Dual Design on Hierarchically Hollow MoTe2/C with Ion/Electron Channel Engineering for High-Performance Sodium Storage.

Transition-metal tellurides have been investigated as novel anode materials for application in sodium-ion batteries (SIBs) due to their rich active sites and unique and controllable layered nanostructures. However, the weak structural strength and inferior intercalation/deintercalation kinetics inhibit the development of transition-metal tellurides. In this work, MoTe2/C composites with two different hollow nanostructures are designed and prepared. By adjustment of the precursor structure, MoTe2/C-2 exhibits superior sodium-storage performance because of its uniquely hollow nanostructure with self-assembled 2D flexible nanosheets grown on the external surface. MoTe2/C-2 delivers a higher specific capacity (276 mAh g-1 at 0.1 A g-1 after 300 cycles), much more than MoTe2/C-1 (201 mAh g-1 at 0.1 A g-1 after 300 cycles), and exhibits a long-time cycling performance (131 mAh g-1 at 1 A g-1 after 2000 cycles). The excellent sodium-storage performance derived from the rational structure design is beneficial for shortening the ion paths, facilitating the sodiation/desodiation process, and reinforcing the intrinsic structural stability, thus boosting the reaction kinetics and prolonging the cycling life. Meanwhile, the assembled full-cell maintains 101 mAh g-1 at 0.1 A g-1 after 50 cycles and lights an electric watch. The findings provide several new views for preparation of more transition-metal tellurides with multi-ion/electron migration channel engineering.

Fluorine-Terminated Self-Assembled Monolayers Grafted Graphite Anode Inducing a LiF-Dominated SEI Inorganic Layer for Fast-Charging Lithium-Ion Batteries.

The electrochemical kinetic processes of Li+ ions, including the desolvation of the Li+ ions from the electrolyte to the solid electrolyte interphase (SEI), the transportation of desolvated Li+ ions across the SEI, and the charge transfer at the interface between the SEI and graphite, determine the rate performance and cycling stability of the graphitic anode in lithium-ion batteries (LIBs). In this work, fluorine-terminated self-assembled monolayers were grafted on the surface of spherical graphite particles to regulate the chemical composition and structure of SEI formed on the graphite surface in the presence of conventional ester electrolytes. The comprehensive characterization and first-principles calculation results illustrate that a uniform LiF-dominated SEI film can be generated on the as-functionalized graphite anode due to the carbon-fluorine bonds' cleavage of fluorine-terminated self-assembled monolayers. The LiF-dominated SEI film is particularly beneficial for desolvated lithium-ion transport across the SEI, affording LiCoO2//graphite full cells with substantially enhanced fast-charging capability and cycle stability. This strategy should be potentially useful for modifying other anode materials to regulate the interfacial chemistry between the anode and electrolyte in lithium-ion batteries.

Entropy-Driven Morphology Regulation of MAX Phase Solid Solutions with Enhanced Microwave Absorption and Thermal Insulation Performance.

Morphology regulation and composition design have proved to be effective strategies for the fabrication of desirable microwave absorbers. However, it is still challenging to precisely control the microstructure and components of MAX phases. Herein, an entropy-driven approach, a transition from irregular grains (low entropy) to sheet structure (high entropy), is proposed to modulate the morphology of MAX phases. The theoretical calculation indicates that the morphology evolution can be ascribed to the enlarged energy difference between (11_00) and (0001) facets. The enriched structural defects and optimized morphologies yield significant dipolar polarization, interfacial polarization, multiple reflections, and scattering, which all enhance the electromagnetic wave absorption performance of (V0.25 Ti0.25 Cr0.25 Mo0.25 )2 GaC. Specifically, its minimum reflection loss can reach up to -47.12 dB at 12.13 GHz, and the optimal effective absorption bandwidth is 4.56 GHz (2.03 mm). Meanwhile, (V0.25 Ti0.25 Cr0.25 Mo0.25 )2 GaC shows also pronounced thermal insulation properties affording it good reliability in the harsh working environment. This work offers a novel approach to designing and regulating the morphology of the high entropy MAX phase, and also presents an opportunity to elucidate the relationship between entropy and electromagnetic wave absorption performance.

