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Vanadium and niobium oxides have been identified as promising electrodes for electrochemical energy storage applications as their constituent transition metals can undergo multiple reduction steps leading to high specific capacities during cycling. MXenes are attractive precursors for these compounds due to their tunable compositions and 2D nanoscale morphology. Herein, we demonstrate the synthesis of a wide range of solid-solution (NbyV2–y)-AlC MAX phases, their chemical etching to produce (NbyV2–y)-CTx MXenes, and the subsequent oxidation of MXenes to form respective oxides. We show that the formation of solid solutions facilitated the etching kinetics of MAX phase powder and accelerated MXene formation compared to pure vanadium and niobium carbides. Oxidation of V2CTx and Nb2CTx produced bilayered vanadium oxide (BVO) with a crumpled nanosheet morphology and nanostructured amorphous Nb2O5 (nANO) nanospheres, respectively. For oxides derived from solid-solution MXenes, scanning electron microscopy imaging revealed the growth of nANO on the surface of BVO nanosheets. Electrochemical cycling of (NbyV2–y)-CTx-derived oxides in Li-ion cells revealed varying intercalation-like behavior with electrodes derived from V2CTx showing redox processes and nANO exhibiting pseudocapacitive response. The CV curves of solid-solution MXene-derived oxides demonstrated primarily BVO/nANO composite-like behavior, with key exceptions. The cells containing Nb0.25V1.75CTx-derived oxide showed a large capacity of 296.8 mA h g–1 driven by significant electrochemical activity at all potentials along the sweep possibly stemming from niobium doping into BVO structure. The Nb1.00V1.00CTx-derived oxide electrode delivered a specific capacity of 298 mA h g–1 with contributions from both, BVO and nANO phases. The improved electrochemical stability of (Nb1.00V1.00)-CTx-derived oxide electrodes compared to an electrode prepared by physically mixing Nb2CTx-derived oxide with V2CTx-derived oxide with the same Nb/V molar ratio was attributed to the stabilizing effect of the BVO/nANO heterointerface. Our work indicates that the use of solid-solution MXenes as precursors is an attractive strategy to synthesize oxides with compositions, morphologies, and properties that cannot be produced otherwise. [ABSTRACT FROM AUTHOR]

MXenes offer high metallic conductivity and redox capacitance that are attractive for high‐power, high‐energy storage devices. However, they operate limitedly under high anodic potentials due to irreversible oxidation. Pairing them with oxides to design asymmetric supercapacitors may expand the voltage window and increase the energy storage capabilities. Hydrated lithium preintercalated bilayered V2O5 (δ‐LixV2O5·nH2O) is attractive for aqueous energy storage due to its high Li capacity at high potentials; however, its poor cyclability remains a challenge. To overcome its limitations and achieve a wide voltage window and excellent cyclability, it is combined with V2C and Nb4C3 MXenes. Asymmetric supercapacitors employing lithium intercalated V2C (Li‐V2C) or tetramethylammonium intercalated Nb4C3 (TMA‐Nb4C3) MXenes as the negative electrode, and a δ‐LixV2O5·nH2O composite with carbon nanotubes as the positive electrode in 5 m LiCl electrolyte operate over wide voltage windows of 2 and 1.6 V, respectively. The latter shows remarkably high cyclability—capacitance retention of ≈95% after 10 000 cycles. This work highlights the importance of selecting appropriate MXenes to achieve a wide voltage window and a long cycle life in combination with oxide anodes to demonstrate the potential of MXenes beyond Ti3C2 in energy storage. [ABSTRACT FROM AUTHOR]

While transition‐metal oxides such as α‐MoO3 provide high capacity, their use is limited by modest electronic conductivity and electrochemical instability in aqueous electrolytes. Two‐dimensional (2D) MXenes, offer metallic conductivity, but their capacitance is limited in aqueous electrolytes. Insertion of partially solvated cations into Ti3C2 MXene from lithium‐based water‐in‐salt (WIS) electrolytes enables charge storage at positive potentials, allowing a wider potential window and higher capacitance. Herein, we demonstrate that α‐MoO3/Ti3C2 hybrids combine the high capacity of α‐MoO3 and conductivity of Ti3C2 in WIS (19.8 m LiCl) electrolyte in a wide 1.8 V voltage window. Cyclic voltammograms reveal multiple redox peaks from α‐MoO3 in addition to the well‐separated peaks of Ti3C2 in the hybrid electrode. This leads to a higher specific charge and a higher rate capability compared to a carbon and binder containing α‐MoO3 electrode. These results demonstrate that the addition of MXene to less conductive oxides eliminates the need for conductive carbon additives and binders, leads to a larger amount of charge stored, and increases redox capacity at higher rates. In addition, MXene encapsulated α‐MoO3 showed improved electrochemical stability, which was attributed to the suppressed dissolution of α‐MoO3. The work suggests that oxide/MXene hybrids are promising for energy storage. [ABSTRACT FROM AUTHOR]

Chemically preintercalated dopamine (DOPA) molecules were used as both a reducing agent and a carbon precursor to prepare δ‐V2O5·nH2O/C, H2V3O8/C, VO2(B)/C, and V2O3/C nanocomposites via hydrothermal treatment or hydrothermal treatment followed by annealing under Ar flow. We found that the phase composition and morphology of the produced composites are influenced by the DOPA:V2O5 ratio used to synthesize (DOPA)xV2O5 precursors through DOPA diffusion into the interlayer region of the δ‐V2O5·nH2O framework. The increase of DOPA concentration in the reaction mixture led to a more pronounced reduction of vanadium and a higher fraction of carbon in the composites' structure, as evidenced by X‐ray photoelectron spectroscopy and Raman spectroscopy measurements. The electrochemical charge storage properties of the synthesized nanocomposites were evaluated in Li‐ion cells with nonaqueous electrolytes. δ‐V2O5·nH2O/C, H2V3O8/C, VO2(B)/C, and V2O3/C electrodes delivered high initial capacities of 214, 252, 279, and 637 mAh g–1, respectively. The insights provided by this investigation open up the possibility of creating new nanocomposite oxide/carbon electrodes for a variety of applications, such as energy storage, sensing, and electrochromic devices. [ABSTRACT FROM AUTHOR]