Method for electrolysis of water at variable current density

Abstract

The present invention relates to a method for alkaline water electrolysis in an electrolysis device and an electrolysis device configured for carrying out said method, the electrolysis device comprising at least one electrolysis cell having an anode compartment provided with an anode, a cathode compartment provided with a cathode and a separator arranged between the anode compartment and the cathode compartment, the method comprising selecting a threshold current density to limit migration of hydrogen generated in the cathode compartment through the separator to the anode compartment at operating current densities up to the threshold current density and to limit migration of oxygen generated in the anode compartment through the separator to the cathode compartment at operating current densities above the threshold current density.

Description

[Technical Field]

[0001] The present invention relates to a method for electrolyzing water with a variable current density, particularly a method for alkaline water electrolysis. [Background technology]

[0002] Global efforts are underway to replace fossil fuels with renewable energy in order to reduce carbon dioxide emissions associated with energy production through the combustion of fossil fuels. Many renewable energy sources, such as wind and solar energy, are typically intermittent or fluctuating, often resulting in an imbalance between available renewable energy and the energy required by homes and industries at a given time. Hydrogen gas is used in a variety of industrial applications, including the generation of electrical energy in fuel cells. At an industrial scale, hydrogen is mainly produced from natural gas through processes such as steam reforming, and therefore, this hydrogen and carbon dioxide are not very suitable for reducing the "carbon footprint" in energy production. However, when using renewable energy sources as an alternative to steam reforming of natural gas, hydrogen can also be produced by electrolysis of water, which does not involve carbon emissions. Therefore, hydrogen gas obtained by electrolysis of water using renewable energy is expected to play a significant role as a hydrogen source in many industrial processes and as a storage medium for renewable energy. Alkaline water electrolysis is a particularly promising variation of water electrolysis for industrial-scale hydrogen production because it does not require expensive precious metal-based catalysts. Alkaline water electrolysis is typically performed in an electrochemical cell at a temperature range of 40–90°C, with an anode compartment containing the anode and a cathode compartment containing the cathode separated by a suitable separator, such as a diaphragm or membrane. An alkaline aqueous solution with a pH higher than 7, such as a KOH aqueous solution, is supplied to the cell as the electrolyte, and a current flows between the electrode in the cathode compartment and the electrode in the anode compartment, i.e., between the cathode and the anode, with a potential difference (cell voltage) of typically 1.6–2.4V. Under these conditions, water is separated into its constituent elements, with gaseous hydrogen produced at the cathode and gaseous oxygen at the anode. In an alkaline medium, the hydrogen evolution reaction at the cathode can be summarized as follows: 4 H2O + 4 e - → 4 OH - + 2 H2(1)

[0003] The hydroxide ions generated in the cathode compartment move to the anode compartment through the separator, and the oxygen evolution reaction at the anode can be summarized as follows: 4 OH - → O2 + 2 H2O + 4 e- (2)

[0004] In industrial processes, multiple cells are typically arranged in a series stack configuration, i.e., a large number of electrolytic cells connected in series. The mixture of electrolyte and gas generated in each compartment is removed from each cell, the gaseous product is separated from the liquid electrolyte, and the liquid electrolyte is recycled back into the cell compartments of the stack, thus enabling the cell / stack to operate continuously. As can be seen from equations (1) and (2) above, twice the molar equivalent amount of water is generated in the cathode compartment than in the anode compartment. Therefore, to maintain the electrolyte balance in both compartments, the anode solution (electrolyte in the anode compartment) and the cathode solution (electrolyte in the cathode compartment) are usually mixed together after gas-liquid separation and then sent back to the electrolytic device.

[0005] Industrial-scale stacks for alkaline water electrolysis, containing numerous electrochemical / electrolytic cells individually connected in series, are optimized in terms of cell voltage and hydrogen purity for operation at nominal loads with high current densities. Especially when relying on renewable energy, the availability and / or cost of electrical energy fluctuates during operation, so it is desirable to be able to perform electrolysis even at current densities far below the nominal load for which the system was designed.

