Electrochemical cell

Abstract

The aim of the invention is to protect the membrane in an electrochemical cell from being damaged without impairing the function of the cell. This is achieved in that a support grating (55) is provided in the first liquid volume (41) and / or in the second liquid volume (42), and the membrane (43) rests against the support grating (55). The support grating (55) is equipped with a plurality of openings (56) which connect the membrane (43) to the liquid volume (41, 42) in which the support grating (55) is provided.

Description

[0001] Electrochemical cell

[0002] The invention relates to an electrochemical cell, preferably an electrochemical equalization cell for changing an oxidation state of an electrolyte of a redox flow battery, wherein a first half-cell with a liquid volume and a second half-cell with a second liquid volume are provided in the electrochemical 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 electrochemical cell, the first liquid volume is at least partially filled with an electrolyte and the second liquid volume is at least partially filled with an electrolyte.

[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.

[0004] The positive and negative electrolytes are circulated separately through the half-cells. A single cell is therefore often referred to as a flow cell. The positive and negative electrodes are usually made of porous graphite felts through which the electrolyte can flow.

[0005] In a flow cell, the positive and negative electrolytes are usually circulated through the respective half-cells at a slight overpressure relative to ambient pressure, typically 0 to 1 bar above ambient pressure. Thus, the membrane is subjected to essentially the same pressure on both sides of a cell, and the membrane is essentially pressure balanced.

[0006] 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.

[0007] 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.

[0008] 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+ referred to) 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+ ) - whereby the negative electrolyte has a more negative electrochemical potential than the positive electrolyte.

[0009] 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 +

[0010] 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).

[0011] In addition to the desired charge / discharge reactions, parasitic reactions can also occur, which lead to an ineffective charge / discharge process.

[0012] 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:

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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 through 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. A disadvantage of this equalizing cell is the increased energy consumption for generating the UV light in the photochemical cell, which reduces the overall efficiency of the redox flow battery.

[0017] It is also known to design a balancing cell like a flow cell in the cell stack of a redox flow battery. An example of such a balancing cell can be found in US Pat. No. 8,916,281 B2. In such a balancing cell, an electrolyte is pumped through one half-cell of the balancing cell, and a charge-balancing electrolyte is pumped through the other half-cell of the balancing cell. A disadvantage of such a balancing cell is that the charge-balancing electrolyte must also be pumped through the balancing cell, which increases energy consumption and reduces the overall efficiency of the redox flow battery. Furthermore, the redox reaction in the balancing cell often results in the generation of gas as a by-product. Pumping a liquid-gas mixture can lead to cavitation in a pump, which can damage or destroy the pump.

[0018] In both types of equalization cell, a pressure difference can develop across the membrane. In the case of an equalization cell, as described in US 8,877,365 B2, this is due to the unpressurized container for hydrochloric acid on one side and the electrolyte pumped under excess pressure on the other side. In the case of an equalization cell as described in US 8,916,281 B2, this is due to any difference in the delivery pressures of the electrolyte and the charge-balancing electrolyte, or if the charge-balancing electrolyte is pumped without a pump. Such a pressure difference stresses the membrane and can lead to deformation or even rupture of the membrane, which in both cases can affect the function of the equalization cell. If the membrane ruptures, it can also lead to mixing of the electrolyte with the charge-balancing liquid, which can contaminate the electrolyte and impair the performance of the redox flow battery.In the worst case, mixing leads to damage to the cell stack or loss of function of the redox flow battery.

[0019] However, similar problems can also occur in other electrochemical cells, such as in individual electrochemical cells of a cell stack of a redox flow battery, in particular a pressure difference across a membrane of an electrochemical cell, which can mechanically stress and damage the membrane.

[0020] It is therefore an object of the present invention to protect the membrane in an electrochemical cell against damage without impairing the function of the cell.

