WO2008097184A1

WO2008097184A1 - Tuning of ion mobility in ceria-containing materials

Info

Publication number: WO2008097184A1
Application number: PCT/SE2008/050136
Authority: WO
Inventors: Natalia V. Skorodumova; Sergei I. Simak; Börje Johansson
Original assignee: Skorodumova Natalia V; Simak Sergei I; Johansson Boerje
Priority date: 2007-02-05
Filing date: 2008-02-04
Publication date: 2008-08-14

Abstract

An oxygen storage (17) comprises a volume (21) of a ceria-containing material, and means (31) for applying an external field (28) over the volume (21). The external field (28) gives rise to an increased ion mobility in the volume (21). The external field (28) comprises preferably an electric field, but may also comprise a magnetic field. The electric field has preferably a field strength giving a potential difference of at least IV over the ceria-containing material volume (21). The oxygen storage (17) is advantageously comprised in a catalyst device (10), an oxygen sensor, or a fuel cell. Also, a method of tuning ion mobility in a volume (21) of a ceria-containing material comprises the step of applying an external field (28) over the volume (21). Furthermore, the tuning of ion mobility can further be utilised in a method of restoring a volume (21) of a ceria-containing material from contaminations.

Description

TUNING OF ION MOBILITY IN CERIA-CONTAINING

MATERIALS

TECHNICAL FIELD

The present invention relates in general to devices and methods utilising ceria-containing material, and in particular to devices utilising and methods related to ion mobility in ceria-containing materials.

BACKGROUND

Development of materials providing high conductivity of oxygen ions is urged by a number of important technological applications, such as exhaust gas car catalysts, oxygen sensors and solid oxide fuel cells. Oxides with the cubic fluorite structure, e.g. ceria (CeCh), are known to be good ionic conductors, especially when they are doped with cations of lower valence than the host cations. In these oxides oxygen ions are transported by oxygen vacancies and, therefore, appropriate doping leads to the appearance of an optimal number of vacancies able to support oxygen transport through the oxide. In modern exhaust gas catalysts, introduced in the 1970's to meet stringent requirements on the emission limits for CO and NO, ceria is widely used as an oxygen storage, where its ability to easily take up and release oxygen, depending on oxygen pressure and temperature, is utilized. Under oxygen lean conditions ceria releases oxygen securing CO and NO_x conversion whereas in oxygen rich atmosphere ceria readily oxidizes.

Up to now oxygen storage capacity (OSC) has mostly been improved by optimizing the composition of ceria-containing materials as an appropriate doping increases the amount of mobile oxygen vacancies. Some examples of such materials presently used in applications are Ce(Zr)Ch, Ce(Sm,Gd)02-x

[2,3]. The operation temperatures for oxygen storage in the modern three way catalysis (TWC) are rather high, about 500-700 ⁰C. Oxygen mobility is high at such high temperatures and the optimal regime of CO and NOx conversion is normally tuned to these high temperatures in order to obtain an efficient conversion for steady-state conditions. However, temperature does not rise to the steady-state operation level immediately as one starts the car. It takes some time until the temperature reaches the target temperature range. During this "cold start" regime OSC is rather low and CO and NOx conversion is far from being complete. Due to this problem most damaging gases get into the atmosphere during the first period after the car start. Optimization of OSC at this regime is still a challenge and no satisfactory solution is presented within prior art.

Chemical deactivation is another problem oxygen storage materials encounter. Most harmful modes are sulphur and phosphorous poisoning. Harmful ions are bonding to the oxygen storage surface, reducing the oxygen exchange efficiency. Restoration of oxygen storages from contamination can typically only be performed if the catalysts are removed from the exhaust system. In prior art, there are no efficient approaches for methods avoiding such poisoning or for in-situ restoring of the OSC by removing harmful contamination .

SUMMARY

An object of the present invention is to provide oxygen storage devices and methods presenting easily tuneable ion mobility. A further object of the present invention is to provide in-situ restoring of oxygen storage devices.

The above objects are achieved by devices and methods according to the enclosed patent claims. In general words, in a first aspect, an oxygen storage, comprises a volume of a ceria-containing material, and means for applying an external field over said volume. The external field comprises preferably an electric field, but may also comprise a magnetic field. The electric field has preferably a field strength giving a potential difference of at least 1 V over the ceria-containing material volume. The oxygen storage is advantageously comprised in a catalyst device, an oxygen sensor, or a fuel cell.

