WO2021234725A1

WO2021234725A1 - Electrochemical preparation method for vanadium electrolyte and its application thereof

Info

Publication number: WO2021234725A1
Application number: PCT/IN2021/050449
Authority: WO
Inventors: Anil Verma; Nishant BERIWAL
Original assignee: Indian Institute Of Technology Delhi
Priority date: 2020-05-20
Filing date: 2021-05-10
Publication date: 2021-11-25

Abstract

The present disclosure discloses an electrochemical preparation method for a vanadium electrolyte, the method comprising: (a) contacting at least one vanadium precursor with aqueous sulfuric acid to obtain a solution; and (b) introducing the solution into a part of an electrochemical reactor and allowing a reaction to complete to form the vanadium electrolyte, wherein the reactor has an operating voltage in the range of 200 mV to 1700 mV. The present disclosure also provides an electrochemical reactor for preparing the vanadium electrolyte through a cost-effective and an energy efficient one step method. Also provided is a vanadium redox flow battery system utilizing the prepared vanadium electrolyte for electrical energy storage.

Description

ELECTROCHEMICAL PREPARATION METHOD FOR VANADIUM

ELECTROLYTE AND ITS APPLICATION THEREOF

FIELD OF INVENTION

[001] The present disclosure broadly relates to a method of preparing vanadium electrolyte and particularly refers to an electrochemical method of preparing vanadium electrolyte suitable for use in all-vanadium redox flow battery (VRFB) systems.

BACKGROUND OF INVENTION

[002] Redox flow batteries (RFBs) have received extraordinary attention due to their simple operating conditions (room temperature and atmospheric pressure), decoupled scale-up for power and energy capacity, longer cycle life than normal secondary batteries, reduced self-discharging, excellent reliability, and safety vanadium redox flow battery (VRFB) is a very special kind of secondary redox flow battery (RFB) that utilizes vanadium as an active material on the positive and negative side of the electrodes, which eliminates the issue of electrolyte contamination due to crossover of ionic species. VRFBs employ the redox couples V0₂ ⁺/V0²⁺ as catholyte and V²⁺/V³⁺ as anolyte in aqueous acid solutions stored in two external tanks. These batteries preferably require vanadium electrolyte in +3.5 oxidation state as a fuel for energy generation and storage operated through a charge-discharge cycle.

[003] Most of the vanadium precursor materials are quite expensive. In particular, the vanadium electrolyte constitutes a major portion of a VRFB cost. For example, 40% to 41% of the total cost of a system of 10kW/120kWh accounts for the cost of vanadium and electrolyte production cost (Noack, Jens, et al. "Techno-economic modeling and analysis of redox flow batery systems." Energies 9.8 (2016): 627).

[004] Generally, the hydrated form of vanadium sulphate VOSO4.XH2O is used for preparing vanadium electrolyte for the VRFB system. Many methods, such as electrolytic process, chemical reduction, and addition of reducing agents have been proposed for synthesizing vanadium electrolyte solution for VRFB systems which involve complex ways of separation of mixed electrolyte solution along with other chemical impurities.

[005] KR20190124865A discloses a method for preparing vanadium electrolyte by phase transformation of ammonium metavanadate (AMV) to ammonium polyvanadate (PMV) by carrying out a heat treatment at 230-280°C temperature. US5587132A discloses a combined route of purification and reduction of V2O5 and ammonium metavanadate (NH4NO3) followed by successive purification steps at a temperature of 400°C-700°C to prepare a vanadium electrolyte solution.

[006] The present research in the development of vanadium electrolyte is associated with many setbacks with respect to high temperature requirements, multiple purification steps, cell damage issues, and high cost of vanadium precursors used which further make VRFB systems an expensive option for commercial applications. Thus, in order to address the above said problems, there is a dire need to develop an economically viable and high-performance vanadium electrolyte production process to achieve a broader acceptance of VRFBs.

SUMMARY OF THE INVENTION

[007] In first aspect of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte, the method comprising: (a) contacting at least one vanadium precursor with aqueous sulfuric acid to obtain a solution; and (b) introducing the solution into a part of an electrochemical reactor and allowing a reaction to complete to form the vanadium electrolyte, wherein the reactor has an operating voltage in the range of 200 mV to 1700 mV.

