1. Field of the Invention
The present invention relates to a structure of a fuel cell stack in which fuel cell units are stacked to be connected in series.
2. Description of the Related Art
An amount of energy per volume and weight that can be generated by a fuel cell apparatus is much larger than that of a conventional secondary battery, and the fuel cell apparatus can be used repeatedly by refilling the fuel cell apparatus with a fuel. Thus, it is expected that the fuel cell apparatus is applicable for use in a digital camera, a mobile telephone, a laptop personal computer, or the like as the power source apparatus of portable electronic equipment, which can be used continuously for a long period of time.
U.S. Pat. No. 5,514,486 discloses an air breathing fuel cell stack in which hydrogen gas is reacted with oxygen in the atmosphere. Herein, a polymer electrolyte membrane is adopted as an electrolyte membrane of a fuel cell unit, and a porous fuel flow field plate and a porous oxygen flow field plate (oxygen supply layer) are adopted as a fuel supply layer and an oxidizer diffusion layer of the fuel cell unit, respectively. A catalyst layer is provided on both surfaces of the polymer electrolyte membrane. The fuel flow field plate for diffusing hydrogen gas over an entire surface of one catalyst layer is provided so as to be stacked on the one catalyst layer, and the oxygen flow field plate for diffusing oxygen in the atmosphere over the entire surface of the other catalyst layer is provided so as to be stacked on the other catalyst layer.
Hydrogen gas supplied to the catalyst layer through the fuel flow field plate generates protons by a catalyst reaction, and enters the polymer electrolyte membrane. On the other hand, oxygen supplied to the catalyst layer on an opposite side through the oxygen flow field plate is combined with the protons passed through the solid polymer electrolyte membrane by a catalyst reaction to generate water molecules. An electromotive force of the fuel cell unit is generally 1 V or less. Therefore, in a case where an output voltage equal to or higher than 1 V is required, the fuel cell units are stacked to be connected in series to assemble a fuel cell stack.
In a case of forming the air breathing fuel cell stack, an opening is provided on the side of a side surface of the fuel cell unit to introduce oxygen in the atmosphere into the stack. Oxygen is introduced through the side opening of the oxygen supply layer and diffused to the catalyst layer formed on a surface of the electrolyte membrane on an oxidizer electrode side through one major surface of the oxygen supply layer. Then, the water generated in the catalyst layer passes through a path reverse to that of oxygen mainly as water vapor and is discharged to the atmosphere from the side opening of the oxygen supply layer.
In the fuel cell stack in which fuel cell units having thicknesses equal to each other are stacked as disclosed in U.S. Pat. No. 5,514,486, generation efficiency tends to decrease in the fuel cell units provided in the middle of the stack, compared with fuel cell units at both ends. In the fuel cell units having poor radiation, which are provided in the middle of the stack, temperature of the oxygen supply layer increases, compared with that of the fuel cell units having satisfactory radiation, which are provided at both ends of the stack.
In general, when the temperature of the oxygen supply layer increases, water vapor partial pressure increases as compared with the case where the temperature of the oxygen supply layer is low. Therefore, oxygen partial pressure decreases. In the middle fuel cell units in which oxygen partial pressure has decreased due to an increase in temperature, a failure occurs in supplying oxygen to the electrolyte membrane, thereby decreasing the electromotive force, compared with the fuel cell units at both ends of the stack, in which the oxygen partial pressure remains high. Thus, when an attempt is made to maintain the output voltage of the entire fuel cell stack, the fuel cell stack needs to be operated with an output current low enough to leave an allowance to the fuel cell units at both ends of the stack so that a failure does not occur in supplying oxygen in the middle fuel cell units in which the temperature tends to increase. Specifically, the middle fuel cell units in which the generation efficiency has decreased due to a difference in temperature may limit the output current of the entire fuel cell stack.
Further, in the fuel cell stack in which a plurality of fuel cell units is stacked, each of the fuel cell units in the middle of the stack is influenced by the water vapor discharged from adjacent fuel cell units on both sides thereof, and the concentration of water vapor in the vicinity of the opening of the middle fuel cell units is likely to become higher than those of the fuel cell units at both ends of the fuel cell stack. Consequently, discharge efficiency of water vapor through the opening decreases, with a result that a failure is likely to occur in supplying of oxygen in the fuel cell units in the middle of the stack.
As a method of keeping the temperature of the fuel cell units in the middle of the stack low, there is a method of placing an air-cooling fan or the like to forcefully cool the fuel cell units in the middle of the stack. However, when a mechanism related to the forceful cooling is provided, the number of components increases to enlarge an outer size of the fuel cell apparatus, and consequently, an arrangement space for a fuel tank is reduced.
Also a method of operating the fuel cell stack while keeping the output current of the fuel cell stack extremely low is considered. However, when the output current is suppressed, a current range that can originally be output by the fuel cell unit cannot be used sufficiently, thereby resulting in an increase in the area of the fuel cell unit to ensure a current. Thus, the outer size of the fuel cell apparatus is enlarged to result in reducing the arrangement space of a fuel tank. Needless to say, in any of the methods, cost per output of a fuel cell increases remarkably.