WO2015040278A1

WO2015040278A1 - Carbon-containing composites and their electrochemical applications

Info

Publication number: WO2015040278A1
Application number: PCT/FI2014/050711
Authority: WO
Inventors: Mikko Karttunen; Pertti Kauranen; Matti Koponen; Lisa Wikström; Thomas TINGELÖF
Original assignee: Teknologian Tutkimuskeskus Vtt
Priority date: 2013-09-17
Filing date: 2014-09-17
Publication date: 2015-03-26
Also published as: FI20135934A

Abstract

The present invention concerns a carbon-polymercomposite material containing at least one thermoplastic polymer, at least two carbon fillers, one beingselected fromgraphites, and at least one additive selected from flow modifiers.The invention also concerns a moulded composite structure, such as a plate,moulded from said composite material, a process for compounding and moulding said material into said structure, as well as the use of this composite in electrochemical applications.

Description

CARBON-CONTAINING COMPOSITES AND THEIR ELECTROCHEMICAL APPLICATIONS

Background of the Invention

Field of the Invention

The present invention concerns carbon-polymer composite materials, as well as a process for compounding such a composite material and moulding it into the desired shape. This material can be used in various applications, but is particularly suitable for use in electrochemical applications, such as fuel cells.

Description of Related Art

Graphite components are commonly used for electrochemical applications like stationary fuel cells and flow batteries, as well as heat exchangers. Due to its properties, graphite is highly suitable for use even in corrosive environments. Thus, graphite materials can be shaped into, e.g. bipolar plates for use in fuel cells, which plates are mainly responsible for distribution of reactants and other materials, as well as electron transport, but also provide mechanical support for membrane electrodes and separation of unit cells in stacks. The electrical conductivity of these plates and other similar components is essential, as well as the chemical resistance of the plate material. Since pure graphite is brittle and expensive to machine (or shape), graphite components have lately been replaced in said electrochemical applications by graphite-polymer composites, which exhibit a higher strength than the corresponding pure graphite components, and at least a similar durability. Common composites include graphite-thermoset composites, prepared for example using an epoxy or a vinyl ester polymer as the thermoset component. Such a composite has been described in, among others, US 6248467 Bl and WO1999019389A1. These materials, however, tend to exhibit poor dimensional tolerances, and they are more difficult to recycle and remould than thermoplastic materials. Other common composites are graphite-thermoplastic composites, prepared for example using polypropylene as the plastic component. Mixtures of more than one thermoplastic have also been investigated, e.g. by Mighri F. et al. (Polym. Eng.Sci. 2004, 44: 1755- 1765). These simple thermoplastic-carbon composites, however, tend to exhibit poor strengths at high temperatures. For example, the mentioned polypropylene cannot withstand high temperatures (generally limited to 70-80°C), which limits the range of applications. For the above described electrochemical applications, high thermal and chemical stability and good electric and thermal conductivity is, however, needed.

As described above, the stability of the mentioned composites can vary depending on the choice of materials. However, also the contents of the materials have an effect. Polyarylene sulphides have been found to provide the composites with relatively high stabilities, and these can endure high temperatures. For example, polyphenylene sulphide (PPS) is one of the most stable, chemically resistant, and temperature resistant engineering thermoplastics. High contents of carbon should be used to prepare electrically highly conductive materials from this polymer, but such high carbon contents generally give the resulting PPS compounds high melt viscosities, low strengths and low durabilities compared to the pure PPS materials.

The production and moulding of these graphite-polymer composites is cost-effective compared to the production of the conventional graphite components. Compression moulding is commonly used (see e.g. EP 1736295 Al, JP 2008078023 A and JP

2006019227 A), but this technology offers a poor precision due to variations in the stage of closing the mould.

Particularly for fuel cell applications, precision is, however, essential. These fuel cells generally include the above mentioned cell plates that in turn include a complex network of channels that guide liquid flow. Poor precision in forming these channels would result in poor function of the fuel cell. Therefore, injection moulding would be a more advantageous alternative, due to its higher precision compared to compression moulding. Injection moulding, however, also has higher requirements regarding the melt flow characteristics, particularly melt viscosity, of the composite, and the temperature resistance, which the common graphite-polymer composites have not been able to fulfil. For example, graphite-polypropylene composites can be injection moulded but the operating temperature is limited to 90 °C.

