Perspectives of high temperature electrolysis using SOEC

 

S. H. Jensen and M. Mogensen

Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark

 

 

1. Abstract

Inefficient conversion technologies as well as improper energy storage systems are major barriers for a wider application of renewable energy such as wind, photovoltaic and hydropower. Reversible Solid Oxide Cells (SOC) that can be used both as Solid Oxide Fuel Cells (SOFC) and as Solid Oxide Electrolyser Cells (SOEC), have the potential to become a cost effective way to solve the conversion problem: SOEC can split H2O into H2 and O2. When need is H2 can be utilized in SOFC with high efficiency.

SOEC has also the potential of splitting carbon dioxide into carbon monoxide and oxygen. This means that electrolysis of a mixture of steam and carbon dioxide results in a mixture of hydrogen and carbon monoxide (syngas). A number of other carbon energy carriers may be produced from syngas. The two simplest are methanol and methane, but also gasoline may be produced. Here the production of H2 and CH4 using high temperature electrolysis of steam and CO2 are investigated. For an optimized system, assuming an electricity price of 3.6 US$/GJ, H2 production price will be 4.8 US$/GJ equivalent to 29 US$/barrel crude oil. CH4 production price is estimated to 7.8 US$/GJ equivalent to 48 US$/barrel crude oil.

 

2. Introduction

Renewable energy has received increasingly interest over the last decades. If renewable energy is to be implemented in the energy infra structure high delivery stability is demanded. SOEC can produce chemical energy carriers that are easy to store such as H2, CH4 or CH3OH when energy from renewable energy sources is available.[1] When society needs energy and no renewable energy source is available, the chemical energy carrier can be utilized in SOFC to produce electricity.

 

A system consisting of a heat exchanger and a reversible SOC has a lot of advantages compared to other conversion techniques. Here is listed a few for the SOEC part:

Because water electrolysis is increasingly endothermic with temperature, electricity demand can be significantly reduced, if the formation of hydrogen is taking place at high temperatures (600-1000 °C). The electric energy need is reduced because the unavoidable joule heat of an electrolysis cell is utilized in the water (steam) splitting process at high temperature.

If heat is available from sources such as heat of geothermal (e.g. on Island), solar or nuclear origin, this will further reduce the electric energy demand for hydrogen production by steam electrolysis. All heat sources with temperatures above 100 °C (the boiling point of water) are extremely beneficial since electric energy for steam rising will be saved.

SOEC can split carbon dioxide into carbon monoxide and oxygen. This means that electrolysis of a mixture of steam and carbon dioxide results in a mixture of hydrogen and carbon monoxide called syngas. By catalytic reactions a number of other energy carriers may be produced from syngas. The two simplest are methanol and methane. The latter is the main constituent in natural gas.

The preferred catalyst for CH4 formation is Ni. Since the negative electrode of a SOC is partly made of Ni it is possible to produce CH4 within the cell.[2] The entropy change for CH4 production from CO2 and H2O is nearly zero. This means that the overall efficiency for a conversion of electricity to CH4 and back again can be very high if the reaction kinetics is high, since only small reaction entropy losses occur.

 

The catalytic reaction to form CH4 or CH3OH from syngas can also be done in the heat exchanger after the cell. (See system description in Figure 4) This means that the energy for H2O vaporization can be produced within the system. A combination of the two ways to produce CH4 may prove to be the best production method, since it seems to optimize efficiency and production speed.

 

Also, the energy losses due to the sluggishness of the electrochemical reactions are in principle the lower, the higher the temperature is. This principle seems to a large degree realised in practise through the significant improvements of the SOFC technology due to the extensive international development efforts. Thus, SOEC is probably more efficient than the already commercialised low temperature electrolysers, and today’s SOFC should be tested in the SOEC mode in order to assess the commercial potential of the technology in this application.

3. Theoretical background

The principle of SOC is shown in Figure 1. The cell basically consists of three different layers. The middle layer (white) is an oxide ion-conducting electrolyte that is gastight. The topmost layer is the positive electrode and the down most is the negative electrode. The electrodes are porous, electron and oxide ion conducting in order to get the gasses into the reaction sites and to get a high three phase boundary where the three species (gas molecules, oxide ions and electrons) can meet and react. A cell voltage is established over the electrodes when gasses with different oxygen partial pressures are fed to the electrodes as described by Nernst equation.[3] The left figure shows SOC in fuel cell mode, where H2 is fed to the cell and reacts with oxide ions to form H2O. If the electrodes are connected through an external circuit with a light bulb, electrons will flow from the down most electrode to the topmost electrode through the circuit as long as gas is fed to the electrodes. In electrolysis mode the reaction is the reverse as in fuel cell mode. Here the electrons are forced to the negative electrode by an external voltage supply (indicated as a wind mill) where H2O is split and H2 and O2 is formed.

