FR2858464A1 - Device for catalytic recombination of gas during charging of alkaline battery with zinc electrode using metal sponge to support catalyst and to dissipate heat during recombination - Google Patents

Abstract

Device for the catalytic recombination of gas formed during the charging of an alkaline battery with a zinc anode is made up of a catalytic mass in contact with a reticulated, honeycomb metal sponge fulfilling the roles of the catalyst support and the heat dissipation structure. An independent claim is also included for an alkaline battery with a zinc electrode incorporating this catalytic recombination device.

Description

- 1

  Catalytic recombination device for alkaline accumulator gases with zinc anode

  The present invention relates to the field of electrochemical generators, and more particularly that of alkaline storage batteries with zinc anode.

  s It is known that accumulators with aqueous electrolyte consume water during their operation, and more specifically during the overcharging necessary for a complete charge of the accumulator, which causes a decomposition of the water of the electrolyte. , in hydrogen and oxygen.

  There are different ways to manage this water consumption, including: * limiting the overload but at the risk of insufficient charging of the battery; * by using a large excess of electrolyte so as to limit the frequency of the additions of water, a situation which can however only apply to stationary accumulator batteries, because of excess weight and resulting volume.

  These solutions do not prevent the need for periodic user interventions, more or less frequent.

  For a long time, maintenance-free, so-called sealed alkaline accumulators have been manufactured, which nevertheless remain equipped with a safety valve opening in case of excess of the internal pressure of the element.

  These accumulators implement the principle of the recombination of the decomposition gases of water. Common examples are the nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) alkaline accumulators of cylindrical or prismatic formats which are used in portable electric and electronic equipment (telephones, computers, etc.).

  The negative electrode is oversized in capacitance with respect to the positive electrode in a ratio which most often varies from about 1.2 to about 1.5.

  When the nickel positive electrode is fully charged, the cell voltage increases, marking the onset of oxygen evolution, which results from the electrochemical oxidation of the water.

  When overloading this cathode, the negative electrode continues to charge.

  The oxygen formed at the positive electrode will diffuse towards the anode of cadmium or metal hydrides and recombine, either with the metal cadmium, or with the hydrogen adsorbed in the metal hydride. This diffusion is facilitated by the use of an oxygen permeable separator and the use of a reduced amount of electrolyte.

  In alkaline accumulator, the reactions observed at the negative electrode are as follows, where M is the metal entering the reaction: at the positive electrode: 2 OH - H20 + 2 02 + 2e [1] at the electrode negative: M +% O02 - MO [2] MO + 2e '+ H20-M + 2OH [3] French Patent No. 2,788,887 describes the principle of alkaline secondary electrochemical generators with zinc anode, as well as a simple and economical technology implementation, to achieve high levels of performance, especially in terms of life cycle.

  The invention that is the subject of said document relates more particularly to the use of a zinc negative electrode of pasted plasticized type, the active mass of which consists of a mixture comprising at least zinc oxide, a fine powder of conductive ceramic, and a plastic binder.

  According to this technology, the pulverized anodic active mass, which is obtained after mixing the various constituents and a diluent, is introduced into a three-dimensional collector, advantageously constituted by a crosslinked copper foam.

  Alkaline zinc anode accumulators, such as nickel-zinc (NiZn) or silver-zinc (AgZn) produced by the assembly of zinc electrodes manufactured as described above, and cathodes of nickel or silver also of pasted type - plasticized in a nickel foam support, have excellent cycling ability, and offer comparable or better performance than other alkaline secondary generators with nickel positive electrodes with, in addition, the advantage of less cost and the absence of heavy metals.

  The NiZn or AgZn accumulators of this technology can operate in "open" mode or in "semi-sealed" or "sealed" mode.

  The general operating principles that apply to alkaline NiCd and NiMH batteries also apply to zinc anode accumulators.

  In particular, the negative zinc electrode is overcapacitive with respect to the positive electrode.

  However, in the case of nickel-zinc accumulators made according to the technology described in French Patent 2,788,887, the overcapacity of the zinc electrode does not exceed about 20% of the capacity of the nickel electrode, this being a striking a significant difference from what is usually described in the literature, where the zinc anode commonly has an overcapacity of 250 to 500%, in order to artificially reduce the depth of discharge of the anode and to increase its duration operating in cycling.

