Method for operating an electrolyzer to produce hydrogen and oxygen

Abstract

The present invention relates to a method for operating an electrolyzer (10) for producing hydrogen and oxygen using a diaphragm (22) permeable to OH ions separating an anode chamber (14) from a cathode chamber (16), comprising at least the following method steps: a) temporarily operating the cathode chamber (16) dry; b) temporarily diffusing water molecules from the anode chamber (14) into the cathode chamber (16) by means of the diaphragm (22); c) varying a pressure difference (42) between the cathode chamber (16) and the anode chamber (14) by means of a throttle valve (46); and d) adjusting the humidification / wetting of the cathode chamber (16) by adjusting the predetermined pressure difference (42).

Description

[Technical field]

[0001] The present invention relates to a method for operating an electrolyser for the production of hydrogen and oxygen using a diaphragm permeable to OH ions separating the anode and cathode compartments from each other, and further to the use of said method for operating an electrolyser or an alkaline electrolyser with temporary water transport through the diaphragm for humidification / wetting of the cathode compartment. [Background technology]

[0002] The electrolyser generally comprises a polymer diaphragm permeable to OH ions and two electrodes on opposite sides of the diaphragm, which is usually made of a polymer material. An aqueous electrolyte, i.e. for example a KOH solution, is fed into the chamber, for example on the oxygen-producing or anode side. The side opposite the diaphragm, which separates the chamber, is optionally also passed through by the electrolyte to produce hydrogen. Particularly in unsteady operation, locally occurring temperature peaks can weaken, in particular the polymer diaphragm material, which can lead, inter alia, to a thinning of the diaphragm and, as a result, to failure due to excessive gas diffusion or the formation of holes. As originally known from [1], a cathode arranged on the hydrogen side tends to locally dry out structures close to the diaphragm, especially during shutdown. A scenario of failure during restart is that, in particular, thermal equilibrium at the diaphragm has not yet been achieved and local temperature peaks due to the lack of good cooling water can lead to the failure of the diaphragm.

[0003] Additionally, dry operated cathodes, due to the lack of water, have a lower thermal capacity than cathodes operated in a flooded mode which causes the cell and stack to cool faster.

[0004] US Pat. No. 5,399,433 shows an embodiment which is advantageously operated dry on the cathode side (hydrogen side), i.e. there is no supply of KOH electrolyte, in order, inter alia, to keep costs for drying the hydrogen low. There, it is proposed to use an aqueous alkaline solution starting from a dry cathode for the electrolytic production of hydrogen. The device comprises two half-cells: an anode half-cell and a cathode half-cell separated by an anion exchanger diaphragm, the surface of which in contact with the cathode half-cell forms a diaphragm-electrode-unit (MEA), in which the alkaline solution is only in the anode half-cell.

[0005] In the case of dry cathodes or in the case of dry operation of the cathode chamber according to US Pat. No. 5,299,366, there is no water circulation on the cathode side, i.e. on the hydrogen side. When the electrolyser is shut down, non-uniform temperatures can form on the diaphragm. The cathode side, i.e. the cathode chamber, is filled only with hydrogen gas and therefore can accumulate less heat than in the case of a cathode chamber filled with water or another medium. Heating during restart of the electrolyser can therefore only take place via the anode side, whereas the water splitting to OH- takes place in the diaphragm on the cathode side. Since polymer diaphragms with poor thermal conductivity are used here, local overheating can be expected in fast start-up processes. This is due to the different degrees of electrical contact of the individual areas, where the water splitting starts preferentially. As long as the water flow through the diaphragm for the splitting of OH- is not uniform, the catalytic surface can only operate uniformly to a limited extent and can form local hot spots that can lead to a deterioration in the medium term and, in the short term, to damage to the diaphragm. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] European Patent No. 2451992B2 [Non-patent literature]

[0007] [Non-Patent Document 1] P. Millet et al., “Cell Failure Mechanisms in PEM Water Electrolyzers”, International Journal of Hydrogen Energy, Vol. 37, No. 22, November 2012, pp. 17478-17487, https: / / doi.org / 10.1016 / j.ijhydene.2012.06.017 Summary of the Invention

