CA1105990A

CA1105990A - Porous electrode

Info

Publication number: CA1105990A
Application number: CA305,965A
Authority: CA
Inventors: Marinus Alfenaar
Original assignee: Electrochemische Energieconversie NV
Current assignee: Electrochemische Energieconversie NV
Priority date: 1977-06-24
Filing date: 1978-06-22
Publication date: 1981-07-28
Also published as: NL7706998A; FR2395615A1; GB2000363B; SE7807135L; IT7849949A0; BE868386A; DE2827971A1; IT1105272B; GB2000363A; US4237195A; JPS5418048A

Abstract

ABSTRACT

An improved porous electrode for use in electrochemical cells, particularly for use in fuel cells. The electrode is of the kind compri-sing a porous layer containing carbon and a non-polar resin such as poly-tetrafluoroethylene, and an electrically conducting screen acting as a collector. To prevent early collector deterioration the collector material is covered with an electrically conductive protective layer, which con-sists either of an electrically conductive resin or of a non-conductive resin in which conductive particles, such as graphite particles, have been incorporated.

Description

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BACKGROUND OF THE INVENTION
-1. General nature of the invention.
This invention relates to porous electrodes for use in electro-chemical cells, particularly for use in fuel cells.

2, Description of the Prior Art.
A known kind of electrode which is particularly suitable for use in fuel cells comprises a porous layer containing carbon and a non-polar resin and an electrically conducting fluid-permeable lattice-work acting as a collector. Such an electrode is described in the published Netherlands patent application 72 14900.
During operation the fuel used penetrates into the pores of the posous layer, which usually contains a catalytically active material that catalyses the galvanic combustion of the fuel in the porous layer. The generation of electric current occurs in the porous layer and the current generated is collected by the electrically conducting lattice-work, and led off therefrom. The electrode is in contact with a suitable electrolyte which closes the circuit within the fuel cell and by which the reaction products of reactions occurring at the cathode and anode can also be led off, Z0 A particular disadvantage of known porous fuel-cell electrodes is the detachment of the collector which occurs during operation, e.g.
as a consequence of corrosion, which collector consists in practice of, for example, metal gauze with a wire thickness of approximately 150 ~m and a mesh size of approximately 700 ~m, or a correspondingly perforated metal plate. Expanded metal may also be used as a collector. As a result of the coTrosion the internal resistance of the electrode increases and the strength of adhesion between the collector and the porous layer decreases. This adhesive strength may decrease even after a relatively short operating time so that the porous layer detaches from the collector and the electrode becomes completely unusable.

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To suppress the corrosion, the collector is made of corrosion-resistant metals such as nickel, silver, gold or platinum. Usually nickel is used, as this is the least expensive~ N:Lckel however is not completely free from the disadvantages referred to.
It has previously been suggested in US Patent Specification w~`C~

3,671,317 to incorporate in an electrode a metal lattice-work,~in order to combat corrosion, is surrounded by an enclosing layer of a thermoplastic material, e.g. polyvinylidene fluoride, with around it, a layer of fluoro-carbon polymer, e.g. polytetrafluoro-ethylene, containing conductive particles such as carbon. In such a construction however the metal lattice-work does not act as a collector and this results in a very high internal resistance of the electrode and unfavourable efficiency of current gene-ration~
It has also previously been proposed in French patent specifica-tion 2,215,710 to affix the collector to the porous layer by means of a synthetic lacquer, e.g. an epoxy resin containing graphite. However, the side of the collector not treated with the lac~uer remains exposed to corrosion, SUMMARY OF THE INVENTION
It is an object of the present invention to provide a porous electrode, e~g. for a fuel cell, in which the collector corrosion is sub-stantially reduced without total or even considerable loss of the collector effect and in which the fluid flow through the collector meshes or holes remains practically unaffected. The porous electrode of the invention, com-prises a porous layer containing carbon and a non-polar resin and an electrically conductive fluid-permeable lattice-work acting as a collector, and to prevent early collector deterioration the surface of the collector material is covered with an electrically conductive collector-covering resin layer.

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In particular the present invention provides a porous electrode comprising a porous layer containing carbon and a non-polar resin and an electrically conductive fluid-permeable screen or grid acting as a collector, wherein the surface of said collector is covered with an electrically conductive collector-covering resin layer.
The present invention also provides a method for making a porous electrode, said electrode comprising a porous layer containing carbon and a non~polar resin and an electrically conductive fluid permeable screen or grid acting as a collector, the surface of said collector being covered with an electrically conductive collector-covering resin layer, said method comprising the following steps:
a) providing an electrically conductive screen or grid as a collector thereto, b) applying an electrically conductive resin layer to said collector while the resin is still in flowing form and allowing said resin to stiffen or set only partly, c) applying a porous layer containing carbon and a non-polar resin thereto, and d~ completing the stiffening or setting.
~s the non-polar resin in the porous layer suitable resin, particularly synthetic resin, may be used. Various resins for this purpose are known, e.g, polypropylene and more especially polytetrafluoro-ethylene.

