EP4677667A1

EP4677667A1 - A method for regenerating electrolytes of an all-iron flow battery

Info

Publication number: EP4677667A1
Application number: EP23724069.2A
Authority: EP
Inventors: Andrii BONDAR; Kyrylo USENKO; Anatolii BESPALIUK; Yaroslav KOLOSOVSKYI; Yuliia DREMLIUHA; Volodymyr MATIUKHA; Vasyl SKRYPNYCHUK
Original assignee: RFlo LLC
Current assignee: RFlo LLC
Priority date: 2023-03-10
Filing date: 2023-03-28
Publication date: 2026-01-14
Also published as: UA129666C2; WO2024191401A1

Abstract

The claimed invention relates to a field of energy industry and concerns a method for regenerating electrolytes in an all-iron flow battery system by including, into a positive electrolyte circuit, a hydrogen-iron battery that is powered by hydrogen generated during charging at a negative electrode of the all-iron battery, and by Fe³⁺ ions which are formed during charging at its positive electrode, and an iron-oxygen electrolyzer that, by means of a current that is generated by the hydrogen-iron battery operation, reduces Fe³⁺ ions in the negative electrolyte and generates hydrogen protons in order to restore its acidity. This implementation of the method enables to reduce excessive Fe³⁺ released at the positive electrode to Fe²⁺ in the positive electrolyte, to maintain stable pH value in the negative electrolyte, and to reduce the negative electrolyte Fe³⁺ ions which transfer from the positive electrolyte through a membrane to Fe²⁺ ions, thereby decreasing corrosion of the negative electrode. The claimed method avoids an imbalance of the state of charge of both the positive electrolyte and the negative electrolyte.

Description

A METHOD FOR REGENERATING ELECTROLYTES OF AN ALLIRON FLOW BATTERY

FIELD OF THE INVENTION

The claimed invention relates to the energy industry, particularly, to electrochemical technologies for energy accumulation and storage, and it relates to a method for regenerating electrolytes in an all-iron flow battery system.

PRIOR ART

Currently, one of the most promising technologies for long-term energy storage is the use of redox flow batteries, especially in view of the demand for using renewable energy sources continues to increase. These sources, e.g., solar and wind energies, have a non-predictable nature that is caused by weather or seasonal factors and require reliable mechanisms for storing and integrating their energy into power grids. Flow batteries differ from other electrochemical energy storage technologies in that they utilize liquid electrolyte solutions which are supplied from outside and comprise reagents that participate in reversible electrochemical reactions. A general operation principle of the flow batteries implies the use of redox reactions both for accumulating the energy in a form of chemical potential in liquid electrolyte solutions and for converting the accumulated chemical energy back to the electrical energy. In terms of a structure, the flow batteries consist of a flow cell that is separated by a separator such as an ion-exchange separator into compartments having positive and negative electrodes arranged therein. The compartments are filled with positive and negative electrolytes, and each of the compartments is connected to a container for storing the corresponding electrolyte, while enabling a closed circulation of the electrolytes between the containers and the cell compartments. The formed closed electrochemical chamber may accumulate and convert the electrical energy to the chemical energy and vice versa for multiple times, thereby providing its reliability and scaling possibilities. Therefore, the flow batteries are used in network storage systems for decreasing peak loads, balancing loads, controlling charge, providing backup power etc.

Currently, the most promising option is to use iron-based flow batteries due to the abundance of raw materials, their affordability, as well as environmentally friendly production and usage. Electrochemical reactions involving iron ions which are dissolved in the corresponding electrolytes take place on the positive and negative electrodes of the all-iron flow battery. The positive electrode uses a Fe²⁺/Fe³⁺ redox pair, while the negative electrode uses a Fe²⁺/Fe° redox pair. Iron chlorides or sulfates solutions (e.g., FeC12, FeClj, FeSCh, FesfSO^)^) may be used as redox active compounds of the electrolytes of the all-iron battery, and these solutions are abundant and provide consistency of the electrolyte, since the very same electrolyte may be used both for the negative electrolyte and for the positive electrolyte, thereby decreasing cross-contamination issues. Owing to the electron configuration of iron, it may be deposited into a uniform structure during its galvanic deposition onto a substrate of the negative electrode, thereby providing a stable morphology of the electrode. Therefore, the all-iron flow batteries decrease the use of toxic raw materials as compared to other redox flow batteries, thereby decreasing an environmental hazard correspondingly.

The main problem in the implementation of the continuously working alliron flow battery is a side process of hydrogen evolution at the surface of the iron electrode during battery charging which results in an imbalance of the state of charge of the electrolytes and, thus, in decreasing the number of hydrogen ions in the negative electrolyte which, in turn, increases its pH. Also, during discharging of the all-iron flow battery, the hydrogen corrosion process of metallic iron takes place, thereby forming additional gaseous hydrogen and further increasing pH of the negative electrolyte. One more side process is the diffusion of Fe³⁺ ions from the positive electrolyte compartment to the negative electrolyte compartment followed by the reduction reaction at a surface of the metallic iron. Both the mentioned side reactions result in decreasing the metallic iron amount at the positive side electrode and, thus, in decreasing its state of charge (SOC).

