What is a fuel-cell and how does it work?

Asked by: Steve Parent

Answer

Perhaps the simplest way to describe the process a fuel cell uses to produce electricity is to compare it to reverse electrolysis. Everyone is familiar with the classic experiment of electrolysis, where direct current is conducted to an anode and a cathode submerged in a liquid high in electrolytes (free ions as from salts for example). The effect is that the electrical energy separates the hydrogen and oxygen with the atoms of oxygen collecting at the anode and the atoms of hydrogen collecting at the cathode.

In a fuel cell, the process is somewhat reversed. Gaseous hydrogen and oxygen flow separately around either side of two electrolytic plates serving as anodes and cathodes. These are separated by a thin polymer membrane which serves as a filter. As free hydrogen and oxygen atoms collect on the plates, they are chemically attracted to each other, but the polymer membrane prevents all but the small hydrogen protons from passing through. The potential created by shearing off the hydrogen proton from its electron is usable electricity and is carried around the membrane in an external circuit. The byproducts of the fuel cell are heat (from the proton electron separation), and water.

It sounds like the greatest thing since sliced bread except there are present limitations: A single fuel cell produces just a fraction of a volt, so many fuel cells must be stacked together to produce the desired amount of electricity; The output of the fuel cell is directly proportional to the purity of the hydrogen. Since it requires more energy to extract pure hydrogen than is produced by the fuel cell, they are impractical as an energy source for most applications; Alternative hydrogen fuel sources for fuel cells, such as methanol, natural gas, or petroleum as more efficient when burned in traditional electrical production methods; Finally, the membrane filter is very expensive because it is platinum covered. The platinum coating acts as a catalyst to induce the disassociation of the hydrogen protons from their electrons to facilitate the electrical potential.
Answered by: Stephen Portz, M.A., Technology Teacher, Space Coast Middle School

A fuel-cell generates electricity by combining hydrogen with oxygen. It is exactly the opposite of electrolysis. Hydrogen is fed to one electrode and oxygen to the other. The proton from the hydrogen migrates through the electrolyte to the oxygen. The electron from the hydrogen goes through the electrical circuit to the cathode, or oxygen side with the proton, to make water.

There are several different fuel-cell constructions. A proton-exchange-membrane (PEM), Phosphoric Acid , molten-carbonate to name a few. It isn't necessary to have pure hydrogen or oxygen. Fuels that have hydrogen can be used. Such as gasoline, natural gas, methanol, etc... The fuel can either have the hydrogen removed to feed into the cell (reformed) or fed directly, depending on the type of cell construction.

Fuel cells have been around for a long time. The first fuel cell was built in 1839 by Sir William Grove. Only recently have they gained attention, because they have very low emission of pollutants, and don't necessary need a hydrocarbon fuel (gasoline) to operate. In fact, you could run a fuel cell by converting the hydrogen back and forth from water to gas. Run the cell when you need it, and plug it into electricity to split the hydrogen back to a gas from water. Applications include the space shuttle, stationary power plants, car and bus power supplies, and even power for hand held devices like cell phone.
Answered by: Scott Grasmick, B.A., health physicist