Balancing reactor for an electrochemical energy storage system

Abstract

Electrochemical balancing reactor (40) for an electrochemical energy storage system (21), the balancing reactor (40) comprising: - a first compartment (41) for circulating a first electrolyte solution (24) containing permanganate ions, the first compartment (41) comprising a first electrode (43) configured to be in contact with the first electrolyte solution (24) in the first compartment (41), - a second compartment (42), configured to contain the first electrolyte solution (24) in a non-circulating manner, the second compartment (42) comprising a second electrode (44) configured to be in contact with the first electrolyte solution (24) in the second compartment (42), - a separator (46) extending between the first compartment (41) and the second compartment (42), the separator (46) comprising a microporous membrane,- means (45) for applying an electrical voltage between the first electrode (43) and the second electrode (44). Figure to be published with the abbreviation: 2,

Description

Title of the invention: Balancing reactor for an electrochemical energy storage system Technical field of the invention

[0001] The invention belongs to the field of electrochemical storage of electrical energy, in particular for stationary and / or long-term storage applications.

[0002] More specifically, the invention is part of the field of circulation batteries, and more specifically of zinc-permanganate circulation batteries.

[0003] The invention relates to a balancing reactor for an electro-chemical energy storage system, as well as an energy storage system comprising such a reactor. Prior art

[0004] Energy storage is essential for the stability of an electrical grid. Indeed, on an electrical grid, at any given time, every source of electricity generation injects electricity into the grid, which must be compensated by an equivalent consumption. Any imbalance between total production and total consumption has detrimental effects on all production and consumption resources connected to that grid.

[0005] A buffer, in the form of reversible electricity storage, is therefore essential to balance the grid and compensate for any difference between electricity production and consumption. This buffer or electricity storage becomes all the more essential when intermittent production methods, such as photovoltaic or wind power sources, are integrated into the grid. It can also be advantageous to store surplus electricity, for example at peak sunlight hours, on photovoltaic panels, for use when this photovoltaic source is no longer available, for example, at night.

[0006] Centralized storage systems such as pumped storage hydroelectric plants (PSHPs) now exist, connected to the electrical grid. However, the possibilities for installing new PSHPs are limited by the geographical characteristics required for such an installation.

[0007] New decentralized electrochemical storage systems, using lithium-ion batteries for example, are beginning to be installed on the grid to supplement existing storage and stabilize the electricity grid. However, the high demand for batteries, particularly linked to the very rapid growth of electric vehicles, is creating constraints on the supply of raw materials. necessary for the manufacture of these batteries, linked in part by their low abundance on Earth.

[0008] New generation batteries, called circulation batteries, or "Redox Flow Batteries" in English, are beginning to be developed for such applications. Such a battery is characterized by storing the active material necessary for the electrochemical reaction of storing and releasing electrical energy in the form of a liquid electrolyte, generally in aqueous solution, in tanks located away from the main reactor.

[0009] Circulation batteries offer a number of advantages over conventional batteries. For example, they can use a wider range of electrolytes, and the aqueous nature of the electrolytes, which makes them non-flammable, gives them superior safety.

[0010] Furthermore, their energy storage capacity can be increased without changing their rated power, simply by increasing the amount of electrolyte stored in the tanks, whereas conventional batteries require multiple tanks to increase capacity. The marginal cost of stored energy can therefore be significantly reduced, particularly for large installations, since such a modification only requires adding active material and increasing the size of the tanks. The operating principle of such a circulation battery is illustrated in [Fig.1]. [Fig.1] represents a hybrid circulation battery 1, in which part of the material storing energy in electrochemical form, i.e. the electrolytes, is contained in the main reactor.

[0011] The circulating battery 1 comprises a main reactor 2, a first reservoir 3 containing a first electrolyte solution 4 and a second reservoir 5 containing a second electrolyte solution 6.

