SOLID STATE FUEL CELL AND PROCESS FOR THE PRODUCTION THEREOF

Abstract: Solid state fuel cell for oxidation of CH4, where the solid state electrolyte is preferably formed by YSZ (yttrium stabilized zirconium oxide). A surfactive metal oxide is applied onto the electrolyte followed by application of a thin layer of CeO2-based ceramics serving as an anode. If desired, MgO can be added in order to improve the volume stability.

Description: Title: Solid State Fuel Cell and Process for the Produc¬ tion Thereof. Technical Field The invention relates to a solid state fuel cell for 5 oxidation of CH 4 , where the solid state electrolyte is preferably formed by YSZ (yttrium oxide stabilized zir¬ conium oxide) . Background Art A conventional solid state fuel cell comprises a solid Λ state electrolyte, a solid state cathode, and a cermet anode. The operating temperature of a stack of solid state cells is approximately 1000 β C. The electrolyte contains usually a Y 2 0 3 -Zr0 2 mixture oxide, a socalled yttrium oxide stabilized zirconium oxide (YSZ) . The ca- thode is usually composed of LaMn0 3 doped with Sr, Mg or Ca, and the anode is usually composed of a mixture of fine Ni-particles and YSZ. The nickel-YSZ-oxide cermet anode is usually produced by mixing NiO with YSZ (Zr x _ x Y x 0 2 -x) . The latter oxide mixture is sintered on the electrolyte. When the fuel cell operates, and H 2 or CH 4 is transferred to the anode, NiO is reduced to Ni. Such a cermet anode must be porous in such a manner that the fuel gas can escape and react with 0 -ions from the YSΣ electrolyte at the same time as it releases electrons to the nickel metal. Accordingly, the anode reaction can only take place in a transition area between the three phases YSZ, Ni , and fuel gas. The anode reaction is as follows : CH 4 (gas) + 40"(YSZ electrolyte) → C0 2 (gas)+8e~(in the Ni metal) . A conductive path or percolation path must be provided through the Ni—phase in order to allow removal of the electrons, said path being ensured by the % by volume of Ni exceeding 35%. However, when Ce0 2 —based anodes are used, a reduction by way of H 2 or CH 4 results in formation of understoichio- etric Ce0 2 _ x capable of conducting both electrons and oxygen ions. An oxidation of H 2 or CH 4 can on such an anode be carried out on the entire surface, i.e. the gas transition zone, e~ and 0 simultaneously being avail¬ able everywhere on said surface. The latter is of parti¬ cular importance in connection with oxidation of CH 4 presenting the main component in natural gas . The sur- face density, i.e. the reaction rate, in connection with understoichiometric Ce0 2 _ x is up to a 100 times higher than in connection with a Ni-YSZ cermet anode. According to a conventional way of adhereing a thin layer of ceramics, such as Ce0 2 , onto another material, such as YSZ, the Ce0 2 powder is suspended in a suitable, viscous liquid spread over the already produced YSZ fol¬ lowed by heating to 1200 to 1600 β C for 1 to 5 hours. Subsequently, the Ce0 2 powder is sintered to an approxi¬ mately compact layer capable of adhering to YSZ. During the heating, the Ce0 2 and YSZ diffuse into one another, Ce0 2 diffusing faster than YSZ, whereby porosities arise about the original YSZ surface due to the socalled Kir- kendall effect. The porosities reduce the mechanical strength with the result that the structure cracks. When the sintering temperature is kept low and the sintering period is short, the formation of porosities can indeed be reduced, but the sintering of the Ce0 2 electrode is, however, poor and results- in a mechanically weak structure . The diffusing of Ce0 2 into YSZ involves the additional problem that a poor electrical conductivity is achieved in the Ce0 2 -YSZ mixture area. Accordingly, a voltage drop arises in said area due to the high resistance. Description of the Invention The object of the invention is to provide a solid state fuel cell for oxidation of CH 4 with a Ce0 2 -based elec¬ trode being mechanically stable and in good electric contact with the electrolyte. A solid state fuel cell of the above type is according to the invention characterised in that a thin layer of Ce0 2 -based ceramics is fastened to the electrolyte and serves as an anode, a surfactive metal oxide also being applied between the electrolyte and the ceramic layer. By adding NiO or another transition metal oxide which is surfactive on YSZ to the Ce0 2 , Ce0 2 is prevented from diffusing into YSZ. Alternatively, a layer of such a metal oxide powder suspended in a dispersion medium can be painted on the YSZ electrolyte followed by applica¬ tion of the Ce0 2 powder. By the expression surfactive is meant that the metal oxide, such as Mn0 2 or