RU2343600C1 - Device for catalytic gas recombination in alkaline accumulator with short zinc anode - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention is attributed to device for catalytic gas recombination (DCGR) in alkaline accumulator with short zinc anode. In this invention device for DCGR produced during charging alkaline accumulator with zinc anode features catalytic mass applied on porous cellular foamed metal acting as catalyst carrier and heat-dissipating structure. The specified catalytic mass contains carbon black that includes platinum-group metal and hydrophobic binder which combination is heat-treated to provide hydrophobic binder agglomeration in specified catalytic mass. The said device is preferably connected with one of accumulator terminals or with some metal component that is part of accumulator casing cover to allow for produced heat dissipation.

EFFECT: maintaining limited pressure in charging and discharging cycles for a long period of time, especially in structures supposing lack of technical service.

13 cl, 1 dwg, 4 ex

Description

The present invention relates to the field of electrochemical generators, in particular to alkaline batteries with a zinc anode.

It is known that water electrolyte batteries consume water during their operation, and more precisely, during recharging of the battery necessary to fully charge it, which causes the electrolyte to decompose into hydrogen and oxygen.

There are various ways to control this water flow, namely:

- limiting recharge, at the risk of not sufficiently recharging the battery;

- using a large excess of electrolyte in order to reduce the frequency of adding water, such a scheme, however, can only be used in stationary batteries, due to the excess weight and volume to which it leads.

These solutions do not avoid the need for periodic, more or less frequent user intervention.

Alkaline batteries have been manufactured for a long time and do not require maintenance, the so-called sealed, which, however, are equipped with a safety valve that opens when there is an excess of pressure inside the battery.

These batteries use the principle of recombination of gases from the decomposition of water. Good examples are alkaline nickel-cadmium batteries (NiCd) and batteries based on nickel and metal hydride (NiMH), cylindrical or prismatic, which are equipped with portable electrical and electronic devices (telephones, computers, ...).

The negative electrode in them has a larger capacity than the positive electrode in a ratio that most often varies from about 1.2 to 1.5.

When the positive nickel electrode is fully charged, the battery voltage rises, causing the onset of oxygen evolution, which is the result of electrochemical oxidation of water.

When recharging the cathode, the negative electrode continues to charge.

Oxygen generated at the positive electrode diffuses to the anode from cadmium or metal hydrides and is reunited either with metallic cadmium or with hydrogen adsorbed in the metal hydride. This diffusion is facilitated by the use of an oxygen permeable dividing wall and the use of a reduced amount of electrolyte.

In an alkaline battery, the reactions observed on the negative electrode are as follows, where M is the metal involved in the reaction:

French patent 2788887 describes the principle of secondary alkaline electrochemical generators with a zinc anode, in which a simple and economical implementation technology allows to achieve a high level of perfection, in particular in the sense of recharging.

The essence of the invention of this document relates in a narrower sense to the use of a negative electrode made of sponge and plasticized zinc, the active mass of which consists of a mixture containing at least zinc oxide, fine conductive ceramic powder and a plastic binder.

According to this technology, the active anode mass in the form of a paste obtained from various components and a diluent is embedded in a three-dimensional manifold consisting of porous foamed copper.

Zinc anode alkaline batteries such as nickel-zinc (NiZn) or silver-zinc (AgZn) made by assembling zinc electrodes made as described above and sponge-plasticized nickel or silver cathodes in a sponge nickel carrier have excellent the ability to recharge and show characteristics comparable or superior to those of other secondary alkaline generators with nickel positive electrodes, and in addition, they have the advantage that at a lower cost not RAT heavy metals.

NiZn or AgZn batteries using this technology can operate according to the “open”, “semi-hermetic”, or “sealed” principle.

The general principles of operation that apply to NiCd and NiMH alkaline batteries are equally applicable to zinc anode batteries. It should be noted that the negative electrode of zinc has an excess capacity compared with the positive electrode,

However, in the case of nickel-zinc batteries made according to the technology described in French patent 2788887, the excess capacity of the zinc electrode does not exceed about 20% of the capacity of the nickel electrode, which is a significant difference compared to what is usually described in the literature, where The zinc anode usually has an excess capacity of 250 to 500% in order to artificially reduce the depth of discharge of the anode and increase the duration of its operation in the recharge mode.