[0012] The main reactor 2 comprises a first compartment 7 and a second compartment 8 separated by an ion exchange membrane 9, in which circulate respectively the first electrolyte solution 4 and the second electrolyte solution 6, brought by a first feed circuit 10 and a second feed circuit 11 equipped respectively with a first pump 12 and a second pump 13.

[0013] The main reactor 2 is represented in a simplified manner and thus comprises only one cell formed from the first compartment 7 and the second compartment 8 separated by the membrane 9, whereas in practice, the reactor comprises a plurality of such cells connected in series to each other and supplied by the first supply circuit 10 and the second supply circuit 11, the first reservoir 3 and the second reservoir 5 being common and shared by the plurality of cells.

[0014] Among hybrid flow batteries, some use, for one of the electrolytes, an aqueous zinc-based solution, and an electrode containing zinc. Some examples of zinc-based hybrid flow batteries are zinc-bromine batteries or zinc-iron batteries. The article by Khor, P. Leung, MR Mohamed, C. Flox, Q. Xu, L. An, RGA Wills, JR Morante, AA Shah, “Revzew of zinc-based hybridflow batteries: From fundamentals to applications”, (Materials Today Energy, Volume 8, 2018, Pages 80-108, ISSN 2468-6069, 12 / 12 / 2017) provides a summary of these zinc flow batteries.

[0015] The use of zinc as the active material at the negative electrode, the anode, has the advantage of being compatible with an aqueous electrolyte, of using a metal much more abundant than the elements used for lithium-ion batteries (i.e., lithium, nickel, cobalt, etc.) and of being able to store a high charge in a fairly small quantity of zinc (819 mAh / g (milliampere-hour per gram of zinc), or 5854 mAh / cm3 (milliampere-hour per cubic centimeter of electrolyte)), with an interesting electrochemical potential (-1.199 V (Volts) in basic medium compared to a standard hydrogen electrode).

[0016] The use of permanganate as an active material in alkaline solution at the positive electrode, the cathode, using the manganate / permanganate couple (MnO42 / MnO4 ), is particularly interesting because it allows the use of an abundant active material with a high electrochemical potential in aqueous medium (+0.558 V compared to a standard hydrogen electrode).

[0017] The use of the zinc / permanganate couple for a circulation battery has been described in particular in the article by Colli, Alejandro N. and Peljo, Pekka and Girault, Hubert H., "High energy density MnO 4~ / MnO 42~ redox couple for alkaline redoxflow batteries".

[0018] Such a circulation battery implementing these two couples is described for example in document CN 11053478 A.

[0019] A circulation battery combining these two electrochemical couples in an alkaline medium according to the following equation 1 thus makes it possible to have an interesting voltage of 1.8 V by using abundant components.

[0020] [Chem. 1]

[0021] 2 MnO4 + Zn ~ 2 MnO? + Zn2+

[0022] More specifically, during discharge, the zinc in the negative electrode is oxidized to produce zincate ions, and the permanganate ions are reduced to produce manganate ions, giving the overall reaction 1 described above. The reverse reactions take place during battery charging.

[0023] An example of a zinc electrode for such a battery is described for example in patent application FR 3091042 Al.

[0024] However, these batteries do not give complete satisfaction.

[0025] Indeed, during charging, a parasitic reaction may take place at the negative electrode, according to equation 2 below.

[0026] [Chem. 2]

[0027] 2 H2O + 2e H2 + 2 OH

[0028] This side reaction consumes manganate ions at the positive electrode to supply the electrons needed for reaction 2, without depositing the equivalent amount of zinc metal at the negative electrode. Therefore, during the next discharge, the amount of zinc metal available for discharge will be correspondingly reduced.

[0029] A second chemical, and not electrochemical, self-discharge reaction also occurs naturally in the battery, according to equation 3 below. This reaction also consumes metallic zinc without consuming the equivalent amount of permanganate ions.

