Rheology Engineering for Dry-Spinning Robust N-Doped MXene Sediment Fibers toward Efficient Charge Storage.

MXene nanosheets are believed to be an ideal candidate for fabricating fiber supercapacitors (FSCs) due to their metallic conductivity and superior volumetric capacitance, while challenges remain in continuously collecting bare MXene fibers (MFs) via the commonly used wet-spinning technique due to the intercalation of water molecules and a weak interaction between Ti3 C2 TX nanosheets in aqueous coagulation bath that ultimately leads to a loosely packed structure. To address this issue, for the first time, a dry-spinning strategy is proposed by engineering the rheological behavior of Ti3 C2 TX sediment and extruding the highly viscose stock directly through a spinneret followed by a solvent evaperation induced solidification. The dry-spun Ti3 C2 TX fibers show an optimal conductivity of 2295 S cm-1 , a tensile strength of 64 MPa and a specific capacitance of 948 F cm-3 . Nitrogen (N) doping further improves the capacitance of MFs to 1302 F cm-3 without compromising their mechanical and electrical properties. Moreover, the FSC based on N-doped MFs exhibits a high volumetric capacitance of 293 F cm-3 , good stability over 10 000 cycles, excellent flexibility upon bending-unbending, superior energy/power densities and anti-self-discharging property. The excellent electrochemical and mechanical properties endow the dry-spun MFs great potential for future applications in wearable electronics.

Nacre-Inspired Strong MXene/Cellulose Fiber with Superior Supercapacitive Performance via Synergizing the Interfacial Bonding and Interlayer Spacing.

MXene fibers are promising candidates for weaveable and wearable energy storage devices because of their good electrical conductivity and high theoretical capacitance. Herein, we propose a nacre-inspired strategy for simultaneously improving the mechanical strength, volumetric capacitance, and rate performance of MXene-based fibers through synergizing the interfacial interaction and interlayer spacing between Ti3C2TX nanosheets. The optimized hybrid fibers (M-CMC-1.0%) with 99 wt % MXene loading exhibit an improved tensile strength of ∼81 MPa and a high specific capacitance of 885.0 F cm-3 at 1 A cm-3 together with an outstanding rate performance of 83.6% retention at 10 A cm-3 (740.0 F cm-3). As a consequence, the fiber supercapacitor (FSC) based on the M-CMC-1.0% hybrid delivers an output capacitance of 199.5 F cm-3, a power density of 1186.9 mW cm-3, and an energy density of 17.7 mWh cm-3, respectively, implying its promising applications as portable energy storage devices for future wearable electronics.

Design of Vertically Aligned Two-Dimensional Heterostructures of Rigid Ti3C2TX MXene and Pliable Vanadium Pentoxide for Efficient Lithium Ion Storage.

Designing a thick electrode with appropriate mass loading is a prerequisite toward practical applications for lithium ion batteries (LIBs) yet suffers severe limitations of slow electron/ion transport, unavoidable volume expansion, and the involvement of inactive additives, which lead to compromised output capacity, poor rate perforamnce, and cycling instability. Herein, self-supported thick electrode composed of vertically aligned two-dimensional (2D) heterostructures (V-MXene/V2O5) of rigid Ti3C2TX MXene and pliable vanadium pentoxide are assembled via an ice crystallization-induced strategy. The vertical channels prompt fast electron/ion transport within the entire electrode; in the meantime, the 3D MXene scaffold provides mechanical robustness during lithiation/delithiation. The optimized electrodes with 1 and 5 mg cm-2 of V-MXene/V2O5 respectively deliver 472 and 300 mAh g-1 at a current density of 0.2 A g-1, rate performance with 380 and 222 mAh g-1 retained at 5 A g-1, and reliability over 800 charge/discharge cycles.

2D material-based peroxidase-mimicking nanozymes: catalytic mechanisms and bioapplications.