[0006] The separator used in alkaline water electrolysis remains permeable to both hydrogen and oxygen gases, so during operation, hydrogen diffusion occurs from the cathode to the anode compartment and oxygen diffusion occurs from the anode to the cathode compartment based on the concentration gradient. To meet current safety requirements, the electrolysis apparatus must be operated so that the oxygen-to-hydrogen (OTH) concentration in the cathode compartment and the hydrogen-to-oxygen (HTO) concentration in the anode compartment do not exceed 4 vol% (preferably 2 vol%) (in either case, the lower explosion limit of the mixture is approximately 4 vol%). During nominal high current density operation, residual gas moving from the opposite compartment through the separator is considerably diluted by the large amount of gas produced at the anode and cathode, respectively, so meeting these safety requirements is generally not a problem. However, at low current densities and with little "desired gas" produced at each electrode, the gas moving from the opposite compartment can lead to HTO or OTH ratios exceeding the above safety thresholds.

[0007] European Patent Application No. 3604642 (EP 3604642 A1) and Australian Patent Application No. 2019374584 (AU 2019374584 A1) (based on International Application WO 2020 / 095664 A1) describe a water electrolysis system and method for producing hydrogen using an alkaline water electrolysis apparatus equipped with a gas-liquid separation tank, the separation tank having an inlet for a mixture of gas and electrolyte, the inlet being positioned above the liquid surface of the electrolyte in the separation tank. By using separate separation tanks for oxygen and hydrogen, respectively, and by appropriately controlling the liquid levels in the separation tanks, it is possible to improve the purity of the hydrogen produced. Furthermore, a dedicated purification apparatus can be used.

[0008] U.S. Patent Application No. 2013 / 337368 (US 2013 / 337368 A1) describes a separator for a water electrolysis cell comprising an anode separator element and a cathode separator element, which are spaced apart from each other to form an internal bypass channel. By supplying an electrolyte that does not contain dissolved gases to this bypass channel, an anode compartment and a backwash flow to the anode compartment are established, and the movement of each gas species through the separator can be reduced.

[0009] Therefore, the object of the present invention is to provide a method for electrolyzing water using alkaline water electrolysis that optimizes performance in terms of cell voltage and gas purity over a wide range of current densities that can change during operation.

[0010] [Overview of the prefecture] Various aspects of the present invention are described in the appended claims.

[0011] The present invention relates to a method for electrolyzing water using an electrolytic apparatus, wherein the electrolytic apparatus comprises at least one electrolytic cell, the electrolytic cell having an anode compartment provided with an anode, a cathode compartment provided with a cathode, and a separator disposed between the anode compartment and the cathode compartment, and the method is as follows: To supply alkaline anode solution to the anode section and alkaline cathode solution to the cathode section, By applying a variable current to an electrolytic cell at a variable operating current density, water electrolysis is performed, generating hydrogen gas at the cathode and oxygen gas at the anode. Discharge from the cathode section, which contains a mixture of cathode liquid and hydrogen gas, and from the anode section, which contains a mixture of anode liquid and oxygen gas. The process involves separating hydrogen gas from the cathode solution and oxygen gas from the anode solution. Includes, At operating current densities up to the threshold current density, the movement of hydrogen generated in the cathode section to the anode section through the separator is restricted, and At operating current densities exceeding the threshold current density, the movement of oxygen generated in the anode compartment to the cathode compartment through the separator is restricted. A threshold current density is selected, and water electrolysis is performed.

[0012] When the electrolysis of water is performed with a variable current density, for example, due to the use of renewable energy as an electrical energy source, the current density will vary between the minimum operating current density and the maximum operating current density. Therefore, the present invention proposes defining the threshold current density within the range of operating current density (i.e., above the minimum operating current density of the electrolytic device and below the maximum operating current density), thereby subdividing the operating current density range into two ranges: a low current density range from the minimum operating current density to the threshold current density, and a high current density range above the threshold current density and up to the maximum current density. The present invention further proposes that the movement of gas through the separator should be actively influenced in different ways in each of the low and high current density ranges, and in particular, that such movement should be restricted. In the low current density range, safety considerations are especially important, and the present invention proposes actively restricting the movement of hydrogen generated in the cathode section through the separator to the anode section compared to movement based purely on the concentration gradient, in order to avoid the HTO ratio in the anode section reaching critical levels. In the high current density range, safety considerations are less critical, and operation can be optimized in terms of the purity of the hydrogen produced. Therefore, this invention proposes to actively restrict, as much as possible, the movement of oxygen generated in the anode section to the cathode section through the separator, compared to movement based purely on the concentration gradient.