[0030] [Chem. 3]

[0031] Zn + 2H2O + 2OH H2 + [Zn(OH)4]2

[0032] Reactions 2 and 3 result in an imbalance of charges between the negative and positive compartments. The negative compartment will gradually become depleted of zinc metal at the end of the charge, and it will contain increasing amounts of permanganate at the end of the charge, at the expense of the amount of manganate. The storage capacity of the zinc / permanganate battery therefore decreases progressively with each charge cycle due to the parasitic reaction 2, and over time due to self-discharge 3.

[0033] A known solution involves regenerating the positive compartment by draining the negative compartment and replacing it with a fresh permanganate solution. However, this solution is cumbersome and requires the installation to be shut down to allow for the renewal process, without providing electricity storage.

[0034] Another solution consists of using a rebalancing reactor by reducing part of the permanganate solution with the dihydrogen produced by reactions 2 and 3, as described in patent application bearing application number FR2307129. An alternative solution to this problem is proposed here. Presentation of the invention

[0035] The invention aims to remedy these drawbacks by providing a circulation battery that allows the use of zinc and permanganate without progressive loss of capacity and does not require renewal of electrolytes.

[0036] To this end, the invention relates to an electrochemical balancing reactor for an electrochemical energy storage system, the balancing reactor comprising:

[0037] - a first circulation compartment of a first electrolyte solution containing permanganate ions, the first compartment comprising a first electrode configured to be in contact with the first electrolyte solution in the first compartment,

[0038] - a second compartment, configured to contain the first solution of electrolyte in a non-circulating manner, the second compartment comprising a second electrode configured to be in contact with the first electrolyte solution in the second compartment,

[0039] - a separator extending between the first compartment and the second compartment, the separator comprising a microporous membrane,

[0040] - means for applying an electrical voltage between the first electrode and the second electrode.

[0041] Such a reactor is capable of regenerating manganate ions in the electrolyte solution by an electrochemical reaction, thus allowing the rebalancing of the electrolytes of the electrochemical energy storage system in a controlled manner and without direct intervention or replacement of the electrolyte.

[0042] The first electrode may include a carbon felt.

[0043] Such an electrode offers a large reaction surface, promoting the formation reaction of manganate ions.

[0044] The carbon felt can extend into the first compartment so as to be traversed by the first electrolyte solution circulating in the first compartment.

[0045] Such an electrode further promotes the formation of manganate ions by ensuring that the solution in contact with the electrode is continuously renewed.

[0046] The second electrode may comprise a metal plate or grid.

[0047] Such an electrode offers a small contact area, promoting the formation of dioxygen at the expense of the formation of undesirable permanganate.

[0048] The metal plate or grid can be made of a metal or alloy that is stable in a basic environment, for example nickel or 316L steel.

[0049] Such a characteristic guarantees the stability of the electrode for long-term use.

[0050] The second electrode may comprise a nickel grid produced from woven nickel wire.

[0051] Such an electrode is stable over time and allows the detachment of gas bubbles generated by the reaction, further promoting the formation of dioxygen.

[0052] The second compartment may include a vent for venting a gas generated at the second electrode.

[0053] Such a characteristic allows the dioxygen formed at the second electrode to be continuously evacuated, promoting the dioxygen formation reaction.

[0054] The invention also relates to an electrochemical energy storage system comprising:

[0055] - a first reservoir containing a first electrolyte solution containing permanganate ions and manganate ions,

[0056] - a second reservoir containing a second electrolyte solution containing zincate ions,

[0057] - a main reactor adapted to allow the transfer of protons or anions hydroxyl, and electrons between the first electrolyte solution and the second electrolyte solution according to at least one redox reaction,

[0058] - a first circulation circuit of the first electrolyte solution between the first tank and main reactor,

[0059] - a second circulation circuit of the second electrolyte solution between the second tank and main reactor, and

[0060] - a balancing reactor as above, in which the first compartment is inserted fluidly into the first circulation circuit.

[0061] Such an energy storage system makes it possible to benefit from the advantages obtained by the use of zinc / zincate and manganate / permanganate electrochemical couples, while compensating for the effects of parasitic and discharge reactions and without external intervention.

[0062] The main reactor may include a zinc-based negative electrode, for example made of zinc metal.

