DE102011106036A1 - Process for the preparation of alkaline polymer electrolyte membranes for galvanic elements - Google Patents

Abstract

The present invention relates to a process for producing an alkaline polymer electrolyte membrane for galvanic elements, polymer electrolyte membranes prepared in this way and galvanic elements containing these polymer electrolyte membranes and the use of these polymer electrolyte membranes in, for example, metal / air cells, metal / metal oxide cells , Metal / metal hydride cells and alkaline fuel cells.

Description

The present invention relates to a process for producing an alkaline polymer electrolyte membrane for galvanic elements, polymer electrolyte membranes prepared in this way and galvanic elements containing these polymer electrolyte membranes and the use of these polymer electrolyte membranes in, for example, metal / air cells, metal / metal oxide cells , Metal / metal hydride cells and alkaline fuel cells.

Ion-conducting polymers in non-aqueous (ie aprotic) electrolyte systems are known in the art (cf., for example EP-A-1079455 ). Furthermore, ion-conducting polymers in aqueous (ie protic) electrolyte systems are known in the art. For example, describe U.S. Patent 6,858,670 and US 2003/0228522 A1 Polyvinyl alcohol-based polymer electrolytes for alkaline batteries, wherein the polymer electrolytes have a glass fiber reinforcement to provide sufficient mechanical stability. In addition, the alkaline batteries described in the prior art and commercially available generally contain a separator of, for example, polypropylene, which serves in particular for the electrical isolation of anode and cathode. Also, nonwovens are mostly used for storing electrolytes in conventional batteries. However, such separators and nonwoven fabrics lead to resistances during operation of the batteries, which leads to a reduction in the cell performance of these batteries.

Thus, it is an object of the present invention to provide a novel polymer electrolyte system which is said to have excellent physicochemical properties such as excellent mechanical stability even without auxiliaries such as glass fibers and at the same time excellent electrochemical properties such as excellent specific ionic conductivity To provide methods for producing such polymer electrolyte systems for galvanic elements with high power densities.

This object is achieved by the embodiments characterized in the claims.

• (a) Erwärmen von 4 bis 25 Gew.-% Polymer in Wasser auf eine Temperatur T₁ von 80 bis 100°C;
• (b) Zugeben einer wässrigen 1 bis 20 M Alkalihydroxid-Lösung;
• (c) gegebenenfalls Zugeben von 0,5 bis 30 Gew.-%, bezogen auf das Polymer, eines pulverförmigen Materials, ausgewählt aus anorganischen Oxiden, Carbonaten und Sulfaten, mit einem mittleren Teilchendurchmesser d₅₀ < 100 nm;
• (d) Erwärmen der nach Schritt (b) oder (c) erhaltenen Lösung oder Suspension auf eine Temperatur T₂ von 80 bis 110°C, wobei T₂ ≥ T₁ ist;
• (e) Aufbringen der Lösung oder Suspension auf einen Träger in einer für eine Schichtdicke von 10 bis 1000 μm geeigneten Menge und Abkühlenlassen der aufgebrachten Lösung oder Suspension unter Erhalt der Polymerelektrolyt-Membran auf dem Träger;
• (f) gegebenenfalls Trocken der Polymerelektrolyt-Membran unter CO₂-Ausschluß zur Einstellung des Wassergehalts.

In particular, there is provided a process for producing a polymer electrolyte membrane for galvanic elements, comprising the steps to be carried out in the following sequence:

• (a) heating 4 to 25% by weight of polymer in water to a temperature T ₁ of 80 to 100 ° C;
• (b) adding an aqueous 1 to 20 M alkali hydroxide solution;
• (c) optionally adding from 0.5 to 30% by weight, based on the polymer, of a powdery material selected from inorganic oxides, carbonates and sulphates, having an average particle diameter d ₅₀ <100 nm;
• (d) heating the solution or suspension obtained after step (b) or (c) to a temperature T ₂ of 80 to 110 ° C, wherein T ₂ ≥ T ₁ ;
• (e) applying the solution or suspension to a support in a suitable amount for a layer thickness of 10 to 1000 microns and allowing the applied solution or suspension to cool to obtain the polymer electrolyte membrane on the support;
• (f) optionally drying the polymer electrolyte membrane under CO ₂ exclusion to adjust the water content.

