REINFORCED ION EXCHANGE MEMBRANE

Abstract: The invention is a reinforced ion exchange membrane for application in electrochemical cells. More specifically a membrane is provided having an ion conducting section or multiple ion conducting sections and a section, or sections that are non-ion conducting. The method according to the invention has several advantages over the existing technologies. Expensive proton conducting material is only used in the active areas, and not used for sealing the cell thus reducing material costs. Compared to non-reinforced membranes or macro reinforced membranes only a small amount of the expensive proton conductor has to be used.

Description: Reinforced ion exchange membrane Subject of the invention is a reinforced ion exchange membrane for application in electrochemical cells. More specifically a membrane is provided having a ion conducting section or multiple ion conducting sections and a section, or sections that are non-ion conducting. Ion exchange membranes are used in many applications like electrolyses; electrodialyses and proton exchange membrane (PEM) fuel cells. A PEM fuel cell comprises typically a proton conducting membrane with a catalyst-containing electrode on both sides. Such an assembly is called a MEA (membrane electrode assembly). Typically these MEA's are placed between electrically conducting plates, often called bi-polar plates to form a single fuel cell, or if more of these cells are stacked, such an assembly is called a fuel cell stack. The main functions of the membrane are proton conduction and gas separation between anode and cathode. Often proton conducting polymer foils are used as membrane. Minimum thickness is determent by the mechanical strength required. Increase of the proton conductivity is often necessary, but feasible ways of increasing conductance like thickness reduction or decrease of molecular weight, also reduce the strength, and are therefore limited. For certain applications like chlorine production membranes with macro reinforcements like woven fabrics are used, but these membranes still need a relative thick proton conducting film on or both sides. These restrictions can be eliminated or reduced by using a non-proton conducting reinforcement in the proton conducting material. Such a material is known from US 5547551 and US 5599614. In this material porous expanded PTFE is used as reinforcement with Nafion (Nafion is a registered trade mark of E. I. DuPont de Nemours and Co., inc) as the proton conducting polymer matrix. These reinforced membranes are thin, have attractive water transport properties, and excellent proton conductivity. Drawback of this material is the high cost of expanded PTFE and the moderate mechanical properties like tensile strength and creep resistance of PTFE. US5716437 teaches that porous PE can be used as reinforcement for proton conducting membranes. Advantages over US5547551 and US 5599614 are reduced cost and improved mechanical properties. Drawback of US5716437 is that the whole reinforcement is impregnated with expensive proton conductor, including parts of the reinforcement were proton conductivity is superfluous, or even unwanted. US5716437 provides no solution for sealing the electrochemical cell. The invention has as its objection to provide a product in which drawbacks of the technologies described above have been eliminated. According to the invention, the porous reinforcement having a thickness between 1 and 100 micron is made partially non-porous prior to application of the proton conductor. According to one embodiment of the invention this is achieved by application of sufficient time temperature and pressure to the areas to be made non-porous. In another embodiment of the invention the areas to be made non porous are impregnated with a non proton conducting polymer, in again an other embodiment of the invention the reinforcement is made non-porous by laminating and pressing an non porous thermoplastic foil on to the porous reinforcement at a temperature above the melting point of the applied non porous thermoplastic foil, the non-porous thermoplastic foil and the reinforcement being preferably of the same polymer or the same family of polymers. The areas of the reinforcement that will be outside the active areas of the electrochemical cell are accordingly made non porous or almost non porous. Subsequently the porous area of the reinforcement is brought in contact with a solution containing a proton conducting polymer or proton conducting polymer precursor. Prior to the impregnation, air, present in the open pores of the reinforcement, is preferably removed and substituted for a fast diffusing gas like He, a soluble gas like C02 or preferably by the vapor of a suitable solvent. Impregnation takes place at a temperature below the boiling point of the solvent in the solution and preferably close to this boiling point. The solution can be applied on one side of the reinforcement or on both sides. The solvent is removed from the reinforcement by drying or by coagulation in a non-solvent. After drying the impregnated membrane receives temperature treatment at a level just below the Tg of the proton conductor and below the Tm (melting point) of the reinforcement. Several modifications according to the invention are possible. Although a single step impregnation process is preferred, multiple impregnation steps can be applied if so desired. If a two-step impregnation process is applied it is, according to the invention attractive to use a solution that contains conductive carbon black to provide a preferably non porous, electrical conducting outer layer on both sides of the reinforced membrane. Prior to impregnation and preferably prior to the making part of the reinforcement non-porous and non-proton conducting, the reinforcement can be treated to improve wetting with the solution and impregnation Most known treatments can be used like corona treatment, plasma treatment and grafting For filling the reinforcement a high polymer concentration in the solution is preferred, however the maximum practical viscosity limits the maximum concentration Surprisingly it was found that the substitution of a "good" solvent like iso propanol for a "poor 5 solvent like water improves the impregnation However this is only possible if the reinforcement has received a proper pre-treatment According to the invention the reinforced membrane can contain fillers like Tι02 SnO2 and Sι02 (silica) Preferably these fillers are applied prior to the impregnation process, more preferred these fillers are added during the production of the reinforcement It was found that applying controlled temperature and pressure to the impregnated, or almost completely impregnated membrane could improve membrane properties Small voids were removed yielding a completely transparent reinforced membrane For this purpose the membrane was pressed at 120 °C for 20 seconds at a pressure of 500 000 Pa Alternatively the impregnated membrane was densified by running it trough the nip between two polished rolls at a temperature of 120°C at a line pressure of 500N/m The method according to the invention has several advantages over the existing technologies Expensive proton conducting material is only used in the active areas, and not used for sealing the cell thus reducing matenal costs Compared to non-reinforced membranes or macro reinforced membranes only a small amount of the expensive proton conductor has to be used EXAMPLE A porous PE-foil, Solupor type 8P07A of DSM Solutech B V was mounted taut in a frame of 100x100mm (outer size), and 50x50mm (inner size) between two 2 HDPE foil having the same size as the frame On top of the foil a second frame with the same size was placed Both stainless steel frames were treated with a solution of 0,5% amino-siloxaan in di-butyl ether At a temperature of 125°C the porous foil and the two non porous foils were pressed together and subsequently cooled to a temperature below the melting point, yielding a frame of non porous PE around on area of porous PE In figure 1 this intermediate product is shown schematically Subsequently 1 cc af a solution containing 5% Nafion 1100 (Solution Technology) was poured on the porous PE (1) and dried in an inert atmosphere at a temperature increasing up to 125°C The membrane was subsequently treated with Demi-water, 3 % H202, H2SO4 1M and again demi-water On both sides of the reinforced membrane an ink was coated comprising 52% 1- propanol, 8% Pt on carbon black and 40% of a 5 % solution of Nafion 1100 (Solution Technologies) The ink was dried at 60°C, and on top of the electrodes, graphite fiber papers (50 * 50 mm) were pressed This MEA was placed between two cell plates (3) (see figure 2) Subsequently the PE edges of the cell plates and the non-porous PE edges of the MEA were welded together using friction welding EXAMPLE 2 A roll porous PE foil, Solupor type 8P07A made by DSM Solutech B V was unwound with a speed of 20m/mιn buy means of a heated (140°C) set of rollers the solupor was densified in specific areas, wile the non densified Solupor was not brought into contact with the hot roll De partly densified Solupor was led trough a bath containing propanol and was dried subsequently at a temperature of 80°C mainly propanol vapor containing atmosphere After the vapor treatment the non porous areas are coated with a layer of 200 micron of a pre heated (80°C) Nafion 1100 solution (10% I Nafion in propanol By a combination of radiation heating and convection heating the solvent was removed. The foil received a heat treatment comprising a hot (120°C) calendaring step, and rewinding of the product.

