Fast Transport Paths Research Articles

Controllable synthesis of cobalt oxide nanoflakes on three-dimensional porous cobalt networks as high-performance cathode for alkaline hybrid batteries

Herein we report porous three-dimensional cobalt networks supported CoO nanoflakes by the combination of successive electro-deposition methods. The electrodeposited Co networks have average large pores of ∼5μm and all the branches are composed of interconnected nanoparticles. CoO nanoflakes with thickness of ∼15nm are uniformly coated on the Co networks forming self-supported Co/CoO composite films. The as-prepared Co/CoO composite films possess combined properties of porous structure and strong mechanical stability. As cathode for alkaline hybrid batteries, the Co/CoO composite films exhibit good electrochemical performances with high capacity of 83.5mAhg−1 at 1Ag−1 and stable high-rate cycling life (65mAhg−1 at 10Ag−1 after 15,000 cycles). The hierarchical porous architecture provides positive roles in the enhancement of electrochemical properties, including fast electronic transportation path, short diffusion of ions and high contact area between the active material and the electrolyte.

Synthesis of pyrite/carbon shells on cobalt nanowires forming core/branch arrays as high-performance cathode for lithium ion batteries

Construction of self-supported porous metal sulfide arrays is critical for the development of high-performance electrochemical energy storage devices. Herein we report hierarchical porous Co/FeS2–C core/branch nanowires arrays by the combination of facile solution-based methods. FeS2–C nanoflakes branch is uniformly coated on the Co nanowire core forming composite core/branch arrays. The as-prepared Co/FeS2–C core/branch nanowire arrays possess combined properties of highly porous structure and strong mechanical stability. As cathode of lithium ion batteries, the Co/FeS2–C core/branch nanowire arrays exhibit good electrochemical performances with initial discharge capacity of 850 mAh g−1 at 0.25 °C and stable high-rate cycling life (539 mAh g−1 after 70 cycles at 0.25 °C). The hierarchical core/branch architecture provides positive roles in the enhancement of electrochemical properties, including fast transportation path of electron, short diffusion of ions and high contact area between the active material and electrolyte.

The Hidden Pathways in Dense Energy Materials - Oxygen at Defects in Nanocrystalline Metals.

Highly abundant oxygen-rich line defects (blue) can act as fast oxygen transport paths. These defects show similar chemistry and therefore similar catalytic activity to the materials surface. These results provide the opportunity to design and produce simple scalable structures as catalysts, whose functionality derives from internal defects rather than from the materials surfaces.

Nickel cobaltite nanograss grown around porous carbon nanotube-wrapped stainless steel wire mesh as a flexible electrode for high-performance supercapacitor application

Nickel cobaltite nanograss with bimodal pore size distribution (small and large mesopores) is grown on various electrode substrates by one-pot hydrothermal synthesis. The small pores (<5nm) in the nanograss of individual nanorods contribute to large surface area, while the large pore channels (>20nm) between nanorods offer fast transport paths for electrolyte. Carbon nanotubes (CNTs) with high electrical conductivity wrap around stainless steel (SS) wire mesh by electrophoresis as an electrode scaffold for supporting the nickel cobaltite nanograss. This unique electrode configuration turns out to have great benefits for the development of supercapacitors. The specific capacitance of nickel cobaltite grown around CNT-wrapped SS wire mesh reaches 1223 and 1070Fg−1 at current densities of 1 and 50Ag−1, respectively. CNT-wrapped SS wire mesh affords porous and conductive networks underneath the nanograss for rapid transport of electron and electrolyte. Flexible CNTs connect the nanorods to mitigate the contact resistance and the volume expansion during cycling test. Thus, this tailored electrode can significantly reduce the ohmic resistance, charge-transfer resistance, and diffusive impedance, leading to high specific capacitance, prominent rate performance, and good cycle-life stability.

