Vacancies play a pivotal role in determining the physical and chemical properties of materials. Introducing vacancies into two-dimensional (2D) materials offers a promising strategy for developing high-performance electrode materials for electrochemical energy storage. Herein, a facile top-down strategy is employed to create V-based MXenes with tunable vacancy concentrations, achieved by designing the precursor (V1-xCrx)2AlC (x = 0.05, 0.1, 0.3) MAX phase and precisely controlling the etching process. Systematic investigations reveal that introducing a moderate concentration of Cr-induced vacancies significantly enhances both the capacitance and rate performance of V-based MXenes. Specifically, V1.9CTz achieves a capacitance of 760 F g-1, far exceeding the 420 F g-1 of vacancy-free V2CTz MXene. In contrast, an excessively high vacancy concentration leads to deteriorated electrochemical performance and compromised structural stability. This work illustrates that defect engineering is a powerful approach to tailor the electrochemical properties of MXenes, offering a framework for designing next-generation MXene-based energy storage systems.

Understanding how electrochemically active sites are utilized under high-rate conditions remains a fundamental challenge for MXene-based energy storage systems. In particular, the dynamic interplay between proton transport and surface redox reactions, which governs the accessibility and effective utilization of active sites, is still poorly understood. Herein, we investigate the charge-storage mechanism of Ti3CNTx MXene in acidic electrolytes by systematically tuning surface terminations and structural features via thermal annealing. The optimized sample (annealing at 300 °C) delivers a high specific capacitance of 470 F g−1 at 2 A g−1 and maintains excellent rate capability. Comprehensive characterization reveals that annealing induces coupled modifications in surface chemistry (reduction of F terminations and increase in O groups), electronic structure (increased Ti valence state), and porosity (formation of mesoporous channels), collectively facilitating proton transport and surface redox kinetics. More importantly, we introduce the concept of dynamic saturation of active sites, defined as a rate-dependent steady-state condition in which the occupation of active sites is governed by a dynamic balance between proton insertion and extraction processes. In situ EQCM measurements, together with electrochemical analysis, demonstrate that the progressive activation of active sites during initial cycling evolves into a stable dynamic equilibrium, enabling efficient utilization of active sites even at high rates. This work provides a mechanistic framework linking structural evolution, proton transport, and redox kinetics in MXenes, offering new insights for the rational design of high-rate pseudocapacitive materials.

The sustainable utilization of natural resources and growing demand for various electronic devices have promoted the development of safe, stable, and rechargeable aqueous zinc-ion batteries (AZIBs). However, a stable cathode material is crucial for ZIBs in an aqueous electrolyte, since it is more difficult for divalent Zn2+ to be reversibly inserted and extracted between active materials than it is for monovalent metal ions. In this work, a tailored multi-defect MXene, Mo1.74CTz, of a complete chemical formula of Mo1.74±0.06CO0.95±0.02(OH)0.63±0.01F0.3±0.03.0.2±0.05H2Oads (Mo1.74CTz), is assembled as cathode in AZIBs. It achieved 75% capacity retention and nearly 100% Coulombic efficiency even after up to 100 000 cycles as the intrinsic structural stability and many vertical holes of the Mo1.74CTz MXene contributed to alleviating the MXene collapse under repeated charge and discharge. Meanwhile, the Mo1.74CTz-based AZIBs exhibited good performance with a specific capacity of 200 mAh g-1 at a current density of 0.2 A g-1, which greatly exceeds previous reports of pure MXene-based cathodes in AZIBs. This work will aid in finding new solutions for sustainable energy development, which will pave the way for AZIBs as an alternative to lithium-ion batteries (LIBs) in the future.

Nb4C3Tx MXene has shown extraordinary promise for various applications owing to its unique physicochemical properties. However, it can only be synthesized by the traditional HF-based etching method, which uses large amounts of hazardous HF and requires a long etching time (> 96 h), thus limiting its practical application. Here, an ultra-efficient and environmental-friendly H2O-assisted supercritical etching method is proposed for the preparation of Nb4C3Tx MXene. Benefiting from the synergetic effect between supercritical CO2 (SPC-CO2) and subcritical H2O (SBC-H2O), the etching time for Nb4C3Tx MXene can be dramatically shortened to 1 h. The as-synthesized Nb4C3Tx MXene possesses uniform accordion-like morphology and large interlayer spacing. When used as anode for Li-ion battery, the Nb4C3Tx MXene delivers a high reversible specific capacity of 430 mAh g(-1) at 0.1 A g(-1), which is among the highest values achieved in pure-MXene-based anodes. The superior lithium storage performance of the Nb4C3Tx MXene can be ascribed to its high conductivity, fast Li+ diffusion kinetics and good structural stability.

