The present invention relates to a polymer electrolyte fuel cell that generates electricity using a fuel and an oxidant.
As portable compact electronic devices such as cell phones, personal digital assistants (PDAs), notebook computers and camcorders become multifunctional and consume more electric power, they are required to have more electric power and longer continuous operating time. In order to meet this requirement, batteries having a higher energy density are in strong demand as power sources. Currently, lithium secondary batteries are mainly used as high energy density batteries. They are expected to have an energy density of about 500 Wh/L, 200 Wh/kg in around 2005, which is considered to be the limit of the energy density of lithium secondary batteries. Under the circumstances, early commercialization of polymer electrolyte fuel cells (PEFCs) is awaited as an alternative to lithium secondary batteries.
Among PEFCs, active research and development is carried out particularly on direct type fuel cells, which generate electricity by supplying, instead of hydrogen obtained by reforming an organic fuel such as methanol or dimethyl ether, the organic fuel directly to a fuel cell. This is because direct type fuel cells have a high theoretical energy density and a relatively simple system structure. Additionally, organic fuels such as methanol and dimethyl ether are easier to store than hydrogen.
A direct type fuel cell as described above typically includes membrane an electrode assemblies (MEA) and separators. The MEA includes a hydrogen ion conductive polymer electrolyte membrane and a pair of catalyst layers and a pair of diffusion layers arranged on both sides of the polymer electrolyte membrane. Electricity is generated by supplying water and an organic fuel such as methanol or dimethyl ether directly to an anode (fuel electrode) and an oxidant such as oxygen or air to a cathode (air electrode). Chemically speaking, the supplied organic fuel such as methanol or dimethyl ether reacts with water in the anode to produce carbon dioxide, hydrogen ions (protons) and electrons. The protons migrate through the polymer electrolyte membrane to the cathode. Meanwhile, in the cathode, oxygen, protons and electrons traveling through an external circuit combine to produce water.
In order to realize the commercialization of direct fuel cells, however, the phenomenon commonly referred to as “crossover” must be overcome in which the organic fuel such as methanol supplied to the anode passes through the polymer electrolyte membrane to the cathode, without reacting. The most widely used polymer electrolyte membrane for direct type fuel cells is perfluoroalkyl sulfonic acid ion exchange membrane having proton conductivity, heat resistance and oxidation resistance. The polymer electrolyte constituting a polymer electrolyte membrane of this type includes a main chain of polytetrafluoroethylene (PTFE), which is hydrophobic, and a side chain of a perfluoro group having hydrophilic sulfonic acid group fixed at the terminal of the perfluoro group. Accordingly, an organic fuel such as methanol having both hydrophilic and hydrophobic parts is a suitable solvent because the organic fuel easily passes through the polymer electrolyte membrane.
The crossover described above not only decreases the fuel utilization efficiency but also causes the catalytic activity of cathode to decrease and an increase in overvoltage at the cathode, resulting in significant degradation of electricity generation characteristics.
In order to suppress such crossover, Japanese Laid-Open Patent Publication No. 2002-231265 discloses to place a palladium membrane or a palladium alloy membrane having proton diffusibility and proton permeability on one surface of either anode or cathode. It also discloses to place the same between two polymer electrolyte membranes. Japanese Laid-Open Patent Publication No. Hei 10-40936 discloses a direct type fuel cell in which an oxidation catalyst layer for catalytic combustion of methanol is sandwiched between two polymer electrolyte membranes.
It is, however, still difficult for the direct type fuel cell having the structure disclosed in Japanese Laid-Open Patent Publication No. 2002-231265 to exhibit excellent electricity generation characteristics without impairing the fuel utilization efficiency. The palladium membrane or the palladium alloy membrane described above prevents the supplied organic fuel such as methanol from passing through the polymer electrolyte membrane. Since, however, the membrane has a high resistance, the proton conductivity is significantly reduced and the electricity generation characteristics at the higher current density side are lowered.
In the case where an oxidation catalyst layer is sandwiched between two polymer electrolyte membranes as disclosed in Japanese Laid-Open Patent Publication No. Hei 10-40936, the permeation of methanol to the cathode can be prevented, but the amount of methanol which contributes for electricity generation at the anode is reduced, resulting in reduced fuel utilization efficiency. Moreover, carbon dioxide produced by catalytic combustion cannot be removed to the outside, and therefore the amount of fuel supplied to the anode, i.e. the rate for supplying fuel to the anode, is limited, resulting in significant degradation of electricity generation characteristics.
In addition to the above, micro defects usually exist in catalyst layers, and the organic fuel supplied to the anode such as methanol passes through the defects and reaches directly to the polymer electrolyte membrane, without reacting. As a result, the amount of organic fuel that crosses over the polymer electrolyte membrane is increased, reducing the fuel utilization efficiency to cause an increase in overvoltage at the cathode. This results in significantly degraded electricity generation characteristics.
In view of the foregoing, the present invention is intended to solve the problems described above. Accordingly, the objects of the present invention are to prevent the crossover of organic fuel such as methanol and to provide a direct fuel cell having excellent electricity generation characteristics without impairing the fuel utilization efficiency.