Electrochemical equalization cell

Abstract

To control the water management in an electrochemical equalization cell (40) with aqueous electrolytes in a way that is efficient, reliable and involves little expenditure on equipment, it is provided that the reaction chamber (57) and the equalization chamber (58) of the second electrochemical half-cell (50b) of the equalization cell (40) are connected to one another below the respective liquid levels (FS1, FS2) of the charge-equalization electrolyte (44) by at least one opening (56) such that the charge-equalization electrolyte (44) can flow back and forth between the reaction chamber (57) and the equalization chamber (58) by way of the at least one opening (56).

Description

[0001] Electrochemical balancing cell

[0002] The present invention relates to an electrochemical equalization cell for changing the oxidation state of an electrolyte, preferably a redox flow battery, wherein a first half-cell with a first reaction chamber and a second half-cell with a second reaction chamber 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 reaction chamber is separated from the second reaction chamber by a membrane, and wherein, during operation of the equalization cell, the first reaction chamber contains the electrolyte, the first electrode of the first half-cell is in contact with the electrolyte, and the second reaction chamber contains an aqueous charge equalization electrolyte, and wherein a charge equalization electrolyte container is provided,which is divided into a reaction chamber containing charge-balancing electrolyte and a compensation chamber containing charge-balancing electrolyte, wherein the second electrode is in contact with the charge-balancing electrolyte in the reaction chamber, so that the reaction chamber forms the reaction space of the second half-cell. The invention also relates to a method for regulating the water balance in such an electrochemical compensation 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. V 2+ / V 3+ , Fe 3+ / Fe 2+ vs. Br2 / Br, Fe 3+ / Fe 2+ vs. Cr^ / 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.

[0006] 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 lv 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 111 or V 3+ referred to) - whereby the negative electrolyte has a more negative electrochemical potential than the positive electrolyte.

[0007] 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 +

[0008] 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).

[0009] In addition to the desired charge / discharge reactions, parasitic reactions can also occur, which lead to an ineffective charge / discharge process.

[0010] 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:

[0011] 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.

[0012] 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.

[0013] 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 7V 3+ and V 4+ A / 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 reduced by a redox reaction, or an over-reduced electrolyte is oxidized by a redox reaction.

[0014] US 8,877,365 B2 shows an electrochemical 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 aqueous 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. Due to the redox reaction, Fe is oxidized in the over-oxidized 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. In a photochemical cell, the chlorine gas is combined with the hydrogen gas produced on the anode side of the redox flow battery through a parasitic reaction to form hydrochloric acid using energy input from UV light. The hydrochloric acid is returned to the anode side of the equalization cell. The disadvantage of this equalization cell is, on the one hand, the increased energy consumption for generating UV light in the photochemical cell, which reduces the overall efficiency of the redox flow battery. On the other hand, water loss from the aqueous acid solution inevitably occurs during operation of the equalization cell, which changes the acid concentration in the electrolyte. A further problem with such an equalization cell is that osmosis and electroosmosis can lead to water exchange with the electrolyte on the cathode side via the membrane of the equalization cell.These effects depend heavily on the current conditions in the equalization cell. This water exchange can lead to strongly fluctuating electrolyte levels on both sides of the membrane. This water exchange can also occur when the equalization cell is not in use. If the electrolyte level is too low on one side of the membrane, part of the membrane surface could be exposed, increasing the current density in the wetted membrane. This can damage the membrane or other parts of the equalization cell. In extreme cases, the electrical connection could be interrupted, preventing the equalization cell from functioning. Last but not least, the electrolyte system could overflow on one side of the membrane. It is therefore essential to control water consumption on both sides of the membrane in an electrochemical equalization cell with aqueous electrolytes.

[0015] For example, a separate water tank could be provided, connected to the electrolyte in a half-cell of the equalization cell via a water circulation pump. The water circulation pump is actively controlled to keep the water balance in the equalization cell under control. However, this increases the equipment complexity of the equalization cell and the redox flow battery, and the additional components increase the susceptibility to failure. Last but not least, the water level in the water tank must be checked regularly and refilled if necessary.

