WO2018108895A1

WO2018108895A1 - Cleaning method for an electrolyte liquid of a redox flow battery

Info

Publication number: WO2018108895A1
Application number: PCT/EP2017/082406
Authority: WO
Inventors: Adam Harding Whitehead; Martin Harrer; Peter Pokorny
Original assignee: Gildemeister Energy Storage Gmbh
Priority date: 2016-12-13
Filing date: 2017-12-12
Publication date: 2018-06-21
Also published as: AT519236B1; AT519236A4

Abstract

The aim of the invention is to reduce the concentration of impurities of an electrolyte liquid which is suitable for a redox flow battery. According to the invention, this is achieved by circulating an electrolyte liquid consisting of a mixture of negative and positive electrolyte liquids, preferably in a ratio of 50:50, from a first tank through negative half cells of a cell stack of a cleaning redox flow battery, whereby the electrolyte liquid flows through the negative half cells. A voltage is applied to the cell stack of the cleaning redox flow battery, and the electrolyte liquid is electrochemically reduced in the negative half cells. At least some of the impurities of the electrolyte liquid are layered on negative electrodes of the negative half cells.

Description

Cleaning process for an electrolyte fluid of a redox flow battery

The subject invention describes a method for reducing contaminants of an electrolyte fluid suitable for a redox flow battery.

A redox flow battery is an electrochemical-based power generation and storage system typically consisting of tanks for storing positive and negative electrolyte fluids, and pumps and lines for circulating the electrolyte fluids through one or more cell stacks, each of which is comprised of a number of cells , The cells of the cell stack are each formed by a positive half cell and a negative half cell, wherein the positive and negative half cells of a cell are separated by a semipermeable membrane, typically an ion exchange membrane. The positive half cell contains a frame-mounted positive electrode through which the positive electrolyte liquid flows. The negative half-cell contains a negative electrode in a frame, through which the negative electrolyte liquid flows. In a vanadium redox flow battery, the charged positive electrolytic liquid is vanadium having the oxidation number +4 (also referred to as V ^IV ) and vanadium having the oxidation number +5 (also referred to as V ^v ). In the charged state, the negative electrolyte liquid consists of vanadium having the oxidation number +2 (also referred to as V ") and vanadium having the oxidation number +3 (also referred to as V ¹ ") - whereby the negative electrolyte liquid is "more negative" than the positive electrolyte liquid Thus, the average oxidation number of the entire electrolyte liquid (negative and positive total) is +3.5, and both the positive and the negative electrolyte liquid may contain sulfuric acid and other additives, and the positive and negative electrodes are mostly porous Gra Between individual adjacent cells of the cell stack, bipolar electrode plates are arranged, which are usually made of a composite material of carbon and plastic, located on the axial outer sides of the axially outer cells of the cell stack At the electrode plates current collector, over which an electrical contact is led to the outside, in order to tap electrical voltage (discharging the redox flow battery) or to create an electrical voltage can (charging the redox flow battery). The cell stack is completed on the axial outer sides of each of an end plate, by which the cell stack is held together.

Usually, the vanadium used in a vanadium electrolytic liquid is found in chemical association with other elements. In the production of vanadium electrolyte fluids, it is important that impurities that improve the performance of the Vanadium redox battery influence, be kept as low as possible. In particular, impurities due to hydrogen catalysts such as copper (Cu), silver (Ag), gold (Au), arsenic (As), antimony (Sb) and platinum group elements in the electrolyte liquid are to be reduced as much as possible, since excessive hydrogen evolution during loading - Drives can significantly reduce the efficiency of the vanadium redox battery.

