With the rapid development of mobile electronics and electric vehicles, future electrochemical capacitors (ECs) need to store as much energy as possible in a rather limited space. As the core component of ECs, dense electrodes that have a high volumetric energy density and superior rate capability are the key to achieving improved energy storage. Here, the significance of and recent progress in the high volumetric performance of dense electrodes are presented. Furthermore, dense yet porous electrodes, as the critical precondition for realizing superior electrochemical capacitive energy, have become a scientific challenge and an attractive research focus. From a pore-engineering perspective, insight into the guidelines of engineering the pore size, connectivity, and wettability is provided to design dense electrodes with different porous architectures toward high-performance capacitive energy storage. The current challenges and future opportunities toward dense electrodes are discussed and include the construction of an orderly porous structure with an appropriate gradient, the coupling of pore sizes with the solvated cations and anions, and the design of coupled pores with diverse electrolyte ions.

While pseudocapacitive electrodes have potential to store more energy than electrical double-layer capacitive electrodes, their rate capability is often limited by the sluggish kinetics of the Faradic reactions or the poor electronic and ionic conductivity. Unlike most transition metal oxides, MoO2 is a very promising material for fast energy storage, attributed to its unusually high electronic and ionic conductivity; the 1-dimensional tunnel is ideally suited for fast ionic transport. Here we report our findings in preparation and characterization of ultrathin MoO2 sheets with oriented tunnels as pseudo-capacitive electrode for fast charge storage/release. A composite electrode consisting of MoO2 and 5 wt.% GO demonstrates a capacity of 1,097 C g-1 at 2 mV s-1 and 390 C g-1 at 1,000 mV s-1 while maintaining ~80% of the initial capacity after 10,000 cycles at 50 mV s-1, due to minimal change in structural features of the MoO2 during charge/discharge except a small volume change (~14%), as revealed from operando Raman spectroscopy, X-ray analyses, and DFT calculations. Further, the volume change during cycling is highly reversible, implying high structural stability and long cycling life.

Wearable electrochemical biosensors for sweat analysis present a promising means for noninvasive biomarker monitoring. However, sweat-based sensing still poses several challenges, including easy degradation of enzymes and biomaterials with repeated testing, limited detection range and sensitivity of enzyme-based biosensors caused by oxygen deficiency in sweat, and poor shelf life of sensors using all-in-one working electrodes patterned by traditional techniques (e.g., electrodeposition and screen printing). Herein, a stretchable, wearable, and modular multifunctional biosensor is developed, incorporating a novel MXene/Prussian blue (Ti3 C2 Tx /PB) composite designed for durable and sensitive detection of biomarkers (e.g., glucose and lactate) in sweat. A unique modular design enables a simple exchange of the specific sensing electrode to target the desired analytes. Furthermore, an implemented solid-liquid-air three-phase interface design leads to superior sensor performance and stability. Typical electrochemical sensitivities of 35.3 µA mm-1 cm-2 for glucose and 11.4 µA mm-1 cm-2 for lactate are achieved using artificial sweat. During in vitro perspiration monitoring of human subjects, the physiochemistry signals (glucose and lactate level) can be measured simultaneously with high sensitivity and good repeatability. This approach represents an important step toward the realization of ultrasensitive enzymatic wearable biosensors for personalized health monitoring.

Two-dimensional transition-metal carbides/carbonitrides (MXenes) with both superb electrical conductivity and hydrophilicity are promising for fabricating multifunctional nanomaterials and nanocomposites. However, the construction of three-dimensional (3D) and lightweight MXene macroscopic assemblies with excellent electrical conductivity and mechanical performances has not been realized due to the weak gelation capability of MXene sheets. Herein, we demonstrate an efficient approach for constructing highly conductive 3D Ti3C2T x porous architectures by graphene oxide assisted hydrothermal assembly followed by directional freezing and freeze-drying. The resultant hybrid aerogels exhibit aligned cellular microstructure, in which the graphene sheets serve as the inner skeleton, while the compactly attached Ti3C2T x sheets present as shells of the cell walls. The porous and highly conductive architecture (up to 1085 S m-1) is highly efficient in endowing epoxy nanocomposite with a high electrical conductivity of 695.9 S m-1 and an outstanding electromagnetic interference (EMI)-shielding effectiveness of more than 50 dB in the X-band at a low Ti3C2T x content of 0.74 vol %, which are the best results for polymer nanocomposites with similar loadings of MXene so far. The successful assembly methodology of 3D and porous architectures of Ti3C2T x would greatly widen the practical applications of MXenes in the fields of EMI shielding, supercapacitors, and sensors.

