Tuesday, March 30, 2010

Gold Based Nanoparticle Catalysts Loaded on Carbon Key to More Powerful Methanol Fuel Cells

Carbon-supported, gold and gold-based, highly monodispersed, highly electrocatalytic, monometallic, binary, and ternary nanoparticles are useful in making more powerful methanol fuel cells according to a team of State University of New York inventors led by Professor Chuan-Jian Zhong 

The Research Foundation of the State University of New York-Binghamton (SUNY-Binghamton, NY) garnered U.S. Patent 7,687,428 for a method of synthesizing and processing carbon-supported, gold and gold-based multimetallic nanoparticles for use as catalysts in fuel cells.  

Carbon-loaded, gold-based nanoparticle catalysts useful as anode catalysts for the electrocatalytic methanol oxidation reaction (MOR) as well as the oxygen reduction reaction (ORR), according to a team of  SUNY- Binghamton inventors Professor Chuan-Jian Zhong, Jin Luo, Nancy N. Kariuki, Linyang Wang, Peter Njoki and Derrick Mott

 Gold and Platinum (AuPt) catalysts may be prepared by either a two-phase protocol or by a thermal decomposition/reduction protocol. The prepared nanoparticles having different bimetallic ratios are assembled on carbon black support materials and activated by thermal treatment. This approach provides good control of nanoparticle size, composition and/or surface properties. Electrocatalytic MOR activities of the prepared and activated AuPt nanoparticle provided in accordance with the methods of the invention are present in both acidic and alkaline electrolytes.
 
Direct methanol fuel cells (DMFCs) are increasingly considered an attractive power source for mobile applications because of the high energy density, the fuel portability, and the easily renewable feature of methanol. The fuel portability of methanol is particularly important in comparison with the difficulties of storing and transporting hydrogen.

The readily-obtainable energy density (approximately 2000 Wh/kg) and operating cell voltage (0.4 V) for methanol fuel cells is presently lower than the theoretical energy density (approximately 6000 Wh/kg) and the thermodynamic potential (approximately 1.2 V) for such fuel cells. These problems are largely caused by poor activity of the anode catalysts and "methanol cross-over" to the cathode electrode. These problems account for a loss of about one-third of the available energy at the cathode and another one-third at the anode.

Recently, gold at nanoscale sizes was found to exhibit unprecedented catalytic activities, both for CO oxidation and for electrocatalytic activity for CO and methanol oxidation. Studies show that nanoscale gold-based bimetallic materials may provide a synergistic catalytic effect for the methanol oxidation reaction (MOR) at the anode in methanol oxidation fuel cells. For example, the synergistic catalytic effect of gold-platinum (AuPt) nanoparticles might suppress adsorbed poisonous species by changing the electronic band structure to modify the strength of the surface adsorption.

While bimetallic AuPt is a known electrocatalyst for oxygen reduction in alkaline fuel cells, few reports concern utilizing AuPt nanoparticles with controllable size and composition in fuel cell catalyst applications. In such bimetallic systems, Pt could provide the main hydrogenation or dehydrogenation sites, while Au together with Pt could speed up the removal of poisonous species. In the past, decreasing activation energy to facilitate oxidative desorption and suppressing adsorption of CO were believed to lead to sufficiently-high adsorptivity to support catalytic oxidation in alkaline electrolytes.

However, it has recently been shown that catalysts prepared by impregnation from Pt and Au precursors provided results similar to those of monometallic Pt catalysts, suggesting that the presence of Au did not significantly affect the catalytic performance of Pt. This is attributed to phase-segregation of the two metals due to their miscibility gap. As such, only Pt participates in the adsorption of CO and the catalytic reaction. How the bimetallic catalytic properties depend on nanoparticle preparation and composition is an important area for the development of new or improved catalysts for fuel cell research.

The SUNY- Binghamton scientists developed a new method of preparing carbon-loaded, gold-based, nanoparticle catalysts. Such catalysts are known to be useful anode catalysts for the electrocatalytic methanol oxidation reaction (MOR) as well as the oxygen reduction reaction (ORR).  The gold/platinum catalysts may be prepared by either a two-phase protocol or by a thermal decomposition/reduction protocol. The prepared nanoparticles are assembled on carbon black support materials and activated by thermal treatment. Nanoparticles having different bimetallic ratios are assembled on carbon black support materials and activated by thermal treatments at different temperatures.

The SUNY-Binghamton approach provides a better control of nanoparticle size, composition and/or surface properties in comparison with traditional approaches such as co-precipitation, deposition-precipitation, and impregnation. Electrocatalytic MOR activities of the prepared and activated AuPt nanoparticle provided in accordance with the methods can be used in both acidic and alkaline electrolytes.

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