Self-supported porous CoO semisphere arrays as binder-free electrodes for high-performance lithium ion batteries

Construction of hierarchical porous metal oxides arrays is critical for the development of high-performance electrochemical energy storage devices. Herein we report porous CoO semisphere arrays using an electrodeposited polystyrene template method. Interestingly, the as-prepared porous CoO semisphere arrays consist of bowl-like semispheres with diameters of ∼500nm and show highly porous structure. As a preliminary test as anode for lithium ion batteries (LIBs), the porous CoO semispheres arrays exhibit superior electrochemical performances with good cyclability (695.5mAhg−1 after 150 cycles at 0.5Ag−1) and high-rate capability. The porous semispherical architecture provides positive roles in the enhancement of electrochemical properties, including fast electronic transportation path, short diffusion of Li ion and high sufficient interfacial contact area between the active material and the electrolyte. The developed methodology enables a new fabrication of porous metal oxides with applications in electro-catalysis, optical and electrochemical devices.

The Oxygen Transport Properties of La2NiO4+ δ Infiltrated La0.6Sr0.4Co0.2Fe0.8O3- δ By Electrical Conductivity Relaxation

La2NiO4+ δ(LNO) infiltration in La0.6Sr0.4Co0.2Fe0.8O3- δ (LSCF) cathodes have performed excellent cell electrochemical properties in our previous work. However, the effect of the infiltration phase on oxygen transport properties is still lacking and the mechanism for the enhancement of electrochemical properties is unclear. In this work, the electrical conductivity relaxation (ECR) is used to characterize the oxygen transport properties in La2NiO4+ δ(LNO) covered La0.6Sr0.4Co0.2Fe0.8O3- δ (LSCF)materials. The ECR experiment was performed at 700oC in different oxygen particle pressure in the range of 0.21 to 0.02 atm. The ECR data suggested that there are fast oxygen transport path along the hetero-structured interface La0.6Sr0.4Co0.2Fe0.8O3- δ (LSCF)/La2NiO4+ δ(LNO). The interface oxygen exchange coefficient k was estimated by assuming the invariable oxygen flux and the ignorable oxygen diffusion in LNO layer due to its minimal relative thickness. The calculated k value was 7.94-3.31 cm﹒s-1, which is larger than that value of the surface of single LSCF or LNO by 3-4 times. XPS analysis shown that Co and Sr diffused from LSCF into LNO layer and an additional low energy state Co peak appeared after ECR measurement resulting from Co disproportionate reaction probably, which are the possible reasons for the enhancement of surface and interface exchange coefficients.

TiO2Nanotube Arrays Composite Film as Photoanode for High-Efficiency Dye-Sensitized Solar Cell

A double-layered photoanode made of hierarchical TiO2nanotube arrays (TNT-arrays) as the overlayer and commercial-grade TiO2nanoparticles (P25) as the underlayer is designed for dye-sensitized solar cells (DSSCs). Crystallized free-standing TNT-arrays films are prepared by two-step anodization process. For photovoltaic applications, DSSCs based on double-layered photoanodes produce a remarkably enhanced power conversion efficiency (PCE) of up to 6.32% compared with the DSSCs solely composed of TNT-arrays (5.18%) or nanoparticles (3.65%) with a similar thickness (24 μm) at a constant irradiation of 100 mW cm−2. This is mainly attributed to the fast charge transport paths and superior light-scattering ability of TNT-arrays overlayer and good electronic contact with F-doped tin oxide (FTO) glass provided from P25 nanoparticles as a bonding layer.

High-performance polypyrrole functionalized PtPd electrocatalysts based on PtPd/PPy/PtPd three-layered nanotube arrays for the electrooxidation of small organic molecules

