Stabilizing Ti3C2Tx MXene flakes in air by removing confined water.

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 Ti[Formula: see text]C[Formula: see text]T[Formula: see text] MXene monoflakes have exceptional thermal stability at temperatures up to 600[Formula: see text]C in air, while multiflakes readily oxidize in air at 300[Formula: see text]C. Density functional theory calculations indicate that confined water between Ti[Formula: see text]C[Formula: see text]T[Formula: see text] 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 Ti[Formula: see text]C[Formula: see text]T[Formula: see text] films at 600[Formula: see text]C, resulting in substantial stability improvement in multiflake films (can withstand 600[Formula: see text]C in air). These findings provide fundamental insights into the kinetics of confined water and its role in Ti[Formula: see text]C[Formula: see text]T[Formula: see text] 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.

In situ turning defects of exfoliated Ti3C2 MXene into Fenton-like catalytic active sites.

Controllable in situ formation of nanoclusters with discrete active sites is highly desirable in heterogeneous catalysis. Herein, a titanium oxide-based Fenton-like catalyst is constructed using exfoliated Ti3C2 MXene as a template. Theoretical calculations reveal that a redox reaction between the surface Ti-deficit vacancies of the exfoliated Ti3C2 MXene and H2O2 molecules facilitates the in situ conversion of surface defects into titanium oxide nanoclusters anchoring on amorphous carbon (TiOx@C). The presence of mixed-valence Tiδ+ (δ = 0, 2, 3, and 4) within TiOx@C is confirmed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) characterizations. The abundant surface defects within TiOx@C effectively promote the generation of reactive oxygen species (ROS) leading to superior and stable Fenton-like catalytic degradation of atrazine, a typical agricultural herbicide. Such an in situ construction of Fenton-like catalysts through defect engineering also applies to other MXene family materials, such as V2C and Nb2C.

Palladium metallene confined on MXene with increased hydroxyl binding strength for highly efficient ethanol electrooxidation.

Rational design and synthesis of high-performance electrocatalysts for ethanol oxidation reaction (EOR) is crucial to large-scale commercialization of direct ethanol fuel cells, but it is still an incredible challenge. Herein, a unique Pd metallene/Ti3C2Tx MXene (Pdene/Ti3C2Tx)-supported electrocatalyst is constructed via an in-situ growth approach for high-efficiency EOR. The resulting Pdene/Ti3C2Tx catalyst achieves an ultrahigh mass activity of 7.47 A mgPd-1 under alkaline condition, as well as high tolerance to CO poisoning. In situ attenuated total reflection-infrared spectroscopy studies combined with density functional theory calculations reveal that the excellent EOR activity of Pdene/Ti3C2Tx catalyst is attributed to the unique and stable interfaces which reduce the reaction energy barrier of *CH3CO intermediate oxidation and facilitate oxidative removal of CO poisonous species by increasing the Pd-OH binding strength.

Directional electronic tuning of Ni nanoparticles by interfacial oxygen bridging of support for catalyzing alkaline hydrogen oxidation.

Metallic nickel (Ni) is a promising candidate to substitute Pt-based catalysts for hydrogen oxidation reaction (HOR), but huge challenges still exist in precise modulation of the electronic structure to boost the electrocatalytic performances. Herein, we present the use of single-layer Ti3C2Tx MXene to deliberately tailor the electronic structure of Ni nanoparticles via interfacial oxygen bridges, which affords Ni/Ti3C2Tx electrocatalyst with exceptional performances for HOR in an alkaline medium. Remarkably, it shows a high kinetic current of 16.39 mA cmdisk-2 at the overpotential of 50 mV for HOR [78 and 2.7 times higher than that of metallic Ni and Pt/C (20%), respectively], also with good durability and CO antipoisoning ability (1,000 ppm) that are not available for conventional Pt/C (20%) catalyst. The ultrahigh conductivity of single-layer Ti3C2Tx provides fast transmission of electrons for Ni nanoparticles, of which the uniform and small sizes endow them with high-density active sites. Further, the terminated -O/-OH functional groups on Ti3C2Tx directionally capture electrons from Ni nanoparticles via interfacial Ni-O bridges, leading to obvious electronic polarization. This could enhance the Nids-O2p interaction and weaken Nids-H1s interaction of Ni sites in Ni/Ti3C2Txenabling a suitable H-/OH-binding energy and thus enhancing the HOR activity.

Chemically coupling SnO2 quantum dots and MXene for efficient CO2 electroreduction to formate and Zn-CO2 battery.

