Method for producing a solid electrolyte

Abstract

Method for producing a solid electrolyte with a multilayer structure, especially for a solid oxide fuel cell, comprising the steps: a) Providing a support layer (10) made of a porous material; b) Application of two or more layers (21, 22, 23) of an ion-conducting material by wet chemical deposition and subsequent thermal treatment at a temperature below 1000 °C, wherein the temperature of the thermal treatment decreases from the first layer (21) to the last layer (22; 23), and wherein the mean size of the applied particles decreases from the first layer (21) to the last layer (22; 23); c) Deposition of one or more non-porous layers (30; 31, 32) of an ion-conducting material by vapor deposition at a temperature below 1000 °C.

Description

[0001] The present invention relates to a method for producing a solid electrolyte with a multilayer structure, in particular for a solid oxide fuel cell.

[0002] The electrolytes used in solid oxide fuel cells are characterized by being electrically insulating, but exhibiting conductivity for ions, especially for oxide ions, such as in the case of the commonly used yttrium-stabilized zirconium oxide, or for protons, such as in the case of yttrium-doped barium zirconate.

[0003] The production of solid electrolytes from such oxide ceramic materials using vacuum plasma spraying (VPS) is known from the prior art (see, e.g., P. Szabo et al., ECS Transactions 2009 (25) 175-185). Solid electrolytes produced using this method have a thickness in the range of 35 to 50 µm. The production of significantly thinner layers, which would be desirable with regard to optimizing the conductivity of the solid electrolyte, is not possible using VPS.

[0004] Methods for producing correspondingly thinner solid electrolytes are known in which nanoparticles of oxide ceramic materials are sintered (see, e.g., F. Han et al., Journal of Power Sources 2012 (218) 157-162). However, the high temperatures of over 1,300 °C used in these processes, which are necessary to achieve sufficient stability, are disadvantageous for several reasons. Firstly, under atmospheric conditions, these high temperatures can lead to corrosion of the metallic substrate materials used, thus impairing the electrochemical functionality. Furthermore, the high temperatures cause shrinkage of the sintered layer, which, especially in larger solid electrolytes (i.e., for larger cells), leads to deformations and geometric deviations from a flat layer. In extreme cases, shrinkage can also cause cracks in the sintered layer.

[0005] In an article by N. Jordan Escalona, ​​the fabrication of high-temperature fuel cells via physical vapor deposition is described (Publications of the Jülich Research Centre, Energy & Environment Series, Volume 32 (2009), pages 21 to 41). In this process, electrolyte and sr barrier layers are deposited onto a surface-conditioned anode substrate made of nickel oxide and yttrium-stabilized zirconium oxide using PVD.

[0006] US patent 2011 / 0200910 A1 discloses a solid oxide fuel cell (SOFC) with a multilayer structure comprising a porous metal ceramic anode, an anodic functional layer, a double-layer electrolyte with a cerium oxide layer and a bismuth oxide layer, and a porous ceramic cathode.

[0007] WO 2007 / 045113 A1 describes a composite element of at least one thin film consisting of at least two different layers of an oxide ceramic, oxide ceramic and metallic or metallic material.

[0008] The invention is therefore based on the objective of proposing a method for producing a sufficiently thin and gas-tight solid electrolyte, with which the above-mentioned problems can be avoided.

[0009] This problem is solved according to the invention by a method according to claim 1.

[0010] The porous support layer onto which the layers of the solid electrolyte according to the invention are applied is preferably electrically conductive and can, in particular, be an anode or a cathode for a solid oxide fuel cell, or also an electrochemically neutral layer. The porosity of the support layer is typically in the range of less than 200 nm.

[0011] By applying one or more layers to the substrate according to the invention using wet chemical deposition and subsequent thermal treatment, the porosity is reduced, thereby enabling the subsequent application of one or more non-porous layers by means of vapor deposition, which ensure the gas tightness of the solid electrolyte. This result could not be achieved by vapor deposition directly onto the porous substrate.