Flexible Fluorinated Graphene/Poly(vinyl Alcohol) Films toward High Thermal Management Capability.

Graphene is widely used in heat dissipation, owing to its inherently high in-plane thermal conductivity and excellent mechanical properties. However, its poor cross-plane thermal conductivity limits its use in some electronic applications. The electron distribution of graphene and the interaction with the base material can be greatly altered by introducing F, the most electronegative element, giving fluorinated graphene oxide (FG) with a high thermal conductivity. Herein, FG is prepared by grafting F atoms onto the surface of graphene oxide in a low-temperature solid-phase reaction with poly(vinylidene fluoride) as a fluorine source. This method can effectively avoid the use of dangerous substances such as HF and F2. The FG dispersion and aqueous poly(vinyl alcohol) (PVA) solution are sequentially vacuum-filtered to obtain the FG/PVA composite film. After natural drying and hot-pressing, the thermal conductivity of the N-FG/PVA film is enhanced by the hydrogen bond between F of FG and the hydroxyl group of PVA. The in-plane and cross-plane thermal conductivity of an N-FG/PVA film containing 10.4 wt % FG are 7.13 and 1.42 W m-1 k-1, respectively. The film has a tensile strength of 60 MPa and an elongation at a break of 28%, which is promising for the thermal management of flexible electronic devices.

Removal of organophosphorus agents via atomically dispersed Co on nitrogen-doped graphene: catalytic degradation and adsorption.

In this work, we disperse Co atomically on three-dimensional networks of N-doped graphene (3DNG) through the impregnation of 3DNG with Co(Ac)2·4H2O solution followed by rapid pyrolysis. The structure, morphology and composition of the as-prepared composite, namely ACo/3DNG, are characterized. The atomically dispersed Co and enriched Co-N species afford the ACo/3DNG with unique catalytic activity for hydrolysis of organophosphorus agents (OPs), and the network structure and super-hydrophobic surface of 3DNG ensures excellent physical adsorption capacity. Thus, ACo/3DNG demonstrates good capability for removal of OPs pesticides from water.

Vapor-phase derived ultra-fine Bismuth nanoparticles embedded in carbon nanotube networks as anodes for sodium and potassium ion batteries.

Bismuth (Bi) is a promising material as the anode for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) due to its characteristics such as reasonable price and high theoretical volumetric capacity (3800 mAh cm-3). Nevertheless, considerable drawbacks have hindered the practical applications of Bi, including its relatively low electrical conductivity and inevitable volumetric change during the alloying/dealloying processes. To solve these problems, we proposed a novel design:Bi nanoparticles were synthesized via a single-step low-pressure vapor-phase reaction and embedded onto the surfaces of multi-walled carbon nanotubes (MWCNTs). After being vaporized at 650℃ and 10-5 Pa, Bi nanoparticles less than 10 nm were uniformly distributed in the three-dimensional (3D) MWCNT networks to form a Bi/MWNTs composite. In this unique design, the nanostructured Bi can reduce the risk of structural rupture during cycling, and the structure of the MWCMT network is beneficial in shortening the electron/ion transport path. In addition, MWCNTs can improve the overall conductivity of the Bi/MWCNTs composite and prevent particle aggregation, thus improving the cycling stability and rate performance. As an anode material for SIB, the Bi/MWCNTs composite has demonstrated excellent fast charging performance with a reversible capacity of 254 mAh/g at 20 A/g. A capacity of 221mAhg-1 after cycling at 10 A/g for 8000 cycles has also been achieved for SIB. As an anode material for PIB, the Bi/MWCNTs composite has delivered excellent rate performances with a reversible capacity of 251 mAh/g at 20 A/g. A specific capacity of 270mAhg-1 after cycling at 1Ag-1 for 5000 cycles has also been achieved for PIB.

Oxidative Cleavage of ß-O-4 Linkage in Lignin via Co Nanoparticles Embedded in 3DNG as Catalyst.