[0008] According to the invention, a method for operating an electrolyser for producing hydrogen and oxygen with a diaphragm permeable to OH ions and separating the anode and cathode compartments from each other comprises the following method steps: a) subjecting the cathode chamber to a temporary dry operation; b) allowing water molecules to temporarily diffuse from the anode chamber into the cathode chamber by means of the diaphragm; c) varying the pressure difference between the cathode and anode chambers by means of a throttle valve; d) a method step of adjusting the humidification / wetting of the cathode chamber by adjusting a predetermined pressure difference; A method is proposed which includes:

[0009] With the solution proposed according to the invention, the collection of water from the anode chamber into the cathode chamber, also called drainage, is controlled by purposefully influencing the pressure difference between the anode and cathode chambers, so that the part of the cathode chamber which is temporarily operated dry, in particular around the diaphragm, is regularly and sufficiently humidified, thereby providing a uniform temperature level and preventing premature damage over the life of the diaphragm material of the diaphragm separating the anode and cathode chambers.

[0010] In the method proposed according to the invention, in an advantageous embodiment, the diaphragm which separates the anode and cathode compartments from one another and which is preferably embodied as a diaphragm made of a polymer material provides for a controlled diffusion of water, preferably fully demineralized water.

[0011] In an advantageous further configuration of the method proposed according to the invention, fully demineralized water, preferably with a conductivity of <0.1 μS, is used for the controlled water diffusion from the anode chamber into the cathode chamber.

[0012] Alternatively, in the method proposed according to the invention, for the case of operating an alkaline electrolytic cell, a KOH solution may be used as the liquid medium instead of fully demineralized water.

[0013] The method proposed according to the invention advantageously comprises a pressure p K When the molar transport rate H 2 O / H 2 We propose to increase the molar transport rate H 2 O / H 2 is called the mole fraction, i.e., it represents the diffusion motion from a mole of water molecule to a mole of hydrogen molecule.

[0014] In the method proposed according to the invention, in an advantageous embodiment, the pressure p in the cathode chamber K is varied in the second range by correspondingly adjusting the throttle valve, resulting in pressure fluctuations.

[0015] In an advantageous further embodiment of the idea on which the present invention is based, the pressure p K The fluctuation of the pressure in the cathode chamber and the resulting pressure peak in the cathode chamber are the molar transport rate H 2 O / H 2 Affects.

[0016] The method proposed according to the invention makes it possible to influence the temperature in the cathode chamber via water transport into the cathode chamber.

[0017] Furthermore, the invention relates to the use of the method for operating an electrolytic cell or an alkaline electrolytic cell with temporary water transport by a diaphragm for humidifying / wetting the cathode compartment. Effect of the Invention

[0018] The solution proposed according to the invention allows one side of the PEM or AEM electrolysis cell or electrolysis stack, in particular the cathode side, to be operated dry only temporarily during operation, thereby keeping the hydrogen drying costs and the energy costs required for this low. On the other hand, the solution proposed according to the invention allows the cathode side to be temporarily at least partially flooded or humidified or moistened, which allows for faster load changes, in particular when increasing the hydrogen production, and also prevents rapid non-uniform cooling of the electrolysis cell or electrolysis stack. Advantageously, the invention allows for flooding or humidification, preferably on the cathode side, so that the hydrogen production can be controlled in an adaptive manner, which is achieved by the targeted metering of fully demineralized water, preferably with a conductivity of <0.1 μS. The solution proposed according to the invention allows for a controllable water diffusion from the anode side to the cathode side by means of a diaphragm. This control, i.e. the control of the water diffusion from the anode chamber to the cathode chamber, is carried out via a change in the pressure difference between the cathode chamber and the anode chamber.

[0019] The solution proposed according to the invention advantageously uses the fact that during ion transport through the polymer diaphragm separating the anode and cathode compartments, water molecules are also entrained from the anode compartment into the cathode compartment. This "drainage" depends on the pressure difference between the cathode and anode sides of the electrolysis cell. By varying the pressure difference on the cathode side relative to the anode side, the water transport into the cathode compartment and thus the humidification or wetting of the cathode can be adjusted.