In a second aspect, a method of tuning ion mobility in a volume of a ceria- containing material comprises the step of applying an external field over the volume.

In a third aspect, the tuning of ion mobility according to the second aspect can further be utilised in a method of restoring a volume of a ceria- containing material from contamination.

One advantage with the present invention is that ion mobility in ceria- containing materials is easily tuneable, providing operational advantages for the arrangements in which the ceria-containing materials are comprised.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

FIG. 1 is a diagram illustrating calculated electron bands of ceria;

FIG. 2 is a plot of the electron density difference between ceria with and without applied external field;

FIG. 3 a diagram illustrating calculated electron bands of ceria when an external field is applied;

FIG. 4 is a diagram illustrating calculated electron bands of partially reduced ceria;

FIG. 5 illustrates schematically an embodiment of a catalyst device; FIG. 6 is a diagram illustrating conversion rates of different exhaust constituent depending on air- fuel ratio;

FIG. 7 illustrates schematically an embodiment of a catalyst device according to the present invention; FIG. 8 is a flow diagram of steps of an embodiment of a method according to the present invention;

FIG. 9 is a flow diagram of steps of another embodiment of a method according to the present invention; FIG. 10 illustrates a possible course of events of temperature and applied external field in a catalyst device provided with an oxygen storage device according to the present invention;

FIG. 1 1 is a flow diagram of steps of an embodiment of another method according to the present invention; FIG. 12 illustrates schematically an embodiment of an oxygen sensor according to the present invention; and

FIG. 13 illustrates schematically an embodiment of a fuel cell according to the present invention.

DETAILED DESCRIPTION

The oxygen storage capacity (OSC) of ceria originates from its ability to undergo reversible transformations between two different oxidation states of different stoichiometries. In its most oxidized state, ceria adopts the stable form CeU2. When exposed to an oxygen depleted environment, Ceθ2 readily releases oxygen, eventually transforming to its most reduced, oxygen poor form Ce2U3. Such behaviour becomes possible due to the ability of cerium atoms to instantly and drastically adjust their electronic configuration to adapt to their local environment. In fact the process of oxygen release leading to oxygen vacancy formation in the oxide is coupled to the electronic localization/ derealization transition of cerium 4f electrons [I].

Cerium oxide is an ionic compound where the cerium atoms contribute with four electrons (including f-electrons) to the p-orbitals of oxygen. Results of a theoretical calculation of electron bands 103 of ceria are illustrated in Fig. 1.

The highest occupied valence band 104 of stoichiometric Ceθ2, i.e. below the Fermi level EF, is due to the p-states of oxygen and the lowest unoccupied band 105 is due to the f-states of cerium. A conduction band 106 appears at even higher energies. A detailed investigation has shown that the oxygen vacancy formation process, the elementary step in the reduction process utilized in ceria oxygen storage, is assisted by a simultaneous condensation of two electrons into the localized f- level traps on two cerium atoms. The cerium therefore changes the valence from +4 to +3. The energy of oxygen vacancy formation is rather high in stoichiometric Ceθ2. By ab intio calculation methods, the energy is estimated to be about 4.1 eV, and experimental data presents a value of about 4.7 eV. This indicates that oxygen with filled p-states is relatively unwilling to leave the oxide.

At the same time, if the occupation of the 0 p-orbitals 104 can be partially reduced, oxygen release would be greatly facilitated. In stoichiometric ceria, this effect can according to the present invention be achieved by a forced electron localization onto the f-orbitals 105 of Ce. First-principles calculations performed, using a full-potential LMTO method, have proven that this can be done by an application of an external field, magnetic or electric. In Fig. 2 a plot of the electron density difference between Ceθ2 with and without applied magnetic field is shown. Areas of charge depletion 106 are illustrated with hatchings from upper right to lower left. Areas of charge accumulation 107 are illustrated with hatchings from upper left to lower right. Charge depletion around the oxygen atoms and charge accumulation around cerium atoms can clearly be seen. The application of the magnetic field leads to a removal of electrons from the p-orbitals of oxygen and to their localization at Ce atoms. The energy of oxygen vacancy formation in this case becomes negative indicating the tendency even to a spontaneous oxygen release.