The cleavage of ß-O-4 linkage in lignin is one of the key steps for oxidative conversion of lignin to low-molecular-weight aromatics. Herein, Co nanoparticles embedded in three-dimensional network of nitrogen-doped graphene (Co/NG@3DNG-X) were prepared through an immersion-pyrolysis procedure, in which X denotes the pyrolysis temperature. The detailed characterization of Co/NG@3DNG-X shows that the Co nanoparticles are coated with a few layers of nitrogen-doped graphene (NG) sheets that are further embedded in 3DNG matrix. The catalytic activities of the Co/NG@3DNG-X for the oxidative cleavage of ß-O-4 linkage in lignin model compounds with O2 as oxidant are explored. It is demonstrated that catalytic activities of as-prepared Co/NG@3DNG-X can be tuned by varying the pyrolysis condition, and the Co/NG@3DNG-900 shows the highest catalytic activity, which is attributed to the enriched Co-Nx species, the strong surface basicity, the high specific surface and the mesoporous motif of 3DNG network. More pronouncedly, the Co/NG@3DNG-900 can also effectively catalyze the oxidative cleavage of organosolv lignin, generating certain monomeric aromatics. Additionally, the intrinsic magnetic property of Co nanoparticles makes the Co/NG@3DNG-X be easily recovered from the reaction mixture, and the as-coated thin NG layer can protect Co nanoparticle from oxidation condition, which putting together afford the Co/NG@3DNG-X with good reusability and stability.

Catalytic Oxidation of Veratryl Alcohol Derivatives Using RuCo/rGO Composites.

Chemoselectively oxidizing Cα -OH to C=O has been considered as a key step for the oxidative depolymerization of lignin. In this work, we design and prepare a series of composites of RuCo alloy nanoparticles and reduced graphene oxide (RuCo/rGO) with different Ru to Co ratios and explore their catalytic activities in the oxidation of veratryl alcohol derivatives, which usually serve as the model compounds for studying lignin oxidation. It is illustrated that the Ru to Co ratio determines the morphology and average size of the RuCo alloy nanoparticles on rGO, and the overall catalytic activities of the composites. The RuCo alloy nanoparticles on rGO with Ru to Co ratios of 1 : 0 to 1.2 : 1 show a unique flower-shaped morphology that increases the exposure of the active sites and thus promotes their contact with the substrates. The RuCo/rGO composites exhibit high catalytic activities for the oxidation of Cα -OH to aldehydes at 100 °C for 2 h. Additionally, the Co component affords the RuCo/rGO composites with magnetic properties that make the separation and recovery of the catalyst simple. Given the high catalytic performances and easy recovery, the RuCo/rGO composites would be potentially useful for the depolymerization of lignin.

Hydrolysis of Organophosphorus Agents Catalyzed by Cobalt Nanoparticles Supported on Three-Dimensional Nitrogen-Doped Graphene.

Catalytic chemical degradations and many other methodologies have been explored for the removal and/or degradation of organophosphorus agents (OPs) that are often used as pesticides, nerve agents, and plasticizers. To explore more efficient and recyclable catalysts for the removal and/or degradation of OPs, we fabricate the composites of cobalt nanoparticles and three-dimensional nitrogen-doped graphene (Co/3DNG). We demonstrate that OPs can be hydrolyzed efficiently at ambient temperature by the Co/3DNG. Because of the unique structural and chemical properties of the supporting matrix 3DNG and active species Co-N, the catalytic activities of Co/3DNG composites are much higher than those of bare 3DNG, Co nanoparticles, or the Co nanoparticles physically mixed with 3DNG. We conclude that in the Co/3DNG composites, the interaction between 3DNG and Co stabilizes and distributes well the Co nanoparticles and affords the active catalytic species Co-N.

Core-Shell PMIA@PVdF-HFP/Al2O3 Nanofiber Mats In Situ Coaxial Electrospun on LiFePO4 Electrode as Matrices for Gel Electrolytes.