Electrochemical conversion of CO2 into formate is a promising strategy for mitigating the energy and environmental crisis, but simultaneously achieving high selectivity and activity of electrocatalysts remains challenging. Here, we report low-dimensional SnO2 quantum dots chemically coupled with ultrathin Ti3C2Tx MXene nanosheets (SnO2/MXene) that boost the CO2 conversion. The coupling structure is well visualized and verified by high-resolution electron tomography together with nanoscale scanning transmission X-ray microscopy and ptychography imaging. The catalyst achieves a large partial current density of -57.8 mA cm-2 and high Faradaic efficiency of 94% for formate formation. Additionally, the SnO2/MXene cathode shows excellent Zn-CO2 battery performance, with a maximum power density of 4.28 mW cm-2, an open-circuit voltage of 0.83 V, and superior rechargeability of 60 h. In situ X-ray absorption spectroscopy analysis and first-principles calculations reveal that this remarkable performance is attributed to the unique and stable structure of the SnO2/MXene, which can significantly reduce the reaction energy of CO2 hydrogenation to formate by increasing the surface coverage of adsorbed hydrogen.

Size/Shape-Controllable Carbonized Wood Electrodes Enabled by an MXene Shell with Spatial Confinement and a Traction Effect on the Wood Cell Wall for Shape-Customizable Energy Storage Devices.

The shrinkage and collapse of wood cell walls during carbonization make it challenging to control the size and shape of carbonized wood (CW) through pre- or postprocessing (e.g., sawing, cutting, and milling). Herein, a shape-adaptive MXene shell (MS) is created on the surface of the wood cell walls. The MS limits the deformation of wood cell walls by spatial confinement and traction effects, which is supported by the inherent dimensional stability of the MS and the formation of new C-O-Ti covalent bonds between the wood cell wall and MS. Consequently, the volumetric shrinkage ratio of CW encapsulated by the MS (CW-MS) is significantly reduced from 54.8% for CW to 2.6% for CW-MS even at 800 °C. The harnessing of this collapse enables the production of CW-MS with prolonged stability and high electric conductivity (384 S m-1). These properties make CW-MS suitable for energy storage devices with various designed shapes, matching the increasingly compact and complex structures of electronic devices.

Construction of V2O5@MXene Cathodes toward a High Specific Capacity Aqueous Aluminum-Ion Battery.

Aqueous aluminum-ion batteries (AAIBs) are considered a strong candidate for the new generation of energy storage devices. The lack of suitable cathode materials has been a bottleneck factor hindering the future development of AAIBs. In this work, we design and construct a highly effective cathode with dual morphologies. Two-dimensional (2D) layered MXene materials possessed good conductivity and hydrophilicity, which are used as the substrates to deposit rod-shaped vanadium oxides (V2O5) to form a three-dimensional (3D) cathode. The cathode design provides a strong boost for the rapid electrochemical activities of rod-shaped V2O5 by embedding/extracting both protons (H+) and aluminum-ion (Al3+). As a result, the V2O5@MXene cathode based AAIB delivers an ultrahigh initial specific capacity of 626 mAh/g at 0.1 A/g with a stable cycle performance up to 100 cycles. This work is a breakthrough for the development of cathode materials for AAIBs.

Solidify Eutectic Electrolytes via the Added MXene as Nucleation Sites for a Solid-State Zinc-Ion Battery with Reconstructed Ion Transport.

Stationary energy storage infrastructure based on zinc-ion transport and storage chemistry is attracting more attention due to favorable metrics, including cost, safety, and recycling feasibility. However, splitting water and liquid electrolyte fluidity lead to cathode dissolution and Zn corrosion, resulting in rapid attenuation of the capacity and service life. Herein, a new architecture of solid-state electrolytes with high zinc ionic conductivity at room temperature was prepared via solidification of deep eutectic solvents utilizing MXene as nucleation additives. The ionic conductivity of MXene/ZCEs reached 6.69 × 10-4 S cm-1 at room temperature. Dendrite-free Zn plating/stripping with high reversibility can remain for over 2500 h. Subsequently, the fabricated solid-state zinc-ion battery with eliminated HER and suppressed Zn dendrites exhibited excellent cycling performance and could work normally in a range from -10 to 60 °C. This design inspired by eutectic solidification affords new insights into the multivalent solid electrochemistry suffering from slow ion migration.

Highly Elastic, Robust, and Efficient Hydrogel Solar Absorber against Harsh Environmental Impacts.

