Electrochemical cell and method for operating such a cell

Abstract

The invention relates to an electrochemical cell (10a; 10b; 10c), in particular an electrolysis cell, comprising at least one first electrode (12a; 12b; 12c), at least one other electrode (14a; 14b; 14c), and at least one electrolyte (16a; 16b; 16c) which is positioned between the at least one first electrode (12a; 12b; 12c) and the at least one other electrode (14a; 14b; 14c). According to the invention, the electrolyte (16a; 16b; 16c) is configured on the basis of a chemically induced mechanical stress on the electrolyte (16a; 16b; 16c).

Description

[0001] Description

[0002] Electrochemical cell and method for operating such a cell

[0003] State of the art

[0004] An electrochemical cell, in particular an electrolysis cell, having at least one first electrode, at least one further electrode and at least one electrolyte arranged between the at least one first electrode and the at least one further electrode has already been proposed.

[0005] Disclosure of the invention

[0006] The invention is based on an electrochemical cell, in particular an electrolysis cell, with at least one first electrode, at least one further electrode and with at least one electrolyte which is arranged between the at least one first electrode and the at least one further electrode.

[0007] It is proposed that the electrolyte be designed depending on a chemically induced mechanical stress of the electrolyte. The chemically induced mechanical stress typically occurs during operation of the electrochemical cell and is particularly dependent on the activity of the ions transported by the electrolyte. The mechanical stress is caused, for example, by an at least partial chemical reduction of a material of the electrolyte, which in particular causes a change in the lattice structure and / or an extension of the electrolyte. The chemically induced mechanical stress can be expressed, for example, as tensile stress, compressive stress, shear stress, or the like.A design of the electrolyte can, for example, relate to a layer thickness, a chemical composition, in particular a doping level, an internal structure of the electrolyte, for example, consisting of different layers, or the like. The electrolyte is preferably designed such that the chemically induced mechanical stress within the electrolyte and / or at at least one, in particular each, interface between the electrolyte and one of the electrodes is at least smaller in magnitude than a voltage limit, in particular at each intended operating point of the electrochemical cell. The voltage limit is preferably at most the value, preferably at most 2 / 3 of the value, particularly preferably at most 1 / 3 of the value, of a tensile strength of the material from which the electrolyte is made.For example, the voltage limit is at most 300 MPa, preferably at most 200 MPa, particularly preferably at most 100 MPa, in particular at most 50 MPa, but can also be higher depending on the material of the electrolyte. The operating point includes, for example, an operating temperature and / or an electrical current density of the electrochemical cell. Preferably, an internal structure of the electrolyte, a shape of the electrolyte, a layer thickness of the electrolyte, a chemical composition of the electrolyte, a doping level of the electrolyte, or the like is designed such that the chemically induced voltage at the most unfavorable intended operating point, in particular a maximum permissible operating temperature and / or a maximum permissible electrical current density, is at least smaller in magnitude than the voltage limit.Preferably, the maximum permissible operating temperature is at least 500°C, preferably at least 600°C, particularly preferably at least 700°C. Preferably, the maximum permissible electrical current density is at least 3 A / m. 2 , preferably at least 2 A / m 2 , particularly preferably at least 1.5 A / m 2 .

[0008] The electrochemical cell is particularly preferably designed as a solid oxide cell, in particular as a solid oxide electrolysis cell (SOEC) and / or as a solid oxide fuel cell (SOFC). The electrolyte is preferably made of a ceramic material that is preferably conductive for oxygen ions, such as doped ceria or doped zirconium dioxide. The at least one first electrode is preferably designed as a main reactant electrode, which is preferably intended for direct contact with a reactant to be split, such as water vapor or carbon dioxide, or a reactant to be oxidized, in particular a fuel such as hydrogen, methane, ammonia or the like. The main reactant electrode is preferably made of a ceramic material, for example doped ceria, or a cermet, for example doped ceria with a nickel matrix. The main reactant electrode is preferably coated with Ga 3+, alternatively or additionally with La 3+ , Yb 3+ , Y 3+ , Ca 3+ and / or Sm 3+ doped. The at least one further electrode is preferably designed as an air electrode, which is provided for the removal or provision of the ions to be transported by the electrolyte, in particular oxygen. The air electrode can be made of any material that appears appropriate to a person skilled in the art and is known from the prior art. When the electrochemical cell is designed and / or operated as a fuel cell, the electrolyte is preferably provided to conduct ions from the air electrode to the main reactant electrode containing the starting material to be oxidized. When the electrochemical cell is designed and / or operated as an electrolysis cell, the electrolyte is preferably provided to conduct ions of the starting material to be split from the main reactant electrode to the air electrode.

