Triboelectric nanogenerators (TENGs) represent a cutting-edge class of devices for energy conversion and self-powered sensing. The selection of appropriate triboelectric and conductive materials is critical in determining the performance of TENGs. In recent years, MXenes, particularly Ti₃C₂ MXene, have emerged as promising candidates for triboelectric/conductive materials in TENGs. To elucidate the multifaceted roles of MXenes, this review examines their applications from a materials science perspective. The applications are categorized into four types based on the functional layers of TENGs where MXenes are applied: (1) MXene films as conductive layers, (2) MXene films as triboelectric layers, (3) MXene nanosheets as fillers in polymer-based triboelectric layers, and (4) MXene films as charge trapping layers. The rationale and advantages of utilizing MXenes in each application are analyzed and elucidated. Owing to their unique combination of properties, including electronegativity, electrical conductivity and flexibility, MXenes demonstrate remarkable versatility in all functional layers, either as pure films or composite films. Systematic analysis reveals that MXene composite films are particularly promising for the applications. This review represents the first comprehensive attempt to classify MXene applications in TENGs and articulate their inherent advantages, thereby providing a foundation for the design and development of high-performance MXene-based TENGs.

Rapid advancements in modern electronic devices necessitate the development of materials that can simultaneously provide efficient energy storage and effective microwave absorption. Herein, a novel composite material was prepared via the self-assembly of Fe₃O₄ nanoparticles on the surface of V₂C MXene. This composite exhibited characteristics suitable for supercapacitor and electromagnetic absorption applications, highlighting the synergistic relationship between energy storage and electromagnetic wave absorption capability. The electrochemical tests revealed that the specific capacity of the V₂C MXene/Fe₃O₄ composite (42.67 mAh·g⁻¹) considerably improved compared with those of the raw materials. The prepared V₂C MXene/Fe₃O₄//V₂C MXene/Fe₃O₄ symmetric supercapacitor (SSC) demonstrated an energy density of 44.8 Wh·L⁻¹, a power density of 959.4 W·L⁻¹, and a capacity retention of 80.14% after 8000 cycles. Moreover, the V₂C MXene/Fe₃O₄ composite exhibited an optimal reflection loss (RL) of −42.4 dB in the Ku band, with an effective absorption bandwidth of 1.9 GHz (14.6–16.5 GHz). This composite material has broad application potential in modern electronic devices owing to its high energy storage capacity and effective electromagnetic wave absorption. This dual functionality improves device performance and offers a compact solution for energy storage and effective microwave absorption.

Mo₂Ga₂C is a novel MAX phase. The powders of Mo₂Ga₂C can be synthesized by established technology. However, the powders are difficult to be sintered to obtain dense bulk samples. In this paper, the Mo₂Ga₂C powders were sintered by spark plasma sintering (SPS)technology. The compositions and microstructures of sintered samples were characterized to explore the impacts of sintering parameters. The suitable parameters to sintering Mo₂Ga₂C by SPS was at the temperature of 700 ℃ for 20 min with the axial pressure of 30 MPa. The obtained samples had the relative density of 71.81%. Longer holding time was more beneficial to the densification of Mo₂Ga₂C than higher sintering temperature. Higher axial pressure had a negative effect on the densification of samples. Compared with hot pressing, SPS can make bulk samples of Mo₂Ga₂C at lower temperature and short time. However, the obtained samples had lower relative density.

