Further improvements to lithium-ion and emerging battery technologies can be enabled by an improved understanding of the chemistry and working mechanisms of interphases that form at electrochemically active battery interfaces. However, it is difficult to collect and interpret spectra of interphases for several reasons, including the presence of a variety of compounds. To address this challenge, we herein present a vibrational spectroscopy and X-ray diffraction data library of ten compounds that have been identified as interphase constituents in lithium-ion or emerging battery chemistries. The data library includes attenuated total reflectance Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction data, collected in inert atmospheres provided by custom sample chambers. The data library presented in this work (and online repository) simplifies access to reference data that is otherwise either diffusely spread throughout the literature or non-existent, and provides energy storage researchers streamlined access to vital interphase-relevant data that can accelerate battery research efforts.

Li-metal batteries performance limitations are associated with physico-chemical processes at electrode/electrolyte interfaces, which lead to high impedance, electrochemical instability, and inhomogeneous Li plating and stripping. In fact, battery performance is largely determined by thermodynamic, kinetic, and mechanical properties of such electrochemical interfaces and interphases. These electrochemical systems tend to operate far away from equilibrium. A thin passive film, the so-called solid electrolyte interphase (SEI) layer, that forms at the electrode/electrolyte interface during battery assembly/formation, which gradually reforms during its operation is critical for its basic function and lifetime. In this work, we overview and exploit the nanoscale spatial resolution, chemical selectivity, and surface sensitivity of near-field infrared nanospectroscopy to characterize electrode/liquid and solid electrolyte interfaces. Near-field infrared measurements in combination with standard surface characterization tools reveal that intrinsic molecular, structural, and chemical heterogeneities at the interface. This work provides a unique insight into the mechanisms of early-stage interphase formation at electrochemically active buried interfaces, and an experimental diagnostic means to aid in the development of methods to control local nanoscale variations in electrolyte chemistry, structure, and ionic conductivity at the surface of the electrode. We have carried out ex situ and in situ near-field IR spectroscopy studies of local chemical composition and composition and structure at nanometer resolution of the SEI layer formed between Li metal and GEN2 or DME-TTE: LiFSI locally highly concentrated electrolyte. We show nanometer-scale inhomogeneities in Li metal SEI formed upon immediate contact with electrolyte and we discuss their implications for the electrode passivity. Acknowledgements. This research was supported by the US Department of Energy (DOE)’s Vehicle Technologies Office under the Battery Materials Research program directed by Tien Duong. The work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract no. DE-AC02-05CH11231. This research also used resources of the Advanced Light Source, a U.S. DOE Office of Science User Facility under contract no. DE-AC02-05CH11231; in particular, Beamline 2.4 was used. Figure 1

Electrochemical conversion of CO2 to CO is demonstrated with symmetric-structured metal supported solid oxide cells (MS-SOC). Perovskite Pr0.5Sr0.4Mn0.2Fe0.8O3−δ (PSMF) and Pr6O11 catalysts were infiltrated into the MS-SOC cathode and anode, using 3 cycles with firing at 850 °C and 8 cycles with firing at 800 °C, respectively. Upon reduction during operation, the perovskite PSMF was transformed to Ruddlesden–Popper structure with a highly efficient electrocatalytic activity. The impact of operating temperature (600–800 °C) and overpotential (0–1.8 V) on the CO2 conversion was investigated. The highest CO2 conversion of 57.2% was achieved at 750 °C and 1.8 V. During extended operation for 150 h at 750 °C and 1.2 V, a cell demonstrated relatively stable performance, with initial current density of 535 mA cm−2 and CO2 conversion of 23%. Degradation mechanisms were studied by posttest characterization.

Further technological advancement of both lithium-ion and emerging battery technologies can be catalyzed by an improved understanding of the chemistry and working mechanisms of the solid electrolyte interphases (SEIs) that form at electrochemically active battery interfaces. However, collecting and interpreting spectroscopy results of SEIs is difficult for several reasons, including the chemically diverse composition of SEIs. To address this challenge, we herein present a vibrational spectroscopy and X-ray diffraction data library of ten suggested SEI chemical constituents relevant to both lithium-ion and emerging battery chemistries. The data library includes attenuated total reflectance Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction data, collected in inert atmospheres afforded by custom designed sample holders. The data library presented in this work (and online repository) alleviates challenges with locating related work that is either diffusely spread throughout the literature, or is non-existent, and provides energy storage researchers streamlined access to vital SEI-relevant data that can catalyse future battery research efforts.

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.

