Solid oxide electrolysis cell with cathode, and method

Abstract

The invention relates to a solid oxide cell with an electrolyte (2), wherein gas-permeable electrodes (1, 3) are attached on opposite sides of the electrode (2), wherein the fuel electrode (1) comprises Sr2Fe2-xMoxO6-δ or Sr2Fe2-xMox-yMyO6-δ, where M = nickel (Ni), and / or cobalt (Co), and / or titanium (Ti), and / or manganese (Mn) and / or copper (Cu). The invention also relates to a method for operating the solid oxide cell.

Description

[0001] Solid oxide electrolysis cell with cathode and process

[0002] Description

[0003] The invention relates to a solid oxide cell (SOC), which includes solid oxide electrolysis cells (SOECs). The invention also relates to a method for operating the solid oxide cell.

[0004] A typical SOC consists of a gas-tight electrolyte surrounded by two porous electrodes. In electrolysis mode, for example, H2O, CO2, or a gas mixture of both can be converted into H2, CO, or synthesis gas. Thus, electrical energy can be converted into chemical energy.

[0005] When using SOC technology, a variety of gases, such as H2, H2O, CO, CO2, hydrocarbon-based gases, etc., can be used in both electrolysis and fuel cell operation.

[0006] The object of the present invention is to further develop a solid oxide electrolysis cell. In particular, a high-performance solid oxide electrolysis cell with a long service life is to be created.

[0007] This object can be achieved by a solid oxide cell having the features of the first claim. A method for operating the cell comprises the features of the independent claim. The dependent claims relate to advantageous embodiments.

[0008] To achieve this goal, a solid oxide cell has an electrolyte, with gas-permeable electrical conductors located on opposite sides of the electrolyte as electrodes, or anode and cathode. During electrolysis, the cathode is also called the fuel electrode for fuel reduction. Meanwhile, oxygen is produced at the anode, which is why it is also called the oxygen electrode. The two electrodes can be attached directly or indirectly to the electrolyte.

[0009] The fuel electrode can be made of Sr2Fe2- x Mon x Oe-ö exist or Sr2Fe2- x Mon x Oe-ö. The fuel electrode can be made of Sr2Fe2- x Mon x .yM y O6-ö with M = nickel (Ni), and / or cobalt (Co), and / or titanium (Ti), and / or manganese (Mn) and / or copper (Cu), or Sr2Fe2- x Mon x .yM y O6-ö. The fuel electrode can be made of Sr2- z M' z Fe2-xMOxO6-ö exist or Sr2-zM' z Fe2-xMo x O6-ö, where M' = Gd, Nd, Pr, La or Sm.

[0010] Sr2Fe2-xMo x C>6-ö and Sr2Fe2-xMo x -yMyO6-ö are also abbreviated as SFM. A composite electrode can also be made from SFM and a second material.

[0011] The fuel electrode is the electrode at which a fuel or other substance is produced by reduction from one or more starting materials during electrolysis. For example, hydrogen can be produced from carbon dioxide by reducing water vapor and / or carbon monoxide. If the solid oxide cell is operated as an electrolysis cell, the anode is the oxygen electrode.

[0012] In fuel cell operation, an oxygen-containing gas is fed to the oxygen electrode to generate electricity.

[0013] The fuel electrode can consist of only one layer. The other electrode can also consist of only one layer, i.e. of only one material. The other electrode is called the oxygen electrode. The oxygen electrode and / or the fuel electrode can comprise more than one layer. Each layer of an electrode can be formed from several starting materials. The fuel electrode can, for example, be made of a Sr2Fe2- x Mon x O6-ö and a powder consisting of another starting material. Each layer of an electrode can therefore consist of a composite material or comprise a composite material. Each electrode can initially be manufactured from one or more starting materials and subsequently doped or infiltrated with another substance.

[0014] The oxygen electrode and / or the fuel electrode may comprise a layer that is electrically conductive and additionally catalytically active. An electrochemical reaction may take place in the catalytically active layer. The electrochemical reaction may, for example, be the conversion of water vapor to hydrogen with the absorption of electrons.

