3D self-supported hierarchical core/shell structured MnCo₂O₄@CoS arrays for high-energy supercapacitors

MnCo₂O₄ architectures assembled from well-aligned nanowire or nanosheet arrays have been vertically grown on Ni foam with robust adhesion by in situ hydrothermal growth. Interestingly, the nanostructures of MnCo₂O₄ can be easily tailored from nanowires to nanosheets by the addition of NH₄F at moderate concentrations in a mixed solvent. Further, unique 3D self-supported hierarchical core/shell MnCo₂O₄@CoS nanowire or nanosheet (MCO-NW@CS or MCO-NS@CS) arrays have been further engineered and synthesized via a subsequent electrodeposition process, which shows that interconnected CoS ultrathin layers as the shell have been uniformly immobilized on the MnCo₂O₄ arrays to form a novel core/shell structure. The synergic effect of the unique structure and heterogeneity is advantageous to increase the interfacial area, improve the charge accumulation and storage, accelerate the electron and electrolyte ion transfer, and enhance the electrochemical activity of the two components (MnCo₂O₄ and CoS) when used as an integrated electrode for (asymmetric) supercapacitors. As a result, the MCO-NW@CS or MCO-NS@CS electrodes display greatly improved electrochemical performance including remarkable specific capacitance, outstanding rate capability and good cycling stability. Moreover, the solid-state asymmetric supercapacitor using MCO-NS@CS as the positive active electrode and activated carbon (AC) as the negative active electrode has also achieved high energy density (55.1 W h kg^-1 at 477.3 W kg^-1). The hierarchical MCO-NW@CS or MCO-NS@CS electrodes with outstanding structural characteristics hold great promise not only in high capacitance applications but also in high energy and power density fields. In addition, to have a deeper understanding of their electrochemical behavior, a combined experimental and density functional theory (DFT) calculation study is also introduced.