1. Field of the Invention
The present invention relates to a fuel cell stack having stacked unit fuel cells through which a supply and drainage passage for a coolant (or cooling liquid) is provided, and in particular, those for preventing electric short circuits due to coolant in the cells.
2. Description of the Related Art
Typically, solid polymer-type fuel cells have a unit fuel cell in which an anode and a cathode are provided on either side of an electrolyte membrane which is a polymer ionic exchange membrane (i.e., cation exchange membrane). Each unit fuel cell is placed between separators for supporting the unit fuel cell, and generally, a specific number of unit fuel cells are stacked to obtain a fuel cell stack.
In this kind of fuel cell stack, a fuel gas supplied to the anode, for example, a hydrogen gas, is ionized to hydrogen ions on catalytic electrodes, and the hydrogen ions are transferred to the cathode via an electrolyte membrane which is humidified to have an appropriate level of humidity. During this process, electrons are generated and flow to an external circuit, providing DC (direct current) electric energy. An oxidizing gas such as oxygen or air is supplied to the cathode, and the hydrogen gas, electrons, and oxygen gas react at the cathode, thereby generating water.
In most cases, the fuel cell stack employs an internal manifold structure so as to supply a reaction gas such as a fuel gas or an oxidizing gas and a coolant for cooling the fuel cell stack to the anode and cathodes of each of the stacked unit fuel cells. Specifically, this manifold structure has a plurality of communicating holes formed through all of the stacked unit fuel cells and separators. When a reaction gas is supplied to the relevant communicating hole, the reaction gas is distributed and supplied to the unit fuel cells, and simultaneously, the used reaction gas is exhausted to the communicating hole provided for exhausting the reaction gas. Similarly, a coolant is supplied through a communicating hole, and the supplied coolant is used for heat exchange and then drained through a communicating hole provided for draining the coolant. Japanese Unexamined Patent Application, First Publication No. Hei 9-63627 discloses an example of such a system.
In the above-explained fuel cell stack, a high voltage is secured by connecting the stacked unit fuel cells in series. Therefore, generally, a power output terminal plate made of a metal having a high conductivity is attached to an end portion of the fuel cell stack in the stacking direction. In the case of employing the above-explained internal manifold structure, communicating holes for the reaction gas and communicating holes for the coolant should be provided in the power output terminal plate, as formed in the above-described unit fuel cell and separator.
However, a power output terminal plate made of metal may corrode because the vicinity of each communicating hole for coolant is exposed to the coolant. Additionally, as metal having a high conductivity, such as Cu, is used, an electric short circuit via the coolant (i.e., cooling liquid) may occur. In order to solve this problem, pure water, which is an effective insulator, may be used. However, in this case, freezing may occur in cold areas. Furthermore, communicating holes provided for the reaction gas may also corrode due to the water content included in the reaction gas.
In addition, a power output terminal plate made of stainless steel, which has a high resistance to corrosion, may increase power loss because such a plate has a high electric resistance.