Transcript
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†gt19@st-andrews.ac.uk, ‡jtsi@st-andrews.ac.uk
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
top related