Here, we report novel high-performance polypyrrole (PPy) functionalized PtPd electrocatalysts based on PtPd/PPy/PtPd three-layered nanotube arrays (TNTAs) for the electrooxidation of small organic molecules, such as methanol, ethanol and formic acid, which are superior fuels for direct alcohol fuel cells (DAFCs) or direct formic acid fuel cells (DFAFCs). The unique hollow structures, array structures and sandwich-like structures of PtPd/PPy/PtPd TNTAs will provide fast transport and short diffusion paths for electroactive species as well as a large exposed surface area for efficient interaction with electroactive species. In particular, the unique sandwich-like structure of PtPd/PPy/PtPd TNTAs results in electron delocalization among the Pt 4f orbitals, Pd 3d orbitals and PPy π-conjugated ligands, and in electron transfer from the PPy to Pt and Pd atoms, leading to higher contents of metallic Pt and Pd and synergistic effects for electrocatalytic reactions. Because of the above merits, the designed PtPd/PPy/PtPd TNTAs exhibit significantly improved catalytic activity and durability for the electrooxidation of small organic molecules compared with those of PtPd TNTAs and commercial Pt/C and Pd/C catalysts. High-Performance Polypyrrole Functionalized PtPd Electrocatalysts. Direct alcohol fuel cells are preferable to conventional hydrogen-powered devices because small, liquid organic molecules such as methanol and ethanol are easier to store and transport than hydrogen gas. Gao-Ren Li and co-workers from Sun Yat-sen University in China have devised a new type of oxidation catalyst for substrates based on hollow nanotubes containing sandwiched metal-polymer complexes. As these systems normally use expensive precious metals their activity and durability are of the utmost importance. The researchers deposited a three-layered material — a platnium-palladium composite (PtPd), a conductive polymer, and another PtPd coating — onto a zinc oxide nanorod template using electrochemical methods. They then dissolved the template to produce durable nanotube assemblies with exposed metal surfaces and short diffusion paths that can oxidize methanol, ethanol, and formic acid much more efficiently than conventional catalysts. This performance enhancement is ascribed to their unique structure and favorable metal–polymer electronic interactions leading to higher uptake of the catalytic metals.

Design of Pd/PANI/Pd Sandwich-Structured Nanotube Array Catalysts with Special Shape Effects and Synergistic Effects for Ethanol Electrooxidation

Low cost, high activity, and long-term durability are the main requirements for commercializing fuel cell electrocatalysts. Despite tremendous efforts, developing non-Pt anode electrocatalysts with high activity and long-term durability at low cost remains a significant technical challenge. Here we report a new type of hybrid Pd/PANI/Pd sandwich-structured nanotube array (SNTA) to exploit shape effects and synergistic effects of Pd-PANI composites for the oxidation of small organic molecules for direct alcohol fuel cells. These synthesized Pd/PANI/Pd SNTAs exhibit significantly improved electrocatalytic activity and durability compared with Pd NTAs and commercial Pd/C catalysts. The unique SNTAs provide fast transport and short diffusion paths for electroactive species and high utilization rate of catalysts. Besides the merits of nanotube arrays, the improved electrocatalytic activity and durability are especially attributed to the special Pd/PANI/Pd sandwich-like nanostructures, which results in electron delocalization between Pd d orbitals and PANI π-conjugated ligands and in electron transfer from Pd to PANI.

Oxygen tracer diffusion along interfaces of strained Y2O3/YSZ multilayers

Heterophase boundaries can offer fast transport paths in solid electrolyte materials. In recent studies an enhancement of the ionic conductivity was indeed observed in micro-/nanoscaled Y(2)O(3)-stabilised ZrO(2) (YSZ) composites and hetero multilayers of thin films. As space charge regions can be neglected due to high charger carrier concentrations, we assume that strain and microstructural changes at the heterophase boundaries are responsible for the observed conductivity effects. In order to obtain independent information on the role of heterophase boundaries for fast transport in strained solid electrolytes, systematic measurements of the (18)O-tracer diffusion coefficient in nanoscaled YSZ/Y(2)O(3) multilayers were performed. Multilayer samples were prepared by Pulsed Laser Deposition (PLD) on (0001) Al(2)O(3) substrates and characterised by X-Ray Diffraction (XRD), Scanning Electron Microscopy (HRSEM) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). To separate interface and bulk transport from the total oxygen diffusivity of the multilayer system, the (average) thickness of the YSZ-layers in the multilayers was varied from 45 nm to 12 nm. Upon decreasing the thickness of the YSZ layers, respectively increasing the density of parallel interfaces, the total diffusion coefficient of the multilayer system is increased by a factor of 2 compared to bulk YSZ. The experimental results agree well with formerly published data for ionic conductivity measurements. They also support a negligible contribution of partial electronic conductivity in the multilayer.

