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MXenes have demonstrated potential for various applications owing to their tunable surface chemistry and metallic conductivity. However, high temperatures can accelerate MXene film oxidation in air. Understanding the mechanisms of MXene oxidation at elevated temperatures, which is still limited, is critical in improving their thermal stability for high-temperature applications. Here, we demonstrate that Ti3C2Tx MXene monoflakes have exceptional thermal stability at temperatures up to 600 °C in air, while multiflakes readily oxidize in air at 300 °C. Density functional theory calculations indicate that confined water between Ti3C2Tx flakes has higher removal energy than surface water and can thus persist to higher temperatures, leading to oxidation. We demonstrate that the amount of confined water correlates with the degree of oxidation in stacked flakes. Confined water can be fully removed by vacuum annealing Ti3C2Tx films at 600 °C, resulting in substantial stability improvement in multiflake films (can withstand 600 °C in air). These findings provide fundamental insights into the kinetics of confined water and its role in Ti3C2Tx oxidation. This work enables the use of stable monoflake MXenes in high-temperature applications and provides guidelines for proper vacuum annealing of multiflake films to enhance their stability. [ABSTRACT FROM AUTHOR]

Traditionally, rechargeable lithium metal battery systems relied on simple metal cationic species to enable metal nucleation and deposition. However, this mechanism is less applicable to room‐temperature rechargeable aluminum batteries (RABs), which utilize complex ionic species. This work takes advantage of two different MXenes, Mo2Ti2C3Tx and Ti3C2Tx, which have different metal‐termination group bond strengths, interlayer spacings, and surface termination compositions, to amplify and visualize the differences between two promising RAB electrolytes (i.e., urea/AlCl3 and EMImCl/AlCl3). This work allowed us to deduce the following: 1) despite lower coulombic efficiencies, smoother Al deposits are observed when the urea/AlCl3 electrolyte is used; 2) unknown side reactions occur in all the different scenarios, however, they are likely less reversible in the EMImCl/AlCl3 electrolyte; 3) metal‐halogen exchange reactions can occur in all scenarios, forming Al(OF)x and AlFx species; 4) the free urea in urea/AlCl3 electrolyte behaves differently from the urea in aqueous solutions. Overall, due to the characteristics of aluminum, RABs involve vastly different internal mechanisms, resulting in the need for audacious attempts toward system understanding. [ABSTRACT FROM AUTHOR]

Water quality monitoring has become more critical in recent years to ensure the availability of clean and safe water from natural aquifers and to understand the evolution of water contaminants across time and space. The conventional water monitoring techniques comprise of sample collection, preservation, preparation, tailed by laboratory testing and analysis with cumbersome wet chemical routes and expensive instrumentation. Despite the high accuracy of these methods, the high testing costs, laborious procedures, and maintenance associated with them don't make them lucrative for end end-users and field testing. As the participation of ultimate stakeholders, that is, common man for water quality and quantity can play a pivotal role in ensuring the sustainability of our aquifers, thus it is essential to develop and deploy portable and user-friendly technical systems for monitoring water sources in real-time or on-site. The present review emphasizes here on possible approaches including optical (absorbance, fluorescence, colorimetric, X-ray fluorescence, chemiluminescence), electrochemical (ASV, CSV, CV, EIS, and chronoamperometry), electrical, biological, and surface-sensing (SPR and SERS), as candidates for developing such platforms. The existing developments, their success, and bottlenecks are discussed in terms of various attributes of water to escalate the essentiality of water quality devices development meeting ASSURED criterion for societal usage. These platforms are also analyzed in terms of their market potential, advancements required from material science aspects, and possible integration with IoT solutions in alignment with Industry 4.0 for environmental application. [ABSTRACT FROM AUTHOR]

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 (Ti3C2Tx, Mo2TiC2Tx, and Ti2CTx) 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, Ti3C2Tx and Ti2CTx, 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. [ABSTRACT FROM AUTHOR]

Rechargeable magnesium batteries (RMBs) are promising next‐generation low‐cost and high‐energy devices. Among all RMBs, anode‐free magnesium metal batteries that use in situ magnesium‐plated current collectors as negative electrodes can afford optimal energy densities. However, anode‐free magnesium metal batteries have remained elusive so far, as their practical application is plagued by low Mg plating/stripping efficiency due to nonuniform Mg deposition on conventional anode current collectors. Herein, for the first time, an anode‐free Mg‐metal battery is developed by employing a 3D MXene (Ti3C2Tx) film with horizontal Mg electrodeposition. The magnesiophilic oxygen and reactive fluorine terminations in MXene enable an enriched local magnesium‐ion concentration and a durable magnesium fluoride‐rich solid electrolyte interphase on the Ti3C2Tx film surface. Meanwhile, Ti3C2Tx MXene exhibits a high lattice geometrical fit with Mg (≈96%) to guide the horizontal electrodeposition of Mg. Consequently, the developed Ti3C2Tx film achieves reversible Mg plating/stripping with high Coulombic efficiencies (>99.4%) at high‐current‐density (5.0 mA cm−2) and high‐Mg‐utilization (50%) conditions. When this Ti3C2Tx film is coupled with a pre‐magnesized Mo6S8 cathode, the anode‐free Mg‐metal full‐cell prototype exhibits a volumetric energy density five times higher than its standard Mg‐metal counterpart. This work provides insights into the rational design of anode current collectors to guide horizontal Mg electrodeposition for anode‐free Mg metal batteries. [ABSTRACT FROM AUTHOR]