DigiSim is based on a fully implicit finite difference (IFD) method suggested by Manfred Rudolph [J. Electroanal. Chem. 314 (1991) 13; J. Electroanal. Chem. 338 (1992) 85; "Physical Electrochemistry: Principles, Methods and Applications," I. Rubinstein (ed.), Marcel Dekker, 1995, pp. 81]. Rudolph's work expanded upon ideas originally put forth by Newman [J.S. Newman, "Electrochemical Systems," Prentice Hall, 1973, p. 414]. Subsequent modifications by Feldberg and Rudolph [J. Electroanal. Chem. 378 (1994) 31; J.Electroanal. Chem. 397 (1995) 1; J. Electroanal. Chem. 413 (1996) 25] led to the current algorithm which is robust as well as computationally efficient. Explicit finite difference (EFD) methods promoted by Feldberg (among others) and used by Gosser in his CV simulation program, were much simpler to encode than the IFD methods but are extremely inefficient. An EFD simulation of a system with coupled fast homogeneous reactions can literally take many orders of magnitude longer than the IFD simulation of the same system.
The method for entering mechanisms in earlier versions of DigiSim was based on the idea that all multi-electron transfer reactions are made up of two or more one-electron transfer reactions. Two common multi-electron mechanisms are the EE mechanism (two sequential one electron transfer reactions) and the ECE mechanism (two one electron transfer reactions with an intervening chemical reaction). Inherent in this argument is the idea that there must be some change in the system concomitant with, or subsequent to, the first electron transfer reaction to allow the second electron transfer reaction to occur at the same potential.
DigiSim 3 can now also be used for multielectron reactions A + ne = B where 1 < n < 9. The assumption is that the reaction is governed by the Butler-Volmer expression written as:
The implicit assumption underlying this treatment is that when n > 1, no significant concentrations of intermediate reactants are produced. (Another way of thinking about it is that each redox reaction occurs more easily than the previous one.) We suggest that this feature only be used in the reversible mode (ks = 104 cm/s); it is also likely that n £ 2.
DigiSim recognizes data generated by BASi®, EG&G, and Cypress software. There is also a generic format with the extension .use. Experimental data from other sources can be converted to any of these formats and loaded into DigiSim. However, there are some rules concerning the data that must be followed, the most important of which is that the potential step (i.e., the difference between adjacent data points) is constant throughout the data file.
No. DigiSim cannot simulate mechanisms that involve adsorption. Proper characterization of adsorption involves the choice of an appropriate isotherm (and there are many to choose from) as well as the proper description of potential dependent rate constants and equilibrium constants. However, DigiSim is not restricted to mechanisms based on semi-infinite diffusion, as it can simulate both finite diffusion, a variety of geometries and the rotating disk electrode.
No. DigiSim can only simulate diffusion to those electrode geometries where diffusion can be described by a single dimensional variable (linear, cylindrical [or hemicylindrical], spherical [or hemispherical]). Cyclic voltammetric responses at a disk microelectrode can be approximated in DigiSim by using a hemispherical electrode of the appropriate radius; the CV responses at a band electrode can be approximated using a hemispherical electrode of the appropriate radius.
No. It is up to the user to propose a mechanism and to enter initial estimates for thermodynamic and kinetic parameter values. Once these have been entered, the fitting routine in DigiSim can be used to optimize the values of user-specified parameters to obtain the best fit between the experimental and simulated data. This process requires that the user has a good intuitive understanding of the characteristic voltammograms for various mechanisms. This understanding can be developed using DigiSim by examining the current responses for various mechanisms as the parameter values are varied.
No. A complicated experimental voltammogram can probably be reasonably well described by a number of different mechanisms. It is important to examine the changes in the voltammograms over a range of scan rates and concentrations. Even if a series of experimental voltammograms run over a range of scan rates are well fit by a single set of simulation parameter values, the sensivity of the fit to each of the parameters of interest must be explored.