The starting material for the vanadium electrolyte liquid is usually V ^v , for example vanadium pentoxide (V ₂ 0 ₅ ) or ammonium metavanadate (NH ₄ V0 ₃ ) is used. However, since these materials are removed in mines, the quality, ie the degree of contamination of the starting material, can vary widely. Before further treatment, the starting material is often chemically cleaned in order to achieve a first reduction of the impurities, as disclosed, for example, in EP 0713257 A1. Usually, this cleaning is done by setting various parameters such as pH and temperature. With this method, sulphates, hydroxides or oxides are selectively deposited, whereupon the prepurified starting material is dissolved in sulfuric acid (H ₂ S0 ₄ ). Subsequently, the solution is reduced, wherein a chemical reduction by means of hydrogen (H ₂ ), carbon monoxide (CO), sulfur dioxide (S0 ₂ ), hydrogen peroxide (H ₂ 0 ₂ ), etc. can take place. In the course of this reduction, a mixture of V ¹ "and V ^{IV is produced} in equal proportions, as disclosed, for example, by CN 102354762 A. Starting from this mixture of V '" and V ^IV , a further chemical reduction can result in negative electrolytic liquid (V "). and V '"containing) or by a chemical oxidation positive electrolyte fluid (V ^IV and V ^v containing) are generated. Before that, moreover, as disclosed in EP 1406333 A1, the prepurified electrolyte liquid can be filtered in order to remove particles. EP 2576719 A1, for example, shows a filter series which contains chelate resin. Using the disclosed filter bank, for example, platinum group elements (ruthenium Ru, rhodium Rh, palladium Pd, osmium Os, iridium Ir, and platinum Pt) can be reduced to less than 4.5 ppm by weight in the electrolyte fluid. However, even lower concentrations would be advantageous in order to additionally reduce the parasitic evolution of hydrogen occurring during operation of a redox flow battery. All of the above methods have in common that with it the critical, usually metallic, impurities of the electrolyte liquid can not be reduced to a concentration of less than 1 ppm by weight.

It is thus an object of the subject invention to provide a method by which the concentration of impurities of an electrolyte liquid suitable for a redox flow battery can be further reduced.

This object is achieved according to the invention by the electrolyte liquid consists of a mixture of negative and positive electrolyte liquid, preferably in the ratio 50:50, and the electrolyte liquid from a first tank by negative half-cells of a Zellstacks a cleaning redox flow battery is circulated, whereby the electrolyte liquid passes through the negative half-cells, wherein a voltage to the cell stack of the cleaning redox flow battery is applied and the electrolyte liquid is reduced electrochemically in the negative half-cells and thereby at least a portion of the contaminants is coated by the electrolyte liquid on negative electrodes of the negative half-cells. This effect is thermodynamically conditioned and usually undesirable in normal operation of a redox flow battery, since the impurities clogged by the deposition of the usually porous negative electrodes pores and serve as hydrogen catalysts. Impurities are substances that are undesirable in the electrolyte fluid and, if appropriate, can also impair the proper operation of a redox flow battery using the contaminated electrolyte fluid. According to the invention, however, this effect is used to clean an electrolyte fluid. As negative (and also positive) electrodes, various suitable electrochemically sufficiently stable electrically conductive materials can be used - mats made of carbon or graphite fibers are often used. The positive electrodes are used for the oxidation of the electrolyte liquid and should therefore be composed of a material with a slight overpotential, thus a more efficient electrochemical reaction and thus a faster deposition of impurities on the negative electrodes is possible, since the low overpotential higher electrical currents to the Cells of cleaning redox flow battery can be created. Thus, in the course of the cleaning process, the impurities are coated by the electrolyte liquid on the negative electrodes of the negative half cell of the cleaning redox flow battery, which frees the electrolyte fluid from the impurities. The thus purified electrolyte liquid can then be used for the proper operation of a redox flow battery.

To carry out the process according to the invention, a conventional redox flow battery can be used as the cleaning redox flow battery. There may also be a commonly used voltage that would otherwise be applied to charge the redox flow battery to the cell stack - usually 1, 0-1, 6V per cell in each cell stack. Thus, the cleaning redox flow battery can be used as in charging, but no separate negative and positive electrolyte liquids are circulated through the half-cells, but a mixture of negative and positive electrolyte liquid is circulated as the electrolyte liquid to be cleaned by the negative half-cells. Of course, the negative and positive electrolytic fluids (usually present before mixing) should of course be largely uncharged, since otherwise no charging process can be efficiently "simulated" during the cleaning process and also during the mixing of charged negative and positive electrolyte fluids to the purifying electrolyte liquid an undesirable thermal reaction occurs per se. For measuring the proportion of impurities currently present in the electrolyte liquid, known methods, such as inductively coupled plasma periodic mass spectrometry (ICPMS), can be used, wherein the proportion of impurities in the electrolyte liquid in the tanks or at other arbitrary point of the cycle of the cleaning redox flow battery can be measured.

