Now UK scientists have made a material that stores more hydrogen while researchers in China have found a way to release the gas more quickly. 

Martin Schröder at the University of Nottingham, UK, and colleagues, have made a porous solid for hydrogen storage with significantly increased hydrogen capacity. Ping Wang and colleagues at the Chinese Academy of Sciences, Shenyang, have discovered that hydrogen release from ammonia borane, a material with high hydrogen storage capability, can be accelerated by mechanical milling with magnesium hydride.

'Hydrogen represents an important potential energy source with zero carbon emissions at the point of use,' explains Schröder. The main barrier to its use as a vehicle fuel is the enormous storage volumes needed when it is carried in its molecular form, so how to increase capacity in any storage material is a key issue.

Schröder's solid is a copper(II)-based metal-organic polymer made up of three polyhedral cages that fit together to provide a hollow framework. The polymer can take up 10 wt% hydrogen at 77 bar and 77 Kelvin. 'This uptake is amongst the highest to date for this class of porous material and is a major contribution to the 2010 target of 6.5 wt% for a whole storage system set by the US Department of Energy,' says Schröder.

 

The cage arrangement promotes hydrogen adsorption at high and low pressures maximizing the obtainable storage capacity

 

 

Another key requirement for hydrogen storage systems is fast hydrogen charge and discharge rates to meet consumer expectations for refueling. Wang worked with ammonia borane, which has exceptional hydrogen storage capacity but a slow release rate. His milling technique speeds up hydrogen release. More than 8 wt% hydrogen can be released within four hours at 100°C, the lowest temperature obtained in any hydride system tested so far, says Wang. Low temperatures are important for controlling hydrogen release and spent fuel regeneration. 

'Promoting hydrogen release by mechanically milling solid ammonia borane is not new,' explains Wang, 'but our studies show a completely different chemical activation mechanism that doesn't take place via alkali metal amidoboranes.' According to Wang, hydrogen is released through a destabilising solid phase reaction between the hydridic H^- in magnesium hydride and the protonic H⁺ in ammonia borane.  

Schröder's structure is notable and its properties could help guide the search for new systems, says Matthew Rosseinsky, an expert on materials for energy storage and generation at the University of Liverpool, UK. 

Schröder says that the next challenge is to increase the strength of hydrogen binding within his material to enable storage at the higher, more ambient temperatures needed for automobile-based applications. For Wang, understanding how magnesium hydride destabilises ammonia borane is key to designing systems with better capacity and kinetic performance.

Janet Crombie