This invention relates to polymer electrolyte membranes.
Polymer electrolyte membranes are useful for various applications, such as fuel cells, electrolyzers, and batteries. In particular, high-temperature proton-exchange membrane (PEM) fuel cells offer several advantages. The proton-exchange membrane combines in one material the function of electrolyte and separator. Additionally, proton-exchange membranes are readily fabricated in thin flexible films and therefore allow the fabrication of thin devices with variable shape. It is desirable to operate fuel cells at temperatures higher than 100° C. at moderate relative humidity to minimize anode catalyst poisoning by carbon monoxide and to enhance reaction kinetics at the electrodes and thereby increase fuel cell efficiency. In addition, high-temperature proton exchange membranes that operate at moderate to high temperature (120° C.-200° C.) can provide higher water electrolysis efficiency, since the electrical efficiency of steam electrolysis increases with temperature, owing to the decrease in both thermodynamic (open circuit) potential and electrode polarization (so that the kinetics at the electrodes are considerably faster). However, commercially available perfluorinated hydrocarbon sulfonated ionomers are known to be chemically unstable at temperatures higher than 80° C.-100° C. and therefore cannot be used for this promising application.
Sulfonated polymers have been extensively investigated for use in polymer electrolyte membranes. Representative examples of the state of the art in this field include US 2006/0030683, U.S. Pat. No. 6,632,847, U.S. Pat. No. 6,869,980, U.S. Pat. No. 6,955,712, US 2005/0037265, U.S. Pat. No. 6,933,068, and US 2002/0091225. Blends or co-polymers including sulfonated polymers have also been investigated for use in polymer membranes, including electrolyte membranes, as in U.S. Pat. No. 6,264,857, U.S. Pat. No. 5,219,679, and EP 0,337,626. However, despite these extensive investigations, it remains challenging to provide polymer electrolyte membranes suitable for demanding applications requiring high proton conductivity, thermal and electrochemical stability, and high mechanical strength for various temperature and humidity conditions. More specifically, polymer electrolyte membranes should have excellent chemical and electrochemical stability up to 150° C.-200° C., high proton conductivity, and excellent mechanical properties. A significant challenge is to develop polymer membranes for which all the requirements-high proton conductivity, thermal and electrochemical stability, and mechanical strength-are met under variable temperature and humidity conditions.
While several reports claiming high-temperature polymer membranes have been made, data relating to fuel cells or electrolyzers at high temperatures is typically not provided. Some examples of materials proposed for high-temperature membranes include sulfonated polyimide membranes, sulfonated polyphenyleneoxide, sulfonated polyquinoxalines, sulfonated polyphenylenes, sulfonated polyetheretherketone (PEEK), sulfonated polyethersulfones, blends of fluorinated sulfonated polyetherethersulfones and polybenzimidazole, blends of sulfonated polyetherketone and polybenzimidazole, sulfonated aromatic polymers supported on porous polybenzoxazole, and so on. In general, much current work is focused on developing high-temperature polymer electrolyte membranes for PEM fuel cells for which it is especially desirable to operate the fuel cell at moderate humidity to simplify water management and fuel cell stack design. On the other hand, polymer membranes for water electrolyzers need to operate in the presence of water in the liquid phase at high temperature (>120° C.), need to have very large area, require excellent mechanical properties, and need to be able to operate over tens of thousands of hours without significant degradation.
Accordingly, it would be an advance in the art to provide polymer electrolyte membranes suitable for such demanding applications, especially for high temperature operation.
Sulfonated polymer compositions including a polymer or copolymer derived from the monomer 2,2′-di-(4,4′dihydroxyphenyl)pentafluoropropanesulfonic acid are provided. These compositions provide high ionic conductivity due to the strongly acidic functional group. Such compositions can provide improved polymer electrolyte membranes, especially in preferred embodiments including polybenzimidazole and polyacrylonitrile in the composition. Such improved polymer electrolyte membranes can provide high ion conductivity in combination with improved thermal and mechanical stability, and are especially suitable for high temperature operation. Applications of such membranes include fuel cell electrolytes, electrolyzer electrolytes and battery electrolytes.
a-d show a synthesis process suitable for making an embodiment of the invention.
The examples of
In preparing polymer compositions according to the invention, alkali salts of the acid of
a-d show an exemplary synthesis procedure for fabricating an embodiment of the invention. In summary, condensation of 2-ketopentafluoropropanesulfonic acid 63 with phenol neat 64 at 115° C. affords the potassium salt 65 of 2,2-(4,4′-hydroxyphenyl)pentafluoropropanesulfonic acid in greater than 80% yield after deprotonation with potassium bicarbonate (
Example 2: Polymer 72 (1.70 g, 2.32 mmol) was dissolved in 50 ml of methanol and acidified by the addition of 300 mg (3.0 mmol) of sulfuric acid in 5 ml of methanol. The colorless solution was dialyzed over 30 hours in 3500 M.W. cutoff tubing, decanted, and freeze dried to afford 1.48 (2.13 mmol) of the acid form of polymer 72 as a colorless powder.
Example 3: Polymer 67 (0.25 g) was dissolved in dimethylacetamide (2.0 g). The resulting solution was applied onto a 1″×2″ glass substrate. After evaporating the solvent overnight at 50° C., the membrane was peeled off from the substrate and soaked in 1 M sulfuric acid overnight. The polymer membrane was then repeatedly washed with water and dried in a vacuum plate at 40° C. The resulting membrane was tested for its proton conductivity at 120° C. and 50% relative humidity by AC impedance analysis. Under these conditions the membrane conductivity was found to be 5 mS/cm.
The performance of polymer electrolyte membranes according to embodiments of the invention can be improved by combining compositions as described above with other materials to provide co-polymers and/or mixtures. More specifically, membranes including sulfonated polymers of the invention can be blended with polybenzimidazole (PBI) to enhance mechanical stability in water at 150° C. Preferably, polyacrylonitrile (PAN) is also included with PBI in such blends. The advantages provided by a blend including PBI and PAN are indicated by the following example.
Example 4: A polymer membrane was prepared from a blend of the sulfonic acid polymer of
Sulfonated polymers of the invention can be formulated with phosphoric acid, triazole, low molecular weight imidazoles, phosphotungstic acid and other strong inorganic acids to enhance proton conductivity at low relative humidities. Co-polymerization of sulfonated polymers of the invention with other monomers can be used to select the degree of hydrophobic/hydrophilic character of the composition.
Copolymers according to the invention can be fabricated. For example, a copolymer can be prepared by condensation of the monomer of
Polymer compositions according to the invention have numerous applications, including fuel cells, electrolyzers, batteries, energy storage devices, chemical sensors, electrochromic devices and electrochemical devices. Especially noteworthy applications include fuel cell electrolyte membranes and lithium ion conductors for batteries.
This application claims the benefit of U.S. provisional application 60/720,348, filed on Sep. 22, 2005, entitled “High Temperature Polymer Electrolyte Membranes”, and hereby incorporated by reference in its entirety. This application also claims the benefit of U.S. application Ser. No. 09/872,770, filed on Jun. 1, 2001, and entitled “Polymer Composition”.
Number | Date | Country | |
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60720348 | Sep 2005 | US |