D09.06.09.presentation

1

(La,Sr)TiO3 perovskites for high

temperature steam electrolysis

George Tsekouras† and John TS Irvine‡

School of Chemistry, University of St Andrews, United Kingdom†[email protected], ‡[email protected]

RelHy High Temperature Electrolysis Workshop

Karlsruhe, Germany

9th June 2009University

ofSt Andrews

Universityof

St Andrews

2

Outline

• Electrolysis Research at the University of St Andrews

• High Temperature Steam Electrolysis

• Motivation and Scope

• Results and Discussion

• Conclusions and Future Work

• Acknowledgements

3

Electrolysis Research at the University of St Andrews

• High temperature steam electrolysis (H20 + 2e- H2 + O2-)

• Dr George Tsekouras (Research Fellow)

• (La,Sr)TiO3 perovskite-based cathodes

• Ms Xuedi Yang (3rd year PhD student)

• (La0.75Sr0.25)0.95Cr0.5Mn0.5O3 perovskite-based cathodes

• Yang, X. and Irvine, J.T.S., J. Mater. Chem., 18 (2008) 2349

• Operation in hydrogen-free steam

• High temperature CO2 electrolysis (CO2 + 2e- CO + O2-)

• Dr Gael Corre (Postdoctoral Research Fellow)

• La0.8Sr0.2Cr0.5Mn0.5O3 perovskite-based cathodes

• Bidrawn, F., Kim, G., Corre, G., Irvine, J.T.S., Vohs, J.M. and Gorte, R.J., Electrochem.

Solid-State Lett., 11 (2008) B167

• Ms Xiangling Yue (1st year PhD student)

• Ni/YSZ cermet-based cathode benchmark initially

• Consider Ni-free ceramic cathodes later on

4

High Temperature Steam Electrolysis

• Solid oxide electrolysis cell (SOEC)

• A solid oxide fuel cell (SOFC) operated in reverse

• Steam separated into H2(g) and O2(g) using thermal and electrical energy

• SOEC operating temperature ~ 900 °C

• Clean route to pure H2(g) provided renewable electricity used (e.g. solar,

hydro, wind)

YSZ

O2-

H2O

H2

O2

Cathode:

2 H2O (g) + 4 e- 2 H2(g) + 2 O2-

Anode:

2 O2- O2(g) + 4 e-

5

Motivation and Scope

• Shown previously1-5 for (La,Sr)TiO3:

• promising as anode material in SOFC

• high conductivity under reducing conditions (due to Ti3+)

• Suggests suitability of LSTs as cathode material in SOEC

• Consider defect chemistries:

• A-site deficiency

• La0.2Sr0.7TiO3

• Oxygen excess

• LaxSr1-xTiO3+δ

• x = 0.3

1. J. Canales-Vazquez, J.C. Ruiz-Morales, J.T.S. Irvine and W. Zhou, J. Electrochem. Soc., 152 (2005) A1458.

2. J. Canales-Vazquez, S.W. Tao and J.T.S. Irvine, Solid State Ionics, 159 (2003) 159.

3. O.A. Marina, N.L. Canfield and J.W. Stevenson, Solid State Ionics, 149 (2002) 21.

4. R. Mukundan, E.L. Brosha and F.H. Garzon, Electrochem. Solid-State Lett., 7 (2004) A5.

5. J.C. Ruiz-Morales, J. Canales-Vazquez, C. Savaniu, D. Marrero-Lopez, W. Zhou and J.T.S. Irvine, Nature, 439 (2006) 568.

