GB2479449A - Polymeric material - Google Patents

Abstract

A material comprises an electronically conducting polymer (ECP) and an ionically conducting polymer (ICP) which are interlinked. Preferably the ionically conducting polymer (ICP) is hydrophilic and the electronically conducting polymer is polyacetylene, PEDOTTSS or polyaniline. Advantageously the material may be used as a membrane electrode assembly where the ionically conducting material is a membrane suitable for use in electrochemical cells including electrolysers and fuel cells and junctions between biological systems and electronic devices such as a nerve-contacting prosthesis.

Description

MEMBRANES AND ELECTROCHEMICAL DEVICES INCLUDING THEM

Field of the Invention

This invention relates to polymer membranes and their use in electrochemical systems.

Background of the Invention

Electrochemical devices, including membrane-electrode assemblies (MEA5) for use in fuel cells, electrolysers, photo-voltaic devices and interfaces between nervous tissue and electronic systems in prosthetic devices and applications involving functional electrical stimulation depend upon the transmission of and proper control of both ions and electrons.

Ionic conducting polymers (ICP) are materials in which the conduction process is principally dependent on ion transfer. Conventional solid ICP are typified by Nafion®, a fluorocarbon-based cationic (proton) conductor which has become the industry standard material for the production of solid polymer fuel cells and electrolysers.

GB2380055A discloses hydrophilic ICP which allow the transmission of ions of various types, most importantly protons (specifically as hydronium ions in cationic ionomeric conductors (CE), but also including OH groups in alkaline-based ionic conductors (AE)). These ionically conducting materials have allowed the production of improved fuel cell and electrolyser MEA5 in which both the ionic properties and the hydraulic properties of the ion-conducting membrane can be controlled.

A MEA requires that the electrons (freed or re-combined), during the reactions at the catalyst at the MEA surface, should be transported into or out of the system, and this is achieved using metallic conductors brought into contact with the ionomeric component, normally by pressing. MEAs made using metallic electronic conductors suffer from a number of problems, including: (i) corrosion of the metallic conductor, or using titanium, a frequent but expensive choice, (ii) occupation of a significant proportion of the surface area of the system by the conducting electrode, thus preventing reactants from reaching the ionomer/catalyst interface, (iii) making and maintaining continuous contact between the metallic conductor and the membrane, normally requiring pressure and restricting the design of the cell, and (iv) the resulting cell is heavy and mechanically rigid.

The resulting structures are complex and heavy because of the need to make the cell body sufficiently rigid to maintain the clamping force necessary to achieve good contact between the components. This results in costly, heavy and complex engineering devices which are not naturally vibration-resistant and which do not satisfy the needs of many industrial applications including lightweight transport systems.

Such conventional junction structures also fail to meet the needs of active neural implants, where it is necessary to transfer charge over the phase boundary of a metallic (electrode) and living tissue. Because metallic conduction is based on electron flow, and neural tissue conduction on ion flow, a transition has to occur which requires an electrode interface.

Electronic conducting polymers (ECP) are well known, and are understood to mean materials in which the conduction process is principally dependent upon electron transfer. ECP include polyacetylene which has achieved electrical conductivities of i07 S/m approximating to that of common metals, while commercial materials supplied as dispersions in water, e.g. polyethylenedioxythiophene:polystyrene suphonate (PEDOT:PSS, commercially available as Clevios 500®), have a conductivity of 3x10451m and exceed the conductivity of graphite commonly used as a conductor in fuel cells.

Summary of the Invention

The present invention is based on the finding that a membrane material, e.g. for a MEA, can be produced by forming an interpenetrating bond between an ECP and a hydrophilic ICP. A MEA can thus be produced in which all the principal components (except the catalyst) are replaced by polymeric materials.

Importantly, such MEAs do not require external pressure to maintain contact between the components, and the invention permits the construction of lightweight MEAs and cells.