[0021] This object is achieved according to the invention in that a support grid is provided in the first liquid volume and / or the second liquid volume of the electrochemical cell, and the membrane rests against the support grid. A plurality of recesses are provided on the support grid, connecting the membrane to the liquid volume in which the support grid is arranged. This support grid, on the one hand, mechanically supports the membrane, so that the membrane is not exposed to undue mechanical stresses and deformations. On the other hand, the recesses ensure that the function of the electrochemical cell is not impaired.

[0022] It is particularly advantageous if at least one recess in the support grid, preferably at least 50% of the recesses or all recesses, is designed with a non-circular cross-sectional area. In this context, it is advantageous if the at least one recess has a longitudinal extent and a transverse extent, wherein the ratio between longitudinal extent and transverse extent is greater than 1.1, preferably greater than 1.5. The non-circular cross-sectional area can ensure that gas bubbles in the (charge balancing) electrolyte, which are forced into an approximately spherical shape due to their surface tension, cannot completely block a recess. This allows the connection between the membrane and the electrolyte or the charge balancing electrolyte in the liquid volume in which the support grid is arranged to be maintained.

[0023] Preventing a recess from becoming blocked by gas bubbles in the (charge-balancing) electrolyte can also be assisted if the cross-sectional area of ​​the at least one recess on a side of the support grid facing the membrane is smaller than the cross-sectional area of ​​the at least one recess on a side of the support grid facing away from the membrane. This is achieved because a large cross-sectional area is more difficult and less frequently blocked by a gas bubble than a smaller cross-sectional area.

[0024] In one embodiment of a half-cell of the electrochemical cell with a container forming the second liquid volume and with a support grid in the container, it is advantageous if a gas bubble retaining part is arranged in the lower region of the container and in the region of the support grid, wherein the gas bubble retaining part is spaced from the support grid by a gap. Gas bubbles collect below the gas bubble retaining part and grow into larger gas bubbles until the large gas bubbles force their way through the gap. Large gas bubbles rising upward through the gap and along the support grid move gas bubbles into recesses or entrain such gas bubbles, thus clearing the recesses of gas bubbles.

[0025] The present invention will be explained in more detail below with reference to Figures 1 to 12, which show exemplary, schematic and non-limiting advantageous embodiments of the invention.

[0026] Fig.1 shows the basic operating principle of a redox flow battery,

[0027] Fig.2 a redox flow battery with a cell stack,

[0028] Fig.3 shows the structure of a cell stack of a redox flow battery,

[0029] Fig.4 a redox flow battery with a compensation cell,

[0030] Fig.5 an embodiment of a compensation cell,

[0031] Fig.6 and 7 each show a compensation cell with a support grid,

[0032] Fig.8a and 8b different views of a support grid,

[0033] Fig.9 a detailed view of the recesses on a support grid,

[0034] Fig.10 an embodiment of a compensation cell with gas bubble removal,

[0035] Fig.11 an embodiment of a compensation cell with a compensation cell stack with several flow cells and

[0036] Fig.12 an embodiment of a compensation cell with several parallel flow cells

[0037] 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.

[0038] A single electrochemical cell 2 consists of two electrochemical 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 sulfonate-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 connections 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).

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] The invention is further described using the example of a vanadium redox flow battery 1, wherein vanadium V in different oxidation states is present as redox elements in both electrolytes 15a, 15b.

[0049] 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 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.

[0050] 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.

[0051] Fig. 4 shows an example of an electrochemical equalization cell 40 in the form of a flow cell 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.

[0052] 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.

[0053] 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).

[0054] 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 15a, 15b. For example, a DC voltage of 5V to 12V is applied as the voltage.