[0020] When the pressure in the cathode chamber is changed over time, a pressure peak occurs on the cathode side through a change in the second range, and this pressure peak corresponds to the molar transport rate H 2O / H 2 may have an impact on

[0021] Such a fast pressure change can be achieved, for example, by means of an electrically controllable throttle valve, advantageously located downstream of the water separator and on the cathode side of the electrolysis cell or stack.

[0022] Furthermore, the solution proposed according to the invention makes it possible to influence the temperature level on the cathode side by regulating the water transport from the anode side to the cathode side.

[0023] The embodiments of the invention will be explained in more detail on the basis of the drawings and the following description. [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 shows an electrolytic cell with a cathode chamber operated dry and an anode chamber equipped with a pumped circuit; [Diagram 2] 1 is a graph plotting the relationship between the molar ratio of water to hydrogen and the pressure difference between the anode and cathode chambers. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] In the following description of embodiments of the invention, identical or similar components are provided with the same reference numerals, and individual repeated description of these components is omitted. The figures only show the subject matter of the invention in a schematic manner.

[0026] 1 shows an electrolysis cell 10 with an electrolysis stack 12. The electrolysis cell 10 comprises an anode chamber 14 and a cathode chamber 16, which are separated from each other by a diaphragm 22 that is permeable to OH ions, preferably configured as a polymer diaphragm 23. In the cathode chamber 16, which is temporarily operated dry, gaseous hydrogen is discharged at a hydrogen outlet 20 into a second water separator 30.

[0027] In the anode chamber 14 of the electrolyser 10, the gaseous oxygen formed in the anode chamber 14 is discharged in an oxygen outlet 18 into a first water separator 26 which is part of a pump circuit 24. The pump circuit 24, in which the first water separator 26 is integrated, further comprises a pump 28, via which the water circulating in the pump circuit 24 reaches again into the anode chamber 14 in a water supply 32. The opening of the water supply 32 is located in the bottom 34 of the anode chamber 14. The pump circuit 24 is operated substantially at a pressure level of approximately 1 bar in order to reduce the necessary sealing costs.

[0028] 1, downstream of the second water separator 30 on the cathode side is a throttle valve 46. The throttle valve 46 is preferably located at a mounting position 48 at the outlet of the second water separator 30, downstream of the cathode chamber 16.

[0029] For the case in which the electrolyser 10 is operated with water according to the representation in FIG. 1, fully demineralized water with a conductivity of <0.1 μS is preferably used. On the other hand, there is also the possibility of operating the electrolyser 10 according to FIG. 1 as an alkaline electrolyser 10. For the latter case, instead of fully demineralized water, a KOH solution is used as the circulating liquid medium in the anode chamber 14. The electrolysis stack 12 shown in FIG. 1 comprises an anode chamber 14 for producing oxygen and a cathode chamber 16 for producing hydrogen. The anode chamber 14 is part of a pump circuit 24 with a first water separator 26, so that oxygen can be separated and water can be transported again via a pump 28 into the anode chamber 14. On the cathode side, hydrogen is formed in the cathode chamber 16, which is likewise guided through a water separator, in this case a second water separator 30, so that the hydrogen obtained is as dry as possible. Regulating the pressure on the hydrogen side via a throttle valve 46, e.g. to 30 bar, reduces the compression energy costs of the downstream compression unit compared to the case where the aim is to store hydrogen under pressure, which is usually the case due to the low molar weight of hydrogen.

[0030] To improve the functioning of the electrolyser 10 proposed according to the invention with fully demineralized water or alkaline with KOH solution, it is necessary to temporarily wet or humidify the cathode side, i.e. the cathode chamber 16. For this purpose (see the diagram in FIG. 2), at a specific operating point given by a specific temperature and a specific current density, for example 45° C. and 0.8 A / cm 2 At an operating point of 0.8 A / cm 2 , it is observed that water molecules are entrained from the anode chamber 14 through the diaphragm 22 into the cathode chamber 16 during ion transport by the diaphragm 22, which is provided as a polymer diaphragm 23. At a particular operating point 38, for example, at a temperature of 45° C. and a current of 0.8 A / cm 2 in FIG. 2 At the operating point 38 as shown for a current density of H 2 O / H 2 A decrease 50 in the molar ratio 40 of the cathode chamber 16 is observed. The water diffusion from the anode chamber 14 to the cathode chamber 16 can be influenced by the variation of the pressure difference 42 between the cathode chamber 16 and the anode chamber 14. Depending on the magnitude of the pressure difference 42, the water transport into the cathode chamber 16 can therefore be influenced and thus the humidification or wetting of the cathode can be adjusted. Therefore, the proposed method according to the invention, utilizing the drainage water from the anode chamber 14 into the cathode chamber 16, can achieve the humidification or wetting of the components of the cathode chamber 16 at specific intervals, in particular the diaphragm 22 formed as a polymer diaphragm 23.