[0015] In addition, the oxygen electrode and / or the fuel electrode can comprise an electrically conductive, porous layer, which is advantageously not catalytically active. The catalytically active layer can be located between the catalytically non-active layer and the electrolyte layer. This allows the use of materials to be optimized. The oxygen electrode and / or the fuel electrode can comprise a current collector layer. The current collector layer can be a third layer of an electrode. The current collector layer can be less gas-permeable than the one or two other layers mentioned. The current collector layer can be located on the outside of an electrode, i.e., at the maximum distance from the electrolyte layer. The current collector layer can be made of gold, platinum, or nickel, for example.

[0016] Electrodes can be not only electrically conductive, but preferably also ionically conductive. The material of an electrode that is both electrically and ionically conductive is also called MIEC. The material can be, for example, a cermet. The material can, for example, comprise two phases: an electrically conductive phase and an ionically conductive phase. The material can, for example, be a (mixed) conductive oxide.

[0017] The symbol "ö" represents an oxygen deficiency in the electrode material. This means that there may be an oxygen deficiency in the crystal structure of the electrode material. This means that the electrode material does not contain the maximum number of oxygen atoms normally specified in the chemical formula.

[0018] With such a fuel electrode made of Sr2Fe2- x Mon x Oe-ö or at least Sr2Fe2- x Mon xOe-ö, a particularly powerful and long-lasting solid oxide cell can be created, especially for use as an electrolysis cell. This also applies to a fuel electrode made of Sr2Fe2- x Mon x . yMyOe-ö exists or the Sr2Fe2- x Mon x .yM y O6-ö. x = 0.5 to 1 has proven particularly suitable for obtaining a long-term stable, high-performance solid oxide cell.

[0019] Z = 0 to 0.4 has proven particularly suitable for obtaining a long-term stable, high-performance solid oxide cell. Preferably, z > 0.

[0020] It is true that ö > 0. It is preferable that ö is at most 1.

[0021] Y less than 0.5 at-% is useful to achieve higher performance. Y greater than 0.5 is not excluded, but at least does not lead to a noticeable increase in performance and may also cause other problems. The proportion of Sr2Fe2- x Mon x Oe-ö or Sr2Fe2- x Mon x -yM y O6-ö in the fuel electrode material or in a layer of the fuel electrode is preferably more than 50 wt.% in order to create a long-term stable and efficient solid oxide cell. This proportion of Sr2Fe2- x Mon x Oe-ö or Sr2Fe2- x Mon x .yM y For example, O6-ö can be at least 60 wt%.

[0022] The proportion of a second substance in the fuel electrode material, the Sr2Fe2- x Mon x Oe-5or Sr2Fe2- x Mon x.y MyO6-ö, or in a layer of the fuel electrode containing Sr2Fe2- x Monx Oe-ö or Sr2Fe2- x Mon x.y The MyO6-ö content can preferably be up to 30 wt.% or up to 50 wt.% to create a long-term stable and efficient solid oxide cell. More than 50 wt.% is generally not technically feasible.

[0023] Gadolinium-doped cerium oxide (GDC) can be selected as a particularly suitable second starting material or substance in order to create a long-term stable and efficient solid oxide cell. Such a fuel electrode or a layer of the fuel electrode can, for example, be made of a Sr2Fe2- x Mon x Oe-ö powder and a powder consisting of GDC. The proportion of the powder consisting of Sr2Fe2- x Mon x The ratio of the powder consisting of Oe-ö is then preferably more than 50 wt.% and can, for example, be at least 70 wt.%.

[0024] It is possible that cobalt, titanium, manganese, copper, and / or nickel are infiltrated into the fuel electrode or into a layer of the fuel electrode. It is possible that the fuel electrode or a layer of the oxygen electrode is doped with cobalt, titanium, manganese, copper, and / or nickel. For example, a fuel electrode or a layer of a fuel electrode made of Sr2Fe2- x Mon x Oe-ö or Sr2Fe2- x Mon x.y MyO6-ö. The Sr2Fe2- x Mon x Oe-ö and / or Sr2Fe2- x Mon x.y The fuel electrode made of MyO6-ö may have been doped or infiltrated with cobalt, titanium, manganese, copper and / or nickel.

