1. Department of Electrical and Computer Engineering, University of Toronto, 10 King’s College Road, Toronto, Ontario M5S 3G4, Canada
2. Department of Electrical and Computer Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States
3. Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136−791, South Korea
4. School of Materials Science and Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei, Anhui Province 230009, People’s Republic of China