EP4662060A1

EP4662060A1 - Catalyst device for lead-acid battery, and lead-acid battery

Info

Publication number: EP4662060A1
Application number: EP24709833.8A
Authority: EP
Inventors: Kotaro Kobayashi
Original assignee: WL Gore and Associates GK
Current assignee: WL Gore and Associates GK
Priority date: 2023-02-10
Filing date: 2024-01-23
Publication date: 2025-12-17
Also published as: CN120677059A; WO2024165936A1; JP2026505409A; KR20250149713A

Abstract

Disclosed is a catalyst device for a lead-acid battery and a lead- acid battery comprising the catalyst device. A catalyst layer (120) mounted in a cavity (112) of the device, and a porous layer (140) including thermoplastic material is mounted in the cavity to cover a face of the catalyst layer and is configured to prevent thermal runaway of the catalyst device. The porous layer has a planar size greater than that of the catalyst layer. The catalyst layer is mounted in the cavity within a retaining arrangement (130) configured to retain the position of the catalyst layer within the cavity and with the at least one face covered by the porous layer, to ensure registration of the catalyst layer and porous layer in use.

Description

CATALYST DEVICE FOR LEAD-ACID BATTERY, AND LEAD-ACID BATTERY

FIELD

The present invention relates to catalyst devices for lead-acid batteries, more preferably a catalyst device for reducing gas build-up in, and loss of electrolyte solution from a lead-acid battery, to improve battery safety and longevity.

BACKGROUND

Lead-acid batteries, in particular, automotive lead-acid batteries, generally employ an open structure in which an electrolyte solution such as dilute sulfuric acid can freely flow. Lead-acid batteries having such a structure, when charged, generate oxygen and hydrogen gas and therefore comprise a vent (vent port) for discharging these gases. Otherwise, the gas pressure inside the battery would increase, which may lead to deformation and breakage of the battery.

Such leakage of gases through the vent leads to a decrease in electrolyte solution. The decrease in electrolyte solution results in an insufficient chemical reaction of battery and leads to a reduced charge capacity and discharge capacity.

In response to these problems, it is known to provide a catalyst device exposed to the enclosure containing the electrolyte, to promote recombination of hydrogen and oxygen gas.

JP 2017-201594 (Nihon Gore Co Ltd) discloses a catalyst part for a lead-acid battery, the catalyst part including a catalyst layer including a catalyst to accelerate a reaction for generating water or water vapor from oxygen and hydrogen, and an arrangement through which at least part of the water or water vapor is condensed and/or flowed back to the inside of the battery. The catalyst part can reduce gas release from an electrolyte solution and a decrease in electrolyte solution due to the leakage, thus providing a lead-acid battery having a long life.

US 7326489 (Philadelphia Scientific) discloses a catalyst device to recombine decomposed gases from electrolyte solution. A catalyst material is housed within a container and is separated from the battery enclosure at an open end of the chamber by a microporous member. The microporous member allows gas to pass, but not liquid. Hydrogen and oxygen can pass through the microporous member to the catalyst, where it recombines and the resulting water vapour can pass back into the battery via the microporous member. CN102800831 A and CN202977567U (Zhejiang Narada Power Source etal.) disclose battery vent plugs each with a housing having a lower part that contains a catalyst material and a porous hydrophobic filter layer separating the catalyst from the battery electrolyte enclosure.

As described above, common lead-acid batteries, when charged, outwardly release oxygen and hydrogen gas through a vent, leading to a decrease in electrolyte solution in the lead-acid batteries. The decrease in electrolyte solution ultimately results reduced charge and discharge capacity. The increased concentration of dilute sulfuric acid in an electrolyte solution may cause a positive plate to corrode, and the decreased electrolyte solution level may expose electrode plates and still further reduce the charge/discharge capacity. Increased acid concentration can lead to corrosion within the batter, for example between a negative plate and a strap.

Furthermore, the decrease in electrolyte solution may also lead to sulfation and penetration shortcircuiting. Sulfation is a phenomenon where lead sulfate generated by discharge may not be sufficiently resolved into lead dioxide and lead by charge to form a bulk crystal of lead sulfate. Such a bulk crystal, which is difficult to reduce into a metal lead, reduces battery performance and shortens battery life. In addition, such a bulk crystal is involved also in penetration short-circuiting. Bulk crystals grow on an electrode into a needle crystal referred to as “dendrite”. If the dendrite continues to grow, it may reach the other electrode and thereby cause a short circuit. This is penetration short-circuiting, which makes it impossible to charge and discharge the battery anymore.

Particularly in recent years, automobiles having idling stop systems for improving fuel economy have been increasingly used. Lead-acid batteries used in idling stop vehicles supply power to all the devices such as an air-conditioner and a fan during the idling stop. Thus, the lead-acid batteries tend to be undercharged and used at a low state of charge as compared with conventional starting lead-acid batteries, exacerbating sulfation and penetration short-circuiting.

The risk of sulfation and penetration short-circuiting can be reduced by reducing or preventing the decrease in electrolyte solution. This has led to the adoption of catalyst devices such as those discussed above.

The recombination of hydrogen and oxygen gas is a highly exothermic reaction (-286 kJmor¹) and excessive rates of the H2 + % C>2 -> H2O recombination reaction at the catalyst, caused by overheating or overcharging of a battery for example, can result in overheating of the catalyst. In general, an increase in catalyst temperature may result in an increase in catalyst activity and thus cause a further increase in reaction speed, leading to thermal runaway. Such thermal runaway can risk ignition of the gasses within the battery and thus risk of explosion or fire.

US 7326489 discloses that the catalyst device may be provided with a shut-down function. Specifically, the container around the catalyst material may melt to physically cover the catalyst and quench the recombination reaction. A drawback of the device disclosed is that melting of the container material risks loss of containment of the catalyst material in the container. The catalyst material may then be exposed and continue to overheat, or debris may be released into the electrolyte resulting in shortcircuiting and a potentially dangerous release of electrical energy. Furthermore, for the shut-down to occur, sufficient heat must be conducted from the catalyst material to the container to melt the container, which may take time and allow thermal runaway to progress.

PCT/JP2018/043437 (W. L. Gore & Associates G.K.) discloses a catalyst having a porous thermoplastic membrane in contact with the catalyst material. The melting point of the membrane is 160°C or less, whereby the membrane can melt to reduce the porosity and thereby reduce the rate or completely quench the recombination reaction. However, for some applications, such as certain automotive applications, registration of the thermoplastic membrane with respect to the catalyst (i.e. position of the across the catalyst surface) may change, which risks that some catalyst surface remains exposed to the catalyst enclosure.

