Douglas N. Bennion

Electrochemical removal of copper ions from very dilute solutions

A device for concentrating electropositive cations using porous, fixed, flow-through, carbon electrodes is described. A feed of 667μg of copper per ml of solution was reduced to less than 1μg of copper per ml of solution. The flow rate was 0.20 cm3/cm2/min through a bed 6 cm thick. Capital cost for the cell is the controlling factor. A preliminary economic analysis indicates that the value of copper recovered will more than pay for the installation and operation of the cell, even for fairly small units.

Silver/Silver Chloride Electrodes: Surface Morphology on Charging and Discharging

Abstract : Transport modes were determined from examinations of morphology for electrochemical oxidation and reduction within electrodes consisting of beds of silver spheres 37.2 micrometers in diameter in 1 N KCl and subjected to 5.0 mA/sq cm applied current density. Oxidation proceeded via silver dissolution, probably at dislocation sites, followed by diffusion and, then, deposition of AgCl in characteristic, bulbed mounds which grow together to form layers of approximately uniform thickness. AgCl film thickness, for the case of partially covered underlying silver, was about 3,500 Angstroms, and distance from silver dissolution pits to AgCl deposition sites was found to be increased, from 4,000 to 40,000 Angstroms, as the local transfer current density became larger within the sphere bed. Reduction of anodically formed AgCl on partially covered silver proceeded by an opposite path: solution and diffusion of AgCl, and deposition of silver on preferred sites of surrounding bare silver surface. (Author)

Design Fundamentals of High Power Density, Pulsed Discharge, Lead Acid Batteries I . Experimental

In this paper the design of a battery with maximum specific power to be discharged for 0.01 s or less is explored. Key elements of the design are bipolar construction, using thin components with high electronic conductivity in the bipolar separator and high ionic conductivity in the electrolyte, and the use of an electrochemical couple with high open-circuit potential and fast electrode kinetics. Bipolar lead-acid stacks were assembled which showed specific powers of 100--800 kW/kg with current densities of up to 10--40 A/cm{sup 2} for up to 100 {mu}s. Single lead-acid cell tests showed that acid concentration, separator thickness and conductivity, discharge potential, and PbO{sub 2} formation time all had a major impact on the cell power output. Tests showed that these cells are capable of well over a million shallow discharge/charge cycles. Evidence indicates that PbSO{sub 4} formation severely reduces cell current densities after 200--400 {mu}s of discharge. In the first 200 {mu}s, H{sub 2}SO{sub 4} concentration depletion at the reaction interface appears to be a factor in current decline.

A Secondary, Nonaqueous Solvent Battery

A room-temperature battery with nonaqueous organic solvent was developed. The cell contains a lithium negative, a graphite positive, and lithium perchlorate--dimethyl sulfite electrolyte. The cell has an open-circuit voltage of 4.5 V, and delivers 10 mA/cm/sup 2/ at 4 V. (RWR)

Engineering Analysis of Shape Change in Zinc Secondary Electrodes

Shape change, the redistribution of active material over the zinc electrode surface as a result of cell cycling, is hypothesized to be caused by convective flows driven primarily by membrane pumping. A mathematical model is formulated based on the convective flow hypothesis for the zinc--silver oxide secondary cell. The numerical solutions predict redistribution of zinc material over the zinc electrode, fluid flow rates, and variations of current distribution and cell potential with the number of cycles. These calculated results can be compared to experimental results. The results suggest that shape change can be eliminated if the convective flow in the zinc electrode compartment parallel to the electrode surface is stopped. (11 figs.)Abstract : Shape change, the redistribution of active material over the zinc electrode surface as a result of cell cycling, is hypothesized to be caused by convective flows driven primarily by membrane pumping. A mathematical model is formulated based on the convective flow hypothesis for the zinc-silver oxide secondary cell and tested by actual cell experiments. The numerical solutions predict redistributions of zinc material over the zinc electrode, fluid flow rates, variations of current distribution, and cell potential with the number of cycles. These numerical calculations are compared quantitatively to the experimental observations. The results show that shape change can be eliminated if the convective flow in the zinc electrode compartment parallel to the electrode surface is stopped. (Author)

Analysis of Porous Electrodes with Sparingly Soluble Reactants

A model is developed for the operation of a porous electrode in which slightly soluble reactants are present. Numerical techniques are used to predict current distribution and total electrode polarization for the case of a uniform porous structure. This technique is extended to the case where nonuniformities in reactant conversion are produced by a nonuniform current distribution within the electrode. In addition, a simulation of electrode behavior on repeated cycling is obtained. The implications of nonuniform reactant conversion and mass transfer limitations for real battery electrodes are discussed. The results of the numerical calculations indicate that diffusion‐limited currents within the porous electrode are possible for certain input parameters. The high polarization at constant current corresponding to a limiting current may be obtained after some time of discharge, but before all of the theoretically available active material is used. In the cycling simulation, significant changes in the relative distribution of reactants and products were observed as a function of depth in the electrode. These changes caused differences in the total electrode polarization from cycle to cycle.