 

 

Figure 1: The principle of a Solid Oxide Cell (SOC). The cell basically consists of three different layers. The middle layer (white) is an ion-conducting electrolyte that is gastight. The topmost layer is the positive electrode and the down most is the negative electrode. The left figure shows SOC in fuel cell mode, where H2 is fed to the cell and reacts with oxide ions to form H2O. The reaction continues as long as electrons are allowed to pass through the light bulb in the external circuit and gasses are fed to the electrodes. The right figure shows SOC in electrolysis mode where the reaction is the reverse as in fuel cell mode. Here electrons are forced to the negative electrode by an external voltage supply, indicated as a windmill. This forces oxide ions (taken from H2O) to migrate through the electrolyte from the negative down most electrode to the topmost.

 

 

 

The overall reaction of the water electrolysis is:

 

                                                                                                       

The reaction at the negative electrode is:

                                                                                               

and at the positive electrode:

                                                                                                            

 

The minimum electric energy supply necessary for the electrolysis process is equal to the change in free energy (Gibbs free energy)

                                                                                                               

where DH is the enthalpy change, T is the temperature in Kelvin and DS is the entropy change by the reaction. The total energy necessary for the electrolysis reaction is DH. TDS is the heat necessary for the reaction to take place. The relation between DG and the equilibrium potential (no current through the external circuit) for the cell is

                                                                                                                       

where n is the number of electrons involved in the reaction, F is faradays constant and  is the equilibrium voltage, which sometimes is called the electromotive force. The value of  is dependent on the actual partial pressures of the reactants and products as described by the Nernst equation:

                                                                                               

where is at standard pressure, () R is the ideal gas constant, is the oxygen partial pressure at the positive electrode, are the partial pressures of H2 and H2O respectively at the negative electrode.

If equation is inserted in we get 

                                                                                                     

Since both  and  are positive for the reaction in equation and approximately independent of T it is seen that  is decreasing with increasing temperature.

Hydrogen is formed at the negative electrode whenever a potential difference, V, larger than  is applied to the electrodes of the cell, and steam supply is sustained to the negative electrode. The electric energy demand, given in units of kWh/Nm3 is illustrated in Fig. 1. The electrical power is determined as -IV, since the current, I, is taken to be negative in electrolysis mode and positive in fuel cell mode.

The passage of current will generate heat inside the cell due to the internal cell resistance in an amount equal to (-V)I. If a voltage equal to DH/nF is applied to the electrodes, the joule heat deposited pr. unit time in the cell is

                                                                   

This is exactly the heat removal pr. unit time by the steam electrolysis process. For this reason DH/nF is called thermo neutral voltage Etn.

Considerable Joule heat is unavoidable when electrolysis is done at a practical timescale with overall economy optimised. For this reason SOEC is an interesting technology since the produced joule heat can be utilized in the highly endothermic electrolysis proces.

Figure 2: Thermodynamics of H2O electrolysis, after ref. [4].

 

The internal area specific resistance (ASR) for SOEC decreases rapidly with temperature following an Arrhenius expression.[5] Furthermore we have

 

                                                                                                              

 

where i is the current density. Since both ASR and DG (and thus ) decrease with temperature, i increase significantly with temperature when the cell voltage is kept at Etn.  Since the area specific H2 production rate is proportional to i it can be imagined that increasing the cell temperature will decrease the H2 production price.

 

The optimal cell voltage is, in case only electrical energy is supplied (no external heat source), close to Etn; if the cell voltage is increased above Etn excess heat is produced. Due to heat losses to the surroundings and in the heat exchanger, nFV has to be slightly higher than DH.

for the reaction in equation . This is the necessary cell voltage if no external heat source is available and the inlet gas is heated in an ideal heat exchanger (see next section). If a heat source at temperatures above the boiling point of water (100 °C) or steam is available the energy needed for H2O evaporation is not necessary and the operating voltage can be decreased to ca.

 

                                                                                                 

 

where is the evaporation enthalpy of H2O at 0.1 Mpa. The detailed economical optimisation of cell voltage must take into account the thermal loss from the electrolyser to the surroundings, degradation speed of the cell as a function of cell over-voltage (-V) and production speed cf. eq. . In fact the real important parameters for the production price are the investment cost and the cost of electricity as shown in the next section.