This problem is being resolved by scientists and developers in various ways, from addition of agents and additives to the electrolytes for inhibiting the hydrogen evolution (see, e.g., B. S. Jayathilake et al 2018 J. Electrochem. Soc. 165 A1630 - Improvements to the Coulombic Efficiency of the Iron Electrode for an All-Iron Redox-Flow Battery) to providing the battery with supplementary hydrogen circulation circuits and supplementary tanks with electrolytes for reduction of hydrogen ions in a posilyte and iron ions in a negolyte (pat. US11228052B dated January 18, 2022). During implementation of these solutions, they appear to be rather complex and require precise regulation of the number of agents, flow rate of the electrolytes or pressure, or precise control of the hydrogen flow etc., thereby complicating this energy accumulation and storage technology in real production conditions.

Thus, a more practical solution would be to provide methods and systems for electrolyte balance restoration, where additional balancing electrochemical cells are introduced into the flow battery system, as well as conditions and their interaction order are created, in order to prevent the formation of excessive gaseous hydrogen or to convert it back to protons by means of a supplementary electrochemical reaction.

For example, a solution disclosed in a pat. US10581103 B2 dated March 3, 2020 teaches an all-iron flow battery system with a balancing cell introduced therein, the cell consists of three chambers which are separated by ion-selective and bipolar membranes, wherein side compartments have electrodes and are filled with a first electrolyte solution, and a middle one is filled with a second electrolyte solution, and either the first compartment or the second compartment of the flow battery are in a fluid communication with side cells. An operation method of this system implies elimination of sharp pH fluctuations in the electrolytes and prevention of gaseous hydrogen evolution by alternating application of a potential to the electrodes of a rebalancing cell, while providing conversion of water to protons and hydroxide ions at the bipolar membrane; as a result, the ions migrate to an aqueous solution of the electrolyte in the middle chamber, and the hydroxide ions migrate to the first electrolyte solution in one of the side chambers. Depending on the communication between the electrochemical balancing cell and one of the compartments of the flow battery, the first electrolyte solution is transferred either to the negative compartment of the flow battery or to the positive compartment of the flow battery.

A drawback of this electrolyte regeneration method is the fact that its implementation requires thorough mechanisms for tracking an imbalance within the circuits of the flow battery and their timely switching between the chambers of the rebalancing cell, and it does not provide a reliable performance of this technology.

The use of an electrochemical balancing element that absorbs the gaseous hydrogen and that can be connected to the flow battery system in order to avoid an imbalance of electrolytes during its operation is taught in a pat. JP6549572B2 dated July 24, 2019. An operation method of the balancing element comprises receiving the hydrogen-containing gas as a side product of the flow, e.g., all-iron battery, contacting the hydrogen-containing gas with the first electrode, contacting the negative electrolyte of the all-iron battery with the second electrode, supplying a voltage to the first electrode and second electrodes in an amount that is sufficient to provide balancing, wherein hydrogen cations are passed through the membrane that is arranged between the first electrode and the second electrode to the negative electrolyte in order to restore pH balance therein. Said element comprises a first electrode comprising a gaseous diffusion electrode and a hydrogen oxidation catalyst, and a second electrode that contacts with a negative electrolyte, a membrane through which hydrogen cations pass from the membrane to the negative electrolyte, and the membrane is arranged between the first electrode and the second electrode. This element substantially is a secondary flow hydrogenmetal battery that, by absorbing parasitic hydrogen released in the main battery during the reaction with the negative electrolyte, may further generate or accumulate the charge, however, the use of only one electrolyte in the liquid circuit has an indirect and insufficient influence on provision of a full balancing of the state of charge of the metal flow battery, since in real conditions it is hard to provide the transfer of the entire hydrogen amount and, thus, equalization of pH of the negative electrolyte and of SOC of the positive electrolyte occurs incompletely.

A patent US11527771 B2 dated December 13, 2022 discloses a method and a system for rebalancing electrolytes in a redox flow battery system having cell compartments which are in a fluid communication with electrochemical balancing elements comprised in a circuit of each of the electrolytes and which comprise elements for directing gaseous hydrogen to them. Trickle bed reactors are used as the electrochemical balancing elements, and multi-layered hydrogen catalysts are arranged in the reactors. A method of rebalancing electrolytes in a redox flow battery system comprises directing gaseous hydrogen generated at a negative side of the redox flow battery system to a catalyst surface, and contacting the gaseous hydrogen with the electrolyte comprising metal ions on the catalyst surface, wherein the metal ions are chemically reduced by the gaseous hydrogen at the catalyst surface, and the state of charge and pH of the electrolytes remain balanced. A drawback of this solution is that the hydrogen generated at the side of the negative electrolyte is not sufficient for balancing the entire system, and to this end, it is implied to use additional external sources for storing and supplying hydrogen and electrolytes, thereby complicating the circuits of the system and requiring a complex regulation of modes for supplying hydrogen and electrolytes to the reactors, and to this end, the system is equipped with an extensive system of sensors which enable a controller to control the hydrogen supply and the circulation of the electrolytes. Said drawback complicates the system and impugns its reliability. Furthermore, use of two reactors with catalysts which comprise precious metals to decrease the hydrogen absorption makes this technology more expensive.

