3D Printing of Additive-Free 2D Ti3C2Tx (MXene) Ink for Fabrication of Micro-Supercapacitors with Ultra-High Energy Densities

Direct Ink Writing: A 3D Printing Technology for Diverse Materials

Additive manufacturing (AM) has gained significant attention due to its ability to drive technological development as a sustainable, flexible, and customizable manufacturing scheme. Among the various AM techniques, direct ink writing (DIW) has emerged as the most versatile 3D printing technique for the broadest range of materials. DIW allows printing of practically any material, as long as the precursor ink can be engineered to demonstrate appropriate rheological behavior. This technique acts as a unique pathway to introduce design freedom, multifunctionality, and stability simultaneously into its printed structures. Here, a comprehensive review of DIW of complex 3D structures from various materials, including polymers, ceramics, glass, cement, graphene, metals, and their combinations through multimaterial printing is presented. The review begins with an overview of the fundamentals of ink rheology, followed by an in-depth discussion of the various methods to tailor the ink for DIW of different classes of materials. Then, the diverse applications of DIW ranging from electronics to food to biomedical industries are discussed. Finally, the current challenges and limitations of this technique are highlighted, followed by its prospects as a guideline toward possible futuristic innovations.

Dual Defocused Laser Pyrolysis: A Lasing-Centric Strategy for Defect and Morphological Optimization in Microsupercapacitor Electrodes

Laser-induced graphene (LIG) has shown great potential for controllable and scalable realization of microsupercapacitors (MSCs). However, as is well-known, LIG electrodes suffer from low charge storage capacity and conductance. In this paper, a lasing-centric method is presented for defect control and morphological enhancement in LIG electrodes through unique dual laser pyrolysis. This method encompasses dual lasing pyrolysis, one for the synthesis of defocused LIG, and another for the decoration of Ru nanoparticles to enhance electrochemical performance. Fundamentally, the investigation simultaneously optimizes for defocused lasing distance and lasing speed, which to the best of the author's knowledge, has not been previously reported. The defocused LIG electrode exhibits a remarkably improved electrochemical capacitance of over 25 times (114 mF cm^-2 ) compared to the one based on focused laser-induced graphene (FLIG). As a device demonstration, a flexible and self-healable MSC has been fabricated based on DFLIG/Ru-PEDOT/Au electrodes, exhibiting a high areal specific capacitance (25.7 mF cm^-2 ), excellent electrochemical stability (91% retention of specific capacitance after 8000 cycles), and good self-healing performance (85.6% retention of specific capacitance after two cut-heal cycles). By enhancing material properties via dual defocused laser pyrolysis, this work presents a strategy for highly controllable and scalable realization of electrodes in micro-energy storage devices.

MXene chemistry, electrochemistry and energy storage applications

The diverse and tunable surface and bulk chemistry of MXenes affords valuable and distinctive properties, which can be useful across many components of energy storage devices. MXenes offer diverse functions in batteries and supercapacitors, including double-layer and redox-type ion storage, ion transfer regulation, steric hindrance, ion redistribution, electrocatalysts, electrodeposition substrates and so on. They have been utilized to enhance the stability and performance of electrodes, electrolytes and separators. In this Review, we present a discussion on the roles of MXene bulk and surface chemistries across various energy storage devices and clarify the correlations between their chemical properties and the required functions. We also provide guidelines for the utilization of MXene surface terminations to control the properties and improve the performance of batteries and supercapacitors. Finally, we conclude with a perspective on the challenges and opportunities of MXene-based energy storage components towards future practical applications.

When MXene (Ti3C2Tx) meet Ti/PbO2: An improved electrocatalytic activity and stability

Stable electrode materials with high catalytic activity are urgently required for electrochemical degradation of refractory organic pollutants in wastewater treatment. Herein, high conductive MXene (Ti₃C₂T_x) was firstly fabricated by electrophoretic deposition (EPD) as an interlayer for preparing a novel PbO₂ electrode. The well-conducted Ti₃C₂T_x interlayer significantly improved the electrochemical performance of the EPD-2.0/PbO₂ (EPD time was 2.0 min) electrode with the charge transfer resistance decreased by 9.51 times, the inner active sites increased by 5.21 times and the ∙OH radicals generation ability enhanced by 4.07 times than the control EPD-0/PbO₂ anode. Consequently, the EPD-2.0/PbO₂ electrode achieved nearly 100% basic fuchsin (BF) and 86.78% COD removal efficiency after 3.0 h electrolysis. Therefore, this new PbO₂ electrode presented a promising potential for electrochemical degradation of BF and the new Ti₃C₂T_x middle layer could also be used to fabricate other efficient and stable anodes, such as SnO₂, MnO₂, TiO₂, etc.

3D Printing of Stretchable, Adhesive and Conductive Ti3C2Tx-Polyacrylic Acid Hydrogels

Stretchable, adhesive, and conductive hydrogels have been regarded as ideal interfacial materials for seamless and biocompatible integration with the human body. However, existing hydrogels can rarely achieve good mechanical, electrical, and adhesive properties simultaneously, as well as limited patterning/manufacturing techniques posing severe challenges to bioelectronic research and their practical applications. Herein, we develop a stretchable, adhesive, and conductive Ti3C2Tx-polyacrylic acid hydrogel by a simple pre-crosslinking method followed by successive direct ink writing 3D printing. Pre-polymerization of acrylic acid can be initiated by mechanical mixing with Ti3C2Tx nanosheet suspension, leading to the formation of viscous 3D printable ink. Secondary free radical polymerization of the ink patterns via 3D printing can achieve a stretchable, adhesive, and conductive Ti3C2Tx-polyacrylic acid hydrogel. The as-formed hydrogel exhibits remarkable stretchability (~622%), high electrical conductivity (5.13 S m−1), and good adhesion strength on varying substrates. We further demonstrate the capability of facilely printing such hydrogels into complex geometries like mesh and rhombus patterns with high resolution and robust integration.

