Dual functionalized fluorinated polymer based stable cation exchange membrane and its preparation method thereof

Abstract

The present invention discloses preparation route for a novel partially fluorinated polymer based stable cation exchange membrane with repeating unit of formula I Reported cation exchange membrane is prepared by dehydrofluorination of polyvinylidene fluoride followed by grafting of 2-acrylamido-2-methyl-1- propanesulfonic acid or any other sulfonic acid functionalized monomer with dehydrofluorinated polyvinylidene fluoride, in which functionalized silica precursor (3-mercaptopropyl trimethoxysilane) is incorporated by acid catalyzed sol-gel process to improve the charged density sulphonic acid groups in the membrane matrix. Reported partial fluorinated polymer based cation exchange membrane showed excellent acid stability and assessed as a potential membrane separator for electrodialysis, electrolysis reversal, water electrolysis, fuel cell, and storage redox flow batteries or other electro-membrane processes. This invention also encompasses that cation exchange membrane useful in harassed acidic / oxidative conditions along with excellent thermal and mechanical stabilities.

Description

[0001] TITLE OF THE INVENTION DUAL FUNCTIONALIZED FLUORINATED POLYMER BASED STABLE CATION EXCHANGE MEMBRANE AND ITS PREPARATION METHOD THEREOF FIELD OF INVENTION The present invention relates to stable cation exchange membranes and its process of preparation thereof by side chain grafting of 2-acrylamido-2-methyl-1- propanesulfonic acid, or any other sulphonic acid functionalized monomer with grafted dehydro-fluorinated polyvinylidene fluoride, and further incorporation of (3-mercaptopropyl)trimethoxysilane or any other silica precursor by acid catalysed sol-gel process, that resulting into grafting of dual functional group with fluorinated polyvinylidene fluoride polymeric matrix that leads to enhance the functional group density in the membrane matrix, and thus membrane conductivity. Prepared stable cation exchange membranes are promising candidates for practical application in electrodialysis, electro-deionization, chlor-alkali process, electrolysers for water splitting, proton exchange membrane fuel cell, energy storage battery or any other electrochemical processes using cation exchange membrane as a separator. BACKGROUD OF THE INVENTION The cation exchange membranes contain negatively charged groups chemically attached with polymer matrix. The following moieties are used as fixed charges in cation-exchange membranes: SO3H, -PO3H2, -COOH, -H2PO2 etc. These fixed anionic groups are in electrical equilibrium with mobile cations (counter-ions), and mobile anions (co-ions) are completely excluded from the polymer matrix because of their electrical charge. Thus, due to the exclusion of the co-ions, a cation- exchange membrane permits transfer of cations only. The stable cation exchange membranes are generally used as separator for various electrochemical separations, energy conversion, and energy storage devices including: electrodialysis (removal of salt from the water, separation of ionic component from organic molecules, selective separation of ions and based on other applications), electrolysis (water splitting, brine splitting for chlor-alkali production, thermochemical process for H2production, etc.), electro-electrodialysis (in-situ ion exchange and ion substitution), proton exchange membrane (PEM) fuel cells, redox flow batteries, etc. Superior membrane stability (thermal, mechanical and chemical) and excellent physicochemical and electrochemical properties are essential requirements to judge its suitability for proposed electro-membrane applications. Properties of cation exchange membrane depends on nature main polymer matrix, concentration of functional groups in the membrane matrix, homogenous distribution of charged functional groups, and hydrophobic-hydrophilic characteristics responsible for the phase separation to achieve the formation of ion-conducting channels and thus good ionic conductivity. For practical applications like fuel cell, water electrolyzers, electrodialysis and other related electro-membrane process, highly desired properties of cation exchange membrane are: ^ High permselectivity - cation exchange membrane should be highly permeable for the cation, but should be impermeable for anion. For ideal cation exchange membrane, permselectivity should nearly equal to unity. ^ High proton / cation conductivity – cation exchange membrane should exhibit high proton / cation conductivity, while electronic conductivity should be extremely minimum. ^ Good mechanical and form stability - the membrane should be mechanically strong and should have a low degree of swelling or shrinking in transition from dilute to concentrated ionic solutions. ^ High chemical stability - the membrane should be stable over a pH range from 0 to 14 and in the presence of oxidizing agents. Electric potential driven separation process utilizing cation exchange membranes, allow the selected transport of cations. During practical applications, scale formation or fouling on the membrane interfaces, can be avoided by periodic acid wash. Thus, cation exchange membranes should be highly stable or durable in acidic solutions. In addition, for any electro-membrane process like fuel cell or water electrolysis, cation exchange membranes are sandwiched between cathode and anode, where the environment is oxidative in nature due to formation of free radicals. Reference may be made to US7790837 wherein E.I. duPont DeNemours & Co (Dupont) under the trade name “Nafion” disclosed perflurosulfonic acid membranes, for elecrodialysis, electrolysis or fuel cell applications. Nafion®is a copolymer of tetrafluroethylene (TFE) grafted with sulfonyl fluoride vinyl ether (PSEPVE), exhibits excellent stabilities and ~0.1 S / cm conductivity. Reference may be made to US4324636 wherein Asahi Chemical Industry disclosed commercial cation exchange membrane, made by either styrene-divinyl benzene copolymer or perflurocarboxilic acid polymers different plasticizers (high density polyethylene, polyvinylidene fluoride or polyvinylfluoride). Reported cation exchange membrane exhibits low mass (fluid) transport, and good conductivity during electro-membrane