Electrochemical device and method of operation thereof

Abstract

The invention relates to an electrochemical device for reversible adsorption of gases, comprising an electrochemical stack (1) having an electrochemical cell (2), wherein the cell (2) comprises an adsorption electrode (3) having an electrochemically active material for reversible electrochemical adsorption of a gas, a counterelectrode (4) and a separator (5) between the adsorption electrode (3) and the counterelectrode (4). The stack (1) has an inlet (15) for admitting a gas mixture and an outlet (16) for discharging the adsorbed gas or the processed gas mixture. There is a reference electrode (6) between the adsorption electrode (3) and the counterelectrode (4). A voltage monitoring unit (12) is connected to the adsorption electrode (3), the counterelectrode (4) and the reference electrode (6), wherein the voltage monitoring unit (12) measures a first voltage (Ua) between the reference electrode (6) and the adsorption electrode (3) and a second voltage (Ug) between the reference electrode (6) and the counterelectrode (4). If the first voltage (Ua) and the second voltage (Ug) are at variance beyond a defined extent, an error message is issued by the voltage monitoring unit (12).

Description

[0001] R. 415137

[0002] - 1 -

[0003] title

[0004] Electrochemical device and method for operating the same

[0005] The invention relates to an electrochemical device for the reversible adsorption of gases, such as is used, for example, to filter out carbon dioxide from exhaust gases or other gas mixtures and to bind it reversibly.

[0006] State of the art

[0007] Electrochemical cells used for separating gases from gas mixtures typically comprise two electrodes: an adsorption electrode and a counter electrode. An electrical voltage can be applied between these electrodes. Charge equalization between the electrodes is achieved by an ionic liquid surrounding both electrodes. At the adsorption electrode, for example, carbon dioxide from a gas mixture is bound through electrochemical processes. The counter electrode acts as an electron donor, providing the electrons necessary for the separation reaction. Gas separation is reversible; that is, switching off the electrical voltage leads to a controlled release of the gases adsorbed at the adsorption electrode.The gas stream supplied to the adsorption electrode originates, for example, from combustion processes or other production processes that generate gas mixtures as exhaust gases or byproducts. This process is used in particular to remove carbon dioxide (CO2) from the gas mixture and subsequently store it or use it in other ways.

[0008] In practical applications, several electrochemical cells are typically stacked and electrically connected in series. Supply channels within the stack supply the gas mixture to the individual cells and remove the processed output gas. Since R. 415137

[0009] - 2 - Since the adsorption electrodes have only a limited capacity for absorbing the gas to be filtered out, the supply of the gas mixture is stopped after a certain time and the voltage between the electrodes is reversed. This causes the bound gas to desorb from the adsorption electrode and be drawn off from the stack.

[0010] When operating an electrochemical cell with two electrodes, for example in the electrochemical conversion of carbon dioxide, it can happen that one or both electrodes require a higher voltage during operation than would be expected based on their initial properties. This is usually a consequence of aging due to chemical degradation of the electrode active materials. Since the cell can overheat and be damaged when operating at elevated voltages, it is important to know the overvoltages of the individual electrodes and adjust the operating current accordingly, which usually means reducing it or even switching it off completely.

[0011] Advantages of the invention

[0012] The electrochemical device according to the invention for the reversible adsorption of gases comprises an electrochemical stack with an electrochemical cell, wherein the cell has an adsorption electrode with an electrochemically active material for the reversible electrochemical adsorption of a gas, furthermore a counter electrode and a separator arranged between the adsorption electrode and the counter electrode. The stack has an inlet for introducing a gas mixture and an outlet for releasing the adsorbed gas or the processed gas mixture. A reference electrode is arranged between the adsorption electrode and the counter electrode.A voltage monitoring unit is connected to the adsorption electrode, the counter electrode, and the reference electrode, the voltage monitoring unit measuring a first voltage between the reference electrode and the adsorption electrode and a second voltage between the reference electrode and the counter electrode. R. 415137.

[0013] - 3 -

[0014] The electrochemical device, or electrochemical stack, operates by applying an electrical voltage between the adsorption electrode and the counter electrode. This allows gas, particularly carbon dioxide, to be reversibly bound to the adsorption electrode as long as an electrical voltage is applied and the corresponding current is flowing. The adsorption of gases from the gas mixture can continue until the capacity of the adsorption electrode is exhausted. Since the electrodes each experience different levels of damage from overvoltages, the operating voltage of each electrode, and thus its overvoltage, can be determined individually with the aid of a reference electrode. This allows the cell to be operated with a maximum current without the overvoltages of the two electrodes exceeding the damage threshold.

