This document summarizes the status, problems, and perspectives of solid oxide electrolysis cells (SOECs) for high-temperature electrolysis. SOECs are attractive because electrolysis is more efficient at higher temperatures, electrochemical processes are faster, and materials are inexpensive. While the technology was explored in the 1970s-80s, new interest has emerged due to energy and climate issues. Risø National Laboratory has developed SOECs using anode-supported nickel-zirconia cells and achieved high performance and reproducibility at the pre-pilot scale. However, further cell performance improvements are needed before comprehensive stack and system development. Other SOEC materials and designs are being explored including alternative electrolytes and hydrogen/o

1. SOEC – Status, Problems
and Perspectives
Mogens Mogensen
Fuel Cells and Solid State Chemistry Department
Risø National Laboratory for Sustainable Energy
The Technical University of Denmark
DK-4000 Roskilde, Denmark
Presented at workshop on
High temperature electrolysis limiting factors
Karlsruhe, June 9 -10, 2009
1

2. Acknowledgements
The colleagues at Risø – DTU provided most of the following for me.
Especially the following persons contributed:
Sune D. Ebbesen
Anne Hauch
Søren Højgaard Jensen
Torben Jacobsen
2

3. Outline
• Why high temperature electrolysis?
• Principle and structure of SOEC
• SOEC materials
• Performance and durability
• Poisoning
• Leak current density through the YSZ
• List of some problems
• Risø’s visions on synthetic fuels
• Economic estimates
• When?
3

4. Why electrolysis at
high temperature?
Because:
• Electrolysis is a heat consuming process. The Joule heat contributes to
the splitting of the water and CO2 molecules. Thus, the higher the
temperature, the less electrical energy is need for the splitting.
• The rate of the electrochemical processes is much faster at high
temperature. More m3 H2 per m2 cell per minute gives a better economy.
• The solid oxide electrolyzer cell (SOEC) consists of relatively
inexpensive materials and may be produced using low cost processes.
• This was realized already 30 years by e.g. Dornier and Westinghouse,
but the technology was not developed.
Thus, SOEC has now attracted new interest due to energy crisis and
climate problems.
4

5. Thermodynamics
H2O → H2 + ½O2
300 1.55
1/(2·n·F) · Energy demand (Volt)
Total energy demand (ΔHf )
250 1.30
Energy demand (KJ/mol)
Electrical
200 energy de 1.04
mand (ΔG
f)
Liquid
Gas
150 0.78
100 0.52
a nd (TΔS f )
50 Heat dem 0.26
0 0.00
0 100 200 300 400 500 600 700 800 900 1000
Temperature (ºC)
Energy (“volt”) = Energy (kJ/mol)/2F
5

6. Thermodynamics
• H2O formation energies
6
Source: NIST chemistry webbook

7. Thermodynamics
300
CO2 → CO + ½O2 1.55
Total energy demand (ΔHf )
1/(2·n·F) · Energy demand (Volt)
250 1.30
Energy demand (KJ/mol)
Electri
cal ene
200 rgy de 1.04
mand
(Δ Gf )
150 0.78
ΔS f )
100 and (T 0.52
He at d em
50 0.26
.
0 0.00
0 100 200 300 400 500 600 700 800 900 1000
Temperature (ºC)
7

8. Thermodynamics
300
Electrical energy demand (Δ Gf) 1.55
1/(2·n·F) · Energy demand (Volt)
250 C O2 → 1.30
CO + ½
Energy demand (KJ/mol)
O2
H 2O → H
2 + ½O
200 2 1.04
150 0.78
750ºC – 900ºC
100 0.52
ΔGH2O→H2 +½O2 = ΔGCO2 →CO +½O2
50 0.26
.
0 0.00
0 100 200 300 400 500 600 700 800 900 1000
Temperature (ºC)
8

9. Principle of SOEC
0.7 V 1.5 V
850 °C EMF ca. 1.1 V
Working principle of a reversible Solid Oxide Cell (SOC) The
cell can be operated as a SOFC (A) and as a SOEC (B).
9

10. Anode-supported SOFC
Cathode current collector,
LSM, ~40µm
Electrochemically active cathode
layer, LSM/YSZ, ~20µm
Electrolyte, YSZ, ~10µm
Electrochemically active anode
layer, Ni/YSZ, ~15µm
Anode current collector (support),
Ni/YSZ, ~250µm
10

