GB1571299A - Electrochemical electrode - Google Patents

Description

(54) ELECTROCHEMICAL ELECTRODE (71) We, PHILIPS ELECTRONIC AND ASSOCIATED INDUSTRIES LIMI TED, of Abacus House, 33 Gutter Lane, London, EC2V 8AH, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to an electrochemical electrode suitable for use in a electrode primary cell or a secondary cell, or in a fuel cell, the electrochemically active part of which electrode consists of a hydride-forming intermetallic compound, and to a primary cell, a secondary cell or a fuel cell including such an electrode.

Electrodes in which the electrochemical active part consists of a hydride-forming intermetallic compound and primary cells, secondary cells and fuel cells including such electrodes, are known per se. Then known elec trodes use intermetallic compounds which form hydrides having hydrogen equilibrium pressures which exceeds one atmosphere at about 20 C.

Examples of such intermetallic compounds are, for example, Lank, and Ti2Ni. The hydrogen equilibrium pressure at ambient temperature of the hydrides of these intermetallic compounds are approximately 3.5 atmospheres and 1 atmosphere respectively. Within the group of intermetallic compounds derived from Lank5, in which lanthanum and/or nickel can be partly replaced by other metals, intermetallic compounds are known whose equilibrium pressure of the hydrides at ambient temperature is below one atmosphere, however the high price of the rare earth metals is a handicap for large scale use. The use in primary cells in secondary cells of hydrideforming intermetallic compounds having a hydrogen equilibrium pressure at ambient temperature of the hydride which exceeds one atmosphere is restricted to systems which are shut off from the environment. In a system which is not shut off from the environment, such a hydride would release hydrogen into the ambient medium, which might result in a rapid discharge of, for example, a secondary cell from the charged state. This risk is also present if a leak occurred in a system which is shut off from the ambient medium.

With systems which are shut off from the ambient medium, it is furthermore advisable to make provisions which prevent a nonpermissible overpressure in the system, such as safety valves which become operative at a given pressure. Of course, hydrogen is also lost when these valves become operative. For these reasons there is a need for intermetallic compounds whose hydrides have a hydrogen equilibrium pressure at an ambient temperature which is below one atmosphere and which are cheaper compared with intermetallic compounds comprising rare earth metals. To enable their use at a temperature above 200 C, it is even desirable that the equilibrium pressure is so low at about 20" C, that also at the temperature at which the compound is used, the equilibrium pressure remains below one atmosphere.

The invention provides an electrochemical electrode consisting of a support bearing the electrochemically active part of the electrode, which part consists of a hydride-forming intermetallic compound and/or a hydride of said intermetallic compound, wherein said intermetallic compound consists of a metal from the group constituted by titanium, zirconium and hafnium, and a metal from the group constituted by copper, chromium, cobalt, nickel, vanadium, manganese and iron, wherein the hydride of the intermetallic compound has an endothermic heat of formation from the intermetallic compound of at least 10kCal per gram molecule of hydrogen, and wherein the inter metallic compound is not a compound of titanium and nickel.

The heat of formation of a hydride defined by the general formula AB;tHrn from an intermetallic compound ABn and gaseous hydrogen may be written 8 H (ABnH = A H (AHi,'2m) + A H (BnHt2rn) A H (ABn) for n > 1 and A H (AWH,n) = A (AH + A H (BnH,) - A H {ABnj where m X= 1+n nm and y = 1+n for n < 1 where A = Ti, Zr or Hf, and B = Cu, Co, Cr, Ni, V, Mn or Fe.

A H (AB nHrn) is equal to the heat of formation of the hydride, this can be calculated by filling in a value for m which applies in the case of full hydrogenation; usually this is between 1 and 2 hydrogen atoms per atom of metal. The heat of formation of the binary hydrides and the intermetallic compounds are usually calculated empirically or can be estimated on the basis of known thermodynamic data, or by an estimate based on thermodynamic data, if necessary using empirical model of the relevant reactions.

