Publications

The effects of the partial pressure of oxygen and temperature on the oxygen reduction on a submerged gold electrode in a lithium carbonate melt were investigated using cyclic voltammetry and impedance analysis. The values for the mass-transfer parameters, Do 1/2 Co, obtained from cyclic voltammetry and impedance analysis were in good agreement. The reaction orders for oxygen at 800�C were calculated to be about 0.3 for the exchange current density and 0.5 for the product Do 1/2 Co; these values are consistent with the mechanism proposed in the literature for oxygen reduction in Li2CO3 melt.

A mathematical model for the charge and discharge of a sealed nickel-cadmium (Ni-Cd) battery is presented. The model is used to study the effect of transport properties of the electrolyte and kinetic parameters of the electrode reactions on the cell performance during the charge and discharge period. The model can also be used to demonstrate the changes of cell performance during cycling. Some comparisons between model predictions and experimental results indicate that the model predictions appear to fit the experimental data well. Sensitivity analyses illustrate that the sealed nickel-cadmium battery operates under activation control. It is also shown theoretically that oxygen generated on the positive electrode during charge is reduced electrochemically on the negative electrode.

A mathematical model of a hydrogen/oxygen alkaline fuel cell is presented that can be used to predict polarization behavior under various potential loads. The model describes the phenomena occurring in the solid, liquid, and gaseous phases of the anode, separator, and cathode regions, assuming a macrohomogeneous, three‐phase porous electrode structure. The model calculates the spatial variation of the partial pressures of oxygen, hydrogen, and water vapor, dissolved oxygen and hydrogen concentrations, electrolyte concentration, and the solid‐ and solution‐phase potential drops. By developing a complete model of the alkaline fuel cell, the interaction of the various transport and kinetic resistances can be more accurately investigated under conditions that simulate actual fuel cells. The model predicts that the solution‐phase diffusional resistance of dissolved oxygen is a major limitation to achieving high performance at low cell potentials, while the ohmic drop in the solid electrodes contributes the most resistance at high cell potentials. Other limitations to achieving high power densities are indicated, and methods to increase the maximum attainable power density are suggested. These performance indications can help future research and the design of alkaline fuel cells.

A mathematical model of a starved lead-acid cell has been developed to study the dynamic behavior of the cell during discharge. Concentrated binary electrolyte theory and a volume-averaging technique were used to model the transport of electrolyte. The model can be used to predict cell voltage and profile of: acid concentration, overpotential, porosity, reaction rate, and electrode capacity, as functions of time. The effects of separator thickness and its porosity were examined with respect to cold-cranking amperage and reserve capacity of the battery. The separator was found to be a significant factor governing performance.

Effects of partial pressure of CO2 and temperature on oxygen reduction kinetics in lithium carbonate melt were examined using electrochem. impedance spectroscopy (EIS) and cyclic voltammetry. The impedance spectra were analyzed by a complex nonlinear least-squares (CNLS) method, using the Randles-Ershler equivalent circuit model, to estimate the electrode kinetic and mass transfer parameters, such as the charge transfer resistance and Warburg coefficient The cyclic voltammetric measurements indicated that the oxygen reduction process in lithium carbonate melt is "reversible" up to 200 mV/s. Values of the diffusion parameter, D01/2CO determined by cyclic voltammetry concurred with those estimated by EIS method. The reaction order with respect to carbon dioxide and the activation energy for the exchange c.d. were determined to be -0.52 and 132 kJ/mol, resp. Also, the reaction order with respect to carbon dioxide and the activation energy for DO1/2COwere calculated to be -0.8 and 185 kJ/mol, resp.

Electrolysis of hydrogen sulfide to its constituents in a solution containing equimolar concentrations of NaOH and NaHS has been carried out at 80°C. In a double-compartment cell employing Nafion membrane as a separator, both crystalline elemental sulfur and high-purity hydrogen have been produced at high current efficiencies. Only minimal, if any, passivation of the anode by sulfur product was observed. According to solution composition, electrolysis could result in gas evolution at the anode, passivation of the anode by sulfur deposition, or oxidation of sulfide (S2-) or polysulfide (Sx2-) to sulfur oxyanions. However, in an optimized solution, electrolysis gave only anodic sulfur via bisulfide (HS-) and sulfide oxidation. Voltammetric and chronopotentiometric studies showed that sulfide, bisulfide, and polysulfide oxidation occurred at about the same potential.

A five-point finite-difrerence procedure is presented which can be used to solve partial differential equations involving time or time-like derivatives and two spatial conditions (i.e. parabolic partial differential equations). Fourth-order accuracy is obtained by approximating the time derivative by five-point central finite differences and solving the resulting system of equations implicitly. The 1- and 2-D diffusion equations are solved to illustrate the procedure. © 1990.