Concentrated VRFB electrolyte composition and method of making same

Abstract

A method for producing a concentrated VRFB electrolyte composition is disclosed. The method includes the steps of (a) mixing vanadium oxide, water, and an aqueous H2SO4 solution in an amount sufficient to produce a reaction mixture containing vanadium sulfate at a concentration of 3.0M or greater and a total sulfate concentration at least twice the concentration of the vanadium sulfate, (b) maintaining the reaction mixture at an initial temperature that substantially avoids precipitation of reaction solids, (c) allowing the reaction mixture to reach ambient temperature, and (d) increasing the viscosity of the reaction mixture to produce the concentrated VRFB electrolyte composition. A concentrated VRFB electrolyte composition is also disclosed. It is believed that the concentrated VRFB electrolyte provides advantages such as reduced transportation costs, logistics, and on-site capital costs at facilities that use VRFBs.

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims the benefit under 35 USC § 119(e) of provisional patent application SN63 / 312,999, filed February 23, 2022, the contents of which are incorporated herein by reference. The invention in one aspect relates to a method for producing a concentrated VRFB electrolyte composition. In another aspect, the invention relates to the concentrated VRFB electrolyte composition itself and / or the concentrated VRFB electrolyte composition produced by this method. In another aspect, the invention relates to a composition that can be used to form a VRFB electrolyte composition. In another aspect, the invention relates to the use of a concentrated VRFB electrolyte composition to produce a VRFB electrolyte composition. [Background technology]

[0002] Vanadium redox flow batteries (also referred to interchangeably throughout this specification as "VRFBs") are emerging energy storage systems that have the potential to effectively store renewable energy and make stationary energy storage viable in commercial environments. VRFBs are an alternative to Li-ion batteries, especially in the field of large-scale energy storage, and are capable of holding large energy capacities suitable for industrial use, have a lifespan of more than 20 years, and are inherently safe with no risk of thermal runaway, making them attractive candidates for industrial decarbonization.

[0003] For the VRFB industry, vanadium electrolyte is one of the key components of flow batteries. It is typically manufactured using a wet chemistry approach, which requires the addition of chemical reagents. Vanadium redox flow batteries are now being deployed worldwide as large-scale energy storage solutions, and the use of higher purity VRFB electrolytes improves the battery's performance and lifespan.

[0004] In the future, large-scale energy storage is believed to have the potential to play a fundamental role in decarbonizing industry and creating a more sustainable future, providing the ability to stabilize and better utilize renewable energy sources. VRFBs are expected to play a key role in the energy transition over the coming decades, and in commercial applications, the highest quality VRFB electrolytes are expected to promote performance and long cycle life.

[0005] Despite the great potential of VRFB, there is room for improvement.

[0006] A particular issue that does not appear to be satisfactorily addressed is the distribution of the VRFB electrolyte to the facilities that operate the VRFB. There are two options currently in use, each with its own deficiencies.

[0007] The first option is to ship the finished VRFB electrolyte to the VRFB facility. The problem with this option is the high cost of shipping the product, which is mostly water. In addition, this option involves complex logistics, including transporting hazardous liquids and filling the finished VRFB into small containers (e.g., tote bags). These logistics are necessary because shipping the finished VRFB in large shipping containers or tankers is generally not permitted due to weight restrictions and excessive sloshing / splashing of the product.

[0008] A second option is to ship the chemicals necessary to produce the finished VRFB electrolyte to the VRFB facility. However, this option requires each VRFB facility to incur the capital costs and inconvenience associated with the complex handling and weighing of the components necessary to produce the VRBB electrolyte on-site. Even if this electrolyte preparation equipment is only located on-site temporarily, the cost of moving this complex equipment is significant, and the space available on-site for this additional equipment may be limited.

[0009] Thus, there remains a need in the art for a VRFB electrolyte product that avoids or mitigates the above-mentioned problems. It would be particularly advantageous if such a product could be manufactured without the need for complex additional equipment. Summary of the Invention

[0010] It is an object of the present invention to obviate or mitigate at least one of the above-mentioned disadvantages of the prior art.

[0011] Another object of the present invention is to provide a novel method for producing a concentrated VRFB electrolyte composition.

[0012] Accordingly, in one of its aspects, the present invention provides a method for making a concentrated VRFB electrolyte composition comprising the steps of: Step (a) mixing vanadium oxide, water, and an aqueous H2SO4 solution in amounts sufficient to produce a reaction mixture containing a vanadium sulfate concentration of at least 3.0 M and a total sulfate concentration at least twice the concentration of said vanadium sulfate. Step (b) maintaining said reaction mixture at an initial temperature that substantially avoids precipitation of reaction solids. Step (c) allowing the reaction mixture to reach ambient temperature. Step (d) increasing the viscosity of the reaction mixture to produce the concentrated VRFB electrolyte composition.

[0013] In another aspect, the present invention provides novel concentrated VRFB electrolyte compositions.

[0014] 1. A composition comprising vanadium sulfate at a concentration of 3.0M or greater and a total sulfate concentration that is greater than or equal to two times the concentration of the vanadium sulfate, wherein the concentrated VRFB electrolyte composition is in the form of a gel or semi-solid and / or has a viscosity of 100 mPa s or greater at 20° C.

[0015] In yet another aspect, the present invention provides a concentrated VRFB electrolyte composition for use in a VRFB system, the composition comprising vanadium sulfate at a concentration of 3.0 M or greater and a total sulfate concentration at least two times the concentration of the vanadium sulfate.

[0016] In yet another aspect, the present invention provides a novel method for producing a VRFB electrolyte composition.

[0017] Thus, the present inventors have developed a novel method for producing a concentrated VRFB electrolyte composition. This novel method is believed to avoid or reduce one or more of the above-mentioned shortcomings and / or problems. Specifically, the present inventors have conceived a method for producing a concentrated VRFB composition, in which components conventionally used to produce a VRFB electrolyte composition are instead used in amounts and under conditions where a semi-solid (preferably gel-like) composition is produced. This concentrated VRFB composition has a significantly lower water content than a conventional ready-to-use VRFB electrolyte composition. This feature of the concentrated VRFB electrolyte composition is believed to avoid or reduce the shipping costs and logistical issues discussed above with respect to a ready-to-use, transported VRFB electrolyte composition. In addition, the concentrated VRFB electrolyte composition is in a semi-solid (preferably gel) physical form, e.g., in a preferred embodiment, the concentrated VRFB electrolyte composition is substantially non-pourable at ambient temperatures (e.g., 20°C to 25°C). This feature of the concentrated VRFB composition is believed to avoid or reduce the excessive weight and sloshing / splashing issues discussed above with respect to a ready-to-use, transported VRFB electrolyte composition. Moreover, this feature of the concentrated VRFB electrolyte composition is believed to obviate or reduce the problems discussed above with respect to the capital costs and inconvenience associated with the complex handling and metering of ingredients required to manufacture VRFB electrolytes on-site at a VRFB facility using base chemical components.