11. Ni - YSZ supported cell
LSM/YSZ electrode
YSZ electrolyte
Ni/YSZ electrode
Ni/YSZ support
11

12. Risø DTU SOCs
Active cell area 4x4 cm2 or 18x18 cm2
12

13. Risø DTU SOFC
• SOFC R&D since 1989 in cooperation with Danish & European industry partners
• Up-scaling from laboratory scale production to pre-pilot scale production of SOFC
(capacity: 1000 12x12cm2 cells per week) in cooperation with TOFC
• SOFCs produced at the pre-pilot shows:
High performance & reproducibility
- ASR = 0.18 ± 0.03 Ωcm2 @ 850°C
Reliability
- high mechanical strength of cells
Inexpensive production methods
- tape casting, spray, screen-printing
TOFC started a 5 MW
SOFC stack capacity
pilot production line
on April 28th, 2009. 13

14. Technology status
• Europe: EIfER and Risø DTU + TOFC have built and tested small
(< 1 kW) stacks
• USA: INL + Ceramatec have built and tested 15 kW system
• Others?
The cell performance (and thus the stack performance) call for
further improvement before a comprehensive stack and system
development activity should start
14

15. Other SOC
materials and design
• Other O2- conducting electrolytes instead of doped zirconia: Higher
ionic conductivity (e.g. LSGM - Ishihara)
• Proton conductor: Pure H2 (in later presentations)
• Ceramic cathodes (H2 – electrodes) : Better stability than Ni (e.g.
Doped SrTiO3 - Irvine)
• Many kinds of anodes (O2 – electrodes) proposed and tested, e.g.
LSM, LSCF and LSC. Contradicting reports about performance and
durability.
• Cathode supported, electrolyte supported, and anode supported
cells tested. Advantages/disadvantages complicated.
15

16. Performance of
reversible SOC
World record in electrolysis
16

17. Durability of Risø SOECs
1.2
1
Cell Voltage (V)
0.8
0.6
0.4
0.2
0
0 200 400 600 800 1000 1200 1400
Time (h)
−0.5 A/cm2, 850°C, p(H2O) = 0.5 atm and p(H2) = 0.5 atm, steam
utilization 28%. 17

18. - 0.5 A/cm2 , 850 °C, pH2O = 0.7 atm
1.25 1.25
3t33 - pretreated glass sealing
Normal3t30 minus offset
glass seal 1.2
Au voltage3t36 (Au foil)
seal
1.2 cell
Cell voltage (V)
Cell voltage (V)
1.15 1.15
1.1 1.1
1.05 Pre-treated glass seal 1.05
1 1
0 200 400 600 800 1000 1200 1400 0 100 200 300 400 500 600
Time (h) Time (h)
18

19. Durability of Risø SOECs
Test Temp. i p(H2O) Ustart Uend ΔU
No
time (h) (oC) (A/cm2) (atm) (V) (V) (mV/100 h)
3test19 766 850 -0.25 0.70 1.004 1.017 2
3test27 68 950 -2.0 0.90 1.528 2.219 1016
3test30 1316 850 -0.5 0.50 1.143 1.175 2
3test32 475 950 -1.0 0.90 1.059 1.185 27
Acceptable ΔU < 0.2 mV/100 h !
19

20. Electron microscopy
for long-term tested SOEC
Impedance spectroscopy ⇒ passivation due to the H2 electrode
SEM/TEM/EDS for long-term (1510 h) tested SOEC:
H2 electr. – Ref. cell TEM image
No delamination
between layers
Intact electrolyte
Satisfying electrode
microstrucure 0.5 μm
4 μm
Si-containing
impurities in the H2
H2 electr. - Tested cell Ni Zr Al Si
electrode
←SEM of ref. and tested
SOEC - 1316 h of EL
TEM & EDS map - from
4 μm same 1316 h EL test →
20