 

The overall reaction for CO2 electrolysis is

 

                                                                                                         

 

and for this reaction. Thus the necessary cell voltage for syngas production is ca. 1.48 V if no external heat source is available. Like reaction reaction is very endothermic which means that if heat can be supplied at the negative electrode the cell voltage can be reduced significantly.

At temperatures below about 700 °C syngas may react to form methane, CH4. The catalytic process is

 

                                                                                              

 

This process is extremely exothermic which means it produces a lot of heat. If the reaction is taking place at the negative electrode the produced heat can be utilized to lower the cell voltage. Reaction requires an appropriate catalyst. Dispersed metallic nickel is the catalyst of preference for this. Fortunately, the SOEC negative electrode is usually made of a composite of Ni and YSZ (yttria stabilised zirconia), a Ni-YSZ-cermet, and thus, it is possible to form CH4 directly in the SOEC by electrolysis of steam and CO2 mixtures.2

This means that the overall reaction in the SOEC will be

                                                                                             

The minimum cell voltage required for reaction is given as

                                                                                                               

where n = 8. Note that heat for steam rising is included in equation . for eq. , where is the change in free energy for reaction at 1000 K (where H2O is steam) and where the reactants and products are at 0.1Mpa. Thus the cell voltage found in equation may not be the economically optimized cell voltage cf. equation and discussion below eq. .

The possible concentration of methane that can be obtained without producing equilibrium carbon is decreasing with increasing temperature, increasing with pressure, and decreasing with steam to carbon ratio.[6],[7] If carbon is formed at the three-phase boundary it will slow down the kinetics of the electrodes.

At 650 °C, 0.1 Mpa, app. 17% methane and 83% H2 (dry gas) can be produced at a negative electrode potential, V = -1.28 V vs. air without producing equilibrium carbon. At 15 Mpa and 650 °C, a mixture of 85% methane and 15% hydrogen dry gas with small concentrations of CO and CO2 can be produced without producing equilibrium carbon, at V = -1.08 V vs. air. It is believed that methane could be produced in SOEC at these pressures, with acceptable costs. The 15 Mpa is taken as an example in analogy to the production of ammonium, which is normally synthesised at pressures around 15 Mpa.[8]

4. System description

In order to estimate the production price for H2 production the system sketched in Figure 3 is analysed. A heat exchanger is used in order to save expenses for heating the feed gas to working temperature of the SOC. Reversed Osmosis Water is fed through the heat exchanger to the cell. Here it is split into H2 and O2 where O2 has migrated trough the gastight electrolyte.  On the way out, H2 and O2 is giving off the heat to the H2O in the heat exchanger. The detailed economic assumptions and results of the calculation are given in the next section.

 

The system analysed for CH4 production is sketched in Figure 4. Here CO2 and H2O are fed through the heat exchanger to the cell. At the cell it is split into syngas and O2. At 0.1 Mpa and 650 oC only small amounts of CO and H2 is catalysed into H2O and CH4 at the electrode. If the pressure is increased the CH4 concentration can be increased significantly. If all the catalytic reaction is taking place at the cell the steam formation from reaction can be split into H2 and O2 and the H2 can then be used in reaction . This gives a very low steam to carbon ratio, where nearly all the steam is utilized, and thus a high CH4 concentration. The low steam to carbon ratio could give problem with carbon formation at the electrodes, but this can be avoided by increasing the pressure.

Figure 3: System for H2 production by electrolysis of steam. The drawing is not to scale. Reversed Osmosis Water is fed through the heat exchanger to the cell. Here it is split into H2 and O2 where 2O-2 migrates trough the gastight electrolyte and O2 is formed at the positive electrode as long as steam is fed to the negative electrode and .  On the way out, H2 and O2 is giving off the heat to the H2O in the heat exchanger. The normal working temperature for SOC is between 600 oC and 1000 oC.

Figure 4: System for production of CH4 by electrolysis of steam and CO2. CO2 and H2O are fed through the heat exchanger to the cell. Here it is split into H2 and CO and O2.  At 0.1 Mpa and 650 oC only small amounts of CO and H2 is catalysed into H2O and CH4 at the negative electrode. Therefore a Ni catalyst is placed downstream in the heat exchanger, where the temperature is lower. If the pressure is increased, more CH4 can be produced at the negative electrode. The exothermic formation of CH4 from syngas produces heat that can be utilised within the system.  If the heat is produced at the negative electrode, the heat can be used to reduce the cell voltage. If the heat is produced downstream at the catalyst the heat can be used for steam rising.

