_5+δ (NBC10) 0.20 NdBa_0.80Co₂O_5+δ (NBC20)

NdBa

_1-X

Co

₂

O

₅₊

_δ

Synthesis

• SOLID STATE REACTIVE FIRING: 1100 °C 12h (x2) • Intermediate regrinding

Characterization

• XRD (phase purity / cell refinement) • SEM (microsctructure)

• Con+_{Titration (oxygent content)}

• TG-DTA (oxygen uptake-release)

Chemical compatibility

• 50:50 wt% mixtures with LSGM and CGO Electrolytes

Electrical Characterization

• Preparation of symmetric cells • EIS measurements

FINAL REMARKS

COMPATIBILITY vs ELECTROLYTES SYNTHESIS & CHARACTERIZATION

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20 30 40 50 60 70 80 Int ens ity ( a. u. ) 2θ (°) NBC0 NBC5 * NBC10 NBC20 * *

characterization: crystal structure

Sample Cell a (Å) c (Å) Vol (Å3₎

NBC0 _Tetragonal P4/mmm 3.9046(1) 7.6169(2) 116.13 NBC5 3.8994(1) 7.6167(3) 115.81 NBC10 3.8967(2) 7.6149(4) 115.63 NBC20 3.8960(2) 7.6136(4) 115.56 0.00 0.05 0.10 0.15 0.20 3.80 3.82 3.84 3.86 3.88 3.90 Vol c/2 a , c / 2 [ Å] x in NdBa_1-xCo₂O_5+δ (mol) a 115.0 115.5 116.0 116.5 117.0 Vo l ( Å 3 ) 0.00 0.05 0.10 0.15 0.20 3.80 3.82 3.84 3.86 3.88 3.90 Vol c/2 a , c / 2 [ Å] x in NdBa_1-xCo₂O_5+δ (mol) a 115.0 115.5 116.0 116.5 117.0 Vo l ( Å 3 ) 0.00 0.05 0.10 0.15 0.20 3.80 3.82 3.84 3.86 3.88 3.90 Vol c/2 a , c / 2 [ Å] x in NdBa_1-xCo₂O_5+δ (mol) a 115.0 115.5 116.0 116.5 117.0 Vo l ( Å 3 ) 0.00 0.05 0.10 0.15 0.20 3.80 3.82 3.84 3.86 3.88 3.90 Vol c/2 x in NdBa_1-xCo₂O_5+δ (mol) a 115.0 115.5 116.0 116.5 117.0

Lattice parameters were refined

through whole powder pattern fitting using the Pawley method Results in agreement with the behaviour of PrBa_1-xCo₂O_5+δ (Pang 2014) *NdCoO₃ FINAL REMARKS ELECTROCHEMICAL CHARACTERIZATI COMPATIBILITY vs ELECTROLYTES SYNTHESIS & CHARACTERIZATION

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NBC0 NBC5 NBC10 NdCoO₃ T = 1100 °C 10 µm

characterization: microstructure

NBC10 FINAL REMARKS ELECTROCHEMICAL CHARACTERIZATI COMPATIBILITY vs ELECTROLYTES SYNTHESIS & CHARACTERIZATION

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+ + + + + + + + + → + + → + 3 2 2 4 3 2 2 3 2 2Fe Co Fe Co Fe Co Fe Co Sample dissolution in HCl containing a known excess of FeCl₂ + + + + ₊ 4 _→ 3 ₊ 3 2 Ce Fe Ce Fe Titration with CeSO₄ Using Ferroin as indicator Co3+ Co4+ Fe3+ Ce3+ Co2+ Fe3+

characterization: oxygen content

FINAL REMARKS

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characterization: oxygen content

[n] in Con+ [Co 3+ ] [Co4+] 5+δ NBC0 3.14 1.72 0.28 5.64 NBC5 3.17 1.66 0.34 5.62 NBC10 3.20 1.60 0.40 5.60

Manthiram A, Kim J-H et al. ‘Crystal chemistry and properties of mixed ionic-electronic conductors’ Journal of Electroceramics 2011, 27, 93.

Defect Chemistry

FINAL REMARKS

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100 200 300 400 500 600 700 800 900 1000 98,5 99,0 99,5 100,0 100,5 T G ( %) T (°C) TG-DTA Analysis • In Air, up to 850 °C

• Heating and cooling rates 3 °C/min

100 200 300 400 500 600 700 800 900 1000 98,5 99,0 99,5 100,0 100,5 T G ( %) T (°C) NBC 0 NBC 5 NBC 10

characterization: TG-DTA analysis

J.-H. Kim et al.