SUMMARY OF THE INVENTION

An objective of the claimed invention is to improve the existing technologies and to provide a practical simplification of the method for regenerating electrolytes of the all-iron battery by implementing a divided provision of the consistency of each of the electrolytes by means of mutually dependent electrochemical balancing elements, while providing a use of the entire gaseous hydrogen flow. A technical effect being achieved is the control of acidity parameters, corrosion degree and state of charge of electrolytes without any external interference with the system operation in order to increase the cyclic operation stability of the all-iron flow battery.

The objective is achieved by providing a method for regenerating electrolytes that is performed by pumping the electrolytes in an all-iron flow battery system by means of the circulation pumps from the compartments of the cell, along the electrolyte circuits, to the corresponding tanks, while at the same time ensuring charging of the all-iron battery. The all-iron battery is charged by supplying a current having a given density to its electrodes, and the electrode of the negative electrolyte compartment acts as an anode (-), while the electrode of the positive electrolyte compartment acts as a cathode (+). During the charging process of the all-iron flow battery, target processes are oxidation of Fe²⁺ ions to Fe³⁺ at the cathode surface and reduction of Fe ions in the negative electrolyte to metallic iron (Fe°) that is galvanically deposited onto the anode surface. Said reactions enable to convert the incoming electrical energy to an accumulated electrochemical form. When these reactions take place at the corresponding electrodes, reaction products, namely the Fe -enriched positive electrolyte and the Fe -depleted negative electrolyte, are continuously pumped out of the compartments to the electrolyte tanks, wherefrom, while moving together with the initial electrolyte present therein, they are supplied again to the corresponding compartments. This results in updating the state of charge (SOC) of each of the electrolytes in the compartments of the cell.

However, these side processes which take place during the charging result in that the capacity of the all-iron battery decreases with each charging cycle and it will be determined by the capacity of the imbalanced positive electrolyte, thereby decreasing the overall current (coulombic) efficiency of the battery operation.

With consideration of the above-mentioned information, an intermediate step, at the same time, is pumping of the electrolytes through additional electrochemical electrolytes balancing elements which are introduced into the alliron battery system.

According to the claimed invention, the negative electrolyte is pumped from the corresponding compartment of the all-iron flow battery through a cathodic portion of the iron-oxygen electrolyzer that is a portion of the negative electrolyte circuit, while the positive electrolyte is pumped from the corresponding compartment of the all-iron flow battery through a cathodic portion of the hydrogen-iron battery that is a portion of the positive electrolyte circuit. Therewith, the gaseous hydrogen, after its evolution at the anode surface as a result of decomposition of an acid that is comprised in the negative electrolyte, is isolated from the negative electrolyte and directed to an anodic portion of the hydrogeniron battery, namely to its gaseous diffusion electrode, where it is catalytically oxidized to H⁺ protons which then diffuse through the membrane to the cathodic portion, where the positive electrolyte flows to the tank and the Fe³⁺ ions are reduced to Fe ions which results in restoration of the initial iron concentration in the positive electrolyte flow.

The current that is generated by the hydrogen-iron battery is at least partially supplied to the electrodes of the electrolyzer, thereby providing its operation. At the anode of the electrolyzer, distilled water is electrochemically decomposed, thereby forming oxygen and H⁺ ions which then diffuse through the membrane to the cathodic portion of the electrolyzer, while at the surface of the electrode of the cathodic portion of the electrolyzer, the Fe ions are reduced to Fe ions which transfer to the negative electrolyte flow when it is pumped through the cathodic chamber of the electrolyzer to the negative electrolyte tank.

The hydrogen-iron battery and the electrolyzer operate continuously during the charging process of the all-iron battery, and the operation current is determined by the limiting flow of reagents at the electrodes of these batteries, so the increased regeneration efficiency may be achieved by harmonization of ratios of active areas of the electrodes and membranes. Therewith, the current that is generated by the hydrogen-iron battery is determined by the intensity of the hydrogen flow from the negative electrolyte compartment; while the current that is consumed by the electrolyzer is determined by the concentration of Fe³⁺ ions in the negative electrolyte.

In order to implement the method, the all-iron flow battery system consists of: a cell of the all-iron flow battery, the cell is divided by a membrane into compartments having electrodes arranged therein, and the compartments are filled with a negative electrolyte and a positive electrolyte respectively; a negative electrolyte tank and a positive electrolyte tank, and each of the compartments is connected to the tanks via a pipeline so as to form separate closed circuits; circulation pumps which are arranged in the circuits at inlets of the corresponding compartments of the cell of the all-iron battery; electrochemical elements for electrolytes balancing which are arranged in the circuits at outlets of the compartments; a gaseous hydrogen transfer tool for transferring the gaseous hydrogen from the negative electrolyte compartment.