The boom in nanotechnology brings new insights into the development of artificial enzymes (nanozymes) with ease of modification, lower manufacturing cost, and higher catalytic stability than natural enzymes. Among various nanomaterials, two-dimensional (2D) nanomaterials exhibit promising enzyme-like properties for a plethora of bioapplications owing to their unique physicochemical characteristics of tuneable composition, ultrathin thickness, and huge specific surface area. Herein, we review the recent advances in several 2D material-based nanozymes, such as carbonaceous nanosheets, metal-organic frameworks (MOFs), transition metal dichalcogenides (TMDs), layered double hydroxides (LDHs), and transition metal oxides (TMOs), clarify the mechanisms of peroxidase (POD)-mimicking catalytic behaviors, and overview the potential bioapplications of 2D nanozymes.

Universal Strategy for Preparing Highly Stable PBA/Ti3C2Tx MXene toward Lithium-Ion Batteries via Chemical Transformation.

Prussian blue analogues (PBAs) are believed to be intriguing anode materials for Li+ storage because of their tunable composition, designable topologies, and tailorable porous structures, yet they suffer from severe capacity decay and inferior cycling stability due to the volume variation upon lithiation and high electrical resistance. Herein, we develop a universal strategy for synthesizing small PBA nanoparticles hosted on two-dimensional (2D) MXene or rGO (PBA/MX or PBA/rGO) via an in situ transformation from ultrathin layered double hydroxides (LDH) nanosheets. 2D conductive nanosheets allow for fast electron transport and guarantee the full utilization of PBA even at high rates; at the meantime, PBA nanoparticles effectively prevent 2D materials from restacking and facilitate rapid ion diffusion. The optimized Ni0.8Mn0.2-PBA/MX as an anode for lithium-ion batteries (LIBs) delivers a capacity of 442 mAh g-1 at 0.1 A g-1 and an excellent cycling robustness in comparison with bare PBA bulk crystals. We believe that this study offers an alternative choice for rationally designing PBA-based electrode materials for energy storage.

Enhanced Jahn-Teller distortion boosts molybdenum trioxide's superior lithium ion storage capability.

Upgrading the energy density and cycling life of current lithium ion batteries is urgently needed for developing advanced portable electronics and electric vehicles. Amorphous transition metal oxides (TMO) with inherent lattice disorders exhibit enormous potential as electrode materials owing to their high specific capacity, fast ion diffusion, and excellent cyclic stability. Yet, challenges remain in their controllable synthesis. In this study, the amorphous phase is induced into α-MoO3 crystal nanobelts at room temperature with the aid of Jahn-Teller effect via enhanced lattice distortion triggered by the accumulation of low-valent molybdenum centers. The optimized HI-MoO3-36 h exhibits high reversible capacities of 886.0 at 0.1 A g-1 and 491.1 mA h g-1 at 1.0 A g-1, respectively, along with outstanding stability retaining 83.4% initial capacity after 100 cycles at 0.1 A g-1. The crystal engineering strategy proposed in this work is believed to be a salutary reference towards the synthesis of high-performance TMO anodes for energy storage applications.

Site-Selective Transformation for Preparing Tripod-like NiCo-Sulfides@Carbon Boosts Enhanced Areal Capacity and Cycling Reliability.

Flexible power supply systems for future wearable electronics desperately require high areal capacity (Ca) and robust cycling reliability due to the limited surface area of the human body. Transition metal sulfides are preferred as cathode materials for their improved conductivity and rich redox centers, yet their practical applications are severely hindered by the sluggish charge transport kinetics and unavoidable capacity decay due to the phase transformation during charge/discharge processes. Herein, we develop a site-selective transformation strategy for preparing tripod-like NiCo-sulfides@carbon (T-NCS@C) arrays on carbon cloth. The mass loading of active materials is balanced with charge (electron and ion) transport efficiency. The optimized T-NCS@C delivers a superior Ca of 494 µA h/cm2 (corresponding to 235 mA h/g) at 3 mA/cm2. Due to the protection of the carbon layer that is derived from transformed metal-organic framework (MOF) sheath, the T-NCS@C displays excellent stability with 92% retention over 5000 charge/discharge cycles. The flexible full cell adopting Fe2O3 as the anode and T-NCS@C as the cathode exhibits an improved Ea (areal energy density) of 389 µW h/cm2 at a Pa (areal power density) of 4.22 mW/cm2 together with robust cycling reliability.