2D MXene (Ti3CNTx) has been considered as the most promising electrode material for flexible supercapacitors owing to its metallic conductivity, ultra-high capacitance, and excellent flexibility. However, it suffers from a severe restacking problem during the electrode fabrication process, limiting the ion transport kinetics and the accessibility of ions in the electrodes, especially in the direction normal to the electrode surface. Herein, we report a NH3-induced in situ etching strategy to fabricate 3D-interconnected porous MXene/carbon dots (p-MC) films for high-performance flexible supercapacitor. The pre-intercalated carbon dots (CDs) first prevent the restacking of MXene to expose more inner electrochemical active sites. The partially decomposed CDs generate NH3 for in situ etching of MXene nanosheets toward 3D-interconnected p-MC films. Benefiting from the structural merits and the 3D-interconnected ionic transmission channels, p-MC film electrodes achieve excellent gravimetric capacitance (688.9 F g-1 at 2 A g-1) and superior rate capability. Moreover, the optimized p-MC electrode is assembled into an asymmetric solid-state flexible supercapacitor with high energy density and superior cycling stability, demonstrating the great promise of p-MC electrode for practical applications.

MXenes as an emerging 2D materials have attracted significant attention in the fields of energy storage, sensing, etc. However, superlong time-consuming (several days) and low-production (similar to 1 g) of dangerously F-contained acid etching method intrinsically make it inapplicable for large-scale manufacture of MXenes. Herein, we pro-pose a supercritical etching method for the mass preparation of various MXenes assisted by supercritical carbon dioxide. The enhanced thermal power of supercritical carbon dioxide molecules helps significantly accelerate the etching rate of the MAX phase and increase the layer spacing of MXenes. As a result, five typical MAX materials (Ti3AlC2, Nb2AlC, Ti2AlC, Mo2Ga2C, and Ti3AlCN) can be exfoliated into the corresponding MXenes (Ti(3)C(2)Tx, Nb(2)CTx, Ti(2)CTx, Mo(2)CTx, and Ti(3)CNTx) with the yield of similar to 1 Kg within 2-5 h. We further developed a Ti(3)C(2)Tx- based Na-ion battery with a remarkable capacity of 100 mAh g(-1) at 100 mA g(-1) (70 mAh g(-1) at 1000 mA g(-1)) and excellent coulombic efficiency of nearly 100%. The proposed supercritical etching strategy provides a platform for the commercial production of MXenes.

Ramie carbon from the massively three-harvests-one-year and naturally channel-structured ramie straw is typically emerging as the excellent waste-biomass-utilization target toward high-performance electrochemical energy storage devices. However, the traditional activation strategies less fully optimize the pore distribution for the purpose of optimal energy storage capability. Herein, we reported the hierarchically interconnected three-dimensional ramie porous carbon based on waste ramie straw by internally-externally molecules-scissored activation strategy with pre-embedded KOH and re-added KOH molecules coordinately (in-ex-RPC). Benefiting from the synergistically internal-external activation strategy, this in-ex-RPC displays an excellent specific capacitance of 300 F g-1, which is much better than that of ramie porous carbon singly through pre-embedded KOH activation (in-RPC, 194 F g-1) or directly adding KOH activation (ex-RPC, 213 F g-1). Based on it, the aqueous symmetric supercapacitors display a cyclic capacity of 90.9%-retaining after 20,000 cycles at 5 A g-1. Therefore, this work may provide an effective strategy for energy storage applications of waste ramie straw and for further promoting waste biomass utilization.

MXenes have shown great potential as an emerging two-dimensional (2D) material for micro-supercapacitors (MSCs) due to their high conductivity, rich surface chemistry, and high capacity. However, MXene sheets inherently tend to lay flat on the substrate during film formation to assemble into compact stacked structures, which hinders ion accessibility and prolongs ion transport paths, leading to highly dependent electrochemical properties on the thickness of the film. Here, we demonstrate a vertically aligned Ti3C2Tx MXene based micro-supercapacitor with an excellent electrochemical performance by a liquid nitrogen-assisted freeze-drying method. The vertical arrangement of the 2D MXene sheets allows for directional ion transport, enabling the vertical-MXene based MSCs to exhibit thickness-independent electrochemical properties even in thick films. In addition, the MSCs displayed a high areal capacitance of 87 mF cm-2 at 10 mV s-1 along with an excellent stability of ∼87.4% after 10 000 charge-discharge cycles. Furthermore, the vertical-MXene approach proposed here is scalable and can be extended to other systems involving directional transport.

