Alkaline water electrolysis method and alkaline water electrolyzer

Abstract

An alkaline water electrolyzer (200) comprising an electronic controller (Cont), a stack of electrolysis cells (Stck), each electrolysis cell comprising an anode and a cathode, the electrolyzer configured to contain an electrolyte made of an anolyte (AnKOH) and a catholyte (CathKOH), the electrolyzer comprising a system (Sys) controlled by the electronic controller (Cont), the system is configured to maintain a concentration of an impurity in the electrolyte within a target range by measuring a characteristic indicative of the concentration of the impurity in the electrolyte and adding an amount of the impurity to the electrolyte in response to the measured concentration of the impurity.

Description

Technical Field

[0001] The technical field of the present invention is the technical field of alkaline water electrolysis for hydrogen production. Background Art

[0002] Hydrogen (H2) is an energy carrier that is receiving a great deal of attention due to its potential to be produced in an environmentally friendly manner and support the emergence of low-carbon emission industrial processes and transportation modes.

[0003] For hydrogen production, electrochemical water splitting is a well-known sustainable and pollution-free method.

[0004] This method can be implemented by alkaline water electrolysis, in which the electrolysis of water is carried out by passing a direct electric current between an anode and a cathode immersed in an aqueous alkaline electrolyte.

[0005] At the cathode, electrons provided by the direct electric current react with water to produce hydrogen and hydroxide ions according to the following reaction:

[0006] 2H2O + 2e - → 2H2 + 2OH -

[0007] At the anode, hydroxide ions release their excess electrons to produce water and oxygen according to the following reaction:

[0008]

[0009] These electrodes are separated by a thin porous foil (commonly referred to as a diaphragm), which is an electronically non-conductive separator that separates the product gases and allows hydroxide ions (OH - ) to be transferred from one electrode to the other.

[0010] Ionic conductivity is provided by the aqueous alkaline electrolyte, typically a solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH).

[0011] Alkaline water electrolysis is a mature technology suitable for industrial scale. In addition, performing water electrolysis under alkaline conditions allows non-noble metal oxides such as cobalt and nickel derivatives to be used for the anode and cathode, due to their good OER (oxygen evolution reaction) activity in alkaline solutions, which provides an alternative to more expensive noble metals such as ruthenium or iridium.

[0012] Figure 1 An electrolysis unit 100 configured to split water is shown, which has a container Cont containing an aqueous alkaline electrolyte El, and a cathode Cat and an anode An immersed in the aqueous alkaline electrolyte.

[0013] The cathode and anode are separated by a diaphragm Dia and connected to a DC power supply DC, which applies a flow of current I between the anode and the cathode through the electrolyte El and the diaphragm Dia. This current I supplies electrons e to the cathode and discharges electrons e from the anode. - And discharges electrons e from the anode. - .

[0014] The diaphragm allows hydroxide ions OH - to pass through, but contains oxygen O2 and hydrogen H2 on the anode side and the cathode side of the electrolysis unit respectively, thus facilitating the collection and purification of the gases generated by the reactions detailed above.

[0015] The electrolyte on the anode side is called anolyte El An , and the electrolyte on the cathode side is called catholyte El Ca .

[0016] In this example, the electrodes (anode and cathode) are made of or coated with Raney nickel, which serves as an electrolysis catalyst. The electrolyte is made of potassium hydroxide KOH, and the diaphragm is an anion exchange membrane (AEM), such as a membrane made of made of.

[0017] Figure 1 A single cell is shown; industrial electrolyzers designed for large-scale hydrogen production typically include dozens of such cells, which are fluidly connected in parallel and electrically connected in series in the form of a so-called "stack".

[0018] In the actual operation of the electrolyzer, the electrolyte contains impurities that originate from the deionized water and fresh potassium hydroxide used to make the electrolyte itself, as well as the components (valves, pumps, pipes, gas / liquid separators...) of the hydraulic circuit used to circulate the electrolyte in the cell stack.

[0019] The two reactions detailed above are the main and expected reactions that occur during hydrogen production by water electrolysis.

[0020] However, due to the presence of a certain amount of impurities, many unintended reactions also occur, some of which are harmful to the efficiency of the entire process and the components of the electrolyzer itself (especially to the cathode).

[0021] The contamination of the electrolyte usually accumulates slowly until it is necessary to at least partially replace the electrolyte with fresh electrolyte during the periodic maintenance of the electrolyzer; this is especially true when the components (valves, pumps, pipes...) of the hydraulic circuit contain iron.

[0022] Such operations are expensive, affect sustainability because the used potassium hydroxide will be treated as chemical waste, interfere with the normal operation of the electrolyzer, and reduce the efficiency of hydrogen production due to impurities.

[0023] Object of the Invention

[0024] In view of the above problems, the applicant considered optimizing the function of the electrolytic cell with respect to the impurity concentration in the electrolyte.