Two-dimensional nanomaterials-added dynamism in 3D printing and bioprinting of biomedical platforms: Unique opportunities and challenges

The nanomaterials research spectrum has seen the continuous emergence of two-dimensional (2D) materials over the years. These highly anisotropic and ultrathin materials have found special attention in developing biomedical platforms for therapeutic applications, biosensing, drug delivery, and regenerative medicine. Three-dimensional (3D) printing and bioprinting technologies have emerged as promising tools in medical applications. The convergence of 2D nanomaterials with 3D printing has extended the application dynamics of available biomaterials to 3D printable inks and bioinks. Furthermore, the unique properties of 2D nanomaterials have imparted multifunctionalities to 3D printed constructs applicable to several biomedical applications. 2D nanomaterials such as graphene and its derivatives have long been the interest of researchers working in this area. Beyond graphene, a range of emerging 2D nanomaterials, such as layered silicates, black phosphorus, transition metal dichalcogenides, transition metal oxides, hexagonal boron nitride, and MXenes, are being explored for the multitude of biomedical applications. Better understandings on both the local and systemic toxicity of these materials have also emerged over the years. This review focuses on state-of-art 3D fabrication and biofabrication of biomedical platforms facilitated by 2D nanomaterials, with the comprehensive summary of studies focusing on the toxicity of these materials. We highlight the dynamism added by 2D nanomaterials in the printing process and the functionality of printed constructs.

Roles of Metal Ions in MXene Synthesis, Processing and Applications: A Perspective

With a decade of effort, significant progress has been achieved in the synthesis, processing, and applications of MXenes. Metal ions play many crucial roles, such as in MXene delamination, structure regulation, surface modification, MXene composite construction, and even some unique applications. The different roles of metal ions are attributed to their many interactions with MXenes and the unique nature of MXenes, including their layered structure, surface chemistry, and the existence of multi-valent transition metals. Interactions with metal ions are crucial for the energy storage of MXene electrodes, especially in metal ion batteries and supercapacitors with neutral electrolytes. This review aims to provide a good understanding of the interactions between metal ions and MXenes, including the classification and fundamental chemistry of their interactions, in order to achieve their more effective utilization and rational design. It also provides new perspectives on MXene evolution and exfoliation, which may suggest optimized synthesis strategies. In this respect, the different effects of metal ions on MXene synthesis and processing are clarified, and the corresponding mechanisms are elaborated. Research progress on the roles metal ions have in MXene applications is also introduced.

3D Printed Template-Assisted Assembly of Additive-Free Ti3C2Tx MXene Microlattices with Customized Structures toward High Areal Capacitance

Ti₃C₂T_x MXene is a promising material for electrodes in microsupercapacitors. Recent efforts have been made to fabricate MXene electrodes with designed structures using 3D printing to promote electrolyte permeation and ion diffusion. However, challenges remain in structural design diversity due to the strict ink rheology requirement and limited structure choices caused by existing extrusion-based 3D printing. Herein, additive-free 3D architected MXene aerogels are fabricated via a 3D printed template-assisted method that combines 3D printed hollow template and cation-induced gelation process. This method allows the use of MXene ink with a wide range of concentrations (5 to 150 mg mL^-1) to produce MXene aerogels with high structural freedom, fine feature size (>50 μm), and controllable density (3 to 140 mg cm^-3). Through structure optimization, the 3D MXene aerogel shows high areal capacitance of 7.5 F cm^-2 at 0.5 mA cm^-2 with a high mass loading of 54.1 mg cm^-2. It also exhibits an ultrahigh areal energy density of 0.38 mWh cm^-2 at a power density of 0.66 mW cm^-2.

Three-Dimensional Ti3C2Tx MXene-Prussian Blue Hybrid Microsupercapacitors by Water Lift-Off Lithography

The construction of electrochemical energy-storage devices by scalable thin-film microfabrication methods with high energy and power density is urgently needed for many emerging applications. Herein, we demonstrate an in-plane hybrid microsupercapacitor with a high areal energy density by employing a battery-type CuFe-Prussian blue analogue (CuFe-PBA) as the positive electrode and pseudocapacitive titanium carbide MXene (Ti₃C₂T_x) as the negative electrode. A three-dimensional lignin-derived laser-induced graphene electrode was prepared as the substrate by laser exposure combined with an environmentally friendly water lift-off lithography. The designed hybrid device achieved enhanced electrochemical performance thanks to the ideal match of the two types of high-rate performance materials in proton-based electrolytes and the numerous electrochemically active sites. In particular, the device delivers a high areal capacitance of 198 mF cm^-2, a wide potential window (1.6 V), an ultrahigh rate performance (75.8 mF cm^-2 retained even at a practical/high current density of 100 mA cm^-2), and a competitive energy density of 70.5 and 27.6 μWh cm^-2 at the power densities 0.74 and 52 mW cm^-2, respectively. These results show that the Ti₃C₂T_x/CuFe-PBA hybrid microsupercapacitors are promising energy storage devices in miniaturized portable and wireless applications.

A binder jet 3D printed MXene composite for strain sensing and energy storage application

Polymer composite materials have been proven to have numerous electrical related applications ranging from energy storage to sensing, and 3D printing is a promising technique to fabricate such materials with a high degree of freedom and low lead up time. Compared to the existing 3D printing technique for polymer materials, binder jet (BJ) printing offers unique advantages such as a fast production rate, room temperature printing of large volume objects, and the ability to print complex geometries without additional support materials. However, there is a serious lack of research in BJ printing of polymer materials. In this work we introduce a strategy to print poly(vinyl alcohol) composites with MXene-surfactant ink. By ejecting highly conductive MXene particles onto a PVOH matrix, the resulting sample achieved conductive behaviour in the order of mS m^-1 with demonstrated potential for strain sensing and energy storage. This work demonstrates that BJ printing has the potential to directly fabricate polymer composite materials with different end applications.