[0013] Actively restricting gas movement through the separator during operation can be achieved by various means. For example, the permeability of oxygen and hydrogen through the separator is affected by the mechanical and electrochemical properties of the separator. For instance, it is possible to change the pore size distribution of the separator by applying a mechanical action to it. However, such solutions tend to increase costs due to the increased complexity of the cell design. Therefore, it is preferable to leave the separator unchanged. Rather, the inventors have observed that, in addition to gas movement through the separator based on the concentration gradient, gas movement can be actively influenced by making the electrolyte pressure in one compartment higher than the pressure in the other compartment. Therefore, according to a preferred embodiment of the present invention, at operating current densities up to the threshold current density, the liquid pressure on the separator in the anode compartment is higher than the liquid pressure on the separator in the cathode compartment, thus restricting the movement of hydrogen from the cathode compartment to the anode compartment.

[0014] Similarly, at operating current densities exceeding the threshold current density, the hydraulic pressure on the separator in the cathode compartment is higher than the hydraulic pressure on the separator in the anode compartment, restricting the movement of oxygen from the anode compartment to the cathode compartment.

[0015] In the context of this invention, differential pressure is measured as the difference between the hydraulic pressure in the cathode compartment and the hydraulic pressure in the anode compartment. Therefore, a positive differential pressure means that the pressure in the cathode compartment is higher than the pressure in the anode compartment, while a negative differential pressure means that the pressure in the anode compartment is higher than the pressure in the cathode compartment.

[0016] The differential pressure between the cathode and anode compartments can be generated by various means. For example, electrolyte recirculation is typically achieved using cathode and anode fluid pumps. To achieve the desired fluid pressure, adjustable valves (adjustable according to current density) can be used at the outlets of the cathode and anode compartments. Alternatively, the fluid levels in the hydrogen and oxygen separation vessels can be adjusted to influence the fluid pressure inside the anode and cathode compartments via their respective discharge lines.

[0017] In one embodiment of the present invention, mechanical pressure is applied to the cathode to press it against the separator, and subsequently the separator against the anode. This embodiment is preferred in conventional zero-gap cell configurations, where the cathode elastic element ensures that the electrode is firmly pressed against the separator, resulting in minimized cell voltage and reduced energy consumption. A zero-gap cell configuration can also be achieved by using an elastic element in the anode compartment to press the anode against the separator, and then the separator against the cathode. However, in the context of the present invention, the use of a cathode elastic element is preferred. Using a cathode elastic element allows for efficient current transfer between the cathode and the cathode current collector or bipolar plate without the need to weld the elastic element to either the cathode or the current collector / bipolar plate. In particular, when a negative differential pressure is applied, the presence of an elastic element in the cathode compartment can help prevent the separator from detaching from the anode electrode. If the separator detaches from the anode electrode, the cell voltage increases, which impairs the cell's performance.

[0018] In one embodiment of the method according to the present invention, the ratio of hydrogen to oxygen (HTO) in the anode compartment is kept below 4 volume percent. Preferably, hydrogen movement is restricted to a level where the HTO ratio in the anode compartment is below 2 volume percent, more preferably below 1.5 volume percent.

[0019] In one embodiment of the method according to the present invention, the ratio of oxygen to hydrogen (OTH) in the cathode compartment is kept below 4% by volume. Preferably, the transfer of oxygen is limited to a level well below 2% by volume, more preferably well below 1.5% by volume, of the OTH ratio in the cathode compartment.

[0020] After separating hydrogen gas from the cathode liquid and oxygen gas from the anode liquid, the cathode liquid is recycled to the cathode compartment and the anode liquid is recycled to the anode compartment. In alkaline water electrolysis, usually the same electrolyte is used as the anode liquid and the cathode liquid, and the alkaline concentration in the electrolyte remains the same in each of the anode compartment and the cathode compartment. However, as described above, since the amounts of water generated or consumed in the anode compartment and the cathode compartment are different, if the alkaline concentrations in both compartments are unbalanced, the operating time will be too long.