Therewith, a positive electrolyte balancing element is a hydrogen-iron battery that consists of a cathodic chamber with a porous electrode arranged therein, and the porous electrode is inserted into a positive electrolyte circuit, and an anodic chamber that is separated from the cathodic chamber by an ion-exchange membrane, the anodic chamber comprises a gaseous diffusion electrode that is connected to the gaseous hydrogen transfer tool, and a hydrogen oxidation catalyst layer is deposited onto the gaseous diffusion electrode of the anodic chamber from the ion-exchange membrane side. A negative electrolyte balancing element is an iron-oxygen electrolyzer that consists of a cathodic chamber comprising a corrosion-resistant electrode that is inserted into a negative electrolyte circuit, and an anodic chamber that is separated from the cathodic chamber by an ion-exchange membrane, the anodic chamber comprises a distilled water electrolyte and comprises an electrochemical oxygen evolution electrode. Therewith, the ironoxygen electrolyzer is electrically connected to the hydrogen-iron battery so as to enable its use as a power source.

According to a possible exemplary embodiment of the invention, an active area of the electrodes is the same as an active area of the membrane of the all-iron flow battery, an active area of the electrodes of the hydrogen-iron battery is 0.5- active area of the electrodes of the electrolyzer is 0.1-5% of the active area of the electrodes of the all-iron battery.

In order to provide effective rebalancing, according to a possible exemplary embodiment of the invention, the density of the current that is supplied to the ironoxygen electrolyzer from the hydrogen-iron battery is 0.1-1% of the current density on the cell of the all-iron battery.

Therewith, an excessive current that is generated during operation of the hydrogen-iron battery may be used to fulfill internal needs of the system, while the oxygen that is formed during the operation of the iron-oxygen electrolyzer may be exhausted out of the system.

The claimed technical solution enables to compensate an influence of the side processes of the all-iron flow battery operation by using the hydrogen-iron battery that is powered by hydrogen that is generated during charging at the negative electrode of the all-iron battery, and by Fe³⁺ ions which are formed during charging at the positive electrode, and the iron-oxygen electrolyzer that, due to the external current that is received from the hydrogen-iron battery, reduces the Fe³⁺ ions in the negative electrolyte and generates hydrogen protons in order to restore its acidity.

DESCRIPTION OF THE FIGURES

In order to provide a more complete understanding of the invention and advantages thereof, the following description provides an explanation of possible exemplary embodiments on the invention with a reference to the appended figures, wherein identical designations denote identical parts:

Fig. 1 illustrates a schematic view of the all-iron battery system;

Fig. 2 illustrates a schematic view of the hydrogen separator chamber; Fig. 3 illustrates a schematic view of the electrolyzer structure;

Fig. 4 illustrates a schematic view of the hydrogen-iron battery structure;

Fig. 5 illustrates a plot depicting a capacity ratio of the negative electrolyte as a function of an initial capacity, when the capacity is lowered as a result of hydrogen losses;

Fig. 6 illustrates a plot depicting the state of charge of the electrolytes as a function of the state of the process during charging without hydrogen losses and with consideration of the hydrogen losses.

Main designations:

1. Cell of the all-iron flow battery

2. Negative electrolyte compartment

3. Positive electrolyte compartment

4. Membrane of the cell of the all-iron flow battery

5. Negative electrolyte tank

6. Positive electrolyte tank

7. Negative electrolyte circulation circuit

8. Positive electrolyte circulation circuit

9. Negative electrolyte pump

10. Positive electrolyte pump

11. Iron-oxygen electrolyzer

12. Hydrogen-iron battery

13. Gaseous hydrogen transfer tool for transferring the gaseous hydrogen from the negative electrolyte

14. Chamber of the separator

15. Transfer tube for transferring the negative electrolyte with the gaseous hydrogen 16. Directing tube for directing the gaseous hydrogen to the hydrogen-iron battery

17. Directing tube for directing the negative electrolyte to the circulation circuit

18. Anodic chamber of the iron-oxygen electrolyzer

19. Cathodic chamber of the iron-oxygen electrolyzer

20. Ion-exchange membrane of the iron-oxygen electrolyzer

21. Oxygen evolution electrode

22. Distilled water electrolyte flow chamber

23. Fe³⁺ ions reduction electrode

24. Current conductor

25. Anodic chamber of the hydrogen-iron battery

26. Cathodic chamber of the hydrogen-iron battery

27. Ion-exchange membrane of the hydrogen-iron battery

28. Hydrogen chamber of the hydrogen-iron battery

29. Gaseous diffusion electrode

30. Catalyst layer

31. Positive electrolyte flow chamber

32. Current conductor

INVENTION IMPLEMENTATION POSSIBILITY

In order to implement a method for regenerating electrolytes, an all-iron flow battery system is used, the system consists of main elements such as a cell (1) of the all-iron battery, the cell is divided into compartments (2, 3) by a membrane (4). The compartments (2, 3) are connected to a negative electrolyte tank (5) and to a positive electrolyte tank (6) respectively, thereby forming separate closed electrolyte circuits (7, 8) which are equipped with circulation pumps (9, 10) at inlets of the compartments (2, 3). At an outlet of the compartment (2), the negative electrolyte circulation circuit (7) is directed through a cathodic portion of an iron- oxygen electrolyzer (11), while at an outlet of the compartment (3), the positive electrolyte circulation circuit (8) is directed through a cathodic chamber of a hydrogen-iron battery (12). A gaseous hydrogen transfer tool for transferring the gaseous hydrogen and directing it to an anodic chamber of the hydrogen-iron battery (12) is arranged in the negative electrolyte circuit (7).

