Publications

Numerical difficulty arises when DAEs are solved with inconsistent initial values of dependent variables. It can cause the solution failures of many popular DAEs solvers. An initialization subroutine, DAEIS (Differential Algebraic Equation Initialization Subroutine), has been developed to handle this issue. In DAEIS, consistent initial values of dependent variables fo index-1 DAEs are obtained through solving a set of nonlinear equations based on the initialization settings. To achieve good efficiency and robustness of the initialization computation, a nonlinear equation solver, GNES, has been specially built for DAEIS. The usage of DAEIS is demonstrated through handling inconsistent initializations for combined continuous/discrete simulations. © Elsevier Science Ltd.

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.

Electrochemical impedance and self-discharge studies were carried out to investigate lithium intercalation into bare and Ni-coated KS10 graphite. Values of the charge-transfer resistances, exchange current densities, surface film resistances, and lithium-ion diffusion coefficients as functions of the state of charge (SOC) all favored the 10 wt% Ni composite KS10 graphite over bare KS10 graphite when these materials were used as the negative electrode in a Li-ion cell with mixed organic electrolyte. The charge-transfer resistances were always lower and gave rise to between 26 and 27% larger exchange current densities, which increased from 137 to 614 mA/g as the SOC increased. The surface film resistances for Ni composite KS10 were between 0.02 and 0.05 $Ømega$ g, slightly smaller than those of 0.03 to 0.08 $Ømega$ g for bare KS10, and both surface film resistances decreased with increasing SOC. The lithium-ion diffusion coefficients were always slightly larger, ranging between 1.09 × 10 -9 and 6.7 × 10 -9 cm 2 /s. Results from the self-discharge study also favored the 10 wt% Ni composite KS10, which exhibited less capacity loss over a 10 day period compared to bare KS10.

A mathematical model is presented for the lithium intercalation of a single spinel particle as a microelectrode under the stimulus of a cyclic linear potential sweep. The model includes both lithium diffusion within the particle and kinetics at the particle/electrolyte interface. The model is used to predict that peak current densities depend linearly on the scan rate to a certain power with a constant term, which is different from the predicted peak current density for a conventional redox system.

Mathematical modeling of electrodeposition is a process that yields information about the plating system of interest. The process consists of first determining the composition of the plating bath of interest by using thermodynamic information. The second step consists of specifying or determining the electrochemical reactions that occur on the electrodes and the chemical reactions that occur on the electrodes and in the bath. The third step consists of specifying the governing equations (material balance equations) for the concentrations of the species in the bath. Next, reaction-rate expressions must be specified for the electrochemical reactions that occur at the electrodes. Finally, the geometry of the plating bath must be specified. Since the material balance equations for species in the bath depends on fluid flow, the flow conditions in the tank must be specified. In some cases, the material balance equations for the concentration of species in the bath and the momentum balance equations for the fluid flow must be solved simultaneously because the electrodeposition process can give rise to density changes at the surface of the working electrode. These density changes cause the hydrodynamics in the bath to change. Sparging and stirring of the bath also affect the flow conditions at the work piece. The hydrodynamic effects are sometimes lumped together and described by a hydrodynamic boundary layer. Similarly, the mass-transfer effects in plating baths are often lumped together and represented by a diffusion layer. These concepts of boundary layers and diffusion layers have been used to simplify the mathematical modeling of electrodeposition.

It is shown that an electrodeposited Zn-Ni-Cd alloy coating produced from sulfate electrolyte inhibits the discharge of hydrogen on carbon steel. The newly developed ternary alloys have approximately ten times higher corrosion resistance when compared to a Zn-Ni alloy. Hydrogen permeation characteristics of Zn-Ni-Cd alloy coatings were studied and compared with those of a bare and a Zn-Ni alloy coated steel. The transfer coefficient, $\alpha$, exchange current density, i o , thickness dependent adsorption-absorption rate constant, k″, recombination rate constant, k 3 , surface hydrogen coverage, $þeta$ H , were obtained by applying a mathematical model to experimental results. Alloys obtained from baths containing higher concentration than 3 g/L of CdSO 4 in the sulfate plating bath are seen to have superior permeation inhibition properties compared to the Zn-Ni alloy coating and bare steel. The hydrogen permeation current was zero under normal corroding conditions for Zn-Ni-Cd alloy and it increased to 0.3 $μ$A/cm 2 at a cathodic over-potential of 250 mV. The hydrogen permeation current density for steel and Zn-Ni alloy under similar conditions were 62.1 and 1.3 $μ$A/cm 2 , respectively.

The electrochemical monitoring technique is used to measure the solubility and the diffusion coefficient of oxygen in a new proton exchange membrane that is being developed by Cape Cod Research, Inc. Using the method of least squares, the data were fit to an analytical solution of Fick s second law to determine D and co. Values of 0.40 x 10(-6) cm(2)/s and 4.98 X 10(-6) mol/cm(3) were obtained for the diffusion coefficient and solubility, respectively of the Cape Cod membrane. These values are significantly less than those of Nafion 117 tested under identical conditions. (C) 2000 The Electrochemical Society. S0013-4651(99)08-003-9. All rights reserved.

Comparisons are made between the finite difference method (FDM) and the control volume formulation (CVF). An analysis of truncation errors for the two methods is presented. Some rules-of-thumb related to the accuracy of the methods are included. It is shown that the truncation error is the same for both methods when the boundary conditions are of the Dirichlet type, the system equations are linear and represented in Cartesian coordinates. A technique to analyze theaccuracy of the methods is presented. Two examples representing different physical situations are solved using the methods. The FDM failed to conserve mass for a small number of nodes when both boundary conditions include a derivative term (i.e. either a Robin or Neumann type boundary condition) whereas the CVF method did conserve mass for these cases. The FDM is more accurate than the CVF for problems with interfaces between adjacent regions. The CVF is (AX) order of accuracy for a Neumann type boundary condition whereas the FDM is (AX)2 order. (C) 2000 Elsevier Science Ltd.

Hydrogen permeation characteristics of a new Zn-Ni-P alloy were studied and compared with that of a Zn-Ni alloy. The Zn-Ni-P alloy was deposited from an acid sulfate bath containing 0.5 M nickel sulfate (NiSO4), 0.2 M zinc sulfate (ZnSO4), 0.5 M sodium sulfate (Na2SO4), and 100 g/L sodium hypophosphite (NaH2PO2) at pH 3. The permeation characteristics of the alloy were studied and compared qualitatively with that of Zn-Ni alloy under cathodically polarized and corroding conditions. The Zn-Ni-P alloy had better permeation inhibition characteristics in terms of permeation efficiency through the alloy. The lyer-Pickering-Zamanzadeh (IPZ) model was used to quantitatively estimate the various kinetic parameters associated with hydrogen permeation for Zn-Ni-P alloy under polarized conditions. The Zn-Ni-P alloy had superior permeation inhibition properties compared to the Zn-Ni alloy. © 2000, NACE International.