applications such as electrodialysis, water electrolysis, and PEM fuel cells. Reference may be made to EP2470290A1wherein commercial electrodialysis membranes are produced by polymerizing one or more monofunctional ionogenic monomers with at least one multifunctional monomer in the pores of a porous substrate. Reported low operating cost electrodialysis membranes with low electrical resistance and high permselectivity are useful in the desalination of water and purification of waste water. Reference may be made to US8758954B2 wherein membranes weakly acidic or weakly basic groups are prepared by: applying curable composition to a support; curing the composition for less than 30 seconds to form a membrane; and optionally removing the membrane from the support. Crosslinking agent with at least two acrylic groups, and membranes are particularly useful for reverse electrodialysis. References may be made to articles Desalination, 1988, 46, 327 and J. Membrane Science, 1999, 156, 61 wherein sulfonation of poly sulfone or polyethersulfone was achieved to prepare stable cation exchange membrane. Reference may be made to US4625000 wherein sulfonation of poly(arylene ether sulphone) Udel P-1700 (PSU) and Vitrex PES 5200P (PES) was achieved on the aromatic ring system using chlorosulfonic acid as a sulfonating agent. Reference may be made to US3709841 wherein sulfonation of polyether sulfones was successfully achieved by chlorosulfonic acid or oleum in an inert solvent (chlorinated hydrocarbons). Reference may be made to US-5087345-A wherein stable cation exchange membrane was reported by reinforcing at least one layer of a fluorocarbon polymer on a woven fabric followed by coating inorganic material. Coating containing inorganic particles uniformly and strongly adheres with the surface for a prolonged period of time. Prepared membrane was used for the electrolysis of an alkali metal chloride at a low cell voltage. Reference may be made to US5203982A wherein cation exchange membranes are prepared by homogeneous solutions comprising at least one water soluble polymerizable monomeric onium styrene sulfonate McKee type salt and / or substantially water soluble monomeric, polymerizable derivative thereof. Reported membranes were especially used for electrodialysis. Reference may be made to US5013765 wherein sulfonation of aromatic polymer was achieved by sulfur trioxide in concentrated sulfuric acid, using a solvent. Prepared cation exchange membranes were often unstable in strong oxidative, acid and base environments. Temperature and mechanical instabilities of these membranes limit their durability and practical applicability. Reference may be made to JP2002348389A wherein fluoropolymer based cation exchange membrane is prepared by a low cost procedure and comprises a long- chain branched polytetrafluoroethylene structure and has oxyhydrofluorocarbon side chains having sulfonic acid groups, represented by [-OCH2 CF2 CF2 SO3H], and bonded thereto. The membrane exhibited 0.5-2.0 meq / g ion-exchange capacity and 0.05-0.25 S cm-1ionic conductivity at 25oC. Reference may be made to US-4983264-A wherein cation-exchange membrane was made using one or more fluorinated polymer with cation-exchangeable groups in water miscible organic solvent. Reference may be made to US-4,661,218 wherein fluoropolymer based cation exchange membrane was prepared comprising at least three integrally laminated layers, in which main layer was made of a perfluorocarbon polymer with carboxylic acid groups, was reported with 1.4 to 2.0 meq / g dry resin ion exchange capacity, depending on the membrane thickness or cation exchange resin volume. References may be made to an article Journal of New Material Electrochemical System, 1998, 1, 47 wherein inventors disclosed preparation method of fluorinated cation exchange membranes by the polymerization of monomers and further functionalization. Reported cation exchange membrane was suitable for the chlor- alkali industries and fuel cell application. References may be made to articles Progress Polymer Science, 2004, 29, 75; and 2005, 30, 644 wherein inventors disclosed preparation of perfluorinated membranes by co-polymerization of tetrafluoroethylene with the vinylether monomer by Dow Chemical Company. Perfluorinated cation exchange membrane also has also been commercialized as “Flemion” by the Asahi Glass Company. Reference may be made to European patent application number 0025644 B1 wherein cation exchange membrane made of a fluorinated copolymer produced by copolymerizing a fluorinated olefin such as CFZ=CFZ and a fluoro- vinyl either having a sulfonic acid group or a functional group convertible to sulfonic group, such as and hydrolyzing the fluorinated copolymer to convert -SO2F groups into - SO3H groups. These cation exchange membranes were used for electrolysis of an aqueous solution of an alkali metal chloride due excellent acid and alkali resistance. Modification of polymer or grafting of functional groups also offers versatile route to architect the polymer wettability, morphology, and performance due to simple preparative route and thus cost. Physical blending of conducting polymer with non- conducting polymer matrix, hydrophilicity and conductivity of the resultant material can be improved, but membrane heterogeneity and leaching-out of conducting polymer are serious concerns. Reference may be made to an article J. Membrane Science, 2007, 294, 8 wherein inventors disclosed good chemical, mechanical, and thermal stability of partially fluorinated polyvinylidene fluoride co-hexafluoropropylene (PVDF-co-HFP). Due to partial fluorinated nature, PVDF-co-HFP was used prepare cation exchange membrane after grafting or addition of acidic functional groups, which impart the ionic / proton conductivity. References may be made to the articles Fuel Cells 2006, 6, 331, Eur. Polym. J. 2008, 44, 932, J. Power Sources 2003117, 14 and J. Power Sources 2006, 154, 51, wherein inventors have disclosed chemical grafting of acidic functional group to PVDF-co-HFP, and blending of PVDF-co-HFP with various hydrophilic conducting materials such as Nafion™ or hydrophilic inorganic particles. Reference may be made to an article ACS Appl. Mater. Interfaces 2015, 7, 28524 wherein inventor disclosed chemical grafting of 