[0018] In a highly preferred embodiment, the formation of the concentrated VRFB electrolyte composition is performed with mixing in the presence of nucleation centers (or sites). The resulting concentrated VRFB composition is highly advantageous because: (1) it can be formed relatively quickly, (2) it can lock up a relatively large amount of free electrolyte to form a stable gel, and (3) it has a relatively fast redissolution rate when water (preferably deionized water) is added to form the VRFB electrolyte on-site at the VRFB facility.

[0019] A parameter called densification level (DL) can be conveniently used to quantify the level of water removal from the reaction mixture in step (a) to the semi-solid concentrated VRFB (e.g., gel-form) electrolyte in step (d). DL = (amount of water removed / amount of total electrolytes) x 100% In a preferred embodiment, the DL is 30% or more, more preferably about 30% to about 70%, more preferably about 40% to about 65%, more preferably about 50% to about 65%, more preferably about 50% to about 60%, and preferably 50%. [Brief description of the drawings]

[0020] Embodiments of the present invention will now be described with reference to the accompanying drawings, in which like reference numbers indicate like parts. [Figure 1] 1 shows an outline of the steps of a preferred embodiment of the present invention. [Diagram 2] 1 shows schematic details of a preferred osmotic module used in a preferred embodiment of the present invention. [Diagram 3] 1 shows schematic details of a preferred osmotic module used in a preferred embodiment of the present invention. [Figure 4] Various results are shown in the examples below. [Diagram 5] Various results are shown in the examples below. [Figure 6] Various results are shown in the examples below. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The present invention relates to a method for producing a concentrated VRFB electrolyte composition, the method comprising the steps of: (a) mixing vanadium oxide, water and an aqueous H2SO4 solution in an amount sufficient to produce a reaction mixture containing vanadium sulfate at a concentration of 3.0 M or greater and a total sulfate concentration at least twice the concentration of the vanadium sulfate, (b) maintaining the reaction mixture at an initial temperature that substantially avoids precipitation of reaction solids, (c) allowing the reaction mixture to reach ambient temperature, and (d) increasing the viscosity of the reaction mixture to produce a concentrated VRFB electrolyte composition. A preferred embodiment of the method can include any one or any two or more combinations of any of the following features: Vanadium oxide includes V2O5. Vanadium oxide includes V2O3. Vanadium oxide includes mixtures of V2O5 and V2O3. Step (a) comprises combining vanadium oxide, water, and an aqueous H2SO4 solution in amounts sufficient to produce a reaction mixture containing vanadium sulfate at a concentration in the range of about 3.0 M to about 5.0 M and a total sulfate concentration that is at least two times the concentration of the vanadium sulfate. Step (a) comprises combining vanadium oxide, water, and an aqueous H2SO4 solution in amounts sufficient to produce a reaction mixture containing vanadium sulfate at a concentration in the range of about 3.5 M to about 3.7 M and a total sulfate concentration that is at least two times the concentration of the vanadium sulfate. Step (a) comprises combining vanadium oxide, water and an aqueous H2SO4 solution in amounts sufficient to produce a reaction mixture containing vanadium sulfate at a concentration of about 3.5 M and a total sulfate concentration at least twice the concentration of the vanadium sulfate. In step (a), the total sulfate concentration is 2 to 4 times the concentration of vanadium sulfate. In step (a), the total sulfate concentration is 2 to 3.5 times the concentration of vanadium sulfate. In step (a), the total sulfate concentration is 2 to 3 times the concentration of vanadium sulfate. In step (a), the total sulfate concentration is 2.5 to 3 times the concentration of vanadium sulfate. The initial temperature in step (b) is about 50° C. or higher. The initial temperature in step (b) is in the range of about 60°C to about 120°C. The initial temperature in step (b) is in the range of about 80°C to about 120°C. The initial temperature in step (b) is in the range of about 100°C to about 110°C. The initial temperature in step (b) is about 105°C. The ambient temperature in step (c) is in the range of about 15°C to about 40°C. The ambient temperature in step (c) is in the range of about 20°C to about 30°C. The ambient temperature in step (c) is in the range of about 22°C to about 28°C. The ambient temperature in step (c) is in the range of about 23°C to about 25°C. Prior to step (c), the reaction mixture is dispensed into a storage vessel. Prior to step (c), the reaction mixture is dispensed into a transport container. The concentrated VRFB electrolyte composition is in the form of a gel product. Prior to step (c), the reaction mixture is subjected to a sub-process to produce a purified reaction mixture which is subjected to step (d). The purification sub-process includes the following steps: Step (i) providing a reaction mixture from step (b) comprising a precipitation valence, the reaction mixture containing at least one impurity that precipitates from the reaction mixture when the valence of the reaction mixture is at or below the precipitation valence. Step (ii) reducing the reaction mixture to a valence less than the precipitating valence, such that at least one impurity precipitates from the reaction mixture as a precipitate. Step (iii) mechanically separating the precipitate from the reaction mixture to produce a purified reaction mixture. Prior to step (ii), the reaction mixture is subjected to a dilution step to produce a diluted reaction mixture. The dilution step results in a diluted reaction mixture containing a vanadium sulfate concentration of less than 3.0 M and a total sulfate concentration of at least two times the concentration of the vanadium sulfate. The dilution step results in a diluted reaction mixture containing a vanadium sulfate concentration of less than 3.0 M and a total sulfate concentration of at least two times the concentration of the vanadium sulfate. The dilution step results in a diluted reaction mixture containing a vanadium sulfate concentration of less than 2.8M and a total sulfate concentration of at least two times the concentration of the vanadium sulfate. The dilution step results in a diluted reaction mixture containing vanadium sulfate at a concentration in the range of about 1.2 M to about 2.5 M and a total sulfate concentration that is greater than or equal to two times the concentration of vanadium sulfate. The dilution step results in a diluted reaction mixture containing vanadium sulfate at a concentration in the range of about 1.2 M to about 2.0 M and a total sulfate concentration that is greater than or equal to two times the concentration of vanadium sulfate. The dilution step results in a diluted reaction mixture containing vanadium sulfate at a concentration in the range of about 1.4 M to about 1.8 M and a total sulfate concentration that is greater than or equal to two times the concentration of vanadium sulfate. The dilution step results in a diluted reaction mixture having a total sulfate concentration that is 2-4 times the concentration of vanadium sulfate. The dilution step results in a diluted reaction mixture having a total sulfate concentration that is 2 to 3.5 times the concentration of vanadium sulfate. The dilution step results in a diluted reaction mixture having a total sulfate concentration that is 2-3 times the concentration of vanadium sulfate. The dilution step results in a diluted reaction mixture having a total sulfate concentration that is 2.5 to 3 times the concentration of vanadium sulfate. The diluted reaction mixture is subjected to steps (ii) and (iii) to produce a diluted purified reaction mixture. The diluted purified reaction mixture is subjected to a concentration step to produce a purified reaction mixture. The purified reaction mixture has a vanadium sulfate concentration of 3.0M or greater and a total sulfate concentration of 2 times the vanadium sulfate concentration. The purified reaction mixture has a vanadium sulfate concentration in the range of about 3.0M to about 5.0M and a total sulfate concentration that is at least two times the vanadium sulfate concentration. The purified reaction mixture has a vanadium sulfate concentration in the range of about 3.0M to about 4.0M and a total sulfate concentration that is at least two times the vanadium sulfate concentration. The purified reaction mixture has a vanadium sulfate concentration of about 3.3M and a total sulfate concentration that is greater than or equal to two times the vanadium sulfate concentration. The purified reaction mixture has a total sulfate concentration that is 2 to 4 times the concentration of vanadium sulfate. The purified reaction mixture has a total sulfate concentration that is 2 to 3.5 times the concentration of vanadium sulfate. The purified reaction mixture has a total sulfate concentration 2-3 times the concentration of vanadium sulfate. The purified reaction mixture has a total sulfate concentration that is 2.5 to 3 times the concentration of vanadium sulfate. The concentration step is carried out using reverse osmosis to produce an aqueous liquid extracted from the diluted purified reaction mixture. The extracted aqueous liquid is used in the dilution step to produce a diluted reaction mixture. The extracted aqueous liquid is used as the sole diluent in the dilution step to produce a diluted reaction mixture. -Water is present in the extracted aqueous liquid. The dilution and concentration steps take place in the permeation module. The osmosis module is configured to pass aqueous liquid from the diluted purified reaction mixture through a reverse osmosis membrane into the reaction mixture prior to step (ii) to produce (1) a diluted reaction mixture on one side of the reverse osmosis membrane and (2) a purified reaction mixture on the other side of the reverse osmosis membrane. The osmosis module is configured to use a pressure gradient to regulate the rate of movement of an aqueous liquid through a reverse osmosis membrane. · Step (d) involves contacting the reaction mixture with a nucleation center. · Nucleation centers contain a mixture of liquid and solid. · Nucleation centres contain solids. Nucleation centers include carbon-based woven or non-woven fabrics, paper or scrim materials. · The nucleation centres contain particulate material. The nucleation centers comprise porous particulate material. The nucleation centers comprise a microporous particulate material. · The nucleation centres contain carbon felt. The nucleation centers contain carbon powder. · Nucleation centers include zeolites. The nucleation centers include vanadium salts, such as poorly crystalline vanadium salts (e.g., VOSO4 salts). · The nucleation centre contains VOSO4 salt. · The nucleation centres contain poorly crystalline VOSO4 salts. The nucleation centers (or sites) are added to the reaction mixture of step (d) in an amount of about 0.1 mg / mL or more, more preferably in an amount in the range of about 0.1 mg / mL to about 20 mg / mL, more preferably in an amount in the range of about 0.5 mg / mL to about 20 mg / mL, more preferably in an amount in the range of about 1 mg / mL to about 20 mg / mL, more preferably in an amount in the range of about 0.1 mg / mL to about 15 mg / mL, more preferably in an amount in the range of about 1 mg / mL to about 10 mg / mL. Step (d) is carried out with mixing of the reaction mixture. The mixing in step (d) is carried out for about 15 minutes or more, more preferably in the range of about 15 minutes to about 180 minutes, more preferably in the range of about 15 minutes to about 150 minutes, more preferably in the range of about 30 minutes to about 120 minutes, more preferably in the range of about 30 minutes to about 90 minutes, more preferably in the range of about 30 minutes to about 60 minutes, more preferably for 60 minutes. The concentrated VRFB electrolyte composition produced in step (d) has a DL of 30% or more, more preferably about 30% to about 70%, more preferably about 40% to about 65%, more preferably about 50% to about 65%, more preferably about 50% to about 60%, and preferably 50%.