[0021] When the same electrolyte is used as both the anode and cathode solution, such imbalance can be avoided by mixing the anode and cathode solutions after hydrogen / oxygen gas separation and recycling the mixture to the cathode and anode sections, respectively. When the electrolytic device is operated at a high current density, the risk of imbalance between the cathode and anode solutions increases, so recycling the mixture of cathode and anode solutions is more effective at high current densities than at low current densities. Therefore, in one embodiment of the present invention, after separating hydrogen gas from the cathode solution and oxygen gas from the anode solution, the cathode and anode solutions are recycled separately at operating current densities up to the threshold current density, i.e., the cathode solution is recycled to the cathode section and the anode solution is recycled to the anode section, and at current densities exceeding the threshold current density, the cathode and anode solutions are mixed at least intermittently and the mixture is recycled to the cathode and anode sections, respectively. Therefore, at operating current densities exceeding the threshold current density, the anode and cathode liquids can be continuously mixed, and the mixture is divided into feed recycled to the cathode section and feed recycled to the anode section. According to this embodiment, at operating current densities exceeding the threshold current density, if an imbalance between the cathode and anode liquids is detected, the mixing of the recycled cathode and anode liquids can be performed only intermittently, as long as the imbalance is detected. This operating mode ensures optimized gas purity during low current density operation by keeping the loops between the anode and cathode liquids separated. The resulting accumulation of alkali concentration imbalance is considered acceptable. Low current density operation is usually an exception to the high current density operation mode and is associated with relatively low water production and consumption, and therefore the degree of imbalance is less compared to the high current density mode. During high current density operation, a balanced and optimal alkali concentration is maintained in both the anode and cathode sections. Gas contamination resulting from the mixing of anode and cathode liquids is considered acceptable. This is because, at high current densities, the gas production rate is high, and undesirable gases are significantly diluted.

[0022] Preferably, the operating current density in the method of the present invention is in the range up to 25 kA / m 2 In principle, the minimum operating current density can be very low (as long as it exceeds 0 kA / m 2 ), and the maximum operating current density is about 25 kA / m 2 Preferably, the operating current density is in the range of 1 to 20 kA / m 2 .

[0023] Within the range of the operating current density, the threshold current density that separates the low-current density operating mode and the high-current density operating mode is selected within the range of 2 to 10 kA / m 2 , preferably within the range of 3 to 6 kA / m 2 . For example, the threshold current density can be 4 kA / m 2 .

[0024] Preferably, the alkaline water electrolysis according to the method of the present invention is carried out at an absolute liquid pressure in the range up to 100 bar, usually in the range up to 50 bar.

[0025] According to one embodiment of the present invention, the liquid pressure difference between the anode compartment and the cathode compartment is maintained in the range of -100 to 100 mbar, preferably in the range of -50 to 50 mbar, for example in the range of +20 to -20 mbar. The minimum liquid pressure difference as an absolute value (i.e., not considering the positive or negative sign) is 1 mbar, preferably 10 mbar.

[0026] In one embodiment, the separator is a diaphragm.

[0027] Preferably, the alkaline anode liquid and the alkaline cathode liquid contain an aqueous solution of an alkali metal hydroxide. Particularly preferably, the alkali metal hydroxide is selected from sodium hydroxide or potassium hydroxide.

[0028] The present invention also relates to an electrolytic apparatus for carrying out the method according to the present invention. The electrolytic apparatus is configured such that, at operating current densities up to a threshold current density, hydrogen generated in the cathode section moves to the anode section through the separator, and at operating current densities exceeding the threshold current density, oxygen generated in the anode section moves to the cathode section through the separator, which is done, for example, by establishing an appropriate differential pressure between the cathode section and the anode section of the cell.

