Technical Papers
New Fabricating Method of Solid Oxide Fuel Cell and Performances*
Yoichi Kawagoe**, Sadahiro Namie**,Takanao Kumakura**, Koki Shiozaki**
To adapt SOFC system for marine power, the improvement of electric power generation performance and its quick response to large road change of the cell elementare required. The purpose of this study is to establish new manufacturing method of the cell element for the improvement of cell performances, and to verify its availability. In the experiment, test cells were prepared using electrolyte membrane with various sizes of roughness on the surface. The electric performances were measured and compared with each other by using hydrogen as fuel. Effect of interfacial structures between electrolyte membrane and electrode layers on the electric performances was investigated.
The main results are as follows： the electric performances of the test cell were improved by the new manufacturing method. It was possible to reduce operating temperature of the cell, which contributes to getting higher reliability of the SOFC system. The facts mainly caused by 3-dimensional extension of real contact area and electrochemical reaction sites between electrolyte and electrodes. To investigate adaptability of natural gas as fuel for the SOFC system, reforming reaction and electric performance were also examined using methane as fuel. It was certified that the steady power generation with methane could be obtained.
1. Introduction
A fuel cell is a device to convert fuel chemical energy directly into electric energy without converting it into heat energy. It can increase energy conversion efficiency and does not exhaust air pollutant nor generate any noise and vibration and, therefore, it is expected to be a promising power generating device of next generation high in harmonization with the environment. The main part of the device bearing the role of energy conversion is called the cell, and consists of electrodes on the fuel and air sides and electrolyte. The fuel cells are roughly classified into five types according to the types of the electrolyte materials. The authors have experimentally studied on the fuel cell of solid oxide electrolyte type (hereafter referred to as SOFC) for the purpose to extract the technical problems and take measures against them in applying the device in marine engines1)〜4).
The SOFC type works at higher temperature than the other types of fuel cells and, therefore, the catalyst to accelerate cell reaction is not required and running cost can be suppressed to a low level. Also, it can use LNG and heavy oil as fuel through reforming reaction and reduce total size because of high power density and, therefore, it is considered suitable for the power source of ships. In addition, it is advantageous in such a respect that the overall efficiency can be increased by re-utilizing heat of exhaust gas for co-generation.
* Translated from Journal of MESJ Vol.33, No.3 (Manuscript received Aug.29, 1997)
** Ship Research Institute(Shinkawa, Mitaka City)
On the other hand, some problems with strength such as peeling by a difference in expansion coefficient of electrolyte and electrodes at high temperatures and damages by vibration are expected to occur and, therefore, selection of the materials becomes difficult.
The SOFC is usually composed of a stack formed by pilling up several tens of unit cells, and used for power generation. In order to apply it to marine engines, however, its compactness and reliability are mandatory. This study newly proposes the method of manufacturing cells by firing electrodes after chemically finishing electrolyte surface in order to increase the basic power generation performance and load change follow-up capability of a unit cell. By this method, the boundary surface between the electrolyte and electrodes is connected three-dimensionally, contact area is increased substantially, and thus the acceleration of cell reaction is expected.
In order to demonstrate this aspect, in the experiment, multiple cells were prepared using those electrolyte membranes of which electrolyte surface roughnesses were different from each other. Hydrogen was supplied as fuel and its basic performances were measured and compared to study about the effects of the surface roughness of the electrolyte on (1) Basic power generation characteristics of cells, (2) Internal resistance of cells, and (3) Load change follow-up capability. For the power generation characteristics, the experiments were also performed while the working temperature was varied in order to study the possibility of reduction in the working temperature of the cell.
