ELECTROCATALYST

The present invention relates to an electrocatalyst that may be used in a fuel cell, and their use in catalyst inks, catalysed electrodes, catalyst coated membranes and membrane electrode assemblies.

A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen or methanol, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.

Electrocatalysts used in fuel cells typically comprise platinum or a platinum alloy.

The metal may be unsupported, but is usually supported on a high surface area carbon such as a furnace carbon black or an acetylene black. Supporting the metal increases the available surface area of the metal and reduces the amount of metal required to achieve desired activity.

The present inventors have sought to prepare electrocatalysts that are suitable for use in fuel cells. The inventors have surprisingly found that catalysts wherein a metal particle is encapsulated by carbon are active for the anodic fuel cell reaction. This is very surprising because it is generally understood that the catalytic reactions occur at the surface of the metal. The inventors believe the catalysts may have advantageous properties when compared to known fuel cell catalysts, e.g. they may have better tolerance to poisons such as carbon monoxide and hydrogen sulphide. Alternatively, it may be possible to use metals that are not traditionally used in fuel cell environments because the carbon encapsulation protects the metal from sintering and/or oxidation.

Accordingly, the present invention provides an electrocatalyst for use in a fuel cell comprising a plurality of metal particles, wherein at least 90% of the surface area of the metal particles is covered by carbon.

Preferably at least 95% of the surface area is covered with a layer of carbon, most preferably all of the surface of the metal is covered with a layer of carbon. The carbon may be present as graphitic sheets and the term "a layer of carbon" includes both a single graphitic sheet and multiple graphitic sheets.

The one or more metal particles suitably comprise one or more transition metals, which are optionally alloyed with further metals. The metal particles preferably comprise one or more metals from Groups 8 and Ib of the Periodic Table (i.e. the Groups containing platinum group metals and Fe, Co, Ni and Cu), which are optionally alloyed with further metals. In a preferred embodiment, the metal particles comprise platinum or palladium, preferably platinum, hi a particularly preferred embodiment, the metal particles consist essentially of platinum or platinum alloyed with one or more metals chosen from precious metals such as ruthenium or gold, or base metals such as molybdenum, tungsten, chromium, tin or titanium.

In a first embodiment of the invention, the electrocatalyst consists essentially of a metal particle, wherein the particle is substantially covered with carbon. The particle suitably has an average diameter of from lnm to lOOnm, preferably from lnm to 50nm.

In a second and preferred embodiment of the invention, the electrocatalyst consists essentially of metal particles dispersed on a carbon support material, wherein the particles are substantially covered with carbon. The particles suitably have an average diameter of from lnm to lOOnm, preferably from lnm to 50nm, most preferably from 1 to lOnm. The carbon support material is suitably a high surface area carbon black, such as a furnace black or an acetylene black. The surface area of the carbon support is suitably at least 5OmVg, preferably at least 200m ² /g. Suitably, the amount of metal in the electrocatalyst is from 10 to 80wt% based on the weight of the carbon support material.

The covering of carbon on the metal particles is suitably less than 1 Onm in thickness. The inventors believe that thicker carbon layers are likely to reduce the activity of the electrocatalyst.

Methods of preparing carbon encapsulated metal particles are disclosed in, for example, WO 03/057626, WO 03/057359 and Lu et al, Chem. Commun., 2005, 98-100. These methods may be adapted for forming the electrocatalysts of the present invention.

In a first method, colloidal particles containing a source of metal are foπned in a liquid medium. The colloidal particles are stabilised by a surfactant. The colloidal particles contain a source of carbon such as cyanide, isocyanide, cyanate or isocyanate ligands. The particles are separated from the liquid medium. To form an electro catalyst according to the first embodiment of the invention, the particles are pyrolysed in an inert gas. To form an electrocatalyst according to the second embodiment of the invention, the particles are dispersed on a carbon support material and are then pyrolysed in an inert gas. A suitable pyrolysis temperature is at least 800 ⁰ C.

