Electrochemical balancing cell

Abstract

The aim of the invention is an efficient charge balancing in an electrolyte (15a, 15b) of a redox flow battery (1). This is achieved in that during the operation of an electrochemical balancing cell (40), a balancing voltage (VA) is applied between the two electrodes (47, 48), said balancing voltage producing a half-electrolysis of water in the second liquid volume (42) in the charge balancing electrolyte (44) and a chemical reduction or chemical oxidation of the redox element in the electrolyte (15a, 15b) in order to change the oxidation stage of the redox element, wherein the charge balancing electrolyte (44) is stored in a charge balancing electrolyte container (45) at a higher level than the liquid level of the charge balancing electrolyte (44) in the second liquid volume (42). The electrochemical balancing cell (40) is equipped with a supply line (51) which connects the charge balancing electrolyte container (45) to the lower region of the second liquid volume (42), and the electrochemical balancing cell (40) is equipped with a discharge line (52) which connects the upper region of the second liquid volume (42) to the charge balancing electrolyte container (45).

Description

[0001] Electrochemical balancing cell

[0002] The present invention relates to an electrochemical equalization cell for changing an oxidation state of an electrolyte with a redox element of a redox flow battery, wherein a first half-cell with a first liquid volume and a second half-cell with a second liquid volume are provided in the equalization cell, wherein the first half-cell comprises a first electrode and the second half-cell comprises a second electrode and an electrical equalization voltage can be applied to the first and second electrodes,and wherein the first liquid volume is separated from the second liquid volume by a membrane, and wherein, during operation of the equalization cell, the first liquid volume contains the electrolyte and the first electrode is in contact with the electrolyte, and the second liquid volume contains an aqueous charge-balancing electrolyte different from the electrolyte, and the second electrode is in contact with the charge-balancing electrolyte, and wherein a circulation pump is provided which, during operation of the equalization cell, at least temporarily circulates the electrolyte through the first liquid volume of the first half-cell. The invention also relates to a method for changing an oxidation state of an electrolyte with a redox element of a redox flow battery having such an electrochemical equalization cell.

[0003] A redox flow battery is an electrochemical energy storage device for electrochemically based energy storage and typically consists of storage tanks for storing positive and negative electrolytes, as well as pumps and pipes for circulating the electrolytes through one or more cell stacks containing a number of individual cells. The individual cells of the cell stack are each formed by a positive half-cell and a negative half-cell arranged side by side, with the positive and negative half-cells of an individual cell separated from each other by a semipermeable membrane, typically an ion exchange membrane. The semipermeable membrane is, for example, a cation and / or anion exchange membrane, e.g., Nation®. 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. The positive and negative electrolytes are circulated separately through the half-cells. The positive and negative electrodes are usually made of porous graphite felts, through which the electrolyte can flow.

[0004] Electrode plates, such as bipolar plates, are arranged between adjacent individual cells of the cell stack as current collectors. These plates are usually made of a composite material made of carbon and plastic. Current collectors are located on the axial outer sides of the axially outer individual cells of the cell stack. These current collectors provide an electrical contact to the outside, allowing an electrical voltage to be tapped across the entire cell stack (discharging the redox flow battery) or to be applied to the cell stack (charging the redox flow battery). The cell stack is closed off on each axial outer side by an end plate, which holds the cell stack together.

[0005] An electrolyte is a liquid and essentially comprises a redox pair consisting of a first redox element and a second redox element, each in the form of a redox-active element or ion, or combinations of redox-active elements and / or ions with different electrical charges (oxidation numbers). There are a variety of redox-active elements or ions, or combinations of redox-active elements and / or ions, that can be used as redox pairs in a redox flow battery. The redox elements of a redox pair are usually dissolved in the electrolyte liquid. The electrolyte liquid is usually an aqueous acid, such as aqueous sulfuric acid. Redox flow batteries with a wide variety of combinations of redox pairs are known. Some non-exhaustive examples of known combinations of redox pairs are V 2+ / V 3+ vs. V 4+ / V 5+(in a vanadium redox flow battery), V 2+ / V 3+ vs. Br7CIBr2, Br2 / Br vs. S / S 2 ', Br / Br2 vs. Zn 2+ / Zn, Ce 4+ / Ce 3+ vs.

[0006] V 2+ / V 3+ , Fe 3+ / Fe 2+ vs. Br2 / Br, Fe 3+ / Fe 2+ vs. Cr 3+ / Cr 2+ , Mn 2+ / Mn 3+ vs. Br2 / Br, Fe 3+ / Fe 2+ vs. Ti 2+ / Ti 4+ and others. The redox elements can be contained in the electrolyte in various chemical compounds, for example in the form of sulfates, such as vanadium sulfate, or chlorides, such as vanadium chloride. The vanadium-based redox flow battery is the most widely used, which is why the following discussion will primarily focus on the vanadium redox flow battery, without restricting its generality.