Electrochemical oxidative coupling of methane (E-OCM) with Sr2Fe1.5Mo0.5O6-5 (SFM) catalyst is demonstrated with metal-supported solid oxide cells (MS-SOC). SFM anode and Pr6O11 cathode catalysts are loaded into a porous symmetric-architecture cell by infiltration and firing. The most effective chelating agent (citric acid/ethyl glycol) and optimal firing/ reducing temperatures (850 degrees C/750 degrees C) for the SFM catalyst precursor solution are selected and confirmed by cell testing. Operating temperature, cell voltage, and oxygen concentration at the cathode greatly affect the methane conversion rate and product selectivity by controlling the oxygen ion flow. CH4 conversion of 85.8% is obtained, with C2H4, C2H6, and H2 concentrations of 10.5%, 12.3%, and 25.6% at 800 degrees C, respectively, in the product exhaust gas. Reasonable stability of the current density and methane conversion is demonstrated during 200 h operation. This research demonstrates technical progress in catalyst and device development for the E-OCM reaction to synthesize valuable chemicals.(c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

MXenes are among the most widely researched materials due to a unique combination of high electronic conductivity and hydrophilic surface, confined in a 2D structure. Therefore, comprehensive characterization of individual MXene flakes is of great importance. Here we report on nanoscale Raman imaging of single-layer and few-layer flakes of Ti3C2Tx MXene deposited on a gold substrate using tip-enhanced Raman scattering (TERS). TERS spectra of MXene monolayers are dominated by an intense peak at around 201 cm-1 and two well-defined peaks at around 126 and 725 cm-1. Absolute intensities of these peaks decrease with increasing number of layers, though the relative intensity of the 126 and 725 cm-1 bands as compared to the 201 cm-1 band increases. The peak positions of the main MXene bands do not significantly change in flakes of different number of layers, suggesting weak coupling between the MXene layers. In addition, we observed stiffening of the 201 cm-1 vibration over the wrinkles in MXene flakes. Using TERS for nanoscale spectroscopic characterization of Ti3C2Tx allows fast Raman mapping with deep subdiffraction resolution at the laser power density on the sample about an order of magnitude lower as compared to confocal Raman measurements. Finally, we demonstrate very high environmental stability of stoichiometric single-layer MXenes and show that the intensity of TERS response from the single- and few-layer flakes of Ti3C2Tx can be used to track early stages of degradation, well before significant morphological changes appear.

Transitional metal carbides and nitrides (MXenes) have promise for incorporation into multifunctional composites due to their high electrical conductivity and excellent mechanical and tribological properties. It is unclear, however, to what extent MXenes are also able to improve the mechanical properties of the composites and, if so, what would be the optimal flake size and morphology. Herein, Ti3C2Tx MXene is demonstrated to be indeed a good candidate for mechanical reinforcement in polymer matrices. In the present work, the strain-induced Raman band shifts of mono-/few-/multilayer MXenes flakes have been used to study the mechanical properties of MXene and the interlayer/interfacial stress transfer on a polymer substrate. The mechanical performance of MXene was found to be less dependent upon flake thickness compared to that of graphene. This enables Ti3C2Tx MXene to offer an efficient mechanical reinforcement to a polymer matrix with a flake length of >10 μm and a thickness of 10s of nanometers. Therefore, the degree of exfoliation of MXenes is not as demanding as other two-dimensional (2D) materials for the purpose of mechanical enhancement in polymers. In addition, the active surface chemistry of MXene facilitates possible functionalization to enable a stronger interface with polymers for applications, such as strain engineering and mechanical enhancement, and in materials including membranes, coatings, and textiles.