Another way to increase the CH4-concentration in the outlet gas is to use a catalyst of dispersed metallic nickel placed down stream in the heat exchanger, where the temperature is lower. Using this technique the steam to carbon ratio will be higher, but this also means that the problems of avoiding carbon formation at the electrodes would be smaller. If no CH4 is formed at the negative electrode, the necessary cell voltage will be between 1.29 V and 1.47 V depending on the ratio between CO2 and H2O in the inlet gas. On the other hand this means that the current density and production rate will be significantly higher.

If CO2 is fed into the heat exchanger after the SOC, problems with carbon formation at the negative electrode can be avoided completely. However the steam to carbon ratio at the reaction site (the Ni catalyst) will be quite high. For this reason this option is not considered a feasible option.

Two things are important considering CH4 production compared to H2 production. 1: The exothermic formation of CH4 from syngas produces the heat for steam rising. 2: CH4 contains more than 3 times more energy per mole than H2, so the energy stored per volume is more than 3 times higher. This means that storage of CH4 will be cheaper than storage of H2.

5. Experimental data

In the analysis, the cell is assumed to be a DK-SOFC 2nd generation cell[9],[10] with a working area of 16 cm2. ASR after correction for steam utilization[*] for different temperatures is shown in Figure 5

 

 

Figure 5. Conversion corrected area specific resistance (CCASR) for standard DK-SOFC cells as a function of current density i, at different temperatures. Positive current densities refer to fuel cell mode, negative to electrolysis mode. It can be seen that the cell kinetic is almost as good in electrolysis mode as in fuel cell mode. 

Degradation is a severe problem of the tested SOC in electrolysis mode so far. The tested cells only last for 100 h or so. Another type of SOEC has proven high sustainability: (1000 oC,

i = 0.3 A/cm2 for 1000 h) without degradation.[11] It should be noted that the kinetics is far slower for these cells than for the DK-SOFC 2nd generation cell that we tested. The degradation of DK-SOFC 2nd generation cells tested in fuel cell mode is far less than in electrolysis mode. Long-term test (1 year) in fuel cell mode has shown limited degradation.9. Investigations into the reason for the fast degradation rate of the DK-cells are in progress.

 

6. Description and results of economical assumptions and calculations

A fuel cell stack with 90 cells of 16 cm2 have a cell area of 1429 cm2 and is assumed to cost 300 US$ or 2100 US$/m2 cell area[12]. Energy loss in heat exchanger, , is assumed to be 5% of the enthalpy changes in each side of the heat exchanger, i.e.

 

                                                               

 

The investment cost is taken to be 3 times the cost for one fuel cell stack e.g. 900 US$ or 6300 US$/m2 cell area. Depreciation time is taken to be 10 years and the interest rate 5% per year. The production price of electricity is taken to be 1.3 US cents per kWh (this is at least realistic for hydroelectric power and geothermal power e.g. at Iceland) It is assumed that the system is operated 50% of the time during the 10 years (low for hydroelectric but high for photo voltaic power plants). Cell voltage is taken to be 1.48 V = Etn e.g. when no external heat source is available. H2O utilization was taken to be 71%. Reversed osmotic (RO) water is assumed to costs 2.3 US$/m3. With these assumptions the production prize is analysed for the temperatures and CCASR in electrolysis mode given in Figure 5. The result is shown in table 1.

 

Table 1: H2 production rate and price at different temperatures. In percentage of production price is given: Depreciation of investment, RO water, Heat exchanger loss, electricity for evaporation and electricity for splitting H2O into H2 and O2. Cell voltage is taken to be 1.48 V and H2O utilization is chosen to be 71% in the calculations. The calculations are based on the kinetics shown in Figure 5. (1Nm3 H2 corresponds to 44.6 mol H2) The main part of production price is electricity for evaporation and splitting of H2O at 1000 oC and 850 oC, where at 750 oC it is depreciation of investment.

Cell temperature [oC] H2 outlet/m2 cell area [Nm3/hour]

Total price

[US cents/

Nm3 H2]

Total price

[US $/GJ]

Depreciation of Investment [%] RO water [%] HE-loss [%] Evaporation Electricity [%]

Reaction electricity

[%]

1000 14.7 6.1 4.8 19 3 2 20 56
850 6.6 7.6 6.0 35 2 1 17 45
750 2.8 11.4 8.9 55 2 1 11 31

 

 

It should be noted that the oxygen partial pressure was taken to 0.1 Mpa, in the above calculation. This means that pure oxygen was produced at the cathode. If the oxygen is collected and sold, the price per Nm3 H2 may be lowered further.