While no material is exclusively restricted to one or the other mode of conduction, for practical purposes ratios of electronic:ionic conduction greater than 20:1 (for an ECP) and 1:20 (for an ICP) make it possible to construct working devices of acceptable efficiency.

A further development provides hydrophilic ECP materials which allow the passage of water and so can be used to make electrolyser MEAs in which the polymeric conductor forms a continuous or near continuous layer covering all the surface area of the MEA. This reduces the resistive losses normally associated with the use of an ECP having a conductivity less than is available from metallic systems.

Clearly, metallic conductors can be combined with an ECP to improve the overall efficiency of the MEA, e.g. in one example a thin interpenetrated layer of ECP is formed on the surface of the polymeric membrane (preferably a hydrophilic ionomeric membrane) and a metallic conductor in the form of discrete wires or a mesh is then laid onto the ECP, after which a second layer of ECP is deposited over the metallic conductor, making good electrical contact and offering a measure of corrosion protection for the metal components.

A hydrophilic electronic polymer may be produced by processes similar to those described in GB2380055A, e.g. by dissolving or mixing the monomers forming the electrically conductive material in/with monomers chosen to provide hydrophilic properties (e.g. vinyl pyrrolidone and/or HEMA and/or acrylonitrile) and polymerising and crosslinking the resulting monomer mixture by suitable means including gamma irradiation, UV irradiation in the presence of a suitable UV initiator and crosslinking agent or by thermal polymerisation.

Aspects of the invention include: (1) the production of electronically conducting polymeric materials containing suitable electronic conduction components (e.g. for the purpose of example only, polyacetylene, PEDOT:PSS or polyaniline) and additionally comprising hydrophilic and/or hydrophobic monomers, the whole being crosslinked by the use of radiation (gamma or UV) to form a polymeric material in which the hydraulic, electrical and gas transmission properties can be separately controlled.

(2) the formation of a junction between an ICP and an ECP by the polymerisation and crosslinking of a suitable monomer mixture against the surface of a pre-existing ionically or electronically conducting material.

(3) a junction as in (2) above in which the junction is formed between a hydrophilic ICP and an ECP by the interpenetration of monomers from one material into the surface of the other.

(4) the production of a membrane electrode assembly for use in an electrochemical cell in which the primary contact between the ionic material and the electronic conductor is made using a polymeric electronic conductor.

(5) an MEA comprising an ionically conducting membrane, a catalyst layer and an electronic conductor partly or entirely composed of a

suitable ECP.

(6) an MEA as in (5) above in which the junction between the polymeric components is an interpenetrated junction so that the resulting MEA operates substantially without the need for external pressure or support to maintain its structure and the electrical contacts between the components and layers thereof.

(7) a junction as in (1), (2) and (3) above by which the ionic activity of viable nervous tissue can be transformed into an electronic signal for detection and use by electronic equipment including but not limited to the control of prosthetic devices.

In summary, the invention provides a method for the formation of junctions between ionically conducting polymeric materials and electronically conducting polymers in which the junction may preferentially consist of an interpenetrated region by the polymerisation of one polymer from an initial monomer mixture in contact with the second material. Embodiments of the invention are lightweight, substantially non-metallic membrane electrode assemblies for use in electrochemical cells including electrolysers and fuel cell, and junctions between biological systems and electronic devices.

A material of the invention preferably has a conductivity of at least x I 2 SIm. It can have a density of no more than 1.8 g/cc.

Descrirtion of the Invention The following Examples illustrate the invention, and show the production of ionic/electronic interfaces. Abbreviations that are not otherwise defined herein are explained in GB2380055A.

There are two routes to the production of electronic-electronic interfaces for each of the two applications given as examples. Thus there are four examples set out below.

Production Routes The two routes arise because it is possible to take either a hydrophilic ionic material or an electronic material as the base structure (substrate). In the first instance, the electronic polymeric material is polymerised against the base material; in the second, it is the ionic material that is introduced as a monomer or pre-polymer and polymerised in situ against the substrate. Such products may appear to be identical although further analysis may show differences in performance.