In the "open" mode, the end of battery charging is accompanied by the release of oxygen on the positive electrode, then, when charging continues, and hydrogen on the negative electrode. Periodic addition of water is required corresponding to the amount of decomposed electrolyte.

In the "semi-hermetic" mode, the battery is equipped with a valve that opens at low pressure from 10 to 20 kPa. The oxygen formed partially combines with the metal zinc of the anode by the reaction:

Zn + 1/2 O ₂ → ZnO

ZnO + 2e- + H ₂ O → Zn + 2OH-

Zinc oxide itself is in equilibrium with the soluble form of zinc in an alkaline medium - zinc acid according to the following simplified equation:

ZnO + 20H- + H ₂ O↔Zn (OH) ₄ ^2-

In the "hermetic" mode, the total set of gases formed must again react in order to avoid an excessive increase in internal pressure.

The principle of operation of a sealed nickel-zinc battery described above has certain limitations for various reasons:

- excess and uncontrolled charge, which leads to an excess of oxygen formation, which, due to the kinetics of reaction [1], enters reactions [2] and [3];

- the consequence of the above phenomenon is enhanced by the delayed diffusion of oxygen to the negative electrode, which is fully charged and begins to produce hydrogen:

H ₂ O + e- → OH- + 1/2 H ₂ ,

Zinc metal is thermodynamically unstable, and tends to corrode with the formation of hydrogen:

Zn + 2H ₂ O → Zn (OH) ₂ + H ₂ ↑

The method of utilization of the generated gases - oxygen and hydrogen - depends on the battery and its performance, an increase in internal pressure promotes the recombination of gas in the electrode region, with the polarity opposite to the electrode where this gas is formed, but an increase in internal pressure is acceptable only to a limited extent in some types buildings.

So, a cylindrical battery with a metal cover and a body can withstand pressure up to 2000 kPa, while prismatic batteries allow a maximum pressure of 500 to 1000 kPa, depending on the size of the battery, the material and the method of attaching the cover to the case. For safety reasons, the covers of the recombination batteries are equipped with valves. They are adjusted to a pressure of about 1500 kPa for cylindrical batteries and up to 200 kPa for prismatic forms.

Hydrogen generation and utilization are a particularly important aspect of the functioning of sealed nickel-zinc batteries.

Various solutions have been proposed to limit the increase in pressure due to the formation of hydrogen, including:

- the use of catalysts, for example, based on silver, included in the positive electrode and contributing to the oxidation of hydrogen during charging by the reaction:

H ₂ + 2OH- → 2H ₂ O + 2-

- the use of a third electrode associated with a positive electrode and providing oxidation of hydrogen;

- the use of a catalytic structure consisting of carbon and platinum, located on a metal manifold or grid of carbon, designed to ensure the recombination of hydrogen and oxygen.

These various solutions, however, are not completely satisfactory either for reasons of limited kinetics of hydrogen oxidation or for reasons of complex construction.

One of the limitations in using the catalytic structure of hydrogen and oxygen recombination is the thermal load during the operation of the system. In fact, the reaction between hydrogen and oxygen is very exothermic and can lead to a significant increase in temperature and the formation of "hot spots" that are harmful to the normal functioning of the catalyst. As a result, it is necessary to quickly remove the heat generated during the recombination reaction.

In addition, and this represents another difficulty related to the practical use of catalytic structures, the water generated by the recombination of hydrogen and oxygen should not limit the gas access to the catalytic zones.

The aim of the present invention is to satisfy all these requirements: the inventors have indeed developed catalytic structures using porous metals as a basis, and a method of their application adapted to the considered use.

This goal is achieved using a device for the catalytic recombination of gases in alkaline batteries with a zinc anode so that the alkaline battery with a zinc anode contains a device that is described in the claims.

The invention relates to a device for the catalytic recombination of gases generated during charging of an alkaline battery with a zinc anode, characterized in that it consists of a catalytic mass in contact with a porous cellular foam metal that acts as a catalyst substrate and a heat dissipating structure, wherein said catalytic mass contains carbon black containing a platinum group metal and a hydrophobic binder, the combination of which is subjected to heat treatment heel to provide sintering of the hydrophobic binder of said catalytic mass.

Foamed metals are already widely used in the production of alkaline batteries as substrate collectors for electrodes. These foamed metals are made on the basis of an organic porous bubbly substrate with open pores. Preferred substrates are foamed, so-called technical polyurethanes, showing good structure regularity.

