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Process principle

Current status :

High temperature solid oxide electrolyser cells (SOEC) have a great potential for hydrogen production, as SOECs can split H2O into H2 and O2 in a very efficient and eco-nomical way. When coupled to an external heat source like solar, geothermal or nuclear, a very high efficiency can be achieved without any greenhouse gas emissions.

Advantages : • Low overall energy demand • High efficiency, increasing with temperature • Use of several high temperature steam sources • Splitting of CO2 into CO and O2 for syngas production • Broad know-how from SOFC technology available

Challenges :

Due to the high operation temperature of 700 or 800 °C to 1000 °C, the SOEC components have to meet specific requirements for a cost effective hydrogen production : • Electrolyte : chemically stable and gastight with high

ionic and low electronic conductivity • Electrodes : porous, chemically stable in highly

reducing/oxidizing environments with good electronic conduction and CTE (coefficient of thermal expansion) close to the electrolyte

• Interconnects : chemically stable in reducing/oxidizing environments

High Temperature Electrolysis (HTE)

I E A / H I A T A S K 2 5 : H I G H T E M P E R A T U R E H Y D R O G E N P R O D U C T I O N P R O C E S S

High Temperature Electrolysis Process description Process description : Steam is dissociated at the cathode. Simultane-ously, oxygen ions migrate through the electro-lyte material. Oxygen molecules form on the an-ode surface by releasing electrons.

Heat source Heat source : Solar, Nuclear, Geothermal, Industrial waste : to provide low temperature heat needs (vaporization) only solar or nuclear for the high temperature heat supplies to the electrolyser.

Conditions Conditions : 700 °C to 1000 °C

Materials Materials : Gastight electrolyte: Zirconia, doped with Y2O3, Sc2O3, Gd2O3, LaGO3 Porous cathode : Ni-YSZ remains the state of the art cathode material, new developments like (LaSr)TiO3 Porous anode : LSM-YSZ as standard material, LSF-YSZ and others in development

Total efficiency (thermal to hydrogen) Total efficiency (thermal to hydrogen) : 40-50 % vs. ~30 % for conventional electrolysis4

Cost evaluation : Cost evaluation : 2.0 to 3.5 €/kg H21, 9 for nu-

clear heat source

Overall reaction H2O H2 + ½O2

(endothermic reaction)

Cathode reaction:

H2O + 2e- H2 + O2-

Anode reaction: O2- ½O2 + 2e-

Cathode Anode steam electrode air electrode

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For the extrapolation to the industrial scale, INL per-formed a conceptual high temperature electrolysis plant design. This calculation is based on coupling an electrolyser with a high temperature gas-cooled 600 MWth nuclear reactor. At an overall process effi-ciency of 50 %, hydrogen production could achieve 2.4 kg H2 per second at 850 °C. Due to the fact that the hydrogen product will have to be compressed for storage and distribution, a high temperature heat ex-changer will supply superheated steam to the cells at 750-950 °C and a pressure of about 5 MPa. The wa-ter consumption is calculated as 21.7 kg/s.5

Flow-sheet

I E A / H I A T A S K 2 5 : H I G H T E M P E R A T U R E H Y D R O G E N P R O D U C T I O N P R O C E S S

Autothermal : The SOEC should be operated at the thermoneutral voltage (1.29V) or slightly above. The energy for the water splitting is completely supplied by the electric power, providing the electric potential (ΔG) and the heat (TΔS).11

Principle of the SOEC process

The key components of a SOEC are a dense ionic conducting electrolyte and the porous anode and cathode. Steam is fed to the cathode (steam electrode) and an electrical potential is applied to the SOEC. Water molecules dissociate to form H2 gas and oxy-gen ions at the triple-phase boundary. The hydrogen gas diffuses to the surface and gets collected, the oxygen ions are transported through the dense elec-trolyte to the porous anode (air electrode), where they are oxidized to oxygen gas and thus release electrons. The total energy demand (ΔH) for SOEC hydrogen production is expressed by :

ΔH = ΔG + TΔS where ΔG is the electrical energy demand and TΔS is the thermal energy demand. An increase in operating temperature decreases the electrical energy demand but increases the thermal energy demand. The total energy demand changes only little with temperature. Thus, the SOEC provides the opportunity to use sev-eral heat sources for an economical hydrogen produc-tion.7, 10

