Room-Temperature Assembled MXene-Based Aerogels for High Mass-Loading Sodium-Ion Storage

MXene-based composite aerogels with bifunctional ferrous ions for the efficient degradation of phenol from wastewater

Herein, MXene-based composite aerogel (MXene-Fe2+ aerogel) are constructed by a one-step freeze-drying method, using Ti3C2Tx MXene layers as substrate material and ferrous ion (Fe2+) as crosslinking agent. With the aid of the Fe2+ induced Fenton reaction, the synthesized aerogels are used as the particle electrodes to remove phenol from wastewater with three-dimensional electrode technology. Combined with the dual roles of Fe2+ and the highly conductive MXene, the obtained particle electrode possesses extremely effective phenol degradation. The effects of experiment parameters such as Fe2+ to MXene ratio, particle electrode dosage, applied voltage, and initial pH of solution on the removal of phenol are discussed. At pH = 2.5, phenol with 50 mg/L of initial concentration can be completely removed within 50 min at 10 V with the particle electrode dosage of 0.56 g/L. Finally, the mechanism of degradation is explored. This work provides an effective way for phenol degradation by MXene-based aerogel, which has great potential for the degradation of other organic pollutants in wastewater.

Ultrarapid Gelation of Porous Ti3C2Tx MXene Monoliths Induced by Ionic Liquids

Gelation is a promising method to assemble 3D macroscopic structures from MXene sheets for various applications. However, the fine control and scalable manufacturing of 3D MXene monoliths remains a great challenge. Herein, the controllable gelation of Ti3C2Tx MXene initiated by various ionic liquids (ILs) is first proposed, where the IL serve as linkers to bond the nanosheets together through electrostatic and hydrogen bonding interactions, forming 3D monoliths with well-adjustable structure. Furthermore, density functional theory calculations and experiments further reveal the cross-linking effect of different ILs. Typically, 3D porous structure with high specific surface area, suitable pore size, and improved electrolyte affinity is designed through the cross-linking of Ti3C2Tx with 1-vinyl-3-ethylimidazole bromide ([C2VIm]Br-Ti3C2Tx). Due to the strong coupling, the as-synthesized monolith possesses excellent rate performance and high energy density. The methodology is quite flexible, controllable, and universal that provides a new perspective for promoting innovative applications of 2D materials.

Recent Progress in Improving Rate Performance of Cellulose-Derived Carbon Materials for Sodium-Ion Batteries

Cellulose-derived carbon is regarded as one of the most promising candidates for high-performance anode materials in sodium-ion batteries; however, its poor rate performance at higher current density remains a challenge to achieve high power density sodium-ion batteries. The present review comprehensively elucidates the structural characteristics of cellulose-based materials and cellulose-derived carbon materials, explores the limitations in enhancing rate performance arising from ion diffusion and electronic transfer at the level of cellulose-derived carbon materials, and proposes corresponding strategies to improve rate performance targeted at various precursors of cellulose-based materials. This review also presents an update on recent progress in cellulose-based materials and cellulose-derived carbon materials, with particular focuses on their molecular, crystalline, and aggregation structures. Furthermore, the relationship between storage sodium and rate performance the carbon materials is elucidated through theoretical calculations and characterization analyses. Finally, future perspectives regarding challenges and opportunities in the research field of cellulose-derived carbon anodes are briefly highlighted.