Therefore, there exists a need for carbon composite materials that have high thermal and chemical stability, while maintaining the good electrical conductivity commonly provided by graphite. High thermal stability of these composites enables their use at higher application temperatures (e.g. at 100 °C to 200 °C). Thus, new applications can be found for electrically conductive thermoplastic composites.

Summary of the Invention

It is an object of the present invention to provide a carbon composite material, which exhibits high electrical conductivity, high stability and strength, even at high temperatures. Particularly, it is an object of the present invention to provide such a carbon composite material in a cost-effective manner.

A further object of the invention is to provide a process for compressing such a material, and for moulding it with high precision.

These and other objects, together with the advantages thereof over known materials and processes, are achieved by the present invention, as hereinafter described and claimed.

The invention is based on improving the electric conductivity of conventional graphite- polymer composite materials. This improved conductivity is achieved by using a combination of at least two carbonaceous fillers, including graphite powder, and a polyarylene sulphide as the polymer. Since the thermal and chemical stability of the polymer is critical for the feasibility of the composite materials, a polyarylene sulphide, such as polyphenylene sulphide (PPS), is used, preferably in powder form to enable homogeneous polymer-carbon premixing before compounding. To make the material more suitable for use with injection moulding, the melt viscosity of the composite is decreased, for example by using titanates as flow modifiers (or coupling agents).

More specifically, the composite material of the present invention is characterized by what is stated in the characterizing part of Claim 1 , while the composite structure is

characterized by what is stated in Claim 10.

Further, the process of the present invention is characterized by what is stated in Claim 13 and the use of the composite in electrochemical applications is characterized by what is stated in Claim 16.

Considerable advantages are obtained by means of the invention. Thus, the present invention provides a composite material with high stability and strength, even at high temperatures. This material also has a sufficiently good electric conductivity to be used as a replacement of pure graphite in electrochemical applications. Further, the material has a sufficiently low melt viscosity to allow processing in high temperatures, such as by injection moulding.

Particularly, the present invention provides means for achieving both high strength and electrical conductivity, simultaneously, in the same material.

Next, the invention will be described more closely with reference to the attached drawings and a detailed description. Brief Description of the Drawings

Figure 1 is a schematic drawing of a preferred application for the present invention, i.e. of the unit of a fuel cell that includes a bipolar plate.

Figure 2 shows the measured resistivities of polyphenylene sulphide (PPS) composites with carbon nanotube filler (CNT), and with or without titanate additive.

Figure 3 shows the measured resistivities of polyphenylene sulphide (PPS) composites with different grades of graphite filler and with 0.75w-% or 3w-% titanate additive.

Figure 4 shows the measured resistivities of polyphenylene sulphide (PPS) composites with graphite filler, 3w-% titanate additive, and optional further carbon filler in the form of CNT in different contents.

Figure 5 shows the measured resistivities of polyphenylene sulphide (PPS) composites with graphite filler, 1.5w-% or 3w-% titanate additive, and optional further carbon filler in the form of CNT. Figure 6 shows the required specific energy input and torque required when compounding by Berstorff ZE 25 x 48D twin screw extruder various PPS composites including varying contents of graphite, varying contents of CNT, and optionally 3w-% titanate.

Figure 7 shows the results of steady state testing of an in-house PPS composite, at a current density of 0.2 Acm-2, ambient pressure and 200 °C, with BASF standard flow field plates for reference. Detailed Description of Preferred Embodiments of the Invention

The present invention concerns a carbon-polymer composite material containing at least one thermoplastic polymer as well as at least two carbonaceous fillers.

Due to its advantageous properties, one of the carbonaceous fillers is graphite, preferably in the form of graphite powder (generally used in either spherical or flake form), most suitably spherical graphite powder. The average particle size (diameter) of this graphite is preferably <300μιη, more preferably 30-200μιη, and most suitably 50-150μιη.

This graphite is typically the main filler, i.e. it typically forms at least 50 w-% of the total amount of carbonaceous fillers, preferably 60-95 w-%.