Two-dimensional MXene materials have demonstrated attractive electrical and electrochemical properties in energy storage applications. Adding stretchability to MXene remains challenging due to its high mechanical stiffness and weak intersheet interaction, so the assembling techniques for mechanically stable MXene architectures require further development. We report a simple fabrication by harnessing the interfacial instability to generate higher dimensional MXene nanocoatings capable of programmed crumpling/unfolding. A sequential patterning approach enabled the design of sequence-dependent MXene textures across multiple length scales, which were utilized for controllable wetting surfaces and high-areal-capacitance electrodes. We next transferred the crumpled MXene nanocoating onto an elastomer to fabricate an MXene/elastomer electrode with high stretchability. The accordion-like MXene can be reversibly folded/unfolded and still preserve efficient specific capacitances. We further fabricated asymmetric MXene supercapacitors, and the devices demonstrated efficient electrochemical performance and large deformability (180° bendability, 100% stretchability). Our texturing techniques can be applied to large MXene families for designing stretchable architectures in wearable electronics.

Transition metal carbides and nitrides (MXenes), a family of two-dimensional (2D) inorganic compounds, are materials composed of a few atomic layers of transition metal carbides, nitrides, or carbonitrides. Ti3C2, the first 2D layered MXene, was isolated in 2011. This material, which is a layered bulk material analogous to graphite, was derived from its 3D phase, Ti3AlC2 MAX. Since then, material scientists have either determined or predicted the stable phases of >200 different MXenes based on combinations of various transition metals such as Ti, Mo, V, Cr, and their alloys with C and N. Extensive experimental and theoretical studies have shown their exciting potential for energy conversion and electrochemical storage. To this end, we comprehensively summarize the current advances in MXene research. We begin by reviewing the structure types and morphologies and their fabrication routes. The review then discusses the mechanical, electrical, optical, and electrochemical properties of MXenes. The focus then turns to their exciting potential in energy storage and conversion. Energy storage applications include electrodes in rechargeable lithium- and sodium-ion batteries, lithium-sulfur batteries, and supercapacitors. In terms of energy conversion, photocatalytic fuel production, such as hydrogen evolution from water splitting, and carbon dioxide reduction are presented. The potential of MXenes for the photocatalytic degradation of organic pollutants in water, such as dye waste, is also addressed, along with their promise as catalysts for ammonium synthesis from nitrogen. Finally, their application potential is summarized.

