Electrode for an electrochemical cell and a method for manufacturing an electrochemical cell with such an electrode

Abstract

Electrode for an electrochemical cell, in which an electrode is applied in a planar fashion to a surface of a solid oxide electrolyte and a counter electrode is applied to the surface of the solid oxide electrolyte opposite the electrode, and the base material of the electrode is a composite whose metallic and catalytically active component contains a nickel phase formed from the NiO as a component of the electrode starting material by reduction of the NiO in a hydrogen-containing atmosphere, and whose ceramic component is formed with a doped cerium oxide and a decomposition product of a spinel formed with a transition metal, wherein the transition metal is selected from Ni, Mn, Fe, Co and Cr, and wherein the electrode is an anode in a solid oxide fuel cell, a cathode in a solid oxide electrolysis cell or an electrode in a solid oxide sensor, characterized in thatthat the electrode is formed from a multilayer structure in which the individual layers are metallurgically bonded to one another and have different consistencies and porosities, and a first layer formed on the electrolyte surface has a lower porosity, a lower nickel content and a lower spinel content than at least one further layer with which the electrode is formed.

Description

[0001] The invention relates to an electrode for an electrochemical cell with an oxygen ion conductor as a solid electrolyte, a method for manufacturing an electrochemical cell with such an electrode, and its use.

[0002] Electrochemical cells are typically used to convert chemical energy into electrical energy or vice versa. This process also involves chemical redox reactions, which usually have a negative impact on the cell's resistance, and thus on its efficiency and lifespan, during operation. This particularly affects the electrode in contact with the fuel gas.

[0003] This also applies to electrochemical cells in which the electrolyte is formed with a solid oxide and which are operated at elevated temperatures above 700 °C.

[0004] In the electrochemical cell designs known up to that point, the achievable power density is limited by the required sintering temperature above 1250 °C for the electrode materials used, which consist of NiO and a solid electrolyte phase. At lower sintering temperatures, sufficient adhesion of the electrode material cannot yet be achieved. Furthermore, at these high sintering temperatures, a smaller electrochemically active interface forms between the nickel and solid electrolyte particles in the electrode, due to the larger particle size and the higher sintering temperature.

[0005] Furthermore, the mechanical stability or strength of the electrolyte material is reduced by diffusion of Mn from the opposite air electrode, which is often co-sintered, into stabilized ZrO2 at high sintering temperatures.

[0006] Another disadvantage of known electrochemical cells with a solid oxide electrolyte is the increased activity of the electrode for steam reforming of methane in the inlet area of ​​the electrochemical cell, which causes local cooling of the electrochemical cell. This results in large temperature gradients during internal reforming, which in turn can lead to increased mechanical stresses, even to the point of destroying the electrochemical cell.

[0007] To counteract these problems, electrodes made from a NiO-CGO (nickel oxide-cerium gadolinium oxide) material were used. However, the addition of transition metal oxides leads to an increase in the polarization resistance of the electrochemical cell with such an electrode.

[0008] It is therefore an object of the invention to provide electrochemical cells that operate in fuel cell mode as well as in electrolysis mode (also for gas mixtures consisting of water vapor / carbon dioxide) with increased redox stability, reduced activity for internal reforming and improved long-term catalytic stability and thus reduced power degradation.

[0009] According to the invention, this problem is solved with an electrode for an electrochemical cell having the features of claim 1. A method for producing electrochemical cells with such an electrode is defined in claim 9, and uses are defined in claim 13. Advantageous embodiments and further developments of the invention can be realized with features specified in dependent claims.

[0010] In the cell according to the invention, one electrode is applied over a surface of a solid oxide electrolyte, and another electrode is applied over a surface of the solid oxide electrolyte opposite the electrode. The base material of the electrode on the side that is H 2 or exposed to a hydrogen-containing chemical compound, in particular a gas mixture containing H₂, is a composite whose metallic and catalytically active component contains a nickel phase formed from the NiO component of the electrode starting material by reduction in a hydrogen-containing atmosphere. The ceramic component is formed with a doped cerium oxide and with spinel formed from at least one transition metal selected from Ni, Mn, Fe, Co, and Cr.

[0011] Advantageously, a spinel according to the chemical formula (Cu) can be used in the electrode material. x Ni 1-x ) y (Mn 1-z M'z The electrode material should contain 2O4 with 0 ≤ x ≤ 1.0, 0.8 ≤ y ≤ 1.2, 0 ≤ z ≤ 1 and M' = Fe, Co or Cr. The spinel should be present in the electrode material in a proportion ranging from 0.01 vol% to 15 vol%. Advantageously, the proportion should be 10 vol%, particularly advantageously 5 vol%, and most advantageously 0.5 vol%.