[0025] Accordingly, the applicant has determined a method for alkaline water electrolysis taking into account the impurity concentration in the electrolyte, and a system designed to implement this method. Summary of the Invention

[0026] To this end, a first aspect of the present invention relates to a method for alkaline water electrolysis using an electrolytic cell comprising a stack of electrolysis units, each electrolysis unit comprising an anode and a cathode, the electrolytic cell being configured to contain an electrolyte made of anolyte and catholyte, the method comprising the steps of: measuring at least one characteristic representing the concentration of impurities in the electrolyte, and adjusting the concentration of impurities in the electrolyte based on at least one measured characteristic of the electrolyte so as to maintain the concentration of impurities within a target range by reducing the concentration of impurities in the electrolyte or delivering a certain amount of impurities to the electrolyte.

[0027] The main advantage of this method is to achieve good performance by better controlling the quality of the electrolyte.

[0028] Other advantages include enhanced sustainability, reduced electrolyte consumption, more regular operation of the electrolytic cell, and better hydrogen production operating parameters due to better control of the presence of impurities in the electrolyte and on the electrodes.

[0029] Additional non-limiting features according to the first aspect of the present invention, taken alone or in any technically feasible combination:

[0030] - The anode may comprise nickel, the impurity may be iron, and the target range may be defined as being between 0.2 mg / l and 6 mg / l;

[0031] - The step of adjusting the concentration of impurities in the electrolyte may include the step of replacing at least a portion of the electrolyte included in the electrolytic cell;

[0032] - The method may include the step of injecting a certain amount of impurities into the electrolyte based on a first one of the at least one measured characteristics;

[0033] - At the step of injecting a certain amount of impurities into the electrolyte, the certain amount of impurities may be injected into the anolyte;

[0034] - The method may include the step of diluting the electrolyte based on a second one of the at least one measured characteristics;

[0035] - The method may include the step of filtering out impurities from the electrolyte based on a second property among at least one measured property;

[0036] - The step of filtering out impurities from the electrolyte may include a sub-step of diluting the electrolyte and a sub-step of supplementing the electrolyte with an alkaline element to compensate for the dilution of the electrolyte and the loss of the alkaline element during the step of filtering out impurities from the electrolyte;

[0037] - The electrolyte may be diluted with deionized water, and the alkaline element supplementation may include adding a KOH solution to the electrolyte; and

[0038] - The step of filtering out impurities from the electrolyte may include a sub-step of passing the electrolyte through a filter membrane that filters out the impurities from the electrolyte.

[0039] A second aspect of the present invention relates to an alkaline water electrolyzer comprising an electronic controller, a stack of electrolysis units, each electrolysis unit including an anode and a cathode, the electrolyzer being configured to contain an electrolyte made of an anolyte and a catholyte, the electrolyzer further including a system controlled by the electronic controller, the system being configured to maintain the concentration of impurities in the electrolyte within a target range by measuring a property representing the concentration of impurities in the electrolyte and adding a certain amount of impurities to the electrolyte in response to the measured impurity concentration.

[0040] Additional non-limiting features according to the first aspect of the present invention, employed alone or in any technically feasible combination:

[0041] - The anode may contain nickel, the impurity may be iron, and the target range may be defined as being between 0.2 mg / l and 6 mg / l;

[0042] - The system may include: a first measuring device configured to measure a first value of a property representing the concentration of impurities in the electrolyte; and a device configured to inject a solution containing a certain amount of impurities into the electrolyte, the electronic controller further being configured to command the device to inject the solution containing a certain amount of impurities into the electrolyte based on the measured first value;

[0043] - The system may include: a second measuring device configured to measure a second value of a property representing the concentration of impurities in the electrolyte; and a dilution device configured to dilute the catholyte, the electronic controller further being configured to command a guiding device to guide the electrolyte towards the dilution device based on the measured second value; and

[0044] - The system may include: a second measuring device configured to measure a second value representing a characteristic of the concentration of impurities in the electrolyte; and a filter configured to filter out impurities from the electrolyte, the electronic controller being further configured to command a guiding device to guide the electrolyte towards the filter based on the measured second value, the guiding device being configured to guide the catholyte towards the filter based on the measured second value. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] When considered in conjunction with the accompanying drawings, many other features and advantages of the present invention will become apparent upon reading the following detailed description, in which:

[0046] - Figure 1 A conventional water electrolysis cell is shown;

[0047] - Figure 2 An electrolytic cell of a first type according to the present invention is shown;

[0048] - Figure 3 An electrolytic cell of a second type according to the present invention is shown;

[0049] - Figure 4 A first method according to the present invention is shown; and

[0050] - Figure 5 A second method according to the present invention is shown. DETAILED DESCRIPTION

[0051] As a specific application of electrolyte content control, the applicant has conducted a large number of experiments on the concentration of iron as an impurity in the electrolyte and the performance of an alkaline water electrolysis unit, mainly when the electrodes (anode and cathode) are made of or coated with Raney nickel used as an electrolysis catalyst and the electrolyte is made of potassium hydroxide KOH.

[0052] Other tests involved other anode / cathode configurations, such as pure nickel mesh / Raney nickel or pure nickel mesh / PGM-coated mesh, where PGM represents platinum group metals.

[0053] It appears that during unit operation, the iron concentration tends to decrease, regardless of the initial iron concentration in the electrolyte.

