Cleaning method for a vanadium electrolyte of a redox flow battery

Description

[0001] The present invention describes a method for reducing impurities in a vanadium electrolyte liquid suitable for a redox flow battery.

[0002] A redox flow battery is an electrochemical energy generation and storage system. It typically consists of tanks for storing positive and negative electrolytes, as well as pumps and pipes for circulating the electrolytes through one or more cell stacks, each of which consists of a number of cells. The cells of the cell stack are each formed by a positive half-cell and a negative half-cell, with the positive and negative half-cells of a cell being separated from each other by a semipermeable membrane, typically an ion exchange membrane. The positive half-cell contains a positive electrode located in a frame through which the positive electrolyte flows. The negative half-cell contains a negative electrode located in a frame through which the negative electrolyte flows.In a vanadium redox flow battery, the positive electrolyte in the charged state consists of vanadium with an oxidation number of +4 (also referred to as V IV<) and vanadium with an oxidation number of +5 (also referred to as V V<). The negative electrolyte in the charged state consists of vanadium with an oxidation number of +2 (also referred to as V II<) and vanadium with an oxidation number of +3 (also referred to as V III<) – making the negative electrolyte more "negative" than the positive electrolyte. The average oxidation number of the entire electrolyte (negative and positive considered together) is therefore +3.5. Both the positive and negative electrolytes can also contain sulfuric acid and other additives. The positive and negative electrodes are usually made as porous graphite mats through which the electrolyte can flow.Bipolar electrode plates, usually made of a composite material of carbon and plastic, are arranged between individual adjacent cells of the cell stack. Current collectors are located on the electrode plates on the axial outer sides of the axially outer cells of the cell stack. These current collectors provide electrical contact to the outside to tap electrical voltage (discharging the redox flow battery) or to apply electrical voltage (charging the redox flow battery). The cell stack is terminated on the axial outer sides by a negative end plate and a positive end plate, which hold the cell stack together.

[0003] The vanadium used in a vanadium electrolyte is usually found in chemical combination with other elements. During the production of vanadium electrolyte, it is important to keep impurities that affect the performance of the vanadium redox battery as low as possible. In particular, impurities from hydrogen catalysts, such as copper (Cu), silver (Ag), gold (Au), arsenic (As), antimony (Sb), and platinum group elements in the electrolyte must be reduced as much as possible, since excessive hydrogen evolution during operation can significantly reduce the efficiency of the vanadium redox battery. Consequently, it is advantageous to remove impurities from an electrolyte before it is used in a redox flow battery for energy storage.

[0004] The starting material for the vanadium electrolyte liquid is usually VV<, e.g. vanadium pentoxide (V 2 O 5 ) or ammonium metavanadate (NH 4 VO 3 ). However, since these materials are mined in mines, the quality, i.e. the degree of impurities in the starting material, can vary greatly. Before further treatment, the starting material is often chemically purified in order to achieve an initial reduction in impurities, as disclosed, for example, in EP 0713257 A1. This purification is usually carried out by adjusting various parameters such as the pH value and the temperature. With this method, sulfates, hydroxides or oxides are selectively separated, after which the pre-purified starting material is dissolved in sulfuric acid (H 2 SO 4 ). The solution is then reduced, whereby a chemical reduction can take place using hydrogen (H 2 ), carbon monoxide (CO), sulfur dioxide (SO 2 ), etc.During this reduction, a mixture of V III< and V IV< is produced in equal proportions, as disclosed, for example, in CN 102354762 A. Starting from this mixture of V III< and V IV<, negative electrolyte liquid (containing V II< and V III<) can be produced by further chemical reduction, or positive electrolyte liquid (containing V IV< and VV<) can be produced by chemical oxidation. Before this, the pre-purified electrolyte liquid can also be filtered to remove particles, as disclosed in EP 1406333 A1. EP2576719 A1, for example, shows a filter series that includes chelate resin. When using the previously mentioned filter methods, elements of the platinum group (ruthenium Ru, rhodium Rh, palladium Pd, osmium Os, iridium Ir and platinum Pt), for example, can be reduced in the electrolyte liquid to a proportion of less than 4.5 ppm by weight.Since even lower concentrations of impurities are advantageous in order to further reduce the parasitic hydrogen evolution occurring due to these impurities during the operation of a redox flow battery, the method according to AT 519236 A4 was developed, with which it is possible to reduce the critical, usually metallic, impurities of the electrolyte liquid to a concentration of less than 1 ppm by weight.

