This command opens a CV-Property Dialog consisting of 3 property pages:
This page consists of 3 sections:
•Charge Transfer Reaction
1. Adding a Charge-Transfer Reaction
Click on the first empty line headlined Charge-Transfer Reaction. Enter the name of the species involved in the charge transfer process in the appearing dialog box
oxidized species + n e- = reduced species (kf and kb computed from Butler-Volmer or Marcus equation (the latter option applies only to n= 1))
oxidized species + n e- => reduced species (kb=0, kf computed from Butler-Volmer or Marcus equation the latter option applies only to n= 1)
oxidized species + n e- <= reduced species (kf=0, kb computed from Butler-Volmer or Marcus equation the latter option applies only to n= 1)
Charge transfer reactions are always formulated to proceed as a reduction from the left to the right. Whether a charge transfer step does really proceed as reduction or oxidation is determined by the relation between initial and standard potential and the direction of the potential scan entered on the Property Page: Simulation Parameters and Chemical Reactions.
•the species name of oxidized and reduced form must not be equal or empty
•any character (including space) can be used in a species name but the first character must not be X, Y or Z
•the number of electrons must be an integer ranging from 1 to 9
Activate the Enable Adsorption checkbox if adsorption reactions need to be taken into account for the species involved in the underlying charge-transfer reaction. The adsorption parameters can be entered on the Property Page: Surface Reactions (see below) as soon as the input on the Property Page: Chemical Reactions has been completed. Note that E° and Keq must have been specified for each entered reaction and the required analytical concentrations must have been entered. Otherwise, access to the Property Page: Surface Reactions remains disabled.
CT-Reactions involving adsorbed species are written in red as shown in the following picture:
View video clip to see an example.
Linking the heterogeneous rate constant of a CT reaction to the surface coverage of an adsorbed species. A heterogeneous rate constant, ks, written in red has been linked to the surface coverage of an adsorbed species. Click here for more details.
Checkbox: Enable termolecular CT-Reaction
This enables the user to enter a termolecular charge transfer reaction (two species + electrode) of the following form
Ox + P + e = Red + Q
where P or Q might be an empty string as well. If both, P and Q are empty the CT-Reaction is treated as a "normal" bimolecular charge transfer.
Ox + H3O+ = HRed + H2O
have attracted increasing interest during the last couple of years.
In order to accomplish that the heterogeneous rate constant, ks, has the usual unit of cm/s it is assumed in DigiElch that the concentration of species P and Q is dimensionless. For this purpose the "real-world concentrations" of P and Q must be divided by 1 mol/l.
2. Adding a Homogeneous Chemical Reaction
Click on the first empty line headlined Chemical Reaction and enter the name of the involved species in the appearing dialog box:
Second-order chemical reactions comprising up to four species can be modeled:
reversible reactions (Keq = kf/kb):
Species1 + Species2 = Species3 + Species4
irreversible reactions (kb=0, independetly of Keq):
Species1 + Species2 => Species3 + Species4
•if the name of a species starts with X, Y or Z it is considered a buffer/excess component. The concentration of such a species does not change in the course of the simulation
•excess components or empty strings can be entered only for Species 2 and/or Species 4
•(pseudo-) first-order reaction: Species 2 and Species 4 are empty strings (or excess components)
•second-order reaction: no species name or only the name of Species 2 or 4 is empty or that of an excess component
Note that numbers are not considered stoichiometric numbers. The notation “2A” is therefore not an abbreviation for "A + A" but simply interpreted as a name of a species. For this reason the reaction “A+ + A- = 2A” (where “2” is a stoichiometric number) must be entered as “A+ + A- = A + A”
3. Editing or Removing a Reaction Equation
Reaction equations can be edited/removed simply by clicking on these reaction equation. The appearing dialog box provides the following options:
4. Meaning of the Chemical/Electrochemical Parameters
oEo (V), ks (cm/s)
oButton α and λ (eV)
oKeq, kf, kb
odepends on whether a first- or second-order chemical reaction has been entered.
Note that DigiElch is able to recognize thermodynamically superfluous reactions (TSR). For example, in the case of the mechanism shown above the equilibrium constant of the second-order cross reaction (third chemical reaction) and the standard potential of the second charge transfer reaction is unambiguously characterized by the remaining equilibrium constants. In order to prevent the user from entering a Keq- or E°-value that violates thermodynamics, these input fields are blocked after having completed the input for the other reaction equations. If, for instance, the user wants to enter Keq for the second-order cross reaction the input field for one of the remaining equilibrium constants must be emptied. If the mouse cursor has left the emptied field the input associated with Keq of the second-order cross reaction becomes active. After entering the Keq-value the input field of the reaction which are TSR's now will be blocked. Blocked ("read only") input fields are displayed on a grey background.
5. Meaning of the Species Parameters
•Canal (mol/l), Cinit (mol/l)
If electrode geometry is Spherical (Hg) the user has the choice to specify whether a particular species is forming an amalgam (i.e. diffusing into the mercury drop) or not.