Versatile MXene Gels Assisted by Brief and Low-Strength Centrifugation

Due to the mutual repulsion between their hydrophilic surface terminations and the high surface energy facilitating their random restacking, 2D MXene nanosheets usually cannot self-assemble into 3D macroscopic gels with various applications in the absence of proper linking agents. In this work, a rapid spontaneous gelation of Ti3C2Tx MXene with a very low dispersion concentration of 0.5 mg mL-1 into multifunctional architectures under moderate centrifugation is illustrated. The as-prepared MXene gels exhibit reconfigurable internal structures and tunable rheological, tribological, electrochemical, infrared-emissive and photothermal-conversion properties based on the pH-induced changes in the surface chemistry of Ti3C2Tx nanosheets. By adopting a gel with optimized pH value, high lubrication, exceptional specific capacitances (~ 635 and ~ 408 F g-1 at 5 and 100 mV s-1, respectively), long-term capacitance retention (~ 96.7% after 10,000 cycles) and high-precision screen- or extrusion-printing into different high-resolution anticounterfeiting patterns can be achieved, thus displaying extensive potential applications in the fields of semi-solid lubrication, controllable devices, supercapacitors, information encryption and infrared camouflaging.

Room-temperature assembled 3D macro-porous Ti3C2Tx/RGO hybrid hydrogel and the application as the self-standing electrode for sodium-ion storage

Assembling two-dimensional (2D) MXene nanosheets into monolithic three-dimensional (3D) structures is an efficient pathway to transfer the nanoscale properties to practical applications. Nevertheless, the majority of the preparation schemes described in the literature are carried out at relatively high temperatures, which inevitably leads to the notorious high-temperature oxidation issue of MXenes. Preparing MXene-based hydrogels at lower temperatures or even room temperature is of great research importance. In this study, we report a novel and efficient room-temperature gelation method for fabricating 3D macro-porous Ti3C2Tx MXene/reduced graphene oxide (RGO) hybrid hydrogels, using anhydrous sodium sulfide (Na2S) as the primary reducing agent and l-cysteine as the auxiliary crosslinker. This room-temperature preparation technique successfully prevents the oxidation issue of MXenes and generates porous aerogels with excellent structural robustness after freeze-drying. As the self-standing anode for sodium-ion storage, the optimized 3D Ti3C2Tx MXene/RGO electrode possesses a specific capacity of 152 mAh/g at 0.1 A/g and good cycling stability with no significant capacity degradation after 500 cycles, which is significantly higher than that of the vacuum-filtered MXene film. This work demonstrates a straightforward room-temperature gelation method for constructing 3D MXene-based hydrogels to avoid the oxidation of MXenes, and casts new insight on the mechanism of the graphene oxide (GO)-assisted gelation.

Interfacial Mo-S-C Bond with High Reversibility for Advanced Alkali-Ion Capacitors: Strategies for High-Throughput Production

The high-throughput scalable production of low-cost and high-performance electrode materials that work well under high power densities required in industrial application is full of challenges for the large-scale implementation of electrochemical technologies. Here, motivated by theoretical calculation that Mo-S-C heterojunction and sulfur vacancies can reduce the energy band gap, decrease the migration energy barrier, and improve the mechanical stability of MoS2 , the scalable preparation of inexpensive MoS2-x @CN is contrived by employing natural molybdenite as precursor, which is characteristic of high efficiency in synthesis process and energy conservation and the calculated costs are four orders of magnitude lower than MoS2 /C in previous work. More importantly, MoS2- x @CN electrode is endowed with impressive rate capability even at 5 A g-1 , and ultrastable cycling stability during almost 5000 cycles, which far outperform chemosynthesis MoS2 materials. Obtaining the full SIC cell assembled by MoS2- x @CN anode and carbon cathode, the energy/power output is high up to 265.3 W h kg-1 at 250 W kg-1 . These advantages indicate the huge potentials of the designed MoS2- x @CN and of mineral-based cost-effective and abundant resources as anode materials in high-performance AICs.