[0005] It is therefore an object of the present invention to provide a method by which the concentration of impurities in an electrolyte liquid suitable for a redox flow battery can be efficiently reduced.

[0006] Independent claims 1 and 3 define the solutions to the stated problem according to the invention. Claim 1 describes a purification method for reducing impurities in a vanadium electrolyte liquid suitable for a redox flow battery, wherein the vanadium electrolyte liquid consists of a mixture of negative and positive vanadium electrolyte liquid of the redox flow battery, preferably in a 50:50 ratio, and the vanadium electrolyte liquid is circulated from a first tank through negative half-cells of a cell stack or multiple cell stacks of a purification redox flow battery, whereby the vanadium electrolyte liquid passes through the negative half-cells, wherein a voltage is applied to the cell stack or multiple cell stacks of the purification redox flow battery, and the vanadium electrolyte liquid in the negative half-cells is electrochemically reduced.and that at least a portion of the impurities of the vanadium electrolyte liquid is deposited on negative electrodes of the negative half-cells, wherein the vanadium electrolyte liquid, after passing through all negative half-cells of the one cell stack or the plurality of cell stacks, passes through positive half-cells of the one cell stack or the plurality of cell stacks of the purification redox flow battery via a connecting device that connects the negative half-cells and the positive half-cells of a cell stack without passing through a second tank, wherein the vanadium electrolyte liquid, after passing through the positive half-cells of the cell stack, is fed to a second tank and stored in the second tank. The technical effect resulting from this is,that the electrolyte fluid can be freed of impurities after passing through the negative half-cells of one or more cell stacks and the positive half-cells of one or more cell stacks once. With this "single-pass method," a purification process for electrolyte fluid can be carried out more quickly and energy-efficiently. The effect of the deposition of impurities in the electrolyte fluid on the negative electrodes of the negative half-cells is thermodynamically determined and is usually undesirable during normal operation of a redox flow battery, as the impurities clog existing pores by depositing on the generally porous negative electrodes and also serve as hydrogen catalysts. Impurities are substances thatwhich are undesirable in the electrolyte and may also impair the proper operation of a redox flow battery using the contaminated electrolyte. However, according to the invention, this effect is utilized to purify the electrolyte. Various suitable, electrochemically sufficiently stable, electrically conductive materials can be used as negative (and also positive) electrodes – mats made of carbon or graphite fibers are often used. The positive electrodes serve to oxidize the electrolyte and should therefore be composed of a material with a low overpotential, which enables a more efficient electrochemical reaction and thus faster deposition of the contaminants onto the negative electrodes.Because the low overpotential allows higher electrical currents to be applied to the cells of the purifying redox flow battery, the impurities in the electrolyte are deposited onto the negative electrodes of the negative half-cell of the purifying redox flow battery during the purifying process, thus freeing the electrolyte of impurities. The purified electrolyte can then be used for the proper operation of a redox flow battery.