Gel electrolytes show certain advantages over conventional liquid and solid electrolytes, but their mechanical strength and surface adhesion to the electrode remain to be improved. To address the challenges, we design and fabricate herein the core-shell nanofiber mats in situ on the LiFePO4 electrode as matrices for gel electrolytes, in which the core is poly(m-phenylene isophthalamide) (PMIA) nanofiber and the shell are composite of Al2O3 nanoparticles and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP). The mechanical property of the core-shell polymeric nanofiber mats and their surface interaction with LiFePO4 electrode are characterized complementarily using dynamic thermomechanical analysis and scanning electron microscopy. The electrochemical properties of the gel electrolytes based on the as-prepared matrices after being loaded with lithium salt solution are studied systematically on half coin cells. It is found that the ultimate strength of the core-shell PMIA@PVdF-HFP/Al2O3 mat can reach 6.70 MPa, 2 times higher than that of the PVdF-HFP/Al2O3 nanofiber mat. Meanwhile, the shell PVdF-HFP/Al2O3 can ensure manifest surface affinity to the LiFePO4 electrode and enhance lithium-ion conductance. Thus, the as-assembled LiFePO4 half coin cells using PMIA@PVdF-HFP/Al2O3 gel electrolyte show good electrochemical performances, especially the long cycle stability with the capacity retention of 96.6% after 600 cycles under 1C.

Hydrated vanadium pentoxide/reduced graphene oxide composite cathode material for high-rate lithium ion batteries.

As well-known, hydrated vanadium pentoxide (V2O5·nH2O) has a larger layer spacing than orthogonal V2O5, which could offer more active sites to accommodate lithium ions, ensuring a high specific capacity. However, the exploration of V2O5·nH2O cathode is limited by its inherently low conductivity and slow electrochemical kinetics, leading to a significant decrease in capability. Herein, we prepared V2O5·nH2O/reduced graphene oxide (rGO) composite with low rGO content (8 wt%) via a simple yet effective dual electrostatic assembly strategy. When used as the cathode material for lithium-ion batteries (LIBs), V2O5·nH2O/rGO manifests a high reversible capacity of 268 mAh g-1 at 100 mA g-1 and especially an excellent rate capability (196 mAh g-1 at 1000 mA g-1 and 129 mA h g-1 at 2000 mA g-1), surpassing those of the V2O5/carbon composites reported in the literatures. Notably, the remarkable performance should be referable to the synergetic effects between one-dimensional V2O5·nH2O nanobelts and two-dimensional rGO nanosheets, which provide a short transport pathway and enhanced electrical conductivity. This strategy opens a new opportunity for designing high-performance cathode material with excellent rate performance for advanced LIBs.

Morphology Design of Co-electrospinning MnO-VN/C Nanofibers for Enhancing the Microwave Absorption Performances.

To enhance microwave loss abilities, constructing composites with one-dimensional (1D) structure is an excellent scheme. In this work, a high-efficiency microwave absorber of MnO nanograins decorated vanadium nitride/carbon nanofibers (MnO-VN/C NFs) was successfully prepared for the first time via co-electrospinning technology and subsequent nitriding treatment. Studying in detail the specific relationship between nitriding time and the morphology of the as-prepared NFs, the precipitations of MnO nanoparticles with tailored structures were attached on the surface of VN/C NFs to optimize their electromagnetic parameters. When the nitriding time was 2.0 h at 600 °C, the MnO-VN/C NFs displayed good microwave absorption performances: the minimum reflection loss (RL) value was -63.2 dB at 8.8 GHz, and the bandwidth of RL < -10 dB was up to 6.4 GHz from 11.6 to 18 GHz at the thickness of 2.8 mm. Meanwhile, the absorption bandwidth (RL< -10 dB) could cover the whole X and Ku band by adjusting the thickness, respectively. The outstanding performances could be attributed to the good impedance matching and various loss pathways including conductive loss and interfacial and dipole polarizations. In these regards, MnO-VN/C NFs are likely to be utilized as a high-efficiency microwave absorber. And the strategy in this work can provide great help to design other 1D structural microwave absorbers with a broader absorbing band.

A High-Performance Primary Nanosheet Heterojunction Cathode Composed of Na0.44 MnO2 Tunnels and Layered Na2 Mn3 O7 for Na-Ion Batteries.