The polymer used in step (a) according to the invention is not subject to any particular restriction, but should be stable electrochemically and in the alkaline range and be soluble in water at temperatures ≥ 20 ° C. The proportion of the polymer is 4 to 25 wt .-%, preferably 5 to 15 wt .-%, more preferably 5 to 10 wt .-%, particularly preferably <10 wt .-%, based on the water content. The heating to a temperature T ₁ of 80 to 100 ° C, preferably 90 to 100 ° C, more preferably 95 to 98 ° C, is preferably carried out with stirring until substantially complete dissolution of the polymer, As polymers, for example, polyvinyl alcohol, polyvinylpyrrolidone, Polyacrylates, polymethacrylates, polycarboxylates and mixtures or copolymers of these can be used. Preferably, a polyvinyl alcohol-based polymer, more preferably a polyvinyl alcohol having a molecular weight> 100,000, more preferably> 120,000, for example about 145,000 and / or with a degree of hydrolysis> 99%, is used. The water used in step (a) is not particularly limited, and preferably high-purity water having a specific resistance of preferably <15 MΩ, for example, <16.2 MΩ can be used.

In step (b), an aqueous 1 to 20 M, preferably 5 to 6 M alkali hydroxide solution is added to the reaction mixture of step (a), the addition preferably being carried out with stirring and maintaining the temperature T ₁ of step (a). The addition takes place for example over a period of 5 to 120 minutes. As the alkali hydroxide, LiOH, NaOH, KOH or CsOH can be used. As the water for producing the alkali hydroxide solution, for example, the high-purity water used in the step (a) can be used.

After addition of the alkali metal hydroxide solution according to step (b) optionally in a step (c) 0.5 to 30 wt .-%, preferably 5 to 20 Wt .-% powdery material, based on the polymer, having an average particle diameter d ₅₀ <100 nm, preferably <50 nm, more preferably <25 nm, in particular be added to further improve the mechanical properties of the polymer electrolyte membrane. This addition is preferably carried out at a temperature in the range of 90 to 100 ° C, preferably 95 to 98 ° C. As the powdery material, inorganic oxides ("nanoscale oxides"), for example, magnesium oxide, alumina, silica, titania and zirconia, carbonates, for example, calcium carbonate, or sulfates, for example, barium sulfate, are used. Titanium dioxide is preferably used with a particle diameter of, for example, <25 nm. For example, the addition of nanoscale oxides such as titanium dioxide can accelerate the crosslinking of the polymer matrix and thus further improve the viscoelastic properties of the polymer electrolyte membranes according to the invention.

The heating according to step (d) of the solution or suspension obtained after step (b) or (c) to a temperature T ₂ of 80 to 110 ° C, preferably 95 to 105 ° C is preferably carried out with stirring, for example for a period of 5 to 120 minutes or until the presence of a substantially homogeneous reaction mass or solution or suspension.

In step (e), the reaction mass or the solution or suspension of step (d) is applied to a support or a corresponding support in a thickness of 10 to 1000 μm, preferably 10 to 500 μm, more preferably <300 μm. applied appropriate amount. The application is carried out by methods known in the art such as casting, tips, screen printing, impregnation, spin coating or "solution casting method". The carrier is not particularly limited and may be, for example, a plastic, a glass, a metal, a metal alloy or an oxide. Alternatively, the reaction mass may be applied to a corresponding cylinder for continuous film drawing by solvent casting technology known in the art.

In the case of producing a polymer electrolyte membrane per se, the support preferably has a surface from which the polymer electrolyte membrane can be removed after cooling and optionally drying without damage, the surface being made of, for example, the above materials for a support can exist. By appropriate shape specification of the carrier, the polymer electrolyte membrane according to the invention can also take any desired shape.