[008] In second aspect of the present disclosure, there is provided a vanadium redox flow battery comprising: (a) a positive half-cell containing a positive half cell solution comprising the vanadium electrolyte; and (b) a negative half-cell containing a negative half-cell solution comprising the vanadium electrolyte, wherein the vanadium electrolyte is prepared by an electrochemical preparation method, the method comprising: (1) contacting at least one vanadium precursor with aqueous sulfuric acid to obtain a solution; and (2) introducing the solution into a part of an electrochemical reactor and allowing a reaction to complete to form the vanadium electrolyte, wherein the reactor has an operating voltage in the range of 200 mV to 1700 mV.

[009] In third aspect of the present disclosure, there is provided a vanadium redox flow battery comprising: (a) a positive half-cell containing a positive half cell solution comprising the vanadium electrolyte; and (b) a negative half-cell containing a negative half-cell solution comprising the vanadium electrolyte, wherein the vanadium electrolyte is prepared by an electrochemical preparation method, the method comprising: (1) contacting at least one vanadium precursor with aqueous sulfuric acid to obtain a solution; and (2) introducing the solution into a part of an electrochemical reactor and allowing a reaction to complete to form the vanadium electrolyte, wherein the reactor has an operating voltage in the range of 200 mV to 1700 mV, and wherein the positive half-cell solution has vanadium concentration in the range of 1 M - 2 M and the negative half-cell solution has a vanadium concentration in the range 1 M to 2 M.

[0010] These and other features, aspects, and advantages of the present subject matter will be better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

[0011] The following drawings form a part of the present specification and are included to further illustrate aspects of the present disclosure. The disclosure may be better understood by reference to the drawings in combination with the detailed description of the specific embodiments presented herein. [0012] Figure 1 depicts the scheme of an electrochemical reactor for preparing vanadium electrolyte, in accordance with an embodiment of the present disclosure.

[0013] Figure 2 depicts the scheme of a conventional vanadium redox flow battery (VRFB) system used to utilize vanadium electrolyte, in accordance with an embodiment of the present disclosure.

[0014] Figure 3 depicts the plot for applied voltage (V) vs time (min) data of the experiment conducted in the electrochemical reactor for conversion of vanadium pentoxide (V2O5) to y³⁵⁺ vanadium electrolyte, in accordance with an embodiment of the present disclosure. [0015] Figure 4 depicts the potential (V) vs. time (h) data plot describing the charge-discharge cycle of the conventional vanadium redox flow battery (VRFB) system, in accordance with an embodiment of the present disclosure.

[0016] Figure 5 depicts the potential (V) vs capacity (Ah) data plot describing the charge storing capacity of the conventional vanadium redox flow battery (VRFB) system, in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Those skilled in the art will be aware that the present disclosure is subject to variations and modifications other than those specifically described. It is to be understood that the present disclosure includes all such variations and modifications. The disclosure also includes all such steps, features, compositions, and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any or more of such steps or features. Definitions

[0018] For convenience, before further description of the present disclosure, certain terms employed in the specification, and examples are delineated here. These definitions should be read in the light of the remainder of the disclosure and understood as by a person of skill in the art. The terms used herein have the meanings recognized and known to those of skill in the art, however, for convenience and completeness, particular terms and their meanings are set forth below.

[0019] The articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.

[0020] The terms “comprise” and “comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as “consists of only”.

[0021] Throughout this specification, unless the context requires otherwise the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated element or step or group of element or steps but not the exclusion of any other element or step or group of element or steps.

[0022] The term “including” is used to mean “including but not limited to”. “Including” and “including but not limited to” are used interchangeably.

[0023] Ratios, concentrations, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a concentration range of about 1M to 2M should be interpreted to include not only the explicitly recited limits of about 1M to about 2M, but also to include sub-ranges, such as 1.1M to 1.9M, 1.3 to 1.7M, and so forth, as well as individual amounts, including fractional amounts, within the specified ranges, such as 1.15M, 1.24M, and 1.625M.

[0024] The term “electrolyte” used herein refers to a solid, liquid, or less frequently a gaseous substance that conducts electricity by the movement of ions.

[0025] The term “trivalent vanadium” used herein refers to vanadium in +3 oxidation state.