[0007] In a vanadium-based redox flow battery, the positive electrolyte in the charged state consists of a redox pair in the form of vanadium with the oxidation number +4 (also known as V IV or V 4+ and vanadium with the oxidation number +5 (also known as V v or V 5+ The negative electrolyte, in the charged state, consists of a redox pair in the form of vanadium with the oxidation number +2 (also known as V" or V 2+ referred to) and vanadium with the oxidation number +3 (also known as V IH or V 3+ referred to) - whereby the negative electrolyte has a more negative electrochemical potential than the positive electrolyte.

[0008] This redox flow battery design is well known, for example, from WO 2018 / 087270 A1 or WO 2014 / 131702 A1 . During operation of a redox flow battery, electrical energy is delivered to a consumer or absorbed from an energy source. In the example of a vanadium-based redox flow battery, the following well-known chemical reaction (redox reaction) occurs during charging / discharging in a single cell of the redox flow battery: y5+ _|_ y2+ y4+ _|_ y3 +

[0009] During the charging process, the reaction taking place is triggered by an external current or voltage source, which applies an electrical voltage to a single cell (or the entire cell stack).

[0010] In addition to the desired charge / discharge reactions, parasitic reactions can also occur, which lead to an ineffective charge / discharge process.

[0011] In addition to impurities, hydrogen reduction in an aqueous electrolyte can cause a significant parasitic effect and lead to an imbalance in the charging process, for example in the form of:

[0012] Hydronium ions HaO + are reduced to hydrogen gas H2. Impurities in the electrolyte, such as Cu, Sb, Ag, Ni, or platinum group metals such as Pt, Pd, Ir, Rh, etc., act as catalysts for hydrogen reduction and enhance the parasitic effect of hydrogen formation. This leads, for example, to a reduced charge of the negative electrolyte during charging.

[0013] It is also possible that water electrolysis of the aqueous electrolyte occurs at both electrodes, producing both hydrogen gas at the cathode and oxygen gas at the anode.

[0014] Such parasitic reactions lead to an imbalance of the charge states of the charge carriers (redox pair) in the two electrolytes, and thus, for example, to different ratios of V 2+ A / 3+ and V 4+ / V 5+This leads to one electrolyte being overcharged compared to the other. An imbalance of charge states leads to a limited battery capacity of the redox flow battery and is fundamentally undesirable. In addition to limited battery capacity, an imbalance of charge states can also lead to undesirable corrosion of parts of the redox flow battery and damage a cell stack. To reduce or compensate for a charge imbalance in the electrolytes of a redox flow battery, it is known to use a balancing cell in which an over-oxidized electrolyte is chemically reduced by a redox reaction, or an over-reduced electrolyte is chemically oxidized by a redox reaction.

[0015] US 8,877,365 B2 shows a balancing cell for a redox flow battery with the redox elements Fe and Cr. The negative electrolyte of the redox flow battery is located on the cathode side of the balancing cell. An anode in hydrochloric acid (HCl) is arranged on the anode side of the balancing cell. The cathode side is separated from the anode side by an ion-selective membrane. An electrical voltage between the anode of the balancing cell and the cathode of the balancing cell causes a redox reaction. The redox reaction causes Fe to be oxidized in the overoxidized negative electrolyte at the cathode of the balancing cell. 3+ to Fe 2+reduced. Chlorine gas is produced at the anode during the redox reaction. The chlorine gas is combined in a photochemical cell with the hydrogen gas produced on the anode side of the redox flow battery by a parasitic reaction, using energy input from UV light, to form hydrochloric acid. The hydrochloric acid is returned to the anode side of the equalizing cell. The disadvantage of this equalizing cell is primarily the increased energy consumption for generating the UV light in the photochemical cell, which reduces the overall efficiency of the redox flow battery.

[0016] It is also known to design a balancing cell like a flow cell of the cell stack of the redox flow battery. An example of such a balancing cell can be found in US 8,916,281 B2, which describes a Fe / Cr redox flow battery. In such a balancing cell, an electrolyte is pumped through a half-cell of the balancing cell, with Fe 3+ in the electrolyte to Fe2+ is reduced and at the same time a balancing electrolyte is pumped through the other half cell of the balancing cell, whereby Fe 2+ in the balancing electrolyte to Fe 3+ oxidized. However, the balancing electrolyte must be chemically reduced again to maintain its charge balancing ability. This requires an additional step, making charge balancing more complex.

[0017] In S. Rudolph et al., "On-line controlled state of charge rebalancing in vanadium redox flow battery", Journal of Electroanalytical Chemistry 703 (2013) 29-37 and in N. Poli et al., "Novel electrolyte rebalancing method for vanadium redox flow batteries", Chemical Engineering Journal 405 (2021) 126583, a balancing cell is described for the charge balancing of the positive electrolyte of a vanadium redox flow battery. In both cases, the positive electrolyte is pumped through both half-cells of a balancing cell. In such balancing cells, the redox reaction in the electrolyte produces gaseous byproducts, such as oxygen O2. The associated difficulty is that a liquid-gas mixture cannot easily be circulated using a conventional circulation pump. In a conventional centrifugal pump, gas in the pumped liquid can lead to cavitation, which can damage or destroy the circulation pump.One solution to this problem would be the use of other pump types, such as positive displacement pumps, for example diaphragm pumps or peristaltic pumps. However, such pump types often have the disadvantage of significantly shorter service lives, which is detrimental to the desired operating time for a redox flow battery, and they often also require more energy to operate, which in turn reduces the overall efficiency of the redox flow battery. The use of degassers on the half-cell with the compensating liquid would also be conceivable, but this increases the system complexity due to additional components and is therefore just as undesirable. If oxygen is returned to the tank for the positive electrolyte as a gaseous by-product, as in Poli et al.As shown, it is necessary to ensure that oxygen does not enter the tank for the negative electrolyte, because the oxygen would chemically oxidize the negative electrolyte, which would impede charge equalization. In many redox flow batteries, however, the gas spaces of both tanks are connected, meaning that the described charge equalization in such redox flow batteries would not function, or would function only to a very limited extent.

