Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-02-29
2002-05-14
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C428S305500, C428S308400, C428S311510, C428S422000, C521S027000, C429S006000
Reexamination Certificate
active
06387230
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of ionically conducting separators. The invention particularly describes a novel inorganic-organic composite membrane especially useful as a proton conducting membrane for use in electrochemical devices, such as fuel cells.
BACKGROUND OF THE INVENTION
Conventional cation and proton conducting membranes typically comprise a sheet of a homogeneous polymer, a laminated sheet of similar polymers, or a blend of polymers. A variety of polymers have been demonstrated to be cation conductors and some of the membranes produced using these polymers are highlighted in Table I. All of these membranes, with the exception of the Gore Select™ membrane, are homogeneous polymers. The Gore Select™ membrane is a polymer blend.
TABLE I
Polymers Used as Ion Conductors
Source
Name
Polymer Structure
DuPont
Nafion ®
Perfluoro side chains on a PTFE
backbone
Dow
Perfluoro side chains on a PTFE
backbone
W. L. Gore
Gore Select ™
Perfluoro side chains on a PTFE
backbone in a matrix
Ballard
Trifluorostyrene backbone with
derivatized side chains
Maxdem
Poly-X ™
Polyparaphenylene backbone
DAIS Corp.
Sulfonated side chains on a styrene-
butadiene backbone
Assorted
Sulfonated side chains grafted to PTFE
and other backbones
Two of these materials, the membranes from DuPont and Dow, have relatively similar compositions and structures. These structures are illustrated in FIG.
1
. Both of the polymers are perfluorosulfonic acids (PFSA's), which are solid organic super-acids, and both membranes are produced as homogeneous sheets. The active ionomer component of the Gore blend is also a PFSA material.
All of those polymer materials rely on sulfonate functionalities (R—SO
3
—) as the stationary counter charge for the mobile cations (H
+
, Li
+
, Na
+
, etc.), which are generally monovalent. The most commonly proposed mechanism for this conduction, through essentially solvated cations, is illustrated in
FIG. 2
, which is a schematic drawing of the commonly proposed structure for perfluorosulfonic acid (PFSA) polymers, as typified by NAFION (a registered trademark of Dupont of Wilmington Del.). One difficulty associated with this approach to cation conductivity is that the polymer membrane requires the presence of water for conductivity. As shown in
FIG. 3
increasing water content increases conductivity at all temperatures. This dependence on water is the weak point of membranes that rely on sulfonic acid groups for their conductivity. As long as proton exchange membranes (PEM) membranes are kept hydrated, they function well, but when they dry out, resistance rises sharply.
The need for a PEM source of moisture besides the water generated at the cathode to maintain the amount of water in the membrane to maintain conductivity in PEM fuel cell membranes has been recognized for as long as PEM fuel cells have been known. A wide variety of methods have been developed to keep membranes supplied with water. These methods typically require adding water as either vapor or liquid to the gas streams entering the cell or adding water directly to the membrane.
There are a number of reasons that water is so easily lost from PEMs, even as it is being generated at the cathode. The vapor pressure of water over a saturated PEM is nearly as great as it is over pure water. This means that at a temperature of 100° C., a full atmosphere of water vapor is required to keep the membrane saturated.
The water carrying power of gaseous oxidizer streams are quite substantial. It is difficult to operate a fuel cell with an air flow of less than twice the amount required to supply a stoichiometric amount of air for oxidation of the fuel (commonly termed two-fold stoichiometry). If a fuel cell is operated at ambient pressure, operating at a temperature of 55° C. will result in the exiting air stream carrying all of the water produced by the cell at two-fold stoichiometry. Operating at temperatures above 55° C. with the same air flow will cause a PEM membrane to become progressively drier. Increasing the operating pressure of the cell or stack will permit operation at higher temperatures, but the price of higher pressure is increased parasitic power losses.
If a proton-conducting membrane could be developed with improved water retention or a reduced dependence on free moisture for proton conduction it would be possible to operate a proton conducting membrane fuel cell with less water, with no water, or at higher temperatures. This would provide simpler, lighter fuel cell stack designs.
There is a related problem that only applies to direct methanol fuel cells (DMFC's) which is referred to as “methanol crossover.” Typical PFSA fuel cell membranes have a higher affinity for methanol than they do for water, as is clearly illustrated in FIG.
4
. In a DMFC, the crossover process relates to the permeation of absorbed methanol through the membrane from anode to cathode. In general, it has been found that rate of methanol crossover through a PEM is proportional to the methanol concentration in the fuel feed stream. Therefore, a proton conducting membrane that requires less water to maintain its conductivity will also exhibit a reduced methanol flux.
Methanol crossover substantially impedes the performance of direct methanol fuel cells. First, methanol that crosses over represents lost fuel value and, therefore, a lower fuel efficiency. Furthermore, when that methanol arrives on the other side of the PEM, it is oxidized by the cathodic electrocatatyst which depolarizes the electrode. Oxidation of methanol at the cathode increases the amount of air, or oxygen, that the cell or stack requires, since a molecule of methanol oxidizing on the cathode requires the same 1½ molecules of oxygen (O
2
) as one being consumed at the anode. Since none of the energy from this oxidation is being extracted as electricity, it all ends up as waste heat, increasing the cooling load on the cell. A proton conducting membrane with substantially reduced methanol crossover would represent a significant improvement in DMFC's.
Alternatives to polymer proton conductors include oxide proton conductors. A wide variety of metal oxides are proton conductors, generally in their hydrated or hydrous forms. These oxides include hydrated precious metal containing oxides, such as RuOx H
2
O)
n
and (Ru-Ti)O
x
(H
2
O), acid oxides of the heavy post transition elements, such as acidic antimony oxides and tin oxides, and the oxides of the heavier early transition metals, such as Mo, W, and Zr. Many of these materials are also useful as mixed oxides. Some oxides which do not fit this description may be useful as well, such as silica (SiO
2
) and alumina (Al
2
O
3
), although these are generally used as, or with, modifiers.
The number of metal oxides with the potential to serve as proton conductors is too large to fully discuss in detail here. This group, which can be summarized as those elements forming insoluble hydrated oxides that are not basic, includes not only known proton conductors, but oxide superacids that will furnish a multitude of free protons in the presence of an aqueous medium. These are shown in bold in FIG.
5
. Many other elements which are not included in this list may be useful in conjunction with these elements as modifiers. An example of this is the inclusion of phosphorus in the structure of Keggin ions which consist primarily of a tungsten or molybdenum oxide framework. While the compounds encompassed in the description above have some degree of proton mobility, not all of those oxides have adequate proton mobility to be useful as components in composite membranes. Some particularly useful examples are discussed below.
Zirconium phosphate, specifically &agr;-zirconium phosphate, whose structure is shown in
FIG. 6
, is known to be an excellent proton conductor when tested as a powder at ambient temperature. Under these conditions the compound is hydrated (Zr(HPO
4
)
2
(H
2
O), and most of the conductivity is the result of protons migrating over t
Cisar Alan J.
Murphy Oliver J.
Bell Bruce F.
Lynntech Inc.
Streets Jeffrey L.
Streets & Steele
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