[0022] The present invention also relates to a composition comprising vanadium sulfate at a concentration of 3.0 M or greater and a total sulfate concentration of at least two times the concentration of vanadium sulfate. Preferred embodiments of this composition can include any one or any two or more combinations of any of the following features: The composition is in the form of a gel. The composition is in semi-solid form. The composition has a viscosity of 100 mPa·s or more at 20°C. The composition comprises a concentration of vanadium sulfate as described in paragraph 0022 for the purified reaction mixture. The composition comprises a total sulfate concentration as described in paragraph 0022 for the purified reaction mixture.

[0023] The present invention also relates to a concentrated VRFB electrolyte composition for use in a VRFB system, the composition comprising vanadium sulfate at a concentration of 3.0 M or greater and a total sulfate concentration at least two times the concentration of the vanadium sulfate, including when produced by the method described above. Preferred embodiments of the composition can include any one or any two or more combinations of any of the following features: The concentrated VRFB electrolyte composition is in the form of a gel. Concentrated VRFB electrolyte compositions are in semi-solid form. The concentrated VRFB electrolyte composition has a viscosity of 100 mPa·s or more at 20°C. The concentrated VRFB electrolyte composition comprises a concentration of vanadium sulfate as described in paragraph 0022 for the purified reaction mixture. The concentrated VRFB electrolyte composition comprises a total sulfate concentration as described in paragraph 0022 for the purified reaction mixture.

