One kilogram of hydrogen yields 120 megajoules – that’s nearly three times as much energy as the same mass of petrol. By volume, though, petrol wins: a litre of petrol can provide 32 megajoules while a litre of liquid hydrogen yields just eight megajoules. Storing huge volumes of hydrogen for fuel cells isn’t so much a problem for stationary power generators or even large vehicles. But for standard cars, where space is at a premium, carting around a giant tank of hydrogen is simply not an option.
How hydrogen is stored, too, is important from a safety perspective. Compressed hydrogen gas can be explosive in a car accident. So researchers are finding ways to store hydrogen at high density – to yield the most power per tank – but not high pressure, so it’s safer to cart around.
Promising candidates come in the form of metallicorganic frameworks, or MOFS. MOFS are materials that look a bit like molecular scaffolds, with metal atoms joined up with organic molecules.
Gaps in the crystal pattern can snare other molecules, such as carbon dioxide, oil or hydrogen.
With limitless number of combinations of metals and organic molecules, MOFS can be customised to do just about anything – from capturing carbon dioxide from coal-fired power plant emissions to cleaning oil from water – and, of course, storing hydrogen.
When hydrogen molecules are trapped by some MOFS, they seem to calm down. An ideal MOF packs loads of hydrogen molecules in – boosting density – while stopping them from jiggling around too much, which eases pressure.