Solid oxide fuel cells (hereinafter referred to as “SOFCs”) can be considered as third-generation fuel cells and have utilized zirconium oxide, to which yttria has been added to stabilize the crystalline structure thereof, as their electrolyte. This material has oxygen ion conductivity, but is characterized in that it can provide the desired conductivity for fuel cells in the high temperature range of 800 to 1000° C. For this reason, the operating temperature of SOFC is usually 800° C. or higher, and the electrodes are made of conductive materials that can withstand this high temperature. For example, the air electrode to which air is supplied is generally made of LaSrMnO3, and the fuel electrode to which hydrogen is supplied is generally made of a Ni—ZrO2 mixture.
In planar-type SOFCs according to the prior art, a fuel electrode or an electrolyte support is thinly coated with another electrode or an electrolyte to make an electrolyte-electrode assembly (hereinafter referred to as “EEA”) which is then inserted with an interconnector made of a conductive metal, which electrically connects the air electrodes and fuel electrodes of the underlying and overlying unit cells and in which gas channels for introducing fuel and air into the respective electrodes are formed on both sides, thereby manufacturing a cell. This planar-type solid oxide fuel cell is advantageous in that the thickness of the EEA layer is thin, but it is difficult to control the uniformity of the thickness or flatness of the EEA layer because of the characteristics of ceramics, thus making it difficult to increase the size of the cell. Also, when the EEA layer and the interconnector are stacked in alternation to stack the unit cells, a gas-sealing material is used at the edge of the cell to seal gas introduced into the cell. The glass-based material that is used as the sealing material starts to soften from about 600° C., but the solid oxide fuel cells are generally operated at a temperature higher than 800° C. in order to obtain the desired efficiency. This increases the risk of a gas leak because of the softening of the sealing material, and thus the glass material for sealing needs to be improved to be commercially viable.
An attempt to overcome the shortcomings of such planar type cells with the development of a unit cell and a stack using a flat tube-type support is disclosed in U.S. Pat. Nos. 6,416,897 and 6,429,051. In these cases, however, an interconnector creating an electrical connection with a gas channel for supplying air or fuel to the outside of the planar type cell is additionally used for stacking. Although this increases the mechanical strength of the stack and widens the contact area between the unit cells to increase power density, the interconnector is made of a metal and so mechanical and thermal stress disadvantageously occurs between the EEA layers made of a ceramic material during high-temperature operation.
To overcome this shortcoming of the metallic interconnector, monolithic unit cells have been proposed in which channels for two kinds of gases are formed in a unit cell support itself or a support stack to omit the gas channel function of the interconnector and reduce the thickness of the cell. Typical examples thereof include a monolithic stack of segmented flat tubular cells, in which the cells are segmented in the lengthwise direction of the flat tube and are electrically connected (U.S. Pat. No. 5,486,428). However, these cells have disadvantages in that a ceramic plate for air channels should additionally be used to form air channels and in that structures for electrical connection and gas supply are complicated, which makes it not easy to increase the size of the stack.
In addition, fuel cell stacks developed to date have used methods in which unit cells are electrically connected only in series (Korean Patent Application No. 10-2008-10176, Korean Patent Application No. 10-2008-30004, etc.). A problem of such methods is that a deterioration in the performance of a certain cell leads to a deterioration in the overall performance of the stack, so that all the cells need to be perfectly made and operated, which is difficult to achieve.