Claims: WHAT IS CLAIMED IS A proton conducting membrane made from a porous film of non proton conducting polymer impregnated with a proton conducting polymer, characterized in that the impregnation of the proton conductor is restricted to the active area of the MEA, and that the area around the proton conducting region is made non porous and non proton conducting A electrochemical cell comprising an MEA, this MEA containing a proton conducting membrane made from a porous film of a non proton conducting polymer impregnated with a proton conducting polymer, characterized in that the impregnation of the proton conductor is restricted to the active area of the MEA, and that the area around the proton conducting region is made non porous and non proton conducting A product according to any of the preceding claims, characterized in that the electrochemical cell is a PEM fuel cell A product according to any of the preceding claims, characteπzed in that pπor to impregnation with the proton conductor the area around the active area was made non-porous and non-proton conducting A product according to any of the preceding claims characteπzed in that the matenal in the non-proton conducting area is the same, or belongs to the same group of polymers as the reinforcement A product according to any of the preceding claims, characterized in that the non-proton conducting edge around the active area comprises the same polymer, or a polymer belonging to the same family of polymers as the edge of the cell plates A product according to any of the preceding claims, characterized in that the polymer of the reinforcement is PE A product according to any of the preceding claims, characteπzed in that the polymer of the reinforcement is PVDF A product according to any of the preceding claims, characterized in that the non proton conducting area of the reinforcement is made non porous by pressing at increased temperature A product according to any of the preceding claims, characterized in that the impregnated proton conducting part of the membrane is densified pressing or rolling at a sufficiently high temperature and pressure A product according to any of the preceding claims, characterized in that the production is done in a roll-to-roll process A product according to any of the preceding claims, characterized in that lamination of the gas diffusion layers in done in a continuous process A product according to the previous claim, characterized in that a double belt pres or belt calander is used A product according to any of the preceding claims, characterized in that seals are integrated in the non-porous, non-proton conducting edge around the active area A product according to any of the preceding claims, characteπzed in that prior to impregnation the reinforcing material has received a wetting improving treatment like corona treatment, plasma treatment or crafting A product according to any of the preceding claims, characterized in that prior to impregnation air in the reinforcement is removed and replaced by another gas

Inventors: MIDDELMAN ERIK, MIDDELMAN, ERIK
Applicants: MIDDELMAN ERIK, NEDSTACK HOLDING B V, MIDDELMAN, ERIK, NEDSTACK HOLDING B.V.
Application year: 2001
Main classification: H01M8
IPC codes: C08J5, H01M2, H01M8