An employee lowers a fuel cell from the orbiter Discovery onto a work stand at the Kennedy Space Center in Cape Canaveral, Fla., Jan. 5, 2006. Fuel cells make power for the orbiter by mixing hydrogen and oxygen to produce electricity. (Kim Shiflett/Associated Press/NASA)

They're nothing new, but the fuel cell has been tabbed by many as a key piece of our energy future. For the automotive industry, they could mean emissions-free vehicles for the masses.

Cars powered by hydrogen-based fuel cells have been making the rounds at car shows around the world. But they're not getting much beyond the car-show circuit yet — and won't until they're economically viable.

While showing a lot of promise, fuel cells still face some major challenges.

What is a fuel cell?

Let's start with what it's not. A fuel cell is not a battery. Batteries store power in a closed system. A fuel cell creates power through a chemical reaction. Most commonly, fuel cells combine hydrogen and oxygen to produce electricity. That reaction leaves byproducts — water and heat.

There is no combustion as in a gas-powered engine, so the process is clean, quiet and efficient. Fuel cells produce power two to three times more efficiently than in the burning of fuel.

Sir William Grove built the first fuel cell in 1838 after he discovered that by arranging two platinum electrodes with one end of each immersed in a container of sulfuric acid and the other ends separately sealed in containers of oxygen and hydrogen, a constant current would flow between the electrodes.

Fuel cells languished as a source of energy until they were pressed into service for the American space program in the 1960s. They were used to help power Gemini and Apollo rockets because they were cheaper than trying to harness solar energy and safer than nuclear power. They are used to provide electricity and water for the space shuttle.

How does it work?

Every fuel cell has two electrodes — one positive (cathode) and one negative (anode). They are sandwiched around an electrolyte, a liquid substance that conducts electricity.

Hydrogen fuel is fed into the anode and air enters the fuel cell through the cathode. With the help of a catalyst, the hydrogen atom splits into a proton and an electron. They take different paths to the cathode. The proton passes through the electrolyte. The electrons create a separate current that can be harnessed before they return to the cathode. The electrons are then reunited with the hydrogen and oxygen in a molecule of water.

The waste heat from a fuel cell can be used to provide hot water or space heating for a home or office.

What are the different kinds of fuel cells?

There are many.

NASA has used the alkaline fuel cell (AFC) on space missions since the 1960s. These cells can achieve power-generating efficiencies of up to 70 per cent. They use potassium hydroxide as the electrolyte and operate at just between 150 and 200 degrees C. They are very susceptible to carbon contamination, so require pure hydrogen and oxygen. Their platinum electrodes make these cells very expensive. Like any container filled with liquid, they can leak.

Proton Exchange Membrane (PEM) fuel cells show the most promise for use in homes and cars. They work with a polymer electrolyte in the form of a thin, permeable sheet. One of the problems is they are not yet very efficient, converting less than 50 per cent of its fuel into power. However, operating temperatures are quite low — about 80 degrees C. The solid, flexible electrolyte will not leak or crack but their fuels must be purified. The cell also uses a platinum catalyst on both sides of the membrane, which raises costs.

Phosphoric Acid fuel cells (PAFC) use phosphoric acid as the electrolyte. They are 40 to 80 per cent efficient and operate at temperatures between 150 to 200 degrees C. Existing phosphoric acid cells have outputs up to 200 kW, and 11 MW units have been tested. The cells use platinum electrode-catalysts and internal parts must be able to withstand the corrosive acid. They are currently used to supply power to office buildings, hotels, hospitals and electric utilities.

Other fuel cell types include Molten Carbonate and Sold Oxide, which operate at high temperatures and are used for industrial applications.

When will fuel-cell powered cars be commercially available?

Honda and Toyota appear to be the leaders in getting cars equipped with fuel cells to market — but only in a limited way.

Honda will begin leasing its FCX Clarity to "a limited number of retail consumers" in southern California during the summer of 2008. The cost for a three-year lease will be about $600 US a month.

Toyota received approval from the Japanese government in early June 2008 to begin producing its FCHV-adv vehicle. The Fuel-Cell Hybrid Vehicle is powered by hydrogen, and it can go 830 kilometres between fill-ups, which is twice the range of previous prototypes.

Toyota expects to make the vehicle available for leasing in Japan later in 2008. The company says the vehicle has been tested in cold weather conditions and runs in temperatures as low as –30 C.

Fuel-cell vehicles are normally offered for lease because the cost of the technology makes ownership a very expensive proposition.

The cars' only emission is water.

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