[0016] Another approach to controlling the water balance in an equalization cell is to balance the mass flows of water in the equalization cell as much as possible through appropriate design. This allows a water equilibrium to be achieved between both sides of the membrane during operation of the equalization cell. However, when the equalization cell is not in use, the conditions change and the water equilibrium would be lost, which in turn leads to water migrating to one side of the membrane and raising the liquid level of the electrolyte there. While this could be absorbed by a sufficiently large electrolyte tank, it would still lead to considerable volume fluctuations of the electrolyte on both sides of the membrane. The electrolyte tank would then have to be significantly oversized, which in turn increases the cost of the equalization cell.

[0017] There is therefore a need for an efficient, reliable and minimally complex control of the water balance of a balancing cell with aqueous electrolyte.

[0018] This is achieved according to the invention in that the reaction chamber and the equalization chamber are connected to one another below the respective liquid levels of the charge equalization electrolyte by at least one opening, so that charge equalization electrolyte can flow back and forth between the reaction chamber and the equalization chamber via the at least one opening.

[0019] This makes it possible for water loss in the reaction chamber, which occurs during operation of the equalization cell due to electroosmosis through the membrane, osmosis through the membrane and due to the redox reaction in the equalization cell which occurs due to the equalization voltage and which causes half a water electrolysis reaction in the reaction chamber, to be at least partially compensated for by charge equalization electrolyte flowing from the equalization chamber into the reaction chamber through at least one opening which connects the reaction chamber to the equalization chamber, whereby the electrolyte concentration in the charge equalization electrolyte in the reaction chamber is increased until the direction of osmosis through the membrane is reversed and water migrates through osmosis from the electrolyte in the first reaction space of the first half-cell through the membrane into the reaction chamber.This allows, when the equalization cell is at a standstill, water to migrate by osmosis from the electrolyte in the first reaction chamber of the first half-cell through the membrane into the reaction chamber of the second half-cell, whereby the volume of the charge equalization electrolyte in the reaction chamber increases and charge equalization electrolyte flows from the reaction chamber into the equalization chamber through at least one opening connecting the reaction chamber to the equalization chamber, until a hydrostatic pressure acting in the reaction chamber and the equalization chamber equalizes.

[0020] The water balance in the equalization cell according to the invention thus regulates itself automatically and requires no active intervention, which increases the reliability and availability of the water balance control. Furthermore, water consumption in the equalization cell can be significantly reduced, enabling longer operating times of the equalization cell without maintenance. Last but not least, the charge equalization electrolyte container can also be made smaller due to the low fluctuations in the liquid levels. The invention implements a hydrostatic osmosis damper in the equalization cell, which dampens the effect of osmosis-driven water migration through the membrane.

[0021] Advantageously, a first gas chamber is formed above the liquid level of the charge-balancing electrolyte in the reaction chamber, and a second gas chamber is formed above the liquid level of the charge-balancing electrolyte in the equalization chamber, wherein the first gas chamber and the second gas chamber are connected to each other by at least one connecting opening. Due to this connection, the pressure in the two gas chambers is approximately equal, thus the exchange of charge-balancing electrolyte between the reaction chamber and the equalization chamber is unaffected by any pressure differences between the two gas chambers. This improves the function of controlling the water balance.

[0022] In an advantageous embodiment, a wall extends into the reaction chamber and / or the compensation chamber in the region of and above the at least one opening. This wall prevents sediments or solids sinking downwards due to gravity from settling in the area of ​​the opening and blocking it.

[0023] The present invention will be explained in more detail below with reference to Figures 1 to 9, which show exemplary, schematic and non-limiting advantageous embodiments of the invention.

[0024] Fig.1 shows 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 a compensation cell according to the invention,

[0028] Fig.5 possible mass flows in a compensation cell that can lead to water loss,

[0029] Fig.6 the conditions in a compensation cell according to the invention during operation of the compensation cell,

[0030] Fig.7 the conditions in a compensation cell according to the invention when the compensation cell is at a standstill,

[0031] Fig.8 shows an alternative embodiment of a compensation cell according to the invention and Fig.9 shows a compensation cell according to the invention with a wall above the opening.

[0032] 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.

[0033] 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).

[0034] An electrical load 14 can take any form. Based on the electrical voltage, current, or power requirements of the electrical load 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, a person skilled in the art can design or select a cell stack 10, or a parallel and / or serial connection of multiple cell stacks, and redox pairs in a redox flow battery 1.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] However, the present invention is not limited to a specific design 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 merely serve to facilitate understanding of the invention. The invention will be described below, without restricting its 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.