It was ascertained that if the quantity A H (AB Hm) is equal to or more negative than - 10k cal/gram mole H2, the hydrogen equilibrium pressure at ambient temperature (approximately 200 C) of the hydride is sufficiently smaller than one atmosphere to enable the hydride to be used at higher temperatures, for example, up to approximately 70" C.

The following Table lists a number of intermetallic compounds, the hydrides which are formed therefrom by complete hydrogenation and the A H calculated in this manner per gram molecule of H2: TABLE

Intermetallic AH grammol.H2 Compound #H (kcal /g.at) Hydride (kcal) ZrV2 1. 1 ZrV2 H4 -29 ZrCr2 -3.9 ZrCr2H4 15 ZrMn2 -5.2 ZrMn2H4 -16 Zr2 Fe -6.4 Zr2 FeH6 -20 Zr2 Co -10 Zr2 CoH6 -12 Zr, Ni -12 Zr,NiH6 -14 Zr, Cu -7.6 Zr2CuH6 -18 Zr,Fe -4.8 Zr, FeH6 -24 Zr4 Fe 4.0 Zr4 FeH7,5 -26 TiCr, -2.7 TiCr,H4 -13 TiMn 3.8 TiMnH5 -15 TiMn2 -3.3 TiMn2H4 -12 Ti2 Co -5.2 Ti2 CoH6 -12 Ti Cu -5 Ti2CuH6 15 HfV2 -0.6 HfV2H4 -22 HfCr, -3.0 HfCr2H4 12 HfMn2 -4.1 HfMn2H4 -14 Hf2 Fe -5.4 Hf, FeH6 -13 2Co -9.0 Hf2 CoH6 -10 Hf2Ni -11.0 Hf2NiH6 -10 Hf, Mn -3.2 Hf,MnH6 -21 The equilibrium pressures of the hydrides listed in the Table are approximately 10-' atmosphere or less at 200 C. For comparison it should be noted that the equilibrium pressure of LaNi at a calculated A H/gram molecule H2 of approximately - 6 KCal, is approximately 3.5 atmosphere at 20 C and of Ti2Ni at a calculated A H/gram molecule H2 of approximately - 9 kCal, is approximately 1 atmosphere at 200 C.

An electrode according to the invention for use in combination with an aqueous electrolyte may, for example, be produced as follows: After preparation, which may, for example, consist in fusing the required metals in a required ratio, the intermetallic compound is pulverized by charging it with hydrogen and then liberating the absorbed hydrogen. This process may be repeated a few times. Thereafter the powder is applied to a metal support, for example with the aid of a binder. To this end the powder, which may consist wholly or partly of the hydride of the intermetallic compound is suspended in an organic solvent in which the binder can be disolved. Suitable organic solvents are, for example: toluene, xylene, propanol.

Thereafter an organic binder is added to the suspension obtained, in a next step this binder must be removed by firing without leaving any residue. A suitable binder for this purpose, is for example, polystyrene or nitrocellulose in a quantity of, for example, 20 grams per 100 ml of solvent. Thereafter a strip of metal gauze is uniformly coated on both sides or on one side, depending on the requirement, with the suspension and dried. The metal gauze may, for example, consist of nickel or stainless steel.