[0007] To carry out the method according to the invention, a conventional redox flow battery can be used as the cleaning redox flow battery. A commonly used voltage that is otherwise applied to the cell stack for charging the redox flow battery—typically 1.0–1.6 V per cell in the respective cell stack—can also be used. However, separate negative and positive electrolyte fluids are not circulated through the half-cells; instead, a mixture of negative and positive electrolyte fluids is circulated through the negative half-cells as the electrolyte fluid to be cleaned.The negative and positive electrolytes (which are usually present separately before mixing) should be largely uncharged, as otherwise a charging process cannot be efficiently "simulated" during the cleaning process. Furthermore, an undesirable thermal reaction occurs when charged negative and positive electrolytes are mixed to form an electrolyte to be cleaned. Known methods such as periodic inductively coupled plasma mass spectrometry (ICP-MS) can be used to measure the proportion of contaminants currently present in the electrolyte. The proportion of contaminants in the electrolyte can be measured in the tanks or at any other point in the circuit of the cleaning redox flow battery.

[0008] A heat exchanger may also be provided in the first tank and / or the second tank to dissipate thermal energy generated during the cleaning process.

[0009] Once the impurities in the electrolyte have been reduced to the desired level, the electrolyte can be oxidized and thus raised to the desired redox potential to create a positive electrolyte. This can be achieved, for example, by dilution with water or sulfuric acid. To create a negative electrolyte from the purified electrolyte, the purified electrolyte can be chemically or electrochemically reduced. Processes for oxidizing and reducing electrolyte are well known and therefore will not be described in detail here.

[0010] In order to prevent hydrogen production caused by impurities deposited on the negative electrodes, as well as recontamination of the electrolyte fluid by dissolving the impurities deposited on the negative electrodes, the negative electrodes of the negative half-cells of the cleaning redox flow battery can be cleaned during or after the cleaning process to remove the coated impurities. This can therefore be carried out after the electrolyte fluid cleaning process has been carried out or during an interruption in the cleaning process. This cleaning of the negative electrodes can be carried out chemically, e.g. using an oxidizing agent such as positively charged electrolyte fluid, hydrogen peroxide H 2 O 2 , or electrochemically. The cleaning redox flow battery must be in idle mode.If a positively charged electrolyte is used to clean the negative electrodes, the preferably pure positive electrolyte will absorb the contaminants. This can only continue until the positive electrolyte contains a certain degree of contamination, at which point the positive electrolyte can be cleaned or disposed of.

[0011] The described cleaning process primarily removes metallic contaminants by depositing them on the negative electrodes. The cleaning process can be continued until the impurity content of the electrolyte reaches or falls below one or more of the following limits: 0.5 ppm by mass of Cu; 1 ppm by mass of As, Pb, Sb; 0.1 ppm by mass of Rh, Ru, Au, Ag, and other Pt group elements. As a side effect, when applying the process, other substances such as Sn, Pb, and Bi, which do not impair proper operation when using the electrolyte of a redox flow battery, are also deposited on the negative electrodes and thus removed from the electrolyte.

[0012] However, the proportion of sulfur dioxide SO2 in the electrolyte fluid is also reduced, not by deposition on the negative electrodes, but by oxidation or reduction. Sulfur dioxide SO2 also leads to increased hydrogen formation in a vanadium electrolyte fluid during operation, which is why reducing sulfur dioxide SO2 has a beneficial effect.

[0013] According to the invention, the purification process is applied to a vanadium electrolyte. The vanadium electrolyte is thus formed as divalent vanadium V II< by the electrochemical reduction of the electrolyte in the negative half-cells, which serves as an indicator of a successfully initiated purification process. An active purification process can be assumed from a concentration of 0.001 MV II<.

[0014] Other electrolyte fluids, such as iron-chromium electrolyte fluids (thus suitable for an iron-chromium redox flow battery), can also be purified in the manner described, but this is not part of the protection sought. It is important that the positive and negative electrolyte fluids are miscible, i.e., they are largely chemically similar or simply have a different oxidation state (such as V 2+< and V 3+< , VO 2+< and VO 2+< in the case of the vanadium redox flow battery).