[0024] The present invention also relates to a method for producing a VRFB electrolyte composition comprising the step of contacting the concentrated VRFB electrolyte composition described above with an aqueous liquid. Preferred embodiments of the method can include any one or any two or more combinations of any of the following features: -The aqueous liquid is water. The aqueous liquid is added in an amount to produce a VRFB electrolyte composition containing a vanadium sulfate concentration of less than 3.0M and a total sulfate concentration of at least two times the concentration of the vanadium sulfate. The aqueous liquid is added in an amount to produce a VRFB electrolyte composition containing a vanadium sulfate concentration of less than 3.0M and a total sulfate concentration of at least two times the concentration of the vanadium sulfate. The aqueous liquid is added in an amount to produce a VRFB electrolyte composition containing a vanadium sulfate concentration of less than 2.8M and a total sulfate concentration of at least two times the concentration of the vanadium sulfate. The aqueous liquid is added in an amount to produce a VRFB electrolyte composition containing vanadium sulfate at a concentration in the range of about 1.2M to about 2.5M and a total sulfate concentration that is at least two times the concentration of vanadium sulfate. The aqueous liquid is added in an amount to produce a VRFB electrolyte composition containing vanadium sulfate at a concentration in the range of about 1.2M to about 2.0M and a total sulfate concentration that is at least two times the concentration of vanadium sulfate. The aqueous liquid is added in an amount to produce a VRFB electrolyte composition including a vanadium sulfate concentration in the range of about 1.4M to about 1.8M and a total sulfate concentration that is at least two times the concentration of the vanadium sulfate. The aqueous liquid is added in an amount that produces a VRFB electrolyte composition containing a total sulfate concentration that is 2-4 times the concentration of vanadium sulfate. The aqueous liquid is added in an amount that produces a VRFB electrolyte composition containing a total sulfate concentration that is 2-3.5 times the concentration of vanadium sulfate. The aqueous liquid is added in an amount to produce a VRFB electrolyte composition including a total sulfate concentration that is 2-3 times the concentration of the vanadium sulfate, and / or the aqueous liquid is added in an amount to produce a VRFB electrolyte composition including a total sulfate concentration that is 2.5-3 times the concentration of the vanadium sulfate.

[0025] In step (a) of the process, it is preferred to use only vanadium oxide in the form of V(III) and V(IV). During step (a), in preferred embodiments, it is preferred to maintain the reaction mixture at a temperature of about 50° C. These preferred embodiments help to reduce or eliminate the risk of precipitation of V2O5 during the process. In preferred embodiments, the precipitation rate is believed to be very slow.

[0026] In a preferred embodiment, in step (a), the components are used in amounts such that the concentrations of vanadium sulfate and sulfuric acid are at least twice the concentrations of these chemicals in the ready-to-use VFRB electrolyte. Such conditions in step (a) are believed to facilitate an osmosis driven dilution / concentration process in a highly preferred embodiment of the method.

[0027] In a preferred embodiment, the reaction mixture produced in step (a) is subjected to a sub-process to produce a purified reaction mixture before being subjected to step (c). Purification of the flow battery electrolyte can be carried out using a variety of processes.

[0028] In a preferred embodiment, the purification process may be carried out at elevated temperature or at ambient temperature (i.e. before or after step (b)).

[0029] In a highly preferred embodiment, this sub-process comprises the following steps: Step (i) providing a reaction mixture from step (b) that contains at least one impurity that comprises a precipitating valence and that precipitates from the reaction mixture when the valence of the reaction mixture is equal to or less than the precipitating valence. Step (ii) reducing the reaction mixture to a valence less than the precipitating valence, such that at least one impurity precipitates from the reaction mixture as a precipitate. Step (iii) mechanically separating the precipitate from the reaction mixture to produce a purified reaction mixture.

[0030] Additional details and preferred embodiments for carrying out this sub-process can be found in U.S. Patent No. 10,333,164 [Sullivan], the entire contents of which are incorporated herein (e.g., as though some or all of the contents of Sullivan could be included herein for purposes of disclosure and claim support).

[0031] Examples of other preferred embodiments of the purification sub-process can be found in US Pat. No. 8,394,529 (Keshavarz et al. (Keshavarz)) and in various references cited on the cover page of Sullivan.

[0032] Since this sub-process may result in the production of hydrogen gas during the reduction step (i.e., step (ii) above), it is preferred to subject the reaction mixture to a dilution step prior to the reduction step (i.e., step (ii) above). Such a dilution step is also believed to reduce the viscosity of the reaction mixture to facilitate downstream processing.

[0033] In a preferred embodiment, the dilution step is performed using an osmotic membrane. Preferably, in one embodiment, water spontaneously passes through a selective reverse osmosis (RO) membrane from a low concentration solution to a high concentration solution. In a preferred embodiment, the concentrated solution may be a highly concentrated aqueous solution of sulfuric acid (H2SO4), preferably greater than 10 M H2SO4, which may be used as a draw solution to pass water through the selective RO membrane from a low concentration solution to a high concentration solution. It is preferable to have equal concentrations (e.g., similar osmotic pressures) on both sides of the RO membrane. Preferably, a pressure gradient can be applied to increase the water transfer rate. This would allow for the use of smaller RO modules. The RO process can be performed at elevated temperatures or at ambient temperatures. Highly concentrated electrolytes may not be stable over long periods at ambient temperatures, but precipitation kinetics are slow enough to allow for some processing at these lower temperatures.

[0034] The diluted reaction mixture is then subjected to a reduction step (i.e., step (ii) above). Once the reaction mixture is diluted, it may be subjected to a reduction step (i.e., step (ii) above) which results in the precipitation of any impurities in the reaction mixture. These impurities are then mechanically removed (i.e., step (iii)), thereby producing a diluted purified reaction mixture.

[0035] In a preferred embodiment, the diluted purified reaction mixture is then subjected to a concentration step to produce a purified reaction mixture having concentrations of vanadium sulfate and sulfuric acid within the ranges defined in step (a) above.

[0036] In a preferred embodiment, the dilution and concentration steps are carried out in a permeation module. In a highly preferred embodiment, the permeation module is configured to pass aqueous liquid from the diluted purified reaction mixture through a reverse osmosis membrane into the reaction mixture prior to step (ii) to produce (1) a diluted reaction mixture on one side of the reverse osmosis membrane and (2) a purified reaction mixture on the other side of the reverse osmosis membrane. For further details of the fluid flow patterns in the permeation module, see Figures 2 and 3.

[0037] The module shown in Figures 2 and 3 may be considered a forward osmosis (FO) module that may be used to separate water from dissolved solutes (it operates in a similar manner as a reverse osmosis (RO) membrane). The driving force is the concentration difference (osmotic pressure P gradient), and it is believed that no hydraulic pressure P is required. In a preferred embodiment, a concentrated aqueous solution of sulfuric acid (H2SO4) is contacted with a semipermeable membrane to move water from a less concentrated solution ("feed") to a more concentrated solution ("receive") (see Figure 2).

