Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
2000-09-12
2003-10-28
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C429S006000, C429S006000, C429S047000, C429S304000, C429S306000, C429S309000
Reexamination Certificate
active
06638659
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to novel membrane electrode assemblies, improved membranes for use in such membrane electrode assemblies, and fuel cells employing such membrane electrode assemblies.
2. Brief Description of the Related Art
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Fuel cells are attractive electrical power sources, due to their higher energy efficiency and environmental compatibility compared to the internal combustion engine. The most well-known fuel cells are those using a gaseous fuel (such as hydrogen) with a gaseous oxidant (usually pure oxygen or atmospheric oxygen), and those fuel cells using direct feed organic fuels such as methanol. In contrast to batteries, which must be recharged, electrical energy from fuel cells can be produced for as long as the fuels, e.g., methanol or hydrogen, and oxidant, are supplied. Thus, a considerable interest exists in the design of improved fuel cells to fill future energy needs.
While a number of different types of electrochemical cells have been employed in the manufacture of fuel cells, arguably ion exchange membrane (IEM) cells have received the most attention. An IEM cell typically employs a membrane comprising an ion-exchange polymer. This ion-exchange polymer membrane serves as a physical separator between the anode and cathode, while also serving as an electrolyte. IEM cells can be operated as electrolytic cells for the production of electrochemical products, or operated as fuel cells for the production of electrical energy.
In some IEM cells, a cation exchange membrane is used wherein protons are transported across the membrane as the cell is operated. Such cells are often referred to as proton exchange membrane (PEM) cells. For example, in a cell employing the hydrogen/oxygen couple, hydrogen molecules (fuel) at the anode are oxidized donating electrons to the anode, while at the cathode the oxygen (oxidant) is reduced accepting electrons from the cathode. The H
+
ions (protons) formed at the anode migrate through the membrane to the cathode and combine with oxygen to form water. In many fuel cells, the anode and/or cathode comprises a layer of electrically conductive, catalytically active particles (usually in a polymeric binder) on the proton exchange membrane. The resulting structure (sometimes also including current collectors) is referred to as a membrane electrode assembly (MEA).
In one approach to the construction of an ion exchange membrane, perfluornated sulfonic acid polymers such as Nafion® (and other ion exchange materials) are incorporated into films, for example porous polytetrafluoroethylene (PTFE), to form composite membranes,:as described for example in U.S. Pat. No. 5,082,472, to Mallouk, et al.; JP Laid-Open Pat. Application Nos. 62-240627, 62-280230, and 62-280231; U.S. Pat. No. 5,094,895 to Branca, U.S. Pat. No. 5,183,545 to Branca et al.; and U.S. Pat. No. 5,547,551 to Bahar, et al. (each of the foregoing references being incorporated herein in their entirety).
In another approach to construction of an ion exchange membrane, a composite membrane is prepared, for example, by precipitation of a water-insoluble, inorganic conductor such as zirconium hydrogen phosphate into a porous Nafion® membrane (See,. e.g., CT/US96/03804 to Grot, et al.). or incorporation of phosphotungstic acid into a Nafion® membrane (See, e.g.,., S. Malhotra, et al., in “Journal of the Electrochemical Society,” Vol. 144, No. 2, L23-L26, 1997—although the resulting membrane was said to demonstrate high conductivity at elevated temperature, the composite membrane lacked sufficient strength at reduced thickness for hydrogen fuel cell applications).
Fuel cells that employ IEMs and direct organic fuels such as methanol frequently suffer from so-called “crossover” of fuel through the membrane. The term “crossover” refers to the undesirable transport of fuel through the membrane from the fuel electrode, or anode, side to the oxygen electrode, or cathode side of the fuel cell. After having been transported across the membrane, the fuel will either evaporate into the circulating oxygen stream or react with the oxygen at the oxygen electrode. Fuel crossover diminishes cell performance for two primary reasons. Firstly, the transported fuel cannot react electrochemically to produce useful energy, and therefore contributes directly to a loss of fuel efficiency (effectively a fuel leak). Secondly, the transported fuel interacts with the cathode, i.e., the oxygen electrode, and lowers its operating potential and hence the overall cell voltage. The reduction of cell voltage lowers specific cell power output, and also reduces the overall efficiency.
Fuel cells that employ IEMs and hydrogen as a fuel also suffer from disadvantages. Certainly, the difficulty of on-board storage and refueling of hydrogen is a major concern in the application of hydrogen fuel cells in vehicles. One approach for surmounting this obstacle has been to utilize the hydrogen fuel obtained through steam reforming of gasoline. Unfortunately, hydrogen fuel from steam reforming of gasoline usually contains a trace amount of carbon monoxide, which results in severe poisoning of anode catalysts. Operating the fuel cell at high temperature can effectively alleviate the carbon monoxide poisoning of anode catalysts. However, at elevated temperature, membranes comprising perfluorinated sulfonic acid polymers such a Nafion® quickly lose ionic conductivity at ambient pressure due to dehydration. Operation at high temperatures with such membranes thus requires that the cells be pressurized.
One particularly useful group of cation-exchange membrane materials for PEM cells is perfluorinated sulfonic acid polymers such as Nafion®, available from E.I. duPont de Nemours & Co. Such cation-exchange polymers have good conductivity and chemical and thermal resistance, which provide long service life before replacement. However, increased proton conductivity is desired for some applications, particularly for fuel cells, which operate at high current densities.
PEMs must have enough strength, minimum fuel crossover, and high ionic conductance at elevated temperature to be useful in fuel cell applications using hydrogen fuel from partial oxidation or steam reforming of hydrocarbons or other sources. Membrane thickness has been reduced in an effort to improve conductance. However, reduction in thickness results in insufficient membrane strength, necessitating use of additional reinforcing materials, and an increase in crossover. For example, pure Nafion® membranes have not provided sufficient strength at reduced thicknesses. To increase the strength additional reinforced materials are needed.
PEMS also require effective catalysts associated with the membranes to provide for reactivity with the fuel source and resulting products of catalysis. Typically, a catalyst layer is applied to the membrane using, for example, a combination of temperature, pressure, and perhaps an adhesive. Such layered structure may be placed between two porous substrates.
Most recently, an alternative low-platinum-loading catalyst layer structure has been developed by Wilson at LANL (M. S. Wilson, U.S. Pat. Nos., 5,211,984 and 5,234,777 (1993)) and Grot (U.S. Pat. 5,330,860) to make membrane electrode assemblies. In this structure, recast ionomer (Nafion®) is used instead of PTFE to bind the catalyst layer structure together, and the low-loading catalyst layer is applied to the membrane, rather than to the gas diffusion structure. Such (PTFE-free) layers have been described as “thin-film” catalyst layers, because the high performance is obtained with a very low catalyst loading (0.12-0.16 mg Pt/cm
2
) in a thin layer (<10 &mgr;m thick). By virtue of their thinness and the high ionomer contents achievable with these catalyst layers, high catalyst utilizations are obtained and the continuity and integrity of the catalyst layer/membrane interface is grea
Fenton James M.
Kunz H. Russell
Lin Jung-Chou
Chaney Carol
Edwards & Angell LLP
University of Connecticut
Yuan Dah-Wei D.
LandOfFree
Membrane electrode assemblies using ionic composite membranes does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Membrane electrode assemblies using ionic composite membranes, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Membrane electrode assemblies using ionic composite membranes will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3172729