However, the value of the magnetic field needed to drive this localization is large (> 10 T), which makes magnetic fields less probable for application in real devices. Fortunately, the same effect can be achieved by applying a reasonably small electric field to the oxide film. A bias of the order of 1.5-5.5 V would be sufficient for a thin volume in the range of 100 nm. In this case, the chemical potentials are different at the two sides of the film as illustrated in Fig. 3. Curve 101 illustrates the situation at the positive potential and curve 102 illustrates the situation at the negative potential. If the oxide film is connected to metal electrodes, electrons occupying the p-states of oxygen would leave the oxide for the metallic drain. In turn, this should lead to a spontaneous oxygen release at the electrode to which a positive bias is applied. Thus, applying the electric field one can increase the oxygen release rate above the usual one for any chosen pressure and temperature conditions.

This has been also been experimentally supported, where a change in behaviour was identified when a voltage of 3 V was applied over an 80 nm thick volume. Also, with thicker samples in the order of 1 10 μm, changed behaviour was present when a voltage of 4.5 kV was applied. For substantially pure Ceria, the effect is thus believed to be noticeable at a field strength of above 35 kV/mm. However, since the voltage range at which a substantial effect was achieved is heavily dependent on the electronic structure of the material, such a voltage will differ considerably between different kinds of materials, and probably also between different doping or impurity degrees. For many materials of this kind, however, it is believed that fields over 5 kV/mm would be enough. The change in properties will also appear within an interval in an increasing amount, and small improvements will be present at voltages lower than at which a maximum effect is noticed.

In the case of strictly stoichiometric ceria, the release process should start immediately when the bias is applied. In reality, however, ceria is almost always partially reduced, especially at surfaces, interfaces and grain boundaries. This implies that a certain amount of Ce3+ ions already is present. The electronic structure of a partially reduced ceria looks slightly different from that shown in Fig 1. Such electronic structure 108 is illustrated in Fig. 4. The difference due to a small occupied f-peak 109 situated -1.2 eV above the p-band of oxygen. In order to force electrons to leave oxygen orbitals, one needs to shift the highest occupied electron state of the system down by about 1.3- 1.5 eV or more. Nonetheless, for partially reduced ceria, the general effect of the electric field remains the same. The comparison of the vacancy formation energies for different vacancy charge states have been performed by ab initio methods [4] by means of subtracting or adding extra electrons to the ceria unit cell. It has been shown that it is considerably easier to form an oxygen vacancy in positively charged unit cells, which simulate the application of a positive bias to the oxide film [4].

Also other materials based on ceria, e.g. ceria doped with other cations, as well as materials comprising ceria in different amounts will present the above illustrated possibility to tune the oxygen release. The efficiency and the potential needed to obtain any changes in oxygen mobility will differ between different materials.

One may furthermore note that the application of a bias would also result in the appearance of a p-type conductivity in the oxide film.

An oxygen storage based on the above ideas is well suited to be incorporated in a catalyst, e.g. a three way catalyst. Fig. 5 illustrates schematically a typical catalyst device 10. Exhaust gas 1 1, typically comprising hazardous gases, such as hydrocarbons, CO and NO_x, enters the catalyst device 10 and flows through a reaction structure 12. The actual catalytic reaction takes place at the surface of the reaction structure 12, and harmless gases, such as H2O, CO2 and N2, leave the catalyst through an output 13. The reaction structure 12 is illustrated in a magnified portion revealing the open large- area monolithic structure of a support 14. The support 14 is typically made from metallic material, e.g. stainless steel, or from ceramic material, e.g. cordierite. The support provides a multitude of small channels 15. A typical diameter of the channels is 1 mm. A part of the support 14 is magnified even more in Fig. 5. There, it is seen that the support 14 is covered with a coating

16, typically based on or at least containing ceria 21, operating as an oxygen storage 17, and comprises furthermore active catalyst material 20 at a surface 18. The coating 16 is typically a porous oxide, e.g. AI2O3, having a large surface. A typical thickness of the coating is 40 micrometer and the surface area is typically 100 m²/g. The active catalyst material 20 can e.g. be Pt metal and is provided in small volumes distributed over the coating surface.