"Porous and Yet Dense" Electrodes for High-Volumetric-Performance Electrochemical Capacitors: Principles, Advances, and Challenges

With the ever-rapid miniaturization of portable, wearable electronics and Internet of Things, the volumetric performance is becoming a much more pertinent figure-of-merit than the conventionally used gravimetric parameters to evaluate the charge-storage capacity of electrochemical capacitors (ECs). Thus, it is essential to design the ECs that can store as much energy as possible within a limited space. As the most critical component in ECs, "porous and yet dense" electrodes with large ion-accessible surface area and optimal packing density are crucial to realize desired high volumetric performance, which have demonstrated to be rather challenging. In this review, the principles and fundamentals of ECs are first observed, focusing on the key understandings of the different charge storage mechanisms in porous electrodes. The recent and latest advances in high-volumetric-performance ECs, developed by the rational design and fabrication of "porous and yet dense" electrodes are then examined. Particular emphasis of discussions then concentrates on the key factors impacting the volumetric performance of porous carbon-based electrodes. Finally, the currently faced challenges, further perspectives and opportunities on those purposely engineered porous electrodes for high-volumetric-performance EC are presented, aiming at providing a set of guidelines for further design of the next-generation energy storage devices.

MXene supported surface plasmons on telecommunications optical fibers

MXenes, an emerging class of two-dimensional materials, exhibit characteristics that promise significant potential for their use in next generation optoelectronic sensors. An interplay between interband transitions and boundary effects offer the potential to tune the plasma frequencies over a large spectral range from the near-infrared to the mid-infrared. This tuneability along with the 'layered' nature of the material not only offer the flexibility to produce plasmon resonances across a wide range of wavelengths, but also add a degree of freedom to the sensing mechanism by allowing the plasma frequency to be modulated. Here we show, numerically, that MXenes can support plasmons in the telecommunications frequency range and that surface plasmon resonances can be excited on a standard MXene coated side polished optical fiber. Thus, presenting the tantalising prospect of highly selective distributed optical fiber sensor networks.

Highly Dispersed, Adhesive Carbon Nanotube Ink for Strain and Pressure Sensors

Carbon nanotubes (CNT) with prominent electrical and mechanical properties are ideal candidates for flexible wearable devices. However, their poor dispersity in solvents greatly limits their applications as a conductive ink in the fabrication of wearable sensors. Herein, we demonstrate a kind of CNT-based conductive dispersion with high dispersity and adhesiveness using cellulose derivatives as the solvent, in which γ-aminopropyl triethoxy silane as a cross-linking agent reacts with cellulose to form copolymer networks, and simultaneously it also acts as an initiator to induce the self-polymerization of dopamine. Based on the conductive CNT ink, we also demonstrated textile-based strain sensors by stencil printing and sponge-based pressure sensors by the dipping method. The textile-based strain sensors could respond to external stimuli promptly. Then, the strain sensors were encapsulated via polydimethylsiloxane with the expansion of working ranges from less than 20 to nearly 70%. The encapsulated textile sensors exhibited excellent sensing performance as wearable strain sensors to monitor human motions including smile, throat vibration, finger folding, wrist bending, and elbow twisting. The sponge sensors hold high sensitivity and excellent durability as well. The conductive CNT-based ink provides an alternative idea in the development of flexible wearable devices.

Perspective on Micro-Supercapacitors

The rapid development of portable, wearable, and implantable electronic devices greatly stimulated the urgent demand for modern society for multifunctional and miniaturized electrochemical energy storage devices and their integrated microsystems. This article reviews material design and manufacturing technology in different micro-supercapacitors (MSCs) along with devices integrate to achieve the targets of their various applications in recent years. Finally, We also critically prospect the future development directions and challenges of MSCs.

3D Printed MXene Aerogels with Truly 3D Macrostructure and Highly Engineered Microstructure for Enhanced Electrical and Electrochemical Performance

Assembling 2D materials such as MXenes into functional 3D aerogels using 3D printing technologies gains attention due to simplicity of fabrication, customized geometry and physical properties, and improved performance. Also, the establishment of straightforward electrode fabrication methods with the aim to hinder the restack and/or aggregation of electrode materials, which limits the performance of the electrode, is of great significant. In this study, unidirectional freeze casting and inkjet-based 3D printing are combined to fabricate macroscopic porous aerogels with vertically aligned Ti₃ C₂ T_x sheets. The fabrication method is developed to easily control the aerogel microstructure and alignment of the MXene sheets. The aerogels show excellent electromechanical performance so that they can withstand almost 50% compression before recovering to the original shape and maintain their electrical conductivities during continuous compression cycles. To enhance the electrochemical performance, an inkjet-printed MXene current collector layer is added with horizontally aligned MXene sheets. This combines the superior electrical conductivity of the current collector layer with the improved ionic diffusion provided by the porous electrode. The cells fabricated with horizontal MXene sheets alignment as current collector with subsequent vertical MXene sheets alignment layers show the best electrochemical performance with thickness-independent capacitive behavior.

Microscale Curling and Alignment of Ti3C2T x MXene by Confining Aerosol Droplets for Planar Micro-Supercapacitors

Additive manufacturing techniques have revolutionized the field of fabricating micro-supercapacitors (MSCs) with a high degree of pattern and geometry flexibility. However, traditional additive manufacturing processes are based on the functionality of microstructural modulation, which is essential for device performance. Herein, Ti₃C₂T _x MXene was chosen to report a convenient aerosol jet printing (AJP) process for the in situ curling and alignment of MXene nanosheets. The aerosol droplet provides a microscale regime for curling MXene monolayers while their alignment is performed by the as-generated directional stress derived from the quasi-conical fiber array (CFA)-guided parallel droplet flow. Interdigital microelectrodes were further developed with the curled MXene and a satisfying areal capacitance performance has been demonstrated. Importantly, the AJP technique holds promise for revolutionizing additive manufacturing techniques for fabricating future smart microelectronics and devices not only in the microscale but also in the nanoscale.