[0018] It is therefore an object of the present invention to provide an efficient charge equalization in an electrolyte of a redox flow battery which does not have the disadvantages of the prior art.

[0019] This object is achieved in that, during operation of the equalization cell, a compensation voltage is applied between the two electrodes, which causes a half water electrolysis reaction in the charge equalization electrolyte in the second liquid volume as a partial reaction of a redox reaction, which releases either oxygen or hydrogen and which causes a chemical reduction or chemical oxidation of the redox element in the electrolyte in the first liquid volume as a counter-reaction of the redox reaction in order to change the oxidation state of the redox element, wherein the charge equalization electrolyte is stored in a charge equalization electrolyte container and a liquid level of the charge equalization electrolyte in the charge equalization electrolyte container is higher than the liquid level in the second liquid volume, wherein a supply line is provided in the equalization cell,which connects the charge balancing electrolyte container to the second liquid volume, wherein the supply line opens in the lower region of the liquid volume and the supply line is connected to the charge balancing electrolyte in the charge balancing electrolyte container, and wherein a discharge line is provided in the equalization cell, which connects the second liquid volume to the charge balancing electrolyte container, wherein the discharge line opens in the upper region of the liquid volume and the discharge line is connected to the charge balancing electrolyte in the charge balancing electrolyte container.

[0020] The equalizing cell is specifically designed with an aqueous charge-balancing electrolyte and is operated in such a way that half of the water electrolysis takes place in the half-cell containing the charge-balancing electrolyte, releasing gas (oxygen or hydrogen). This half-water electrolysis is not completed in the first half-cell of the equalizing cell; instead, the ions and electrons produced during water electrolysis are used in the first half-cell to chemically reduce or chemically oxidize the redox element in the electrolyte. This is possible because chemical reduction or chemical oxidation is more reactive than completing water electrolysis. A further advantage is that the circulation of the charge-balancing electrolyte is self-regulating. With a more vigorous redox reaction, more gas bubbles are formed and the circulation of the charge-balancing electrolyte is increased.This automatically pumps more unused charge balancing electrolyte into the second half-cell.

[0021] The circulation driven by the gas bubbles is improved if the discharge line is connected to a riser pipe in the charge-balancing electrolyte tank. The riser pipe extends a predetermined height into the charge-balancing electrolyte tank and the riser pipe in the charge-balancing electrolyte tank opens into the charge-balancing electrolyte. This reduces the amount of gas bubbles in the charge-balancing electrolyte in the charge-balancing electrolyte tank.

[0022] Water loss in the charge-balancing electrolyte can be easily compensated by providing a water tank connected to the charge-balancing electrolyte tank via a water supply line. A water circulation pump is provided in the water supply line to pump water from the water tank to the charge-balancing electrolyte tank as needed. This can also be done fully automatically using a level sensor in the charge-balancing electrolyte tank.

[0023] The present invention will be explained in more detail below with reference to Figures 1 to 6, which show exemplary, schematic and non-limiting advantageous embodiments of the invention.

[0024] Fig.1 the basic operating principle of a redox flow battery,

[0025] Fig.2 a redox flow battery with a cell stack,

[0026] Fig.3 shows the structure of a cell stack of a redox flow battery,

[0027] Fig.4 shows a compensation cell according to the invention with pumpless delivery of the charge compensation electrolyte, Fig.5 shows a further embodiment of a compensation cell according to the invention and

[0028] Fig.6 an inventive arrangement of several compensation cells in a compensation cell stack.

[0029] Fig. 1 shows a schematic structure of a redox flow battery 1 using a single cell 2 of a cell stack 10 to explain the well-known functional principle of a redox flow battery 1. For better explanation and illustration, Fig. 1 shows only a single cell 2 of a cell stack 10 of a redox flow battery 1, whereby a cell stack 10 will generally have a plurality of single cells 2 arranged side by side in the cell stack 10.