2-acrylamido-2-methyl-1- propanesulfonic acid with PVDF-co-HFP by radical polymerization in presence of initiator to prepare the cation exchange membrane. Reported cation exchange membrane showed relatively low conductivity due limited grafting density, which limits membranes applications. The majority of partial fluorinated polymer based cation exchange membranes are prepared by blending of organic polymer matrices with sulfonic-acid, aromatic and / or acid-base functional groups or containing inorganic particle additives, such as zeolites and silica. Generally, cation exchange membranes are made by sulfonating aromatic polymers. These membranes are unstable under strong oxidative, and acidic conditions, similar to operating environment of fuel cell, or membrane electrolysis. Further, thermal and mechanical instabilities of these membranes are serious concerns. Membrane instability or low durability are also the inherent limitations of cation exchange membrane, and responsible for complication of preparative procedures, and high membrane cost, useful of electro-dialytic separation processes and membrane based energy devices. Thus, there is an urgent requirement for the development and commercialization of partially fluorinated acidic and oxidative stable cation exchange membranes with excellent physicochemical and electrochemical properties. In addition, use of hazardous chemicals during grafting of functional groups is also an serious problem, Therefore, macromolecular modification of low cost partial fluorinated polymer, simple preparation methodology without the use of hazardous chemicals has been explored to achieve the goal for the preparation of cation exchange membrane for electro-dialytic separation processes and membrane based energy devices such as membrane electrolyzers, fuel cell, and conducting separator for the redox flow batteries. SUMMARY OF THE INVENTION The present invention relates to a dual functionalized fluorinated polymer based stable cation exchange membrane for electrochemical process; comprising a sol-gel matrix comprising a) sulfonated organosilicon compound and b) functionalized monomer grafted fluorinated polymer with a molar ratio between 1:5 to 1:15; Characterized in that the said cation exchange membrane is stable in harassed acidic and oxidative conditions and possesses excellent thermal and mechanical stabilities. Wherein dual functionalized group are selected from sulfonic acid, phosphonic acid and combination thereof. Wherein sol-gel membrane matrix comprises of 50 to 90 mol % of functionalized monomer grafted fluorinated polymer and 10 to 30 mol% of sulfonated organosilicon compound and the sulfonated organosilicon compound is selected from (3-mercaptopropyl trimethoxysilane), (3- mercaptopropyl triethoxysilane) and (3-mercaptopropyl tripropoxysilane). In an another embodiment, the present invention a dual functionalized fluorinated polymer based stable cation exchange membrane is in the form of film of the desired thickness using suitable casting process for unsupported membrane or reinforced type of membrane using suitable woven type of fabric, wherein cation exchange membrane is having membrane thickness between 10 to 250 µm and with or without any suitable support such as woven type of fabric. More specifically, the present invention relates to the preparation of dual functionalised partial fluorinated polymer based stable cation exchange membrane. Method for the preparation of cation exchange membrane homogeneous in nature having formula (I) as repeating unit involve the following steps: . More specifically, the present invention of membrane matrix having formula (I) is dual functionalised partial fluorinated polymer based stable cation exchange membrane wherein sulphonated dehydrofluorinated polyvinylidene fluoride (DPVDF) crosslinked wo silica moiety, both are grafted by –SO3H group. In accordance to present invention, the functionalized monomer grafted fluorinated polymer comprises compound of co-polymerization of: a) reactive and hydrophilic acrylic monomer, and b) partially dehydro- fluorinated polyvinylidene fluoride polymer. Wherein, the reactive and hydrophilic acrylic monomer is selected from 2-Acrylamido-2- methylpropane sulfonic acid, 2-Acrylamido-2-methylpropane phosphonic acid or combination thereof. The compound of co-polymerization of: a) reactive and hydrophilic acrylic monomer, and b) partially dehydro-fluorinated polyvinylidene fluoride polymer is represented by formula (II): Therefore, fluorinated acidic and oxidative stable cation exchange membranes as disclosed in present invention has an ion exchange capacity of 0.50 to 1.50 meq / g and the ionic conductivity of membrane varies 3.0 to 8.00×10-2S cm-1. Further, the membrane swelling ratio has been improved and under treatment of hot water at 60 °C for 24 hours the swelling ratio ranges from 5-15% and membrane water uptake ranges from 20 to 35% by wt. As per one more embodiment, the fluorinated acidic and oxidative stable cation exchange membranes membrane of present invention has stability in acidic environment and therefore ion exchange capacity and conductivity loss is less than 7.0% under highly acidic i.e.10 M H2SO4at 30oC for 120 hours environment. Further, the present invention also provides a process for the preparation of dual functionalized fluorinated polymer based stable cation exchange membrane for electrochemical process having general formula (I) mentioned above, comprising of steps: Step i) Reacting polyvinylidene fluoride in solvents with alkali at temperature between 15 to 40oC to obtain partially dehydrofluorinated polyvinylidene fluoride, wherein alkali optionally is selected from non- limiting agents like sodium hydroxide, potassium hydroxide or similar thereof. Step ii) precipitating out partially dehydrofluorinated polyvinylidene fluoride in water, Step iii) co-polymerization by reacting partially dehydrofluorinated polyvinylidene fluoride and 2-acrylamido-2-methyl-1-propansulfonic acid in 1:5 to 1:15 molar ratio using Azobisisobutyronitrile as radical initiator in presence of solvent and under nitrogen atmosphere at temperature 20-80oC to obtain co-polymerized sulfonic acid functionalized monomer grafted fluorinated polymer represented by formula (II): Step iv) Sol-gel preparation