3D Printing for Solid-State Energy Storage

Ever-growing demand to develop satisfactory electrochemical devices has driven cutting-edge research in designing and manufacturing reliable solid-state electrochemical energy storage devices (EESDs). 3D printing, a precise and programmable layer-by-layer manufacturing technology, has drawn substantial attention to build advanced solid-state EESDs and unveil intrinsic charge storage mechanisms. It provides brand-new opportunities as well as some challenges in the field of solid-state energy storage. This review focuses on the topic of 3D printing for solid-state energy storage, which bridges the gap between advanced manufacturing and future EESDs. It starts from a brief introduction followed by an emphasis on 3D printing principles, where basic features of 3D printing and key issues for solid-state energy storage are both reviewed. Recent advances in 3D printed solid-state EESDs including solid-state batteries and solid-state supercapacitors are then summarized. Conclusions and perspectives are also provided regarding the further development of 3D printed solid-state EESDs. It can be expected that advanced 3D printing will significantly promote future evolution of solid-state EESDs.

MXene-Copper/Cobalt Hybrids via Lewis Acidic Molten Salts Etching for High Performance Symmetric Supercapacitors

MXenes have attracted great interests as supercapacitors due to their metallic conductivity, high density, and hydrophilic nature. Herein we report Ti₃ C₂ -Cu/Co hybrids via molten salt etching in which the existence of metal atoms and their interactions with MXene via surficial O atoms were elucidated by XAFS for the first time. The electrochemical investigation of Ti₃ C₂ -Cu electrode demonstrated the pseudocapacitive contribution of Cu and a splendid specific capacitance of 885.0 F g^-1 at 0.5 A g^-1 in 1.0 M H₂ SO₄ . Symmetric supercapacitor Ti₃ C₂ -Cu//Ti₃ C₂ -Cu was demonstrated with operating voltage of 1.6 V, areal capacitance of 290.5 mF cm^-2 at 1 mA cm^-2 , and stability over 10 000 cycles. It delivered an areal energy density of 103.3 μWh cm^-2 at power density of 0.8 mW cm^-2 , based on which a supercapacitor pouch was fabricated. It provides deeper insights into the molten salt mechanism and strategies for designing MXene-based materials for electrochemical energy storage.

Synthesis and Electronic Applications of Particle-Templated Ti3C2Tz MXene-Polymer Films via Pickering Emulsion Polymerization

MXene/polymer composites have gained widespread attention due to their high electrical conductivity and extensive applications, including electromagnetic interference (EMI) shielding, energy storage, and catalysis. However, due to the difficulty of dispersing MXenes in common polymers, the fabrication of MXene/polymer composites with high electrical conductivity and satisfactory EMI shielding properties is challenging, especially at low MXene loadings. Here, we report the fabrication of MXene-armored polymer particles using dispersion polymerization in Pickering emulsions and demonstrate that these composite powders can be used as feedstocks for MXene/polymer composite films with excellent EMI shielding performance. Ti₃C₂T_z nanosheets are used as the representative MXene, and three different monomers are used to prepare the armored particles. The presence of nanosheets on the particle surface was confirmed by X-ray photoelectron spectroscopy and scanning electron microscopy. Hot pressing the armored particles above T_g of the polymer produced Ti₃C₂T_z/polymer composite films; the films are electrically conductive because of the network of nanosheets templated by the particle feedstocks. For example, the particle-templated Ti₃C₂T_z/polystyrene film had an electrical conductivity of 0.011 S/cm with 1.2 wt % of Ti₃C₂T_z, which resulted in a high radio frequency heating rate of 13-15 °C/s in the range of 135-150 MHz and an EMI shielding effectiveness of ∼21 dB within the X band. This work provides a new approach to fabricate MXene/polymer composite films with a templated electrical network at low MXene loadings.

2D Silicene Nanosheets for High-Performance Zinc-Ion Hybrid Capacitor Application

Supercapacitors possessing fast-charging characteristics and long lifespan are becoming increasingly important for powering portable and smart energy storage devices, and combining capacitive and battery-type materials into an integrated device is an effective method for increasing the overall performance of capacitors. Silicene is being designed as a cathode for the development of enhanced capacitance and ultra-cycle stable zinc-ion hybrid capacitors. Possessing a maximum areal capacity of 14 mF cm^-2, a maximum power density of 9 mW cm^-2, capacitance retention of 112% even after 10 000 cycles, and an unexpectedly high energy density of 23 mJ cm^-2, this achievement of the zinc-ion hybrid capacitor would be superior to that of previously reported silicon-based supercapacitors. The DFT calculations further reveal that Zn ions dominate the capacitive behavior of the silicene electrode. The support association between silicene and zinc-ion hybrid capacitors so that they can take advantage of each other's strengths, which takes electrochemical energy technology to a stage, offering a straightforward proposal for integration and implementation of silicon-based materials.

