Construction of core-shell cobalt sulfide/manganese molybdate nanostructure on reduced graphene oxide/Ni foam as an advanced electrode for high-performance asymmetric supercapacitor

Development of an Amorphous Nickel Boride/Manganese Molybdate Heterostructure as an Efficient Electrode Material for a High-Performance Asymmetric Supercapacitor

The exploration of heterostructure materials with unique electronic properties is considered a desirable platform for fabricating electrode/surface interface relationships for constructing asymmetric supercapacitors (ASCs) with high energy density. In this work, a heterostructure based on amorphous nickel boride (Ni_XB) and crystalline square bar-like manganese molybdate (MnMoO₄) was prepared by a simple synthesis strategy. The formation of the Ni_XB/MnMoO₄ hybrid was confirmed by powder X-ray diffraction (p-XRD), field emission scanning electron microscopy (FE-SEM), field-emission transmission electron microscopy (FE-TEM), Brunauer-Emmett-Teller (BET), Raman, and X-ray photoelectron spectroscopy (XPS). In this hybrid system (Ni_XB/MnMoO₄), the intact combination of Ni_XB and MnMoO₄ leads to a large surface area with open porous channels and abundant crystalline/amorphous interfaces with a tunable electronic structure. This Ni_XB/MnMoO₄ hybrid shows high specific capacitance (587.4 F g^-1) at 1 A g^-1, and it even retains a capacitance of 442.2 F g^-1 at 10 A g^-1, indicating superior electrochemical performance. The fabricated Ni_XB/MnMoO₄ hybrid electrode also exhibited an excellent capacity retention of 124.4% (10000 cycles) and a Coulombic efficiency of 99.8% at a current density of 10 A g^-1. In addition, the ASC device (Ni_XB/MnMoO₄//activated carbon) achieved a specific capacitance of 104 F g^-1 at 1 A g^-1 and delivered a high energy density of 32.5 Wh.kg^-1 with a power density of 750 W·kg^-1. This exceptional electrochemical behavior is due to the ordered porous architecture and the strong synergistic effect of Ni_XB and MnMoO₄, which enhances the accessibility and adsorption of OH^- ions that improve electron transport. Moreover, the Ni_XB/MnMoO₄//AC device exhibits excellent cyclic stability with a retention of 83.4% of the original capacitance after 10000 cycles, which is due to the heterojunction layer between Ni_XB and MnMoO₄ that can improve the surface wettability without causing structural changes. Our results show that the metal boride/molybdate-based heterostructure is a new category of high-performance and promising material for the growth of advanced energy storage devices.

Enhanced Supercapacitor Performance Using a Co3 O4 @Co3 S4 Nanocomposite on Reduced Graphene Oxide/Ni Foam Electrodes

To avoid an enormous energy crisis in the not-too-distant future, it be emergent to establish high-performance energy storage devices such as supercapacitors. For this purpose, a three-dimensional (3D) heterostructure of Co₃ O₄ and Co₃ S₄ on nickel foam (NF) that is covered by reduced graphene oxide (rGO) has been prepared by following a facile multistep method. At first, rGO nanosheets are deposited on NF under mild hydrothermal conditions to increase the surface area. Subsequently, nanowalls of cobalt oxide are electro-deposited on rGO/Ni foam by applying cyclic-voltammetry (CV) under optimized conditions. Finally, for the synthesis of Co₃ O₄ @Co₃ S₄ nanocomposite, the nanostructure of Co₃ S₄ was fabricated from Co₃ O₄ nanowalls on rGO/NF by following an ordinary hydrothermal process through the sulfurization for the electrochemical application. The samples are characterized by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The obtained sample delivers a high capacitance of 13.34 F cm^-2 (5651.24 F g^-1 ) at a current density of 6 mA cm^-2 compared to the Co₃ O₄ /rGO/NF electrode with a capacitance of 3.06 F cm^-2 (1230.77 F g^-1 ) at the same current density. The proposed electrode illustrates the superior electrochemical performance such as excellent specific energy density of 85.68 W h Kg^-1 , specific power density of 6048.03 W kg^-1 and a superior cycling performance (86% after 1000 charge/discharge cycles at a scan rate of 5 mV s^-1 ). Finally, by using Co₃ O₄ @Co₃ S₄ /rGO/NF and the activated carbon-based electrode as positive and negative electrodes, respectively, an asymmetric supercapacitor (ASC) device was assembled. The fabricated ASC provides an appropriate specific capacitance of 79.15 mF cm^-2 at the applied current density of 1 mA cm^-2 , and delivered an energy density of 0.143 Wh kg^-1 at the power density of 5.42 W kg^-1 .