Surface Modification of a MXene by an Aminosilane Coupling Agent

MXenes, two-dimensional (2D) transition metal carbides and/or nitrides, possess surface termination groups such as hydroxyl, oxygen, and fluorine, which are available for surface functionalization. Their surface chemistry is critical in many applications. This article reports amine functionalization of Ti3C2Tx MXene surface with [3-(2-aminoethylamino)-propyl]trimethoxysilane (AEAPTMS). Characterization techniques such as X-ray photoelectron spectroscopy verify the success of the surface functionalization and confirm that the silane coupling agent bonds to Ti3C2Tx surface both physically and chemically. The functionalization changes the MXene surface charge from −35 to +25 mV at neutral pH, which allows for in situ preparation of self-assembled films. Further, surface charge measurements of the functionalized MXene at different pH values show that the functionalized MXene has an isoelectric point at a pH around 10.7, and the highest reported positive surface charge of +62 mV at a pH of 2.58. Furthermore, the existence of a mixture of different orientations of AEAPTMS and the simultaneous presence of protonated and free amine groups on the surface of Ti3C2Tx are demonstrated. The availability of free amine groups on the surface potentially permits the fabrication of crosslinked electrically conductive MXene/epoxy composites, dye adsorbents, high-performance membranes, and drug carriers. Surface modifications of this type are applicable to many other MXenes

Distinguishing electronic contributions of surface and sub-surface transition metal atoms in Ti-based MXenes

MXenes are a rapidly-expanding family of 2D transition metal carbides and nitrides that have attracted attention due to their excellent performance in applications ranging from energy storage to electromagnetic interference shielding. Numerous other electronic and magnetic properties have been computationally predicted, but not yet realized due to the experimental difficulty in obtaining uniform surface terminations (Tx), necessitating new design approaches for MXenes that are independent of surface terminations. In this study, we distinguished the contributions of surface and sub-surface Ti atoms to the electronic structure of four Ti-containing MXenes (Ti2CTx, Ti3C2Tx, Cr2TiC2Tx, and Mo2TiC2Tx) using soft x-ray absorption spectroscopy. For MXenes with no Ti atoms on the surface transition metal layers, such as Mo2TiC2Tx and Cr2TiC2Tx, our results show minimal changes in the spectral features between the parent MAX phase and its MXene. In contrast, for MXenes with surface Ti atoms, here Ti3C2Tx and Ti2CTx, the Ti L-edge spectra are significantly modified compared to their parent MAX phase compounds. First principles calculations provide similar trends in the partial density of states derived from surface and sub-surface Ti atoms, corroborating the spectroscopic measurements. These results reveal that electronic states derived from sub-surface M-site layers are largely unperturbed by the surface terminations, indicating a relatively short length scale over which the Tx terminations alter the nominal electron count associated with Ti atoms and suggesting that desired band features should be hosted by sub-surface M-sites that are electronically more robust than their surface M-site counterparts

Evidence of a magnetic transition in atomically thin Cr2TiC2Tx MXene

Two-dimensional (2D) transition metal carbides and nitrides known as MXenes have shown attractive functionalities such as high electronic conductivity, a wide range of optical properties, versatile transition metal and surface chemistry, and solution processability. Although extensively studied computationally, the magnetic properties of this large family of 2D materials await experimental exploration. 2D magnetic materials have recently attracted significant interest as model systems to understand low-dimensional magnetism and for potential spintronic applications. Here, we report on synthesis of Cr2TiC2Tx MXene and a detailed study of its magnetic as well as electronic properties. Using a combination of magnetometry, synchrotron X-ray linear dichroism, and field- and angular-dependent magnetoresistance measurements, we find clear evidence of a magnetic transition in Cr2TiC2Tx at approximately 30 K, which is not present in its bulk layered carbide counterpart (Cr2TiAlC2 MAX phase). This work presents the first experimental evidence of a magnetic transition in a MXene material and provides an exciting opportunity to explore magnetism in this large family of 2D materials

Sustainable Reuse and Recycling of Spent Li‐Ion batteries from Electric Vehicles: Chemical, Environmental, and Economical Perspectives

Abstract The rapidly increasing adoption of electric vehicles (EVs) worldwide is causing high demand for production of lithium‐ion batteries (LIBs). Tremendous efforts have been made to develop different components of LIBs in addition to design of battery pack architectures as well as manufacturing processes to make better batteries with affordable prices. Nonetheless, sustainable use of LIBs relies on the availability and cost of rare metals, which are naturally concentrated in a few countries. In addition, toxic electrolytes used in LIBs pose concerns on environmental impacts if LIBs are not handled properly after decommissioned from EVs. Therefore, it is paramount to realize effective utilization of spent LIBs, where their remaining capacities can be reused in less demanding applications. Finally, electrode materials and other valuable components of LIBs can be recovered via recycling, completing their circular life cycle. In this review, available options of LIBs after their retirement from EV applications, including battery second use, repair of electrode materials by direct regeneration, and material recovery by hydrometallurgical or pyrometallurgical processes are discussed. Throughout the review, the discussion is based around current available technologies, their environmental impacts, and economic feasibility as well as provided examples of pilot and industrial scale adoption of the processes

Modified MAX Phase Synthesis for Environmentally Stable and Highly Conductive Ti3C2 MXene

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. <br /

Titanium carbide MXene shows an electrochemical anomaly in water-in-salt electrolytes

Identifying and understanding charge storage mechanisms is important for advancing energy storage, especially when new materials and electrolytes are explored. Well-separated peaks in cyclic voltammograms (CVs) are considered key indicators of diffusion-controlled electrochemical processes with distinct Faradic charge transfer. Herein, we report on an electrochemical system with separated CV peaks, accompanied by surface-controlled partial charge transfer, in 2D Ti3C2Tx MXene in water-in-salt electrolytes. The process involves the insertion/desertion of desolvation-free cations, leading to an abrupt change of the interlayer spacing between MXene sheets. This unusual behavior increases charge storage at positive potentials, thereby increasing the amount of energy stored. This also demonstrates new opportunities for the development of high-rate aqueous energy storage devices and electrochemical actuators using safe and inexpensive aqueous electrolytes