Electricity is essentially the flow of electrons. Fuel cells use a chemical reaction to send electrons on a round trip that forms an electric circuit. Loads connected to that circuit -- lights, motors, computers -- use the electricity to do their work.

In the most common process, constant streams of hydrogen and oxygen are combined in the fuel cell to produce electricity. The byproduct, water, is the only waste exhaust. Some fuel cell technologies also produce considerable heat.

Hydrogen molecules are pushed into the anode side of the fuel cell, while air flows into the opposite side, the cathode. Their chemical reaction produces electricity, water and heat.

What's in a fuel cell

There are five distinct layers inside a typical fuel cell. On one side, where pressurized hydrogen enters the cell, is a negatively charged electrode, called an anode. This layer is backed by a thin catalyst. On the opposite side, where pressurized air enters the cell, is a positively charged electrode, known as a cathode. This layer also is paired with a catalyst. Between the anode and the cathode is a solid electrolyte membrane.

How fuel cells work

Hydrogen fuel cells use various types of electrolyte. Most fuel cell companies today are developing products based on a proton exchange membrane (PEM) electrolyte. Here's how this technology works:

Hydrogen molecules are pushed into the anode side of the fuel cell. Each pure hydrogen molecule is made up of two hydrogen atoms. Each atom consists of a proton and an electron. When it contacts the catalyst, a hydrogen molecule breaks into two protons and two electrons. The protons have a positive charge, and the electrons have a negative charge.

Meanwhile, air flows into the opposite side of the fuel cell, the cathode. The catalyst there splits each oxygen molecule into two oxygen atoms, which have a negative charge.

Opposites attract. To reach the oppositely-charged oxygen atoms, the positively charged hydrogen protons pass through the electrolyte -- thus the name, proton exchange membrane. Each atom of oxygen pairs up with two hydrogen protons to form a water molecule. Now comes the best part.

Will work for protons

All that's missing are the electrons, which also want to travel from the anode to the cathode, but can't pass through the electrolyte membrane. This conundrum is whereby the electricity is produced.

Think of the positive charge in the cathode as a "hole" that only electrons can fill. The electrons are so strongly attracted to the cathode, they are willingly conducted out of the cell on an external wire that eventually carries them back into the cell on the opposite side.

As electrons return to the cell's cathode they neutralize the charge imbalance and fill the hole. Along their journey the electrons power whatever loads are connected to the external circuit.

Like a battery? Yes and no

Like a battery, the fuel cell has a positive terminal (the cathode) and a negative terminal (the anode). And like a battery, the electricity from a fuel cell is direct current (DC). Unlike a battery, the chemical reaction can be perpetuated. As long as there is a supply of hydrogen and oxygen the circuit remains charged with electricity.

A single fuel cell produces about three fourths of a volt, which isn't enough to do very much work. To get adequate voltage, fuel cells are stacked up and wired together. The negative terminal of one cell is connected to the positive terminal of the next, and so on. The last negative terminal and the far opposite positive terminal become the points where a load is connected.

A fuel cell's only "exhaust" is water. There usually is enough heat produced in the chemical reaction that the water exits as vapor (steam), but it soon cools and turns into hot water. The near-boiling water, along with the radiated heat from the fuel cell, can be used for other purposes, such as comfort-heating a space.

Many fuel cell technologies

Fuel cell manufacturers are developing technologies along many paths. They use various membranes, as well as different electrochemical processes. Membranes can be solid or liquid. Solid oxide fuel cells (SOFC) use a membrane that works in the opposite direction from PEM fuel cells, passing oxygen ions from the cathode through a solid membrane to the anode side of the fuel cell. Alkaline fuel cells use a membrane of liquid potassium hydroxide, the chemical found in flashlight batteries.

Many fuel cells are attached to a reformer, a device that converts a hydrocarbon fuel, such as methanol, into hydrogen for use by the fuel cell. Fuel cells also can use fuels other than hydrogen that are less complicated to produce and safer to handle.