[0063] The balancing reactor can be electrically connected in series in a bipolar manner to the main reactor by the first electrode and the second electrode.

[0064] Such a feature makes it possible to supply the balancing reactor directly from the energy storage system, and in particular its charging system, during the storage phase.

[0065] The balancing reactor may include an electrical shunt or an electronic shunt connected between the first electrode and the second electrode, said electrical shunt or electronic shunt being configured to regulate the balancing reactor during a charge of the electrochemical energy storage system.

[0066] Such a feature makes it possible to control the current flowing in the balancing reactor, for example to reserve its activity for the charging phases of the storage device and reduce losses during the discharge phase. by allowing the amount of manganate produced in the rebalancing reactor to be regulated. Brief description of the figures

[0067] [Fig. 1] is a schematic representation of the operation of a battery with traffic,

[0068] [Fig.2] is a schematic view of an electrochemical energy storage system, of the circulating battery type, according to the invention,

[0069] [Fig.3] is a schematic view of a battery rebalancing reactor of the [Fig.2],

[0070] [Fig.4] [Fig.5] are schematic views of two electrochemical systems of energy storage systems in which the balancing reactor is bipolarly connected to the main reactor,

[0071] [Fig.6] is a graphical representation of the current, voltage and charge electrical in a test setup of the rebalancing reactor of [Fig.3], and

[0072] [Fig.7] is a graphical representation of the evolution of the absorbance spectrum of a electrolyte solution in the rebalancing reactor test setup of [Fig.3]. Detailed description of the invention

[0073] A circulation battery 21 according to the invention is shown in [Fig.2]. The circulation battery 21 is an electrochemical energy storage system, designed to receive energy in electrical form for storage, and then release it in the form of electrical energy.

[0074] The example shown in [Fig.2] is simplified, particularly with regard to the main reactor, which is shown as a single unit in [Fig.2] for the sake of clarity.

[0075] The circulating battery 21 comprises a main reactor 22, a first reservoir 23 containing a first electrolyte solution 24 and a second reservoir 25 containing a second electrolyte solution 26.

[0076] According to the invention, the circulating battery 21 also includes an electro-chemical balancing reactor 40.

[0077] The first electrolyte solution 24 is an aqueous solution containing manganate MnO4 and permanganate MnO42 ions in a basic medium, contained in the first reservoir 23 and circulating in a first supply circuit 30 to the main reactor 22 at the positive electrode and then returning to the first reservoir 23 by a first return circuit 32.

[0078] The first electrolyte solution 24 is, for example, an alkaline solution of sodium manganate or potassium manganate.

[0079] The first electrolyte solution 24 fills at least partially the first reservoir 23, and, in the case of partial filling, a sky forms above solution 24.

[0080] The first supply circuit 30 takes the first electrolyte solution 24 from the first reservoir 23 and conveys it to an inlet of the main reactor 22.

[0081] The first return circuit 32 carries the first electrolyte solution 24 from an outlet of the main reactor 22, through the balancing reactor 40, to the first tank 23.

[0082] The return circuit 32 opens into the first reservoir 23 and the first electrolyte solution 24 circulates in the return circuit 32 to the first reservoir 23 if necessary.

[0083] Together, the first supply circuit 30 and the first return circuit 32 form a first circulation circuit of the first electrolyte solution.

[0084] The first electrolyte solution 24 is driven through the first feed circuit 30, the main reactor 22, the balancing reactor 40 and the first return circuit 32 by the action of at least one first pump 35.

[0085] Each first pump 35 is mounted on the first supply circuit 30 or on the first return circuit 32. The first pump 35 is, for example, a circulation pump or a peristaltic pump.

[0086] The second electrolyte solution 26 is an aqueous solution containing zincate ions [Zn(OH)4]2 in a basic medium, contained in the second reservoir 25 and circulating in a second supply circuit 31 to the main reactor 22 at the negative electrode, then returning to the second reservoir 25 by a second return circuit 33.