[0012] The cerium oxide should be stabilized with a metal oxide from the rare earth group, in particular gadolinium oxide or samarium oxide.

[0013] The electrode can be particularly advantageously formed from a multilayer structure in which the individual layers are metallurgically bonded to one another and exhibit different consistencies and porosities. A first layer, formed on the surface of the solid electrolyte, can have a lower porosity, a lower nickel content, and a lower spinel content than at least one further layer with which the electrode is formed.

[0014] The electrode can be particularly advantageously formed with at least three layers, of which the first layer is an electrode layer, a centrally arranged second layer is an active electrode layer, and a third layer is an electrode contact layer. The second layer has a higher porosity, a higher nickel content, and a higher spinel content than the first layer, and the third layer has a higher nickel content and a lower spinel content than the second layer. The first layer should have a porosity of no more than 15 vol%, a maximum nickel content of 15 vol%, and a maximum spinel content of 10 vol%; the second layer should have a porosity of at least 30%, a nickel content of at least 20 vol% up to a maximum of 70 vol%, and a maximum spinel content of 15 vol%; and the third layer should have a porosity of at least 30 vol% and a nickel content exceeding 70 vol%.-% and have a maximum spinel content of 5 vol.%.

[0015] Furthermore, the electrode should have a maximum total thickness of 100 µm.

[0016] To produce an electrochemical cell with an electrode according to the invention, a viscous suspension containing the composite, whose catalytically active component includes a nickel phase, and whose ceramic component contains a doped cerium oxide, as well as a powder mixture formed with at least one transition metal selected from Ni, Mn, Fe, Co, and Cr, and at least one liquid, is applied to a surface of a support or the solid oxide electrolyte with a constant layer thickness. Subsequently, a heat treatment is carried out in which liquid and / or organic components are removed and sintering is achieved with the simultaneous formation of a metallurgical bond with the respective surface of the support or solid oxide electrolyte.

[0017] During the sintering of the electrode material, a partial decomposition of the mixed oxide occurs, and cations diffuse into the NiO and CGO. During the initialization (first commissioning) of the respective electrochemical cell under a reducing atmosphere (in hydrogen / water vapor) at elevated temperature (especially in the range of 800°C to 950°C), decomposition of the spinel (Cu) then takes place. x Ni 1-x ) y (Mn 1-z M' zThe reaction of NiO₂⁻ into oxide components and metallic copper, as well as the reduction of NiO₂ to metallic nickel, leads to the formation of nickel alloys. The oxide components react with CGO, and the copper forms a nickel alloy with the nickel. This results in improved adhesion between individual particles in the electrode material. Furthermore, the decomposition of the mixed oxide phase allows for greater porosity at a lower required sintering temperature. The resulting Ni-Cu alloy also provides increased sulfur resistance.

[0018] The suspension can be applied to the surface of the support or the solid oxide electrolyte before heat treatment, for example by screen printing, spin coating, wet powder spraying, doctor blade process, ink-jet printing, 3D printing.

[0019] To form a multi-layered electrode, the multiple layers should be applied to the surface of the support or solid oxide electrolyte by successive application and drying, each layer having a different consistency, before carrying out the heat treatment.

[0020] Various liquids and / or, if necessary, an organic binder can be used to prepare the suspension. A surfactant can also be included in the suspension. It is advantageous if the powder mixture is homogeneously distributed in the suspension so that it can be applied to the respective surface, resulting in a homogeneous distribution of the powdered materials throughout the volume of the electrode or the individual layers that make up the electrode. Organic components can be included in the suspension to influence the porosity; these can act as placeholders. The porosity can be influenced solely or additionally by the particle size of the powder mixture used. Larger particle sizes increase the porosity.

[0021] For the solid electrolyte, Y₂O₃-stabilized ZrO₂, Yb₂O₃-stabilized ZrO₂, or Sc₂O₃-stabilized ZrO₂ with added CeO₂ can be used as the material. An electrode of an electrochemical cell that comes into contact with oxygen, air, or another oxygen-containing chemical compound or gas mixture containing oxygen can be formed with (La,Sr)(Mn,Fe,Co)O₃ or (La,Sr)(Fe,Co)O₃ as a perovskite and Y₂O₃, Sc₂O₃, or Y₂O₃-stabilized ZrO₂, or Gd₂O₃ or Sm₂O₃-stabilized CeO₂, as is also the case in the prior art.

[0022] The electrochemical cell equipped with an electrode according to the invention should exhibit sufficient conductivity for ions and electrons at an operating temperature between 700 °C and 950 °C after reduction in an H2 / H2O gas mixture.