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The collector-covering layer can derive its electrical con-ductivity from the resin itself. Electrically conductive resins are well-known.~Such a resin is commercially available under the Trade Mark ~URATKORROPREN 44 of the firm Clouth AG.
As such resins are fairly expensive, it may be preferable to use a non-conductive resin in which electrically conductive particles, e.g. o~ metal, are incorporated. The metal should preferably have a high specific conductivity and be inert under the conditions in which the fuel cell containing the electrode is operated. Examples are platinum, gold, iridium, silver, nickel, or alloys of two or more of these ~etals. Nickel-chromium steel and similar alloys are also suitable. The electrically conductive particles are however preferably carbon particles, particularly graphite particles. With carbon in the collector-covering layer particular-ly good adhesion with the porous layer, which also contains carbon, is ensured. Graphite has the advantage over other forms of carbon that it is not significantly attacked during operation of the electrode.
The particles may be substantially spherical in shape, but preferably they will be elongate in form, particularly fibreshaped. The use of particles with an elongate form, particularly in the form of fibres, results in the internal resistance of the electrode being lower than if spherical particles are used. If substantially spherical particles are used, their diameter is preferably from 0.1 to 50 ~m, and particularly $rom 0.1 to 5 ~ m.
The resin in the collector-covering layer may be thermoplastic, thermosetting or rubber-like. Examples of suitable resins are chlorinated rubbers, polyethylene, ethylene vinylacetate copolymer, polypropylene, polyurethane resins, polyvinyl chloride, chlorinated polyethylene, epoxy resins and phenol resins; ethylene-propylene copolymer rubbers, styrene-acrylonitrile copolymer, polybutadiene, butyl rubber or sulphur rubbers can also be used, as also can modified or unmodified polyesters, polyvinyl esters and polyacrylates. The resin preferably has some degree of elasti-city.

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If an alkaline electrolyte is used in the electrochemical cell, the resin used is preferably an epoxy resin, which has a high resistance to alkali. I~ an acid electrolyte is used, an epoxy resin can again be used, or alternatively e polyurethane, polyvinyl chloride, a chlorinated rubber or chlorinated polyethylene. It may be desirable to treat the collector metal in advance with an agent to improve the adhesion, e.g.
wi~h a primer.
The concentration o~ conductive particles in the resin can lor example be 10-75% by weight of the combination of resin and particles, and is pre~erably 50-70% by weight.
The thickness of the covering layer will preierably not be greater than is necessary in order to obtain the good adhesion desired between the collector and the porous layer, the reason being to keep the adverse e~fect on the porosity o~ the collector as low as possible.
L5 A thickness of ~rom 10 to 20 um is usually adequate, although, if desired, thinner or thicker layers, e.g. from 5 to 50 ~ m, may be used, The resin, whether containing conductive particles or not, may be applied to the collector in any suitable way, e.g. by spraying, moulding or dipping. In a pre~erred ~orm o~ production the resin will then be allowed to stif~en or set only partially and then the porous layer will be applied, a~ther which the stilfening or setting can be completed.
It is of advantage to first make the porous layer and the col-lector surrounded by the covering layer, and then to press the collector on the electrolyte side at least partly into the layer composition obtained.
By this procedure a strong electrode with good porosity properties is ob-tained. It is also possible to form the porous layer in situ by deposltion on the collector surrounded by the covering layer. In another pre~erred procedure the resin is applied without conductive particles to the collec-tor and then, while the resin is still plastic, the conductive particles are scattered thereon and the collector with its covering layer is pressed at least partly into the previously prepared porous layer, the electrical-ly ~s~ ~
conductive particles thus being pressed into the still plastic resin in one operation.
The electrodes according to the invention can be made in any suitable way. The catalytically active material can, ~or example, be mixed in powder form with carbon in powder form and polytetra~luoro-ethylene in powder form and ii' necessary also a pore-$ormer, i'ollowed by moulding oi' the whole to iorm an electrode at an elevated temperature, e.g. in a suitable prèssing mould, a~ter which the pore-i'ormer can be leached, e.g.
with hot water. Salts such as sodium sulphate, sodium carbonate, ammonium carbonate and the like can be used as the pore-iormer.
The catalytic material in powder ~orm may, i'or fuel-cell elec-trodes to be used as anodes, be a commercially available platinum black or palladium black or a mixture o$ the two, or another suitable material such as nickel. ~or electrodes to be used as cathodes, the catalytically active material is frequently silver powder or a noble metal. The inven-tion is, however, not restricted to specii'ic catalytic materials, which can in fact be totally absent, or to specii'ic methods oi' making the porous layer. The porous layer may also consist o~ a catalytically active material which is dispersed in iinely divided iorm in a porous matrix oi' carbon and, i'or example, polytetrai'luoro-ethylene. The catalytically active material also may not be in powder i'orm but porouslv cohesive, e.g. by sintering powder o~ the particular catalytically active material, usually a metal.
The porosity of the porous layer may be the same over the entire thickness oi' the porous layer, or may increase or decrease in a direction oi thickness. The porous layer may also consist o~ two or more layers in which the porosity is the same over the entire thickness in each of the constituent layers but the porosity difiers from one constituent layer to the other.
It is possible for a porous layer to be provided on only one side of the collector but the collector surrounded by a covering layer may also be provided with a porous layer on both sides.

