Effect of Pressure and Gas Concentration in Fuel Cells

The Gibbs free energy changes vary with temperature, pressure and gas concentration in fuel cells. Take into account the following generation reaction:

jJ + kK → mM

Where k moles of K react with j moles of J to generate m moles of M. Both the reactants and products have an associated 'activity'. We can call this 'activity' a, aj and ak for the reactants and am for the product activity. When gases behave close to ideal conditions (as is the case with fuel cells), we know that:

a=P/P0

Where P is the pressure/partial pressure of the gas and the standard pressure is P0 (around 0.1 MPa). This simple equation is useful because fuel cells are, in a general way, gas reactors. When dissolved chemicals are involved, the activity can be linked to the molarity or strength of the solution. The case of water in fuel cells is complex to deal with, but in steam form, it can be stated that the activity of water is equal to the partial pressure of water divided by the standard pressure of the steam.

These activities modify the Gibbs free energy change of reaction. When the activity of the reactants increase, the Gibbs free energy becomes more negative (more energy is released). When the activity of the products increase, the reverse effect happens to the Gibbs free energy (less energy is liberated).

Another concept to take note off is the effect on the Electromagnetic Field (EMF). When the activity of the reactants increase, so does the the EMF.

Reference:

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

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