The further carbonaceous filler(s) is/are selected from any carbonaceous materials that can be processed and which can promote the electrical conductivity (i.e. not diamond) of a composite, particularly from materials formed of pure or essentially pure carbon, such as carbon black, carbon fibres, carbon nanofibres, carbon nanotubes, graphene or a type of graphite that is different from the graphite used as the main carbonaceous filler. For example, spherical graphite can be used as the main filler, whereby the further filler(s) can be selected from flaky graphite, carbon black, carbon fibres, carbon nanofibres, graphene and carbon nanotubes.

The optionally used carbon black is preferably selected from powders having an average diameter of primary particles of 10-20nm. Similarly, the optionally used carbon fibres preferably have an average length of 5-1000μιη and an average diameter of 5-15μιη, while the optionally used carbon nanotubes preferably have an average length of 1-50μιη and an average diameter of 8-100nm. The carbon nanofibres are naturally of nano-size. Of these further fillers, carbon nanotubes (CNT) can be used in contents of up to 10w-% of the entire composite material, carbon black can be used in contents of up to 20w-% of the composite material, and carbon fibres can be used in contents of up to 10w-% of the composite material. The total content of carbonaceous filler in the resulting composite is preferably >50w-%, more preferably 50 - 95w-%, and most suitably 60-80w-%. A high carbon filler content is needed to provide a sufficient electric conductivity (commonly of the level of 10 S/cm or higher).

According to a preferred embodiment of the invention, graphite (preferably as purely spherical graphite or a mixture of spherical and flaky graphite) is mixed with either carbon nanotubes or with both carbon black and carbon fibres, to form the filler mixture of the composite.

Thus, the composite material of the invention can contain, for example, 75w-% graphite powder, 2w-% carbon black and 0.5w-% carbon fibres, or 67w-% graphite powder and 2.5w-% carbon nanotubes, calculated based on the total weight of the composite material. Combinations of two or more carbon materials in polymer composites have been found to provide synergistic effects (for example a lower resistivity). The further carbon filler(s) provides a lowered contact resistance between the graphite particles forming the main filler. At least one thermoplastic polymer is used, as a binder, to facilitate plastic processing type of manufacturing of the composite article. This thermoplastic is preferably selected from polyarylene sulphides, more preferably being polyphenylene sulphide. Most suitably, this polyarylene sulphide is used as the only organic polymer. The content of thermoplastic polymer in the composite material is preferably 10-45w-%, more preferably 10-35w-%, and most suitably 15-30w-%, although it is mainly guided by the required content of carbonaceous filler, since high carbon contents cause the desired increased electrical conductivities (or lower resistivities). To provide a further possibility of using higher carbon contents in the composite material, and to provide a lower viscosity, flow modifier(s) (or coupling agents) can be added to said composite material. The term "flow modifier" is intended to mean a group of additives that lower the melt viscosity of materials, such as composites. The same group of additives can be called "coupling agents" due to their further capability of influencing the interaction between particles and polymers, most likely by facilitating the breakdown of agglomerates. The breakdown of agglomerates also causes an increase in electrical conductivity.

Preferably at least one flow modifier is selected from the group of titanates. Titanate flow modifiers in the composition reduce the melt viscosity allowing high temperature processing and, thus, making the material more suitable for injection moulding. Examples of suitable titanates include organotitanates, preferably organotitanates based on the titanium(IV) atom, more preferably divalent or multivalent organotitanates, most suitably with one or more C_1-18 ligands attached to the titanium atom, particularly two or more C_1-18 ligands. These C_1-18 ligands, as well as optional further ligands, commonly include also ether groups, hydroxyl groups, ester groups, halogens (-C1, -Br or -I), amine groups or phosphate groups within the ligands, or more than one type of such functional groups.

Examples of ligands that can be attached to the titanium atom include the following:

-alkoxy;

-aryloxy;

-alkylate;

-alkoxy-halogen;

-aryloxy-halogen;

-alkoxy-amine;

-aryloxy-amine;

-P0₂-alkoxy;

- P0₂-alkoxy-halogen;

- P0₂-OH;

wherein the alkoxy and the alkylate each independently can be linear or branched, can include 1-18 carbon atoms, and in case the alkoxy or alkylate contains 2-18 carbon atoms, the group can further contain one or more double or triple bonds. Typical examples of titanates that have been found suitable for the present purpose ( flow modifiers in materials intended for electrochemical applications) are the simple hydroxy-acetate-titanate, more complex alternatives

