Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Making catalytic electrode – process only
Reexamination Certificate
2000-11-14
2002-09-03
Bell, Bruce F. (Department: 1741)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Making catalytic electrode, process only
C204S282000, C204S284000, C204S290070, C204S290140, C204S294000, C429S047000, C429S047000, C429S047000
Reexamination Certificate
active
06444602
ABSTRACT:
A gas diffusion electrode (GDE) consumes or is depolarized by a gas feed while allowing direct electronic transfer between the solid and gas phase. Together with the electrolyte, the GDE provides a path for ionic transfer, which is just as critical. GDEs are typically constructed from a conductive support, such as a metal mesh, carbon cloth, or carbon paper. This support is often called a web. The web is coated with hydrophobic wet-proofing layers, and finally, a catalytic layer is applied most commonly to one face. While the catalytic layer can consist of very fine particles of a precious metal mixed with a binder, many employ the methods similar to that in Petrow, et al., 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 nobel metal. Thus called a “supported” catalyst, this methodology has shown superior performance and utilization of the catalyst in electrochemical applications. However, the application of this supported catalyst as well as wet proofing layers to the web engages the need for a well-dispersed mix.
Often, GDEs are cited as key components in Fuel Cells. Here, the anode is typically depolarized with hydrogen while the cathode is depolarized with oxygen or air. The resulting products are energy in the form of electricity, some heat, and water. Examples of acid or alkaline fuel cells are well known. However, some have also realized that the energy-producing quality of a fuel cell can be adapted to industrial electrochemical processes and thus save energy and hence reduce operating costs.
GDEs also may allow the creation of a commodity directly from a gaseous feedstock. For example, Foller, et al. (The Fifth International Forum on Electrolysis in the Chemical Industry, Nov. 10-14, 1991, Fort Lauderdale, Fla., Sponsored by the Electrosynthesis Co., Inc.) describe the use of a GDE to create a 5 wt. % hydrogen peroxide in caustic. In this case, oxygen is the feedstock and a specific carbon black (without noble metals) is the feedstock and a specific carbon black (without noble metals) is the catalyst. A typical chlor-alkali cell uses two solid electrodes to produce sodium hydroxide and chlorine. In this case, both the anode and cathode expend energy to evolve gas, and special measures are taken to keep the resulting hydrogen away from the chlorine due to a potentially explosive mixture. The typical chlor-alkali cathode can be replaced with an oxygen-depolarized cathode, as has been shown by Miles et al. in U.S. Pat. No. 4,578,159 and others. A cell run in such a manner saves approximately one volt, and the hydrogen/chlorine problem is eliminated. Aqueous hydrochloric acid is an abundant chemical by-product. One can recover the high-value chlorine by oxidizing solutions of HCl, and thus recycle the chlorine as a feedstock to the chemical plant.
Electrolysis becomes extremely attractive when the standard hydrogen-evolving cathode is substituted with an oxygen-consuming gas diffusion electrode due to the significant drop in energy consumption. The ability of the gas diffusion electrode to operate successfully in this and the preceding examples is acutely dependent on the structure of the gas diffusion electrode: for in all these cases, the electrode serves as a zone for liquid-gas-solid contact, as a current distributor, and most importantly, as a liquid barrier. The use of solid polymer electrolytes has greatly expanded the field of electrochemistry. As summarized above, electrochemical processes depend on the transfer of ionic and electronic charge through the use of an anode, cathode, and an ionic liquid electrolyte. However, with the advent of the solid polymer electrolyte fuel cell, the traditional liquid phase has been replaced with a membrane composed of a polymer electrolyte that transfers ionic charge under typical electrolytic conditions. One can deposit a catalyst layer directly on the membrane, or attach a gas diffusion electrode to one or both faces of the conducting membrane. Such an assembly can be called a membrane electrode assembly (MEA), or for fuel cell applications, a PEMFC (proton exchange membrane fuel cell).
These solid polymer electrolytes are often composed of ion-conducting membranes that are commercially available. For example, in addition to the previously mentioned Nafion (a cation exchange membrane), Asahi Chemical and Asahi Glass make perfluorinated cation exchange membranes whereby the ion exchange group(s) are carboxylic acid/sulfonic acid of carboxylic acid. These companies produce cation exchange membranes with only the immobilized sulfonic acid group as well. Non-perfluorinated ion exchange membranes are available through Raipore (Hauppauge, N.Y.) and other distributors such as The Electrosynthesis Co., Inc. (Lancaster, N.Y.). Anion exchange membranes typically employ a quaternary amine on a polymeric support and are commercially available as well.
Nafion is typically employed in some fuel cells. For the hydrogen/air (O
2
) fuel cell, hydrogen and oxygen are fed directly to the anode and cathode respectively, and electricity is generated. For these “gas breathing” electrodes to perform, the gas diffusion electrode structure must be highly porous to allow three phase contact between the solid electrode, the gaseous reactant, and the liquid or near liquid electrolyte. In addition to providing a zone for three-phase contact, the gas diffusion electrode structure aids in making electrical contact to the catalyst, enhances transport of reactant gasses into the zone, and provides for facile transport of product away from the zone (e.g. water vapor).
In addition to a gaseous hydrogen fuel and gaseous air (O
2
) oxidant, others employ a mixed phase system such as the methanol/air(O
2
) fuel cell. Here, liquid methanol is oxidized at the anode while oxygen is reduced at the cathode. Another utilization for ion-conducting membranes and gas diffusion electrodes includes the electrochemical generation of pure gasses (for example see Fujita et al. in
Journal of Applied Electrochemistry
. vol. 16, page 935, (1986), electro-organic systhesis [for example see Fedkiw et al. in
Journal of the Electrochemical Society
, vol. 137, no. 5, page 1451 (1990)], or as transducers in gas sensors [for example see Mayo et al. in
Analytical Chimica Acta
, vol. 310, page 139, (1995)].
Typically, these electrode/ion-conducting membrane systems are constructed by forcing the electrode against the ion conducting membrane. U.S. Pat. No. 4,272,353, No. 3,134,697; and No. 4,364,813 all disclose mechanical methods of holding electrodes against the conducting membrane. However, the effectiveness of a mechanical method for intimately contacting the electrode to the polymer membrane electrolyte may be limited since the conducting membrane can frequently change dimensions with alterations in hydration and temperature. Swelling or shrinking can alter the degree of mechanical contact.
Thus, an alternative method of contacting the electrodes with the polymer membrane electrolyte involves direct deposition of a thin electrode onto one or both sides of the conducting polymer substrate. Nagel et al. in U.S. Pat. No. 4,326,930 disclose a method for electrochemically depositing platinum onto Nafion. Others have employed chemical methods whereby a metal salt is reduced within the polymer membrane [for example see Fedkiw et al. in
Journal of the Electrochemical Society
, vol. 139, no. 1, page 15 (1192)].
In both the chemical and electrochemical methods, one essentially precipitates the metal onto the ion conducting membrane. This precipitation can be difficult to control due to the nature of the ion-conducting polymer membrane, the form of the metal salt, and the specific method employed to precipitate the metal. As the goal of a thin, porous, and uniform metal layer is often not met via precipitation, practitioners have turned to other deposition methods. For example, ion beam assisted deposition techniques are disclosed in
Allen Robert J.
De Castro Emory S.
DeMarinis Michael
Shaikh Khaleda
Bell Bruce F.
Bierman, Muserlian and Lucas
DeNora S.p.A.
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