The most widely used methods are to inform the foamed organic material of the conductivity by applying an electronic conductor, and then deposit the metal using one or more electrochemical processes, then remove all organic substances by heat treatment, and finally deoxidize and anneal the deposited metal, alloy or several metals forming a final porous structure, which should essentially or completely preserve the initial porosity. Specifically, these methods make it possible to obtain foamed nickel, copper or alloys based on these metals, which can be used in the framework of the present invention.

In the framework of the present invention, for the recombination of the gases generated during charging of the battery, the used foamed metal plays a dual role: on the one hand, it serves as a substrate for the reaction catalyst, on the other hand, it contributes to the removal of heat generated by the recombination of hydrogen and oxygen.

As for heat dissipation, it is provided by radiation, convection, and thermal conductivity. This dispersion is the better, the greater the thermal conductivity of the foamed metal itself. To improve this characteristic, it is recommended in one embodiment of the invention to use foamed copper, since this metal has excellent thermal conductivity.

For such an embodiment, it would be advantageous to use foamed copper or copper alloys, which can be inexpensively produced industrially, as disclosed in French Patent No. 2737507.

In addition, it is necessary that the foamed metal be chemically inert under the conditions of its use and, in particular, both under the conditions of the catalytic reaction and reacting gases, and in the alkaline electrolyte of the battery. For this purpose, a protective layer can be applied to the surface of the cells of the foamed metal of all metals or alloys that do not satisfy these conditions.

In particular, when using foamed copper, it is considered common for the surface of its cells to be coated with an outer layer that protects the copper from corrosion that occurs in the presence of oxygen. This protective layer can be, for example, a nickel layer, which can be applied mainly by electrolysis and which shows a high quality long-term coating, provides effective chemical protection and good temperature characteristics.

Since the circulation of gases in the space located between the top of the electrodes and the battery cover is limited, it is important to ensure that the gases can penetrate into the catalytic structure, which can be achieved, among other things, so that the distance between the device and the thermal collector, which allows the removal heat to the outside of the battery would be as little as possible.

The advantage of the foam type structure with respect to a flat or curved substrate or a substrate with a less developed surface, such as an all-mesh metal, is that it has a higher working density of cells per unit surface than the working density of a less processed surface, and very good access to the inside of the structure.

Therefore, it is possible to implement the catalytic device in such a way that the catalyst is fixed in the cells of the foam substrate by any conventional method and covers the cells of the specified foam substrate, fully preserving its increased porosity, which will allow easy circulation of oxygen and hydrogen inside the substrate.

Dissipation due to thermal conductivity, of course, is the main way to remove the heat generated by the recombination of gases. In the framework of the invention and in order to contribute to the effectiveness of this dispersion method, it is possible to attach in any way, in particular, at one of its edges, a closed catalytic structure to one of the battery terminals, which will act as a heat removal collector, in order to facilitate the "radiator" effect, performed by electrodes attached to the battery terminals and which are in contact with external air. The part of the catalytic structure that is thus in contact with one of the terminals of the battery can be freed from the deposition of a catalyst for better heat transfer between two metal surfaces. It can also be laminated to provide a better contact surface.

Similarly, it is possible to attach, in any generally known manner, especially by soldering, all or part of the foamed metal, in particular an edge or side free of catalyst, to a metal part or plate that can make up all or part of the battery cover to help remove heat outward from the battery.

In the case of using a case and a cover made of plastic materials (nylon, BS, NORYL®, ...) a metal part or plate can be inserted into the cover and communicate with the space outside the case.

The foamed porous metal used as a catalyst support in the device according to the invention can be selected in a wide range of pore sizes and, in particular, from class 30 PP1 (pore per linear inch) inclusive (average pore diameter of about 0.8 mm) to class 90 PP1 inclusive (average pore diameter of about 0.2 mm).

The foamed metals of the invention can be used in a very wide density range; The main restrictions imposed on the material relate, on the one hand, to the need to maintain sufficient open porosity in this substrate structure, and, on the other hand, to have a sufficiently efficient heat sink network. The metal or alloy is chosen, taking into account both of these points equally.

For the initial thicknesses of the foamed metal before possible pressing, usually in the range of one to three millimeters, it is possible to use densities in the range of 200 to 1500 mg / cm ^{2 of the} apparent surface.