Fabrication, Performance, and Potential Applications of MXene Composite Aerogels

Aerogel, known as one of the remarkable materials in the 21st century, possesses exceptional characteristics such as high specific surface area, porosity, and elasticity, making it suitable for a diverse range of applications. In recent years, MXene-based aerogels and MXene composite aerogels as functional materials have solved some limitations of traditional aerogels, such as improving the electrical conductivity of biomass and silicon aerogels, further improving the energy storage capacity of carbon aerogels, enhancing polymer-based aerogels, etc. Consequently, extensive research efforts have been dedicated to investigating MXene-based aerogels, positioning them at the forefront of material science studies. This paper provides a comprehensive review of recent advancements in the preparation, properties, and applications of MXene-based composite aerogels. The primary construction strategies employed (including direct synthesis from MXene dispersions and incorporation of MXene within existing substrates) for fabricating MXene-based aerogels are summarized. Furthermore, the desirable properties (including their applications in electrochemistry, electromagnetic shielding, sensing, and adsorption) of MXene composite aerogels are highlighted. This paper delves into a detailed discussion on the fundamental properties of composite aerogel systems, elucidating the intricate structure-property relationships. Finally, an outlook is provided on the opportunities and challenges for the mass production and functional applications of MXene composite aerogels in the field of material engineering.

Aqueous MXene inks for inkjet-printing microsupercapacitors with ultrahigh energy densities

Although inkjet-printing technology has achieved significant development in preparing scalable and adaptable energy storage devices for portable and micro devices, searching for additive-free and environmentally friendly aqueous inks is a significant challenge. Hence, an aqueous MXene/sodium alginate-Fe²⁺ hybrid ink (denoted as MXene/SA-Fe) with solution processability and suitable viscosity is prepared for direct inkjet printing microsupercapacitors (MSCs). The SA molecules are adsorbed on the surface of MXene nanosheets to construct three-dimensional (3D) structures, thus effectively alleviating the two notorious problems of oxidation and self-restacking of MXene. Concurrently, Fe²⁺ ions can compress the ineffective macropore volume and make the 3D structure more compact. Moreover, the hydrogen and covalent bonding formed between the MXene nanosheet, SA, and Fe²⁺ effectively protects the oxidation of MXene and thus increases its stability. Thus, the MXene/SA-Fe ink endows the inkjet-printed MSC electrode with abundant active sites for ion storage and a highly conductive network for electron transfer. As a demonstration, the MXene/SA-Fe ink is used to direct inkjet-printed MSCs with an electrode spacing of 310 μm, which exhibit remarkable capacitances of 123.8 mF cm^-2 (@5 mV s^-1), good rate capability, an extraordinary energy density of 8.44 μWh cm^-2 at a power density of 33.70 μW cm^-2, long-term cycling stability of 91.4 % capacitance retention after 10,000 cycles, and surprising mechanical durability with 90.0 % of its initial capacitance retained after 10,000 bending cycles. Therefore, MXene/SA-Fe inks are expected to create various opportunities for printable electronics.

In Situ Growth of Iron Sulfide on Fast Charge Transfer V2 C-MXene for Superior Sodium Storage Anodes

Due to the upstream pressure of lithium resources, low-cost sodium-ion batteries (SIBs) have become the most potential candidates for energy storage systems in the new era. However, anode materials of SIBs have always been a major problem in their development. To address this, V₂ C/Fe₇ S₈ @C composites with hierarchical structures prepared via an in situ synthesis method are proposed here. The 2D V₂ C-MXene as the growth substrate for Fe₇ S₈  greatly improves the rate capability of SIBs, and the carbon layer on the surface provides a guarantee for charge-discharge stability. Unexpectedly, the V₂ C/Fe₇ S₈ @C anode achieves satisfactory sodium storage capacity and exceptional rate performance (389.7 mAh g^-1  at 5 A g^-1 ). The sodium storage mechanism and origin of composites are thoroughly studied via ex situ characterization techniques and first-principles calculations. Furthermore, the constructed sodium-ion capacitor assembled with N-doped porous carbon delivers excellent energy density (135 Wh kg^-1 ) and power density (11 kW kg^-1 ), showing certain practical value. This work provides an advanced system of sodium storage anode materials and broadens the possibility of MXene-based materials in the energy storage.

