Experimental and theoretical insights into colossal supercapacitive performance of graphene quantum dots incorporated Ni3S2/CoS2/MoS2 electrode

Transition metal sulfides (TMS) are promising electrode materials for supercapacitor applications due to their unmatchable advantages. To overcome the deficiency of individual metal sulfides, the combination of one or more pseudocapacitive transition metals with conductive carbon material can be a plausible solution. In this work, a simple and one-step hydrothermal synthesis method is used to fabricate the flower-like Ni3S2/CoS2/MoS2 (NCMS) and Ni3S2/CoS2/MoS2-GQDs (NCMS-G) directly on Ni foam and used it as a binder-free electrode for high-performance supercapacitor applications. The NCMS-G-2.5 electrode possessed colossal specific capacitance (2364 Fg- 1 at the current density of 2 Ag-1), excellent rate capability (55 % at the current density of 10 Ag-1), as well as good cycling stability (93 % retention after 10,000 cycles at the current density of 20 Ag-1). Furthermore, assembled NCMS-G-2.5//rGO asymmetric supercapacitor (ASC) device delivered an excellent energy density of 84.8 Wh kg- 1 and superior cycling performance (98 % capacitance retention after 10,000 cycles). We have investigated the interactions between Ni3S2/CoS2/MoS2 and carbon based nanostructure GQDs employing stateof-the-art Density Functional Theory (DFT) simulations. The interactions between Ni3S2/CoS2/MoS2 and GQDs is not only long range van der Waals interactions, but also chemical interactions involving charge transfer from metal 3d orbital to C 2p orbital of GQDs are also present. Due to this synergic effect, there is enhancement in electronic states near Fermi level which may improve the conductivity of the hybrid structure. Enhanced quantum capacitance, improved conductivity and increased surface area for the hybrid structure may attribute towards superior charge storage performance for the hybrid structures which support experimental data. It is expected that the fabricated, unique, 3D flower NCMS-G-2.5 has potential prospects for developing advanced and high-performance energy storage devices.

Automated summary

Transition metal sulfides (TMS) are promising electrode materials for supercapacitor applications. To overcome the deficiency of individual metal sulfides, the combination of pseudocapacitive transition metals with conductive carbon material can be a plausible solution. In this work, flower-like Ni3S2/CoS2/MoS2 (NCMS) and Ni3S2/CoS2/MoS2-GQDs (NCMS-G) structures were fabricated directly on Ni foam using a hydrothermal synthesis method. The NCMS-G-2.5 electrode showed high specific capacitance, excellent rate capability, and good cycling stability. Furthermore, an assembled NCMS-G-2.5//rGO asymmetric supercapacitor (ASC) device exhibited excellent energy density and superior cycling performance. The interactions between Ni3S2/CoS2/MoS2 and carbon based nanostructure GQDs were investigated using Density Functional Theory (DFT) simulations, revealing enhanced electronic states near Fermi level and improved conductivity for the hybrid structure. The fabricated 3D flower NCMS-G-2.5 has potential prospects for developing advanced and high-performance energy storage devices.