[0026] The term “tetravalent vanadium” used herein refers to vanadium in +4 oxidation state.

[0027] The term “pentavalent vanadium” used herein refers to vanadium in +5 oxidation state.

[0028] The term “vanadium concentration” used herein refers to concentration of vanadium in +3.5 oxidation state.

[0029] The term “carbon paper” used herein refers to flat sheets consisting of carbon microfibers. It is a porous layer that lays between the catalyst coating layer and fed gas layer.

[0030] The term “graphite felt” used herein refers to rayon or polyacrylonitrile based soft, flexible and high temperature resistible material comprising carbon content in the form of graphite.

[0031] The term “solid electrolyte membrane” used herein refers to a solid sheet like dense porous material with proton conduction and cation exchanging properties generally used in fuel cells. Examples include nafion 117, nafion 115, etc.

[0032] The term “Ah” used herein refers to ampere-hour which is a unit for specifying the storage capacity of a battery. For example, a battery of 1 Ah capacity can provide a current of 1 ampere for 1 hour. 1 Ah also corresponds to 3,600 coloumbs of electric charge.

[0033] The term “coulombic efficiency” used herein refers to charge efficiency with which electrons are transferred in batteries. It is calculated as :

% - (Qout/Qin) * 100 wherein, q_c is the coulombic efficiency %, Q_out is the amount of charge that exits a battery during a discharge cycle, and Q;, is the amount of charge that enters the battery during the charge cycle.

[0034] The term “at most 200mA/cm ” used herein refers to any current density more than 0 and less than 200.0. It includes current densities selected from the range of 0.1-199.9mA/cm², 5-195mA/cm², 50-150mA/cm² etc.

[0035] Ratios, concentrations, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited.

[0036] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the preferred methods, and materials are now described. All publications mentioned herein are incorporated herein by reference.

[0037] The present disclosure is not to be limited in scope by the specific embodiments described herein, which are intended for the purposes of exemplification only. Functionally-equivalent products, compositions, and methods are clearly within the scope of the disclosure, as described herein.

[0038] As discussed in the background of the present disclosure, vanadium electrolyte for VRFB systems can be produced by an electrolytic process and/or a chemical reduction process, and/or a process requiring the addition of reducing compounds, however, such methods are energy intensive, time taking and complicated. Most of the processes result in a mixed electrolyte that requires further processing before it is used in a VRFB. Some of the conventionally used methods also require additional liquid and/or solid chemical/reducing agent, which circulates in the VRFB system as an impurity even after purification. In other processes, high temperature operation is also needed along with liquid/solid chemicals/reducing agents which make the reactant synthesis process more complicated and costlier for synthesizing vanadium electrolyte solution using V2O5 for VRFB system.

[0039] To address the problems mentioned above, the present disclosure provides an electrochemical preparation method for a vanadium electrolyte, the method comprising: (a) contacting at least one vanadium precursor with aqueous sulfuric acid to obtain a solution; and (b) introducing the solution into a part of an electrochemical reactor and allowing a reaction to complete to form the vanadium electrolyte, wherein the reactor has an operating voltage in the range of 200 mV to 1700 mV.

[0040] The present disclosure discloses a simple one-step electrochemical preparation method, which uses vanadium pentoxide (V2O5) in sulphuric acid solution. Since the cost of conventionally used VOSO4.XH2O is much higher than vanadium pentoxide (V2O5) powder, preparation of low-cost vanadium electrolyte from V2O5 powder strengthens the practical use of VRFB system of the present disclosure. Moreover, the major disadvantage related to the low solubility of V2O5 which prevents direct production of its high concentration solutions has been overcome in the present disclosure by employing an electrochemical preparation methodto achieve maximum dissolution of V2O5. The reaction involved in the reduction of vanadium precursor V2O5 having vanadium in +5 oxidation state to V0²⁺ and V³⁺ having vanadium in +4 and +3 oxidation states respectively is as below:

