Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
1999-04-30
2002-04-02
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S047000, C429S047000, C429S047000, C429S314000, C429S315000
Reexamination Certificate
active
06365294
ABSTRACT:
BACKGROUND OF THE INVENTION
Direct methanol and H
2
/O
2
proton exchange membrane (PEM) fuel cells are promising power generators for terrestrial and space applications where high energy efficiencies and high power densities are required. A critically important component of these devices is the proton conducting membrane. For a cationexchange membrane to be used in such fuel cells, a number of requirements are to be met, including: (I) High ionic (protonic) conductivity, (ii) dimensional stability (low/moderate swelling), (iii) low electro-osmotic water flow, (iv) mechanical strength and chemical stability over a wide temperature range, (vi) a high resistance to oxidation, reduction, and hydrolysis, and (vi) low hydrocarbon fuel cross-over rates (e.g., low methanol cross-over for direct methanol fuel cells). To date, those membranes reported in the open literature that conduct ions (protons) at moderate temperatures also possess a high methanol permeability and those membranes that do not transport methanol have a low proton conductivity.
Over the past decade, numerous membrane materials have been examined for use in hydrogen/oxygen and direct methanol fuel cells, including perfluorosulfonic acid membranes, such as Dupont's Nafion® (see, for example, Ticianelli, Derouin, Redondo, and Srinivasan, 1988
, J. Electrochem. Soc
., 135, 2209), radiation-grafted copolymers of poly(styrene sulfonic acid) with either low-density poly(styrene), poly(tetrafluoroethylene)/poly (perfluoropropylene), or poly(tetrafluoroethylene) (Guzman-Garcia, Pintauro, Verbrugge, and Schneider, 1992
, J. Appl. Electrochem
., 22, 204), &ggr;-radiation-grafted cation-exchange membranes where styrene/divinylbenzene was grafted into poly(fluoroethylene-co-hexafluoropropylene) (Büchi, Gupta, Haas, and Scherer, 1995
, Electrochim
. Acta, 40, 345) and sulfonated styrene-ethylene/butylene-styrene triblock polymer (Wnek, Rider, Serpico, Einset, Ehrenberg, and Raboin, 1995, in
Proton Conducting Membrane Fuel Cells
I, S. Gottesfeld, G. Halpert, and A. Landgrebe, Eds., PV 95-23, The Electrochemical Society Proceedings Series, pp. 247-251). These polymers operate in a hydrated, water swollen state, which is necessary forfacile proton conduction. Unfortunately, the electro-osmotic water flows and methanol (liquid fuel) cross-over rates in these polymers are high. Additionally, some of the polymers are not chemically stable during long-time fuel cell operation (HO
2•
radicals formed at the anode during oxygen reduction degrade the polymer).
Reinforced composite ion-exchange membranes have been used as proton-exchange materials in PEM fuel cells, where an ion-exchange polymer (normally a sulfonated perfluorinated polymer) is impregnated into a microporous polytetrafluoroethylene film (U.S. Pat. No. 5,525,436; Kolde, Bahar, Wilson, Zawodzinski, and Goftesfeld, 1995, “Proton Conducting Membrane Fuel Cells I,” Electrochemical Society Proceedings, Vol. 95-23, p. 193). These composite membranes, which are identified by the GORE-SELECT trademark, are characterized by a high proton conductance and good mechanical properties, as is the case for homogeneous sulfonated perfluorinated polymer membranes. The methanol cross-over rates in homogeneous perfluorinated polymer membranes as well as the GORE-SELECT™ membranes, however, are unacceptably high at methanol liquid feed concentrations greater than or equal to about 1.0 M.
Another material being examined as a fuel cell proton-exchange membrane is acid-doped polybenzimidazole (PBI) (U.S. Pat. No. 5,525,436). At elevated temperatures (greater than 100° C.) these membranes exhibited good proton conductivity with low methanol cross-over rates. In contrast with traditional proton-exchange materials and the polyphosphazene membranes described in this patent application, the PBI membranes can not be used in a liquid feed methanol fuel cell because the acid dopant will leach out of the membrane and into the liquid methanol solution that is in contact with the membrane during fuel cell operation, resulting in a loss in proton conductivity.