This Page can be entered only if
•there is at least one CT-reaction for which the check box Enable Adsorption has been activated as shown above.
•a value for E° (V) and Keq must have been entered for each charge-transfer and chemical reaction, respectively.
•the required analytical concentrations, Canal (mol/l), of initially present species must have been entered.
•the diffusion modus must not be Semi-Infinite 2D
If only a single CT-reaction of the form Ox + e = Red has been entered on the Property Page: Chemical Reactions, the Property Page: Surface Reactions looks as follows:
Note that all relevant parameters are zero by default. Consequently, the activation of the Enable Adsorption check box has no effect on the simulated CV if the parameters on the Property Page: Surface Reactions have not been modified.
You cannot directly add a new CT-Reaction of Adsorbed Species or Adsorption Reaction on this page. This must be done by entering a new Charge-Transfer-Reaction on the Property Page: Chemical Reactions and activating the Enable Adsorption checkbox for this reaction.
1. Defining a Reaction of Adsorbed Species
Unlike chemical reactions on the Property Page: Chemical Reactions, the definition of reactions between adsorbed and desorbed species is currently restricted to two predefined types:
•Type 1: Red1* + Ox2 = Ox1* + Red2
•Type 2: A* + P = Q + S
Click on the mouse button on the first empty line headlined Reaction of Adsorbed Species and select the reaction type from the appearing combo box.
Then use the combo boxes in the appearing dialog box to define the species involved in this reaction
Note that any species required for formulating such a reaction must have been defined on the Property Page: Chemical Reactions before. This should be accomplished by entering (on the Property Page: Chemical Reactions) an analogously defined reaction equation for the desorbed species even if the reaction between the desorbed species is negligibly slow (kf ~ 0).
2. Meaning of Parameters in CT-Reactions of Adsorbed Species
The charge-transfer parameters in this section refer to the direct reduction of the adsorbed species. The latter are marked by an "*".
•Eo* (V), ks* (1/s)
•α* and λ (eV)*
3. Meaning of Parameters in Adsorption Reaction
4. Meaning of Parameters in Reaction of Adsorbed Species
•Keq, kf, kb
5. Meaning of Parameters in Adsorbed Species
View video clip to see an example.
A value of Cdl (F) written in magenta indicates that a time/potential dependent double layer capacity has been entered by the user. The polynomial coefficients describing the dependence of the double layer capacity as function of the electrode potential can be edited by clicking with the right mouse button while the cursor is localized over the input field associated with Cdl (F).
1. Scan Parameters:
•Scan segment, Estart (V), Eend (V), v (V/s)
Estart - values plotted on a grey background are automatically filled in. They are "read only" and cannot be edited/modified. This ensures a "smooth scan" where the starting potential of a scan is always equal to the end potential of the previous scan.
•Check Box: use the same value of v(V/s) in each scan segment
•Check Box: apply background correction
•Potential steps (V)
Removing scan segments:
Depending on the selected option for Diffusion the following geometry options are available
When selecting Spherical (Hg) the user has the choice to specify whether a particular species is going to form an amalgam (i.e. diffusing into the mercury drop) or not.
5. Experimental Conditions:
•Ru (Ohm), Cdl (F)
6. Model Parameters
•Noise level (%)
•Expansion Factor x-grid (perpendicular to electrode), Truncation error (%)
oThe first one is a particular implementation of the box method which is the only simulation technique reported in the literature that yields exponential converges for the flux error towards zero when refining the grid expansion factor. The importance of the Expansion factor and the Relative truncation error for the accuracy of the simulated flux has been discussed in detail in a series of papers listed on the ElchSoft Homepage. The user is strongly advised to deal with these papers when working with DigiElch. Using the default setting for the Relative truncation error ensures that the flux error becomes independent of the selected grid expansion factor to the greatest possible extend.
oThe second simulation technique implemented into DigiElch is an adaptive grid simulator. The starting grid is identical to that described above for the box method. However, the adaptive grid simulator does not result in exponential convergence for the simulated flux error. The error level originating from using an exponentially expanding space grid is therefore usually much larger than the truncation error. Consequently, the accuracy of the simulated flux can be guaranteed only when using a sufficiently small Local FEM Error (see more below). Also note that the adaptive grid simulator is not fully implemented. The following simulations cannot be executed with this tool:
▪simulation of two-dimensional diffusion systems
▪simulations involving amalgam forming species
•Xmax / SQRT(Dt)
•Expansion factor y-grid (parallel to electrode) , Boxes for electrode
8. FEM- (Adaptive Grid-) Simulation
•Local FEM Error
9. Level of Multi-Core CPU-Support
10. Simulation Name
There is a single-line edit control on the bottom of the Property Page: Simulation Parameters that can be used for giving the simulation a more meaningful name as that automatically generated by DigiElch. The name referring to the active simulation is indicated in the frame window of the simulation document after closing the CV-Properties Dialog. When exporting a simulation the default file name is either “simulation name.use” or “simulation name.txt ”. For this reason, the simulation name must not contain ‘\’ or any other character leading to problems in file names.