Robust and Aligned Carbon Nanotube/Titania Core/Shell Films for Flexible TCO‐Free Photoelectrodes

Carbon nanotube (CNT)/semiconducting oxide hybrids are an ideal architecture for light-harvesting devices, in which the CNTs are expected to not only act as a scaffold but also provide fast transport paths for photogenerated charges in the oxide. However, the current potential of CNTs for charge transport is largely suppressed due to the nanotubes not being interconnected but isolated by the low conductive oxide coatings. Herein, a flexible and conductive CNT/TiO(2) core/shell heterostructure film is reported, with aligned and interconnected CNTs wrapped in a continuous TiO(2) coating. Without using additional transparent conducting oxide (TCO) substrates, this unique feature of the film boosts the incident photon-to-electron conversion efficiency to 32%, outperforming TiO(2) nanoparticle electrodes fabricated on TCO substrates. Moreover, the film shows high structural stability and can generate a stable photocurrent even after being bent hundreds of times.

Preparation, characterization, conductivity studies of novel solid polymer electrolytes based on blend of poly (AN-co-VEC) and EVA

Novel solid polymer electrolytes (named as SRx (x=1–6)) containing trifluoromethane sulfonates (LiCF3SO3) are prepared based on the blend of poly (acrylonitrile-co-vinyl ethylene carbonate) poly (AN-co-VEC) and ethylene vinyl acetate copolymer (EVA). The structural properties of the polymer electrolytes SRx are systematically investigated by varieties of techniques, and the electrochemical performance including ion conductivity and Li+ transference number are studied by electrochemical impedance spectroscopy (EIS). The surface morphology of solid polymer electrolyte are subjected to transmission electron microscopy (TEM) and scanning electron microscopy (SEM) measurements, by which highly collected spherical grains are found on the picture of SEM and TEM. It is noteworthy that the special structure probably affords fast transport paths for Li+, enhancing ionic conductivity (=6.0×10−5S cm−1) and Li+ transference number (t+=0.74), and the former is higher than that of conventional solid polymer electrolytes which is up to now mostly 10−7 to 10−8. The reason may be due to excellent solubility of VEC for Li+ salt, besides, the spherical structure of the solid polymer electrolyte may play an important role. This is perhaps the first report of VEC used as host copolymer for the solid polymer electrolytes.

Could graphene construct an effective conducting network in a high-power lithium ion battery?

This study is trying to demonstrate whether graphene is able to construct an effective conducting network for both electron and ion transports in cathode system of a high-power lithium ion battery (LIB), not based on a coin cell, but by employing a commercial soft-packaged 10Ah battery pack as a model system. Compared with the cells using commercial conductive additive (7wt% carbon black and 3wt% conductive graphite), a 10Ah cell using only 1wt% graphene and 1wt% carbon black shows lower internal resistance and higher energy density due to the excellent conductivity of graphene. However, owing to the fact that the planar structure of the graphene sheets blocks fast Li+ ion transport, the steric effect resulted heavy polarization occurs at a relatively high charge/discharge rate (>3C). That is, although flexible and planar graphene helps construct an effective electron transfer network, it retards Li+ ion transport. Thus, for an energy-storing LIB with a low working charge/discharge rate, graphene additive shows apparent superiority over commercial ones even with much less addition fraction and may find its real commercial applications; while, for a high-power LIB which works at higher charge/discharge rate, fast ion transport path is required to be effectively constructed before a real application. Simulation results indicate that further work should be focused on the adjustment of electrode pore structure and modification of graphene steric structure accordingly to construct an unimpeded ion conducting network and provide high speed path for Li+ ion transport.