NiO, is easy to distribute uniformly on the YSZ surface as well as that said oxide does not clot in spots on the YSZ sur¬ face during the heating process. The measure can alter¬ natively be performed by admixing up to 30% by weight of NiO into the Ce0 2 -based ceramic powder. It is assumed, that a very lower content of NiO suffizes. Alternative, assumed suitable auxiliary oxides are oxides of the transition metals V, Cr, Mn, Fe, Co, Cu, Zn, Nb, Ta and of Ga, Ge, In, Sn, Sb, Pb , and Bi. The auxiliary oxide has partly the effect that the YSZ surface is completely covered thereof and accordingly prevents Ce0 2 from diffusing into YSZ, and partly the effect that it adheres the two materials to one another by a low transition surface tension to both YSZ and Ce0 2 . A proviso applying to the auxiliary oxide layer for be¬ ing usable in a solid state fuel cell is that said auxi¬ liary oxide layer serving as an adhesive layer must be penetrable to oxygen ions when the fuel cell is operat- ing. When NiO is used as auxiliary oxide, the NiO is during the starting phase presumably reduced into metal¬ lic Ni due to the effect of hydrogen. As the molar vol¬ ume of Ni is as low as 6.6 cm 3 /mole, and the molar volume of NiO is 11.2 cm 3 /mole, a significant contraction takes place, whereby Ce0 2 is brought into direct contact with YSZ in such a manner that 0 ions can move approximately freely from YSZ into Ce0 2 . When the Ni resulting from the reduction of NiO appears on Ce0 2 surface as particles of a diameter of approxi— mately 1 μm or more, problems are likely to arise in connection with the oxidation of CH* , as Ni is a well- known cracking catalyst. Ni exposed into CH Λ at 1000 β C is accordingly assumed to cause a separation of carbon gra¬ dually destroying the anode. The latter is apparently the case in the cermet anode described in EP Patent Ap¬ plication No. 388,558, said application not mentioning either that CH 4 can be used as fuel gas. The separation of carbon can be avoided by ensuring that almost no Ni is visible on the Ce0 2 surface, for instance by only applying Ni as a layer onto, the YSZ electrolyte and not admixing it into Ce0 2 . A carbon separation can alterna- tively be avoided by treating said surface with H 2 S in the starting phase. When the visible Ni is subjected to H 2 S at 1000 β C, a NiS layer is formed on the Ni particles with the result that the cracking-catalytic properties disappear. In this manner it is possible to add up to 35 to 40% by weight of Ni , whereby the conductivity of an electrode of understoichiometrie Ce0 2 - x can be improved because a treatment by means of H 2 S removes the ability of Ni to separate carbon from CH 4 in connection with cracking. Compared to an Ni-YSZ cermet electrode, the Ni-Ce0 2 _ x electrode treated with H 2 S is in addition tolerant to a possible content of sulphur in CH 4 . A substantial problem of the conventional Ni-YSZ cermet anode is that it is deactivated when subjected for a prolonged period of time to fuel gas having a sulphur content of the magni¬ tude 100 ppm. Ni-free Ce0 2 -based electrodes are also assumed to be tolerant to a sulphur content in the fuel gas . A particular problem related to Ce0 2 -based electrodes is that an understoichiometric Ce0 2 _ x electrode can break because when some of the 0 -ions are removed from the crystal lattice to form Ce0 2 _ x at the same time as some of the Ce + * ions are reduced into Ce +3 , the crystal lattice expands partly because less negative 0 are present to keep the positve Ce ions together, and partly because a Ce +3 ion is larger than a Ce + * ion. A solution to this problem has apparently been found. When Ce0 2 is doped with a tri- or divalent metal oxide, such as Y 2 0 3 or CaO, solid solutions of the type Ce,_ y Y y 02-γ. or Ce 1 _ x Ca x 0 2 - x are formed. Once an excess of oxy¬ gen ions, i.e. oxygen ion vacances, has been introduced into a Ce 1 _ x Ca x 0 2 - x crystal lattice, it is difficult to form additional oxygen ion vacances by way of reducing Ce + * into Ce +3 . The process is as follows: H 2 + 2Ce + *+0 2 ~ -B- 2Ce +3 + vacance + H 2 0, where the vacances form part as species on equal terms with the ions of the crystal lattice, i.e. already in¬ troduced vacances from a CaO doping displaces the equi¬ librium to the left. When the formation of Ce +3 and vacances is stopped by the addition of CaO, the electronic conductivity of the Ce0 2 - based electrode is reduced because the electronic con¬ ductivity is provided by the outermost and loosely ar¬ ranged electron of the Ce +3 ion. Accordingly, a lower concentration of Ce +3 results in a lower conductivity. Materials, such as Nb 2 0 5 in which Nb +5 is more easil reduced than Ce + *, can be added in order to increase the electronic conductivity. The outermost and loosely ar¬ ranged electron of Nb + * is also mobile. One way of in¬ creasing the volume stability of Ce0 2 is to dope with metal oxides, where the metal ion is smaller than Ce +4 . As a result, local tensions, i.e. deformations, are in¬ troduced into the Ce0 2 capable of absorbing the local in¬ creases of the volume arising during the formation of Ce + * by reaction with Ce +3 . Materials which may be doped in Ce0 2 in order to improve the conductivity are for instance Nb 2 0 5 , Ta 2 0 5 , Ti0 2 , P 2 0 5 , As 2 0 5 , Bi 2 0 3 Zn0, ln 2 0 3 , S0 3 and Sb 2 0 5 or mixtures thereof or materials presenting the same mixture by ad¬ dition of Ce0 2 followed by a heat treatment, such as the carbonates of the same materials or for instance Ce(S0 4 ) 2 instead of S0 3 or CeP0 3 instead of P ∑ O ^ . Materials to be doped in Ce0 2 in order to improve the volume stability by reduction are metal oxides, where the metal ion is mono-, di-, or trivalent, such as Na 2 0 , K 2 0, MgO, CaO, SrO, BaO, Sc 2 0 3 , Y 2 0 3 , La 2 0 3 , ln 2 0 3 , Ga 2 0 3 as well as tri- and divalent lanthanides, Ti 2 0 3 , ZnO , Al 2 0 3 . Oxides with metal ions smaller than Ce + * are ap¬ parently particularly suited. The invention also relates to a process of producing a solid state fuel cell. The process according to the in— vention is characterised by fastening to the electrolyte a thin layer of Ce0 2 —based ceramic powder admixed a tran¬ sition metal oxide which is surfactive on the elec¬ trolyte. As a result a suitable process for producing the fuel cell is obtained. Brief Description of the Drawings Fig. 1 illustrates a conventional solid state fuel cell, Fig. 2 illustrates a solid state fuel cell according to the invention with a Ce0 -based anode, Fig. 3 illustrates a fuel cell which can be repeated in a stack, and Fig. 4 illustrates a stack of cells with a manifold for fuel gas and air, respectively. 8 Description of the Preferred Embodiments of the Inven¬ tion Fig. 1 illustrates a conventional solid state fuel cell, where the fuel gas is H 2 . The fuel electrode, the anode, is formed by an Ni-YSZ cermet. The electrolyte is a YSZ electrolyte and the air electrode, the cathode, is formed by La(Sr)Mn0 3 . The thickness of both the electrode and the electrolyte is typically 100 μm . However, consi¬ derable variation possibilities exist of from a few μ and up to 0.3 mm for the electrolyte, and 1 mm for the electrodes. The Ni-YSZ anode is very poor as anode for CH 4 oxidation. According to the invention a Ce0 2 —based ceramics is used instead, cf. Fig. 2, said ceramics serving as an anode, a surfactive metal oxide also being applied between the electrolyte and the ceramic layer. In this manner a me¬ chanically stable structure is obtained. The surfactive metal oxide can for instance be oxides of V, Cr, Mn, Fe , Co, Ni, Cu, Zn, Nb or Ta. In order to improve the volume stability it is in addition possible to add alkali metal oxides, MgO, CaO, SrO, BaO, Sc 2 0 3 , Ti 2 0 3 , CnO, ln 2 0 3 , Ga 2 0 3 , La 2 0 3 or oxides of rare earth metals. In order to improve the electronic conductivity, it is possible to add Nb 2 0 5 , Ta 2 0 5 , P 2 0 5 , As 2 0 5 , ZnO, ln 2 0 3 , Bi 2 0 3 , S0 3 , Sb 2 0 5 or mixtures thereof. Fig. 3 illustrates a single fuel cell which can be re¬ peated in a stack. At the top, the fuel cell comprises a terminal plate 1 of electrically conductive material. The terminal plate 1 is provided with some longitudinal channels for the feeding of fuel gas. A fuel cell elec¬ trode 2 of Ce0 2 —based ceramics is provided below the terminal plate 1 and serves as an anode. The YSZ elec¬ trolyte 3 is provided below the fuel electrode 2. An additional terminal plate 5 with longitudinal channels for the feeding of an air flow is provided below the air electrode 4. The channels in the terminal plate 5 extend transverse to the channels in the terminal plate 1. Dur¬ ing an operation, the voltage across the cell, i.e. from one terminal plate to the other, is approximately 0.7 V. According to the invention it is advantageous to struc¬ ture a cell stack in connection with a manifold for fuel gas and air, respectively, as illustrated in Fig. 4. Some elongated channels 10, 11 of wavy electrically con¬ ductive material are provided between the cells for fuel gas and air, respectively. The wavy layers are arranged such that the flow direction of the air extends trans¬ verse to the flow direction of the fuel gas . A common inlet opening 6 and a common outlet opening 7 are pro¬ vided in connection with said manifolds for fuel gas and a common inlet opening 8 and a common outlet opening 9 are provided for air.

