Excellent electrochemical performance of LiFe0.4Mn0.6PO4 microspheres produced using a double carbon coating process

Composite LiFe0.4Mn0.6PO4/C microspheres are considered advanced cathode materials for electric vehicles and other high-energy density applications due to their advantages of high energy density and excellent cycling stability. LiFe0.4Mn0.6PO4/C microspheres have been produced using a double carbon coating process employing traditional industrial techniques (ball milling, spray-drying and annealing). The obtained LiFe0.4Mn0.6PO4 microspheres exhibit a high discharge capacity of around 166 mA h g-1 at 0.1 C and excellent rate capabilities of 132, 103, and 72 mA h g-1 at 5, 10, and 20 C, respectively. A reversible capacity of about 152 mA h g-1 after 500 cycles at a current density of 1 C indicates an outstanding cycling stability. The excellent electrochemical performance is attributed to the micrometer-sized spheres of double carbon-coated LiFe0.4Mn0.6PO4 nanoparticles with improved electric conductivity and higher Li ion diffusion coefficients, ensuring full redox reactions of all nanoparticles. The results show that the advanced high-energy density cathode materials can be produced using existing industry techniques.

Enhancing the carbon yield of cellulose based carbon fibres with ionic liquid impregnates

Here we show that ionic liquids (ILs), protic or aprotic in nature containing a phosphate anion, can be used as effective impregnating compounds resulting in a 50% improvement of the carbon yield of cellulose based carbon fibres and a 70 °C reduction in the onset of the depolymerization temperature. Using 13C NMR and FTIR spectra, we characterize the carbonized fibres with and without IL impregnates. The oxidative step in the formation of carbon fibres from cellulose precursors is very important in determining the final material properties, as such we examine this stage and show that the IL reduces the onset of the cellulose depolymerization temperature while improving the oxidative stability. This study highlights the ability of ILs to act as novel impregnates which can successfully reduce the formation of tar and volatile substances during carbonization of cellulose based carbon fibres resulting in an improved carbon yield and significant cost savings due to reduced maintenance and wear of equipment. This journal is

High N-content holey few-layered graphene electrocatalysts: Scalable solvent-less production

Few layered nitrogen doped graphene (NG) attracts great interest in energy storage and conversion applications due to its electronic and catalytic properties. However, its bulk production cannot be envisioned by the current synthetic methods. Here we report a facile, solvent-less, low cost and high yield process for the synthesis of NG. Mechanochemical solid-state exfoliation allows scalable synthesis of holey and crumple nitrogen-doped few-layered graphene from graphite with controlled high concentration N doping and a high surface area through ball-milling. By adjusting the ratio of starting materials, the nitrogen content can be modulated from 4.87 to 17.83 at.%. Furthermore, the types of nitrogen-containing species in few-layered graphene can also be controlled. The resultant NG exhibits superior oxygen reduction reaction performance and more reliable stability than commercial Pt/C catalysts. This journal is

Highly sensitive SnO2 nanofiber chemiresistors with a low optimal operating temperature: synergistic effect of Cu2+/Au co-doping

Metal oxide chemiresistors (MOCs) with a low optimal operating temperature, high sensitivity and fast response/recovery are highly promising for various applications, but remain challenging to realize. Herein, we demonstrate that SnO2 nanofibers after being co-doped with Cu2+ and Au show considerably enhanced sensing performances at an unexpectedly decreased operating temperature. A synergistic effect occurs when the two dopants are introduced together. Co-doping may form a novel strategy to the development of ultrasensitive MOCs working at a low optimal temperature. This journal is © the Partner Organisations 2014.

