Stamping of Flexible,Coplanar Micro‐Supercapacitors Using MXene Inks

The fast growth of portable smart electronics and internet of things have greatly stimulated the demand for miniaturized energy storage devices. Micro‐supercapacitors (MSCs), which can provide high power density and a long lifetime, are ideal stand‐alone power sources for smart microelectronics. However, relatively few MSCs exhibit both high areal and volumetric capacitance. Here rapid production of flexible MSCs is demonstrated through a scalable, low‐cost stamping strategy. Combining 3D‐printed stamps with arbitrary shapes and 2D titanium carbide or carbonitride inks (Ti₃C₂T_x and Ti₃CNT_x, respectively, known as MXenes), flexible all‐MXene MSCs with controlled architectures are produced. The interdigitated Ti₃C₂T_x MSC exhibits high areal capacitance: 61 mF cm^?2 at 25 µA cm^?2 and 50 mF cm^?2 as the current density increases by 32 fold. The Ti₃C₂T_x MSCs also showcase capacitive charge storage properties, good cycling lifetime, high energy and power densities, etc. The production of such high‐performance Ti₃C₂T_x MSCs can be easily scaled up by designing pad or cylindrical stamps, followed by a cold rolling process. Collectively, the rapid, efficient production of flexible all‐MXene MSCs with state‐of‐the‐art performance opens new exciting opportunities for future applications in wearable and portable electronics.     

Polysiloxane Cross-Linked Mechanically Stable MXene-Based Lithium Host for Ultrastable Lithium Metal Anodes with Ultrahigh Current Densities and Capacities

The applications of lithium metal anode are limited by uncontrollable lithium dendrite growth and infinite volume changes during cycling. These fundamental issues are exacerbated at high cycling current densities and capacities. Herein, a mechanically stable and resilient lithium metal host is fabricated by covalently cross-linking a highly-conductive and lithiophilic MXene/silver nanowire scaffold through a silylation reaction between MXene nanosheets and polysiloxane. Compared with the control sample (an MXene scaffold assembled by weak van der Waals forces), the covalently cross-linked MXene scaffold displays excellent mechanical strength and resilience, which is conducive to buffer the large internal stress fluctuations generated during rapid and deep lithium plating-stripping and guaranteed that the integrated framework structure is maintained during long-term charging-discharging cycles. When used in a symmetric cell, the lithium composite anode based on the covalently cross-linked MXene host affords an unprecedented cyclic lithium plating-stripping stability of a record-high 3000 h lifespan at an ultrahigh current density (20 mA cm⁻²) and areal capacity (10 mAh cm⁻²). When this composite anode is coupled with a LiNi_0.5Co_0.2Mn_0.3O₂ cathode, the full cell delivers an ultrahigh rate of 10 C for up to 1000 cycles, with an average capacity decay of 0.043% per cycle and a stable Coulombic efficiency of 98.7%.     

Flexible MXene-Based Composites for Wearable Devices

In recent decades, flexible and wearable devices have been extensively investigated due to their promising applications in portable mobile electronics and human motion monitoring. MXene, a novel growing family of 2D nanomaterials, demonstrates superiorities such as outstanding electrical conductivity, abundant terminal groups, unique layered-structure, large surface area, and hydrophilicity, making it to be a potential candidate material for flexible and wearable devices. Numerous pioneering works are devoted to develop flexible MXene-based composites with various functions and designed structures. Therefore, the latest progress of the flexible MXene-based composites for wearable devices is summarized in this review, focusing on the preparation strategies, working mechanisms, performances, and applications in sensors, supercapacitors, and electromagnetic interference shielding materials. Moreover, the current challenges and future outlooks are also discussed.     

Controlled Assembly of MXene Nanosheets as an Electrode and Active Layer for High-Performance Electronic Skin

MXenes are an emerging class of 2D transition metal carbides and nitrides. They have been widely used in flexible electronics owing to their excellent conductivity, mechanical flexibility, and water dispersibility. In this study, the electrode and active layer applications of MXene materials in electronic skins are realized. By utilizing vacuum filtration technology, few-layer MXene electrodes are integrated onto the top and bottom surfaces of the 3D polyacrylonitrile (PAN) network to form a stable electronic skin. The fabricated flexible device with Ti₃C₂T_x MXene electrodes outperforms those with other electrodes and exhibits excellent device performance, with a high sensitivity of 104.0 kPa⁻¹, fast response/recovery time of 30/20 ms, and a low detection limit of 1.5 Pa. Furthermore, the electrode and the constructed MXene/PAN-based flexible pressure sensor exhibit robust mechanical stability and can survive 240 bending cycles. Such a robust, flexible device can be enlarged or folded like a jigsaw puzzle or origami and transformed from 2D to 3D structures; moreover, it can detect tiny movements of human muscles, such as movements corresponding to sound production and intense movements during bending of fingers.     