Advantageously, a second in a second tank, consisting of a mixture of negative and positive electrolyte liquid, preferably in the ratio 50:50 existing second electrolyte liquid is circulated by positive half-cells of the cleaning redox flow battery, whereby the second electrolyte liquid passes through the positive half-cells. This reduces the (first) electrolyte fluid and oxidizes the second electrolyte fluid. Thus, of course, only the (first) electrolyte fluid will be purified in the first place. However, the impurities present can pass through the ion exchange membrane, with which impurities can pass from the second electrolyte liquid into the (first) electrolyte liquid (and vice versa). This allows impurities originally in the second electrolyte liquid to be deposited on the electrodes of the negative half cell.

However, the (first) electrolyte liquid and the second electrolyte liquid may also be continuously or batch-mixed during the cleaning process. This speeds up the process of cleaning the entire electrolyte fluid.

This mixing during the cleaning process can be carried out by recycling (continuously or batchwise) a portion, preferably 10%, of the (first) electrolyte liquid after passing through the negative half-cells into the second tank and a portion, preferably 10% of the second electrolyte liquid after Passing through the positive half-cells is returned to the first tank. This method is easy to implement, so the reduction of contamination is continuous and can take some time and many changes. When the (first) electrolytic liquid and the second electrolytic liquid are sufficiently cleaned, the first electrolytic liquid and the second electrolytic liquid can be mixed together.

It can also be provided in the first tank and / or in the second tank, a heat exchanger, which serves the dissipation of thermal energy, which arises during the cleaning process.

In order to accelerate the cleaning process, the electrolyte liquid can be circulated from a first tank through the negative half-cells, sulfuric acid being separated from a first tank. A second tank is circulated through the positive half cells and an increased voltage, which is greater than the usual voltage, preferably 2.5 V per cell, is applied to the positive half cells and the negative half cells. In addition, in the positive half-cell, a catalyst for reducing an existing overpotential may be provided in the context of an oxygen evolution. This accelerates the oxidation, which allows a faster reaction rate and thus cleaning rate. When using a catalyst, a lower voltage can be applied to the cell stack, thus reducing the possibility of overloading components of the cell stack. This catalyst may, for example, consist of platinum Pt, iridium (IV) oxide Ir0 ₂ , lead (IV) oxide Pb02, etc. So it is the entire electrolyte liquid pumped from a tank through the negative half-cells and reduced. The applied voltage oxidizes the sulfuric acid in the positive half-cells. This increases the efficiency of the process and requires fewer recirculations than when the electrolyte liquid is pumped through the negative half cells and the second electrolyte liquid through the positive half cells. Thus, the total duration of the cleaning process can be reduced by about a factor of 10.

Alternatively, the electrolyte liquid may be circulated from a first tank through the negative half cells, with pure electrolyte liquid consisting of a mixture of negative and positive electrolyte liquid, preferably in the ratio 50:50, circulated by a second tank through the positive half cells. When using pure electrolyte liquid instead of the sulfuric acid, it is not necessary to apply an increased voltage to the cell stacks. A commonly used voltage that can otherwise be used to charge the redox flow battery, typically 1, 0-1, 6V per cell in each cell stack, is sufficient.

Advantageously, however, in the context of the cleaning process, the electrolyte liquid can also be pumped from a first tank through the negative half cells of the cell stack, pumped from the negative half cells into a second tank and pumped further and from the second tank through the positive half cells of the cell stack. Thus, the (to be cleaned) electrolyte liquid is circulated alternately through the negative half-cells and through the positive half-cells. With a sufficiently low recirculation rate, with this method, a single recirculation may be sufficient to reduce the electrolyte fluid contaminants to the desired level.

