Nanoenergetic Composites with Fluoropolymers: Transition from Powders to Structures.

Over the years, nanoenergetic materials have attracted enormous research interest due to their overall better combustion characteristics compared to their micron-sized counterparts. Aluminum, boron, and their respective alloys are the most extensively studied nanoenergetic materials. The majority of the research work related to this topic is confined to the respective powders. However, for practical applications, the powders need to be consolidated into reactive structures. Processing the nanoenergetic materials with polymeric binders to prepare structured composites is a possible route for the conversion of powders to structures. Most of the binders, including the energetic ones, when mixed with nanoenergetic materials even in small quantities, adversely affects the ignitability and combustion performance of the corresponding composites. The passivating effect induced by the polymeric binder is considered unfavorable for ignitability. Fluoropolymers, with their ability to induce pre-ignition reactions with the nascent oxide shell around aluminum and boron, are recognized to sustain the ignitability of the composites. Initial research efforts have been focused on surface functionalizing approaches using fluoropolymers to activate them further for energy release, and to improve the safety and storage properties. With the combined advent of more advanced chemistry and manufacturing techniques, fluoropolymers are recently being investigated as binders to process nanoenergetic materials to reactive structures. This review focuses on the major research developments in this area that have significantly assisted in the transitioning of nanoenergetic powders to structures using fluoropolymers as binders.

Reversible Al Metal Anodes Enabled by Amorphization for Aqueous Aluminum Batteries.

Aqueous aluminum metal batteries (AMBs) are regarded as one of the most sustainable energy storage systems among post-lithium-ion candidates, which is attributable to their highest theoretical volumetric capacity, inherent safe operation, and low cost. Yet, the development of aqueous AMBs is plagued by the incapable aluminum plating in an aqueous solution and severe parasitic reactions, which results in the limited discharge voltage, thus making the development of aqueous AMBs unsuccessful so far. Here, we demonstrate that amorphization is an effective strategy to tackle these critical issues of a metallic Al anode by shifting the reduction potential for Al deposition. The amorphous aluminum (a-Al) interfacial layer is triggered by an in situ lithium-ion alloying/dealloying process on a metallic Al substrate with low strength. Unveiled by experimental and theoretical investigations, the amorphous structure greatly lowers the Al nucleation energy barrier, which forces the Al deposition competitive to the electron-stealing hydrogen evolution reaction (HER). Simultaneously, the inhibited HER mitigates the passivation, promoting interfacial ion transfer kinetics and enabling steady aluminum plating/stripping for 800 h in the symmetric cell. The resultant multiple full cells using Al@a-Al anodes deliver approximately a 0.6 V increase in the discharge voltage plateau compared to that of bare Al-based cells, which far outperform all reported aqueous AMBs. In both symmetric cells and full cells, the excellent electrochemical performances are achieved in a noncorrosive, low-cost, and fluorine-free Al2(SO4)3 electrolyte, which is ecofriendly and can be easily adapted for sustainable large-scale applications. This work brings an intriguing picture of the design of metallic anodes for reversible and high-voltage AMBs.

A Defect Engineered Electrocatalyst that Promotes High-Efficiency Urea Synthesis under Ambient Conditions.

Synthesizing urea from nitrate and carbon dioxide through an electrocatalysis approach under ambient conditions is extraordinarily sustainable. However, this approach still lacks electrocatalysts developed with high catalytic efficiencies, which is a key challenge. Here, we report the high-efficiency electrocatalytic synthesis of urea using indium oxyhydroxide with oxygen vacancy defects, which enables selective C-N coupling toward standout electrocatalytic urea synthesis activity. Analysis by operando synchrotron radiation-Fourier transform infrared spectroscopy showcases that *CO2NH2 protonation is the potential-determining step for the overall urea formation process. As such, defect engineering is employed to lower the energy barrier for the protonation of the *CO2NH2 intermediate to accelerate urea synthesis. Consequently, the defect-engineered catalyst delivers a high Faradaic efficiency of 51.0%. In conjunction with an in-depth study on the catalytic mechanism, this design strategy may facilitate the exploration of advanced catalysts for electrochemical urea synthesis and other sustainable applications.

Decomposition and Energy-Enhancement Mechanism of the Energetic Binder Glycidyl Azide Polymer at Explosive Detonation Temperatures.

