British scientists have proposed a way of making it easier and cheaper to run hydrogen fuel-cell vehicles by filling them with ammonia.
The researchers have developed a way to cut the costs of making hydrogen from ammonia, which can be transported and stored much more easily than hydrogen in tanks similar to those already used by filling stations for liquid petroleum gas (LPG).
This method of cracking ammonia using relatively cheap sodium rather than an expensive catalyst could pave the way for fuel-cell vehicles to make their own hydrogen from a widely available chemical, rather than carrying a tank of very high-pressure hydrogen and requiring expensive new infrastructure.
Alternatively, combining a small amount of hydrogen with the rest of the ammonia would enable it to be burned in an optimised but conventional internal combustion engine (ICE), the researchers from the ISIS Neutron Source facility in Oxfordshire claim.
Several major car manufacturers are planning to launch commercial hydrogen fuel-cell vehicles in the next few years, including Toyota, Honda and General Motors. But there are major fears about the difficulty of creating a hydrogen fuel infrastructure – as well as the perceived dangers of the potentially explosive gas.
Prof Bill David, whose research team at ISIS has built a 20cm3 proof-of-concept reactor, said ammonia could be stored at much lower pressures than the 700 atmospheres typically used to store hydrogen for use in vehicles.
‘We would store the ammonia at 20 atmospheres so it would be straightforward to store it on forecourts,’ he told The Engineer. ‘If you’ve got the equivalent of LPG tanks on your forecourts you don’t need to worry about expensive hydrogen infrastructure.’
He added that an ammonia-fuelled ICE could be almost as efficient as some current engine designs. ‘The ammonia’s performance is not as good as petrol or diesel but it’s in the same ballpark. I reckon we can easily get 40 miles to the gallon with a tank of ammonia, so it has a range way beyond what you’re going to get with an electric car.’
Old idea, new solution
Current methods of producing hydrogen from ammonia rely on precious metal catalysts such as ruthenium at very high temperatures. The ISIS solution – which was actually first recorded by British chemist Arthur Titherley in 1894 – is to react the ammonia with sodium to produce sodium amide, which then breaks down into nitrogen, hydrogen and the original sodium.
Although the reaction still requires temperatures of 400°C, this is lower than those needed for a catalyst-based reaction. A constant flow of ammonia would mean only a small amount would need to be heated at any one time, making it possible for a conventionally sized car battery to start the process.
Ammonia is one of the world’s most widely produced inorganic chemicals. Conventionally it is manufactured from hydrocarbons but using biogas or hydrogen made using renewable energy would offer a low-carbon alternative.
David admitted that converting hydrogen into ammonia and then back again created inefficiencies but said it was a better way of storing the fuel than using large amounts of energy to compress it. ‘If you want to make hydrogen compact then ammonia is probably the best way,’ he said.
The first ammonia-burning vehicle was a bus created in Belgium in 1943, and several other ammonia-fuelled cars have been built in recent years, notably the adapted Toyota GT-86 sports car produced by Italian tyre firm Marangoni.
But David said the advantage of turning some of the ammonia into hydrogen was that it made it easier to combust the fuel and reduced the need for expensive catalysts.
Prof Robert Steinberger-Wilckens, chair in hydrogen and fuel cell research at the University of Birmingham, said using sodium and ammonia in place of hydrogen would increase the weight of the fuel by a factor of at least four, but that the overall weight may still be less than that of a high-pressure hydrogen tank.
‘I believe this solution could work well for small, portable applications,’ he told The Engineer via email. ‘I doubt the extra energy required for first producing ammonia and the cracking it at high temperatures will be justified for any applications requiring larger flows of hydrogen.’
The ISIS Neutron Source, which is operated by the Science and Technology Facilities Council, enabled the researchers to prove the exact nature of chemical reactions taking place as the ammonia reacted with the sodium to produce hydrogen. Their research is published today in The Journal of the American Chemical Society.