[0087] The second electrolyte solution 26 is, for example, a solution of sodium zincate or potassium zincate.

[0088] The second electrolyte solution 26 partially fills the second reservoir 25, and forms in the second reservoir 25 a sky comprising in particular dihydrogen H2, produced by the parasitic reaction 2 and by the self-discharge reaction 3 previously described, and separated from the liquid phase by gravimetric separation.

[0089] The second supply circuit 31 takes the second electrolyte solution 26 from the second reservoir 25 and conveys it to an inlet of the main reactor 22.

[0090] The second return circuit 33 carries the second electrolyte solution 26 from an outlet of the main reactor 22 to the second tank 25, without passing through the balancing reactor 40. The second return circuit 33 opens into the second reservoir 25 and the second electrolyte solution 26 flows from the outlet of the second return circuit 33 into the second reservoir 25.

[0091] Together, the second supply circuit 32 and the second return circuit 33 form a second circulation circuit for the second electrolyte solution.

[0092] The second electrolyte solution 26 is driven through the second feed circuit 31, the main reactor 22 and the second return circuit 33 by the action of at least one second pump 36.

[0093] Each second pump 36 is mounted on the second supply circuit 31 or on the second return circuit 33. The second pump 36 is, for example, a circulation pump or a peristaltic pump.

[0094] The main reactor 22 comprises a first compartment 27 and a second compartment 28 separated by an ion exchange membrane 29, of cationic or anionic type.

[0095] The first compartment 27 receives the circulation of the manganate and permanganate ion solution and the positive electrode while the second compartment 28 receives the circulation of the zincate ion solution and the negative electrode.

[0096] The positive electrode consists of a carbon felt 34 through which the aqueous solution containing manganate and permanganate ions circulates.

[0097] The negative electrode for the Zn / Zn2+ couple can be a zinc metal plate or a zinc electrode made from a mixture of zinc oxide, calcium zincate, an electronically conductive additive, and a binder, as described in document FR 3091042 AL

[0098] The negative electrode can also be made of the carbon felt 34 on which the reduction of a potassium zincate solution is carried out, as in the example shown in [Fig.2]. In this case, the second electrolyte solution 26 is more specifically a saturated or supersaturated aqueous solution of potassium zincate or sodium zincate.

[0099] The carbon felt 34 constitutes a high surface contact electrode with the two electrolyte solutions 24, 26 and offers good permeability for the circulation of electrolyte flows.

[0100] Membrane 29 allows ion exchange between compartments 27, 28, while blocking electron flow. Membrane 29 is, for example, of the type marketed by Fumasep under the name FKE-50 or of the type marketed by Chemours under the name Nafion 115 (registered trademark).

[0101] The balancing reactor 40, shown in detail in [Fig.3], is adapted to regenerate the manganate ions consumed by the parasitic reaction 2 and the self-discharge reaction 3, by implementing an electrochemical reaction to reduce the excess permanganate ions in the first electrolyte solution.

[0102] For this purpose, the balancing reactor 40 includes a first compartment 41 in which the first electrolyte solution 24 circulates, and includes a first electrode 43 in contact with the first electrolyte solution 24 in the first compartment 41.

[0103] The electro-chemical regeneration reaction takes place at the first electrode 43 of the balancing reactor 40, according to the following equation 4:

[0104] [Chem. 4]

[0105] [MnO4] + e ' [MnO4]2

[0106] The balancing reactor 40 also includes a second compartment 42 equipped with a second electrode 44, said second compartment 42 also containing the first electrolyte solution, in a non-circulating manner. The second electrode 44 is in contact with the first electrolyte solution 24 in the second compartment 42.

[0107] By "in a non-circulating manner", it is understood that the quantity of electrolyte solution in the second compartment is substantially constant and that there is no perceptible flow of said solution into and out of the second compartment.