Ammonium Intercalation Induced Expanded 1T-Rich Molybdenum Diselenides for Improved Lithium Ion Storage.

Transition metal dichalcogenides (TMDs), particularly molybdenum diselenides (MoSe2), have the merits of their unique two-dimensional (2D) layered structures, large interlayer spacing (∼0.64 nm), good electrical conductivities, and high theoretical capacities when applied in lithium-ion batteries (LIBs) as anode materials. However, MoSe2 remains suffering from inferior stability as well as unsatisfactory rate capability because of the unavoidable volume expansion and sluggish charge transport during lithiation-delithiation cycles. Herein, we develop a simultaneous reduction-intercalation strategy to synthesize expanded MoSe2 (e-MoSe2) with an interlayer spacing of 0.98 nm and a rich 1T phase (53.7%) by rationally selecting the safe precursors of ethylenediamine (NH2C2H4NH2), selenium dioxide (SeO2), and sodium molybdate (Na2MoO4). It is noteworthy that NH2C2H4NH2 can effectively reduce SeO2 and MoO42- forming MoSe2 nanosheets; in the meantime, the generated ammonium (NH4+) efficiently intercalates between MoSe2 layers, leading to charge transfer, thus stabilizing 1T phases. The obtained e-MoSe2 exhibits high capacities of 778.99 and 611.40 mAh g-1 at 0.2 and 1 C, respectively, together with excellent cycling stability (retaining >90% initial capacity at 0.2 C over 100 charge-discharge cycles). It is believed that the material design strategy proposed in this paper provides a favorable reference for the synthesis of other transition metal selenides with improved electrochemical performance for battery applications.

Recent Advances in Molybdenum-Based Materials for Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries as power supply systems possessing a theoretical energy density of as high as 2600 Wh kg-1 are considered promising alternatives toward the currently used lithium-ion batteries (LIBs). However, the insulation characteristic and huge volume change of sulfur, the generation of dissolvable lithium polysulfides (LiPSs) during charge/discharge, and the uncontrollable dendrite formation of Li metal anodes render Li-S batteries serious cycling issues with rapid capacity decay. To address these challenges, extensive efforts are devoted to designing cathode/anode hosts and/or modifying separators by incorporating functional materials with the features of improved conductivity, lithiophilic, physical/chemical capture ability toward LiPSs, and/or efficient catalytic conversion of LiPSs. Among all candidates, molybdenum-based (Mo-based) materials are highly preferred for their tunable crystal structure, adjustable composition, variable valence of Mo centers, and strong interactions with soluble LiPSs. Herein, the latest advances in design and application of Mo-based materials for Li-S batteries are comprehensively reviewed, covering molybdenum oxides, molybdenum dichalcogenides, molybdenum nitrides, molybdenum carbides, molybdenum phosphides, and molybdenum metal. In the end, the existing challenges in this research field are elaborately discussed.

Mechanistic insight in site-selective and anisotropic etching of prussian blue analogues toward designable complex architectures for efficient energy storage.

Engineering coordination compounds, e.g., prussian blue (PB) and its analogues (PBAs), with designable complex nanostructures via chemical etching holds great opportunities for improving energy storage performances by adjusting topological geometry, selectively exposing active sites, tuning electronic properties and enhancing accessible surface area. Unfortunately, it remains ambiguous particularly on site-selective and anisotropic etching behaviors. Herein, for the first time, we propose that two distinct regions are formed inside NiCo PBA (NCP) cubes due to the competition between classical ion-by-ion crystallization and non-classical crystallization based on aggregation. Such a unique structure ultimately determines not only the etching position but also the anisotropic pathway by selectively exposing unprotected Ni sites. According to this principle, complex PBA architectures, including nanocages, open nanocubes (constructed by six cones sharing the same apex), nanocones, and chamfer nanocubes can be intentionally obtained. After thermal annealing, NCP nanocones are converted to morning glory-like porous architectures composed of NiO/NiCo2O4 heterostructures with a mean particle size of 5 nm, which show improved rate performance and cycling stability.