Nanometric La 0.9Sr 0.1Ga 0.8Mg 0.2O 3− x ceramic prepared by low-pressure reactive spark-plasma-sintering

It is proposed a processing route to obtain dense (≥90%) nanometric La 0.9Sr 0.1Ga 0.8Mg 0.2O 3− x (LSGM) electrolyte ceramic by reacting under spark-plasma-sintering (SPS) conditions a relatively poor mixture of intermediate powdered La/Sr- and Ga/Mg-based products synthesized separately by precipitation and hydrothermal methods, respectively. Samples with particle size of 9–20, 37–93, and 120–240 nm were measured from the electrical conductivity viewpoint. A fast transport path along grain boundaries was not observed and, a higher nano particle size improves total conductivity in our samples. Results suggest that it is necessary to make a clear distinction between particle size from microscopy observation and crystallite size from structural (e.g. XRD) measurements when looking at the general conductivity – grain size dependence. This dependence is complex and the use of an unconventional, far from equilibrium technique such as SPS might influence it. Currently, a good understanding is missing, but pending on materials, technology specifics and the complex relationship between them, some of the usually promoted ideas from literature such as the necessity of high mixing levels in the precursor powders, of high pressure application during sintering, of low particle size (nano) in the sintered ceramic for a higher conductivity, of high purity phase and so on require careful consideration.

Controlled Positioning of Large Interfacial Nanocavities via Stress‐Engineered Void Localization

The Kirkendall effect has intensively been exploited at the nanoscale for the fabrication of hollow nanostructures. [1,2] This fabrication strategy generally starts from a core–shell nanostructure where the core material is a faster diffusing species. Preferred outward atomic diffusion across the interface leads to a net inward injection of vacancies, which are likely to accumulate, supersaturate, and finally condense into a single void. If the core material is only partly consumed, a core–void–shell nanostructure can be produced. [3–7] Investigations on the formation of this ‘‘intermediate’’ state are crucial for a deep understanding of the vacancy coalescence and void evolution process induced by the nanoscale Kirkendall effect. Moreover, nanostructures with controlled porosity would enable greater control of the local chemical environment, which is important for molecule probing, drug delivery and catalysis applications. [2–4] Generally the nucleation of Kirkendall voids is favored at a core–shell interface due to the high defect content and stress there. In most cases of spherical core–shell nanoparticles, uniformly distributed interfacial voids are divided by filamentlike bridges, which act as fast transport paths for the delivery of remaining core material into the shell. [2a,2f,4] In materials with one or more macroscopic dimensions such as nanowires and films, the nanoscale Kirkendall effect usually initiates the formation of multiple voids randomly scattered at the interface during the diffusion evolution. [5] Void localization has been noticed in some systems. [5b,6,7] For example, an asymmetric

A novel approach to counter denial of service attacks against transport network resources

A new generation of optical transport networks (OTNs) and wavelength division multiplexing (WDM) networks is currently emerging. Their control plane for fast transport path creation, protection, and restoration is inevitable for cost effectiveness. Such automatically switched optical networks (ASONs) are susceptible to denial of service (DoS) attacks at their user network interface (UNI) and external network node interface (E-NNI) access points. Attacks against transport resources aim to prevent users from setting up transport connections, such as paths in OTNs or wavelengths in WDM networks. We present novel concepts for detecting such attacks and identifying the attackers, covering both 'simple' and distributed DoS (DDoS) attacks. We have analyzed a variety of potential attack scenarios by means of simulation to validate the detection methods and the framework for effectiveness. Our results indicate that the framework exhibits good detection rates for the DDoS attacks mentioned earlier. After discussing the problem and methods chosen to address it, we explain the results of our analysis.