[0108] The second electrode 44 is the site of one or two electrochemical counter-reactions, from among the reactions 5 and 6 below, depending on the pH of the solution:

[0109] [Chem. 5]

[0110] 4OH O2 + 2H2O + 4e [YES] [Chem. 6]

[0112] 2H2O O2 + 4H+ + 4e

[0113] The balancing reactor 40 further includes means for applying an electrical voltage 45 between the first electrode 43 and the second electrode 44, so as to implement the above reactions.

[0114] The voltage applied between the first electrode and the second electrode is sufficient to generate an electric current and cause reactions 4, 5 and 6 above.

[0115] The degree of rebalancing, i.e. the quantity of manganate ions regenerated, is determined by the total charge delivered by this electric current, which is the product of the current intensity by the duration of circulation.

[0116] The balancing reactor 40 also includes a separator 46 disposed between the first compartment 41 and the second compartment 42, said separator comprising in particular a microporous membrane. By microporous, it is understood that the membrane has a porosity whose characteristic size is on the order of one-tenth of a micrometer to tens of micrometers (1 micrometer = 10⁶ meters).

[0117] The microporous membrane makes it possible to greatly slow down the exchange of electrolyte solution between the first compartment and the second compartment, without preventing the passage of ionic current between the first electrode and the second electrode.

[0118] The microporous membrane is chosen to be stable in contact with the electrolyte solution, and in particular not to corrode.

[0119] The microporous membrane comprises, for example, a polypropylene microporous membrane.

[0120] The polypropylene microporous membrane, for example, has a thickness of approximately 25 micrometers, with a porosity of 40%, and an air permeability (measured according to the Gurley method described in ISO 5636-5) of 620 seconds.

[0121] The microporous membrane can be coated with a surfactant to promote the wettability of the electrolyte in contact with the separator 46.

[0122] The microporous membrane is for example a membrane marketed under the brand name Celgard (registered trademark), under reference 3401.

[0123] To promote the electrochemical reduction of permanganate ions into manganate ions, the first electrode 43 is chosen to have a large reaction surface and to be stable in contact with the first electrolyte solution 24.

[0124] The first electrode 43 is preferably a carbon felt through which the first electrolyte solution 24 can flow, so as to supply the first compartment 41 with electrolyte to be rebalanced.

[0125] The combination of an electrolyte circulation through the first compartment 41 and the use of a first electrode with a large reaction surface promotes the electrochemical reduction 4 converting permanganate ions into manganate ions, at the expense of a reduction of water or hydroxyl ions into dihydrogen.

[0126] Conversely, the second compartment 42 is designed to promote an electrochemical oxidation of water or hydroxyl ions into dioxygen, according to relations 5 and 6, to the detriment of an electrochemical oxidation of manganate ion into permanganate ions which would go against the balance sought.

[0127] To this end, the first electrolyte solution 24 is received in a non-circulating manner in the second compartment 42, so as to limit the supply of manganate ions and thus minimize the electrochemical oxidation reaction of manganate. The first electrolyte solution 24 in the second compartment 42, however, remains in contact with the electrolyte to be regenerated, which circulates in the first compartment 41, through the microporous membrane of the separator 46.

[0128] The microporous nature of the membrane, i.e. that the dimensions of the porosity are on the order of micrometers, makes it possible to limit the diffusion of electrolyte between the two compartments, without introducing a significant resistance.

[0129] The second electrode 43 advantageously has a small reaction surface, and comprises for example a metal plate or grid.

[0130] The second electrode 43 is for example composed of a good electronic conductor stable in contact with the first electrolyte solution 24 and stable at the potential of production of dioxygen, for example nickel or 316L steel.

[0131] The second electrode 43 is preferably a nickel grid produced from woven nickel wire. The mesh of the grid must be sufficiently large so as not to trap the oxygen bubbles produced and not to prevent these bubbles from escaping for removal.

[0132] In order to facilitate the production of dioxygen, the second compartment 42 includes, for example, a vent 47 allowing the evacuation of the dioxygen bubbles produced.

[0133] The combination of a non-circulating electrolyte solution in the second compartment and an electrode with a small reaction surface promotes the electrochemical oxidation of water or hydroxyl ions into dioxygen.