[0038] In a preferred embodiment, step (d) of the method includes contacting the reaction mixture (whether or not it has been purified according to the preferred embodiments above) with nucleation centers. Such contacting is believed to reduce the time required to complete step (d) to produce the concentrated VRFB electrolyte from hours (or days) to minutes.

[0039] The following results are from a modelling study using nucleation centers for different purposes. [Table 1]

[0040] For details, see T. Van Nguyen and Y. Li, Electrochem. Soc. (ECS) Presentations, A03-0204 and A03-0205 (2021). 239 th ECS Meeting with the 18 th See International Meeting on Chemical Sensors (IMCS) (May 30-June 3, 2021) (confex.com). See also “A Solid / Liquid High-Energy-Density Storage Concept for Redox Flow Batteries and Its Demonstration in an H2-V System”, T. Van Nguyen and Y. Li, Paper ID APEN-MIT-2021_023, Applied Energy Symposium: MIT A+B August 11-13, 2021 · Cambridge, USA. See also Yuanchao Li and Trung Van Nguyen 2022 J. Electrochem. Soc. 169 110509. See also International Publication No. WO 2021 / 108244A1 [Nguye et al.].

[0041] In a preferred embodiment, the nucleation centres (or sites) are located within the electrolyte reservoir, which may be advantageous if the nucleation centres (or sites) comprise a carbon-based material such as carbon felt.

[0042] In a preferred embodiment, the nucleation centers (or sites) are introduced from a small reservoir located upstream of the electrolyte vessel, which may be advantageous when the nucleation centers (or sites) comprise a zeolite and / or a vanadium salt, such as a highly crystalline or poorly crystalline vanadium salt (e.g., VOSO4 salt).

[0043] In a preferred embodiment, the nucleation centers (or sites) are vanadium salts, such as low crystallinity vanadium salts (e.g., VOSO4 salts). This embodiment is believed to be particularly advantageous because the nucleation centers can be produced in situ and does not require the addition of foreign materials during step (d). Additionally, the use of low crystallinity vanadium salts (e.g., VOSO4 salts) as nucleation centers (or sites) is believed to promote gel formation and improve redissolution times.

[0044] In a preferred embodiment, the nucleation centers (or sites) are added to the reaction mixture of step (d) in an amount of about 0.1 mg / mL or more, more preferably in an amount in the range of about 0.1 mg / mL to about 20 mg / mL, more preferably in an amount in the range of about 0.5 mg / mL to about 20 mg / mL, more preferably in an amount in the range of about 1 mg / mL to about 20 mg / mL, more preferably in an amount in the range of about 0.1 mg / mL to about 15 mg / mL, more preferably in an amount in the range of about 1 mg / mL to about 10 mg / mL.

[0045] In a preferred embodiment, step (d) of the method comprises contacting the reaction mixture (whether or not purified according to the preferred embodiment above) with nucleation centers while mixing. The term "mixing" as used in this context is intended to have a broad meaning and includes providing mechanical energy to the reaction mixture. Sources of mechanical energy are not particularly limited and include devices that add kinetic energy to the reaction mixture.

[0046] In a preferred embodiment, the device that imparts kinetic energy to the reaction mixture may be a solid-solid mixer or a solid-liquid mixer. Non-limiting examples of such devices include kneaders, extruders, tumblers, ribbon mixers, mullers, and the like.

[0047] In a preferred embodiment, the device that imparts kinetic energy to the reaction mixture may be a liquid-liquid mixer or a liquid-gas mixer, non-limiting examples of such devices include static mixers, mechanically mixed tanks, jet mixers, etc.

[0048] In a preferred embodiment, the device that adds kinetic energy to the reaction mixture may be magnetic-based. A non-limiting example of such a device is a vessel configured to generate a rotating magnetic field in the reaction mixture, placing a magnetic element (e.g., a rod, plate, etc.) in the reaction mixture to cause stirring.

[0049] In another preferred embodiment, the device that adds kinetic energy to the reaction mixture is a stirred reactor, such as a continuous stirred tank reactor (CSTR), also known as a vat- or backmix reactor, a mixed flow reactor (MFR), or a continuous-flow stirred tank reactor (CFSTR).

[0050] In a preferred embodiment, the mixing in step (d) is carried out for about 15 minutes or more, more preferably in the range of about 15 minutes to about 180 minutes, more preferably in the range of about 15 minutes to about 150 minutes, more preferably in the range of about 30 minutes to about 120 minutes, more preferably in the range of about 30 minutes to about 90 minutes, more preferably in the range of about 30 minutes to about 60 minutes, more preferably for 60 minutes.

[0051] A semi-solid concentrated VRFB (e.g., in gel form) electrolyte is formed in or upstream of the electrolyte reservoir in step (d) as a result of denaturing the reaction mixture of step (a) by removal of water to produce the semi-solid concentrated VRFB (e.g., in gel form) electrolyte in step (d).

[0052] After the semi-solid concentrated VRFB (e.g., in gel form) is formed in the electrolyte container, the electrolyte container can be loaded onto a transport vehicle (e.g., truck, rail car, etc.) and transported to the VRFB site. The semi-solid concentrated VRFB (e.g., in gel form) has a reduced weight compared to the ready-to-use VRFB electrolyte, allowing larger volumes of equivalent electrolyte to be transported with fewer shipping volumes.

[0053] At the VRFB site, the semi-solid concentrated VRFB electrolyte (e.g., in gel form) can be contacted with an aqueous liquid (e.g., water) to produce a ready-to-use VRFB electrolyte. The time required for dissolution can be shortened by using poorly crystalline salts as nucleation centers (or sites), as described above. Despite this, dissolution is generally considered to be faster than precipitation.

[0054] It is also possible to reduce the time required by heating the mixture of concentrated VRFB electrolyte and aqueous liquid, which can be accomplished by initiating a "formation charge" prior to dissolution of all the salt. EXAMPLES

[0055] Embodiments of the present invention are illustrated with reference to the following examples, which should not be used to limit or interpret the scope of the invention.

[0056] (Densification process) A forward osmosis (FO) apparatus, the schematic of which is shown in FIG. 4, was used to densify the electrolyte.

[0057] FO devices use an osmotic process to separate water from dissolved solutes using a semipermeable membrane, the driving force being the osmotic pressure gradient between the receiver solution (high concentration) and the feed solution (low concentration): the high osmotic pressure of the receiver solution induces a net water flow across the membrane from the low osmotic pressure of the feed solution.