In a second method, a metal salt and a carbon source (e.g. poly( vinyl alcohol)) are mixed together in deionised water at room temperature. The solution is sprayed as a fine mist into a closed vessel containing a saturated ammonia solution. A solid precipitates and is collected. To form an electrocatalyst according to the first embodiment of the invention, the solid is pyrolysed in an inert gas. To foπn an electrocatalyst according to the second embodiment of the invention, the precipitate is contacted with a carbon support material, which is then pyrolysed in an inert gas. A suitable pyrolysis temperature is at least 800 ⁰ C.

In a third method, a carbon forming agent (e.g. polyvinyl alcohol), furfuryl alcohol or sucrose) is mixed with a pre-formed catalyst material such as platinum particles dispersed on a high surface area carbon support (e.g. HiSpec™ materials available from Johnson Matthey pic). The mixture is dried and then pyrolysed in an inert gas at a temperature of at least 800 ⁰ C to form an electrocatalyst according to the second embodiment of the invention.

In a further aspect the present invention provides an electrocatalyst ink comprising a catalyst according to the invention dispersed in a liquid medium. The ink suitably comprises aqueous and/or organic solvents, optional polymeric binders and optional proton-conducting polymers. Methods of making electrocatalyst inks are disclosed in EP 731 520.

In a further aspect the present invention provides an electrode comprising a catalyst according to the invention deposited on an electronically conducting substrate. The catalyst can be deposited onto a substrate using well known techniques, such as those disclosed in EP 731 520. The catalyst may be formulated into an ink and the ink may be deposited onto an electronically conducting substrate using techniques such as spraying, printing and doctor blade methods. Suitable substrates include carbon fibre papers and filled carbon fibre non- woven webs, such as those disclosed in EP 791 974. The catalysed electrodes may be used in fuel cells with acid electrolytes such as proton exchange membrane (PEM) fuel cells or phosphoric acid fuel cells, or they may be used in alkaline electrolyte fuel cells.

In PEM fuel cells, the electrolyte is a proton conducting polymer membrane. Electrocatalysts may be deposited onto one or both faces of the membrane to form a catalysed membrane. In a further aspect the present invention provides a catalysed membrane comprising a catalyst according to the invention deposited on an ion-conducting polymer membrane. The catalyst can be deposited onto the membrane using well known techniques. The catalyst may be formulated into an ink and either directly deposited onto the membrane or deposited onto a decal blank for subsequent transfer to a membrane. Suitable membranes are well known to those skilled in the art and include perfluorinated sulphonic acid membranes such as Nafion®, Fleinion® and Aciplex®.

In PEM fuel cells, the membrane is interposed between two catalyst layers, and each catalyst layer is in contact with an electronically conducting substrate. This five-layer assembly is known as a membrane electrode assembly. In a yet further aspect the present invention provides a membrane electrode assembly comprising a catalyst according to the invention. The membrane electrode assembly may be prepared by a process wherein an electrode according to the invention is combined with an ion-conducting membrane. Alternatively, the membrane electrode assembly may be prepared by a process wherein a catalysed membrane according to the invention is combined with an electronically conducting substrate. Preferably the electrocatalyst according to the invention is present in the anode of the membrane electrode assembly.

For a more complete understanding of the invention, reference is made to the schematic drawings wherein:

Fig. 1 is a schematic diagram showing an electrocatalyst according to the first embodiment of the invention. Fig. 2 is a schematic diagram showing an electrocatalyst according to the second embodiment of the invention. The features shown in the schematic diagrams are not to scale.

Figure 1 shows a metal particle (1) encapsulated by a layer of carbon (2). The diameter of the metal particle is, e.g. 1-lOOnm and the thickness of the carbon layer is less than lOnm, preferably about 2nm.

Figure 2 shows a carbon support material (3), wherein metal particles (4) are dispersed on the support material (3). The metal particles (4) are encapsulated by a layer of carbon (5). The average diameter ofthe metal particles is, e.g. 1-lOnm and the thickness of the carbon layer is less than lOnm, preferably about 2nm.