High performance asymmetric supercapacitors based on Ti₃C₂T MXene and electrodeposited spinel NiCo₂S₄ nanostructures

Spinel metal sulfides have been investigated for a wide range of applications mostly in electrochemical energy storage owing to their better electronic conductivity and high reversible redox activity. Herein, we report a facile fabrication approach for the binder-free supercapacitor electrodes based on spinel NiCo₂S₄ (NCS) on various substrates such as Cu-foil (CF), Ni-foam (NF), and vertical graphene nanosheets grown on carbon tape (VG) via a single step-controlled electrodeposition technique. The obtained electrodeposited NiCo₂S₄ grown on Cu-foil (denoted as CF-NCS) in symmetric assembly shows a high specific capacitance of 167.28 F g^-1 compared to NCS grown on Ni-foam and VG substrates, whereas, symmetric NiCo₂S₄ grown on a VG substrate device exhibits better cycling performance (81% for 3000 cycles) compared to CF-NCS and NF-NCS. Furthermore, an asymmetric supercapacitor was assembled in combination with MXene (Ti₃C₂T ) as a negative electrode (denoted as TCX). As a result, the CF-NCS//TCX device exhibits a high areal capacitance of 48.6 mF cm^-2 at 2 mA cm^-2 of current density. We also report good specific capacitance of 54.57 F g^-1 at 2 A g^-1; in addition, the CF-NCS//TCX assembly delivers maximum areal and gravimetric energy density of 14.86 mWh cm^-2 and 14.86 Wh kg^-1 respectively. In contrast, the VG-NCS//TCX device showed improved cycling stability with 85% of capacitance retention over 5000 cycles owing to its highly porous structure and multiple conductive networks in the VG substrate and provides structural stability to NCS with fast ion diffusion. This experiment favors 2D MXene as a capacitive electrode that provides a replacement for carbon-based electrodes in asymmetric assembly with superior electrochemical performance. Hence, the hierarchical NCS structure grown on the various substrates in combination with MXene serve as a promising material for energy storage application.