JPS6157657B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to primary or secondary cell electrodes in which the electrochemically active part consists of a hydride-forming intermetallic compound, and to primary, secondary and fuel cells equipped with such electrodes. BACKGROUND OF THE INVENTION Electrodes whose electrochemically active parts consist of hydride-forming intermetallic compounds and cells and fuel cells equipped with such electrodes are known per se. Generally it relates to intermetallic compounds that form hydrides with hydrogen equilibrium pressures greater than 1 atmosphere at room temperature. Examples of such intermetallic compounds are e.g. _LaNi5 and
Ti ₂ Ni. The hydrogen equilibrium pressures of these intermetallic compound hydrides at ambient temperature are approximately
3.5 atm and 1 atm. lantern and/
Or nickel can be partially replaced by other metals. Among the group of metal compounds derived from LaNi ₅ , intermetallic compounds are known whose equilibrium pressure of hydrides at ambient temperature is lower than 1 atm, but the high price of rare earth metals prohibits large-scale use. It becomes an obstacle. The use in batteries of intermetallic compounds that produce hydrides with a hydrogen equilibrium pressure of more than 1 atmosphere of hydride at ambient temperature is limited to systems isolated from the environment. In systems that are not shielded from the surroundings, such hydrides are
It will release hydrogen into the surrounding medium. However, this results in, for example, rapidly emptying the secondary battery in the charged state. This risk also exists if a leak occurs in a system that is isolated from the surrounding media. It is further expedient to provide for the prevention of impermissible overpressures in devices, such as valves, which become actuated by a given constant pressure, by means of a system that is isolated from the surrounding medium. Of course, hydrogen is also lost when these valves become activated. For these reasons, there is a need for intermetallic compounds whose hydrides have a hydrogen equilibrium pressure at ambient temperature below one atmosphere and are cheaper compared to intermetallic compounds consisting of rare earth metals. To enable their use at high temperatures, it is precisely desirable that the equilibrium pressure be maintained below 1 atmosphere even at the operating temperature since the equilibrium pressure at ambient temperature is low. Titanium, with a heat of formation of hydrides formed from intermetallic compounds per hydrogen molecule equal to or lower than -10 K Cal., excluding compounds of titanium and nickel;
An intermetallic compound forming a hydride of a metal selected from the group consisting of zirconium and hafnium and a metal selected from the group consisting of copper, cobalt, chromium, nickel, vanadium, manganese and iron. This requirement is met according to the invention by an electrode characterized in that the electrochemically active part is constituted by: The heat of formation of a hydride defined by the general formula ABnHm formed from the intermetallic compound ABn and gaseous hydrogen can be written as: That is, for n1 For n<1 Here, A=Ti, Zr or Hf, B=Cu, Co,
Cr, Ni, V, Mn or Fe. ΔH(ABnHm) is equal to the heat of formation of the hydride, which can be calculated by entering the value for m that applies in the case of complete hydrogenation. Usually this is a number between 1 and 2 hydrogen atoms per metal atom. The heat of formation of binary hydrides and intermetallic compounds is usually calculated experimentally or can be estimated on the basis of known thermodynamic data or by drawing experimental models. If △H resulting from this equation is equal to or negatively larger than -10 KCal./molecule of hydrogen, the hydrogen equilibrium pressure at ambient temperature (approximately 20°C) of the hydride will exceed approximately 20°C. For example, 70℃
It was determined that the pressure was sufficiently lower than 1 atm to enable its use. The following table lists a number of intermetallic compounds, the hydrides formed therefrom in complete hydrogenation and the ΔH calculated in this way per molecule of _H2 .

