Phase and morphology control in the synthesis of Co₃O₄ nanosphere/α-Co(OH)₂ nanosheet hybrids for application in supercapacitors

In this study, we developed a facile chemical precipitation approach for the synthesis of heterogeneous Co3O4 nanosphere/Co(OH)2 nanosheet hybrids in the presence of polyvinylpyrrolidone (PVP) as a phase-controlling reagent. The crystalline phase and morphology of the product varied upon changing the chain length of PVP (from 8k to 120k), with short-chain PVP favoring the formation of Co3O4 nanospheres and long-chain PVP favoring the synthesis of Co(OH)2 nanosheets. Accordingly, heterogeneous Co3O4/Co(OH)2 hybrids of various blending ratios were readily prepared in the presence of PVP of different molecular weights. The molecular weight of PVP also affected the electrochemical properties of the Co3O4 nanosphere/Co(OH)2 nanosheet composites. When using PVP of moderate chain length (58k), the resultant Co3O4/Co(OH)2 composite exhibited the optimal supercapacitive performance, characterized by an excellent specific capacitance of 771.2 F g−1 at 1 A g−1, and retained approximately 68.5% of this capacitance when operated at a high current density of 10 A g−1. Furthermore, this composite displayed an excellent charge/discharge cycling life at a current density of 4 A g−1, with a capacitance retention of 93.3% after 3000 repeated cycles.