A Moisture-Driven Actuator Based on Polydopamine-Modified MXene/Bacterial Cellulose Nanofiber Composite Film

As a new 2D material, MXene (Ti₃C₂T_x) shows great potential as a smart multifunctional humidity-responsive actuator due to its high hydrophilicity and conductivity but suffers from ambient oxidation and mechanical brittleness. Inspired by the mussels, the authors overcome these weaknesses by designing and fabricating a nacre-like and lamellar-structured composite film that consists of polydopamine-modified MXene and bacterial cellulose nanofibers, which shows improved properties as a moisture-driven actuator. The actuator has high conductivity (2848 S cm^–1), excellent tensile strength (406 MPa), and toughness (15.3 MJ m^–3). Moreover, the actuator is highly sensitive to moisture with the advantages of fast response (1.6 s), large deformation (176°), and high actuation force output (6.5 N m^–2). It is additionally demonstrated that the actuator works as the electrical switch, robotic arm, and motor in a moisture-driven manner. Overall, it is believed that this work improves the drawbacks of current MXene-based actuators, laying the groundwork for their wider applications as moisture-driven devices.     

Neuro-Receptor Mediated Synapse Device Based on Crumpled MXene Ti3C2Tx Nanosheets

Artificial synapse devices can simulate the neuro-transmission in a completely electronic way, but the neuro-biochemical responses are still a challenge for them. Here, a novel three-terminal (3T) neuro-receptor-mediated (acetylcholine receptor (AChR) as a proof-of-concept) synapse device (NR-S) based on the solution–MXene interface is presented. It is demonstrated that the synaptic plasticity behavior triggered by neuro-transmitter (ACh) and the pathogenic autoantibody (AChR-ab) induced neuronal damage that can be detected and recorded. The improved sensitivities, including the linear responses to ACh in an extremely wide range (1 am to 1 µm ) and ultra-low (1 am ) limit of detection, are obtained using crumpled MXene. Furthermore, the ability of the proposed NR-S to determine the tiny neuronal injury caused by only 10 ng mL⁻¹ AChR-ab is conceptually proven. Collectively, the novel 3T NR-S has good application prospects in the field of the neuromorphic chip for not only realizing the bionic simulation of the chemically modulated or injured neuro-transmission but also offering an efficient experimental platform for neuro-biochemistry studies.     

A Novel Approach for Achieving High‐Efficiency Photoelectrochemical Water Oxidation in InGaN Nanorods Grown on Si System: MXene Nanosheets as Multifunctional Interfacial Modifier

MXene nanosheets with attractive electrical conductivity and tunable work function have been adopted as multifunctional interfacial modifier between InGaN nanorods and Si for photoelectrochemical water oxidation for the first time. Compared to bare InGaN/Si systems, MXene interfacial layers give rise to an ultralow onset potential of 75 mV versus reversible hydrogen electrode (RHE), which is the highest ever reported for InGaN‐ or Si‐based photoanodes by interfacial modification. Furthermore, the modified photoanode exhibits a significantly enhanced photocurrent density (7.27 mA cm^?2) at 1.23 V versus RHE, which is about 10 times higher than that achieved with the InGaN/Si photoanode. The detailed mechanism demonstrates that the formed type‐II band alignment in InGaN/MXene heterojunction and an Ohmic junction at the MXene/Si interface make MXene an ideal electron‐migration channel to enhance charge separation and transfer process. This synergetic effect of MXene can significantly decrease the charge resistance at semiconductor/Si and semiconductor/electrolyte hetero‐interfaces, eventually resulting in the fast hole injection efficiency of 82% and superior stability against photocorrosion. This work not only provides valuable guidance for designing high‐efficiency photoelectrodes through the integration of multiscale and multifunctional materials, but also presents a novel strategy for achieving high‐performance artificial photosynthesis by introducing interfacial modifier.     

二维纳米材料MXene的研究进展

MXene是一类新型碳/氮化物二维纳米层状材料,一般是利用化学刻蚀的手段通过选择性刻蚀掉前驱体MAX相中的A原子层而得到。其通式可表示为M_(n+1)X_nT_x,其中M代表早期过渡族金属,X代表碳和/或氮,T_x代表MXene在刻蚀过程中产生的附着在其表面的官能团(-OH、-F、=O、等)。采用一定的手段将多层MXene剥落,可获得类石墨烯形貌的单层MXene。MXene除了具备传统二维材料的性能外,还兼具良好的导电性、亲水性、透光性、柔韧性以及能量储存性能,在复合材料、润滑剂、环境污染治理、电池、电容器、催化、传感器、抗菌等领域具有潜在的应用价值。文章总结了MXene的制备、结构、性能和应用等方面的最新成果,并展望了其今后的研究方向。     

Boosting the Photocatalytic Ability of Cu2O Nanowires for CO2 Conversion by MXene Quantum Dots

MXene quantum dots (QDs) are emerging 0D nanomaterials. Here, a new heterostructure is developed based on a 1D photoactive semiconductor and a 0D MXene QD for improved photocatalytic reduction of CO₂ into methanol. Specifically, Ti₃C₂ QDs are incorporated onto Cu₂O nanowires (NWs) through a simple self‐assembly strategy. It is demonstrated that Ti₃C₂ QDs not only significantly improve the stability of Cu₂O NWs but also greatly improve their photocatatlytic performance by enhancing charge transfer, charge transport, carrier density, light adsorption, as well as by decreasing band bending edge and charge recombination. The energy level diagram derived from both experimental measurements and theoretical calculations provide further insights of such hierarchical photocatalysis system.