Prospects and Challenges of MXenes as Emerging Sensing Materials for Flexible and Wearable Breath-Based Biomarker Diagnosis

A fully integrated, flexible, and functional sensing device for exhaled breath analysis drastically transforms conventional medical diagnosis to non-invasive, low-cost, real-time, and personalized health care. 2D materials based on MXenes offer multiple advantages for accurately detecting various breath biomarkers compared to conventional semiconducting oxides. High surface sensitivity, large surface-to-weight ratio, room temperature detection, and easy-to-assemble structures are vital parameters for such sensing devices in which MXenes have demonstrated all these properties both experimentally and theoretically. So far, MXenes-based flexible sensor is successfully fabricated at a lab-scale and is predicted to be translated into clinical practice within the next few years. This review presents a potential application of MXenes as emerging materials for flexible and wearable sensor devices. The biomarkers from exhaled breath are described first, with emphasis on metabolic processes and diseases indicated by abnormal biomarkers. Then, biomarkers sensing performances provided by MXenes families and the enhancement strategies are discussed. The method of fabrications toward MXenes integration into various flexible substrates is summarized. Finally, the fundamental challenges and prospects, including portable integration with Internet-of-Thing (IoT) and Artificial Intelligence (AI), are addressed to realize marketization.

A Multi-Scale Structural Engineering Strategy for High-Performance MXene Hydrogel Supercapacitor Electrode

MXenes as an emerging two-dimensional (2D) material have attracted tremendous interest in electrochemical energy-storage systems such as supercapacitors. Nevertheless, 2D MXene flakes intrinsically tend to lie flat on the substrate when self-assembling as electrodes, leading to the highly tortuous ion pathways orthogonal to the current collector and hindering ion accessibility. Herein, a facile strategy toward multi-scale structural engineering is proposed to fabricate high-performance MXene hydrogel supercapacitor electrodes. By unidirectional freezing of the MXene slurry followed by a designed thawing process in the sulfuric acid electrolyte, the hydrogel electrode is endowed with a three-dimensional (3D) open macrostructure impregnated with sufficient electrolyte and H+ -intercalated microstructure, which provide abundant active sites for ion storage. Meanwhile, the ordered channels bring through-electrode ion and electron transportation pathways that facilitate electrolyte infiltration and mass exchange between electrolyte and electrode. Furthermore, this strategy can also be extended to the fabrication of a 3D-printed all-MXene micro-supercapacitor (MSC), delivering an ultrahigh areal capacitance of 2.0 F cm-2 at 1.2 mA cm-2 and retaining 1.2 F cm-2 at 60 mA cm-2 together with record-high energy density (0.1 mWh cm-2 at 0.38 mW cm-2 ).

Flexible Photodriven Actuator Based on Gradient-Paraffin-Wax-Filled Ti3C2Tx MXene Film for Bionic Robots

Due to their high flexibility and adaptability, bionic robots have great potential in applications such as healthcare, rescue, and surveillance. The flexible actuator is an essential component of the bionic robot and determines its performance. Even though much progress has been achieved in bionic robot research, there still exists a great challenge in preparing a flexible actuator with a large stroke, high sensitivity, fast response, low triggering power, and long lifetime. This study presents a flexible actuator based on a paraffin wax and Ti₃C₂T_x MXene (PW-MX) film composite. Such a flexible actuator delivers an excellent actuation performance, including a large curvature change (2.2 × 10² m^-1), high thermal sensitivity (4.6 m^-1/°C), low triggering power of light (76 mW/cm²), wavelength selectivity, fast response (0.38 s), and long lifetime (>20000 cycles). Due to the high thermal sensitivity and the strong infrared absorption of the PW-MX film, crawling motion of an inchworm robot based on PW-MX film can be triggered by infrared irradiation from the human finger. To mimic living organisms with bioluminescence, we prepared a PW-MX actuator with green fluorescence by doping PW-MX film with CdSe/ZnS quantum dots. The integration of luminescent function enables the PW-MX actuator to deliver information under light stimulation and to camouflage under a background of green foliage actively. With its merits of ease of fabrication and high actuation performance, the flexible PW-MX actuator is expected to lend itself to more applications in the future.

Direct Ink Writing of Highly Conductive MXene Frames for Tunable Electromagnetic Interference Shielding and Electromagnetic Wave-Induced Thermochromism

3D printing of MXene frames with tunable electromagnetic interference shielding efficiency is demonstrated. Highly conductive MXene frames are reinforced by cross-linking with aluminum ions. Electromagnetic wave is visualized by electromagnetic-thermochromic MXene patterns. The highly integrated and miniaturized next-generation electronic products call for high-performance electromagnetic interference (EMI) shielding materials to assure the normal operation of their closely assembled components. However, the most current techniques are not adequate for the fabrication of shielding materials with programmable structure and controllable shielding efficiency. Herein, we demonstrate the direct ink writing of robust and highly conductive Ti₃C₂T_x MXene frames with customizable structures by using MXene/AlOOH inks for tunable EMI shielding and electromagnetic wave-induced thermochromism applications. The as-printed frames are reinforced by immersing in AlCl₃/HCl solution to remove the electrically insulating AlOOH nanoparticles, as well as cross-link the MXene sheets and fuse the filament interfaces with aluminum ions. After freeze-drying, the resultant robust and porous MXene frames exhibit tunable EMI shielding efficiencies in the range of 25-80 dB with the highest electrical conductivity of 5323 S m^-1. Furthermore, an electromagnetic wave-induced thermochromic MXene pattern is assembled by coating and curing with thermochromic polydimethylsiloxane on a printed MXene pattern, and its color can be changed from blue to red under the high-intensity electromagnetic irradiation. This work demonstrates a direct ink printing of customizable EMI frames and patterns for tuning EMI shielding efficiency and visualizing electromagnetic waves.

Air-Stable Conductive Polymer Ink for Printed Wearable Micro-Supercapacitors

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.