After the contaminants of the electrolyte liquid have been reduced to the desired degree, the electrolyte liquid can be oxidized and thus raised to the desired redox potential to produce a positive electrolyte liquid. This can be done for example by dilution with water or sulfuric acid. In order to produce a negative electrolyte liquid from the purified electrolyte liquid, the cleaned electrode can rolytflüssigkeit be reduced chemically or electrochemically. Methods for oxidizing and reducing electrolyte liquids are well known and will therefore not be described further here.

In order to prevent hydrogen production caused by the impurities deposited on the negative electrodes, as well as recontamination of the electrolyte liquid by dissolving the impurities deposited on the negative electrodes, during or after the cleaning process, the negative electrodes of the negative half cells of the cleaning -Redox flow-through battery to be cleaned to remove the coated impurities. This can therefore be carried out after the execution of the cleaning process of the electrolyte liquid or during an interruption of the cleaning process. This cleaning of the negative electrodes can be carried out chemically, for example using an oxidizing agent such as, for example, a positively charged electrolyte liquid, hydrogen peroxide H ₂ O ₂ , or electrochemically. The purge redox flow battery must be idle. When a positively charged electrolyte liquid is used to clean the negative electrodes, the, preferably pure, positive electrolyte liquid naturally picks up the impurities. Of course, this can only take place until the positive electrolyte liquid has a certain degree of impurities, whereupon the positive electrolyte liquid can be subjected to purification, or even disposed of.

The cleaning method described above primarily removes metallic contaminants by coating them on the negative electrodes. The cleaning process can be carried out until the proportion of the impurities in the electrolyte liquid reaches or falls below one or more of the following limits: 0.5 mass ppm Cu; 1 mass ppm As, Pb, Sb; 0.1 mass ppm Rh, Ru, Au, Ag and other elements of the Pt group. As a side effect when using the method, other, but when using the electrolyte liquid of a redox flow battery, the proper operation ebendieser non-interfering substances, such as Sn, Pn, Bi deposited on the negative electrode and thus removed from the electrolyte liquid.

However, the proportion of sulfur dioxide S0 _{2 of} the electrolyte liquid is reduced, but not by deposition on the negative electrodes, but by oxidation, or reduction. Sulfur dioxide S0 ₂ leads in a vanadium electrolyte liquid during operation also to increased hydrogen formation, which is why the reduction of sulfur dioxide S0 _{2 represents} an advantageous effect.

The cleaning method can be applied in particular to a vanadium electrolyte liquid. The vanadium electrolyte liquid is thus by the electrochemical

Reduction of the electrolyte liquid in the negative half-cells as divalent vanadium V, which serves as an indicator of a successfully initiated cleaning process. It can be assumed from shares of 0.001 MV "of an active cleaning process.

Of course, other electrolyte fluids, such as iron-chromium electrolyte fluids (thus suitable for an iron-chromium redox flow-through battery) may also be cleaned in the manner described. It is important that the positive and negative electrolyte liquid are miscible, ie are chemically substantially similar or have only a different oxidation state (such as V ²⁺ and V ³⁺ , V0 ²⁺ and V0 ₂ ⁺ in the case of the vanadium redox flow battery ).

The subject invention will be explained in more detail below with reference to Figures 1 to 5, which show by way of example, schematically and not by way of limitation advantageous embodiments of the invention. It shows

1 shows a redox flow battery with a cell stack,

2 shows a section through the cell stack,

3a and b a first embodiment of the cleaning process,

4a and b show a second embodiment of the cleaning method,

5a and b, a third embodiment of the cleaning process.