Thinking small to store more From mobile devices to the power grid, the needs for high-energy density or high-power density energy storage materials continue to grow. Materials that have at least one dimension on the nanometer scale offer opportunities for enhanced energy storage, although there are also challenges relating to, for example, stability and manufacturing. In this context, Pomerantseva et al. review fundamental processes of charge storage that apply specifically to nanostructured materials and briefly explore potential manufacturing processes. The authors also consider some of the skepticism, such as that found in the battery community, to the use of these materials. Science, this issue p. eaan8285 BACKGROUND Nanomaterials offer greatly improved ionic transport and electronic conductivity compared with conventional battery and supercapacitor materials. They also enable the occupation of all intercalation sites available in the particle volume, leading to high specific capacities and fast ion diffusion. These features make nanomaterial-based electrodes able to tolerate high currents, offering a promising solution for high-energy and high-power energy storage. However, there are still many challenges associated with their use in energy storage technology and, with the exception of multiwall carbon-nanotube additives and carbon coatings on silicon particles in lithium-ion battery electrodes, the use of nanomaterials in commercial devices is very limited. After decades of development, a library of nanomaterials with versatile chemical compositions and shapes exists, ranging from oxides, chalcogenides, and carbides to carbon and elements forming alloys with lithium. This library includes various particle morphologies, such as zero-dimensional (0D) nanoparticles and quantum dots; 1D nanowires, nanotubes, and nanobelts; 2D nanoflakes and nanosheets; and 3D porous nanonetworks. Combined with lithium and beyond lithium ions, these chemically diverse nanoscale building blocks are available for creating energy storage solutions such as wearable and structural energy storage technology, which are not achievable with conventional materials. ADVANCES The success of nanomaterials in energy storage applications has manifold aspects. Nanostructuring is becoming key in controlling the electrochemical performance and exploiting various charge storage mechanisms, such as surface-based ion adsorption, pseudocapacitance, and diffusion-limited intercalation processes. The development of new high-performance materials, such as redox-active transition-metal carbides (MXenes) with conductivity exceeding that of carbons and other conventional electrode materials by at least an order of magnitude, open the door to the design of current collector–free and high-power next-generation energy storage devices. The combination of nanomaterials in hybrid architectures, such as carbon-silicon and carbon-sulfur, together with the development of versatile methods of nanostructuring, overcome challenges related to large volume change typical for alloying and conversion materials. These examples indicate that nanostructured materials and nanoarchitectured electrodes can provide solutions for designing and realizing high-energy, high-power, and long-lasting energy storage devices. OUTLOOK The limitations of nanomaterials in energy storage devices are related to their high surface area—which causes parasitic reactions with the electrolyte, especially during the first cycle, known as the first cycle irreversibility—as well as their agglomeration. Therefore, future strategies aim to develop smart assembly of nanomaterials into architectures with controlled geometry. Moreover, combining nanomaterials with complementary functionalities, such as high electronic conductivity of graphene or MXenes with high operating voltage and high redox activity of oxides, is necessary. Building sophisticated electrode architectures requires innovative manufacturing approaches, such as printing, knitting, spray deposition, and so on. Already-developed techniques such as 3D printing, roll-to-roll manufacturing, self-assembly from solutions, atomic layer deposition, and other advanced techniques should be used to manufacture devices from nanomaterials that cannot be made by conventional slurry-based methods. Such manufacturing approaches can also enable long-sought flexible, stretchable, wearable, and structural energy storage and harvesting solutions for Internet of Things and other disruptive technologies. Nanomaterials for energy storage applications. The high surface-to-volume ratio and short diffusion pathways typical of nanomaterials provide a solution for simultaneously achieving high energy and power density. Furthermore, the compatibility of nanomaterials with advanced manufacturing techniques—such as printing, spray coating, roll-to-roll assembly, and so on—allows for the design and realization of wearable, flexible, and foldable energy storage devices. Lithium-ion batteries, which power portable electronics, electric vehicles, and stationary storage, have been recognized with the 2019 Nobel Prize in chemistry. The development of nanomaterials and their related processing into electrodes and devices can improve the performance and/or development of the existing energy storage systems. We provide a perspective on recent progress in the application of nanomaterials in energy storage devices, such as supercapacitors and batteries. The versatility of nanomaterials can lead to power sources for portable, flexible, foldable, and distributable electronics; electric transportation; and grid-scale storage, as well as integration in living environments and biomedical systems. To overcome limitations of nanomaterials related to high reactivity and chemical instability caused by their high surface area, nanoparticles with different functionalities should be combined in smart architectures on nano- and microscales. The integration of nanomaterials into functional architectures and devices requires the development of advanced manufacturing approaches. We discuss successful strategies and outline a roadmap for the exploitation of nanomaterials for enabling future energy storage applications, such as powering distributed sensor networks and flexible and wearable electronics.

