Preparation and capacitance behaviors of cobalt oxide/graphene composites

In this study, cobalt oxide (<TEX>Co₃O₄</TEX>)/graphene composites were synthesized through a simple chemical method at various calcination temperatures. We controlled the crystallinity, particle size and morphology of cobalt oxide on graphene materials by changing the annealing temperatures (200, 300, <TEX>400∘C</TEX>). The nanostructured <TEX>Co₃O₄</TEX>/graphene hybrid materials were studied to measure the electrochemical performance through cyclic voltammetry. The <TEX>Co₃O₄</TEX>/graphene sample obtained at <TEX>200∘C</TEX> showed the highest capacitance of 396 <TEX>Fg⁻¹</TEX> at 5 <TEX>mVs⁻¹</TEX>. The morphological structures of composites were also examined by scanning electron microscopy and transmission electron microscopy (TEM). Annealing <TEX>Co₃O₄</TEX>/graphene samples in air at different temperatures significantly changed the morphology of the composites. The flower-like cobalt oxides with higher crystallinity and larger particle size were generated on graphene according to the increase of calcination temperature. A TEM analysis of the composites at <TEX>200∘C</TEX> revealed that nanoscale <TEX>Co₃O₄</TEX> (~7 nm) particles were deposited on the surface of the graphene. The improved electrochemical performance was attributed to a combination effect of graphene and pseudocapacitive effect of <TEX>Co₃O₄</TEX>.