[0029] The present invention will be described in detail below in relation to specific preferred embodiments and corresponding drawings. [Brief explanation of the drawing]

[0030] [Figure 1] This is a schematic exploded view of the components of an electrolytic cell in an electrolytic apparatus suitable for performing the method according to the present invention. [Figure 2] This is a schematic diagram of an electrolytic apparatus suitable for carrying out the method according to the present invention. [Modes for carrying out the invention]

[0031] Figure 1 shows a schematic exploded view of the main components of an electrolytic cell, which form part of the electrolytic cell stack of an alkaline water electrolyzer usable to carry out the method of the present invention. The electrolyzer includes a cell stack, which contains a number of individual electrolytic cells 10 electrically connected in series. Figure 1 shows only the components of one electrolytic cell 10. In this stack, the continuity of components is repeated as shown by the black dots. Each electrolytic cell 10 is separated by a bipolar plate 11 and comprises an anode compartment 12 and a cathode compartment 13, which are separated by a diaphragm 14. The anode compartment 12 comprises a circumferential anode frame 15, which houses an anode current collector 16 and an anode (anode electrode) 17. The anode current collector 16 provides mechanical and electrical connections between the anode side of the bipolar plate 11 and the anode 17. The anode current collector 16 may be an elastic element, but is preferably a rigid element, and may be welded to the anode side of the bipolar plate 11 and to the back side of the anode 17. The cathode compartment 13 comprises a circumferential cathode frame 18, which houses the cathode current collector 19 and the cathode (cathode electrode) 20. The cathode current collector 19 is preferably an elastic element that presses the cathode 20 against the diaphragm 14.

[0032] The cell components further include channels for supplying alkaline electrolytes to the cathode and anode compartments (only one channel 21 is visible in Figure 1), and fluid channels 22 and 23 for discharging a mixture of hydrogen / cathode solution and a mixture of oxygen / anodic solution, respectively. By using two channels for electrolyte supply, one channel 21 can supply the anode compartment 12 of the stack, and the other channel (not visible in Figure 1) can supply the cathode compartment 13 of the stack. After leaving the stack, the electrolytes can be recirculated separately through fluid channels 22 and 23, or remixed before re-entering the stack of cell 10. As will be described in more detail below, this establishes separate recirculation modes depending on the actual operating conditions.

[0033] The elements of the stack may further include openings (not shown in Figure 1) for tie rods to clamp various elements together to form an assembled stack. Depending on the specific application, the stack may contain a small number of electrolytic cells (e.g., 5 to 20 cells) to a large number of cells (e.g., more than 100 cells).

[0034] To visualize the operating scheme of the alkaline water electrolysis apparatus according to the present invention, Figure 2 shows a schematic diagram of an electrolysis apparatus 24 suitable for performing the method according to the present invention. The electrolysis apparatus 24 comprises a stack 25, which may have a number of electrolytic cells 10 (e.g., the electrolytic cells shown in Figure 1). The electrical energy source is indicated by reference numeral 26. The electrical energy source can be any kind of electrical energy that provides direct current (DC) at the minimum voltage required to perform water electrolysis. The DC source of electrical energy supplied to the electrolysis apparatus 24 can be derived from an inherently AC or DC energy source. Typically, a transformer and / or rectifier is provided to obtain the desired DC source. Preferably, the electrical energy source is a source that provides a variable current, such as a renewable energy source. The variable current in the sense of the present invention is not alternating current (AC) but direct current (DC) with an intensity that can change over time, but the intensity is such that the current density at the electrode level can fluctuate during operation. The current is supplied to the stack 25 via wires 27, 28. During operation, the current density j is continuously or periodically monitored by an appropriate sensor 29, and the control unit 30 sets the measured current density to a predetermined threshold current density j. T Compare this to the low current density operating mode (i.e., threshold current density j T At current densities up to (j), the movement of hydrogen generated in the cathode compartment through the separator to the anode compartment establishes a higher oxygen pressure in the anode compartment than the hydrogen pressure in the cathode compartment (Δp = p H2 - p O2This is actively restricted by establishing a negative differential pressure Δp between the cathode and anode sections of the stack (< 0). This can be achieved, for example, by controlling appropriate valves 31, 32 located at the gas outlets, including fluid channels 22, 23, via a control unit 30 (Figure 1), where fluid channels 22, 23 discharge to the initial portions of recirculation lines 22a, 23a for the cathode and anode liquids, respectively. In this way, hydrogen movement is restricted and optimal oxygen purity is ensured. This operating mode may involve a slight penalty to production capacity when high H2 purity is required. This is because the initial hydrogen purity is reduced by the oxygen moving into the cathode section, due to some of the oxygen moving into the cathode section, which can be eliminated in subsequent hydrogen purification steps by combining hydrogen and oxygen using a catalyst, but this comes with a slight loss of production capacity. In this low current density scheme, a dual circulation mode for the electrolyte can be established by maintaining separate circulation paths for the cathode liquid (i.e., the electrolyte supplied to the cathode compartment) and the anode liquid (i.e., the electrolyte supplied to the anode compartment). The associated electrolyte imbalance between the cathode and anode compartments is acceptable in this low current regime because, at low current densities, it takes a long time for the imbalance to be established, and the steady state of equilibrium is less imbalanced than at high current densities. However, in the dual circulation regime, optimal gas purity is maintained because the anode and cathode liquids are kept separate.