MXene-based microsupercapacitors (MSCs) have promoted the development of on-chip energy storage for miniaturized and portable electronics due to the small size, high power density and integration density. However, restricted energy density and operating voltage invariably create obstacles to the practical application of MSCs. Here, we report a symmetric MXene-based on-chip MSC, achieving an ultrahigh energy density of 75 mWh cm-3 with high operating voltage of 1.2 V, which are almost the highest values among all reported symmetric MXene MSCs. The adjustment strategy of acetone on the viscosity and surface tension of MXene ink, along with the natural sedimentation strategy, can effectively prevent the orderly stacking of MXene sheets. Further, we developed an all-in-one Si-electronics with three series MSCs through laser-etching technology, obviously presenting high integration capacity and processing compatibility. Thus, this work will contribute to the development of high integration all-in-one electronics with high energy density MXene-based MSCs.

Shape-controlled nano-silvers have great promise for the practical applications in devices due to their unique electronic properties. Although variously complicated nano-silver structures have been reported, the accurate control of one-dimension (1D) oriented assembly of Ag crystals remains challenging. Here, we innovatively fabricated Ag nano-lines (AgNLs) with pair directed arrays along the parallel boundary of the templated nanofibers. The multi-stage template mechanism of polyvinyl pyrrolidone (PVP) molecules based on electrospinning and UV irradiation played an indispensable effect in the coherent single-nanoparticle assembly of Ag nanocrystals in the nanofibers. Using the electrochemically assisted analysis, we found the special electron conduction and sensibility of water molecules in the AgNLs. Further, based on the tightly connected and gapped assembly characteristics of the AgNLs, we integrated the AgNLs arrays as the nanoscale humidity sensors which behaved the different sensing properties at low, middle and high relative humidity (RH). Our research demonstrated the application of AgNLs in the humidity-relative fields and provided a new strategy for fabricating the nanoscale 1D directed non-contact humidity sensors.

MXene with excellent flexibility, metallic conductivity, and ultra-high capacitance, makes a promising electrode for flexible supercapacitor. But the serious restacking phenomenon between MXene layers undesirably limits the ion transport kinetics and substantially reduces ion storage sites, badly restricting the rate capability and storage capacity of supercapacitor. Here, we constructed a free-standing, flexible, structurally 3D-interconnected and hydronium ion penetrable MXene/Graphdiyne nanotube (MG) composite film by employing graphdiyne nanotubes (GDY-NTs) with inherent in-plane pores for horizontal-vertical intercalation among MXene layers. Benefiting from the above synergistic effect, this composite film presents a greatly-improved capacitance of 337.4 F g(-1) (337.4 C g(-1)) and an obviously-enhanced rate capability of 73 %-remaining at 100 mV s 1, which is much better than those of the pure Ti(3)C(2)Tx films (230.8 F g 1, 55 %-remaining). Based on it, we developed an asymmetric solid-state flexible supercapacitor with a high energy density of 19.7 Wh kg(-1) at the power density of 750 W kg(-1) and a capacitance retention of 88.2 % after 10 000 cycles at 8 A g(-1). Evidently, this work provides a new route to solve the restacking issue of MXene for high-performance flexible supercapacitor.

Printed electronics are expected to facilitate the widespread distributed wearable electronics in the era of the Internet of things. However, developing cheap and stable electrode inks remains a significant challenge in the printed electronics industry and academic community. Here, overcoming the weak hydrophilicity of polyaniline, a low-cost, easy-fabricating, and air-stable conducting polymer (CP) ink is devised through a facile assemble-disperse strategy delivering a high conductivity in the order of 10-2 S cm-1 along with a remarkable specific capacitance of 386.9 F g-1 at 0.5 A g-1 (dehydrated state). The additive-free CP ink is directly employed to print wearable micro-supercapacitors (MSCs) via the spray-coating method, which deliver a high areal capacitance (96.6 mF cm-2 ) and volumetric capacitance (26.0 F cm-3 ), outperforming most state-of-the-art CP-based supercapacitors. This work paves a new approach for achieving scalable MSCs, thus rendering a cost-effective, environmentally friendly, and pervasive energy solution for next-generation distributed electronics.