[0030] A single cell 2 consists of two half-cells 2a, 2b, which form a positive reaction chamber 3a and a negative reaction chamber 3b. The two half-cells 2a, 2b, or the positive reaction chamber 3a and the negative reaction chamber 3b, are separated by a semipermeable, in particular ion-selective, membrane 4. The reaction chambers 3a, 3b are formed, for example, in recesses 6a, 6b of frames 5a, 5b. An electrode 7a, 7b is arranged in each of the frames 6a, 6b, in the reaction chambers 3a, 3b, or in the recesses 6a, 6b, which is in contact with the respective electrolyte 15a, 15b located in the half-cell 2a, 2b. Electrolytes 15a, 15b with different electrical charges (positive and negative electrolyte) flow through the recesses 6a, 6b and, if applicable, the electrodes 7a, 7b arranged therein of an individual cell 2.Each of the electrolytes 15a, 15b contains a redox pair with specific, time-varying concentrations (depending on the state of charge) of redox elements. The semipermeable, particularly ion-selective, membrane 4 can be made, for example, of sultanate-modified polytetrafluoroethylene (PTFE), with the trade name Nation™, and enables ions to achieve charge equalization between the positive reaction space 3a and the negative reaction space 3b (or between the electrolytes 15a, 15b contained therein). A redox flow battery 1 also includes power terminals 11, 12 for tapping an electrical cell stack voltage Vz applied to the cell stack 10 via a consumer 14 (discharging the redox flow battery 1) or for applying an electrical cell stack voltage Vz to the cell stack 10 (charging the redox flow battery 1).

[0031] An electrical consumer 14 can have any form. Based on the electrical voltage, current, or power requirement of the electrical consumer 14, a cell stack 10 can be configured in a redox flow battery 1 to provide the necessary electrical voltage and / or the necessary electrical current. Redox flow batteries 1 are often used as stationary energy storage devices, for example, to serve as emergency power systems for industrial plants, storage systems for renewable energy (photovoltaics, wind power), and the like. Consequently, depending on the application, the person skilled in the art can configure a cell stack 10, or a parallel and / or serial connection of several

[0032] Design or select cell stacks and redox pairs in a redox flow battery 1.

[0033] The electrolytes 15a, 15b are stored in storage tanks 13a, 13b and are circulated from there through the cell stack 10 by means of circulation pumps 9a, 9b, specifically through a half-cell 2a, 2b of a single cell 2 of the cell stack 10. For this purpose, a supply line 16a, 16b and a discharge line 17a, 17b are provided for each electrolyte 15a, 15b, which are connected via electrolyte connections 22a, 22b, 23a, 23b to the corresponding electrolyte channels 18a, 18b, 19a, 19b (see Fig. 3) in the cell stack 10.

[0034] In a cell stack 10 with several adjacent individual cells 2, an electrode plate 8, such as a bipolar plate, is arranged between each two adjacent individual cells 2. At the outer ends of the cell stack 10, a power connection 11, 12 can be located on the outer electrode plates 8 or on the outer half-cells 2a, 2b (or electrodes 7a, 7b) of the cell stack 10, which can be electrically contacted from the outside.

[0035] The typical structure of a cell stack 10 of a redox flow battery 1 is explained in more detail with reference to Figs.2 and Fig.3.

[0036] A cell stack 10 of a redox flow battery 1 comprises at least one individual cell 2, generally a plurality of individual cells 2, which in turn are each formed from two frames 5a, 5b of half-cells 2a, 2b. A frame 5a, 5b is preferably made of a plastic, such as an elastomer, such as a polyolefinic thermoplastic elastomer (TPE or TPO), such as Santoprene®, or a thermoplastic vulcanate (TPV), in particular using an injection molding process. In the stack direction R (in the direction in which the individual cells 2a, 2b are arranged next to one another), between two frames 5a, 5b of an individual cell 2 in the cell stack 10, a semipermeable membrane 4, typically an ion exchange membrane (either a cation or anion exchange membrane, e.g. Nation®), is arranged.The membrane 4 separates the reaction spaces 3a, 3b, recesses 6a, 6b of the half-cells 2a, 2b of a single cell 2, the electrodes 7a, 7b arranged therein, and the electrolyte liquids 15a, 15b located therein. Between each two individual cells 2 adjacent in the stack direction R, an electrode plate 8, e.g., a bipolar plate, is arranged in the cell stack 10. The electrode plate 8 is, as shown in Fig. 3, inserted into mutually facing recesses 32 in the frames 5a, 5b. The frames 5a, 5b have central recesses 6a, 6b that run through in the stack direction R, each forming a reaction space 3a, 3b and in which electrodes 7a, 7b, e.g., mats made of carbon fiber, are arranged, as shown in Fig. 3.The differently charged electrolytes 15a, 15b are pumped through the recesses 6a, 6b in the frames 5a, 5b through the individual cells 2, with an electrolyte 15a, 15b with a different electrical charge flowing through the electrode 7a, 7b of each half-cell 2a, 2b of an individual cell 2. The electrolytes 15a, 15b are supplied and removed from the outside via electrolyte connections 22a, 22b, 23a, 23b and are then distributed internally via an electrolyte channel system provided in the frames 5a, 5b with electrolyte channels 18a, 18b, 19a, 19b. The electrolyte connections 22a, 22b, 23a, 23b are provided, for example, on an end plate 24 of the cell stack 10, as shown in Fig.3, although other arrangements of the electrolyte connections 22a, 22b, 23a, 23b, for example on an end frame 20, are also possible.