[0043] As already explained at the beginning, during operation of a redox flow battery 1, parasitic processes or parasitic chemical reactions lead to a charge imbalance in the aqueous 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, resulting in the overall system being overoxidized. Or the negative electrolyte 15b being overcharged and the positive electrolyte 15a being undercharged, resulting in the overall system being 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 an electrolyte 15a, 15b must either be chemically reduced or chemically oxidized in order to change its oxidation state.

[0044] The invention relates to an electrochemical change in the oxidation state of an electrolyte 15a, 15b by means of an electrochemical balancing cell 40 with water balance control. During operation, a redox reaction takes place in the electrochemical balancing cell 40, which either chemically oxidizes or chemically reduces one of the aqueous electrolytes 15a, 15b as needed.

[0045] Fig. 4 shows an electrochemical equalization cell 40 for changing the oxidation state of an electrolyte 15a, 15b of a redox flow battery 1 according to the invention. In the illustrated embodiment, for example, the positive electrolyte 15a is overcharged (overoxidized) and is chemically reduced in the electrochemical equalization cell 40. However, it would also be conceivable for the positive electrolyte 15a to be overreduced and chemically oxidized in the electrochemical equalization cell 40.

[0046] The equalizing cell 40 consists of a first electrochemical half-cell 50a with a first reaction chamber 41 and a second electrochemical half-cell 50b with a second reaction chamber 42, which are separated from each other by a membrane 43.

[0047] The membrane 43 is a semipermeable, in particular ion-selective, membrane and can be made, for example, from sulfonate-modified polytetrafluoroethylene (PTFE), with the trade name Nation™. The membrane 43 enables ions to achieve charge equalization between the first reaction chamber 41 and the second reaction chamber 42 (or between the electrolytes contained therein). The first electrochemical half-cell 50a can be designed like a flow-through half-cell 2a, 2b of the cell stack of the redox flow battery 1 and can comprise a frame 46 that forms the first reaction chamber 41. The frame 46 is made, for example, of plastic. However, the first half-cell 50a can also be designed in any other way, in particular also in a design other than a flow-through half-cell.

[0048] The first reaction chamber 41 of the first electrochemical half-cell 50a contains the electrolyte 15a, 15b, the oxidation state of which is to be changed in the electrochemical equalization cell 40.

[0049] A first electrode 47 is arranged in the first electrochemical half-cell 50a, for example, in a recess in the frame 46. The first electrode 47 is in contact with the aqueous electrolyte 15a, 15b in the first reaction chamber 41.

[0050] The electrolyte 15a, 15b is preferably circulated through the first reaction chamber 41, either permanently or intermittently if the electrochemical equalization cell 40 is integrated into a redox flow battery 1, in order to equalize the charge in the electrolyte 15a, 15b as needed. For this purpose, a circulation pump 51 can be provided, which is arranged in an electrolyte line 52. The electrolyte line 52 is connected to an electrolyte tank 13a, 13b of the redox flow battery 1, in which the electrolyte 15a, 15b is stored. Instead of a separate circulation pump 51, the circulation pump 9a, 9b of the redox flow battery 1 could also be used. For the preparation of an electrolyte 15a, 15b, for example for later use in a redox flow battery 1, in an electrochemical equalization cell 40, the electrolyte 15a, 15b does not necessarily have to be circulated.In particular, in such an embodiment, the first electrochemical half-cell 50a of the electrochemical balancing cell 40 could also be designed like a second electrochemical half-cell 50b described below.

[0051] The second electrochemical half-cell 50b comprises a charge-balancing electrolyte container 45 containing charge-balancing electrolyte 44. A second electrode 48 is arranged at least partially within the charge-balancing electrolyte 44 in the charge-balancing electrolyte container 45. The charge-balancing electrolyte container 45 thus forms the second reaction chamber 42 of the second electrochemical half-cell 50b. The second electrode 50b is in contact with the charge-balancing electrolyte 44 in the charge-balancing electrolyte container 45.

[0052] 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 15a, 15b. For example, a DC voltage of 5V to 12V is applied. One electrode 47 thus serves as the cathode and the other electrode 48 as the anode, although the polarity can also be reversed.

[0053] An electrode 47, 48 can be designed as desired, for example flat or as a rod electrode or as a grid electrode.