SUMMARY OF THE INVENTION

A first aspect of the invention relates to a catalyst device for a lead-acid battery, the catalyst device comprising: a housing, defining a cavity; a catalyst layer mounted in the cavity, wherein the catalyst layer comprises catalyst for accelerating a reaction for generating water or water vapor from oxygen and hydrogen; and a porous layer including thermoplastic material mounted in the cavity, and wherein the porous membrane has a planar size greater than that of the catalyst layer; and wherein a face of the catalyst layer is in contact with the porous layer and covers the at least one face of the catalyst layer; wherein the catalyst layer is mounted in the cavity within a retaining arrangement configured to retain the position of the catalyst layer within the cavity and with the at least one face covered by the porous layer.

The porous layer comprising thermoplastic material (hereinafter referred to as the “porous layer”) is in use disposed between an electrolyte enclosure of a lead-acid battery and the catalyst layer. Hydrogen and oxygen generated within the battery is able to permeate through the porous layer to the catalyst layer, to react in a recombination reaction and form water vapour. The water vapour permeates into the enclosure and condenses to maintain the electrolyte level in the battery. If the temperature of the catalyst layer rises to above a melting point or glass transition temperature of the thermoplastic material, the porosity of the porous layer is reduced to control the rate of the recombination reaction and moderate the temperature of the catalyst layer.

Due to manufacturing tolerances, relative movement of the catalyst layer and porous layer in relation to one another and the housing cannot be entirely eliminated. Furthermore, some clearance between the housing and the layers may facilitate ease of manufacture of the device. The retaining arrangement retains the position of the catalyst layer such that the face of the catalyst layer in contact with the porous layer remains covered within the range of any motion of the catalyst layer or polymer layer within the cavity. Accordingly, the possibility of misregistration of the catalyst layer and porous layer (by which we mean a part of the face of the catalyst layer adjacent to a face of the porous layer not being covered by the porous layer) is reduced or eliminated.

A porous layer may in some embodiments also undergo a degree of shrinkage when the thermoplastic material is heated above the glass transition temperature or melting point, and the larger area of the porous layer accommodates such shrinkage and the face of the catalyst layer remains covered. Contact with the catalyst layer may create friction with the porous layer and resist shrinkage.

By “layer” we refer to a structure having parallel or generally parallel opposite faces each having dimensions greater than a thickness of the layer between the faces. A layer may have one or more peripheral surfaces extending around a periphery and between the faces thereof. A layer as disclosed herein will typically be planar, when mounted in the cavity.

By “cavity” we include a recess or opening sized to receive and retain a catalyst layer and a porous layer. The cavity may be of any shape or configuration, but is conveniently circular in cross section.

The retaining arrangement may comprise one or more retaining formations, to prevent or substantially prevent lateral motion of the catalyst layer.

The retaining arrangement may extend around a periphery of the catalyst layer. One or more retaining formations of the retaining arrangement may extend around at least a part of the catalyst layer.

An inner profile of the retaining arrangement may comprise one or more said retaining formations.

The housing may comprise the retaining arrangement. For example, the retaining arrangement may comprise one or more retaining formations defined by the housing, and wherein the one or more retaining formations extend into the cavity.

The retaining arrangement may comprise a retainer, wherein the retainer is mounted within the cavity in substantially fixed relation to the housing. The one or more retaining formations may be defined by a retainer.

The retaining arrangement may extend around a periphery of the catalyst layer. An inner profile of the retaining arrangement may comprise one or more said retaining formations. For example, the retaining arrangement may comprise a retainer in the form of a ring having a shape generally the shape of the catalyst layer. An inner profile of the ring may function as the retaining formation. That is to say, the catalyst layer may fit closely within the ring

By “ring” we include any structure that extends around the periphery of the catalyst layer, and can be round, square, polygonal, etc. The catalyst layer may have a corresponding shape to that of the ring, or may have a different shape sized to fit within and be retained by the ring.

It will be understood that lateral clearances may be provided between catalyst layer and the retaining arrangement, for example to accommodate manufacturing tolerances or to facilitate assembly; such that the catalyst layer and the porous layer may each be capable of a range of lateral motion within the cavity with respect to the housing.

The range of lateral motion of the porous layer and all the catalyst layer within the cavity is desirably minimised, however, For example be less than 1 mm, less than 0.5 mm, or less than 0.1 mm, in some embodiments.

The porous layer may comprise a peripheral surface region that extends beyond the range of lateral motion of the porous layer in relation to the catalyst layer around the entire periphery of the catalyst layer. For example, where the cavity, catalyst layer and porous layer are generally circular, the centres of the catalyst layer and porous layer may be axially misaligned by up to a first lateral distance. The radius of the porous layer may exceed the radius of the catalyst layer by greater than the first lateral distance, thus, misregistration of the porous layer and catalyst layer is prevented.

The peripheral surface region may at least partially overlay the retaining arrangement.

That is to say, one or more faces of the retaining arrangement may be oriented towards the peripheral surface region of the porous layer, and the porous layer may extend laterally too cover at least a part or all of said faces of the retaining arrangement.

As disclosed herein, the porous layer my in some employments accordingly be bonded to, adhered to, welded to or otherwise joined to the retaining arrangement. The porous layer may in use at least partially seal against the underlying retaining arrangement.

It will be understood that the housing, cavity, porous layer and catalyst layer may each have any suitable shape. The catalyst layer, porous layer and cavity (typically also the housing) are conveniently coaxial and/or circular. The catalyst layer may have a catalyst layer thickness. The retaining arrangement may have a depth (wherein the depth is a distance in a direction between the faces of the catalyst layer when the catalyst layer is mounted in the cavity).

The depth of the retaining arrangement may be greater than equal to or less than the catalyst layer thickness.

The inventors have found that the depth of the retaining arrangement is advantageously similar to or less than the thickness of the catalyst layer.

In some environments, the depth of the retaining arrangement is greater than the catalyst layer thickness, wherein he depth of the retaining arrangement is less than 0.5 millimetres, or less than 0.2 millimetres greater than the catalyst layer thickness.

In some impediments, the depth of the retaining arrangement is less than the catalyst layer thickness where in the depth of the retaining arrangement is at least around 0.2 mm last then or 0.5 mm less than less than the catalyst layer thickness.