[0009] “Intended” should be understood in particular to mean specially programmed, designed and / or equipped. The fact that an object is intended for a specific function should be understood in particular that the object fulfills and / or performs this specific function in at least one application and / or operating state. The design according to the invention advantageously makes it possible to keep the risk of damage to the electrochemical cell during operation of the electrochemical cell low. In particular, the risk of cracking in the electrolyte and / or total failure of the electrochemical cell low can be advantageously kept low. Furthermore, the electrochemical cell can be operated advantageously reliably and with low wear even at advantageously high electrical current densities, and in particular at a relatively low oxygen partial pressure at the main reactant electrode.

[0010] It is further proposed that the doping level of the electrolyte increases from the first electrode to the further electrode. The electrolyte is preferably coated with Ga 3+doped. The doping level can increase stepwise or continuously, in particular linearly. The doping level is preferably lowest at the main reactant electrode. The doping level is preferably highest at the air electrode. The doping level is preferably designed such that a decreasing activity of the ions transported by the electrolyte within the electrolyte is counteracted, preferably such that the decreasing activity of the ions transported by the electrolyte within the electrolyte is compensated or overcompensated. The at least one first electrode, the electrolyte and the at least one further electrode are preferably arranged one behind the other along a stacking direction. The doping level preferably increases along the stacking direction.The stacking direction is preferably at least substantially perpendicular to a main extension plane of the electrolyte, the at least one first electrode and / or the at least one further electrode. The doping level is constant in one, in particular every, plane perpendicular to the stacking direction, preferably within the scope of a manufacturing accuracy of the electrolyte. The term "substantially perpendicular" is intended here to define in particular an orientation of a direction relative to a reference direction, wherein the direction and the reference direction, in particular viewed in a projection plane, enclose an angle of 90° and the angle has a maximum deviation of in particular less than 8°, advantageously less than 5° and particularly advantageously less than 2°.A "main extension plane" of a structural unit is to be understood in particular as a plane that is parallel to a largest side surface of a smallest imaginary cuboid that just completely encloses the structural unit, and in particular runs through the center of the cuboid. By means of the design according to the invention, the activity of the ions within the electrolyte can advantageously be kept constant. In particular, any chemical reduction of the electrolyte material that occurs can advantageously be kept homogeneous. In particular, mechanical stresses caused by an inhomogeneous reduction of the electrolyte material can advantageously be kept low. In particular, mechanical stress at an interface between the electrolyte and the air electrode can advantageously be kept low.It is further proposed that the electrolyte comprise a first layer and at least one further layer, wherein the layers have different doping levels. Preferably, the layer with the lower doping level is arranged on the at least one first electrode. Preferably, the further layer is arranged on the at least one further electrode. Preferably, the layers of the electrolyte are arranged one behind the other along the stacking direction. The electrolyte can have exactly two, three, four or more layers, each with different doping levels. Within one, in particular each, layer, the doping level is preferably constant within the scope of a manufacturing accuracy of the layer. The configuration according to the invention advantageously makes it easy to produce an electrolyte with an increasing doping level.In particular, the quality and cost / production time of the electrolyte can be advantageously easily adjusted by changing the number of layers.

[0011] It is further proposed that a, in particular average, doping level of the electrolyte is greater than 10%, in particular based on an atomic fraction of the electrolyte. The average doping level is preferably an atomic fraction averaged over a possibly inhomogeneous spatial distribution of the foreign atoms and / or over differently doped layers of the electrolyte. In particular, a local doping level, in particular a doping level of a layer of the electrolyte, can be less than or greater than the average doping level of the electrolyte. Preferably, the average doping level and / or a minimum doping level of the electrolyte is greater than 12%, particularly preferably greater than 14%. Preferably, the average doping level and / or a maximum doping level of the electrolyte is less than 30%, particularly preferably less than 20%, particularly preferably less than 16%.The inventive design allows the chemically induced mechanical stress to be kept advantageously low. In particular, the chemically induced mechanical stress can be reduced by up to 30 MPa per 0.1 Gd cation fraction.