  In "open" mode, the end of the charge of the accumulator is accompanied by an evolution of oxygen at the positive electrode, and then of hydrogen at the negative electrode when the charge is prolonged. A periodic addition of water is necessary, corresponding to the decomposed amount of electrolyte.

  In "semi-sealed" mode, the accumulator is equipped with a valve that opens at a low pressure, between 10 and 20 kPa. The oxygen formed partially recombines with the metallic zinc of the anode, according to the reaction: Zn +% O2 - ZnO ZnO + 2 e + H2O - Zn + 2 OH The zinc oxide is itself in equilibrium with the soluble form zinc in an alkaline medium, zincate, according to the following simplified equation: ZnO + 2 OH- + H20 _ Zn (OH) 42 In "sealed" mode, all the gases formed must recombine to avoid an excessive increase in internal pressure.

  The principle of operation of a sealed nickel-zinc battery as described above finds its limits for various reasons: * an excessive and uncontrolled load which will lead to an excess of oxygen production, the kinetics of the reaction [1] Overriding those of reactions [2] and [3]; * consequence of the phenomenon described above, aggravated by a slow diffusion of oxygen to the negative electrode, it is fully charged, and there is then a release of hydrogen: H20 + e - OH + 1 / 2H2 metallic zinc is thermodynamically unstable, and tends to corrode with the formation of hydrogen: Zn + 2 H 2 O - Zn (OH) 2 + H 2 "The mode of management of the formed gases, oxygen and hydrogen, is a function of the design of the accumulator and its embodiment, the internal pressure increase favoring the recombination of gas at the electrode of opposite polarity to that in which it is formed, but being only acceptable within narrow limits in certain types. of housing.

  Thus, a cylindrical shaped element with metal housing and cover withstands pressures above 2000 kPa, while prismatic elements will accept maximum pressures of between 500 and 1000 kPa, depending on the dimensions of the accumulator, the nature of the materials and the mode of connection box / lid. For safety reasons, valves equip the covers of recombination accumulators. They are set at about 1500 kPa for cylindrical elements, and up to 200 kPa for prismatic formats.

  The formation of hydrogen and its management is a particularly important aspect of the operation of a sealed nickel zinc battery.

  Various solutions have been proposed for limiting the increase in pressure due to the formation of hydrogen, among which: the use of silver-based catalysts, for example, incorporated in the positive electrode, and allowing the oxidation of hydrogen upon charging according to the reaction: H2 + 2 OH- - 2 H20 + 2e * the use of a third electrode, connected to the positive electrode, and ensuring the oxidation of hydrogen; the use of a catalytic structure consisting of carbon and platinum, deposited on a metal collector or a carbon fabric, responsible for ensuring the recombination of hydrogen and oxygen.

  These various solutions are however not entirely satisfactory, either because of limited hydrogen oxidation kinetics or because of a complex construction.

  One of the limitations to using a catalytic recombinant hydrogen and oxygen structure is the thermal management constraint of the system. Indeed, the reaction between hydrogen and oxygen is very exothermic and can lead to a significant increase in temperature, and the formation of "hot spots", damaging to the proper functioning of the catalyst. It is therefore necessary to ensure rapid evacuation of the calories produced during the recombination reaction.

  Moreover, and this constitutes another difficulty relating to the practical implementation of catalytic structures, the water formed during the recombination of hydrogen and oxygen must not limit the access of the gases to the catalytic sites. .

  The object of the present invention is to meet these various requirements, its authors having for this purpose developed catalytic structures using as supports metal foams, and an implementation adapted to the intended use.

  Metal foams are nowadays widely used in the industry of alkaline accumulators, as support-collectors of electrodes. These foams are made from a porous organic crosslinked porous substrate with open pores. The preferred substrates are so-called technical polyurethane foams with good structural regularity.

