GB2120005A - A method and device for the recombination of hydrogen and oxygen released in electric cells by aqueous electrolytes - Google Patents

Abstract

A method and a device 7 for the catalytic recombination of the hydrogen and oxygen formed in electrical accumulators are described. The disadvantages which have occurred hitherto in prior art, for example the possibility of too great an increase in the temperature of the catalyst, are avoided because the catalyst is flooded by the electrolyte at least during the recombination phase. As a result of this flooding of the catalyst layer the heat of reaction is well dissipated and, in addition, loss of water due to the escape of (hot) water vapour does not occur. The device may also be located external to the accumulator. <IMAGE>

Description

SPECIFICATION A method and device for the recombination of hydrogen and oxygen released in electric cells by aqueous electrolytes The invention relates to a method and device for the recombination of hydrogen and oxygen released in electric accumulator cells by aque ous electrolytes.

In the case of rechargeable batteries a loss of water which lowers the level of the electro lyte is caused by electrolysis of water forming hydrogen and oxygen as a result of self discharge, by conflicting reactions during charging, but mainly as a result of over charging. If a battery is not regularly topped up with water, damage to the battery can occur as a result.

It is known practice for the hydrogen and oxygen gases which are generated by corro sion and electrolysis to be recombined again on a catalyst to form water and for the water formed to be conducted back into the electro lyte.

The chemical reaction of hydrogen and oxy gen to form water is associated with the release of a great quantity of heat so that the catalyst can be heated above the boiling point of water and the recombined water emerges from the recombination zone in the form of water vapour. Since the quantity of heat re leased from the catalyst during an intense generation of gas in the battery is very great and since, secondly, the water vapour stream ing from the catalyst does not dissipate a sufficient quantity of heat, it must be ensured with all recombination elements of this type that the catalyst is not overheated and sub jected to thermal destruction. If the catalyst is fitted in a plastic holder or mount, the plastics material can also be damaged by heat in the event of insufficient heat dissipation.A more frequent occurrence is partial overheating of the the catalyst, which occassionally leads to the ignition of explosion of the gases and thus to the destruction of the battery.

To avoid this, it is proposed in German Patent Specification 22 13 219 that the cata lyst material be attached to a body having good thermal conductivity, preferably copper, in order to ensure a uniform distribution of temperature in the recombination element and thus avoid partial, localised, overheating. It is also known practice for the quantity of reac tion gases conducted to the catalyst to be restricted by frit, which is gas-permeable to a limited extent, by throttle valves or by other means (German Offenlegungsschrift 29 04 842; British Patent Specification No.

1,492,970). In German Auslegeschrift 12 22 154 a method is described which fully utilizes the steep rise in the temperature of the recom bination element, which is independent of the ambient temperature, during recombination of the gases in order to limit or stop the charging current and thus electroysis of the electrolyte.

In this manner the temperature of the catalyst can be kept below the ignition temperature of the gas mixture. A further method of heat dissipation is described in German Offenlegungschrift 23 26 169 by water-repellent recombination elements floating on the electrolyte.

These solutions have the disadvatnage that they are very costly in terms of design or necessarily involve a very high expenditure on control. The reduction of the mass flow of reaction gases to the catalyst considerably impairs the efficiency of the hydrogen/oxygen recombination in the case of open systems. In gastight batteries a great increase in pressure occurs automatically, making high demands on material and dimensioning of the cell case or box. A severe reduction in charging current depending on the catalyst temperature is not advisable with traction or vehicle batteries.

Traction batteries are charged using a charge factor (i.e. percent excess charge) of 1.2 to 1.3 so that the availability of the energy accumulator would be drastically limited if the charging current were severely reduced. Moreover, in order to protect them from overheating, each individual cell would have to be monitored which would necessarily involve an incalculable expenditure on control means. An improvement in heat dissipation by having recombination elements floating on the elctrolyte also involves several disadvantages. In the event of intense gassing in the cell a large quantity of gas will pass through the catalyst without being recombined or else the recombination elements float on a cushion of gas above the electrolyte so that heat dissipation is interrupted by the electrolyte.As a result of the free mobility of the floating bodies, channels can form preventing a large part of the gas from recombining.