[0134] Thus, the imbalance between the quantity of manganate ions and the quantity of permanganate ions in the first electrolyte solution 24 caused by reactions 2 and 3 is rebalanced by the electro-chemical reactions implemented in the balancing reactor 40.

[0135] The first compartment 41 of the balancing reactor can be placed in series in the first supply circuit 30 or in the first return circuit 32, as shown in [Fig.2], the first electrolyte solution 24 comprising the permanganate ions entering at one end of the first compartment 41 and exiting at the other end.

[0136] Alternatively, the balancing reactor 40 can be placed in a separate feed circuit from that of the main reactor 22 and equipped with its own pump, drawing and discharging the first electrolyte solution 23 into the first tank 23.

[0137] The second compartment 42 is initially filled with the first electrolyte solution, which does not then circulate during the operation of the balancing reactor 40.

[0138] As indicated above, the main reactor 22 is represented in a simplified manner and comprises, in [Fig.2], only one cell formed from the first compartment 27 and the second compartment 28 separated by the membrane 29, whereas in practice, the main reactor 22 comprises a plurality of such cells electrically connected in series to each other and supplied by the first supply circuit 30 and the second supply circuit 31, the first reservoir 23 and the second reservoir 25 being common and shared by the plurality of cells.

[0139] Moreover, the balancing reactor 40 is advantageously shared between a plurality of such cells, and preferably by all the cells of the main reactor 22.

[0140] The different cells of the main reactor 22 and the balancing reactor can be mounted fluidically "in parallel", i.e. supplied by diverging supply circuits from the electrolyte reservoir and emptying into convergent return circuits back to the reservoir.

[0141] One embodiment is schematically represented in [Fig.4], with such an assembly with several cells 22a, 22b, 22c of the main reactor 22 and the balancing reactor mounted in parallel.

[0142] Furthermore, in the embodiment of [Fig.4], the balancing reactor 40 is also placed in continuity with the bipolar stack of cells 22a, 22b, 22c of the main compartment 22 of the circulating battery.

[0143] In this embodiment, an electrical shunt 48 is placed between the two electrodes 43, 44 of the rebalancing reactor 40, to control the current used for the electrochemical reaction, the residual current passing through the electrical shunt 48.

[0144] Alternatively, as shown in [Fig.5], an electronic shunt 49, which allows control of the current through it, can be used instead of the electrical shunt 48, to more easily control the amount of rebalancing carried out by the rebalancing reactor 40.

[0145] During the charging of the circulating battery in the configuration of Figures 5 and 6, the resistance of the electrical shunt 48 or electronic 49 is adjusted to allow a portion of the charging current to pass through the electrochemical balancing reactor 40 in order to regenerate the electrolyte.

[0146] During discharge, the resistance of the electrical shunt 48 or electronic 49 is reduced to be as low as possible so as to short-circuit the rebalancing reactor 40 and avoid losses during energy restitution.

[0147] These effects were demonstrated by the applicant in an experiment monitoring the concentrations of manganate and permanganate ions in the aqueous electrolyte solution in a rebalancing reactor 40 as described above, in an assembly shown in [Fig.3], not integrated into an energy storage system.

[0148] The first compartment 41 was placed in a circulation circuit of a sodium permanganate solution, containing manganate and permanganate ions, driven by a peristaltic pump. The second compartment 42 contains the same solution, which was put in place before the test.

[0149] The measurement of manganate and permanganate ion concentrations in the solution circulating in the first compartment was carried out by UV / visible spectrometry based on Beer-Lambert's law.

[0150] Small amounts of the solution circulating in the rebalancing reactor were taken from the circuit at different time intervals to perform a UV / visible spectrum of this solution and to monitor the progress of the reduction of permanganate.

[0151] A current of 2 mA / cm2 (milliamperes per square centimeter) was supplied through the rebalancing reactor during the test, and the resulting voltage between electrodes 43, 44 was measured over time.