Several oxidation and reduction reactions are utilized in a three-way catalyst. The main overall reactions are:

2 CO +O₂ → 2 CO₂ 4 C_xHy + (4x+y) O₂ → 4x CO₂ + 2y H₂O

2 NO + 2 CO → N₂ + CO₂

These reactions depend on the temperature and the composition of the exhaust gas. Fig. 6 is a diagram the conversion rate of the different constituent, NO_x 121, CO 122 and HC 123, depending on the air-fuel ratio.

By selecting appropriate operation conditions, the catalyst can be designed to give an optimum conversion rate at the steady-state exhaust temperature. The temperature determines basically the kinetics of diffusion in the catalyst and operation at non-optimized temperatures gives typically a non-optimum conversion rate.

Ceria is used in the three-way catalyst as a promoter for the catalytic reaction. The basic operation of ceria is to function as an oxygen storage capacity. When the air-to-fuel ratio fluctuates during an engine cycle, the oxygen storage capacity operates by storing excess oxygen under oxidizing, or lean, conditions, whereas the oxygen storage capacity operates by releasing oxygen under reducing, or rich, conditions. The oxygen storage and releasing capacity is strongly controlled by the ion mobility of the ceria. During cold start, the temperature of the catalyst is low and the oxygen storage capacity is low. A non-optimum conversion is achieved.

According to the present invention, ion mobility in oxygen storage devices comprising ceria can be controlled by other means than temperature. This makes it possible to increase the ion mobility under e.g. cold start conditions and thereby compensating the low temperature. When the temperature increases towards normal operation temperature, the ion mobility can successively be adapted to be substantially optimum at all temperatures. Fig. 7 illustrates a three-way catalyst using an oxygen storage 17 with tuneable ion mobility according to the present invention. Two electrodes are provided around the support 14. A negative electrode 24 is placed in connection with the support 14, and a positive electrode 26 is placed above the volume 21 of a ceria-containing material. In alternative embodiments, one or both of the electrodes 24, 26 may be provided without electrical contact to the volume 21. The volume 21 of ceria-containing material operates as an oxygen storage 17 of the catalyst device. By applying a voltage difference between the electrodes 24, 26, an external field 28 is applied over the volume 21. According to the discussion further above, such an external field changes the ion mobility in ceria. A voltage supply 30 connected to the electrodes 24, 26 constitutes in the present embodiment together with the electrodes 24, 26 the means 31 for applying an external field, and is preferably controllable, to provide variations of the strength of the external field, e.g. depending on the present operation temperature. A control unit 32 is illustrated in Fig. 7.

In a further preferred embodiment, a sensor 34, is connected to the means 31 for applying an external field. The sensor 34 is arranged to measure conditions of the gas entering and/or leaving the catalyst 10. The sensor 34 can typically be a temperature sensor and/or a sensor responsive to gas composition. The output from the sensor 34 is connected to the control unit 32, which is arranged to control the output of the voltage supply 30 in accordance with the sensor output. The catalyst device 10 can thereby be tuned to operate with an optimum oxygen storage capacity at any temperature.

A method for tuning the ion mobility according to the present invention becomes very comprehensive. Fig. 8 illustrates a flow diagram of steps of an embodiment of a method according to the present invention. The method of tuning ion mobility in a volume of a ceria-containing material begins in step 200. In step 220 an external field is applied over the volume. The external field comprises preferably an electric field, preferably having a field strength of at least 5 kV/mm over said volume. More preferably, the electric field has a field strength of at Ieast35 kV/mm over the volume. The external field may also comprise a magnetic field. The procedure ends in step 299.

Fig. 9 illustrates another embodiment of a method for tuning the ion mobility according to the present invention. The method of tuning ion mobility in a volume of a ceria-containing material begins in step 200. In step 220 an external field is applied over the volume. In step 230, a surrounding temperature or a surrounding gas composition, i.e. surrounding conditions is detected. In step 240, a strength of the external field is varied in response to a result of the detection in step 230. The procedure ends in step 299. The procedure is here illustrated as a single row of events, however, anyone skilled in the art understands that such controlling of the external field preferably occurs continuously or intermittently. This is indicated by the dotted line 250.