21. Cells without glass seals
Cell A
1025 P a s s iv a t io n A c t iv a t io n D e gra da t io n 0.6
Cell voltage (black) and
In-plane voltage (mV)
1000
Cell voltage (mV)
Cell voltage
975
0.3
corresponding in-plane voltage
950 In-plane voltage
0.0
(gray) at the Ni/YSZ electrode
925 measured during co-electrolysis of
900
0 200 400 600 800 1000 1200
-0.3
1400
steam and carbon dioxide 850 ºC
Electrolysis time (h) and -0.25 A/cm2. The gas
Cell B
975 0.4 composition to the negative Ni/YSZ
electrode was 45 % H2O – 45 %
In-plane voltage (mV)
In-plane voltage 0.3
Cell voltage (mV)
950
CO2 – 10 % H2, while pure oxygen
0.2
925 Cell voltage
was supplied to the LSM/YSZ
0.1
electrode
900 0.0
0 100 200 300 400 500 600
Electrolysis time (h)
Cell C
925 0.1
In-plane voltage (mV)
Cell voltage (mV)
Cell voltage
0.0
-0.1
In-plane voltage
900 -0.2
0 100 200 300 400
Electrolysis time (h) 21

22. CO2 electrolysis - impurities
850 °C, 0.25 A/cm2
1250 * The increase in cell voltage after 295 and 363 hours of electrolysis w as caused by a sensor break in the oven
temperature controle causing a low ering of the cell temperature to 795ºC and 835ºC respectively
1200
Cell voltage (mV)
1150
CO2 - CO as provided
1100
1050
* Clean CO2 - CO
1000
*
Degradation from 25 - 600 h: 1 m V / 1000 h
950
0 100 200 300 400 500 600
Electrolysis time (h)
22

23. Calculated electronic
leakage through YSZ
The leak current
density, ieh, as a
function of
electrolyte
thickness at
four
temperatures at
a cell voltage of
1.3 V.
23

24. Calculated electronic
leakage through YSZ
Electronic leak
current density,
ieh, as a function
of cell voltage
for a 20 μm thick
YSZ electrolyte
24

25. List of some problems
• Durability of the H2O- and the CO2-electrode at high current
density (2 - 4 A/cm2) must be improved
• Develop cheap gas cleaning
• Redox tolerance of the Ni-YSZ-electrode should be improved or
even better, an all ceramic cathode (H2O, CO2) should be
developed.
• Pressurized operation to be developed
• Costs should be further decreased
• A most efficient way of cost reduction is further reduction of
area specific resistance of the SOEC
25

26. Visions for synfuels from
electrolysis of steam and carbon dioxide
1. Big wind turbine parks off-shore in the North sea, couple to a
large SOEC system producing methane, which is fed into the
existing natural gas net-work in Denmark
2. Large SOEC systems producing DME, synthetic gasoline and
diesel in Island, Canada, Greenland … driven by geothermal
energy and hydropower. Danish companies might build and
own these factories.
3. The target market should be replacement of natural gas and
liquid fuels for transportation
4. All the infrastructure exists!!
26

27. Gasoline production using
SOEC
e-
Fischer-Tropsch-catalyst
850 °C H2 + Gasoline 25°C
+ 2O2- - CO
H2O + Heat exchange H2O +
O2 CO2 CO2 25°C
Heat exchange
O2 O2 25°C
27

28. Economy assumptions for H2
production using SOEC
Electricity 1.3US¢/kWh
Heat 0.3US¢/kWh
Investment 4000 $/m2 cell area
Demineralised Water 2.3 $/m3
Cell temperature 850 ° C
Heat reservoir temperature 110 °C
Pressure 1 atm
Cell voltage* 1.29 V (thermo neutral potential)
Life time 10 years.
Operating activity 50%
Interest rate 5%
Energy loss in heat exchanger 5%
H2O inlet concentration 95% (5% H2)
H2O outlet concentration 5% (95% H2)
28

29. H2 production –
economy estimation
29

30. Commercial SOEC systems
- when ?
• The SOEC R&D effort at Risø will approach 20 man-year in
2009
• Strongly increasing international interest in SOEC R&D
• In spite of this the estimated time before commercial
production: > 10 y, unless the energy situation becomes much
worse. It may take a few years before we have solved the life
time problems at high current density. Afterwards
demonstration over several years is necessary before
commercialization is possible. Energy supply problems in the
transport sector may increase the R&D effort.
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31. Thank you for your attention!
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