DE10342162A1 - Material used e.g. in the production of anodes for high temperature fuel cells is made from a rare earth-calcium chromite-titanate - Google Patents

Abstract

Material is made from a rare earth-calcium chromite-titanate or a rare earth-calcium chromite-titanate in a mixture with MexZr1-xO2 or MeCe1-xO2 (where Me = alkaline earth metal or rare earth). An independent claim is also included for a process for the production of the above material comprising heating treating a powder mixture containing CaCO3, CrO3 and TiO2 at 800[deg] C.

Description

The The invention relates to a material for electrochemical applications in the high temperature range, a manufacturing process for one such material and uses for this material.

Of the material according to the invention is particularly advantageous for suitable for use in high-temperature fuel cells. As already expressed, the material should also at high temperatures can be used which can be in the range between 700 and 1000 ° C. He has to be chemical be stable and not decay.

Out In the prior art are different materials and manufacturing technologies for different Elements and items for corresponding electrochemical applications known.

So For example, anodes of high temperature fuel cells made of materials that have good electron conducting properties should and should be possible high electrical currents can guide which means that a sufficiently high electrical conductivity must be given. additionally should you for Oxygen ions should be well conductive and have a large surface area for activated oxygen required in an electrocatalytic oxidation of a fuel is, have.

So For example, cubic stabilized zirconia (8YSZ) is used for anodes used by high-temperature fuel cells. For the production of solid electrolyte are mixtures of nickel stabilized with YSZ, called Ni-YSZ cermets for example in S.P.S. Badwal "Stability of Solid Oxide Fuel Cell Components "; Solid State Ionics 143 (2001) pages 39-46.

With Such materials but problems arise, especially in Use of hydrocarbons, such as methane as fuel for high-temperature fuel cells. In this case, partial oxidation of methane occurs during the current flow within the fuel cell, which leads to decomposition and carbon free sets. The carbon precipitates on the anode so that reduces the catalytic activity becomes.

this Disadvantage is counteracted by the fact that the fuel methane in excess water vapor is added, leaving the methane in hydrogen and carbon dioxide can be split.

there occur extremely unfavorable temperature differences at the inlet and outlet of the high temperature fuel cell, caused by the endothermic reaction of water vapor and methane become.

One suitable material for such anodes should therefore be suitable to be on the surface of such Anodes provide sufficient oxygen to ensure reforming and oxidation of methane can, without a carbon deposit on the surface of Anodes occurs.

For the adaptation the thermal expansion coefficient an anode material to the solid electrolyte are the Anode material often additions of solid electrolyte material added. These are how already addressed the so-called nickel-YSZ cermet in which the proportion of Nickel usually has been set in the range of 40% by volume. Accordingly The electrical behavior of the anode material is essential determined by the high nickel content.

The Metallic nickel, however, tends to contact with an oxidizing agent for its own oxidation, whereby once the electrical conductivity is significantly reduced and this process until complete also mechanical destruction lead the entire fuel cell can.

Around to counteract these disadvantages of Ni cermets have become others Materials, as stated below, proposed.

La _l-x (Sr, Ca) _x CrO ₃ J. Steir, J. van Herle and AJ McEvoy, "Stability of Calcium Substituted Lanthanum Chromites Used as SOFC Anodes for Methane Oxidation", Journal of the European Ceramic Society 19 (1999) Pages 897 to 902; RT Baker, IS Metcalfe, "Activity and deactivation of La _0.8 Ca _0.2 CrO ₃ in dry methane using temperature-programmed technique", Applied Catalysis A: 126 (1995) pages 297 to 317;
La _0.8 Sr _0.2 Cr _0.97 V _0.03 O ₃ P. Vernoux, M. Guillodo, J. Fouletier, A. Hammou "Alternative anode material for gradual methane reforming in solid oxide fuel cells", Solid State Ionics 135 (2000) pages 425 to 431;
ZrO ₂ (Y ₂ O ₃ + Nb ₂ O ₅ ) PR Slater, JTS Irvine, "Niobium based tetragonal tungsten bronzes as potential anodes for solid oxide fuel cells: syntheses and electrical characteristics", Solid State Ionics 120 (2000) pages 125 to 134;
Kaiser, JL Bradley, PR Slater, JTS Irvine, "Tetragonal tungsten bronze type phases (Sr _lx Ba _x ) _0.6 Ti ₀ (Sr _lx Ba _x ) _0.6 Ti _0.2 Nb _0.8 O ₃ . ₂ Nb _0.8 O _3-0 : Material characterization and performance as SOFC anodes ", Solid State Ionics 134 (2000) pages 519 to 524;
La _0.6 Sr _0.4 Co _0.2 Fe _0.8 O ₃ A. Hartley, M. Sahibzada, M. Weston, IS Metcalfe and D. Mantzavinos "La _0.6 Sr _0.4 Co _0.2 Fe _0.8 O ₃ as the anode and intermediate temperature solid oxide fuel cells ", Catalysis Today 55 (2000) pages 197 to 204; M. Weston and IS Metcalfe "La _0.6 Sr _0.4 Co _0.2 Fe _0.8 O ₃ as of anode for direct methane activation in SOFCs", Solid State Ionics 113-115 (1998) pages 247-251.