[0015] In a first advantageous embodiment, the voltage monitoring unit issues an error message as soon as the first voltage deviates from the second voltage by more than a predefined value. Alternatively or additionally, the voltage monitoring unit can also issue an error message as soon as the sum of the first voltage and the second voltage deviates from a reference voltage by more than a predefined value.

[0016] In a further advantageous embodiment, the reference electrode is embedded in the separator and electrically insulated from the adsorption electrode and the counter electrode. This allows for voltage measurement between the reference electrode, which can be set to a reference potential, and each of the two cell electrodes without requiring additional space.

[0017] The adsorption electrode is preferably designed for adsorbing carbon dioxide. Such devices or electrochemical cells are known from the prior art and operate in the manner described. The voltage between the adsorption electrode and the counter electrode is provided by a voltage source that is connected to the stack and adjustable so that a constant current can be applied. If several electrochemical cells are present, they are advantageously connected in series so that a constant current flows through all electrochemical cells. R. 415137

[0018] - 4 -

[0019] The following steps are carried out in the inventive method for operating in this electrochemical device:

[0020] Supplying a gas mixture via the inlet into the stack, wherein the gas mixture at the adsorption electrode contains the adsorbable gas;

[0021] Applying a constant current between the adsorption electrode and the counter electrode;

[0022] Monitoring of the first electrical voltage applied between the adsorption electrode and the reference electrode and the second voltage applied between the reference electrode and the counter electrode, issuing an error message if the voltage difference between the first voltage and the second voltage exceeds a predefined value.

[0023] As the adsorption electrode approaches saturation, an increasingly higher voltage is required to maintain the current between the adsorption electrode and the counter electrode. The voltage thus serves as an early indicator of the exhaustion of the adsorption capacity, allowing for timely intervention to stop the supply of the gas mixture to the stack and reverse or interrupt the voltage between the cell electrodes, i.e., the adsorption electrode and the counter electrode. With the voltage removed, the gas is desorbed from the adsorption electrode and can be drained from the stack.

[0024] Aging can cause the voltage between the reference electrode and one of the cell electrodes to increase. In a further advantageous embodiment of the invention, if this occurs, the current through the cell(s) can be reduced until the overvoltage is again within a tolerable range, in order to prevent damage to the cell.

[0025] drawing

[0026] The drawing depicts an electrochemical device for the reversible adsorption of gases. It shows

[0027] Fig. 1 shows a cross-section through two electrochemical cells forming part of an electrochemical stack in such a device, R. 415137

[0028] - 5 -

[0029] Fig. 2 is an enlarged view of the section of Fig. 1 designated II, showing the voltage monitoring unit; Fig. 3a shows the electrochemical device according to the invention during adsorption operation; and

[0030] Fig. 3b shows the same device as in Fig. 3a after the adsorption has ended.

[0031] Description of the exemplary implementations

[0032] Figure 1 shows a cross-section through two electrochemical cells 2, which are part of a stack 1 and thus a device for the reversible adsorption of gases. Each electrochemical cell 2 consists of an adsorption electrode 3 and a counter electrode 4, separated from each other by a separator 5. The adsorption electrode 3 and the counter electrode 4 are surrounded by a suitable electrolyte solution, the separator 5 being permeable only to certain ions. An external electrical contact 7 serves to electrically connect the adsorption electrode 3. The counter electrode 4 is similarly connected to another electrical contact 8. This allows an external electrical voltage to be applied between the adsorption electrode 3 and the counter electrode 4. The adsorption electrode 3 includes channels (not shown in the drawing) through which a gas mixture can be introduced.To increase the capacity of the electrochemical system, several electrochemical cells 2 are stacked on top of each other, forming a stack 1, which is surrounded by a housing 10 that accommodates the electrical contacts 7, 8, so that they can be contacted from the outside.