The above variation of the ion mobility is advantageously utilized during cold start of a catalyst, e.g. a three way catalyst. Fig. 10 illustrates a possible course of events of temperature and applied external field in a catalyst device provided with an oxygen storage device according to the present invention. At the very beginning, at tθ, the temperature 1 10 is low and thereby the ion mobility of an oxygen storage without external field is low and typically too low for producing an appropriate exhaust conversion at lean operation conditions. By applying an external field 120, the ion mobility can, however, be tuned to a higher level, allowing the catalytic reaction to operate appropriately also at lower temperatures. Since the exhaust producing process typically is a cyclic procedure, the exhaust conditions vary rapidly between rich conditions and lean conditions. Since the increased ion mobility is needed primarily at the lean conditions, the external field is preferably allowed to vary in registry with the exhaust production cycle, as seen in Fig. 10.

When the process proceeds, e.g. at tl, the catalyst warms up, as indicated by the temperature curve 110, and the spontaneous ion mobility increases.

The maximum applied external field 120 can thereby be reduced in order to provide a suitable amount of oxygen. When the catalyst device has reached normal operation temperatures, at t2, the external field can be turned off and the catalyst device operates as usual.

In the above example, temperature has been used as indicator for controlling the applied external field. However, in other configurations, monitoring of the actual result, i.e. the gases leaving the catalyst device could be used as an indicator of whether an external field is needed or not. The tuning of ion mobility of the catalyst can also be utilized in order to compensate for changes in incoming exhaust compositions. This can readily be achieved by monitoring the incoming gas composition instead. Such monitoring may then also provide for the periodic variation seen in Fig. 10.

In a chamber of a real catalyst ceria is in contact with many different chemicals. Some of them can contaminate the oxygen storage and cause at least partial deactivation. The most damaging contaminants are considered to be phosphorous and sulphur, which cause electron redistribution, delocalizing Ce f-electrons to fill up their own p-shells. This leads to making the neighbouring oxygen vacancies inactive for oxygen transport. Even small concentration of contaminations is able to block the surface layers of ceria- containing materials leading to a noticeable degradation of oxygen storage capacity.

The possibility to tune the ion mobility according to the present invention can also be used for removing contaminants and restoring the oxygen storage capability of ceria. Fig. 11 illustrates a flow diagram of steps of an embodiment of a method according to the present invention. The procedure of restoring a volume of a ceria-containing material from contaminations starts in step 260. In step 262, contamination ion mobility in connection with the volume is tuned. This is performed according to any of the ion mobility tuning methods described here above, and as indicated by the dotted box 220. The procedure ends in step 269. The application of an external field is able to change the electron distribution in the ceria. An electron redistribution can be forced, moving back the electrons from the contaminants p-shells to become delocalized Ce f-electrons. The contaminants will thereby increase their tendency to leave the ceria, and by applying an appropriate external field, spontaneous emission of contaminants will result.

It is thus possible to restore a contaminated oxygen storage device by means that are already present for the intended use. This means that restoring the oxygen storage from contaminants can be performed without any dismounting of the catalyst device. This means that restoring procedures can be performed frequently, e.g. every time a catalyst device is to be used or shut down.

As a conclusion, an application of an external field could assist in tuning the oxygen release rates for ceria-containing materials as well as to remove poisoning contaminants.

An oxygen storage with tuneable ion mobility can be used also in other applications. A volume of ceria can e.g. be used in oxygen sensor applications. As mentioned further above, when ceria is exposed for oxygen- rich atmosphere, the ceria is oxidized to its highest oxidized state CeCb. When ceria is present in an oxygen-poor atmosphere, a reduction takes place, ate least of the surface of ceria towards Ce2θ3. Such oxidation changes are accompanied by changes in electrical properties. By measuring the electrical properties of ceria, it is thereby possible to achieve a measure that is dependent on the oxygen content to which the ceria is exposed. Fig. 12 illustrates an embodiment of such an oxygen sensor 50. A volume 21 of a ceria-containing material is provided in contact with a gas volume 51 , in which the oxygen content is to be monitored. The volume 21 of a ceria- containing material thereby acts as an oxygen storage device. A monitor 52 observes electrical properties of the volume 21 of a ceria-containing material.