In the case of electrode material or an electrode, for example, a porous zinc anode, a magnesium anode or an aluminum anode, the application of the solution or suspension of step (d) by means of, for example, spraying, brushing, knife coating, dip coating, optionally under reduced pressure take place, wherein a composite structure between electrode material or electrode and polymer electrolyte membrane is obtained with almost optimal contact surface. Alternatively, the electrodes can be applied directly to the polymer electrolyte membrane in the form of a suitable paste, dispersion or powder by suitable printing methods, for example with the aid of a doctor blade or similar methods. By such a preferred method step, contact resistances between the electrode and the polymer electrolyte membrane can be minimized, and thus a reduction in the performance of the galvanic element can be avoided.

Furthermore, the viscosity of the solution or suspension of step (d) before application, for example by means of volatile solvents such as methanol, ethanol, propanol or isopropanol, can be adjusted so that the application to the carrier, in particular to an electrode can be further optimized. In a preferred embodiment, first the electrode material or the electrode can be impregnated with the solution or suspension obtained in step (d) with a low viscosity, wherein in particular by the low viscosity, the application and the contacting with respect to a contact surface to be increased between electrode material or Electrodes are improved. Thereafter, the solution or suspension of higher viscosity obtained in step (d) may be applied to the impregnated electrode material or the impregnated electrode for producing the polymer electrolyte membrane. As a result of the impregnation and thus improved contact zone between the electrode and the electrolyte, for example in a Zn / air cell, the Zn ions formed by the oxidation at the anode can migrate more easily into the electrolyte.

In a preferred embodiment, the electrodes of a galvanic element comprising an anode and a cathode, for example in the case of a zinc / air battery, a porous zinc electrode or a zinc paste as an anode and a porous gas diffusion electrode with a carbon-bonded electrocatalyst such as manganese dioxide, as above carried out impregnated. When using a zinc paste, the zinc powder used as the starting material for the electrode may be additionally mixed with powdery polymer such as polyvinyl alcohol.

After cooling, preferably to room temperature, the inventive Polymer electrolyte membrane obtained on the desired support, wherein the polymer electrolyte membrane is formed by crosslinking in situ.

In order to adjust the water content, the polymer electrolyte membrane according to the invention can be dried under CO ₂ exclusion, preferably under an inert gas atmosphere, according to step (f), the water content being in the range from 20 to 85% by weight, based on the total polymer electrolyte membrane, preferably from 60 to 70 wt .-% may be. Drying under CO ₂ exclusion is required to avoid carbonate deposits on the membrane, which can lead to reduced performance of the polymer electrolyte membrane. After drying, the polymer electrolyte membrane according to the invention may preferably be welded to air and moisture-tight for transport and / or storage with conventional methods and devices.

A further subject of the present invention is a polymer electrolyte membrane obtainable by the method described above per se or applied to electrode material or to an electrode.

Another object of the present invention relates to a polymer electrolyte membrane, wherein the polymer is polyvinyl alcohol having a molecular weight of> 100,000, preferably> 120,000, and / or with a degree of hydrolysis> 99%.

In particular, the polymer electrolyte membrane of the present invention has excellent mechanical stability and viscoelastic properties as well as excellent specific ionic conductivity. For example, the polymer electrolyte membrane according to the invention exhibits a specific ionic conductivity of up to 0.2 S / cm, which is significantly higher than the specific ionic conductivity of polymer electrolyte membranes known in the prior art, which for example have values of only up to 0, 14 S / cm show. Furthermore, the production of galvanic elements is cheaper and significantly simplified by the inventive method.

The polymer electrolyte membrane according to the invention per se or applied to electrode material or to an electrode may be in a galvanic element, preferably selected from metal / air cells, for example zinc / air cells, metal / metal oxide cells such as zinc / nickel oxide. Cells or zinc / silver oxide cells, and alkaline fuel cells, which can be operated with hydrogen, methanol, ethanol or formic acid as fuel and oxygen or air, can be used. When using the polymer electrolyte membrane of the invention in a battery such as zinc / air cells advantageously no separator is required, resulting in a significant reduction of resistances in the galvanic element.

