Composite electrolyte membranes for fuel cells and methods...

Details Composite electrolyte membranes for fuel cells and methods... Composite electrolyte membranes for fuel cells and methods...

1999-12-20

2004-04-06

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S317000

Reexamination Certificate

active

06716548

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to composite electrolyte membranes for fuel cells and methods of making same. More specifically, the present invention is directed to proton-conducting membranes lot fuel cell applications. The present invention further describes materials which reach high intrinsic proton conductivity, and are suitable for use as electroiytic membranes in methanol fuel cells.
BACKGROUND OF THE INVENTION
The need for pollution control stimulated the development of polymer electrolyte membrane fuel calls (PEMFC) and attracted an increasing interest particularly for the automotive and stationary power applications [1-3]. For example, Daimler Benz presented in 1998 a fuel cell powered car, NECAR II with a total electric power close to 50 kW. A fuel cell is an almost ideal energy source yielding a very high thermal efficiency and an essentially zero release of atmospheric pollutants. In transport applications, the direct methanol fuel cell (DMFC) is presently considered as the most appropriate and promising. Up to now, only perfluorinated ionomers (PFI) membranes were considered to meet the requirements of polymer electrolyte membrane (PEM) fuel cells, namely, a high proton conductivity, a high stability in the cell operating conditions and a high durability. Presently the commercial solid polymer electrolyte material used in PEMFC is either perfluorinated NAFION (Du Pont) or NAFION-like polymers [4] supplied by Dow, Asahi Glass (FLEMION) and Asahi Chemicals (ACIPEX). Unfortunately, these PEMFC limit large scale application due to a number of drawbacks. First of all, these ionomers are very expensive. For example, the manufacturer's price for the NAFION membranes (Dupont de Nemours) scale exceeds 600 US$/m
2
. Other membranes of this kind (DOW membranes, RAI membranes, . . . ) are still more expensive (up to 2000 US$/m
2
). In fact, such membranes have been used for a long time in H
2
fuel cells for application where cost was not a main criterion (e.g. spacecraft, submarines etc). In addition, a significant drawback of these materials is their high permeability to methanol which allows an easy transport of this fuel from the anode to the cathode. This phenomenon, also called methanol crossover, reduce significantly the cell performance and must be diminished if not eliminated before DMFC can be commercialized.
Currently the necessity to reduce the cost of PEM stimulates the development of new proton conducting polymers. New studies are also undertaken in order to rationalize the most efficient combination of properties of the perfluorinated ionomer (PFI) polymer, which make them efficient proton conductors, and develop new polymers with similar properties by a less expensive chemistry. As a result PFI NAFION membrane has been extensively studied and tested in low temperature fuel cell systems [5]. In this context, Ballard Advanced Materials' group for the development of PEM membranes [6] recently developed a membrane based on a trifluorostyrene monomer called BAM3G (Ballard Advanced Material 3rd generation), which has demonstrated excellent performance and longevity of several thousand hours of operation.
The remarkable properties of PFI polymers lie in the combination of the high hydrophobicity of the fluorinated polymer backbone and high hydrophilicity of the sulfonic acid branches. The hydration of the PFI membrane is crucial for the performance of PEMFC since proton conductivity decreases drastically with dehydration. For instance, with NAFION membrane, which loses water above 80° C., the conductivity drops to very low values above this temperature.
One more limitation associated with NAFION type PFI membranes [2,4] is the methanol crossover when used in the direct methanol fuel cell (DMFC). This results in a decreased fuel cell performance due to depolarization of the oxygen reducing cathode. A further drawback of the perfluorinated polymers is that they are not environmentally friendly, a criteria that will be important when fuel cells become mass-produced.
The above mentioned disadvantages of PFI membranes induced many efforts to synthesize PEM based on hydrocarbon-type polymers and brought about the emergence of partially fluorinated and fluorine free ionomer membranes as alternatives to NAFION membranes. Among them the membranes based on aromatic polyether ether ketone (PEEK) were shown to be of promise for fuel cell application [7-9], as they possess good mechanical properties, thermal stability, toughness and some conductivity, depending on sulphonation degree. Nevertheless, the proton conductivity of PEEK or SPEEK has yet to reach a level sufficient to enable an adequate performance in a fuel cell.
Efforts have thus been made to improve the proton conductivity to composite membranes. For example, the addition of solids such as zeolites or tin-mordenite was aimed at improving the performance of the composite membranes. Unfortunately, their presence in the membranes does not impart thereto a high enough proton conductivity to make them useful as a solid electrolyte in polymer electrolyte membrane fuel cells (Mikhailenko et al. 1997, Microporous Mat. 11:37-44).
“Polymer Material for Electrolytic Membranes in Fuel Cells” by Yen et al., U.S. Pat. No. 5,795,496 is one such example of SPEEK with the aim of using it in fuel cells. The materials described in Yen et al. have low methanol permeability but high proton conductivity, and made from inexpensive, readily available materials. According to that invention, proton conducting membranes are formed based on a sulfonic acid-containing polymer. One preferred material is PEEK or PES. This invention is said to overcome disadvantages associated with the high cost of NAFION membranes and with its methanol permeability problems which allows for a substantial amount of fuel crossover across the membrane by using materials which were inexpensive starting materials and which enhanced protection against fuel crossover. In a particular embodiment, PEEK was sulfonated with H
2
SO
4
to give H-SPEEK, a polymer which is soluble in an organic solvent and water mixture. While the inventors found that sulfonic acid increases the proton conducting performance of PEEK (the sulfonate groups are responsible for proton conductivity), it degrades the physical structure of the resulting membrane. Hence, the inventors developed a trade-off between the amount of sulfonation and appropriate physical structure by sulfonating less than one out of every three benzene rings. PBS was treated in an analogous manner. Yen et al. also teach methods of modifying the morphology of the processed polymers to limit the transport of methanol across the membrane (to reduce the free volume) by using zeolites tin motdeioite or the like. Unfortunately, such solids do not impart high enough proton conductivity to the composite membrane. However, Yen et al. do not teach a composite electrolyte membrane which reaches a high enough proton conductivity to be useful in PEM fuel cells.
During the last two decades solid electrolytes have attracted substantial attention owing to both their great potential in several electrochemical technologies, such as fuel cells, batteries and sensors, and the academic, interest in the phenomenon of fast ionic mobility in solids In spite of the fact, that a vast number of various proton conductors have already been identified, the development of chemically stable superionic conductors still remains one of the prime objectives among the current directions of research in solid state electrochemistry and materials science. Currently considerable efforts are being devoted to proton conducting salts of oxo acids including various hydrated and anhydrous phosphates
Heteropolyacids (HPAs) are known as the most conductive solids among the inorganic solid electrolytes. Nevertheless, the use of HPAs or other solid inorganic acids in polymers and their effect on membranes for fuel cells, for example, has not been significantly addressed to show that t

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