Applications The two applications that have been selected are: construction of an MEA for use in (for example) a fuel cell or electrolyser which requires the inclusion of a catalyst (normally but not exclusively as a dispersion of solid particles) at the junction and optionally a metallic conductor in the form of an array of wires or a mesh. The second example is the production of an array of small area contacts for use in neurological applications when the size of each element is of paramount importance; production involves the use of a mask to restrict the polymerisation of the second material to small defined areas of controlled shape, size and distribution.

Example I illustrates the production of an interface suitable for use in an MEA that removes the conventional requirement for the application of external pressure in order to achieve good electrical contact between the components.

Example IA: an ionic base layer (substrate or membrane) onto which an electronically conducting layer is polymerised to form part of a MEA The ionic substrate can consist of any suitable ionic polymer, this may be hydrophilic (as described in GB2380055A when it can be either an AE or a CE material), or an essentially non-hydrophilic' ionic conducting material such as a fluorocarbon (e.g. Nafion®).

(i) Place the substrate polymer in a frame sealed at the edges so that liquid applied to the working' surface cannot leak round to the rear of the substrate.

(ii) Hydrate the substrate to a controlled degree. The degree of penetration of any monomer applied to the working surface will be dependent upon the hydration status of the substrate, while the subsequent expansion of the substrate will produce interfacial stresses if the polymerisation is carried out while it is in a state of hydration significantly different from that ultimately used. For this reason the substrate is normally fully hydrated, but it should be noted that the rate of ingress of any material applied to the "working" surface will be affected by the level of hydration.

(iii) For the purposes of this example it is assumed that the substrate is a hydrophilic cationic exchange polymer, when it should be hydrated in DD water.

(iv) A suitable catalyst material in small or nano-particle form, e.g. of platinum (for use with a proton exchange polymer Nafion® or AN-VP-AMPSA co-polymer) or stainless steel (for use with an anionic exchange polymer) is distributed onto the "working" surface.

(v) If an additional metallic conductor is required, then it may be placed onto the "working" surface of the substrate polymer.

(vi) Prepare a sample of Clevios 500 (PEDOT:PSS) which is available commercially as a colloidal suspension in water (i.e. an aqueous dispersion) by the addition of 5 to 50 wt% (preferably 15 wt%) of a monomer mixture comprising of AN-VP as a 50-50 mixture plus a suitable catalyst and crosslinking agent (preferably allyl methacrylate). The components are miscible in the water contained in the colloidal suspension, and the resulting viscous liquid can be polymerised and crosslinked using either gamma radiation from a 60Co source or by UV radiation in presence of a

suitable catalyst.

(vii) Apply a layer of the monomer mixture prepared in (v) above to the exposed working surface, and after a defined time delay to allow the monomer to diffuse into the surface layer of the substrate, polymerise the initially liquid monomer mixture using UV radiation from a high output discharge tube, or from 20 kRads to 2.5 MRads of gamma radiation from a 60Co source (preferably 100 kRads).

(viii) During the polymerisation process the initially liquid monomer mixture was found to polymerise to form an elastic layer, encapsulating the catalyst and conductor structure (if used). The interface consists of an interpenetrated region whose thickness will depend upon the degree of hydration of the substrate and the properUes of the applied monomer mixture prior to polymerisation.

In the case illustrated it has been found that a time delay of 10 minutes at ambient temperature prior to a UV polymerisation process lasting 30 minutes was sufficient to produce an IPN interlayer approximately 150 pm in thickness.

The resulting structure demonstrated that the two layers, substrate polymer and newly polymerised layer, were intimately joined, while the catalyst and the conductor structure (where used) were firmly attached to the substrate.