Solar distillation is a promising approach for addressing water scarcity, but relentless stress/strain perturbations induced by wind and waves would inevitably cause structural damage to solar absorbers. Despite notable advances in efficient solar absorbers, there have been no reports of compliant and robust solar absorbers withstanding practical mechanical impacts. Herein, an elastic and robust hydrogel absorber that exhibited a high level of evaporation performance was fabricated by introducing ion-coordinated MXene nanosheets as photothermal conversion units and mechanically enhanced fillers. The ion-coordinated MXene nanosheets acting as strong cross-linking points provided excellent elasticity and robustness to the hydrogel absorber. As a result, the evaporation rate of hydrogel absorber, with a high initial value of 2.61 kg m-2 h-1 under one sun irradiation, remained at 2.15 kg m-2 h-1 under a 100% tensile strain state and 2.40 kg m-2 h-1 after 10 000 stretching-releasing cycles. This continuous and stable water desalination approach provides a promising device for actual seawater distillation.

In Situ MXene Anchored Structure for Highly Durable Solar Steam Generation.

As a promising fresh water harvesting technology, interfacial solar steam generation has attracted growing interest. Efficient solar absorption and long-term operational performance are critical requirements of this technology. However, developing robust evaporators to promote practical applications under extreme conditions is still a grand challenge. Herein, we propose a light-assisted strategy to in situ prepare a Ti3C2Tx MXene anchored structure (MXAS) for enhanced solar evaporation with superior mechanical properties (compressive strength of 78.47 MPa, which can withstand a pressure of 3.92 × 106 times its own weight). Light irradiation enlarges the interlayer spacing of MXene and improves the solar absorption capability. Under one sun, the three-dimensional MXAS evaporator exhibits a steam generation rate of 2.48 kg m-2 h-1and an evaporation efficiency of 89.3%, and it demonstrates long-term durability when testing in seawater. This strategy provides valuable insights into the potential application of a high-performance water evaporation system.

Synergistic Effect of Ti3C2Tx MXene Nanosheets and Tannic Acid-Fe3+ Network in Constructing High-Performance Hydrogel Composite Membrane for Photothermal Membrane Distillation.

Photothermal membrane distillation (PMD) has emerged as a promising and sustainable approach for seawater desalination and wastewater purification. However, the wide application of the technique is severely impeded by low freshwater production and membrane fouling/wetting issues. Herein, we developed an advanced hydrogel-engineered membrane with simultaneously enhanced photothermal conversion capacity and desired fouling and wetting resistance for PMD. By the synergies of photothermal Ti3C2Tx MXene nanosheets and the tannic acid-Fe3+ network in the hydrogel, the membrane was endowed with excellent surface self-heating ability, yielding the highest freshwater production rate (1.71 kg m-2 h-1) and photothermal efficiency among the fabricated hydrogel composite membranes under 1 sun irradiation. Meanwhile, the PMD membrane could robustly resist oil-induced fouling and surfactant-induced wetting, significantly extending the membrane lifespan in treating contaminated saline water. Furthermore, when desalinating real seawater, the membrane exhibited superior durability with a stable vapor flux and excellent ion rejection (e.g., 99.24% for boron) for 100 h.

Supertough MXene/Sodium Alginate Composite Fiber Felts Integrated with Outstanding Electromagnetic Interference Shielding and Heating Properties.

The development of multifunctional MXene-based fabrics for smart textiles and portable devices has garnered significant attention. However, very limited studies have focused on their structure design and associated mechanical properties. Here, the supertough MXene fiber felts composed of MXene/sodium alginate (SA) fibers were fabricated. The fracture strength and bending stiffness of felts can be up to 97.8 MPa and 1.04 N mm2, respectively. Besides, the fracture toughness of felts was evaluated using the classic Griffith theory, yielding to a critical stress intensity factor of 1.79 MPam. In addition, this kind of felt presents outstanding electrothermal conversion performance (up to 119 °C at a voltage of 2.5 V), high cryogenic and high-temperature tolerance of photothermal conversion performance (-196 to 160 °C), and excellent electromagnetic interference (EMI) shielding effectiveness (54.4 dB in the X-band). This work provides new structural design concepts for high-performance MXene-based textiles, broadening their future applications.

Ultrarapid Gelation of Porous Ti3C2Tx MXene Monoliths Induced by Ionic Liquids.