Polyphosphazenes, whose basic structure is shown in
FIG. 1
, are an interesting class of polymers that combine the attributes of a low glass transition temperature polymer (a high degree of polymer chain flexibility) with high-temperature polymer stability. From a synthetic viewpoint polyphosphazenes are the most highly developed of all the inorganic-backbone polymer systems (see, for example, Potin, and DeJaeger, 1991
Eur. Polym. J
., 27, 341). With appropriate functionalization of the phosphorous-nitrogen backbone, an unlimited number of specialty polymers can be synthesized. Thus, by the proper choice of R1 and R2 in the figure below, base polymers can be synthesized for eventual use in proton exchange membrane fuel cells (where the base polymer is chemically manipulated by the addition of sulfonate ion-exchange sites and/or chemical crosslinks).
Polyphosphazenes (without fixed ion-exchange groups) have been used as pervaporation and gas separation membranes (see, for example, Peterson, Stone, McCaffrey, and Cummings, 1993
, Sep. Sci. and Techn
., 28,271) and as solvent-free solid polymer electrolyte membranes in lithium batteries, where there are no fixed charges attached to the polymer (Blonsky, Shriver, Austin, and Allcock, 1984
, J. Am. Chem., Soc
., 106, 6854). No one has yet used sulfonated polyphosphazene cation-exchange membranes as proton conductors in fuel cells (where water sorption is needed for trans-membrane proton transport).
From both theoretical predictions and experimental measurements, it is known that a proton-exchange membrane for solid polymer electrolyte (SPE) fuel cell applications requires a high concentration of ion-exchange groups and some water content for proton conduction. There are limitations, however, to the ion-exchange group concentration in the film, imposed by the required solvent transport properties of the membrane, the polymer chemistry, and the osmotic stability of the polymer. Thus, as the ion-exchange capacity of the polymer increases, water (and polar solvent) sorption by the polymer increases, resulting in unwanted polymer swelling (which may weaken the mechanical properties of the film) and unacceptably high liquid fuel (e.g., methanol) cross-over rates. It is also undesirable if the membrane water content were too low; a membrane's ionic conductivity decreases dramatically when the average number of water molecules per ion-exchange site is less than six and a low polymer water content may also affect adversely the electrochemical kinetics of oxygen reduction during fuel cell operation.
Water and polar solvent (e.g., methanol) uptake in fuel cell proton-exchange membranes are difficult to control because many PEM materials are not crosslinked and the polymer's water/methanol content is dependent on both the membrane's ion-exchange capacity and the polymer crystallinity (which itself decreases with an increase in the number of fixed ion-exchange groups). Sulfonated polyphosphazene membranes (with SO
3
−
ion-exchange groups attached to the polymer) offer a much wider range of possible structures and water/methanol transport rates because the number of ion-exchange groups in the membrane can be adjusted independently of the degree of crosslinking. With a suitably sulfonated and crosslinked polyphosphazene membranes, the problems of unwanted water transport and methanol cross-over that are common to traditional PEM materials can be overcome, yet the membrane conductance can be maintained sufficiently high, since crosslinking limits swelling and water/methanol absorption and transport.
In addition to chemical crosslinking, there is another method by which the mechanical and transport properties of a polyphosphazene-based cation-exchange membrane can be altered and improved for SPE fuel cell applications, that being the blending of a sulfonated polyphosphazene with a non-sulfonated polymer. One can blend the sulfonated phosphazene with either a non-sulfonated polyphosphazene
Pintauro Peter N.
Tang Hao
Carbo Michael D.
Michael D. Carbo, P.L.C.
The Administrators of The Tulane Educational Fund
Ventola II Ronald J.
Weiner Laura
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