1. School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University 1 , Beijing 100044, China
2. State Key Laboratory of Tribology, Tsinghua University 2 , Beijing 100084, China
3. College of Engineering, Shantou University 3 , 243 Daxue Road, Shantou, 515063 Guangdong, China
4. Energy Conversion Research Center, Doshisha University 4 , Kyoto 630-0321, Japan