Strain Sensing Coatings for Large Composite Structures Based on 2D MXene Nanoparticles

Real-time strain monitoring of large composite structures such as wind turbine blades requires scalable, easily processable and lightweight sensors. In this study, a new type of strain-sensing coating based on 2D MXene nanoparticles was developed. A Ti₃C₂T_z MXene was prepared from Ti₃AlC₂ MAX phase using hydrochloric acid and lithium fluoride etching. Epoxy and glass fibre-reinforced composites were spray-coated using an MXene water solution. The morphology of the MXenes and the roughness of the substrate were characterised using optical microscopy and scanning electron microscopy. MXene coatings were first investigated under various ambient conditions. The coating experienced no significant change in electrical resistance due to temperature variation but was responsive to the 301-365 nm UV spectrum. In addition, the coating adhesion properties, electrical resistance stability over time and sensitivity to roughness were also analysed in this study. The electromechanical response of the MXene coating was investigated under tensile loading and cyclic loading conditions. The gauge factor at a strain of 4% was 10.88. After 21,650 loading cycles, the MXene coating experienced a 16.25% increase in permanent resistance, but the response to loading was more stable. This work provides novel findings on electrical resistance sensitivity to roughness and electromechanical behaviour under cyclic loading, necessary for further development of MXene-based nanocoatings. The advantages of MXene coatings for large composite structures are processability, scalability, lightweight and adhesion properties.

Development of 3D-Printed MWCNTs/AC/BNNTs Ternary Composite Electrode Material with High-Capacitance Performance

Activated carbon (AC) and multiwalled carbon nanotubes (MWCNTs) have been extensively investigated in recent decades as electrical double-layer capacitor (EDLC) electrode materials for supercapacitors, owing to their superior capacitive properties and cycling stability performance. However, in the modern electronics industry, ternary electrode materials have been designed to develop high-performance and efficient energy storage devices. EDLC-based ternary materials are of great importance, where all the present components participate both individually and as a multicomponent electrode system to promote high-electrochemical performance electrode materials. In this study, we have incorporated an optimized content of boron nitride nanotube (BNNT) powder into a binary material composed of AC and MWCNTs to enhance their electrochemical performance using a pneumatic printer. The printed MWCNTs/AC/BNNTs ternary composite electrode material has shown a maximum specific capacitance of 262 F g−1 at a minimum current density of 1 A g−1, with a capacitance retention of 49.61% at a maximum current density of 10 A g−1. These results demonstrate that the printable MWCNTs/AC/BNNTs ternary composite electrode material is a potential candidate for the development of high-performance supercapacitors.

Flocculation of MXenes and Their Use as 2D Particle Surfactants for Capsule Formation

MXenes, transition metal carbides or nitrides, have gained great attention in recent years due to their high electrical conductivity and catalytic activity, hydrophilicity, and diverse surface chemistry. However, high hydrophilicity and negative ζ potential of the MXene nanosheets limit their processability and interfacial assembly. Previous examples for modifying the dispersibility and wettability of MXenes have focused on the use of organic ligands, such as alkyl amines, or covalent modification with triethoxysilanes. Here, we report a simple method to access MXene-stabilized oil-in-water emulsions by using common inorganic salts (e.g., NaCl) to flocculate the nanosheets and demonstrate the use of these Pickering emulsions to prepare capsules with shells of MXene and polymer. Ti₃C₂T_z nanosheets are used as the representative MXene. The salt-flocculated MXene nanosheets produce emulsions that are stable for days, as determined by optical microscopy imaging. The incorporation of a diisocyanate in the discontinuous oil phase and diamine in the continuous water phase led to interfacial polymerization and the formation of capsules. The capsules were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM), confirming the presence of both polymer and nanosheets. The addition of ethanol to the capsules led to the removal of the toluene core and retention of the shell structure. The ability to assemble MXene nanosheets at fluid-fluid interfaces without the use of ligands or cosurfactants expands the accessible material constructs relevant for biomedical engineering, water purification, energy storage, electromagnetic electronics, catalysis, and so on.

A wearable battery-free wireless and skin-interfaced microfluidics integrated electrochemical sensing patch for on-site biomarkers monitoring in human perspiration

In this study, an ultra-high sensitive, flexible, wireless, battery-free, and fully integrated (no external analysis equipment) electrochemical sensing patch system, including a microfluidic-sweat collecting unit, was newly developed for the on-site monitoring of the [K⁺] concentration in human sweat. Multiwalled carbon nanotube (MWCNT) and MXene-Ti₃C₂T_X based hybrid multi-dimensional networks were applied to obtain a high surface activation area and faster charge transfer rate, strongly adsorbing the valinomycin membrane to protect the ionophore for effective transshipment and immobilization of the [K⁺]. Furthermore, the controllable porosity of carbon-based materials can accelerate the kinetic process of ion diffusion. This hybrid nanonetwork structure effectively enhanced electrochemical stability and sensitivity, addressing the noise and signal drifting problems experienced with low concentration detection. The fabricated sensor exhibited a high ion concentration sensitivity of 63 mV/dec with excellent selectivity, amplified to 173 mV/dec with the integrated amplification system. The Near Field Communication (NFC) is used to transmit measurements to a smartphone wirelessly. A microfluidic channel was integrated with the electrochemical sensor patch to efficiently collect sweat on the human skin surface and mitigate the sensor surface contamination problem. Furthermore, the developed sensing patch can also be applied to other biomarkers on-site detection after modifying the working electrode with the corresponding selective membranes.