  The most widely used manufacturing methods consist of making the organic conductive foam by an electron conductive deposit, subsequently metallizing it by electrochemical deposition (s), then eliminating all organic matter by thermal treatment, and finally deoxidizing and annealing. the metal, alloy or deposited metals, constituting the final crosslinked structure, which must retain its porosity 40 substantially or completely open original. They make it possible in particular to produce foams of nickel, copper or alloys based on these metals, which can be used in the context of the invention.

  In the context of the present invention, for the recombination of the gases formed during the charging of the accumulator, the metal foam used has a dual role: it serves on the one hand to support the catalyst of the reaction, and it also contributes to the evacuation of the calories produced during the recombination of hydrogen and oxygen.

  As regards the heat dissipation, this is ensured by radiation, convection and / or conduction. This dissipation is all the better as the metal constituting the metal foam is itself a good thermal conductor. In order to optimize this characteristic, it is advantageous, in one of the embodiments of the invention, to use a copper foam, this metal constituting an excellent thermal conductor.

  For such an embodiment, it will be advantageous to use foams of copper or copper alloys, such as those which can be industrially produced economically according to the process described in French Patent No. 2,737,507.

  It is furthermore necessary for the metal foam to be chemically inert under its conditions of use, and particularly with respect to the catalytic reaction and the reactive gases, as well as in the alkaline electrolyte of the accumulator. A protective layer may for this purpose be applied to the surface of the mesh of the foam, on any metal or alloy that does not meet these conditions.

  Thus, it is particularly appropriate, for an implementation of a copper foam, that the surface of its mesh is coated with a surface layer protecting copper from corrosion occurring in the presence of oxygen. This protective layer may, for example, be a nickel layer, which may advantageously be made by electrolysis, and have a good continuous coating quality, provide effective chemical protection, and good temperature behavior.

  Although the flow of gases in the space between the top of the electrodes and the battery cover is limited, it is important to ensure that the gases can enter the catalytic structure, which will otherwise be preferably designed in such a way that the distance between the device and a thermal discharge collector towards the outside of the accumulator is as small as possible.

  The advantage of a foam-type structure over a smaller planar or tortuous surface or developed surface, such as expanded metal, is to provide a large mesh density per unit area, thus a large surface area. developed, and a very large access to the heart of the structure.

  Thus, it will be possible to realize the catalytic device in such a way that the catalyst is fixed on the meshes of the support foam by any appropriate means, and covers the meshes of said foam, while maintaining a high porosity which will allow ensure easy circulation of oxygen and hydrogen within it.

  Dissipation by conduction is of course the major mode of evacuation of the calories produced during the recombination of gases. In the context of the invention, and to promote the efficiency of this dissipation mode, it is advantageous to connect, especially at one of its ends, the catalyst-coated structure to one of the terminals of the accumulator, which will act as a thermal evacuation manifold, in order to benefit from the "radiator" effect provided by the electrodes fixed to terminals coming out of the accumulator, and thus in contact with the outside air. The part of the catalytic structure placed in contact with one of the terminals of the accumulator may advantageously be devoid of catalyst deposition for better heat transfer between two metal surfaces. It can also be laminated to provide a better contact surface.

  It may also be fixed by any appropriate means, and in particular solder, all or part of the metal foam, in particular an end or a rim without catalyst, to a metal part or plate that may form all or part of the battery cover, to promote a removal of calories to the outside of the accumulator.

  In the case where a housing and a cover made of plastic material (nylon, ABS, NORYL, ...) are used, the metal part or plate may be crimped into the cover, and communicate with the outside of the housing.

  The crosslinked metal foam used as the catalyst support in the device according to the invention can be chosen from a wide range of pore sizes, and in particular of the linear PPI grade (Pore By linear Inch) included (mean pore diameter about 0.8 mm) at grade 90 PPI included (average pore diameter of about 0.2 mm).

  Metal foams according to the invention can be used in a very broad range of densities, the main constraints which are essential in this respect being, on the one hand, maintaining a sufficient open porosity in this support structure, and on the other hand to have a sufficiently efficient thermal drainage network, the nature of the chosen metal or alloy also having an influence in this field.

