Nickel hydroxide obtained by high-temperature two-step synthesis as an effective material for supercapacitor applications

Hybrid supercapacitors with nickel hydroxide electrode are widely used as modern power sources for electrovehicles, ignition of different electric engines, etc. Nickel hydroxide for supercapacitor use must satisfy special features which are quite different from those requested for battery application. The aim of this work is to improve the promising two-stage high-temperature method by altering hydrolysis condition (hot and cold) in order to obtain Ni(OH)₂ with improved electrochemical activity. Ni(OH)₂ samples have been investigated by PXRD, TG, DSC, SEM, TEM, cyclic voltammogramm, and galvanostatic charge-discharge cycling. It has been established from PXRD, TG, and DSC analyses that material obtained by hydrolysis at high temperature is a highly crystalline β-Ni(OH)₂ characterized by high thermal stability. Materials prepared by cold hydrolysis are a highly defective β_bc-Ni(OH)₂, with 6.3 % water content and a lower thermal stability. It has been shown that samples prepared by hot hydrolysis have a high redox reversibility and electrochemical cycling stability, but a lower electrochemical capacity. This suggests that the electrochemical processes are localized in the thin layer at the particle surface. Cyclic voltammograms of samples prepared by cold hydrolysis exhibit gradual activation of the active material, anyhow resulting in higher capacity. By means of the galvanostatic charge-discharge curves at different current densities, the specific capacities of the samples have been calculated. The sample prepared by cold hydrolysis has higher specific capacities than the sample prepared by hot hydrolysis.     

Crystal Engineering of Supramolecular 1,4-Benzene Bisamides by Side-Chain Modification – Towards Tuneable Anisotropic Morphologies and Surfaces

Benzene bisamides are promising building blocks for supramolecular nano-objects. Their functionality depends on morphology and surface properties. However, a direct link between surface properties and molecular structure itself is missing for this material class. Here, we investigate this interplay for two series of 1,4-benzene bisamides with symmetric and asymmetric peripheral substitution. We elucidated the crystal structures, determined the nano-object morphologies and derived the wetting behaviour of the preferentially exposed surfaces. The crystal structures were solved by combining single-crystal and powder X-ray diffraction, solid-state NMR spectroscopy and computational modelling. Bulky side groups, here t-butyl groups, serve as a structure-directing motif into a packing pattern, which favours the formation of thin platelets. The use of slim peripheral groups on both sides, in our case linear perfluorinated, alkyl chains, self-assemble the benzene bisamides into a second packing pattern which leads to ribbon-like nano-objects. For both packing types, the preferentially exposed surfaces consist of the ends of the peripheral groups. Asymmetric substitution with bulky and slim groups leads to an ordered alternating arrangement of the groups exposed to the surface. This allows the hydrophobicity of the surfaces to be gradually altered. We thus identified two leitmotifs for molecular packings of benzene bisamides providing the missing link between the molecular structure, the anisotropic morphologies and adjustable surface properties of the supramolecular nano-objects.     

Preparation and characterization of fibres from a thermotropic liquid crystal polyester with non-coplanar biphenylene units

Abstract

Fibres were prepared from the nematic melt of poly(2,2′-dimethyl-4,4′-biphenylene phenylterephthalate). The phenyl substitution in the terephthalic acid moiety in combination with the non-coplanar biphenyl moiety prevent crystallization of this thermotropic polyester. Oriented fibres were prepared by two different methods. Similarly to other thermotropic liquid crystal polymer fibres, chain orientation can be achieved by an elongational flow in a down draw process. This polyester also allows tensile deformation of the spun fibres around the glass transition temperature. In both routes similar degrees of orientation and mechanical properties were obtained. Tensile moduli in the order of 40–45 GPa and tensile strengths up to 550–650 MPa were obtained. The orientation function values were determined to be in the order of 0·8–0·9. A comparison with other unsubstituted thermotropic liquid crystal polyesters at the same level of orientation revealed that the moduli are the same, although the substituents increase the chain diameter. This result may be attributed to an increase of the apparent shear modulus due to an interlocking mechanism of the rigid lateral substituents.     

Polymer electrolytes—relaxation and transport properties

Relaxations in polymer electrolytes were studied in poly(ethylene oxide) and epoxidized natural rubbers both filled with lithium perchlorate. Impedance relaxation was investigated over a wide range of salt concentration at room temperature. Imaginary part of impedance as a function of frequency exhibits generally one maximum and one minimum. These two extreme values rule properties in the DC limit. They can be related to transport properties and degree of dissociation. It turns out that DC conductivity is dominantly ruled by transport coefficient.     

Turbulence structure of dilute polymer and surfactant solutions in artificially roughened pipes

Pressure drop and velocity profile measurements are presented for turbulent flows of drag reducing fluids. The investigation was done in two rough pipes, known as k- and d-type rough pipes. The results are compared with those obtained in hydraulically smooth pipe of identical diameter. The spatial arrangement of the roughness elements in the pipe determines the parallel shift in the elastic sublayer and in the core region of the dimensionless turbulent velocity profile. The slopes of the velocity profiles in these regions remain unaffected by the arrangement which is an indication that the hydrodynamic influence of the roughness is restricted to the near-wall region. The drag reducing surfactant solution exhibited a drag reduction in the smooth as well as in the rough pipes which was higher than that given by Virk's maximum drag reduction asymptote. For this solution no influence of the roughness on the turbulence was detected when the dimensionless roughness height in viscous units was less than 12.     

Phase Behavior of Polymer Blends

Major emphasis is placed on the phase behavior of miscible polymer blends. To understand the complex phase behavior of blends, a refined version of an equation-of-state theory is discussed. This theory makes the simultaneous occurrence of upper critical solution temperature and lower critical solution temperature in blends of high molar mass polymers conceivable. The kinetics of isothermal phase dissolution as it emanates from different experimental routes is discussed in terms of CahN′s linearized theory of phase separation. The rate of phase dissolution varies as a function of quench depth, which indicates the rate is directed by both the thermodynamic driving force and the mobility.     

Determination of the chemical diffusion coefficient of lithium in multiwall carbon nanotubes

Electrochemical intercalation of lithium into multiwall carbon nanotubes produced by the arc-electric technique has been carried out in button cells to estimate the solid state diffusion coefficients of lithium. An extension of the Galvanostatic Intermittent Titration Technique (G.I.T.T.) of Weppner and Huggins and further developed by Honders is employed. The results are interpreted providing that the lithium species enter the nanotubes perpendicularly to their walls and thus give rise to a necklace structure as observed by Transmission Electron Microscopy (TEM). Finally, the results are compared with the data obtained for different kinds of carbon: carbon fiber, petroleum coke and graphite. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.