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March 27, 2008
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Hitachi Maxell Developed New High Active Catalyst
Enabling Higher Performance Fuel Cells
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—Achieving approximately 4.8 times*1 higher oxygen reduction
current per unit area compared to that of platinum— |
Tokyo, March 27, 2008 --- Hitachi Maxell, Ltd. (President:Yoshito Tsunoda) today announced development of a new catalyst*2 that is used in oxygen reduction reaction at cathode of polymer electrolyte fuel cell (PEFC*3). The new catalyst is gold-platinum (AuPt) nanoparticle 2 to 3 nanaometer in size and the new AuPt catalyst generates approximately 4.8 times*1 higher oxygen reduction current per unit area than that of commercial platinum catalyst.
PEFC is a promising clean energy source for automobiles, homes, and mobile devices. At present, platinum (Pt) is generally used as catalyst for oxygen reduction reaction in the PEFC. However, platinum is an extremely expensive precious metal, therefore, an important R & D theme is reduction of material cost of the PEFC by minimizing utilization amount of the Pt with further improvement of catalytic activity of the Pt.
Increase in surface area of the catalyst by size reduction is an effective way of improving the catalytic activity. Also, it has been reported that addition of base metals such as iron (Fe), cobalt (Co) and nickel (Ni) to Pt improves the oxygen reduction reaction activity*4. However, there is problem that these kinds of base metals are easy to dissolve in acidic media where the catalyst is working in PEFC.
Maxell has developed a new catalyst for oxygen reduction reaction in PEFC. The new catalyst is composed of Pt and Au that is resistant to acidic media. Au-Pt phase diagram exhibits a very considerable miscibility gap. Moreover, it was difficult to synthesize Au particles with size less than 5 nanometer due to its relatively low melting point*5. By applying in-house-developed nano-level particle synthetic technology, Maxell has succeeded in developing a new catalyst with a high-activity structure in which Au and Pt are not fully alloyed. Using citric acid as a reducing agent, the AuPt catalyst 2 to 3 nanometer in size was synthesized at 373 K. Compared with Pt catalyst, this new AuPt catalyst achieves an approximately 4.8 times higher oxygen reduction current per unit area. It is supposed that the structure in which Au and Pt are not fully alloyed, as evaluated by X-ray diffraction analysis, improved the oxygen reduction reaction activity.
This success represents a large step closer to the practical use of fuel cells for applications requiring large current, such as power sources for automobiles and homes. Further, Maxell will present this new technology for synthesizing highly active AuPt catalyst at the 101st catalysis conference held at the Tower Hall Funabori (Edogawa-ku, Tokyo) on March 29.
Maxell will continue research and development based on the nano-technology with a view to practical application in polymer electrolyte fuel cell and direct methanol fuel cell*6.
| *1 |
Evaluation using rotating disk electrode method in 0.5 mol/l sulfuric acid aqueous solution at 35 degrees Celsius saturated with oxygen gas. Oxygen reduction current was normalized by specific surface area of the catalysts. All potentials were referred to normalized hydrogen electrode (NHE). |
| *2 |
Decreases activation energy in oxygen reduction reaction and increases the reaction rate. |
| *3 |
A low-temperature-operation fuel cell that uses solid polymer electrolyte membrane and generates electricity by chemical reaction of hydrogen and oxygen. |
| *4 |
T. Toda, H. Igarashi, H. Uchida, and M. Watanabe, J. Electrochem. Soc., 146, 3750 (1999). |
| *5 |
Generally, metals with low melting points readily form large particles, and metals with high melting points readily form small particles. The melting points of gold (Au) and platinum (Pt) are 1,064 and 1,770 degree Celsius, respectively. |
| *6 |
A low-temperature-operation fuel cell that uses solid polymer electrolyte membrane and generates electricity by using methanol and water as anode fuel and oxygen as cathode one. |
・Main Features
| 1. |
Synthesis of gold-platinum (AuPt) catalysts with a particle size in 2 to 3 nanometer. |
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By applying nano-level particle synthetic technology, Maxell synthesized AuPt catalyst 2 to 3 nanometer in size. |
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| 2. |
4.8 times higher oxygen reduction current per unit area compared to that of Pt catalyst. |
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The key to this improvement lies in the synthesis of the AuPt catalyst at low temperature enabling nano-sized catalyst and nano-structure in which Au and Pt are not fully alloyed. |
| ・ |
Electron Microscope Photograph of Gold-Platinum (AuPt) Catalyst Deposited on a Conductive Carbon Support |
An electron microscope photograph of a gold-platinum (AuPt) catalyst deposited on a conductive carbon support. The dark grey or black areas are the gold-platinum catalysts, and the light grey areas are the carbon support. The gold-platinum catalyst particles are 2 to 3 nanometer in size.
| ・ |
Comparison of Oxygen Reduction Activity in Comparison with Commercial Platinum (Pt) Catalyst |
The oxygen reduction reaction activity of commercial platinum (Pt) and gold-platinum (AuPt) catalysts deposited on the same conductive carbon support (specific surface area: 800 m2/g) was evaluated. The oxygen reduction currents were normalized by unit surface area of the catalysts. Oxygen reduction currents 4.8 and 3.2 times higher than those of commercial platinum catalyst at 0.6 V and at 0.8 V versus normalized hydrogen electrode (NHE), respectively, were observed. |
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