[0054] The charge-balancing electrolyte 44 can, in the simplest case, 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)ZI(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.The charge balancing electrolyte 44 preferably does not comprise redox elements (such as vanadium) contained in the electrolyte 15a, 15b, whose oxidation state is to be changed, in order to operate the water electrolysis efficiently.

[0055] A redox reaction takes place in the 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 takes place.

[0056] During a chemical reduction of an over-oxidized electrolyte 15a, 15b, the chemical reaction takes place in the half-cell 50b with an acidic charge balancing electrolyte 44

[0057] HJJ ~^ —O ? + 2H + + 2e and with a basic charge balancing electrolyte 44 the 2 chemical reaction 2OH 2H,O + —O, + 2e . As a counter-reaction, the other 2

[0058] Half cell 50a assuming a vanadium-based electrolyte 15a, 15b with an acidic charge balancing electrolyte 44, the chemical reaction 2K 3+ + 2e 2V 2+ and with a basic charge balancing electrolyte 44 the chemical reaction 2VÖ 2+ +2e -^2VO2 + In both cases, the electrolyte 15a, 15b is consequently chemically reduced.

[0059] During a chemical oxidation of an over-reduced electrolyte 15a, 15b, the chemical reaction 2H ++2e —^H2ab and with a basic charge balancing electrolyte 44 the chemical reaction 27 / 2O+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

[0060] 2PO2 + -^2 ? 2+ +2e and with a basic charge balancing electrolyte 44 the chemical reaction 2K 2+ 2K 3+ + 2e . In both cases, the electrolyte 15a, 15b is chemically oxidized. The polarity of the compensation voltage VA is reversed.

[0061] 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.

[0062] To explain the fundamental problems associated with the water balance of the equalizing cell 40 and the invention, Fig. 5 illustrates the essential mechanisms that influence the water balance of the equalizing cell 40. Water balance refers, in particular, to the water content, as well as the dynamic change in the water content, in the electrolytes 15a, 15b and charge equalizing electrolyte 44 involved in the redox reaction.

[0063] In the equalization cell 40, the redox reaction takes place with half the water electrolysis of the charge equalization electrolyte 44, whereby gaseous oxygen O2 (as in the example in Fig. 4 and Fig. 5) or hydrogen H2 is produced in the second reaction chamber 42 at the second half-cell 50b. The ions H produced during the water electrolysis +(as in the example of Fig. 4 and Fig. 5) or OH' migrate through the semipermeable membrane 43 into the first reaction chamber 41 of the first half-cell 50a. Thus, water is lost in the second reaction chamber 42 through half the water electrolysis, which is indicated by the arrow WE. A further portion of the water in the second reaction chamber 42 can be lost through evaporation, which is indicated by the arrow E. However, evaporation E does not necessarily have to occur. Due to the compensating voltage VA applied to the electrodes 47, 48, an electric field is created which leads to electroosmosis across the membrane 43, whereby water from the second reaction chamber 42 migrates through the membrane 43 into the first reaction chamber 41, which is indicated by the arrow EO. With the polarity of the compensating voltage VA reversed, the direction of the electroosmosis can also be reversed, which is indicated by the dashed line in Fig. 5. Last but not least, osmosis also takes place via membrane 43.Through osmosis, water flows through the semipermeable membrane 43 toward the higher particle concentration to create a concentration balance. Osmosis can therefore fundamentally occur in both directions, as indicated by the double arrow O. The arrows WE, E, EO, O thus represent mass flows that influence the water balance in the equalization cell 40. However, these mass flows depend on the operating conditions in the equalization cell 40, such as the flowing electrical current i, the acid content (base content) in the electrolyte 15a, 15b and charge equalization electrolyte 44, temperature, etc. These mass flows also lead to the liquid levels in the two half-cells 50a, 50b, particularly in the second reaction chamber 42 of the second half-cell 50b, fluctuating significantly.

[0064] When the equalization cell 40 is not in operation, i.e., when the equalization voltage VA=0, at least osmosis O can still occur. Evaporation E is also fundamentally possible. Thus, when the equalization cell 40 is not in operation, water can migrate from the electrolyte 15a, 15b into the second reaction chamber 42 due to osmosis, or water can migrate from the second reaction chamber 42 into the electrolyte 15a, 15b. Thus, even when the equalization cell 40 is not in operation, a strongly fluctuating liquid level in the second reaction chamber 42 is to be expected.

