Method for electrolyser regeneration

Abstract

The invention relates to a method for the regeneration of an electrolyser, which is designed to produce hydrogen and oxygen by means of electrical energy and which comprises an electrolytic cell (1), the electrolytic cell (1) having a cathode chamber (2) and an anode chamber (3) which are separated from one another by a selective 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), a voltage being applied between the cathode electrode and the anode electrode during the operation of the electrolyzer, the anode electrode (8) being made of a porous material, and the anode electrode (8) being made of a porous material. And the anode chamber (3) is filled with water or an aqueous electrolyte during the operation of the electrolyzer. In order to carry out the method, the following steps are carried out: reducing the 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 water or aqueous electrolyte from the anode chamber (3); introducing hydrogen into the anode chamber (3); -refilling the anode chamber (3) with water or an aqueous electrolyte.

Description

Technical Field

[0001] The present invention relates to a method for the regeneration of an electrolyzer for generating hydrogen and oxygen from water by means of electrical energy. Background Art

[0002] Electrolyzers are generally used to decompose water into hydrogen and oxygen by means of an electric current. Hydrogen can be used as an energy carrier for a variety of applications, in particular for storing chemical energy for periods of several months to several years and converting it back into electrical energy, for example, by means of a fuel cell when needed. An electrolyzer comprises one or more electrolytic cells having a cathode chamber and an anode chamber separated from each other by a selectively permeable membrane. The anode chamber is filled with water or, especially when using an anion exchange membrane (AEM), with an alkaline electrolyte. The cathode chamber is either filled with water or, when using an AEM, also operates dry, i.e., without separate water filling. Such an electrolyzer is known from WO 2009 / 007691 A2.

[0003] In an alkaline electrolyzer, the membrane is coated with electrodes on both sides: an anode electrode is coated on the side facing the anode chamber, and a cathode electrode is coated on the side facing the cathode chamber. These two electrodes together with the membrane form a so-called membrane electrode assembly (MEA). A DC voltage of about 1.8 V is applied between the two electrodes, causing an ion flow to pass through the membrane. Here, hydrogen and hydroxide ions (OH-) are generated at the cathode:

[0004] 4H2O + 4e - → 2H2 + 4OH -

[0005] The OH- ions diffuse through the membrane into the anode chamber, where they recombine into water and oxygen while releasing electrons, thus closing the current loop:

[0006] 4OH - → O2 + 2H2O + 4e -

[0007] Therefore, the membrane must be selectively permeable to both water and OH- anions.

[0008] During the operation of the electrolyzer, energy losses inevitably occur: in addition to losses on the system side (such as conversion losses in the power supply, power of the electric pump, and water treatment), most of the losses occur in the stack itself. This is manifested by the working voltage of the electrolytic cell being higher than the theoretical decomposition voltage of water, which is 1.23 V. The higher the working voltage, the higher the losses in the electrolytic cell, and the less efficiently the electrolyzer operates.

[0009] The electrode in the anode chamber has a porous structure that acts as a catalyst to decompose water molecules. Here, the role of this electrode - like that of the cathode electrode - undergoes specific aging phenomena, which can be divided into reversible and irreversible aging processes. Both phenomena are characterized by a slow degradation of specific electrochemical properties. Here, the efficiency of the electrolyzer, i.e., the ratio of the hydrogen gas flow produced to the input electrical power, decreases. At the same time, the operating voltage during operation increases relative to the operating voltage in the new state (BOL: Begin Of Life), which is undesirable and should be minimized.

[0010] In addition, the cause of degradation is oxygen microbubbles. These oxygen microbubbles form in the porous anode catalyst during operation and partially cover the anode catalyst, which reduces the catalytically active area and increases losses. Due to the microporous structure of the catalyst, these microbubbles can only be removed insufficiently by rinsing with water or an aqueous electrolyte. As a decomposition process, it can only be achieved by diffusion transport into the anode solution when the electrolyzer is shut down, or by reacting with hydrogen gas diffusing through the membrane from the cathode. Both processes are very slow as diffusion processes, so regeneration requires the electrolyzer to be shut down for several hours to several days. Summary of the Invention