[0058] In this case, the feed solution has a V of about 1.55 M 3.5+The vanadium electrolyte composition contained 2M H2SO4 (V(III) to V(IV) molar ratio of 1:1). The water content was calculated to be about 51M or 66% by weight, and the density of the vanadium electrolyte composition was 1.4 g / cm 3 The receptor solution was 15M to 17M concentrated sulfuric acid.

[0059] Nafion® membranes were used instead of less stable commercial FO membranes such as cellulose triacetate (CTA) membranes due to the presence of strong acids on both sides. The device was adapted from a standard flow battery, with interdigitated flow fields and a Nafion® membrane sandwiched between two sheets of porous carbon felt that act as a porous support substrate for the membrane. This configuration is similar to the membrane electrode assembly (MEA) of a flow battery. Both the receptor and feed solutions were recirculated through the FO cell until the feed solution reached the desired densification level.

[0060] The graduated cylinder in Figure 4 was utilized to record and measure the amount of water withdrawn.

[0061] After the osmosis process removes the water, the electrolyte concentrations of V(III) and V(IV) increase. When their concentrations exceed their solubility, the supersaturated solution can be precipitated as a solid. Based on the literature data in Figure 5, the electrolyte in the as-received LCE is stable at RT because V(IV) in the electrolyte is unsaturated and V(III) is slightly supersaturated. When the electrolyte is densified from 0 to 50% by removing the water, both V(IV) and V(III) become supersaturated, causing them to precipitate.

[0062] (Coprecipitation of V(III) and V(IV)) The results shown in Figure 5 imply that both V(III) and V(IV) are supersaturated at 50% DL. To identify which species precipitates first, an electrochemical method was used. The vanadium electrolyte was densified to 50% in the FO cell of Figure 4 and then mixed with 10 mg / mL of low-crystalline V3.5+ nucleation material (produced in 50% densified electrolyte and precipitated by activated carbon felt as nucleation center). The open circuit voltage (OCV) change during the precipitation process was monitored by a two-electrode system consisting of a graphite rod as the working electrode and a mercury sulfate reference electrode (MSE) as the reference electrode.

[0063] The Nernst equation for the V(IV) / V(III) redox couple can be written as follows: E=E θ +RT / F*ln([V(IV)] / [V(III)] This relationship indicates that an increase in OCV is due only to a decrease in [V(III)] (when V(III) precipitates from solution), and a decrease in OCV is due to a decrease in [V(IV)] (when V(IV) precipitates).

[0064] The OCV response shown in Figure 6 fluctuates by first increasing and then decreasing. From the Nernst equation and the fact that V(III) was by far supersaturated, it can be inferred that V(III) is the first species to precipitate, followed by V(IV).

[0065] After the first round of precipitation of V(III) and V(IV), some of the free water molecules are fixed within the precipitate, resulting in a supersaturation of both species. This process is repeated as shown in the oscillatory pattern in Figure 6. The precipitation process may continue to occur within the liquid part of the gel network until the system reaches some equilibrium level.

[0066] (V 3.5+ Precipitation and dissolution processes of electrolyte compositions As mentioned above, low crystalline gels are advantageous because they can be quickly redissolved.

[0067] Three cases were performed. In all three cases, the densified electrolyte was densified to 60% V 3.5+ It was an electrolyte.

[0068] In case A, 10 mg / mL of low-crystalline V 3.5+ A solid was added to the densification solution as a nucleation material and applied with stirring. Case B had the same nucleation material density but with stirring. Case C had no nucleation material and no stirring.

[0069] When mixed and allowed to stand, the slurry in Case B eventually formed a gel with very low fluidity. Cases A and C also eventually formed a precipitate. In contrast to Case B, the precipitate in Cases A and C was a highly crystalline solid. Although Cases A and C each represent an advance in the art, they have slower dissolution rates and the crystalline solids lock in less free water within the precipitate.

[0070] The redissolution performance of the two types of precipitates was investigated. Deionized (DI) water was added to each of the precipitates of Case A, Case B, and Case C to restore the total vanadium concentration to 110% of its original level (i.e., 10% higher than the original concentration). The redissolution rate of Case B (low crystalline gel) was much faster than that of Case A and Case C (high crystalline solid, respectively), i.e., 40 seconds vs. 1.5 hours. The short dissolution rate of Case B (low crystalline gel) can significantly simplify the electrolyte recovery process at the installation site.

[0071] Therefore, when exemplified, Case B is believed to be the most preferred embodiment for commercial purposes.

[0072] (Possible gel formation mechanism) The following provides a possible mechanism for gel formation that is believed to explain the observed results: However, it should be understood that the present invention is not to be defined or limited to any particular theory or mode of operation.

[0073] Case B above is believed to be the most preferred embodiment for commercial purposes since it (1) can precipitate relatively quickly, (2) can fix all of the relatively large amount of free electrolyte to form a stable gel, and (3) has a relatively fast re-dissolution rate when DI water is added back to the original concentration level (VRFB electrolyte).

[0074] After nucleation material is added to a supersaturated electrolyte and the mixture is stirred and allowed to sit for a period of time, larger visible particles settle to the bottom of the container while sub-nanoparticles (sub-NPs) are suspended in the solution due to their small size. These suspended sub-NPs continue to grow and when they become abundant enough to form an interconnected network capable of immobilizing a large proportion of the free water molecules, a gel is formed.

[0075] Due to density differences, as the sub-NPs increase in size and become denser, the sub-NPs will flow to the bottom, while the lighter water molecules will flow to the top. This can cause phase separation, turning the solution into a slurry (a mixture of solid and liquid). Thus, complete gel formation requires a sufficient number of suspended sub-NPs to grow and form a network before phase separation occurs.

[0076] This possible mechanism hypothesizes that it is the sub-NPs suspended in the solution that trigger the slurry precipitation to form a gel network, rather than the larger nucleation material located at the bottom of the solution. If this is correct, the mixing operation can be terminated once sufficient sub-NPs have been produced, thus shortening the stirring stage considerably. Two solutions with the same densification level and nucleation material density, but different volumes, are applied to the same vial size to test this concept.