The gaseous hydrogen transfer tool for transferring the gaseous hydrogen from the negative electrolyte may be provided at the outlet of the negative electrolyte compartment (2) and represented as a separator that consists of an intermediate chamber (14), a transfer tube (15) for transferring the negative electrolyte with the gaseous hydrogen formed therein, a directing tube (16) for directing the gaseous hydrogen to the anodic chamber of the hydrogen-iron battery (12), and a directing tube (17) for directing the negative electrolyte to the circulation circuit (7).

The iron-oxygen electrolyzer (11) is a flow cell that consists of two sealed chambers (18, 19) which are separated by an ion-exchange membrane (20). The chamber (18) is an anodic chamber and comprises an electrochemical oxygen evolution electrode (21) that is connected to an aqueous electrolyte flow chamber (22). The chamber (19) that is functionally a cathodic chamber comprises a Fe³⁺ ions reduction corrosion-resistant electrode (23) that is a flow chamber which the electrolyte from the negative electrolyte circulation circuit (7) is pumped through. Therewith, the electrode (23) is connected to a flat graphite current conductor electrode (24).

The hydrogen-iron battery is a cell that consists of two sealed chambers (25, 26) which are separated by an ion-exchange membrane (27). The chamber (25) operates as an anodic chamber and comprises a hydrogen gas chamber (28) which the gaseous hydrogen is supplied to, a gaseous diffusion flat porous carbon electrode (29) that is connected thereto and is coated with a catalyst layer (30) at the membrane (27) side. The chamber (26) operates as a cathodic chamber and comprises a positive electrolyte flow chamber (31) that is connected to the positive electrolyte circulation circuit (8) and is a corrosion-resistant (e.g., graphite) porous electrode that has a surface with the process of electrochemical reduction of Fe³⁺ ions occurring thereat and that is connected to a flat graphite current conductor electrode (32).

According to the below-mentioned exemplary embodiments of the claimed method, an aqueous solution of iron chloride with the addition of ammonium chloride and boric acid is used as the negative side electrolyte of the all-iron battery, while an aqueous solution of iron chloride with the addition of ammonium chloride is used as the positive side electrolyte. Therewith, a molar ratio of said substances in the solutions is as follows, depending on a cycle state: in the charged state negative electrolyte: 0.2-2 M of FeCl₂, 2-3 M of NH₄C1, 0.2-0.4 M of H3BO3 positive electrolyte: 0.4-1 M of FeCl₂, 0.6-2 M of FeCh, 2-3 M ofNH₄Cl in the discharged state negative electrolyte: 1-3 M of FeCl₂, 2-3 M of NH₄C1, 0.2-0.4 M of H3BO3 positive electrolyte: 1-3 M of FeCl₂, 2-3 M of NH₄C1

Main reactions during operation of the all-iron battery are as follows: at the negative electrode during charging - Fe²⁺ + 2e‘ = Fe, during discharging - Fe - 2e‘ = Fe²⁺ at the positive electrode during charging - Fe²⁺ - e' = Fe³⁺ during discharging - Fe³⁺ + e' = Fe²⁺ Therewith, the following side reactions take place at the negative electrode: during charging

(1 ) 2H⁺ + 2e’ = H₂

(2) Fe³⁺ +e'= Fe²⁺ during discharging

In order to start the regeneration of electrolytes according to the claimed method, the pumps (9, 10) are activated in the all-iron battery system to pump the electrolytes along the circuits (7, 8) and the charging process of the all-iron battery is started.

During the charging process of the all-iron battery, the negative electrolyte from the compartment (2) of the all-iron battery (1) is pumped through the sealed cathodic chamber (19) of the electrolyzer (11). The pumping is performed through the flow chamber (23) which substantially represents the porous graphite electrode whereat the process of reduction of Fe³⁺ ions to Fe²⁺ takes place. Therewith, the electrode (23) is made of the corrosion-resistant material and may be a porous flow electrode that is equipped with the flat graphite current conductor electrode (24).

At the same time, distilled water or water vapor is pumped through the anodic sealed chamber (18) of the electrolyzer via the flow chamber (22), and the distilled water or the water vapor is decomposed at the surface of the electrode (21) arranged in this chamber, while forming ions H⁺ which transfer through the membrane (20) to the negative electrolyte flow in the electrode (23) and the gaseous oxygen that is exhausted outside. An acid-resistant Nation membrane having a thickness from 50 to 500 pm manufactured by Chemours may be used as the membrane (20), while as the electrode catalyst for this reaction, oxygen evolution reaction catalysts such as iridium oxide, as well as a more abundant ruthenium oxide on the electrode titanium carrier, may be used.