[0012] It is further proposed that the electrochemical cell comprise an electron barrier arranged between the electrolyte and the at least one first electrode. The electron barrier is preferably provided to keep an electron concentration in the electrolyte below a limit value. The electron barrier is preferably provided to counteract, in particular to prevent, a transfer of electrons from the at least one first electrode into the electrolyte. The electron barrier preferably has a layer thickness along the stacking direction of less than 10% of a maximum extension of the electrolyte in this direction. The electron barrier is preferably ion-conducting, in particular oxygen-conducting. The electron barrier is made, for example, from yttria-stabilized zirconium dioxide (YSZ) or from scandium-doped yttria-stabilized zirconium dioxide (ScYSZ).By means of the design according to the invention, the extent of a reduction of the electrolyte material can advantageously be kept low.

[0013] Furthermore, it is proposed that the electrolyte layer thickness be greater than 5 μm. The electrolyte layer thickness is preferably a maximum extension of the electrolyte along the stacking direction. The layer thickness is preferably less than 15 μm, more preferably less than 10 μm. The inventive design advantageously allows the mechanical stability of the electrolyte to be maintained at a high level, particularly even during a reduction in the electrolyte material and after the formation of cracks in the electrolyte. In particular, the risk of total failure of the electrochemical cell can be advantageously kept low.

[0014] Furthermore, it is proposed that the electrolyte be made of gadolinium-doped cerium dioxide, or CGO for short. The inventive design advantageously minimizes ohmic loss of the electrolyte, particularly when increasing the electrolyte layer thickness.

[0015] Furthermore, a method for low-stress operation, in particular electrolysis operation, of an electrochemical cell, in particular according to the invention, is proposed, wherein in at least one method step of the method, at least one operating parameter of the electrochemical cell is adjusted as a function of a mechanical stress limit value of the electrochemical cell, in particular the one already mentioned or a further one. In low-stress operation, the chemically induced mechanical stress is preferably kept below the stress limit value. To carry out the method, the electrochemical cell is preferably part of an electrochemical system which comprises at least one fluid supply unit for supplying the electrochemical cell with the reactant and at least one control or regulating unit for adjusting the at least one operating parameter.A “control or regulating unit” should be understood in particular to mean a unit with at least one control electronics unit. A “control electronics unit” should be understood in particular to mean a unit with a processor unit and with a memory unit as well as with an operating program stored in the memory unit. A limit value of the at least one operating parameter is preferably stored in the memory unit of the control or regulating unit, so that if the limit value of the operating parameter is maintained, the voltage limit value is maintained. Alternatively or additionally, a dependency of the chemically induced mechanical stress on the at least one operating parameter is stored in the memory unit of the control or regulating unit, for example in the form of a table, a mathematical function, a characteristic curve or the like.The control or regulating unit preferably regulates the at least one operating parameter as a function of the voltage limit value. The inventive design advantageously allows the electrochemical cell to be operated with low wear. In particular, the risk of cracking in the electrolyte and / or total failure of the electrochemical cell can be advantageously kept low. Furthermore, a parameter range of the at least one operating parameter, within which a design of the electrochemical cell must meet a requirement regarding the chemically induced mechanical stress for safe operation, can advantageously be kept small. In particular, an advantageously large number of differently designed electrochemical cells can advantageously be operated safely using the inventive method.

[0016] It is further proposed that, in at least one method step of the method, an operating temperature of the electrochemical cell is set as a function of an electrical parameter of the electrochemical cell. Preferably, the control or regulating unit sets the operating temperature lower, the higher the electrical parameter is. Preferably, the control or regulating unit compensates for additional chemically induced mechanical stress caused by an increase in the electrical parameter by lowering the operating temperature. Preferably, the control or regulating unit increases the operating temperature when the electrical parameter is lowered in order to increase the activity of the ions in the electrolyte. The electrical parameter is, for example, an electrical current density, an absolute value of an electrical current, or the like.Preferably, the control or regulating unit sets a target value for the operating temperature as a decreasing, in particular linearly decreasing, function of the electrical parameter. The electrical parameter can be detected by a sensor element of the control or regulating unit and / or set by a higher-level control or regulating system. The operating temperature is preferably set by adjusting a flow rate of a process fluid through the air electrode, for example by means of a valve, a fan, a compressor, or the like of the electrochemical system, and / or by adjusting a heat transfer to the process fluid upstream of the air electrode, for example by means of a heat exchanger and an adjustable bypass of the electrochemical system that bypasses the heat exchanger.Due to the design according to the invention, the electrochemical cell can advantageously be operated with low wear and at the same time efficiently, in particular even at relatively high current densities.

