Pseudocapacitors have the potential to achieve high energy and high power density simultaneously, a holy grail for electrochemical energy storage. MXene-based pseudocapacitors have made major progress in the last decade, achieving better energy and power density than carbon supercapacitors using the double-layer charge storage mechanism. However, one obstacle facing pseudocapacitors is their shorter lifetime. In MXene-based pseudocapacitors, which showed up to 500,000 cycles lifetime in aqueous electrolyte at room temperature, this concern is pronounced particularly at high temperatures due to the limited stability of the active material in aqueous solutions. This work shows that Ti 3 C 2 T x MXene electrodes in 5 M H 2 SO 4 possess excellent rate capabilities from -50 °C to 100 °C but also a sufficient lifetime at 70 °C when using a float test holding at -0.9 V vs. Hg/Hg 2 SO 4 . Post-mortem characterization using X-ray photoelectron spectroscopy and Raman spectroscopy showed negligible signs of oxidation in the bulk of the film. This work suggests sufficient stability of Ti 3 C 2 T x MXene as a negative electrode in protic aqueous electrolytes across a wide temperature range rooted in thermodynamics, making it promising for pseudocapacitor energy storage.
The first step to wider adoption of two-dimensional (2D) materials is understanding their fundamental properties by employing characterization methods, among which Raman spectroscopy plays a unique role, being a fast and nondestructive tool. The number, frequencies, and intensities of the modes (or bands) in the Raman spectrum have been used to identify the 2D materials' crystal lattice, bonding, and even number of layers. MXenes, 2D transition metal carbides, nitrides, and carbonitrides, span diverse chemistries and structures, but only a few Raman spectra have been reported. This work is the first systematic experimental Raman spectroscopy study of the MXene family. We explore the vibrational spectra and provide peak assignments for ten MXenes with varying structures (from 2 to 4 atomic layers of transition metal) and compositions─Ti2CTx, Nb2CTx, Mo2CTx, V2CTx, Ti3C2Tx, Mo2TiC2Tx, Ti3CNTx, Nb4C3Tx, V4C3Tx, and Mo2Ti2C3Tx (terminated with −F, −OH, and ═O) based on the experimental results and previously reported computational studies. We discuss the effects of MXene layer thickness, surface terminations, and MXene's metallic properties on Raman scattering. Additionally, we employ polarized Raman spectroscopy to identify out-of-plane vibrations and explain the higher frequency region of the spectra. Finally, we demonstrate how electrochemical reactions affect molecular Raman scattering through the change in surface terminations. By creating the Raman spectra library of the most frequently used MXenes, we open the door for the use of Raman spectroscopy for fingerprinting and in situ studies of various MXenes.
MXenes, a large family of two-dimensional materials, have attracted interest due to their large chemistry space and diverse chemical, electrical, mechanical, and optical properties. MXenes follow the general formula M n+1 X n T x (n = 1-4) with M representing an early transition metal, X—carbon and/or nitrogen, and T—surface terminations (=O, –OH, and –F). In particular, MXenes’ metallic conductivities and redox-active surfaces make them attractive for electrochemical energy storage. Like with many other 2D materials, Raman spectroscopy has proven to be a crucial tool for MXene characterization. More recently, in situ Raman was used to elucidate structural changes in MXene electrodes during electrochemical cycling with a subset of aqueous electrolytes. Confined electrolytes (water-in-salt, PEG, etc.) have shown promise in various electrochemical systems, with recent results pointing to new charge storage mechanisms in MXenes. Further exploration is needed to understand the effect of the cations, anions, and concentrations of confined electrolytes on charge storage mechanisms. This work focuses on using in situ electrochemical Raman spectroscopy to analyze different confined electrolyte systems in MXene cells. The findings demonstrate the potential of using MXenes in aqueous electrochemical devices.
It is highly important to implement various semiconducting, such as n- or p-type, or conducting types of sensing behaviors to maximize the selectivity of gas sensors. To achieve this, researchers so far have utilized the n-p (or p-n) two-phase transition using doping techniques, where the addition of an extra transition phase provides the potential to greatly increase the sensing performance. Here, we report for the first time on an n-p-conductor three-phase transition of gas sensing behavior using Mo2CTx MXene, where the presence of organic intercalants and film thickness play a critical role. We found that 5-nm-thick Mo2CTx films with a tetramethylammonium hydroxide (TMAOH) intercalant displayed a p-type gas sensing response, while the films without the intercalant displayed a clear n-type response. Additionally, Mo2CTx films with thicknesses over 700 nm exhibited a conductor-type response, unlike the thinner films. It is expected that the three-phase transition was possible due to the unique and simultaneous presence of the intrinsic metallic conductivity and the high-density of surface functional groups of the MXene. We demonstrate that the gas response of Mo2CTx films containing tetramethylammonium (TMA) ions toward volatile organic compounds (VOCs), NH3, and NO2 is ∼30 times higher than that of deintercalated films, further showing the influence of intercalants on sensing performance. Also, DFT calculations show that the adsorption energy of NH3 and NO2 on Mo2CTx shifts from -0.973, -1.838 eV to -1.305, -2.750 eV, respectively, after TMA adsorption, demonstrating the influence of TMA in enhancing sensing performance.