V₂0₅ + 2H⁺ - _► 2V0₂ ⁺ + H₂0

V0₂ ⁺ + 2H⁺ + e - _► V0²⁺ + H₂0

V0²⁺ + 2H⁺ + e - _► V³⁺+ H₂0

[0041] The reactions (ii) and (iii) are fine tuned in order to obtain V0²⁺ and V³⁺ aqueous solution in equimolar concentrations so to account for the formation of pure v³⁵⁺ electrolyte. The vanadium (V³⁵⁺) electrolyte solution obtained in the process does not need any separation or purification step as the process does not require any liquid and/or solid chemical/reducing agent to be added. The obtained y³⁵⁺ can directly be used in all-vanadium redox flow battery or hybrid vanadium redox flow battery systems. The process does not require any elevated temperature and is energy efficient. The present disclosure substantiates the efficiency of electrochemical preparation method of vanadium electrolyte in terms of a less complicated, cost efficient, easily scalable, energy feasible due to low overpotential, minimized electrode etching, as well as a less time taking process due to fast reaction.

[0042] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte, the method comprising: (a) contacting at least one vanadium precursor with aqueous sulfuric acid to obtain a solution; and (b) introducing the solution into a part of an electrochemical reactor and allowing a reaction to complete to form the vanadium electrolyte, wherein the reactor has an operating voltage in the range of 200 mV to 1700 mV.

[0043] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte, the method comprising: (a) contacting at least one vanadium precursor with aqueous sulfuric acid to obtain a solution; and (b) introducing the solution into a part of an electrochemical reactor and allowing a reaction to complete to form the vanadium electrolyte, wherein the reactor has an operating voltage in the range of 200 mV to 1700 mV, and wherein the vanadium electrolyte is an equimolar mixture of trivalent vanadium and tetravalent vanadium in sulfuric acid.

[0044] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte, the method comprising: (a) contacting at least one vanadium precursor with aqueous sulfuric acid to obtain a solution having pentavalent vanadium; and (b) introducing the solution into a part of an electrochemical reactor and allowing a reaction to complete to form the vanadium electrolyte, wherein the reactor has an operating voltage in the range of 200 mV to 1700 mV. In another embodiment of the present disclosure, the pentavalent vanadium is provided by V2O5 powder. [0045] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein the at least one vanadium precursor is V2O5 powder. In another embodiment of the present disclosure, V2O5 powder has a maximum particle size of 500 microns.

[0046] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte, the method comprising: (a) contacting at least one vanadium precursor with aqueous sulfuric acid to obtain a solution is carried out at a temperature in the range of 15 to 30 °C and at atmospheric pressure of 1.01 bar; and (b) introducing the solution into a part of an electrochemical reactor and allowing a reaction to complete to form the vanadium electrolyte, wherein the reactor has an operating voltage in the range of 200 mV to 1700 mV.

[0047] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte, the method comprising: (a) contacting at least one vanadium precursor with aqueous sulfuric acid to obtain a solution is carried out at a temperature in the range of 15 to 30 °C and at atmospheric pressure of 1.01 bar, wherein at least one vanadium precursor is vanadium pentoxide powder; and (b) introducing the solution into a part of an electrochemical reactor and allowing a reaction to complete to form the vanadium electrolyte, wherein the reactor has an operating voltage in the range of 200 mV to 1700 mV.

[0048] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte, the method comprising: (a) contacting at least one vanadium precursor with aqueous sulfuric acid to obtain a solution is carried out at a temperature in the range of 15 to 30 °C and at atmospheric pressure of 1.01 bar, wherein at least one vanadium precursor is vanadium pentoxide powder, and wherein the aqueous sulfuric acid has a concentration in the range of 4 M - 8 M; and (b) introducing the solution into a part of an electrochemical reactor and allowing a reaction to complete to form the vanadium electrolyte, wherein the reactor has an operating voltage in the range of 200 mV to 1700 mV. In another embodiment of the present disclosure, the aqueous sulfuric acid has a concentration in the range of 4 M - 6 M. In another embodiment of the present disclosure, the aqueous sulfuric acid has a concentration in the range of 4 M - 5 M.

[0049] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein the at least one vanadium precursor has a weight percentage in the range of 7 % - 14 % with respect to the solution.

[0050] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein introducing the solution into a part of the electrochemical reactor and allowing a reaction to complete to form the vanadium electrolyte is carried out a temperature in the range of 40-50°C.

[0051] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein the reactor is operated at a current density of at most 200mA/cm².