Electrostatically Assembling 2D Nanosheets of MXene and MOF-Derivatives into 3D Hollow Frameworks for Enhanced Lithium Storage.

As an essential member of 2D materials, MXene (e.g., Ti3 C2 Tx ) is highly preferred for energy storage owing to a high surface-to-volume ratio, shortened ion diffusion pathway, superior electronic conductivity, and neglectable volume change, which are beneficial for electrochemical kinetics. However, the low theoretical capacitance and restacking issues of MXene severely limit its practical application in lithium-ion batteries (LIBs). Herein, a facile and controllable method is developed to engineer 2D nanosheets of negatively charged MXene and positively charged layered double hydroxides derived from ZIF-67 polyhedrons into 3D hollow frameworks via electrostatic self-assembling. After thermal annealing, transition metal oxides (TMOs)@MXene (CoO/Co2 Mo3 O8 @MXene) hollow frameworks are obtained and used as anode materials for LIBs. CoO/Co2 Mo3 O8 nanosheets prevent MXene from aggregation and contribute remarkable lithium storage capacity, while MXene nanosheets provide a 3D conductive network and mechanical robustness to facilitate rapid charge transfer at the interface, and accommodate the volume expansion of the internal CoO/Co2 Mo3 O8 . Such hollow frameworks present a high reversible capacity of 947.4 mAh g-1 at 0.1 A g-1 , an impressive rate behavior with 435.8 mAh g-1 retained at 5 A g-1 , and good stability over 1200 cycles (545 mAh g-1 at 2 A g-1 ).

Identifying the active site of ultrathin NiCo LDH as an efficient peroxidase mimic with superior substrate affinity for sensitive detection of hydrogen peroxide.

Nanozymes have been extensively investigated to imitate protein enzymes in biomimetic chemistry and the identification of the active site is believed to be the pre-requisite before one can effectively regulate their activity. Herein, ultrathin NiCo LDH nanosheets are synthesized via a fast co-precipitation at room temperature and can be stably dispersed in water without any additives of surfactants or organic solvents. By tuning the ratio between Ni and Co in LDH nanosheets, the activity is tuned and their peroxidase-like activity is determined by Co sites that show higher affinity to both 3,3',5,5'-tetramethylbenzidine (TMB) and hydrogen peroxide (H2O2) due to the strong Lewis acidity of Co3+ and the low redox potential of Co3+/Co2+. Together with their small crystallite size, ultra-thin thickness and tunable composition, NiCo LDH is used as a nanozyme for highly sensitive colorimetric detection of H2O2 and the limit of detection (LOD) reaches 0.48 µM.

Nitrogen doped carbon nanofiber derived from polypyrrole functionalized polyacrylonitrile for applications in lithium-ion batteries and oxygen reduction reaction.

Nitrogen doped carbon nanofiber films (NCNFs) is successfully synthesized via the carbonization of polypyrrole (PPY) functionalized electrospun polyacrylonitrile (PAN) nanofibers. Benefitting from the nitrogen-doped one-dimensional (1D) nanostructure, the as-obtained NCNFs show multifunctional applications as electrode materials for lithium ion batteries (LIBs) and catalysts for oxygen reduction reaction (ORR). For LIB application, the obtained NCNFs could be directly used as flexible, free-standing and binder-free electrode, which exhibits high specific capacity up to 698.9mAhg-1 (higher than that of carbon nanofiber derived from PAN), long life over 160 cycles, and good rate capability (148.8mAhg-1 at 2Ag-1). In addition, a preliminary ORR catalytic activity study has demonstrated that the NCNFs exhibit good catalytic activity. Therefore, these properties endorse NCNFs as auspicious candidates for both LIBs and ORR.