1. School of Chemical & Biological Engineering and Research Center for Energy Conversion & Storage, Seoul National University, Seoul 151-744, South Korea
2. Fuel Cell Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea
3. School of Advanced Materials Engineering, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul 136-702, South Korea