[0055] 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 second liquid volume 42. In the reaction space of each electrochemical half-cell 50a, 50b of the equalization cell 40, an electrolyte is contained or an electrolyte is passed through, once the electrolyte 15a, 15b of the redox flow battery 1, whose oxidation state is to be changed, and once the charge equalization electrolyte 44. The electrolyte contained in the respective half-cell 50a, 50b is in contact with the electrode 47, 48 of the half-cell 50a, 50b.The electrolyte 15a, 15b, whose oxidation state is to be changed, is circulated at least temporarily through the first half-cell 50a, specifically through the first liquid volume 41 of the first electrochemical half-cell 50a. A separate circulation pump 35 and corresponding lines 36 can be provided for this purpose, or the already existing circulation pump 9a, 9b of the redox flow battery 1 can also be used.

[0056] When using the electrochemical equalization cell 40, the first liquid volume 41 of the first half-cell 50a contains the electrolyte 15a, 15b whose oxidation state is to be changed and which at least partially fills the first liquid volume 41. The second liquid volume 42 of the second half-cell 50b contains a charge equalization electrolyte 44, which at least partially fills the second liquid volume 42. The charge equalization electrolyte 44 is circulated, for example, 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. A circulation pump 46 and corresponding lines 49 can be provided for circulating the charge equalization electrolyte 44.

[0057] In the embodiment of Fig. 5, one half-cell 50a is designed as described above for Fig. 4. The other half-cell 50b is formed by a container 51 in which the charge-balancing electrolyte 44 is arranged. In this embodiment, the charge-balancing electrolyte 44 is not circulated from a charge-balancing electrolyte container 45, as in Fig. 4. One of the electrodes 47, 48 of the balancing cell 40, in the embodiment of Fig. 5, the anode 48, is at least partially arranged in the charge-balancing electrolyte 44 and is in contact with the charge-balancing electrolyte 44.

[0058] A half-cell 50a, 50b of the equalizing cell 40 can, similar to a half-cell 2a, 2b of the cell stack 10 of the redox flow battery 1, comprise, in an advantageous embodiment, a frame in which an electrode 47, 48 is arranged in a recess. The frame can be made of plastic. Channels can be formed in the frame in order to supply and discharge an electrolyte 15a, 15b or the charge equalizing liquid to and from the first or second liquid volume 41, 42. The electrode 47, 48 can be designed in any desired manner, for example, flat, as a rod electrode, or as a grid electrode.

[0059] 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.

[0060] The function of the compensation cell 40 is the same in both embodiments according to Fig.4 and Fig.5 and is briefly explained below using an exemplary embodiment for the purpose of understanding.

[0061] A redox reaction takes place in the equalizing cell 40, whereby when the equalizing voltage A is applied in the half-cell 50b with the charge equalizing electrolyte 44, a half electrolysis reaction of a water electrolysis takes place and in the half-cell 50a with the electrolyte 15a, 15b as a counter-reaction of the redox reaction, a chemical oxidation or reduction of the electrolyte 15a, 15b.

[0062] During a chemical reduction of an over-oxidized electrolyte 15a, 15b, the chemical reaction 2H2O O2+ 4H + 4e occurs in the half-cell 50b with an acidic charge-balancing electrolyte 44, and the chemical reaction +2e~ occurs with a basic charge-balancing electrolyte 44. As a counter-reaction, the other Half cell 50a assuming a vanadium-based electrolyte 15a, 15b with an acidic charge balancing electrolyte 44, the chemical reaction 2K 3+ + 2e ~^ 2V 1+ and with a basic charge balancing electrolyte 44 the chemical reaction 21 ) 2 + 2e ~^ 2VO2. In both cases, the electrolyte is chemically reduced.

[0063] During a chemical oxidation of an over-reduced electrolyte 15a, 15b, the chemical reaction 2H + +2e / / 2ab and with a basic charge balancing electrolyte 44 the chemical reaction + 20H . As a counter-reaction, the chemical reaction 2VO221 ) takes place in the other half-cell 50a, assuming a vanadium-based electrolyte 15a, 15b with an acidic charge-balancing electrolyte 44. 2+ 2e and with a basic charge balancing electrolyte 44 the following chemical reaction 2K 2+ -^- 2K 3+ +2e~ . In both cases, the electrolyte is consequently chemically oxidized. The polarity of the equalization voltage VA is reversed. 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. 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 during the redox reaction.