Sulfur-impregnated, sandwich-type, hybrid carbon nanosheets with hierarchical porous structure for high-performance lithium-sulfur batteries

Sandwich-type hybrid carbon nanosheets (SCNMM) consisting of graphene and micro/mesoporous carbon layer are fabricated via a double template method using graphene oxide as the shape-directing agent and SiO2 nanoparticles as the mesoporous guide. The polypyrrole synthesized in situ on the graphene oxide sheets is used as a carbon precursor. The micro/mesoporous strcutures of the SCNMM are created by a carbonization process followed by HF solution etching and KOH treatment. Sulfur is impregnated into the hybrid carbon nanosheets to generate S@SCNMM composites for the cathode materials in Li-S secondary batteries. The microstructures and electrochemical performance of the as-prepared samples are investigated in detail. The hybrid carbon nanosheets, which have a thickness of about 10-25 nm, high surface area of 1588 m2 g-1, and broad pore size distribution of 0.8-6.0 nm, are highly interconnected to form a 3D hierarchical structure. The S@SCNMM sample with the sulfur content of 74 wt% exhibits excellent electrochemical performance, including large reversible capacity, good cycling stability and coulombic efficiency, and good rate capability, which is believed to be due to the structure of hybrid carbon materials with hierarchical porous structure, which have large specific surface area and pore volume.

Evolution of the electrochemical capacitance of transition metal oxynitrides with time: the effect of ageing and passivation

A number of transition metal nitrides and oxynitrides, which are actively investigated today as electrode materials in a wide range of energy conversion and storage devices, possess an oxide layer on the surface. Upon exposure to ambient air, properties of this layer progressively change in the process known as "ageing". Since a number of electrochemical processes involve the surface or sub-surface layers of the active electrode compounds only, ageing could have a significant effect on the overall performance of energy conversion and storage devices. In this work, the influence of the ageing of tungsten and molybdenum oxynitrides on their electrochemical properties in supercapacitors is explored for the first time. Samples are synthesised by the temperature-programmed reduction in NH3 and are treated with different gases prior to exposure to air in order to evaluate the role of passivation in the ageing process. After the synthesis, products are subjected to controlled ageing and are characterised by low temperature nitrogen adsorption, X-ray photoelectron spectroscopy and transmission electron microscopy. Capacitive properties of the compounds are evaluated by performing cyclic voltammetry and galvanostatic charge and discharge measurements in the 1 M H2SO4 electrolyte. © 2014 the Partner Organisations.

Performance enhancement of single-walled nanotube-microwave exfoliated graphene oxide composite electrodes using a stacked electrode configuration

We report the development of a stacked electrode supercapacitor cell using stainless steel meshes as the current collectors and optimised single walled nanotubes (SWNT)-microwave exfoliated graphene oxide (mw rGO) composites as the electrode material. The introduction of mw rGO into a SWNT matrix creates an intertwined porous structure that enhances the electroactive surface area and capacitive performance due to the 3-D hierarchical structure that is formed. The composite structure was optimised by varying the weight ratio of the SWNTs and mw rGO. The best performing ratio was the 90% SWNT-10% mw rGO electrode which achieved a specific capacitance of 306 F g-1 (3 electrode measurement calculated at 20 mV s-1). The 90% SWNT-10% mw rGO was then fabricated into a stacked electrode configuration (SEC) which significantly enhanced the electrode performance per volume (1.43 mW h cm-3, & 6.25 W cm-3). Device testing showed excellent switching capability up to 10 A g-1, and very good stability over 10000 cycles at 1.0 A g-1 with 93% capacity retention. © the Partner Organisations 2014.

High internal phase emulsion (HIPE) xerogels for enhanced oil spill recovery

Oil spills cause serious damage to the aquatic ecosystem and require quick cleanup. Herein we report high internal phase emulsion (HIPE) xerogels for the first time as oil absorbents for enhanced oil spill recovery. The HIPE xerogels absorb diesel from the water-oil mixture in 20-30 seconds. The absorption capacity of the HIPE xerogels ranges from 20 to 32 times for different kinds of oils, and the oils can be recovered simply by being squeezed out, with a recovery rate around 80%. They can be reused at least 40 times without obvious deterioration in oil separation properties from 0 to 45 °C. These novel xerogels are suitable for practical use in oil spill reclamation and wastewater treatment.