Of course, it is possible, without going beyond the scope of the present invention, to perform a catalytic gas recombination device by applying several layers of foamed metal, of which at least one is coated with a catalyst.

Catalysts deposited on a foam metal substrate in order to form a catalytic recombination device according to the invention are those that allow the reaction between oxygen and hydrogen to be catalyzed. We can talk about catalysts based on platinum group metals, such as, in particular, platinum and palladium, while these metals can be combined with forms of coal or graphite, in particular carbon black.

The catalytic mass consists of a mixture of carbon black containing a platinum group metal and a hydrophobic binder, the combination of which is subjected to heat treatment to ensure sintering of the hydrophobic binder of the specified catalytic mass.

Preferably, the catalytic mass consists of a mixture of carbon black on which platinum is deposited. It is preferable to incorporate the catalytic mass into the foamed metal by rolling, pressing, or spraying.

The invention also relates to an alkaline battery with a zinc anode, characterized in that it contains inside its housing a device for the catalytic recombination of gases generated during charging of the system, consisting of a catalytic mass deposited on the surface of a porous foamed cellular metal, which acts as a catalyst substrate and structure thermal dissipation.

Preferably, the catalytic gas recombination device is attached to one of the terminals or to the metal part of the battery cover.

Four examples of the implementation of the metal catalytic structure for recombination are described below, which allow to evaluate the benefits of the present invention. These examples should be understood in an illustrative sense, as in no way limiting the scope of the invention.

Example 1

The carbon black on which platinum is applied is mixed in a ratio of 10% w / w with an oil product with a boiling point of 200 ° C. PTFE (polytetrafluoroethylene) is added in the form of an aqueous suspension with a content of 60% in a ratio of 40% w / w. solid matter. The mixture is kneaded to obtain a paste containing a catalytic mass.

In addition, a strip of foamed nickel of class 45 PPI is cut out (average pore size of about 0.6 mm), 2.5 mm thick, 100 mm long and 15 mm wide, having a density of 50 mg / cm ^{2 of} apparent surface.

The resulting paste is rolled into a 1 mm thick sheet and a strip of 100 mm long and 5 mm wide is cut out. This strip is placed on a strip of foamed metal, centered on it and rolled together until the paste penetrates into the foamed metal. The whole is treated under nitrogen at 300 ° C. for 10 minutes to allow sintering of the hydrophobic binder.

The resulting structure is folded in the form of a spiral and placed in an element of a prismatic NiZn battery with a capacity of 30 A × H containing a reduced volume of electrolyte. The battery cover is equipped with a pressure measuring device that allows you to monitor changes in the internal pressure of the cell during charging and discharging cycles.

One of the edges of the foamed metal is attached to one of the battery terminals so as to exclude the possibility of contact between the spiral and the upper part of the separators, in order to eliminate any risk of polarization of the catalytic structure by the ion gradient. The risk of such contact can be avoided by means of separators made of organic material.

Then the battery case is closed. The battery is tested during cyclic operation in the C / 4 mode by current, which is 7.5 A for a 30 A × H battery, without monitoring the cell voltage at the end of charging. The drawing shows the change in pressure inside the battery without a catalytic structure for recombination of gases according to the invention - curve 1A and with a catalytic structure for recombination of gases according to the invention - curve 1B. It was found that the catalytic structure significantly improves the recombination of hydrogen with oxygen and allows you to maintain low internal pressure. Curves 2A and 2B in the drawing correspond to the values of the battery voltage during cycles without a catalytic structure and with the specified structure.

After more than 10,000 hours of operation, there is no noticeable increase in pressure inside the battery due to a loss of catalyst activity. The pressure limits at which it is possible to use the catalytic structure according to the invention are compatible with the operation of a prismatic nickel-zinc battery with a plastic housing that can be equipped with a safety valve designed to open at 2 bar (about 200 kPa) in a sealed mode (without technical service).

Example 2

An aqueous suspension of carbon black is prepared on which palladium is applied in a ratio of 10% w / w. To the water-carbon black mixture, PTFE is added in the form of an aqueous suspension in a ratio of 30% w / w.

The suspension is filtered and a mixture of carbon black - PTFE is washed. After drying, the resulting powder is suspended in water and dispersed by means of a sonicator.