The object of the invention is therefore to find a method for the recombination of gasses, in which the measures which are costly in terms of design and control engineer ing can be omitted and in which there is no danger of the catalyst overheating with the consequences mentioned above. Also, the efficiency of the method should be high, i.e. to ensure that, if possible, all the hydrogen and oygen gases evolved during charging and over-charging are recombined to form water and pass back completely into the electrolyte.

According to the invention there is provided a method of recombining hydrogen and oxy gen released in accumulator cells from an aqueous electrolyte, on a waterproof or water repellent catalyst carrier zone, wherein the catalyt carrier zone is flooded by the electro lyte at least during a recombination phase and the mixture of hydrogen and oxygen gases which is to be recombined is conducted through the flooded catalyst carrier zone. Ex cellent heat dissipation and an immediate absorption of the formed water in the electrolyte are achieved by this flooding of the catalyst with the electrolyte. The catalyst carrier zone, which is fixed in the cell by a suitable holder or casing, can consist of individual particles, combined between suitable walls to form a porous mass of catalyst carrier, can form one cohesive moulding or can be composed of a plurality of such mouldings.Examples of the carrier material of catalyst carriers of this type are metal or ceramic powder, the selected grain size of which is greater than that of the pores of the wall so that the powder cannot fall out of the case, fibrous felts, non-woven fabrics, sintered compacts, metal and plastic foams or stacks or batches of several felt or foam bodies.

The actual catalyst is deposited on the carrier material in accordance with known methods. The highest possible degree of homogeneity in the catalyst carrier has been found to be advantageous. In particular, the pores are to have a small band width in relation to the pore diameter. This prevents the gas from passing through a few large-volume pores and, as a result, from avoiding frequent contact with active points on the catalyst.

The catalyst carrier is to be designed with the largest surface area possible in order to force the gas to follow the longest possible and highly tortuous path and make frequent contact with active points on the catalyst to obtain the required high yield during recombination. Carriers having at least 60% and preferably 80 to 95% porosity, pore diameters of 0.02-1 mm, and preferably 0.03 to 0.2 mm, and a "bubble point", as specified in DIN 12741 orASTM E 128-61, of 0.002 to p.1 bar, and preferably 0.01 to 0.06 bars, have proved advantageous. The term, bubble point, means the pressure at which the first gas bubble of an inert gas passes through the carrier. In addition, the catalyst-charged carrier requires intense waterproofing or water-repellent finishing to allow the reaction of the gas mixture in the catalyst to proceed.The water-proofing can be accomplished in a conventional manner, for example by treatment with a PTFE dispersion or by other known methods.

The advantage of this invention is that costly separation of the electrolyte/gas mixture is unnecessary. Likewise, it is unnecessary to take precautions, such as separation of the gas/liquid e.g. by frit or the like, which are intended to prevent droplets of the electrolyte from damaging the catalyst carrier material or de-activating the ctalyst. At the same time the elctrolyte which is constantly circulating around the catalyst layer ensures excellent cooling so that overheating of the recombination unit and the damage associated therewith can be excluded; in addition, the water which is formed during the chemical reaction is directly re-absorbed in the electrolyte.

Preferred carrier materials for the catalyst are electrolyte-resistant plastics material or synthetic material in the form of felts or fibres and/or an electrolyte resistant metal, for example nickel powder and/or any other resistant inorganic material. Particularly suitable electrolyte-resistant synthetic materials are homopolymers or copolymers of polyethylene (PE), polypropylene (PP), perfluoroethylene (PTFE) and the like.

Suitable catalysts are those which are normally used for recombination, in particular the metals of the platinum group which, in accordance with the known methods, are applied in a finely divided form to the carrier. When using organic fibres as carriers, it has occassionally been found advantageous to coat the fibres with a layer of an inactive metal which is inert to the electrolyte, for example nickel, before the catalyst is applied. Better thermal conduction within the carrier body and an improved adherence of the catalyst to the carrier are achieved by using these measures.

The manufacture of the carrier catalyst based on a synthetic carrier can take place, for example, as follows: A bulk quantity of fibriform PTFE (fibre length approximately 3 yo 8 mm, fibre diameter approximately 0.1 mm) is provided with a nickel layer of approximately 10 mg of nickel per gram of PTFE fibre, in accordance with known methods. The catalyst is then applied to the nickei/PTFE mixture by impregnation with metal salt solutions. A platinum/ palladium mixture having a weight ratio of 1:3 is used as the catalyst; the quantity of catalyst metal per gram of PTFE is preferably 2 to 6 mg of mixture. An additional waterproofing, which extends beyond the waterrepellent action of the PTFE, of the active centres for protection from the corrosion attack of the electrolyte by impregnation with PTFE dispersion and drying has been found to be advantageous. The catalyst volume has a bulk density of 0.7 to 0.8 g/cm3 and a porosity of 60 to 70%.