The iron-oxygen electrolyzer (11) operates in a flow mode, while pumping both electrolytes being the negative electrolyte of the all-iron battery and the distilled water.

Oxygen, after its evolution in the anodic chamber (18), is pumped out by the distilled water flow to a distilled water container (not shown in the Figures), where it is isolated from the liquid as a result of a buoyant force and exhausted out of the system.

When applying the current to the electrodes of the electrolyzer (11) during charging of the all-iron battery (1), the following reactions take place:

- in the anodic chamber (18) which the distilled water is supplied to, at the surface of the electrode (21):

2H₂O - 4e* = O₂ + 4H⁺

- in the cathodic chamber (19) that is connected to the negative electrolyte circulation circuit (7), at the electrode (23):

Fe³⁺ + e' = Fe²⁺

At the same time, the gaseous hydrogen, after its evolution in the negative electrolyte compartment (2) of the battery, is transferred by the electrolyte flow out of the compartment and supplied to the transfer tool (13) therefor which is the sealed chamber (14) of the separator, and the sealed chamber is embedded either into the circuit (7) or directly into the negative electrolyte compartment (2) having the transfer tube (15) for transferring the negative electrolyte with the gaseous hydrogen, the gaseous hydrogen directing tube (16) and the directing tube (17) for directing the negative electrolyte back to the circuit (7) inserted into the compartment. At the outlet of the negative electrolyte compartment, the negative electrolyte with hydrogen bubbles is supplied along the tube (15) to the separator chamber (14), where the bubbles, due to the buoyant force, are separated from the electrolyte flow, and the gaseous hydrogen diffuses along the pneumatic tube (16) to the hydrogen chamber (28) of the hydrogen-iron battery (12). The negative electrolyte without gas bubbles is transferred along the tube (17) to the circuit (7) and continues to circulate therein.

The hydrogen chamber (28) is arranged in the sealed chamber (25) and comprises the gaseous diffusion anode electrode (29), i.e., the flat porous carbon electrode coated with the catalyst layer (30) from the membrane (27) side. Platinum nanoparticles which are deposited onto an electrically conductive corrosion-resistant carrier such as carbon nanoparticles may be used as the catalyst. Acid-resistant cation exchange membranes (e.g., Nafion) having the thickness from 50 to 500 pm, as well as acid-resistant anion exchange membranes of the same thickness, may be used as the ion-exchange membrane.

The second sealed chamber (26) of the hydrogen-iron battery that operates as the cathodic chamber is connected to the positive electrolyte circulation circuit (8) of the all-iron battery (1) and comprises the corrosion-resistant, e.g., graphite porous electrode (31) which the electrolyte of the all-iron battery (1) flows through and which has the surface whereat the process of electrochemical reduction of Fe³⁺ ions takes place, and which is connected to the flat graphite current conductor electrode (32).

During the hydrogen-iron battery (12) operation and charging of the all-iron battery (1), the following reactions take place:

- in the anodic chamber (25) which the hydrogen from the negative electrolyte compartment (2) is supplied to, at the anode (29) at the surface of the catalyst (30);

H₂ - 2e' = 2H⁺ - in the cathodic chamber (26) that is connected to the positive electrolyte circulation circuit (8):

Fe³⁺ + e = Fe²⁺ which is accompanied by generation of the electrical current that is used directly for the electrolyzer (11) operation.

In order to confirm that it is possible to achieve the objective of the invention, the following technical parameters were used in experimental tests: capacity of the pumps (9, 10) in a range from 0.1 to 10.0 ml/min/cm , while providing a certain ratio of pumping rates of the negative electrolyte and the positive electrolyte in a range, e.g., from 0.5 to 2.0; current density during charging of the all-iron battery in a range from 5 to 100 mA/cm , having fixed values in a range from 25 to 75 mA/cm , namely, 25 mA/cm², 50 mA/cm², 75 mA/cm²; areas of each of the electrodes of the all-iron battery in a range from 100 to 2000 cm², and exemplary values are in a range from 400 to 800 cm², having fixed values of 400 cm², 600 cm², 800 cm²; pumping rate of the electrolytes, thus, in a range from 0.5 to 5.0 ml/min/cm , and exemplary values are in a range from 2.0 to 3.0 ml/min/cm , having fixed values of 2.0 ml/min/cm², 2.5 ml/min/cm², 3.0 ml/min/cm².

At said parameters, the voltage measured at each of the cells of the all-iron battery was: from 1.25 to 1.6 V during charging, with typical values at the beginning of the charging being 1.25 V, 1.35 V, 1.45 V; and typical values at the end of the charging process being 1.3 V, 1.45 V, 1.6 V. from 0.8 to 1.2 V during discharging, with typical values at the beginning of the discharging being 0.9 V, 1.05 V, 1.2 V; and typical values at the end of the discharging being 0.8 V, 0.9 V, 1.0 V.