MXenes produced by Lewis acid molten salt (LAMS) etching of MAX phases have attracted the community's attention due to their controllable surface chemistry. However, their delamination is challenging due to the hydrophobicity of the produced multilayer MXene and strong interactions between the halogen-terminated MXene sheets. The current delamination method involves dangerous chemicals such as n-butyllithium or sodium hydride, making scale-up difficult and limiting the practical application of this class of MXenes. In this work, we present a simple and efficient method for the delamination of MXenes from the LAMS synthesis while maintaining their surface chemistry. LiCl salt and anhydrous polar organic solvents are used for delamination. Films produced from the delaminated MXene are flexible and have an electrical conductivity of 8000 S/cm, which is maintained after a week of exposure to 95% humidity. This successful delamination, preservation of inherent surface properties, and stability under high-humidity conditions dramatically expand the range of MXene chemistries available for research and potential applications.
MXenes are being heavily investigated in biomedical research, with applications ranging from regenerative medicine to bioelectronics. To enable the adoption and integration of MXenes into therapeutic platforms and devices, however, their stability under standard sterilization procedures must be established. Here, we present a comprehensive investigation of the electrical, chemical, structural, and mechanical effects of common thermal (autoclave) and chemical (ethylene oxide (EtO) and H2O2 gas plasma) sterilization protocols on both thin-film Ti3C2Tx MXene microelectrodes and mesoscale arrays made from Ti3C2Tx-infused cellulose–elastomer composites. We also evaluate the effectiveness of the sterilization processes in eliminating all pathogens from the Ti3C2Tx films and composites. Post-sterilization analysis revealed that autoclave and EtO did not alter the DC conductivity, electrochemical impedance, surface morphology, or crystallographic structure of Ti3C2Tx and were both effective at eliminating E. coli from both types of Ti3C2Tx-based devices. On the other end, exposure to H2O2 gas plasma sterilization for 45 min induced severe degradation of the structure and properties of Ti3C2Tx films and composites. The stability of the Ti3C2Tx after EtO and autoclave sterilization and the complete removal of pathogens establish the viability of both sterilization processes for Ti3C2Tx-based technologies.
Extending applications of Ti3C2Tx MXene in nanocomposites and across fields of electronics, energy storage, energy conversion, and sensor technologies necessitates simple and efficient analytical methods. Raman spectroscopy is a critical tool for assessing MXene composites; however, high laser powers and temperatures can lead to the materials' deterioration during the analysis. Therefore, an in-depth understanding of MXene photothermal degradation and changes in its oxidation state is required, but no systematic studies have been reported. The primary aim of this study was to investigate the degradation of the MXene lattice through Raman spectroscopic analysis. Distinct spectral markers were related to structural alterations within the Ti3C2Tx material after subjecting it to thermal- and laser-induced degradation. During the degradation processes, spectral markers were revealed for several specific steps: a decrease in the number of interlayer water molecules, a decrease in the number of −OH groups, formation of C–C bonds, oxidation of the lattice, and formation of TiO2 nanoparticles (first anatase, followed by rutile). By tracking of position shifts and intensity changes for Ti3C2Tx, the spectral markers that signify the initiation of each step were found. This spectroscopic approach enhances our understanding of the degradation pathways of MXene, and facilitating enhanced and dependable integration of these materials into devices for diverse applications, from energy storage to sensors.
Polyvinylidene fluoride (PVDF) is a semicrystalline polymer used in thin-film dielectric capacitors because of its inherently high dielectric constant and low loss tangent. Its dielectric constant can be increased by the formation and alignment of its β-phase crystalline structure, which can be facilitated by 2D nanofillers. 2D carbides and nitrides, MXenes, are promising candidates due to their notable dielectric permittivity and ability to increase interfacial polarization. Still, their mixing is challenging due to weak interfacial interactions and poor dispersibility of MXenes in PVDF. This work explores a novel method for delaminating Ti3C2Tx MXene directly into organic solvents while maintaining flake size and quality, as well as the use of a non-solvent-induced phase separation method for producing both dense and porous PVDF-MXene composite films. A deeper understanding of dielectric behavior in these composites is reached by examining MXenes with both mixed and pure chlorine terminations in PVDF matrices. Thin-film capacitors fabricated from these composites display ultrahigh discharge energy density, exceeding 45 J cm-3 with 95% efficiency. The PVDF-MXene composites are also processed using a green and sustainable solvent, propylene carbonate.
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.
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