Trapping Lithium Selenides with Evolving Heterogeneous Interfaces for High-Power Lithium-Ion Capacitors

Transition metal selenides anodes with fast reaction kinetics and high theoretical specific capacity are expected to solve mismatched kinetics between cathode and anode in Li-ion capacitors. However, transition metal selenides face great challenges in the dissolution and shuttle problem of lithium selenides, which is the same as Li-Se batteries. Herein, inspired by the density functional theory calculations, heterogeneous can enhance the adsorption of Li₂ Se relative to single component selenide electrodes, thus inhibiting the dissolution and shuttle effect of Li₂ Se. A heterostructure material (denoted as CoSe₂ /SnSe) with the ability to evolve continuously (CoSe₂ /SnSe→Co/Sn→Co/Li₁₃ Sn₅ ) is successfully designed by employing CoSnO₃ -MOF as a precursor. Impressively, CoSe₂ /SnSe heterostructure material delivers the ultrahigh reversible specific capacity of 510 mAh g^-1 after 1000 cycles at the high current density of 4 A g^-1 . In situ XRD reveals the continuous evolution of the interface based on the transformation and alloying reactions during the charging and discharging process. Visualizations of in situ disassembly experiments demonstrate that the continuously evolving interface inhibits the shuttle of Li₂ Se. This research proposes an innovative approach to inhibit the dissolution and shuttling of discharge intermediates (Li₂ Se) of metal selenides, which is expected to be applied to metal sulfides or Li-Se and Li-S energy storage systems.

NiFe-LDH/MXene nano-array hybrid architecture for exceptional capacitive lithium storage

Layered double hydroxides (LDHs) have great advantages in the domain of energy storage because of their exchangeable anions and large specific surface area. Nevertheless, the shortcomings of their poor electrical conductivity, easy stacking of nanosheets, and large volume variation in the cycling processes lead to unsatisfactory cycling stability and rate performance, which severely limits their further application. Therefore, we generated homogeneous nanoarrays of NiFe-LDH on the surface of Ti₃C₂T_x-MXene by a refluxing process. The resulting NiFe-LDH/MXene-500 hybrid material was applied as an anode of a lithium-ion battery (LIB) and exhibited a discharge capacity of 894.8 mA h g^-1 at 200 mA g^-1 (over 300 cycles) and could maintain a reversible capacity of 547.1 mA h g^-1 even at 1 A g^-1. With the addition of MXene, the volume increases of the NiFe-LDH/MXene hybrid materials were also significantly alleviated. The thickness of the NiFe-LDH/MXene-500 electrode only increased by 31% after 50 cycles, which was far better than the prepared NiFe-LDH electrode. On the hand, the synergistic interaction of NiFe-LDH and MXene could stabilize the structure, reduce the activation barrier of ion/electron diffusion, and promote electron transfer in the electrode. MXene with high conductivity can be used as electrical and ionic conductance media to promote the transformation reaction of NiFe-LDH. According to the detailed kinetic analysis, the capacitance control behavior is the main electrochemical reaction of NiFe-LDH/MXene electrodes.

MXene-Based Porous Monoliths

In the past decade, a thriving family of 2D nanomaterials, transition-metal carbides/nitrides (MXenes), have garnered tremendous interest due to its intriguing physical/chemical properties, structural features, and versatile functionality. Integrating these 2D nanosheets into 3D monoliths offers an exciting and powerful platform for translating their fundamental advantages into practical applications. Introducing internal pores, such as isotropic pores and aligned channels, within the monoliths can not only address the restacking of MXenes, but also afford a series of novel and, in some cases, unique structural merits to advance the utility of the MXene-based materials. Here, a brief overview of the development of MXene-based porous monoliths, in terms of the types of microstructures, is provided, focusing on the pore design and how the porous microstructure affects the application performance.

