Fabrication of hierarchical NiCo2O4@NiCo2S4 core/shell nanowire arrays by an ion-exchange route and application to asymmetric supercapacitors

Highlights: • The method was hydrothermal and ion-exchange route process. • They showed high specific capacitance of 3176 F g⁻¹ at a current density of 2 A g⁻¹ • The high capacitance can retain 86.52% even at a current density of 10 A g⁻¹ • The diffusion-controlled process was dominant in the total capacitance. • The asymmetric supercapacitor device exhibited high energy density of 196.8 Wh kg⁻¹ at 752.33 W kg⁻¹ Although supercapacitors have been extensively investigated due to the good electrochemical performance, the awful energy density, poor cycle stability and low rate performance still remain challenges in applications. In this article, we successfully design and synthesize the hierarchical NiCo₂O₄@NiCo₂S₄ core/shell nanowire arrays on Ni foam by a special interface ion-exchange process. As supercapacitor material, it exhibits a high specific capacitance of 3176 F g⁻¹ at a current density of 2 A g⁻¹ and can retain 86.52% at a high current density of 10 A g⁻¹, indicating excellent rate capability. In addition, an asymmetric supercapacitor is assembled with NiCo₂O₄@NiCo₂S₄ as the cathode electrode and porous carbon as the anode electrode and KOH solution as the electrolyte, showing a super energy density of 196.8 Wh kg⁻¹ at 752.33 W kg⁻¹ and a high power density of 5625.12 W kg⁻¹ at 162.45 Wh kg⁻¹. After 15000 cyclic voltammetry cycles at a scan rate of 60 mV s⁻¹, 137% of initial capacitance is maintained. Theoretical analysis reveals that the diffusion-controlled reaction is dominant in the total capacitance of NiCo₂O₄@NiCo₂S₄ electrode, which may be the reason for the high specific capacitance and excellent rate capability. Thereby, these electrochemical performances demonstrate that the as-fabricated NiCo₂O₄@NiCo₂S₄ core/shell nanowire arrays are the promising candidates for high-performance supercapacitors.