Three Co (III) Complexes Based on Double Ligands: Crystal Structures and Their Derivatives Applied as Supercapacitor Electrode Materials

Three Co (III) complexes based on double ligands {[Co(2,2’-bpy)(2,5-HPda) (2,5-Pda)]Cl·2H2O, namely Co-1; [Co(2,2’-bpy)(2,5-Pda)2]·2H2O, namely Co-2; [Co(2,2’-bpy)(2,5-HPda)(2,5-Pda)]NO3·H2O, namely Co-3} were synthesized by hydrothermal methods. Single crystal X-ray diffraction (XRD), X-ray photoelectron spectrometer (XPS) and energy dispersive spectrometer (EDS) were applied to confirm the components of the three complexes and the oxidation states of the central atom. Optical micrograph, powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR) and ultraviolet-visible spectrum (UV-vis) were used to characterize these complexes. The thermal stabilities of the three complexes were further evaluated by thermo gravimetric analysis (TG). Afterwards, Co-3 was calcined under a flow of nitrogen at 420 °C for 2 h and the composites (C(N,O)/CoxOy) were formed. When the composites were used as electrode materials for supercapacitors, the results showed an excellent specific capacitance of 544 F·g−1 in 6 M KOH solution at a current density of 2 A·g−1. The cycle tests showed that the specific capacitance retained well after 2000 cycles at 20 A·g−1 current density.