Ammonia economy

We’re now used to the phrase ‘the hydrogen economy’ being bandied around. According to Tue Johannessen, from the department of chemical engineering at the Technical University of Denmark, the hydrogen economy might not turn out to be based on hydrogen at all.

Because of the difficulties of handling the gas, he says, it might be better to use ammonia as a base.

Hydrogen is still at the root of Johannessen’s research, because of its use in fuel cells to generate electricity. However, the problems of storing hydrogen are acute. He says that complex hydrides, based on alanates or borates, are the most promising storage media, but so far most have had too low a hydrogen denisty, too slow kinetics, or not enough reversibility to be of practical use. Moreover, all hydrides react with oxygen and water, making them difficult to handle.

Ammonia, meanwhile, can be easily converted into hydrogen through a well-known reaction — the reverse of its synthesis — at temperatures of around 650K. Once again, however, storage is a problem. ‘It is not desirable to drive around with liquid ammonia fuel in a pressurised container,’ Johannessen says. But here, there is a ready answer to the storage problem — solid metal ammine complexes.

The Danish team is looking at magnesium chloride (MgCl2) as a storage medium. Its ammine, Mg(NH3)Cl2, contains 9.1% hydrogen by weight in the form of ammonia, and has a low ammonia pressure at room temperature, meaning that it can be handled safely at room temperature, unlike any hydride.

Moreover, the ammonia storage is readily reversible. By combining the system with an ammonia decomposition catalyst, hydrogen can be delivered below 620K.

Johannessen and his colleagues believe that the system can be used as a practical fuel store for a proton exchange membrane (PEM) fuel cell.