0

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0 100 200 300 400 500 600 700 800 900 1000Temperature (°C)

Spe

c. E

nerg

y (k

Wh/

m3 H

2)

Q=TΔS (Heat demand)

ΔG (electric energy demand)

ΔH (total energy demand)

Liqu

idw

ater

Steam

Thermodynamics of steam electrolysis : The electrolysis becomes increasingly endothermic with temperature11

Flow-sheet for nuclear cycle

Allothermal : The SOEC is operated below the thermoneutral voltage. The heat requirement (TΔS) for the water splitting is partly supplied by an external high tem-perature heat source.11

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Up to now for the SOEC technology materials from the SOFC has been used with quite promising results. For future projects the materials have to be adapted and optimised with respect to long term stability, effi-ciency, cost reduction as well as the presence of highly reducing and oxidizing environments. The ma-terial selection also depends on the working tempera-ture and compatibility with other components.6,10 The electrode materials have to be favourable to gas trans-port and electrochemical activity. Noble metals are excluded due to the costs. New materials under development :10, 13, 14, 15

Electrolyte : LSGM, GDC, SDC, LaGO3 proton con ducting parasites

Cathode : only limited studies on alternative materials are available, like SDC-Ni, (LaSr)TiO3

Anode : electronically conducting mixed oxides with perovskite structure (LSF, LSC)

Expected efficiency

Cost evaluation

Gas cooled nuclear reactors such as the VHTR and solar central receiver systems are consid-ered for high temperature heat supply (900°-1000°C) for allothermal operation. Such high temperatures are not required for autothermal operation mode. Low temperature heat (150°-200° C) for steam generation can be supplied from various sources like geothermal or waste heat.1,3,5,12

I E A / H I A T A S K 2 5 : H I G H T E M P E R A T U R E H Y D R O G E N P R O D U C T I O N P R O C E S S

In allothermal mode, the investment cost for a process will be relatively high, due to the neces-sary increase in the total active area (lower current density) and the necessity of more sophisticated high temperature heat exchanger devices. The lower overall efficiency of the autothermal pro-cess may be compensated by reducing capital in-vestment, due to smaller electrolysis units (higher current density) and omission of external high temperature devices. For this reason mainly auto-thermal operation of the SOEC has been analyzed resulting in production costs of 1.1-1.8 €/kg H2 depending on the electricity price.9 Considering variations in the electricity cost for different nu-clear reactors, hydrogen could be produced be-tween 2.0 and 2.4 €/kg for Sodium cooled fast reactor (SFR) and 2.4 to 3.0 €/kg for European pressurised reactor (EPR) respectively.1 The influ-ence of the electricity cost has been found to be the key factor1,9. Hydrogen production costs are tightly related to techno-economic models.

Materials

Description of heat sources Experimental, existing prototypes Cylindrical design was chosen for the prototypes of the HOT ELLY project in the 1980s.4, 8 Current investiga-tions focus on planar designs.2,7,9,10,11 Single and multi-ple cell experiment studies have been performed. Within the Hi2H2 project for single cells with an active cell area of 16 cm², maximum current density of -3.6 A/cm2 at a cell voltage of 1.48 V and hydrogen produc-tion of 1.34 kg/m2h was reached.9 Idaho National Laboratory (INL) have demonstrated a 15 kW inte-grated laboratory scale (ILS) facility with a hydrogen production rate of 0.9 Nm3/h.2

Characteristics of the two operation modes for HT electrolysis compared to conventional water electrolysis (HOT ELLY)11

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Main initiatives European development : Hi2H2 consortium, www.hi2h2.com,

EU Project Relhy : www.relhy.net

Denmark : Risoe DTU : www.risoe.dk

France : CEA : www.cea.fr

Germany : DORNIER - Doenitz (D) in 1979 and 1986, DLR Stuttgart, www.dlr.de Eifer : www.eifer.uni-karlsruhe.de