[0077] When stirred and left to sit, the larger nucleation particles settle to the bottom, resulting in different diffusion distances between the ions in solution and the particles that have sunk to the bottom of the vial in the two cases with different solution volumes. If the suspended sub-NPs determine the final gel formation, then the two cases should show similar slurry / gel formation rates regardless of the diffusion distance. Alternatively, if the slurry / gel formation starts with larger nucleation material at the bottom of the vial, then the case with the shorter diffusion distance (i.e., smaller volume) should have a faster slurry / gel formation rate since the vanadium ions can reach the surface of the nucleation material faster.

[0078] Both cases were found to have the same slurry and gel formation rates, thus proving the concept. This mechanism depends only on the density of the sub-NPs in solution and not on the total amount of electrolyte, which could allow for the use of less electrolyte in high-throughput applications.

[0079] While the present invention has been described with reference to exemplary embodiments and examples, the description is not intended to be construed in a limiting sense. Thus, various modifications of the exemplary embodiments, as well as other embodiments of the invention, will become apparent to those skilled in the art upon reference to this description. It is therefore intended that the appended claims cover any such modifications or embodiments.

[0080] All publications, patents, and patent applications mentioned in this specification are incorporated by reference in their entirety to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference in its entirety.

Claims

1. 1. A method for producing a concentrated VRFB electrolyte composition, comprising: Vanadium oxide, water and H2O are mixed in an amount sufficient to produce a reaction mixture containing a vanadium sulfate concentration of at least 3.0 M and a total sulfate concentration of at least two times the concentration of said vanadium sulfate. ２ (a) mixing an aqueous SO4 solution; (b) maintaining the reaction mixture at an initial temperature that substantially avoids precipitation of reaction solids; Step (c) of allowing the reaction mixture to reach ambient temperature; (d) increasing the viscosity of the reaction mixture to produce the concentrated VRFB electrolyte composition; A method for providing the above.

2. The vanadium oxide is V ２ O ５ and V ２ O ３ The method of claim 1 , further comprising one or both of the following:

3. Step (a) comprises reacting vanadium oxide, water and H2O in amounts sufficient to produce a reaction mixture containing vanadium sulfate at a concentration ranging from about 3.0 M to about 5.0 M and a total sulfate concentration at least twice the concentration of the vanadium sulfate. ２ 3. The method of claim 1 or 2, comprising mixing an aqueous solution of SO4.

4. 4. The method according to claim 1, wherein in step (a), the total sulfate concentration is 2 to 4 times the concentration of the vanadium sulfate, the total sulfate concentration is 2 to 3.5 times the concentration of the vanadium sulfate, the total sulfate concentration is 2 to 3 times the concentration of the vanadium sulfate, or the total sulfate concentration is 2.5 to 3 times the concentration of the vanadium sulfate.

5. 5. The method of any of claims 1 to 4, wherein the initial temperature in step (b) is about 50°C or higher; the initial temperature in step (b) is in the range of about 60°C to about 120°C; the initial temperature in step (b) is in the range of about 80°C to about 120°C; the initial temperature in step (b) is in the range of about 100°C to about 110°C; or the initial temperature in step (b) is about 105°C.

6. 6. The method of any of claims 1 to 5, wherein the ambient temperature in step (c) is in the range of about 15°C to about 40°C; the ambient temperature in step (c) is in the range of about 20°C to about 30°C; the ambient temperature in step (c) is in the range of about 22°C to about 28°C; or the ambient temperature in step (c) is in the range of about 23°C to about 25°C.

7. The method according to any one of claims 1 to 6, wherein prior to step (c), the reaction mixture is dispensed into a storage container; or the reaction mixture is dispensed into a transport container.

8. 8. The method of claim 1, wherein the concentrated VRFB electrolyte composition is in the form of a gel product.

9. Prior to step (c), said reaction mixture is subjected to a sub-process to produce a purified reaction mixture that is subjected to step (d), wherein said sub-process of purification comprises: (i) providing a reaction mixture from step (b) that contains at least one impurity that comprises a precipitating valence and that precipitates from said reaction mixture when the valence of said reaction mixture is equal to or less than said precipitating valence; (ii) reducing the reaction mixture to a valence less than the precipitating valence such that at least one impurity precipitates from the reaction mixture as a precipitate; (iii) mechanically separating the precipitate from the reaction mixture to produce the purified reaction mixture; The method according to any one of claims 1 to 8, comprising:

10. Prior to step (ii), the reaction mixture is subjected to a dilution step to produce a diluted reaction mixture, the dilution step resulting in a diluted reaction mixture comprising vanadium sulfate at a concentration of less than 3.0 M and a total sulfate concentration of at least twice the concentration of the vanadium sulfate, or the dilution step resulting in a diluted reaction mixture comprising vanadium sulfate at a concentration of less than 3.0 M and a total sulfate concentration of at least twice the concentration of the vanadium sulfate, or the dilution step resulting in a diluted reaction mixture comprising vanadium sulfate at a concentration of less than 2.8 M and a total sulfate concentration of at least twice the concentration of the vanadium sulfate.

10. The method of claim 9, wherein the diluting step results in a diluted reaction mixture comprising vanadium sulfate at a concentration ranging from about 1.2 M to about 2.5 M and a total sulfate concentration that is at least twice the concentration of the vanadium sulfate, or wherein the diluting step results in a diluted reaction mixture comprising vanadium sulfate at a concentration ranging from about 1.2 M to about 2.0 M and a total sulfate concentration that is at least twice the concentration of the vanadium sulfate, or wherein the diluting step results in a diluted reaction mixture comprising vanadium sulfate at a concentration ranging from about 1.4 M to about 1.8 M and a total sulfate concentration that is at least twice the concentration of the vanadium sulfate.

11. 11. The method of claim 10, wherein the diluting step results in the diluted reaction mixture having a total sulfate concentration that is 2 to 4 times the concentration of the vanadium sulfate, or the diluting step results in the diluted reaction mixture having a total sulfate concentration that is 2 to 3.5 times the concentration of the vanadium sulfate, or the diluting step results in the diluted reaction mixture having a total sulfate concentration that is 2 to 3 times the concentration of the vanadium sulfate, or the diluting step results in the diluted reaction mixture having a total sulfate concentration that is 2.5 to 3 times the concentration of the vanadium sulfate.

12. The diluted reaction mixture is subjected to steps (ii) and (iii) to produce a diluted purified reaction mixture, and the diluted purified reaction mixture is subjected to a concentrating step to produce a purified reaction mixture, further comprising: the purified reaction mixture having a vanadium sulfate concentration of 3.0 M or greater and a total sulfate concentration of 2 times or greater than the vanadium sulfate concentration; or the purified reaction mixture having a vanadium sulfate concentration in the range of about 3.0 M to about 5.0 M and a total sulfate concentration of 2 times or greater than the vanadium sulfate concentration; or the purified reaction mixture having a vanadium sulfate concentration in the range of about 3.0 M to about 4.0 M and a total sulfate concentration of 2 times or greater than the vanadium sulfate concentration.