[0015] The present invention is described below with reference to the Figuren 1 bis 4 which show exemplary, schematic and non-limiting advantageous embodiments of the invention. Fig.1 a redox flow battery with a cell stack, Fig.2 a section through the cell stack, Fig.3 a first embodiment of the cleaning process, Fig.4 a second embodiment of the cleaning process.

[0016] With reference to Fig.1 and 2The well-known structure of a conventional redox flow battery 1 according to the prior art is 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, i.e., positive half-cells 42 and negative half-cells 41 are arranged alternately in the cell stack 2. A semipermeable membrane 6, typically an ion exchange membrane (cation and / or anion exchange membrane, e.g., Nafion®), is arranged between the positive half-cell 42 and the negative half-cell 41 of a cell 4. An electrode plate 7, e.g., a bipolar plate, is arranged between two adjacent cells 4. A positive electrode 422 is arranged in each frame 401 of the positive half-cells 42, and negative electrodes 412 are arranged in each frame 401 of the negative half-cells 41.The positive electrodes 422 and negative electrodes 412 are typically designed as mats made of carbon or graphite fibers. During normal operation, differently charged electrolyte fluids are pumped through the cells 4 by means of pumps 71, 72 via recesses 80 in the frames 401 of the positive half-cells 41 and the negative half-cells 42 or cells 4. In a cell 4 or the respective positive half-cell 42, the positive electrode 422 is flowed through by the positive electrolyte fluid, and the negative electrode 412 of the negative half-cell 41 is flowed through by the negative electrolyte fluid. In 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 in the half-cells (e.g. V 2+< and V 3+< , VO 2+< and VO 2 +< ).

[0017] Fig.1 also shows the tanks 91, 92 of a redox flow battery 1, in which the electrolyte fluids for operation are usually stored. During normal operation, i.e. during energy generation or energy storage, the electrolyte fluids are circulated between the negative half-cells 41 or positive half-cells 42 and the negative or positive tanks 91, 92 using the pumps 71, 72. The negative or positive tanks 91, 92 can be spatially separate containers, but can also be formed, for example, as two compartments separated by a partition wall in a common container. The cell stack 2 is closed off at the two axial ends by a negative end plate 60 and a positive end plate 61, for example made of plastic.The negative end plate 60 and the positive end plate 61 are clamped by clamping means 50, consisting of through-reaching bolts 51, nuts 52, washers 53 and springs 54, and thus press the frames 401 of the negative half-cells 41 and positive half-cells 42 of the cell stack 2 together. An electrical connection 19 can be provided on each of the negative end plate 60 and the positive end plate 61, via which the current collectors 3 inside the redox flow battery 1 on both sides of the redox flow battery 1 can be connected to an external circuit. For reasons of clarity, the electrical connection 19 is only shown in . Fig. 1 shown, and the connection between current collector 3 and electrical connection 19 is not visible in the figures. Furthermore, in the exemplary embodiment shown, the electrolyte fluid connections for the supply and removal of the electrolyte fluids are provided on the end plates 60. A positive inlet 921 serves to supply the positive half-cells with electrolyte fluid (i.e. positive electrolyte fluid during normal operation), and a positive outlet 922 serves to return the electrolyte fluid to the positive tank 92 after it has flowed through the positive half-cells 42. Analogously, a negative inlet 911 serves to supply the negative half-cells 41 with electrolyte fluid (i.e. negative electrolyte fluid during normal operation), and a negative outlet 912 serves to return the electrolyte fluid to the negative tank 91 after it has flowed through the negative half-cells 41.In order to prevent possible settling of the, for example, elastic, frames of the cells 4 due to the contact pressure, spacers 8 can be provided between the negative end plate 60 and the positive end plate 61 in order to ensure a constant distance 8' between the negative end plate 60 and the positive end plate 61.