[0012] Because all steps of the process according to the invention can be carried out at a temperature below 1000 °C, i.e., in the range of the normal operating temperature of a solid oxide fuel cell, corrosion of the materials used and deformation of the layers due to thermal stresses can be significantly reduced or completely avoided. This particularly enables the production of large-area solid oxide fuel cells with the solid electrolyte according to the invention.

[0013] The wet-chemical application in step b of the process according to the invention particularly comprises the application of a suspension, a colloidal solution, or a sol of particles made of ion-conducting material onto the carrier layer or onto a preceding layer of the ion-conducting material. The suspension or solution preferably comprises an organic binder, which is eliminated during the subsequent thermal treatment. Alternatively, it is also conceivable to first wet-chemically apply and thermally treat only particles made of an inert material that has no conductivity, and to subsequently introduce the ion-conducting material into the layer(s), e.g., by infiltration.

[0014] The particles used for wet chemical application preferably have a medium size in the range of 1 to 100 nm, more preferably from 5 to 50 nm.

[0015] According to the invention, step b comprises the application of two or more layers, wherein the temperature of the thermal treatment decreases from the first to the last layer. The thermal treatment of the first layer can be carried out, in particular, at a temperature of 900 to 1000 °C, and the treatment of the last layer, in particular, at a temperature of 600 to 700 °C. The higher temperature of the first layer results in good adhesion to the substrate layer and high mechanical stability, while the lower temperature of the last layer leads to the finest possible porous structure, i.e., to the lowest possible surface roughness for the subsequent vapor deposition.

[0016] A decreasing porosity of the wet-chemically applied layers is additionally achieved by decreasing the average size of the applied particles from the first to the last layer, for example from approximately 50 nm to approximately 5 nm.

[0017] The two or more layers in step b of the method according to the invention can comprise either the same or different ion-conducting materials. However, the use of the same material is generally preferred.

[0018] The gas-phase deposition in step c of the process can include, in particular, all types of physical vapor deposition (PVD) or chemical vapor deposition (CVD). Examples of these processes are EBPVD (electron beam PVD), PLD (pulsed laser deposition), sputtering, PECVD (plasma-enhanced CVD), MOCVD (metal-organic CVD), and ALD (atomic layer deposition). In most cases, particles with a size in the range of 1 to 500 nm are assumed, especially those with a size of 1 to 100 nm.

[0019] The gas phase deposition is preferably carried out at a temperature of 400 to 800 °C. Under these conditions, a substantially gas-tight layer of the solid electrolyte can be produced.

[0020] It may be provided that, as part of step c of the process, two or more layers of different ion-conducting materials are applied by means of vapor deposition.

[0021] The ion-conducting material(s) applied in step c can be the same or different from the ion-conducting material(s) applied in step b. For example, several layers of a first ion-conducting material can be applied in step b, and in step c one layer of this first ion-conducting material and another layer of a second ion-conducting material.

[0022] The ion-conducting material(s) used in the process according to the invention are preferably selected from oxide ceramics, in particular from doped zirconium oxides, such as yttrium-stabilized zirconium oxide (YSZ), from doped cerium oxides, such as gadolinium-doped cerium oxide (GDC), yttrium-doped barium zirconate (BZY), yttrium-doped barium cerate (BCY), doped bismuth(III) oxide, and doped lanthanum gallates. For certain applications of the solid electrolyte, however, cation-conducting materials can also be used, in particular proton-conducting or lithium-, sodium-, or silver-conducting materials.

[0023] The present description further discloses a solid electrolyte with a multilayer structure, in particular for a solid oxide fuel cell, which is produced according to the inventive method.

[0024] Advantages and preferred embodiments of the solid electrolyte have already been described in connection with the manufacturing process according to the invention.

