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Basic Principles of a Hydrogen Fuel Cell

The basic functioning of a hydrogen fuel cell (FC) is simple. One way of looking at it is to say that the hydrogen is burned or combusted as according to the following reaction:

                                                                       2H2 + O2 → 2H2O

However, instead of heat energy being produced, it is electrical energy that is released.

At the anode of an acid electrolyte FC, the hydrogen gas suffers ionization, liberating electrons and creating H+ ions (protons):

                                                                       2H2 → 4H+ + 4e-

Where energy is released from that reaction. In the cathode, oxygen reacts with electrons that are taken from the electrode, and protons from the electrolyte, to generate water.

                                                                 O2 + 4e- + 4H+ → 2H2O

For the reaction to continue, electrons generated at the anode have to pass through an electrical circuit to the cathode. Also, protons have to pass through the electrolyte. An acid is a fluid with free protons and works for that purpose very well. Some polymers can also contain mobile protons. Such materials are named proton exchange membranes.

By comparing the last two equations, it is clear that two hydrogen molecules will be needed for each oxygen molecule. It needs to be noted that the electrolyte must allow only protons to pass through it. Otherwise, if electrons were to pass through that layer, they would be lost and not complete their path around the circuit.

The overall reaction is similar in an alkaline electrolyte fuel cell. In this kind of FC, the OH- ions are available and mobile. In the anode, hydrogen reacts with these, liberating energy, electrons and generating water.

                                                              2H2 + 4OH- → 4H2O + 4e-

In the cathode, electrons from the electrode react with oxygen and water in the electrolyte, forming new OH- ions.

                                                              O2 + 4e- + 2H2O → 4OH-

For these reactions to proceed without a problem, the hydroxyl ions need to pass through the electrolyte and an electrical circuit needs to be present for the electrons to go from the anode side to the cathode side. When comparing the last two equations, it can be noticed that twice as much hydrogen is needed as oxygen. Also, even though water is consumed at the cathode, twice as much of it is generated at the anode.

Reference:


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