New routes to hydrogen storage materials, which could offer alternative fuel for cars, have been developed by two teams of scientists in the US and Singapore.

Hydrogen is an important energy source as it reacts with oxygen to release energy with the only by-product being water. However, at atmospheric pressure it is gaseous, and therefore needs to be stored at high pressure to reduce the storage volume.  By using a solid material with a high hydrogen content, the volume required for hydrogen storage is considerably reduced, and the need for high pressure eliminated.

 

Ammonia borane could provide a source of hydrogen for fuelling the cars of the future

 

Ammonia borane (NH₃BH₃) has a high hydrogen content and is stable at room temperature, but has, in the past, proven difficult to prepare in high yield. Now, Tom Autrey and co-workers at the Pacific Northwest National Laboratory, Richland, US, have developed a new one-pot synthetic method to this solid material.¹

Autrey's method requires in situ production of ammonium borohydride (NH₄BH₄) by the addition of NH₄X and MBH₄ salts (X = Cl, F, M = Na, Li,) in liquid ammonia, followed by removal of the majority of the ammonia, then addition of tetrahydrofuran (THF) which causes the NH₄BH₄ to decompose to ammonia borane in high yield.

As Autrey explains, 'to be a viable hydrogen storage material economic routes to synthesis and regeneration are of the utmost importance'. At the moment hydrogen release from ammonia borane is not reversible, therefore Autrey says the 'long-term challenge is to regenerate ammonia borane from the spent storage material'.

Another problem with ammonia borane is that its decomposition leads to the production of the volatile compound borazine as a by-product. Borazine can poison proton exchange membrane fuel cells. This issue has been addressed by another team, led by Ping Chen at the National University of Singapore.²

Chen proposes the use of sodium aminoborane (NaNH₂BH₃) as an alternative to ammonia borane as it does not release borazine on decomposition.  Traditionally sodium aminoborane is made using a mechano-synthetic route which requires additives to aid milling. But these additives cause a reduction in the hydrogen density of the product.

Chen's wet-chemical method allows pure sodium aminoborane to be made. He proposes two routes, the faster of which involves adding sodium hydride (NaH) to a solution of ammonia borane (NH₃BH₃) in THF. The reaction occurs within 10 minutes at -3 °C, giving solid sodium aminoborane which can be filtered off.

Zhitao Xiong, a member of Chen's team, says the most important aspect of this work is that 'it opened the road to a new class of materials comprising alkali or alkaline earth metal cation and [NH₂BH₃]^- anion for storing hydrogen'.

Todd Marder at Durham University, UK, welcomes both teams' research, saying 'the study of materials which can store and release, under mild conditions, a significant percentage of their weight as hydrogen, is certainly one which is of considerable importance. However, given that the hydrogen release is not reversible, a major effort to develop economical methods for regeneration of ammonia borane is required if such materials are to result in commercially viable technologies for common use as hydrogen sources for automobiles.'

Vikki Chapman