Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
2000-09-25
2002-04-30
Bell, Bruce F. (Department: 1741)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C429S047000, C204S283000, C204S284000, C204S293000, C502S101000, C502S180000, C502S185000, C502S313000
Reexamination Certificate
active
06379834
ABSTRACT:
STATE OF THE ART
As mankind expands his presence and activity throughout the world, he is often limited by the availability of electrical energy to support his endeavors. Fuel Cells offer one solution to this dilemma by directly deriving electricity from chemical feedstocks such as oxygen and hydrogen. The Fuel Cell approach also offers the potential to reduce pollution problems inherent in direct combustion technology. Applications for Fuel Cells include power for vehicular traction, stationary power for home and industry, and power supplies for marine use. However, pure hydrogen fuel is not always available, and the development of distribution means for hydrogen is uncertain.
In order for the Fuel Cell technology to realize the potential as a generic energy source, flexibility in the choice of fuel is needed. Large-scale technology such as Solid Oxide Fuel Cells (SOFC) and Phosphoric Acid Fuel Cells (PAFC) achieve some feed flexibility by operating at high temperatures, and thus “burn” some of the anode contaminants that typically result from deriving hydrogen from carbon-containing feedstocks such as methane or propane. Both PAFC and SOFC technology are not amenable to the smaller scales (approximately <200 Kwatts) envisioned for automotive, and other applications cited above.
The Polymer Electrolyte Membrane Fuel Cell (PEMFC) is often cited as the appropriate energy source for applications requiring less than around 200 kWatts, and also for devices needing as little as a few hundred watts. This class of fuel cell operates at less than 180° C., and more typically around 70° C. due to the limitations in the stability of the polymer electrolyte membrane. There is great enthusiasm behind the PEMFC approach based on this system's lack of liquid electrolyte, ease of construction, and high specific power as a function of volume or mass.
In order to impart some fuel flexibility for the PEMFC, an additional fuel-reforming component is needed. The “reformer” converts hydrogen-containing substances such as methane, propane, methanol, ethanol, and gasoline into hydrogen gas, carbon monoxide, and carbon dioxide through either a steam reformation reaction, partial oxidation, or a combination of both. Reformer technology has now advanced to the state whereby commercially units are available. For example, a newly formed company Epyx (Acorn Park, Cambridge, Mass.) offers a fuel processor that converts gasoline into hydrogen. Johnson Matthey PLC (London, UK) offers a HotSpot™ fuel processor that converts methanol using a combination of steam reforming and partial oxidation. For both these technologies, the untreated output is hydrogen and approximately 1-2% carbon monoxide. Through additional clean-up, the carbon monoxide can be reduced to around 50 ppm or less.
Platinum has long been acknowledged as the best anode catalyst for hydrogen. Early fuel cells employed particles of platinum black mixed with a binder as a component in gas diffusion electrodes. The use of platinum black for hydrogen has been largely supplemented by the highly disperse and very active catalysts created by the methods similar to that found in Petrow and Allen, U.S. Pat. No. 4,082,699. This patent teaches the use of using finely divided carbon particles such as carbon black as the substrate for small (tens of angstroms) particles of the noble metal. Thus called a “supported” catalyst, this methodology has shown superior performance and utilization of: the catalyst in electrochemical applications. However, while supported platinum catalysts have demonstrated high activity for hydrogen oxidation, this proclivity for facile kinetics is severely retarded with carbon monoxide concentrations of only a few ppm.
Thus, with a fuel processor technology producing hydrogen streams containing around 50 ppm CO and platinum-based gas diffusion anodes being poisoned slowly with as little as 1 ppm, there is a clear need for a CO tolerant catalyst. The current state-of-the-art CO tolerant electrocatalyst is a platinum ruthenium bimetallic alloy (Pt:Ru) and is available commercially in supported form (E-TEK, Inc., Natick, Mass.). The mechanism for CO tolerance is believed to involve the nucleation of oxygen containing species (OH
ads
) on the ruthenium site such that platinum-adsorbed CO can participate in a bimolecular reaction with the activated oxygen thereby freeing the platinum site for hydrogen oxidation. However, the ruthenium site is also prone to poisoning by CO at higher concentrations of CO, and the important nucleation of oxygen containing species is then inhibited (H. A. Gasteiger, N. M. Markovic, and P. N. Ross;
J. Physical Chemistry,
Vol. 99, No. 22, 1995, p 8945). Although Pt:Ru has been optimized and thoroughly studied to show that an alloy composed of Pt:Ru in the atomic ratio of 1:1 yields the best tolerance to CO, this bimetallic catalyst functions only at around 10 ppm CO or less because of the eventual poisoning of the ruthenium site.
A recent monograph reviewing bimetallic electrocatalysts has summarized several important facts in the preparation and activity of electrocatalysts (P. N. Ross: “The Science of Electrocatalysis on Bimetallic Surfaces,” in
Frontiers in Electrochemistry
Vol. 4, J. Lipowski and P. N. Ross Jr., Wiley-Interscience, New York, N.Y., 1997). The activity of a bimetallic catalyst is dependent on electronic and structural effects. Electronic properties are determined by the electron configuration of the alloying elements while structural properties are determined by both the selection of alloying elements and the method of preparation of the alloy itself. This last observation is important in the design of CO tolerant catalysts. For example, a Pt:Ru alloy prepared by sputtering a bulk alloy, annealing a bulk alloy, or depositing a submonolayer of ruthenium on platinum all yield fundamentally different catalytic properties (P. N. Ross, p 19). The precept that alloy formation methodology influences catalyst function follows from the creation of three zones in every bimetallic catalyst: metal “A”, metal “B”, and an intermixed zone “A-B”. The distribution of these zones determines activity.
Another important property noted by Ross in the monograph is that the phenomenon of surface segregation in bimetallic alloys has often been neglected. Surface segregation is the enrichment of one element at the surface relative to the bulk, and in our case would be dominated by platinum in an alloy of 4d elements with the exception of silver and tin (Ross, p. 51).
In summary, there is ample evidence to show that electrocatalysts can differ in their activity due to preparation methods. Another difference arises from dissimilarities between the bulk and surface compositions of the alloy. For these two reasons, we expect even greater contrasts to occur between bimetallic alloys prepared as bulk metals compared to alloys prepared as very small (10 to 300 Å) supported particles.
Molybdenum has been observed to play a catalytic role in the oxidation of small organic molecules otherwise known as “C1” molecules (to designate one carbon atom). As early as 1965, a molybdenum platinum black complex was implicated in the catalytic oxidation of formaldehyde and methanol in sulfuric acid (J. A. Shropshire;
Journal of the Electrochemical Society,
vol. 112, 1965, p. 465). Although the molybdenum was added as a soluble salt, it was reduced and deposited onto the platinum black electrode. Later on, several others took note of this property of molybdenum and tried to intentionally create platinum alloys. H. Kita et al. confirm that a platinum molybdenum complex formed through reduction of the metal salt onto the surface of the platinum foil electrode can catalyze methanol oxidation (H. Kita et al.;
J. Electroanalytical Chemistry,
vol. 248, 1988, p. 181). H. Kita extended this work to creating a membrane electrode assembly (MEA) of chemically deposited platinum and molybdenum on Nafion, to be used in a PEMFC. As before, the fuel here is methanol (H. Kita et al.;
Electrochemistry in Transition,
Oliver Murphy
Allen Robert J.
De Castro Emory S.
Giallombardo James R.
Bell Bruce F.
Bierman, Muserlian and Lucas
De Nora S.p.A.
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