The monitor 52 is in the present embodiment connected to a front electrode 53 provided in contact with the volume 21 at the side facing the gas volume 51. The monitor is further connected to a base electrode 54 provided in contact with a side of the volume 21 opposite to the gas volume 51. The observed electrical property can e.g. be electric conductivity. According to the present invention, two external field applying electrodes 24, 26 are provided. In the present embodiment, the base electrode 54 is also used as one 24 of the field applying electrodes. The other electrode 26 is provided within the gas volume 51. In the present embodiment, one electrode 26 is provided in electrical contact with the volume 21. In alternative embodiments both electrodes may be in electrical contact, none of the electrodes 24, 26 may be in electrical contact, or only electrode 26 may be in electrical contact with the volume 21. By applying an external electric field between the electrodes 24, 26, the ion mobility within the volume 21 can be increased. An increased ion mobility in turn means that the oxidation/ reduction of the ceria is facilitated, and an increased oxygen sensitivity is expected from the oxygen sensor.

Fig. 13 illustrates another application of an oxygen storage according to the present invention. An embodiment of a fuel cell 60 is illustrated. A volume

21 of a ceria-containing material is used as an oxygen storage device and is provided with an anode 62 at a surface facing a gas volume 67 having a hydrogen rich atmosphere. Hydrogen molecules 65 impinging on the surface of the oxygen storage device and may react with the oxygen ions that are provided at the surface of the volume 21, forming water molecules 66. The basic reaction is:

H₂ + O²- → 2 H₂O + 2 e- . Two electrons are thus provided at the anode 62.

The vacancy in the volume 21 after the oxygen ion is occupied by another oxygen ion, due to diffusion in the volume 21 enabled by a certain ion mobility. Successive replacements of oxygen ions will move 70 the vacancy to the opposite side of the volume 21. This corresponds to a migration of oxygen 64 from the opposite side towards the anode.

A cathode 61 is provided at a surface of the volume 21 facing another gas volume 67 instead having an oxygen rich atmosphere. Oxygen molecules dissociate at the surface and oxygen ions are incorporated in the structure of the volume 21 occupying the vacancy originally created at the anode side. This process requires a supply of 2 electrons per oxygen ion:

¹A O₂ + 2 e- → O²- .

The overall reaction results in formation of water from oxygen and hydrogen and the provision of two excess electrons at the anode and the need of two electrons at the cathode. By connecting the anode and the cathode by electrical leads 68, 69, a current will flow from the anode to the cathode. This is the basic operation of a fuel cell. However, in order to have a reasonable ion mobility within the volume 21, the fuel cell has to be operated at very high temperatures.

According to the present invention, by applying an external electrical field over the volume 21, the ion mobility can be increased. In the present embodiment, two electrodes 24, 26 are provided with a voltage from a voltage supply 30 in order to create the electrical field over the volume 21. In the present embodiment, none of the electrodes 24, 26 are provided in electrical contact with the volume 21. In alternative embodiments one or both of the electrodes may be in electrical contact with the volume 21. The increased ion mobility opens up for reduced operation temperatures, and operation temperatures not too much above room temperature may be possible to achieve.

Prior art experiments have studied the improved cathode performance of mixed conducting Lao.6Sro.4Coo.sFeo.2θ3-x used in solid oxide fuel cells [5]. It has been shown that the electrochemical resistance of mixed conducting model cathode can be reduced drastically by a short but strong dc polarization of the cell. However, ceria containing materials were not discussed.

External electrical fields are widely used in other applications involving different oxides. One example is correlated oxide systems in microelectronics, in particular, for the design of modem electronic switches. These devices work by modulating the electrical charge carrier density, and hence electrical resistance, of a thin semiconducting channel through the application of an electrical field. Electric-field-tuned metal-insulator transitions have been studied in heteroepitaxial Ag/ CeCb/ Lao.67Cao.33MnCh junctions, which exhibit reproducible switching between a high resistance state (HRS) with insulating properties and a semiconducting or metallic low resistance state (LRS) with resistance ratio up to 10⁵ [6]. Switching between

HRS and LRS is obtained by the application of a positive and negative threshold voltage of about 3 eV. Oxygen release or ion mobility was, however, not in the focus of this study.