【table】

[Table] The equilibrium pressures of the hydrides listed in this table are approximately
Reaching 10 ^-1 atmospheres or less. For comparison,
In the calculated △H/H ₂ Kawara molecule of about -6kcal
It should be noted that the equilibrium pressure of LaNi ₅ reaches about 3.5 atm, and the equilibrium pressure of Ti ₂ Ni in the calculated △H/H ₂ molecule of about -9 kcal reaches about 1 atm. . The electrode used in conjunction with the aqueous electrolyte solution according to the present invention can be manufactured, for example, as follows.
That is, for example, after preparation, which consists in melting the desired metals in the appropriate ratios, the intermetallic compound is reduced to a powder by charging it with hydrogen one or more times, liberating it from the hydrogen. let This powder is then applied to a metal support, for example by means of a binder. For this purpose, this powder consisting of all or part of an intermetallic hydride is
This binder is suspended in an organic solvent in which it can also be dissolved. Suitable organic solvents include, for example:
These are toluene, xylene, and propanol. An organic binder is then added to the suspension obtained, and in the next step this binder must be removed by calcination without leaving any residue. A suitable binder for this purpose is
For example, polystyrene or nitrocellulose in an amount of 20 g per 100 ml of solvent. Afterwards, strips of metal gauze are applied as required.
Both or one side thereof is uniformly coated with this suspension and dried. This metal gauze is made of nickel or stainless steel, for example. It can be used as a perforated metal plate or a perforated metal support. The binder is first burned out in a furnace and the powder is then sintered. For this purpose, a constant temperature is maintained for some time at a temperature at which the binder evaporates or decomposes. Generally this temperature is 250-300°C for the binder. Sintering is preferably carried out in vacuum or in a reducing atmosphere at a temperature just below the melting point. Intermetallic powders can also be applied to metal gauze by electrophoresis. Here the powder is suspended in a polar organic solvent such as methanol. A metal support is placed in this suspension and connected, for example, to the cathode. After obtaining the desired layer thickness, it is sintered as described above. The resulting porous layer can then be impregnated with a solution of macromolecular material. However, the macromolecular substance is either hydrophilic and made insoluble in water in a subsequent operation, for example by heat treatment or by infrared radiation, or it is insoluble in water and the macromolecular substance is made insoluble in water. In the next operation, which must, of course, remain insoluble in water,
Either it becomes hydrophilic, for example due to aging. By using this measure, the bond of the electrode material can be maintained during charging and discharging of the electrode. Macromolecular substances having alcoholic hydroxyl groups that can be made insoluble in aqueous electrolyte solutions by physical treatment, such as heat treatment, with or without the presence of auxiliary substances that promote or cause curing or crosslinking. It can be particularly used as a hydrophilic macromolecular substance. For example, polypynyl alcohol appears to be particularly suitable for use in aqueous electrolyte solutions as electrodes according to the invention. In that case, for example, ammonium chloride or sodium bisulfide can be used as auxiliary substances. Then heat treatment in air for 10~
It consists of heating in an oven to 120-150°C for 20 minutes. Water-insoluble macromolecular substances that can be used for this preparation are, for example, aging cellulose acetobutyrate, which can be dissolved in organic solvents. After impregnation, the macromolecular material is aged, for example with an alcoholic solution. The sintering operation described above ensures sufficient hydrogen transfer between the particles of the intermetallic compound. This hydrophilic macromolecular material serves as a binder that ensures permanent cohesion of the porous layer during charging and discharging. The hydrophilic properties of this binder allow the aqueous electrolyte solution to penetrate into the sintered layer allowing ionic transport. Generally, water-permeable binders do not increase the internal resistance of the electrochemical device, or increase it very little. According to another method, which is particularly suitable for testing desired electrode materials, an electrochemically active powder material is mixed with nickel or copper powder and pressed to form an electrode. A primary battery equipped with an electrode according to the present invention can be configured as follows, for example. This anode or cathode contains a hydride of one of the intermetallic compounds according to the invention as electrochemically active material. The cathode, ie the lower potential pole, can be composed of, for example, NiOCH or MnOOH. The electrolyte can consist of an alkaline solution such as a KOH solution. The structure of a secondary battery with an electrode according to the invention differs in that the intermetallic compound, which is part of the cathode, the lower potential pole, is not converted into a hydride, if desired, until the moment the battery is charged. However, the structure is not essentially different from that of a primary battery. When the battery described above is discharged, the following reactions