Summary of Activation Overvoltage

In low- and medinm-temperature fuel cells, activation overvoltage is the most important irreversibility and cause of voltage drop, and occurs mainly at the cathode. Activation overvoltage at both electrodes is important in cells using fuels other than hydrogen, such as methanol. At higher temperatures and pressures the activation overvoltage becomes less important. 

Whether the voltage drop is significant at both electrodes or just the cathode, the size of the voltage drop is related to the current density i by the equation 

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