[0052] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein the electrochemical reactor comprises: (i) a catalyst coated anode made up of carbon paper; (ii) a cathode made up of a material selected from carbon paper/felt or graphite felt; and (iii) a solid electrolyte membrane, wherein the cathode is fed with the solution and the catalyst coated anode is fed with hydrogen gas.

[0053] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein the electrochemical reactor comprises: (i) a catalyst coated anode made up of carbon paper; (ii) a cathode made up of graphite felt with a pore size in the range of 10-100 micrometers; and (iii) a solid electrolyte membrane, wherein the cathode is fed with the solution and the catalyst coated anode is fed with hydrogen gas. [0054] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein the electrochemical reactor comprises: (i) a catalyst coated anode made up of carbon paper; (ii) a cathode made up of a material selected from carbon paper/felt or graphite felt; and (iii) a solid electrolyte membrane, wherein the cathode is fed with the solution and the catalyst coated anode is fed with hydrogen gas at a pressure of 1 atm.

[0055] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein the electrochemical reactor comprises: (i) a catalyst coated anode made up of carbon paper, wherein the catalyst is selected from platinum- based catalyst, ruthenium-based catalyst, or palladium-based catalyst; (ii) a cathode made up of a material selected from carbon paper/felt or graphite felt; and (iii) a solid electrolyte membrane, wherein the cathode is fed with the solution and the catalyst coated anode is fed with hydrogen gas.

[0056] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein the electrochemical reactor comprises: (i) a catalyst coated anode made up of carbon paper; (ii) a cathode made up of a material selected from carbon paper/felt or graphite felt; and (iii) a solid electrolyte membrane selected from a group consisting of nafion 117, nafion 115, and combinations thereof, wherein the cathode is fed with the solution and the catalyst coated anode is fed with hydrogen gas.

[0057] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein the electrochemical reactor comprises: (i) a catalyst coated anode made up of carbon paper, wherein the catalyst is selected from platinum- based catalyst, ruthenium-based catalyst, or palladium-based catalyst; (ii) a cathode made up of a material selected from carbon paper/felt or graphite felt with a pore size in the range of 10-100 micrometers; and (iii) a solid electrolyte membrane selected from a group consisting of nafion 117, nafion 115, and combinations thereof, wherein the cathode is fed with the solution and the catalyst coated anode is fed with hydrogen gas at a pressure of 1 atm.

[0058] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein the vanadium electrolyte is suitable for use in a vanadium redox flow battery without further reduction.

[0059] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein the electrochemical reactor comprises: (i) a catalyst coated anode made up of carbon paper; (ii) a cathode made up of a material selected from carbon paper/felt or graphite felt with a pore size in the range of 10-100 micrometers; and (iii) a solid electrolyte membrane, wherein the cathode is fed with the solution and the catalyst coated anode is fed with hydrogen gas, and wherein the vanadium electrolyte is suitable for use in a vanadium redox flow battery without further reduction.

[0060] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein the electrochemical reactor comprises: (i) a catalyst coated anode made up of carbon paper; (ii) a cathode made up of a material selected from carbon paper/felt or graphite felt with a pore size in the range of 10-100 micrometers; and (iii) a solid electrolyte membrane, wherein the cathode is fed with the solution and the catalyst coated anode is fed with hydrogen gas, and wherein the vanadium electrolyte has a total vanadium concentration in the range of 1 M to 2 M with respect to the vanadium electrolyte. In another embodiment of the present disclosure, the vanadium electrolyte has a total vanadium concentration in the range of 1.3 M to 1.7 M with respect to the vanadium electrolyte.

[0061] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein the vanadium electrolyte has a total vanadium concentration in the range of 1 M to 2 M with respect to the vanadium electrolyte. In another embodiment of the present disclosure, the vanadium electrolyte has a total vanadium concentration in the range of 1.3 M to 1.7 M with respect to the vanadium electrolyte.

[0062] In an embodiment of the present disclosure, there is provided an electrochemical preparation method for a vanadium electrolyte as described herein, wherein the vanadium electrolyte is prepared in a time period in the range 80 - 150 minutes. In another embodiment of the present disclosure, the vanadium electrolyte is prepared in a time period in the range 82 - 110 minutes. In another embodiment of the present disclosure, the vanadium electrolyte is prepared in a time period in the range 83 - 90 minutes.