Individual substances in the system La _la Ca _a Cr _lb Ti _b O _3-δ G.Pudmich, BA Boukamp, M. Gonzalez-Cuenca, W. Jung, W. Zipprich and F.Tietz, "Chromites / titanate based perovskites for application as anodes in solid oxide fuel cells ", Solid state Ionics, 135 (2000) pages 433 to 438;
Sr _{II, 5 ×} M _× TiO _X _ _δ WO 00/64814 A1.

at These proposed materials, which have a sufficiently high electrical Reached conductivity, But there were stability problems on. They were generally insufficiently stable in reduction and also the thermal expansion was usually too big.

Other Materials known from the prior art were in contrast to sufficiently stable, but their electrical conductivity inadequate.

at The perovskite oxides can undergo electrocatalytic activity Substitutions known elevated become. However, this is accompanied by an increased thermal expansion, but that undesirable and leads to mechanical stresses, which in turn cause destruction can.

It is therefore an object of the invention to provide a material for electrochemical Applications in the high temperature range above 700 ° C available too thermally, mechanically and chemically stable is and an increased electric conductivity having.

According to the invention this Task with a material according to claim 1 solved. It can be produced by a method according to claim 4 and suitable uses are given in claims 9 and 10.

advantageous Embodiments and developments of the invention can with in the subordinate claims designated characteristics can be achieved.

The material according to the invention for electrochemical applications in the high temperature range can be formed solely from a rare earth calcium chromite titanate or in a mixture with Me _x Zr _lx O ₂ or Me _x Ce _lx O ₂ , in which Me is an alkaline earth or rare earth element to be included.

A preferred rare earth-calcium chromite titanate is (La _x Ca _{lx) l-α} (Cr _ly Ti _{y) l-β} O _3-δ, in which case x = 0-1, y = 0-1, α = 0 -0.3 and β = 0-0.1.

The A lattice sites (La + Ca) as well as the B lattice sites (Cr + Ti) should be particularly preferably occupied substoichiometric, because of that the necessary grain size of the particles in the electrode layer and adhesive strength of the layers can be achieved on the solid electrolyte.

The Rare earth calcium chromite titanate can be found on the B-site with low levels (0 - 0.2) Ni, Fe, Ru, or Cu may be doped.

The for the material used according to the invention complex oxides based on rare earth calcium chromite titanates with perovskite structure achieve a sufficiently high electrical conductivity, which also in the mentioned high temperatures is maintained. Besides, they are sufficient chemically stable and for Oxygen ions conductive, so that the problems mentioned at the electrochemical conversion of hydrocarbons, such as methane not occur when the material for an anode of high-temperature fuel cells is used.

This also applies to the thermal expansion coefficient considering the thermal expansion coefficient of materials for Solid electrolyte to.

Of Furthermore, it is stable to reduction and it also occurs in an oxidizing the atmosphere no chemical reactions that affect the actual function can, on.

Such Reactions also occurred at temperatures up to 1300 ° C with a Electrolytes do not open.

He was ^{stable to} reduction at 1000 ° C and an oxygen partial pressure of 10 ^-14 Pa.

One Inventive material For example, from or using calcium carbonate, Chromium oxide and titanium oxide in powder form, in a heat treatment in air or an atmosphere containing oxygen a solid-state reaction be formed. additionally For example, a rare earth oxide may also be preferred in such a powder mixture Lanthanum oxide be included.

The Solid state reaction could already be initiated at a temperature of 800 ° C and a sintering The powder components occurred at temperatures from at least 1000 ° C.

As with the in 1 As can be seen in the diagram, with lanthanum calcium chromite titanates electrical conductivities above 10 Scm ^-1 at temperatures of 1000 ° C. in air and with the in 2 shown diagram in a H ₂ / H ₂ O / Ar atmosphere with pH ₂ O / pH ₂ = 0.01 can be achieved.

The relationship between temperature and electrical conductivity is shown in the diagram for various lanthanum calcium chromite titanates 3 clarified.