Multitasking MXene Inks Enable High-Performance Printable Microelectrochemical Energy Storage Devices for All-Flexible Self-Powered Integrated Systems

The future of mankind holds great promise for things like the Internet of Things, personal health monitoring systems, and smart cities. To achieve this ambitious goal, it is imperative for electronics to be wearable, environmentally sustainable, and safe. However, large-scale manufacture of self-sufficient electronic systems by exploiting multifunctional materials still faces significant hurdles. Herein, multitasking aqueous printable MXene inks are reported as an additive-free high-capacitance electrode, sensitive pressure-sensing material, highly conducting current collector, metal-free interconnector, and conductive binder. By directly screen printing MXene inks, MXene-based micro-supercapacitors (MSCs) and lithium-ion microbatteries (LIMBs) are delicately fabricated on various substrates. The as-prepared MSCs exhibit ultrahigh areal capacitance of 1.1 F cm^-2 and the serially connected MSCs offer a record voltage of 60 V. The quasi-solid-state LIMBs deliver a robust areal energy density of 154 μWh cm^-2 . Furthermore, an all-flexible self-powered integrated system on a single substrate based on the multitasking MXene inks is demonstrated through seamless integration of a tandem solar cell, the LIMB, and an MXene hydrogel pressure sensor. Notably, this integrated system is exceptionally sensitive to body movements with a fast response time of 35 ms. Therefore, this multipurpose MXene ink opens a new avenue for powering future smart appliances.

3D-Printed Zn-Ion Hybrid Capacitor Enabled by Universal Divalent Cation-Gelated Additive-Free Ti3C2 MXene Ink

The construction of aqueous Zn-ion hybrid capacitors (ZICs) reconciling high energy/power density is practically meaningful yet remains a grand challenge. Herein, a high-capacitance and long-life ZIC is demonstrated by 3D printing of a Ti₃C₂ MXene cathode, affording optimized carrier transport, facile electrolyte penetration, and ample porosity. The 3D-printable additive-free MXene ink with desirable rheological property is derived by a fast gelation process employing a trace amount of divalent cations, which overcomes the tedious post-treatments required for additive removal. The thus-fabricated 3D-printed (3DP) MXene cathode results in a dual-ion storage mechanism to synergize pseudocapacitive behavior of H⁺ and electrical double-layer capacitive behavior of Zn²⁺, which is systematically probed by a wide suite of in situ/ex situ electroanalytic characterizations. The 3DP MXene cathode accordingly exhibits a favorable areal capacitance of 1006.4 mF cm^-2 at 0.38 mA cm^-2 and excellent rate capability (184.4 F g^-1 at 10 A g^-1), outperforming the state-of-the-art ZICs. More impressively, ZIC full cells comprising a 3DP MXene cathode and a 3DP Zn anode deliver a competitive energy/power density of 0.10 mWh cm^-2/5.90 mW cm^-2 as well as an ultralong lifespan (86.5% capacity retention over 6000 cycles at 10 mA cm^-2).

Synthesis of new M-layer solid-solution 312 MAX phases (Ta1−xTix)3AlC2 (x = 0.4, 0.62, 0.75, 0.91 or 0.95), and their corresponding MXenes

Synthesis of a new solid solution (Ta,Ti)₃C₂T_x MXene from the new quaternary (Ta,Ti)₃AlC₂ MAX phase system, with variable Ti : Ta ratios, has been demonstrated.

Microstructure and electrochemical properties of high performance graphene/manganese oxide hybrid electrodes

Hybrids consisting of 2D ultra-large reduced graphene oxide (RGO) sheets (∼30 μm long) and 1D α-phase manganese oxide (MnO₂) nanowires were fabricated through a versatile synthesis technique that results in electrostatic binding of the nanowires and sheets. Two different hybrid (RGO/MnO₂) compositions had remarkable features and performance: 3 : 1 MnO₂/RGO (75/25 wt%) denoted as 3H and 10 : 1 MnO₂/RGO (90/10 wt%) denoted as 10H. Characterization using spectroscopy, microscopy, and thermal analysis provided insights into the microstructure and behavior of the individual components and hybrids. Both hybrids exhibited higher specific capacitance than their individual components. 3H demonstrated excellent overall electrochemical performance with specific capacitance of 225 F g⁻¹, pseudocapacitive and electrochemical double-layer capacitance (EDLC) contributions, charge-transfer resistance <1 Ω, and 97.8% capacitive retention after 1000 cycles. These properties were better than those of 10H; this was attributed 3H's more uniform distribution of nanowires enabling more effective electronic transport. Thermal annealing was used to produce reduced graphene oxide (RGO) that exhibited significant removal of oxygen functionality with a resulting interlayer spacing of 0.391 nm, higher D/G ratio, higher specific capacitance, and electrochemical properties representing more ideal capacitive behavior than GO. Integrating ultra-large RGO with very high surface area and MnO₂ nanowires enables chemical interactions that may improve processability into complex architectures and electrochemical performance of electrodes for applications in electronics, sensors, catalysis, and deionization.

Tuning the microstructure of ultra-large reduced graphene oxide (RGO) 2D sheets and manganese oxide (MnO₂) 1D nanowires to produce a hybrid material enabled achieving excellent electrochemical capacitive behavior for energy storage.

Advancements in Therapeutics via 3D Printed Multifunctional Architectures from Dispersed 2D Nanomaterial Inks

2D nanomaterials (2DNMs) possess fascinating properties and are found in multifarious devices and applications including energy storage devices, new generation of battery technologies, sensor devices, and more recently in biomedical applications. Their use in biomedical applications such as tissue engineering, photothermal therapy, neural regeneration, and drug delivery has opened new horizons in treatment of age-old ailments. It is also a rapidly developing area of advanced research. A new approach of integrating 3D printing (3DP), a layer-by-layer deposition technique for building structures, along with 2DNM multifunctional inks, has gained considerable attention in recent times, especially in biomedical applications. With the ever-growing demand in healthcare industry for novel, efficient, and rapid technologies for therapeutic treatment methods, 3DP structures of 2DNMs provide vast scope for evolution of a new generation of biomedical devices. Recent advances in 3DP structures of dispersed 2DNM inks with established high-performance biomedical properties are focused on. The advantages of their 3D structures, the sustainable formulation methods of such inks, and their feasible printing methods are also covered. Subsequently, it deals with the therapeutic applications of some already researched 3DP structures of 2DNMs and concludes with highlighting the challenges as well as the future directions of research in this area.

