Description The present invention relates to a material having a high thermal and electric conductivity, particularly but not exclusively adapted for manufacturing bipolar plates for fuel cells.

It is known that fuel cells convert the hydrogen energy into electric energy which can be employed to power electric motors, for example.

Presently fuel cells are divided into different distinct classification families depending on the type of electrolyte used: alkaline cells, phosphoric acid cells, melted-carbonate cells, with solid oxides and with a protonic-exchange membrane (usually known with the acronym PEM).

The operation heart of fuel cells is the oxidation- reduction process of hydrogen and oxygen molecules to obtain water molecules by releasing electrons and therefore producing a current flow.

In cells with a protonic-exchange membrane the electrolyte (enabling transportation of ions H+) consists for example of a fluorinated polymer containing acid sulphonic groups.

The Hs ions are in fact transferred into and through the electrolyte exactly by means of the acid sulphonic groups.

This material is known under the name of NAFIONO for example and is marketed by Dupont.

In addition fuel cells generally contemplate the presence of a catalyst, generally a platinum, palladium or also nickel catalyst capable of facilitating dissociation of the hydrogen'atom.

It is to be noted that in order to enable an optimal operation of the catalyst, the catalyst surface shall be constantly renewed, i. e. cleared of the reaction products in such a manner that a new hydrogen may reach the catalyst itself thereby allowing the reaction.

For the purpose appropriate conductive plates are presently used that consist of bipolar plates of a parallelepiped shape with thickness included between 1 and 2 mm and provided with appropriate canalisations exactly intended for enabling removal of the reaction products and arrival of new molecules for a good operation of the fuel cell.

These bipolar plates must necessarily act as electric current collectors and also allow a good dissipation of the produced heat.

There are presently different techniques for manufacturing bipolar plates. According to one of these techniques the plates are made up of a compound consisting of a matrix with a thermosetting base in which graphite fillers are buried in percentages until a maximum of 73%.

While these bipolar plates with a thermosetting matrix are widely used in the manufacture of fuel cells with a protonic-exchange membrane, they however have some limits

and operating drawbacks.

First of all the thermosetting matrix of which they are made does not allow recycling of the material.

In addition, the material itself is very brittle so that it is impossible to increase the amount of the graphite filler and often even extraction of the finished product from the mould becomes a problem. In other words, use of this compound involves many production rejects connected with a veritable breaking of the finished product.

In addition, still due to the material nature, moulding of the bipolar plates with an already defined shape is substantially impossible. In other words, elements having a parallelepiped shape are generally moulded and they are then worked to remove material in order to finish them and make the necessary canalisations for elimination of the reaction products at the catalyst, for example.

The above operations greatly increase costs of these elements, which in turn greatly affects the final cost of the fuel cell.

In fact each set consisting of the electrolyte, catalysts and bipolar plates is able to generate rather low potential differences, approximately of 0.4-0. 8 volts, so that in each fuel cell the presence of a plurality of these sets is necessary, each with its associated plates.

Under this situation the present invention aims at 'providing a new material having lower production costs than those of the bipolar plates with a thermosetting base.

Another aim of the invention is to provide a material

enabling production of a bipolar plate impervious to hydrogen and oxygen flows at the pressures and temperatures of use of the fuel cell.

It is a further aim of the invention to obtain a material that can be directly manufactured with the appropriate shapes ensuring a correct flowing of the gases by a single injection and moulding operation thereby avoiding possible further working operations for material removal.

It is a still further aim of the invention to obtain a material enabling a correct heat dissipation and also having low resistivity values in a direction orthogonal to the moulding flow.

Further'aims of the invention reside in enabling recycling of the material, increasing the mechanical features of the material itself and also allowing the per cent filler content of same to be increased.

The foregoing and further aims that will become more apparent in the course of the following description are substantially achieved by a material, particularly intended for manufacturing bipolar plates for fuel cells, in accordance with what stated in the appended claims.

Further features and advantages will be best understood from the detailed description of a preferred but. not exclusive embodiment of a material in accordance with the invention.

It is to be pointed out first of all that the material intended for manufacturing bipolar plates for fuel cells comprises at least one plastic matrix, that is no longer made of a thermosetting material, but comprises thermoplastic material. Choice of the organic matrix has fallen on polyolefins having a more or less crystalline

base and on polyesters and/or polyamides.

By way of example only, one of the thermoplastic materials usable as the matrix appeared to be polypropylene due to its good workability at low temperatures.

Use of a matrix with a thermoplastic base enables a cheap production of the manufactured article and simultaneously, the observance of the required shapes during the injection and moulding steps, in addition to usability at the work conditions (generally temperatures included between 50 and 100°C, gas pressures ranging from the atmospheric pressure to about 2 bars, in a mainly aqueous medium).

Then, a filler comprising graphite and/or carbon black is distributed in the thermoplastic matrix. Obviously the filler may only consist either of graphite or of carbon black, may comprise percent combinations of the same products or also other materials, provided they are conductive.

The filler is added in such percentages, form and sizes that an electric and thermal conductivity of the material is allowed.

From a percentage point of view, it has been noticed that the filler (comprising graphite) shall constitute at least 70% by weight of the material and shall preferably be included between 75 and 95% of same.

In particular the optimal per cent values by weight of the filler appears to be between 80% and 90%.