x, δ

6

Solid State Synthesis

• Stoichiometric amounts of La2O3, SrCO3 and TiO2 powders

• Thorough grinding and mixing via planetary ball-milling

• Powders pressed into pellets for firing

• 3x thermal treatments

• 1x at 1300 °C

• 2x at 1450 °C

La0.2Sr0.7TiO3 La0.3Sr0.7TiO3+δδδδ

Ti (atom %) 30.1 30.3

Sr (atom %) 19.9 21.7

La (atom %) 5.9 9.1

La:Ti 0.20 0.30

Sr:Ti 0.66 0.72

• La, Sr and Ti stoichiometries confirmed

via SEM/EDX of dense pellets

7

XRD Spectroscopy

• Note: oxidised powders

• Cubic perovskite crystal structure confirmed

0

20

40

60

80

100

120

20 30 40 50 60 70 80 90

2ΘΘΘΘ/°

Rel

ativ

e In

ten

sity

* *

**

* vaseline

La0.2Sr0.7TiO3

La0.3Sr0.7TiO3+δ

d (Å) V (Å3)

La0.2Sr0.7TiO3 3.90072 59.3520

La0.3Sr0.7TiO3+δ 3.90501 59.5481

8

SEM of pellets and powders

• La0.2Sr0.7TiO3 • La0.3Sr0.7TiO3+δ

• pellet • pellet

• powder • powder

9

SEM: effect of milling time on La0.2Sr0.7TiO3 particle size

• 2 hrs ball-milling• 1 hr ball-milling

• Difficulty to mill La0.2Sr0.7TiO3 powder to small particle size

• Problem during screen-printing ink formulation and device fabrication

• Possibly overcome by:

• longer milling time

• alternative synthesis (e.g. sol-gel)

10

Electrical Properties

0

2

4

6

8

10

12

14

300 450 600 750 900 1050

T (K)

σσ σσ (

S)

La0.3Sr0.7TiO3+δ

La0.2Sr0.7TiO3

* 5%H2/Ar *

3.3

3.4

3.5

3.6

-19 -18 -17 -16 -15

log pO2σσ σσ

(S

)

La0.2Sr0.7TiO3

* 900 °C *

0

1

2

3

4

5

6

7

-19 -18 -17 -16 -15

log pO2

σσ σσ (

S)

La0.3Sr0.7TiO3+δ

* 900 °C *

• Semiconductor-to-metal

transitions:

• La0.2Sr0.7TiO3: 330 K

• La0.3Sr0.7TiO3+δ: 840 K

• Both LST compositions displayed n-type behaviour

desired for an SOEC cathode

11

SOEC architecture

YSZ

LSMLSM/YSZ

LST/YSZLST

• Planar

• YSZ electrolyte-supported

• Composite layers for combined electrical and ionic conductivity

• Pure layers for current collection

• LSM: La0.8Sr0.2Mn0.95O3

12

SOEC preparation

YSZ

LSMLSM/YSZ

LST/YSZLST

• YSZ electrolyte:

• 1450 °C sinter, 12 hrs

• Screen-printing inks:

• Composition:

• Ceramic

• Vehicle (terpineol + poly(vinyl butyrate))

• Dispersant (KD-1)

• Formulation:

• Dispersion of ceramic into acetone using

ultrasonic probe

• Addition of organic vehicle, stirring to

evaporate acetone

• Screen-printing mesh:

• 325 wires/inch

• Mesh opening = 50 µm

• Printed layers dried at 100 °C prior

to subsequent print

• Electrodes fired at 1200 °C, 2 hrs

• LST/YSZ composite:

• 50:50 mol.%

• LSM/YSZ composite:

• 50:50 wt.%

13

High temperature steam electrolysis

• Carrier gases:

• 5%H2/Ar

• Ar

YSZ

LSM/YSZ

LST/YSZLST

H2O

H2

O2

H2O

LSM

• Carrier gases bubbled through water at room

temperature yielding ~ 3%H2O

14

SEM of SOEC

• La0.2Sr0.7TiO3-based cathode

• 12 µm

• La0.3Sr0.7TiO3+δ-based cathode

• 15 µm

• LSM-based anode

• 8 µm

15

• 10 min, - 1.4 V bias, in 3%H2O/Ar

Effect of SOEC operation

• Improvement in performance observed with SOEC operation (results for

La0.3Sr0.7TiO3+δ-based SOEC shown):