 

Figure 6. H2 production price is plotted for different cell temperatures as a function of cell voltage. The right most point is the ones that are analysed in table 1. 1Nm3 H2 corresponds to 44.6 mol H2. Note that it is only at 850 oC and 1000 oC it is feasible to lower the cell voltage below 1.48 V. This is due to the low production rate at 750 oC.

 

If steam is available the cell voltage may be lowered to 1.29 V cf. eq. . Given the conversion corrected ASR in Figure 5, H2 production price as a function of cell voltage is shown in Figure 6. It is seen that it is not economically feasible to reduce the cell voltage if the cell is running at 750 oC. This is because at lower cell voltage the production rate is decreased and thus there is less hydrogen to pay off the investment. At 1000 oC the production rate is so high (cf. table 1) that it is more optimal to decrease the cell voltage and thus using less electricity pr mole H2, though the price reduction is quite small.

 for separating CO2 from air (CO2 sequestration) is 22.1 kJ/mol, corresponding to 0.50 GJ/tonne. Taking electricity price to be 1.3 US cents per kWh this corresponds to 1.8 US$/tonne CO2. The technology for CO2 separation from air (sequestration) is quite new, and for this reason the price for CO2 separation from air is estimated to be 20 US$/tonne.

From Figure 6 it is seen that price reduction is limited if the cell voltage is lowered below

1.48 V e.g. thermo neutral voltage for syngas production. The calculations were carried out for H2 but the trends also applies to CH4 production. For this reason the cell voltage is taken to be 1.48 V in the calculation of production price for CH4. The kinetics (conversion corrected ASR) shown in Figure 5 is again used for steam electrolysis. The reaction kinetics for and is not known. Hence the following assumption is made: The steam to carbon ratio is taken to be equal to 2 and it is assumed that the ratio between CO2 and H2O is constant (which means that whenever 1 CO2 is split into CO + ½O2, 2 H2O are split into 2 H2 and 1 O2 ) It is also assumed that all the H2 and CO formed by electrolysis is reacting to form CH4. The utilization of H2O and CH4 is again taken to be 71%, meaning that the composition of the outlet gas is 71% CH4, 19% H2O and 10% CO2, (71% CH4 can be formed within the cell, though it will be at a higher pressure, cf. section 3) The production price is estimated and the result is shown in table 2.

 

Table 2: Estimated CH4 production rate and price at different temperatures. In percentage of production price is given: Depreciation of investment, RO water, CO2 sequestration, Heat exchanger loss, electricity for evaporation and electricity for splitting H2O into H2 and O2. Cell voltage is taken to be 1.48 V and H2O and CO2 utilization is chosen to be 71% in the calculations. The calculations are based on the kinetics shown in Figure 5 (with some assumptions for the kinetics for reaction and , please see the text). (1Nm3 H2 corresponds to 44.6 mol H2) The main part of production price is electricity for evaporation of H2O and splitting of H2O and CO2 at 1000 oC and 850 oC, where at 750 oC it is depreciation of investment.  


Cell Temp. [oC] CH4 outlet/m2 cell area [Nm3/hour]

Total price

[US cents/

Nm3 CH4]

Total price

[US$/GJ]

Depreciation of Investment [%] RO water [%] CO2 seques-tration [%] HE-loss [%]

Evaporation Electricity [%]

Reaction electricity

[%]

1000 3.68 28 7.8 17 1 14 1 18 49
850 1.65 34 9.5 31 1 12 1 15 40
750 0.70 49 13.7 51 1 8 1 10 29

 

 

In March 2000, OPEC decided to stabilise oil prices within a range of 22-28 US$/barrel of crude oil corresponding to 3.6-4.58 US$/GJ. The price for H2 production at 1000 oC taken from table 1 is 4.8 US$/GJ equivalent to 29 US$/barrel crude oil.  The electrical energy cost used in the calculations (1.3 US cents/kWh) corresponds to 3.6 US$/GJ. With the above assumptions the increase in price for converting electric energy into chemical bound energy in H2 is 1.17US$/GJ so the increase in price for being able to store the energy will be 32%. It should be noted that the optimum fraction of the chemically bound energy that can be converted back into electricity is about 60%, using a SOFC. The rest will be released as heat. Since SOC for H2 is fully scaleable the SOC can be installed in houses and the released heat can be used for room heating.

