1. Research Group of New Energy Materials and Devices, State Key Laboratory of Coal and CBM Co-Mining, North University of China, Taiyuan 030051, P. R. China
2. State Key Laboratory of Coal and CBM Co-Mining, Jincheng 048012, P. R. China
3. State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, P. R. China