-0.066

-0.064

-0.062

-0.06

0 100 200 300 400 500 600

t (s)

I (A

cm

-2)

* 900 °C *

-2

-1

0

1

2

3

2.5 3.5 4.5 5.5 6.5 7.5

Z' (ΩΩΩΩ cm2)

-Z"

( ΩΩ ΩΩ c

m2 )

3%H2O/5%H2/Ar

3%H2O/5%H2/Ar after I-V

3%H2O/5%H2/Ar after CE

* 900 °C *3%H2O/5%H2/Ar

3%H2O/5%H2/Ar after I-V

3%H2O/5%H2/Ar after CE

16

Effect of SOEC operation cont.

0

0.4

0.8

1.2

1.6

-0.12 -0.1 -0.08 -0.06 -0.04 -0.02 0

I (A cm-2)

- E

(V

)

L2S7T10 ArLa0.2Sr0.7TiO3 in 3%H2O/Ar

* 900 °C *

0

0.4

0.8

1.2

1.6

-0.12 -0.1 -0.08 -0.06 -0.04 -0.02 0

I (A cm-2)

- E

(V

)

L3S7T10 ArLa0.3Sr0.7TiO3+δ in 3%H2O/Ar

* 900 °C *

• Hysteretic I-V curves observed during SOEC testing

• Cathode reduction and improved performance during measurement

• Cyclic voltammetry (CV), 10 mV s-1

17

SOEC operation in hydrogen-free steam

0

0.4

0.8

1.2

1.6

-0.115 -0.085 -0.055 -0.025 0.005

I (A cm-2)

- E

(V

)

Series1

Series2

La0.2Sr0.7TiO3 in 3%H2O/5%H2/Ar

La0.2Sr0.7TiO3 in 3%H2O/Ar

* 900 °C *

0

0.4

0.8

1.2

1.6

-0.115 -0.085 -0.055 -0.025 0.005

I (A cm-2)

- E

(V

)

L3S7T10 5%H2

L3S7T10 Ar

La0.3Sr0.7TiO3+δ in 3%H2O/5%H2/Ar

La0.3Sr0.7TiO3+δ in 3%H2O/Ar

* 900 °C *

0

0.4

0.8

1.2

1.6

-0.115 -0.085 -0.055 -0.025 0.005

I (A cm-2)

- E

(V

)

L2S7T10 Ar

L3S7T10 Ar

La0.2Sr0.7TiO3 in 3%H2O/Ar

La0.3Sr0.7TiO3+δ in 3%H2O/Ar

* 900 °C *

• Performance largely independent of the absence or presence of hydrogen

• Under more aggressive electrolysis conditions, slightly better performance was

observed in hydrogen-free steam

• Oxygen-excess La0.3Sr0.7TiO3+δ demonstrated slightly better performance

compared to A-site deficient La0.2Sr0.7TiO3, due to a lower steam electrolysis

onset potential

18

Conclusions

• Both A-site deficient La0.2Sr0.7TiO3 and oxygen-excess La0.3Sr0.7TiO3+δ

demonstrated n-type electrical properties sought for SOEC cathode application

• (La,Sr)TiO3 materials demonstrated electrolysis performance largely

independent of the absence or presence of hydrogen

• Opens up possibility of high temperature electrolysis of hydrogen-free steam.

Future Work

• Monitor H2 production directly using mass spectroscopy

• Set up humidifier for generation of high steam

• Obtain dew point sensor for accurate determination of steam

• Introduce catalysts to LST surface to improve electrolysis performance

19

Acknowledgements

• Prof John Irvine (supervision)

• Dr Cristian Savaniu (electrical property measurements)

• Dr David Miller (solid state synthesis)

• Dr Samir Boulfrad (screen-printing and device testing)

• Dr Mark Cassidy (ink formulations)

• EPSRC-funded SUPERGEN Consortium XIV: Delivery of Sustainable Hydrogen