[0031] This temporary wetting or humidification of the polymer diaphragm 23 on the cathode side makes it possible to avoid local overheating during the fast start-up process of the electrolyzer 10. By the method proposed according to the invention, i.e. by utilizing water diffusion from the anode chamber 14 into the cathode chamber 16, a temperature equalization or splitting of OH ions can be achieved, so that a homogenous functioning behavior can be achieved and local hot spots associated with the occurrence of overheating, which leads to quality degradation in the medium term and diaphragm damage in the short term, can be prevented.

[0032] As can be seen from the diagram in FIG. 2, as the pressure difference 42 between the cathode and the anode increases, the molar ratio H 2 O / H 2 40 or how the mole fraction reduction 50 is accomplished.

[0033] The solution proposed according to the invention, in particular by means of a throttle valve 46 arranged downstream of the cathode chamber 16, allows the cathode pressure p prevailing in the cathode chamber 16 to be reduced. K In the case of variations in the second range, it is possible to achieve pressure peaks occurring on the cathode side, which in turn correspond to the above-mentioned molar ratio H 2 O / H 2 40. Pressure variations of this kind are achieved by suitable control, in particular of an electrically controllable throttle valve 46, which is downstream of the second water separator 30 on the cathode side.

[0034] The invention is not limited to the embodiments described herein and the aspects set forth herein, but rather many variations within the scope of the art are possible within the scope of the claims. [Explanation of symbols]

[0035] 10 Electrolytic cell 14 Anode chamber 16 Cathode chamber 22 Diaphragm 40 Molar transport rate 42 Pressure difference between the anode and cathode chambers 46 Throttle valve

Claims

1. A method for operating an electrolyser (10) for producing hydrogen and oxygen, using a diaphragm (22) permeable to OH ions and separating an anode compartment (14) and a cathode compartment (16) from each other, comprising the following method steps: a) a method step of temporarily dry operating said cathode chamber (16); b) a method step of temporarily diffusing water molecules from the anode chamber (14) into the cathode chamber (16) by means of the diaphragm (22); c) varying the pressure difference (42) between the cathode chamber (16) and the anode chamber (14) by means of a throttle valve (46); d) the method step of regulating the humidification / wetting of said cathode chamber (16) by adjusting a predetermined pressure differential (42); A method comprising:

2. 2. A method according to claim 1, characterised by controlled water diffusion, preferably fully demineralised water, from the anode chamber (14) into the cathode chamber (16).

3. 3. The method according to claims 1 and 2, characterized in that for controlled water diffusion, fully demineralized water with a conductivity of <0.1 μS is used.

4. 2. The method according to claim 1, characterized in that in alkaline operation of the electrolytic cell (10), a KOH solution is used.

5. The pressure p in the cathode chamber (16) Ｋ When the molar transport rate (40) H ２ O / H ２ The method according to claims 1 to 4, characterized in that

6. The pressure p in the cathode chamber (16) Ｋ 6. The method of claim 5, further comprising: reducing the flow rate of the throttle valve (46) in the range of seconds by correspondingly controlling the throttle valve (46).

7. The pressure p in the cathode chamber (16) Ｋ The fluctuation of the molar transport rate H ２ O / H ２ 7. The method according to claims 5 and 6, characterized in that it affects (40).

8. 2. The method of claim 1, further comprising influencing the temperature in the cathode chamber (16) via the water transport into the cathode chamber (16).

9. 9. The method according to claim 1, wherein the electrolytic cell (10) or alkaline electrolytic cell (10) is operated with temporary water transport through a diaphragm (22) for humidification / wetting of the cathode chamber (16).

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