Ashok K. Vijh

Sparking voltages and side reactions during anodization of valve metals in terms of electron tunnelling

Abstract When valve metals are anodized to high voltages, the oxide growth is either accompained or overtaken by side reactions and sparking which indicate electronic currents through the oxide. The nature of these phenomena has been discussed in terms of a tunnelling model. It has been concluded that side reactions such as oxygen evolution do not per se give rise to the sparking and dielectric breakdown. It has been suggested that for sparking to occur, the normal tunnelling mechanism for the oxygen evolution must change over to a mechanism involving avalanche multiplications. It has been shown that the suggested model and related considerations provide useful interpretative insights into the phenomenology of side reactions and sparking voltages on all valve metals. Specifically, explanations have been put forward for low anodization efficiency on silicon as well as for certain departures from “ideal” film-forming behaviour observed for the case of Zr and W, in the context of discussion of anodic behaviour of valve metals in general.

Non-noble metal-carbonized aerogel composites as electrocatalysts for the oxygen reduction reaction

Non-noble metal-based electrocatalysts have been examined for their electrocatalytic activity toward the reduction of oxygen. These materials were prepared from highly porous polyacrylonitrile microcellular foams containing a salt of iron or cobalt, followed by carbonisation. In common with Pt/C, iron or cobalt-carbonized aerogel nanocomposites show good electrocatalytic activity for the oxygen reduction in acidic solutions.

A New Fuel Cell Electrocatalyst Based on Carbonized Polyacrylonitrile Foam The Nature of Platinum‐Support Interactions

X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) have been conducted on a new fuel cell electrocatalytic material based on a highly porous carbonized polyacrylonitrile (PAN) microcellular foam with very low platinum loading ({approximately}13 to 23 {micro}g/cm{sup 2}). TEM images of this material clearly show the existence of nanometer size platinum particles which are homogeneously distributed in the highly porous carbonized PAN matrix. An XPS study of Pt-loaded PAN indicates that C 1s, O 1s, and N 1s peaks shift to lower binding energies, compared to virgin PAN. It was concluded that special metal-support interaction exists, through the formation of a charge-transfer complex between platinum and pyridine-type nitrogen atoms of PAN support. This interaction leads to the enhancement of the catalytic activity, and the improvement of the long-time stability of this electrocatalyst.

Electrochemical preparation and characterization of conducting copolymers: poly (aniline-co-N-butylaniline)

Abstract The solubility and processability of conducting polymers are, in many cases, the most important properties for their wide applications. To obtain a soluble conducting polymer, a series of poly(aniline-co-butylamline) copolymers, from the monomers aniline and N-butylaniline, has been prepared in aqueous solution by electrochemical methods. The mechanism of the electropolymerization has been studied. The ratio of the two monomers has a great influence on the copolymerization process and the electrochemical behavior of the copolymers. The copolymers have been characterized by electrochemical and spectroscopic methods. Their electrochemical properties were compared with those of homopolymers, polyaniline and poly (N-butylaniline).

A New Fuel Cell Electrocatalyst Based on Highly Porous Carbonized Polyacrylonitrile Foam with Very Low Platinum Loading

In recent years, interest in the development of a polymer electrolyte fuel cell (PEFC) has considerably increased due to its advantages (e.g., high power density) compared with other fuel cell systems, and its potentials to become the power plant for the next generation, nonpolluting automobile engine. For large scale application of fuel cells, low platinum loading electrode structures are necessary. An electrode based on a highly porous carbonized polyacrylonitrile foam material with very low platinum loading ({approximately} 13 {micro}g/cm{sup 2}) was prepared. The performance of this electrode for oxygen reduction was evaluated and compared with bulk platinum and glassy carbon electrodes.