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The invention is applicable both to electrodes which when in use are in contact with a liquid phase on both sides and to gas-diffusion electrodes.
In gas-diffusion electrodes the layer containing the catalyst, which layer is sufficiently porous to pass ~as and liquids, will pre~e-rably be in contact with a layer which stops liquids but passes gas.
During operation the layer which passes both gas and liquids is then in contact with the electrolyte, e.g. a sodium hydroxide or potassium hydroxide solution or a phosphoric acid solution, and the layer which only passes gas is in contact with the gas. For the anode, the gas consists of the gaseous fuel, e.g. hydrogen. For the cathode, the gas consists of oxygen or a gas containing molecular oxygen, e.g. air.
An advantage of the electrode according to the invention lies in the ~act that a cheaper metal can be used for the collector material. In-stead of nickel or a noble metal it is possible, for example, to use steelor chromium steel without having the electrode rendered unserviceable within a short time.

BRIEF DESCRIPTION OF THE FIGURES

Figure l is schematic top view of part of an embodiment to the electrode according to the invention and Figure 2 is a representation of a part section at right angles to the surface of the electrode. Individual numbers always refer to the same parts.

DETAILED DESCRIPTION OF THE FIGURES

Nickel or steel wires 1, 2, 3 and 4 form part of the collector gauze of the electrode. Instead of wire gauze it is also possible to use a perforated nickel or steel plate, or expanded metal. The collector wires are surrounded by a collector-covering layer. The collector wires provided with the covering layer are embedded in a porous layer made up of ~l~Sg~

constituent layers 6, 7 and 8. When the electrode i9 in operation the electrolyte phase is at 9. In that case, o~ course, some electrolyte has penetrated into the pores of the electrode. Furthermore, when the electrode is in operation,the gaseous-phase i9 at 10.

DESCRIPTION OF A PREFERRED EMBODIMENT

In a particular embodiment, the thickness o~ the wires 1-4 is about 300 ~m, the poroslty o~ the collector gauze about 150 ~m re-sulting in a gauze thickness o~ about 50%.
The term porosity as used here and hencei'orth ls dei'ined as the ratio between the volume occupied by the pores, or the volume notoccupied by the particular material, and the total volume oi the layer concerned.
The collector-covering layer 5 consists o~ core varnish obtained under the Trade Mark ELASTOLUX BLAN~ V2037 o$ the ~irm Tollens, (an epoxy resin) containing 50% by weight (oi' the resin + graphite) oi' graphite particles. The thickness oi' layer 5 is 15 ~m.
The porous layer oi' the electrode is made up o~ constituent layers 6, 7 and 8. Layer 6 consists o~ a mixture oi 90~ by weight o~
carbon and 10% by weight oi' polytetra~luoro-ethylene. The polytetra-i'luoro-ethylene content can, however, vary but it should pre~erably be between 8 and 15% by weight. Layer 6 is approx. 40 ~m thick, but oi course less in the vicinity o~ collector wires 1 and 3. Layer 6 may contain catalytically active material but that is not the case in the present example. The porosity of layer 6, apart ~rom the micro pores in the carbon particles, which pores are not oi' any importance ~or the working oi' the electrode, is 30%. It will always pre~erably be between 25 and 35%. The pore width is 1 to 10 ~ m. This porosity depends on the particle size oi' the polytetrai'luoro-ethylene powder with which the layer is made and on the pressure used in making the electrode. The same applies to the porosities o$ layers 7 and 8, hereinai'ter described.

