Since their discovery in 2011, the number of 2D transition metal carbides and nitrides (MXenes) has steadily increased. Currently more than 40 MXene compositions exist. The ultimate number is far greater and in time they may develop into the largest family of 2D materials known. MXenes’ unique properties, such as their metal‐like electrical conductivity reaching ≈20 000 S cm−1, render them quite useful in a large number of applications, including energy storage, optoelectronic, biomedical, communications, and environmental. The number of MXene papers and patents published has been growing quickly. The first MXene generation is synthesized using selective etching of metal layers from the MAX phases, layered transition metal carbides and carbonitrides using hydrofluoric acid. Since then, multiple synthesis approaches have been developed, including selective etching in a mixture of fluoride salts and various acids, non‐aqueous etchants, halogens, and molten salts, allowing for the synthesis of new MXenes with better control over their surface chemistries. Herein, a brief historical overview of the first 10 years of MXene research and a perspective on their synthesis and future development are provided. The fact that their production is readily scalable in aqueous environments, with high yields bodes well for their commercialization.
- Award ID(s):
- 1740795
- PAR ID:
- 10156808
- Date Published:
- Journal Name:
- Chemical Society Reviews
- Volume:
- 47
- Issue:
- 23
- ISSN:
- 0306-0012
- Page Range / eLocation ID:
- 8744 to 8765
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Batteries and supercapacitors have emerged as promising candidates for next-generation energy storage technologies. The rapid development of new two-dimensional (2D) electrode materials indicates a new era in energy storage devices. MXenes are a new type of layered 2D transition metal carbides, nitrides, or carbonitrides that have drawn much attention because of their excellent electrical conductivity, electrochemical and hydrophilic properties, large surface area, and attractive topological structure. This review focuses on various synthesis methods to prepare vanadium carbide MXenes with and without etchants like hydrofluoric acid, lithium fluoride, and hydrochloric acid to remove the ‘A’ layers of the MAX phase. The goal is to demonstrate the utilization of a less toxic etching method to achieve MXenes of comparable properties to those prepared by traditional methods. The influence of intercalation on the effect of high interlayer spacing between the MXene layers and the performance of MXenes as supercapacitor and battery electrodes is also addressed in this review. Lastly, the gaps in the current knowledge for vanadium carbide MXenes in synthesis, scalability, and utilization in more energy storage devices were discussed.more » « less
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Two-dimensional transition metal carbides, nitrides, and carbonitrides, known as MXenes, hold potential in electrocatalytic applications. Tungsten (W) based-MXenes are of particular interest as they are predicted to have low overpotentials in hydrogen evolution reaction (HER). However, incorporating W into the MXene structure has proven difficult due to the calculated instability of its hypothetical MAX precursors. In this study, we present a theory-guided synthesis of a W-containing MXene, W2TiC2Tx, derived from a non-MAX nanolaminated ternary carbide (W,Ti)4C4-y precursor by selective etching of one of the covalently bonded tungsten layers. Our results indicate the importance of W and Ti ordering and the presence of vacancy defects for the successful selective etching of the precursor. We confirm the atomistic out-of-plane ordering of W and Ti using density functional theory, Rietveld refinement, and electron microscopy methods. Additionally, the W-rich basal plane endows W2TiC2Tx MXene with a remarkable HER overpotential (~144 mV at 10 mA/cm2). This study adds a tungsten-containing MXene made from a covalently bonded non-MAX phase opening more ways to synthesize novel 2D materials.
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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 (Ti3C2T
x ) 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. -
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Abstract MXenes are a rapidly growing family of 2D transition metal carbides and nitrides that are promising for various applications, including energy storage and conversion, electronics, and healthcare. Hydrofluoric‐acid‐based etchants are typically used for large‐scale and high‐throughput synthesis of MXenes, which also leads to a mixture of surface terminations that impede MXene properties. Herein, a computational thermodynamic model with experimental validation is presented to explore the feasibility of fluorine‐free synthesis of MXenes with uniform surface terminations by dry selective extraction (DSE) from precursor MAX phases using iodine vapors. A range of MXenes and respective precursor compositions are systematically screened using first‐principles calculations to find candidates with high phase stability and low etching energy. A thermodynamic model based on the “CALculation of PHAse Diagrams” (CALPHAD) approach is further demonstrated, using Ti3C2I2as an example, to assess the Gibbs free energy of the DSE reaction and the state of the byproducts as a function of temperature and pressure. Based on the assessment, the optimal synthesis temperature and vapor pressure are predicted and further verified by experiments. This work opens an avenue for scalable, fluorine‐free dry synthesis of MXenes with compositions and surface chemistries that are not accessible using wet chemical etching.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c8cs00649k