[0011] The regeneration method for an electrolyzer according to the present invention has the following advantages: rapid regeneration of the porous catalytic electrode is achieved, and the electrolyzer can be put back into use after a short shutdown. This improves economic efficiency and thus reduces the cost of the hydrogen produced thereby. The method is applicable to an electrolyzer configured to produce hydrogen and oxygen by means of electrical energy and including an electrolytic cell, wherein the electrolytic cell has a cathode chamber and an anode chamber separated from each other by a selectively permeable membrane. The membrane is coated with a cathode electrode on the side facing the cathode chamber and with an anode electrode on the side facing the anode chamber, and a voltage is applied between the cathode electrode and the anode electrode during the operation of the electrolyzer, wherein the anode electrode is made of a porous material, and the anode chamber is filled with water or an aqueous electrolyte during the operation of the electrolyzer. The method according to the present invention includes the following steps:

[0012] - Lower the voltage between the anode electrode and the cathode electrode to 0 V;

[0013] - Lower the pressure in the anode chamber to less than 2 bar (0.2 MPa);

[0014] - Remove water or an aqueous electrolyte from the anode chamber;

[0015] - Introduce hydrogen gas into the anode chamber;

[0016] - Refill the anode chamber with water or an aqueous electrolyte.

[0017] The purpose of the method is to remove the oxygen microbubbles from the anode electrode. To make the anode accessible, after cutting off the voltage, first the water or electrolyte is removed, for example by flushing with an inert gas such as nitrogen. Subsequently, the anode chamber is filled with hydrogen, where it is ensured that the entire anode electrode is loaded with hydrogen. The high diffusion rate of hydrogen ensures that oxygen microbubbles in even the smallest pores can be reached and react with hydrogen under catalysis to form water. Here, in order to catalytically initiate this reaction, a bifunctional anode catalyst, i.e., having both oxygen evolution reaction (OER) activity and oxygen reduction reaction (ORR) activity, is advantageous. A catalytically active single-cell component such as a nickel support structure in the catalyst region is also advantageous.

[0018] Since the total volume of the oxygen microbubbles is very small, due to the large thermal mass of the stack composed of multiple electrolytic cells, the released reaction enthalpy does not cause a significant temperature increase, thus not damaging the anode electrode. Subsequently, the anode chamber can be immediately refilled with water, or the residual unreacted hydrogen can be removed in advance by flushing again with an inert gas. Thereby, the anode electrode can be regenerated in a short time without the electrolyzer having a long downtime.

[0019] In an advantageous configuration, hydrogen is introduced into the anode chamber over a period of 5 to 30 seconds. This is sufficient to fill the hydrogen and diffuse it throughout the electrode layer, because hydrogen easily diffuses into the smallest pores due to its very high diffusion constant. Subsequently, the hydrogen can be flushed out of the anode chamber with nitrogen or other inert gases to avoid contaminating the oxygen that is generated on the anode side during operation and may be required for other applications.

[0020] At the start of the regeneration process, advantageously the voltage between the anode electrode and the cathode electrode is reduced to 0 V over a time interval of 10 to 30 seconds, so that the reactions on both electrodes slow down and the larger oxygen bubbles are flushed away by the anode water. Correspondingly, after the regeneration is completed, advantageously the voltage between the electrodes is also increased over a time interval of 10 to 30 seconds until the operating voltage is reached.

[0021] In a further advantageous expansion, the method is applied to an electrolyzer having an anion exchange membrane (AEM) that is selectively permeable to hydroxide ions and water. Description of the Drawings

[0022] In the drawings, Figure 1 An electrolyzer with its main components that can be regenerated by the method according to the invention is schematically shown. Detailed Description