[0063] In an embodiment of the present disclosure, there is provided a vanadium redox flow battery comprising: (a) a positive half-cell containing a positive half cell solution comprising the vanadium electrolyte prepared by the method as described herein; and (b) a negative half-cell containing a negative half-cell solution comprising the vanadium electrolyte prepared by the method as described herein.

[0064] In an embodiment of the present disclosure, there is provided a vanadium redox flow battery comprising: (a) a positive half-cell containing a positive half cell solution comprising the vanadium electrolyte prepared by the method as described herein, wherein the positive half-cell solution has vanadium concentration in the range of 1 M - 2 M; and (b) a negative half- cell containing a negative half-cell solution comprising the vanadium electrolyte prepared by the method as described herein, wherein the negative half-cell solution has a vanadium concentration in the range 1 M to 2 M.

[0065] Although the subject matter has been described in considerable detail with reference to certain examples and implementations thereof, other implementations are possible.

EXAMPLES

[0066] The disclosure will now be illustrated with working examples, which is intended to illustrate the working of disclosure and not intended to take restrictively to imply any limitations on the scope of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice of the disclosed methods and compositions, the exemplary methods, devices and materials are described herein. It is to be understood that this disclosure is not limited to particular methods, and experimental conditions described, as such methods and conditions may apply.

[0067] The working examples as depicted in the forthcoming sections highlight the process to the reduction of vanadium precursor V2O5 having vanadium in +5 to v³⁵⁺ electrolytic solution in an electrochemical reactor and further evaluates the performance of a conventional VRFB system utilizing the prepared vanadium electrolyte of the present disclosure.

Materials and Methods

[0068] For the purpose of the present disclosure, vanadium pentoxide (V2O5) powder was procured from of up to 99.9% purity. The sulfuric acid solution used for dissolving V2O5 powder was obtained from. The objectives and benefits of the present method of the disclosure are made clear with the help of accompanying Figures 1 and 2. Figure 1 depicts an electrochemical reactor that is used for preparing the vanadium electrolyte of the present disclosure. Figure 2 depicts a conventional vanadium redox flow battery (VRFB) system which further utilizes the prepared vanadium electrolyte for electrical energy storing purpose.

Scheme for the electrochemical reactor for preparing vanadium electrolyte [0069] Figure 1 reveals a labelled scheme of an electrochemical reactor employed for preparing a vanadium electrolyte in a continuous manner by reduction of a vanadium precursor solution. The labelled scheme is described using the following designators : 1: dissolution vessel; 2: pump; 3: membrane; 4: catalyst coated gas diffusion layer; 5, 6, 7, 8: gaskets; 9: anodic graphite plate; 10: cathodic graphite plate; 11: hydrogen gas inlet; 12: hydrogen gas outlet; 13: feed inlet; 14: electrolyte outlet; 15: carbon electrode; and 16: cathode chamber of the electrochemical cell.

[0070] Vanadium precursor solution was prepared by dissolving a definite amount of V2O5 powder in sulphuric acid solution. The dissolved vanadium pentoxide solution was sent to the electrochemical reactor having an anode, cathode, and a solid electrolyte membrane. The anode was made up of carbon paper with a platinum catalyst coated hydrogen gas diffusion layer positioned in close proximity with the Nafion-117 solid electrolyte membrane. The cathode was made up of thermally treated graphite felt having a pore size in the range of 10-100 micrometers. It was separated from the membrane by a cathode chamber to accommodate the flow of the precursor solution. The dissolved solution containing V2O5 precursor was circulated through the cathode chamber of the electrochemical cell with the help of a pump. The continuous circulation was done in order to collect vanadium electrolyte having vanadium in +3.5 oxidation state in a pure form so that it can directly be used in a VRFB system without further reduction or purification.

