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1. Effect of simultaneous electrical and mechanical stressing on porosity of Ti ₃ C ₂ T _x MXene films

   https://doi.org/10.1088/1361-6439/acbfc4  

   Sharp, Logan C; Al-Mamun, Nahid Sultan; Wetherington, Maxwell; Haque, Aman (March 2023, Journal of Micromechanics and Microengineering)

   Abstract MXenes are atomically layered carbides and nitrides of transition metals that have potential for micro-devices applications in energy storage, conversion, and transport. This emerging family of materials is typically studied as nanosheets or ultra-thin films, for which the internal defects are mostly nanoscale flake-flake interface separation type. However, micro-devices applications would require thicker films, which exhibit very high density of microscale pores. Electrical conductivity of thicker MXenes is significantly lower than nanosheets, and the physics of defect size and density control are also different and less understood. Current art is to perform high temperature annealing to improve the electrical conductivity, which can structurally alter or degrade MXene. The key contribution of this study is a room-temperature annealing process that exploits the synergy between electrical pulses and compressive mechanical loading. Experimental results indicate over a 90% increase in electrical conductivity, which reflects a decrease in void size and density. In the absence of compressive loading, the same process resulted in a conductivity increase of approximately 75%. Analytical spectroscopy and microscopy indicated that the proposed multi-stimuli process kept the MXene composition intact while significantly decreasing the void size and density. 

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2. 2D molybdenum and vanadium nitrides synthesized by ammoniation of 2D transition metal carbides (MXenes)

   https://doi.org/10.1039/c7nr06721f  

   Urbankowski, Patrick; Anasori, Babak; Hantanasirisakul, Kanit; Yang, Long; Zhang, Lihua; Haines, Bernard; May, Steven J.; Billinge, Simon J.; Gogotsi, Yury (January 2017, Nanoscale)

   MXenes are a rapidly growing class of 2D transition metal carbides and nitrides, finding applications in fields ranging from energy storage to electromagnetic interference shielding and transparent conductive coatings. However, while more than 20 carbide MXenes have already been synthesized, Ti 4 N 3 and Ti 2 N are the only nitride MXenes reported so far. Here by ammoniation of Mo 2 CT x and V 2 CT x MXenes at 600 °C, we report on their transformation to 2D metal nitrides. Carbon atoms in the precursor MXenes are replaced with N atoms, resulting from the decomposition of ammonia molecules. The crystal structures of the resulting Mo 2 N and V 2 N were determined with transmission electron microscopy and X-ray pair distribution function analysis. Our results indicate that Mo 2 N retains the MXene structure and V 2 C transforms to a mixed layered structure of trigonal V 2 N and cubic VN. Temperature-dependent resistivity measurements of the nitrides reveal that they exhibit metallic conductivity, as opposed to semiconductor-like behavior of their parent carbides. As important, room-temperature electrical conductivity values of Mo 2 N and V 2 N are three and one order of magnitude larger than those of the Mo 2 CT x and V 2 CT x precursors, respectively. This study shows how gas treatment synthesis such as ammoniation can transform carbide MXenes into 2D nitrides with higher electrical conductivities and metallic behavior, opening a new avenue in 2D materials synthesis. 

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3. The Evolution of MXenes Conductivity and Optical Properties Upon Heating in Air

   https://doi.org/10.1002/smtd.202300568  

   Shamsabadi, Ahmad A.; Fang, Hui; Zhang, Danzhen; Thakur, Anupma; Chen, Cindy Y.; Zhang, Aixi; Wang, Haonan; Anasori, Babak; Soroush, Masoud; Gogotsi, Yury; et al (July 2023, Small Methods)

   Abstract MXenes, a family of 2D transition‐metal carbides and nitrides, have excellent electrical conductivity and unique optical properties. However, MXenes oxidize in ambient conditions, which is accelerated upon heating. Intercalation of water also causes hydrolysis accelerating oxidation. Developing new tools to readily characterize MXenes’ thermal stability can enable deeper insights into their structure–property relationships. Here, in situ spectroscopic ellipsometry (SE) is employed to characterize the optical properties of three types of MXenes (Ti₃C₂T_x, Mo₂TiC₂T_x, and Ti₂CT_x) with varied composition and atomistic structures to investigate their thermal degradation upon heating under ambient environment. It is demonstrated that changes in MXene extinction and optical conductivity in the visible and near‐IR regions correlate well with the amount of intercalated water and hydroxyl termination groups and the degree of oxidation, measured using thermogravimetric analysis. Among the three MXenes, Ti₃C₂T_xand Ti₂CT_x, respectively, have the highest and lowest thermal stability, indicating the role of transition‐metal type, synthesis route, and the number of atomic layers in MXene flakes. These findings demonstrate the utility of SE as a powerful in situ technique for rapid structure–property relationship studies paving the way for the further design, fabrication, and property optimization of novel MXene materials. 