With reference to Figures 1 and 2, the well-known construction of a conventional redox flow battery 1 according to the prior art will be explained. A cell stack 2 of a redox flow battery 1 comprises a plurality of cells 4. Each cell is formed from a positive half-cell 42 and a negative half-cell 41, so there are 2 positive half-cells 42 and negative half-cells 41 arranged alternately in the cell stack. Between the positive half cell 42 and the negative half cell 41 of a cell 4, a semipermeable membrane 6, typically an ion exchange membrane (cation and / or anion exchange membrane, eg Nafion®), is arranged in each case. Between two adjacent cells 4, an electrode plate 7, for example a bipolar plate, is arranged. In the frame 401 of the positive half-cells 42, a positive electrode 422 are respectively arranged, in the frame 401 of the negative half-cells 41 each negative electrodes 412 are arranged. The positive electrodes 422 and negative electrodes 412 are usually designed as mats of carbon or graphite fibers. About recesses 80 in the frame 401 of the half-cells 40, or cells 4 are pumped in normal operation by means of the pumps 71, 72 electrically differently charged electrolyte liquids through the cells 4, wherein in a cell 4, or the respective positive half-cell 42 respectively the positive Electrode 422 is flowed through by the positive electrolyte liquid and the negative electrode 412 of the negative half cell 41 by the negative electrolyte liquid. For some types of redox Flow batteries 1, such as a vanadium redox flow battery or a vanadium polyhalite battery, the two electrolyte liquids are chemically similar or have only a different oxidation state (eg V ²⁺ and V ³⁺ , V0 ²⁺ and V0 ₂ ⁺ ).

1 also shows the tanks 91, 92 of a redox flow battery 1, in which usually the electrolyte liquids are stored for operation. In normal operation, i. In the course of power generation or energy storage, the electrolyte liquids are circulated using the pumps 71, 72 between the negative half-cells 41 and positive half-cells 42 and the negative and positive tanks 91, 92, respectively. The negative or positive tanks 91, 92 may be spatially separate containers, but may also be formed, for example as two compartments divided by a dividing wall, in a common container. The cell stack 2 is completed at the two axial ends by an end plate 60, for example made of plastic. The end plates 60 are driven by tensioning means 4, e.g. passing through bolts 41, which clamped by means of nuts 52, washers 53 and springs 54, and thus compress the frame 401 of the half-cells 40 of the cell stack 2 together. Furthermore, an electrical connection 19 can be provided on the end plates 60, via which the current collectors 3 in the interior of the redox flow battery 1 can be connected to an external circuit on both sides of the redox flow battery 1. For reasons of clarity, the electrical connection 19 is shown only in FIG. 1, and the connection between the current collector 3 and the electrical connection 19 can not be seen in the figures. Furthermore, in the exemplary embodiment shown, the electrolyte liquid connections for the supply and removal of the electrolyte liquids are provided on the end plates 60. In this case, a positive inflow 921 is used to supply the positive half-cells with electrolyte liquid (in normal operation, ie positive electrolyte liquid) and a positive outflow 922 to return the electrolyte liquid after flowing through the positive half-cells 42 into the respective negative or positive tank 91, 92. Analog serves a negative inflow 91 1 to the negative half-cells 41 with electrolyte liquid (in normal operation so negative electrolyte liquid) to supply and a negative outflow 912 to return the electrolyte liquid after flowing through the negative half-cells 41 in the respective negative or positive tank 91, 92. In order to prevent a possible setting of, for example elastic, frames of the cells 4 by the contact pressure, spacers 8 may be provided between the end plates 60 in order to ensure a constant distance 8 'between the end plates 60.

For the operation of the redox flow battery 1, it is desirable to keep the impurities in the positive and / or negative electrolyte liquids low, preferably below 1 ppm by weight. Impurities can be As, Pb, Sb, Rh, Ru, Au, Ag, etc. The cleaning method according to the invention can be carried out until the electrolyte liquid 101 as impurity 1 1 less than 0.5 mass ppm Cu and / or below 1 ppm by mass As, Pb, Sb, and / or less than 0.1 ppm by mass each of Rh, Ru, Au, Ag, and / or other platinum group elements. To clean the electrolyte liquid according to the invention, the procedure is as follows.