[0033] The gas mixture flows through the electrochemical cells 1, whereby the gas to be filtered out is bound to the adsorption electrode 3, and the remaining gas mixture exits the stack 1. The adsorption electrode 3 is coated, for example, with suitable polymers that have reversible binding sites for the gas, such as carbon dioxide. If an electrical voltage is applied between the adsorption electrode 3 and the counter electrode 4, carbon dioxide molecules are deposited onto the polymers as long as the electrical voltage is maintained. This process continues until the R. 415137

[0034] - 6 -

[0035] The binding sites of the polymers are occupied and the capacity of cells 1 is therefore exhausted. After interrupting the gas supply, the voltage at cells 1 can be reversed, which releases the adsorbed carbon dioxide. This can then be stored or used in other ways.

[0036] The electrochemical cells forming stack 1 are connected in series. This means that an electrical voltage can be applied to the stack, and the resulting electric current flows through all electrochemical cells 2. A constant current is conducted through the stack and thus through all electrochemical cells, enabling the adsorption of gases. Since the adsorption electrodes 3 have a finite capacity, fewer adsorption sites are available for CO2 molecules as the charge increases. This must be compensated for by an increasing electrical voltage due to the constant current. During operation, however, care must be taken to ensure that this voltage does not exceed a certain value in order to limit the heating of cells 1 and prevent potential damage.

[0037] Fig. 2 shows an enlarged version of the section labeled II from Fig. 1. A reference electrode 6 is arranged in the separator 5. This electrode is embedded in the separator 5 so that it is electrically insulated from the adsorption electrode 3 and the counter electrode 4. The reference electrode 6 is at a reference potential, so that the voltage U can be measured using a voltage monitoring unit 12. a between the reference electrode 6 and the adsorption electrode 3 and the voltage U g The voltage between the reference electrode 6 and the counter electrode 4 can be measured. These voltages can be measured and monitored at each individual electrochemical cell 2 via taps 11 on the electrical contacts 7, 8 and on the reference electrode 6. This allows for the early detection of an overvoltage at one of the cell electrodes 3, 4 or between the cell electrodes 3, 4.

[0038] Figure 3a schematically depicts a device according to the invention during operation of the electrochemical system, for example, to remove carbon dioxide from a gas mixture. The stack 1 comprises a plurality of electrochemical cells 2 connected electrically in series, with the voltage being supplied via a voltage source 25. The stack R. 415137

[0039] - 7 -

[0040] Stack 1 has suitable connections and electrical wiring so that the applied current flows through all cells 2. Stack 1 has an inlet 15 into which a supply line 18 for the gas mixture containing the gas to be filtered opens. The gas mixture can, for example, be the exhaust gas of an internal combustion engine from which the carbon dioxide is to be removed. A shut-off valve 24 is arranged in the supply line 18 to interrupt the gas supply if necessary. For the discharge of the processed gas mixture, Stack 1 has an outlet 16 to which a discharge line 19 is connected. Stack 1 includes structures for guiding the gas mixture to supply the incoming gas mixture to all electrochemical cells 2 and to direct the processed gas mixture from the cells 2 to the outlet 16. The processed gas mixture is introduced via a directional control valve 20 into an outlet 21 and can, for example, be released into the environment.Stack 1 includes a voltage monitoring unit 12 with an evaluation unit that measures the voltage at each individual electrochemical cell 2.

[0041] If a voltage increase is registered in one or more electrochemical cells 2 during a constant current flowing through the stack 1, this indicates that the adsorption capacity of the adsorption electrode 3 in the corresponding electrochemical cells 2 is exhausted or will soon be exhausted. When the voltage reaches a predetermined limit, the supply of the gas mixture is interrupted by closing the shut-off valve 24. The current flowing through the stack 1 is then interrupted or reversed. The gas bound in the adsorption electrode, for example, carbon dioxide, is thereby desorbed and exits the stack 1 via the discharge line 19. In this case, the directional control valve 20 is moved to the switching position shown in Fig. 3b, so that the desorbed gas is directed via a gas line 22 into a gas storage tank 23, where the gas is stored.Once the desorption of the gas is complete, the directional control valve 20 is returned to the position shown in Fig. 3a, the shut-off valve 24 is opened, and a current is again passed through the stack 1 so that the adsorption process begins again.

[0042] Voltage monitoring at the electrochemical cells 2 eliminates the need for gas sensors for carbon dioxide within stack 1 and, if necessary, also in the discharge line 19. This allows for precise monitoring of stack 1 and thus precise control of the adsorption process. (R. 415137)

[0043] - 8 -

[0044] Desorption process in electrochemical stack 1 is possible without the need to install a large number of gas sensors.