[0025] In the solid electrolyte, the total thickness of all layers of the ion-conducting material(s) is 0.5 to 10 µm, preferably 1 to 5 µm. Despite the multilayer structure, the total thickness is therefore significantly less than that of a solid electrolyte produced by vacuum plasma spraying.

[0026] The total thickness of the wet-chemically applied layers is preferably from 0.1 to 5 µm, more preferably from 0.5 to 3 µm, and the total thickness of the layers applied by gas phase deposition is preferably from 0.5 to 5 µm, more preferably from 0.5 to 2 µm.

[0027] As already described in connection with the inventive method, the solid electrolyte comprises several wet-chemically applied layers, the porosity of which decreases with distance from the support layer.

[0028] In particular, the wet-chemically applied layer adjacent to the carrier layer can have a pore size in the range of 100 to 300 nm, and the wet-chemically applied layer furthest from the carrier layer can have a pore size in the range of 1 to 150 nm, preferably 1 to 50 nm.

[0029] The solid electrolyte preferably exhibits a leakage rate of less than 5·10 -3 Pa·m 3 ·s -1 on, which is achieved due to the non-porous layer(s) applied by means of gas phanesen deposition.

[0030] The solid electrolyte can also be used in a solid oxide electrolysis cell, or in other electrochemical applications such as solid phase batteries, etc.

[0031] These and other advantages of the invention are described in more detail with reference to the figures and the following exemplary embodiments.

[0032] They show in detail: Fig. 1: Electron micrograph of a first embodiment of a solid electrolyte; Fig. 2: Electron micrograph of a second embodiment of a solid electrolyte; and Fig. 3: Electron micrograph of a third embodiment of a solid electrolyte.

[0033] According to a first embodiment of the invention, a solid electrolyte with a multilayer structure for a solid oxide fuel cell can be produced according to the following method: In step a, an electrically conductive support layer is provided, e.g. made of the material NiCrAl-LST-NiO. The support layer has a homogeneous surface with a defined porosity of less than 200 nm.

[0034] In step b, particles of yttrium-stabilized zirconium oxide (YSZ) with an average particle size of 5 to 50 nm are applied to the substrate layer using a wet chemical process. This application takes the form of an aqueous suspension with an organic binder (e.g., polyvinyl alcohol). The suspension is applied in a layer thickness of less than 1 µm using a spraying or dipping process. The applied suspension is then thermally treated at a temperature of approximately 900 °C.

[0035] Step b is repeated by applying a second layer of YSZ using a wet chemical process, but with the difference that the thermal treatment of this second layer takes place at a temperature of approximately 700 °C. This results in a significantly finer porosity of less than 5 nm.

[0036] In step c, a layer of gadolinium-doped cerium oxide (GDC) is applied to the second wet-chemical layer by physical vapor deposition (PVD) at a temperature in the range of 600 to 800 °C. The PVD layer has a non-porous structure and ensures the gas tightness of the solid electrolyte.

[0037] An electron micrograph of the first embodiment of the solid electrolyte produced according to this method is shown in the Fig. 1 shown, where the individual layers are labelled with the following reference symbols: 10 Substrate layer made of NiCrAl-LST-NiO 21. First wet chemical layer made of YSZ, coarsely porous, thickness approx. 1 µm 22. Second wet-chemical layer of YSZ, finely porous, thickness approx. 1 µm 30 PVD layers made of GDC, approximately 2 µm thick

[0038] A solid oxide fuel cell with a solid electrolyte according to this embodiment has a leakage rate below the relevant limit of 5·10 -3 Pa·m 3 ·s -1 on.

[0039] A second embodiment of a solid electrolyte can be produced according to the following method: Steps a and b are carried out as in the first embodiment. A layer of YSZ is first deposited onto the second wet-chemical YSZ layer by physical vapor deposition, and then a layer of gadolinium-doped cerium oxide (GDC) is deposited on top of this, also by PVD.