[0065] To control the water balance of the equalizing cell 40, the charge equalizing electrolyte container 45 is divided according to the invention into a reaction chamber 57 and an equalizing chamber 58. Charge equalizing electrolyte 44 is contained in both chambers 57, 58, so that a liquid level FS1, FS2 is established in each of the two chambers 57, 58. In the reaction chamber 57, the second electrode 48 is arranged in the charge equalizing electrolyte 44 and thus forms the reaction space 42 of the second half-cell 50b in the charge equalizing electrolyte container 45. The charge equalizing electrolyte 44 in the reaction chamber

[0066] 57 is in contact with the membrane 43. The reaction chamber 57 and the compensation chamber

[0067] 58 in the charge-balancing electrolyte container 45 are connected to one another by an opening 56, preferably in the lower (in the direction of gravity) region of the charge-balancing electrolyte container 45, so that normally an exchange of the charge-balancing electrolyte 44 between the reaction chamber 57 and the balancing chamber 58 only takes place via the opening 56. However, the opening 56 could also be arranged higher up. It is also conceivable to arrange several openings 56 one above the other (in the direction of gravity). The opening 56 is in any case arranged below the liquid level FS1 in the reaction chamber 57 and below the liquid level FS2 in the balancing chamber 58 in order to enable balancing of the charge-balancing electrolyte 44.

[0068] In the embodiment according to Fig.4, the separation of the charge equalization electrolyte container 45 is effected by a partition wall 55 in the charge equalization electrolyte container 45. The opening 56 is provided in the partition wall 55, preferably in the lower (in the direction of gravity) region of the partition wall 55.

[0069] In an embodiment with a partition wall 55, the side of the partition wall 55 facing the reaction chamber 57, which is in contact with the charge balancing electrolyte 44 in the reaction chamber 57, could be designed as a second electrode 48.

[0070] The opening 56 may be designed as a hole, slot, multiple holes, multiple slots, etc.

[0071] The effect of separating the charge equalization electrolyte container 45 into two chambers 57, 58 connected to each other by an opening 56 is explained below.

[0072] In the following considerations, it is assumed that the equalization cell 40 is in operation, a compensation voltage VA is applied, and the redox reaction is running. If the occurring mass flows result in a loss of water in the reaction chamber 57, the hydrostatic pressure of the charge-balancing electrolyte 44 acting in the equalization chamber 58 will cause charge-balancing electrolyte 44 to flow from the equalization chamber 58 through the opening 56 into the first chamber 57 (Fig. 6) to compensate for the loss. This not only introduces additional water into the reaction chamber 57, but also an additional amount of the substance dissolved in the water of the charge-balancing electrolyte 44 (acid, such as sulfuric acid, or base).As more and more water is lost in the reaction chamber 57 toward the electrolyte 15a, 15b in the reaction chamber 41 of the first half-cell 50a, the electrolyte concentration in the charge-balancing electrolyte 44 in the reaction chamber 57 increases. This can reverse the direction of osmosis toward the reaction chamber 57, or the osmotic migration of water from the electrolyte 15a, 15b into the reaction chamber 57 increases. This first slows the water loss in the reaction chamber 57 and then compensates for the water loss until equilibrium is reached.

[0073] When the equalization cell 40 is not in operation, osmosis is the only (significant) driving force of water migration, apart from possible loss through evaporation. If water is lost from the reaction chamber 57 into the electrolyte 15a, 15b when the equalization cell 40 is switched off, this loss is compensated for by the mechanism explained above. If water migrates from the electrolyte 15a, 15b into the reaction chamber 57 due to osmosis when the equalization cell 40 is switched off, the volume of the charge-balancing electrolyte 44 in the reaction chamber 57 increases and the electrolyte concentration decreases, thereby reducing the density of the charge-balancing electrolyte 44 in the first chamber 57. A certain amount of the charge balancing electrolyte 44 of the reaction chamber 57 will flow into the balancing chamber 58 (Fig.7) until a hydrostatic equilibrium is established between the reaction chamber 57 and the balancing chamber 58.

[0074] In the equalization cell 40, both during operation of the equalization cell 40 and when the equalization cell 40 is at a standstill, different and dynamically changing liquid levels FS1, FS2 of the charge equalization electrolyte 44 in the reaction chamber 57 and the equalization chamber 58 are established, as shown in Fig. 6 and Fig. 7. This occurs essentially due to the different densities of the charge equalization electrolytes 44 in the reaction chamber 57 and the equalization chamber 58, due to different electrolyte concentrations of the charge equalization electrolyte 44, and due to the acting hydrostatic pressure, which is known to be density-dependent.