[0025] The proportion of cobalt, titanium, manganese, copper, and / or nickel in the fuel electrode or the fuel electrode layer can, in technically reasonable amounts, be up to 40 at.%, 30 at.%, or 50 at.%. This can further improve the electrochemical properties of a fuel electrode. The data refer to the materials containing Sr2Fe2. x Mon x Oe-öOr from or Sr2Fe2- X M o x-y MyOe-ö at least include.

[0026] A barrier can be located between the fuel electrode and the electrolyte to improve the properties of the solid oxide cell. The barrier can, for example, comprise or consist of doped cerium oxide. The barrier can comprise or consist of gadolinium-doped cerium oxide (GDC) or samarinium-doped cerium oxide (SDC). The barrier is generally a layer located between two other layers. The doping can be at least 10 wt.% and / or up to 20 wt.%.

[0027] The electrolyte can consist of a gas-tight, ceramic material that can conduct oxygen ions at least at sufficiently high temperatures, such as 600°C and above, and has an insulating effect on electrons.

[0028] The electrolyte may consist of or comprise yttrium-stabilized zirconia (YSZ) or scandium dioxide-stabilized zirconia (ScSZ) or samarine- (SDC) or gadolinium-doped cerium oxide (GDC) or strontium- or magnesium-doped lanthanum gallium oxide (LSGM).

[0029] Typical oxygen electrode materials include doped lanthanide perovskites such as Sr-doped LaMnOs (LSM), Sr-doped La(Fe,Co)O3 (LSCF) and Sr-doped La(Co)O3 (LSC).

[0030] A separating layer may be present between the electrolyte and the fuel electrode. The separating layer may, for example, be a cerium compound between the electrolyte and the fuel electrode.

[0031] The problems of long-term stability in SOC systems can be solved by implementing an electrically and preferably also ionically conductive fuel electrode containing at least Sr2Fe2- x Mon xOe-ö should at least be included, remedied or at least reduced.

[0032] Materials suitable for solid oxide cells can be selected so that their thermal expansion coefficients are similar to those of the other materials, further improving long-term stability. YZS, LSGM, or GDC have sufficiently similar thermal expansion coefficients to the material Sr2Fe2- x Mon x Oe-ö of the oxygen electrode. To a gas-permeable layer, the Sr2Fe2- x Mon x Oe-ö can therefore be adjacent to a layer of YZS, LSGM or GDC in order to obtain a long-term stable solid oxide cell.

[0033] The solid oxide cell is usually operated at temperatures from 600°C or 650°C.

[0034] The solid oxide cell is typically operated at temperatures up to 900°C. The solid oxide cell can be plate-shaped. The anode, electrolyte, and cathode can then be shaped like flat plates. An anode compartment can be adjacent to the anode, which can be separated from a cathode compartment adjacent to the cathode. The anode compartment can have a gas outlet. The cathode compartment can have an inlet and an outlet for gas. During electrolysis, water vapor, carbon dioxide, a mixture of the two gases, and / or other fuels such as N2O can be introduced into the cathode compartment, which can be converted using the new material. The water vapor can be passed through the cathode compartment along the cathode. Oxygen can then be formed in the anode compartment.

[0035] For example, a plate-shaped interconnector can be used to electrically connect two plate-shaped solid oxide cells. An interconnector is an electrically conductive connecting part that can be made of metal.

[0036] The solid oxide cell can be tubular. For example, one electrode can be a tube. The outside of this electrode can be coated with the electrolyte. The outside of the electrolyte can be coated with the other, outer electrode. An electrically conductive interconnector can be connected to the tubular, inner electrode and led through the coatings to the outside. The interconnector is then electrically separated from the other, outer electrode and therefore does not touch the outer electrode.

[0037] To manufacture the fuel electrode, an electrode material paste can be made from powders and organic substances. This paste can be used to produce the electrode layer using various processes, such as screen printing.