Claims: Claims . 1. Solid state fuel cell for oxidation of CH 4 where the solid state electrolyte is preferably formed by YSZ (yttrium stabilized zirconium oxide) , c h a r a c - t e r i s e d in that a thin layer of Ce0 2 —based cera¬ mics is fastened to the electrolyte and serves as an anode, a surfactive metal oxide being applied between the electrolyte and the ceramic layer. 2. A fuel cell as claimed in claim 1, c h a r a c — t e r i s e d by adding a metal oxide to the Ce0 2 -based ceramics, said metal oxide being surfactive on the elec¬ trolyte. 3. A fuel cell as claimed in claim 1, c h a r a c ¬ t e r i s e d by initially applying a powder of the surfactive metal oxide onto the electrolyte followed by application of the Ce0 2 powder. 4. A fuel cell as claimed in claims 1 to 3 , c h a r ¬ a c t e r i s e d by Nb 2 0 5 , Ta 2 0 5 , Ti0 2 , P 2 0 5 , As 2 0 5 , ZnO, ln 2 0 3 , Bi 2 0 3 , S0 3 or Sb 2 0 5 or mixtures thereof being added to increase the electron conductivity. 5. A fuel cell as claimed in claims l to 3, c h a r ¬ a c t e r i s e d by alkali metal oxides MgO, CaO, SrO, BaO, Sc 2 0 3 , Ti 2 0 3 , ZnO, ln 2 0 3 , Ga 2 0 3 , Y 2 0 3 or La 2 0 3 , or oxides of the rare earth metals (lanthanides) being ad- ded to improve the volume stability. 6. A fuel cell as claimed in claims l to 5, c h a r ¬ a c t e r i s e d by the Ce0 2 -based layer of ceramics being of a thickness of approximately 100 μm. 7. A process of producing a solid state fuel cell as claimed in claim 1, c h a r a c t e r i s e d by fast¬ ening to the electrolyte a thin layer of Ce0 2 -based cera¬ mic powder admixed a transition metal oxide which is surfactive on the electrolyte. 8. A process as claimed in claim 7, c h a r a c t e r¬ i s e d by the used surfactive transition metal oxide being oxides of V, Cr, Mn, Fe , Co, Ni , Cu, Zn, Nb or Ta. 9. A process as claimed in claim 7, c h a r a c t e r- i s e d by the used surfactive transition metal oxide being NiO. 10. A process as claimed in claim 9, c h a r a c t e r¬ i s e by mixing up to 30% by weight of NiO into the Ce0 2 -based ceramic powder. 11. A process as claimed in claims 7 to 10, c h a r ¬ a c t e r i s e d by applying a powder of surfactive metal oxide onto the electrolyte followed by application of the Ce0 2 powder. 12. A process a claimed in claims 7 to 10, c h a r - a c t e r i s e d by adding Nb 2 0 5 , Ta 2 0 5 , Ti0 2 , P 2 0 5 , As 2 0 5 , ZnO, ln 2 0 3 , Bi 2 0 3 , S0 3 or Sb 2 0 5 to increase the electron conductivity. 13. A process as claimed in claims 7 to 10, c h a r ¬ a c t e r i s e d by adding alkali metal oxides MgO, CaO, SrO, BaO , Sc 2 0 3 , Ti 2 0 3 , ZnO, ln 2 0 3 , Ga 2 0 3 , Y 2 0 3 or La 2 0 3 , or oxides of rare earth metals to improve the volume stability. 14. A process as claimed in claims 7 to 13 , c h r ¬ a c t e r i s e d by treating the Ni—Ce0 2 - x electrode with H 2 S. 15. A process as claimed in claim 14, c h a r a c - t e r i s e d by adding such an amount of NiO that an Ni—content of up to 60% by volume is achieved. 16. A process as claimed in claim 14, c h a r a c ¬ t e r i s e d by adding such an amount of iron oxide or cobalt oxide that an iron or cobalt content of up to 60% by volume is achieved. AMENDED CLAIMS [received by the International Bureau on 12 June 1992 (12.06.92); original claims 1-16 replaced by amended claims 1-15 (3 pages)] Claims . 