[0018] For the operation of the redox flow battery 1, it is desirable to keep the impurities in the electrolyte liquid to be purified low, preferably below 1 ppm by weight. Impurities can be As, Pb, Sb, Rh, Ru, Au, Ag, etc. The purification method according to the invention can be carried out until the electrolyte liquid 101 contains, as impurity 11, less than 0.5 ppm by mass of Cu and / or less than 1 ppm by mass of As, Pb, Sb and / or less than 0.1 ppm by mass of Rh, Ru, Au, Ag and / or other platinum group elements. Preferably, the purification method according to the invention can be carried out until the electrolyte liquid 101 contains, as impurity 11, less than 0.1 ppm by mass of Cu and / or less than 0.1 ppm by mass of As, Pb, Sb and / or less than 0.01 ppm by mass of Rh, Ru, Au, Ag and / or other platinum group elements. According to the invention, the following procedure is used to clean the electrolyte fluid.

[0019] According to the invention, a vanadium electrolyte liquid is used as the electrolyte liquid 101 to be purified. The electrolyte liquid 101 consists of a ratio of V III< :V IV< of approximately 50:50, as can be achieved, for example, by mixing positive and negative electrolyte liquid of a vanadium redox flow battery as in Fig.1 shown or can also be produced by methods known in the prior art. Thus, a certain degree of impurities 11 is present in the electrolyte liquid 101, which must be reduced. For the application of the cleaning method according to the invention, a redox flow battery 1, as shown in Fig.1 and Fig.2 described, whereby a voltage V otherwise normally used for charging can be applied at a level of e.g. 1.6V per cell.

[0020] In the proceedings under Fig. 3 The electrolyte liquid 101 to be cleaned is stored in a first tank 91'. The first tank 91' can be the tank of a cleaning redox flow battery 1', ie a commercially available redox flow battery 1, as in Fig.1 shown. The cleaning redox flow battery 1' can be connected to a second tank 92'. Through appropriate connections, the electrolyte fluid stored in the first tank 91' can be circulated via the cleaning redox flow battery 1' as described below. If the cleaning redox flow battery 1' is used to clean electrolyte fluid, the positive and negative electrolyte fluids are not circulated individually; instead, the electrolyte fluid to be cleaned consists of a mixture of positive and negative electrolyte fluids.

[0021] To reduce impurities 11 in the electrolyte liquid 101 to be cleaned, the electrolyte liquid 101 is circulated through the cleaning redox flow battery 1'. The electrolyte liquid 101 to be cleaned is circulated from a first tank 91' through negative half-cells 41 of a cell stack 4 or multiple cell stacks 4 of the cleaning redox flow battery 1', causing the electrolyte liquid 101 to pass through the negative half-cells 41. A voltage is applied to the cell stack 4 or multiple cell stacks 4 of the cleaning redox flow battery (1'), and the electrolyte liquid 101 in the negative half-cells 41 is electrochemically reduced. At least some of the impurities 11 in the electrolyte liquid 101 are deposited on negative electrodes 410 of the negative half-cells 41.After passing through the negative half-cells 41 of one cell stack 4 or the multiple cell stacks 4 of the cleaning redox flow battery 1', the electrolyte liquid 101 to be cleaned passes through the positive half-cells 42 of one cell stack 4 or the multiple cell stacks 4 of the cleaning redox flow battery 1' without passing through a second tank. This essentially means that the electrolyte liquid exiting the negative half-cells 41 is directed directly into the positive half-cells 42. For this purpose, a connecting device 10 is provided, which connects the negative half-cells 41 and the positive half-cells 42 of the cell stack 4 or the multiple cell stacks 4 to one another. The connecting device 10 can be integrated in the cleaning redox flow battery 1', for example in the frame 401 of the half-cells 41, 42, but can also be arranged externally on the cleaning redox flow battery 1'.

