Method for regenerating an electrolyser

Abstract

The invention relates to a method for regenerating an electrolyser which is designed for producing hydrogen and oxygen with the aid of electrical energy and which comprises an electrolysis cell (1), the electrolysis cell (1) having a cathode chamber (2) and an anode chamber (3) which are separated from one another by a selectively permeable membrane (6). The membrane (6) is coated on the side facing the cathode chamber (2) with a cathode electrode (7) and on the side facing the anode chamber (3) with an anode electrode (8), between which an electrical voltage is applied during operation of the electrolyser, the anode electrode (8) consisting of a porous material and the anode chamber (3) being filled with water or an aqueous electrolyte solution during operation of the electrolyser. To carry out the method, the following steps are performed: - lowering the electrical voltage between the anode electrode (8) and the cathode electrode (7) to 0 V; - lowering the pressure in the anode chamber (3) to less than 2 bar (0.2 MPa); - removing the water or the aqueous electrolyte solution from the anode chamber (3); - introducing hydrogen gas into the anode chamber (3); - refilling the anode chamber (3) with water or with aqueous electrolyte solution.

Description

[0001] title

[0002] Process for regenerating an electrolyzer

[0003] The invention relates to a method for regenerating an electrolyzer as used to produce hydrogen and oxygen from water using electrical energy.

[0004] State of the art

[0005] Electrolyzers are typically used to split water into hydrogen and oxygen using electrical current. Hydrogen can be used as an energy carrier for a variety of applications, particularly to store chemical energy over periods of months and years and, when needed, for example, using a fuel cell to convert it back into electrical energy. The electrolyzer comprises one or more electrolysis cells that have a cathode compartment and an anode compartment separated by a selectively permeable membrane. The anode compartment is filled with water or—particularly when using anion exchange membrane (AEM)—with an alkaline electrolyte solution. The cathode compartment is either filled with water or—when using an AEM—can also be operated dry, i.e., without separate water filling.Such an electrolyzer is known from WO 2009 / 007691 A2.

[0006] In an alkaline electrolyzer, the membrane is coated with an electrode on both sides: an anode electrode on the side facing the anode compartment, and a cathode electrode on the side facing the cathode compartment. The two electrodes, together with the membrane, form a so-called membrane-electrode assembly (MEA). A direct voltage of approximately 1.8 V is applied between the two electrodes, causing an ion current to flow through the membrane. This produces hydrogen and hydroxide ions (OH-) at the cathode:

[0007] The OH" ions diffuse through the membrane into the anode compartment and recombine there to form water and oxygen, releasing electrons, so that the circuit is closed:

[0008] The membrane must therefore have selective permeability for both water and OH" anions.

[0009] Energy losses inevitably occur during electrolyzer operation: In addition to system-side losses, such as conversion losses in the power supply, electrical pumping power, and water treatment, a large portion of the losses occur in the cell stack itself. This results in an operating voltage of the electrolysis cell that exceeds the theoretical decomposition voltage of water of 1.23 V. The higher the operating voltage, the higher the losses in the electrolysis cell and the less effective the electrolyzer is.

[0010] The electrode in the anode compartment has a porous structure that acts catalytically to break down the water molecules. The electrode's action—like the cathode electrode—is subject to certain aging phenomena, which can be divided into reversible and irreversible aging processes. Both phenomena are characterized by a gradual degradation of certain electrochemical properties. This leads to a deterioration of the electrolyzer's efficiency, i.e., the ratio of hydrogen flow produced to electrical power supplied. This is accompanied by an increase in the operating voltage during operation compared to the operating voltage when new (BOL: Beginning of Life), which is undesirable and should be minimized.

[0011] One of the causes of degeneration is oxygen microbubbles. These form during operation in the porous anode catalyst and partially cover it, reducing the catalytically effective surface and increasing losses. Due to the fine-pored structure of the catalyst, these microbubbles can only be inadequately removed by flushing with water or an aqueous electrolyte solution. The only possible degradation processes are either diffusive transport into the anode solution when the electrolyzer is switched off or reactive conversion with hydrogen, which diffuses from the cathode through the membrane. Both processes are very slow as diffusion processes, so that regeneration requires an electrolyzer downtime of several hours to several days.