MXene-based absorbers have shown promising application prospects because of their sophisticated structural design and clever material composites. However, the intrinsic MXene materials themselves have not achieved significant breakthroughs in microwave absorption (MA) performance. Therefore, the development of novel and efficient pure MXene absorbing materials is imperative to address inherent mismatches in electromagnetic parameters, highlighting the urgent need in this area. Here, a straightforward strategy involving etching time modulation is proposed to customize the electromagnetic wave (EMW) absorption properties of delaminated Mo₂CT_x MXene. The impact of varying etching degrees on the EMW absorption capabilities of Mo₂CT_x MXenes was systematically investigated through controlled etching durations of Mo₂Ga₂C MAX phase. Among them, the sample etched for 12 h achieved an effective absorption bandwidth (EAB) of 4.4 GHz at an ultrathin thickness of 1.3 mm, and the strongest reflection loss (RL) value was as high as −60.7 dB when the sample etching time was increased to 24 h. The improvement in absorbing performance was attributed to the dielectric loss and polarization process induced by terminal functional groups and surface-rich defects, which optimized impedance matching. This work establishes that intrinsic Mo₂CT_x MXene materials with superior absorbing properties outperform traditional pure MXenes, providing a strong basis for advancing Mo-based MXene absorptive materials.

Two-dimensional MXenes are generally prepared by the etching of acid solutions. The as-synthesized MXenes are terminated by acid group anions (F^–, Cl^–, etc.), which affect the electrochemical performance of MXenes. Here, we report a novel method to prepare Mo₂C MXene from Mo₂Ga₂C by the hydrothermal etching of alkali solutions. Highly pure Mo₂C MXene was successfully synthesized by the etching of NaOH, while the etchings of LiOH and KOH were failed. The concentration of NaOH, temperature, and time strongly affect the purity of as-prepared MXene. Pure Mo₂C MXene could be synthesized by the etching of 20 M NaOH at 180 ℃ for 24 h. After intercalation by hexadecyl trimethyl ammonium bromide at 90 ℃ for 96 h, few-layer Mo₂C MXene was obtained. The Mo₂C MXene made by NaOH etching after intercalation exhibited excellent performance as anode of lithium-ion battery, compared with general Mo₂C MXene made by HF etching and the Mo₂C MXene reported in literature. The final discharge specific capacity was 266.73 mAh·g⁻¹ at 0.8 A·g⁻¹, which is 52% higher than that Mo₂C made by HF etching (175.77 mAh·g⁻¹). This is because Mo₂C MXene made by NaOH etching has lager specific surface area, lower resistance, and pure O/OH termination without acid anion termination. This is the first report to make Mo₂C MXene by alkali etching and the samples made by this method exhibited significantly better electrochemical performance than the samples made by general HF etching.

Nowadays, photocatalytic technologies are regarded as promising strategies to solve energy problems, and various photocatalysts have been synthesized and explored. In this paper, a novel CdS/MoO₂@Mo₂C-MXene photocatalyst for H₂ production was constructed by a two-step hydrothermal method, where MoO₂@Mo₂C-MXene acted as a binary co-catalyst. In the first hydrothermal step, MoO₂ crystals with an egged shape grew on the surface of two-dimensional (2D) Mo₂C MXene via an oxidation process in HCl aqueous solution. In the second hydrothermal step, CdS nanorods were uniformly assembled on the surface of MoO₂@Mo₂C-MXene in ethylenediamine with an inorganic cadmium source and organic sulfur source. The CdS/MoO₂@Mo₂C-MXene composite with MoO₂@Mo₂C-MXene of 5 wt% exhibits an ultrahigh visible-light photocatalytic H₂ production activity of 22,672 μmol/(g·h), which is ~21% higher than that of CdS/Mo₂C-MXene. In the CdS/MoO₂@Mo₂C-MXene composite, the MoO₂ with metallic nature separates CdS and Mo₂C MXene, which acts as an electron-transport bridge between CdS and Mo₂C MXene to accelerate the photoinduced electron transferring. Moreover, the energy band structure of CdS was changed by MoO₂@Mo₂C-MXene to suppress the recombination of photogenerated carriers. This novel compound delivers upgraded photocatalytic H₂ evolution performance and a new pathway of preparing the low-cost photocatalyst to solve energy problems in the future.

