|Title of Invention||
A COMPOSITE MEMBRANE FOR USE IN ELECTROCHEMICAL APPARATUSES AND PROCESSES
|Abstract||A composite membrane for use in electrochemical apparatuses and processes comprising two thin films of a resin of thermoplastic ion exchange polymer material, such as herein described, the films being of predetermined thickness to provide two. outer layers; a porous substrate of predetermined thickness, constituting a middle layer interposed between the two outer layers, to form a sandwich structure, said middle layer material being selected from (I) porous _felt of randomly oriented elass fibers or thin fiber glass mat of oriented fibers (2) porous non- woven/woven carbon (non conductive) tissue/mat (3) high temperature resistant polymeric woven mesh, resistant to acid -alkali conditions, the said two outer layers being embedded in the middle layer on either side of the said middle layer.|
|Full Text||This invention relates to a composite membrane for use in electrochemical apparatuses and processes
This invention proposes a solid polymer electrolyte composite possessing good ion conduction properties and mechanical strength for an electrochemical apparatus. The composites proposed herein have similar conductivities as the conventional pure membrane of similar thickness and are cost effective. They are suitable for polymer electrolyte membrane fuel celts and direct methanol fuel ceils.
Polymer electrolyte membrane fuel cells (PEMFC) are ideal sources for quiet, efficient and lightweight power. PEMFC's have a polymer electrolyte membrane disposed between a positive electrode (cathode) and a negative electrode(anode). The polymer electrolyte membrane is composed of an ion exchange polymer (i.e. ionomer).
The polymer electrolyte membrane plays a vital role in PEMFC's and has the important function
to conduct the protons, formed at the anode by interaction of the reactant and
metal catalyst, to the cathode where the oxidant gains electrons and reacts with
protons to form water
it serves as a separator for the two reactant gases
The electrochemical reaction occurs at each of the two junctions ( 3 phase boundaries) where one of the electrodes, elec. olyte (polymer membrane) and reactant gas interface. Hence, the ion exchange membrane should serve as a good proton transfer membrane and must have low permeability for reactant gases to avoid crossover phenomena that reduce performance of the fuel cell. This is especially important in fuel cell applications in which the reactant gases are under pressure and the cell is operated at high temperatures eg direct methanol fuel cells.
Conventionally, solid ion conducting membrane electrolytes for use in fuel ceils and other electrochemical devices are selected from commercially available
perfl uorosulphonic acid membranes sold under the trade names Nafion® ( E.I. Dupont de Nemours and Co.) and Aciplex®(Asahi chemical Industry, Japan) . For use in fuel cells the membranes of thickness of 170u.m or less are used. Despite their potential for many applications, PEMFC's have not yet been commercialized due to high overall cost of the membrane (US $800-1000 An2). An effective way to reduce the cost of these membranes is to use thinner membranes so that weight of the resin per unit area is lowered. Thin membranes also offer less resistance and hence better performances can be achieved. But thin membranes pose handling difficulties, durability and longevity can decrease and the liability of reactant gas crossover through the membrane increases leading to lower cell performances.
Incorporation of various inorganic fillers into the membrane to decrease fuel crossover is known. The problem of making thin membranes with inorganic fillers, to increase conductivity has however not been addressed. Ion exchange membrane in which silica is dispersed throughout the membrane is also known, but the use of microporous silica substrate has not been considered.
Also known is a process for making composite polymer with porous substrates like expanded poly(tetrafluoroethylene) and porous hydrocarbon substrate such as polyolefin with ion exchange polymer provided by perfluorosulphonic acid polymer. The microporous polymeric sheets have pores extending from one side to the another. These membranes have lower ionic conductivity than the conventional unreinforced perfluorosulphonic acid ion conducting polymers as they are very thin. Equally known is a method of formation of composite solid polymer electrolyte membranes which have a porous polymer substrate interpenetrated with an ion conducting material. The porous substrate consists of poly(benzazole) polymers like poly(benzoxazole), poly(benzothiazole)or poly(benzimidazole) and polyaramid polymer(Kevlar). The ion conducting polymer can be perfluorosulphonic acid or any hydrocarbon based polymer electrolyte. The use of other porous substrates has not been considered.
A method of preparing a composite membrane, comprising porous substrate of randomly oriented individual fibers with e .ibedded ion conducting polymer within the porous substrate for, use in fuel cells is documented. Free standing dimensionally stable composite membrane with greater handlability can be prepared using this procedure, which is based on the solvent casting technique. Woven glass mats and carbon mats (woven and non oriented mats) have not been considered. The procedure is suitable only for solvent casting technique where the polymer is available as the solution. The limitations of the process are those associated with solvent casting technique.
