1. Department of Materials Science & Tsukuba Research Center for Energy Materials Science (TREMS), Institute of Pure and Applied Sciences, University of Tsukuba , 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573 , Japan
2. Department of Chemistry, Faculty of Mathematics and Natural Science, Universitas Sumatera Utara . Jl. Dr. T. Mansur No. 9, Padang Bulan, Medan Baru, Medan, Sumatera Utara 20222 , Indonesia