High electrical conductivity and all-open microstructure characteristics intrinsically endow both graphene and MXenes with superior electrochemical energy storage capability. However, the above two-dimensional (2D) thicker electrodes (>20 μm) severely dilute their unique rapid electronic-ionic transferring characteristic, posing a paradox of high gravimetric and high volumetric capacitive properties due to massively excessive macropores or an unduly restacked issue. Herein, we elaborately construct novel monolithic NH2-graphene and Ti3C2Tx MXene (NG@MX) composites through dual-functional induced self-assembly with the help of both covalent and hydrogen bonding interactions. Notably, much thicker monolithic NG@MX electrodes (>90 μm) fabricated by a conventional roll-coating method without any further compaction treatment can simultaneously deliver two times gravimetric (gra.) and volumetric (vol.) performance than those of pure graphene (in vol.) or MXene (in gra.) materials. Moreover, monolithic NG@MX-based supercapacitors can remarkably present two times energy density as that of graphene and four times as MXene, respectively. Such greatly enhanced electrochemical properties are closely related to the appropriate equilibrium of the volumetric density and the open structure, which can effectively guarantee the rapid transfer of both electrons and ions in the thick monolithic NG@MX electrodes. Undoubtedly, dual-functional chemical bonding-induced self-constructing NG@MX monoliths efficiently solve the long-existing gra. and vol. capacitive paradox of the thicker 2D materials used in supercapacitors, which will guide the design of high-performance capacitive materials and promote their practical application in electrochemical energy storage.

MXenes are attracting growing attentions from scientific community owing to their decent electric and ionic conductivity, highly accessible surface area, and the presence of redox-active sites. Herein, an acid molecular scissor is proposed to artificially tailor Ti3CNTx MXene at the atomic scale and create defective nanosheets and redox-active sites. The tailored Ti3CNTx MXene exhibits a significantly improved electrochemical performance with the specific capacitance reaching 376 F g(-1) and 230.68 mF cm(-2), much higher than that of original Ti3CNTx MXene (237 F g(-1) and 168.27 mF cm(-2)). The tailored Ti3CNTx MXene was further assembled into a micro-supercapacitor, which demonstrates a high volumetric capacitance of 250 F cm(-3), a high energy density of 12.46 mW h cm(-3) at a power density of 0.43 W cm(-3). This work offers a new strategy to reinvent MXenes at the atomic scale with largely enhanced redox-active sites for energy storage, catalysis, solid lubricants and elec-tromagnetic interference shielding.

In article number 2100956, Haitao Zhang, Weiqing Yang, Jun Chen, and co-workers invent a low-cost, scalable, and air-stable conducting polymer ink via a facile assemble-disperse strategy for printed micro-supercapacitors, which exhibits remarkable energy storage capacity as a power source for wearable bioelectronics in the era of Internet of Things.

The excellent intrinsic properties of two-dimensional MXenes, such as hydrophilic, good electrical and thermal conductivity, excellent film forming ability, render them potential candidates for many innovations in different areas. High-quality MXene plays an important role in enhancing the devices’ performances in various applications. However, the synthesis of high-quality MXenes has not been well documented. Herein, we summarize the up-to-date preparation strategies of MXene and compare their respective advantages and disadvantages. We particularly outline the criteria of high-quality MXenes, and focus on the influence of different synthetic methods on the surface functional groups, morphology (size and defect), layer spacing of MXene, etc. The challenges on MXenes’ preparation and their future perspectives are also provided. We anticipate this perspective will provide prospective guidance for synthesizing desirable high-quality MXenes.

Ti3CNTx MXenes with unique electrical conductivity can be widely applied for supercapacitors and electromagnetic shielding. However, its relatively low-yield quaternary nitrogen-containing Ti3AlCN ceramics precursor (less than 50％), due to the inevitable Al segregation during the synthesizing process, significantly hindered its widely commercial applications. Herein, we employed the controllable AlN-oversaturation precursor strategy to precisely tune the phase transition point of quaternary Ti3AlCN ceramics to obtain high-yield Ti3AlCN precursor for the purpose of high conductivity Ti3CNTx MXenes. Combined energy dispersive X-ray spectrometer (XRD) with X-ray photoelectron spectroscopy (XPS) characterizations, the yield of the quaternary nitrogen-containing Ti3AlCN ceramics was evidently proved to be up to 70％, which is 1.4 times than that of previously reported works. Such relatively high-yield quaternary Ti3AlCN is mainly ascribed to the elimination of Al segregation. Based on it, we further developed accordion-like two-dimensional (2D) MXene via hydrofluoric acid etch and vacuum freeze-dry. This novel accordion-like 2D Ti3CNTx MXene possesses high electrochemical capacitive properties (209 F/g). Therefore, this controllable AlN-oversaturation precursor strategy will pave a way to exploit costly high-yield MAX ceramics precursor for high conductivity MXenes and also play a powerful role in promoting their practical applications including electrical and magnetic engineering fields.