Supramolecular ionic networks with superior thermal and transport properties based on novel delocalized di-anionic compounds

Supramolecular ionic networks based on highly delocalized dianions having (trifluoromethane-sulfonyl)imide, (propylsulfonyl)methanide and (cyano-propylsulfonyl)imide groups were developed and their physical properties were examined in detail. Most of the synthesized compounds were semi-crystalline possessing Tm values close to 100°C; however, amorphous networks were also obtained using aromatic asymmetric dianions. Rheological measurements in temperature sweep tests at a constant frequency confirmed two different behaviors: a fast melting close to the Tm for semi-crystalline materials and a thermoreversible network for liquid transition for the amorphous supramolecular ionic networks. It was found that the amorphous ionic networks showed significantly higher ionic conductivity (10^-3 S cm^-1 at 100°C) than the crystalline ionic networks (10^-6 S cm^-1) and previously reported amorphous citrate ionic networks (10^-5 S cm^-1). The supramolecular ionic networks containing hydrophobic (trifluoromethanesulfonyl)imide groups demonstrated improved water stability and higher thermal stability than the previously synthesized carboxylate ones. Noticeably, the obtained amorphous supramolecular ionic networks combine not only high ionic conductivity and thermal stability, but also self-healing properties into the same material.

On the security of compressed sensing-based signal cryptosystem

With the development of the cyber-physical systems (CPS), the security analysis of the data therein becomes more and more important. Recently, due to the advantage of joint encryption and compression for data transmission in CPS, the emerging compressed sensing (CS)-based cryptosystem has attracted much attention, where security is of extreme importance. The existing methods only analyze the security of the plaintext under the assumption that the key is absolutely safe. However, for sparse plaintext, the prior sparsity knowledge of the plaintext could be exploited to partly retrieve the key, and then the plaintext, from the ciphertext. So, the existing methods do not provide a satisfactory security analysis. In this paper, it is conducted in the information theory frame, where the plaintext sparsity feature and the mutual information of the ciphertext, key, and plaintext are involved. In addition, the perfect secrecy criteria (Shannon-sense and Wyner-sense) are extended to measure the security. While the security level is given, the illegal access risk is also discussed. It is shown that the CS-based cryptosystem achieves the extended Wyner-sense perfect secrecy, but when the key is used repeatedly, both the plaintext and the key could be conditionally accessed.

Ionic transport through a composite structure of N-ethyl-N-methylpyrrolidinium tetrafluoroborate organic ionic plastic crystals reinforced with polymer nanofibres

The incorporation of polyvinylidene difluoride (PVDF) electrospun nanofibres within N-ethyl-N-methylpyrrolidinium tetrafluoroborate, [C2mpyr][BF4] was investigated with a view to fabricating self-standing membranes for various electrochemical device applications, in particular lithium metal batteries. Significant improvement in mechanical properties and ionic conduction was demonstrated in a previous study, which also demonstrated the remarkably high performance of the lithium-doped composite material in a device. We now seek a fundamental understanding of the role of fibres within the matrix of the plastic crystal, which is essential for optimizing device performance through fine-tuning of the composite material properties. The focus of the current study is therefore a thorough investigation of the phase behaviour and conduction behaviour of the pure and the lithium-doped (as LiBF4) plastic crystal, with and without incorporation of polymer nanofibres. Analysis of the structure of the plastic crystal, including the effects of lithium ions and the incorporation of PVDF fibres, was conducted by means of synchrotron XRD. Ion dynamics were evaluated using VT solid-state NMR spectroscopy. ATR-FTIR spectroscopy was employed to gain insights into the molecular interactions of doped lithium ions and/or the PVDF nanofibres in the matrix of the [C2mpyr][BF4] composites. Preliminary measurements using PALS were conducted to probe structural defects within the pure materials. It was found that ion transport within the plastic crystal was significantly altered by doping with lithium ions due to the precipitation of a second phase in the structure. The incorporation of the fibres activated more mobile sites in the systems, but restricted ion mobility with different trends being observed for each ion species in each crystalline phase. In the presence of the fibres a strong interaction observed between the Li ion and the pyrrolidinium ring disappeared and formation of the second phase was prevented. As a result, an increased number of mobile lithium ions are released into the solid solution structure of the matrix, simultaneously removing the blocking effect of the second phase. Thus, ion conduction was remarkably improved within the Li-doped composite compared to the neat Li-doped plastic crystal.

