The rate of species production and destruction is:

(20–130)

where is the source or sink of the species (molar flux), is the stoichiometric coefficient, is the current density (), is the number of electrons per mole of fuel, and is the Faraday constant.

Using the local current information, the Ansys Fluent SOFC With Unresolved Electrolyte Model applies species fluxes to the electrode boundaries. By convention [473], the current density is positive when it flows from the electrode into the electrolyte solution. The current densities are positive at the anodes and negative at the cathodes.

The reaction at the cathode electrode is:

(20–131)

or

(20–132)

(20–133)

The reaction at the anode electrode is:

(20–134)

(20–135)

(20–136)

(20–137)

Note that the total enthalpy includes the formation enthalpy for each species only when the volumetric reactions are enabled. Once the volumetric reactions are enabled, the SOFC model solver can correctly compute the reaction heat.

In practice, if carbon monoxide () is present in the anode fuel stream, it may be oxidized to generate carbon dioxide (). This effect is modeled by introducing a / split factor as follows:

(20–138)

where amd are species mole fractions of and , respectively.

The molar source terms for each anode-side species are:

(20–139)

(20–140)

(20–141)

(20–142)

By default, Equation 20–138 is used to compute the / split factor, however, you can define your own split factor in the user-defined function called h2_co_split_func() (see User-Accessible Functions for the Solid Oxide Fuel Cell With Unresolved Electrolyte Model).

Since electrolysis is a reversed fuel cell process, the SOFC module can also be used to model the electrolysis. In this case, power is supplied to an electrolyzer to convert water vapor into hydrogen and oxygen. Water vapor is fed through the anode electrodes to the active electrolyte region, and the following electrochemical reactions take place:

(20–143)

(20–144)

Note that, conventionally, the negative voltage is supplied to the cathode side in power consuming devices such as electrolyzers. Ansys Fluent adopts the inverse notation where the negative voltage is supplied to the anode side, while cathode remains positively charged. The main reason for this discrepancy is that the same infrastructure is used for both the electrolysis and fuel cells models. Usage of the terms "anode" and "cathode" in this manual and in the GUI should be interpreted according to conventions for power-supplying devices.

In electrolysis, the activation overpotentials have the opposite sign of what is used in fuel cells. This means that the cell voltage is higher than the open circuit voltage, since power is added to overcome the activation overpotentials. The ionic conductivity in the electrolyte is a function of temperature, as in the case of SOFC. The molar reaction rates for hydrogen, oxygen and water vapor are computed based upon the current density :