Typically, solid oxide fuel cells (SOFC) employ an electrolyte of ion-conductive oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (MEA). The electrolyte electrode assembly is interposed between a pair of separators (bipolar plates). In use, generally, a predetermined numbers of the separators and the electrolyte electrode assemblies are stacked together to form a fuel cell stack.
In a fuel cell stack, in order to obtain the output voltage efficiently, the fuel cells need to be stacked together under the desired pressure. Further, in order to prevent leakage of the reactant gas such as the fuel gas and the air as much as possible, reactant gas manifolds need to be sealed reliably by applying pressure to the reactant gas manifolds in the stacking direction.
In this regard, for example, as shown in FIG. 16, a flat plate type solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2006-339035 (hereinafter referred to as the conventional technique 1) includes a cell stack 1a and four manifolds M1 to M4 provided around the cell stack 1a. A fuel gas and an oxygen-containing gas are supplied to, and discharged from each of unit cells 2a through the manifolds M1 to M4.
A first pressure applying mechanism 3a applies pressure to the cell stack 1a, and a second pressure applying mechanism 4a applies pressure to the manifolds M1 to M4. The first pressure applying mechanism 3a includes a compression spring 5a as pressure applying means, and the second pressure applying mechanism 4a includes a compression spring 6a as pressure applying means.
Further, in a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2007-073359 (hereinafter referred to as the conventional technique 2), as shown in FIG. 17, a large number of units 3b each formed by sandwiching a power generation cell 1b between a pair of separators 2b are stacked together. At both ends (upper and lower ends) in the stacking direction of the fuel cell, an upper tightening plate 4b and a lower tightening plate 5b are provided. A large circular hole 6b is formed at the center of the upper tightening plate 4b. The circular hole 6b is larger than the outer shape of the power generation cell 1b, and a weight 7b is placed in the circular hole 6b. 
The upper tightening plate 4b and the lower tightening plate 5b are tightened together by a plurality of bolts 8b to apply a tightening load in the stacking direction to the units 3b. By a load applied by the weight 7b, a plurality of power generating elements of the units 3b tightly contact each other.
Further, a cell stack disclosed in Japanese Laid-Open Patent Publication No. 2009-500525 (PCT) (hereinafter referred to as the conventional technique 3) includes at least one electrochemical cell interposed between a first end plate connected to an electrically conductive bolt and a second end plate connected to another electrically conductive bolt.
The cell stack includes a housing, means for fixing the cell stack to this housing to support the cell stack, and means for applying a mechanical load at a constant level to the entire fuel cell stack. The means for applying the load at the constant level includes at least one elastic pad inserted into a space between the cell stack and a wall of the housing. For example, the elastic pad is a silicone pad, and has insulating property.