[0017] It is further proposed that, in at least one process step of the method, reactant utilization, in particular steam utilization, of the electrochemical cell be kept below a limit value. Particularly preferably, the reactant utilization limit value is less than 85%, preferably less than 75%, particularly preferably less than 65%, in particular at most 60%. Preferably, the reactant utilization limit value is more than 10%, preferably more than 30%, particularly preferably more than 50%. The inventive design allows an oxygen partial pressure at the main reactant electrode to be advantageously kept high.

[0018] The electrochemical cell according to the invention and / or the method according to the invention are not intended to be limited to the application and embodiment described above. In particular, the electrochemical cell according to the invention and / or the method according to the invention may have a number of individual elements, components, units, and method steps that differs from the number stated herein to fulfill a function described herein. Furthermore, in the value ranges specified in this disclosure, values ​​within the stated limits are also to be considered disclosed and can be used arbitrarily.

[0019] Drawings

[0020] Further advantages will become apparent from the following description of the drawings. The drawings illustrate three exemplary embodiments of the invention. The drawings, the description, and the claims contain numerous features in combination. Those skilled in the art will also expediently consider the features individually and combine them into useful further combinations.

[0021] They show:

[0022] Fig. 1 is a schematic representation of an electrochemical cell according to the invention,

[0023] Fig. 2 is a schematic representation of a further embodiment of an electrochemical cell according to the invention,

[0024] Fig. 3 is a schematic representation of an alternative embodiment of an electrochemical cell according to the invention,

[0025] Fig. 4 is a schematic flow diagram of a method according to the invention,

[0026] Fig. 5 is a schematic diagram of an operating temperature of an electrochemical cell as a function of an electrical parameter in the context of the method according to the invention and

[0027] Fig. 6 is a schematic diagram of a logarithmically plotted oxygen partial pressure as a function of reactant utilization by an electrochemical cell. Description of the embodiments

[0028] Figure 1 shows an electrochemical cell 10a, in particular an electrolysis cell. The electrochemical cell 10a is preferably designed as a solid oxide cell, in particular as a solid oxide electrolysis cell. The electrochemical cell 10a comprises at least one first electrode 12a. The at least one first electrode 12a is preferably intended for direct contact with a reactant, in particular an electrolysis reactant. The electrolysis reactant preferably comprises more than 50%, in particular more than 75%, preferably at least 85% water vapor, in particular based on volume and / or mass. The at least one first electrode 12a is preferably made of a cermet made of gadolinium-doped cerium dioxide, CGO for short, with a nickel matrix. The electrochemical cell 10a comprises at least one further electrode 14a. The at least one further electrode 14a is preferably intended to transport oxygen released from the electrolysis reactant.The further electrode 14a can be made of any material deemed appropriate by a person skilled in the art. The electrochemical cell 10a comprises at least one electrolyte 16a. The electrolyte 16a is made of gadolinium-doped CGO. The electrolyte 16a is arranged between the at least one first electrode 12a and the at least one further electrode 14a. The first electrode 12a, the electrolyte 16a, and the further electrode 14a are arranged one behind the other along a stacking direction 32a. A respective main extension plane of the first electrode 12a, the electrolyte 16a, and the further electrode 14a preferably runs at least substantially perpendicular to the stacking direction 32a.