for matrix by mixing ((3-mercaptopropyl) trimethoxysilane) in step iii) in presence of acid catalyst at temperature between 40 to 90oC, Step v) casting of membrane film by drying the step iv) mixture at temperature between 20 to 50oC. Step vi) Dipping cast membrane in Hydrogen peroxide solution for time between 0.5 to 10 Hr to oxidize thiol group to sulfonic acid and to obtain a sol-gel membrane matrix represented by formula (I): Wherein, in the step iv) for Sol-gel preparation concentration selected between 10 to 30 mol% of ((3-mercaptopropyl) trimethoxysilane) and 50 to 90 mol % of co- polymerized sulfonic acid functionalized monomer grafted fluorinated polymer represented by formula (II). And a suitable mineral acid catalyst selected from hydrochloric acid , sulfuric acid, phosphoric acid, which can be used in concentration selected between 0.5 to 10% w / v of reaction mass. Further, non-limiting examples of solvents used in preparation protocol of membrane matrix having formula (I) are selected from dimethyl acetamide, isopropanol, N-methylpyrrolidone and combination thereof. In accordance to one more embodiment of present invention, it provides a simple process for the preparation of homogeneous and stable cation exchange membrane having formula (I) as repeating unit comprising the following general steps: i. In a vessel, desired amount polyvinylidene fluoride was dissolved in dimethyl acetamide under continuous stirring at room temperature (>15oC), wherein polyvinylidene fluoride and dimethyl acetamide are mixed in any desired ratio; ii. clear solution obtained in step (i) was charged with 0.01-0.25 molar NaOH solution in isopropanol until the solution becomes dark brown colour followed by continuous stirring for >1 / 2 hour; iii. obtained solution of step (ii), was precipitated in water, and resultant dehydroflourinated PVDF was filtered, washed with water to remove excess of NaOH, and dried; iv. dehydroflourinated PVDF obtained in step (iii) was dissolved in N- methylpyrrolidone (NMP) at 20-40 °C in a vessel, and 2-acrylamido-2-methyl-1- propansulfonic acid was added along with AIBN (free radical initiator) under nitrogen atmosphere at 30-80 °C with stirring, to obtain sulphonic acid grafted polyvinylidene fluoride; v. to the mixture, desired amount of silica precursor ((3-mercaptopropyl) trimethoxysilane) and few drops of HCl were added to the mixture under stirring at >50 °C; wherein the ratio of sulphonated dehydrofluorinated polyvinylidene fluoride (DPVDF) to silica precursor ranges from 1:5 to 1:15 by weight / molar. vi. cation exchange membrane prepared in step (v) was dried and dipped into H2O2 ( >1 hour) for oxidizing –SH group into –SO3H group, followed by washing with water. In yet another embodiment of the present invention, wherein polyvinylidene fluoride and dimethyl acetamide are mixed in the ratio >1.5:10 (w / v). In yet another embodiment of the present invention, wherein the ratio of sulphonated dehydrofluorinated polyvinylidene fluoride (DPVDF) to silica precursor ranges from 20-30% (w / w). In yet another embodiment of the present invention, wherein sulphonated dehydrofluorinated polyvinylidene fluoride copolymer and silica moiety both are grafted with –SO3H group. In yet another embodiment of the present invention, wherein partially fluorinated polymer (DPVDF) based stable cation exchange membrane was prepared by chemical grafting of 2-acrylamido-2-methyl-1-propanesulfonic acid or any other functionalized monomer with dehydrofluorinated polyvinylidene fluoride. In yet another embodiment of the present invention, wherein the co-polymerization of dehydrofluorinated polyvinylidene fluoride (DPVDF) and 2-acrylamido-2-methyl- 1-propanesulfonic acid (AMPS) or any other sulfonic acid functionalized monomer by free radical copolymerization in the presence of initiator. In yet another embodiment of the present invention, wherein dehydrofluorination of PVDF in dimethyl acetamide (DMAc) was achieved with NaOH solution in isopropanol to create the double bonds in PVDF responsible for chemical grafting of AMPS. In yet another embodiment of the present invention, wherein sulphonated dehydrofluorinated polyvinylidene fluoride was prepared by chemical grafting of AMPS or any other sulfonic acid functionalized monomer with DPVDF by free radical copolymerization in the presence of initiator. In yet another embodiment of the present invention, wherein improvement in the density of charged groups (sulphonic acid group) in the polymer matrix by inclusion of silica precursor (3-mercaptopropyl trimethoxysilane or any other) by acid catalysed sol-gel process followed by oxidation of mercapto group in to sulphonic acid group using any oxidising agent (such as hydrogen peroxide). In yet another embodiment of the present invention, wherein homogeneous dispersion of silica precursor within sulphonated DPVDF polymer by strong hydrogen bonding. In yet another embodiment of the present invention, wherein chemical grafting of 2-acrylamido-2-methyl-1-propanesulfonic acid or any other phosphonic (-PO3H2) acid functionalized monomer with DPVDF polymer matrix. In yet another embodiment of the present invention, wherein silica precursor may be functionalized with –SO3H, or –PO3H2 or both groups for significantly improvement in membrane conductivity. In yet another embodiment of the present invention, wherein the membrane has an ion exchange capacity of 1to 1.4 meq / g. In yet another embodiment of the present invention, wherein the membrane has conductivity of 4 to 6×10-2S cm-1. In yet another embodiment of the present invention, wherein membrane swelling ratio (under treatment of hot water at 60°C for 24 hours) ranges from 5-15%. In yet another embodiment of the present invention, wherein membrane ion exchange capacity and conductivity loss is less than 4.5% under highly acidic (10 M H2SO4 at 30oC for 120 hours) enviroment. In yet another embodiment of the present invention, wherein copolymer (DPVDF) with following chemical structure was subjected to the functionalization by grafting of sulphonic acid functionalized monomer (for example AMPS): In yet another embodiment of the present invention, wherein cation exchange membrane based on partial fluorinated DPVDF copolymer grafted with AMPS having the chemical structure:

[0002] Fo DPVDF In yet another embodiment of the present invention, wherein sulphonated dehydrofluorinated polyvinylidene fluoride copolymer contains at least one –SO3H or –PO3H2group, while other functional group are attached with silica moiety, as described in the following chemical structure formula (I): In yet another embodiment of the present invention, wherein homogeneous cation exchange membrane made by partially fluorinated sulphonated DPVDF copolymer and functionalized silica precursor attached via strong hydrogen bonding. In yet another embodiment of the present invention, wherein cation exchange membrane with high charge density due to grafting of charged groups (–SO3H or – PO3H2) exhibited excellent physicochemical and electrochemical properties along with good stabilities (chemical, mechanical, and thermal). In yet another embodiment of the present invention, wherein cation exchange membrane according to the present invention showed high efficiency during electrolysis, electrodialysis, or other electro-membrane applications, and superior stabilities (thermal and acid). Thus, the industrial significance of the cation exchange membrane of the present invention is extremely high. BRIEF DESCRIPTION OF THE DRAWINGS For a more detailed understanding of the present invention, reference is made to the following description taken in conjunction with the accompanying drawing. It is to be understood; however, each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. FIGURE NO.1 is a schematic illustration of chemical reaction for the preparation of dual functionalised partial fluorinated polymer based stable resistant cation exchange membrane as per present invention. FIGURE NO. 2 depicts two-compartment membrane cell for the study of i-V characteristics of present invention disclosed cation exchange membrane. FIGURE NO. 3 shows Current-voltage (i-V) curve of stable cation exchange membrane (150 g functionalized PVDF + 30 ml MPTS) for present invention disclosed cation exchange membrane . FIGURE NO.4 shows schematic presentation of electrodialysis process for water desalination using present invention disclosed cation exchange membrane. DETAILED DESCRIPTION OF THE INVENTION Present invention concerns with the development of functionalised fluorinated polymer based cation exchange membranes for electrolysis, redox flow battery, fuel cell, electrodialysis and other electro-membrane applications, including electro- driven separation, and electrochemical energy devices. Cation exchange membrane reported in this invention exhibits excellent stabilities (chemical, mechanical, and dimensional), extremely low swelling ratio, high permselectivity, ionic conductivity, ion-exchange capacity, moderate water content, and these are the essential requirements for any commercial cation exchange membrane for diversified energy and water applications. Preparative protocols adopted for the preparation of cation exchange membrane are very simple, scalable, and less expensive. These attractive features will significantly contribute to economy, and cost of electro-membrane processes including electrodialysis and various energy devices. In the present invention, preparative protocols for the cation exchange membrane include, dehydro-fluorination under treatment with alkali of PVDF by introducing unsaturation, free radical copolymerization of dehydro-fluorinated PVDF (DPVDF) and 2-acrylamido-2-methyl-1-propansulfonic acid (AMPS) in presence of free radical initiator 2,2-azobis(2-methylpropionitrile (AIBN), and acid catalysed sol-gel processing of obtained viscous solution with drop-wise addition of silica precursor (3-mercaptopropyl) trimethoxysilane) (MPTS) in acidic media (of few drops of HCl). The degree of grafting of AMPS with DPVDF, i.e. molality of –SO3H group in the polymer matrix, was tuned and finally controlled by stoichiometric concentration of AMPS in the polymer matrix. It was observed that with increase in AMPS (more than stoichiometric) concentration, –SO3H group grafted DPVDF membrane showed restricted electrochemical properties, and ionic conductivity, necessary for high performance cation exchange membrane. Further, these membrane properties were enhanced by inclusion silica precursor (MPTS) via acid catalysed sol-gel process to achieve homogenous membrane forming polymeric solution. Obtained thin film membrane contained mercapto groups, which were oxidized to –SO3H group by oxidizing agent (H2O2). Obtained cation exchange membrane was homogeneous in nature due to hydrogen bonding between MPTS and DPVDF. Preparation protocols of cation exchange membrane are eco-friendly in nature without any requirement of hazardous chemicals or sophisticated equipment. Price of commercial chemicals, simple membrane forming procedure, confirmed cost- effective nature of the process and membrane. In present invention, chemical stable cation exchange membrane was prepared by co-polymeriztion DPVDF with varied amount of AMPS (12-24 mmol) in presence of free radical initiator (AIBN). To prepare homogeneous cation exchange membrane, MPTS was introduced in the polymer matrix by acid catalyzed sol-gel process, and prepared membrane forming material possess formula (I), as repeating unit in the polymer matrix. In present invention and referring to figure no.1 reaction scheme, the process for preparation of chemical stable cation exchange membrane was prepared by co- polymeriztion DPVDF with varied amount of AMPS in presence of free radical initiator (AIBN) comprising of following steps: i. In a vessel, desired amount polyvinylidene fluoride (PVDF) was dissolved in dimethyl acetamide under continuous stirring at room temperature (>15 oC); ii. clear solution obtained in step (i) was charged with 0.05 molar NaOH solution in isopropanol until the solution became dark brown colour followed by continuous stirring for >0.5 hour; iii. obtained solution of step (ii), was precipitated in water, and resultant dehydroflourinated PVDF (DPVDF) was filtered, washed with water to remove excess of NaOH, and dried; iv. dehydroflourinated PVDF obtained in step (iii) was dissolved in N- methylpyrrolidone (NMP) at room temperature (>15oC) in a vessel, and 2-acrylamido-2-methyl-1-propansulfonic acid (AMPS) was added along with AIBN (free radical initiator) under neutral atmosphere at 60 + / -20 oC in stirring for 4-8 hours, to obtain sulphonic acid grafted polyvinylidene fluoride (Intermediate I), refer as compound having Formula (II); v. to the mixture, desired amount of silica precursor ((3-mercaptopropyl) trimethoxysilane) and few drops of HCl were added and mixture was stirred for another 1-4 hours at >70oC resulting into formation of matrix of Intermediate-II; vi. obtained viscous solution was transformed into thin film of the desired thickness by doctor blade casting method on a glass plate; vii. cation exchange membrane prepared in step (vi) was dried and dipped into H2O2 ( >1 hours) for oxidizing –SH group into –SO3H group, followed by washing with water to obtain a final cation exchange membrane product having chemical structure formula (I). In another embodiment the present invention the process for preparation of chemical stable cation exchange membrane comprising following steps: i. In a vessel, desired amount of PVDF was dissolved in dimethyl acetamide under continuous stirring at room temperature (>15oC); ii. to the solution obtained in step (i), NaOH solution (0.01-0.25 M) was added under continuous stirring (>1 / 2 hour). Solution colour turns to dark brown; iii. resultant solution of step (ii), was precipitated in water, and obtained dehydroflourinated PVDF was filtered, washed, and dried; iv. dehydroflourinated PVDF obtained in step (iii) was dissolved in NMP at room temperature (>15oC) in a vessel, and 2-acrylamido-2-methyl-1- propansulfonic acid (AMPS) was added along with AIBN (free radical initiator) under inert atmosphere at 30-80oC with stirring, to obtain sulphonic acid grafted polyvinylidene fluoride; v. to the mixture, desired amount of silica precursor ((3-mercaptopropyl) trimethoxysilane) and a few drops of HCl were added and mixture was stirred for another 1-4 hours at >50oC; vi. obtained viscous solution was