11~59~) Layer 7 is 80 ~m thick and also consists of a mixture of car-bon and polytetrafluoro-ethylene. This layer further contains 0.86mg/cm of silver as catalytically active material.The polytetrafluoro-ethylene content will preferably be between 15 and 30% by weight and in this example is 21% by weight. The porosity is 20% and preferably always between 20 and 25%.
Layer 8 is 180 ~m thick on average and consists entirely of poly-tetrafluoro-ethylene. Its average porosity is 50% and the pore width, as in the case,of layers 6 and 7, is between 1 and 10 ~m.

Example I
An electrode of this kind was used in a continuous 3,000-hour life test a3 the air electrode in a hydrogen-air fuel cell with a 40% wt.
potassium hydroxlde solution as the electrolyte. The operating temperature was 65 C, the current density 100 mA/cm . The increase of the electrode's internal resistance was found by means of regular current/voltage measure-ments. The rate of increase is a measure for the aging of the electrode due to the reduction of the adhesion between the collector and the porous layer.
The rate of increase of the internal resistance of the electrode was 0.02 ohm.cm2/1,000 hours.

Example II
The life test as described in Example I was applied to an electrode identical to that in Example I except that the collector-cove-ring layer consisted of enamel lacquer obtained under the Trade Mark SIKKENS M45-61 of the firm Sikkens (an epoxy-urea resin) containing 50%
by weight of graphite particles. The rate of increase of the resistance of the electrode was again about 0.02 ohm.cm /1,000 hours.

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COMPARATIVE EXPERIMENT
The life test as described in Example I was applied to an electrode identical to that the Example I except that the collector-covering layer was omitted. The rate of increase oi the internalresistance S o~ the s1ectrode ~5: 0.4 ohr.or /1,000 hours, :~' ., ;
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A porous electrode comprising a porous layer containing carbon and a non polar resin and an electrically conductive fluid-permeable screen or grid acting as a collector, wherein the surface of said collector is covered with an electrically conductive collector-covering resin layer.

2. An electrode as claimed in claim 1, wherein the non polar resin is polytetrafluoro-ethylene.

3. An electrode as claimed in claim 1, wherein the collector-covering layer contains an electrically conductive resin.

4. An electrode as claimed in claim 1, wherein the collector-covering layer consists of a non-electrically conductive resin in which electrically conductive particles have been incorporated.

5, An electrode as claimed in claim 4, wherein the electrically conducting particles are carbon particles.

6, An electrode as claimed in claim 5, wherein the carbon particles are graphite particles,

7, An electrode as claimed in claim 4, wherein the electrically conductive particles are metal particles.

8. An electrode as claimed in claim 4, wherein the diameter of the electrically conductive particles is from 0.1 to 50 µm.

9. An electrode as claimed in claim 8, wherein the diameter of the electrically conductive particles is from 0.1 to 5 µm.

10. An electrode as claimed in claim 4, wherein the electrically conductive particles have the form of fibres.

11. An electrode as claimed in claim 1, wherein the resin in the collector-covering layer is at least to some extent elastic in character.

12. An electrode as claimed in claim 4, wherein the resin in the covering layer is an epoxy resin.

13. An electrode as claimed in claim 4, wherein the resin in the covering layer is a polyurethane, polyvinyl chloride, chlorinated rubber or chlorinated polyethylene.

14. An electrode as claimed in claim 4, wherein the concentration of the electrically conducting particles in the resin is 10-75% by wt.
based on the resin plus the electrically conducting particles.

15. An electrode as claimed in claim 14, wherein the concentration of the electrically conductive particles in the resin is 50-70% by wt.
based on the resin plus the electrically conductive particles.

16. An electrode as claimed in claim 1, wherein the thickness of the covering layer is from 5 to 50 µm.

17. An electrode as claimed in claim 16, wherein the thickness of the covering layer is from 10 to 20 µm.

18. An electrochemical cell containing one or more electrodes as claimed in claim 1.

19. A method for making a porous electrode, said electrode comprising a porous layer containing carbon and a non-polar resin and an electrically conductive fluid-permeable screen or grid acting as a collector, the surface of said collector being covered with an electrically conductive collector-covering resin layer, said method comprising the following steps:

a) providing an electrically conductive screen or grid as a collector thereto, b) applying an electrically conductive resin layer to said collector while the resin is still in flowing form and allowing said resin to stiffen or set only partly, c) applying a porous layer containing carbon and a non-polar resin thereto, and d) completing the stiffening or setting.

20. A method for making an electrode as claimed in claim 19, wherein the resin is applied without conductive particles to the collector, the conductive particles are scattered thereon while the resin is still plastic and the collector with its covering layer is pressed at least partly into the previously prepared porous layer.