Gelation is a promising method to assemble 3D macroscopic structures from MXene sheets for various applications. However, the fine control and scalable manufacturing of 3D MXene monoliths remains a great challenge. Herein, the controllable gelation of Ti3C2Tx MXene initiated by various ionic liquids (ILs) is first proposed, where the IL serve as linkers to bond the nanosheets together through electrostatic and hydrogen bonding interactions, forming 3D monoliths with well-adjustable structure. Furthermore, density functional theory calculations and experiments further reveal the cross-linking effect of different ILs. Typically, 3D porous structure with high specific surface area, suitable pore size, and improved electrolyte affinity is designed through the cross-linking of Ti3C2Tx with 1-vinyl-3-ethylimidazole bromide ([C2VIm]Br-Ti3C2Tx). Due to the strong coupling, the as-synthesized monolith possesses excellent rate performance and high energy density. The methodology is quite flexible, controllable, and universal that provides a new perspective for promoting innovative applications of 2D materials.

Green Synthesis and Biosafety Assessment of MXene.

The rise of MXene-based materials with fascinating physical and chemical properties has attracted wide attention in the field of biomedicine, because it can be exploited to regulate a variety of biological processes. The biomedical applications of MXene are still in its infancy, nevertheless, the comprehensive evaluation of MXene's biosafety is desperately needed. In this review, the composition and the synthetic methods of MXene materials are first introduced from the view of biosafety. The evaluation of the interaction between MXene and cells, as well as the safety of different forms of MXene applied in vivo are then discussed. This review provides a basic understanding of MXene biosafety and may bring new inspirations to the future applications of MXene-based materials in biomedicine.

Multilayer Core-Shell Fiber Device for Improved Strain Sensing and Supercapacitor Applications.

1D fiber devices, known for their exceptional flexibility and seamless integration capabilities, often face trade-offs between desired wearable application characteristics and actual performance. In this study, a multilayer device composed of carbon nanotube (CNT), transition metal carbides/nitrides (MXenes), and cotton fibers, fabricated using a dry spinning method is presented, which significantly enhances both strain sensing and supercapacitor functionality. This core-shell fiber design achieves a record-high sensitivity (GF ≈ 4500) and maintains robust durability under various environmental conditions. Furthermore, the design approach markedly influences capacitance, correlating with the percentage of active material used. Through systematic optimization, the fiber device exhibited a capacitance 26-fold greater than that of a standard neat CNT fiber, emphasizing the crucial role of innovative design and high active material loading in improving device performance.

Interfacial Modulation of Ti3C2Tx MXene by Cellulose Nanofibrils to Construct Hybrid Fibers with High Volumetric Specific Capacitance.

The intrinsic poor rheological properties of MXene inks result in the MXene nanosheets in dried MXene microfibers prone to self-stacking, which is not conducive to ion transport and diffusion, thus affecting the electrochemical performance of fiber-based supercapacitors. Herein, robust cellulose nanofibrils (CNF)/MXene hybrid fibers with high electrical conductivity (916.0 S cm-1) and narrowly distributed mesopores are developed by wet spinning. The interfacial interaction between CNF and MXene can be enhanced by hydrogen bonding and electrostatic interaction due to their rich surface functional groups. The interfacial modulation of MXene by CNF can not only regulate the rheology of MXene spinning dispersion, but also enhance the mechanical strength. Furthermore, the interlayer distance and self-stacking effect of MXene nanosheets are also regulated. Thus, the ion transport path within the fiber material is optimized and ion transport is accelerated. In H2SO4 electrolyte, a volumetric specific capacitance of up to 1457.0 F cm-3 (1.5 A cm-3) and reversible charge/discharge stability are demonstrated. Intriguingly, the assembled supercapacitors exhibit a high-volume energy density of 30.1 mWh cm-3 at 40.0 mW cm-3. Moreover, the device shows excellent flexibility and cycling stability, maintaining 83% of its initial capacitance after 10 000 charge/discharge cycles. Practical energy supply applications (Power for LED and electronic watch) can be realized.

MXene-Derived TiO2/Starbon Nanocomposite as a Remarkable Electrode Material for Coin-Cell Symmetric Supercapacitor.

