1. Field of the Invention
Aspects of the present invention relate to a polymer electrolyte membrane, a method of preparing the same and a fuel cell employing the same, and more particularly, to a polymer electrolyte membrane having excellent ionic conductivity and durability, a method of preparing the same and a fuel cell employing the same.
2. Description of the Related Art
Fuel cells can be classified into polymer electrolyte membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), and other kinds depending on the type of electrolyte used. The working temperature of fuel cells and their constituent materials vary depending on the type of electrolyte used.
A PEMFC is small and lightweight, but can achieve a high output density. Furthermore, a power generation system can be easily constituted using PEMFCs.
A basic PEMFC may include an anode (fuel electrode), a cathode (oxidizing agent electrode), and a polymer electrolyte membrane interposed between the anode and the cathode. The anode may include a catalyst layer to promote the oxidation of a fuel. The cathode may include a catalyst layer to promote the reduction of an oxidizing agent.
The fuel supplied to the anode may generally be hydrogen, a hydrogen-containing gas, a mixture of methanol vapor and water vapor, an aqueous methanol solution, etc. The oxidizing agent supplied to the cathode may generally be oxygen, an oxygen-containing gas, or air.
Fuel is oxidized to produce protons and electrons at the anode of the PEMFC. The protons migrate to the cathode through the electrolyte membrane and the electrons migrate to an external circuit (load) through a conductive wire (or current collector). The electrons are supplied to the cathode from the external circuit through another conductive wire (or current collector). At the cathode of the PEMFC, the migrated protons react with the electrons and oxygen to produce water. The migration of electrons from the anode to the cathode via the external circuit is the result of electric power.
In a PEMFC, the polymer electrolyte membrane acts as an ionic conductor for the migration of protons from the anode to the cathode and also acts as a separator to prevent contact between the anode and the cathode. The polymer electrolyte membrane therefore requires sufficient ionic conductivity, must provide electrochemical safety, have high mechanical strength, and have thermal stability at its operating temperature, and should be easily formed into thin layers.
Generally, materials for the polymer electrolyte membrane include a sulfonated perfluorinated polymer with fluorinated alkylene in the backbone and fluorinated vinylether side chains with sulfonic acid at its terminal, such as NAFION®, manufactured by DUPONT. The polymer electrolyte membrane absorbs an appropriate amount of water and provides excellent ionic conductivity.
However, such a polymer electrolyte membrane may not provide satisfactory methanol permeability and is typically expensive to produce. Also, the polymer electrolyte membrane may experience a lower ionic conductivity at operating temperatures of 100° C. or higher due to the loss of moisture by evaporation. It may therefore be difficult to operate a PEMFC using this type of polymer electrolyte membrane under atmospheric pressure at a temperature of about 100° C. or higher. Therefore, the use of polymer electrolyte membranes that include NAFION® may be limited to PEMFCs that can operate at 100° C. or lower, such as, for example, PEMFCs that operate at about 80° C.
A fuel cell operating at a temperature of 100° C. or higher can have less of a problem of catalyst poisoning by CO compared to a fuel cell operating at a low temperature. Also, a higher operating temperature of a fuel cell increases the activity of fuel cell catalysts, enabling a higher output. On the other hand, the durability of a polymer electrolyte membrane may be lessened at high temperatures, and the ionic conductivity may deteriorate due to dryness.
When NAFION® is used as a polymer electrolyte membrane to be operated at a high temperature of 100° C. or more, the ability of the NAFION® to contain water may deteriorate. Hence, forming the polymer electrolyte membrane by adding a heteropoly acid or using an inorganic material such as silica, etc. has been carried out. When adding a heteropoly acid, the operating temperature of a fuel cell may be increased a little bit, but heteropoly acids typically dissolve in water. Also, if the above forming method of the polymer electrolyte membrane is used, the ionic conductivity of the polymer electrolyte membrane is low because the polymer electrolyte membrane does not have a good ability to contain water at 130° C. or higher.
U.S. Pat. No. 5,525,436 describes a solution to the problem of dryness of a polymer electrolyte membrane carried out by impregnating a strong acid such as phosphoric acid, sulfuric acid, etc., into polybenzimidazole. As the amount of impregnated phosphoric acid in an electrolyte membrane increases, the ion conductivity increases. However, the mechanical strength of the electrolyte membrane deteriorates.
When a strong acid such as phosphoric acid, sulfuric acid, or the like is impregnated into a polymer, a hydrophilic polymer containing a hydroxyl group, an amino group, or an acid group may be used as the polymer. The hydrophilic polymer may form a hydrogen bond with the strong acid or form a strong bond by forming a complex with the strong acid to improve the strong acid retention capacity.
However, such a hydrophilic polymer easily dissolves in a strong acid, and thus the use of the hydrophilic polymer is limited. Also, hydrophilic polymer typically swell easily, and thus do not provide sufficient mechanical strength.