1. Field of the Invention
The present invention relates to fuel electrodes for solid oxide fuel cells and a process for the production thereof.
2. Related art statement
Fuel cells have recently been noted as power generators, and are a very promising technique, for generating power since the fuel cells can directly convert chemical energy possessed by a fuel into electrical energy, and are not limited by the Carnot's cycle. Further, the cells have essentially high energy conversion efficiency, can use of a variety fuels (naphtha, natural gas, methanol, coal reformed gas, heavy oil, etc) with low public nuisance, and their power generating efficiency is not influenced by the installation scale.
Particularly, since the solid oxide fuel cell (hereinafter referred to as SOFC) operates at high temperature) of about 1,000.degree. C., electrochemical reaction on its electrodes is extremely high. Thus, SOFC expensive noble metal catalysts such as platinum, has small polarization, and relatively high output voltage, consequently, its energy converting efficiency is very high compared with other fuel cells. Further, since SOFC is entirely constituted by solids, it has stability and long life.
Since a voltage loss of the SOFC is great due to polarization of the fuel electrode, a primary problem is how to lower the polarization by enhancing activation of the fuel electrode to elevate output of the SOFC. At present, a cermet of nickel-zirconia is popularly used as a material for fuel electrodes. However, in an operating temperature range of the SOFC (for example, 900.degree. to 1,000.degree. C.), flocculation and sintering of nickel as a reaction catalyst proceed to accelerate activated polarization. In order to prevent such phenomena, a variety of techniques have been proposed.
One of them is to form a porous skeleton around nickel grains by EVD (electrochemical vapor deposition) after coating and drying of a nickel slurry on a solid electrolyte (Unexamined Japanese patent application publication No. 61-225,778). However, since this technique is a vapor deposition, reacting conditions are complicated, a processing speed is low, a processing time is long, and a cost is high. Therefore, productivity is low. In addition, existing SOFCs cannot cope with a current large scale and large area tendency.
Another technique is also proposed to coat nickel grains with an electron-conductive oxide by forming a fuel electrode, impregnating the fuel electrode with a solution of an organic metallic compound capable of forming the electron-conductive oxide through thermal decomposition, and thermally decomposing it (Unexamined Japanese patent application No. 1-302,669). However, this technique cannot exhibit high catalytic power possessed by nickel because the nickel grains are coated with a film.
Yet another technique is proposed to form a nickel film on a porous skeleton preliminarily formed from zirconia by impregnating the porous skeleton with a nickel slurry (Unexamined Japanese patent application No. 2-72,558). Furthermore, another more popular technique is proposed to calcine a mixture of nickel oxide and yttria-stabilized zirconia, mill the mixture, and form and fire a film (The 56th annual meeting of the electrochemical society of Japan, 1G31, 1989). However, it is difficult for these techniques to increase the reacting area of nickel.
Further, in any of the above techniques, high temperatures of not less than 1,200.degree. C. are necessary to form a fuel electrode having high performance, which exerts an adverse influence upon other parts of the SOFC and poses a limitation upon constituent materials.