Gas diffusion electrodes are used as cathodes, for example, both in zinc / air batteries and in fuel cells. The catalyst layer of the electrode is treated with the solution or suspension of step (d), for example a PVA / KOH solution. In this way, the so-called three-phase zone of a gas diffusion electrode, consisting of the solid catalyst, the electrolyte and the gaseous oxygen, is significantly increased. The transport of hydroxide ions, which are formed at the cathode of an alkaline galvanic cell by electron uptake (reduction), from the reaction zone of this three-phase zone in the alkaline electrolyte is thus improved. By using a polymer electrolyte membrane according to the invention, the gas diffusion electrode hardly contains liquid electrolytes. It is known that in conventional batteries by reactions between the alkaline liquid electrolyte and the carbon dioxide from the air can form solid carbonate deposits in the porous structure, which have a negative impact on the life of the gas diffusion electrode, which is when using the polymer electrolyte membrane according to the invention can be avoided.

The construction of, for example, a zinc / air cell with a polymer electrolyte based on, for example PVA is thus significantly different from the structure of a conventional cell. The cell with, for example, a PVA membrane according to the invention contains as an additional solid component said membrane, which in case of damage by z. As zinc dendrites or carbonation can be expanded, examined and optionally replaced. In a conventional cell, the separator is usually pasted on the gas diffusion electrode, so that replacement without damaging the gas diffusion electrode is hardly possible. Above all, the polymer electrolyte membrane according to the invention offers additional protection for the user against leakage of the electrolyte (strongly alkaline solution of potassium hydroxide) and the carbonation is slowed down. Thus, for example, a zinc / air cell with the polymer electrolyte membrane according to the invention is also safer than a conventional zinc / air cell.

1 Figure 4 is a graph of the discharge of four Zn / air cells at 10 mA discharge current.

2 is a graphical representation of the discharge behavior of three Zn / air cells in Fully charged state (0% DOD) at 0-40 mA discharge current.

The following examples serve as further explanation of the present invention without being limited thereto.

EXAMPLE 1 Production of an Electrolyte Membrane According to the Invention from Polyvinyl Alcohol, Potassium Hydroxide, Water and Titanium Dioxide and Installation in a Conventional Zinc / Air Battery

The embodiment relates to an electrolyte membrane based on polyvinyl alcohol and a concentrated aqueous solution of potassium hydroxide, which is modified by the addition of nanoscale titanium dioxide. According to these exemplary methods, another polymer such as polyvinylpyrrolidone or poly (meth) acrylates, as well as other alkalis may be used. Also, the additive titanium dioxide as a powdery material can be replaced by other nanoscale powdery materials that are stable in strong alkaline solution such as magnesium oxide, zirconium dioxide or calcium carbonate.

In this embodiment, the polymer electrolyte membrane is incorporated into a conventional zinc / air battery. This consists in a conventional manner of a zinc anode and a porous gas diffusion electrode with an active electrocatalyst such. B. manganese dioxide, at which the oxygen reduction takes place. The electrolyte used is a highly concentrated potassium hydroxide solution which has a viscosity comparable to that of an aqueous solution. As a separator between the electrodes, a commercially available porous membrane of polypropylene (PP), for example, from Celgard Type 5550 is used, which is impregnated with the liquid electrolyte in the manufacturing process of the battery. It is shown that the electrolyte is fixed by using the polymer electrolyte membrane of the present invention, thereby achieving a higher specific capacity of the battery. In addition, the manufacturing process for the entire battery is simplified.