Owing to its large capacity and high average potential, the structure and reversible O-redox compensation mechanism of Na2 Mn3 O7 have recently been analyzed. However, capacity fade and low coulombic efficiency over multiple cycles have also been found to be a problem, which result from oxygen evolution at high charge voltages. Herein, a Na0.44 MnO2 ⋅Na2 Mn3 O7 heterojunction of primary nanosheets was prepared by a sol-gel-assisted high-temperature sintering method. In the nanodomain regions, the close contact of Na0.44 MnO2 not only supplies multidimensional channels to improve the rate performance of the composite, but also plays the role of pillars for enhancing the cycling stability and coulombic efficiency; this is accomplished by suppressing oxygen evolution, which is confirmed by high-resolution (HR)TEM, cyclic voltammetry, and charge/discharge curves. As the cathode of a Na-ion battery, at 200 mA g-1 after 100 cycles, the Na0.44 MnO2 ⋅Na2 Mn3 O7 heterojunction retains an 88 % capacity and the coulombic efficiency approaches 100 % during the cycles. At 1000 mA g-1 , the Na0.44 MnO2 ⋅Na2 Mn3 O7 heterojunction has a discharge capacity of 72 mAh g-1 . In addition, the average potential is as high as 2.7 V in the range 1.5-4.6 V. The above good performances indicate that heterojunctions are an effective strategy for addressing oxygen evolution by disturbing the long-range order distribution of manganese vacancies in the Mn-O layer.

Graphene quantum dots in photodynamic therapy.

Graphene quantum dots (GQDs) have shown great promise in a variety of medical applications. Recently, it has been found that GQDs are also beneficial for photodynamic therapy (PDT). However, the findings of GQDs as PDT agents have been controversial in the literature. Herein, we investigate the photoactivity of single-atomic-layered GQDs by examining their ability to generate singlet oxygen (1O2) under irradiation and their effects on the photoactivity of photosensitizers. We demonstrate that the GQDs with lateral sizes of ∼5 or 20 nm are photo-inactive for they cannot generate 1O2 under irradiation of either a 660 nm laser (105 mW cm-2) or a halogen light. Moreover, the GQDs inhibit the photoactivity of two classical photosensitizers, namely, methylene blue and methylene violet. The stronger interaction between the GQDs and the photosensitizer results in greater inhibition of GQDs. Besides, the large-sized GQDs exhibit stronger inhibition than the small-sized GQDs. The inhibitory effect of the GQDs on the photoactivity of photosensitizers is consistent with their photo-cytotoxicity. These results indicate that the single-atomic-layered GQDs are not potential PDT agents, but they may be helpful for photosensitizers by delivering them into the cells. The discrepancy between the current work and the literature is probably associated with the GQDs used.

Dual Role of Graphene Quantum Dots in Active Layer of Inverted Bulk Heterojunction Organic Photovoltaic Devices.

Graphene quantum dots (GQDs) have shown broad application prospects in the field of photovoltaic devices due to their unique quantum confinement and edge effects. Here, we prepared GQDs by a photon-Fenton reaction as reported in our previous work, which has great advantage in the preparation scale. The photoelectric properties of the inverted hybrid solar cells based on poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C61 butyric acid methylester (PCBM):GQDs and P3HT:GQDs with different contents of GQDs as the active layers are demonstrated, as well as their morphology and structure by atomic force microscopy images. Then, the different roles of GQDs played in the ternary (P3HT:PCBM:GQDs) and binary (P3HT:GQDs) hybrid solar cells are studied systematically. The results indicate that the GQDs provide an efficient excition separation interface and charge transport channel for the improvement of hybrid solar cells. The preliminary exploration and elaboration of the role of GQDs in hybrid solar cells will be beneficial to understand the interfacial procedure and improve device performance in the future.

Hierarchical TiO2-x nanoarchitectures on Ti foils as binder-free anodes for hybrid Li-ion capacitors.

Hybrid Li-ion capacitor (LIC) draws more attention as novel energy storage device owing to its high power density and high energy density. Designing three-dimensional electrode materials is beneficial for improving electrochemical performance of LICs. Herein, an improved hydrothermal method combined with an ion-exchange reaction is used to manufacture oxygen vacancies (OVs)-doping TiO2 (TiO2-x) nanowires/nanosheets (NWS) on Ti-foil. Then TiCl4 treatment is performed to form TiO2-x NWS/nanocrystallines (NWSC). These-obtained hierarchical nanoarchitectures assumes enrich electro-active sites and contact areas, which can improve electron transference and structural stability. The TiO2-x NWSC is used as binder-free anode for Li-ion battery and achieves high specific capacity (300 mAh g-1 at 0.1 A g-1), excellent rate capability (102 mAh g-1 at 5 A g-1) and long cycle stability (44% after 1000 cycles at 1 A g-1). LICs assembled with a TiO2-x NWSC anode and an activated carbon cathode have an energy density of 44.2 W h kg-1 at the power density of 150 W kg-1. Therefore, the TiO2-x NWSC is a potential candidate for high energy and high power electrochemical energy storage devices.