[0064] From the design of the electrochemical equalization cell 40, it can be seen that the membrane 43 is subjected to pressure when there is a pressure difference between the electrolyte 15a, 15b in the half-cell 50a and the charge equalization electrolyte 44 in the other half-cell 50b. In the design according to Fig. 5, the electrolyte 15a, 15b is typically circulated at an overpressure of up to 1 bar relative to the ambient air, whereas the charge equalization electrolyte 44 in the container 51 is at ambient pressure. The hydrostatic pressure acting in the charge equalization electrolyte 44 in the container 51 at least does not completely compensate for the differential pressure. In a design according to Fig. 4, the charge equalization electrolyte 44 can, for example, be circulated without a pump, which in turn exerts a differential pressure on the membrane 43. Even in the case of different pressures during circulation of the electrolyte 15a, 15b and the charge equalization electrolyte 44, a differential pressure acts on the membrane 43 of the equalization cell 40.

[0065] The differential pressure acting on the membrane 43 of the equalizing cell 40 mechanically stresses and deforms the membrane 43. This can compromise the seal between the membrane 43 and the other parts of the equalizing cell 40. In the worst case, the membrane 43 can rupture. Both of these can impair the function of the equalizing cell 40. Mixing can also occur between the electrolyte 15a, 15b and the charge equalizing electrolyte 44, which can subsequently impair the performance and function of the redox flow battery 1.

[0066] To prevent this, according to the invention, a support grid 55 is provided in the first liquid volume 41 and / or in the second liquid volume 42. The support grid 55 is advantageously arranged at least in the liquid volume 41, 42 in which the lower pressure is expected during operation of the equalizing cell 40. The membrane 43 of the equalizing cell 40 rests against the support grid 55, and the support grid 55 thus prevents the membrane 43 from deforming due to a pressure difference. At the very least, the support grid 55 reduces pressure-induced deformation. However, the electrolyte 15a, 15b or the charge equalizing electrolyte 44 must be in direct contact with the membrane 43 to enable the redox reaction in the equalizing cell 40.Therefore, a plurality of recesses 56 are provided on the support grid 55, which connect the membrane 43 to the liquid volume 41, 42 in which the support grid 55 is arranged, and thus to the electrolyte 15a, 15b or the charge-balancing electrolyte 44 in the liquid volume 41, 42. This is illustrated by way of example in Fig. 6 in an embodiment of the balancing cell 40 as shown in Fig. 5. In this embodiment, the support grid 55 is arranged in the second liquid volume 42, i.e., in the container 51 for the charge-balancing electrolyte 44.

[0067] The support grid 55 is made of a sufficiently mechanically stable and electrolyte-resistant material, for example, a metal, coated metal, or plastic. The support grid 55 is naturally dimensioned, for example, in terms of thickness D or material, so that it is sufficiently mechanically stable with respect to the expected and known pressure difference. This means that the support grid 55 deforms under the expected acting pressure difference only to the extent permitted by the membrane 43. Once the structure of the compensation cell 40 is known, the support grid 55 can be easily designed mechanically.

[0068] On the side of the support grid 55 facing the membrane 43, the cross-sectional areas of the recesses 56 should be as small as possible to prevent the membrane 43 from deforming into the recesses 56 due to the acting differential pressure. The cross-sectional areas of the recesses 56 on this side should be less than 5 mm 2 , preferably less than 2.5mm 2 , preferably less than 1 mm 2 , or even smaller.

[0069] The cross-sectional areas of the recesses 56 can in principle have any shape, for example circular, elliptical, slit-shaped, in the shape of a polygon, etc.