In the different embodiments above, the electrodes have been placed within or without direct electric contact with the ceria containing volumes. By allowing a gap between the electrodes and the volume may have constructional advantages. However, e.g. an air gap will set higher requirements on the applied voltages in order to reach the requested field strength within the volume. Such considerations have to be made when different applications are designed. The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.

REFERENCES

[1] N.V. Skorodumova, S.I. Simak, B.I. Lundqvist, LA. Abrikosov, B.

Johansson, "Quantum origin of the oxygen storage capability of ceria", Phys.

Rev. Lett. 89, 166601 (2002).

[2] A. Trovarelli, Catalysis by Ceria and Related materials, Catalytic Science Series, V.2. Imperial College Press, p.508, 2002.

[3] II. Inaba, H Tagawa, "Ceria-based solid electrolytes", Solid State Ionics

83, 1-16 (1996).

[4] Y. Juang, J. P. Adams, M. van Schilfgaarde,, "Theoretical study of environmental dependence of oxygen vacancy formation in Ceθ2^M, Applied Physics Letters, 87, 141917(2005)

[5] F. S. Baumann, J. Fleig:, M. Konuma, U. Starke, H U Habermeier, J.

Maier, "Strong performance improvement of Lao.6Sro.4Coo.sFeo.2θ3-deita SOFC cathodes by electrochemical activation", Journal of the Electrochemical

Society, 152, A2074-A2079 (2005). [6] R. Fors, S.I. Khartsev, A.M. Grishin, "Giant resistance switching in metal- insulator-manganite junctions: evidence for Mott transition", Phys. Rev. B

71. 045305 (2005).

Claims

1. Method of tuning ion mobility in a volume (21) of a ceria-containing material, characterised by the step of applying (220) an external field (28) over said volume (21), said external field comprises at least one of an electric field and a magnetic field.

2. Method according to claim 1, characterised in that said external field (28) comprises an electric field.

3. Method according to claim 2, characterised in that said electric field has a field strength within said volume (21) of at least 5kV/mm.

4. Method according to claim 3, characterised in that said electric field has a field strength within said volume (21) of at least 35kV/mm.

5. Method according to any of the claims 1 to 4, characterised in that said external field comprises a magnetic field.

6. Method according to any of the claims 1 to 5, characterised by varying (240) a strength of said external field (28).

7. Method according to claim 6, characterised by detecting (230) at least one of a surrounding temperature and a surrounding gas composition, and in that said varying (240) of a strength of said external field (28) is performed in response to a result of said detecting (230).

8. Method of restoring a volume (21) of a ceria-containing material from contaminations, characterised by tuning (262) contamination ion mobility in connection with said volume (21) according to any of the claims 11 to 15.

9. Oxygen storage (17), comprising a volume (21) of a ceria-containing material, characterised by means (31) for applying an external field (28) over said volume (21), said external field comprises at least one of an electric field and a magnetic field.

10. Oxygen storage according to claim 9, characterised in that said external field (28) comprises an electric field.

11. Oxygen storage according to claim 10, characterised in that said electric field has a field strength within said volume (21) of at least 5kV/mm.

12. Oxygen storage according to claim 11, characterised in that said electric field has a field strength within said volume (21) of at least 35kV/mm.

13. Oxygen storage according to any of the claims 9 to 12, characterised in that said external field comprises a magnetic field.

14. Oxygen storage according to any of the claims 9 to 13, characterised in that said means (31) for applying an external field (28) is arranged for providing variations of a strength of said external field (28).

15. Oxygen storage according to claim 14, characterised by a sensor (34) connected to said means (31) for applying an external field (28), for sensing at least one of temperature and gas composition, and in that said means (31) for applying an external field (28) being arranged for providing said variations of a strength of said external field (28) in response to an output from said sensor (34).

16. Catalyst device (10), comprising an oxygen storage (17) according to any of the claims 9 to 15.

17. Oxygen sensor (50), comprising an oxygen storage (17) according to any of the claims 9 to 15.

18. Fuel cell (60), comprising an oxygen storage (17) according to any of the claims 9 to 15.