Proton Ceramic Cell (PCC) electrical performance and reliability under reversible (electrolysis) operation : status and prospects

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Proton Ceramic Cell (PCC) electrical performance and reliability under reversible (electrolysis) operation : status and prospects

NRC Energy, Mining and Environment (EME) Portfolio

Proton Ceramic Cell (PCC) Electrical

Performance & Reliability under Reversible

(Electrolysis) Operation: Status and Prospects

NRC Energy, Mining and Environment (EME) Portfolio

Yeong Yoo, Research Officer

12 July 2013

PPCC 2013 International Workshop on Protonic Ceramic Fuel Cells Status & Prospects, 10-12 July 2013, Montpellier, France

Presentation Outline

1. Introduction

2. Barium cerate based proton conducting SOFC

3. Proton conducting electrolyte based

Reversible SOFC

4. Chemical stability of BCZY

5. Performance degradation in SOEC

6. Summary

7. Acknowledgments

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1. Introduction: About NRC

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Fig. 1. Schematic Diagram of Fuel Cell Mode in Proton

Conductor based SOFCs

Ni-BCY Anode BSCF-BCY Cathode

BCY Electrolyte H+ 2e- H₂  2H+ _{+ 2e} -Fuel: H₂ H₂O Air: 1/2O₂ H+ H+ H+ H2O

Overall: H

+

1/2O

 H

O

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1)

Highest Proton Conductivity

10

_{~ 10}

S/cm at 1000~600 ˚C

2) High transport number of protons

at 500-600 ˚C

3) but, poor mechanical strength and

low chemical stability under CO

and steam to form BaCO

(or

Ba(OH)

) and CeO

4) Therefore, solid solutions with

Barium Zirconates

5) Comparison of chemical stability

and cell performance of anode

supported cells using BCG, BCY,

and BCZY proton- conducting

electrolytes

H

Fueled SOFCs based on proton conducting electrolyte

Source: H. Iwahara et al., Solid State Ionics 168 (2004) 299-310.

Fabrication Procedures

Experimental

BCG

_/BCY

_/BCZY

_{electrolyte by}

wet powder spray

Pressing

Anode

Co-firing

BSCF

_{, PSN, LSCM, LSCF}

cathodes by wet spray

Electrolyte

Sintering

Cathode

Characterization

Single cell assembly

(20 mm dia.)

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Ni diffusion in BCG and BCY

Fig. 2. Scanning electron micrograph of as-sintered surface of BCG electrolyte on NiO-BCG anode (a) and BCY electrolyte on NiO-BCY anode (b), sintered at 1250 ℃.

Results and Discussion

(a)

(b)

BaGd

NiO

Chemical reactions to form secondary phases in

BCZY electrolyte co-sintered on NiO-BCZY anode

Ba(Ce

Zr

Y

)O

+ yNiO = yBaNiO

+ Ba

(Ce

Zr

Y

)O

(1)

Ba(Ce

Zr

Y

)O

= BaO + 0.8(Ce

Zr

)O

+ 0.1Y

O

(2)

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Fig. 3. Scanning electron micrographs and EDX profiles of the fractured surfaces of

(a) BCY//NiO-BCY and (b) BCZY//NiO-BCZY bilayers sintered at 1250 and 1350℃, respectively.

Y Ni _Ce O Ba Ni diffusion O Ce Ba Ni Zr Y Negligible Ni diffusion

(a) BCY//NiO-BCY

(b) BCZY//NiO-BCZY

Ni diffusion and secondary phase formation in BCY sintered on NiO-BCY

Fig. 4. SEM image (a) and EDS maps for O (b), Ni (c), Y (d), Ba (e), and Ce (f) from the fractured surface of BCY thin-film electrolyte on a porous NiO-BCY anode sintered at 1200 °C for 2 h in air.

Kα1 Kα1 Lα1 Lα1 Lα1

(a)

(d)

(e)

(f)

(c)

(b)

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Densification of BCZY sintered on NiO-BCZY anode

(a) (b)

Fig. 5. SEM photos of the surface of BCZY on NiO-BCZY sintered at (a) 1300, (b) 1325, (c) 1350, and (d) 1400 °C for 2 h in air.

10 m 10 m 10 m 10 m

(a)

(d)

(b)

(c)

Fig. 6. OCVs of the single cells under 75% H

in N

as fuel and air

as oxidant as a function of fuel gas flow rate at 600 °C.

