Hydrogen fuel cell vehicles use liquid hydrogen that is converted onboard to generate electricity. This is then used to power electric motors, which drive the wheels.
The conversion process turns the hydrogen into water, which is emitted from the tailpipe.
Fuel cell vehicles (FCVs) contain a hydrogen storage tank - usually behind the rear seats.
Hydrogen filling stations are still relatively uncommon, although Japan, South Korea, the United States and western Europe have some in operation and more planned.
The hydrogen is stored as a highly compressed liquid, which is why the on-board storage tank tends to be bulky.
The hydrogen is then fed to the positive terminal in the fuel cell stack. Oxygen from the air is fed to the negative terminal.
The hydrogen splits into protons and electrons and it’s the movement of the latter that sends power to the electric motors. The protons and electrons recombine at the negative terminal to produce water.
Most of the electricity required for propulsion is supplied by the fuel cell, although there’s a high-output battery that stores electricity generated from regenerative braking.
This can supplement the power from the fuel cell, and a power control unit governs the flow of electricity.
Currently, there aren’t many FCVs available to the general public, although recent models include the Toyota Mirai and Hyundai ix35 Fuel Cell. Both have a range of around 300 miles.
Although FCVs emit only water, hydrogen can’t simply be dug up out of the ground or scooped out of the air with big nets.
Most of it is converted from natural gas, oil or coal, and the conversion process generates carbon dioxide as a byproduct; up to 12 tons of CO2 are produced for every ton of hydrogen, which makes it somewhat controversial.
Hydrogen can be generated using green energy, but this is not widely done.