Example IB: an electronically conductive base layer (substrate or membrane) onto which an ionically conducting layer is polymerised to form part of a MEA The electronically conductive polymer used as the base or substrate was PEDOT:PSS (Clevios 500) which is available commercially as a colloidal suspension in water (i.e. an aqueous dispersion). This was prepared as supplied, by applying the liquid dispersion to a glass or rigid polythene sheet and removing the water in accordance with the manufacturer's recommendations to form a thin polymeric layer which became the base layer or substrate for the ensuing stages.

A monomer mixture as described in GB2380055A comprising a mixture of AN-VP-AMPSA incorporating a suitable UV initiator and a crosslinking agent (allyl methacrylate) was applied to the substrate and polymerised using UV radiation from a high output discharge tube, or between 20 kRads to 2.5 MRads of gamma radiation from a 60Co source (preferably 100 kRads).

As in the previous example (items (vi) and (v) above), suitable catalyst may be applied to the substrate prior to the addition of the monomer mixture, together with metallic conductors if required as part of the MEA structure.

The resulting structure demonstrated that the two layers, substrate polymer and newly polymerised layer, adhered well, while the catalyst and the conductor structure (where used) were firmly attached to the substrate.

Example 2 illustrates the production of a distribution of small contact structures in MEA that removes the conventional requirement for the application of external pressure in order to achieve good electrical contact between the components.

Example 2A

As in IA, where an ionic base layer (substrate or membrane) is prepared and onto which an electronically conducting layer is polymerised. As before, the ionic substrate can consist of any suitable ionic polymer; this may be hydrophilic (as described in GB2380055A when it can be either an AE or a CE material), or an essentially non-hydrophilic' ionic conducting material such as a fluorocarbon (e.g. Nafion®).

Example 2B

As in IB, where an electronically conductive base layer (substrate or membrane) is employed and onto which an lonically conducting layer is polymerised.

Because the objective is the production of an array of small (down to nano-scale) elements, the primary difference between this procedure and that set out in Example I is the use of a perforated mask to restrict the polymerisation activity to defined areas. This process is well known and used in the electronics industry for the production of complex components for integrated circuits.

The use of a mask effectively restricts the method of polymerisation to UV because the thickness of mask necessary to prevent unwanted polymerisation when using ionising radiation would be prohibitive. The process has been employed using a mask containing only 3 holes, and has demonstrated the ability to restrict polymerisation of the second layer to defined areas. Following polymerisation, the surplus monomer mixture was removed by repeated washing with water.

It is known that the hydrophilic ionic polymers can be prepared by thermal polymerisation using a suitable thermal initiator, and thus an alternative method of restricting the polymerisation process to defined areas would be to apply heat selectively at the points where polymerisation is required. This may produce a less well defined structure.

Claims (11)

1. CLAIMS1. A material comprising an electronically conducting polymer (ECP) and an ionically conducting polymer (ICP) which are interlinked.
2. 2. A material according to claim 1, obtainable by polymerisation of monomers to form the ECP in contact with the ICP.
3. 3. A material according to claim 1, obtainable by polymerisation of monomers to form the ICP in contact with the ECP.
4. 4. A material according to claim 2 or claim 3, wherein the polymerisation is conducted in defined areas only.
5. 5. A material according to any preceding claim, wherein the polymers comprise, at their junction, an interpenetrated region.
6. 6. A material according to any preceding claim, which has a conductivity of at least 5 x 102 S/rn.
7. 7. A material according to any preceding claim, wherein the ICP is hydrophilic.
8. 8. A material according to claim 1, substantially as described in theExamples.
9. 9. A membrane-electrode assembly comprising, as the membrane, a material according to any of claims I to 8.
10. 10. A prosthetic device comprising, at a surface intended to contact viable nerves, a material according to any of claims I to 8.
11. II. Use of a material according to any of claims I to 8, wherein one of the polymers is present in the form of an array of discrete areas thereof, as a multi-contact neural-electronic interface.