Recently, fuel cell has been attracting attention as a power generating device of the next generation, which can contribute to solution of the problems related to environments and energy, now having been increasingly becoming serious social problems, because of its high power generation efficiency and compatibility with the environments.
Fuel cells fall into several categories by electrolyte type. Of these, a polymer electrolyte fuel cell (PEFC), being more compact and generating higher output than any other type, is considered to be a leading fuel cell type in the future for various purposes, e.g., small-size on-site facilities, and as movable (i.e., power source of vehicles) and portable applications.
Thus, PEFCs have inherent advantages in principle, and extensively developed for commercialization. PEFCs normally uses hydrogen as the fuel. Hydrogen is dissociated into proton (hydrogen ion) and electron in the presence of catalyst provided on the anode side. Of these, the electron is passed to the outside to produce power, and recycled back to the system on the cathode side. On the other hand, the proton is passed to the proton-conducting membrane, through which it moves towards the cathode side. On the cathode side, the proton, electron recycled back from the outside and oxygen supplied from the outside are bonded to each other in the presence of catalyst, to produce water. Thus, a PEFC by itself is a very clean energy source which generate power while it is producing water from hydrogen and oxygen.
Hydrogen to be supplied to a fuel cell is normally produced by an adequate method, e.g., methanol reforming to extract hydrogen. However, the direct fuel type fuel cell has been also extensively developed. It is directly supplied with methanol or the like, from which the proton and electron are produced in the presence of catalyst, where water is normally used together with methanol.
In the fuel cell, the proton-conducting membrane is responsible for transferring the proton produced on the anode to the cathode side. As described above, flow of the proton takes place in concert with that of the electron. It is therefore necessary to conduct a sufficient quantity of the proton at high speed, for a PEFC to produce high output (or high current density). Therefore, it is not too much to say that performance of the proton-conducting membrane is a key to performance of the PEFC. The proton-conducting membrane also works as the insulation film which electrically insulates the anode and cathode from each other and as the fuel barrier membrane which prevents the fuel to be supplied to the anode side from leaking to the cathode side, in addition to transferring the proton.
The proton-conducting membranes for the current PEFCs are mainly of fluorine resin-based ones, with a perfluoroalkylene as the main skeleton, and partly with sulfonic acid group at the terminal of the perfluorovinyl ether side chains. Several types of these fluorine resin-based membranes have been proposed, e.g., Nafion R membrane (Du Pont, U.S. Pat. No. 4,330,654), Dow membrane (Dow Chemical, Japanese Patent Application Laid-Open No.4-366137), Aciplex R membrane (Asahi Chemical Industries, Japanese Patent Application Laid-Open No.6-342665), and Flemion R membrane (Asahi Glass).
The fluorine resin-based membrane is considered to have a glass transition temperature (Tg) of around 130° C. under a wet condition. The so-called creep phenomenon occurs as temperature increases from the above level to cause problems, e.g., changed proton-conducting structure in the membrane to prevent the membrane from stably exhibiting the proton-conducting performance, and modification of the membrane to a swollen morphology, or jelly-like morphology to make it very fragile and possibly cause failure of the fuel cell. Moreover, the sulfonic acid group tends to be eliminated when the membrane of wet morphology is exposed to high temperature, to greatly deteriorate its proton-conducting performance. For these reasons, the maximum allowable temperature for stable operation for extended periods is normally considered to be 80° C.
A fuel cell, depending on the chemical reaction for its working principle, has a higher energy efficiency as it operates at higher temperature. In other words, a fuel cell operating at higher temperature becomes more compact and lighter for the same output. Moreover, a fuel cell operating at high temperature allows to utilize its waste heat for cogeneration to produce power and heat, thus drastically enhancing its total energy efficiency. It is therefore considered that operating temperature of a fuel cell is desirably increased to a certain level, normally to 100° C. or higher, in particular 120° C. or higher.
The catalyst in service on the anode side may be deactivated by impurities in the hydrogen fuel (e.g., carbon monoxide), a phenomenon known as catalyst poisoning, when it is not sufficiently purified. This is a serious problem which can determine serviceability of the PEFC itself. It is known that the catalyst poisoning can be avoided when the fuel cell operates at sufficiently high temperature, and the cell is preferably operated at high temperature also from this point of view. Moreover, the active metals for the catalyst itself will not be limited to pure noble metals, e.g., platinum, but can be extended to alloys of various metals, when the fuel cell can operate at sufficiently high temperature. Therefore, operability at high temperature is advantageous also viewed from reducing cost and widening applicable resources.
For the direct fuel type fuel cell, various approaches to extract the proton and electron from the fuel directly and efficiently have been studied. It is a consensus that production of sufficient power is difficult at low temperature, and possible when temperature is increased to, e.g., 150° C. or higher.
Thus, operability of PEFCs at high temperature is demanded from various aspects. Nevertheless, however, its operating temperature is limited to 80° C. at present by the heat resistance consideration of the proton-conducting membrane, as discussed above.
