Electrode Reaction

The subnode defines the electrode kinetics for a charge transfer reaction that occurs on an electrolyte-electrode interface boundary. Use multiple nodes to model multiple reactions, for instance in mixed potential problems.

The parent node may be either an Internal Electrode Surface or an Electrode Surface.

Note that the Electrode Kinetics and Stoichiometric Coefficients sections described below are not available for all Electrochemistry interfaces.

The , Eeq (SI unit: V), is used in the electrode kinetics expressions in the following section (via the definition of the overpotential), or for setting up primary current distribution potential constraints.

The , dEeq/dT (SI unit: V/K), is used when calculating the reversible heat source of the electrode reaction, which in turn can be used for coupling to heat transfer physics. Note that dEeq/dT parameter value has no impact on the equilibrium potential variable.

The settings of this section will define the , iloc (SI unit: A/m2), at the interface between the electrolyte and the electrode. Note that iloc for all built-in kinetics expression types will depend on the overpotential, which in turn depend on the Equilibrium potential defined in the previous section.

For all expressions the i0 (SI unit: A/m2) is a measure of the kinetic activity.

These kinetics expression are typically used for secondary current distribution problems.

The is valid when the overpotentials of the reactions are small (<<25 mV). The linearized version can also be used to troubleshoot a model with convergence problems.

The αa (dimensionless), and αc (dimensionless), parameters will impact how much iloc will change upon changes in the overpotential.

This kinetics expression type neglects the cathodic (negative) term in the Butler-Volmer equation. It is only valid for electrode reactions with high anodic overpotentials (>>100 mV).

The Αa (SI unit: V), defines the required increase in overpotential to result in a tenfold increase in the current density.

This kinetics expression type neglects the anodic (positive) term in the Butler-Volmer equation. It is only valid for electrode reactions with significant cathodic overpotentials (<<-100 mV).

The Αc (SI unit: V), describes the required decrease in overpotential to result in a tenfold increase in the current density magnitude. Αc should be a negative value.

This kinetics expression type is typically used in tertiary current distribution problems. One or both of the CO (dimensionless) and CR (dimensionless) parameters may be concentration dependent, and should typically be defined so that CO = CR at equilibrium.

This kinetics expression type is typically used in tertiary current distribution problems for reactions occurring far away from the equilibrium potential.

The kinetics expression type defines an irreversible electrode reaction where the kinetics is so fast that the only factor limiting the reaction rate is the transport of a species to the reacting surface.

The node will set the at the boundary, and balance the fluxes of the species participating in the reaction and the current densities according to the Stoichiometric Coefficients settings.

This choice imposes a zero overpotential for the electrode reaction by applying a constraint on the potential variables in order to comply with the equilibrium potential. Use this kinetics for very fast reactions.

In the Secondary Current Distribution interface the condition set by this expression type is mathematically identical to what is applied when a Primary Current Distribution is chosen on the interface top node. The expression type can hence be used to mix primary and secondary current distributions on different electrodes. The Thermodynamic equilibrium (primary condition) cannot not be used when defining the kinetics for multiple electrode reactions at the same electrode in the Secondary Current Distribution interface.

Use the to impose an upper limit on the local current density magnitude. The feature can be used to model additional mass transport limitations that are not already included in the local current density expression.

For enter a value for ilim (SI unit: A/m2).

Specify the nm in the electrode reaction and the (vc1, vc2, and so forth) for each of the involved species according to the following generic electrochemical reaction:

Set νi as positive (νred) for the reduced species and negative (νox) for the oxidized species in an electrochemical reaction. The number of participating electrons, n, should be positive.

If the concentration of a species in the charge conservation model for the electrolyte is based on an algebraic expression (such as the electroneutrality condition, or the water auto ionization), the stoichiometric coefficient for this species cannot be set explicitly. The stoichiometric coefficient will instead be set implicitly, based on the number of electrons and the stoichiometric coefficients of the other species participating in the reaction.

	An easy way to determine the stoichiometric coefficients for a reaction is to write the reaction as a reduction reaction (with the electrons on the left), irrespectively on the expected actual direction of the reaction in the model. The species on the left side then have negative coefficients and the species on the right have positive coefficients.

The section provides two options: and to calculate the reversible heat source of the electrode reaction, which in turn can be used for coupling to heat transfer physics.

The parameter, dEeq/dT (SI unit: V/K), can be specified in case of selection. Note that dEeq/dT parameter value has no impact on the equilibrium potential variable.

The parameter, Etherm (SI unit: V), can be specified in case of selection.