[0070] However, it has been shown that circular cross-sectional areas can lead to blocking of recesses 56. As explained above, the redox reaction produces a gaseous by-product (e.g., oxygen or hydrogen), which can lead to gas bubbles 57 being present in the electrolyte 15a, 15b or in the charge balancing electrolyte 44 in the respective half-cell 50a, 50b. The gas bubbles 57 have a tendency to collect in the recesses 56 and block the recesses 56, as indicated in Fig. 6. As the recesses 56 become increasingly blocked, this can impair the function of the balancing cell 40 because less electrolyte 15a, 15b or charge balancing electrolyte 44 reaches the membrane 43, or, in the case of complete blockage, can even completely prevent the desired redox reaction in the balancing cell 40.

[0071] To effectively prevent this, it is advantageous if at least one recess 56, preferably more than 50% of the recesses 56, preferably all recesses 56, have a non-circular cross-sectional area. Due to the surface tension of the gas bubbles, the gas bubbles are forced into an approximately spherical shape. If gas bubbles 57 collect in a recess 56 and a larger gas bubble 57 grows in the recess 56, then a portion of the recess 56 always remains free due to the non-circular cross-sectional area and the spherical shape of the gas bubble 57, so that electrolyte 15a, 15b or charge-balancing electrolyte 44 always reaches the membrane 43 through the recess 56. If the gas bubble 57 in the recess 56 becomes larger and larger due to growth, the gas bubble 57 is pushed out of the recess 56 by the surface tension before it can block the recess 56.

[0072] It has therefore proven advantageous if the cross-sectional area of ​​at least one of the recesses 56, at least on the side facing away from the membrane 43, is elongated with a longitudinal extent L and a transverse extent B (transverse to the longitudinal extent), wherein the ratio between the longitudinal extent L and the transverse extent B is greater than 1.1, preferably greater than 1.25, more preferably greater than 1.5. The upper limit of this ratio results from manufacturing restrictions and restrictions on the mechanical stability of the support grid 55. With a ratio below 1.1, it is no longer possible to reliably prevent the recess 56 from being completely filled by gas bubbles. A suitable cross-sectional shape is, for example, elliptical or in the form of a longitudinal groove (as in Fig. 9). The orientation of the at least one non-circular recess 56 in the support grid 55 is irrelevant.

[0073] However, not all cross-sectional areas of the recesses 56 in the support grid 55 need to be of the same shape or size, nor do they need to be aligned the same. The recesses 56 also do not necessarily need to be arranged in a regular pattern.

[0074] Fig.9 shows an arrangement of recesses 56 on a support grid 55 with elongated cross-sectional areas in the form of longitudinal grooves with longitudinal extension L and transverse extension B. The ratio L / B is greater than 1.5.

[0075] It is also conceivable that the cross-sectional area of ​​at least one recess 56 in the support grid 55 is smaller on the side facing the membrane 43 than on the side facing away from the membrane 43. The cross-sectional area of ​​at least one recess 56 can also change continuously (discretely or continuously) through the thickness of the support grid 55. It is also possible for the cross-sectional area of ​​at least one recess 56 in the support grid 55 to be designed differently on the side facing the membrane 43 than on the side facing away from the membrane 43. Such a design is shown in Figs. 8a and 8b. Fig. 8a shows the side of the support grid 55 facing away from the membrane 43 and Fig. 8b shows the side of the support grid 55 facing the membrane 43. On the side facing away from the membrane 43, the recesses 56 are circular with a large cross-sectional area.On the side facing the membrane 43, the cross-sectional areas of the recesses 56 are smaller and elongated compared to the opposite side. The change in cross-sectional area is stepped in this design, but could also be continuous, even extending in several steps. The part with the recesses 56 with the smaller cross-sectional areas could also be designed as a separate insert, inserted into the support grid 55, e.g., in a recess.