The rapid development and further modularization of miniaturized and self‐powered electronic systems have substantially stimulated the urgent demand for microscale electrochemical energy storage devices, e.g., microbatteries (MBs) and micro‐supercapacitors (MSCs). Recently, planar MBs and MSCs, composed of isolated thin‐film microelectrodes with extremely short ionic diffusion path and free of separator on a single substrate, have become particularly attractive because they can be directly integrated with microelectronic devices on the same side of one single substrate to act as a standalone microsized power source or complement miniaturized energy‐harvesting units. The development of and recent advances in planar MBs and MSCs from the fundamentals and design principle to the fabrication methods of 2D and 3D planar microdevices in both in‐plane and stacked geometries are highlighted. Additonally, a comprehensive analysis of the primary aspects that eventually affect the performance metrics of microscale energy storage devices, such as electrode materials, electrolyte, device architecture, and microfabrication techniques are presented. The technical challenges and prospective solutions for high‐energy‐density planar MBs and MSCs with multifunctionalities are proposed.

The last decade has witnessed rapid progress in the development of implantable and wearable bio(chemical) sensors, which allow for real-time, continuous health monitoring. Among different device co...

The journey of smart textile-based wearable technologies first started with the attachment of sensors to fabrics, followed by embedding sensors in apparels. Presently, garments themselves can be transformed into sensors, which demonstrates the tremendous growth in the field of smart textiles. Wearable applications demand flexible materials that can withstand deformation for their practical use on par with conventional textiles. To address this, we explore the potential reasons for the enhanced performance of wearable devices realized from the fabrication of carbon nanostructured fibers with the use of graphene, carbon nanotubes and other two-dimensional materials. This review presents a brief introduction on the fabrication strategies to form carbon-based fibers and the relationship between their properties and characteristics of the resulting materials. The likely mechanisms of fiber-based electronic and storage devices, focusing mainly on transistors, nano-generators, solar cells, supercapacitors, batteries, sensors and therapeutic devices are also presented. Finally, the future perspectives of this research field of flexible and wearable electronics are discussed. The present study supplements novel ideas not only for beginners aiming to work in this booming area, but also for researchers actively engaged in the field of fiber-based electronics, dealing with advanced electronics and wide range of functionalities integrated into textile fibers.

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 (Ti3C2Tx and Ti3CNTx, respectively, known as MXenes), flexible all-MXene MSCs with controlled architectures are produced. The interdigitated Ti3C2Tx 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 Ti3C2Tx MSCs also showcase capacitive charge storage properties, good cycling lifetime, high energy and power densities, etc. The production of such high-performance Ti3C2Tx 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 allMXene MSCs with state-of-the-art performance opens new exciting opportunities for future applications in wearable and portable electronics.

The family of two-dimensional (2D) metal carbides and nitrides, known as MXenes, are among the most promising electrode materials for supercapacitors thanks to their high metal-like electrical conductivity and surface-functional-group-enabled pseudocapacitance. A major drawback of these materials is, however, the low mechanical strength, which prevents their applications in lightweight, flexible electronics. A strategy of assembling freestanding and mechanically robust MXene (Ti3 C2 Tx ) nanocomposites with one-dimensional (1D) cellulose nanofibrils (CNFs) from their stable colloidal dispersions is reported. The high aspect ratio of CNF (width of ≈3.5 nm and length reaching tens of micrometers) and their special interactions with MXene enable nanocomposites with high mechanical strength without sacrificing electrochemical performance. CNF loading up to 20%, for example, shows a remarkably high mechanical strength of 341 MPa (an order of magnitude higher than pristine MXene films of 29 MPa) while still maintaining a high capacitance of 298 F g-1 and a high conductivity of 295 S cm-1 . It is also demonstrated that MXene/CNF hybrid dispersions can be used as inks to print flexible micro-supercapacitors with precise dimensions. This work paves the way for fabrication of robust multifunctional MXene nanocomposites for printed and lightweight structural devices.

The exciting combination of high electrical conductivity, high specific capacitance and colloidal stability of two-dimensional Ti3C2Tx MXene (referred to as MXene) has shown great potential in a wide range of applications including wearable electronics, energy storage, sensors, and electromagnetic interference shielding. To realize its full potential, recent literature has reported a variety of solution-based processing methodologies to develop MXenes into multifunctional architectures, such as fibres, films and aerogels. In response to these recent critical advances, this review provides a comprehensive analysis of the diverse solution-based processing methodologies currently being used for MXene-architecture fabrication. A critical evaluation of the processing challenges directly affecting macroscale material properties and ultimately, the performance of the resulting prototype devices is also provided. Opportunities arising from the observed and foreseen challenges regarding their use are discussed to provide avenues for new designs and realise practical use in high performance applications.