Without wishing to be bound by theory, it is believed that having the depth of the retaining arrangement similarto or less than the thickness of the catalyst layer assists in maintaining the porous layer in contact with the at least one face of the catalyst layer, such that the porous layer can more effectively moderate the rate of reaction on the catalyst layer when temperatures become excessive, in use.

The thermoplastic material will inherently have a melting point or glass transition temperature, herein referred to as the first temperature. Around or above the first temperature, the thermoplastic material begins to lose its elastic modulus and the porosity of the porous layer is reduced. In effect, when the thermoplastic material begins to deform or melt, the pores of the porous material coalesce or become blocked.

In preferred embodiments, the retaining arrangement will be dimensionally stable to a second temperature that is higher than the first temperature. In the event of thermal runaway, the retaining arrangement thereby remains dimensionally stable while the porous material moderates or quenches the recombination reaction.

By “dimensionally stable” we mean that the retaining arrangement does not melt, deform or decompose below the second temperature. For example, the retaining arrangement may comprise a polymeric material, having a melting point, glass transition temperature or decomposition temperature above the first temperature. For the avoidance of doubt, thermal expansion does not constitute a loss of dimensional stability. Where the retaining arrangement comprises a thermoplastic material and the second temperature is a melting point or glass transition temperature of the retaining arrangement thermoplastic material, the second temperature may in some embodiments be the same as or substantially the same as the first temperature. For example, the retaining arrangement thermoplastic material may be the same thermoplastic material as that of the porous later. In such cases while the first and second temperatures may be the same, the bulk material of the retaining arrangement may remain sufficiently dimensionally stable while the porosity of the porous layer reduces. Where the first and second temperatures are the same or similar, this may assist in adhering the porous layer to the retaining arrangement.

In preferred embodiments, the housing will be dimensionally stable to a third temperature. The third temperature may be higher than the first temperature. The second and third temperatures may in some embodiments be the same.

The porous layer may comprise a porous membrane. The porous layer, or the porous membrane, may be perforated.

In some embodiments, the porous layer may comprise an expanded thermoplastic polymer membrane, such as expanded polyethylene, polyurethane, or the like.

The glass transition temperature or melting point of the thermoplastic polymer material of the porous layer may be 200°C or less, or 180°C or less, or 160°C or less, or 140°C or less. The glass transition temperature or melting point of the thermoplastic polymer material of the porous layer may be in the range from 50°C to 200°C, or in the range from 60°C to 180°C, in the range from 100°C to 180°C, in the range from 100°C to 160°C, in the range from 100°C to 140°C The glass transition temperature may be from 50°C to 180°C, or in the range from 60°C to 160°C, or in the range from 80°C to 140°C.

The skilled person will understand that the thermoplastic material may comprise one thermoplastic polymers or in some embodiments more than one thermoplastic polymer. The skills person will furthermore understand that around all above the glass transition temperature (in the case of an amorphous or a semi-crystalline polymer) and/or the melting point (in the case of a crystalline or semicrystalline thermoplastic polymer), the thermoplastic polymer begins to lose its elastic modulus and becomes dimensionally unstable. Typically a polymer above the glass transition or temperature or melting point becomes flowable. Additionally, for a given type of polymer material the glass transition temperature or melting point may depend on a number of factors such as the average molecular weight or range of molecular weights of the polymer, the ratio of monomers present in a copolymer, or the like. The skilled person will therefore be able to select a desired melting point of glass transition temperature for a particular purpose (for example a maximum temperature allowable for a particular battery enclosure, or catalyst material) by suitable selection of thermoplastic material composition.

The catalyst device may comprise a porous layer in contact with both faces of the catalyst layer. The catalyst device may comprise at least two porous layers, wherein a first porous layer is in contact with a first face of the catalyst layer and a second porous layer is in contact with an opposite second face of the catalyst layer, each porous layer having a planar size greater than that of the catalyst layer.

The catalyst device may comprise at least two porous membranes. Each porous layer may comprise more than one porous membrane.

The or each porous layer may be laminated to and in contact with corresponding face of the catalyst layer.

By “laminated” to, or a “laminate”, we refer to structure formed by applying force between and through the thickness of respective layers, to bind the layers together. A laminated structure will typically be formed by the application of pressure, for example by pinch rollers or the like, and heat. Lamination may cause melting or flowing between the materials being laminated, or in some important may cause chemical reaction to occur for example of an adhesive material that between.

The each porous layer may be bonded to and in contact with the corresponding face of the catalyst layer. For example, in some embodiments adhesive such as contact adhesive may be used to bond the porous layerto the catalyst layer. Adhesive may be used discontinuously, for example in dots, to reduce the risk of blocking the poles of the porous layer or covering too great an area of the face of the catalyst layer. Bonding may alternatively comprise melting or thermally welding.

In some embodiments, alternatively or additionally, some or all of the peripheral regions of at least two of the porous membranes are bonded to one another. For example, the porous membranes may extend over and around both said retainer and the catalyst layer therein, and be bonded to one another.

The at least two porous membranes may form an envelope or pouch around the catalyst layer and retainer, in some embodiments.

The or each porous layer may be laminated or bonded to the retaining arrangement. A porous layer may for example be laminated to both the catalyst layer and a retainer around the catalyst layer, on one or both sides thereof.

The catalyst device may comprise a diffusion limiting layer, adapted to regulate rate of diffusion or flow of gas to the catalyst layer.

The diffusion limiting layer may be adjacent to (in a direction of diffusion of gas towards the catalyst layer) the porous layer. The porous layer may be between the diffusion limiting layer and the catalyst layer. The diffusion limiting layer may be in contact with the porous layer on a face opposite to the face of the porous layer in contact with the catalyst layer. The diffusion limiting layer may be laminated to or bonded to the porous layer.

The skilled person will understand that in use gas within a lead acid battery may diffuse from the electrolyte enclosure to the catalyst layer of a device in accordance with the invention in fluid communication therewith. In some operating conditions, such as at elevated temperature, pressure within the battery enclosure may exceed ambient pressure. In embodiments of the device incorporating also a vent to vent such excess pressure from the battery, a flow of gas from the enclosure may occur, wherein the catalyst device may be located across a flow pathway gas from the battery enclosure to an outside environment.

The diffusion or flow limiting effect of the diffusion limiting layer may be reflected by a comparatively high Gurley airflow number, such as a Gurley airflow number of 50 second or more, or 100 seconds or more.

The diffusion limiting layer may comprise a porous fluoropolymer layer.

The porous fluoropolymer layer may be hydrophobic.

The said porous fluoropolymer layer may comprise a fluoropolymer membrane. The fluoropolymer membrane may be an expanded fluoropolymer membrane, such as expanded polytetrafluoroethylene.