[0152] Figure 6 shows the evolution of the intensity I (constant, equal to 2 mA / cm²) of the current through the balancing reactor 40, voltage, total load Q (in mA.h, milliampere hours) delivered by said current, and voltage E (in Volts) between the two electrodes of the balancing reactor 40, as a function of time t (in hours).

[0153] The increase in voltage (in absolute value) between the electrodes of the balancing reactor as a function of time indicates an increase in manganate concentration in the negative compartment (the first compartment) of the reactor.

[0154] Figure 7 shows the absorbance spectra A (unitless) of the solution alkaline sodium permanganate as a function of X wavelength (in nanometers), on the UV / visible spectrum. The spectra are measured before the test (denoted So), and after the test (denoted Sf), i.e., after 20 hours of circulation in the rebalancing reactor 40.

[0155] These results show that the absorbance of the wavelength bands associated with permanganate decreases and that the absorbance of those associated with manganate increases after the implementation of reactor 40.

[0156] Thus, the efficiency of the rebalancing reactor 40 is established for the reduction of permanganate to manganate in the electrolyte solution 24.

Claims

Demands

1. Electrochemical balancing reactor (40) for an electrochemical energy storage system (21), the balancing reactor (40) comprising: - a first compartment (41) for circulating a first electrolyte solution (24) containing permanganate ions, the first compartment (41) comprising a first electrode (43) configured to be in contact with the first electrolyte solution (24) in the first compartment (41), - a second compartment (42), configured to contain the first electrolyte solution (24) in a non-circulating manner, the second compartment (42) comprising a second electrode (44) configured to be in contact with the first electrolyte solution (24) in the second compartment (42), - a separator (46) extending between the first compartment (41) and the second compartment (42), the separator (46) comprising a microporous membrane,- means (45) for applying an electrical voltage between the first electrode (43) and the second electrode (44).

2. Balancing reactor (40) according to claim 1, wherein the first electrode (43) comprises a carbon felt.

3. Balancing reactor (40) according to claim 2, wherein the carbon felt extends into the first compartment (41) so as to be traversed by the first electrolyte solution (24) circulating in the first compartment (41).

4. Balancing reactor (40) according to any one of claims 1 to 3, wherein the second electrode (44) comprises a metal plate or grid.

5. Balancing reactor (40) according to claim 4, wherein the metal plate or grid is made of a metal or alloy stable in basic media, for example nickel or 316L steel.

6. Balancing reactor (40) according to claim 4, wherein the second electrode (44) comprises a nickel grid produced from woven nickel wire.

7. Balancing reactor (40) according to any one of claims 1 to 6, wherein the second compartment (42) includes a vent (47) for venting a gas generated at the second electrode (44).

8. An electrochemical energy storage system (21) comprising: - a first reservoir (23) containing a first electrolyte solution (24) containing permanganate and manganate ions, - a second reservoir (25) containing a second electrolyte solution (26) containing zincate ions, - a main reactor (22) adapted to permit the transfer of protons or hydroxyl anions and electrons between the first electrolyte solution (24) and the second electrolyte solution (26) according to at least one redox reaction, - a first circulation circuit (30, 32) for the first electrolyte solution (24) between the first reservoir (23) and the main reactor (22), - a second circulation circuit (31, 33) for the second electrolyte solution (26) between the second reservoir (25) and the main reactor (22), and - a reactor balancing (40) according to one of the preceding claims,in which the first compartment (41) is fluidly inserted into the first circulation circuit (30, 32).

9. Electro-chemical energy storage system (21) according to claim 8, wherein the balancing reactor (40) is electrically connected in series in a bipolar manner to the main reactor (22) by the first electrode (43) and the second electrode (44).

10. Electrochemical energy storage system (21) according to claim 9, wherein the balancing reactor (40) comprises an electrical shunt (48) or an electronic shunt (49) connected between the first electrode (43) and the second electrode (44), said electrical shunt (48) or electronic shunt (49) being configured to regulate the balancing reactor (40) during a charge of the electrochemical energy storage system (21).

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