Synthesis of porous Co3O4/Reduced graphene oxide by a two-step method for supercapacitors with excellent electrochemical performance

Porous Co3O4/reduced graphene oxide (RGO) composites with excellent supercapacitive performance were successfully synthesized on nickel foam (NF) by electrophoretic deposition followed by thermal reduction, and a hydrothermal method. The effects of the introduction of RGO and molar ratio of hexamethylenetetramine (HMT) and Co(NO3)2 on morphology and electrochemical performance of the deposits were investigated. The morphological evolution of Co3O4 prepared on RGO with the increase in molar ratio was followed: dispersed flower-like clusters composed of fine nanosheets (1:2) → coarse flower-like clusters (1:1) → a honeycomb-like structure consisting of fine nanosheets (2:1) → two layers of structures (initial honeycomb-like structure covered with coarse flower-like structure in 3:1 and 4:1). A highest specific surface area was obtained in the sample prepared in the HMT/Co(NO3)2 M ratio of 2:1. The introduction of RGO contributed to the increase in specific surface area of the deposits. For the three-electrode testing system, the specific capacitance of the Co3O4/RGO deposit at the molar ratio of 2:1 reached the highest values of 1138.11 F g−1 at a current density of 1 A g−1, 800.71 F g−1 at a scanning rate of 5 mV s−1, which were higher than those of Co3O4. This Co3O4/RGO deposit also demonstrated the outstanding cycling stability (the specific capacitance of the 80.67% of initial value was retained after 5000 cycles). For the two-electrode testing system, the as-fabricated asymmetric supercapacitor device achieved a high specific capacitance of 108.87 F g−1 at a current density of 1 A g−1 with a stable operational voltage of 1.45 V and a high energy density of 647.05 Wh⋅kg−1.