What Limits the Current in a Fuel Cell?

Hydrogen reacts at the anode and releases energy. However, that doesn't mean that the reaction proceeds at an unlimited rate just because energy is liberated.

Activation energy must be involved so that the process can get over the "energy hill". If the probability of a molecule having sufficient energy is low, then the reaction won't proceed at normal speeds. This is the case for fuel cell reactions, with the exception of situations which involve high temperatures.

The 3 best known ways of dealing with slow reaction rates are: the use of catalysts, increasing the electrode area and raising the temperature.

The first two solutions can be employed to any chemical reaction. The third, however, is present in fuel cells only. When considering the following reaction

2H2 + 4OH- → 4H2O + 4e-

It can be noticed that fuel gas and OH- ions coming from the electrolyte need to be present alongside the activation energy. Additionally, the "meeting" between H2 and OH- ions has to happen on the surface of the electrode, since the generated electrons need to be removed.

The reaction that involves fuel/oxygen (gas) with the electrolyte (solid or liquid) and the electrode is commonly called the three phase contact.

The rate at which reactions occur depend on the the area of the electrode. Thus, these are usually porous. Additionally, to help the case, high temperatures and a catalyst are commonly employed together in the assembly and functioning of fuel cells.

Reference:

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

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