[0054] This can be explained by (i) iron binding to the NiOOH layer formed on the anode during operation and (ii) iron deposition on the cathode surface.

[0055] At the same time, it was observed that the iron content in KOH significantly affects the performance of the unit: an increase in the unit voltage during operation is associated with a concomitant decrease in the iron concentration.

[0056] In contrast, the activity of the cathode is independent of the iron content in the electrolyte.

[0057] Thus, the overpotential that appears at the anode (defined as the difference between the equilibrium potential of a given reaction and the potential at which the catalyst operates at a specific current under specific conditions) is interpreted as being positively affected by the iron incorporated in the NiOOH layer formed on the anode surface.

[0058] The exact mechanism for the performance enhancement due to iron at the anode is not fully understood, but some possible explanations are as follows: (a) The incorporation of iron into the anode increases the activity of the catalyst and improves the OER (oxygen evolution reaction) for generating gaseous oxygen O2, (b) Iron facilitates the oxidation of Ni 3+ to form highly reactive Ni 4+ ions in the NiOOH matrix, (c) The synergistic effect with iron sites, which contributes to the OER kinetics as they have the optimal bond energy for adsorbing OER intermediates; and the synergistic effect with electrochemically generated Fe 4+ ions and Ni 4+ ions, which establish a synergistic action between them under OER conditions, where Fe 4+ stabilizes the oxyl radicals formed as intermediates, and Ni 4+ ions promote O - O coupling to facilitate O2 evolution.

[0059] Such mechanisms can be found in the relevant literature.

[0060] On the other hand, iron dissolved in the KOH electrolyte has a toxic effect on the cathode.

[0061] In fact, the iron deposited on the cathode tends to coat the Raney nickel surface of the electrode with iron, which is not as effective a catalyst as Raney nickel in decomposing water H2O into hydrogen gas H2 and hydroxide ions OH - . This effect is also particularly harmful when the cathode is made of platinum group metals (ruthenium, rhodium, palladium, osmium, iridium, and platinum).

[0062] We see that in order to limit the harmful effect of iron on the cathode, the iron concentration in the electrolyte must be restricted, but the same concentration must be high enough to positively affect the reaction at the anode.

[0063] The applicant has determined a specific range of iron concentration in the electrolyte, which is designed as the target range below and satisfies two constraints: An iron concentration including between 0.25 mg / l and 6 mg / l, preferably between 0.5 mg / l and 3 mg / l, and more preferably between 0.5 mg / l and 2.5 mg / l allows the anode to effectively function as a catalyst while preventing the deterioration of the cathode function.

[0064] This mainly relates to the case where KOH is used as the electrolyte, but can be extended to other alkaline solutions, such as NaOH-based electrolytes, which have substantially the same chemical properties as KOH-based electrolytes.

[0065] Furthermore, the observation results on the applicability of the above iron concentration range for Ni-based cathodes and PGM-based cathodes (PGM stands for platinum group metals, including platinum, palladium, rhodium, ruthenium, iridium, and osmium) have been confirmed, so at least such electrodes can also be used to form the cathode.

[0066] Electrolytic Cell - First Type, Dilution

[0067] In order to maintain operating conditions favorable for hydrogen production during alkaline water electrolysis, the applicant has proposed Figure 2 the electrolyzer 200 shown, which is capable of continuously maintaining the iron concentration of the electrolyte within a given iron concentration range.

[0068] The electrolyzer includes a stack Stck of electrolysis units, each electrolysis unit being similar in its structure and functional principle to Figure 1 the unit 100 shown and being configured to function as described below.

[0069] A first hydraulic circuit is dedicated to the anolyte An KOH , the stack Stck includes an anolyte inlet In An and an anolyte outlet Out An , the anolyte is brought into the stack through this inlet, and the anolyte An KOH (KOH - liquid) and the generated oxygen O2 are retrieved from the stack through this outlet.

[0070] After leaving the stack, the KOH liquid - oxygen mixture, indicated as "O2 + KOH" in the drawing, is brought to the oxygen - liquid separator Sep O2 , the oxygen - liquid separator is configured to separate the mixture into gaseous oxygen O2 to be stored separately and anolyte An An to be re - fed to the anolyte inlet In KOH .

[0071] On the way back to the stack, as a device configured to inject iron into the electrolyte, the anolyte three - way valve V Iron is configured to inject the iron - containing solution Sol Iron into the anolyte upon receiving a control signal Sig Iron .

[0072] Upstream of the anolyte inlet In An , a measuring device Meas for measuring the characteristic representing the iron concentration in the anolyte is arrangedAn , for example, to measure the anolyte three-way valve V iron and the anolyte inlet In An of the anolyte between them (i.e., exactly upstream of the stack) to obtain the iron concentration of the anolyte entering the stack. Alternatively, the measuring device Meas An can be located at any other position considered appropriate by the practitioner.

