1. Division of Energy Science and Engineering, Graduate School of Engineering, Kyoto University, Sakyo‐Ku, Kyoto 606‐01, Japan
2. Tokyo Gas Company, Limited, Fundamental Technology Research Laboratory, 1‐16‐25 Shibaura, Minato‐Ku, Tokyo 105, Japan
3. Department of Materials Science, School of Engineering, The University of Shiga Prefecture, 2500 Hassaka, Hikone, Shiga 522, Japan
4. Department of Fundamental Energy Science, Faculty of Energy Science, Kyoto University, Sakyo‐ku, Kyoto 606‐01, Japan