Publications

A simple technique based on finite differences is presented for obtaining symbolic solutions for boundary value problems (BVPs). The governing equations for the node points are expressed in matrix form and the dependent variables (e.g. concentration) at both the boundaries (both at x = 0 and x = 1) are taken as unknown constants. The solution is obtained by finding the matrix inverse using Maple. The unique aspect of the technique presented here is that the solution obtained is valid for various boundary conditions (both linear and nonlinear) and geometries. Both linear ordinary differential equations (ODEs) and partial differential equations (PDEs) with linear and non-linear boundary conditions are treated in this paper. Solutions analytical in time are obtained for PDEs. (C) 2000 Elsevier Science Ltd.

A semianalytical solution technique is presented for solving Laplace s equation to obtain primary and secondary potential and current density distributions in electrochemical cells. The potential distribution inside a rectangle with the electrodes facing each other between two insulators is presented to illustrate the method. It is shown that the method yields analytic equations for the potential and the potential gradient along the lines. The unique attribute of the technique developed is that the solution once obtained is valid for nonlinear boundary conditions also. The procedure is applied to some realistic problems encountered in electrochemical engineering to illustrate the utility of the technique developed. 2000 The Electrochemical Society. All rights reserved.

A review is presented of the mathematical models that have been developed to describe the phenomena that occur in the active material in the nickel electrode. The review includes models that describe the reaction thermodynamics, proton diffusion, electron conductivity, semiconductor effects, and the reactions in the solid phase. The appropriateness of these models and their usefulness in predicting phenomena observed in nickel-based batteries are discussed.

The authors have developed a mathematical methods course for seniors and first-year chemical engineering graduate students that uses the matrix exponential and Maple to solve initial value problems, boundary value problems, and partial differential equations. A brief description of some of what they cover in their course is presented.

A Pd-encapsulated graphite electrode was used as the negative electrode in Li-ion cells. Through dispersion of ultrafine nanoparticles of palladium on the surface of graphite, the interfacial properties of the carbon surface were modified. The presence of the palladium dramatically reduces the initial irreversible capacity of the graphite in propylene carbonate (PC)-based electrolyte. Palladium suppresses the solvated lithium ion intercalation and improves the charge-discharge performance and initial coulombic efficiency of graphite. For example, 10-wt.% of Pd-nanoparticles dispersed on the surface of graphite increases the initial charge-discharge coulombic efficiency from 59% to 80.3%. Electrochemical impedance spectroscopy (EIS) indicates that palladium dispersed on graphite increases the ohmic conductivity and also improves the Li insertion rate into graphite. However, an excess amount of palladium on graphite leads to a decrease in the charge-discharge efficiency due to the consumption of lithium by the formation of Li2PdO2.

The performance of a system consisting of a proton exchange membrane (PEM) fuel cell coupled to a centrifugal air compressor is simulated. Two modes of operation of the system are investigated: one in which the speed of the compressor is constant, and the other in which the compressor speed is varied with the electric load on the fuel cell stack. The operating characteristics of the compressor and the PEM fuel cell stack and their influence on the system efficiency are analyzed for a step change in the stack current. The effects of the fuel cell stack back-pressure and the electric load on the compressor power consumption and the system efficiency are studied. It is found that the system efficiency is higher when the fuel cell stack is operated at a constant oxygen gas stoichiometry by varying the compressor speed instead of at a constant compressor speed. The system model can be used to determine the rotation speed of the compressor for various electric loads.

A shrinking core model is presented for the galvanostatic discharge$\backslash$nof a metal hydride particle. A quantitative criterion for when the$\backslash$nshrinking core can be completely neglected or approximated by a pseudosteady-state$\backslash$nsolution is presented. The effect of shrinking of the core on the$\backslash$ndischarge behavior of a metal hydride particle is also studied.

Tlte beliavior of galvanized carbon steel samples was studied in Ca(OH)2 solutions, simulating the alkaline environment in reinforced concrete. Under shorter periods, it was seen that a passive layer was formed on the surface of the zinc coating. Tlie film, however, was not stable for large periods of time. Tliis was revealed by the long duration tests where the passive layer was disrupted and the carbon substrate was protected sacricially by zinc dissolution. The presence of chlorides accelerated the passive layer breakdown. The role of calcium nitrite in inhibiting the corrosion process of zinc was also studied. It was found that zinc was not protected by nitrite in the presence of chloride ions. Tlie inhibitor, however, significantly reduced the corrosion rate of the underlying steel. Copyright ©2000, American Concrete institute. All rights reserved,.

Electrochemical impedance spectroscopy was used as an in situ technique to determine the average particle size of metal hydride electrodes. Using this, the pulverization of bare and cobalt-encapsulated LaNi4.27Sn0.24 alloy was studied as a function of charge-discharge cycles. In the case of bare alloy, pulverization causes an exponential decay in particle size with cycling. Cobalt-encapsulated alloys do not undergo much pulverization with cycling. Bode responses obtained for bare alloy electrodes indicate the increase in particle to particle resistance with cycling. Alloy oxidation, which is responsible for the increase in particle to particle resistance is absent in the case of cobalt encapsulated alloy. Surface analysis indicates the presence of alloy segregation for bare LaNi4.27Sn0.24. Decrease in particle size and increase in bare alloy resistance is accompanied with severe decay in electrode discharge capacity.

Modeling of secondary lithium batteries is reviewed in this paper. The models available to simulate the electrochemical and thermal behavior of secondary lithium batteries are discussed considering not only their electrochemical representation (transport phenomena and thermodynamics of the system), but also the mathematical techniques that have been used for solving the equations. A brief review of the governing equations for porous electrodes, and methods for solving these equations is also given. © 2000 Elsevier Science Ltd. All rights reserved.