[0073] The second hydraulic circuit is dedicated to the catholyte Cath KOH , the stack Stck includes a catholyte inlet In Cat and a catholyte outlet Out Cat , and the catholyte (designated as Cat KOH ) is brought into the stack through this inlet, and the catholyte Cath KOH (KOH liquid) and the generated hydrogen H2 are retrieved from the stack through this outlet.

[0074] After leaving the stack, the KOH liquid-hydrogen mixture indicated as "H2+KOH" in the drawing is brought to the hydrogen-liquid separator Sep H2 , and the hydrogen-liquid separator is configured to separate the mixture into gaseous hydrogen O2 to be stored separately and catholyte Cath Cat to be re-supplied to the catholyte inlet In KOH .

[0075] On the way back to the stack, a guiding three-way valve V Dir is arranged as a guiding device, which is configured to determine one of two paths to deliver the catholyte to the inlet In of the stack Cat .

[0076] The first path is the default path, and when the iron concentration is within a given iron concentration range, the catholyte usually takes this default path, and this default path directly brings the catholyte back to the catholyte inlet In Cat , that is, without further modifying the composition of the catholyte.

[0077] When it is necessary to reduce the iron concentration in the catholyte, at the guiding three-way valve V Dir receives a signal Sig Dir (this signal controls the valve to direct the catholyte towards the second path), the catholyte takes the second path.

[0078] The second path includes a dilution device V Dil , and this dilution device is configured to, according to the signal Sig DilDrain the volume Evac of the catholyte from the electrolyzer and inject a certain volume of fresh KOH solution Sol KOH into the electrolyzer to dilute the catholyte and bring the diluted catholyte to the inlet In of the stack Stck Cat . The fresh KOH solution should have an iron concentration lower than that of the recycled catholyte to effectively reduce the iron concentration of the catholyte, and preferably have a KOH concentration close to or equal to that of the recycled catholyte to keep the operating parameters of the electrolyzer constant. If the electrolyte is based on another base different from KOH, that other base must be used instead of KOH.

[0079] The dilution device is configured such that the volume of the drained catholyte and the volume of the injected KOH solution are the same in order to keep the volume of the electrolyte in the electrolyzer constant. Such a device can be made of a conventional electrically controlled valve.

[0080] Upstream of the catholyte inlet In Cat , a measuring device Meas for measuring a characteristic representing the iron concentration in the catholyte is arranged Cat , for example, to measure this characteristic of the catholyte between the second catholyte three-way valve V KOH and the catholyte inlet In Cat (i.e., exactly upstream of the stack) to obtain the iron concentration of the catholyte entering the stack.

[0081] The balance line Bal hydraulically connects the hydrogen - liquid separator Sep H2 to the oxygen - liquid separator Sep O2 to balance the internal pressure between the anode electrolyte side and the cathode electrolyte side of the cells of the stack during the operation of the electrolyzer.

[0082] A measuring device Meas for measuring a characteristic representing the iron concentration in the electrolyte is arranged, for example, connected to the balance line Bal El .

[0083] In addition, the electronic controller Cont is configured to control the three - way valves V Iron , V Filt , V DIW and V KOH via signals Sig Iron , Sig Dir , Sig DIW and Sig KOH respectively, to adjust the iron concentration in the electrolyte by causing addition or reduction of iron from the electrolyte.

[0084] The generation of these signals is based on the measuring devices Meas An , Meas Catand / or Meas El and are based on measurement results of these measuring devices, which are functionally connected to the electronic controller Cont.

[0085] The characteristic representing the iron concentration can be the concentration of iron in any given oxidation state in the electrolyte.

[0086] Element V Iron 、V Dir 、V Dil 、Meas An 、Meas Cat and Meas El form a system Sys, which is configured to maintain the iron concentration in the electrolyte within a target iron concentration range.

[0087] Depending on the operating characteristics of the electrolytic cell, it may not be necessary to inject iron into the electrolyte or filter iron out of the electrolyte; in such a case, only the relevant parts of the system Sys may be installed on the electrolytic cell.

[0088] Such a case may occur, for example, when the iron provided by the elements of the hydraulic circuit is sufficient to maintain the iron concentration in the electrolyte above the lower limit of the target range, or when binding iron to the anode and depositing iron on the cathode prevent the iron concentration from exceeding the upper limit of the target range, especially when the elements of the hydraulic circuit do not release iron or only release a small amount of iron into the electrolyte.

[0089] Meas An 、Meas Cat and Meas El are configured to measure a characteristic representing the iron concentration at a specific section of the hydraulic circuit, but the practitioner may choose other sections of the hydraulic circuit to characterize the electrolyte, for example, for the accessibility of the measuring device or to characterize the electrolyte at a section of the hydraulic circuit that is considered more suitable for characterizing the iron concentration.

[0090] The electronic controller Cont may include a system for controlling the conventional functions of the hydrogen production electrolytic cell, such as controlling pumps and valves and receiving measurements from sensors for safety issues or feedback from the operating process to control the fluid circulation in the device, and may further be configured as described above to regulate the iron concentration in the electrolyte.

[0091] Electrolytic Cell - Second Type, Filtration

[0092] By Figure 3 The second option shown by the electrolytic cell 300 only differs from the first option in the characteristics and functions of the second path taken by the catholyte on its way back to the stack. For other sections of the electrolytic cell, reference may be made to the description of the first option and Figure 2 description.