[0038] The fuel electrode can be manufactured from powder by sintering in a high-temperature furnace. An example of such a manufacturing process is described below.

[0039] To prepare a LaO.58SrO.4CrO.2FeO.8O3-δ powder for the oxygen electrode, a modified Pechini method was used in which previously dried La2O3 with a purity of 99% from Sigma-Aldrich was added to a precursor powder mixture.

[0040] The investigated Sr2Fe2- x Mon xOe-O (x = 0.5, 1.0) powders were synthesized by the solid-state synthesis route. Powder precursors SrCO3 (from Aldrich, 99% purity), Fe2O3 (from Alfa Aesar, 99% purity), and MoO3 (from Alfa Aesar, 99% purity) were weighed proportionally to the desired stoichiometry and ball-milled at 250 rpm for 4 h with the addition of zirconium dioxide balls and isopropanol (from VWR, 99% purity). After drying at room temperature, the powders were annealed in air at 1100-1200 °C for 8 h. To obtain an average particle size of 1 pm, the powders were ball-milled again. The Sr2Fe2MoOe-ö powder was reduced in 100% H2 at 900 °C for 6 hours to obtain a single-phase powder.For the SFMx-GDC composites, commercially available Ceo,8Gdo,20i,9 powder (from SOFCMAN, 99.5% purity) was used. SFMx and GDC were mixed in a weight ratio of 7:3, ground in acetone, and dried overnight.

[0041] The powder mixture was then ball-milled at 1200 rpm for 10 minutes. The corresponding slurries were prepared by mixing the powders with a solution of terpineol and 3% ethylcellulose in a 1:1 weight ratio. Electrolyte-supported button cells SFMx(-GDC)|GDC|8YSZ|GDC|LSCF were fabricated by screen printing the prepared slurries onto commercially available 250 μm-thick 8YSZ electrolytes (from Kerafol GmbH, Eschenbach in der Oberpfalz, Germany). The GDC barrier layers were fired at 1350 °C for 2 h in air. The SFMx-GDC fuel electrode was sintered at 1150 °C, and the LSCF oxygen electrode was sintered at 1080 °C for 3 h in air. A 5 pm gold contact layer (from Heraeus GmbH, Hanau, Germany) was added to ensure acceptable cell contact with the current collector.Furthermore, the Sr2Fe2MoOe-ö material was modified by substituting the molybdenum with Ni, Co, Cu, Ti, and Mn and prepared in the same manner as described above. The perovskite oxides Sr2Fe2-xMo were obtained. x -yMyO6-ö (where M = Ni, Co, Cu, Ti, or Mn) were mixed and tested with and without GDC as fuel electrode materials. Very high electrochemical performance and exceptionally low degradation rates, especially under CO2 electrolysis conditions, were demonstrated compared to state-of-the-art Ni-YSZ fuel electrodes as well as several recently developed fuel electrode materials.

[0042] It can be observed that fuel electrodes with SrFeMoO.sOe-6 and Sr2FeMoO.65Nio.3sO6-6 outperform Ni-YSZ regardless of the gas atmosphere. For 80% CO2 + 20% CO at 900 °C, Ni-YSZ cells achieve a current density of -0.63 A cm 2 at 1.5 V, while SFM current densities of -1 .08 A cm2 to -1 .23 A cm 2 This exceeds the state-of-the-art Ni-YSZ technology in CO2 electrolysis by around 50%. Long-term tests under constant current load of -0.3 A cm -2 at 900 °C for up to 500 hours are shown in Figure 2. Compared to modern Ni-cermet fuel electrodes, the SFM-based materials show lower degradation.

[0043] Perovskite oxides Sr2Fe2- x Mon x Oe-öOr Sr2Fe2- x Mon x .yM y O6-ö with M = nickel (Ni), and / or cobalt (Co), and / or titanium (Ti), and / or manganese (Mn), and / or copper (Cu) were prepared and investigated. For example, the following materials were synthesized and studied in detail: S^FeMoOe-ö (Sr2Fe.oMoi.oOe-ö), S^Fe.sMoo.sOe-ö, Sr2FeMoO6-0-Ce0.9Gd0.1O19, Sr2Fe1.5Mo0.5O6-0-Ce0.9Gd0.1O19, and Sr2FeMoo.65Nio.3sO6-ö.

