Friday, April 30, 2010

Carbon Nanotubes Decorated with Platinum Nanoparticles Is French Recipe for Robust Fuel Cell Electrode


Scientists at the French Atomic Agency (CEA) have cooked up more powerful fuel cell catalysts using carbon nanotubes and platinum nanoparticles.  They use solvents and mechanical mixing to produce and disperse catalytic platinum  nanoparticles on carbon nanotubes to prepare longer life electrodes for use in fuel cells. Before use, the nanotubes may also be heat treated at 2000.degree. C. for about two hours to remove a catalyst residue allowing their synthesis.

In U.S. Patent 20100104926, Commissariat A L'energie Atomique (CEA) (Paris, FR) inventors Bertrand Baret  (Palaiseau, FR), Henri-Christian Perez  (Courcouronnes, FR) and Pierre-Henri Aubert  (Menucourt, FR),  reveal a method of dispersing platinum catalyst on carbon nanotube supports to create a more robust fuel cells. The French recipe reduces the amount of platinum used per square centimeter (cm2) for fuel cell electrodes from milligrams to micrograms (mu.g.). 

The key method relates to the preparation of a catalytic composition that comprises a carbonated structuring material associated with a catalyst. The invention comprises mixing a solution of a first solvent including the carbonated structuring material and a solution of a second solvent including the catalyst, and agitating the resulting mixture up to the precipitation of the catalyst on the carbonated structuring material. The catalyst and the structuring material are not soluble in the mixture of the first and second solvents. The composition thus obtained can be used after filtration as a material for an electrode in a fuel cell. 

The solutions can be obtained by mechanical stirring, and optionally by ultrasonic treatment. In particular, the ultrasonic treatment of a solution comprising the structured material in the form of carbon nanotubes is advantageous, because it serves to separate the aggregates of aligned carbon nanotubes for which a simple stirring would not have been sufficient. Moreover, this treatment has the effect of breaking the nanotubes and reducing their original size. The average size of the nanotubes obtained depends on the duration of the dispersive treatment.

FIG. 4 is a TEM image of a composite of Pt-1 platinum nanoparticles/carbon nanotubes. It shows a view of a drop of dispersion observed by TEM. Nanotubes nearly completely covered with platinum nanoparticles are obtained (dark spots in the picture, size about 2 to 3 nanometers (nm), on the surface of the tubes).



FIG. 5 is a TEM image of a composite of Pt-2 platinum nanoparticles/carbon nanotubes in a mass proportion of 4/5, intended to be filtered subsequently to form an electrode having a theoretical maximum content of pure platinum of 56 .mu.g/cm2.

The filtration of 10 mL of dispersion of the composite obtained (nanotubes/nanoparticles) on a 2.3 cm2 carbon felt disk, gives a difference in mass of 0.33 mg, corresponding to a filtration yield of 91% of the mass of platinum. The effective density of platinum nanoparticles (with organic coating) is 63 .mu.g/cm2, corresponding to a density of pure platinum (without coating) of about 51 .mu.g/cm2.

FIG. 25 is an image obtained by scanning electron microscopy of a composite of Pt-0 nanoparticles on a mixture of carbon fibers and nanotubes in a mass proportion of 1/60, the composite then being intended to be filtered to prepare an electrode having a theoretical pure platinum content of about 9 .mu.g/cm2

The inventive method is suitable for preparing a catalytic composition from a dispersion of a carbonated structuring material in a first solvent and the addition of a solution of a second solvent comprising the catalyst, said catalyst being insoluble at least in the final resulting mixture. It is obviously desirable for the structuring material to be insoluble in the mixture of solvents.

This is an original method to the knowledge of the inventors, and is effective for combining the catalytic element with a carbonated element for the production of electrodes usable in fuel cells and/or for other conventional electrochemical applications. 

In the context of the present invention, a "carbonated structuring material" corresponds in particular to the materials typically employed in fuel cells. Such a material is said to be structuring in the sense that the catalyst is deposited on it. Such a material is generally in the form of a set of nanoparticles. It is advantageous for the smallest dimension of the particles to be between 5 nm and 10 .mu.m, and for their largest dimension to be not more than 5 mm and generally equal to or higher than 1 .mu.m.

The carbonated materials are preferably multiwall carbon nanotubes synthesized in the laboratory by chemical vapor deposition (CVD) of aerosol. This synthesis is suitable for obtaining nanotubes with controlled lengths. They are aligned, hence not tangled, and can therefore be dispersed very easily in liquid medium, for example in isopropanol without additive, under the effect of a treatment by stable ultrasound (using a power probe or simply in a laboratory ultrasonic tank). 

In general, the size of the catalyst particles selected is lower than that of the particles of structuring material, so that the particles of structuring material are advantageously larger than the catalyst particles in at least one of their dimensions, for example the length. Typically, these are nanometer-sized catalyst particles. Preferably, the largest dimension of the catalyst particles does not exceed about 20% of the smallest dimension of the carbonated structural material.

The metal is often selected from noble metals and alloys and more particularly platinoids and platinoid alloys. Platinoids correspond to the family of platinum, iridium, palladium, ruthenium and osmium. In a nonlimiting manner, platinum is nevertheless preferred in this family. Platinoid alloys comprise at least one platinoid. It may be a natural alloy such as osmiridium (osmium and iridium) or an artificial alloy such as an alloy of platinum and iron, platinum and cobalt, or even platinum and nickel
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