Mass Transfer or Concentration Loss

Let us consider the Nernst equation (Equation 4.49) for real thermodynamic fuel cell voltage given by Equation 4.49. 

In this equation, the Nernst voltage loss term represents the voltage loss owing to the variation in reactant and product gas concentrations in the supply gas stream. If we just have only single reactant specie for one electrode side, the Nernst voltage is given as 

Concentration overpotential or loss is estimated as the change in Nernst voltage loss owing to the variation of reactant gas concentration from the bulk gas flow stream to the gas concentration at the electrode reaction surface as follows  

From Equation 6.131a for the limiting current density with mass transfer limited by diffusion resistance, the reactant concentration in the bulk gas flow in the channels is written as 

Substituting Equation 6.135 into Equation 6.128b, we have 

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The kinetic losses, ohmic losses, mass transfer losses, and short circuit and crossover losses are illustrated in Figure 5.27. The dominant losses are typically activation losses and ohmic losses. At high current density, the mass transfer or concentration losses dominate. In the illustration, the internal losses are shown as constant however, the internal losses also depend on the current density. 

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