Replacing existing inert binders with energetic ones in composite explosives is a novel way to improve the explosive performance, on the proviso that energetic binders are capable of releasing chemical energy rapidly in the detonation environment. Known to be a promising candidate, the reaction mechanism of glycidyl azide polymer (GAP) at typical detonation temperatures higher than 3000 K has been theoretically studied in this work at the atomistic level. By analyzing and tracking the cleavage of characteristic chemical bonds, it was found that at the detonation temperature, GAP was able to release a large amount of energy and small molecule products at a speed comparable to commonly used explosives in the early reaction stage, which was mainly attributed to the decomposition of azide groups into N2 and the main chain breakage into small fragments. Moreover, N2 generation was found to be accelerated by H atom transfer at an earlier reaction step. The dissociation energy of the main chain was lowered with structure deformation so as to facilitate the fragmentation of the GAP chain. Based on this analytical study of reaction kinetics, GAP was found to have higher reactivity at the detonation temperature than at lower temperatures. The small molecules' yield rate is of the same order of magnitude as an explosive detonation reaction, indicating that GAP has the potential to improve the performance of composite explosives. Our study reveals the chemical decomposition mechanism of a typical energetic binder, which would aid in the future design and synthesis of energetic binders so as to achieve both sensitivity-reducing and energy-enhancing performance goals simultaneously.

Theoretical studies on the structures, material properties, and IR spectra of polymorphs of 3,4-bis(1H-5-tetrazolyl)furoxan.

Theoretical studies on the structures, densities, and heats of formation of conformational isomers of 3,4-bis(1H-5-tetrazolyl)furoxan (H2BTF) were performed based on density functional theory (DFT) calculations. Two stable planar conformational isomers, the face-to-back and the back-to-face conformers, and one stable slightly twisted conformer, the back-to-back conformer, were predicted for H2BTF at the M06-2X/6-311 + G(d,p) level of theory. The face-to-back conformer was calculated to be the most stable conformational isomer on the potential energy surface. No stable face-to-face conformer, whether planar or tilted, was identified in the calculations. The Vienna Ab Initio Simulation Package (VASP) was used in combination with molecular dynamics simulation to explore the stable crystal forms and the densities of the stable conformational isomers. Two of them exhibited high densities: the face-to-back conformer with P21 symmetry (2.01 g/cm3) and the back-to-back conformer with Pna21 symmetry (2.05 g/cm3). Their heats of formation were also predicted to be high when calculated at the same DFT level. The detonation pressures and velocities of these polymorphs, as calculated using the EXPLO5 program, are well above those of many advanced high energy density materials, pointing to the potential use of these conformers as novel explosives with good detonation performance. Also, IR spectra are shown to be able to distinguish these denser polymorphs of H2BTF. This study suggests that it could be worth investigating whether denser polymorphs of H2BTF can be grown.

2D Black Phosphorus for Energy Storage and Thermoelectric Applications.

Recent progress in the currently available methods of producing black phosphorus bulk and phosphorene are presented. The effective passivation approaches toward improving the air stability of phosphorene are also discussed. Furthermore, the research efforts on the phosphorene and phosphorene-based materials for potential applications in lithium ion batteries, sodium ion batteries, and thermoelectric devices are summarized and highlighted. Finally, the outlook including challenges and opportunities in these research fields are discussed.

General Approach for MOF-Derived Porous Spinel AFe2O4 Hollow Structures and Their Superior Lithium Storage Properties.

A general and simple approach for large-scale synthesis of porous hollow spinel AFe2O4 nanoarchitectures via metal organic framework self-sacrificial template strategy is proposed. By employing this method, we can successfully synthesize uniform NiFe2O4, ZnFe2O4, and CoFe2O4 hollow architectures that are hierarchically assembled by nanoparticles. When these hollow microcubes were tested as anode for lithium ion batteries, good rate capability and long-term cycling stability can be achieved. For example, high specific capacities of 636, 449, and 380 mA h g(-1) were depicted by NiFe2O4, ZnFe2O4, and CoFe2O4, respectively, at a high current density of 8.0 A g(-1). NiFe2O4 exhibits high specific capacities of 841 and 447 mA h g(-1) during the 100th cycle when it was tested at current densities of 1.0 and 5.0 A g(-1), respectively. Discharge capacities of 390 and 290 mA h g(-1) were delivered by the ZnFe2O4 and CoFe2O4, respectively, during the 100th cycle at 5.0 A g(-1).