[0093] From upstream to downstream, the second path of the second option includes a first cathode electrolyte three-way valve V DIW , an iron filter Filt, and a second cathode electrolyte three-way valve V KOH , and is designed to filter out iron from the cathode electrolyte before returning the cathode electrolyte to the stack Stck.

[0094] As a device configured to be able to dilute the cathode electrolyte, the first cathode electrolyte three-way valve V DIW is configured to supply the cathode electrolyte with deionized water when receiving a control signal Sig DIW .

[0095] The iron filter Filt is configured to filter out iron from the cathode electrolyte Cath mem through a membrane Filt KOH .

[0096] For optimal filtration, it may be possible to first dilute the cathode electrolyte, for example, by adding deionized water by using the first cathode electrolyte three-way valve V DIW according to the operating specifications of the filter (pressure, range of acceptable concentration, etc.).

[0097] After filtration, a portion of the cathode electrolyte is drained by emptying Filt evac , thereby discharging the filtered iron and a certain amount of KOH.

[0098] The purified anode electrolyte obtained by filtering the cathode electrolyte by means of the filter Filt circulates towards the second cathode electrolyte three-way valve V KOH , where it is supplemented with a KOH-based solution in order to compensate for the dilution caused by the deionized water supplied at the first cathode electrolyte three-way valve V KOH and the loss of KOH during filtration at the iron filter Filt.

[0099] Finally, the purified and supplemented cathode electrolyte is conveyed back to the cathode electrolyte inlet In Cat of the stack Stck.

[0100] The components V Iron , V Dir , V DIW , V KOH , Meas An , Meas Cat and Meas El form a system Sys, which is configured to maintain the iron concentration in the electrolyte within a target iron concentration range.

[0101] Electrolysis

[0102] The electrolytic cell described above with the aid of Figure 2 and Figure 3 can be used according to the alkaline water electrolysis method 400 described below, which is designed to regulate the iron concentration in the electrolyte and is shown in Figure 4 .

[0103] At step 410, the electrolyte El, which can be considered as the sum of the anolyte An KOH and the catholyte Cat KOH , circulates within the stack Stck.

[0104] From this point of view, it is convenient to follow two logical branches, one branch dedicated to the anolyte, with steps 420 An to 460 An , and the other branch dedicated to the catholyte, with steps 420 Cat to 460 Cat .

[0105] Furthermore, even though the steps of the method are presented in a sequential form, it should be understood that they actually occur concurrently and continuously.

[0106] At step 420 An , the anolyte leaving the cell stack Stck is brought to the oxygen-liquid separator Sep O2 .

[0107] At step 430 An , a measurement is performed by the device Meas An to measure a characteristic representing the iron concentration in the anolyte (such as the concentration of oxidized iron in the anolyte just upstream of the stack Stck), and the measurement result is sent to the electronic controller Cont by any device known in the art for a similar purpose (e.g., via a cable).

[0108] Characterizing the anolyte just upstream of the stack allows for better control of the composition of the anolyte entering the stack compared to other locations. However, the anolyte can be characterized at any other location deemed appropriate by the practitioner.

[0109] The electronic controller Cont is configured to compare the result of this measurement with a low threshold Thr An defined by the practitioner of the electrolytic cell and stored in the electronic memory within the electronic controller at the test step 440 L .

[0110] The low threshold Thr LDefined by the practitioner so as to ensure that the iron concentration [Fe] remains above the lower limit of the target range, for example as a function of the typical rate of change of the iron concentration and the inertia of the electrolytic cell when taking corrective steps to maintain the concentration in the electrolytic cell.

[0111] As a characteristic representing the iron concentration in the electrolyte, the amount of Fe(II) in the electrolyte can be determined, for example, by spectrophotometry: TPTZ (2,4,6-tris-2-pyridyl-1,3,5-triazine) forms a complex with Fe(II), which can be quantified by colorimetric detection at 594 nm as a measure of the iron concentration. A table established by experiment indicates the equivalence between the measured amount of Fe(II) and the corresponding iron concentration in the anolyte, so that the measured characteristic can be easily converted into the iron concentration in the anolyte.

[0112] If the result of test step 440 An shows that the iron concentration [Fe] is not lower than the low threshold Thr L , then no specific action is taken and the anolyte is returned to the stack without modifying its composition ([ Figure 4 the test step 440 in process 400 An branch "no").

[0113] Conversely, if the result of test step 440 An shows that the iron concentration [Fe] is lower than the low threshold Thr L ([ Figure 4 the test step 440 in process 400 An branch "yes"), then at step 450 An a signal Sig is generated by the electronic controller Cont Iron and this signal is sent to the anolyte three-way valve V Iron .

[0114] At step 460 An , in response to the received signal Sig Iron , the anolyte three-way valve V Iron delivers a quantity of iron-containing solution Sol Iron to the anolyte that is being returned to the stack Stck. In this way, in response to the measured characteristic representing the iron concentration, a quantity of iron is added to the electrolyte.