A perforated metal plate may also be used as a foraminous metal support. The binder is first removed by heating in a furnace, whereafter the powder is sintered. To this end the temperature is kept constant for some time at a value at which the binder evaporates or decomposes. In general this temperature for said binders is 250--300" C. Sintering is effected at a temperature just below the melting point, of the intermetallic compound preferably in vacuo or in a reducing atmosphere. The powder of the intermetallic compound may also be applied to the metal gauze by means of electrophoresis. Herein the powder is suspended in a polar organic solvent such as methanol. The metal support is placed in the suspension and is, for example, connected as cathode. After a layer of the required thickness has been obtained, it is sintered as described above. Thereafter the porous layer obtained may be impregnated with a solution of a macromolecular substance which is either hydrophilic and made water-insoluble in a next operation, for example by a thermal treatment or by radiation, or is water-insoluble and is made hydrophilic in a following operation, for example by saponification, in which the macromolecular substance must, of course, remain water-insoluble. The use of this measure effects that, during charging and discharging of the electrode, the cohesion of the electrode material is retained. Macromolecular substances which are particularly suitable used as hydrophilic macromolecular substances contain alcoholic hydroxyl groups, and can be made insoluble in, for example, an aqueous electrolyte solution by means of a physical treatment such as a thermal treatment either in the presence or absence of an auxiliary substance which promotes or effects hardening or cross-linking. Polyvinyl alcohol, for example, appeared to be particularly suitable for use in an eletcrode according to the invention in an aqueous electrolyte solution. In that case ammonium chloride or sodium hydrosulphide may, for example, be used as an auxiliary substance The thermal treatment then consists of heating in an oven to 120--150" C for 10 to 20 minutes in air. Water-insoluble macromolecular substances which may be used in the production of electrodes according to the invention are, for example, saponifiable cellulose-acetobutyrates which can be dissolved in organic solvents. After impregnating the metal support with the macromolecular substance, the macromolecular substance is saponified with, for example, an alcoholic lye solution.

The sintering operation described above makes it posible for hydrogen transfer to occur between the particles of the intermetallic compound to a sufficient degree. The hydrophilic macromolecular substance serves as a binder which guarantees a permanent cohesion of the porous layer during charging and discharging. Owing to the hydrophilic character of the binder, an aqueous electrolyte solution can penetrate into the sintered layer so that ion transport can occur. In general the hydrophilic binder does not increase or hardly increases the internal resistance of the electrochemical system.

According to another method which is particularly suitable for testing the eligible electrode materials, the electrochemical active pulverulent substances are mixed with nickel or copper powder and are pressed to form an electrode.

A primary cell with an electrode according to the invention, may, for example, be constructed as follows: The anode or negative electrode contains as electrochemical active material a hydride of one of the intermetallic compounds according to the invention. A positive electrode may, for example, consist of NiOOH or MnOOH. The electrolyte may consist of an alkaline solution, such as a KOH solution. The construction of a secondary cell having an electrode according to the invention does not essentially differ from that of a primary cell, except that the intermetallic compound which is part of the anode, is, if so desired, converted into the hydride not before the moment secondary cell is charged. When said primary cells are discharged, the following reactions take place.

ABnHm ABn + mH+ + me (Anode) mNiOOH + mH + mem(Ni(OH)2 (Cathode) When charging a secondary cell, these reactions proceed in the opposite direction. Of course the electrodes described may also be used in a fuel cell, the hydrogen then being provided in gaseous form. Then the following reactions take place at the anode.

m ABn + -H2 AB Hm (Absorption) 2 ABnHm AB, + mH+ + me It is of course also possible to use solvent other than water, and this may be important if, in view of the not so noble character of some intermetallic compounds, there is a risk of a reaction with water, oxides being formed at the same time.

If there is a risk of the electrode material being corroded by the electrolyte, the electrode material may be coated in known manner with a corrosion-resistant hydrogen-permeable coating, for example consisting of palladium or TiN,, by electrolysis, chemical vapour deposition, electroless plating, electrophoresis or any other suitable method.

An embodiment of the invention will now be described with reference to the accompanying drawing, in which Figure 1 is a crosssection of part of an electrode on an enlarged scale, and Figure 2 shows diagrammatically a sectional elevation of a secondary cell including electrodes of the form shown in Figure 1.

Referring to Figure 1, the electrode consists of a metallic wire gauze having cross-wires 1 and 2, and a coating of grains 3 of an intermetallic compound are sintered to each side of this gauze. A hydrophilic macromolecular substance 4 is present between the grains 3.