The development of flexible MXene-based multifunctional composites is becoming a hot research area to achieve the application of conductive MXene in wearable electric instruments. Herein, a flexible conductive polyimide fiber (PIF)/MXene composite film with densely stacked "rebar-brick-cement" lamellar structure is fabricated using the simple vacuum filtration plus thermal imidization technique. A water-soluble polyimide precursor, poly(amic acid), is applied to act as a binder and dispersant to ensure the homogeneous dispersion of MXene and its good interfacial adhesion with PIF after thermal imidization, resulting in excellent mechanical robustness and high conductivity (3787.9 S/m). Owing to the reflection on the surface, absorption through conduction loss and interfacial/dipolar polarization loss inside the material, and the lamellar structure that is beneficial for multiple reflection and scattering between adjacent layers, the resultant PIF/MXene composite film exhibits a high electromagnetic interference (EMI) shielding effectiveness of 49.9 dB in the frequency range of 8.2-12.4 GHz. More importantly, its EMI shielding capacity can be well maintained in various harsh environments (e.g., extreme high/low temperature, acid/salt solution, and long-term cyclic bending), showing excellent stability and durability. Furthermore, it also presents fast, stable, and long-term durable Joule heating performances based on its stable and excellent conductivity, demonstrating good thermal deicing effects under actual conditions. Therefore, we believe that the flexible conductive PIF/MXene composite film with excellent conductivity and harsh environment tolerance possesses promising potential for electromagnetic wave protection and personal thermal management.

The development of efficient, low-cost and highly sensitive chemical sensors that can monitor trace amounts of volatile organic compounds (VOCs) has become indispensable for industrial and biomedical applications. Here, we demonstrate a high-performance and flexible VOC sensor based on a 3D Mxene framework (3D-M). The 3D-M was prepared through an effective electrospinning technique coupled with a self-assembly approach. The resulting 3D-M sensor can perform at room temperature, and displays a high sensitivity to extremely low concentrations of VOCs (ppb level) due to its highly interconnected porous structure, which enable the easy access and diffusion of gas molecules. Furthermore, the sensor also exhibits an ultra-wide sensing range (from 50 ppb up to a saturated vapor), fast response and recovery (less than 2 min), and good reversibility and flexibility (no performance decrease for 1000 bending cycles) for various VOCs (e.g. acetone, methanol and ethanol), demonstrating a great potential for both industrial and biomedical applications.

MXenes have been under a lot of scientific investigation due to the novel characteristics that are inherent to two-dimensional nanostructures. There are a multitude of MXenes being studied and one of the most popular among these would be the titanium carbides. The general formula for titanium carbide is Ti n + 1C n for the nanosheets produced that have undergone much study in the past few years. These studies include how the etching process affects the final MXene sheet and how the post-processing via heat or combining with polymers and/or inorganic compounds influences the mechanical and electrical properties. It is found that different etching techniques can be used to change the electrical properties of the produced MXenes and different post-processing techniques can be used to further change the properties of the nanosheets. The possible application of the titanium carbide MXenes as chemical sensing and energy storage materials will be briefly discussed. MXene nanosheets show promise in such devices due to their high surface area to volume ratio and their specific surface structure with feasible surface functionalization.

Lithium-ion capacitors (LICs) have emerged as promising energy storage devices with both high energy density and high power density. However, due to the mismatch of charge-storage capacity and electrode kinetics between battery-type anodes and capacitor-type cathodes, the application of lithium-ion capacitors has been limited. In this work, interconnected aerogel-like MXene wrapped Fe2O3 nanospheres have been prepared and investigated as battery-type anode materials for lithium-ion capacitors. In this rationally designed hybrid electrode, the Ti3C2Tx MXene matrix is capable of providing fast transport of electrons and suppressing the volume change of Fe2O3. Simultaneously, Fe2O3 hollow nanospheres offer large specific capacity and prevent restacking of the MXene layers, synergizing to boost the electrochemical performances of such hybrid electrodes. Meanwhile, the three-dimensional (3-D) nitrogen and sulphur dual-doped porous carbon (NS-DPC) derived from biomass has also been fabricated as a capacitor-type cathode material for lithium-ion capacitors. Consequently, the lithium-ion capacitors can demonstrate a high energy density of 216 W h kg−1 at a power density of 400 W kg−1 and a high power density of 20 kW kg−1 at an energy density of 96.5 W h kg−1. This work elucidates that both high energy density and power density can be achieved in hybrid lithium-ion capacitors.

