How do fuel cells work?

May 26, 2008 Off By Chris Hinton

A while back I wrote about fuel cell cars that run on hydrogen rather than ye olde fossil fuels. But I wanted to know how these fuel cells actually work? The hydrogen, presumably, isn’t being “burned” in the normal sense, so what’s going on?
Let’s take a look at that most common of electrical sources, the battery.  Batteries usually consist of two metal “poles”, with an acid or salt solution sandwiched between them. The chemical reaction of these components makes electrons collect on the negative (-) terminal of the battery and, when an electrical item is attached to the battery, these electrons are used for power. At the same time, though, the chemical reaction within the battery is continuing, which eventually reduces the difference in charge between the positive (cathode) and negative (anode) poles… and the battery stops producing power.
How about a fuel cell? There are a number of different types of fuel cell, but we’ll look at hydrogen cells here. Fuel cells still work by transferring electrons, but the source of those electrons is different: the electrons are stripped from the hydrogen fuel itself. The fuel cell consists of two catalyst-coated electrodes, separated by a membrane which only allows charged particles to pass through. The two electrodes are also connected to an electrical load, such as a car’s motor. Hydrogen is fed into the cell at one end, where the catalyst prompts the Hydrogen to become positively charged H+. The dropped electron from the Hydrogen is picked up by the electrode (which becomes the negative anode), and used to power the load (i.e. the car). Meanwhile, the positively charged H+ passes through the membrane to the other electrode (the cathode), where it is combined with oxygen from the air, and the returning electrons, to become water.

Fuel Cell DiagramFuel-cell flow diagram – click on the image to enlarge

I did wonder what happens when the catalysts are used up, but then I remembered that catalysts, while playing a part in aiding chemical reactions, are not actually used up in them. So the catalysts just sit there, merrily helping to convert the hydrogen to H+, and then into water.
What amazes me about this setup is how none of the components of the fuel cell are used up in the reaction – as long new fuel is fed in, the cell will never go flat!
This type of fuel cell produces a small amount of Direct Current, so several fuel cells are stacked together to get enough power to run something like a car. It obviously works, with the Honda FCX boasting a generating capacity of 100kw.
So the technology works, and it actually seems very elegant. It is much more efficient than combustion, and much better for the environment. Hydrogen is the most abundant element in the universe, and even if we did somehow run out, we could always find a way to extract more from water or hydrocarbons. Not bad, really.