Intimately coupled hybrid of carbon black/nickel cobaltite for supercapacitors with enhanced energy-storage properties and ultra-long cycle life

Advanced materials and technologies for supercapacitors used in energy conversion and storage: a review

Supercapacitors are increasingly used for energy conversion and storage systems in sustainable nanotechnologies. Graphite is a conventional electrode utilized in Li-ion-based batteries, yet its specific capacitance of 372 mA h g−1 is not adequate for supercapacitor applications. Interest in supercapacitors is due to their high-energy capacity, storage for a shorter period and longer lifetime. This review compares the following materials used to fabricate supercapacitors: spinel ferrites, e.g., MFe2O4, MMoO4 and MCo2O4 where M denotes a transition metal ion; perovskite oxides; transition metals sulfides; carbon materials; and conducting polymers. The application window of perovskite can be controlled by cations in sublattice sites. Cations increase the specific capacitance because cations possess large orbital valence electrons which grow the oxygen vacancies. Electrodes made of transition metal sulfides, e.g., ZnCo2S4, display a high specific capacitance of 1269 F g−1, which is four times higher than those of transition metals oxides, e.g., Zn–Co ferrite, of 296 F g−1. This is explained by the low charge-transfer resistance and the high ion diffusion rate of transition metals sulfides. Composites made of magnetic oxides or transition metal sulfides with conducting polymers or carbon materials have the highest capacitance activity and cyclic stability. This is attributed to oxygen and sulfur active sites which foster electrolyte penetration during cycling, and, in turn, create new active sites.

Fabrication of uniform urchin-like N-doped NiCo2O4@C hollow nanostructures for high performance supercapacitors

Transition metal oxides are commonly used in electrochemical energy storage materials, but there are still many drawbacks that impede a wide range of applications. Heteroatom doping can significantly improve its performance. Herein, we have successfully prepared highly uniform N-doped NiCo₂O₄@C hollow nanostructures for supercapacitors by a two-step hydrothermal treatment associated with successive annealing process. Prepared N-doped NiCo₂O₄@C materials exhibited an admirable specific capacitance of 1028 F g^-1 at a current density of 3 A g^-1, with 625 F g^-1 remaining even at high current density of 20 A g^-1. Besides, this composite showed good electrochemical stability with capacity retention of 84% after 5000 cycles repetitive galvanostatic charge-discharge test at 10 A g^-1. An asymmetric supercapacitor was assembled by the N-doped NiCo₂O₄@C electrode, attached activate carbon (AC) as a counter electrode, exhibiting a high energy density of 26.67 W h kg^-1 at a power density of 402 W kg^-1. The improvement of electrochemical performance is ascribed to the co-doping of nitrogen and carbon atoms. These results indicate that N-doped NiCo₂O₄@C can be employed as an ideal electrode material for electrochemical energy storage.