To prepare the polymer electrolyte membrane in a four-necked flask as a template 25 g of highly pure water (with a resistivity of more than 16.2 MΩ) and 3 g of polyvinyl alcohol ("PVA", M = 145,000 g mol ^-1 , supplier: Merck KG , Darmstadt) in a 500 ml four-necked flask. Polyvinyl alcohol is> 99% hydrolyzed before. The four-necked flask is then installed in a standard stirring apparatus and slowly heated to 90 ° C. under a medium stirring stage (preferably about 250 rpm) in a heating bath liquid (Thermal M, BASF SE) until the entire PVA has dissolved and a homogeneous reaction mass is present. The degree of hydrolysis of PVA is checked by means of an acid / base titration. The electrolyte, a highly concentrated potassium hydroxide solution, is prepared from 5 g of ultrapure water (resistivity X16.2 MΩ) and 4 g of potassium hydroxide (supplier: Sigma-Aldrich Chemie GmbH). The addition of the electrolyte to the homogeneous reaction mass of PVA and water via a dropping funnel, which is infected with the four-necked flask, being added slowly over feinstdosierte droplets in the present reaction mass. During this time, it is preferable to increase the agitation level to about 400 rpm. The mixture is then stirred at about 95 ° C and medium stirring stage (preferably 250 U / min) for about 15 minutes with further heat. If, after this time, a homogeneous reaction mass is present, the previously weighed titanium dioxide (0.5 g with an average diameter of <25 nm, Anastas modification, supplier: Sigma-Aldrich Chemie GmbH) is added and increased stirring (preferably about 400 U / min) until a homogeneous pure white mass is present. The synthesis temperature can then be selected between 95 ° and 110 ° C.

Thereafter, the hot reaction mass is further processed via the "solution casting method". For this purpose, a defined amount of the mass is poured into a Petri dish. The layer thickness of the polymer electrolyte membrane can be adjusted in this way between 10 and 1000 microns. After cooling the reaction mass to room temperature, the drying of the PVA / KOH / TiO ₂ is preferably carried out until the desired mechanical strength is reached. This is adjusted via the water content. In the exemplary embodiment, the water content is 70 percent by weight. The drying takes place under inert gas, so that no potassium carbonate can form from the carbon dioxide of the air and the potassium hydroxide of the polymer electrolyte membrane. This would arise on the surface of the membrane and thus passivate the polymer electrolyte membrane.

The inventive polymer electrolyte membrane based on polyvinyl alcohol prepared in this way can now be incorporated into a zinc / air cell. For example, PVA / KOH / TiO ₂ based polymer electrolyte membranes may be tested for their discharge behavior in a primary Zn / air button cell (eg, from a battery manufacturer such as Varta Microbattery). For this purpose, a commercially available cathode without separator and a Hg-free zinc paste with a 6 M KOH solution containing 2 weight percent ZnO used. Initially, an impedance measurement of the assembled cell is performed in the de-energized state (ie at 0 mA). At a frequency of 100 kHz, an impedance of about 1 ohm is measured. The cell voltage is typically at 1.45 V. Then, a current voltage curve with absorbed a current load of the cell up to 40 mA. With decreasing layer thickness of the polymer electrolyte membrane, a reduction in the impedance or an increase in the cell voltage is observed. Subsequently, the cell is discharged continuously with a current of 10 or 5 mA. The discharge voltage is set to 0.7V. The measurements are carried out at room temperature. At a current density of 10 mA, a charge of 550 mAh is taken from the zinc / air battery. Subsequently, at a current of 5 mA, another charge of 15 mAh is taken. The theoretical amount of charge for a Gram zinc is 820 mAh. Since the amount of Zn used to make the zinc anode is about 850 mg, it achieves a specific capacity of about 665 mAh g ^-1 . In a comparative measurement with a conventional Hg-free zinc / air cell (same electrode materials as above) using a conventional electrolyte and separator (ie, aqueous potassium hydroxide solution and polypropylene), a specific charge amount of about 600 mAh g ^{-1 is} measured.

The specific capacity of a primary zinc / air cell with a PVA-based polymer electrolyte membrane according to the invention is thus about 10% higher than with a cell using a conventional liquid electrolyte. In addition, it must be emphasized that the cell with a polymer electrolyte membrane according to the invention does not contain an additional separator, for example made of polypropylene or nonwoven, as conventional cells.

Example 2: Unloading of four Zn / air cells

A polymer electrolyte membrane according to the invention (8 wt .-% PVA, 10.7 wt .-% KOH, 0.7 wt .-% TiO ₂ , 80.6 wt .-% H ₂ O) having a thickness of about 150 microns is installed in three galvanic elements. As a comparison, a commercially available Hg-free cell with separator (Varta Microbattery) was used. The unloading process is in 1 shown graphically at 10 mA discharge current. In comparison with the commercially available cell with separator, an approximately 8-12% higher capacity (in mAh) was measured on the cells with the polymer membrane according to the invention. This surprisingly higher capacity can be attributed to a more efficient electrolyte management of the cell with the polymer electrolyte membrane according to the invention.

