H.S. Wroblowa

Rechargeable manganese oxide electrodes: Part II. physically modified materials

Abstract Manganese dioxide electrodes are known to be rechargeable only under conditions of shallow depth of discharge not exceeding a fraction of one-electron capacity. Even under these conditions only some 50–100 cycles can be obtained. In the present work “chemically modified” manganese oxide materials are described which have no inherent limitations to their cycle-life even under conditions of deep discharge reaching 80–95% of their theoretical two-electron capacity in each cycle. The unique properties of these materials include also the capability of high current drains and insensitivity to overdischarge and overcharge. Methods of preparation, physico-chemical characteristics and electrochemical behavior of these materials are described. Possible interpretations of the effects of chemical modification on electrode rechargeability are discussed.

Mechanism and kinetics of oxygen reduction on steel

Abstract The mechanism of oxygen reduction on steel was determined as a function of the state of the surface. The steel/alkaline electrolyte interface was found to provide a particularly well defined electrochemical system for rotating ring/disc electrode (RRDE) studies which enabled the quantitative kinetic evaluation of separate reaction steps. Some relevant concepts and equations used in the interpretation of RRDE data were re-examined and/or derived. Measurements of the collection efficiency using the oxygen/peroxyl ion system have shown that the platinum ring commonly used in mechanistic studies of oxygen reduction catalyzes the chemical decomposition of peroxyl ions, thus leading to erroneous mechanistic determinations.

Intermediate products of atmospheric oxygen reduction and the integrity of metal—organic coating interface

Abstract The mechanisms of delamination and undercoating corrosion in systems protected by organic material(s) remain unclear. The observed types of failure have been interpreted in various mechanistic terms having a common assumption that the hydroxyl ions generated at the metal surface as products of the reduction of atmospheric oxygen are responsible for the loss of protective properties. The possibility of formation of superoxide and/or peroxide species and the effects of their chemical interaction with the coating seem to have been neither considered nor studied. They are presently discussed in terms of the concentration and stability of peroxide species and of a possible mechanism of their attack on the organic coating. Results of the recent rotating ring—disc electrode (RRDE) studies of the mechanism of oxygen reduction at steel and zinc surfaces in alkaline media are summarized and compared. Although the actual mechanism differs somewhat, peroxyl ions are present as stable products of oxygen reduction in the entire interfacial potential region corresponding to the oxide-covered surfaces in both the passive and prepassive potential regions. Only at bare and/or adsorbed-hydrogen-covered metal surfaces are hydroxyl ions the only final products. The mechanistic effects of the presence of a number of foreign anions at the zinc and steel interfacial regions were studied using the RRDE method. A possible connection between the presence of certain anions and corrosion protective properties are discussed. Further mechanistic considerations of delamination and undercoating corrosion should take into account the presence of the highly aggressive peroxide and nascent superoxide ions whose decomposition may involve destructive chemical interaction with organic material and/or other components of the coated system.

THE MECHANISM OF OXYGEN REDUCTION ON ZINC

Abstract A systematic investigation of the products of oxygen reduction and their possible effects on the failure of coated systems which was initiated for model steel systems is here continued for the zinc/alkaline electrolyte interface. The analysis of the experimental rotating ring-Zn disc electrode data allowed the pathways of oxygen reduction to be determined as a function of the state of metal surface. Peroxyl ions were found to be stable products of oxygen reduction at oxide covered zinc surfaces, both in the passive and prepassive potential regions. Only at bare metal surfaces, does oxygen reduction lead exclusively to hydroxyl ions.

CANDIDATE MATERIALS FOR THE POSITIVE CURRENT COLLECTOR IN SODIUM-SULFUR CELLS - I. CERAMIC OXIDES

Abstract Chromium oxides doped with various metal oxides and strontium doped lanthanum chromite perovskites were prepared, physically characterized and tested for their suitability as coating materials for positive current collectors in sodium-sulfur cells. All of the materials studied are corrosion resistant. Perovskites show the highest conductivities, whereas only the chromium oxides doped with lower valence metal cations indicated acceptably low resistivities. However, upon exposure to the melt the resistivity of all materials studied increased in time. This has been shown to be caused by the loss of electron hole carriers during sample equilibration with the low oxygen partial pressure in the melt. The rate of the loss of conductivity is sufficiently slow to make the perovskites and chromium oxides doped with lithia or magnesia usable as coating materials for positive current collectors in sodium-sulfur cells.

Candidate materials for the sulfur electrode current collector—II. Aluminum and its alloys

Abstract In the continuing search for materials for the positive current collector/container of the high-temperature sodium-sulfur cell, the electrochemical behavior of aluminum alloys in polysulfide/sulfur melts has been examined. Electrochemical evidence coupled with SEM surface examination and chemical analysis of the melt have shown that aluminum materials studied remain passive within the 1.25–4.5 V potential range (Na/Na + reference). The existence of the transpassive region currently reported in the literature has not been confirmed. Interpretation of the nature of the observed residual currents is suggested. The alloys studied can be used as self-healing substrates of conductive coatings or as matrices of composite materials in contact with the polysulfide/sulfur melts.

Rechargeable Manganese Oxide Electrodes and Cells

Rechargeable batteries are indispensable as subsidiary power plants or as auxiliaries in a large segment of industrial products. The necessity of their further development increases with the rapid expansion of industries using throwaway primary cells and with the concern about energetic and environmental problems. Primary batteries contribute to a considerable energy waste (the energy expended in manufacturing may exceed ten times the energy supplied by the battery during its lifetime), valuable material waste, and waste disposal problems. Secondary batteries presently in use might have to be replaced if the use of toxic lead and cadmium were to be prohibited.

Candidate materials for the sulfur electrode current collector—III. Al/SiC composite materials

Abstract It has been previously found that aluminum and some of its alloys can be used as component materials in high temperature sodium/sulfur cells. The present study describes the electrochemical behavior of composite aluminum/silicon carbide materials in the polysulfide/sulfur melts at 350°C. Some of the composites studied have been found suitable to serve as the material, coating or lining of the positive current collector in Na-S cells.

Electroreduction of oxygen in molten NaOH

Abstract A study of oxygen electroreduction at a bright platinum electrode has been carried out in molten sodium hydroxide at 350°C. Analysis of potentiostatic and potentiodynamic measurements led to the following results: (1) The electroactive species in oxygen-containing NaOH melts at 350°C is sodium peroxide. (2) A general method of quantitative determination of kinetic and mass transport parameters for certain reactions has been described. (3) The mechanism of oxygen reduction in molten NaOH has been suggested. (4) Kinetic and mass-transport parameters characterizing oxygen reduction in molten NaOH have been calculated. (5) The reaction studied seems to be one of the rare examples of relative invariance of the Tafel slope with temperature.