In this study, the synthesis of a MXene (Ti3C2Tx)-derived TiO2/starbon (M-TiO2/Starbon-800 °C) nanocomposite using a facile calcination method is explored. High-temperature exposure transforms layered Ti3C2Tx into rod-like TiO2 and starbon into amorphous carbon. The resulting M-TiO2/Starbon-800 °C nanocomposite exhibits a significantly larger surface area and pore volume compared to its individual components, leading to superior electrochemical performance. In a three-electrode configuration, the nanocomposite achieved a specific capacitance (Csp) of 1352 Fg⁻¹ at 1 Ag⁻¹, while retaining more than 99% of its Csp after 50 000 charge/discharge cycles. Furthermore, when incorporated into a two-electrode symmetric coin cell, it demonstrates a Csp of 115 Fg⁻¹ along with exceptional long cycle life. Moreover, the device shows an energy density (ED) of 51 Whkg-1 and a power density (PD) of 7912 Wkg-1 at 5 Ag-1. The enhanced charge storage is attributed to the formation of a porous structure with a high specific surface area resulting from the interaction between M-TiO2 nanorods and starbon, which facilitates efficient ion penetration.

Activating Ion Channels in Collapsed Hydrogel Derived Densified MXene Films with Cellulose Nanofibers to Overcome the Areal Versus Volumetric Capacitance Trade-Off.

Concomitant achievement of all three performance pillars of a supercapacitor device, namely gravimetric, areal, and volumetric capacitance is a grand challenge. Nevertheless, its fulfilment is indispensable for commercial usage. Although, high compactness is the fundamental requirement to achieve high volumetric performance, it severely affects ion transportation in thick electrodes. Such trade-off makes it extremely challenging to realize very high areal and volumetric performance simultaneously. Here, a collapsed hydrogel strategy is introduced to develop MXene/cellulose nanofiber (CNF) based densified electrodes that offer excellent ion transportation despite a massive increase in areal mass loading (>70 mg cm-2). Quasi-oriented MXene/CNF (MXCF) hydrogels are produced through an electric field-guided co-assembly technique. Ambient dehydration of these hydrogels incorporates numerous pores in the resultant compact electrodes due to crumpling of the MXene sheets, while CNF ensures connectivity among the locally blocked pores in different length scales. The resultant collapsed MXCF densified electrode shows a remarkably high areal capacitance of 16 F cm-2 while simultaneously displaying a high volumetric capacitance of 849.8 F cm-3 at an ultrahigh mass loading of up to 73.4 mg cm-2. The universality of strategy, including the co-assembly of hydrogel and its collapse, is further demonstrated to develop high-performance asymmetric and wearable devices.

N-Terminalized Ti3 C2 Tx MXene for Supercapacitor with Extraordinary Pseudocapacitance Performance.

MXenes have demonstrated significant potential in electrochemical energy storage, particularly in supercapacitors, owing to their exceptional properties. The surface terminal groups of MXene play a pivotal role in pseudocapacitive mechanism. Considering the hindered electrolyte ion transport caused by -F terminal groups and the limited ion binding sites associated with -O terminal groups, this study proposes a novel strategy of replacing -F with -N terminal groups. The modulated MXene-N electrode, featuring a substantial number of -N terminal groups, demonstrates an exceptionally high gravimetric capacitance of 566 F g-1 (at a scan rate of 2 mV s-1 ) or 588 F g-1 (at a discharge rate of 1 A g-1 ) in 1 м H2 SO4 electrolyte, and the potential window is significantly increased. Furthermore, subsequent spectra analysis and density functional theory calculations are employed to investigate the mechanism associated with -N terminal groups. This work exemplifies the significance of terminal modulation in the context of electrochemical energy storage.

MXene-Derived Na+ -Pillared Vanadate Cathodes for Dendrite-Free Potassium Metal Batteries.

Cation-intercalated vanadates, which have considerable promise as the cathode for high-performance potassium metal batteries (PMBs), suffer from structural collapse upon K+ insertion and desertion. Exotic cations in the vanadate cathode may ease the collapse, yet their effect on the intrinsic cation remains speculative. Herein, a stable and dendrite-free PMB, composed of a Na+ and K+ co-intercalated vanadate (NKVO) cathode and a liquid NaK alloy anode, is presented. A series of NKVO with tuneable Na/K ratios are facilely prepared using MXene precursors, in which Na+ is testified to be immobilized upon cycling, functioning as a structural pillar. Due to stronger ionic bonding and lower Fermi level of Na+ compared to K+ , moderate Na+ intercalation could reduce K+ binding to the solvation sheath and favor K+ diffusion kinetics. As a result, the MXene-derived Na+ -pillared NKVO exhibits markedly improved specific capacities, rate performance, and cycle stability than the Na+ -free counterpart. Moreover, thermally-treated carbon paper, which imitates the microscopic structure of Chinese Xuan paper, allows high surface tension liquid NaK alloy to adhere readily, enabling dendrite-free metal anodes. By clarifying the role of foreign intercalating cations, this study may lead to a more rational design of stable and high-performance electrode materials.