The graphite filler utilised in the present invention is

of the synthetic type because this allows a better control in the dimensional distribution of same as compared with natural graphite.

In terms of size the particles of powdered graphite have an average diameter included between 40 and 80-10-6 metres with a preferred value approximately of 60 10-6 metres ; in addition, at least 10-20% (preferably 15%)'of the graphite has a diameter exceeding 100-10-6 metres.

On the other hand a graphite provided with a low shape ratio appeared to be the best for the purposes of the invention. It is known in fact that graphite has a structure consisting of superposed planes in which conductivity is only present along the planes and not at right angles thereto. In other words from the point of view of the electric conductivity, graphite is very anisotropic.

It is also to be pointed out that the material in accordance with the present invention is also adapted to be injected into a mould in a direction substantially transverse to the thickness of the plate to be made.

Now, during this physical injection process, graphite has a tendency to lay in a direction parallel to the plane of injection therefore ensuring a better thermal and electric conductivity along this direction.

On the, other hand, the bipolar plate with a parallelepiped shape must on the contrary ensure an optimal electric and thermal conductivity in a direction perpendicular to its faces, i. e. along its thickness.

Therefore selection of graphite having a shape ratio as low as possible becomes of a fundamental importance; in this way the graphite material will tend to take the most

isotropic possible arrangement in the volume of the bipolar plate, even when injected into a mould.

In addition, in order to ensure the electric conductivity the graphite particles are required to be substantially in contact with each other within the matrix.

It is therefore well apparent how important is the average size of the graphite particles that must be capable of ensuring a substantial contact with each other and consequently allow the electric conductivity.

In fact, it has been ascertained that by use of graphite particles with smaller diameters in the same percentages by weight it was not always possible to ensure the electric conductivity.

Also to be pointed out is the fact that a percent portion of expanded graphite until a maximum of 10% and preferably until a maximum of 5% can be added to the above mentioned synthetic graphite.

The obtained material enables manufacture of conductive components (bipolar plates for heat/current dissipation...) by preparing a mixture and/or a compound of the mentioned material which will be directly supplied in its definitive shape so that any further possible working on the workpiece will be avoided.

For example, techniques involving injection, injection- compression, compression or extrusion can be adopted.

It is also to be pointed out that the finished product will have a heat and current conductivity in a direction longitudinal to the moulding flow as well.

After the above statements, in order to manufacture a bipolar plate, the thermoplastic material and graphite filler are mixed together in a uniform and homogeneous manner trying to impart the mixture the weakest shearing stresses during mixing to avoid the physical graphite properties being altered.

By minimizing the shearing stresses required for melting the polymer to the benefit of the elongation flows that promote dispersion and distribution of the filler, the required amount of graphite (or filler) can be incorporated in a single passage while simultaneously controlling size and integrity of same.

The compound thus. obtained can. be also directly injected into suitable moulds the shape of which matches that of the bipolar plate to be manufactured thereby allowing a finished product already provided with the necessary shapes to be obtained, which finished product will also have higher mechanical features than the bipolar plates made of thermosetting material.

In this connection it is to be noted that the bipolar plate of thermoplastic material is provided with a flat base of parallelepiped shape with a thickness included between 0.5 and 4 mm, preferably of 1.6 mm.

A predetermined number of channels are present on the main face of said bipolar plate, which channels have an extension substantially parallel to the face plane and the hollow depth of which is in the order of 10-3 metres.

In particular, each face has a plurality of channels parallel to each other exactly intended for removal of the reaction products from the catalyst.

The minimum thermal conductivity of the material is higher than 5 Wm~1K~l and is generally higher than 10-20 Wm~lK-l.

In this way the material allows a correct dissipation of the heat generated in the electrochemical process so that the system can operate at a temperature that is as constant as possible'.

The material also has low resistivity values, lower than 5 S2, cm, preferably lower than 250 10-3 Q cm and more preferably lower than 100-10-3 Q cm.

These resistivity values are to be measured in a direction orthogonal to the moulding flow (direction Z); in fact usually graphites present on the market tend to direct their maximum conductivity in a direction longitudinal to the moulding flow (plane X Y) and use of graphites having undoubtedly low (even lower than 5) anisotropy values in current conduction is therefore fundamental.

The bipolar plate thus obtained can be incorporated into the appropriate fuel cells intended for production of electric energy for example, in which at least one protonic-exchange membrane is present which is'associated with a catalyst in turn cooperating with at. least one bipolar plate in accordance with the above description.

The invention achieves important'advantages.

The obtained material also allows a direct moulding of the bipolar plate that must not be further worked. This enables an important saving of materials and production costs.

The material thus made has mechanical features greatly higher than those of the corresponding plates having a thermosetting base, and a much lower cost.

It is also to be noted that the material with a thermoplastic base can be recycled and therefore has less problems in. terms of environmental impact.

In addition, the obtained material is able to be also injection moulded while still ensuring optimal thermal and electric conductivity features in the direction of the plate thickness.

The same material appeared excellent in its work conditions (i. e. with temperatures included between 50 and 100°C, in a mainly aqueous medium and with a gas pressure between 1 and 2 atmospheres).

It is to be pointed out that this material can be used not only for producing bipolar plates for fuel cells, but also for possible further conductive elements in batteries of any nature and others.

In addition, use of the particular matrix has allowed a conductive filler until even 80-90% by weight to be obtained.