As an example, a vanadium electrolyte liquid is used as the electrolyte liquid 101 to be cleaned. The electrolyte liquid 101 consists of a ratio of V '": V ^IV of about 50:50, as for example, by mixing positive and negative electrolyte liquid of a vanadium redox flow battery as shown in Figure 1 arises or by the in the state of Thus, a certain degree of impurities 11 is present in the electrolyte liquid 101, which is to be lessened to a certain extent. For the application of the cleaning method according to the invention as cleaning redox flow battery V, a redox flow battery 1 can be used , as described in Fig.1 and Fig.2, can be applied, whereby an otherwise commonly used for charging voltage V in the amount of, for example, 1, 6V per cell can be applied.

In the process according to FIGS. 3 a and 3 b, the electrolyte liquid 101 to be purified is stored in a first tank 91 'and a second electrolyte liquid 102 is stored in a second tank 92'. The first tank 91 'and the second tank 92' may be the tanks of a purge redox flow battery 1 ', i. a commercial redox flow battery 1, as shown in Fig.1. By means of appropriate connections and / or switching valves, the electrolyte liquids stored therein can be circulated via the cleaning redox flow battery V as described below. This can also be done basically in normal operation of the cleaning redox flow battery 1 ', or the redox flow battery 1, of course, no positive and negative electrolyte liquid are circulated individually, but the electrolyte to be purified as stated above from a mixture of positive and negative Electrolyte fluid exists.

Thus, the electrolyte liquid 101 is circulated through the negative half-cells 41 of the purifying redox flow battery V via the negative inflow 91 1 and the negative flow 912, and the second electrolyte liquid 102 is circulated through the positive inflow 921 and the positive outflow 922 through the positive half-cells 42 of the cleaning redox flow battery 1 'circulated. In the negative half-cell 41 101 V ^{IV is} reduced electrochemically to V ¹ "in the electrolyte liquid, with a subsequent reduction of a part of the V ¹ " to V "by electrochemical means 0.001 M at V ", which is an indicator of the atmosphere required for cleaning. Thus, the impurities 1, typically metallic, are electrochemically or chemically coated on the negative electrodes 412 of the negative half-cells 41 of the cleaning redox flow battery 1 ', eg in the reaction 2V ²⁺ + Cu ²⁺ <-> 2V ³⁺ + Cu. As a purely electrochemical reaction, for example, Cu ²⁺ + 2e ^" -> Cu, whereby this electrochemical reaction proceeds parallel to the usual redox reaction reaction V ³⁺ + e ^" - »V ²⁺ .

Preferably, to accelerate the cleaning process, the electrolyte liquid

101 and the second electrolyte liquid 102 are continuously mixed together during the cleaning process. This is done here by adding a portion during the cleaning process

101 'of the electrolyte liquid 101 is returned to the second tank 92 after passing through the negative half-cells 41. In addition, a portion 102 'of the second electrolyte liquid

102 after passing through the positive half-cells 42 in the first tank 91 'recycled. This mixing can in principle be continuous or batchwise, whereby the rate of mixing can be regulated by a valve (not shown).

In this method, however, thermal problems may occur because the electrolytic liquid is heated by the mixing process because the electrolytic liquid 101 and the second electrolytic liquid 102 reach different redox potentials by the cleaning process. During the cleaning process, a temperature of the electrolyte fluid should not exceed 40 ° C. Therefore, in the first tank 91 'and in the second tank 92', a heat exchanger 93 for discharging thermal energy may be provided as shown in the embodiment of FIG. If the negative and positive electrolytic liquid were not mixed continuously or in batches after passing through the respective half-cells, but were completely mixed with one another, a temperature rise of about 26 ° C. would be expected without the use of a heat exchanger 93. If a base operating temperature of 30 ° C is assumed, then the electrolyte liquid would reach a temperature of 56 ° C and thus exceed the target maximum temperature of 40 ° C. Thus, as mentioned, preferably about 10% of the electrolyte liquid is mixed after each pass through the respective half-cells, whereby the temperature increase can be limited to about 3 ° C. Thus, with an assumed basic operating temperature of 30 ° C, the target maximum temperature of 40 ° C is not reached. Thus, the rate of mixing and the performance of the heat exchanger must be coordinated so as not to reach the target maximum temperature.