This initial report demonstrates the family of two-dimensional transition-metal carbides and carbonitrides (MXenes) as promising plasmonic materials for surface-enhanced Raman spectroscopy (SERS), allowing sensitive detection of analyte molecules. Their surface plasmon resonances, metallic characters, and rich surface chemistries enable both SERS electromagnetic and chemical enhancements. We have synthesized seven MXenes—Nb2C, Mo2C, Ti2C, V2C, Ti3C2, Mo2TiC2, and Ti3CN, of which only Ti3C2 and Nb2C have been explored as pristine substrates for SERS applications. All explored MXenes could detect the 10–7 M concentrations of Rhodamine 6G dye. Ti3C2 and Ti2C show the highest enhancement and were compared to the commercially available substrates, where Ti3C2 outperformed gold nanoparticles. Additionally, the best-performing MXene was tested with a variety of dyes, suggesting the versatility of MXene sensing. Due to high charge carrier densities and tunable surface chemistries, MXenes show potential for SERS sensors that are inexpensive, easy to fabricate and optimize for specific applications. Surface-enhanced Raman spectroscopy is a powerful, non-destructive analytical tool that provides high selectivity and sensitivity and could be used for single-molecule characterization. It allows for fast detection of environmental pollutants and toxins, as well as biosensing and environmental monitoring. However, the prevalence of the application is limited by the cost and stability of commercially available substrates that employ noble metal nanostructures (Ag, Au, Pt). Recent research activities have focused on discovering new enhancing substrates: the Ti3C2 MXene has been gaining interest as a SERS substrate due to its surface plasmon resonance, metallic behavior, and surface terminations, achieving the record-setting enhancement factors. In contrast, this study elucidates the capability of MXenes beyond Ti3C2 to perform as SERS substrates. We showed that Nb2C, Mo2C, Ti2C, V2C, Ti3C2, Mo2TiC2, and Ti3CN MXenes could detect low concentrations of dye molecules. Furthermore, compared to the commercial gold nanoparticle substrate, Ti3C2 showed advantageous performance. There are approximately 50 MXenes that have been synthesized to date. Their high charge carrier densities and tunable surface chemistries open the avenues of SERS detection of various analytes, with the perspective of replacing noble metals with materials made of earth-abundant elements. This would make SERS more accessible for biomedical, environmental, and forensic applications.

One of the primary factors limiting further research and the commercial use of the two-dimensional (2D) MXene titanium carbide (Ti3C2), as well as MXenes in general, is the rate at which freshly made samples oxidize and degrade when stored as aqueous suspensions. Here, we show that including excess aluminum during synthesis of the Ti3AlC2 MAX phase precursor leads to the creation of Ti3AlC2 grains with improved stoichiometry and crystallinity. Ti3C2 nanosheets produced from the improved Ti3AlC2 are of higher quality, as evidenced by their increased resistance to oxidation and an increase in their electrical conductivity to 20,000 S/cm. Our results indicate that defects created during the synthesis of Ti3C2 (and by inference, other MXenes) lead to the previously observed instability. We show that by eliminating those defects results in Ti3C2 that is highly stable in aqueous solutions and in air. Aqueous suspensions of single- to few-layer Ti3C2 flakes produced from the modified Ti3AlC2 have a shelf life of over ten months, compared to one to two weeks for Ti3C2 produced from conventional Ti3AlC2, even when stored in ambient conditions. Freestanding films made from Ti3C2 suspensions stored for ten months show minimal decreases in electrical conductivity and negligible oxidation. Oxidation of the improved Ti3C2 in air initiates at temperatures that are 100-150°C higher than conventional Ti3C2. The observed improvements in both the shelf life and properties of Ti3C2 will facilitate the widespread use of this material.

Alloying is a long-established strategy to tailor properties of metals for specific applications, thus retaining or enhancing the principal elemental characteristics while offering additional functionality from the added elements. We propose a similar approach to the control of properties of two-dimensional transition metal carbides known as MXenes. MXenes (Mn+1Xn) have two sites for compositional variation: elemental substitution on both the metal (M) and carbon/nitrogen (X) sites presents promising routes for tailoring the chemical, optical, electronic, or mechanical properties of MXenes. Herein, we systematically investigated three interrelated binary solid-solution MXene systems based on Ti, Nb, and/or V at the M-site in a M2XTx structure (Ti2-yNbyCTx, Ti2-yVyCTx, and V2-yNbyCTx, where Tx stands for surface terminations) showing the evolution of electronic and optical properties as a function of composition. All three MXene systems show unlimited solubility and random distribution of metal elements in the metal sublattice. Optically, the MXene systems are tailorable in a nonlinear fashion, with absorption peaks from ultraviolet to near-infrared wavelength. The macroscopic electrical conductivity of solid solution MXenes can be controllably varied over 3 orders of magnitude at room temperature and 6 orders of magnitude from 10 to 300 K. This work greatly increases the number of nonstoichiometric MXenes reported to date and opens avenues for controlling physical properties of different MXenes with a limitless number of compositions possible through M-site solid solutions.