[0023] A conventional electrode on an electrochemical cell (G3) can withstand more than 20 redox cycles with a power loss of less than 0.5%.

[0024] The redox stability of an electrochemical cell with the electrode (G6) according to the invention is significantly improved, and a power increase of 0.7% was observed after 25 redox cycles. A power degradation of 0.6% only occurred after 43 redox cycles. After 75 redox cycles, it amounted to only 4.3%. Fig. 3).

[0025] The electrical resistance of an electrochemical cell equipped according to the invention can be reduced by up to 30% compared to conventional cells ( Fig. 1a and Fig. 1b) The polarization resistance of the electrodes can be reduced by up to 60% depending on the respective operating temperature of the cell ( Fig. 2a and Fig. 2b).

[0026] It is also possible to achieve operation with a higher methane content in the fuel compared to previously known possibilities when operating as a fuel cell, and thus increased system efficiency.

[0027] When used in a solid oxide fuel cell, the electrode forms an anode, and when used in a solid oxide electrolysis cell, the electrode forms a cathode. An electrode according to the invention can also be used in a solid oxide sensor.

[0028] The invention will be explained in more detail below by way of example.

[0029] This shows: Fig. 1a a diagram showing the ratios of achievable electrical voltages or specific electrical power per area and electrical current per area for electrochemical cells with conventional and inventive electrodes at different operating temperatures; Fig. 1b a diagram of the electrical surface resistance for electrochemical cells with conventional and inventive electrodes at different operating temperatures; Fig. 2a a diagram of the electrical surface resistance for electrochemical cells with conventional and inventive electrodes at different operating temperatures in comparison during operation in fuel cell operating mode with conventional and inventive anode; Fig. 2b a diagram of the electrical surface resistance of the cathode for electrochemical cells with conventional and inventive electrodes at different operating temperatures in comparison when operating in fuel cell mode with conventional and inventive anode; Fig. 3 a diagram that reflects the change in power density at the same operating point in fuel cell operating mode due to redox cycles or power losses in % for electrochemical cells with conventional and inventive electrodes.

[0030] The following raw materials are used for electrode production: - A powdered charge-compensated composition with respect to the respective concentration of Gd3+ with Ce 0.8 Gd 0.2 O 1.9 or Ce 0.8 Gd 0.1 O 1.95 -(CGO), with a mean particle size d 50 < 2.0 µm, especially with d 50 < 0.5 µm, - A powdered NiO, with a medium particle size d 50 <5.0 µm, especially with d 50 < 2.5 µm - A powdered spinel containing copper, nickel, and manganese, especially Cu 0.6 Ni 0.4 Mn2O4 or Cu 0.8 Ni 0.2Mn2O4 (CNM) with a mean particle size d 50 <5.0 µm, especially with d 50 < 2.5 µm.

[0031] The powders (CGO, NiO, CNM) are mixed together in the appropriate ratio and subjected to various milling, mixing, dispersing, drying, sieving, and rolling steps. They are then processed with an organic binder (e.g., polyvinyl butyral resin (PVB) or ethylcellulose (EC)) and additives (organic wetting, dispersing, and pore-forming agents) to create a screen-printable paste. Three pastes with different solids ratios (CGO:NiO:CNM) are produced. The manufacturing parameters should be selected to achieve the desired properties, such as storage stability, good screen-printing behavior, crack-free drying of the printed layers, and the layer porosity required for electrochemical activity after sintering.

[0032] The pastes are sequentially applied to the electrolyte substrate in several superimposed layers using screen printing and then dried. First, a layer with the lowest NiO content (Ni:CGO:CNM = 12 : 87 : 1 by volume) is printed, followed by a layer with a medium NiO content (Ni:CGO:CNM = 58 : 40 : 2 by volume), and finally a layer with the highest NiO content (Ni:CGO:CNM = 87 : 11 : 2 by volume). The middle layer is then printed last. Subsequently, the layers of the electrode that comes into contact with oxygen or an oxygen-containing gas or gas mixture (the so-called air electrode) are also sequentially printed and dried on the opposite side of the electrolyte substrate. The coated electrolyte substrates with the dried electrode layers are then co-fired in air.The sintering temperature of the CGO / NiO / CNM electrodes is between 1150 °C and 1250 °C. This results in an electrochemical cell with well-adhering layers for the fuel gas and air electrodes. The cell can then be assembled into a cell stack in this state. Before operation, an H₂ / N₂ mixture is introduced into the stack at operating temperature. In the fuel gas electrode (the electrode that comes into contact with H₂ or an H₂-containing gas or gas mixture), NiO is reduced to Ni and spinel to Ni, Cu, and MnO. During the reduction phase, MnO forms a mixed oxide with the CGO, and Cu alloys the Ni present in the electrode. After this process, an electrode with improved redox stability and lower internal electrical resistance is obtained, as is the case, for example, in [reference to a specific application]. Fig. 1- Fig.Figure 3 shows that for high power density and redox stability, it is particularly important that the porosity of the first layer is in the range of 10% - 15%, the second layer, which is located between the first and third layers, is in the range of 30% - 45%, and the third layer is in the range of 35% - 45%.