[0076] As the surface area of ​​the membrane 43 increases, the pressure load on the membrane 43 due to an acting differential pressure increases. To counteract this, the thickness D of the support grid 55 can be increased. However, this also increases the length of the recesses 56 through the support grid 55, which is disadvantageous with regard to a possible blocking of the recesses 56 by the gas bubbles 57. It is therefore advantageous in one embodiment of the support grid 55 according to the invention if the support grid 55 is designed in several parts. For this purpose, the support grid 55 is supplemented by a further support grid 55a, as shown in Fig. 7. The support grid 55 rests against the membrane 43 and the further support grid 55a rests against the support grid 55 on the side facing away from the membrane 43. The further support grid 55a has recesses 56a with a larger cross-sectional area than the support grid 55. The further support grid 55a thus increases the mechanical stability.At the same time, the larger cross-sectional areas of the additional support grid 55a ensure that the risk of blocking of the recesses 56, 56a is reduced. Regarding the shape, position, and orientation of the cross-sectional areas of the recesses 56a of the additional support grid 55a, the above statements regarding the recesses 56 of the support grid 55 apply equally.

[0077] In the same way, further support grids can be arranged if the mechanical load on the membrane 43 makes it necessary.

[0078] The support grid 55 and the at least one further support grid 55a can also be manufactured as a single component. This can result in a design as shown in Figs. 8a and 8b.

[0079] To prevent the blocking of recesses 56 (56a) of the support grid 55, a further measure in the form of a gas bubble remover 52 can be provided as an alternative to the special cross-sectional shape of the recesses 56 (56a) or in addition thereto, as described below with reference to Fig. 10. However, this can only be used effectively in an embodiment of the equalization cell 40 with a container 51 for the charge equalization electrolyte 44, not when the charge equalization electrolyte 44 is circulated, as in an embodiment according to Fig. 4.

[0080] In the embodiment of the equalization cell 40 in Fig. 10, a gas bubble eliminator 52 is arranged in the lower (relative to the direction of gravity) region of the container 51 containing the charge-balancing electrolyte 44 and in the region of the support grid 55. The gas bubble eliminator 52 comprises a gas bubble retaining part 58, which is arranged in the lower (relative to the direction of gravity) region of the container 51 containing the charge-balancing electrolyte 44, but spaced from the support grid 55, forming a gap 59. The gas bubble retaining part 58 is, for example, a piece of sheet metal or a piece of plastic arranged in the container 51. Gas bubbles 57 collect below the gas bubble retaining part 58 and accumulate to form a larger gas bubble 57a.If the buoyancy of the growing gas bubble 57a below the gas bubble retaining part 58 becomes sufficiently large (depending on the gap between the support grid 55 and the gas bubble retaining part 58), the gas bubble 57a forces its way through the gap 59, overcoming the surface tension of the gas bubble 57a. The large gas bubble 57a rising along the support grid 55 moves the gas bubbles 57 in the recesses 56 or picks up these gas bubbles 57 as it moves past and ensures that they detach from the recesses 56. By suitably inclining the gas bubble retaining part 58, it can be ensured that the gas bubble 57a forces its way through the gap 59 and does not detach from the gas bubble retaining part 58 on the other side.

[0081] In a redox flow battery 1, several equalizing cells 40 can of course also be provided, which can also operate in parallel. In an embodiment as shown in Fig. 4, similar to a cell stack 10 of the redox flow battery 1, a equalizing cell stack 60 with several adjacent equalizing cells 40 can be provided, through which the electrolyte 15a, 15b and the charge equalizing electrolyte 44 flow in parallel. A support grid 55 according to the invention is arranged in at least one equalizing cell 40. A equalizing cell stack 60 with several equalizing cells 40 is shown in Fig. 11. However, a equalizing cell stack 60 with several parallel equalizing cells 40 can also be realized with an embodiment of an equalizing cell 40 as shown in Fig. 5, as shown in Fig. 12.