The reaction taking place in a fuel cell is exothermic in nature, by which is meant that temperature within the cell spontaneously increases as the cell starts to work. However, the PEFC must be cooled not to be exposed to high temperature of 80° C. or higher, as limited by the resistance of the proton-conducting membrane to heat. It is normally cooled by water-cooling system, and the PEFC separator is devised to include such a system. This tends to increase size and weight of the PEFC as a whole, preventing it to fully exhibit its inherent characteristics of compactness and lightness. In particular, a water-cooling system as the simplest cooling means is difficult to effectively cool the cell, whose maximum allowable operating temperature is set at 80° C. If it is operable at 100° C. or higher, it should be effectively cooled by use of heat of vaporization of water, and water could be recycled for cooling to drastically reduce its requirement, leading to reduced size and weight of the cell. When a PEFC is used as the energy source for a vehicle, the radiator size and cooling water volume could be greatly reduced when the cell is controlled at 100° C. or higher than controlled at 80° C. Therefore, the PEFC operable at 100° C. or higher, i.e., the proton-conducting membrane having a heat resistance of 100° C. or higher, is strongly in demand.
As described above, the PEFC operable at higher temperature, i.e., increased heat resistance of the proton-conducting membrane, is strongly in demand viewed from various aspects, e.g., power generation efficiency, cogeneration efficiency, cost, resources and cooling efficiency. Nevertheless, however, the proton-conducting membrane having a sufficient proton conductivity and resistance to heat has not been developed so far.
With these circumstances as the background, a variety of heat-resistant proton-conducting membrane materials have been studied and proposed to increase operating temperature of PEFCs.
Some of more representative ones are heat-resistant aromatic-based polymers to replace the conventional fluorine-based membranes. These include polybenzimidazole (Japanese Patent Application Laid-Open No.9-110982), polyether sulfone (Japanese Patent Application Laid-Open Nos.10-21943 and 10-45913), and polyetheretherketone (Japanese Patent Application Laid-Open No.9-87510).
These aromatic-based polymers have an advantage of limited structural changes at high temperature. However, many of them have the aromatic structure directly incorporated with sulfonic acid or carboxylic acid group. They tend to suffer notable desulfonation or decarboxylation at high temperature, and unsuitable for the membranes working at high temperature.
Moreover, many of these aromatic-based polymers have no ion-channel structure, as is the case with fluorine resin-based membranes. As a result, the membranes of these polymers tend to be notably swollen as a whole in the presence of water, causing various problems, e.g., high possibility of separation of the membrane from the electrode joint and broken membrane due to the stress produced at the joint in the membrane-electrode assembly, resulting from the dry and wet conditional cycles which change the membrane size, and possibility of deteriorated strength of the water-swollen membrane, leading to its failure. In addition, each of the aromatic polymers is very rigid in a dry condition, possibly causing damages and other problems while the membrane-electrode assembly is formed.
On the other hand, the following inorganic materials have been proposed as the proton-conducting materials. For example, Minami et al. incorporate a variety of acids in hydrolyzable silyl compounds to prepare inorganic proton-conducting materials (Solid State Ionics, 74 (1994), pp.105). They stably show proton conductivity at high temperature, but involve several problems; e.g., they tend to be cracked when made into a thin film, and difficult to handle and make them into a membrane-electrode assembly.
Several methods have been proposed to overcome these problems. For example, the proton-conducting material is crushed to be mixed with an elastomer (Japanese Patent Application Laid-Open No.8-249923) or with a polymer containing sulfonic acid group (Japanese Patent Application Laid-Open No.10-69817). However, these methods have their own problems. For example, the polymer as the binder for each of these methods is merely mixed with an inorganic crosslinked compound, and has basic thermal properties not much different from those of the polymer itself, with the result that it undergoes structural changes in a high temperature range, failing to stably exhibit proton conductivity, and its proton conductivity is generally not high.
A number of R & D efforts have been made for various electrolyte membranes to solve these problems involved in the conventional PEFCs. None of them, however, have succeeded in developing proton-conducting membranes showing sufficient durability at high temperature (e.g., 100° C. or higher) and satisfying the mechanical and other properties.
In the direct methanol type fuel cell (sometimes referred to as DMFC) which works on methanol as the fuel in place of hydrogen, on the other hand, methanol directly comes into contact with the membrane. The sulfonated fluorine resin-based membrane, e.g., Nafion membrane, now being used has a strong affinity for methanol, possibly causing problems which can lead to failure of the fuel cell when it absorbs methanol, e.g., swelling to a great extent and dissolution in methanol in some cases. Crossover of methanol to the oxygen electrode side can greatly reduce cell output. These problems are common also with the electrolyte membranes containing an aromatic ring. Therefore, the membranes developed so far are neither efficient nor durable also for DMFCs.
It is an object of the present invention to provide a proton-conducting membrane, excellent in resistance to heat, durability, dimensional stability and fuel barrier characteristics, and showing excellent proton conductivity at high temperature, which can solve the problems involved in the conventional PEFCs. It is another object of the present invention to provide a method for producing the same. It is still another object of the present invention to provide a fuel cell using the same.