Piezoelectric-based wearable electronics promise potential applications in human physiological monitoring for disease prediction, diagnose and rehabilitation. Still, it is a big challenge to fulfill excellent compatibility and precisely monitoring of the complexly subtle physiological signals simultaneously. Here, we prepared a three-dimensional hierarchically interlocked PVDF/ZnO nanofiber-based piezoelectric sensor through epitaxial growing ZnO nano rods (NRs) on the surface of electrospun PVDF nanofibers, which enables the fiber-based physiological monitoring electronics (PME) of well flexibility and high gas permeability. Due to synergistic piezoelectric effect of the effectively deformed interlocked ZnO NRs and the uniformly orientated PVDF nanofibers with high electroactive phase, the sensitivities of PME in both pressing and bending modes have been greatly improved 6 times and 41 times than that of pure PVDF nanofibers respectively. On this basis, the designed PME can precisely detect the complexly subtle physiological signals of respiration, wrist pulse and muscle behavior. Moreover, a sensitive gait recognition system was successfully developed based on PME arrays. Therefore, this proposed fiber-based device provides an alternative strategy to monitor the human subtle physiological signals and demonstrates promising potential in the expanded application of healthcare and clinical diagnosis.

Multifunctional micro‐force sensing in one device is an urgent need for the higher integration of the smaller flexible electronic device toward wearable health‐monitoring equipment, intelligent robotics, and efficient human–machine interface. Herein, a novel microchannel‐confined MXene‐based flexible piezoresistive sensor is demonstrated to simultaneously achieve multi‐types micro‐force sensing of pressure, sound, and acceleration. Benefiting from the synergistically confined effect of the fingerprint‐microstructured channel and the accordion‐microstructured MXene materials, the as‐designed sensor remarkably endows a low detection limit of 9 Pa, a high sensitivity of 99.5 kPa−1, and a fast response time of 4 ms, as well as non‐attenuating durability over 10 000 cycles. Moreover, the fabricated sensor is multifunctionally capable of sensing sounds, micromotion, and acceleration in one device. Evidently, such a multifunctional sensing characteristic can highlight the bright prospect of the microchannel‐confined MXene‐based micro‐force sensor for the higher integration of flexible electronics.

Solid-state composite polymer electrolytes (CPEs) usually suffer from intrinsic low ionic conductivity and a solid-solid interface, badly inhibiting their widespread commercial application in all-solid-state Li-metal battery (ASSLMB) energy storage. Herein, a synergetic strategy using strong Lewis acid-base and weak hydrogen bonds was employed for self-assembly in situ construction of three-dimensional (3D) network-structured poly(ethylene oxide) (PEO) and SiO2 CPEs (PEO@SiO2). Ascribed to this synergistically rigid-flexible coupling dynamic strategy, a harmonious incorporation of monodispersed SiO2 nanoparticles into PEO could remarkably reduce crystallinity of PEO, significantly enhancing the ionic conductivity (∼1.1 × 10-4 S cm-1 at 30 °C) and dramatically facilitating solid electrolyte interface stabilization (electrochemical stability window > 4.8 V at 90 °C). Moreover, the PEO@SiO2-based ASSLMBs possess excellent rate capability over a wide temperature range (∼105 mA h g-1 under 2 C at 90 °C), high temperature cycling capacity (retaining 90 mA h g-1 after 100 cycles at 90 °C), and high specific capacity (146 mA h g-1 under 0.3 C at 90 °C). Unambiguously, these high ionic conductivity CPEs along with excellent flexibility and safety can be one of the most promising candidates for high-performance ASSLMBs, evidently revealing that this synergistically rigid-flexible coupling dynamic strategy will open up a way to exploit the novel high ionic conductivity solid-state electrolytes.