Scientists have improved the performance of ammonia borane as a hydrogen storage material - making it more practical for a fuel tank in hydrogen-powered vehicles. The material was enhanced by the addition of catalytic nanoparticles to the structure, allowing it to release hydrogen more cleanly and at lower temperatures. 

Finding ways to store hydrogen to run next-generation fuel cell vehicles is a challenge, since traditional metal canisters filled with compressed or liquefied hydrogen gas are heavy, bulky and expensive. A better solution is to use a solid material, and the most promising candidate for this is ammonia borane (NH₃BH₃) - a waxy solid consisting largely of hydrogen. 

However, there are drawbacks to using this material. Releasing the hydrogen can be tricky, usually requiring heating at over 100°C, which is too hot for polymer-based fuel cells to operate. In addition, the material is prone to become unstable - expanding rapidly or turning into foam - and released hydrogen can be poisoned by other gases released from the heated material. 

Now, Ping Chen and colleagues at the Dalian Institute of Chemical Physics, in Dalian, China, have modified the structure of ammonia borane to eliminate these problems. 'By introducing nanoparticles of cobalt and nickel catalysts into the structure we can hold nearly 6 per cent by weight of hydrogen at a temperature as low as 59°C - with no byproduct and sample foaming,' Chen told Chemistry World. 

Better breakdown

The team worked in collaboration with researchers at the National University of Singaporeand used a 'co-precipitation' method to uniformly distribute a small amount of catalytic nanoparticles throughout the ammonia borane structure. The new material releases hydrogen at the lowest temperature so far - and the high stability makes it an attractive candidate for further investigation as a practical onboard hydrogen storage material. 

'This system lowers the temperature for hydrogen gas release and therefore has potential as an efficient way to supply hydrogen to a fuel cell,' says Tom Autrey at Pacific Northwest National Laboratory in Washington, US, who led the first investigations into the storage potentials of ammonia borane. 'This process also reduces the concentration of borazine - an unwanted impurity in the hydrogen that can arise from AB decomposition.' 

But there is still work to be done, Autrey notes, before this technology is truly able to blossom. Since the process is not yet easily reversible, a system will be needed that allows the fuel tank to easily be recharged with hydrogen.

Lewis Brindley

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