An object of the present invention is to overcome the disadvantages of the existing conventional pure and composite membranes. This invention provides a macro composite of solid polymer electrolyte with improved dimensional stability, handlability with similar ionic conductivity and gas permeability properties as the pure conventional polymer electrolyte. The said polymer composite membrane consists of a non woven felt made of randomly oriented fibers distributed in a binder by a wet lay process or thin woven mats. The randomly oriented fiber felt or the woven fabric mat acts as a matrix and the polymer electrolyte is embedded within the porous matrix. The fibers are oriented randomly in the X and Y direction in the felt. A typical felt may have a nominal mass of the felt with its randomly oriented fibers in range of 45-50g/m2. This fell has a minimum tensile strength of 2277N/m in the longitudinal direction and 700 N/m in the transverse direction. The thickness may be 0.01 mm or more. The woven glass mats can have weights ranging from 24-135 g/m and thickness of the mats ranging from 0.02-0.2 mm. The non woven carbon tissues may have thickness in the range of 0.08mm- 0.2 mm and a mass in the range of 7- 25 g/m".
An ion conducting polymer/ precursor in the melt processable form such as sulphonyl chloride( -SOiCI) or sulphonyl fluoridef-SOiF) are suitable for preparation of the composite membrane. The sulphonic acid membrane is obtained using conventional hydrolysis and ion exchange techniques.
The polymer composite^ are prepared as thin films by molding two thin films oi'ion exchange polymer :ri a thermoplastic form with the randomly oriented fiber felt in between the films at the melt processable temperature of the polymer electrolyte. The advantage of the present composite is that dimensionally -stable and hand lable polymer electrolyte membranes are obtained with lesser amount of polymer electrolyte than the conventionally available membranes of similar thickness. The membrane is amenable to contusions processing as the process for the invention involves melt processable polymer electrolyte and stable matrix as non-oriented fibers/oriented fibers in the telt form or woven mat.
'Hie composite membrane for use in electrochemical apparatuses and processes, according to this invention, comprises two thin films of a resin of thermoplastic ion exchange material, such a«, heroin described, the films of predetermined thickness to provide two outer layers; a porous substrate of predetermined thickness constituting a middle layer interposed between two outer layers, to form a sandwich structure, said middle layer material being selectee! Iroin (1) porous fiberglass tissue in non-woven felt made of randomly oriented eja^s fibers or porous w:>ven mats (2) porous non-woven carbon tissue (.') hi",h temperature resistant polymeric woven mesh, resistant fo acid-alkali conditions, the said two other outer layers being embedded in the middle layer on either wide of said niddle layer
The resin of thermoplastic ion exchange polymer material is perfluorosulphonic acid in sulphonyl fluoride form.
Briefly, the process of making an ion-exchange membrane of the present invention comprises
(1) formation ofihiu films of the ion exchange polymer in its thermoplastic form by extrusion, compression moulding, solvent casting or any other suitable technique for formation of thiu films
(2) formation of a sandwich structure using two films of ion exchange membrane and fiberglass tissue prepared as a non-woven felt made of randomly oriented glass fibers/thin porous glass woven mat my be used.
(3) Other materials that can be used to form sandwich structure are the porous carbon tissue that are made from polyacrylonitrile or thin non-conductive woven porous
carbon material or polymeric woven meshes that with stand high temperature and are stable in the alkaline and the acid conditions.
4. Compression of the sandwich structure at the melt processing temperature of the thermoplastic form of the ion exchange polymer material.
5. Conversion of the thermoplastic form of polymer into the sulphonic acid form by hydrolysis and ion exchange techniques.
The composite membrane comprises a thermoplastic ion exchange material held in the matrix of the reinforcing fiberglass tissue. The composite membrane of the present invention has many applications including electrolysis, fuel cells and batteries and use as superacid catalysts. The composite membrane has good mechanical strength with the properties of the ion exchange material it is formed with.
The composite membrane has uniform thickness. The composite membrane has similar properties as the conventional pure membrane of similar thickness. The composite membrane is impenetrable to non polar gases and bulk flow of liquids. The ion exchange material present in the matrix of the non woven tissue/ woven mat expands and swells when the composite membrane is soaked in water. The ion exchange material is firmly bonded to non woven/ woven fiber mat and hence does not come out. The composite membrane with desired mechanical strength and good conductivity is obtained when the membrane is swollen in water. These membranes have long term stability they may be poisoned by organics and metal ions. The composite membranes can be boiled in acids and hydrogen peroxide to remove these impurities. The composite membrane is stable to boiling and treatment with acid and peroxides.
To prepare the composite membrane compression molding technique may be adopted. The film of the thermoplastic form of the ion exchange material is pressed above the glass transition temperature of the material (The temperature chosen should be below the degradation temperature of the thermoplastic form of the ion exchange material). The thin films can suitably be formed above 200°C , preferably above 220°C and more preferably at 240-250°C to obtain films of varying thickness. The non woven felt / woven
mat is sandwiched between two such thin films and pressed at the glass transition temperature of the ionomer resin in melt processable form. The membrane is then processed to convert to the sulphonic acid form and to remove the organic and metallic impurities to give composite membranes that are ready for use.