  For initial thicknesses of the foam before possible compression, generally between one and three millimeters, densities of between 200 and 1500 mg / cm.sup.2 of apparent surface area can advantageously be used.

  It is of course possible to make a catalytic recombination device by superimposing several strips of foams, at least one of which is coated with catalyst, without departing from the scope of the present invention.

  The catalysts applied in contact with the metal foam support, to constitute a catalytic recombination device according to the invention, are all those which make it possible to catalyze the combination reaction between oxygen and hydrogen.

  It may advantageously be catalysts based on platinum-bearing metals, such as in particular platinum and palladium, and may associate these metals with carbon or graphite, and in particular carbon black.

  Four examples of implementation of recombinant catalytic metallic structures, which make it possible to measure the interest of said invention, are described below by way of non-limiting illustrations of the invention.

Example I:

  10% by weight platinum-containing carbon black is mixed with petroleum having a boiling point of 200 ° C.. PTFE is added in the form of a 60% aqueous suspension. 40% by weight expressed as solids. The mixture is kneaded until a paste constituting the catalytic mass is obtained.

  In addition, a strip of 45 PPI-grade nickel foam (average pore size of about 0.6 mm), thickness 2.5 mm, length 100 mm and width 15 mm and having a density of 50 is cut out. mg / cm 2 apparent area.

  The paste obtained previously is laminated in the form of a sheet of thickness 1 mm, and a strip of length 100 mm and width 5 mm is cut. This strap is placed on the foam strip, centered on it, and the whole is rolled until the paste penetrates into the foam. The whole is treated under nitrogen at 300 ° C. for 10 minutes to sinter the hydrophobic binder.

  The obtained structure is spirally wound and placed in a prismatic NiZn accumulator element with a capacity of 30 Ah, having a reduced electrolyte volume. The battery cover is equipped with a pressure gauge to follow the evolution of the internal pressure of the element during the charging and discharging cycles.

  One of the ends of the foam is connected to one of the polarities of the accumulator, without any possibility of contact between the spiral and the top of the separators, in order to avoid any risk of polarization of the catalytic structure by ionic continuity. It is possible to avoid such a risk of contact by means of a spacer made of organic material.

  The battery box is then closed. The battery is subjected to cycling at a C / 4 ampere rate of 7.5 A for a 30 Ah battery, without control of the voltage of the element at the end of charging. The single figure shows the evolution of the internal pressure of the accumulator without (curve 1A) and with (curve IB) catalytic recombination structure according to the invention. It is found that the catalytic structure significantly improves the recombination of hydrogen with oxygen, and allows to keep a low internal pressure. The curves 2A and 2B of the single figure respectively correspond to the voltage values of the accumulator during the cycles, without catalytic structure and with said structure.

  After more than 10,000 hours of operation, no increase in the internal pressure of the accumulator due to a loss of catalyst activity is observed. The pressure limit values to which the use of the catalytic structure according to the invention is limited are compatible with the sealed (maintenance-free) operation of a prismatic nickel-zinc battery using a plastic housing, which may be equipped with a calibrated safety valve for opening at 2 bar (approx. 200kPa).

Example 2

  A suspension is carried out in carbon black water on which palladium has been deposited at a rate of 10% by weight. PTFE in the form of an aqueous suspension is added to the strongly stirred carbon-water mixture at a rate of 30% by weight.

  The suspension is filtered, and the black carbon - PTFE mixture washed. After drying, the powder obtained is suspended in water and dispersed using a sonifier.

  A 60 PPI grade nickel foam (average pore size about 0.4 mm), 2 mm thick, 50 mm long and 15 mm wide, with a density of 55 mg / cm 2 was used. apparent surface. The dispersion of the catalytic powder previously obtained is pulverized using a spray gun used for thin layer chromatography. Several sprays are carried out with, between each of them, drying which can be achieved with a hot air gun. The operation is performed on both sides of the foam, in order to ensure complete coverage of the mesh of the foam, without however closing its porosity.

  Care was taken to save one end of the strip from any catalyst deposition over a length of 10 mm.

  The structure thus obtained is then dried in an oven at 100 C under air, and then subjected to heat treatment at 300 C under nitrogen for 15 minutes.