12. The method of claim 11, wherein the purified reaction mixture has a vanadium sulfate concentration of about 3.3 M and a total sulfate concentration of about 2 times the concentration of the vanadium sulfate, or the purified reaction mixture has a total sulfate concentration of 2 to 4 times the concentration of the vanadium sulfate, or the purified reaction mixture has a total sulfate concentration of 2 to 3.5 times the concentration of the vanadium sulfate, or the purified reaction mixture has a total sulfate concentration of 2 to 3 times the concentration of the vanadium sulfate, or the purified reaction mixture has a total sulfate concentration of 2.5 to 3 times the concentration of the vanadium sulfate.

13. 13. The method of claim 12, wherein the concentrating step is carried out using reverse osmosis to produce an extracted aqueous liquid from the diluted purified reaction mixture, the extracted aqueous liquid being used in the dilution step to produce the diluted reaction mixture, and the extracted aqueous liquid being used as the sole diluent in the dilution step to produce the diluted reaction mixture.

14. 14. The method of claim 12 or 13, wherein the dilution and concentration steps occur in a permeation module configured to pass aqueous liquid from the diluted purified reaction mixture through a reverse osmosis membrane into the reaction mixture prior to step (ii) to produce (1) the diluted reaction mixture on one side of the reverse osmosis membrane and (2) the purified reaction mixture on the other side of the reverse osmosis membrane, and wherein the permeation module is configured to use a pressure gradient to regulate the rate of movement of aqueous liquid through the reverse osmosis membrane.

15. 15. The method of any of claims 1 to 14, wherein step (d) comprises contacting the reaction mixture with nucleation centers, the nucleation centers comprising a solid, a liquid, or a mixture of a liquid and a solid.

16. The nucleation centers may be porous particulate materials, microporous particulate materials, carbon felt, carbon powder, zeolites, VOSO ４ 16. The method of claim 15, wherein the vanadium salt is selected from the group consisting of a vanadium salt, a highly crystalline vanadium salt or a poorly crystalline vanadium salt, and any mixture of two or more thereof.

17. 17. The method of claim 15 or 16, wherein the nucleation centers are used in an amount of about 0.1 mg / mL or more, in an amount ranging from about 0.1 mg / mL to about 20 mg / mL, in an amount ranging from about 0.5 mg / mL to about 20 mg / mL, in an amount ranging from about 1 mg / mL to about 20 mg / mL, in an amount ranging from about 0.1 mg / mL to about 15 mg / mL, in an amount ranging from about 1 mg / mL to about 10 mg / mL.

18. 18. The method of any of claims 1 to 17, wherein step (d) is carried out with mixing of the reaction mixture, and mixing in step (d) is carried out for about 15 minutes or more, for about 15 minutes to about 180 minutes, for about 15 minutes to about 150 minutes, for about 30 minutes to about 120 minutes, for about 30 minutes to about 90 minutes, for about 30 minutes to about 60 minutes, for 60 minutes.

19. 19. The method of claims 1 to 18, wherein: (i) the concentrated VRFB electrolyte composition produced in step (d) has a densification level (DL) of 30% or greater, which is from about 30% to about 70%, which is from about 40% to about 65%, which is from about 50% to about 65%, which is from about 50% to about 60%, which is 50%, and (ii) the densification level is calculated according to the formula: DL = (amount of water removed / amount of total electrolyte) x 100%.

20. 20. A concentrated VRFB electrolyte composition produced according to the method of any of claims 1 to 19.

21. 21. A method of producing a VRFB electrolyte composition comprising the step of contacting the concentrated VRFB electrolyte composition of claim 20 with an aqueous liquid.

22. The aqueous liquid is added in an amount to produce the VRFB electrolyte composition comprising a vanadium sulfate concentration of less than 3.0M and a total sulfate concentration of at least twice the concentration of the vanadium sulfate, or the aqueous liquid is added in an amount to produce the VRFB electrolyte composition comprising a vanadium sulfate concentration of less than 3.0M and a total sulfate concentration of at least twice the concentration of the vanadium sulfate, or the aqueous liquid is added in an amount to produce the VRFB electrolyte composition comprising a vanadium sulfate concentration of less than 2.8M and a total sulfate concentration of at least twice the concentration of the vanadium sulfate, or the aqueous liquid is added in an amount to produce the VRFB electrolyte composition comprising a vanadium sulfate concentration in the range of about 1.2M to about 2.5M and a total sulfate concentration of at least twice the concentration of the vanadium sulfate, or the aqueous liquid is added in an amount to produce the VRFB electrolyte composition comprising a vanadium sulfate concentration in the range of about 1.2M to about 2.0M and a total sulfate concentration of at least twice the concentration of the vanadium sulfate.

22. The method of claim 21, wherein the aqueous liquid is added in an amount to produce the VRFB electrolyte composition comprising a total sulfate concentration that is 2 to 4 times the concentration of the vanadium sulfate, or the aqueous liquid is added in an amount to produce the VRFB electrolyte composition comprising a total sulfate concentration that is 2 to 3.5 times the concentration of the vanadium sulfate, or the aqueous liquid is added in an amount to produce the VRFB electrolyte composition comprising a total sulfate concentration that is 2 to 3 times the concentration of the vanadium sulfate, or the aqueous liquid is added in an amount to produce the VRFB electrolyte composition comprising a total sulfate concentration that is 2.5 to 3 times the concentration of the vanadium sulfate.

23. 1. A composition (preferably in the form of a concentrated VRFB electrolyte composition for use in a VRFB system), comprising vanadium sulfate at a concentration of 3.0 M or greater and a total sulfate concentration at least two times the concentration of said vanadium sulfate, and in the form of a gel or semi-solid and / or having a viscosity at 20° C. of 100 mPa s or greater.

24. 24. The composition of claim 23, wherein the vanadium sulfate concentration and the total sulfate concentration are each as described in claim 12 for the purified reaction mixture.

25. 25. Use of the composition according to claim 23 or 24 for the manufacture of a VRFB electrolyte for use in a VRFB.

26. 26. The use of claim 25, wherein the VRFB electrolyte composition comprises a vanadium sulfate concentration and a total sulfate concentration, each as set forth in claim 22.

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