[0045] The voltage measurement with the voltage monitoring unit 12 can also be used to measure an overvoltage at one of the cell electrodes 3, 4, which may be a sign of aging due to chemical degradation of the electrode active materials, especially if the voltages U a and U g The operating voltage of cell 2 differs from that of cell 2. Due to the changes in cell 2, a higher voltage is required than in its new state. To prevent overheating, the current through the cell can be adjusted by reducing it or, in extreme cases, switching it off completely. Since the electrodes each experience different levels of damage from overvoltages, the operating voltage of each cell electrode 3, 4 can be individually determined relative to a reference voltage of the reference electrode 6 using the reference electrode 6. This allows cell 2 to continue operating at a maximum current without causing damage to cell 2.

Claims

R. 415137 - 9 - Claims 1. Electrochemical device for the reversible adsorption of gases, comprising an electrochemical stack (1) with an electrochemical cell (2), wherein the cell (2) comprises an adsorption electrode (3) with an electrochemically active material for the reversible electrochemical adsorption of a gas, a counter electrode (4), and a separator (5) arranged between the adsorption electrode (3) and the counter electrode (4), wherein the stack (1) has an inlet (15) for introducing a gas mixture and an outlet (16) for discharging the adsorbed gas or the processed gas mixture, characterized in that a reference electrode (6) is arranged between the adsorption electrode (3) and the counter electrode (4), and a voltage monitoring unit (12) is connected to the adsorption electrode (3), the counter electrode (4), and the reference electrode (6), wherein the voltage monitoring unit (12) provides a first voltage (U a) between the reference electrode (6) and the adsorption electrode (3) and a second voltage (U g ) measures between the reference electrode (6) and the counter electrode (4).

2. Electrochemical device according to claim 1, characterized in that the voltage monitoring unit (12) issues an error message as soon as the first voltage (U) a ) from the second voltage (U g ) deviates by more than a specified value.

3. Electrochemical device according to claim 1, characterized in that the voltage monitoring unit (12) issues an error message as soon as the sum of the first voltage (U) a ) and the second voltage (U g ) deviates from a reference voltage by more than a specified value. R. 415137 - 10 - 4. Electrochemical device according to claim 1, 2 or 3, characterized in that the reference electrode (6) is embedded in the separator (5) and is electrically insulated from the adsorption electrode (3) and the counter electrode (4).

5. Electrochemical device according to one of claims 1 to 4, characterized in that the adsorption electrode (3) is designed for adsorbing carbon dioxide (CO2).

6. Electrochemical device according to one of claims 1 to 5, characterized in that the stack (1) can be connected to a voltage source (25) via which an electrical voltage can be applied between the adsorption electrode (3) and the counter electrode (4).

7. Electrochemical device according to one of claims 1 to 6, characterized in that the stack (1) comprises a plurality of cells (2) whose adsorption electrodes (3) and counter electrodes (4) are electrically connected in series.

8. Method for operating an electrochemical device according to any one of claims 1 to 7, characterized by Supplying a gas mixture via the inlet (15) into the stack (1), wherein the gas mixture contains the gas that can be adsorbed at the adsorption electrode (3), Applying a current between the adsorption electrode (3) and the counter electrode (4), Monitoring of the first electrical voltage (U) applied between the adsorption electrode (3) and the reference electrode (6). a ) and the second voltage (U) applied between the reference electrode (6) and the counter electrode (4). g ), Output an error message if the voltage difference between the first voltage (U) a ) and second voltage (U g ) exceeds a predetermined value.

9. Method according to claim 8, characterized in that the stack (1) comprises a plurality of cells (2), wherein the applied voltage at R. 415137 - 11 - measured by the voltage monitoring unit (12) on several or all cells (2) and the supply of the gas mixture is stopped as soon as the voltage monitoring unit (12) issues an error message 10. Method according to claim 9, characterized in that after the supply of the gas mixture has ceased, the voltage between the adsorption electrode (3) and the counter electrode (4) is reversed.

11. Method according to claim 9 or 10, characterized in that, after the supply of the gas mixture has ceased, the outlet (16) of the stack (1) is connected to a discharge line (19).

12. Method according to claim 8, characterized in that, upon output of an error message, the current flowing through the cell (2) is reduced to such an extent that the difference between the first voltage (U) a ) and the second voltage (U g ) falls below a predetermined limit.

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