Glossary of Terms
anode
An electrode where oxidation occurs. Anodic charge transfer current densities and overpotentials are positive by definition.
cathode
An electrode where reduction occurs. Cathodic charge transfer current densities and overpotentials are negative by definition.
cell voltage
The difference in potential over an electrochemical cell.
charge transfer reaction
A reaction during which charge is transferred from an electron conducting phase (electrode) to an ion conducting phase (electrolyte).
charge transfer current density
The current density at an electrolyte-electrode interface associated with a charge transfer reaction.
concentrated electrolyte
An electrolyte where the concentration of charged species is so high that the interactions (friction forces) between the ions needs to be considered. See also diluted electrolyte.
current collector
A part of an electrochemical device with high electric conductivity for conducting current out of the cell. See also current feeder.
current feeder
A part of an electrochemical device with high electric conductivity for conducting current into the cell. See also current collector.
diluted electrolyte
An electrolyte where the charged species are diluted in a solvent so that the interaction between the ions can be neglected. See also concentrated electrolyte.
electric potential
The potential in the electrode (electron conducting) phase, denoted ϕs (phis).
electrode
An electron conductor.
electrode reaction
See charge transfer reaction.
electrokinetic flow
Transport of fluid or charged particles within a fluid by means of electric fields.
electrolyte
An ion conductor.
electrolyte potential
The potential in the electrolyte (ionic) phase, denoted ϕl (phil).
electroneutrality
A situation where the net charge of any small control volume of a domain is zero. Electroneutrality can usually be assumed for electrolytes, with an exception for the very proximity of the electrolyte-electrode interface, which is due to the charge separation within the electrochemical double layer.
equilibrium potential
The potential difference between the electrode and electrolyte at which the net current density of an electrode reaction is zero.
Faraday’s Law
A mathematical expression relating the flux of a species over an electrode-electrolyte interphase to be proportional to the charge transfer current density.
gas diffusion electrode (GDE)
A porous electrode that also includes gas pores in the porous matrix.
GDE
See gas diffusion electrode (GDE).
Gibbs free energy
A thermodynamic potential that depends on the enthalpy and entropy of the reacting species.
intrinsic volume averages
The physical properties of the fluid, such as density, viscosity, and pressure.
intercalation
The process of ions reacting on the surface of a host particle to form a solid, intercalated, species within the host. The intercalated species can then be transported further into the particle by diffusion processes.
migration
The transport of charged species in an electrolyte due to the electric force imposed by the electric field.
morphology correction parameter
A correction parameter that is used to account for morphology effects of a porous structure.
Nernst–Planck equation
A commonly used equation for describing the transport of charged species in a diluted electrolyte. The equation includes transport due to diffusion, migration and convection. The Nernst–Planck equation can be modeled in COMSOL Multiphysics using the Tertiary Current Distribution, Nernst–Planck interface (and the Transport of Diluted Species interface). The Nernst–Planck equation is often combined with an electroneutrality condition.
Nernst–Planck–Poisson equation
In certain situations, for instance when striving to resolve the electrochemical double layer at an electrode, and electroneutrality cannot be assumed, the Nernst–Planck equation is combined with the Poisson’s equation for describing the charge density.
overpotential
A deviation from the equilibrium potential.
oxidation
A charge transfer reaction where electrons are produced.
perfectly mixed electrolyte.
An electrolyte where no concentration gradients are present, for instance due to stirring. See also secondary current distribution.
primary current distribution
The current distribution that is established when overpotentials can be neglected — that is, when the current distribution is governed by the electrolyte and electrode conductivities only. See also secondary current distribution and tertiary current distribution.
Poisson’s equation
An equation for relating the electric field to the space charge density. Can be modeled in COMSOL using the Electrostatics interface.
pore electrolyte
An electrolyte in the pores of a porous matrix. See porous electrode.
porous electrode
A porous matrix containing both electron conducting and ion conducting media.
potentiostat
A device that monitors and controls the potential of an electrode versus a reference electrode.
reduction
A charge transfer reaction where electrons are consumed.
reference electrode
An electrode used for reference when defining the potentials of the electrodes in a cell. A good reference electrode is stable and does not vary in potential. Typically no net reactions occur at a reference electrode surface, and the net current density is zero.
resistive film
A film with ohmic resistivity, typically formed as a result of electrode reactions.
secondary current distribution
The current distribution that is established when concentration overpotentials can be neglected — that is, when the current distribution is governed by the activation overpotentials and the electrolyte conductivity. See also tertiary current distribution and primary current distribution. Note that COMSOL models using the Secondary Current Distribution interface can in fact be modeling a tertiary current distribution because mass transport activation losses can be present in the electrode reaction current density expressions.
solid-electrolyte interphase (SEI)
A resistive and passivating layer formed on the graphite electrode particles in the negative electrode of a lithium-ion battery.
stoichiometric coefficients
The number of species reacting in a reaction formula. In a redox reaction the oxidized species have negative sign and the reduced species have positive sign by convention.
superficial volume averages
The flow velocities, which correspond to a unit volume of the medium including both pores and matrix. These are sometimes called Darcy velocities, defined as volume flow rates per unit cross section of the medium.
supporting electrolyte
An electrolyte with an excess of charge carriers so that the conductivity can be assumed not to be affected by the currents flowing in the cell.
surface molar flux
The tangential flux in the surface dimension as governed by diffusion according to Fick’s law.
tertiary current distribution
The current distribution that is established when concentration overpotentials cannot be neglected — that is, when the current distribution is governed by the activation and concentration overpotentials, as well as the electrolyte conductivity. The electrolyte conductivity can also be nonconstant due to the currents flowing in the cell. See also secondary current distribution and primary current distribution.