Nickel–cobalt oxide nanosheets asymmetric supercapacitor for energy storage applications

Abstract

Supercapacitors are a promising candidate in applications that necessitate high electrochemical stability and storage energy. In this study, ${\mathrm{NiCo}}_2{\mathrm{O}}_4$ nanosheets were prepared hydrothermally on an ITO substrate and investigated to be utilized as supercapacitor electrodes. The morphology of ${\mathrm{NiCo}}_2{\mathrm{O}}_4$ nanosheets was examined by scanning electron microscopy ($\mathrm{SEM}$) and atomic force microscopy ($\mathrm{AFM}$). The $\mathrm{SEM}$ results showed a 3D-flower-like nanostructure with interconnected nanosheets which was confirmed by the $\mathrm{AFM}$ results. However, X-ray fluorescence (XRF) results showed that the as-prepared sample has stoichiometry of $\mathrm{Nickle}(1):\mathrm{Cobalt}(2)$. The electrochemical measurements of the as-prepared ${\mathrm{NiCo}}_2{\mathrm{O}}_4$ supercapacitor electrode such as cyclic voltammetry ($\mathrm{CV}$) and galvanostatic charge/discharge ($\mathrm{GCD}$) studies were done in a two-electrode system with 1.0 M $\mathrm{KOH}$ and 1.0 M ${\mathrm{H}}_2{\mathrm{SO}}_4$. $\mathrm{CV}$ curves showed quasi-rectangular shape and high electrochemical stability in KOH and H₂SO₄ electrolyte solutions. In addition, the integral areas of $\mathrm{CV}$ curves for both electrolytes are almost identical, indicating efficient charge transfer and ion transport at the electrode/electrolyte interface. Electrochemical impedance spectroscopy ($\mathrm{EIS}$) curves of $\mathrm{KOH}$ and ${\mathrm{H}}_2{\mathrm{SO}}_4$ electrolyte revealed a significant difference. This difference indicates that, the charge transfer in ${\mathrm{H}}_2{\mathrm{SO}}_4$ electrolyte is faster than charge transfer in $\mathrm{KOH}$, resulting in a linear behavior of the EIS curve. A fabricated hybrid asymmetric supercapacitor ($\mathrm{SC}$) composed of ${\mathrm{NiCo}}_2{\mathrm{O}}_4/\mathrm{ITO}$ anode and graphite/ITO cathode delivered a specific capacity of around $235\mathrm{F}/\mathrm{g}$ in $\mathrm{KOH}$ solution and $723\mathrm{F}/\mathrm{g}$ in ${\mathrm{H}}_2{\mathrm{SO}}_4$ electrolyte at 10 mV/s scan rate. The superior electrochemical performances could be attributed to the large surface area that facilitates charge transfer at the electrode/electrolyte interface.