[0115] A quantity of iron-containing solution Sol IronIt is advantageous to deliver it to the anolyte rather than to the catholyte because, while iron is beneficial to the anode, it is harmful to the cathode, and although the anolyte and catholyte are hydraulically connected via the diaphragm and balance line Bal of each electrolysis cell, after injecting iron into the anolyte, the iron concentration is not uniform or at least not immediately uniform between the anolyte and the catholyte.

[0116] The iron-containing solution may consist of an aqueous solution of iron sulfate (FeSO4 or Fe2(SO4)3) at a given concentration.

[0117] This quantity can be determined by an electronic controller based on the volume of the electrolyte in the electrolytic cell, the concentration of iron in the iron-containing solution, and the difference between the iron concentration in the anolyte and the first target iron concentration in the anolyte, where the first target iron concentration is defined by the practitioner and exceeds the low threshold Thr L , preferably in the range of 1 mg / l to 3 mg / l, for example 2 mg / l.

[0118] The electronic controller and the anolyte three-way valve V Iron can be configured to deliver this quantity of the iron-containing solution progressively to avoid local peaks in the iron concentration in the hydraulic circuit.

[0119] While the anolyte is being processed, the catholyte is also being processed.

[0120] In step 420 Cat , the catholyte leaving the stack Stck is brought to the hydrogen-liquid separator Sep H2 .

[0121] In step 430 Cat , measurements are performed by means of the device Meas Cat to measure a characteristic identifying the iron concentration in the catholyte (such as the concentration of oxidized iron in the catholyte just upstream of the stack Stck), and the measurement result is sent to the electronic controller Cont by any device known in the art for a similar purpose (e.g., via a cable).

[0122] Characterizing the catholyte just upstream of the stack allows for better control of the composition of the catholyte entering the stack compared to other locations. However, the catholyte can be characterized at any other location that the practitioner deems appropriate.

[0123] The electronic controller Cont is configured to compare the result of this measurement with a high threshold Thr Cat defined by the practitioner of the electrolytic cell and stored in the electronic memory within the controller at the test step 440 H for comparison.

[0124] High threshold Thr H Defined by the practitioner so as to ensure that the iron concentration [Fe] remains below the upper limit of the target range, for example as a function of the typical rate of change of the iron concentration and the inertia of the electrolytic cell when taking corrective steps to maintain the concentration in the electrolytic cell.

[0125] As a characteristic representing the iron concentration in the catholyte, the amount of Fe(II) in the electrolyte can be determined, for example, by spectrophotometry: TPTZ (2,4,6-tri-2-pyridyl-1,3,5-triazine) forms a complex with Fe(II), which can be quantified by colorimetric detection at 594 nm as a measure of the iron concentration. A table established by experiment indicates the equivalence between the measured amount of Fe(II) and the corresponding iron concentration in the catholyte, so the measured characteristic can be easily converted into the iron concentration in the catholyte.

[0126] If the test result shows that the iron concentration [Fe] is below the high threshold Thr H , then no specific action is taken and the catholyte is returned to the stack without changing its composition ( Figure 4 test step 440 in process 400 Cat branch "no").

[0127] Conversely, if the test result shows that the iron concentration [Fe] is above the high threshold Thr H ( Figure 4 test step 440 in process 400 Cat branch "yes"), then at step 450 Cat a signal Sig is generated by the electronic controller Cont Dir , and depending on the type of the electrolytic cell - the first type or the second type, Sig Dil or Sig DIW and Sig KOH are generated and sent to the pilot three-way valve V Dir and the dilution device V Dil or the first catholyte three-way valve V DIW and the second catholyte three-way valve V KOH . $

[0128] In response to the test at step 450 Cat , at step 460 Cat the iron concentration in the electrolyte is reduced.

[0129] In the case of the first type of electrolytic cell, dilution is used to reduce the iron concentration in the electrolyte. In response to the received signal Sig Dir , the pilot three-way valve V Dir directs the recycled catholyte towards the dilution device VDil Guided. In response to the received signal Sig Dil , the dilution device V Dil discharges the volume Evac of the catholyte from the electrolytic cell and injects a certain volume of fresh KOH solution Sol KOH into the electrolytic cell. The dilution device can operate in a binary manner by diluting or not diluting the catholyte at a fixed rate, or can be configured to discharge a volume of catholyte depending on the signal Sig Dil which can be generated to reflect, for example, the difference between a high threshold Thr H and the measured iron concentration in the catholyte (such difference being calculated by the electronic controller Cont), as a criterion for adjusting the desired dilution level.