  The catalytic structure is placed in a NiZn accumulator similar to that described in Example 1, the end of the uncoated catalyst strip being connected to one of the polarities, by welding on the part of the terminal located at the inside the accumulator. Under cycling conditions identical to those described above, the changes in the internal pressure substantially correspond to those of the curve 1 B of the single figure, remaining less than 2 bar (approximately kPa).

Example 3

  A catalytic structure according to Example 2 is made, the end of the catalyst-free foam being spot-welded onto a steel metal plate 40 which has been previously crimped into the nylon cover of the housing.

  Under cycling conditions identical to those described in Example 1, the evolutions of the internal pressure are similar to those of the curve 1B of the single figure.

Example 4

  A catalytic structure according to the invention is prepared by applying the procedure described in Example 1, the grade 45 PPI nickel foam being here replaced by a copper foam of the same grade, but with a density of 35 mg / cm 2. on which a protective deposit of 20 mglcm2 nickel was made by electrolysis.

  Under operating conditions identical to those detailed in Example 1, it is found that the internal pressure of the accumulator again follows a similar evolution to that described by the curve 1 B of the single figure.

  Naturally, and as is moreover largely the result of the foregoing, the invention is not limited to the particular embodiments which have been described by way of examples. The invention is not limited to the illustrations that have been provided, but embraces all variants thereof.

Claims (16)

  1. A device for catalytic recombination of the gases formed during the charging of an alkaline zinc-anode accumulator, characterized in that it is composed of a catalytic mass arranged in contact with a cross-linked alveolar metal foam ensuring the role of catalyst support and heat dissipation structure.   2. Catalytic recombination device according to claim 1, characterized in that the metal foam is made of nickel, or nickel-based alloy, and has a substantially or completely open porosity.   3. catalytic recombination device according to claim 1, characterized in that the metal foam is made of copper, or copper-based alloy, and has a substantially or completely open porosity.   4. Catalytic recombination device according to claim 1, characterized in that the meshes of the metal foam are coated with a protective deposit intended to ensure a chemical inertia of the foam under the conditions of its use.   5. Catalytic recombination device according to claim 1, characterized in that the catalytic mass, or the catalyst, used is fixed in contact with the meshes of the metal foam by any appropriate means.   6. Catalytic recombination device according to claims 1 and 5, characterized in that the catalytic mass is introduced into the foam by rolling or compression.   7. Catalytic recombination device according to claims 1 and 5, characterized in that the catalytic mass is introduced into the foam 25 by spraying.   8. Catalytic recombination device according to claim 1, characterized in that the catalytic mass comprises a metal of the platinum group.   9. Catalytic recombination device according to claim 1, characterized in that the catalytic mass consists of a mixture of carbon black on which platinum has been deposited, and a hydrophobic binder.   10. The catalytic recombination device according to claims 1, 8 and 9, characterized in that the foam filled by the catalytic mass is heat-treated to sinter the hydrophobic binder of said catalytic mass.   11. Catalytic recombination device according to claim 1, characterized in that the metal foam used has an average pore diameter of between about 0.2 and 0.8 mm. 12. Catalytic recombination device according to claim 1, characterized in that the metal foam used has a density of between 200 and 1500 mg / cm 2 of apparent surface.   13. Catalytic recombination device according to claim 1, characterized in that it is connected by any means to one of the terminals of the accumulator, to promote the heat dissipation of calories provided by the exothermic reaction of recombination of gases.   14. Catalytic recombination device according to claim 1, characterized in that it is connected by any means to the metal lid or to a metal part constituting a part of the battery box cover, to promote the heat dissipation of the cells. calories provided by the exothermic reaction of recombination of gases.   15. Alkaline accumulator with zinc anode characterized in that it comprises, inside its housing, a device for the catalytic recombination of the gases formed during the charging of the system, composed of a catalytic mass arranged in contact with a crosslinked cellular foam, which acts as a catalyst support and a heat dissipation structure.   Alkaline zinc anode accumulator according to claim 15, characterized in that the catalytic gas recombination device is connected by any means to one of the terminals or a metal part of the battery cover.