Switzerland : EMPA : www.empa.ch/h2e

USA : INL (Idaho National Laboratory, www.inl.gov

IEA/HIA task 25 : High Temperature Hydrogen Production Process

[1] R. Rivera-Tinco, C. Mansilla, C. Bouallou, F. Werkhoff, Hydrogen production by high temperature electrolysis cou-pled with EPR, SFR, or HTR: techno-economic study and coupling possibilites, Int. J. Nuclear Hydrogen Production and Applications, Vol. 1, No. 3, 2008,

[2] M.G. McKellar, J.E. OBrien, C.M. Stoots, J.S. Herring, Demonstration and System Analysis of High Temperature Steam Electrolysis for Large-Scale Hydrogen Production Using SOFCs, 8th EUROPEAN SOFC Forum, 2008, Lu-cerne,

[3] Sigurvinsson J, Mansilla C, Lovera P, Werkoff F., Can high temperature steam electrolysis function with geothermal heat ?, Int J Hydrogen Energy 2007;32(9):1174–82,

[4] W. Doenitz, R. Schmidberger, E. Steinheil, R. Streicher, “Hydrogen production by high temperature electrolysis of water vapour”, International Journal of Hydrogen Energy, Vol. 5, pp. 55-63, 1980,

[5] J.S. Herring, P. Lessing, J.E. O’Brien, C. Stoots, J. Hartvigsen, S. Elangovan, Hydrogen production through High Temperature Electrolysis in a Solid Oxide Cell” Second Information Excahnge Meeting on Nuclear Production of Hydrogen, 2-3. October 2003,

[6] U.F. Vogt, J. Sfeir, J. Richter, C. Soltmann, P. Holtappels, B-site substituted lanthanum strontium ferrites as electrode materials for electrochemical applications, Pure Appl. Chem., Vol. 80, No. 11, pp. 2543–2552, 2008,

[7] S.H. Jensen, P.H. Larsen, M. Mogensen, Hydrogen and synthetic fuel production from renewable energy sources, Int. J. Hydrogen Energy 32 (2007), 3253-3257,

[8] W. Doenitz, E. Erdle, “High temperature electrolysis of water vapour – Status of development and perspectives for application”, International Journal of Hydrogen Energy, Vol. 10, pp. 291-295, 1985,

[9] A. Hauch, S.D. Ebbesen, S.H. Jensen, M. Mogensen, Highly efficient high temperature electrolysis, J. Mater. Chem., 2008, 18, 2331-2340,

[10] Meng Ni, Michael K.H. Leung_, Dennis Y.C. Leung, Technological development of hydrogen production by solid oxide electrolyzer cell (SOEC), Int. J. of Hydrogen Energy 33 (2008) 2337-2354,

[11] W. Dönitz, E. Erdle, R. Streicher, High temperature electrochemical technology for hydrogen production, chapter 3, Electrochemical Hydrogen Technologies, edited by Hartmut Wendt, Elsevier 1990,

[12] E. Erdle, J. Gross, V. Meyringer, Possibilities for Hydrogen production by combination of a solar thermal central re-ceiver system and high temperature electrolysis of steam, Solar thermal central receiver systems, Proceedings of third int. workshop, June 23-27, Konstanz, Springer-Verlag, Vol. 2, pp. 727-736, 1986,

[13] G. Tsekouras, J. T.S. Irvine, (La,Sr)TiO3 perovskites as cathode for solid oxide electrolysis cell, International Workshop on High Temperature Electrolysis Limiting Factors, 2009 Karlsruhe,

[14] T. Ishihara, T. Kannou, S. Hiura, N. Yamamoto, T. Yamada, Steam Electrolysis Cell Stack using LaGaO3-based Electrolyte, International Workshop on High Temperature Electrolysis Limiting Factors, 2009 Karlsruhe,

[15] H. Matsumoto, T. Sakaia, S. Matsushitab, T. Ishihara, Intermediate-temperature steam electrolysis using proton-conducting perovskite, International Workshop on High Temperature Electrolysis Limiting Factors, 2009 Karlsruhe

References

High Temperature Electrolysis (HTE)

Contacts :

• Ulrich VOGT, EMPA, ulrich.vogt@empa.ch • François LE NAOUR, francois.le-naour@cea.fr • Pierre BAURENS, pierre.baurens@cea.fr • Sune D Ebbesen, Sune.Ebessen@risoe.dk

https:// www-prodh2-task25.cea.fr

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