[0023] In Figure 1An electrolyzer is shown having main components for supplying and discharging the required gases and liquids. The electrolyzer includes an electrolytic cell 1, which has a cathode chamber 2 and an anode chamber 3 separated from each other by a membrane electrode assembly 5 (MEA). The membrane electrode assembly 5 is composed of a membrane 6, which is coated with a cathode electrode 7 on the side facing the cathode chamber 2 and with an anode electrode 8 on the side facing the anode chamber 3. The anode electrode 8 and the cathode electrode 7 are conductive, and the ion conduction in the porous layer is borne by the electrolyte or, if present, by the ionomer. A DC voltage can be applied between the electrodes. Here, the voltage during operation is typically about 1.8 volts, and the current density through the membrane is about 3 A / cm2. If the electrolyzer is equipped with an anion exchange membrane (AEM: Anion Exchange Membrane), the membrane 6 is permeable to hydroxide ions (OH-) and water (H2O). The anode chamber 3 is filled with water or an aqueous electrolyte - for example, potassium hydroxide solution (KOH-) - which is supplied by an electrolyte pump 24. The cathode chamber 2 is either filled with water or - when using an AEM membrane - can be operated dry, where the required water in the cathode chamber 2 diffuses in from the anode chamber 3 through the membrane 6. After a voltage is applied between the electrodes 7, 8, hydrogen and hydroxide ions (OH- ions) are generated at the cathode electrode 7, where, when using the AEM membrane 6, the required water diffuses from the anode chamber 3 into the cathode chamber 2:

[0024] 4H2O+4e - →2H2+4OH -

[0025] The OH- ions diffuse back through the membrane into the anode chamber 3, where they recombine into water and oxygen with the release of electrons:

[0026] 4OH - →O2+2H2O+4e -

[0027] Water or the electrolyte is continuously introduced into the anode chamber 3 by the electrolyte pump 24 and discharged through the anode outlet line 21 to maintain constant conditions in the anode chamber 3 and to discharge the generated oxygen. The required water is pumped by the electrolyte pump 24 from the gas-liquid separator 20 through the electrolyte line 22. Since water is continuously consumed during the operation of the electrolyzer, more water is supplemented through a water line not shown in the figure when needed. The oxygen-water mixture from the anode chamber 3 is introduced into the gas-liquid separator 20 through the anode outlet line 21, where the oxygen is separated from the water. The oxygen is conveyed through a pressure regulating valve 25 for further use or discharged into the atmosphere, and the pressure in the anode chamber 3 can also be regulated by this pressure regulating valve. As mentioned above, the water generated in the gas-liquid separator 20 is recycled into the anode chamber 3.

[0028] The cathode chamber 2 is also continuously supplied with water to maintain constant conditions and remove the generated hydrogen. The discharged water enters the second gas-liquid separator 10 through the cathode outlet line 9, where the hydrogen is separated from the water and discharged through the pressure regulating valve 12. The pressure in the cathode chamber 2 can be adjusted by the pressure valve 12. The water separated in the gas-liquid separator 20 is pumped back to the cathode chamber 2 by the water pump 14, and the consumed water is also replenished through a supply line not further shown. The hydrogen is collected in a gas tank for further use and further compressed as required. The pressure in the cathode chamber 2 is about 30 bar (3 MPa), significantly higher than the pressure in the anode chamber 3, where it generally does not exceed 2 bar (0.2 MPa) during operation. This facilitates the further use and storage of hydrogen because the hydrogen does not need to be significantly compressed. However, the atmospheric pressure can also be set on both the cathode side and the anode side.

[0029] The electrolyzer is also capable of operating with a dry cathode chamber 2. In this case, the water supply to the cathode chamber 2 is omitted, and the water required at the cathode electrode 7 diffuses entirely from the anode chamber 8 through the membrane 6. The gas-liquid separator 10 still exists to produce anhydrous and thus high-purity hydrogen.

[0030] For implementing the regeneration method described below, there is a nitrogen container 30, from which gaseous nitrogen can be introduced into the anode chamber 3 through the line 31 and the shut-off valve 32 when needed. If it is not desired to mix with the oxygen in the gas-liquid separator 20, the nitrogen can be discharged (if required) through a separate discharge line 27 and the shut-off valve 28. In addition to nitrogen, other chemically inert gases, such as noble gases, can also be used.