A carrier catalyst based on a nickel carrier has proved equally successful, and it can be manufactured as described below: The platinum/palladium catalyst (2-6 mg per gram of nickel powder) is deposited on nickel powder from a hydrochloric acid solution and the raw material is coated with a porous, highly water-repellent layer. The catalyst volume ready for use has a density of 0.5 to 0.7 g/cm3 and a porosity of 90%. Pore diameters of 0.03 ot 0.2 mm have been found to be advantageous.

The invention also provides a device for the catalytic recombination of hydrogen and oxygen, released in electric accumulator cells from an aqueous electrolyte the said device comprising a gastight or closed case having one or a plurality of gas inlet openings in a bottom part and a water-repellent catalyst carrier in the case carrying the catalyst, wherein the interior of the case is adapted to be flooded with the electrolyte during the recombination reaction at least up to the level of the catalyst zone. Good dissipation of heat from the catalyst is ensured by this flooding action. Flooding is normally achieved by locating the gas inlet openings below the level of the electrolyte, but above the accumulator plates. In this case the gas inlet openings are naturally arranged in such a manner that the gases formed can pass into the catalyst zone.

This can be assisted by installing gas conducting devices, such as inclined walls. If controlled conduction of the electrolyte is provided, for example by a circulating pump, the device is arranged in the flow path of the electrolyte. The gas admission side of the device is then the side on which the gas/electrolyte mixture enters the device.

The design and arrangement of embodiments of the recombination unit will now be described in more detail by way of example with reference to the accompanying drawings in which: Figures 1 to 3 illustrate various embodiments of the invention, and Figure 4 reproduces an arrangement of the invention in a battery system having a circulating electrolyte (controlled conduction).

An electrochemical cell having the recombination unit according to the invention is diagrammatically illustrated in Fig. 1. The electrodes 2 of differing polarity and the earth leads 3, 4 are arranged in the conventional manner in the case 1 of the cell. The cell can be connected to a consumer or to a charging set through poles 5, 6 which are led in a gastight manner through the cell cover. Between the leads 3, 4 and the poles 5, 6 is a recombination unit 7 which has a case in the form of a chamber. The chamber is defined by side walls 10 which are impervious to the electrolyte and gas, a cover 13 and a porous or perforated bottom plate 11 (gas inlet openings) which is gas and electrolyte permeable.

A carrier catalyst 8 rests on the bottom plate 11 and is held together in the required density by a second porous or perforated plate 12 so that the catalyst 8 has a "bubble point" of 0.002 to 0.1 bars, and preferably 0.01 to 0.06 bars, for the reaction gases. The plates 11 and 12 at the same time prevent the catalyst 8 from being washed out into the electrode stack or in an outward direction. In addition, the plate 12 divides the recombination chamber 7 into a space for the catalyst 8 and into an empty space 14 which serves as a pressure or electrolyte balance.

As a result of the gas bubbles rising during charging, electrolyte is displaced in the cell in such a manner that displacement of the electrolyte level 15 occurs, which is compensated for or absorbed by the empty space 14. The gases formed collect firstly in a space or chamber 16 above the plates and force the electrolyte in the space 14 to the level 17. In this case the level of electrolyte 15 in the cell space drops to the level 15a, whereby the gases pass through the perforated plate 11 into the catalyst volume 8, where recombination to form water takes place during cooling by the electrolyte. An aperture 18 serves to balance pressure.

Fig. 2 shows a further embodiment of the invention which has the advantage that by means of lateral poles 205, 206, the space above the plate stack can be adapted specifi cally for the recombination unit. The electro lyte level 215, during charging is shown. On the roof-shaped wall 219 which is impervious to gas (Fig. 2) the rising gas bubbles are collected, and flow along the wall and are forced through the perforated plate 211 (gas inlet opening) into the catalyst 208, which is held together by the upper perforated plate 212 and in which the gases are recombined to form water while being cooled by the electrolyte.

