Synthesis of 2D Solid-Solution (Nb_<i>y</i>V_2–<i>y</i>)CT_<i>x</i> MXenes and Their Transformation into Oxides for Energy Storage

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

Nickel Oxide Reduction by Hydrogen: Kinetics and Structural Transformations

We studied the reduction kinetics of bulk NiO crystals by hydrogen and the corresponding structural transformations in the temperature range of 543–1593 K. A new experimental approach allows us to arrest and quench the reaction at different stages with millisecond time resolution. Two distinctive temperature intervals are found where the reaction kinetics and product microstructures are different. At relatively low temperatures, 543–773 K, the kinetic curves have a sigmoidal shape with long induction times (up to 2000 s) and result in incomplete conversion. Low-temperature reduction forms a complex polycrystalline Ni/NiO porous structure with characteristic pore size on the order of 100 nm. No induction period was observed for the high-temperature conditions (1173–1593 K), and full reduction of NiO to Ni is achieved within seconds. An extremely fine porous metal structure, with pore size under 10 nm, forms during high-temperature reduction by a novel crystal growth mechanism. This consists of the epitaxial-like transformation of micrometer-sized NiO single crystals into single-crystalline Ni without any crystallographic changes, including shape, size, or crystal orientation. The Avrami nucleation model accurately describes the reaction kinetics in both temperature regimes. However, the structural transformations during reduction in both nanolevel and atomic level are very complex, and the mechanism relies on both nucleation and the critical diffusion length for outward diffusion of water molecules

Pristine Ti₃C₂T_<i>x</i> MXene Enables Flexible and Transparent Electrochemical Sensors

In the era of the internet of things, there exists a pressing need for technologies that meet the stringent demands of wearable, self-powered, and seamlessly integrated devices. Current approaches to developing MXene-based electrochemical sensors involve either rigid or opaque components, limiting their use in niche applications. This study investigates the potential of pristine Ti3C2Tx electrodes for flexible and transparent electrochemical sensing, achieved through an exploration of how material characteristics (flake size, flake orientation, film geometry, and uniformity) impact the electrochemical activity of the outer sphere redox probe ruthenium hexamine using cyclic voltammetry. The optimized electrode made of stacked large Ti3C2Tx flakes demonstrated excellent reproducibility and resistance to bending conditions, suggesting their use for reliable, robust, and flexible sensors. Reducing electrode thickness resulted in an amplified faradaic-to-capacitance signal, which is advantageous for this application. This led to the deposition of transparent thin Ti3C2Tx films, which maintained their best performance up to 73% transparency. These findings underscore its promise for high-performance, tailored sensors, marking a significant stride in advancing MXene utilization in next-generation electrochemical sensing technologies. The results encourage the analytical electrochemistry field to take advantage of the unique properties that pristine Ti3C2Tx electrodes can provide in sensing through more parametric studies