[0082] In Fig. 11, several equalizing cells 40 are arranged side by side to form an equalizing cell stack 60. Adjacent equalizing cells 40 in the equalizing cell stack 60 are separated from each other by a bipolar plate 63 (as in cell stack 10). The electrodes 47, 48 are designed, for example (as in a cell stack 10) as flow-through electrodes, for example, as graphite felt. End plates 61, 62 are arranged at the axial ends of the equalizing cell stack 60, which hold the equalizing cell stack 60 together. The equalizing voltage A can also be applied via the end plates 61, 62 and / or the electrolyte and charge equalizing electrolyte 44 can be supplied and removed. The structure of a balancing cell stack 60 therefore does not differ significantly from the structure of a cell stack 10. In the embodiment of Fig. 11, the charge balancing electrolyte 44 is pumped without a pump, for example only by the buoyancy force of the resulting gas bubbles 57.

[0083] The invention finds particularly advantageous application in an electrochemical equalization cell 40 as described above. However, there may also be redox flow batteries 1 in whose individual electrochemical cells 2 of a cell stack 10, similar problems as in the described electrochemical equalization cell 40 occur, in particular a pressure difference across a membrane 4 of an individual electrochemical cell 2. Therefore, the support grid 55 can also be provided in at least one electrochemical half-cell 2a, 2b of such an individual electrochemical cell 2. In an electrochemical cell (individual electrochemical cell 2 or electrochemical equalization cell 40), a charge equalization electrolyte 44 is not necessarily contained in the second liquid volume of a second half-cell, but generally an electrolyte (electrolyte 15a, 15b or charge equalization electrolyte 44).

[0084] The invention thus generally relates to an electrochemical cell, preferably an electrochemical balancing cell 40.

Claims

Patent claims 1. Electrochemical cell, preferably an electrochemical equalization cell (40) for changing an oxidation state of an electrolyte 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 cell, 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 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, during operation of the electrochemical cell, the first liquid volume (41) is at least partially filled with an electrolyte (15a,15b) and the second liquid volume (42) is at least partially filled with an electrolyte (15a, 15b, 44), characterized in that a support grid (55) is provided in the first liquid volume (41) and / or in the second liquid volume (42) and the membrane (43) rests against the support grid (55), and in that a plurality of recesses (56) are provided on the support grid (55), which connect the membrane (43) to the liquid volume (41, 42) in which the support grid (55) is arranged.

2. Electrochemical cell according to claim 1, characterized in that at least one recess (56) in the support grid (55), preferably at least 50% of the recesses (56) or all recesses (56), is designed with a non-circular cross-sectional area.

3. Electrochemical cell according to claim 2, characterized in that the at least one recess (56) has a longitudinal extent (L) and a transverse extent (B), wherein the ratio between longitudinal extent (L) and transverse extent (B) is greater than 1.1, preferably greater than 1.

5.

4. Electrochemical cell according to one of claims 1 to 3, characterized in that the cross-sectional area of the at least one recess (56) on a side of the support grid (55) facing the membrane (43) is smaller than the cross-sectional area of the at least one recess (56) on a side of the support grid (55) facing away from the membrane (43).

5. Electrochemical cell according to one of claims 1 to 4, characterized in that the first liquid volume (41) is at least temporarily flowed through by the electrolyte (15a, 15b) during operation of the electrochemical cell.

6. Electrochemical cell according to one of claims 1 to 5, characterized in that the second liquid volume (42) is at least temporarily flowed through by the electrolyte (15a, 15b, 44) during operation of the electrochemical cell.

7. Electrochemical cell according to one of claims 1 to 5, characterized in that the second liquid volume (42) is formed by a container (45) in which a charge balancing electrolyte (44) and the second electrode (48) are arranged in contact with the charge balancing electrolyte (44).

8. Electrochemical cell according to claim 7, characterized in that the support grid (55) is arranged in the container (45).

9. Electrochemical cell according to claim 8, characterized in that in the lower A gas bubble retaining part (58) is arranged in the region of the container (45) and in the region of the support grid (55), wherein the gas bubble retaining part (58) is spaced from the support grid (55) by a gap (59).

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