0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 1.2 0 20 40 60 80 100 120

Fuel Gas (75% H2 in N2) Flow Rate (cc/min)

O p e n C ir c u it V o lt a g e (V ) BSCF-BCY//BCY//NiO-BCY BSCF-BCZY//BCZY//NiO-BCZY Theoretical OCV

Negligible gas leakage through BCZY electrolyte

sintered on NiO-BCZY anode

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Effect of electrolyte densification and composite cathode

Fig. 7. I-V Characteristics of single cells at 600 C under humid 75% H₂ in N₂ (3% H₂O) as the fuel gas and humid air as the oxidant gas at a gas flow rate of 100 mL min-1_.

Durability of BCZY based single cell

Fig. 8. Degradation of a single cell performance of BSCF-BCZY//BCZY, 10 µm thickness //NiO-BCZY at 600 C under humid 75% H₂ in Ar with a gas flow rate of 50 ml min-1_.

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Fig. 9. Schematic Diagram of Electrolysis Mode in Reversible SOFCs

3. Proton conducting electrolyte based

Reversible SOFC

Ni-BCZY Cathode BSCF-BCZY Anode

BCZY Electrolyte H+ 2e- H₂  2H+ _{+ 2e}- H₂ H₂ H₂O H+ H+ H+ 1/2O 2

Overall: H

O

 H

+ 1/2O

Why High Temperature

Steam Electrolysis?

Figure from JAERI-Research—97-064

1) Higher operating

temperature:

 Reduces the

electrical energy

requirement

2) > 100 % electrical

efficiency?

Reversible SOFCs

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Why Proton Conductive Reversible SOFCs?

1) Lower operation temperature, resulting in lower

material costs and lower degradation rates

2) Convenient production of dry and high purity hydrogen

without additional gas separation systems, in comparison

to oxygen ionic conductor-based reversible SOFCs

H. Iwahara et al., Solid State Ionics 3/4 (1981) 359-363

Proton conductor-based Reversible SOFCs

K. Katahira et al., Solid State Ionics 138 (2000) 91-98 Fig. 10. Hydrogen evolution rate vs. current density at

900 C. Electrolyte: (1) SrCe_0.90Sc_0.10O_3-, (2) SrCe_0.95Sc_0.05O_3- (Broken line shows theoretical rate.

Cell: H₂O 1 atm, Pt l electrolyte l Pt, Ar)

Fig. 11. Hydrogen evolution rate vs. current density. Broken line shows theoretical rate.

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Proton conductor-based Reversible SOFCs

Fig. 12. Cross-section of the experimental apparatus.

Fig. 13. Change in hydrogen production rate under various partial pressures of water vapor at 460 C and current density of 2.4 mA cm-2_.

Oxygen ionic conductor-based Reversible SOFCs

Fig. 14. I-V curves of single cells with BSCF-SDC or LSM as an oxygen electrode on YSZ electrolyte/NiO-

YSZ anode, operating at 850 C under SOEC and SOFC modes.

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Proton conductor-based Reversible SOFCs

Fig. 15. I-V curves of reversible SOFCs of Ni-BCZY //BaCe_0.5Zr_0.3Y_0.2O_3-//SSC-BCZY measured at various temperatures with H₂ and 50%H₂O-50%Air

in hydrogen and oxygen electrodes, respectively.

Fig. 16. Impedance spectra of reversible SOFCs with H₂ and 50%H₂O-50%Air in hydrogen

and oxygen electrodes, respectively.

M. Ni et al. Journal of Power Sources 177 (2008) 369-375

Modeling for concentration overpotentials of

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Proton conductor-based Reversible SOFCs

M. Ni et al. Journal of Power Sources 177 (2008) 369-375

Fig. 17. Theoretical modeling results in electrolysis mode: (a) concentration overpotentials of

electrodes and (b) I-V characterisitics

Fig. 18. Theoretical modeling results in fuel cell mode: (a) concentration overpotentials of

Fig. 19. Performance of a BCZY-based single cell in both of fuel cell and electrolysis modes at 600 °C under humid air (2.76% H₂O) at the air electrode and humid 48.5% H₂

in Ar (2.76% H₂O) at the hydrogen electrode with a gas flow rate of 120 ml min-1_, respectively with the ASR calculation using the area of the air electrode.

Y. Yoo et al. Journal of Power Sources 229 (2013) 48-57

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Fig. 20. Optical microscopic image (X100) of the surface of thin BCZY electrolyte deposited on a single cell after cooling from a high temperature steam electrolysis test

at 600 C under humid gas atmosphere.

