Reducing the Activation Overvoltage

We have seen that the exchange current density o is the crucial factor in reducing the activation overvoltage. A crucial factor in improving fuel cell performance is, therefore, to increase the value of io, especially at the cathode. This can be done in the following ways  

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The data summarized in this paper have established that the oxide pyrochlores under discussion substantially reduce the activation energy overvoltages associated with oxygen electrocatalysis. Specifically, it is found that these catalysts, in aqueous alkaline media near ambient temperature, are superior to any other oxygen evolution catalyst and are equal in performance to the best known oxygen reduction catalysts. As bidirectional oxygen electrocatalysts, they appear to be unmatched. 

There are methods for measuring ohmic separately conc can usually be reduced to a negligible value by vigorous stirring. Thus from the value of rj can be found as a function of the current density. This rj, which is related to the rate constant of the reaction, is often called the activation overvoltage. 

The activation overvoltage reduces because of better use of catalyst sites, as noted in Section 3.4.3. 

Till now, we have been familiar with the concept that the activation overvoltage on the cathode is reduced upon increase in pressure and temperature. Upon increasing the pressure, the change in OCV is given by the second term of Eq. (2.74), i.e.. 

Because the potential difference between point (A) and (D) is 0.19 volt, the equilibrium concentration of HFe02 on the cathode should be about 10 5 mol/1. This means that the iron cathode with a hydrogen overvoltage of about 0.2 volt is in an immunity state. If the iron cathode is not polarized, the equilibrium concentration of HFe02 on the iron is about 10 2 mol/1 as determined at point (B), and the iron therefore corrodes. With an activated cathode of a small hydrogen overvoltage such as 0.12 volt, which corresponds to the difference between the potentials at (A) and (E), it becomes necessary to reduce the HFe02 concentration to less than 10 mol/1. 

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