An expanded polymer membrane, such as an expanded polyethylene or an expanded polytetrafluoroethylene membrane as disclosed herein comprises a microstructure of fibrils, typically nodes interconnected by fibrils, and pores extending therebetween.

The porous fluoropolymer layer may have a Gurley number of 100 seconds or more. The porous fluoropolymer layer may have a Gurley number of 200, 300, 400, 700 or 1000 seconds or more.

The expanded porous polytetrafluoroethylene may have a Gurley number of 100 seconds or more. The porous fluoropolymer layer may have a Gurley number of 200, 300, 400, 700 or 1000 seconds or more.

The catalyst device may comprise a porous anti-poisoning layer, capable of absorbing or decomposing a catalytic poison.

The anti-poisoning layer may be adjacent to (in a direction of diffusion of gas towards the catalyst layer) the porous layer. The anti-poisoning layer may contact a face of the porous layer, and optionally laminated or bonded thereto. Where present, the anti-poisoning layer may be adjacent to, in contact with, laminated or bonded to the diffusion limiting layer. The anti-poisoning layer may contact a face of the porous layer, and optionally laminated or bonded thereto.

That is to say, the anti-poisoning layer may be between the porous layer and the diffusion limiting layer, or vice versa.

Examples of a composition that acts as a catalytic poison is dilute sulfuric acid in an electrolyte solution, or sulfide which is generated from dilute sulfuric acid, such as H2S. Materials such as antimony and arsenic may also be present and function as a catalytic poison in lead-acid batteries. The catalytic poison, when coming into contact with a catalyst, reduces its catalytic performance. The substance capable of absorbing or decomposing a catalytic poison may be activated carbon, ZnO, potassium carbonate, or the like, and such materials can absorb or decompose a catalytic poison.

The anti-poisoning layer may comprise a membrane. The membrane may be an expanded polymer membrane. The porous anti-poisoning layer or membrane may comprise a fluoropolymer. The porous anti-poisoning layer or membrane may comprise polytetrafluoroethylene, such as ePTFE. A woven, non-woven or knitted fabric may also be used.

The anti-poisoning layer may be hydrophilic. For example the anti-poisoning layer may comprise a membrane that has been subjected to a hydrophilization treatment, whereby catalytic poisons may be adsorbed more readily. In one example hydrophilization treatment, a metal oxide gel may be used. Specifically, a sol of a hydrophilic metal oxide is provided, and a porous member is immersed in the sol, which gelates afterward. In this manner, the inner surface of pores of the porous member can be modified by the hydrophilic oxide gel. For example, based on the sol-gel process, the surface of the member may be coated with a silica material for hydrophilization. Hydrophilization can also be conducted by a surface treatment with plasma or the like, as known to one skilled in the art.

The anti-poisoning layer may comprise a substance capable of absorbing or decomposing the catalytic poison inside the porous layer capable of absorbing or decomposing the catalytic poison.

The term “inside the porous layer” (or membrane, as the case may be) means that the substance capable of absorbing or decomposing a catalytic poison may be present or arranged in cavities or on or embedded into the surface of the pores of the porous layer, analogously for example to the catalyst inside the catalyst layer illustrated in Figure 8 and discussed in further detail below.

The catalyst device may comprise an outer hydrophobic porous layer having a Gurley number lower than the Gurley number of the diffusion limiting layer. The outer hydrophobic porous layer may have a Gurley number lower than 20 seconds, for example. The outer hydrophobic porous layer may, in use, inhibit or prevent sulfuric acid mist and/or electrolyte solution (typically an aqueous dilute sulfuric acid solution) from coming into direct contact with the catalyst layer, and thereby may increase the life of the catalyst device.

The outer hydrophobic porous layer may comprise a fluoropolymer. The outer hydrophobic porous layer may comprise a membrane. The membrane may be an expanded polymer membrane, such as an expanded fluoropolymer membrane. The membrane may comprise an ePTFE membrane.

The outer hydrophobic porous layer may be positioned so as to be located closer to the electrolyte enclosure of a battery, in use.

For example, in some environments the cavity has an open end (which in use is oriented towards or in fluid communication with an electrolyte enclosure) and the outer hydrophobic porous layer is located closer to or, in some environments across, the open end of the cavity.

The catalyst layer may be porous.

The catalyst layer may include a hydrophobic porous member. In the catalyst layer, hydrogen gas and oxygen gas generated by a battery reaction recombine to form water or water vapor, and the environment in or around the catalyst layer therefore tends to be humid. When a catalyst is covered by water or water vapor, hydrogen gas and oxygen gas are less likely to come into contact with the catalyst, and the catalyst reaction (recombination reaction) tends to be less efficient. Provision of a catalyst layer having a hydrophobic porous member facilitates the release of the water or water vapor generated out of the catalyst layer, and thereby may improve efficiency. The hydrophobicity may in some embodiments also facilitate return of the water or water vapor to the electrolyte enclosure of the battery, in use of the catalyst device.

The catalyst, or a catalyst support supporting the catalyst may be arranged in the cavities or on the surface of the pores of the catalyst layer (or hydrophobic porous member thereof). In this case, the catalyst support, in particular, the catalyst is exposed in the cavities of the catalyst layer, such that hydrogen gas and oxygen gas may contact the catalyst.

The catalyst layer may alternatively comprise a molded, pelletized or sintered powder. The catalyst layer may comprise a molded, pelletized or sintered powdered catalyst support. The catalyst layer may comprise a polymer powder, mixed with powdered catalyst support, wherein the mixture is molded, pelletized or sintered to form the catalyst layer.

The catalyst layer (or hydrophobic porous member thereof) is preferably unreactive with other materials inside the battery, such as salts of sulfuric acid. Indeed other layers of the catalyst device, such as the outer hydrophobic porous layer, the diffusion limiting layer and/or the anti-poisoning layer may furthermore comprise materials or membranes preferably unreactive with other materials inside the battery, such as salts of sulfuric acid.

For example, polypropylene and PTFE can be used, and woven fabrics, nonwoven fabrics, knitted fabrics, and porous membranes thereof may also be used. The catalyst layer may comprise an expanded polymer membrane.

The catalyst layer (or hydrophobic porous member) may comprise porous polytetrafluoroethylene (PTFE), such as ePTFE .

