Oxygen vs Air in PEM Fuel Cells

Most of the time, the designer doesn't have the choice between air and oxygen when assembling a PEM fuel cell. Oxygen is employed in systems that don't depend on air to operate, such as submarine and spacecraft (when that is not the case, air is used). However, the performance of a PEM fuel cell is greatly improved when oxygen is used. That happens because of three factors:

The increase in partial pressure of oxygen makes the 'no loss' open circuit voltage rise, as appointed by the Equation of Nernst (https://en.wikipedia.org/wiki/Nernst_equation).
Use of better catalyst sites reduces the activation over-voltage.
The mass transport or concentration over-voltage losses are reduced by the increase in the limiting current, which is an event caused by the absence of nitrogen(a gas that contributes for this kind of loss at high current densities.

Some results have showed that the change from air to oxygen in a PEMFC can increase performance by as much as 30%.

Reference:

LARMINIE, James; DICKS, Andrew. Fuel Cell Systems Explained. 2. ed. West Sussex, England: Wiley & Sons Ltd., 2003. 418 p.

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