The calculated production price for CH4 at 1000 oC is 7.8 US$/GJ equivalent to 48 US$/barrel crude oil. This is somewhat higher than that for H2 (see table 1 and 2) due to the fact that CO2 captured from the air is assumed to be more expensive than RO water. The production price for CH4 is also higher than that for H2 because the efficiency h[†] for H2 production is 98% and only 76% for CH4 production.

 

7. Conclusion

For an optimized system, with an electricity price of 3.6US$/GJ, the production price for H2 will be 4.8 US$/GJ equivalent to 29US$/barrel crude oil.  If CH4 is produced instead, the price will be 7.8 US$/GJ equivalent to 48 US$/barrel crude oil. The reason why it is interesting to produce CH4 for energy storage instead of H2 is because CH4 contains more than 3 times more energy than H2 per molecule and hence the energy stored in a specific volume at a given pressure will be 3 times smaller. This means that the investment cost for storage and transport facilities can be significantly reduced.

 

8. Acknowledgement

The help from the whole fuel cell group at Risø is greatly appreciated.

 

9. References



[*]Since  change due to the conversion of H2O to H2 at the negative electrode cf. eq. , ASR will not be measured correct, see eq. . The conversion corrected ASR can be used mathematically to predict the current density at a given cell voltage and steam utilization.2

[†] , where E is the electric energy used to form 1 mol H2. E is calculated as the sum of HE-loss, evaporation electricity and reaction electricity.



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[2].   S.H. Jensen, J.V.T. Høgh, R. Barfod, M. Mogensen, “High temperature electrolysis of steam and carbon dioxide”, in Energy technologies for Post Kyoto targets in the medium term. Proceedings. Risø international energy conference, Risø (DK), 19-21 May 2003. Sønderberg Petersen, L. ; Larsen, H. (eds.), Risø National Laboratory (DK). Systems Analysis Department; Risø National Laboratory (DK). Information Service Department. Risø-R-1405(EN) (2003) p. 204-215

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[5].   P.V. Hendriksen, S. Koch, M. Mogensen, Y. L. Liu and P. H. Larsen, “Break down of losses in thin electrolyte SOFCs”, in Proceedings of the 8th International symposium on solid oxide fuel cells (SOFC-VIII), Paris, April 27 -May 2, 2003. Singhal, S.C.; Dokiya, M.; (eds.), (The Electrochemical Society, Pennington, NJ), in press.

[6].   J. R. Rostrup, Catalytic Steam Reforming, Springer-Verlag Berlin Heidelberg, New York, Tokyo, 1984.

[7].   J. R. Rostrup, Steam Reforming Catalysts, Danish technical Press, Copenhagen, Denmark 1975.

[8].   C. J. H. Jacobsen, J. Dohrup, I. Schmidt, Katalyse. Introduktion til kemien bag katalytiske processer, (Catalysis. Introduction to the chemistry behind catalytic processes), Gl. Holte bogtryk, Lyngby, Denmark, 1999.

[9].   N. Christiansen, S. Kristensen, H.H. Larsen, P.H. Larsen, M. Mogensen, P.V. Hendriksen, S. Linderoth, “Status of SOFC Development at Haldor Topsøe / Risø”, in Proc. 5th European SOFC Forum, , Oberrohrdorf (CH), 1-5 July 2002. J. Huijsmans (Eds.), European Fuel Cell Forum, 2002, p. 34-41.

[10].   K Kammer, P.H. Larsen, Y.L. Liu, B. Kindl, M. Mogensen, “Development of Thin-Electrolyte Solid Oxide Fuel Cells”, in Proc. 5th European SOFC Forum, Oberrohrdorf (CH), 1-5 July 2002. J. Huijsmans (Eds.), European Fuel Cell Forum, p. 875-882.

[11].   W. Dönitz, E. Erdle, R. Streicher, “High temperature electrochemical technology for hydrogen production and power generation” in Electrochemical Hydrogen Technologies. Electrochemical production and combustion of hydrogen, Amsterdam (NL) 1990. H. Wendt (eds.), Elsevier,.

[12].   M. Mogensen, C. Bagger. ”SOFC: The key to make renewable energy profitable?” in Program and abstracts of 1998 Fuel cell seminar, Palm Springs, CA (US), 16 - 19 Nov 1998. Courtesy Associates Inc., Washington, DC, 1998, p. 96-99.