[0031] During the operation of the electrolyzer, oxygen is generated at the anode electrode 8, and the oxygen is discharged together with water or the electrolyte. In addition, oxygen microbubbles are formed in the anode electrode 8, and the oxygen microbubbles are intercepted by its porous structure and occupy an increasingly large area of the catalytically active surface of the anode electrode 8 over time, thereby rendering it ineffective. This results in an increase in the working voltage required between the electrodes, and thus higher losses, that is, more electrical energy needs to be input to produce a specific amount of hydrogen.

[0032] To regenerate the anode electrode 8 and remove microbubbles, the following method is employed: for example, the voltage applied between the cathode electrode 7 and the anode electrode 8 is reduced to 0V within 20 seconds to terminate the chemical reaction in the electrolytic cell 1. Subsequently, the pressure in the anode chamber 3 is reduced to approximately 1 bar (0.1 MPa), and water or electrolyte is removed from the anode chamber 3. To avoid an excessive pressure difference, the pressure in the cathode chamber 2 can also be reduced. The liquid is discharged through the electrolyte pump 24 operating as a suction pump, or through a separate additional pump. The liquid can also be removed by purging with nitrogen introduced from the nitrogen container 30 at a slight overpressure of approximately 1.5 bar (0.15 MPa). In the next step, hydrogen is introduced from the gas-liquid separator 10 of the cathode chamber 2 or from another hydrogen container into the anode chamber 3 through the flushing line 15. This continues for approximately 5 to 30 seconds, during which hydrogen is introduced for such a long time until the anode chamber 3 is completely filled and the anode electrode 8 is fully loaded. The introduced hydrogen readily diffuses into the porous cathode electrode 7 and reacts with the oxygen microbubbles to form water. Although the reaction enthalpy released here causes a temperature increase, due to the small amount of oxygen in the microbubbles, the magnitude of the temperature increase is small. Subsequently, the anode chamber 3 can be flushed with nitrogen from the nitrogen container 30 to remove hydrogen, but this step can also be omitted as required. Finally, the anode chamber 3 is refilled with water or an aqueous electrolyte, and the electrolyzer can be used again to produce hydrogen and oxygen.

Claims

1. A method for regenerating an electrolyzer, the electrolyzer being configured to produce hydrogen and oxygen by means of electrical energy and the electrolyzer comprising an electrolytic cell (1), wherein, The electrolytic cell (1) has a cathode chamber (2) and an anode chamber (3) separated from each other by a selective permeable membrane (6). The membrane (6) is coated with a cathode electrode (7) on the side facing the cathode chamber (2) and with an anode electrode (8) on the side facing the anode chamber (3). A voltage is applied between the cathode electrode and the anode electrode during the operation of the electrolyzer. The anode electrode (8) is made of a porous material, and the anode chamber (3) is filled with water or an aqueous electrolyte during the operation of the electrolyzer. It is characterized by the following steps: - Lower the voltage between the anode electrode (8) and the cathode electrode (7) to 0 V; - Lower the pressure in the anode chamber (3) to less than 2 bar (0.2 MPa); - Remove water or an aqueous electrolyte from the anode chamber (3); - Introduce hydrogen into the anode chamber (3); - Refill the anode chamber (3) with water or an aqueous electrolyte.

2. The method according to claim 1, wherein After removing water or an aqueous electrolyte, rinse the anode chamber (3) with nitrogen or an inert gas.

3. The method according to claim 1 or 2, characterized in that, Introduce the hydrogen into the anode chamber (3) in 5 to 30 seconds.

4. The method according to any one of claims 1 to 3, characterized in that Introduce the hydrogen at a pressure of 1.1 to 1.8 bar (0.11 to 0.18 MPa).

5. The method according to any one of claims 1 to 4, characterized in that After introducing the hydrogen and before filling the anode chamber (3) with water or an aqueous electrolyte, rinse the anode chamber with nitrogen or an inert gas.

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

7. The method according to any one of claims 1 to 6, characterized in that, After refilling the anode chamber (3) with an aqueous electrolyte, raise the voltage between the anode electrode (8) and the cathode electrode (7) back to the operating voltage within a time interval of 10 to 30 seconds.

8. The method according to any one of claims 1 to 7, characterized in that, The selective permeable membrane (6) is an anion exchange membrane (AEM) that is selectively permeable to hydroxide ions (OH-) and water (H2O).

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