Ti₃C₂ MXene is a novel two-dimensional carbide with an accordion-like lamellar structure. The flakes of Ti₃C₂ MXene can slide or stack under tension or compression, which can change the number and length of internal conductive paths and directly affect the output electrical signals. Thus, Ti₃C₂ MXene can be used as sensing materials to measure the change of stress/strain. Combined with the elastic substrates, this material is used to make flexible sensors to monitor the motions and health signals of human-beings. This paper described the working principle of flexible stress/strain sensors made by Ti₃C₂ MXene composites. Recent methods to prepare the sensors were reviewed, i.e., electrospinning, filtration/coating, impregnation, screen printing, freeze drying, and freeze thawing. The key factors of the methods, i.e., flexible substrate, sensing parameters, sensor structure, detection limit, cycle times, sensing rage, responsive time and gauge factor, were represented. Some applications of the MXene-based sensors were discussed. Finally, the promising development of Ti₃C₂ MXene flexible sensors was outlooked and the problems to be solved were summarized.

MAX phases (Ti₃SiC₂, Ti₃AlC₂, V₂AlC, Ti₄AlN₃, etc.) are layered ternary carbides/nitrides, which are generally processed and researched as structure ceramics. Selectively removing A layer from MAX phases, MXenes (Ti₃C₂, V₂C, Mo₂C, etc.) with two-dimensional (2D) structure can be prepared. The MXenes are electrically conductive and hydrophilic, which are promising as functional materials in many areas. This article reviews the milestones and the latest progress in the research of MAX phases and MXenes, from the perspective of ceramic science. Especially, this article focuses on the conversion from MAX phases to MXenes. First, we summarize the microstructure, preparation, properties, and applications of MAX phases. Among the various properties, the crack healing properties of MAX phase are highlighted. Thereafter, the critical issues on MXene research, including the preparation process, microstructure, MXene composites, and application of MXenes, are reviewed. Among the various applications, this review focuses on two selected applications: energy storage and electromagnetic interference shielding. Moreover, new research directions and future trends on MAX phases and MXenes are also discussed.

The effect of etching environment (opened or closed) on the synthesis and electrochemical properties of V₂C MXene was studied. V₂C MXene samples were synthesized by selectively etching of V₂AlC at 90 ℃ in two different environments: opened environment (OE) in oil bath pans under atmosphere pressure and closed environment (CE) in hydrothermal reaction kettles under higher pressures. In OE, only NaF (sodium fluoride) + HCl (hydrochloric acid) etching solution can be used to synthesize highly pure V₂C MXene. However, in CE, both LiF (lithium fluoride) + HCl and NaF+HCl etchant can be used to prepare V₂C MXene. Moreover, the V₂C MXene samples made in CE had higher purity and better-layered structure than those made in OE. Although the purity of V₂C obtained by LiF+HCl is lower than that of V₂C obtained using NaF+HCl, it shows better electrochemical performance as anodes of lithium-ion batteries (LIBs). Therefore, etching in CE is a better method for preparing highly pure V₂C MXene, which provides a reference for expanding the synthesis methods of V₂C with better electrochemical properties.

Two-dimensional carbide MXenes (Ti₃C₂T_x and V₂CT_x) were prepared by exfoliating MAX phases (Ti₃AlC₂ and V₂AlC) powders in the solution of sodium fluoride (NaF) and hydrochloric acid (HCl). The specific surface area (SSA) of as-prepared Ti₃C₂T_x was 21 m²/g, and that of V₂CT_x was 9 m²/g. After intercalation with dimethylsulfoxide, the SSA of Ti₃C₂T_x was increased to 66 m²/g; that of V₂CT_x was increased to 19 m²/g. Their adsorption properties on carbon dioxide (CO₂) were investigated under 0–4 MPa at room temperature (298 K). Intercalated Ti₃C₂T_x had the adsorption capacity of 5.79 mmol/g, which is close to the capacity of many common sorbents. The theoretical capacity of Ti₃C₂T_x with the SSA of 496 m²/g was up to 44.2 mmol/g. Additionally, due to high pack density, MXenes had very high volume-uptake capacity. The capacity of intercalated Ti₃C₂T_x measured in this paper was 502 V·v^–1. This value is already higher than volume capacity of most known sorbents. These results suggest that MXenes have some advantage features to be researched as novel CO₂ capture materials.