[0037] The cell stack 10 can be closed off in the stacking direction R by an end frame 20 at each of its two axial ends. An electrically conductive current collector 21 is arranged in the end frame 20, e.g., in a recess on one end face of the end frame 20, and is connected to an externally routed electrical power connection 11, 12. In the illustrated embodiment, the current collector 21 rests against the last electrode plate 8 of the last individual cell 2 to establish electrical contact. However, the current collector 21 or a power connection 11, 12 could also be designed differently. Likewise, the end frame 20 could be omitted from the cell stack 10.

[0038] In the illustrated embodiment, the cell stack 10 is arranged between two rigid end plates 24 and pressed together by clamping means 25. The clamping means 25 are designed, for example, with through-reaching bolts 26, nuts 27, washers 28, and springs 29, as shown in Fig. 2. However, the cell stack 10 can also be held together in other ways; in particular, the clamping means 25 can be designed differently. The two end plates 24 can also be arranged between two pressure plates 30, which are pressed together by the clamping means 25, as shown in Fig. 2. To prevent the frames 5a, 5b from settling due to the contact pressure of the clamping means 25, a spacer 31 can also be provided between the end plates 24.

[0039] However, the present invention is not limited to a specific embodiment of a half-cell 2a, 2b, a single cell 2, a cell stack 10, or the redox flow battery 1. Nor is the invention limited to a specific electrolyte 15a, 15b or to specific redox elements in the electrolytes 15a, 15b. The above statements regarding a redox flow battery 1 serve merely to facilitate understanding of the invention.

[0040] The invention will now be described, without limitation of generality, using the example of a vanadium redox flow battery 1, wherein vanadium V in different oxidation states is present as the redox element in both electrolytes 15a, 15b. As already explained at the outset, during operation of a redox flow battery 1, parasitic processes or parasitic chemical reactions lead to a charge imbalance in the electrolytes 15a, 15b, so that one of the electrolytes 15a, 15b is electrically overcharged or undercharged compared to the other electrolyte 15a, 15b. This leads, for example, to the positive electrolyte 15a being overcharged and the negative electrolyte 15b being undercharged, whereby the overall system is overoxidized. Or the negative electrolyte 15b being overcharged and the positive electrolyte 15a being undercharged, whereby the overall system is overreduced.To correct such a charge imbalance, the oxidation state of at least one of the electrolytes 15a, 15b must be changed. This means that one of the electrolytes 15a, 15b must be either chemically reduced or chemically oxidized to change its oxidation state.

[0041] The invention relates to an electrochemical change in the oxidation state of an electrolyte 15a, 15b by means of an electrochemical balancing cell 40. In the electrochemical balancing cell 40, a redox reaction takes place which either chemically oxidizes or chemically reduces one of the electrolytes 15a, 15b as required.

[0042] Fig. 4 shows an example of an electrochemical equalization cell 40 for changing the oxidation state of an electrolyte 15a, 15b of a redox flow battery 1. In the illustrated embodiment, for example, the positive electrolyte 15a is overcharged (overoxidized) and is chemically reduced in the equalization cell 40. However, it would also be conceivable for the positive electrolyte 15a to be overreduced and chemically oxidized in the equalization cell 40.

[0043] In the compensation cell 40, a first liquid volume 41 and a second liquid volume 42 are provided, which are separated from each other by a membrane 43.

[0044] The membrane 43 is a semipermeable, particularly ion-selective, membrane and can be made, for example, of sulfonate-modified polytetrafluoroethylene (PTFE), trade name Nation™. The membrane 43 enables ions to achieve charge equalization between the first liquid volume 41 and the second liquid volume 42 (or between the electrolytes contained therein).

[0045] The electrochemical equalization cell 40 also includes two electrodes 47, 48, with one electrode 47 serving as the cathode and the other electrode 48 as the anode. An electrical voltage VA is applied to the two electrodes 47, 48 to drive the redox reaction for changing the oxidation state of the electrolyte. For example, a DC voltage of 5V to 12V is applied as the voltage.

[0046] The electrochemical equalization cell 40 in the embodiment according to Fig.4 thus consists, like a single cell 2 of the redox flow battery 1, of two electrochemical half-cells 50a, 50b which are separated from each other by a membrane 43. Each electrochemical half-cell 50a, 50b of the electrochemical equalization cell 40 comprises an electrode 47, 48 and a reaction space in the form of the first liquid volume 41 and the second liquid volume 42. An electrolyte is passed through the reaction space of each electrochemical half-cell 50a, 50b of the electrochemical equalization cell 40: the electrolyte 15a, 15b of the redox flow battery 1, whose oxidation state is to be changed, and the charge equalization electrolyte 44. Each electrode 47, 49 is in contact with one of the electrolytes 15a, 15b or with the charge equalization electrolyte 44.The electrode 47, 48 can be designed as desired, for example flat or as a rod electrode or as a grid electrode.

[0047] When using the electrochemical equalization cell 40, the first liquid volume 41 contains the electrolyte 15a, 15b whose oxidation state is to be changed. The second liquid volume 42 contains a charge equalization electrolyte 44. The charge equalization electrolyte 44 is circulated from a charge equalization electrolyte container 45 through the second liquid volume 42, which can occur both continuously and intermittently during operation of the equalization cell 40.