Separating graphene quantum dots by lateral size through gel column chromatography.

Graphene quantum dots (GQDs) prepared through photo-Fenton reaction of graphene oxide are separated via gel column chromatography. The as-separated GQDs were selectively introduced into the active layer of organic solar cells and achieved an enhancement of power conversion efficiency (PCE).

Flexible micro-supercapacitors assembled via chemically reduced graphene oxide films assisted by a laser printer.

Flexible micro-supercapacitors (MSCs) as power suppliers are important for portable and wearable electronic devices. Despite enormous efforts made, a simple, inexpensive high-throughput technique of graphene-based MSCs is still challenging. In this work, flexible MSCs are fabricated through commercial laser printing of the interdigital configuration of reduced graphene oxide-graphene oxide-reduced graphene oxide (rGO-GO-rGO) where the conductive rGO works as the electrode and the insulated GO serves as the separator. We demonstrate that the as-fabricated MSC devices show high-energy storage capacities, good cyclic stability and remarkable flexibility. The relationship between the geometric parameters (integration level and coverage fraction) and the capacitive performance of the MSCs is studied systematically to build better theoretical guidance for the design of future in-plane MSCs.

Composites of Layered M(HPO4)2 (M = Zr, Sn, and Ti) with Reduced Graphene Oxide as Anode Materials for Lithium Ion Batteries.

Tetravalent metal phosphates (M(HPO4)2, M = Zr, Sn, and Ti) have robust layered structures with interlayer d spacings over 7.5 Å, but show poor electrical conductivity. On the other hand, single-atomic-layered reduced graphene oxide (rGO) sheets exhibit a high electrical conductivity. In this work, the combination of rGO and M(HPO4)2 is explored for their potential as anode materials for lithium ion batteries (LIBs). Specifically, rGO/M(HPO4)2 composites are prepared, and their electrochemical performances are investigated systematically. In comparison with bare M(HPO4)2, the rGO/M(HPO4)2 composites exhibit larger specific capacity, higher rate capability, better cyclic stability, lower voltage for lithium ion insertion and extraction, and improved first Coulombic efficiency. We propose that the superior electrochemical performances of the composites are primarily contributed to the large interlayer space of M(HPO4)2 and the rGO sheets cladded on the surfaces of the layered M(HPO4)2. The attached rGO sheets bridge the layers together forming a network that is beneficial for the electron and ion diffusion within the composites, thus enhancing the discharge/charge rate capability of the composites. In addition, the attached rGO sheets provide extra anchoring sites for Li+; the specific capacity of the composites as anode materials is thus enhanced.

Preparation of nitrogen and sulfur co-doped ordered mesoporous carbon for enhanced microwave absorption performance.

Ordered mesoporous carbon nanomaterials (OMCs) co-doped with homogeneous nitrogen and sulfur heteroatoms were prepared by nanocasting with the pyrrole oligomer catalyzed by sulfuric acid as a precursor and ordered mesoporous silica SBA-15 as a hard-template. By multi-technique approach utilization, it was demonstrated that the N and S co-doped OMCs possessed high ordered mesoporous structures, large surface areas and homogeneous distribution of heteroatoms. As a microwave absorber, the as-prepared materials exhibited a minimum reflection loss (RL) of -32.5 dB at the thickness of 2.5 mm and an absorption bandwidth of 3.2 GHz (RL < -10 dB) in X-band (8.2-12.4 GHz). The good microwave absorption performance was mainly originated from the high electrical conductivity induced by the high surface activity and special structures. And microwave energy can be effectively attenuated through multiple reflections and absorptions in complex conductive network. The design strategy in this work would contribute to the production of a lightweight absorber, presenting a strong absorbency and a wide bandwidth in microwave frequency.