[0012] Advantages of the invention

[0013] The inventive regeneration process for an electrolyzer has the advantage of achieving rapid regeneration of the porous catalytic electrodes, making the electrolyzer available again after a short downtime. This increases efficiency and thus reduces the cost of the hydrogen produced. The process is applied to an electrolyzer designed to produce hydrogen and oxygen using electrical energy and comprising an electrolysis cell. The electrolysis cell has a cathode compartment and an anode compartment separated from each other by a selectively permeable membrane.The membrane is coated with a cathode electrode on the side facing the cathode compartment and with an anode electrode on the side facing the anode compartment, between which an electrical voltage is applied during operation of the electrolyzer. The anode electrode consists of a porous material, and the anode compartment is filled with water or an aqueous electrolyte solution during operation of the electrolyzer. The method according to the invention comprises the following steps:

[0014] - Lowering the electrical voltage between the anode electrode and the cathode electrode to 0 V;

[0015] - reducing the pressure in the cathode chamber to less than 2 bar (0.2 MPa);

[0016] - Removing the water or aqueous electrolyte solution from the anode compartment;

[0017] - Introducing hydrogen gas into the anode chamber;

[0018] - Refilling the anode chamber with water or an aqueous electrolyte solution. The aim of the process is to remove the oxygen microbubbles from the anode electrode. To ensure accessibility to the anode, the water or electrolyte solution is first removed after the electrical voltage is switched off, for example by purging with an inert gas such as nitrogen. The anode chamber is then filled with hydrogen gas, ensuring that the entire anode electrode is exposed to hydrogen gas. The high diffusion rate of the hydrogen guarantees that the oxygen microbubbles are reached even in the smallest pores and, catalytically triggered, react with the hydrogen gas to form water. For the catalytic initiation of the reaction, a bifunctional anode catalyst, i.e., one that is active in both OER and ORR (OER: Oxygen Evolution Reaction; ORR: Oxygen Reduction Reaction), is advantageous.Catalytically active cell components, such as nickel support structures in the catalyst area, are also advantageous.

[0019] Since the total volume of the oxygen microbubbles is small, the released reaction enthalpy, given the large thermal mass of a stack consisting of numerous electrolysis cells, does not lead to a significant temperature increase, so no damage to the anode electrode is to be expected. The anode chamber can then either be immediately refilled with water, or the remaining, unreacted hydrogen can be removed by further purging with the inert gas. The anode electrode can thus be regenerated in a short time, without requiring long downtimes of the electrolyzer.

[0020] In an advantageous embodiment, the hydrogen gas is introduced into the anode chamber for a period of 5 to 30 seconds. This is sufficient for flooding and diffusion into the entire electrode layer, since hydrogen easily diffuses into even the smallest pores due to its very high diffusion constant. The hydrogen can then be flushed out of the anode chamber with nitrogen or another inert gas to prevent contaminants from entering the oxygen generated on the anode side during operation, which may be required for other applications.

[0021] At the beginning of the regeneration process, the voltage between the anode and cathode electrodes is advantageously reduced to 0 V for a time interval of 10 to 30 seconds, so that the reactions at both electrodes slowly subside and larger oxygen bubbles are flushed out with the anode water. Accordingly, after the regeneration is complete, the voltage between the electrodes is advantageously increased for a time interval of 10 to 30 seconds until the operating voltage is reached again.

[0022] In a further advantageous development, the method is used in an electrolyzer which has an anion exchange membrane (AEM) which is selectively permeable to hydroxide ions and water.

[0023] drawing

[0024] In the drawing, Fig. 1 shows a schematic representation of an electrolyzer with its essential components, which can be regenerated using the method according to the invention.