(C₅H₉0)(C₂H₅0)Ti((OP(0)(OC₆H₄Cl))P(0)(OCH=CH₂)₂))₂;

(OCH₂CH(CH₃)CH₂0)TiOP(0)(OCH₂C₆H₄CH₃)OP(0)(OCH₃)((OCH(C₆H₅)₂))OP(OH)- (0)OP(0)(OC₂H₅)₂;

(CH₃)₂CH0₂Ti(OP(0)(OC₃H₁₇)OP(0)(OC₈H₁₇)₂;

(CH₃)₂CHOTi(OC₂H₄NHC₂H₄NH₂)₃;

(OCH₂)₂Ti((OP(0)(OH)OP(0)(OC₈H₁₇)₂))₂;

(CH₃)₂CHOTiOP(0)((OHP(OH(OC₈H₁₇)₂))(OP(0)(OCH₃)(OC₄H₉)₃;

((CH₃)₂CHO))₂Ti((OC₂H₄N(C₂H₄OH)₂))₂. A particularly preferred example of the titanate suitable for use in the context of the present invention is the 2-propanolato-(tris-isooctadecanoato-0)-titanium(IV).

The titanate is preferably used in a content of 0.5-10w-%, more preferably 0.075-5 w-%, of the entire composite material. If further additives are used, their content generally does not exceed 5w-%.

The present invention also concerns a structure made of the above described composite material. Examples of such structures are plates, which are commonly used in, among others, fuel cells (e.g. as bipolar plates), as well as in batteries (including liquid flow batteries) and heat exchangers.

An exemplary construction, or a repeating unit 1 of a fuel cell stack, including said bipolar plate, is shown in Figure 1, which shows the following components:

2 bipolar plate with channels

3 anode

4 cathode

5 electrolyte

A typical plate of this type can have a size of, for example 150 x 150 x 3 mm. Further, the present invention concerns a method for manufacturing such a composite structure, e.g. in the form of a plate, by compounding the above described carbon composite material and injection moulding it into the desired shape.

The compounding is carried out in order to efficiently mix the various substances that are to form the composite. In this step a conventional mixer can be used. The compounding step generally lasts for less than 10 minutes, the time, however, depending on the efficiency of the mixing (generally between 200 and 1200 rpm). Typically, the residence time in compounding step is 3 to 5 min. The polymer is optionally melted prior to or during compounding.

According to a preferred embodiment of the invention, the compounding step is carried out using an extruder, such as a twin-screw extruder, to provide a good distribution of the carbonaceous fillers and the flow modifier(s) in the polymer, as well as to cause the preferred melting of the polymer.

In case of using an extruder, the present composites can suitably be compounded for example at temperatures varying in the range of 300-360°C, preferably 300-330°C and more preferably 315-320°C, and using a screw speed of about 150 rpm - 300rpm. Further, an elevated pressure can be used, such as a pressure of 50-800bar.

The injection moulding is carried out by injecting the compounded composite material into a mould cavity, generally after heating the material to an elevated temperature. The temperature profile of injection moulding machine is typically 115 °C - 335 °C - 340 °C - 340 °C - 345 °C. Temperature profile can be changed depending on the filler loading and the combination of carbon fillers, but the temperature is preferably maintained within the range 100-350°. Preferably the mould is separately heated, for example using water to a temperature of at least 80°C (i.e. from 80 to almost 100°C), or using other means, such as oil, to an even higher temperature (e.g. a temperature of 140°C to 180 °C).

The injection is generally carried out using high pressures, such as a pressure of 100- 800bar, depending on the viscosity of the composite material, while the clamping pressure during the moulding step generally is lower, while being higher than atmospheric pressure. With the given injection pressure range, suitable holding pressures can be, for example 50- 500bar.

According to another preferred embodiment of the invention, a grinding step is carried out after the moulding step, for example using sanding or abrasive blasting, to remove the lower-quality surface layer (containing less graphite) that often is formed on the composite structures, and to provide even lower resistivities.