Elastic Buffering Layer on CuS Enabling High-Rate and Long-Life Sodium-Ion Storage

The latest view suggests the inactive core, surface pulverization, and polysulfide shuttling effect of metal sulfides are responsible for their low capacity and poor cycling performance in sodium-ion batteries (SIBs). Whereas overcoming the above problems based on conventional nanoengineering is not efficient enough. In this work, erythrocyte-like CuS microspheres with an elastic buffering layer of ultrathin polyaniline (PANI) were synthesized through one-step self-assembly growth, followed by in situ polymerization of aniline. When CuS@PANI is used as anode electrode in SIBs, it delivers high capacity, ultrahigh rate capability (500 mAh g^-1 at 0.1 A g^-1, and 214.5 mAh g^-1 at 40 A g^-1), and superior cycling life of over 7500 cycles at 20 A g^-1. A series of in/ex situ characterization techniques were applied to investigate the structural evolution and sodium-ion storage mechanism. The PANI swollen with electrolyte can stabilize solid electrolyte interface layer, benefit the ion transport/charge transfer at the PANI/electrolyte interface, and restrain the size growth of Cu particles in confined space. Moreover, finite element analyses and density functional simulations confirm that the PANI film effectively buffers the volume expansion, suppresses the surface pulverization, and traps the polysulfide.

Metal-Organic Framework Materials for Electrochemical Supercapacitors

Exploring new materials with high stability and capacity is full of challenges in sustainable energy conversion and storage systems. Metal-organic frameworks (MOFs), as a new type of porous material, show the advantages of large specific surface area, high porosity, low density, and adjustable pore size, exhibiting a broad application prospect in the field of electrocatalytic reactions, batteries, particularly in the field of supercapacitors. This comprehensive review outlines the recent progress in synthetic methods and electrochemical performances of MOF materials, as well as their applications in supercapacitors. Additionally, the superiorities of MOFs-related materials are highlighted, while major challenges or opportunities for future research on them for electrochemical supercapacitors have been discussed and displayed, along with extensive experimental experiences.

Tailoring Nitrogen Terminals on MXene Enables Fast Charging and Stable Cycling Na-Ion Batteries at Low Temperature

Sodium-ion batteries stand a chance of enabling fast charging ability and long lifespan while operating at low temperature (low-T). However, sluggish kinetics and aggravated dendrites present two major challenges for anodes to achieve the goal at low-T. Herein, we propose an interlayer confined strategy for tailoring nitrogen terminals on Ti₃C₂ MXene (Ti₃C₂-N_funct) to address these issues. The introduction of nitrogen terminals endows Ti₃C₂-N_funct with large interlayer space and charge redistribution, improved conductivity and sufficient adsorption sites for Na⁺, which improves the possibility of Ti₃C₂ for accommodating more Na atoms, further enhancing the Na⁺ storage capability of Ti₃C₂. As revealed, Ti₃C₂-N_funct not only possesses a lower Na-ion diffusion energy barrier and charge transfer activation energy, but also exhibits Na⁺-solvent co-intercalation behavior to circumvent a high de-solvation energy barrier at low-T. Besides, the solid electrolyte interface dominated by inorganic compounds is more beneficial for the Na⁺ transfer at the electrode/electrolyte interface. Compared with of the unmodified sample, Ti₃C₂-N_funct exhibits a twofold capacity (201 mAh g^-1), fast-charging ability (18 min at 80% capacity retention), and great superiority in cycle life (80.9%@5000 cycles) at - 25 °C. When coupling with Na₃V₂(PO₄)₂F₃ cathode, the Ti₃C₂-N_funct//NVPF exhibits high energy density and cycle stability at - 25 °C.

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.

Stabilizing BiOCl/Ti3C2Tx Hybrids for Potassium-Ion Batteries via Solid Electrolyte Interphase Reconstruction

The synergistic innovation from reasonable material design to electrolyte optimization is the key to improve the performance of anode materials for potassium ion batteries (PIBs). In this work, a two-dimensional...