1. Department of Applied Chemistry, Faculty of Engineering, The University of Tokyo, 7‐3‐1 Hongo, Bunkyo‐ku, Tokyo 113, Japan
2. Research Center for Advanced Science and Technology, The University of Tokyo, 4‐6‐1 Komaba, Meguro‐ku, Tokyo 153, Japan
3. Electric Power Development Company, Limited, 6‐15‐1 Ginza, Chuo‐ku, Tokyo 104, Japan