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4. 2D MXene-containing polymer electrolytes for all-solid-state lithium metal batteries

   https://doi.org/10.1039/C8NA00206A  

   Pan, Qiwei; Zheng, Yongwei; Kota, Sankalp; Huang, Weichun; Wang, Shijun; Qi, Hao; Kim, Seyong; Tu, Yingfeng; Barsoum, Michel W.; Li, Christopher Y. (January 2019, Nanoscale Advances)

   Nanocomposite polymer electrolytes (CPEs) are promising materials for all-solid-state lithium metal batteries (LMBs) due to their enhanced ionic conductivities and stability to the lithium anode. MXenes are a new two-dimensional, 2D, family of early transition metal carbides and nitrides, which have a high aspect ratio and a hydrophilic surface. Herein, using a green, facile aqueous solution blending method, we uniformly dispersed small amounts of Ti 3 C 2 T x into a poly(ethylene oxide)/LiTFSI complex (PEO 20 -LiTFSI) to fabricate MXene-based CPEs (MCPEs). The addition of the 2D flakes to PEO simultaneously retards PEO crystallization and enhances its segmental motion. Compared to the 0D and 1D nanofillers, MXenes show higher efficiency in ionic conductivity enhancement and improvement in the performance of LMBs. The CPE with 3.6 wt% MXene shows the highest ionic conductivity at room temperature (2.2 × 10 −5 S m −1 at 28 °C). An LMB using MCPE with only 1.5 wt% MXene shows rate capability and stability comparable with that of the state-of-the-art CPELMBs. We attribute the excellent performance to the 2D geometry of the filler, the good dispersion of the flakes in the polymer matrix, and the functional group-rich surface. 

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5. Advancements in 2D MXene-based supercapacitor electrodes: synthesis, mechanisms, electronic structure engineering, flexible wearable energy storage for real-world applications, and future prospects

   https://doi.org/10.1039/D4TA00328D  

   Kadam, Sujit Anil; Kadam, Komal Prakash; Pradhan, Nihar R (January 2024, Journal of Materials Chemistry A)

   Hagfeldt, Anders (Ed.)

   Supercapacitors are widely recognized as a favorable option for energy storage due to their higher power density compared to batteries, despite their lower energy density. However, to meet the growing demand for increased energy capacity, it is crucial to explore innovative materials that can enhance energy storage efficiency. Recent research has focused on investigating various electrode materials for use in supercapacitors, with particular attention given to MXenes. MXenes exhibit immense potential for energy storage due to their unique characteristics, including a 2D van der Waals layered structure, small band gaps, hydrophilic surface, excellent electrical conductivity, high specific surface area, and active redox sites on the surface facilitated by transition metals. These attributes collectively contribute to their promising stability, energy and power density, and overall lifespan. This comprehensive review explores a diverse array of topics pertaining to the latest 2D MXene-based supercapacitor electrodes. It encompasses discussions on different synthesis methods, electrode structures, the underlying working mechanisms, and the impact of electrolytes on supercapacitor performance. Additionally, a concise overview of various types of MXene materials is presented, ranging from titanium-based MXenes to niobium-based MXenes, vanadium-based MXenes, molybdenum-based MXenes, and tantalum-based MXenes. Furthermore, this review focuses on electronic structure engineering strategies such as heterostructures based on MXenes, heteroatom-doping based on MXenes, polymer based MXenes, and ternary composites based on MXenes, all of which contribute to improving the electrochemical performance of supercapacitors. The review thoroughly examines the advantages and disadvantages of MXene-based supercapacitor electrodes, offering a comprehensive understanding of their strengths and limitations. Additionally, it discusses the structural stability of MXene-based electrodes after electrochemical testing, as well as their applications in daily human life, particularly focusing on the uses of MXene-based flexible wearable energy storage for real-world applications. In the end, the challenges and prospects of MXenes in supercapacitors are discussed. 

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