Publications

This work presents an improved point defect model for the time-dependent formation of passive oxide films on metal surfaces. Like previous point-defect models, the present model assumes that charged defects, or vacancies, carry current across the growing oxide film. However, we treat the vacancies explicitly as material species which participate in oxide formation and dissolution reactions formulated for arbitrary oxide stoichiometry. The model includes boundary conditions, based on jump mass balances from formal continuum mechanics, that relate vacancy fluxes to the interfacial reaction rates as well as the motion of the film boundaries. Thus, unlike previous models, this model treats the film growth process formally as a moving boundary problem. Casting the equations in dimensionless form yields the key dimensionless groups. The dependence of the film growth rate on these groups can be rationalized in simple physical terms. The predicted trends in film growth rate and current density agree qualitativel y with experimental data for nickel passivation, although the model parameters have not been optimized to achieve good agreement with current density data. The model provides a starting point for incorporating better descriptions of interfacial reaction kinetics within boundary conditions based on rigorous continuum mechanics. © 2001 Elsevier Science Ltd. All rights reserved.

A model for a nickel-metal hydride cell was constructed based on the planar electrode approximation. The mass balances of active species, the kinetics of electrochemical reactions, the ohmic effects of internal resistance, and the energy balance of the whole cell were considered in the model. An empirical approach was utilized to account for the hysteresis potential behavior of the nickel electrode. The model predictions showed favorable agreement with the experimental data. © 2001 Elsevier Science B.V. All right reserved.

A simple theoretical model is presented to simulate the galvanostatic discharge behavior of the Ni-composite graphite electrode. The discharge profiles predicted by using a constant diffusion coefficient (CDC) and by a varied diffusion coefficient (VDC) are compared in this paper. The results show that, the VDC model can be simplified to the CDC for discharge rates less than 2C for a 5 $μ$m particle. Also, an approximate analytical solution is presented for VDC model, which is found to be valid for discharge rates up to 6C. Exchange current and diffusion coefficient for the lithium-diffusion are predicted. © 2001 Elsevier Science B.V.

A nonisothermal model of a nickel-hydrogen cell has been developed with the consideration of multiple phases in the nickel active material. Important mechanisms inside a nickel-hydrogen cell, such as mass balances of active species, kinetics of electrochemical reactions, and the energy balance of the whole cell, etc., have been included in the model. The model predictions under different conditions are presented and analyzed. These predictions showed that nickel phase reactions have significant influences on the behavior of a nickel-hydrogen cell. Some observed phenomena of a nickel-hydrogen cell, e.g., the capacity variation at different temperatures and the KOH concentration change between charge and discharge processes, could be reflected reasonably with the model. (C) 2001 The Electrochemical Society.

Ruthenium oxide-carbon composites with different loadings of RuO2 on carbon have been synthesized by an electroless deposition process. Increase in RuO2 loading results in increasing the specific capacitance of the composite electrode. The effect of temperature treatment on the performance of these materials has been studied in detail. Maximum capacitance was observed after heat treatment at 100 °C for all the composites. Increasing the oxidation temperature further converts the Ru oxides to crystalline form, which leads to poor capacitance values. A maximum capacitance of 260 F/g was obtained for 20wt.% RuO2 loaded carbon treated at 100 °C. The volumetric surface area of the composite remains constant with increased RuO2 loading. Since Ru oxides have a large pseudocapacitance, this increases the volumetric capacitance of the carbon significantly.

It is shown that composite Ru oxide-carbon based supercapacitors possess superior energy and power densities as compared to bare carbon. An electroless deposition process was used to synthesize the ruthenium oxide-carbon composites. Ru is dispersed on the carbon matrix as small particles. The effect of electrochemical oxidation and temperature treatment on the material performance has been studied extensively. Increasing the oxidation temperature reduces the proton transport rate and also increases the degree of crystallinity of the deposits. This adversely affects the performance of the composite. Loading a small amount of Ru oxide (9 wt %) on carbon increases the capacitance from 98 to 190 F/g. © 2001 The Electrochemical Society. All rights reserved.

The separation of variables method is extended to obtain concentration profiles in a particle electrode under galvanostatic boundary conditions. The method is also used to find exact analytical solutions for composite slab and spherical electrodes. Finally, the method is used to obtain a solution for a lithium/polymer cell model that was presented previously by Doyle and Newman. © 2001 Elsevier Science B.V.

Serial simulation is required to predict the behavior of an electrochemical system undergoing many processes. This is demonstrated through the simulation of charge/open-circuit/discharge processes of a thin film nickel hydroxide electrode. The numerical issues involved in this kind of simulation are discussed. An efficient and robust procedure is presented. It can be easily used to achieve the serial simulation of electrochemical processes, e.g., battery cycling, cyclic voltammetry etc.

Two different approaches were used to model the insertion of lithium ions into a carbon particle. In the first approach, a concentration gradient was considered as the driving force (DFM) for diffusion while in the second approach chemical potential driving force was used (CPM). Lithium ion-lithium ion interactions are included in the CPM model but not in the DFM model. These approaches were used to model a lithium foil/1 M LiClO4-propylene carbonate/carbon fiber cell. The model predictions indicate that the lithium ion-lithium ion interactions inside the particle play a significant role in predicting the electrochemical and thermal performance of the cell.

Approximate models are developed, based on second, fourth, and sixth order polynomials, that describe the concentration profile of an electrochemically active species in a spherical electrode particle. Analytical expressions are obtained that describe the way the concentration profiles, surface concentrations, and electrode utilization change during the galvanostatic discharge of an electrode particle. Based on a comparison with an exact analytical model over a wide range of dimensionless current densities, all three approximate models performed extremely well in predicting these quantities. Quantitative criterion for the validity of these models is also developed and shows that the sixth order, four parameter approximate model is the best. These approximate models, or similarly developed models, should find extensive use in simplifying the modeling of complex electrochemical systems without sacrificing much accuracy as shown in Part II of this series for the concentration-dependent diffusion coefficient case. © 2001 The Electrochemical Society. All rights reserved.