Light years ahead

Many existing hydrogen-powered cars need a heavy tank the size of a fridge to store their fuel, hampering their range and efficiency.

Future vehicles could be lighter and have a longer range thanks to world-leading pulsed neutron and muon source ISIS, based at the Rutherford Appleton laboratory near Oxford. Scientists there are using the facility to examine potential lightweight hydrogen storage materials at the atomic level.

One of them, Dr Martyn Bull, explained: 'Not only are current tanks heavy, requiring more of the stored energy to move the vehicle, it also takes a massive amount of energy to compress the hydrogen. So as a green energy technology, while it gives zero emissions from the exhaust, there is a big penalty further back down the supply chain.'

The alternative is to use lightweight solids that can hold hydrogen atoms in nanoscale pores.

According to Bull, a target material should be light and stable at temperatures of 0-100ºC at near atmospheric pressure. It must be able to absorb hydrogen quickly so it does not take a long time to fill, yet release it quickly to power the vehicle. Equally, it must not leak and needs to fulfil manufacturers' stringent safety requirements.

ISIS has a source of neutrons which it sends into the prototype material being investigated. 'We can then detect which way out it comes and measure whether it's lost or gained any energy,' said Bull. 'From that information we can work out the structure of it on the atomic scale and examine how the hydrogen is being taken up, how quickly it's stored and how quickly it can be released.

'When something absorbs hydrogen it expands slightly. We can see where it's going, how strongly it's bound there, and how it responds when we change the temperature or pressure. From that data you can then determine what's going on inside and whether further investigation is worthwhile.'

Prof Bill David, ISIS senior fellow and associate director, is leading a rapid prototyping project to study storage materials. His team starts by selecting an alloy of a lightweight metal element such as lithium, boron or magnesium that will hold hydrogen. It then makes hundreds of small samples and tests them under a variety of pressures before scaling up to a sample that can be tested functionally.

The team has recently published results of successful tests on the lithium-based material Li4BH4(NH2)3. If it were used in a car, the research estimates it could travel about 300 miles, compared with the maximum 200 miles of typical existing hydrogen vehicles.

A parallel project examined the potential of zeolite, a material that is highly porous at the nanoscale, allowing the storage of parallel layers of hydrogen.

'Materials like zeolite and the lithium material have an enormous surface area inside. If you were to take a sample the size of a sugar cube and you were to unwrap it and look at its surface area, it would end up about the size of a football pitch,' said Bull.

'At this scale, hydrogen is held with a chemical and physical bond. In the first layer, the hydrogen is held to the storage material by electrostatic force. The bond between the hydrogen and the storage material is stronger than that between the two hydrogen atoms. But you don't want a bond so strong the hydrogen doesn't want to become unstuck. Once that initial layer is in place, other hydrogen atoms fit in using hydrogen bonding.'

With current concerns about the environmental impact of biofuels and a UN report that shows they are better used for heat and power rather than for transport, the focus has returned to hydrogen to power the vehicles of the near future.

According to Bull, the main challenge to a future hydrogen economy is the supply chain.

'To make it truly zero emissions, the hydrogen must be produced from water or methane using solar power,' he said. 'But if you can prove you can viably store enough hydrogen to make the vehicle worthwhile, the rest will follow. After all, when Henry Ford was building his cars, there weren't that many filling stations.'

Photo credit: Pete Skinner