Therewith, the discharging time was determined by a required usage scenario, but not more than 12 hours. Typical values were 0.5 hours, 1 hour, 3 hours, 6 hours, 9 hours, 12 hours, and the discharging time was determined by a required use scenario and by the state of charge of the all-iron battery at an initial time point.

Parameters of the hydrogen-iron battery were set as follows: current density is from 100 to 1000 mA/cm , and exemplary values are in a range from 200 to 500 mA/cm², having fixed values of 200 mA/cm², 350 mA/cm², 500 mA/cm²; area of the electrodes is from 0.5 to 200 cm , and exemplary values are in a range from 2.5 to 30 cm , having fixed exemplary values of 2.5 cm , 10 cm , 20 cm , 30 cm²; pumping rate of the electrolyte is the same as the pumping rate of the electrolyte in the compartment (3) of the all-iron battery, since it comprises the shared positive electrolyte circulation circuit (8); operation duration is the same as the duration of the charging process of the alliron battery, since the input reagent is hydrogen resulted from its evolution during the charging process of the all-iron battery.

Parameters of the iron-oxygen electrolyzer were set as follows: current density: from 1 to 100 mA/cm², and exemplary values are in a range from 2 to 20 mA/cm², having fixed values of 2 mA/cm², 10 mA/cm², 20 mA/cm²; area of the electrodes is from 2 to 80 cm², and exemplary values are in a range from 8 to 32 cm², having fixed values of 8 cm², 10 cm², 20 cm², 32 cm²; pumping rate of the electrolyte is the same as the pumping rate of the electrolyte in the compartment (2) of the all-iron battery, since it comprises the shared negative electrolyte circulation circuit (7); operation duration of the iron-oxygen electrolyzer is the same as the operation duration of the hydrogen-iron battery, since the input current for the electrolysis is derived from electrochemical processes of the hydrogen-iron battery, thus, it occurs continuously during the charging process of the all-iron battery.

During the tests, the states of charge of the negative electrolyte and positive electrolyte were compared both in the all-iron battery system without any balancing elements and in the all-iron battery system equipped with the hydrogeniron battery and the iron-oxygen electrolyzer.

When the system operates without any balancing elements, due to the side processes (hydrogen evolution in the negative electrolyte compartment and Fe³⁺ ions migration from the positive electrolyte compartment to the negative electrolyte compartment), the decrease of the number of the ferric ions in the positive electrolyte and, thus, the loss of its capacity was observed. It was found that the capacity within the above-fixed operation parameters of the all-iron battery is decreased by 5% per one operation cycle, and after 13th cycle, it causes a drop of the positive electrolyte capacity by half as shown in the scheme of Fig. 5.

At the same time, the negative electrolyte loses 5% of the current for the hydrogen evolution reaction, thereby resulting in the acidity change of the negative electrolyte, while creating the imbalance of the states of charge of the negative electrolyte and positive electrolyte, and, thus, in losing nominal operational conditions and decreasing the all-iron battery efficiency.

When the balancing elements as per the claimed invention are included into the system, they will help to compensate or prevent these processes. In particular, the hydrogen-iron battery utilizes hydrogen resulted from its evolution during the side process in the negative electrolyte compartment in order to decrease the Fe³⁺ concentration in the positive electrolyte and thereby decreasing the state of charge of the positive electrolyte and shifting it towards the state of charge of the negative electrolyte with consideration of losses for the hydrogen evolution. The gaseous hydrogen H₂ is oxidized to H⁺ and returns to the electrolyte. During the process of further positive electrolyte circulation, the H⁺ ions from the electrolyte, due to diffusion and electromigration, pass through the membrane of the all-iron battery and restore the pH in the negative electrolyte.

Said electrochemical processes create the potential difference at the cell of the hydrogen-iron battery and the electrical current that supplies power to the ironoxygen electrolyzer, and its excess may be returned to the external electrical circuit.

In turn, the use of the electrolyzer allows to avoid losing the metallic iron due to the reaction with the Fe ions by reducing these ions in the electrolyzer to Fe²⁺. Therefore, the hydrogen loss in the all-iron battery system is compensated by increasing, by means of the electrolyzer, the number of iron ions in the negative electrolyte that is gradually consumed during the system operation.

Therefore, inclusion of said electrochemical balancing elements into the alliron battery system enables to balance the pH of the negative electrolyte and the state of charge (SOC) of the positive electrolyte, thereby preventing any significant capacity loss and equalizing the electrolyte balance, and the schematic view of the electrolyte balance, both with the use of the hydrogen-iron battery and the ironoxygen electrolyzer in the system and without them, is shown in Fig. 6.

The hydrogen-iron battery and the iron-oxygen electrolyzer operate continuously during the charging process of the all-iron battery, and the current that is generated by the hydrogen-iron battery is determined by the intensity of the hydrogen flow from the negative electrolyte compartment, while the current that is consumed by the electrolyzer is determined by the concentration of Fe³⁺ ions in the negative electrolyte.

