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

Abstract

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.