4.5 Setting Up the Electrochemistry Parameters

You can specify electrochemistry settings for the SOFC With Unresolved Electrolyte Model using the Electrochemistry tab in the SOFC Model dialog.

Figure 4.5.1: The Electrochemistry Tab in the SOFC Model Dialog

In the Electrochemistry tab, you can set the anode and cathode exchange current density, the anode and cathode mole fraction reference values, the concentration exponents, the Butler-Volmer transfer coefficients, and the temperature-dependent exchange current density.

You can specify a value for the exchange current density at the anode (the default value is 1000 Amps/m $^2$) using the Anode Exchange Current Density field. Likewise, you can specify a value for the exchange current density at the cathode (the default value is 100 Amps/m $^2$) using the Cathode Exchange Current Density field.

You can also specify Mole Fraction Reference Values for the fuel cell reactants in the Electrochemistry tab. By default, the reference value for $H_{\rm 2}$ is 0.8, the reference value for $O_{\rm 2}$ is 0.21, and the reference value for $H_{\rm 2}O$ is 0.2.

The Butler-Volmer transfer coefficients can be set in the Electrochemistry tab as well. These coefficients are the $\alpha_{\rm a}$ and $\alpha_{\rm c}$ from Equation  3.4-14 for both the anode and the cathode reactions.

Remember that $\alpha$ has anodic and cathodic values at both the cathode and the anode. By default, the value of $\alpha$ is set to 0.5 because of the nearly universal assumption that there is a symmetric balance between the forward and backward reactions. In most cases, these default values will be sufficient.

If you find yourself changing the Butler-Volmer transfer coefficients, or if you have some other rate-limiting reaction in your fuel cell simulation, you may also want to consider changing the exponents for the stoichiometric coefficients for the fuel cell reactants. These exponents can be specified in the Electrochemistry tab. By default, the exponent values for $H_{\rm 2}$, $O_{\rm 2}$, and $H_{\rm 2}O$ are 0.5.

The Enable Temperature Dependant I_0 option allows the exchange current density to change as a function of temperature in an exponential fashion

where you can provide the values for the constants $A$ and $B$.