Polytetrafluoroethylene, which by nature has excellent properties such as hydrophobicity, chemical resistance, UV resistance, oxidation resistance, and heat resistance, is suitable as a constituent material of a battery. Polytetrafluoroethylene can be made porous by expanding, optionally biaxially, a PTFE sheet at a controlled expansion ratio and temperature conditions, to form a porous ePTFE layer, as known to one skilled in the art. More particularly, expanded porous polytetrafluoroethylene is composed of nodes (knots) and fibrils (small fibers). A catalyst or a catalyst support is held in microcavities (micropores) defined by the nodes and/or the fibrils. The nodes and the fibrils are both made of polytetrafluoroethylene, and the difference between them is thought to be due to the difference in the state of aggregation or crystallization of polytetrafluoroethylene molecules. Generally, it is believed that a node is an aggregate of polytetrafluoroethylene primary particles, whereas a fibril is made of a bundle of crystal ribbons expanding from the node, i.e., the primary particles.

Figure 8 schematically illustrates catalyst particles supported on surfaces of a catalytically inert powdered support. The catalyst material comprising the catalyst and support are inside the ePTFE membrane, between the fibrils and nodes thereof. Such materials may be made in the manner of other expanded polymer materials, by mixing the components thereof in powdered form, then extruding (e.g. to form a tape) and expanding under controlled temperature and expansion ratio conditions, to form an expanded membrane.

The catalyst may be any catalyst for recombining hydrogen and oxygen to form water, and examples include Pd, Pt, and Au. The support supporting a catalyst may be any support having a specific surface area sufficient to support the catalyst in a desired dispersed state. The support can be selected from the group consisting of silica, alumina, zeolite, carbon, oxides and carbides of Group IVB, VB, VIB, VIIB, and VIII transition metals, and combinations thereof. Alternatively, the support may be a carbon material. It is not preferred that a supporting material effect a chemical reaction other than the desired reaction or substances constituting the supporting material be eluted upon contact with condensed water. In this regard, carbon materials are chemically stable and preferred supporting materials. Examples of carbon materials include carbon black (e.g., oil furnace black, channel black, lamp black, thermal black, and acetylene black), activated carbon, coke, natural graphite, and artificial graphite. These may be used in combination. The catalyst layer may further include a substance capable of absorbing or decomposing a catalytic poison, for example as an alternative to the device including a separate anti-poisoning layer. The substance capable of absorbing or decomposing a catalytic poison may be mixed with the catalyst material, or support comprising catalyst material, and/or may be dispersed within the catalyst layer (e.g. within a porous membrane, as disclosed herein).

In a further aspect there is provided a lead-acid battery, the battery comprising an electrolyte enclosure and one or more catalyst devices in accordance with the first aspect, wherein the cavity of the or each catalyst device is in fluid communication with the electrolyte enclosure. The cavity of the or each device may, in use, be positioned above the nominal level of electrolyte liquid (or otherwise isolated from the electrolyte liquid). The housing of the or each device may extend into the enclosure. An open end of each cavity may extend into the enclosure.

The lead-acid battery may include two or more or a plurality of cells, each having an electrolyte enclosure. Each electrolyte enclosure may be provided with one or more catalyst devices in accordance with the invention.

When there are multiple cells, catalytically generated water or water vapor derived from an electrolyte solution in a cell can move to other cells. In this case, the amount of electrolyte solution can differ from cell to cell. At least one catalyst device in each cell may help hydrogen gas and oxygen gas generated in each cell to recombine in the catalyst layer in each cell and help the water or water vapor generated to flow back to the cell (the cell from which the water or water vapor is derived). This is useful for avoiding the difference in the amount of electrolyte solution from cell to cell.

Unless otherwise stated, when applied to a structural feature, material, composition or method, the term “comprise” herein means to include the recited structural feature, material, composition or method, in additional to other structural features, materials, compositions or method steps, etc, or to consist of that structural feature, material, composition or method step.

BRIEF DESCRIPTION OF DRAWINGS

Example embodiments will now be described with reference to the following drawings in which:

Figure 1 is an exploded perspective view of an embodiment of a catalyst device;

Figure 2 is a schematic plan view of the catalyst device of Figure 1 ;

Figure 3 is a schematic cross sectional view of the catalyst device of Figure 1 ;

Figure 4-6 are a schematic cross sectional views of alternative catalyst devices; Figures 7(a)-(e) are schematic plan views of example catalyst devices;

Figure 8 is an illustration of the microstructure of a porous layer having particulate material inside the layer;

Figure 9 shows (A) a test enclosure; (B) a schematic diagram of the supply of feed gas to the test enclosure; and (C) a schematic cross sectional view of a catalytic test device; and

Figure 10-12 show test data for Examples 1-3

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Figure 1 shows an exploded view of a catalyst device 100 for a lead-acid battery. Schematic plan and cross sections views of the device 100 are shown in Figures 2 and 3.

The catalyst device has a housing 110, typically molded from a plastics material such as polypropylene. The housing defines a cavity 1 12 (in the embodiment shown a cylindrical cavity), having an open end (indicated generally as 114). In use, the device is configured to be inserted into an aperture in the housing of a battery, such that the open end 114 of the cavity 112 faces into the enclosure.

The catalyst device 100 has a catalyst layer 120. In the embodiment shown, the catalyst layer is formed of two plies or sub layers 120a, 120b to form a desired thickness of the catalyst later. The catalyst layer 120 is mounted within the cavity 112 and is surrounded by a retaining arrangement, in the form of a ring 130. The ring 130 will typically also be formed of a plastics material, such as polypropylene.

A porous layer 140 is mounted in the cavity. The porous layer has an area that is greater than the area of the upper face 122 (in the orientation of the figures) of the catalyst layer 120. The porous layer 140 is in contact with and covers the upper face 122. Accordingly a peripheral region of the 144, of the face 142 adjacent to the catalyst layer 140 partially overlays a part of the end 132 of the ring 130.

The porous layer 140 comprises a thermoplastic material. A porous polyethylene material may be used, for example, in particular an expanded polyethylene membrane, where the polyethylene has a melting point below that of the polypropylene housing 110 and ring 130.

The catalyst layer 120 is mounted in the cavity 1 10, within the ring 130 and he porous layer 140 is also mounted in the cavity in contact with the face 122 of the catalyst layer 120. The ring 130 constrains lateral movement (i.e. generally parallel to the faces 122, 142), such that within the range of motion possible, the surface 122 remains covered by the porous layer 140. This is illustrated in Figure 2. To assist in assembly of the layers of the device, and due to manufacturing tolerances, there is provided a small clearance between the inner profile 133 of the ring 130 and the outer edges 124 of the catalyst layer 120, which provides for a maximum lateral displacement L1 between the centre of the catalyst layer 126 and the centre 116 of the housing before the catalyst layer abuts the ring. The porous layer 140 similarly has clearance to move laterally to a maximum lateral displacement L2 between the centre of the porous layer 146 and the centre of the housing 116, before abutting the inner wall 1 13 of the cavity 112.