[0035] Figure 2 schematically illustrates the recirculation of the anode and cathode fluids by recirculation lines having line sections 22a, 22b, 22c, and 22d, and by recirculation lines having line sections 23a, 23b, 23c, and 23d, to which the respective electrolyte channels 18 and 19 (Figure 1) are connected. Pumps, preferably separate pumps 33 and 34 for the cathode and anode fluids, may be provided to bring about recirculation.

[0036] In the example shown in Figure 2, the hydrogen / cathode liquid mixture and the oxygen / anode liquid mixture, discharged from the stack via lines 22a and 23a respectively, are supplied to gas-liquid separators 35 and 36. From these gas-liquid separators, hydrogen is discharged via line 37 and oxygen via line 38. The cathode liquid from which hydrogen has been lost is discharged via line 22b of the cathode liquid recirculation line, while the anode liquid from which oxygen has been lost is discharged via line 23b of the anode liquid recirculation line.

[0037] Current density exceeding the threshold current density (j>j T In this configuration, a positive pressure difference is established between the cathode and anode compartments. In this way, the relatively high hydrogen pressure in the cathode compartment becomes higher than the oxygen pressure in the anode compartment, thereby actively restricting the movement of oxygen from the anode to the cathode compartment (Δp = p H2 - p O2 > 0). In this operating mode, optimal hydrogen purity is maintained, but this may again be associated with a slight penalty in production capacity, which is indicated by hydrogen loss associated with hydrogen entering the anode compartment across the diaphragm. The electrolyte concentrations in the cathode and anode compartments can be measured, for example, by conductive sensors 39, 40 located at the inlets 41, 42 of the electrolyte channels for the cathode and anode liquids (in Figure 1, the electrolyte channel for the cathode liquid is indicated as "21," and the electrolyte channel for the anode liquid is not visible in Figure 1), where the line portions 22d, 23d of the recirculation lines for the cathode and anode liquids respectively return to the stack 25. Below a certain threshold for conductivity imbalance, a dual circulation mode similar to low current density may be maintained to maintain optimal gas purity. Above a certain threshold, a mixed circulation mode can be established, in which case the cathode and anode liquids are mixed before being recirculated to the stack. In certain embodiments of the present invention, under high current density conditions, the mixed circulation mode may be the only circulation mode.

[0038] As shown in Figure 2, a mixed circulation mode can be established by appropriate valves 43, 44, 45, and 46, thereby ensuring that the anode and cathode fluids from the stack 25 are supplied to the mixing chamber 47 via lines 22e and 23e, and then recirculated to the stack via 22f and 23f, and to the remaining portions 22d and 23d of the recirculation lines for the cathode and anode fluids, respectively.

[0039] In dual circulation mode, the mixing chamber 47 is bridged via the line sections 22c and 23c of the cathode liquid and anode liquid recirculation lines.