4. Chemical Stability of BCZY

Fig. 21. Scanning electron micrographs of the surface morphology of BCZY electrolyte pellets (a) as sintered and (b) after heating at 200 °C for 24 h

under humid air (3% H₂O) with a gas flow rate of 100 ml min-1_.

Chemical Stability

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Fig. 22. Elemental analysis by SEM/EDS for 5 selected areas on a sintered BCZY pellet after heating at 200 °C for 24 h under humid air (3% H₂O) with a gas flow rate of 100 ml min-1_.

Chemical Stability

Chemical Stability

Fig. 23. (a) X-ray diffraction patterns of BCZY sintered pellets heat-treated under different conditions and (b) reaction products formed at 200 C

under humid air or in boiling water (a)

Ba(Ce

Zr

Y

)O

+ H

O = Ba(OH)

+

0.8(Ce

Zr

)O

+ 0.1Y

O

(5)

Ba(OH)

+ CO

= BaCO

+ H

O (6)

Ba(Ce

Zr

Y

)O

+ CO

= BaCO

+

0.8(Ce

Zr

)O

+ 0.1Y

O

(7)

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Chemical Stability

Fig. 24. Thermodynamics of chemical reactions related to BaCeO₃ and BaZrO₃ under CO₂ and H₂O as a function of temperatures.

5. Performance degradation in SOEC

Long term degradation

Main issue for the viability of SOEC technology & Required

further improvement prior to commercialization.

Higher electrode polarization increase during electrolysis

compared to fuel cell operation.

 Delamination or microstructure degradation at the

electrode/electrolyte interface.

Short-term degradation resulting from high current densities

or high steam concentration at the air/steam electrode.

Cell voltage degradation attributed to ohmic degradation.

Kirkendall voids in the electrolyte layer: Mass transport of

two or more species with different mobility leaves pores

behind.

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5. Performance degradation in SOEC

Degradation processes in electrolysis mode for oxygen

conductor-based SOFCs:

1) Enhanced diffusion processes in the electrolyte

and the formation of Kirkendall voids,

2) Formation of oxygen in the electrolyte,

3) Different oxygen activity in the oxygen electrode

and hence different catalytic activity for oxygen

reduction and oxygen evolution as SOFC and SOEC, respectively,

4) Delamination or microstructure degradation of the electrolyte /

oxygen electrode interface,

5) Enhanced evaporation of Ni(OH)

due to higher p(H

O) in the fuel gas,

6) Enhanced silicon poisoning of the hydrogen electrode due to higher

5. Performance degradation in SOEC

Fig. 25. Calculated evolution of the operation cell voltage for different degradation rates in % of

the initial cell voltage (1.06 V) per 1,000 h.

J. Schefold et al., J. Electrochem. Soc. 159 (2) A137-A144 (2012)

Fig. 26. Cell voltage during operation of a single cell of LSCF//CGO/YSZ//Ni-YSZ with j= -1.0 Acm-2

at 772-784 C .

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1. The anode supported button type single cells for proton

conductor-based reversible SOFCs comprising of BSCF-BCZY cathode // BCZY

(10-15 µm) // NiO-BCZY have been fabricated by

wet powder spraying and

co-firing

.

2. The electrochemical performance and durability of single cells were

characterized at the temperature 600-700 °C under fuel cell and

electrolysis modes. The maximum power density of

493 mW cm

_was

obtained

at 600

C and 0.7 V

.

3. The area specific resistance of the anode supported button cell in fuel

cell mode was about

0.46 Ω

۰cm

_{and in the electrolysis mode was}

_0.26

Ω

_۰cm

_{at 600 °C}

_{, indicating}

_{high efficiency, reversible SOFC}

_.

4. The stability of BCZY electrolyte in water-containing atmospheres was

investigated under humid air at 200

C and by soaking sintered pellets

in boiling water. The surface of BCZY formed BaCO

, Ba(OH)

, and

cerium zirconate, requiring the optimization of BCZY compositions and

sintering conditions by

increasing Zr concentration

to enhance the

stability in water-containing atmospheres at low temperatures.

7. Acknowledgments

• This work was supported by the Program of Energy

Research and development (PERD) Technology and

Innovation (T&I) in Canada.

• The author would like to gratefully acknowledge the

financial support for traveling from NRC-EME portfolio.

• Thanks are also addressed to Mr. Nguon Lim, Mr. D.