Lithium-ion batteries (LIBs) play an important role in modern society. The low capacity of graphite cannot meet the demands of LIBs calling for high power and energy densities. Silicon (Si) is one of the most promising materials instead of graphite, because of its high theoretical capacity, low discharge voltage, low cost, etc. However, Si shows low conductivity of both ions and electrons and exhibits a severe volume change during cycles. Fabricating nano-sized Si and Si-based composites is an effective method to enhance the electrochemical performance of LIB anodes. Using a small size of Si nanoparticles (SiNPs) is likely to avoid the cracking of this material. One critical issue is to disclose different types and electrochemical effects of various coupled materials in the Si-based composites for anode fabrication and optimization. Hence, this paper reviews diverse SiNP-based composites for advanced LIBs from the perspective of composition and electrochemical effects. Almost all kinds of materials that have been coupled with SiNPs for LIB applications are summarized, along with their electrochemical influences on the composites. The integrated materials, including carbon materials, metals, metal oxides, polymers, Si-based materials, transition metal nitrides, carbides, dichalcogenides, alloys, and metal-organic frameworks (MOFs), are comprehensively presented.

Li-metal anode is widely acknowledged as the ideal anode for high-energy-density batteries, but seriously hindered by the uncontrollable dendrite growth and infinitely volume change. To this goal, seeking suitable stable scaffolds for dendrite-free Li anodes with large current density (> 5 mA cm-2) and high Li loading (> 90%) are highly in demand. Herein, a conductive and lithiophilic three-dimensional (3D) MXene/graphene (MG) framework is demonstrated for dendrite-free Li-metal anode. Benefiting from its high surface area (259 m2 g-1) and lightweight nature with uniformly dispersed lithiophilic MXene nanosheets as Li nucleation sites, the as-formed 3D MG scaffold showcases an ultrahigh Li content (~92% of the theoretical capacity), as well as strong capabilities in suppressing the Li-dendrites formation and accommodating the volume changes. Consequently, the MG based electrode exhibits high Coulombic efficiencies (~99%) with a record lifespan up to 2700 h, and is stable for 230 cycles at an ultrahigh current density of 20 mA cm-2. When coupled with Li4Ti5O12 or sulfur, the MG-Li/Li4Ti5O12 full-cell offers an enhanced capacity of 142 mAh g-1 after 450 cycles while the MG-Li/sulfur cell delivers improved rate performance, implying the great potential of this 3D MG framework for building long lifetime, high-energy-density batteries.

In the recent years, two-dimensional (2D) metal carbide and nitride (MXenes) materials have shown characteristics of promising electrocatalytic properties for hydrogen evolution reaction (HER). Here, by using density functional theory calculations, the etching and exfoliating properties of Cr2AlC to Cr2CO2 and electrocatalytic (for HER) properties with and without carbon defect were studied. Results show that etching the pristine Cr2AlC by HF solutions could intend to generate Cr2C MXenes with O* termination, that is, Cr2CO2, and Cr2CF2 and Cr2C(OH)2 will translate into Cr2CO2 even if they were generated first during the etching reactions. The exfoliation energy of multilayer Cr2CO2 into monolayer Cr2CO2 MXene is large (2.78 eV/nm2), and the delamination process requires Li+ (LiF) cation. Carbon vacancy is easily generated during the etching and exfoliation reactions, and the formation energy of carbon vacancy (1.71 and 1.59 eV with and without considering charge correction) of Cr2CO2 is lower than that o...