[0025] Description of the embodiments

[0026] Fig. 1 of the drawing shows an electrolyzer with the essential components for supplying and removing the required gases and liquids. The electrolyzer comprises an electrolysis cell 1 having a cathode compartment 2 and an anode compartment 3, separated from each other by a membrane electrode assembly 5 (MEA). The MEA 5 is formed by a membrane 6 coated with a cathode electrode 7 on the side facing the cathode compartment 2 and an anode electrode 8 on the side facing the anode compartment 3. The anode electrode 8 and the cathode electrode 7 are electrically conductive, and the ionic conduction in the porous layer is provided by the electrolyte or, if present, by an ionomer. A direct current can be applied between the electrodes. The voltage during operation is typically around 1.8 V, and the current through the membrane is approximately 3 A / cm 2If the electrolyzer is equipped with an anion exchange membrane (AEM), the membrane 6 is permeable to hydroxide ions (OH") and water (H2O). The anode compartment 3 is filled with water or an aqueous electrolyte solution - for example, a potassium hydroxide solution (KOH") - which is supplied via an electrolyte pump 24. The cathode compartment 2 is either filled with water or - when using an AEM membrane - can be operated dry, whereby the water required in the cathode compartment 2 diffuses through the membrane 6 from the anode compartment 3. After applying an electrical voltage between the electrodes 7, 8, hydrogen and hydroxide ions (OH") are formed at the cathode electrode 7, whereby the required water diffuses from the anode compartment 3 into the cathode compartment 2 when using an AEM membrane 6:

[0027] The OH" ions diffuse through the membrane back into the anode compartment 3 and recombine there to form water and oxygen, releasing electrons:

[0028] The water or electrolyte solution is continuously fed into the anode chamber 3 by the electrolyte pump 24 and discharged via an anode outlet line 21 to maintain constant conditions in the anode chamber 3 and to remove the oxygen produced. The required water is drawn from a gas-liquid separator 20 by the electrolyte pump 24 via an electrolyte line 22. Since water is continuously consumed during operation of the electrolyzer, additional water is supplied as needed through a water line not shown in the drawing. The oxygen-water mixture from the anode chamber 3 is fed via the anode outlet line 21 into the gas-liquid separator 20, where the oxygen is separated from the water. The oxygen is either passed on for further use or vented into the ambient air via a pressure control valve 25, which can also be used to adjust the pressure in the anode chamber 3.The water accumulating in the gas-liquid separator 20 is - as already mentioned - recirculated into the anode chamber 3.

[0029] The cathode chamber 2 is also continuously supplied with water to maintain constant conditions and to remove the hydrogen produced. The discharged water flows via a cathode outlet line 9 into a second gas-liquid separator 10, where the hydrogen is separated from the water and discharged via a pressure control valve 12. The pressure in the cathode chamber 2 can be adjusted via the pressure valve 12. The water separated in the gas-liquid separator 20 is fed back into the cathode chamber 2 via a water pump 14, with the used water being replenished here via a supply line (not shown). The hydrogen is collected in a gas tank for later use and, if necessary, further compressed. At approximately 30 bar (3 MPa), the pressure in the cathode chamber 2 is significantly higher than in the anode chamber 3, where generally no more than 2 bar (0.2 MPa) prevails during operation.This facilitates the further use and storage of the hydrogen, which requires less compression. It is also possible to set atmospheric pressure on both the cathode and anode sides.

[0030] The electrolyzer can also be operated with a dry cathode chamber 2. In this case, the water supply to the cathode chamber 2 is eliminated, and the water required at the cathode electrode 7 diffuses exclusively through the membrane 6 from the anode chamber 8. The gas-liquid separator 10 is still present to produce anhydrous and thus highly pure hydrogen gas.

[0031] To carry out the regeneration process described below, a nitrogen tank 30 is provided, from which gaseous nitrogen can be introduced into the anode chamber 3 via a line 31 and a shut-off valve 32 as needed. If necessary, the nitrogen can be discharged via a separate discharge line 27 and a shut-off valve 28 if mixing with the oxygen in the gas-liquid separator 20 is not desired. Instead of nitrogen, another chemically inert gas, such as a noble gas, can also be used.