From the in 4 The diagram shows the thermal expansion of La _0.47 Ca _0.4 Cr _0.2 Ti _0.8 O ₃ and stabilized zirconium oxide (YSZ) in the temperature range of 0 to 900 ° C and it is clear that only slight differences in the thermal expansion coefficients These two materials are present so that they can be used with each other in combination even in high temperature fuel cells.

at an X-ray analysis could not interact with a 100 h storage and temperature from 1300 ° C with stabilized zirconia for two examples of lanthanum calcium chromite titanates, each with 50 mol% of stabilized zirconia have been analyzed become.

The corresponding X-ray diagrams are in the 5a to 5c played.

there the lower trace gives the spectrum for that from the respective lanthanum calcium chromite titanate and the stabilized zirconia powder mixture formed before heat treatment and the upper course after annealing in air at temperatures from 1300 ° C again.

In 6 are a scanning electron micrograph and an EDX analysis of the interface of a lanthanum calcium chromite titanate with stabilized zirconia, as an electrolyte material at the interface area after the temperature treatment at 1000 ° C and 5 h reproduced.

As with the in 7 As shown in the diagram, for a lanthanum calcium chromite titanate, its stability could also be demonstrated at high temperatures in an argon / hydrogen / steam atmosphere with an oxygen partial pressure pO ₂ = 10 ^-14 Pa. This is evident from the X-ray spectrum, the upper curve representing the X-ray spectrum before the heat treatment and the lower curve the X-ray spectrum after the heat treatment.

With 8th To illustrate a changing catalytic activity of lanthanum calcium chromite titanates with altered cation composition and A-lattice space and B-lattice space stoichiometry in the oxidation of hydrocarbons with the gas composition specified therein.

A preferred material according to the invention may have the composition La _0.47 Ca _0.4 Cr _0.2 Ti _0.8 O ₃ . It can be prepared from a lanthanum oxide, calcium carbonate, chromium oxide (III) and titanium dioxide, which have been mixed together in appropriate proportions. In this case, a heat treatment is carried out in air at 1350 ° C for 16 h.

Such a material is then stable at a temperature of 1000 ° C in an Ar / H ₂ / H ₂ O atmosphere and an oxygen partial pressure of 10 ^-14 Pa, as shown by the diagram in FIG 7 is clarified.

It has an electrical conductivity at temperatures of 900 ° C, which is 0.3 S · cm ^-1 and an oxygen partial pressure of 10 ^-15 Pa (see. 2 ). The thermal expansion coefficient is (10 - 10.5) × 10 ^-6 K ^-1 (cf. 4 ) and also a significantly increased catalytic activity for the oxidation of propene as a hydrocarbon for lanthanum calcium chromite titanates could be detected (see. 8th ), so that in particular the detection could be obtained as a suitable anode material for hydrocarbons of high-temperature fuel cells.

by virtue of their chemical, thermal and in particular electrical properties can the inventions to the invention materials but also for Gas sensors used alone or together for solid electrolytes become. But there is also the possibility of coatings to produce such a material.

at According to sufficient porosity but can also membranes that for example Oxygen permeable are made of such a material.

Claims (10)

Material for electrochemical applications in the high temperature range, characterized in that the material formed from a rare earth calcium chromite titanate or rare earth calcium chromite titanate in a mixture with Me _x Zr _lx O ₂ or Me _x Ce _lx O ₂ , wherein Me an alkaline earth or Sel tenerdelement is included. Material according to claim 1, characterized in that it consists of (La _{l - x} Ca _x ) _{l - α} (Cr _ly Ti _y ) _{l - β} O _{3 -δ} , where x = 0-1, y = 0,1, α = 0-0.3 and β = 0-0.1 is formed or contained. Material according to one of the preceding claims, characterized characterized in that the rare earth calcium chromite titanate with Ni, Fe, Ru or Cu is doped on the B-square. A method for producing a material according to any one of claims 1 to 3, characterized in that a powder mixture formed at least from CaCO ₃ , CrO ₃ and TiO _{2 is} subjected to a heat treatment with a minimum temperature of 800 ° C for the formation of the material. Method according to Claim 6, characterized that in addition a rare earth oxide is contained in the powder mixture. A method according to claim 6 or 7, characterized in that additionally La ₂ O _{3 is contained} in the powder mixture. Method according to one of claims 6 to 8, characterized that the material is sintered at a temperature of at least 1000 ° C. Method according to one of the methods 6 to 8, characterized in that in the powder mixture additionally Me _x Ce _lx O ₂ or Me _x Zr _lx O ₂ , wherein Me is an alkali or rare earth metal is included. Use of a material according to one of claims 1 to 5 for the production of anodes for high-temperature fuel cells. Use of a material according to one of claims 1 to 5 for the production of gas sensors, gas sensors with solid electrolytes, Oxygen membranes, electrodes or coatings.