Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Teaching & Learning Packages Fuel Cells The principle
Fuel Cells AimsBefore you startIntroductionThe principleHistory of the technologyTypes of fuel cellsSolid oxide fuel cells (SOFCs)• Electrolyte• Electrode materials• Interconnection• Thermodynamics• System and outlookMolten carbonate fuel cells (MCFCs)• Electrolyte• Electrochemistry• ElectrodesProton exchange membrane fuel cells (PEMFCs)• Membrane• Electrodes and membrane-electrode assembly• Efficiency and reaction conditionsFuelling requirementsCase study: Fuelling practicalitiesSummaryQuestionsGoing further
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The principle

Oxygen and hydrogen, when mixed together in the presence of enough activation energy have a natural tendency to react and form water, because the Gibbs free energy of H2O is smaller than that of the sum of H2 and ½O2 added together (Hence, we don’t smoke our pipes on Zeppelins!). If hydrogen and oxygen were combined directly, we would see combustion:

H2 + ½O2 → H2O

Combustion involves the direct reaction of H2 gas with O2. The hydrogen donates electrons to the oxygen . We say that the oxygen has been reduced and the fuel oxidised. This combustion reaction releases heat energy.

The fuel cell separates hydrogen and oxygen with a gas-impermeable electrolyte through which only ions (e.g. H+, O2-, CO32–) can migrate. Hence two half reactions occur at the two electrodes. The type of reactions at the electrodes is determined by the type of electrolyte.

Grove’s fuel cell is one of the simplest examples.

The half-reaction at the anode:
H2 → 2H+ + 2e–

The half-reaction at the cathode:
O2 + 4e–+ 4H+ → 2H2O

The net reaction is the combustion reaction:
H2 + ½O2 → H2O


Activation polarization is caused by the energy intensive activity of the making and breaking of chemical bonds: At the anode, the hydrogen molecules enter the reaction sites so that they are broken into ions and electrons. The resulting ions form bonds with the catalyst atoms and the electrons remain in the vicinity until new hydrogen molecules start bonding with the catalyst, breaking the bond between the earlier ion. The electrons migrate through the bipolar plate if the bonding energy of the ion is low enough and the ions diffuse through the electrolyte. A similar process occurs at the cathode: Oxygen molecules are broken up and react with the electrons from the anode and the protons that diffused through the electrolyte to form water. Water is then ejected as a waste product and the fuel cell runs (can supply a current), as long as fuel and oxygen is provided.

The exact reactions at the electrodes depend upon which species can be transported across the electrolyte. Fuel cells are classified according to the type of electrolyte (see Types of Fuel Cells). The most common electrolytes are permeable for protons and the reactions are as discussed above. The second most common electrolytes, found in solid oxide fuel cells (SOFCs), are permeable for oxide ions and the following half-reactions occur:

The half-reaction at the anode:
H2 + O2– → H2O + 2e–

The half-reaction at the cathode:
O2 + 4e– → 2O2–

The net reaction is the same as before:
H2 + ½O2 → H2O

 

A third type of electrolyte, used for molten carbonate fuel cells at high temperatures conducts carbide ions (CO32–):

The half-reaction at the anode:
H2 + CO32– → H2O + CO2 + 2e–

The half-reaction at the cathode:
½O2 + CO2 + 2e– → CO32–

The net reaction is the combustion reaction:
H2 + ½O2 → H2O

 

We also commonly see alkaline electrolytes, across which OH– is the transported species. In this case the half-reactions would be:

The half-reaction at the anode:
H2 + 2OH– → 2H2O + 2e–

The half-reaction at the cathode:
O2 + 4e– + 2H2O → 4OH–

The net reaction is the combustion reaction:
H2 + ½O2 → H2O

 

 Building a simple fuel cell

 

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