transformed into thin film of the desired thickness by doctor blade casting method on a glass plate; vii. cation exchange membrane prepared in step (v) was dried and dipped into H2O2 for oxidizing –SH group into –SO3H group, followed by washing with water. To the sulphonic acid grafted polyvinylidene fluoride solution in NMP (10-30%, w / v), MPTS silica precursor (10-30% w / w of DPVDF) was added along with a few drops of HCl under constant stirring at 80+ / -10oC for acid catalysed sol-gel processing. Viscous solution was cast as thin film of desired thickness with the help of doctor blade and dried at 70-90oC for >2 hours. Obtained thin film membrane was socked in H2O2 (30% aqueous) for >2 hours to oxidize the –SH group into – SO3H group. Reaction scheme for the preparation of membrane forming material of partial fluorinated cation exchange membrane is illustrated in FIG.1. As important embodiment, reported stable cation exchange membrane preparation process is novel in nature over previously reported procedures for low cost membrane. Significantly, process does not involve any hazardous chemicals or methods. As significant novelty of the reported process, to improve the molality of –SO3H groups in the membrane matrix, we strategically incorporated silica precursor by acid catalysed sol-gel method. The -SH group of the MPTS was oxidized to –SO3H group using strong oxidizing agent (H2O2), which further further increased the amount of the charged functional group (–SO3H) in the membrane matrix, whereas homogeneity of the membrane was retained. In the present invention, reported cation exchange membrane was prepared without any support, and showed excellent thin film forming capability along with high mechanical stability and burst strength of the membrane. Further, with suitable support of woven fabric, reported membrane can be prepared without deterioration in the properties. In addition, different preparative steps for the membrane preparation were achieved either at room temperature (>15oC) or at 60-90oC using free radical initiator. Thus membrane protocols require low energy, without any energy intensive source. Various electrochemical membrane processes with cation exchange membrane separator, require low consumption of electrical energy, because of excellent ionic conductivity and stability of the membrane in harassed (strong acidic or oxidative) environment. Reported cation exchange membrane may be widely used for water electrolysis, electrodialysis, fuel cell, redox flow battery, electrochemical membrane reactor, or any other electrochemical applications. Further, water electrolysis, chlor-alkali process, fuel cell, and redox flow battery applications are based on selective proton / ionic-transport, and utilization of reported cation exchange membrane also may be preferred. Water content in the membrane phase is generally governed by hydration of charged functional groups (-SO3H) or hydrophilic segments, which provide necessary sites for water molecules accommodation, and thus formation of ion conducting channels. This is very critical parameter for the high performance negatively charged membrane. In addition, high water content also may be responsible dimensional instability of the membrane. Thus, proper tuning of water content in the membrane phase, ionic conductivity, and dimensional stability is required. It was observed that optimized water content (20-30%, w / w) is necessary. In the present invention, by tuned hydrophobic-hydrophilic segments in the membrane forming material, we achieved desired level of water content in the membrane phase. The ion exchange capacity (IEC) of a membrane is a measure of fixed charged group concentration in the membrane matrix, and expressed by by meq. / g of dry membrane. This means, x meq. / g of dry membrane contains 1 mmol concentration of fixed charged groups exchangeable to ions or protons. Classical acid-base titration method was used for the estimation of IEC. In other words, concentration of fixed exchangeable charged groups also controls membrane quality in terms of ionic conductivity and hydrophilic nature. In addition, membrane stability and durability depends upon the nature main polymer chain, side grafting of functional charged groups. These structural features were judicially considered during designing of membrane synthesis. Skilled person based upon prior art, knowledge and description, as provided above can prepare membrane, and determine all parameters, which are essential requirement for high performance cation exchange membrane. The membrane permselectivity explains extent of counter-ion transport across the membrane. Membrane permselectivity may be estimated with the help of counter- ion transport number obtained by membrane potential measurement under certain concentration gradient. Cation exchange membrane also may be assessed by ionic conductivity measurement using a potentiostat / galvanostat impedance analyzer. The membrane resistance was determined in terms of membrane conductivity by Nyquist plots by Fit and Simulation method considering membrane thickness and area. For these wide applications, the necessary requirement of successful high performance cation exchange membranes are: ^ High perm selectivity — cation exchange membrane should be high permeable to cation (counter-ion) and impermeable to anion (co-ion). ^ Low electrical resistance — cation exchange membrane should possess low electrical resistance, and thus less potential drop during commercial applications. ^ Good mechanical stability — membrane should have high mechanical stability along with low degree of swelling or shrinking. This is essential for the preparation of membrane electrode assembly, membrane durability. ^ High chemical stability— membrane should exhibit excellent stability over 0-14 pH range in the presence of oxidizing agents. In general for any desired commercial applications, permselectivity of cation exchange membrane should close to one, membrane ionic conductivity should >4.0×10-2S cm-1, membrane should be mechanically strong with low degree of swelling or shrinking, membrane should be stable in strong acidic or alkaline environment even in presence of oxidizing agents. Many cation exchange membranes reported earlier showed poor stabilities (thermal, chemical and mechanical) under harassed environment, and are unsuitable for commercial exploitations. Utilization of fluorinated polymer main chain provides chemical, thermal and mechanical stability to the cation exchange membrane, while side chain grafting of charged functional groups avoids the deterioration in membrane functional nature, and thus ionic conductivity under harassed environment. Further incorporation of silica matrix, not only contributes towards functional charged membrane properties, and ionic conductivity, but also improves the water holding capacity (bound water content) of the membrane matrix. Improved bound water content avoids membrane dehydration and thus retained the ionic conductivity even at elevated temperature (> 100oC). In addition, mechanical strength of the cation exchange membrane may be further improved by reinforcing the membrane making polymer solution on woven type of fabric (PVC, PE, glass or Teflon) as support. In the following examples, only preferred embodiments of the invention are specifically described, and it will be appreciated that variations in compositions and other external parameters of the invention are possible, without departing from the spirit and intended scope of the invention. EXAMPLES Following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention. Examples 1-3 General procedure for the preparation of fluorinated polymer based stable cation exchange membrane 150 gram of PVDF was slowly added to a beaker containing dimethylacetamide (1000 ml) under continuous stirring at 30oC. In a separate beaker 200 mg NaOH (0.05 M) was dissolved in 100 ml isopropanol. Then alkali solution was added drop- wise to the PVDF solution under vigorous stirring at 30oC for 1 hour. The colorless solution became dark brown colored. To precipitate the dehydrofluorinated PVDF (DPVDF), mixture was poured in excess of distilled water (5.0 liter), precipitate