Scaling the production of synthetic 2D materials to industrial quantities has faced significant challenges due to synthesis bottlenecks whereby few have been produced in large volumes. These challenges typically stem from bottom-up approaches limiting the production to the substrate size or precursor availability for chemical synthesis and/or exfoliation. In contrast, MXenes, a large class of 2D transition metal carbides and/or nitrides, are produced via a top-down synthesis approach. The selective wet etching process does not have similar synthesis constraints as some other 2D materials. The reaction occurs in the whole volume; therefore, the process can be readily scaled with reactor volume. Herein, the synthesis of 2D titanium carbide MXene (Ti3C2Tx) is studied in two batch sizes, 1 and 50 g, to determine if large-volume synthesis affects the resultant structure or composition of MXene flakes. Characterization of the morphology and properties of the produced MXene using scanning electron microscopy, X-ray diffraction, dynamic light scattering, Raman spectroscopy, X-ray photoelectron spectroscopy, UV-visible spectroscopy, and conductivity measurements show that the materials produced in both batch sizes are essentially identical. This illustrates that MXenes experience no change in structure or properties when scaling synthesis, making them viable for further scale-up and commercialization.

The MXene field continues to grow and expand as more research groups begin to study this fascinating and very large class of 2D materials. While synthesis and applications of MXenes have been widely discussed in literature, characterization is often overlooked. Due to the large variety of MXene structures and compositions, it is often necessary to use multiple advanced characterization techniques within a single study, and each characterization technique has its own quirks, pitfalls, and benefits when applied to MXenes. This review focuses on the utilization of X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, electron microscopy/spectroscopy and a number of other techniques to understand if the precursor (MAX phase) is suitable for MXene synthesis, confirm successful synthesis of MXene, and finally determine its composition, structure and properties. Researchers should look to this review article as a guide to help them understand which techniques to use for characterization of their MXene samples and leverage the best characterization practices developed to date.

Multifunctional fiber-reinforced composites play a significant role in advanced aerospace and military applications due to their high strength and toughness resulting in superior damage tolerance. However, early detection of structural changes prior to visible damage is critical for extending the lifetime of the part. MXenes, an emerging class of two-dimensional (2D) nanomaterials, possess hydrophilic surfaces, high electrical conductivity and mechanical properties that can potentially be used to identify damage within fiber-reinforced composites. In this work, conductive Ti3C2Tx MXene flakes were successfully transferred onto insulating glass fibers via oxygen plasma treatment improving adhesion. Increasing plasma treatment power, time and coating layers lead to a decrease in electrical resistance of MXene-coated fibers. Optimized uniformity was achieved using an alternating coating approach with smaller flakes helping initiate and facilitate adhesion of larger flakes. Tensile testing with in-situ electrical resistance tracking showed resistances as low as 1.8 kΩ for small-large flake-coated fiber bundles before the break. Increased resistance was observed during testing, but due to good adhesion between the fiber and MXene, most connective pathways within fiber bundles remained intact until fiber bundles were completely separated. These results demonstrate a potential use of MXene-coated glass fibers in damage-sensing polymer-matrix composites.

Two-dimensional transition metal carbides, nitrides and carbonitrides, popular by the name MXenes, are a promising class of materials as they exhibit intriguing optical, optoelectronic and electrochemical properties. Taking advantage of their metallic conductivity and hydrophilicity, titanium carbide MXenes (Ti3C2Tx and others) are used to fabricate solution processable transparent conducting electrodes (TCEs) for the design of three-electrode electrochromic cells. However, the tunable electrochromic behavior of various titanium-based MXene compositions across the entire visible spectrum has not yet been demonstrated. Here, we investigate the intrinsic electrochromic properties of titanium-based MXenes, Ti3C2Tx, Ti3CNTx, Ti2CTx, and Ti1.6Nb0.4CTx, where individual MXenes serve as a transparent conducting, electrochromic, and plasmonic material layer. Plasmonic extinction bands for Ti3C2Tx, Ti2CTx and Ti1.6Nb0.4CTx are centered at 800, 550 and 480 nm, which are electrochemically tunable to 630, 470 and 410 nm, respectively, whereas Ti3CNTx shows a reversible change in transmittance in the wide visible range. Additionally, the switching rates of MXene electrodes with no additional transparent conductor electrodes are estimated and correlated with the respective electrical figure of merit values. This study demonstrates that MXene-based electrochromic cells are tunable in the entire visible spectrum and suggests the potential of the MXene family of materials in optoelectronic, plasmonic, and photonic applications, such as tunable visible optical filters and modulators, to name a few.