occur. ABnHm→ABn+mH ⁺ +me (Anode) mNiOOH+mH ⁺ +me→mNi(OH) ₂ (Cathode) When charging a secondary battery, these reactions proceed in the opposite direction. Of course, the electrode described can also be used in a fuel cell, in which hydrogen is then provided in gaseous form. The following equation then occurs at the anode: ABn+m/2H ₂ →ABnHm (absorption) ABnHm→ABn+mH ⁺ +me It is of course also possible to use other solvents than water, and given the less inert nature of some intermetallic compounds, there is a risk of reaction with water. This is important if the oxides are produced at the same time. In fact, if the electrode material is at risk of being corroded by the electrolyte, it can be treated by electrolysis, chemical vapor deposition, electroless plating, electrophoresis or other methods suitable for this purpose, e.g. A corrosion-resistant, hydrogen-permeable coating of palladium or TiNi ₃ can be provided by known methods. The invention will be further explained with reference to the drawings. Symbols 1 and 2 indicate the vertical and horizontal intersecting lines of the metal wire fabric. A coating of intermetallic particles 3 is sintered onto either side of this fabric 1-2. Hydrophilic macromolecular substances are present between these particles 3. The secondary battery shown schematically in FIG. 2 consists of a metal gauze strip 22 (anode), i.e. the higher potential electrode Ni(OH) ₂ coated with a hydride-forming intermetallic compound according to the invention. An assembly consisting of a porous nickel layer 23 (cathode) containing a lower potential electrode and a polypropylene fiber separator (felt) 24 is provided in a tube 21 of, for example, nickel or nickel-plated steel. Each of the strips 22 and layers 23 are interconnected (not shown) and connected to electrodes 25 and 26, respectively. Embodiment: Zr ₂ Ni obtained from the melt is mixed with nickel carbonyl powder in a ratio of 1:1 to 100μ in a nitrogen atmosphere.
Milled to smaller particle size. Plates with a diameter of 8 mm and a thickness of approximately 1.0 mm were pressed from this mixture at ambient temperature in a press at a pressure of 0.5 ton/cm ² . Each plate weighed approximately 250 mg. This results in a porosity of approximately 40%;
This is important for proper electrolyte impregnation. Made by this process and Zr ₂ Ni 130mg
The plate containing was used as a test electrode in an electrochemical cell. However, this battery also contains Ni(OH) ₂ /
NiOOH counter electrode, Ag/AgCl reference electrode and
A 6HKOH electrolyte was used. The temperature was 22°C. Under these conditions it became evident that the potential of the reversible hydrogen electrode was −1085 mV with respect to the reference electrode at 1 atm H ₂ pressure. When the test electrode was brought to -1080 mV with respect to the reference electrode, it became evident that an anodic current flowed which was initially 30 mA and then rapidly decreased. The strength of the current
When it dropped below 1.5 mA, the potentiostatic charging process was stopped. At that moment, 104 Asec charges were converted. Then the resting potential of the test electrode is changed to Ag/AgCl
Regarding the reference electrode, it was -1041mV. Other than the production of Zr ₂ Ni hydride, there is no method in this device that can generate an anodic current at a controlled potential. It is clear that from this potential, which makes the cathode the lower potential electrode with respect to the reversible hydrogen potential at a H ₂ pressure of 1 atmosphere, a hydride with an equilibrium pressure clearly lower than 1 atmosphere is formed. When the Nernst equation is applied to the measured equilibrium potential, it becomes clear that it corresponds to an equilibrium pressure of approximately 0.035 atmospheres. If the dissipated charge is related to the amount of Zr ₂ Ni, a capacity of 222 Ah/Kg is found. In this regard, it should be noted that by discontinuing the potentiostatic charging process, the material will not be charged to its maximum capacity. In summary, the present invention provides electrodes for primary, secondary batteries, and fuel cells that have -10 kCal per hydrogen molecule.
It consists of an intermetallic compound that forms a hydride with a smaller heat of formation.

[Brief explanation of the drawing]

FIG. 1 is an enlarged cross-sectional view of a part of an electrode according to the present invention, and FIG. 2 is a side vertical cross-sectional view of a secondary battery according to the present invention. DESCRIPTION OF SYMBOLS 1, 2... Vertical and horizontal intersecting lines of metal wire fabric, 3... Particles, 21... Tube body, 22... Metal gauze strips, 2
3...Porous nickel layer, 24...Separator (felt) made of polypropylene fiber, 25,
26...electrode.

Claims (1)

[Scope of Claims] 1. A primary, secondary or fuel cell electrode in which the electrochemically active part is composed of an intermetallic compound that produces a hydride, wherein the electrochemically active part is composed of an intermetallic compound that forms a hydride, except for a compound of titanium and nickel. a metal selected from the group consisting of titanium, zirconium or hafnium, having a heat of formation of a hydride formed from an intermetallic compound per molecule of hydrogen equal to or less than -10K Cal.; and copper; chromium, cobalt,
An electrode for a primary, secondary or fuel cell, characterized in that it consists of an intermetallic compound that forms a hydride with a metal selected from the group consisting of nickel, vanadium, manganese and iron.