[0130] In the case of a second type of electrolytic cell, filtration is employed to reduce the iron concentration in the electrolyte. In response to the received signals Sig Dir 、Sig DIW and Sig KOH , at step 460 Cat (which step includes filtering the catholyte), the three-way valve V Filt is guided to direct the circulating catholyte towards the iron filter Filt, the first catholyte three-way valve V DIW injects deionized water into the catholyte for dilution, and the second catholyte three-way valve V KOH supplements the filtered catholyte with a KOH-based solution. In this way, the first sub-step 460 Cat -1 of catholyte dilution, the second sub-step 460 Cat -2 of membrane filtration, and the third sub-step 460 Cat -3 of KOH supplementation are performed in this order. The dilution level of the catholyte can be determined by the electronic controller according to the operating specifications of the filter, and Sig DIW can be defined to reflect this dilution level. Supplementing the filtered catholyte with a KOH-based solution can be determined by the electronic controller according to the dilution level and the KOH loss due to discharging the portion of the catholyte that has passed through Filt evac to discharge the filtered iron. In this example, KOH represents the alkaline element of the electrolyte; when using another alkaline element such as sodium hydroxide, the corresponding alkaline element is used. The electronic controller can be configured to activate the filtration of the catholyte in order to obtain a second target iron concentration in the catholyte that is defined by the practitioner and is below the high threshold Thr H (preferably in the range of 1 mg / l to 3 mg / l, for example 2 mg / l).

[0131] Advantageously, the iron concentration in the catholyte is reduced rather than that in the anolyte, because although iron is harmful to the cathode, it is beneficial to the anode, and although the anolyte and the catholyte are hydraulically connected via the diaphragms and the balance line Bal of each electrolysis cell, after filtration, the iron concentration is not uniform or at least not immediately uniform between the anolyte and the catholyte. After reducing the iron concentration, the catholyte is returned to the stack Stck.

[0132] Measuring device Meas An 、Meas Cat and Meas El allow obtaining a first value, a second value and a third value representing the characteristics of the iron concentration in the electrolyte, and these values may be different from each other due to the dynamics of the reactions occurring in the electrolytic cell.

[0133] It should be noted that the test conditions of step 440 An and 440 Cat are consistent with each other under normal operating conditions.

[0134] In addition, instead of using two different measuring devices Meas KOH dedicated to the anolyte An KOH and the catholyte Cath An and Meas Cat , a common measuring device can be used, such as the measuring device Meas El connected to the balance line Bal, which is otherwise based on the same principle and is configured as Meas An and Meas Cat .

[0135] In this case, in the above process 400, the steps are the same, except that the measurement performed by Meas El is used instead of the measurements performed by Meas An and Meas Cat .

[0136] Alternatively, any one of the measuring devices Meas An 、Meas Cat and Meas El can be used to measure the characteristics representing the iron concentration in the electrolyte and be used in any one of the test steps 440 An and 440 Cat to compare with the low threshold Thr L and the high threshold Thr H determined by the practitioner.

[0137] Although the above-described method 400 represents an example of continuous operation of electrolyzers 200 and 300 while controlling the iron concentration in the electrolyte, the present invention is not limited to this type of operation, and the applicant also contemplates a batch method.

[0138] Specifically, in Figure 5 the method 500 shown, when it is determined at test step 440 Cat that the iron concentration exceeds the high threshold Thr H then the dilution or filtration operation of step 460 of process 400 Cat is changed to replace 560 Cat a part or all of the electrolyte contained in the electrolyzer so as to bring the iron concentration back into the target range; other than this operation, process 500 is the same as process 400, and the same reference numerals refer to the same steps. The composition and volume of the fresh electrolyte can be calculated by the practitioner based on the volume of the electrolyte on the electrolyzer and the measured iron concentration.

[0139] Processes 400 and 500 are not mutually exclusive, but are in fact complementary: the application of process 400 allows the service life of the electrolyzer to be extended without the need for maintenance or replacement of the electrolyte, but process 500 allows for a better definition of the time when the electrolyte needs to be replaced after the life of the electrolyte has already been extended due to the filtration of process 400.

[0140] An advantage of the present invention is that batch replacements of the electrolyte can be spaced apart and more precisely timed, thus saving costs, time, and manual labor, and thus placing the electrodes in an environment that is conducive to the operation of the electrolyzer.

[0141] In addition, due to the continuous control and correction of the iron concentration in the electrolyte, the electrolyzer can also be used continuously for a long period of time while ensuring optimal function and reducing maintenance costs.

[0142] The above examples describe electrodes, anodes, and cathodes made of or coated with Raney nickel used as an electrolysis catalyst, and an electrolyte substantially made of potassium hydroxide (KOH) (which means that the main component of the electrolyte is a potassium hydroxide solution, as is well known in the art).

[0143] The cathode can be a PGM-based cathode (PGM stands for platinum group metals, including platinum, palladium, rhodium, ruthenium, iridium, and osmium), and the electrolyte can be based on sodium hydroxide (NaOH) (which means that the main component of the electrolyte is a sodium hydroxide solution, as is well known in the art).

[0144] As already mentioned, considering iron as an impurity is only a specific application of the methods and devices described above. Depending on the specific purposes of the practitioner, the method and the device are suitable for application to other types of impurities present in the electrolyte.

[0145] Each example among the examples mentioned in this document can be freely combined within the technical limitations understood by practitioners in the field of the present invention.

[0146] By studying the drawings, the disclosure, and the appended claims, those skilled in the art will be able to understand and realize other variations of the disclosed examples when practicing the claimed invention.