Foamed nickel of class 60 PP1 (average pore size of about 0.4 mm), 2 mm thick, 50 mm long and 15 mm wide, with a density of 55 mg / cm ^{2 of the} apparent surface is used. The dispersion of the pre-obtained catalytic powder is sprayed using a spray gun used for thin layer chromatography. Carry out several sprayings with drying between them, which can be carried out using a gun with hot air. The operation is carried out on both sides of the foamed metal in order to ensure full coverage of the cells of the foamed metal, but without closing the pores. Measures are taken to prevent any contact of the catalyst on one of the edges of a piece of foam metal to a length of 10 mm.

Thus obtained structure is then dried in a drying chamber at 100 ° C in air, then subjected to heat treatment at 300 ° C in nitrogen atmosphere for 15 minutes.

The catalytic structure is placed in a nickel-zinc battery, similar to that described in Example 1, so that the edge of the piece, not coated with a catalyst, is attached to one of the poles by soldering to a terminal located inside the battery. Under cyclic conditions identical to those described above, changes in internal pressure almost correspond to those on curve 1B of the drawing, remaining below 2 bar (about 200 kPa).

Example 3

A catalytic structure is prepared according to Example 2, the edge of the foamed metal, free of catalyst, is spot-wound to a metal plate, which is previously inserted into the nylon cover of the housing.

Under cyclic conditions identical to those disclosed in Example 1, changes in internal pressure are similar to those in curve 1B of the drawing.

Example 4

A catalytic structure is prepared according to the invention, using the procedure described in Example 1, with the replacement of foamed nickel of class 45 PPI with foamed copper of the same class, but with a density of 35 mg / cm ² , which is electrolyzed by applying a nickel protective layer with a density of 20 mg / cm ² .

Under experimental conditions identical to those described in Example 1, it was found that the internal pressure of the battery in this case also changes according to curve 1B in the drawing.

Of course, the invention is not limited to the individual embodiments set forth above, which have been described as examples. The invention is not limited to the illustrations, but includes all possible options.

Claims (13)

1. A device for the catalytic recombination of gases generated during charging of an alkaline battery with a zinc anode, characterized in that it contains a catalytic mass deposited on a porous cellular foam metal that acts as a catalyst substrate and a heat dissipating structure, said catalytic mass containing carbon black, including a platinum group metal and a hydrophobic binder, the combination of which is subjected to heat treatment to ensure sintering of the hydrophobic with yazuyuschego material of said catalytic mass. 2. The device for catalytic recombination according to claim 1, characterized in that the foamed metal contains nickel or an alloy based on nickel and has a significant or completely open porosity. 3. The device for catalytic recombination according to claim 1, characterized in that the foamed metal contains copper or an alloy based on copper and has a significant or completely open porosity. 4. The device for catalytic recombination according to claim 1, characterized in that the cells of the foamed metal are coated with a protective layer designed to provide chemical inertness of the foamed metal in the conditions of its use. 5. The device for catalytic recombination according to claim 1, characterized in that the catalytic mass is embedded in the foamed metal by rolling or pressing. 6. The device for catalytic recombination according to claim 1, characterized in that the catalytic mass is introduced into the foamed metal by atomization. 7. The device for catalytic recombination according to claim 1, characterized in that the catalytic mass consists of a mixture of carbon black on which platinum is deposited. 8. The device for catalytic recombination according to claim 1, characterized in that the foam metal used has an average pore diameter of from about 0.2 to about 0.8 mm. 9. The device for catalytic recombination according to claim 1, characterized in that the used foamed metal has a density of from 200 to 1500 mg / cm ^{2 the} apparent surface. 10. The device for catalytic recombination according to claim 1, characterized in that it is connected to one of the terminals of the battery in order to facilitate thermal dissipation of heat generated during the exothermic gas recombination reaction. 11. The device for catalytic recombination according to claim 1, characterized in that it is attached to a metal cover or a metal part that is part of the battery cover, to facilitate thermal dissipation of heat generated during the exothermic gas recombination reaction. 12. An alkaline battery with a zinc anode, characterized in that it contains inside its body a device for the catalytic recombination of gases generated during charging, consisting of a catalytic mass deposited on a porous foamed cellular metal that acts as a catalyst substrate and a heat dissipating structure. 13. Alkaline battery with a zinc anode according to item 12, wherein the device for catalytic recombination of gases is attached to one of the terminals or the metal part of the battery cover.

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