Transport of Electrochemical Reaction Components

Before reactions can be defined and parameterized for any reactive flow, the chemical composition of a fluid mixture needs to be known. This holds equally true for electrochemical reactions.

Electrochemical reactions in Simcenter STAR-CCM+ can convert reaction components from both regular species and electrochemical species mixtures.

Species

The term species refers to any component of a chemical reaction. For example, in the hydrogen oxidation reaction:

Figure 1. EQUATION_DISPLAY

H_{2} + {\frac{1}{2} O}_{2} \to H_{2} O

(4070)

there are three species: two reactant species—hydrogen ( $H_{2}$ ) and oxygen ( $O_{2}$ ), and one product species—water ( $H_{2} O$ ). In Simcenter STAR-CCM+, the electrically neutral species that you define as multi-component liquid or multi-component gas, are known as species.

Electrochemical Species

Electrically charged species, such as ions, or charged particles consisting of several molecules, are modeled using the electrochemical species model. Ions are atoms, groups of atoms, or molecules, that are charged because they contain a greater or lesser number of electrons than protons. If the number of electrons orbiting a nucleus is greater than the number of protons in that nucleus, this particle is referred to as a negatively charged ion. Conversely, if the number of electrons orbiting a nucleus is less than the number of protons, this particle is referred to as a positively charged ion.

When considering electrochemical reactions, the term electrochemical species refers to charged species such as ions that participate in an electrochemical reaction. For example, in the reaction:

Figure 2. EQUATION_DISPLAY

H^{+} + {O H}^{-} \to H_{2} O

(4071)

there are two electrochemical species, the hydrogen ion ( $H^{+}$ , proton) and the hydroxide ion ( $O H^{-}$ ).

Being charged matter, electrochemical species are affected by Coulomb forces and are therefore transported by electric fields. This process is called migration. As such, electrochemical species modelling can be useful even in the absence of electrochemical reactions, when species shall be separated, or pumped without moving parts interacting with the fluid.

The mass balance equations that are listed in the following sections determine the spatial distribution of reaction components within an electrochemically reacting system. These balances depend on relations that describe the electric potential and velocity, and usually require electroneutrality to be preserved. The velocity is generally determined by Fluid Flow. The governing equations for electric potential can vary depending on the existence of significant electrical current densities.