The composites of the present invention can be applied to, among others, fuel cells, batteries, electrolytic processes, and heat exchangers, preferably fuel cells, redox flow batteries or industrial heat exchangers, with a particularly preferred application being in the field of polymer fuel cells, most suitably high temperature polymer fuel cells using e.g. phosphoric acid doped polybenzimidazole (PBI) as the electrolyte. Fuel cells generally include cell plates that in turn include a complex network of channels that guide liquid and gas flow. Among others due to said channels, precision is essential in the manufacture of these plates. Therefore, injection moulding is a more advantageous procedure in this manufacture than, e.g., the conventional compression moulding. Said fuel cell plates (or bipolar plates) must also be resistant to cracking, chemical reactions and corrosion at a wide range of temperatures, whereby not only the resistivity guides the choice of materials.

Other applications include redox flow batteries, electrolytic processes and industrial heat exchangers. The invention also provides the possibility to operate in advantageous conditions in phosphoric acid fuel cells.

The following non-limiting examples are intended merely to illustrate the advantages obtained with the embodiments of the present invention. EXAMPLES

In the following examples, unless something different is stated, the compounding was carried out using DSM Micro 15cc twin screw compounder with a temperature profile of 300-310-330°C and a screw speed of 200rpm, while the moulding was carried out using an injection moulding apparatus Haake minijet with an injection temperature of 330°C, an injection pressure of 100-800bar, and a mould temperature of 140°C.

Example 1 - Effect of the type of carbon component and titanate flow modifier

Polyphenylene sulphide (PPS, Ryton PR11 (Chevron Philips), powder) was used as the single polymer substance. This PPS was compounded with different carbon substances, and injection moulded into plates of the size 100 x 100 x 3 mm. The resistivities of these composites were tested.

The sample PPS-012 was processed by mixing with a batch mixer Brabender Plasticorder) and compression moulded. The other samples were mixed with a Berstorff twin screw compounder and injection moulded (Demag Ergotech 100/420-120 EL-EXIS S). The lowest resistivities achieved with different filler types are presented in Table 1.

Table 1. Resistivities of PPS-carbon composites

Code Filler Flow Flow Filler Resistivity, Resistivity modifier modifier loading, after grinding, w-% w-%

PPS-012 Carbon No 6.0 39.0 not measured nanotube

PPS-1016 Graphite No - 70.0 1.80 1.10

PPS-1039 Graphite KR238^1} 0.75 74.4 0.50 0.32

PPS-1035 Modified Lica38 ⁾ 0.75 64.5 1.33 not measured graphite

PPS-1059 Graphite + No 77.3 0.59 0.39

carbon fibre PPS-1048 Graphite + KR238 0.75 77.3 0.16 0.1

carbon fibre

PPS-1054 Graphite + KR238 1.5 77.3 0.23 0.18

carbon fibre

^UKR238, Titanate(2-),bis[P,P-dioctyl diphosphato(2-)-0",0""][l,2-ethanediolato(2-)-

0,0']-, dihydrogen, branched and linear . .CAS# 68585-63-7 90%

Titanium IV tetrakis {2,2-bis(2-propenyloxy)methyll-l-butanolato} adduct

2 moles (ditridecyl phosphito-O), dihydrogen . .CAS# 64157-14-8 10%

Lica38, Titanium IV 2,2(bis 2-propenolatomethyl)butanolato, tris(dioctyl)pyrophosphato-0

Graphite-carbon fiber ratio was 75 to 25 (w-%>) in recipes PPS-1016 to PPS 1054.

The used graphite was of the type: KS-5 75TT TIMREX (Spherical). The modified graphite means a special graphite grade with "conductive bridges" (Cond20/96, Graphit Kropfmuehl AG).

The used carbon nanotubes were of the type: Nanocyl NC7000.

The used carbon fibers were of the type: Grafil PU6-1

In the overall results, a small amount (2 wt-%) of CNT (in the graphite - CNT mixture) lowered the contact resistance between graphite particles in the composites, compared to the results obtained with a simple PPS-graphite composite.