[0032] When the electrolyzer is in operation, oxygen is produced at the anode electrode 8, which is carried away with the water or electrolyte solution. In addition, oxygen microbubbles form in the anode electrode 8, which are held there by its porous structure and over time occupy increasingly larger areas of the catalytically active surface of the anode electrode 8, thus rendering it ineffective. This causes an increase in the required operating voltage between the electrodes and thus a higher loss, i.e. more electrical energy must be supplied to produce a certain amount of hydrogen. The following process is used to regenerate the anode electrode 8 and remove the microbubbles: The electrical voltage applied between the cathode electrode 7 and the anode electrode 8 is reduced to 0 V within, for example, 20 seconds in order to end the chemical reactions in the electrolysis cell 1.The pressure in the anode chamber 3 is then reduced to approximately 1 bar (0.1 MPa), and the water or electrolyte solution is removed from the anode chamber 3. To avoid excessively high differential pressure, the pressure in the cathode chamber 2 can also be reduced. The liquid is discharged via the electrolyte pump 24, which then acts as a suction pump, or a separate, additional pump. The liquid can also be removed by purging using nitrogen, which is introduced from the nitrogen container 30 at a slight overpressure of approximately 1.5 bar (0.15 MPa). In the next step, hydrogen from the gas-liquid separator 10 of the cathode chamber 2 or from another hydrogen container is introduced into the anode chamber 3 via the purge line 15. This takes place for approximately 5 to 30 seconds, with the hydrogen being introduced until the anode chamber 3 is completely filled and the anode electrode 8 is fully charged.The introduced hydrogen diffuses extremely easily into the porous cathode electrode 7 and reacts with the oxygen microbubbles to form water. The reaction enthalpy released in this process does lead to heating, but this is only slight due to the small amount of oxygen in the microbubbles. The anode chamber 3 can then be flushed with nitrogen from the nitrogen container 30 to remove the hydrogen, although this step can also be omitted if necessary. Finally, the anode chamber 3 is refilled with water or the aqueous electrolyte solution, and the electrolyzer can be used again to produce hydrogen and oxygen.

Claims

Claims 1. A method for regenerating an electrolyzer designed to produce hydrogen and oxygen using electrical energy and comprising an electrolysis cell (1), wherein the electrolysis cell (1) has a cathode compartment (2) and an anode compartment (3) separated from one another by a selectively permeable membrane (6), wherein the membrane (6) is coated with a cathode electrode (7) on the side facing the cathode compartment (2) and with an anode electrode (8) on the side facing the anode compartment (3), between which an electrical voltage is applied during operation of the electrolyzer, wherein the anode electrode (8) consists of a porous material and the anode compartment (3) is filled with water or an aqueous electrolyte solution during operation of the electrolyzer, characterized by the following steps: Lowering the electrical voltage between the anode electrode (8) and the cathode electrode (7) to 0 V; Reducing the pressure in the anode chamber (3) to less than 2 bar (0.2 MPa); Removing the water or aqueous electrolyte solution from the anode compartment (3); Introducing hydrogen gas into the anode chamber (3); Refill the anode chamber (3) with water or aqueous electrolyte solution.

2. Method according to claim 1, characterized in that the anode chamber (3) is flushed with nitrogen gas or an inert gas after the removal of the water or the aqueous electrolyte solution.

3. Method according to claim 1 or 2, characterized in that the hydrogen gas is introduced into the anode chamber (3) for 5 to 30 seconds.

4. Process according to one of claims 1 to 3, characterized in that the hydrogen gas is introduced at a pressure of 1.1 to 1.8 bar (0.11 to 0.18 MPa).

5. A method according to claims 1 to 4, characterized in that after the introduction of the hydrogen gas and before filling the anode chamber (3) with water or with aqueous electrolyte solution, the anode chamber is flushed with nitrogen gas or an inert gas.

6. Method according to one of claims 1 to 5, characterized in that the electrical voltage between the anode electrode (8) and the cathode electrode (7) is reduced to 0 V within a time interval of 10 to 30 seconds.

7. Method according to one of claims 1 to 6, characterized in that after the anode chamber (3) has been refilled with the aqueous electrolyte solution, the electrical voltage between the anode electrode (8) and the cathode electrode (7) is raised again to a working voltage in a time interval of 10 to 30 seconds.

8. Method according to one of claims 1 to 7, characterized in that the selectively permeable membrane (6) is an anion exchange membrane (AEM) which is selectively permeable to hydroxide ions (OH") and water (H2O).

Images