[0040] An electron micrograph of the solid electrolyte according to the second embodiment is shown in the Fig. 2 is shown and has the following layer structure: 10 Carrier layer made of NiCrAl-LST-NiO 21. First wet-chemical YSZ layer, coarse-pored, thickness approx. 1 µm 22. Second wet-chemical YSZ layer, finely porous, thickness approx. 1 µm 31 PVD layer made of YSZ, thickness approx. 0.5 µm 32 PVD layer made of GDC, thickness approx. 1 µm

[0041] A third embodiment of a solid electrolyte can be produced essentially according to the method of the second embodiment, with the difference that in step b a total of three layers are applied wet chemically, namely a first and a second layer with a higher porosity (up to about 300 nm) and a third layer with a lower porosity (up to about 5 nm).

[0042] An electron micrograph of a third embodiment of a solid electrolyte produced according to this method is shown in the Fig. 3 is shown and has the following layer structure: 10 Carrier layer made of NiCrAl-LST-NiO 21 First wet-chemical YSZ layer, coarse-pored, thickness approx. 0.5 µm 22. Second wet-chemical YSZ layer, coarse-pored, thickness approx. 0.5 µm 23. Third wet-chemical YSZ layer, finely porous, thickness approx. 0.5 µm 31 PVD layer made of YSZ, thickness approx. 0.5 µm 32 PVD layer made of GDC, thickness approx. 1 µm

Claims

[1] Method for producing a solid electrolyte with a multilayer structure, in particular for a solid oxide fuel cell, comprising the steps: a) Providing a support layer (10) made of a porous material; b) Application of two or more layers (21, 22, 23) of an ion-conducting material by wet chemical deposition and subsequent thermal treatment at a temperature below 1000 °C, wherein the temperature of the thermal treatment decreases from the first layer (21) to the last layer (22; 23), and wherein the mean size of the applied particles decreases from the first layer (21) to the last layer (22; 23); c) Deposition of one or more non-porous layers (30; 31, 32) of an ion-conducting material by vapor deposition at a temperature below 1000 °C. [2] Method according to claim 1, wherein the support layer (10) is electrically conductive, and in particular is an anode or a cathode for a solid oxide fuel cell or an electrochemically neutral layer. [3] Method according to claim 1 or 2, wherein the wet chemical application in step b comprises the application of a suspension, a colloidal solution or a sol of particles of ion-conducting material onto the carrier layer (10) or onto a previous layer (22, 23) of the ion-conducting material. [4] Method according to claim 3, wherein the particles have an average size in the range of 1 to 100 nm, preferably from 5 to 50 nm. [5] Method according to one of the preceding claims, wherein in step b the temperature of the thermal treatment decreases from a range of 900 to 1000 °C to a range of 600 to 700 °C. [6] Method according to any of the preceding claims, wherein the two or more layers (21, 22, 23) comprise the same or different ion-conducting materials. [7] Method according to any of the preceding claims, wherein the vapor deposition in step c comprises physical vapor deposition (PVD) or chemical vapor deposition (CVD), in particular EBPVD, PLD, sputtering, PECVD, MOCVD or ALD. [8] Method according to any of the preceding claims, wherein the gas phase deposition is carried out at a temperature of 400 to 800 °C. [9] Method according to any of the preceding claims, wherein step c comprises the application of two or more layers (31, 32) of different ion-conducting materials. [10] Method according to any of the preceding claims, wherein the ion-conducting material(s) applied in step c are the same or different from the ion-conducting material(s) applied in step b. [11] Method according to any of the preceding claims, wherein the ion-conducting material(s) are selected from oxide ceramics, in particular from doped zirconium oxides, such as yttrium-stabilized zirconium oxide (YSZ), from doped cerium oxides, such as gadolinium-doped cerium oxide (GDC), yttrium-doped barium zirconate (BZY), yttrium-doped barium cerate (BCY), doped bismuth(III) oxide and doped lanthanum gallates.

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