Electrolyzers

Last updated: November 9, 2009.

For the last 150 years or so, virtually every car ever made has run on a liquid we rather confusingly call gas. But in the next 150 years, many people think cars will run on a real gas: hydrogen. It sounds like a great idea, but there's hardly any hydrogen in Earth's atmosphere. So if we want large quantities to power the world's cars, trucks, buses, and motorcycles, we'll need to make it ourselves with electrolyzers. What are they and how do they work? Let's take a closer look!

Photo: A hydrogen fuel pump at a filling station in Sacramento, California. Solar panels (far left, top) make the electricity needed to power an electrolyzer, which produces hydrogen from water. You can then pump the hydrogen into a tank in your car. Photo by Keith Wipke courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

What does an electrolyzer do?

In theory, running cars off hydrogen is a great idea: it's the simplest and most common chemical element and it makes up the vast majority (something like three quarters) of the entire matter in the Universe. Plenty for everyone, then! But there's a snag: poke about in the air around you and you won't find much hydrogen at all—only about one liter of hydrogen in every million liters of air. (In volume terms, that's the same as hunting down about two liters of water randomly distrbuted mixed up in every Olympic swimming pool full). So where will all the vast clouds of hydrogen come from to run our global car fleet? Water, the magic substance that covers 70 percent of Earth's surface, is made partly from hydrogen. Split good old H2O into its parts and you get H2 (hydrogen) and O2 (oxygen). How do you do it? With an electrolyzer!

How does an electrolyzer work?

An electrolyzer is a piece of electrochemical apparatus (something that uses electricity and chemistry at the same time) designed to perform electrolysis: splitting a solution into the atoms from which it's made by passing electricity through it. Electrolysis was pioneered in the 18th century by British chemist Sir Humphry Davy (1778–1829), who used a primitive battery called a Voltaic pile to discover a number of chemical elements including sodium and potassium.

An electrolyzer is a bit like a battery working in reverse:

• In a battery, you have chemicals packed into a sealed container with two electrical terminals dipping into them. When you connect the terminals into a circuit, the chemicals undergo reactions inside the container and produce electricity that flows through the circuit. (Read more about this in our main article on batteries.)
• In an electrolyzer, you place a solution in a container and dip two terminals into it. You connect the terminals up to a battery or other power supply and pass electricity through the solution. Chemical reactions take place and the solution splits up into its atoms. If the solution you use is pure water (H2O), you find it quickly splitting up into hydrogen gas (at the negative electrode) and oxygen gas (at the positive electrode). It's relatively easy to collect and store these gases for use in future.

Photo: Demonstrating hydrogen power. Light (from the Sun) hits a solar cell (left), making electricity. An electrolyzer uses the electrical energy to split water into oyxgen and hydrogen ( collected in the test tubes in the middle of the picture). The hydrogen is then fed into a fuel cell (metal box on the right), which produces electricity and lights a lamp (right). Photos by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

The trouble with hydrogen cars

It's easy to see how a world full of hydrogen cars might work. We'd have lots of electrolyzer factories all over the place making hydrogen gas from water. Once made, we'd need to compress and transport the hydrogen to gas stations where people could pump it into their cars, which would be powered by hydrogen fuel cells instead of conventional gasoline engines.

But do you see the problem? Producing hydrogen fuel by electrolysis uses energy—and quite a lot of it: we have to use electricity to split up water. We also use energy transporting hydrogen and compressing it (turning hydrogen gas into a liquid) so cars can carry enough of it in their tanks to go anywhere. That's a real problem because the energy density of hydrogen (the amount of energy it carries per unit of its volume or mass) is only about a fifth that of gasoline, so you need five times more to go as far (assuming your hydrogen car is heavy as your gasoline one, which may not be the case). Another problem is that hydrogen is difficult to store for long periods because its extremely tiny molecules easily leak out of most containers—and since hydrogen is flammable, leaks can cause horrific explosions.

All told, today's hydrogen cars are considerably less efficient than the best electric cars running off batteries and often less efficient than ordinary gasoline or diesel engine vehicles! Some people think the answer is to use solar panels to do the electrolysis of water "for free," like we show in our top picture. But we could just as easily store the same energy in batteries and use those to power our cars instead. Fuel-cell cars sound promising, but if battery cars really are better, hydrogen may turn out to be an expensive distraction from the important business of switching the world from fossil fuels to renewable energy.

Photo: Electric cars, powered by batteries, are generally more efficient than hydrogen fuel-cell cars. Picture by Mike Linenberger courtesy of US National Renewable Energy Laboratory/Department of Energy (DoE/NREL).

Further reading

• Hydrogen economy: Wikipedia's comprehensive introduction to the pros, cons, and practical issues involved in using hydrogen as an alternative to fossil fuels.
• Better transport: David McKay considers the physics behind different forms of "green" transport and concludes "...hydrogen is a hyped-up bandwagon. . I'll be delighted to be proved wrong, but I don't see how hydrogen is going to help us with our energy problems." This is a chapter from his superb book Sustainable Energy—Without the hot air.