Effect of Concentration on Activation Loss

As we have discussed in Chapter 4, electrochemical reaction kinetics is given by the Butler-Volmer equation (Equation 5.76) 

At higher current density where mass transfer loss is predominant, the second term that represents the reduction reaction or product becomes insignificant and the equation can be simplified by dropping this term as 

Equation 6.141 represents the activation overpotential on the basis of the reactant gas concentration in bulk gas flow. The mass transfer loss can be estimated on the basis of the changes in activation overpotential owing to the variation in reactant gas concentration from the bulk flow to the reaction surface as follows  

Equation 6.143 represents the mass transfer loss owing to the activation reaction. 

Considering that the mass transfer loss is dominated by the oxygen concentration in cathode site, the limiting current density is estimated based on Equation 6.131a as 

---