[0029] The electrolyte 16a is designed depending on a chemically induced mechanical stress of the electrolyte 16a. The doping level of the electrolyte 16a increases from the first electrode 12a to the further electrode 14a. The electrolyte 16a has a first layer 18a with a first doping level. The electrolyte 16a comprises at least one further layer 20a with a further doping level. The electrolyte 16a comprises, for example, an additional layer 22a with an additional doping level. The first doping level, the further doping level, and in particular the additional doping level are of different sizes. The first layer 18a, the further layer 20a, and in particular the additional layer 22a are arranged one behind the other along the stacking direction 32a.A respective main extension plane of the first layer 18a, the further layer 20a and in particular the additional layer 22a preferably runs at least substantially perpendicular to the stacking direction 32a. The first layer 18a preferably has the lowest doping level of all layers 18a, 20a, 22a of the electrolyte 16a. The first layer 18a is preferably arranged on the first electrode 12a. The further layer 20a preferably has the highest doping level of all layers 18a, 20a, 22a of the electrolyte 16a. The further layer 20a is preferably arranged on the further electrode 14a. The additional layer 22a preferably has a doping level between the first layer 18a and the further layer 20a and is arranged between the first layer 18a and the further layer 20a.

[0030] The electrochemical cell 10a comprises an electron barrier 24a arranged between the electrolyte 16a, in particular the first layer 18a, and the at least one first electrode 12a. The electron barrier 24a preferably has a maximum extension in the stacking direction 32a that is less than 10% of a maximum extension of the electrolyte 16a in this direction.

[0031] Figures 2 and 3 each show a further exemplary embodiment of the invention. The following descriptions and the drawings are essentially limited to the differences between the exemplary embodiments, whereby with regard to components with the same designation, in particular with regard to components with the same reference numerals, reference can in principle also be made to the drawings and / or the description of the other exemplary embodiments, in particular Figure 1. To distinguish the exemplary embodiments, the letter a is placed after the reference numerals of the exemplary embodiment in Figure 1. In the exemplary embodiments of Figures 2 to 3, the letter a is replaced by the letters b to c. Figures 4 to 6 explain a method according to the invention which can be carried out with each exemplary embodiment of Figures 1 to 3. The reference numerals of Figures 4 to 6 therefore do not have any letters appended to them.Figure 2 shows an electrochemical cell 10b, in particular an electrolysis cell, with at least one first electrode 12b, with at least one further electrode 14b and with at least one electrolyte 16b which is arranged between the at least one first electrode 12b and the at least one further electrode 14b. The electrolyte 16b is designed as a function of a chemically induced mechanical stress of the electrolyte 16b. The electrolyte 16b has a, in particular average, doping level of more than 10%, preferably of at least 14%. The doping level is preferably less than 20%, preferably less than 16%. The electrolyte 16b is made up of a single layer, for example. The doping level is, for example, homogeneous throughout the entire electrolyte 16b.Alternatively, the electrolyte 16b has several layers as in Figure 1, wherein a doping level averaged over all layers is preferably between 10% and 20%, preferably between 14% and 16%.

[0032] Figure 3 shows an electrochemical cell 10c, in particular an electrolysis cell, with at least one first electrode 12c, with at least one further electrode 14c and with at least one electrolyte 16c, which is arranged between the at least one first electrode 12c and the at least one further electrode 14c. The electrolyte 16c is designed as a function of a chemically induced mechanical stress of the electrolyte 16c. A layer thickness 34c of the electrolyte 16c is greater than 5 pm. The layer thickness 34c is preferably less than 15 pm, in particular less than 10 pm. The electrolyte 16c can be single-layered as in Figure 2 or multi-layered as in Figure 1. The electrolyte 16c can have a doping level of at least 10%, in particular of at least 14%, or a doping level less than 10%.

[0033] Figure 4 shows a flow diagram of a method 26 for low-stress operation, in particular electrolysis operation, of the electrochemical cell 10a, 10b, 10c. The method 26 is preferably carried out by a control or regulating unit of an electrochemical system comprising the at least one electrochemical cell 10a, 10b, 10c. In at least one method step of the method 26, the control or regulating unit sets at least one operating parameter of the electrochemical cell 10a, 10b, 10c as a function of a mechanical voltage limit value of the electrochemical cell 10a, 10b, 10c. The method 26 preferably comprises a query 36 in which the control or regulating unit queries an operating point to be set, for example from a higher-level control or regulating system. An electrical parameter to be set, in particular a current density 42 (cf. Figure 5), is preferably determined in the query 36.Preferably, the control or regulating unit sets an operating temperature 44 (see Figure 5) of the electrochemical cell 10a, 10b, 10c as a function of the electrical parameter of the electrochemical cell 10a, 10b, 10c. Preferably, the control or regulating unit determines a setpoint 46 (see Figure 5) of the operating temperature 44 as a function of the electrical parameter. Preferably, the method 26 comprises a temperature output 38, in which the control or regulating unit outputs the setpoint 46, for example, to the higher-level control or regulating system or to an actuator of the electrochemical system. Preferably, the method 26 comprises a reactant utilization output 40, in which a reactant utilization 30 (see Figure 6) of the electrochemical cell 10a, 10b, 10c is kept below a limit value 28 (see Figure 6). For example, the control or regulation unit in the reactant usage output 40 outputs the limit value 28 to the higher-level control or regulation.