1. Solid state fuel cell for oxidation of CH 4 where the solid state electrolyte is preferably formed by YSZ (yttrium stabilized zirconiumoxide) , c h a r a c - t e r i s e d in that a thin layer of Ce0 2 -based cera¬ mics is fastened to the electrolyte and serves as an anode, a surfactive metal oxide also being applied be¬ tween the electrolyte and the ceramic layer. 2. A fuel cell as claimed in claim 1, c h a r a c - t e r i s e d by adding a metal oxide to the Ce0 2 -based ceramics, said metal oxide being surfactive on the elec¬ trolyte . 3. A fuel cell as claimed in claim 1, c h a r a c ¬ t e r i s e d by initially applying a powder of the surfactive metal oxide onto the electrolyte followed by application of the Ce0 2 powder. 4. A fuel cell as claimed in claims 1 to 3, c h a r ¬ a c t e r i s e d by Nb 2 0 5 , Ta 2 0 5 , Ti0 2 , P 2 0s > As 2 0 5 , ZnO, ln 2 0 3 , Bi 2 0 3 , S0 3 or Sb 2 0 5 or mixtures thereof being added to increase the electron conductivity. 5. A fuel cell as claimed in claims 1 to 3 , c h a r ¬ a c t e r i s e d by alkali metal oxides MgO, CaO, SrO, BaO, Sc 2 0 3 , Ti 2 0 3l ZnO, ln 2 0 3 , Ga 2 0 3 , Y 2 0 3 or La 2 0 3 , or oxides of the metals (lanthanides) of rare earths being added to improve the volume stability. 6. A process of producing a solid state fuel cell as claimed in claim 1, c h a r a c t e r i s e d by fast¬ ening a thin layer of Ce0 2 -based ceramic to the electro- lyte, a surface active metal oxide being applied between the electrolyte and the ceramic layer. 7. A process as claimed in claim 6, c h a r a c t e r¬ i s e d by the used surfactive transition metal oxide being oxides of V, Cr, Mn, Fe, Co, Ni , Cu, Zn, Nb or Ta. 8. A process as claimed in claim 6, c h a r a c t e r¬ i s e d by the used surfactive transition metal oxide being NiO. 9. A process as claimed in claim 8, c h a r a c t e r— i s e d by mixing up to 30% by weight of NiO into the Ce0 2 —based ceramic powder. 10. A process as claimed in claims 6 to 9, c h a r ¬ a c t e r i s e d by applying a powder of surfactive metal oxide onto the electrolyte followed by application of the Ce0 2 powder. 11. A process a claimed in claims 6 to 9, c h a r ¬ a c t e r i s e d by adding Nb 2 0 5 , Ta 2 0 5 , Ti0 2 , P 2 0 5 , As 2 0 5 , ZnO, ln 2 0 3 , Bi 2 0 3 , S0 3 or Sb 2 0 5 to increase the electron conductivity. 12. A process as claimed in claims 6 to 9, c h a r ¬ a c t e r i s e d by adding alkali metal oxides MgO, CaO, SrO, BaO, Sc 2 0 3 , Ti 2 0 3 , ZnO, ln 2 0 3 , Ga 2 0 3 , Y 2 0 3 or a 2 0 3 , or oxides of rare earth metals to improve the volume stability. 13. A process as claimed in claims 6 to 12 , c h a r ¬ a c t e r i s e d by treating the Ni-Ce0 2 _ x electrodide with H 2 S. 1θ 14. A process as claimed in claim 13, c h a r a c ¬ t e r i s e d by adding such an amount of NiO that an Ni-content of up to 60% by volume is achieved. 15. A process as claimed in claim 13, c h a r a c ¬ t e r i s e d by adding such an amount of iron oxide or cobalt oxide that an iron or cobalt content of up to 60% by volume is achieved.

Inventors: KINDL BRUNO, MOGENSEN MOGENS, KINDL, BRUNO, MOGENSEN, MOGENS
Applicants: RISOE FORSKNINGSCENTER, FORSKNINGSCENTER RISOE
Application year: 1992
Main classification: H01M8
IPC codes: C23C14, H01M4, H01M8