Analysis of Porous Electrodes with Sparingly Soluble Reactants II . Variable Solution Properties, Convection, and Complexing

The galvanostatic operation of flooded porous electrodes employing metal/metal salt couples is analyzed. A model is developed for a single, circular pore configuration which accounts for the effects of differing equivalent volumes of the solid reactants. The Cd/Cd(OH)/sub 2/ couple in concentrated aqueous potassium hydroxide and the Ag/AgCl couple in concentrated potassium chloride solutions are considered. Overpotential is computed as a function of time for solid-film and solution-diffusion versions of the model. The solid film model shows a linear overpotential-time relationship and nearly uniform current distribution. The solution-diffusion model shows a variety of overpotential-time curves, based on different physical parameters. In general, anodic failure is caused by blockage of pores or by complete coverage of the metal surface by product crytallites. Cathodic failure is caused by low mass transport which leads to limiting currents in the electrode. 41 references.

Engineering analysis of shape change in zinc secondary electrodes. I. Theoretical. [Zn--Ag/sub 2/O secondary cell]

Shape change, the redistribution of active material over the zinc electrode surface as a result of cell cycling, is hypothesized to be caused by convective flows driven primarily by membrane pumping. A mathematical model is formulated based on the convective flow hypothesis for the zinc--silver oxide secondary cell. The numerical solutions predict redistribution of zinc material over the zinc electrode, fluid flow rates, and variations of current distribution and cell potential with the number of cycles. These calculated results can be compared to experimental results. The results suggest that shape change can be eliminated if the convective flow in the zinc electrode compartment parallel to the electrode surface is stopped. (11 figs.)Abstract : Shape change, the redistribution of active material over the zinc electrode surface as a result of cell cycling, is hypothesized to be caused by convective flows driven primarily by membrane pumping. A mathematical model is formulated based on the convective flow hypothesis for the zinc-silver oxide secondary cell and tested by actual cell experiments. The numerical solutions predict redistributions of zinc material over the zinc electrode, fluid flow rates, variations of current distribution, and cell potential with the number of cycles. These numerical calculations are compared quantitatively to the experimental observations. The results show that shape change can be eliminated if the convective flow in the zinc electrode compartment parallel to the electrode surface is stopped. (Author)

Electrochemical concentrating and purifying from dilute copper solutions

The effectiveness of a new process for recovering copper ions from dilute solutions has been tested. Porous, fixed, flow-through graphite electrodes are used in conjunction with a membrane which separates the two feed streams. The concept is demonstrated using a dilute feed of 800μ ml{−1} copper ions which is reduced to less than 1μ ml{−1}, and a concentrated stream of 0.4 M copper which is concentrated to 0.7M. The results show the concept is technically feasible and that it is competitive with existing technology for copper recovery.

Analysis of Porous Electrodes with Sparingly Soluble Reactants III . Short Time Transients

A mathematical model which describes the operation of porous battery electrodes was developed. It includes short time transient behavior of electrolyte concentration, porosity, current distribution, reaction rate, and detailed solution diffusion descriptions of a sparingly soluble reactant. Calculations were made for constant current (50 and 25 mA/cm/sup 2/) charging and discharging of Ag/AgCl electrodes. Results of calculations indicate that the concentration of the electrolyte, KCl, inside the porous electrode plays an important role in the performance of the electrode. During charging, the electrolyte concentration falls in the depth of the electrode; the solution electrical conductivity is thereby decreased. Reaction penetration depth on charging is very shallow, less than 0.3 mm, for the current densities investigated. On discharging, the electrolyte concentration in the depth of the electrode increases. Consequently, the electrical conductivity of the solution as well as the solubility of AgCl increases. A high reaction rate in the depth of the electrode is possible even at high current discharge. Charge utilization over 90 percent is predicted if the charge is stored uniformly by employing a slow charging rate. 12 figures.