A thermochemical approach to the bandgaps of semiconducting and insulating materials

Many materials of practical interest are either semiconducting or insulating in nature. One of the most important quantities characterising these materials, therefore, is the energy bandgap. In the present article, procedures available for estimating the bandgaps of binary, inorganic materials from thermochemical and related data have been briefly reviewed with special emphasis on recent work.It has been shown that heats of formation per equivalent and heats of atomisation per equivalent may be used for approximate prediction of bandgaps of these materials. Theoretical origins of the correlation of bandgaps to heats of formation, heats of atomisation, lattice energies, single bond energies and average bond energies etc., have been indicated.

Oxygen reduction on a new electrocatalyst based on highly porous carbonized polyacrylonitrile microcellular foam with very low platinum loading

A highly porous carbonized polyacrylonitrile microcellular foam with very low platinum loading (approximately 13 to 23 gmg cm2) was prepared for use as a possible electrode material in a fuel cell system. The effective surface area of platinum at the PAN/Pt electrode is estimated to be about 1.4-times the geometric area of the substrate electrode. The average particle size deduced from the electrochemical measurements is 44.9 nm. The electrochemical reduction of oxygen was studied at this material by using the rotating disk electrode method. Scanning electron microscopic images of this material clearly show the existence of nanometer sized platinum particles that are homogeneously distributed. These platinum particles are somewhat “isolated” from each other, within the carbonized PAN matrix that is highly porous with a pore size ranging from ca. 1 to 10 mm. The reaction order for the oxygen reduction on this material is unity. The results show that the O2 reduction at the PAN/Pt electrode involves an overall transfer of four electrons per O2 molecule consumed.

Electrochemistry of poly(aniline-co-N-butylaniline) copolymer: Comparison with polyaniline and poly(N-butylaniline)

Abstract Conducting poly(aniline- co -butylaniline) copolymer was studied by cyclic voltammetry and impedance spectroscopy. Its electrochemical properties and impedance behavior were compared with those of the homopolymers polyaniline and poly( N -butylaniline). The electrochemical stability of poly(aniline- co -butylaniline) copolymer was found to be better than that of polyaniline and comparable with that of poly( N -butylaniline). The impedance behavior of the copolymer was interpreted on the basis of an adatom model. The validity of this model for describing this copolymer electrode in both its oxidized and reduced states was demonstrated. A quantitative estimate of the rate of charge transport in this copolymer as a function of potential was obtained.

The corrosion potentials of some common metals in oxygenated solutions in relation to semi-conductivity of the corrosion films*

Abstract An attempt has been made to explore the role of estimated semiconductivity of corrosion reaction films in determining the magnitudes of open-circuit corrosion potentials of Cu, Sn, Al, Ni, Fe, Zn and Pb in chloride solutions. The experimental data on corrosion potentials, as recently reported by Butler, Francis and McKie, have been interpreted qualitatively in terms of the involvement of the semiconducting corrosion films in the electron-hole participation that must occur in order to sustain the partial electrode reactions on the film-covered metals. Arguments have been put forward to show why presence of a highly insulating corrosion film on a metal tends to lead to more cathodic corrosion potentials, for the particular data of Butler et al . which indicate that the corrosion potentials of metals, under their experimental conditions, are controlled predominantly by the cathodic conjugate reaction of the reduction of oxygen.

Electrochemical Aspects of Electroosmotic Dewatering of Clay Suspensions

ABSTRACT An attempt is made to elucidate the electrochemical aspects of the electroosmotic dewatering (EOD) of clays as reported in some recent work, especially that on interrupted DC power electroosmotic dewatering published by Rabie, Mujumdar and Weber (2). These authors showed that the dewatering by EOD stops after the DC power has been on for several minutes or hours; on interruption of their power and on short-circuiting of the electrodes, conditions can be created again for some further dewatering by DC power EOD. This discovery, of Rabie et al. is interpreted as a fuel cell effect and it is shown that it affords clues to several other electrochemical strategies for the possible enhancement of the efficiency of the EOD by DC power. Further, the open circuit potentials observed by Rabie et al. (2) on the interruption of DC power are given an electrochemical interpretation which leads to quantitative estimates in agreement with the experimental values.