Synthesis of an indium oxide nanoparticle embedded graphene three-dimensional architecture for enhanced lithium-ion storage

Indium oxide nanoparticles were synthesised by using a facile and scalable strategy. The as-prepared nanoparticles (20-40 nm) were in situ and homogeneously distributed in a three-dimensional (3D) graphene architecture subsequently during the fabrication process. The obtained nanocomposite acts as a high capacity anode material for lithium-ion batteries and demonstrates good cycle stability. A drastically enhanced capacity of 750 mA h g^-1 in comparison with that of bare In2O3 nanoparticles can be maintained after 100 cycles, along with an improved high rate performance (210 mA h g^-1 at 1 A g^-1 and 120 mA h g^-1 at 2 A g^-1). The excellent performance is linked with the indium oxide nanoparticles and the unique 3D interconnected porous graphene structure. The highly conductive and porous 3D graphene structure greatly enhances the performance of lithium-ion batteries by protecting the nanoparticles from the electrolyte, stabilizing the nanoparticles during cycles and buffering the volume expansion upon lithium insertion.

Increased ion conduction in dual cation [sodium][tetraalkylammonium] poly[4-styrenesulfonyl(trifluoromethylsulfonyl)imide-co-ethylacrylate] ionomers

© The Royal Society of Chemistry. Solid-state polymer electrolytes, as an alternative to traditional liquid electrolytes, have been intensively investigated for energy conversion and storage devices. The transport rate of single ions is the key to their high performance. For application in emerging sodium batteries, we have developed three dual-cation polymeric ionomers, which contain bulky tetraalkylammonium ions in addition to the sodium ion. The sizes and relative contents of the ammonium ions vary relative to the sodium ion contents. Comparative studies of ion dynamics, thermal properties, phase behaviours and ionic conductivities were carried out, taking advantage of various spectroscopic and thermal chemistry methods. The ion conductivities of the ionomers are greatly enhanced by the introduction of bulky counterions, as a result of the additional free volume and decreased sodium ion association. Raman spectroscopy and thermal analysis as well as the solid-state nuclear magnetic resonance studies are used to probe the conductivity behaviour.

Advanced N-doped mesoporous molybdenum disulfide nanosheets and the enhanced lithium-ion storage performance

With the increasing interest in two-dimensional van der Waals materials, molybdenum disulfide (MoS2) has emerged as a promising material for electronic and energy storage devices. It suffers from poor cycling stability and low rate capability when used as an anode in lithium ion batteries. Here, N-doped MoS2 nanosheets with 2-8 atomic layers, increased interlayer distance, mesoporous structure and high surface area synthesised by a simple sol-gel method show an enhanced lithium storage performance, delivering a high reversible capacity (998.0 mA h g-1, 50 mA g-1), high rate performance (610 mA h g-1, 2 A g-1), and excellent cycling stability. The excellent lithium storage performance of the MoS2 nanosheets might be due to the better electrical and ionic conductivity and improved lithium ion diffusion which are related to their structural characteristics and high concentration N doping. The possible mechanism of the improved performance is proposed and discussed.

Exceptional durability enhancement of PA/PBI based polymer electrolyte membrane fuel cells for high temperature operation at 200°C

The incorporation of phosphotungstic acid functionalized mesoporous silica in phosphoric acid doped polybenzimidazole (PA/PBI) substantially enhances the durability of PA/PBI based polymer electrolyte membrane fuel cells for high temperature operation at 200°C.