As shown in the expanded view of region A of Figure 2, the maximum possible lateral offset between the centres 126, 146 of the layers 120, 140 is thus L1 + L2. Advantageously, the radial thickness of the ring 130 can be made to be equal to or more preferably greater than L1 + L2, to thereby ensure that the face 122 of the catalyst layer 120 cannot become uncovered by the porous layer 140 due to said lateral motion (as might occur during vibrations in use, for example).

It will be understood that the maximum lateral displacements are typically small, in the range of less than 1 mm, less than 0.5 mm of less than 0.1 mm.

When the porous layer 140 is in use exposed to an electrolyte enclosure of a lead-acid battery, hydrogen and oxygen generated within the battery is able to permeate through the porous layer to the catalyst layer 120, to react in a recombination reaction and form water vapour, which water vapour or water may then be returned to the enclosure. If the temperature of the catalyst layer 120 rises to above a melting point or glass transition temperature of the thermoplastic material, the porosity of the porous layer is reduced to control the rate of the recombination reaction and moderate the temperature of the catalyst layer. The porous layer 140 has a greater surface area than the catalyst layer 120, and the peripheral region 144 of the porous layer 140 extends sufficiently beyond the face 122 of the catalyst layer 120 that lateral movement of the porous layer 140 within the cavity 112 and the catalyst layer 120 within the retaining arrangement 130 cannot leave any part of the face 122 exposed. The material selection for the housing 110 and retaining arrangement, ring 130 (in the embodiment shown, polypropylene) and the porous layer 140 (in the embodiment shown, polyethylene thermoplastic material) ensures that when the thermoplastic material of the porous layer 140 begins to melt, the housing and retaining arrangement remain dimensionally stable, such that misregistration cannot occur.

As most clearly shown in the cross sectional view of Figure 3, the depth di2o of the catalyst layer 120 is greater than the depth d o of the ring 130. The porous layer 140 covering the catalyst layer 120, in the embodiment shown a flexible polyethylene membrane, thereby effectively wraps over and around the periphery of the catalyst layer 120 and moderates the rate of the recombination reaction.

In other embodiments (not shown) the depths of porous layer and retaining arrangement substantially the same, or the depth of the retaining arrangement may in some cases be greater. The catalyst device 100 also includes diffusion limiting layers 150, in the embodiment shown in the form of an ePTFE membrane having a Gurley airflow number of around 100 seconds. A diffusion limiting layer 150a is positioned adjacent to the porous layer 140 and is between the catalyst layer 120 and the open end 114 of the cavity 112, so that in use it is between the electrolyte enclosure and the catalyst layer 120 to control the rate of diffusion or flow of gas from the enclosure to the catalyst layer 120.

In the embodiment shown, a further diffusion limiting layer 150b is positioned adjacent to and covering the lower face 123 (in the orientation shown in the figures) of the catalyst layer 120, to regulate diffusion or flow of any gas that passes around the catalyst layer 120 and ring 130 to the lower face 123.

The catalyst device 100 further includes a porous anti-poisoning layer 160 adjacent to the diffusion limiting layer 150b. The anti-poisoning layer 160 includes a substance capable of absorbing or decomposing a catalyst poison such as hydrogen sulfide. An example of a suitable anti poisoning layer 160 is a porous membrane, conveniently an ePTFE membrane, comprising zinc oxide powder inside the porous membrane.

An outer hydrophobic porous layer 170, which in the embodiment shown is formed from ePTFE membrane, is positioned across the open end 114 of the cavity 112 and sealed or bonded to the housing around the periphery of the cavity 112. the outer hydrophobic porous layer 170 has a comparatively lower Gurley air flow number than the diffusion limiting layers 150a, 150b, conveniently in the range of around 20 Gurley seconds. The outer hydrophobic porous layer 170 prevents liquid electrolyte solution from entering the cavity 112, in use.

Whilst in the embodiment shown the various layers are positioned against one another, in alternative embodiments, one or more of the layers may be laminated or otherwise bonded to one another, as disclosed herein, furthermore, more than one of each type of layer may in some cases be used, such as two porous layers adjacent to one another. Some embodiments may emit one or more of the layers of the device 100. For example in some cases the outer layer may function as a diffusion limiting layer, obviating the need for a separate diffusion limiting layer. Optionally, the anti-poisoning layer may also be omitted for some applications of a catalyst device as disclosed here in.

As discussed above, the melting point or the glass transition temperature of the thermoplastic material of the porous layer 140 can be appropriately selected depending on the amount of gas generated in a particular battery application, catalytic performances, and the like. The melting point may for example be in the range from 100°C to 180°C, or around 160°C. Whilst the porous layer 140 of the device 100 comprises polyethylene, thermoplastic material may in other embodiments be polyethylene having a higher or a lower melting point, polypropylene, polyvinyl chloride, polymethyl methacrylate, polystyrene, or polyvinylidene fluoride for example. Table 1 represents the melting point and the glass transition temperature of a range of example thermoplastic materials which may be used. Table 1 : Melting point and glass transition temperature of each thermoplastic resin

Figure 4 shows a cross sectional view of an alternative catalyst device 100A. Features in common with the device 100 are provided with like reference numerals.

The device 100A includes a housing 110, defining a cavity 112. The catalyst device 100A has a catalyst layer 120, each face 122, 123 of which is covered by a porous layer 140 and an adjacent diffusion limiting layer 150. An anti-poisoning layer 160 and an outer porous hydrophobic layer 170 is also provided. The device 100A further includes a retaining arrangement 130 in the form of a ring 130 in the cavity 1 12.

A further catalyst device 100B is illustrated in Figure 5, in which the housing 110B has an open end 114B and a closed end 115B. The catalyst layer 120 is mounted in a recess 130B in the closed end 115B of the housing 110B, with an inner wall 133B of the recess limiting lateral movement of the catalyst layer 120, the recess 130 thereby functioning as a retaining arrangement.

The anti-poisoning layer 160 is adjacent to the diffusion limiting layer 150 but spaced apart therefrom. The anti-poisoning layer 160 and the porous hydrophobic outer layer 170 extend over the open end 114 of the housing 110B.

