Butler-Volmer equation large anodic current

Equation (8.21) can be applied to different charge transfer processes both at the cathode and the anode by fitting different parameter values for a and I, to these processes. The approximation in Equation (8.21) for the Butler-Volmer equatiotf is much better than the Tafel approximation. The reason is that it is exact both for large cirrrents (in one of the directions) and for zero current. The equation can be used as an approximation for the Butler-Volmer equation in both current directions if a different pair of parameters values a and Iq ate used for each direction. 

Large Cathodic Current We have seen from Figure 6.7 that for the large negative values of overpotential r], the partial cathodic current density i approaches i, i i. For these conditions the Butler-Volmer equation (6.45) can be simplified. Analysis of Eq. (6.45) shows that when rj becomes more negative, the first exponential term in the equation (corresponding to the anodic partial current) decreases, whereas the second exponential term (corresponding to the cathodic partial reaction) increases. Thus, under these conditions. 

Physically, the reason for the dramatic difference between performances of cathode and anode active layers is the exchange current density ia at the anode the latter is 10 orders of magnitude higher than at the cathode [6]. Due to the large ia, the electrode potential r]a is small. The anode of PEFC, hence, operates in the linear regime, when both exponential terms in the Butler-Volmer equation can be expanded [178]. This leads to exponential variation of rja across the catalyst layer with the characteristic length (in the exponent)... 

For metals at elevated temperatures, especially liquid metals, the exchange current density as defined by the Butler-Volmer equation is extremely large. Therefore, it is believed that no or very little Zr " or Zr + ion forms at the anode. When the potential difference is not too high, the other metals in the alloy are not oxidised, because they are more noble than zirconium. Similarly, reduction of the alkali or alkali-earth chlorides in the salt does not occur either, because they are less noble than zirconium. 

The Butler-Volmer equation written in the form (44) is frequently used in corrosion. It describes the relationship between current density and potential of an electrode reaction under charge transfer control in terms of three easily measurable quantities P, and P, . For large anodic overvoltages (tl/p 1) Eq. (44)... 

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