was filtered, washed 3-4 times with distilled water, and dried at 80oC in an oven. In next procedure, free radical co-polymerization of DPVDF with 2-acrylamido-2- methyl-1-propansulfonic acid (AMPS) was achieved in the presence free radical initiator. In a typical method, 150 gram DPVDF (3.41 mol) was added to NMP (900 ml), and 111 gram AMPS (0.54 mol) was also added along with 1.20 gram 2,2′- azobis(2-methylpropionitrile) (AIBN) (free radical initiator) to the reaction vessel. Reaction mixture was stirred for 6 hours at 60oC. Then, desired amount of (3- mercaptopropyl) trimethoxysilane (MPTS) (according to Tab.1) was added to the reaction mixture along with hydrochloric acid (15 ml). Then the temperature of reaction mixture was increased to 80oC, and reaction mixture in constantly stirred for 2 hours under neutral atmosphere. Afterwards, obtained viscous solutions of different compositions were transformed into thin film of the desired thickness with the help of doctor blade on the surface of cleaned glass plate, and dried at 70oC, till complete dryness. Dried thin film membrane was peeled off in distilled water, and washed with distilled water. Then resultant membrane was dipped in hydrogen peroxide solution (30%, v / v) for 3 hours at 30oC. Obtained membrane was conditioned by equilibration in HCl (1.0 M), followed by washed with distilled water, for the further application. Properties of cation exchange membranes (of different compositions) were measured and included in Table 1. Table-1 Composition and properties of stable fluorinated polymer based stable cation exchan e membrane 0- 3 33%, w / v (150 g PVDF-33 37.64 1.29 5.78 functionalized P DF 50 l E xamples 4-6 General procedure for acid stable fluorinated polymer based stable cation exchange membrane During membrane operation in strong acidic conditions, electrodialysis of acidic solution, water electrolysis for hydrogen production, or electrochemical Bunsen process (iodine-sulfur cycle) for water splitting, etc. acid stability of a cation exchange membrane is a necessary requirement for the process durability. Acid stability of fluorinated polymer based stable cation exchange membrane was studied by acid treatment of the membrane samples in sulfuric acid (10 M) at 30oC for 120 hours. Acid stability for cation exchange membranes with varied MTPS content was recorded as percentage loss in membrane weight, ion exchange capacity, and ionic conductivity values after the treatment with sulfuric acid and included in Table 2. Table-2 Acid stability test: percentage loss in membrane weight (ϕloss) , ion exchange capacity (IECloss), and ionic conductivity (^lmoss) after acid treatment for cation exchange membranes with varied MPTS content Example MPTS content in ϕlossIECloss^ m sulphonic acid graftedlossPVDF solution (%) (%) (%) (%,w / w) 4 20%, w / v (150 g 1.8 2.3 3.9 functionalized PVDF + 30 ml MPTS) 5 25%, w / v (150 g 2.1 2.6 4.2 functionalized PVDF + 38 ml MPTS) 6 33%, w / v (150 g 2.6 3.0 4.9 functionalized PVDF + 50 ml MPTS) Examples 7-9 General procedure for oxidative stable fluorinated polymer based stable cation exchange membrane Under harassed acidic and oxidative water splitting environment, oxy active radical attack on the membrane matrix is a serious problem, which causes membrane degradation and reduces the durability during practical operation. Oxidative stability of reported membranes was studied in simulated oxidative environment (3 ppm FeSO4 + 3% H2O2 at 80 °C for 3 h), by recording percentage loss in weight, ion exchange capacity, and conductivity of the membrane after oxidative treatment and included in Table 3 for cation exchange membranes with varied MPTS content. Table-3 Oxidative stability test: percentage loss in membrane weight (ϕloss) , ion exchange capacity (IECloss), and ionic conductivity (^lmoss) after acid treatment for cation exchange membranes with varied MPTS content Example MPTS content in sulphonic ϕlossIECloss (%)^m(%)acid grafted PVDF solutionloss(%,w / w) (%) 7 20%, w / v (150 g 2.9 3.4 4.3 functionalized PVDF + 30 ml MPTS) 8 25%, w / v (150 g 3.5 3.7 4.7 functionalized PVDF + 30 ml MPTS) 9 33%, w / v (150 g 3.8 4.2 4.9 functionalized PVDF + 30 ml MPTS) Example 10 General procedure for current-voltage behaviour of stable fluorinated polymer based stable cation exchange membrane The accompanying figure no.2 depicts two-compartment membrane cell for the study of i-V characteristics, wherein 1: Feed solution reservoir; 2: Peristaltic pumps; 3: Anode; 4: Cathode; 5: Calomel electrode connected with salt bridges; 6: Cation exchange membrane; 7: DC power supply. The current–voltage (i–V) curve for cation exchange membrane (150 g functionalized PVDF + 30 ml MPTS) with 4.0 cm2effective area was recorded in equilibration with NaCl solution (0.10 M) in a two-compartment cell. Desired current was imposed in steps across the electrodes (precious metal oxide (Pt, Ru and Rh) coated (6.0 μm) titanium sheets (Titanium Tantalum Products, Chennai, India), and resultant potential across the membrane was recorded using salt bridges and saturated calomel electrodes connected with digital multi-meter (accuracy: 0.001 mV). Experiments were performed under dynamic conditions, by feeding both compartments with fresh experimental solution using pumps (50 cm3 / min). Typical current-voltage characteristics of stable fluorinated polymer based stable cation exchange membrane (150 g functionalized PVDF + 30 ml MPTS) are described in FIG.3. Example 11 Desalination of saline water by electrodialysis using fluorinated polymer based stable cation exchange membrane Saline water with or without chlorine dosing (total dissolved solid: < 5000 ppm), can be generally desalinated by electrodialysis to provide potable water. Generally, saline water obtained either from surface or from ground, is disinfected by chlorination before desalination. In addition, electrodialysis is also used for the separation of non-ionic organic and electrolytes. The accompanying figure no. 4 illustrates the schematic presentation of electrodialysis process for water desalination. An electrodialysis (ED) unit was fabricated with 10 cell pairs of fluorinated polymer based stable cation exchange membrane (25%, w / v (150 g functionalized PVDF + 30 ml MPTS)) and interpolymer anion-exchange membrane (IPA) (interpolymer of polyethylene styrene–divinylbenzene copolymer thin film with quaternary ammonium groups) with ion exchange capacity: 0.9 meq / g; water uptake: 15%, area resistance: 3.2 Ω cm2) with 100 cm2effective membrane area was used desalination of brackish water. There was two electrode wash (EW), concentrated (CC) and desalinated compartments (DC) (FIG.3). Precious metal oxides (titanium-ruthenium-platinum) coated TiO2 sheets of 6.0 mm thickness were fitted in the end plate of the electrodialysis unit as cathode and anode. There was in-built parallel-cum-series flow arrangement, and flow of each compartment was monitored separately. Both EW compartments were interconnected, and recirculated with Na2SO4solution (0.10 M). Initially, saline water (total dissolved solid (TDS): 4000-5000 ppm) was passed through DC (flow rate: 1.0 LPH) and CC (flow rate: 1.0 LPH), using peristaltic pumps. During electrodialysis experiments, constant voltage (12.0 V) was imposed across the electrodes with the help of direct current power supply and corresponding current was recoded. With progress of experiment TDS of DC was reduced to < 500 ppm (drinking water as per World’s Health Organization), while TDS of CC was significantly increased. ADVANTAGES OF THE INVENTION The various advantages of the present product / process are given below. 1. Present invention of product / process reports partial fluorinated polymer based stable cation exchange membrane for diversified practical applications such as in electrodialysis, electrolysis reversal, water electrolysis, fuel cell, and storage redox flow batteries or other electro-membrane processes using cation exchange membrane as a separator 2. This invention also encompasses that reported cation exchange membrane, which is highly stable under acidic / oxidative conditions along with excellent thermal and mechanical stabilities.