Molecular dynamics simulation of diffusion of gases in a carbon-nanotube–polymer composite

Extensive molecular dynamics (MD) simulations were carried out to compute the solubilities and self-diffusivities of CO2 and CH4 in amorphous polyetherimide (PEI) and mixed-matrix PEI generated by inserting single-walled carbon nanotubes into the polymer. Atomistic models of PEI and its composites were generated using energy minimizations, MD simulations, and the polymer-consistent force field. Two types of polymer composite were generated by inserting (7,0) and (12,0) zigzag carbon nanotubes into the PEI structure. The morphologies of PEI and its composites were characterized by their densities, radial distribution functions, and the accessible free volumes, which were computed with probe molecules of different sizes. The distributions of the cavity volumes were computed using the Voronoi tessellation method. The computed self-diffusivities of the gases in the polymer composites are much larger than those in pure PEI. We find, however, that the increase is not due to diffusion of the gases through the nanotubes which have smooth energy surfaces and, therefore, provide fast transport paths. Instead, the MD simulations indicate a squeezing effect of the nanotubes on the polymer matrix that changes the composite polymers' free-volume distributions and makes them more sharply peaked. The presence of nanotubes also creates several cavities with large volumes that give rise to larger diffusivities in the polymer composites. This effect is due to the repulsive interactions between the polymer and the nanotubes. The solubilities of the gases in the polymer composites are also larger than those in pure PEI, hence indicating larger gas permeabilities for mixed-matrix PEI than PEI itself.

An Analysis Of Void Nucleation In Passivated Interconnect Lines Due To Vacancy Condensation And Interface Contamination

AbstractNucleation of voids due to vacancy condensation in passivated aluminum lines is analyzed within the context of classical nucleation theory. A discussion of sources of hydrostatic tensile stress in such lines provides a reasonable upper limit of 2 GPa. The void nucleation rate is then calculated at various sites within the line. Results suggest that nucleation rates are far too low to account for observed rates of voiding. Void nucleation at a flaw at the line/passivation interface is then considered as an alternative nucleation mechanism. Such flaws may be created by contaminants introduced during fabrication of the line. In this case, nucleation is feasible at greatly reduced stresses, well within the observed values. Furthermore, a simple model of void growth indicates that a fast atomic transport path, such as a grain boundary, must intersect the void for an appreciable growth rate. These results suggest that void nucleation in aluminum interconnect lines occurs at flaws at the sidewall of the line and that stress-induced and electromigration-induced voiding can be controlled by eliminating interfacial contamination.

An Analysis of Void Nucleation In Passivated Interconnect Lines Due to Vacancy Condensation and Interface Contamination

AbstractNucleation of voids due to vacancy condensation in passivated aluminum lines is analyzed within the context of classical nucleation theory. A discussion of sources of hydrostatic tensile stress in such lines provides a reasonable upper limit of 2 GPa. The void nucleation rate is then calculated at various sites within the line. Results suggest that nucleation rates are far too low to account for observed rates of voiding. Void nucleation at a flaw at the line/passivation interface is then considered as an alternative nucleation mechanism. Such flaws may be created by contaminants introduced during fabrication of the line. In this case, nucleation is feasible at greatly reduced stresses, well within the observed values. Furthermore, a simple model of void growth indicates that a fast atomic transport path, such as a grain boundary, must intersect the void for an appreciable growth rate. These results suggest that void nucleation in aluminum interconnect lines occurs at flaws at the sidewall of the line and that stress-induced and electromigration-induced voiding can be controlled by eliminating interfacial contamination.

Retardation of oxidation and duplex oxide growth on single crystal nickel after magnesium implantation

Rutherford backscattering spectrometry and channeling have been employed to study the oxidation behaviour of single crystal nickel after magnesium implantation and to probe the structural relationship between the substrate and its oxide following oxidation in pure oxygen at 700°C for various times. A two-tier oxide structure, including an oriented inner oxide layer, was found to form during the oxidation of an unannealed implanted sample and a reduction in oxidation rate was observed. Correlations appear to exist between this oxidation behaviour and the presence of a high concentration of magnesium. Results are discussed in terms of the blocking effect of magnesium on the outward diffusion of nickel cations along fast transport paths, such as grain boundaries. It is concluded that the oriented inner oxide layer of the two-tier oxide grows inwards, presumably via an inward diffusion of oxygen. This oxide growth mechanism becomes important when a high concentration of Mg incorporated into a growing oxide retards the outward diffusion of nickel cations and therefore reduces the usual outward growth of oxide.