[0022] In the negative half-cell 41, V IV< is electrochemically reduced to V III< in the electrolyte liquid 101, with a further portion of the V III< being electrochemically reduced to V II<. In the process, a concentration of more than 0.001M of V" is achieved in the negative half-cell 41, which is an indicator of the atmosphere required for cleaning. Thus, the usually metallic impurities 11 are electrochemically or chemically coated onto the negative electrodes 412 of the negative half-cells 41 of the cleaning redox flow battery 1', e.g., in the reaction 2 V 2+< + Cu 2+< ↔ 2 V 3+< + Cu. As a purely electrochemical reaction, for example, Cu 2+< + 2e -< → Cu can occur, with this electrochemical reaction occurring in parallel to the usual redox reaction V 3+< + e -< → V 2+<.

[0023] According to a preferred embodiment of the invention, which is described in Fig.3 As shown, the electrolyte liquid 101 to be cleaned is circulated via the negative inlet 911 and the negative outlet 912 through the negative half-cells 41 of the cleaning redox flow battery 1' and then, without first passing through a second tank, is circulated via the positive inlet 921 and the positive outlet 922 through the positive half-cells 42 of the cleaning redox flow battery 1' and stored in a second tank 92', which can very particularly preferably be the tank of a cleaning redox flow battery 1'. In this embodiment, the connecting device 10 is a line that connects the negative outlet 912 and the positive inlet 921 to one another.

[0024] The electrolyte liquid 101 is pumped from a first tank 91' via the negative inflow 911 through the negative half-cells 41 of the cell stack 4 of the purifying redox flow battery 1'. The electrolyte liquid 101 is then pumped from the negative half-cells 41 via the negative outflow 912 and via a connecting device 10, without passing through a second tank 92', and via the positive inflow 921 through the positive half-cells 42 of the cell stack 4 of the purifying redox flow battery 1'. The electrolyte liquid 101 is then pumped from the positive half-cells 42 of the first cell stack 4 via the positive outflow 922 into a second tank 92'.

[0025] The connecting device 10 can be configured as any type of conduit for conducting the electrolyte fluid. Preferably, the connecting device 10 can be configured as a hose, which can particularly preferably be made of rubber, plastic, elastomer, or synthetic raw materials. Furthermore, the connecting device can also be configured as a hose, which can be made of a renewable raw material, such as rubber. Preferably, the connecting device 10 can be connected to the cell stacks by means of nozzles or hose nozzles.

[0026] The negative half-cells 41 are connected to the negative inflow 911 and the negative outflow 912, whereby the electrolyte liquid 101 can enter the negative half-cells 41 via the negative inflow 911 and flow out of the negative half-cells 41 via the negative outflow 912. The positive half-cells 42 are connected to the positive inflow 921 and the positive outflow 922, whereby the electrolyte liquid 101 can enter the positive half-cells 42 via the positive inflow 921 and flow out of the positive half-cells 42 via the positive outflow 922.

[0027] The cleaning process can cause the electrolyte to heat up. This heating of the electrolyte is undesirable; during the cleaning process, the temperature of the electrolyte liquid should not exceed 40°C. Therefore, a heat exchanger 93 for dissipating thermal energy can be provided in the first tank 91' and / or the second tank 92', as shown in the exemplary embodiment of Fig. 3a. Thus, assuming a base operating temperature of 30°C, the desired maximum temperature of 40°C is not reached.

[0028] The hydrogen formation caused by impurities 11 in the electrolyte liquid 101, i.e. in the negative half-cell 41, can also result in more vanadium with an oxidation number of +4 being produced in the positive half-cell 42 than the electrolyte liquid 101 originally had before being pumped into the negative half-cell 41. This would result in an imbalance in the charge state of the electrolyte liquid 101 and the oxidation number of the electrolyte liquid 101 would shift from an initial +3.50 towards +4, whereby the extent of this effect depends on the duration of the cleaning process and the initial concentration of the impurities 11 in the electrolyte liquid 101. The hydrogen production depends primarily on how long the negative electrode 412 coated with the impurities 11 is in contact with the electrolyte liquid 101.Therefore, in principle, an even faster reduction of impurities 11 is desirable.