Another example of a catalyst device 100C is shown in Figure 6. The catalyst device 100 see is similar to the catalyst device 100 except that a respective porous layer 140a, 140b covers respective faces 122, 123 of the catalyst layer 120. The porous layers 140a, 140b are joined (e.g. by thermal welding, or bonding with adhesive) to one another around the outside of the ring 130, in region 148, to completely enclose the catalyst layer 120 and the ring 130.

In other embodiments (not shown) respective porous layers can be joined the retaining arrangement, for example around a periphery of a retainer, to encapsulate the catalyst layer.

As shown in the schematic plan view of Figure 7, various relationships between the shape of the catalyst layer and the retaining arrangement is possible. The outer edges 124 of the catalyst layer 120 may have a shape and size that matches that of the inner profile 133 of the retaining arrangement 130 as closely as possible (Figures 7(a) and 7(e)). Alternatively, a clearance or clearances C1 may be provided between the catalyst layer 120 and the retaining arrangement 130 (Figures 7(b) and 7(d)) or between the catalyst layer 120 and an inner wall 113 of the cavity 1 12 (Figure 7(c)). The inner profile 133 of the retaining arrangement 130 may comprise multiple retaining formations 136 extending from the cavity 112 (Figure 7(c)) or the inner profile 133 of a retainer 130 (Figure 7(d)). The inner profile 133 of the retaining arrangement may have a keyed relationship (Figure 7(e)). Furthermore, while the examples shown have generally circularly symmetrical symmetry of the housing and/or the retaining arrangement, the invention is not so limited and other shapes and configurations are contemplated.

EXPERIMENTAL

A test enclosure 200 was provided (Figure 9(A)) and connected to a controlled flow of a feed gas consisting of a stoichiometric mixture of hydrogen and oxygen feed gas (in the mole ratio 2:1). The feed gas is provided from respective gas cylinders via mass flow controllers 210, 220 connected to a common feed line 230 that extends to the enclosure 200. A catalyst test device 100T was inserted into an aperture in the test enclosure wall, with the open end of the cavity in the device housing oriented into the inside of the enclosure 200.

Example catalyst test devices 100T were constructed generally in accordance with the device shown in Figures 1-3, but omitting the retaining ring 30 that is depicted in the Figures. Each device has a type K thermocouple 190 positioned manually through a small aperture the housing, in thermal contact with the ePTFE layer adjacent to the inner wall of the housing, as illustrated in the schematic view of Figure 9(C). The housing, cavity, retaining ring and all layers of the devices were round in cross section.

The catalyst material used was alumina-supported palladium, 5wt%Pd/Alumina (Manufacturer: NECC). The catalyst layer includes two catalysts sheets, each being a filled PTFE sheet, with thickness 565 pm with a composition Pd/Alumina/PTFE=1 .2/68.8/30 (wt%). Thus, total mass of palladium in the catalyst layer was around 0.7 mg.

The porous layers used were expanded polyethylene (ePE), manufactured by Shenzhen Senior Technology Material Co., Ltd, having a melting point of 130°C. Each ePE layer was of thickness 16 pm, a Gurley number of 200 seconds and had a basis weight (mass per area) of 8.9 gm^-2

Flow rate of the 2H2/C>2feed gas was controlled at the following flow rates, each for 5 minute duration, and the flow rate increased stepwise in accordance with Table 2.

Experiments were conducted at ambient temperature within the laboratory, which varied between tests.

Table 2: Feed gas flow rate and duration

The temperature of the catalyst layer was measured throughout.

In normal use of an automotive lead-acid battery, gas flow rates of gas vented from the electrolyte enclosures are typically in the region of 2-4 ml/min. The test enclosure and feed gas flow rates therefore greatly exceed those normally encountered in a lead-acid battery and are considered to illustrate an extreme scenario which may lead to thermal runaway of a catalyst device.

The Gurley value of a porous layer or membrane disclosed herein is evaluated based on JIS P 8117:1998. The Gurley value refers to a time (second(s)) for which 100 cm³ of air vertically passes through a sample having an area of 6.45 cm² at a pressure of 1.29 kPa. The Gurley value is an index of air permeability.

EXAMPLE 1

A first test device, device 1 , was constructed in which the catalyst layer 120 was positioned in registration with the porous layers covering the inner and outer faces of the catalyst layer.

The device was exposed to feed gas flow rates in accordance with Table 2 and the temperature monitored.

Two examples of device 1 were tested and the results are shown in Figure 10.

The temperature of the catalyst layer over time for both runs was the same, within the range of experimental error.

The maximum temperature of the catalyst layer measured by the thermocouple remained below 100°C in both runs, indicating that the polyethylene porous layer functioned to moderate the rate of the recombination reaction and, after around 10 minutes, that the polyethylene layer porosity reduced such that the recombination reaction rate, and thus catalyst layer temperature did not continue to increase with the increased feed gas flow during steps 3-5 of each run. The maximum measured temperature at the location of contact between the thermocouple and the catalyst sheet is lower than the melting point of the porous layers, and this is understood to reflect the relatively thermal conductivity of the catalyst layer. The observed variation in temperatures profiles (which may also be observed in Figure 12, discussed below) are caused by variations in the relative positions of the thermocouple in relation to the catalyst sheet, during manual construction of the test devices. EXAMPLE 2

A second test device, device 2 was constructed in which the catalyst layer 120 was intentionally positioned out of registration with the porous layer, such that a small area of the face of the catalyst layer was not covered by the porous layer.

Results of the tests conducted on device 2 are shown in Figure 11 , together with the results from the first test device 1 for comparison.

In the experiment conducted on test device 2, ignition of the feed gas within the test enclosure occurred after around 9 minutes.

Ignition indicates that a temperature of the exposed region of the face of the catalyst layer was sufficiently high to ignite the feed gas. Again, the measured temperature at the thermocouple is lower and can be understood not to reflect the higher temperature region of the catalyst layer that caused ignition.

EXAMPLE 3

Two further test devices, test devices 4, were constructed generally in accordance with the device of Figure 3, including a polypropylene retaining ring 130 and two ePE layers overlaying both the catalyst layer and the retaining ring.

Two additional test devices, test devices 5, were constructed in which both the polypropylene retaining rings were omitted.

Test runs were conducted on all four devices and the results are shown in Figure 12 and Table 3 below.

Table 3

The runs conducted on the test devices 5 (marked (3) and (4), “no PP ring” in the figure and table) increased to a maximum temperature of around 150°C. This temperature is sufficiently high that a polypropylene housing may begin to deform and risk loss of containment of the catalyst layer and other layers from the device cavity. It is noted that slight cooling of the thermocouple occurs at the high flow rates used in the testing protocol.