Scheme of a conventionally used vanadium redox flow battery (VRFB) system for utilizing the prepared vanadium electrolyte [0071] Figure 2 reveals a labelled scheme of a conventionally used vanadium redox flow battery system employed to utilize the prepared vanadium electrolyte in order to substantiate its performance in electrical energy storage for the purpose of the present disclosure. The labelled scheme is described using the following designators: 17, 18: graphite felt electrode; 19: positive electrolyte tank; 20: negative electrolyte tank; 21, 22: pumping device; 23: membrane; 24: source/load. The prepared vanadium y³⁵⁺ electrolyte was stored in the positive and negative electrolyte tanks from where it was pumped through the pumping device to act as a reactant at both cathode and anode. The concentration of the fed vanadium electrolyte remained 1.5M at all times in the process. The VRFB was then operated in charging mode and discharging mode, as generally done in any secondary battery. The synthesized energy was stored in the source/load in the form of electrical energy which can be utilized for further applications.

[0072] The efficiency of the electrochemical reactor in producing vanadium electrolyte is depicted in Example 1. Example 2 and 3 depict the performance of produced vanadium electrolyte in terms of stored voltage in charge-discharge cycle and charge storing capacity of the VRFB system respectively.

EXAMPLE 1

Method for preparation of vanadium electrolyte

[0073] Vanadium precursor solution was prepared by dissolving vanadium pentoxide (V2O5) powder in sulfuric acid solution at ambient conditions of 15- 30°C temperature and 1.01 bar pressure. V2O5 powder (-13.77 g) was dissolved in a suitable volume (~ 95 mL) of 4.0 M H2SO4 to prepare 100 mL of total solution comprising pentavalent vanadium. The total concentration of vanadium in the 100 mL solution was 1.5 M which contributes to 13.77% (w/v) in the vanadium electrolyte. The reduction of pentavalent vanadium was carried out in the electrochemical reactor operated at 40-50°C temperature. A constant current was applied at the DC source at a current density of 200 mA/cm and with a voltage window of 200 mV to 1700 mV. It can be derived from the voltage vs time data as revealed in Figure 3 that the experiment was completed within 84 minutes of increasing the potential starting from 200 mV up to 1700 mV. A vanadium electrolyte containing vanadium in 3.5+ oxidation state in a total concentration of 1.5M was obtained.

EXAMPLE 2

Utilization of vanadium electrolyte in a conventional vanadium redox flow battery (VRFB) system

[0074] The vanadium electrolyte prepared in Example 1 was subjected to a conventional VRFB system as provided in Figure 2. Figure 4 reveals the charging-discharging cycle observed in the VRFB system with 1.5M vanadium electrolyte in the potential limit of 1000 mV to 1700 mV. In the beginning, charging of VRFB with an opening voltage of 1470 mV up to 1700 mV was observed. Thereafter, the portion of the curve represents the discharging of VRFB with an opening voltage of 1290 mV which falls with the course of discharging to a limit of 1000 mV. Figure 5 reveals potential (V) vs. capacity (Ah) data of VRFB as recorded with 1.5M vanadium electrolyte substantiating the charge storage capacity of the battery. The upper curve represents charging of VRFB to a potential limit of 1700 mV, where the battery achieves a capacity of 0.92Ah. On discharging this battery with the lower potential window of 1000 mV (represented by the bottom curve), the battery discharges to a capacity of 0.85Ah. The battery operates with a coulombic efficiency of -93%.

[0075] In light of the examples as described herein, the selection of present disclosure is to provide a simple one-step electrochemical preparation method to prepare low-cost vanadium electrolyte without using reducing agents while avoiding electrolytic etch or corrosion of the electrodes in use. It is also an object of the present disclosure to provide a vanadium electrolyte solution that can be further applied in all-vanadium redox flow battery or hybrid vanadium redox flow battery systems to efficiently generate electrical energy for storage purposes.

[0076] However, the example has been shown and described with reference to certain preferred embodiments, it is obvious for a person skilled in the art to arrive at various modifications and changes with respect to some or all mentioned embodiments without deviating from the spirit and scope of the invention and compromising on the advantageous aspects of the present disclosure.