Achieving Site-Specificity in Multistep Colloidal Synthesis.

We show that partial inhibition of the emerging Ag domain can be achieved by controlling the growth dynamics. With the symmetry broken by the "fresh" surface, sequentially growth gives (Au sphere)-(Ag wire)-(Ag plate) triblock nanostructures. This new understanding opens doors to sophisticated synthetic designs, broadening the horizon of our search for functional architectures.

Fabrication of flexible thermoelectric thin film devices by inkjet printing.

Ink-jet printing of thermoelectric nanomaterials is successfully used to fabricate flexible thin film TE devices for power generation and cooling.

Synthesis of two-dimensional transition-metal phosphates with highly ordered mesoporous structures for lithium-ion battery applications.

Materials with ordered mesoporous structures have shown great potential in a wide range of applications. In particular, the combination of mesoporosity, low dimensionality, and well-defined morphology in nanostructures may exhibit even more attractive features. However, the synthesis of such structures is still challenging in polar solvents. Herein, we report the preparation of ultrathin two-dimensional (2D) nanoflakes of transition-metal phosphates, including FePO4, Mn3(PO4)2, and Co3(PO4)2, with highly ordered mesoporous structures in a nonpolar solvent. The as-obtained nanoflakes with thicknesses of about 3.7 nm are constructed from a single layer of parallel-packed pore channels. These uniquely ordered mesoporous 2D nanostructures may originate from the 2D assembly of cylindrical micelles formed by the amphiphilic precursors in the nonpolar solvent. The 2D mesoporous FePO4 nanoflakes were used as the cathode for a lithium-ion battery, which exhibits excellent stability and high rate capabilities.

Gas flow induced by ultrasonic cavitation bubble clouds and surface capillary wave.

In this paper, we report a gas flow phenomenon induced by ultrasonic water cavitation and capillary wave in a vibrating hollow tip and reflector system. The cavitation clouds generated a gas suction force and the capillary wave created tunnels through which the gas could go into the liquid. The gas flow rate was measured and compared under different conditions, including applied power, type of reflector, and tip-to-reflector distance. A model was proposed to explain the mechanisms of the gas flow and analyze the results in the experiments.

Fe-based metallopolymer nanowall-based composites for Li-O2 battery cathode.

Metallopolymer nanowalls were prepared through a simple wet-chemical process using reduced graphene oxides as heterogeneous nucleation aids, which also help to form conductive electron paths. The nanowalls grow vertically on graphene surface with 100-200 nm in widths and ∼20 nm in thickness. The Fe-based metallopolymer nanowall-based electrode shows best performance as O2 cathode exhibiting high round-trip efficiencies and stable cycling performance among other transition metal containing metallopolymer counterparts. The electrode delivers discharge-charge capacities of 1000 mAh/g for 40 cycles and maintains round-trip efficiencies >78% at 50 mA/g. The 1(st)-cycle round-trip efficiencies are 79%, 72%, and 65% at current densities of 50, 200, and 400 mA/g, respectively. The NMR analysis of the Fe-based metallopolymer based electrode after 40 cycles reveals slow formation of the side products, CH3CO2Li and HCO2Li.

n-Type carbon nanotubes/silver telluride nanohybrid buckypaper with a high-thermoelectric figure of merit.

n-Type thermoelectric (TE) materials was made from carbon nanotube (CNT) buckypapers. We used silver telluride (Ag2Te) to achieve electron injection to the CNTs. The TE characterizations on more than 50 samples show that the CNTs/Ag2Te hybrids exhibit negative Seebeck coefficients (e.g., n-type) from -30 to -228 µV/K. Meanwhile, the tunneling coupling between the CNTs and Ag2Te increase the electrical conductance to the range of 10,000-20,000 S/m, which is higher than each single component (CNTs or Ag2Te). These n-type TE buckypapers are flexible and robust with ZT values of 1-2 orders of magnitude higher than previously reproted for CNT-based TE materials. In addition, the preparation of such buckypapers are very simple compared to a tranditonal inorganic process, without the need for hot pressing or spark sintering. These n-type TE buckypapers can provide important components for fabricating CNT-based flexible TE devices with good conversion efficiency.

Aqueous-based chemical route toward ambient preparation of multicomponent core-shell nanotubes.