Example 3: Current-voltage curves of two Zn / air cells

A polymer electrolyte membrane according to the invention with a reduced PVA content (4.2% by weight of PVA, 11.2% by weight of KOH, 0.7% by weight of TiO ₂ , 83.9% by weight of H ₂ O. ) and a thickness of about 75 microns is installed in two galvanic elements. For comparison, a commercially available high performance Cochlear cell with separator (Varta Microbattery) was used. The discharge process of these cells in the fully charged state (0% depth of discharge (DOD) at 0 to 40 mA discharge current in 2.5 mA step for 1 min 2 shown graphically. In comparison with the commercially available cell, a voltage higher by 20-25 mV at a 20 mA discharge current was measured on the cells with the polymer membrane according to the invention. This surprisingly higher performance can be attributed to a high ionic conductivity of the polymer electrolyte membrane according to the invention.

Example 4: Discharge of two zinc / air cells

The cells listed in Example 3 were then completely discharged with a 10, 5, 2 and 1 mA discharge current at room temperature. In each case, a capacity of 593 or 602 mAh and an energy quantity of 718 or 730 mWh were measured. In comparison, the cell of the Cochlear type from Varta Microbattery GmbH was deprived of a capacity of 560 mAh and an energy of 674 mAh. The incorporation of a PVA / KOH / TiO ₂ membrane at the same zinc mass of about 850 mg per cell resulted in an approximately 6-8% higher energy yield than the commercially available cell. This was attributed to a more efficient electrolyte management of the cells with a correspondingly thin polymembrane according to the invention.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 1079455 A [0002]
• US 6858670 [0002]
• US 2003/0228522 A1 [0002]

Claims (10)

A process for producing a polymer electrolyte membrane for electroplating elements, comprising the steps (a) characterized in the following order: heating 4 to 25% by weight of polymer in water to a temperature T ₁ of 80 to 100 ° C; (b) adding an aqueous 1 to 20 M alkali hydroxide solution; (c) optionally adding from 0.5 to 30% by weight, based on the polymer, of a powdery material selected from inorganic oxides, carbonates and sulphates, having an average particle diameter d ₅₀ <100 nm; (d) heating the solution or suspension to a temperature T ₂ of 80 to 110 ° C, wherein T ₂ ≥ T ₁ ; (e) applying the solution or suspension to a support in a suitable amount for a layer thickness of 10 to 1000 microns and allowed to cool to obtain the polymer electrolyte membrane on the support; (f) optionally drying the polymer electrolyte membrane under CO ₂ exclusion to adjust the water content. The method of claim 1, wherein the polymer is selected from the group consisting of polyvinyl alcohol, polyvinylpyrrolidone, polyacrylates, polymethacrylates, polycarboxylates, and mixtures or copolymers of these. A process according to claim 2, wherein the polymer is polyvinyl alcohol having a molecular weight> 100,000 and / or with a degree of hydrolysis> 99% and / or in an amount <10% by weight in step (a). A process according to any one of claims 1 to 3, wherein the alkali hydroxide is LiOH, NaOH, KOH or CsOH. A method according to any one of claims 1 to 4, wherein the inorganic oxide is magnesium oxide, alumina, silica, titania or zirconia. Method according to one of claims 1 to 5, wherein the carrier has a surface made of plastic, glass, metal, metal alloy, oxide or electrode material or consists of these materials or is an electrode. A method according to any one of claims 1 to 6, wherein the support is a porous zinc anode, a magnesium anode or an aluminum anode. Polymer electrolyte membrane obtainable by a process according to any one of claims 1 to 7. A polymer electrolyte membrane, wherein the polymer is polyvinyl alcohol having a molecular weight of> 100,000 and a degree of hydrolysis> 99%. Use of the polymer electrolyte membrane according to claim 8 or 9 in a galvanic element selected from metal / air cells, metal / metal oxide cells, metal / metal hydride cells and alkaline fuel cells.

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