[0034] Figure 5 shows a dependence of the setpoint 46 of the operating temperature 44 on the current density 42. The control or regulating unit assigns the setpoint 46 of the operating temperature 44 to the current density 42 based on a negative correlation, in particular based on a characteristic curve with a negative slope.

[0035] Figure 6 shows a dependence of a logarithmically plotted oxygen partial pressure 48 in the first electrode 12a, 14a, 16a on the reactant utilization 30, in particular steam utilization, of the electrochemical cell 10a, 10b, 10c during electrolysis operation of the electrochemical cell 10a, 10b, 10c. The limit value 28 of the reactant utilization 30 is preferably selected such that a minimum of a gradient of the oxygen partial pressure 48 only occurs at a higher value of the reactant utilization 30 than the limit value 28. The limit value 28 is, for example, at most 60%.

Claims

Claims 1. Electrochemical cell (10a; 10b; 10c), in particular an electrolysis cell, with at least one first electrode (12a; 12b; 12c), at least one further electrode (14a; 14b; 14c) and with at least one electrolyte (16a; 16b; 16c) which is arranged between the at least one first electrode (12a; 12b; 12c) and the at least one further electrode (14a; 14b; 14c), characterized in that the electrolyte (16a; 16b; 16c) is designed as a function of a chemically induced mechanical stress of the electrolyte (16a; 16b; 16c).

2. Electrochemical cell (10a; 10b; 10c) according to claim 1, characterized in that a doping level of the electrolyte (16a; 16b; 16c) increases from the first electrode (12a; 12b; 12c) to the further electrode (14a; 14b; 14c) 3. Electrochemical cell (10a) according to one of the preceding claims, characterized in that the electrolyte (16a; 16b; 16c) comprises a first layer (18a) and at least one further layer (20a, 22a), wherein the layers (18a, 20a, 22a) have different doping levels.

4. Electrochemical cell (10a; 10b; 10c) according to one of the preceding claims, characterized in that a, in particular average, doping level of the electrolyte (16a; 16b; 16c) is greater than 10%.

5. Electrochemical cell (10a; 10b; 10c) according to one of the preceding claims, characterized by an electron barrier (24a; 24b; 24c) arranged between the electrolyte (16a; 16b; 16c) and the at least one first electrode (12a; 12b; 12c).

6. Electrochemical cell (10a; 10b; 10c) according to one of the preceding claims, characterized in that a layer thickness (34a; 34b; 34c) of the electrolyte (16a; 16b; 16c) is greater than 5 pm.

7. Electrochemical cell (10a; 10b; 10c) according to one of the preceding claims, characterized in that the electrolyte (16a; 16b; 16c) is made of gadolinium-doped cerium dioxide, abbreviated to CGO 8. Method (26) for low-stress operation, in particular electrolysis operation, of an electrochemical cell (10a; 10b; 10c), in particular according to one of the preceding claims, wherein in at least one method step at least one operating parameter of the electrochemical cell (10a; 10b; 10c) is set as a function of a mechanical voltage limit value of the electrochemical cell (10a; 10b; 10c).

9. Method (26) according to claim 8, characterized in that in at least one method step an operating temperature (44) of the electrochemical cell (10a; 10b; 10c) is set as a function of an electrical parameter of the electrochemical cell (10a; 10b; 10c).

10. Method (26) according to one of claims 8 or 9, characterized in that in at least one method step a reactant utilization (30) of the electrochemical cell (10a; 10b; 10c) is kept below a limit value (28).

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