Room-temperature synthesized V2O5@MnO2 core-shell nanotubes with tunable tunnel dimensions via a facile aqueous-based method are presented. The rational-designed tubular morphology endows them with good permeability of electrolyte ions for maximum utilization of the electroactive sites, while the epitaxial-grown MnO2 imposes mechanical support to V2O5 against structural collapse upon long-term cycling. Hence, specific capacitance as high as 694 F g(-1) is achieved at 1 A g(-1) accompanied by excellent cycling stability (preserved 92% of its initial specific capacitance after 5000 cycles). In addition, functionalization of the V2O5@MnO2 nanotubes with other transition metal oxides results in ternary composites, V2O5@MnO2/M nanotubes (M = Fe2O3, Co2O3/Co(OH)2, Ni(OH)2). The versatility of this synthetic protocol provides a platform to fabricate complex ternary nanocomposites in a more benign way.

Ultrahigh rate capabilities of lithium-ion batteries from 3D ordered hierarchically porous electrodes with entrapped active nanoparticles configuration.

Three dimensional (3D) ordered hierarchically porous electrodes with an entrapped active nanoparticles configuration afford an extremely effective conductive 3D network from the micrometer to the nano meter scale for fast electron and Li-ion transport, and also allow the development of a stable solid electrolyte interphase over the electrode materials, therefore exhibiting extraordinary rate capabilities.

Bio-inspired antireflective hetero-nanojunctions with enhanced photoactivity.

A bio-inspired antireflective hetero-nanojunction structure has been fabricated by the hydrothermal growth of ZnO nanorods on silicon micro-pyramids. It has been shown that this structure suppresses light reflection more effectively resulting in a high photocurrent response and good charge separation simultaneously. The strategy provides a means to enhance solar energy conversion.

Binder-free graphene foams for O2 electrodes of Li-O2 batteries.

We report a novel method to prepare bind-free graphene foams as O2 electrodes for Li-O2 batteries. The graphene foams are synthesized by electrochemical leavening of the graphite papers, followed by annealing in inert gas to control the amount of structural defects in the graphene foams. It was found that the structural defects were detrimental to the processes of the ORR and OER in Li-O2 batteries. The round-trip efficiencies and the cycling stabilities of the graphene foams were undermined by the structural defects. For example, the as-prepared graphene foam with a high defect level (ID/IG = 0.71) depicted a round-trip efficiency of only 0.51 and a 20(th)-cycle discharge capacity of only 340 mA h g(-1) at a current density of 100 mA g(-1). By contrast, the graphene foam electrode annealed at 800 °C with ID/IG = 0.07 delivered a round-trip efficiency of up to 80% with a stable discharge voltage at ~2.8 V and a stable charge voltage below 3.8 V for 20 cycles. According to the analysis on the electrodes after 20 cycles, the structural defects led to the quickened decay of the graphene foams and boosted the formation of side products.

Carbon inverse opal entrapped with electrode active nanoparticles as high-performance anode for lithium-ion batteries.

Enhancing ion and electron transport kinetics together with improving cycle life are important issues to be considered when developing high-performance Li ion batteries. Here we demonstrate a three dimensional ordered macroporous conductive electrode concept by entrapping electrode active nanoparticles in an interpenetrating macroporous carbon inverse opal. The electrodes are featured with simultaneously enhanced ion and electron transport kinetics as well as geometrically constrained active nanoparticles. The electrode can deliver up to 94.17% of theoretical capacity over 1000 discharge/charge cycles at a current density of 2.0 A g(-1), and exhibits good rate capability in the high current density range of 1.0-10.0 A g(-1). We hope that our findings will help pave the way for tailored design of many other sophisticated electrode materials in electrochemistry.

Cu doped V2O5 flowers as cathode material for high-performance lithium ion batteries.

Hierarchical Cu doped vanadium pentoxide (V2O5) flowers were prepared via a simple hydrothermal approach followed by an annealing process. The flower precursors are self-assembled with 1D nanobelts surrounding a central core. The morphological evolution is investigated and a plausible mechanism is proposed. As the cathode material for lithium ion batteries, the Cu doped V2O5 samples exhibit improved electrochemical performance compared to the un-doped ones. Among them Cu0.02V1.98O5 delivered higher reversible specific capacities, better cycling stabilities and excellent rate capabilities, e.g. 97 mA h g(-1) at 20.0 C.