Thursday, February 25, 2010

Shin-Etsu Chemical Reveals More Powerful Nano Catalysts for Direct Methanol Fuel Cells

Smaller nanoparticles are better when it comes to catalyst particles for direct methanol fuel cells (DMFC). 

Shin-Etsu Chemical Co., Ltd. (Chiyoda-Ku, JP) inventors have developed a process for producing an improved platinum-ruthenium (PtRu) electrode catalyst for a fuel cell. The process includes a first support step of producing metallic fine particles having an average particle diameter of 0.1 to 1.5 nanometers provided at regulated particle intervals on an electroconductive carbon carrier, and a second support step of growing a metal identical to or dissimilar to the metal using the metallic fine particles as a nucleus. In the first support step, the metallic fine particles are supported by an immersion method, according to U.S. Patent Application 20100048388.

 The above constitution can provide an electrode catalyst for a fuel cell, which has a high level of percentage support, has a high level of dispersibility, and has improved methanol oxidation activity per weight of the catalyst. Further, when treatment in an atmosphere containing hydrogen is carried out at a low temperature below C., the methanol oxidation activity per active surface area can be improved without lowering the active area, says inventor Shigeru Konishi.

For mobile phones, laptop computers and other devices,  batteries with a higher capacity are desired, but it is quite difficult to increase the capacity of secondary batteries. Thus direct methanol fuel cells (DMFC) using methanol fuel are of great interest for portable electronic devices.

DMFC has the advantage of possible size reduction since they can utilize liquid fuel directly without converting it into hydrogen or the like. Research efforts have been made thereon toward commercial use. However, the problems that the electrolyte membrane has a high methanol permeability and the anode catalyst has a low methanol oxidation activity have slowed the commercial application of DMFC.

Most often PtRu catalysts are used as the anode catalyst. While efforts are made to search for high activity catalysts other than PtRu, no other catalysts have overtaken PtRu. Means for enhancing the activity of PtRu catalysts include use of a supported catalyst in which PtRu nano-particles having a small particle size and a large surface area are dispersed on a carbon support. Notably, commercially available supported catalysts, for example, TEC61E54 (54 wt % PtRu/C, Tanaka Kikinzoku Group, PtRu size 4 nm) still have an insufficient activity, with a further enhancement of activity being needed. To this end, it is desired that PtRu particles be further reduced in particle size (less than 4 nm) and more richly and uniformly supported on a carbon support (high loading and high dispersion). 

After making extensive investigations to attain an improved fuel cell catalysts, the inventors have found that when a process of forming metal nuclei such as Pt on support carbon, then growing PtRu on the metal nuclei (to be referred to as two-stage loading process, hereinafter) is used as means for producing a highly loaded/highly dispersed PtRu-laden catalyst, there is obtainable a catalyst in which PtRu particles having an average particle size of up to 4 nm are loaded on carbon in a well dispersed fashion even at a loading of at least 50 wt %. Although a catalyst having a methanol oxidation activity 2.5 times higher than commercially available TEC61E54 can be prepared by this two-stage loading process, a further improvement in catalyst activity is desired for higher power capacity so that the catalyst may be actually used in fuel cells. 

Continuing further investigations, the inventors found that while formation of metal nuclei on support carbon is conventionally performed by alcohol reduction in a solution containing carbon and metal salt, formation of metal nuclei by an immersion technique is successful in providing the catalyst with a greater specific surface area even at the same loading. Particularly when Ru is used as the metal species, a significantly large specific surface area is available, and a highly loaded/highly dispersed PtRu/C catalyst resulting from PtRu growth thereon has a large specific surface area and an improved methanol oxidation activity per PtRu weight (weight activity). It has also been found that when the catalyst is further treated in a hydrogen-containing atmosphere at a temperature below C., it is improved in methanol oxidation activity per active surface area (specific surface area activity) while maintaining a large active surface area, and also improved in the weight activity of PtRu.  

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