A further embodiment of the invention which also has lateral poles 305, 306 is shown in Fig. 3. The gases formed during charging collect on the wall 319, which is impervious to gas and pass through the perfo rated or porous plate 311 into the catalyst 308 in which the gases are recombined. The recombination unit is always filled with elec trolyte because of the position of the electro lyte level 315. The lower limit of the recombi nation unit is formed by a plate 319 which is perforated on a partial surface 311 and which occupies the entire cross-section of the cell.

The perforated partial surface can occupy up to a quarter of the cell cross-section.

A plate 320, parallel to plate 319 and having a corresponding perforated surface 312, delimits the catalyst 308 which is in the form of a powder bulk, on the top side. In this case the perforated surfaces 311 and 312 are staggered relative to one another in such a manner that the path followed by the gases through the catalyst is maximised. The perfo rated plate 312 and the aperture 316 are used to balance pressure. The advantage of this arrangement lies in the low overall height of the recombination unit and in the long path (from 311 to 312) which the gases must take through the volume and can thus be recom bined to produce a high yield.

The principle of the invention may be trans ferred to recombination units outside the cells and in this connection allows a great number of cells to be operated by the same recombi nation unit. For this purpose the electrolyte of the cells is circulated in such a manner that it floods the recombination chamber and en sures cooling therein. Fig. 4 illustrates this modification. The battery 420 consists of the individual cells 421, which are electrolytically connected to one another by electrolyte distribution channels 423 and electrolyte collection channels 424. Circulation of the electrolyte is effected by the pump 425. The gas formed during charging or over-charging of the cells 421 is conducted together with the electrolyte through the collecting channels 424 into the recombination chamber 407. The catalyst 408 consists of the composition described with reference to Figs. 1 or 2 and is protected from being washed out, as in Fig. 1, by porous or perforated plates 411 and 412. The heat released during the reaction and the water formed are led away by the electrolyte so that overheating and damage to the catalyst volume is avoided. The electrolyte in turn is, if necessary, cooled in a heat exchanger 428 and pumped back into the cells.

Claims (13)

1. A method of recombining hydrogen and oxygen released in electrical accumulator cells from an aqueous electrolyte, on a waterproof or water repellent catalyst carrier zone, wherein the catalyst carrier zone is flooded by the electrolyte at least during a recombination phase and the mixture of hydrogen and oxygen gases which is to be recombined is conducted through the flooded catalyst carrier zone.

2. A method according to Claim 1, wherein an electrolyte flow through the catalyst carrier zone is generated at least during an a cumulator charging process.

3. A method according to Claim 1 or 2, wherein a carrier for the catalyst in the carrier zone has a fibrous structure.

4. A method according to any one of Claims 1 to 3 wherein homopolymers, or copolymers of polytetrafluoroethylene, polyethylene, polypropylene or nickel are used as catalyst carriers.

5. A method according to any one of Claims 1 to 4, wherein the catalyst zone has a bubble point of 0.002 to 0.1 bars.

6. A method according to Claim 5, wherein the bubble point of the catalyst zone is of 0.01 to 0.06 bars.

7. A device for the catalytic recombination of hydrogen and oxygen, released in electric accumulator cells from an aqueous electrolyte, the said device comprising a gastight or closed case having one or a plurality of gas inlet openings in a bottom part and a water repellent catalyst carrier in the case carrying the catalyst, wherein the interior of the case is adapted to be flooded with the electrolyte during the recombination reaction at least up to the level of the catalyst zone.

8. A device according to Claim 7, wherein one or a plurality of apertures is formed in the top part of the case.

9. A device according to Claim 7 or 8, wherein gas inlet openings for the hydrogen are arranged below the level of the electrolyte and above the accumulator plates.

10. A device according to any one of Claims 6 to 9, wherein during the controlled conduction of the electrolyte the device is located in the flow path of the electrolyte.

11. A device for the catalytic recombina- tion of hydrogen and oxygen substantially as described herein with reference to and as illustrated in Figs. 2, 3 or 4 of the accompanying drawings.

12. A method according to any one of Claims 1 to 6, wherein the electrolyte is alkaline.

13. A method of recombining hydrogen and oxygen released in electrical cells from an aqueous electrolyte substantially as described herein with reference to and as illustrated in Fig.1, 2, 3 or 4 of the accompanying drawings.