NiCo₂O₄ nanostructure-decorated PAN/lignin based carbon nanofiber electrodes with excellent cyclability for flexible hybrid supercapacitors

Nanostructured binary metal oxide-decorated carbon nanofibers (CNFs) were utilized as flexible electrodes for hybrid supercapacitor applications. Polyacrylonitrile (PAN)/lignin based carbon nanofibers (PAN/lignin CNFs) with different contents of lignin were prepared by electrospinning, stabilization and carbonization. Afterwards, the NiCo₂O₄ oxides were deposited on the surface of CNFs by a facile hydrothermal method without any toxic reagents. SEM images confirmed that the NiCo₂O₄ (nanosheet and nanoneedle) nanostructures were uniformly grown on the surface of every carbon fiber. The obtained flexible NiCo₂O₄-decorated PAN/lignin CNFs with 50% lignin (NiCo₂O₄@CNF55) electrode exhibited high specific capacitance of the ∼1757 F g⁻¹ at 2 mA cm⁻² and excellent cyclability with ∼138% capacitance retention after 5000 cycles at 7 mA cm⁻², suggesting lower internal resistance and higher electrochemical reversibility. In addition, the NiCo₂O₄@CNFs//N-rGO solid state asymmetric supercapacitor devices were assembled in order to evaluate their practical applications. The NiCo₂O₄@CNF55//N-rGO asymmetric supercapacitor device exhibited a specific capacitance of 134.3 F g⁻¹ at a current densities of 1 A g⁻¹, and possessed a maximum energy density of 47.75 Wh kg⁻¹ with a power density of 799.53 W kg⁻¹.