The operation of the iron-oxygen electrolyzer (11), primarily the power and the current density thereat, is controlled by measuring the current and the voltage at the cell of the hydrogen-iron battery (12) and by monitoring the pH values using pH sensors within the negative electrolyte tank (5) and the positive electrolyte tank (6) respectively. According to these values, a hydrogen amount lost in the system as well as an electricity amount that should be supplied from the hydrogen-iron battery to the electrolyzer cell in order to compensate these losses were calculated.

During the tests, the following ratios between the active areas of the electrodes and the current density on these electrodes which confirmed the strong efficiency of use of the mentioned balancing elements in the hydrogen-iron battery system were determined: the active area of the electrodes of the hydrogen-iron battery being 0.5-10% of the active area of the electrodes of the cell of the all-iron battery, the active area of the electrolyzer being 0.1-5% of the active area of the electrodes of the all-iron battery, the current density on the electrodes of the cell of the all-iron battery being 5-500 mA/cm², the density of the current that is supplied to the iron-oxygen electrolyzer from the hydrogen-iron battery being 0.1-1% of the current density on the cell of the all-iron battery.

After the charging process of the all-iron battery is completed, either the discharging process starts or a pause in the battery operation takes place followed by discharging, depending on the selected operation mode. During the discharging, the polarity of the electrodes of the all-iron battery and the polarity of the current are changed. Therewith, reverse processes take place on the electrodes of the all-iron cell: oxidation of the metallic Fe resulting in formation of Fe²⁺ in the negative electrolyte and reduction of Fe³⁺ to Fe²⁺ in the positive electrolyte while returning the electrical energy to the external electrical circuit. Since the positive electrolyte compartment electrode acts as the anode in this case, the hydrogen evolution and the Fe³⁺ electromigration processes are suppressed, and, thus, there is no need in the regeneration at this step, so the method is ended and the operation of the hydrogen-iron battery and the electrolyzer is terminated.

However, in order to maintain a long-term stability of the regenerated electrolytes to avoid hydrogen corrosion and to preserve the accumulated charge, the electrolytes, during the operation pause of the all-iron battery, may be pumped out from its compartments to the tanks which is performed by changing the pumping direction of the circulation pumps. Furthermore, during the pause and discharging of the all-iron battery, the electrolytes also may be pumped out of the supplementary cells of the hydrogen-iron battery and of the electrolyzer, as well as the corresponding electrolyte flows may be redirected through bypass channels in order to enhance the operation stability of said cells.

Therefore, the claimed method and the parameters obtained in said exemplary embodiments thereof indicate that it may be used for all-iron battery systems in wide ranges of scaling of geometric dimensions of the electrodes and powers depending on the required usage scenario. Therewith, in order to increase the current of the battery, a parallel connection of individual cells may be used, while in order to increase the battery voltage, a series connection of individual cells is used.

The above-described exemplary embodiments of the invention must be used for illustrative purposes only and must not restrict the scope of the invention, since skilled artisans are able to introduce obvious modifications during implementation of the invention without going beyond its essence.

The claimed invention enables to implement the reliable method for regenerating electrolytes in the all-iron flow battery system and to significantly eliminate the imbalance in the state of charge of the positive electrolyte and the negative electrolyte, as well as to increase the overall operation stability and lifetime of the all-iron flow battery.

Claims

1. A method for regenerating electrolytes in an all-iron flow battery system, the method comprising pumping, by means of circulation pumps, a negative electrolyte and a positive electrolyte simultaneously along closed circuits for corresponding electrolytes which pass through electrolyte storage tanks, compartments of a cell of the all-iron battery and balancing elements for restoring an initial concentration of ions in the electrolytes, wherein a hydrogen-iron flow battery is used as an electrochemical positive electrolyte balancing element, an iron-oxygen flow electrolyzer is used as an electrochemical negative electrolyte balancing element, the electrolytes are pumped and a current is supplied to electrodes of the cell of the all-iron flow battery simultaneously, the positive electrolyte is pumped from a positive electrolyte compartment through a cathodic portion of the hydrogen-iron battery, and a gaseous hydrogen is isolated from the negative electrolyte transferring from a negative electrolyte compartment and is directed to an anodic portion of the hydrogen-iron battery, the negative electrolyte is pumped from the negative electrolyte compartment through a cathodic portion of the iron-oxygen electrolyzer, while a distilled water-based electrolyte is pumped through an anodic portion of the iron-oxygen electrolyzer, and a current from electrodes of the hydrogen-iron battery is supplied to electrodes of the electrolyzer.

2. The method for regenerating electrolytes according to claim 1, wherein a density of the current that is supplied to the electrodes of the iron-oxygen electrolyzer from the hydrogen-iron battery is 0.1-1% of a density of the current at the electrodes of the cell of the all-iron battery.

3. The method for regenerating electrolytes according to claim 1, wherein an active area of the electrodes of the hydrogen-iron battery is 0.5-10% of an active area of the electrodes of the cell of the all-iron battery, and an active area of the electrodes of the electrolyzer is 0.1-5% of an active area of the electrodes of the all-iron battery.