[0048] An electrochemical half-cell 50a, 50b of the equalizing cell 40 advantageously consists of a frame, for example made of plastic, in which an electrode 47, 48 is arranged.

[0049] Channels are formed on each electrochemical half-cell 50a, 50b in order to be able to supply and discharge an electrolyte 15a, 15b or the charge balancing electrolyte 44 to and from the first or second liquid volume 41, 42.

[0050] An electrode 47, 48 can be inserted into a recess of a frame of the respective electrochemical half-cell 50a, 50b of the equalizing cell 40, as in a single cell 2 of the cell stack 10 of the redox flow battery 1.

[0051] In the simplest case, the charge-balancing electrolyte 44 can be water. However, for a more efficient charge-balancing process, an aqueous solution of an ionic substance is used, for example, an aqueous sodium chloride (NaCl) or sodium hydroxide (NaOH) solution, or an acidic or basic charge-balancing electrolyte 44, because this increases the electrical conductivity of the charge-balancing electrolyte 44. The charge-balancing electrolyte 44 is, for example, sulfuric acid H2SO4 in an aqueous solution, for example, with a concentration of 2 to 16 M(ol) / I(iter), typically 3 M / L or 6 M / L. The charge-balancing electrolyte 44 is thus, in the most general case, water containing ions. The concentration of the ions determines the electrolyte concentration.A redox reaction takes place in the electrochemical equalization cell 40, whereby when the equalization voltage VA is applied in the half-cell 50b with the charge equalization electrolyte 44, a half electrolysis reaction of a water electrolysis takes place and in the half-cell 50a with the electrolyte 15a, 15b, a chemical oxidation or reduction of the electrolyte 15a, 15b, specifically of a redox element in the electrolyte 15a, 15b.

[0052] During a chemical reduction of an over-oxidized electrolyte 15a, 15b, the following chemical reaction occurs in the half-cell 50b with an acidic charge-balancing electrolyte 44: HJJ -^ —C + 2H + + 2e and with a basic charge balancing electrolyte 44 2 the chemical reaction 2OH 2HJ) + — <9, + 2e ■ As a counter-reaction, the other 2

[0053] Half cell 50a assuming a vanadium-based electrolyte 15a, 15b with an acidic charge balancing electrolyte 44 the following chemical reaction

[0054] 2K 3+ + 2e 2V 2+ and with a basic charge balancing electrolyte 44 the chemical reaction 2PO 2+ +2e“ -^2VO2 + In both cases, the electrolyte is chemically reduced.

[0055] During a chemical oxidation of an over-reduced electrolyte 15a, 15b, the chemical reaction takes place in the half-cell 50b with an acidic charge balancing electrolyte 44 and with a basic charge-balancing electrolyte 44, the chemical reaction 2H2O+2e~ -^H2+2OH~ . As a counter-reaction, in the other half-cell 50a, assuming a vanadium-based electrolyte 15a, 15b with an acidic charge-balancing electrolyte 44, the chemical reaction 2VO2-^2 ? 2++2e“ and with a basic charge balancing electrolyte 44 the chemical reaction

[0056] 2V 2+ 2V i+ + 2e . In both cases, the electrolyte is chemically oxidized. The polarity of the compensation voltage V A is reversed here.

[0057] It can be seen that in the half-cell 50b with the charge-balancing electrolyte 44, only half of the water electrolysis takes place, forming either oxygen O2 or hydrogen H2. However, oxygen O2 and hydrogen H2 are not produced simultaneously. In the half-cell 50a, 50b with the electrolyte 15a, 15b, instead of the other half of the water electrolysis, the chemical reduction or oxidation of the electrolyte 15a, 15b takes place. When using charge-balancing electrolytes 44 with a different chemical basis, such as halides, other gases, such as chlorine gas, can also be formed. The charge balancing electrolyte 44 is stored in a charge balancing electrolyte container 45, wherein the liquid level of the charge balancing electrolyte 44 in the charge balancing electrolyte container 45 is higher than the liquid level in the second liquid volume 42 of the second electrochemical half-cell 50b.A supply line 51 is provided in the equalization cell 40, which connects the charge-balancing electrolyte container 45 to the second liquid volume 42. The supply line 51 opens into the lower region (in the direction of gravity) of the liquid volume 42. The supply line 51 is connected to the charge-balancing electrolyte 44 in the charge-balancing electrolyte container 45. This allows the charge-balancing electrolyte 44 to flow from the charge-balancing electrolyte container 45 via the supply line 51 into the second liquid volume 42. A discharge line 52 is also provided in the equalization cell 40, which connects the second liquid volume 42 to the charge-balancing electrolyte container 45. The discharge line 52 opens into the upper region (in the direction of gravity) of the liquid volume 42 of the second electrochemical half-cell 50b of the equalizing cell 40.The discharge line 52 is connected to the charge-balancing electrolyte 44 in the charge-balancing electrolyte container 45. This allows the charge-balancing electrolyte 44 to flow from the second liquid volume 42 via the discharge line 52 into the charge-balancing electrolyte container 45.