Also, due to the hydrogen formation in the electrolyte liquid 101 caused by impurities 1 1, ie in the negative half cell 41, less vanadium with the oxidation number +2 may be present than vanadium with the oxidation number +5 in the second electrolyte liquid 102. ie in the positive half-cell 42. Thus, an imbalance in the charge state of the electrolyte liquid 101 and the second electrolyte liquid 102 would result and the oxidation number of the electrolyte liquid 101 would shift from +3.50 to +4, the extent of this effect depends on the duration of the application of the cleaning process and the initial concentration of the impurities 1 in the electrolyte liquid 101. To reverse this effect For example, a chemical reduction may be provided, wherein, for example, oxalic acid may be added to the second electrolyte liquid 102. Alternatively, the second electrolyte liquid 102, which is circulated through the positive half cells 42, may be replaced by "fresh" electrolyte liquid The production of hydrogen depends primarily on how long the negative electrode 412 coated with the contaminants 11 is in contact with the Electrolyte liquid 101. Therefore, in principle a still rapid reduction of the impurities 1 1 is desirable because the process just described takes a few weeks, depending on the electrolyte liquid 101 to be cleaned.

A further embodiment of the cleaning method according to the invention is therefore outlined in FIGS. 4a and 4b. Here, the electrolyte liquid 101 is circulated via the negative inflow 91 1 and the negative outflow 912 from a first tank 91 'through the negative half cells 41 of the purifying redox flow battery V. However, due to the positive half cells 42 of the cleaning redox flow battery V, sulfuric acid S stored in a second tank 92 is circulated via the positive inflow 921 and the positive outflow 922. For this purpose, an increased voltage V1, which is greater than the voltage V, is applied to the cleaning redox flow battery 1 ', ie to the positive half cells 41 and the negative half cells 42. Typically, the boosted voltage V1 is about 2.5V per cell. As in the process outlined in Fig. 3a and 3b, the impurities 11 are also coated on the negative electrode 412, ie, for example, the chemical reaction Cu ² + + 2e ^" -> Cu occurs again Reducing one

Be provided overpotentials in the context of oxygen evolution in the positive half-cells 42.

As an alternative to sulfuric acid S, pure electrolyte liquid R, consisting of a mixture of negative and positive electrolyte liquid, preferably in the ratio 50:50, can be used in the cleaning process according to FIG. 4 and thus circulated by the second tank 92 through the positive half cells 42. For the definition of "pure", the same limit values of impurities as are to be achieved for the electrolyte liquid 101 can be considered here, ie 0.5 mass ppm Cu, 1 mass ppm As, Pb, Sb, 0.1 mass ppm. ppm Rh, Ru, Au, Ag and elements of the Pt group.

FIGS. 5a and 5b show a third embodiment of the cleaning method according to the invention. The electrolyte liquid 101 is pumped from a first tank 91 'via the negative inflow 91 1 through the negative half cells 41 of the cell stack 2 of the cleaning redox flow battery V. Further, the electrolyte liquid 101 is pumped from the negative half-cells 41 via the negative drain 912 in the second tank 92 and further pumped from the second tank 92 through the positive inflow 921 in the positive half-cells 42 of the cell stack 2 of the cleaning redox flow battery V. From the positive drain 922, the electrolyte liquid 101 flows back into the first tank 91 '. The pros The cleaning process corresponds to the procedure described with reference to FIG.

In addition, during or after the cleaning process, the negative electrodes 412 of the negative half cells 41 of the cleaning redox flow battery V may be subjected to cleaning to remove the impurities 1 1 coated on the negative electrodes 412.

In addition, the second electrolyte liquid 102 in the positive half cells 42 can be oxidized to a mixture of V ^IV and V ^v .