Claims

CLAIMS We claim, 1. A dual functionalized fluorinated polymer based stable cation exchange membrane for electrochemical process; comprising a sol-gel matrix comprising a) sulfonated organosilicon compound and b) functionalized monomer grafted fluorinated polymer with a molar ratio between 1:5 to 1:15; Characterized in that the said cation exchange membrane is stable in harassed acidic and oxidative conditions and possesses excellent thermal and mechanical stabilities.

2. The dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 1, wherein dual functionalized group are selected from sulfonic acid, phosphonic acid and combination thereof.

3. The dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 1, wherein cation exchange membrane is having membrane thickness between 10 to 250 µm and with or without any suitable support such as woven type of fabric.

4. The dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 1, wherein membrane water uptake ranges from 20 to 35% by wt.

5. The dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 1, wherein membrane swelling ratio under treatment of hot water at 60 °C for 24 hours is ranges from 5-15%.

6. The dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 1, wherein membrane ionic conductivity of varies between 3.0 to 8.00×10-2S cm-1.

17. The dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 1, wherein membrane has an ion exchange capacity of 0.50 to 1.50 meq / g.

8. dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 1, wherein membrane ion exchange capacity and conductivity loss is less than 7.0% under highly acidic condition of 10M H2SO4 at 30oC for 120 hours environment.

9. The dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 1, wherein sol-gel membrane matrix comprises of 50 to 90 mol % of functionalized monomer grafted fluorinated polymer and 10 to 30 mol% of sulfonated organosilicon compound.

10. The dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 1, wherein sulfonated organosilicon compound is selected from (3-mercaptopropyl trimethoxysilane), (3- mercaptopropyl triethoxysilane) and (3-mercaptopropyl tripropoxysilane).

11. The dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 1, wherein sol-gel membrane matrix represented by formula (I):.

212. The dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 1, wherein functionalized monomer grafted fluorinated polymer comprises compound of co-polymerization of: a) reactive and hydrophilic acrylic monomer, and b) partially dehydro-fluorinated polyvinylidene fluoride polymer.

13. The dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 12, wherein reactive and hydrophilic acrylic monomer is selected from 2-Acrylamido-2-methylpropane sulfonic acid, 2- Acrylamido-2-methylpropane phosphonic acid or combination thereof.

14. The dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 12, wherein compound of co-polymerization of: a) reactive and hydrophilic acrylic monomer, and b) partially dehydro- fluorinated polyvinylidene fluoride polymer is represented by formula (II):

15. A process for the preparation of dual functionalized fluorinated polymer based stable cation exchange membrane for electrochemical process as claimed in claim 1, comprising of steps: Step i) Reacting polyvinylidene fluoride in solvents with alkali at temperature between 15 to 40oC to obtain partially dehydrofluorinated polyvinylidene fluoride, Step ii) precipitating out partially dehydrofluorinated polyvinylidene fluoride in water,3Step iii) co-polymerization by reacting partially dehydrofluorinated polyvinylidene fluoride and 2-acrylamido-2-methyl-1- propansulfonic acid in 1:5 to 1:15 molar ratio using Azobisisobutyronitrile as radical initiator in presence of solvent and under nitrogen atmosphere at temperature 20-80oC to obtain co-polymerized sulfonic acid functionalized monomer grafted fluorinated polymer represented by formula (II):Step iv) Sol-gel mixing ((3-mercaptopropyl) trimethoxysilane) in step iii) in presence of acid catalyst at temperature between 40 to 90oC, Step v) casting of membrane film by drying the step iv) mixture at temperature between 20 to 50oC. Step vi) Dipping cast membrane in Hydrogen peroxide solution for time between 0.5 to 10 Hr to oxidize thiol group to sulfonic acid and to obtain a sol-gel membrane matrix represented by formula (I):.

416. The process for the preparation of dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 15, wherein in step iv) for Sol-gel preparation concentration selected between 10 to 30 mol% of ((3-mercaptopropyl) trimethoxysilane) and 50 to 90 mol % of co-polymerized sulfonic acid functionalized monomer grafted fluorinated polymer represented by formula (II).

17. The process for the preparation of dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 15, wherein for step iv) a suitable mineral acid catalyst selected from hydrochloric acid , sulfuric acid , phosphoric acid.

18. The process for the preparation of / dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 15, wherein in step iv) for Sol-gel preparation the acid catalyst concentration is selected between 0.5 to 10% w / v of reaction mass.

19. The process for the preparation of dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 15, wherein solvents are selected from dimethyl acetamide, isopropanol, N- methylpyrrolidone and combination thereof.

20. The process for the preparation of dual functionalized fluorinated polymer based stable cation exchange membrane as claimed in claim 15, wherein in step i) the alkali for partial dehydrogenation of polymer is selected from sodium hydroxide, potassium hydroxide.5

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