1. Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Nam-gu, Pohang 790-784, Korea
2. School of Integrative Engineering, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 156-756, Korea
3. Center for Fuel Cell Research, Korea Institute of Science and Technology, 5 Wolsong-gil, Sungbuk-gu, Seoul 136-791, South Korea
4. Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, United States