The runs conducted on the test devices 4 (marked (1) and (2), “with PP ring”) again indicate that the polyethylene porous layer acted to moderate the rate of the recombination reaction by melting and reducing in porosity near to the end of step 2, but illustrate that the presence of the PP ring 130 improves the effectiveness of the shut-down effect of the ePE layers.

Examples 4 and 5 illustrate the effect of relative dimensions of the catalyst layer and polypropylene retaining ring.

EXAMPLE 4

A further test device was constructed analogous to device 4. Two further devices were constructed with smaller catalyst layer area, to provide a “gap from edge” between the periphery of the catalyst and the polypropylene retaining ring. The thickness of both the catalyst layer and retaining ring was 0.75 mm

Results are shown in table 4, below.

Table 4

These data illustrate that the maximum rate of recorded temperature rise of the catalyst layer, and the maximum recorded temperature, both correlate to the size of the gap or clearance between the catalyst layer and the retainer.

These results suggest that in applications of the inventive device where suppression of the catalyst temperature is preferred over the overall rate of the recombination reaction, a minimum clearance may therefore be desirable.

EXAMPLE 5

A further test device was constructed analogous to device 4. The thickness of both the catalyst layer and retaining ring was 0.75 mm.

Two further devices were constructed with increased thickness retaining ring (0.95 mm and 1.20 mm thickness rings). The results are set out in Table 5, and show a correlation between increased retaining ring thickness or depth (compared to the catalyst layer thickness/depth) and the maximum rate of temperature increase and maximum temperature of the catalyst layer. Table 5

These results suggest that in applications of the inventive device where suppression of the catalyst temperature is preferred over the overall rate of the recombination reaction, a minimum thickness gap or indeed a ring of reduced depth, may be desirable.

Whilst exemplary embodiments have been described herein, these should not be construed as limiting to the modifications and variations possible within the scope of the invention as disclosed herein and recited in the appended claims.

Claims

1 . A catalyst device for a lead-acid battery, the catalyst device comprising: a housing, defining a cavity; a catalyst layer mounted in the cavity, wherein the catalyst layer comprises catalyst for accelerating a reaction for generating water or water vapor from oxygen and hydrogen; and a porous layer including thermoplastic material mounted in the cavity, and wherein the porous membrane has a planar size greater than that of the catalyst layer; and wherein a face of the catalyst layer is in contact with the porous layer and covers the at least one face of the catalyst layer; wherein the catalyst layer is mounted in the cavity within a retaining arrangement configured to retain the position of the catalyst layer within the cavity and with the at least one face covered by the porous layer.

2. The catalyst device of claim 1 , wherein the retaining arrangement comprises one or more retaining formations, to prevent or substantially prevent lateral motion of the catalyst layer; or a retainer, wherein the retainer is mounted within the cavity in substantially fixed relation to the housing .

3. The catalyst device of claim 2, wherein the retaining arrangement comprises a retainer in the form of a ring having a shape generally the shape of the catalyst layer.

4. The catalyst device of any preceding claim, wherein lateral clearances provided between catalyst layer and the retaining arrangement permit a range of lateral motion within the cavity with respect to the housing of the catalyst layer and the porous layer, wherein the porous layer comprises a peripheral surface region that extends beyond the range of lateral motion of the porous layer in relation to the catalyst layer around the entire periphery of the catalyst layer.

5. The catalyst device of any preceding claim, wherein the catalyst layer has a catalyst layer thickness and the retaining arrangement has a depth, and wherein the depth of the retaining arrangement is equal to or less than the catalyst layer thickness.

6. The catalyst device of any preceding claim, wherein thermoplastic material has a melting point or glass transition temperature that is a first temperature, and wherein the retaining arrangement is dimensionally stable to a second temperature that is higher than the first temperature.

7. The catalyst device of any preceding claim, wherein the porous layer comprises an expanded thermoplastic polymer membrane.

8. The catalyst device of claim 7, wherein the thermoplastic of the expanded thermoplastic polymer membrane is polyethylene or polyurethane.

9. The catalyst device of any preceding claim, comprising at least two porous layers, wherein a first porous layer is in contact with a first face of the catalyst layer and a second porous layer is in contact with an opposite second face of the catalyst layer, each porous layer having a planar size greater than that of the catalyst layer.

10. The catalyst device of claim 9, wherein peripheral regions of at least two of the porous membranes are bonded to one another.

11 . The catalyst device of any preceding claim, comprising or more of: a diffusion limiting layer, adapted to regulate rate of diffusion or flow of gas to the catalyst layer; a porous anti-poisoning layer, capable of absorbing or decomposing a catalytic poison; an outer hydrophobic porous layer.

12. The catalyst device of claim 11 , wherein one of more of the diffusion limiting layer, the antipoisoning layer or the outer hydrophobic porous layer comprises: an expanded polymer membrane; an expanded fluoropolymer membrane; or an expanded PTFE membrane.

13. The catalyst device of claim 12, wherein the catalyst device comprises an anti-poisoning layer and wherein the anti-poisoning layer comprises an expanded polymer membrane and a substance capable of absorbing or decomposing a catalytic poison inside the membrane.

14. The catalyst device of any one of claims 11 to 13, wherein the catalyst device comprises both a diffusion limiting layer and an outer hydrophobic porous layer, and wherein outer hydrophobic porous layer has a Gurley number lower than the Gurley number of the diffusion limiting layer.

15. The catalyst device of any one of claims 11 to 14, wherein the catalyst device comprises a diffusion limiting layer and wherein the diffusion limiting layer has a Gurley airflow number of 50 second or more, or 100 seconds or more.

16. The catalyst device of any preceding claim, wherein the catalyst layer is porous.

17. The catalyst device of any preceding claim, wherein the catalyst layer includes a hydrophobic porous member, and catalyst, or a catalyst support supporting the catalyst, arranged in the cavities or on the surface of the pores of the catalyst layer.

18. The catalyst device of claim 17, wherein the hydrophilic porous member comprises ePTFE.

19. A lead-acid battery, the battery comprising an electrolyte enclosure and one or more catalyst devices in accordance with any preceding claim, wherein the cavity of the or each catalyst device is in fluid communication with the electrolyte enclosure.

20. The lead-acid battery of claim 19, comprising a plurality of cells, each having an electrolyte enclosure, wherein each electrolyte enclosure is provided with one or more catalyst devices in accordance any one of claims 1 to 18.