[0029] In Fig. 4 a second preferred embodiment of the cleaning method according to the invention is shown.

[0030] The electrolyte liquid 101 is pumped from a first tank 91' via the negative inflow 911 of the first cell stack 4 through the negative half-cells 41 of the first cell stack 4 of the purification redox flow battery 1'. The electrolyte liquid 101 is then pumped from the negative half-cells 41 of the first cell stack 4 via the negative outflow 912 of the first cell stack 4 and via the negative inflow 911 of the second cell stack 5 through the negative half-cells 41 of the second cell stack 5 of the purification redox flow battery 1'. Furthermore, the electrolyte liquid 101 is pumped from the negative half-cells 41 of the second cell stack 5 via the negative outflow 912 of the second cell stack 5 via a connecting device 10, without passing through a second tank 92', and via the positive inflow 921 of the second cell stack 5 through the positive half-cells 42 of the second cell stack 5 of the purification redox flow battery 1'.Furthermore, the electrolyte liquid 101 is pumped from the positive half-cells 42 of the second cell stack 5 via the positive outflow 922 of the second cell stack 5 and via the positive inflow 921 of the first cell stack 4 through the positive half-cells 42 of the first cell stack 4 of the purification redox flow battery 1'. Furthermore, the electrolyte liquid 101 is pumped from the positive half-cells 42 of the first cell stack 4 into a second tank 92' via the positive outflow 922 of the first cell stack 4.

[0031] Also according to the preferred embodiment shown in Figs. 4a and 4b, a heat exchanger 93 for dissipating thermal energy can be provided in the first tank 91' and / or in the second tank 92' in order not to reach the desired maximum temperature of 40°C at an assumed base operating temperature of 30°C.

[0032] In addition, during or after the cleaning process, the negative electrodes 410 of the negative half-cells 41 of the cleaning redox flow battery 1' can be subjected to cleaning in order to remove the contaminants 11 deposited on the negative electrodes 410.

Claims

1. A purifying method for reducing contaminants (11) in a vanadium electrolyte liquid (101) suitable for a redox flow battery, the vanadium electrolyte liquid (101) consisting of a mixture of negative and positive vanadium electrolyte liquid of the redox flow battery, preferably in a ratio of 50:50, and the vanadium electrolyte liquid (101) being circulated from a first tank (91') through negative half-cells (41) of one or more cell stacks (4) of a purifying redox flow battery (1'), by which the vanadium electrolyte liquid (101) passes through the negative half-cells (41), wherein a voltage is applied to the one or more cell stacks (4) of the purifying redox flow battery (1') and the vanadium electrolyte liquid (101) in the negative half-cells (41) is electrochemically reduced in the process, wherein at least some of the contaminants (11) in the vanadium electrolyte liquid (101) are deposited on negative electrodes (410) of the negative half-cells (41), characterized in that the vanadium electrolyte liquid (101), after passing through the negative half-cells (41) of the one or more cell stacks (4), passes through positive half-cells (42) of the one or more cell stacks of the purifying redox flow battery (1), without passing through a second tank, via a connecting device (10) that connects the negative half-cells (41) and the positive half-cells (42) of a cell stack, and in that the vanadium electrolyte liquid (101) is fed to a second tank after passing through positive half-cells (42) of the one or more cell stacks (4) and is stored in the second tank.

2. The purifying method according to claim 1, characterized in that the negative half-cells (41) are connected to a negative inflow (911) and a negative outflow (912), wherein the vanadium electrolyte liquid (101) is fed into the negative half-cells (41) via the negative inflow (911) and is guided out of the negative half-cells (41) via the negative outflow (912), and the positive half-cells (42) are connected to a positive inflow (921) and a positive outflow (922), wherein the electrolyte liquid (101) is fed into the positive half-cells (42) via the positive inflow (921) and is guided out of the positive half-cells (42) via the positive outflow (922), wherein the connecting device (10) connects the negative outflow (912) and the positive inflow (921) to one another.