Claims

claims

1 . A cleaning method for reducing impurities (1 1) of an electrolyte fluid (101) suitable for a redox flow battery, characterized in that the electrolyte fluid (101) consists of a mixture of negative and positive electrolyte fluid of the redox flow battery, preferably in the ratio 50:50, and the electrolyte liquid (101) is circulated from a first tank (91 ') through negative half cells (41) of a cell stack (2) of a purifying redox flow battery (1'), whereby the electrolyte liquid (101) distributes the negative half cells (41 ) is applied by applying a voltage to the cell stack (2) of the cleaning redox flow battery (1 ') and thereby electrochemically reducing the electrolyte liquid (101) in the negative half cells (41), and at least some of the contaminants ( 1 1) of the electrolyte liquid (101) thereby coated on negative electrodes (412) of the negative half-cells (41) becomes.

2. Cleaning method according to claim 1, characterized in that in a second tank (92 ') located, consisting of a mixture of negative and positive electrolyte liquid, preferably in the ratio 50:50, second electrolyte liquid (102) by positive half-cells (42) the purifying redox flow battery (1) is circulated, whereby the second electrolyte liquid (102) passes through the positive half-cells (42).

3. Cleaning method according to claim 2, characterized in that the electrolyte lytflüssigkeit (101) and second electrolyte liquid (102) during the cleaning process is continuously or batchwise mixed together.

4. A cleaning method according to claim 3, characterized in that for mixing during the cleaning process, a portion (101 ') of the electrolyte liquid (101) after passing through the negative half-cells (41) in the second tank (92') is recycled, and that Part (102 ') of the second electrolyte liquid (102) after passing through the positive half-cells (42) in the first tank (91') is recycled.

5. Cleaning method according to claim 3 or 4, characterized in that in the first tank (91 ') and / or in the second tank (92'), a heat exchanger (93) is provided for the discharge of thermal energy.

6. A cleaning method according to claim 1, characterized in that the electrolyte liquid (101) is circulated from a first tank (91 ') through the negative half-cells (41), sulfuric acid (S) from a second tank (92') through the positive Half cells (42) is circulated, and that an increased voltage (V1), which is greater than the voltage (V), preferably 2.5 V per cell (4), to the positive half-cells (41) and the negative half-cells (42) is created.

7. Cleaning method according to claim 6, characterized in that in the positive half-cells (42) a catalyst for reducing an overpotential is provided.

8. Cleaning method according to claim 1, characterized in that the electrolyte liquid (101) is circulated from a first tank (91 ') through the negative half-cells (41), that pure electrolyte liquid (R) consisting of a mixture of negative and positive electrolytic liquid, preferably in the ratio 50:50, from a second tank (92 ') through the positive half-cells (42) is circulated.

9. Cleaning method according to claim 1, characterized in that the electrolyte liquid (101) is pumped from a first tank (91 ') through the negative half cells (41) of the cell stack (2), of the negative half cells (41) a second tank ( 92 ') is pumped and pumped from the second tank (92') through the positive half-cells (42) of the cell stack (2).

10. Cleaning method according to claim 5 or 9, characterized in that the electrolyte liquid (101) is oxidized after completion of the cleaning process.

1 1. Cleaning method according to one of claims 1 to 10, characterized in that during or after the cleaning process, the negative electrodes (412) of the negative half-cells (41) are subjected to a cleaning in order to remove the impurities (1 1) coated on the negative electrodes (412) ,

12. A cleaning method according to any one of claims 1 to 1 1, characterized in that the cleaning process is carried out until the electrolyte liquid (101) has less than 0.5 mass ppm of Cu as an impurity.

13. A cleaning method according to any one of claims 1 to 12, characterized in that the cleaning process is performed until the electrolyte liquid (101) has less than 1 mass ppm of As, Pb, Sb as an impurity.

14. A cleaning method according to any one of claims 1 to 13, characterized in that the cleaning process is carried out until the electrolyte liquid (101) each has less than 0.1 mass ppm of the following elements: Rh, Ru, Au, Ag and other elements of Pt-group.

15. A cleaning method according to any one of claims 1 to 14, characterized in that the electrolyte liquid (101) is a vanadium electrolyte liquid and that the electrochemical reduction of the electrolyte liquid (101) in the negative half-cells (41) is formed by divalent vanadium V ".