[0058] As already explained above, either oxygen O2 or hydrogen H2 is produced in the second liquid volume 42 through half the water electrolysis, which form gas bubbles 53 in the charge-balancing electrolyte 44. The gas bubbles 53 rise due to the natural buoyancy force (seen in the direction of gravity). Thus, the density of the charge-balancing electrolyte 44 is lower in the upper region of the second liquid volume 42 than in the lower region of the second liquid volume 42. The density will essentially decrease continuously from bottom to top. Once the liquid level of the charge-balancing electrolyte 44 in the charge-balancing electrolyte container 45 is higher than the liquid level in the second liquid volume 42 and the supply line 51 opens into the lower region of the second liquid volume 42, a natural circulation of the charge-balancing electrolyte 44 occurs.This circulation is primarily driven by the higher-density charge-balancing electrolyte 44 in the charge-balancing electrolyte reservoir 45 and in the supply line 51. However, this circulation is also supported by the buoyancy of the gas bubbles 53 and their upward movement. Thus, no circulation pump is required to circulate the charge-balancing electrolyte 44. The circulation can also be considered self-regulating because the circulation increases when more gas bubbles 53 are generated due to a more vigorous redox reaction.

[0059] The gas entrained in the charge equalization electrolyte 44, such as oxygen O2 or hydrogen H2, will collect in the gas space 46 of the charge equalization electrolyte container 45 and can be discharged from there via a discharge opening 55, for example simply released into the environment, or used for other purposes.

[0060] The discharge line 52 may also include a riser pipe 54 that extends a predetermined height h (as seen in the direction of gravity) into the charge-balancing electrolyte 44 in the charge-balancing electrolyte container 45. The riser pipe 54 opens into the charge-balancing electrolyte 44. Such a riser pipe 54 improves the natural circulation of the charge-balancing electrolyte 44 because the charge-balancing electrolyte 44 in the charge-balancing electrolyte container 45 is less contaminated with gas bubbles 53.

[0061] The electrolyte 15a, 15b to be treated in the equalization cell 40 can be circulated from the respective storage tank 13a, 13b through the first half-cell 50a of the equalization cell 40. For this purpose, a separate circulation pump 56 and corresponding lines 49 connecting the storage tank 13a, 13b to the first liquid volume 41 can be provided. However, the existing circulation pump 9a, 9b of the redox flow battery 1 could also be used to circulate the electrolyte 15a, 15b through the first half-cell 50a.

[0062] The circulation of the electrolyte 15a, 15b through the first half-cell 50a of the equalizing cell 40 occurs continuously or intermittently. The circulation of the charge equalizing electrolyte 44 occurs only during operation of the equalizing cell 40, when the redox reaction for charge equalization takes place.

[0063] Due to the semi-electrolysis of water taking place, water from the aqueous charge-balancing electrolyte 44 is consumed. To compensate for this water consumption, a water tank 60 containing water 61 can be provided. The water tank 60 is connected to the charge-balancing electrolyte tank 45 via a water supply line 62. A water circulation pump 63 is arranged in the water supply line 62. A level sensor 64 is arranged on the charge-balancing electrolyte tank 45 and measures the liquid level of the charge-balancing electrolyte 44 in the charge-balancing electrolyte tank 45. If the liquid level drops too far due to water consumption, the water circulation pump 63 is activated, which pumps water 61 from the water tank 60 into the charge-balancing electrolyte tank 45 until a predetermined liquid level in the charge-balancing electrolyte tank 45 is reached again.

[0064] The water 61 in the water tank 60 can be refilled, for example, during regular maintenance of the redox flow battery 1. In Fig. 6, as in a cell stack 10 of the redox flow battery 1, several balancing cells 40 are arranged next to one another and form a balancing cell stack 70. Adjacent balancing cells 40 in the balancing cell stack 70 are separated from one another (as in cell stack 10) by a bipolar plate 71. End plates 72, 73 are arranged at the axial ends of the balancing cell stack 70, which hold the balancing cell stack 70 together.

[0065] The compensation voltage V Aapplied and / or the electrolyte 15a, 15b and charge equalization electrolyte 44 are supplied and removed. The structure of a balancing cell stack 60 therefore does not differ significantly from the structure of a cell stack 10. The charge equalization performance can be increased by the parallel flow of the electrolyte 15a, 15b through the first liquid volume 41 of the respective first electrochemical half-cells 50a of the electrochemical equalization cells 40 and the charge equalization electrolyte 44 through the second liquid volume 42 of the respective second electrochemical half-cells 50b of the electrochemical equalization cells 40.