The composite membrane of the said ion exchange material within the matrix of the non woven fiber reinforced tissue / thin woven mat has the following advantages
1. Composite membranes have good flexibility and mechanical strength than the pure ion exchange material.
2. Composite membranes are cheaper than the ion exchange material by 30-40% depending on the thickness of the films of the thermoplastic form of the ion exchange material used to form the sandwich composite.
3. Composite membranes have lower permeability to methanol.
A thin film of ion exchange irtterial in thermoplastic form (especially the resin of perfluorosulphonic acid in sulphonyl fluoride form like Nafion®resin 1100) was prepared by compression molding of a known weight of the material between 230°C and 250°C. say, at 240°C to give a film of thickness 40um or less. A sandwich structure is then formed by using two films and a fiber glass tissue in a non woven felt made of randomly oriented glass fibers / woven glass mat. The sandwich structure was molded at the melt processable temperature of the thermoplastic form of the ion exchange material at a pressure of 5000psi. This resulted in a composite membrane with the thickness of 120 fim.
The composite membrane is hydrolyzed using a 30% alcoholic solution of NaOH. The membrane is hydrolyzod in the alkaline solution at 80°C for 8hrs. The resultant composite membrane in sodium form is then washed to remove excess alkali and converted to the sulphonic acid form of the membrane by boiling in 1M H2SO4 solution
for 8hrs. The composite membrane in H+ form is then boiled in 3% U202 solution and 1M H2SO4 to remove organic and metallic impurities. The conductivity of the hydrated membrane was similar to that of pure Naflon ® membrane in H+ form.
The composite membrane prepared by the above method had a glass fiber content of 15% and a resin content of 16mg/cm3. The thickness of the membrane was 120 pm.
The above procedure could be -repeated with various thickness of films to give composiiss of varying compositions and varying resin contents. The resin content can be varied from 15mg/cm2 or lower to 40 mg/cmz depending on the strength of the material required and the application.
Thin films of the ion exchange material are prepared as indicated in Example 1. Instead of the glass film, a porous non woven carbon tissue material is used. The sandwich structure is formed by pressing two films of the thin ion exchange material "with the carbon tissue /non woven mat material placed between the two. The thickness of the composite membrane b depentfent on the thickness of the carbon tissue and the thickness of the films. Films of varying thickness from (OO^m to 20Gp*vi can be prepared by this procedure. The composite films are purified as described in example 1.
1. A composite membrane for use in electrochemical apparatuses and processes comprising two thin films of a resin of thermoplastic ion exchange material, such as herein described, the films being of predetermined thickness to provide two outer layers; a porous substrate of predetermined thickness constituting a middle layer interposed between two outer layers to form a sandwich structure, said middle layer material being selected from (1) porous fibre glass tissue in non-woven felt made of randomly oriented glass fibres or porous woven mats or (2) porous non-woven carbon tissue (3) high temperature resistant polymeric woven mesh resistant to acid-alkali conditions, the said two other outer layers being embedded in the middle layer on either side of the said middle layer.
2. The composite membrane as claimed in Claim 1 is a macrocomposite.
3. The composite membrane as claimed in Claim 1 wherein the resin of thermoplastic ion exchange polymermaterial is perfluorosulphonic acid in sulphonyl fluoride form.
4. A composite membrane for use in electrochemical apparatuses and processes substantially as herein described and as illustrated by the Examples.
5. A method of manufacture of a composite membrane for use in electrochemical apparatuses and processes comprising two thin films of a resin of thermoplastic ion exchange polymer material such as herein described, the films being separately heat pressed between 200 - 250 deg. C to a predetermined thickness to provide two outer layers; interposing a middle layer of predetermined thickness between the said two outer layers to form a sandwich structure, said middle layer material being selected from (1) porous fibre glass tissue in non-woven felt made of randomly oriented glass fibres or porous woven mats (2) porous non-woven carbon tissue (3) high temperature resistant polymeric woven mesh, resistant to acid-alkali conditions, the said two other outer layers being embedded in the middle layer on either side of the said middle layer.
6. The method as claimed in Claim 5 wherein the films are separately pressed to varying thickness.
7. The method as claimed in Claim 5 or claim 6 wherein the sandwich structure is pressed under pressure.
8. The method as claimed m Claim 7 wherein the sandwich structure is pressed to give membranes of varying thickness.
Dated this the 20:" December 2002
|Indian Patent Application Number||975/MAS/2002|
|PG Journal Number||08/2007|
|Date of Filing||24-Dec-2002|
|Name of Patentee||SPIC SCIENCE FOUNDATION|
|Applicant Address||CENTER FOR ENERGY RESEARCH, MOUNT VIEW, 64 MOUNT ROAD, GUINDY, CHENNAI - 600 032.|
|PCT International Classification Number||H01M2/14|
|PCT International Application Number||N/A|
|PCT International Filing date|