[0044] The solid oxide cell serves primarily as a solid oxide electrolysis cell. During operation, the electrodes of the solid oxide electrolysis cell are connected to a direct current source. This provides significant advantages compared to the state of the art.

[0045] It shows

[0046] Figure 1 : Voltage-current density diagram;

[0047] Figure 2: Voltage-time diagram;

[0048] Figure 3: Voltage-time diagram;

[0049] Figure 4: Structure of a SOCE.

[0050] Figure 1 shows the voltages applied to the electrodes of solid oxide electrolysis cells and the resulting current densities as a function of the applied voltage. Figure 1 shows the measured current-voltage characteristics of solid oxide electrolysis cells with different fuel electrodes. The characteristics illustrate the relationship between cell potential and the achieved current voltage by changing the potential and measuring the current response relative to the catalytically active area. It follows that a comparison of the materials of the solid oxide electrolysis cells at a voltage point of, for example, 1.5 V also represents a comparison of the achieved power per area.

[0051] According to Figure 1, the Ni-YSZ standard fuel electrode has a performance of - 0.63 A cnv 2in CO2 electrolysis (shown on the right in Figure 1). The fuel electrodes with the materials S^FeMoOe-ö, S^Fe.sMoo.sOe-ö, and the composite material Sr2FeMoO6-0-Ce0.9Gd0.1O19 show higher performance with current densities of -1.10 A CHT each. 2 , -1.08 A cm -2 and -1.10 A CHT 2 (shown further to the left from Ni-YSZ). The composite electrode Ni-GDC is an alternative to Ni-YSZ and is currently being used in several research studies. Industrial use as a replacement for Ni-YSZ has not yet become established. This material is characterized by higher catalytic performance than Ni-YSZ (-1.14 A cm'). 2, Figure 1 , shown to the left of the previously mentioned materials). In addition, GDC, like SFM, is a material with mixed ionic and electronic conductivity (MIEC). In MIEC materials, the electrochemical reactions take place across the three-phase boundary on the entire material surface. The materials Sr2Fe1.5Mo0.5O6-0-Ce0.9Gd0.1O19 and S^FeMo0.esNio.ssOe-ö in Figure 1 outperform Ni-GDC, each with -1.22 A cm' 2 and -1 .23 A cm' 2 (shown on the left in Figure 1).

[0052] The current-voltage curves demonstrate that SFM materials for CO2 electrolysis surpass the industrially used Ni-YSZ, as well as the even more powerful Ni-GDC, in the achieved current density.

[0053] Microstructural changes in Ni-containing Ni-YSZ and Ni-GDC electrodes lead to a loss of cell performance under extended operating conditions, resulting in an increase in cell potential. Operating parameters such as current density, gas composition, temperature, and local redox potential influence cell aging.

[0054] Figures 2 and 3 illustrate the long-term stability of SFM fuel electrodes under test conditions between 500 h and 1000 h. At a current density of -300 mA crn -2 Sr2FeMoO6-0-Ce0.9Gd0.1O19 exhibits a linear response with a potential increase of 4 mV. S^FeMoOe-ö and S^Fe.sMoo.sOe-ö, however, show a potential decrease in the first few days, indicating activation of the material, and a total potential increase of 1 mV and 0.12 mV, respectively. The long-term stability test of S^FeMoo.esNio.ssOe-ö at a current density of -500 mA crn -2Over 1000 h of measurement time illustrates this trend and decreases over the entire time 9 mV kh -1 In comparison, cells with Ni-GDC fuel electrodes have 31 mV kh under the same measurement conditions -1 The initial performance and long-term stability results of SFM fuel electrodes under real-world conditions show that they outperform the industrially used Ni-YSZ and the widely used Ni-GDC.