Advantages of the present disclosure

[0077] The present disclosure discloses an electrochemical preparation method for a vanadium electrolyte wherein the vanadium electrolyte is prepared by the one-step process of subjecting the vanadium precursor solution to the electrochemical reactor. The vanadium precursor selected is a low-cost vanadium pentoxide powder in comparison to conventionally used vanadium sulphate precursor. In electrochemical systems, high purity is important, therefore, vanadium pentoxide powder taken was of 99.9% purity. Owing to its electrochemical nature, the vanadium electrolyte preparation method achieves maximum dissolution of vanadium pentoxide without applying elevated temperatures, which makes it a safer and an energy efficient process in comparison to processes at high temperature. The main advantage of the present disclosure lies in the complete elimination of any successive purification or separation steps as the reduction process is carried without including the addition of reducing agents. Moreover, ¾ reduction is a thermodynamically more favorable process than water electrolysis and does not need high potentials. Since carbon-based material generally corrodes over 1700 mV, the present disclosure operates at much lower potential difference, which is a safe operating voltage window for electrode health. Further, at the anodic side of the electrochemical reactor, platinum with carbon paper is used to facilitate a fast splitting reaction of H2 gas. It is a continuous process and can be scaled up very easily. The present disclosure also discloses appreciable energy storage capacity and coulombic efficiency of the prepared vanadium electrolyte when employed in the vanadium flow redox battery system.

Claims

I/We Claim:

1) An electrochemical preparation method for a vanadium electrolyte, the method comprising:

(a) contacting at least one vanadium precursor with aqueous sulfuric acid to obtain a solution; and

(b) introducing the solution into a part of an electrochemical reactor and allowing a reaction to complete to form the vanadium electrolyte, wherein the reactor has an operating voltage in the range of 200 mV to 1700 mV.

2) The method as claimed in claim 1, wherein the vanadium electrolyte is an equimolar mixture of trivalent vanadium and tetravalent vanadium in sulfuric acid.

3) The method as claimed in claim 1, wherein the solution has pentavalent vanadium.

4) The method as claimed in claim 1, wherein contacting at least one vanadium precursor with aqueous sulfuric acid to obtain a solution is carried out at a temperature in the range of 15 to 30 °C and at atmospheric pressure of 1.01 bar.

5) The method as claimed in claim 1, wherein the at least one vanadium precursor is vanadium pentoxide powder.

6) The method as claimed in claim 1, wherein the aqueous sulfuric acid has a concentration in the range of 4 M - 8 M.

7) The method as claimed in claim 1, wherein the at least one vanadium precursor has a weight percentage in the range of 7 % - 14 % with respect to the solution.

8) The method as claimed in claim 1, wherein introducing the solution into a part of the electrochemical reactor and allowing a reaction to complete to form the vanadium electrolyte is carried out a temperature in the range of 40-50°C.

9) The method as claimed in claim 1, wherein the reactor is operated at a current density of at most 200mA/cm . 10) The method as claimed in claim 1, wherein the electrochemical reactor comprises:

(i) a catalyst coated anode made up of carbon paper;

(ii) a cathode made up of a material selected from carbon paper/felt or graphite felt; and

(iii) a solid electrolyte membrane, wherein the cathode is fed with the solution and the catalyst coated anode is fed with hydrogen gas.

11) The method as claimed in claim 9, wherein the graphite felt has a pore size in the range of 10-100 micrometers.

12) The method as claimed in claim 9, wherein hydrogen gas is fed at a pressure of 1 atm.

13) The method as claimed in claim 9, wherein the catalyst is selected from platinum-based catalyst, ruthenium-based catalyst, or palladium-based catalysts.

14) The method as defined in any one of the claim 1 - 12, wherein the vanadium electrolyte is suitable for use in a vanadium redox flow battery without further reduction.

15) The method as claimed in any one of the claims 1 - 12, wherein the vanadium electrolyte has a total vanadium concentration in the range of 1 M to 2 M with respect to the vanadium electrolyte.

16) The method as claimed in any one of the claims 1 - 12, wherein the vanadium electrolyte is prepared in a time period in the range 80 - 150 minutes.

17) A vanadium redox flow battery comprising:

(a) a positive half-cell containing a positive half-cell solution comprising the vanadium electrolyte prepared by the method as claimed in any one of the claims 1- 15; and

(h) a negative half-cell containing a negative half-cell solution comprising the vanadium electrolyte prepared by the method as claimed in any one of the claims 1- 15. 18) A vanadium redox flow battery as claimed in claim 16, wherein the positive half-cell solution has vanadium concentration in the range of 1 M - 2 M and the negative half-cell solution has a vanadium concentration in the range 1 M to 2 M.