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.

The textural properties and microstructure of konjac glucomannan - tungsten gels induced by DC electric fields

Konjac glucomannan - tungsten (KGM-T) gels were successfully prepared under DC electric fields, in the presence of sodium tungstate. The textural properties and microstructure of the gels were investigated by Texture Analyzer, Rheometer and SEM. Based on the response surface methodology (RSM) results, the optimum conditions for KGM-T gel springiness is 0.32% sodium tungstate concentration, 0.54% KGM concentration, 24.66V voltage and 12.37min treatment time. Under these conditions, the maximum springiness value of KGM-T gel is 1.21mm. Steady flow measurement indicated that KGM-T gel showed characteristic non-Newtonian pseudoplastic behaviour, with low flow behaviour indexes in the shear thinning region. SEM demonstrated the porosity of the freeze-dried samples. These findings may pave the way to use DC electric fields for the design and development of KGM gels and to apply KGM gels for practical applications.

Determination of rate constants using microelectrodes

An understanding of the rate and the mechanism of reaction is of fundamental importance in the many facets of chemistry. Electrochemical systems are further complicated by the heterogeneous boundary, between the solution and the electrode, that the electron passes through before any electrochemical reaction can take place. This thesis concerns the development of methods for analysing electrode kinetics. One part involves the further development of the Global Analysis procedure to include electrodes with a spherical geometry which are traditionally the most popular form of electrodes. Simulated data is analysed to ascertain the accuracy of the procedure and then the known artifacts of uncompensated solution resistance and charging current are added to the simulated data so that the effects can be observed. The experimental analysis of 2-methyl-2-nitropropane is undertaken and comparisons are made with the Marcus-Hush electrochemical theories concerning electrode kinetics. A related section explores procedures for the kinetic analysis of steady state voltammetric data obtained at microdisc electrodes. A method is proposed under the name of Normalised Steady State Voltammetry and is tested using data obtained from a Fast Quasi-Explicit Finite Difference simulation of diffusion to a microdisc electrode. In a second area of work using microelectrodes, the electrochemical behaviour of compounds of the general formula M(CO)3(η3 - P2P1) where M is either Cr, Mo or W and P2P' is bis(2-diphenylphosphinoethyl)phenylphosphine) is elucidated. The development of instrumentation and mathematical procedures relevant to the measurement and quantitation of these systems is also investigated. The tungsten compound represents the first examples where the 17-electronfac+ and mer+ isomers are of comparable stability.

Dielectrophoresis-Raman spectroscopy system for analysing suspended nanoparticles

A microfluidic dielectrophoresis platform consisting of curved microelectrodes was developed and integrated with a Raman spectroscopy system. The electrodes were patterned on a quartz substrate, which has insignificant Raman response, and integrated with a microfluidic channel that was imprinted in poly-dimethylsiloxane (PDMS). We will show that this novel integrated system can be efficiently used for the determination of suspended particle types and the direct mapping of their spatial concentrations. We will also illustrate the system's unique advantages over conventional optical systems. Nanoparticles of tungsten trioxide (WO₃) and polystyrene were used in the investigations, as they are Raman active and can be homogeneously suspended in water.

Dielectrophoresis-raman spectroscopy system for analysing suspended WO3 nanoparticles

Dielectrophoresis (DEP) utilizing a curved microelectrode pattern was developed and integrated with a Raman spectroscopy system. The electrodes were patterned on a Raman transparent quartz substrate, and integrated with a microfluidic channel in poly-dimethylsiloxane (PDMS). This integrated system can be efficiently used for the determination of suspended particles type and the direct mapping of their spatial concentrations. It will be demonstrated that the integration of Raman mapping with dielectrophoretically controlled WO3 particles can be used for studying suspended particles in situ.

Tuneable optical waveguide based on dielectrophoresis and microfluidics

In this work, an array of dielectrophoretic curved microelectrodes patterned in a microfluidic channel and integrated with a multimode rib polymeric waveguide is demonstrated. The microfluidic channel is infiltrated with suspended silica (SiO2) and tungsten trioxide (WO3) nanoparticles. The optofluidic system is found to be sensitive and responds not only to the infiltration of nanoparticle suspensions in the microfluidic channel, but also to the magnitude and frequencies of dielectrophoretic forces applied on the nanoparticles. The nanoparticles can be uniformly concentrated or repelled from the region between the curved microelectrode tips forming either a dense stream of flowing nanoparticles or a region void of nanoparticles in the evanescent sensitive region of the polymeric waveguide. The concentration and repulsion of nanoparticles from this region creates a refractive index gradient in the upper cladding of the polymeric waveguide. These conditions made it possible for light to either remain guided or be scattered as a function of dielectrophoretic settings applied on the nanoparticles. The results demonstrate that we successfully developed a novel tuneable polymeric waveguide based on dielectrophoretic assembly of nanoparticles suspended in fluids.