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St Andrews Centre for Advanced MaterialsSTACAM
High Temperature Steam Electrolysis
John Irvine, Kelcey Eccleston,Angela Kruth, Alan Feighery, Fran Jones, Paul
Connor and Cristian Savaniu
University of St Andrews
St Andrews Centre for Advanced MaterialsSTACAM
Contents
• Hydrogen and its Production • High Temperature Electrolysis• St Andrews work
– Oxide Electrolyser– Reversible fuel cell concept– Solid oxide proton conductors
• Conclusion
St Andrews Centre for Advanced MaterialsSTACAM
Hydrogen Fuel
• Fuel cell vehicles, power generation– Eliminates polluting emissions at point of use
• Secondary energy carrier – Complete fuel cycle must be considered– Primary energy production: fossil fuel,
hydroelectric, solar, wind– Secondary energy production: electricity
St Andrews Centre for Advanced MaterialsSTACAM
The Opportunity• Scotland has 25% of Europe’s Renewable
Capacity• Wind, Wave and Tidal• Possible Net Exporter - New “Oil” Economy
• But not in short term– Capacity is remote– Need to Distribute Energy
• Hydrogen?
St Andrews Centre for Advanced MaterialsSTACAM
Hydrogen Production Methods
• Steam Reforming– Most of world hydrogen production– Produces CO2
• Photoelectrochemical, Biomass• Electrolysis
– Alkaline– Polymer Membrane– Solid Oxide
St Andrews Centre for Advanced MaterialsSTACAM
Hydrogen Production by Electrolysis
• Separating hydrogen and oxygen in water by electric current
• Two electrodes, cathode and anode, separated by an ion-conducting electrolyte
• Different types of electrolysis characterised by the type of electrolyte: Alkaline, Polymer Membrane, Solid Oxide
St Andrews Centre for Advanced MaterialsSTACAM
Disadvantages of Polymer Membrane and Alkaline Electrolysis• Must use precious metal catalysts• Not amenable to high pressure operation• Poor long term stability• Easily contaminated:
– Alkaline electrolyte absorbs CO2
– Must use ultrapure water or polymer membrane accumulate cations.
St Andrews Centre for Advanced MaterialsSTACAM
High Temperature Electrolysis• Heat provides part of total
energy required for electrolysis• ∆H lower in vapour phase than
liquid phase• Higher electrical efficiency• High temperature favours
reaction, reduces overpotential – Thus practical total efficiency
should exceed low temperature• Oxygen ion electrolyte: 750-
1000 ºC• Protonic electrolyte: 450-750 ºC
St Andrews Centre for Advanced MaterialsSTACAM
Solid Oxide Electrolysis
• Reverse operation of SOFC
• Solid electrolyte• No need for ultrapure
water• Can use less expensive
electrode materials• High pressure operation
possible
H2O
H2
O2
e-
O=
H2O
O2
H2
e-
H+
St Andrews Centre for Advanced MaterialsSTACAM
Solid Oxide Electrolysis
• High efficiency with respect to electricity• 80% of production cost from electricity• 2x cost of steam reforming, at present• Economic in regions with large renewable
energy sources– Egypt--hydroelectric– Iceland--geothermal– Scotland--wind, wave?