[0075] According to the invention, the electrolyte concentration in the charge-balancing electrolyte 44 in the reaction chamber 57 can vary significantly, but the liquid level FS1 in the reaction chamber 57 will fluctuate only slightly. To make particularly good use of this effect, it is advantageous if the volume for the charge-balancing electrolyte 44 in the reaction chamber 57 in the charge-balancing electrolyte container 45 is smaller than the volume for the charge-balancing electrolyte 44 in the equalization chamber 58. The smaller the ratio of the volume for the charge-balancing electrolyte 44 in the reaction chamber 57 to the volume for the charge-balancing electrolyte 44 in the equalization chamber 58, the smaller the fluctuations in the liquid levels FS1, FS2 can be expected.

[0076] The water balance in the equalizing cell 40 is thus self-regulating and requires no active intervention. Furthermore, water consumption in the equalizing cell 40 can be significantly reduced, allowing longer operating times of the equalizing cell 40 without maintenance. Last but not least, the charge equalizing electrolyte reservoir 45 can also be made smaller due to the minimal fluctuations in the liquid levels FS1, FS2.

[0077] In order to achieve the most unaffected adjustment of the liquid levels FS1, FS2 of the charge equalization electrolyte 44, it is advantageous if the gas pressure acting on the charge equalization electrolyte 44 in the two chambers 57, 58 is approximately the same.

[0078] In the embodiment of Fig. 4, the reaction chamber 57 and the equalization chamber 58 are provided in a common charge equalization electrolyte container 45, wherein, during operation of the equalization cell 40, a gas space 53, 54 free of charge equalization electrolyte 44 is formed above the respective liquid level FS1, FS2 of the charge equalization electrolyte 44 in the reaction chamber 57 and the equalization chamber 58. The two gas spaces 53, 54 are connected to one another via a connecting opening 63, in that the partition wall 55 does not extend over the entire height of the charge equalization electrolyte container 45. In this embodiment, however, the partition wall 55 extends at least above (in the direction of gravity) the respective maximum expected liquid levels FS1, FS2 of the charge equalization electrolyte 44 in the reaction chamber 57 and the equalization chamber 58.However, a partition wall 55 extending over the entire height of the charge equalization electrolyte container 45 could also be provided, in which a connecting opening 63 is provided in the area of ​​the gas spaces 53, 54, connecting the two gas spaces 53, 54. However, the connecting opening 63 connecting the gas spaces 53, 54 is arranged at least above (in the direction of gravity) the respective maximum expected liquid levels FS1, FS2 of the charge equalization electrolytes 44 in the reaction chamber 57 and the equalization chamber 58. These designs of dividing the charge equalization electrolyte container 45 into reaction chamber 57 and equalization chamber 58 ensure that the pressures in the two gas spaces 53, 54 are approximately equal. Normally, no equalization of the charge equalization electrolytes 44 between the reaction chamber 57 and the equalization chamber 58 takes place via the gas spaces 53, 54.

[0079] Fig. 8 shows another possible embodiment of the charge-balancing electrolyte container 45. In this embodiment, the charge-balancing electrolyte container 45 comprises a first container 45a containing charge-balancing electrolyte 44 and a second container 45b containing charge-balancing electrolyte 44. The first container 45a forms the reaction chamber 57, and the second container 45b forms the balancing chamber 58. The reaction chamber 57 and the balancing chamber 58 are thus separated from each other by the separate containers 45a, 45b. The first container 45a and the second container 45b are connected to one another below the respective liquid levels FS1, FS2 of the charge balancing electrolyte 44, preferably in the lower (in the direction of gravity) region of the respective container 45a, 45b, by at least one line 60, wherein the at least one line 60 forms the at least one opening 56.However, the effect and function of the balancing cell 40, in particular the automatic regulation of the water balance, remains unaffected.

[0080] During operation of the equalization cell 40, a gas chamber 53, 54 is formed in each container 45a, 45b above the respective liquid level FS1, FS2 of the charge equalization electrolyte 44. To approximately equalize the pressures therein, the gas chambers 53, 54 are connected to one another via at least one gas line 62, which forms the connecting opening 63.