Universal in Situ Crafted MOx-MXene Heterostructures as Heavy and Multifunctional Hosts for 3D-Printed Li-S Batteries

The Li-S battery has emerged as a promising next-generation system for advanced energy storage. Notwithstanding the recent progress, the problematic polysulfide shuttling, retarded sulfur redox, and low output of volumetric capacity remain daunting challenges toward its practicability. In response, this work demonstrates herein a universal approach to in situ craft MO_x-MXene (M: Ti, V, and Nb) heterostructures as heavy and multifunctional hosts to harvest good battery performances with synchronous polysulfide immobilization and conversion. Theoretical calculations indicate that the in situ implanted oxides boost the reaction kinetics of polysulfide transformation without affecting the intrinsic conductivity of MXene. As a result, the representative VO_x-V₂C/S electrode enables a high volumetric capacity (offering 1645.98 mAh cm^-3 at 0.2 C) and cycling stability (retaining 631.17 mAh cm^-3 after 1500 cycles at 2.0 C with a capacity decay of 0.03% per cycle). More encouragingly, 3D-printed sulfur electrodes harnessing VO_x-V₂C hosts readily harvest an areal capacity of 9.74 mAh cm^-2 at 0.05 C under an elevated sulfur loading of 10.78 mg cm^-2, holding promise for the development of practically viable Li-S batteries.

Silicon-Based 3D All-Solid-State Micro-Supercapacitor with Superior Performance

The large-scale fabrication of high-performance on-chip micro-supercapacitors (MSCs) is the footstone for the development of next-generation miniaturized electronic devices. In practical applications, however, MSCs may suffer from a low areal energy density as well as a complicated fabrication strategy that is incompatible with semiconductor processing technology. Herein, we propose a scalable fabrication strategy for the realization of a silicon-based three-dimensional (3D) all-solid-state MSC via the combination of semiconductor-based electrode processing, chemical vapor deposition, and hydrothermal growth. The individual Si/C/MnO₂ electrode shows a maximum specific capacitance of 223.74 mF cm^-2, while the symmetric electrodes present a maximum areal energy density of 5.01 μWh cm^-2 at the scan rate of 1 mV s^-1. The full 3D Si/C/MnO₂ MSC delivers a high energy density of 2.62 μWh cm^-2, at a power density of 117.82 μW cm^-2, as well as a long cycle life with capacitance retention >92% after 4000 cycles. Our proposed method enables the fabrication of 3D MSCs based on a thick silicon interdigitated electrode array, holding a great promise for the development of 3D on-chip microscale energy storage devices.

MXene/Activated-Carbon Hybrid Capacitive Deionization for Permselective Ion Removal at Low and High Salinity

Two-dimensional, layered transition metal carbides (MXenes) are an intriguing class of intercalation-type electrodes for electrochemical applications. The ability for preferred counterion uptake qualifies MXenes as an attractive material for electrochemical desalination. Our work explores Ti₃C₂T_x-MXene paired with activated carbon in such a way that both electrodes operate in an optimized potential range. This is accomplished by electrode mass balancing and control over the cell voltage. Thereby, we enable effective remediation of saline media with low (brackish) and high (seawater-like) ionic strength by using 20 and 600 mM aqueous NaCl solutions. It is shown that MXene/activated-carbon asymmetric cell design capitalizes on the permselective behavior of MXene in sodium removal, which in turn forces carbon to mirror the same behavior in the removal of chloride ions. This has minimized the notorious co-ion desorption of carbon in highly saline media (600 mM NaCl) and boosted the charge efficiency from 4% in a symmetric activated-carbon/activated-carbon cell to 85% in a membrane-less asymmetric MXene/activated-carbon cell. Stable electrochemical performance for up to 100 cycles is demonstrated, yielding average desalination capacities of 8 and 12 mg/g, respectively, for membrane-less MXene/activated-carbon cells in NaCl solutions of 600 mM (seawater-level) and 20 mM (brackish-water-level). In the case of the 20 mM NaCl solutions, surprising charge efficiency values of over 100% have been obtained, which is attributed to the role of MXene interlayer surface charges.

Direct synthesis of metastable phases of 2D transition metal dichalcogenides

The different polymorphic phases of transition metal dichalcogenides (TMDs) have attracted enormous interest in the last decade. The metastable metallic and small band gap phases of group VI TMDs displayed leading performance for electrocatalytic hydrogen evolution, high volumetric capacitance and some of them exhibit large gap quantum spin Hall (QSH) insulating behaviour. Metastable 1T(1T') phases require higher formation energy, as compared to the thermodynamically stable 2H phase, thus in standard chemical vapour deposition and vapour transport processes the materials normally grow in the 2H phases. Only destabilization of their 2H phase via external means, such as charge transfer or high electric field, allows the conversion of the crystal structure into the 1T(1T') phase. Bottom-up synthesis of materials in the 1T(1T') phases in measurable quantities would broaden their prospective applications and practical utilization. There is an emerging evidence that some of these 1T(1T') phases can be directly synthesized via bottom-up vapour- and liquid-phase methods. This review will provide an overview of the synthesis strategies which have been designed to achieve the crystal phase control in TMDs, and the chemical mechanisms that can drive the synthesis of metastable phases. We will provide a critical comparison between growth pathways in vapour- and liquid-phase synthesis techniques. Morphological and chemical characteristics of synthesized materials will be described along with their ability to act as electrocatalysts for the hydrogen evolution reaction from water. Phase stability and reversibility will be discussed and new potential applications will be introduced. This review aims at providing insights into the fundamental understanding of the favourable synthetic conditions for the stabilization of metastable TMD crystals and at stimulating future advancements in the field of large-scale synthesis of materials with crystal phase control.