Claims

Patent claims 1. An electrochemical equalization cell for changing an oxidation state of an electrolyte (15a, 15b) with a redox element of a redox flow battery (1), wherein a first electrochemical half-cell (50a) with a first liquid volume (41) and a second electrochemical half-cell (50b) with a second liquid volume (42) are provided in the electrochemical equalization cell (40), wherein the first electrochemical half-cell (50a) comprises a first electrode (47) and the second electrochemical half-cell (50b) comprises a second electrode (48), and an electrical equalization voltage (V A), and wherein the first liquid volume (41) is separated from the second liquid volume (42) by a membrane (43), and wherein, during operation of the electrochemical equalization cell (40), the first liquid volume (41) contains the electrolyte (15a, 15b) and the first electrode (47) is in contact with the electrolyte (15a, 15b), and the second liquid volume (42) contains an aqueous charge equalization electrolyte (44) different from the electrolyte (15a, 15b), and the second electrode (48) is in contact with the charge equalization electrolyte (44), and wherein a circulation pump (9a, 9b, 56) is provided which, during operation of the electrochemical equalization cell (40), at least temporarily circulates the electrolyte (15a, 15b) through the first liquid volume (41) of the first electrochemical half-cell (50a), characterized in that during operation of the electrochemical equalization cell (40) between the two electrodes (47, 48) a compensation voltage (V A) is applied, which causes a half water electrolysis reaction in the charge balancing electrolyte (44) in the second liquid volume (42), which releases either oxygen or hydrogen and which causes a chemical reduction or chemical oxidation of the redox element in the electrolyte (15a, 15b) in the first liquid volume (41) in order to change the oxidation state of the redox element, that the charge balancing electrolyte (44) is stored in a charge balancing electrolyte container (45) and a liquid level of the charge balancing electrolyte (44) in the charge balancing electrolyte container (45) is higher than the liquid level in the second liquid volume (42), that a supply line (51) is provided in the electrochemical balancing cell (40) which connects the charge balancing electrolyte container (45) to the second liquid volume (42),wherein the supply line (45) opens into the lower region of the second liquid volume (42) and the supply line (51) is connected to the charge balancing electrolyte (44) in the charge balancing electrolyte container (45), and in that a discharge line (52) is provided in the electrochemical equalization cell (40), which connects the second liquid volume (42) to the charge balancing electrolyte container (45), wherein the discharge line (52) opens into the upper region of the second liquid volume (42) and the discharge line (52) is connected to the charge balancing electrolyte (44) in the charge balancing electrolyte container (45).

2. Electrochemical equalization cell according to claim 1, characterized in that the discharge line (52) is connected to a riser pipe (54) in the charge equalization electrolyte container (45), the riser pipe (54) projecting a predetermined height (h) into the charge equalization electrolyte container (45) and the riser pipe (54) in the charge equalization electrolyte container (45) opening into the charge equalization electrolyte (44).

3. Electrochemical equalization cell according to claim 1 or 2, characterized in that a water tank (60) is provided which is connected to the charge equalization electrolyte tank (45) via a water supply line (62), wherein a water circulation pump (63) is provided in the water supply line (62) in order to pump water from the water tank (60) into the charge equalization electrolyte tank (45) as required.

4. A method for changing an oxidation state of an electrolyte (15a, 15b) with a redox element of a redox flow battery (1) with an electrochemical equalization cell (40) with a first electrochemical half-cell (50a) with a first liquid volume (41) and a second electrochemical half-cell (50b) with a second liquid volume (42), wherein the first electrochemical half-cell (50a) comprises a first electrode (47) and the second electrochemical half-cell (50b) comprises a second electrode (48), and an electrical equalization voltage (VA) is applied between the first electrode (47) and the second electrode (48), and wherein the first liquid volume (41) is separated from the second liquid volume (42) by a membrane (43), and wherein the first liquid volume (41) contains the electrolyte (15a, 15b) in contact with the first electrode (47), and the second liquid volume (42) one from the electrolyte (15a,15b) different aqueous charge balancing electrolytes (44) in contact with the second electrode (48), and wherein the electrolyte (15a, 15b) is at least temporarily circulated through the first liquid volume (41) of the first electrochemical half-cell (50a), characterized in that the balancing voltage (V, A) in the charge balancing electrolyte (44) in the second liquid volume (42) causes a half water electrolysis reaction which releases either oxygen or hydrogen and which causes a chemical reduction or chemical oxidation of the redox element in the electrolyte (15a, 15b) in the first liquid volume (41) in order to change the oxidation state of the redox element, that the charge balancing electrolyte (44) is stored in a charge balancing electrolyte container (45) and that a liquid level of the charge balancing electrolyte (44) in the charge balancing electrolyte container (45) is higher than the liquid level in the second liquid volume (42), that the charge balancing electrolyte container (45) is connected to the second liquid volume (42) via a supply line (51), wherein the supply line (51) is in the lower region of the second liquid volume (42) and the supply line (51) is connected to the charge balancing electrolyte (44) in the charge balancing electrolyte container (45), that the second liquid volume (42) is connected to the charge balancing electrolyte container (45) by a discharge line (52), wherein the discharge line (52) opens in the upper region of the second liquid volume (42) and the discharge line (52) is connected to the charge balancing electrolyte (44) in the charge balancing electrolyte container (45), and that the released oxygen or hydrogen in the form of gas bubbles (53) in the second liquid volume (42) and in the discharge line (52) rises upwards into the charge balancing electrolyte container (45), so that the charge balancing electrolyte (44) is circulated through the second liquid volume (42).

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