[0055] Figure 4 shows a cross-sectional view of an example of the structure and operation of a solid oxide electrolysis cell with an oxygen electrode 1, an electrolyte 2, and a fuel electrode 3. The electrolyte 2 is located between the oxygen electrode 1 and the fuel electrode 3. An oxygen electrode chamber 4 adjoins the oxygen electrode 1. The oxygen electrode chamber 4 has an outlet 5 for oxygen. A fuel electrode chamber 6 adjoins the fuel electrode 3. The fuel electrode chamber 6 has an inlet 7 for an oxygen-containing gas, such as water vapor. The fuel electrode chamber 6 has an outlet 8 for excess gas. In the fuel electrode chamber 6, a fuel, such as hydrogen, is produced by electrolysis. The electrodes are connected to a direct current source 9.

Claims

Claims 1. Solid oxide cell with an electrolyte (2), wherein gas-permeable electrodes (1, 3) are mounted on opposite sides of the electrolyte (2), characterized in that the fuel electrode (1) is Sr2Fe2- x Mon x Oe-ö or Sr2Fe2- x Mon x . yMyOe-ö with M = nickel (Ni), and / or cobalt (Co), and / or titanium (Ti), and / or manganese (Mn) and / or copper (Cu) or Sr2-zM' z Fe2- x Mon x O6-ö where M' = Gd, Nd, Pr, La or Sm.

2. Solid oxide cell according to the preceding claim, characterized in that x = 0.5 to 1 and / or z = 0 to 0.

4.

3. Solid oxide cell according to one of the preceding claims, characterized in that the proportion of Sr2Fe2- x Mon x Oe-ö or Sr2Fe2- x Mon x .yM y O6-ö in the material of the fuel electrode or in a layer of the fuel electrode is more than 50 wt.%.

4. Solid oxide cell according to the preceding claim, characterized in that the proportion of a second substance in the material of the fuel electrode or in a layer of the fuel electrode is up to 30 wt.%, wherein the material or the layer is Sr2Fe2- x Mon x Oe-ö or Sr2Fe2- x Mon x .yM y O6-ö includes.

5. Solid oxide cell according to the preceding claim, characterized in that gadolinium-doped cerium oxide is present as the second substance.

6. Solid oxide cell according to one of the preceding claims, characterized in that cobalt, titanium, manganese or copper and / or nickel are infiltrated into the fuel electrode.

7. Solid oxide cell according to the preceding claim, characterized in that the proportion of cobalt, titanium, manganese or copper and / or nickel is up to 40 at.% or up to 30 at.%, based on the material of the fuel electrode, which is Sr2Fe2- x Mon x Oe-ö or from or Sr2Fe2- x Mon x .yM y O6-ö at least includes.

8. Solid oxide cell according to one of the preceding claims, characterized in that there is a barrier between the fuel electrode (1) and the electrolyte (2).

9. Solid oxide cell according to the preceding claim, characterized in that the barrier comprises gadolinium-doped cerium oxide (GDC) or samarine oxide-doped cerium oxide (SDC).

10. Solid oxide cell according to the preceding claim, characterized in that the barrier is doped with 10 wt.% to 20 wt.%. 11 . Solid oxide cell according to one of the preceding claims, characterized in that the fuel electrode (1) does not comprise nickel or the proportion of nickel is a maximum of 5 wt.%.

12. Solid oxide cell according to one of the preceding claims, characterized in that the electrolyte (2) consists of a gas-tight ceramic material which can conduct oxygen ions and has an insulating effect on electrons.

13. Solid oxide cell according to the preceding claim, characterized in that the electrolyte consists of yttrium-stabilized zirconium oxide (YSZ) or strontium and magnesium doped lanthanum gallium oxide (LSGM) or gadolinium-doped cerium oxide (GDC).

14. Solid oxide cell according to one of the preceding claims, characterized in that ö is not greater than 1.

15. A method for a solid oxide cell according to any one of the preceding claims, characterized in that the solid oxide cell is operated at a temperature between 600°C and 900°C.

16. Process according to the preceding claim, characterized in that electrolysis is carried out.

17. Process according to the preceding claim, characterized in that CO2 electrolysis is carried out.

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