Thursday, December 24, 2009

Canon Water Repellent Nano-Catalyst Layer Improves Fuel Cell Performance and Lowers Cost

FIG. 8 is an atomic force microscope (AFM) image of Canon's water repellent nano-catalyst layer surface for polymer electrolyte fuel cells to power small electronic equipment, such as mobile phones,  notebook personal computers, or digital cameras.

Canon (Tokyo, JP) inventors Shinnosuke Koji, Kazuya Miyazaki, Yoshinobu Okumura, and  Kaoru Ojima created a water repellent nano-catalyst layer for a polymer electrolyte fuel cell.  The hydrophobic property is imparted even to the inside of fine pores of the catalyst layer to improve water evacuation performance but in such a way that the effective surface area and the catalyst utilization ratio can be increased.

The catalyst shows improved evacuation performance of the water produced during the electrochemical reaction of hydrogen and oxygen.  It has a stable performance over a long period of time. It can be used to manufacture a more stable polymer electrolyte fuel cell “at a low cost,” according to Canon inventors in U.S. Patent Application 20090311578

In the polymer electrolyte fuel cell, in general, a fluororesin-based ion exchange membrane is used as a solid electrolyte of a proton conductor, and a catalyst, such as platinum or platinum-alloy fine particles having high catalyst activation, is used for promoting a hydrogen oxidation reaction and an oxygen reduction reaction. The electrode reaction occurs in a so-called three-phase interface (electrolyte--catalyst electrode--fuel) in a catalyst layer. In this case, there is a problem in that a voltage is gradually reduced as power generation time elapses, and power generation finally stops.

This is caused by a so-called "flooding phenomenon" in which water generated in the reaction is retained in spaces of the catalyst layer and the water fills the spaces in the catalyst layer, thereby inhibiting the supply of a fuel gas serving as a reactant. As a result, a power generation reaction stops. In particular, the flooding phenomenon is liable to occur in the catalyst layer on a cathode side, where the water is generated.

In order to prevent the flooding phenomenon, it is necessary to make the inside of the catalyst layer hydrophobic. There is a generally known method of mixing, with a catalyst layer including catalyst fine particles and a proton-conductive electrolyte, fluororesin-based particles, such as polytetrafluoroethylene (PTFE), together with a solvent or a surfactant. However, this method has a problem in that the three-phase interface is reduced due to the presence of the PTFE particles, so that output power is also reduced.  The Canon water repellant catalyst does not reduce the three-phase interface.

The water repellent coating film includes molecules of a fluorine-based compound with a molecular weight of 10,000 or less. When the molecular weight is larger than 10,000, it is difficult to make the inside of the micro space in the porous catalyst layer hydrophobic.

In order to maximize the reaction surface area, the catalyst forming the catalyst layer includes catalyst particles or catalyst-carrying particles each with a particle diameter of several nm to several tens of nm, or a nano structural body formed of the catalyst nanoparticles. The catalyst layer constitutes a porous body and has fine pores each having a diameter of several nanometers (nm) to several hundreds of micrometers.

The fluorine-based is used as a precursor of the water repellent coating film, thereby enabling the formation of the water repellent coating film also on the inside of the  nanometer and micrometers  pores.  The inside of the micro space is also made hydrophobic, so the catalyst utilization ratio is increased, thereby enabling driving with a high output power for a long time. A nano structural body catalyst may be adopted irrespective of the size or the shape of the catalyst.

Examples of the fluorine-based compound with at least one polar group and with a molecular weight of 10,000 or less include perfluoro alcohol, perfluoro carboxylic acid, Demnum (manufactured by Daikin Industries, Ltd.) used as a lubricating oil, surface treating agents, such as Krytox (manufactured by DuPont) and Novec EGC-1720 (manufactured by 3M).

The catalyst nanoparticles are manufactured from  a platinum oxide, a composite oxide of the platinum oxide and an oxide of a metallic element other than platinum.  Platinum is obtained by performing a reduction treatment of a platinum oxide or a multi-metal platinum composite.   

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