Claims

[1] Electrode for an electrochemical cell in which an electrode is applied over a surface on a surface of a solid oxide electrolyte and a cathode is applied over a surface on the surface of the solid oxide electrolyte opposite the electrode and the base material of the electrode is a composite whose metallic and catalytically active component contains a nickel phase that was formed from the NiO as a component of the electrode starting material by reduction of the NiO in a hydrogen-containing atmosphere, and whose ceramic component is formed with a doped cerium oxide and a spinel formed with at least one transition metal selected from Ni, Mn, Fe and Cr. [2] Electrode according to claim 1, characterized by , that the electrode material contains a spinel according to the chemical formula (Cu x Ni1- x ) y (Mn 1-z M' z)2O4 with 0 ≤ x ≤ 1.0, 0.8 ≤ y ≤ 1.2, 0 ≤ z ≤ 1 and M' = Fe, Co or Cr is contained. [3] Electrode according to any one of the preceding claims, characterized by , that the electrode material contains spinel in a proportion ranging from 0.01 vol.% to 15 vol.%. [4] Electrode according to any one of the preceding claims, characterized by that the cerium oxide is stabilized with a metal oxide from the group of rare earth metals, in particular gadolinium oxide or samarium oxide. [5] Electrode according to any one of the preceding claims, characterized by, that the electrode is formed from a multilayer structure in which the individual layers are metallurgically bonded together and have different consistency and porosity, and a first layer formed on the electrolyte surface has a lower porosity, a lower nickel content and a lower spinel content than at least one further layer with which the electrode is formed. [6] Electrode according to any one of the preceding claims, characterized by, that the electrode is formed with at least three layers, of which the first layer is an electrode layer, a centrally arranged second layer is an active electrode layer and a third layer is an electrode contact layer, wherein the second layer has a greater porosity, a greater nickel content and a greater spinel content than the first layer and the third layer has a greater porosity, a greater nickel content and a greater spinel content than the second layer. [7] Electrode according to the preceding claim, characterized by, that the first layer has a porosity of a maximum of 15%, a maximum nickel content of 15 vol% and a maximum spinel content of 10 vol%, and the second layer has a porosity of at least 30%, a nickel content of at least 20 vol% up to a maximum of 70 vol% and a maximum spinel content of 15 vol%, and the third layer has a porosity of at least 30 vol%, a nickel content above 70 vol% and a maximum spinel content of 5 vol%. [8] Electrode according to any one of the preceding claims, characterized by that the electrode has a maximum total thickness of 100 µm. [9] Method for manufacturing an electrochemical cell with an electrode according to any one of the preceding claims, characterized by, that a viscous suspension in which the composite, whose metallic and catalytically active component contains a nickel phase formed from the NiO as a component of the electrode starting material by reduction in a hydrogen-containing atmosphere, and whose ceramic component contains a doped cerium oxide as well as a powder mixture formed with at least one transition metal selected from Ni, Mn, Fe, Co and Cr formed from spinel and at least one liquid, is applied to a surface of a support or the solid oxide electrolyte with a constant layer thickness and subsequently a heat treatment is carried out in which liquid and / or organic components are removed and sintering with simultaneous formation of a metallurgical bond with the respective surface of the support or solid oxide electrolyte is achieved. [10] Method according to the preceding claim, characterized bythat a maximum temperature of 1250 °C is maintained during heat treatment. [11] Method according to the two preceding claims, characterized by that the suspension is applied to the surface of the support or solid oxide electrolyte by means of screen printing, spin coating, wet powder spraying, doctor blade process, ink-jet printing or 3D printing prior to carrying out the heat treatment. [12] Method according to any one of the three preceding claims, characterized by , that several layers are applied to the surface of the support or solid oxide electrolyte by successive application and drying, each with a different consistency, before carrying out the heat treatment. [13] Use of an electrode according to any one of claims 1 to 8 for use as an anode in a solid oxide fuel cell, as a cathode in a solid oxide electrolysis cell or as an electrode in a solid oxide sensor.