MXenes are a family of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides with a general formula of Mn+1XnTx, in which two, three, or four atomic layers of a transition metal (M: Ti, Nb, V, Cr, Mo, Ta, etc.) are interleaved with layers of C and/or N (shown as X), and Tx represents surface termination groups such as -OH, ═O, and -F. Here, we report the scalable synthesis and characterization of a MXene with five atomic layers of transition metals (Mo4VC4Tx), by synthesizing its Mo4VAlC4 MAX phase precursor that contains no other MAX phase impurities. These phases display twinning at their central M layers which is not present in any other known MAX phases or MXenes. Transmission electron microscopy and X-ray diffraction were used to examine the structure of both phases. Energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and high-resolution scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy were used to study the composition of these materials. Density functional theory calculations indicate that other five transition metal-layer MAX phases (M'4M″AlC4) may be possible, where M' and M″ are two different transition metals. The predicted existence of additional Al-containing MAX phases suggests that more M5C4Tx MXenes can be synthesized. Additionally, we characterized the optical, electronic, and thermal properties of Mo4VC4Tx. This study demonstrates the existence of an additional subfamily of M5X4Tx MXenes as well as a twinned structure, allowing for a wider range of 2D structures and compositions for more control over properties, which could lead to many different applications.

MXene/carbon composite electrodes with high loadings of MXene were prepared via electrospinning. These flexible and free-standing electrodes exhibit high areal capacitance relative to pure carbon nanofibers and MXene-coated fibers and textiles.

AbstractMXenes are a class of two‐dimensional (2D) transition metal carbides, nitrides and carbonitrides that have shown promise for high‐rate pseudocapacitive energy storage. However, the effects that irreversible oxidation have on the surface chemistry and electrochemical properties of MXenes are still not understood. Here we report on a controlled anodic oxidation method which improves the rate performance of titanium carbide MXene (Ti3C2Tx, Tx refers to ‐F, =O, ‐Cl and ‐OH) electrodes in acidic electrolytes. The capacitance retention at 2000 mV s−1 (with respect to the lowest scan rate of 5 mV s−1) increases gradually from 38 % to 66 % by tuning the degree of anodic oxidation. At the same time, a loss in the redox behavior of Ti3C2Tx is evident at high anodic potentials after oxidation. Several analysis methods are employed to reveal changes in the structure and surface chemistry while simultaneously introducing defects, without compromising electrochemically active sites, are key factors for improving the rate performance of Ti3C2Tx. This study demonstrates improvement of the electrochemical performance of MXene electrodes by performing a controlled anodic oxidation.

Pseudocapacitive energy storage in supercapacitor electrodes differs significantly from the electrical double-layer mechanism of porous carbon materials, which requires a change from conventional thinking when choosing appropriate electrolytes. Here we show how simply changing the solvent of an electrolyte system can drastically influence the pseudocapacitive charge storage of the two-dimensional titanium carbide, Ti3C2 (a representative member of the MXene family). Measurements of the charge stored by Ti3C2 in lithium-containing electrolytes with nitrile-, carbonate- and sulfoxide-based solvents show that the use of a carbonate solvent doubles the charge stored by Ti3C2 when compared with the other solvent systems. We find that the chemical nature of the electrolyte solvent has a profound effect on the arrangement of molecules/ions in Ti3C2, which correlates directly to the total charge being stored. Having nearly completely desolvated lithium ions in Ti3C2 for the carbonate-based electrolyte leads to high volumetric capacitance at high charge–discharge rates, demonstrating the importance of considering all aspects of an electrochemical system during development. Effects from electrolytes on supercapacitor electrodes, especially pseudocapacitive materials, are important but often overlooked. Gogotsi and colleagues demonstrate strong influences from electrolyte solvents on charge-storage processes in a titanium carbide and identify a best-performing electrode/electrolyte couple for supercapacitors.

With the development of the Internet of Things (IoT), the demand for thin and wearable electronic devices is growing quickly. The essential part of the IoT is communication between devices, which requires radio-frequency (RF) antennas. Metals are widely used for antennas; however, their bulkiness limits the fabrication of thin, lightweight, and flexible antennas. Recently, nanomaterials such as graphene, carbon nanotubes, and conductive polymers came into play. However, poor conductivity limits their use. We show RF devices for wireless communication based on metallic two-dimensional (2D) titanium carbide (MXene) prepared by a single-step spray coating. We fabricated a ~100-nm-thick translucent MXene antenna with a reflection coefficient of less than -10 dB. By increasing the antenna thickness to 8 μm, we achieved a reflection coefficient of -65 dB. We also fabricated a 1-μm-thick MXene RF identification device tag reaching a reading distance of 8 m at 860 MHz. Our finding shows that 2D titanium carbide MXene operates below the skin depth of copper or other metals as well as offers an opportunity to produce transparent antennas. Being the most conductive, as well as water-dispersible, among solution-processed 2D materials, MXenes open new avenues for manufacturing various classes of RF and other portable, flexible, and wearable electronic devices.