As can be seen particularly from the results of Table 1, the use of a flow modifier lowers the resistivity of the produced composites, whereby the electrical conductivity is improved. Further, only small amounts of flow modifier are required (such as 0.75w-%>). The resistivity of the graphite - carbon fibre mixture (in the PPS composite) was clearly lower (1.7 Ωαη) than that of the reference graphite-PPS composite (3.7 Ωαη) when the total filler loading was 65 wt-%>. The lowest resistivity of all PPS composites was achieved with the loading of 77.25 wt-% of synthetic graphite - carbon fibre mixture. The resistivity of the moulded sample of this mixture (code PPS-1048) was 0.16 Qcm. The qualities of these mouldings were also good. The resistivity of compression moulded composites with the loading of 78.25 w-% of graphite - carbon fibre - carbon black mixture was 0.031 cm. After grinding of the skin of the sample of synthetic graphite - carbon fibre mixture, its resistivity was 0.1 Ωαη. Similarly, after grinding of the skin of the sample of graphite - carbon fibre - carbon black mixture, its resistivity was 0.030 Qcm. Thus, grinding could be used to decrease the resistivities of the composites even further.

Example 2 - Effect of addition of CNT and titanate flow modifier To determine the effect of the addition of a flow modifier on the resistivity of a conductive carbon material, a titanate flow modifier (KR-TTS (Kenrich Petrochemicals), Titanium IV 2-propanolato, tris isooctadecanoato-O) was compounded with carbon nanotubes (CNTs; Nanocyl NC7000, length=l ^m, diameter=9.5nm, 3% and 7%) and polyphenylene sulphide (PPS, Ryton V-l (Chevron Philips), powder), using the above described compounding and moulding conditions, as well as a holding pressure of 50-500bar.

The CNTs were first dissolved in 350 ml isopropanol by ultrasonication. Titanate flow modifier was added (20w-%, calculated based on the CNTs) to the solution and solvent was evaporated by Rotavapor machine. The product was finally dried in a vacuum oven before mixing with the PPS, compounding, injection moulding into a bipolar plate and measuring the resistivity (shown in Figure 2).

In the compounding step, a force of 685N was applied to the 7% CNT with titanate and 1200N to the 7% CNT without titanate.

These results show, among others that the carbon material is essential particularly in providing the sufficiently low through-plane resistivity, while the titanate improves the results even further. The same experiment and measurement was carried out using two different types of graphite (a spherical KS-5 75TT TIMREX, d90= 70 μιη, and a mixture of this spherical type with a flake-type SGA 20M Kropfmuehl, d90= <50 μιη) and the above mentioned titanate (0.75% and 3%). The results are shown in Figure 3. From these results it can be seen that, while a mixture of graphites is more advantageous when used alone, a spherical type of graphite provides an even smaller resistivity when used with titanate, in a sufficient amount, although even minor amounts of titanate will provide significant improvements.

The additional effect of using both graphite (52.5%, 55%, 57.5%, 60%, 67.5% and 70%) and carbon nanotubes (7.5%, 5%, 2.5% and 0%>), with 3% titanate, is shown in Figure 4.

Similar results are shown in Figure 5, with carbon filler, either as pure graphite (60w-%) or as a mixture of graphite and CNT (57.5w-% graphite and 2.5w-% CNT), as well as either 1.5w-% or 3w-% titanate.

These results demonstrate that a mixture of graphite and CNTs is the most advantageous. Example 3 - Compounding trials

Trials were carried out the test the effect of the titanate flow modifier on the compounding of composite materials.

The materials and contents shown in the following Table 2 were used in the composites.

Table 2. Composite materials produced by Berstorff ZE 25 x 48D twin screw extruder

The results are shown in Figure 6. These results demonstrate, among others, that the addition of titanate to the composites will result in a lower requirement for energy input. The materials of Table 2 gave composites, which displayed the electrical conductivities (and resistivities) of the following Table 3.

Table 3. Resistivities and conductivities of composite materials

a prepared using a normal injection speed: 600mm/s

b prepared using a higher injection speed: 700mm/s

Example 4 - Fuel Cell test Single cell testing:

The in-house developed compression moulded PPS composite 1056 was tested in a single cell hardware in a Fuel Cell technologies test station, University Model. The recipe of PPS- 1056 is shown in Table 4, below. This compound was used to prove the performance of carbon filled PPS compound modified with titanate flow modifier. PPS- 1056 compound was grinded and compression moulded in to the sheet form. The required flow field was CNC milled onto the plate. Table 4. Composite material PPS-1056

* carbon black (Ketjenblack EC-600 JD)

In order to accelerate the testing, the cell temperature was 200 °C (40 °C above the recommended operating temperature of the BASF PI 000 Membrane electrode assembly (MEA)). Before the testing the material was cleaned by repeated boiling for 30 minutes in MQ water and rinsing with iso-propanol. In-situ steady-state testing was made 200 °C, a current density of 0.2 A cm^"2, stoichiometries 2 at air cathode, 1.2 at hydrogen anode and ambient pressure.

