1.  Field of the Invention
The present invention relates to a fuel cell stack and a method of supplying reactant gases to the fuel cell stack.  The fuel cell stack comprises units of fuel cells stacked together.  Each of the fuel cells includes a pair of separators and a membrane electrode assembly interposed between the separators.  The membrane electrode assembly includes an anode, a cathode, and an ion exchange membrane of solid polymer electrolyte interposed between the anode and the cathode. 
2.  Description of the Related Art
Generally, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which comprises two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes.  The electrolyte membrane is a polymer ion exchange membrane (proton exchange membrane).  Each of the electrodes comprises a catalyst and a porous carbon.  The membrane electrode assembly is interposed between separators (bipolar plates).  The membrane electrode assembly and the separators make up a unit of the fuel cell for generating electricity.  A plurality of fuel cells are connected together to form a fuel cell stack. 
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the anode.  The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons.  The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current.  An oxygen-containing gas or air is supplied to the cathode.  At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water. 
In the fuel cell stack, temperature of some of the fuel cells tends to be low in comparison with the other fuel cells.  Specifically, one end surface of each of the outermost fuel cells (end cells) in the stacking direction is exposed to the external air, and thus, the fuel cells are likely to be cooled by the external air.  Further, when a manifold is disposed between two of fuel cells stacked together, the fuel cells adjacent to the manifold are likely to be cooled by the manifold.  If the temperature of the fuel cells is lowered significantly, the power generating performance of the fuel cells is lowered.  Further, condensation of water may occur in the fuel cells.  Water produced in the chemical reactions is not smoothly discharged from the fuel cells, and the voltage of the fuel cells is lowered. 
In particular, when the fuel cell stack is operated at a temperature below the freezing point, the difference between the temperature in the fuel cell stack and the external air temperature is large.  Therefore, the temperature in each of the end cells is lowered significantly.  If the operation the fuel cell stack is started at the temperature below the freezing point, water produced at the time of power generation in each of the end cells is cooled below the freezing point.  The frozen water may close reactant gas flow passages (oxygen-containing gas flow passage and/or fuel gas flow passage) or the porous carbon undesirably.  As a result, a shortage of reactant gases may occur in the end cells.  The shortage of the reactant gases gives rise to a voltage drop in the end cells. 
In an attempt to prevent the end cells from being cooled excessively, for example, Japanese laid-open patent publication No.  8-130028 (the prior art 1) discloses a solid polymer electrolyte fuel cell stack which does not have any grooves (coolant passages) in outer separators of end cells for preventing the separators from being cooled excessively and preventing condensation of water. 
Further, Japanese laid-open patent publication No.  8-167424 (prior art 2) discloses a solid polymer electrolyte fuel cell stack which includes heating members heated by an electric current flowing from the solid polymer electrolyte fuel cell stack.  The heating member is disposed at least on each of current collectors in contact with the outer surfaces of outermost separators of the fuel cell stack for preventing end cells from being cooled excessively and preventing condensation of water. 
In the prior arts 1 and 2, in order to prevent the end cells from being cooled excessively, the separator which does not have any grooves (coolant passages) or the heater heated by the electric current flowing from the solid polymer electrolyte fuel cell stack is used.  Therefore, the special designs at opposite ends of the fuel cell stack make it difficult to downsize the fuel cell stack. 
Further, in the prior art 1, the solid polymer electrolyte fuel cell stack requires different types of separators, i.e., the separator which has the groove as the coolant passage, and the separator which does not have the groove.  The requirement for the different types of separators is a burden in the production line, and thus, the production cost is high.