Hydrogen can be used to power vehicles in two ways. It can be burned using a hydrogen internal combustion engine (sometimes called an H2ICE), which is basically a modified gasoline engine. It can also be used in a fuel cell.

Unlike a traditional car engine, a fuel cell does not burn fuel and has few moving parts. Inside the fuel cell, a chemical reaction takes place that converts the chemical energy of a fuel into electricity. Fuel cells are a little bit like batteries, except that you never have to charge them. To keep them operating, all you do is provide a constant supply of fuel. For automobiles, that fuel is usually hydrogen.

Under Pressure

The main advantage of fuel cells is that they are more efficient than combustion engines and gas-electric hybrid powertrains. A recent National Academies of Science report estimated fuel cell vehicles to have 2.4 times the fuel efficiency of conventional gasoline vehicles. Using this multiplier, a vehicle like the Toyota Camry would achieve the equivalent of 65 miles per gallon, far above the 39 mpg attained by the current gasoline-hybrid version.

But fuel cells have their drawbacks. The new technology is extremely expensive, and significant questions remain about fuel cells’ reliability, durability, and performance under various conditions. In addition, the storage of hydrogen in the vehicle presents a major technical challenge. Liquid fuels like gasoline pack a lot of energy into a small amount of space. Hydrogen, in contrast, is a gas, and gas takes up a lot of room. For example, to get the same amount of energy from hydrogen than is found in a gallon of gasoline, you’d need to fill a small room with hydrogen. Unless you have a really big car—or a freeway with living room-sized lanes—this is a problem.

One solution is to pressurize the hydrogen and store it in a smaller space, just like the air in scuba tanks that divers use. But even when hydrogen is stored at high pressures, it still doesn’t yield much energy. Many fuel cell vehicle prototypes store hydrogen 5000 psi (pounds per square inch), and some manufacturers are testing tank pressures as high as 10,000 psi. (This is a lot of pressure: to give you some perspective, the typical soda can is pressurized at 30-50 psi, and a scuba tank at 2500 – 4500 psi.)

Yet even when stored at 10,000 psi, hydrogen only yields about 14 percent the energy per gallon that gasoline does. Less energy in the onboard fuel means current fuel-cell prototypes can’t travel as far as today’s gasoline vehicles, even though fuel-cell vehicles use their fuel more efficiently. While the 2006 Toyota Prius has a real-world range of about 570 miles, current fuel-cell vehicles have a range of between 100 and 250 miles.

Hydrogen’s Distant Future

Another unanswered question is how buyers of fuel-cell vehicles will refuel their cars. Today, very few fueling stations exist; according to the DOE, there are only a few dozen hydrogen stations in the US. Hydrogen is hard to store onboard a vehicle, and it’s also hard to store in tanker trucks, rail cars, and other equipment traditionally used to distribute liquid fuels. So we’ll probably need to rethink our fuel distribution infrastructure in order to supply hydrogen effectively.

One advantage of hydrogen is that it can be made onsite at fueling stations or in people’s homes using electrolyzers or natural gas reformers, so in the future hydrogen may give consumers more choice in locations to fuel their vehicles. But hydrogen infrastructure will take significant amounts of time and financial investment to develop, and many buyers will not want a hydrogen-powered vehicle until the refueling network is fully established.

Hydrogen has great promise as a future motor fuel. If hydrogen is made using environmentally sound methods and used in highly efficient fuel-cell vehicles, then the fuel offers a real solution to the problems of urban smog and climate change. However, much of the technology we need to make hydrogen vehicles a reality does not exist today. Hydrogen is a future solution, and we should be careful not to focus exclusively on hydrogen at the expense of other solutions that can be implemented today.