Sorption
While electrochemical problems predominantly require the modelling of charge converting heterogeneous electrochemical reactions, the concentrations of reactants are also significantly changed by non charge-transferring processes such as sorption.
Sorption is a general term for the mechanisms of adsorption and absorption. Adsorption describes the process of molecules being bound to a surface by means of adhesion. Absorption refers to molecules which enter a bulk volume—in electrochemical applications, this is most commonly an electrolyte that is modelled as a solid in Simcenter STAR-CCM+.
The sorption process can be written in the form of a reaction. For the example of water and steam/vapour at an interface, this sorption process can be written as:
where is the sorption rate constant, is the concentration of the reactant, is the activity of the sorbed species, and is an equilibrium constant, which allows for discontinuous concentrations at interfaces—as in Henry's law.
where is the stoichiometric coefficient of species .
Different methods can be used to parameterize Eqn. (4165).
- Springer
-
To allow for the most general representation, Simcenter STAR-CCM+ models the reaction rate in terms of chemical activities rather than concentrations to account for deviations from ideal solution behavior. As a consequence, Eqn. (4165) becomes:
(4167)where is an equilibrium-related term.
The equilibrium concentration term for the fluid species is computed as:
(4168)where is the equilibrium concentration constant of the reaction.
is the dimensionless solute content at equilibrium and is usually computed in terms of chemical activities :(4169)where is the saturation pressure.
The Springer reaction formulation [836] is tailored to model water transport in Polymer Electrolyte Membrane (PEM) fuel cells. The dimensionless solute content at equilibrium becomes a dimensionless water content at equilibrium, and is expressed as:(4170)where is the rate exponent. For a Multi-Component Gas continuum in Simcenter STAR-CCM+, the dimensionless water content at equilibrium is cropped at , which corresponds to a saturated gas (for example, water vapour). , , , and are coefficients that you specify. The default coefficient values in Simcenter STAR-CCM+ are from Springer [836] at 30°C for water.
- Wu Li Berg
- Similar to the
Springer formulation, the Wu Li Berg reaction formulation [838] is tailored to model water transport in
PEM fuel cells. The dimensionless water content at equilibrium is defined
as: (4171)
- Henry
- Unlike the above
fuel cell related expressions, Henry's law is not formulated in terms of
reference concentrations and dimensionless solute contents—the surface
sorption rate is expressed directly as:(4172)
Sorption: Electrochemical Reaction Heating
The Electrochemical Reaction Heating model accounts for heat contributions which are due to the sorption process (for heat contributions due to reversible and irreversible electrochemical processes, see Eqn. (4143)).