3. A purifying method for reducing contaminants (11) in a vanadium electrolyte liquid (101) suitable for a redox flow battery, the vanadium electrolyte liquid (101) consisting of a mixture of negative and positive vanadium electrolyte liquid of the redox flow battery, preferably in a ratio of 50:50, and the vanadium electrolyte liquid (101) being circulated from a first tank (91') through negative half-cells (41) of a first cell stack (4) of a purifying redox flow battery (1'), whereby the vanadium electrolyte liquid (101) passes through the negative half-cells (41) of the first cell stack (4), wherein the negative half-cells (41) of the first cell stack (4) are connected to negative half-cells (41) of a second cell stack (5), by which the vanadium electrolyte liquid (101) passes through the negative half-cells (41) of the second cell stack (5), wherein a voltage is applied to the first cell stack (4) and the second cell stack (5) of the purifying redox flow battery (1') and the vanadium electrolyte liquid (101) in the negative half-cells (41) of the first cell stack (4) and of the second cell stack (5) is electrochemically reduced in the process, and in that at least some of the contaminants (11) in the vanadium electrolyte liquid (101) are deposited on negative electrodes (410) of the negative half-cells (41) of the first cell stack (4) and of the second cell stack (5), wherein the vanadium electrolyte liquid (101), after passing through the negative half-cells (41) of the first cell stack (4) and of the second cell stack (5), passes through the positive half-cells (42) of the second cell stack (5) of the purifying redox flow battery (1) via a connecting device (10) which connects the negative half-cells (41) of the second cell stack (5) and the positive half-cells (41) of the second cell stack (5) to one another without passing through a second tank.

4. The purifying method according to claim 3, characterized in that the positive half-cells (41) of the second cell stack (5) are connected to positive half-cells (41) of the first cell stack, whereby the vanadium electrolyte liquid (101) passes through the positive half-cells (41) of the first cell stack (4).

5. The purifying method according to claims 3 and 4, characterized in that the vanadium electrolyte liquid (101) is fed to a second tank after passing through the positive half-cells (42) of the first cell stack (4).

6. The purifying method according to any of claims 1 to 5, characterized in that the connecting device (10) is preferably designed as one or more hoses.

7. The purifying method according to any of claims 1 to 6, characterized in that the connecting device (10) does not exceed a length of 100 meters, particularly preferably the connecting device (10) does not exceed a length of 10 meters, most preferably the connecting device (10) does not exceed a length of 1 meter and in the most preferred embodiment the connecting device (10) does not exceed a length of 10 cm.

8. The purifying method according to claims 1 to 7 in each case, characterized in that the vanadium electrolyte liquid (101) is oxidized after completion of the purification process.

9. The purifying method according to any of claims 1 to 8, characterized in that during or after the purification process, the negative electrodes (410) of the negative half-cells (41) are subjected to a cleansing in order to remove the contaminants (11) deposited on the negative electrodes (410).

10. The purifying method according to any of claims 1 to 9, characterized in that the purifying method is carried out until the vanadium electrolyte liquid (101) has less than 0.5 mass ppm Cu as contaminant.

11. The purifying method according to any of claims 1 to 10, characterized in that the purifying method is carried out until the vanadium electrolyte liquid (101) has less than 1 mass ppm As, Pb, Sb as contaminants.

12. The purifying method according to any of claims 1 to 11, characterized in that the purifying method is carried out until the vanadium electrolyte liquid (101) has less than 0.1 mass ppm of each of the following elements: Rh, Ru, Au, Ag and other elements of the Pt group.

13. The purifying method according to any of claims 1 to 12, characterized in that the electrochemical reduction of the electrolyte liquid (101) in the negative half-cells (41) is formed by divalent vanadium V".