Journal of the Electrochemical Society 155 (4) B385-B390 (2008)

Results in agreement with the

behaviour of the parent compound NdBaCo₂O_5+δreported by Kim

FINAL REMARKS

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20 30 40 50 60 70 80 ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ _∗ ∗ ∗ 2θ (°) 1000 °C 5 h 900 °C 12 h as mixed LSGM-NBC0 ∗ 20 30 40 50 60 70 80 2θ (°) GDC-NBC0 1000 °C 5 h 900 °C 12 h as mixed

* BaNdGa₃O₇ and/or BaLaGa₃O₇

20 30 40 50 60 70 80

GDC-NBC0

1100 °C 5 h

2θ (°)

as mixed

Commercial electrolyte powders:

• LSGM: La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ

• GDC: Ce_0.9Gd_0.1O_2-δ

chemical compatibility with electrolytes

FINAL REMARKS

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chemical compatibility with electrolytes

20 30 40 50 60 70 80 1000 °C 5 h 900 °C 12 h as mixed GDC-NBC10 1000 °C 5 h 900 °C 12 h as mixed Int ens ity ( a. u. ) GDC-NBC5 2θ (°) GDC-NBC0 1000 °C 5 h 900 °C 12 h as mixed 20 30 40 50 60 70 80 1000 °C 5 h 900 °C 12 h as mixed LSGM-NBC10 1000 °C 5 h 900 °C 12 h as mixed LSGM-NBC5 * * 2θ (°) 1000 °C 5 h 900 °C 12 h as mixed LSGM-NBC0 * * * * * * * * * * * FINAL REMARKS ELECTROCHEMICAL CHARACTERIZATI COMPATIBILITY vs ELECTROLYTES

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chemical compatibility with electrolytes

20 30 40 50 60 70 80 GDC-NBC0 GDC-NBC5 as mixed as mixed as mixed GDC-NBC10 1000 °C 5h + 1100 °C 5h 1000 °C 5h + 1100 °C 5h 1000 °C 5h + 1100 °C 5h Int ens ity ( a. u. ) 2θ (°) FINAL REMARKS ELECTROCHEMICAL CHARACTERIZATI COMPATIBILITY vs ELECTROLYTES

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• Die pressed CGO (1mm thick, 13 mm ø) • Sintering 1400 °C

• Preapration of NBCx slurry (60% solid content) • Brush painting of electrodes + firing (950 °C) • Current collector deposition + firing (800 °C)

SILVER CURRENT COLLECTOR NBCx CATHODE GDC ELECTROLYTE NBCx CATHODE SILVER CURRENT COLLECTOR

symmetric cells fabrication

10 µm 30 µm 1 mm 30 µm GDC pellet Ag current collector FINAL REMARKS ELECTROCHEMICAL CHARACTERIZATION

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R_Ω R_POL

ω

HF,MF: electrochemical processes LF: mass transport (diffusion)

R_Ω = Electrolyte +

Contact Resistance

R_POL = Cathode Polarization Resistance -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 20.0 20.5 21.0 21.5 22.0 22.5 23.0 0.1Hz 1Hz 10Hz 100Hz 1kHz EIS Test NBC/GDC/NBC 700°C Re Z (Ω) Im Z (Ω ) Test 1 Test 2 Test 3

electrochemical impedance

spectroscopy

FINAL REMARKS ELECTROCHEMICAL CHARACTERIZATION

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0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 -5 -4 -3 -2 -1 0 1 2 3 4 5 NBC0 NBC5 NBC10 ln (A SR [Ω ·cm 2 ]) 1000/T [1/K] Temperature (°C) 800 750 700 650 600 550

NBC0 NBC5 NBC10 Wang et al. Kim et al. Chavez et al. Zhang et al.

Ea [eV] 1,30 1,58 1,78 1,11 1,23 2,09 1,38

ASR

FINAL REMARKS

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0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 -5 -4 -3 -2 -1 0 1 2 3 4 5 NBC0 NBC5 NBC10 Wang et al. Kim et al. Chavez et al. Zhang et al. ln (A SR [Ω ·cm 2 ]) 1000/T [1/K] Temperature (°C) 800 750 700 650 600 550

NBC0 NBC5 NBC10 Wang et al. Kim et al. Chavez et al. Zhang et al.

Ea [eV] 1.30 1.58 1.78 1.11 1.23 2.09 1.38

ASR

FINAL REMARKS

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HF-MF processes analysis

-3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0 1.00 1.05 1.10 1.15 1.20 1.25 ln (RHF ) [ Ω cm 2 ] 1000/T [K-1_] R_HF NBC0 NBC5 NBC10 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0 1.00 1.05 1.10 1.15 1.20 1.25 ln (RHF ) [ Ω cm 2 ] 1000/T [K-1_] R_MF NBC0 NBC5 NBC10 FINAL REMARKS ELECTROCHEMICAL CHARACTERIZATION

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activation energies

FINAL REMARKS

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summary

NdBa_1-xCo₂O_5+δ layered perovskite tolerates Ba deficiency ≤ 0.1 Tetragonal P4/mmm

Oxygen content and Co mean oxidation state increase with increasing deficiency

Ba deficient NdBa_1-xCo₂O_5+δgis chemically stable with GDC electrolyte

up to 1100 C while reacts with LSGM already at 900 C

The electrochemical performances (ASR) worsen increasing Ba deficiency [unexpected]

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..still to do

Evaluation of the Electrical conductivity (hint on charge compensation mechanism)

Evaluation of the Thermal Expansion Coefficient

Deeper Electrochemical characterization

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THANK

YOU

Renato Pelosato, Ph.D.

Dipartimento di Chimica Materiali e Ingegneria Chimica

Politecnico di Milano Milano, Italy