The present work explores the electrochemical energy storage performance of Ti3C2T x (MXene) and 1T-VS2 nanosheet hybrids synthesized by a simple in situ hydrothermal method. Different analytical methods such as X-ray diffraction, field emission scanning electron microscopy, Fourier transform infrared, Raman spectroscopy, and Brunauer–Emmett–Teller were employed to explore the structural and morphological properties and composition of electrode materials. Furthermore, the electrochemical characterization of 1T-VS2/MXene hybrid electrode materials with different concentrations of MXene was investigated systematically. The all pseudocapacitive asymmetric supercapacitor cell was fabricated by combining the best performing 1T-VS2/MXene and MXene, which displayed the highest specific capacitance of 115.7 F/g at a current density of 0.8 A/g in an expanded potential range of 1.6 V. Additionally, the highest energy density achieved was 41.13 W h kg–1 at a maximum power density of 793.50 W kg–1. The asymmetric supercapacitor was able to achieve a high capacitance retention of 85% and a coulombic efficiency of 100% after 5000 galvanostatic charge–discharge cycles. Moreover, the synergistic effect and charge storage kinetics of the 1T-VS2/MXene hybrid pseudocapacitive electrode material were investigated in detail using experimental and density functional theory calculations. Based on the results, we have further demonstrated the usage of 1T-VS2/MXene and MXene as a high-performance energy storage material for supercapacitor application with the dominating intercalation mechanism. The lower diffusion energy barrier for electrolytic ions in the case of hybrid 1T-VS2/MXene supports the higher charge storage. The enhanced density of states and lower diffusion barrier justify the superior charge storage performance for hybrid 1T-VS2/MXene.

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In view of recent environmental concerns, development of sustainable as well as renewable energy sources, such as the production of hydrogen through electro-reduction of water and energy storage devices in particular high power density as well as eco-friendly supercapacitors have become exigency for energy security. Herein, cobalt selenides (CoSe 2 ) and their carbon allotropic heterostructures have been synthesized and explored for bifunctional applications. The designed CoSe 2 /rGO/MWCNT heterostructure with an effective interface construction possessed enhanced HER (η 20 :131 mV, Tafel slope: 52 mV/ dec, stable up to ∼8.5 h) in alkaline pH. Owing to its storage performance, the asymmetric CoSe 2 /rGO/MWCNT//F-MWCNT device also proved to have excellent high capacitance (23.73 F/g at mass normalized current density of 30 A/g), high energy density (44.64 Wh/kg at a power density of 22.32 W/kg @ 6 A/g), and good cycling stability (91.03% capacitance retention after 3000 cycles). The experimental data are also supported by the results from density functional theory simulations in terms of the computed overpotential for HER activity and the quantum capacitance for charge storage performance. The interactions between CoSe 2 and carbon allotropes CNT/rGO are due to charge transfer from the Co 3d orbital of CoSe 2 to the C 2p orbital. Because of this interaction, the density of states shows enhancement near the Fermi level for hybrid structures, which indicates an increase in the conductivity of the material. The theoretical computed overpotential for HER activity follows the trend CoSe 2 > CoSe 2 /MWCNT > CoSe 2 /rGO > CoSe 2 /rGO/ MWCNT, supporting the experimental data. Also, the quantum capacitance is highest for the ternary heterostructure CoSe 2 /rGO/ MWCNT, justifying its superior charge storage performance as obtained from the experimental data. This work provides technological insights into the design of an efficient and cost-effective catalyst for sustainable hydrogen production and flexible energy storage applications.

2-D Ti3C2TX MXene nanosheets are obtained by etching Ti3SiC2 MAX phase that is synthesized by heating the elemental Ti, Si, and C mixture at high temperature. The electron emission behavior of both Ti3C2TX MXene and Ti3SiC2 MAX phase is studied. MXene exhibits excellent field emission characteristics with a turn-on field of 4.7 V μm–1, and that for the Ti3SiC2 MAX phase is 6.5 V μm–1. The turn-on electric field corresponding to an emission current density of 10 μA cm–2 is 5.0 V μm–1 for Ti3C2TX MXene and 7.5 V μm–1 for the Ti3SiC2 MAX phase. The calculated enhancement factor of MXene nanosheets is ∼4280, which is one of the highest reported enhancement factors to date. In order to get theoretical insight into the field emission properties for Ti3C2 and OH-terminated Ti3C2 MXene in comparison to the Ti3SiC2 MAX phase, we have presented the structure and electronic properties from state of the art density functional theory (DFT) simulations. The interaction of – OH with Ti3C2 involves charge transfer from the “Ti” 3d orbital to −OH. The computed work function follows the trend Ti3SiC2 > Ti3C2 > Ti3C2/OH, which supports the maximum field emission in −OH-terminated Ti3C2 MXene and the minimum field emission in the Ti3SiC2 MAX phase.

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