The results from the accelerated testing of the in-house compression moulded composite material 1056 may be seen in Figure 7. The performance of the same setup with standard BASF isostatic graphite flow field plates is given for reference. The performance follows the same trend that has been observed for a large number of PI 000 ME As with the BASF graphite plates at this temperature. After an initial increase due to the activation of the MEA, the cell voltage reaches a plateau and then decreases with time. The voltage decay is attributed to the MEA degradation, not to the flow plates. The cell voltage with the composite plates is about 10 mV lower than the cell voltages that may be obtained with the BASF test plates, but this can be assigned to the lower conductivity of the composite plates. No degradation was visible in the test plates after the testing.

This example shows that the composite plates are as stable as or even more stable than the original graphite plates.

Claims

1. A carbon-polymer composite material containing at least one thermoplastic polymer, characterized in that it contains at least two carbon fillers, at least one being selected from graphites, and at least one additive selected from titanate flow modifiers.

2. The carbon composite material of Claim 1, characterized in that at least one carbon filler is in the form of graphite powder, preferably spherical graphite.

3. The carbon composite material of Claim 2, characterized in that >50w-%, preferably 60-85w-%, of the carbon filler is graphite.

4. The carbon composite material of any preceding claim, characterized in that the further carbon component(s) is/are selected from carbon black, carbon fibers, graphene and carbon nanotubes.

5. The carbon composite material of any preceding claims, characterized in that at least one thermoplastic polymer is polyarylene sulphide, preferably polyphenylene sulphide, most suitably used as the only organic polymer.

6. The carbon composite material of any preceding claim, characterized in that its content of thermoplastic polymer is 10-45w-%, preferably 10-35w-%, most suitably 15- 30w-%.

7. The carbon composite material of any preceding claims, characterized in that the titanate flow modifier is selected from organic Ti(IV) titanates, whereby at least one ligand attached to the titanium atom is an organic Ci_is-ligand, the titanate most suitably being 2- propanolato-(tris-isooctadecanoato-0)-titanium(IV).

8. The carbon composite material of any preceding claim, characterized in that the titanate flow modifier is selected from titanates, which are present in a content of 0.5-1 Ow- % of the entire composite material.

9. The carbon composite material of any preceding claim, characterized in that it contains >50w-% of carbon fillers, preferably 50-85w-%, most suitably 60-70w-%, the remaining substances being thermoplastic polymer and additives.

10. A moulded composite structure, characterized in that it has been moulded from the composite material of any of claims 1 to 9.

11. The composite structure of claim 10, characterized in that it is in the form of a plate.

12. The composite structure of claim 10 or 11, characterized in that it has a resistivity of <2Ωαη, preferably <0.2ΩΟΉ.

13. A method for manufacturing the composite structure of any of claims 10 to 12, characterized by carrying out a compounding step to form the carbon-polymer composite material of any of claims 1 to 9, and an injection moulding step to mould the material into the desired shape.

14. The method of claim 13, characterized by carrying out the compounding step with the material to be compounded at an elevated temperature, such as a temperature of >200°C, and optionally at an elevated pressure, such as a pressure of 50-800bar.

15. The method of claim 13 or 14, characterized by carrying out the injection moulding by injecting the compounded composite material into a mould cavity and by subsequently allowing the material to solidify, the injection preferably taking place at an elevated temperature, more preferably a temperature of 200-350°C, even more preferably a temperature of 250-350°C, most suitably a temperature of 300-350°C, and using pressure for the injection, such as a pressure of 100-800bar.

16. Use of the carbon composite material of any of claims 1 to 9, or of the composite structure of any of claims 10 to 12 in electrochemical applications.

17. The use of claim 16, where the electrochemical applications are selected from fuel cells, batteries, electrolysis processes or heat exchangers, preferably fuel cells, redox flow batteries or industrial heat exchangers, more preferably polymer fuel cells, most suitably high temperature polymer fuel cells using e.g. phosphoric acid doped polybenzimidazole (PBI) as the electrolyte.