The present invention relates to a method for producing a bridged polymer membrane and a fuel cell.
A cell includes an electrolyte and a pair of electrodes separated by the electrolyte. In case of a fuel cell, a fuel such as a hydrogen gas is supplied to one of the electrodes, and an oxidizing agent such as an oxygen gas is supplied to the other electrode, thereby converting chemical energy involved in the oxidation of the fuel into electric energy. The electrolyte permeates hydrogen ions, that is, protons but by does not permeate reactive gases such as the hydrogen gas and the oxygen gas. Typically, a fuel cell has a plurality of single cells, and each of the single cells has an electrolyte and a pair of electrodes separated by the electrolyte.
As the electrolyte for the fuel cell, a solid such as a polymer electrolyte membrane and a liquid such as phosphoric acid are used. Recently, the polymer electrolyte membranes have been receiving attention as the electrolytes for the fuel cell. For example, perfluorosulfonic acid polymers and complexes between basic polymers and strong acids are used as materials for the polymer electrolyte membranes.
Typically, the perfluorosulfonic acid polymer has a framework of perfluorocarbon such as a copolymer of tetrafluoroethylene and trifluorovinyl and a side chain being bonded thereto and having a sulfonic acid group such as a side chain that a sulfonic acid group is bonded to perfluoroalkylene group. The sulfonic acid group releases a hydrogen ion to convert into an anion, and therefore conducts proton.
Polymer electrolyte membranes comprising complexes of basic polymers and strong acids have been developed. International Publication WO96/13872 and its corresponding U.S. Pat. No. 5,525,436 disclose a method for producing a proton conductive polymer electrolyte membrane by immersing a basic polymer such as polybenzimidazoles in a strong acid such as phosphoric acid, sulfuric acid and so on. The fuel cell employing such a polymer electrolyte membrane has the advantage that it can be operated at 100xc2x0 C. or above.
J. Electrochem. Soc., Vol. 142, No. 7,1995, ppL121-L123 describes that immersing a polybenzimidazole in 11M phosphoric acid for at least 16 h impregnates five molecules of phosphoric acid per unit of the polybenzimidazole.
Further, International Publication WO97/37396 and its corresponding U.S. Pat. No. 5,716,727 describe a method for producing a polymer electrolyte membrane by obtaining a solution of a polybenzimidazole dissolved in trifluoroacetic acid, followed by adding phosphoric acid to the solution, and subsequently by removing the solvent.
The whole disclosures of WO 96/13872, J. Electrochem. Soc., Vol. 142, No. 7, 1995, ppL121-L123 and WO97137396 are incorporated herein as reference.
Even though a basic polymer by itself has a sufficient mechanical strength, there are cases that the mechanical strength of the basic polymer decreases to an insufficient degree by impregnating the basic polymer with a strong acid for providing proton conductivity. Therefore, it is desired to further improve the mechanical strength of the basic polymer for applying the complex of the basic polymer and the strong acid to the electrolyte membrane for the fuel cell and so on.
U.S. Pat. No. 5,599,639 describes a basic polymer wherein a sulfonic acid group is introduced into a basic polymer such as polybenzimidazole and so on through a linker such as an alkylene and so on. The basic polymer is required to incorporate water therein for providing proton conductivity so that the basic polymer may be used as the electrolyte membrane for the fuel cell.
However, when the basic polymer is immersed in water, there are cases that the basic polymer swells and that a sufficient mechanical strength is not achieved. The whole disclosure of U.S. Pat. No. 5,599,639 is incorporated herein as reference. The present inventors tried to improve the mechanical strength by shaping a basic polymer into a membrane configuration followed by reacting with an external bridging agent. However, the basic polymer in a gel or solid form did not smoothly react with the external bridging agent.
To solve the aforementioned problem, the present invention is accomplished by shaping a basic polymer into a membrane configuration followed by proceeding a bridging reaction.
According to one aspect of the present invention, there is provided a method for producing a bridged polymer membrane, comprising the steps of: obtaining a liquid medium comprising a basic polymer having an amino group in a repeating unit, a bridging agent, and a solvent; shaping the liquid medium into a membrane configuration to obtain a shaped membrane; and bridging the basic polymer by the bridging agent in the shaped membrane.
Preferably, the bridging agent has at least two epoxy groups or isocyanate groups in the molecule thereof.
Preferably, the liquid medium contains 0.001 to 0.8 mole of the bridging agent per unit of the basic polymer.
Preferably, the basic polymer is selected from the group consisting of polybenzimidazoles, polyimidazoles, polyvinylimidazoles, and polybenzbisimidazoles.
Preferably, the method may further comprise the step of impregnating the basic polymer with a strong acid for providing proton conductivity. The strong acid may be impregnated in the form of the liquid medium. Alternatively, the strong acid may be impregnated after the shaping but before the heating. Alternatively, the strong acid may be impregnated after heating.
Alternatively, the basic polymer may have a strong acid group in the repeating unit in the basic polymer. The presence of the strong acid provides proton conductivity.
According to another aspect of the present invention, there is provided a fuel cell comprising a plurality of single cells, each of the single cells comprising a bridged polymer membrane obtained by the aforementioned method and a pair of electrodes sandwiching the bridged polymer membrane.