[0040] In high current density modes, oxygen production in the anode compartment is sufficiently high to ensure that the HTO ratio remains below a safety threshold. In low current density modes, the HTO ratio in the anode compartment is kept low because the net hydrogen flux from cathode to anode is minimized by the differential pressure Δp. That is, the pure diffusion flux of hydrogen from cathode to anode is mitigated by the convection flux from anode to cathode (mainly oxygen, but also containing hydrogen). A moderate increase in the OTH ratio in the cathode compartment is acceptable because OTH is less critical than HTO for safety purposes. In a preferred embodiment of the present invention, an elastic element (see Figure 1) can be provided in the cathode compartment to ensure that the zero-gap cell configuration can be maintained even with a moderate negative differential pressure Δp.

[0041] It should be understood that the above description of exemplary embodiments is not exhaustive and is intended only as an illustration. Those skilled in the art will be able to make certain additions, deletions, and / or modifications to the embodiments covered by the disclosure, without departing from the spirit or scope of the disclosure.

Claims

1. A method for electrolyzing water using an electrolytic apparatus, wherein the electrolytic apparatus comprises at least one electrolytic cell, the electrolytic cell having an anode compartment provided with an anode, a cathode compartment provided with a cathode, and a separator disposed between the anode compartment and the cathode compartment, and the method is To supply alkaline anode solution to the anode compartment and alkaline cathode solution to the cathode compartment, By applying a variable current to the electrolytic cell at a variable operating current density, water electrolysis is performed, generating hydrogen gas at the cathode and oxygen gas at the anode. Discharge the mixture of the cathode liquid and the hydrogen gas from the cathode compartment, and discharge the mixture of the anode liquid and the oxygen gas from the anode compartment. To separate the hydrogen gas from the cathode liquid and the oxygen gas from the anode liquid, Includes, At operating current densities up to the threshold current density, the movement of hydrogen generated in the cathode section to the anode section through the separator is restricted by applying a higher hydraulic pressure to the separator in the anode section than the hydraulic pressure applied to the separator in the cathode section, and At operating current densities exceeding the threshold current density, the movement of oxygen generated in the anode section to the cathode section through the separator is restricted by applying a higher hydraulic pressure to the separator in the cathode section than the hydraulic pressure applied to the separator in the anode section. A method comprising selecting the threshold current density and performing water electrolysis.

2. The method according to claim 1, wherein mechanical pressure is applied to the cathode to press the cathode against the separator, or to the anode to press the anode against the separator.

3. The method according to claim 1, wherein the ratio of hydrogen to oxygen in the anode compartment is kept below 4 volume percent.

4. The method according to claim 1, wherein the ratio of oxygen to hydrogen in the cathode compartment is kept below 4 volume percent.

5. The method according to claim 1, wherein the hydrogen gas is separated from the cathode liquid and the oxygen gas is separated from the anode liquid, the cathode liquid is recycled to the cathode compartment and the anode liquid is recycled to the anode compartment.

6. After separating the hydrogen gas from the cathode solution and the oxygen gas from the anode solution, At operating current densities up to the threshold current density, the cathode fluid is recycled to the cathode section, and the anode fluid is recycled to the anode section. The method according to claim 1, wherein at operating current densities exceeding the threshold current density, the cathode liquid and the anode liquid are mixed at least intermittently, and the mixture is recycled to the cathode section and the anode section.

7. The operating current density is 25 kA / m³. ２ The method according to claim 1, which is within the range up to [a certain point].

8. The threshold current density is 2 to 10 kA / m ２ The method according to claim 7, selected within the range.

9. The method according to claim 1, wherein the water electrolysis is performed at an absolute hydraulic pressure in the range of up to 100 bar.

10. The method according to claim 9, wherein the hydraulic pressure difference between the anode section and the cathode section is maintained in the range of -100 to 100 mbar.

11. The method according to claim 1, wherein the separator is a diaphragm.

12. The method according to claim 1, wherein the alkaline anode solution and the alkaline cathode solution contain an aqueous solution of an alkali metal hydroxide.

13. The method according to claim 12, wherein the alkali metal hydroxide is selected from sodium hydroxide or potassium hydroxide.

14. An electrolytic apparatus for performing electrolysis of water, the electrolytic apparatus comprising at least one electrolytic cell, the electrolytic cell having an anode section provided with an anode, a cathode section provided with a cathode, and a separator disposed between the anode section and the cathode section, and configured for performing the method according to any one of claims 1 to 13.

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