The secondary cell shown diagrammatically in Figure 2 comprises in a tube 21 of, for example, nickel or nickelplated steel, an assembly composed of metal gauze strips 22 which are coated with a hydride-forming intermetallic compound according to the invention (negative electrode), of porous nickel layers 23 which contain Ni(OH)2 (positive electrode) and a separator of polypropylene fibres (felt) 24. Each of the strips 22 are interconnected (not shown) and each of the layers 23 are interconnected (not shown), the strips 22 and the layers 23 being connected to the poles 25 and 26 respectively.

Zr2Ni obtained from a melt was ground together with carbonyl nickel powder in the weight ratio 1:1 in a nitrogen atmosphere to a particle size smaller than 100 am (The nickel powder was present in order to provide the electrode with sufficient electric conductivity when the zirconium-nickel compound was hydrogenated). At a pressure of 0.5 ton/ cm2, plates having a diameter of 8 mm and approx.mately 1.0 mm thick were pressed from this mixture in a press at the ambient temperature. Each plate weighed approximately 250 mg. From this it follows that the porosity is approximately 40 ', which is important for a proper impregnation with the electrolyte. A plate made according to this procedure and which contains 130 mg Zr2Ni was applied as a test electrode in an electrochemical cell in which also a Ni(OH)2/NiOOH counter electrode, an Ag/AgC1 reference electrode and an aqueous 6N KOH electrolyte was applied.

The temperature was 220 C. It appeared under these conditions that the potential of the reversible hydrogen electrode was - 1085mV with respect to the reference electrode at a H2 pressure of 1 atmosphere. When the test electrode was brought to - 1080mV with respect to the reference electrode, it appeared that a cathodic current flowed which was originally 30 mA and which rapidly decreased thereafter. The potentiostatic charging process was discontinued when the current strength fell to below 1.5 mA; 104 Asec of charge had been converted at that moment. Hereafter the equilibrium potential of the test electrode after the potentiostatic process had been discontinued was - 1041 mV with respect to the Ag/AgC1 reference electrode. Besides the formation of the hydride of Zr2Ni, there are no processes in the system which may cause a cathodic current at the potential applied during the potentiostatic charging process. From this potential, which is anodic with respect to the reversible hydrogen potential at a H2pressure of 1 atmosphere, it appears that a hydride is formed which has an equilibrium pressure which is clearly below 1 atmosphere.

When the Nernst equation is applied to the equilibrium potential measured, it appears that it corresponds to an equilibrium pressure of approximately 0.035 atm. If the charge consumed is related to the quantity of Zr2Ni, then a capacity of 222Ah/kg is found. In this respect it should be noted that by discontinuing the potentiostatic charging process the material is not charged to the maximum capacity.

WHAT WE CLAIM IS: 1. An electrochemical electrode consisting of a support bearing the electroctrochemically active part of the electrode, which part consists of a hydride-forming intermetallic compound and/or the hydride of said intermetallic compound, wherein said intermetallic compound consists of a metal from the group constituted by titanium, zirconium and hafnium, and a metal from the group constituted by copper, chromium, cobalt, nickel, vanadium, manganese and iron, wherein the hydride of the intermetallic compound has an endothermic heat of formation from the intermetallic compound of at least 10 kCal per gram molecule of hydrogen, and wherein the intermetallic compound is not a compound of titanium and nickel.

2. An electrochemical electrode as claimed in Claim 1, wherein the intermetallic compound is an intermetallic compound specified in the Table.

3. A primary cell including an electrochemical electrode as claimed in Claim 1 or Claim 2.

4. A secondary cell including an electrochemical electrode as claimed in Claim 1 or

Claims (1)

1. Claim 2.

   5. A secondary cell including a negative elec trode which is an electrochemical electrode as claimed in Claim 2, substantially as herein described with reference to Figure 2 of the drawing.

   6. A fuel cell including an electrochemical electrode as claimed in Claim 1 or Claim 2.