St Andrews Centre for Advanced MaterialsSTACAM
St Andrews Centre for Advanced MaterialsSTACAM
System
St Andrews Centre for Advanced MaterialsSTACAM
HexisTM Principle: Cell StackA HexisTM stack segment consists of a fuel cell and a current collector. Approximately 50 segments form a stack. Current Collector
Cell
Fuel Cell
Cell Stack
Area 100 cm2
Current/cell approx. 30 amp Voltage/cell approx. 0.7 voltPower/cell approx. 21 wattelConnected in series 1050 wattel
St Andrews Centre for Advanced MaterialsSTACAM
• The current collectors are responsible for gas distribution, heat exchange and making the electrical contact between the segments
• The cell stack also functions as a burner
HexisTM stack segment Advantage of the HexisTM principle
A high degree of integration is the key to low manufacturing costs
Current collectorand internal heatexchanger
CathodeElectrolyteAnode
Air
AfterburningFuel
Cell
St Andrews Centre for Advanced MaterialsSTACAM
Siemens Westinghouse Tubular
St Andrews Centre for Advanced MaterialsSTACAM
Component Materials• Electrolyte:
– Gastight, thin, not electrically conductive– Oxygen ion conductors: Yttria-stabilised zirconia (YSZ)– Proton conductors: Ba-doped cerates (produce dry H2)
• Cathode:– Porous, stable in reducing atmospheres– Transition metal cermets: Ni-YSZ
• Anode:– Porous, stable in oxidizing atmospheres– Pt (high cost)– Electronic conducting mixed oxides: (La,Sr)MnO3,
(La,Sr)CoO3
St Andrews Centre for Advanced MaterialsSTACAM
Component Material Preparation
• YSZ electrolyte– 2.5-cm diameter pressed pellets (2 cm after
sintering)– Tape cast electrolyte cut to 2cm diameter– Sintered 1500 C, 10 hrs
• (La0.8Sr0.2)CoO3 anode– Sol gel powder preparation – Screen printing on YSZ disks
St Andrews Centre for Advanced MaterialsSTACAM
Sintered LSC Layer on YSZ
St Andrews Centre for Advanced MaterialsSTACAM
Sintered LSC Layer on YSZ
St Andrews Centre for Advanced MaterialsSTACAM
LSC on YSZ with Ceria Interlayer
St Andrews Centre for Advanced MaterialsSTACAM
Electrolysis Test SetupReference electrode
Counter electrode
O2 outDry Ar in
Gold ringsTest cell
Furnace
H2O + H2 outWet Ar inWorking electrode
St Andrews Centre for Advanced MaterialsSTACAM
Faraday efficiency
0
0.2
0.4
0.6
0.8
1
1.2
0 0.01 0.02 0.03 0.04 0.05 0.06
I (A)
Eff
icie
ncy
865
920
Changes in pO2 with applied current for 865 C and 920 C
0.00E+00
2.00E-04
4.00E-04
6.00E-04
8.00E-04
1.00E-03
1.20E-03
1.40E-03
1.60E-03
0 0.01 0.02 0.03 0.04 0.05 0.06
I (A)
pO
2 (
atm
)865920
0.1ccmin-1
St Andrews Centre for Advanced MaterialsSTACAM
0
0.2
0.4
0.6
0.8
1
0.0 0.2 0.4 0.6 0.8 1.0 1.2
I/A
V/V
0.0
0.1
0.2
0.3
0.4
0.5
0.6
P/W
Reversible fuel cell
St Andrews Centre for Advanced MaterialsSTACAM
BC10Y electrolyte discPt electrodes
Alumina tubes
OCV thermocouple
I/V electrolysis
OCV pO2
wet Ar gas
dry ArgaspO2 sensor
Au seals
Electrolysis cell: Pt, steam/Ar | BaCe0.9Y0.1O2.95 | Pt, 0.5%H2O/Arelectrolyte thickness ~ 2mm
St Andrews Centre for Advanced MaterialsSTACAM
I/mA
log
(pH
2/atm
)
-0.5
0
0.5
1
1.5
1 2 3
U/V
I/mA
3% steam
47% steam
-15
-10
-5
00.5 1 1.5 2
Observed variation of hydrogen partial pressure in dry gas with applied current at 618 ˚C and 3% steam feed.
I-V curves for cell in galvanodynamicmode at 618 ˚C at two different steam feeds.
St Andrews Centre for Advanced MaterialsSTACAM
Summary• Low Temp Electrolysers
– Work– Available– Expensive
• High Temp Electrolysers– Still need development– promise efficiency and cost gains
• Solid Proton conducting Oxides– offer useful temperature compromise– Stability an issue for best materials so far
• HT Electrolyser LT Fuel Cell– Chance to gain a little back from thermodynamics if theoretical voltages are approached
St Andrews Centre for Advanced MaterialsSTACAM
Acknowledgements
ORS
Scottish Enterprise
EPSRC
EU HIT Proton Network