[0081] In the embodiment of the charge equalization electrolyte container 45 shown in Fig. 9, a wall 61 is provided in the region of at least one opening 56 and above (in the direction of gravity) the opening 56, which extends a short distance into the reaction chamber 57. In this embodiment, the wall 61 is arranged on the partition wall 55. Instead of a wall 61 extending into the reaction chamber 57, or in addition to a wall 61 extending into the reaction chamber 57, a wall 61 extending into the equalization chamber 58 can also be provided (as indicated by dashed lines in Fig. 9). The wall 61 ensures that any sediment or solids in the charge equalization electrolyte 44, which sink due to gravity, are deposited on the wall 61 and do not block the opening 56 located below.Such a wall 61 can of course also be provided in an embodiment of the charge equalization electrolyte container 45 with two separate containers 45a, 45b, wherein the wall 61 in this case protrudes from a respective container wall of the container 45a, 45b into the reaction chamber 57 and / or into the equalization chamber 58.

Claims

Patent claims 1. Electrochemical equalization cell for changing an oxidation state of an electrolyte (15a, 15b), preferably a redox flow battery (1), wherein a first electrochemical half-cell (50a) with a first reaction chamber (41) and a second electrochemical half-cell (50b) with a second reaction chamber (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 (VA) can be applied to the first electrode (47) and second electrode (48), and wherein the first reaction chamber (41) is separated from the second reaction chamber (42) by a membrane (43), and wherein, during operation of the electrochemical equalization cell (40), the first reaction chamber (41) contains the electrolyte (15a,15b) and the first electrode of the first half-cell is in contact with the electrolyte and the second reaction space (42) contains an aqueous charge-balancing electrolyte (44), and wherein a charge-balancing electrolyte container (45) is provided which is divided into a reaction chamber (57) containing charge-balancing electrolyte (44) and a balancing chamber (58) containing charge-balancing electrolyte (44), wherein the second electrode (48) is in contact with the charge-balancing electrolyte (44) in the reaction chamber (57), so that the reaction chamber (57) forms the second reaction space (42) of the second electrochemical half-cell (50b), characterized in that the reaction chamber (57) and the balancing chamber (58) are connected to one another below the respective liquid levels (FS1, FS2) of the charge-balancing electrolyte (44) by at least one opening (56). are,so that charge balancing electrolyte (44) can flow back and forth via the at least one opening (56) between the reaction chamber (57) and the balancing chamber (58).

2. Electrochemical equalization cell according to claim 1, characterized in that the at least one opening (56) is arranged in the lower region of the reaction chamber (57) and the equalization chamber (58).

3. Electrochemical equalization cell according to claim 1 or 2, characterized in that a first gas space (53) is formed above the liquid level (FS1) of the charge equalization electrolyte (44) in the reaction chamber (57) and a second gas space (54) is formed above the liquid level (FS2) of the charge equalization electrolyte (44) in the equalization chamber (58), wherein the first gas space (53) and the second gas space (54) are connected to one another by at least one connecting opening (63).

4. Electrochemical equalization cell according to one of claims 1 to 3, characterized in that a partition wall (55) is provided in the charge equalization electrolyte container (45), which divides the charge equalization electrolyte container (45) into the reaction chamber (57) and the equalization chamber (58), wherein the at least one opening (56) is provided in the partition wall (55).

5. Electrochemical equalization cell according to claim 3 and 4, characterized in that the at least one connecting opening (63) is formed in the partition wall (55).

6. Electrochemical equalization cell according to one of claims 1 to 3, characterized in that the charge equalization electrolyte container (45) comprises a first container (45a) which forms the reaction chamber (57) and a second container (45b) which forms the equalization chamber (58), wherein a line (60) is provided which connects the first container (45a) to the second container (45b) and the line (60) forms the at least one opening (56).

7. Electrochemical equalization cell according to claim 3 and 6, characterized in that a gas line (62) is provided which connects the first gas space (53) in the first container (45a) with the second gas space (54) in the second container (45b) and the gas line (62) forms the at least one connecting opening (63).

8. Electrochemical equalization cell according to one of claims 1 to 7, characterized in that in the region of the at least one opening (56) and above the opening (56) a wall (61) projects into the reaction chamber (57) and / or into the equalization chamber (58).