Claims

1. An alkaline water electrolysis method (400, 500), the alkaline water electrolysis method using an electrolytic cell, the electrolytic cell comprising a stack (Stck) of electrolysis units (100), each electrolysis unit comprising an anode (An) and a cathode (Cat), the electrolytic cell being configured to accommodate an electrolyte (El) made of anolyte (An KOH ) and catholyte (Cath KOH ), the method being characterized by comprising the following steps: - Measurement (420 An , 430 Cat ) represents at least one characteristic of the concentration of impurities in the electrolyte; - Adjust (460 An , 460 Cat ) the concentration of the impurities in the electrolyte based on the at least one measured property of the electrolyte so as to maintain the concentration of the impurities within a target range by reducing (460 Cat ) the concentration of the impurities in the electrolyte or delivering (460 An ) a quantity of the impurities to the electrolyte.

2. The method (400, 500) according to claim 1, wherein the anode comprises nickel, the impurity is iron, and the target range is defined as being between 0.2 mg / l and 6 mg / l.

3. The method (500) according to any one of claims 1 and 2, wherein the step of adjusting the concentration of the impurities in the electrolyte comprises replacing (560 Cat ) a step of including at least a portion of the electrolyte in the electrolytic cell.

4. The method (400, 500) according to any one of claims 1 to 3, comprising the step (460 An ) of injecting a quantity of said impurity into said electrolyte based on a first one of said at least one measured property 5. The method (400, 500) according to claim 4, wherein in the step (460 An ) of injecting the amount of the impurity into the electrolyte, the amount of the impurity is injected into the anolyte (An KOH ).

6. The method (400) according to any one of claims 1 to 5, comprising the step (460 Cat ) of diluting the electrolyte based on a second one of the at least one measured property.

7. The method (400) according to any one of claims 1 to 5, comprising the step (460 Cat ) of filtering out the impurity from the electrolyte based on a second one of the at least one measured property.

8. The method (400) according to claim 7, wherein the step (460 Cat ) of filtering out the impurities from the electrolyte comprises: - Sub-step (460 Cat - 1) of diluting the electrolyte solution; and - A sub-step (460 Cat - 3) of supplementing an alkaline element to the electrolyte to compensate for the dilution of the electrolyte and the loss of the alkaline element during the step of filtering out the impurities from the electrolyte.

9. The method (400) according to claim 8, wherein the electrolyte is diluted with deionized water, and the alkaline element replenishment comprises adding a KOH solution to the electrolyte.

10. The method (400) according to any one of claims 7 to 9, wherein the step (460 Cat ) of filtering the impurities out of the electrolyte comprises a sub-step (460 Cat -2) of passing the electrolyte through a filtration membrane that filters the impurities out of the electrolyte.

11. An alkaline electrolyzer (200; 300), comprising an electronic controller (Cont), a stack (Stck) of electrolysis units (100), each electrolysis unit comprising an anode (An) and a cathode (Cat), the electrolyzer being configured to contain an electrolyte (El) made of anolyte (An KOH ) and catholyte (Cath KOH ), the electrolyzer being characterized in that it further comprises a system (Sys) controlled by the electronic controller (Cont), the system being configured to maintain the concentration of the impurities in the electrolyte within a target range by measuring a characteristic representing the concentration of the impurities in the electrolyte and adding a certain amount of the impurities to the electrolyte in response to the measured impurity concentration.

12. The alkaline water electrolyzer (200; 300) according to claim 11, wherein the anode comprises nickel, the impurity is iron, and the target range is defined as being between 0.2 mg / l and 6 mg / l.

13. The alkaline water electrolyzer (200; 300) according to claim 11 or 12, wherein the system (Sys) comprises: - First measuring device (Meas An ), the first measuring device being configured to measure a first value representing a characteristic of the concentration of the impurities in the electrolyte; and - Device (V Iron ), the device being configured to inject a solution (Sol Iron ) containing a certain amount of the impurity into the electrolyte (El), the electronic controller (Cont) further being configured to command the device (V Iron ) to inject the solution (Sol Iron ) containing the certain amount of the impurity into the electrolyte based on the measured first value.

14. The alkaline water electrolyzer (200) according to any one of claims 11 to 13, wherein the system (Sys) comprises: - Second measuring device (Meas Cat ), the second measuring device being configured to measure a second value representing a characteristic of the concentration of the impurities in the electrolyte (El); and - Dilution device (V Dil ), the dilution device is configured to dilute the cathode electrolyte, The electronic controller is also configured to command the guiding device (V Dir ) to direct the electrolyte (El) towards the dilution device (V Dil ) based on the measured second value.

15. The alkaline water electrolyzer (300) according to any one of claims 11 to 13, wherein the system (Sys) comprises: - Second measuring device (Meas Cat ), the second measuring device being configured to measure a second value representing a characteristic of the concentration of the impurities in the electrolyte (El); and - a filter (Filt), the filter being configured to filter the impurity out of the electrolyte (El), The electronic controller is also configured to command the guiding device (V Dir ) to guide the electrolyte (El) towards the filter (Filt) based on the measured second value.

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