9. Electrochemical equalization cell according to one of claims 1 to 8, characterized in that a circulation pump (9a, 9b, 51) is provided which circulates the electrolyte (15a, 15b) at least temporarily through the first reaction chamber (41) of the first electrochemical half-cell (50a) during operation of the electrochemical equalization cell (40).

10. Method for regulating the water balance in an electrochemical equalization cell (40) for changing an oxidation state of an electrolyte (15a, 15b), preferably a redox flow battery (1), wherein a first reaction space (41) of a first electrochemical half-cell (50a) of the electrochemical equalization cell (40) contains the electrolyte (15a, 15b) and a second reaction space (42) of a second electrochemical half-cell (50b) of the electrochemical equalization cell (40) contains an aqueous charge equalization electrolyte (44), wherein the first reaction space (41) is separated from the second reaction space (42) by a membrane (43), wherein the first electrochemical half-cell (50a) has a first electrode (47) in contact with the electrolyte (15a, 15b) and the second electrochemical half-cell (50b) comprises a second electrode (48) in contact with the charge balancing electrolyte (44), and wherein, during operation of the electrochemical balancing cell (40), an electrical balancing voltage (VA) is applied to the first electrode (47) and second electrode (48), and wherein the second reaction space (41) is formed in a charge balancing electrolyte container (45) which is divided into a reaction chamber (57) containing the charge balancing electrolyte (44) and a balancing chamber (58) containing the charge balancing electrolyte (44), wherein the second electrode (48) is arranged in the reaction chamber (57) so that the reaction chamber (57) forms the second reaction space (42) of the second electrochemical half-cell (50b), characterized in that a liquid formed by electroosmosis through the membrane (43),Osmosis through the membrane (43) and through the redox reaction taking place in the electrochemical equalization cell (40) due to the equalization voltage (VA), which causes a half water electrolysis reaction in the reaction chamber (57), and any water loss in the reaction chamber (57) that may occur due to evaporation of water is at least partially compensated for by charge equalization electrolyte (44) flowing from the equalization chamber (58) into the reaction chamber (57) through at least one opening (56) that connects the reaction chamber (57) to the equalization chamber (58), whereby the electrolyte concentration in the charge equalization electrolyte (44) in the reaction chamber (57) is increased until the direction of osmosis through the membrane (43) is reversed and water flows through osmosis from the electrolyte (15a, 15b) in the first reaction space (57) of the first electrochemical half-cell (50a) through the membrane (43) into the reaction chamber (57) migrated., 11. Method for regulating the water balance in an electrochemical equalization cell (40) for changing an oxidation state of an electrolyte (15a, 15b), preferably a redox flow battery (1), wherein a first reaction space (41) of a first electrochemical half-cell (50a) of the electrochemical equalization cell (40) contains the electrolyte (15a, 15b) and a second reaction space (42) of a second electrochemical half-cell (50b) of the electrochemical equalization cell (40) contains an aqueous charge equalization electrolyte (44), wherein the first reaction space (41) is separated from the second reaction space (42) by a membrane (43), wherein the first electrochemical half-cell (50a) comprises a first electrode (47) in contact with the electrolyte (15a, 15b) and the second electrochemical half-cell (50b) comprises a second electrode (48) in contact with the Charge balancing electrolyte (44),and wherein, when the electrochemical equalization cell (40) is at a standstill, no electrical equalization voltage (VA) is applied to the first electrode (47) and second electrode (48), and wherein the second reaction space (42) is formed in a charge equalization electrolyte container (45) which is divided into a reaction chamber (57) containing charge equalization electrolyte (44) and a compensation chamber (58) containing charge equalization electrolyte (44), wherein the, second electrode (48) is arranged in the reaction chamber (57), so that the reaction chamber (57) forms the second reaction space (42) of the second electrochemical half-cell (50b), characterized in that water migrates by osmosis from the electrolyte (15a, 15b) in the first reaction space (41) of the first electrochemical half-cell (50a) through the membrane (43) into the reaction chamber (57) of the second electrochemical half-cell (50b), whereby the volume of the charge-balancing electrolyte (44) in the reaction chamber (57) increases and charge-balancing electrolyte (44) flows from the reaction chamber (57) into the balancing chamber (58) through at least one opening which connects the reaction chamber (57) to the balancing chamber (58), until a hydrostatic pressure acting in the reaction chamber (57) and the balancing chamber (58) equalizes.

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