|Lithium hydride storage. Hydrogen (H) atoms are shown in green, lithium (Li) atoms in dark grey, nitrogen (N) atoms in blue and boron (B) atoms are in grey and inside the pyramids. Click to enlarge.|
UK scientists have developed a new variety of lithium hydride that shows promise as an on-board storage medium for hydrogen that could support a vehicle range of more than 300 miles. The new quaternary hydride is formed through the reaction of the two potential hydrogen storage materials, LiNH2 and LiBH4, and has an ideal stoichiometry of Li4BN3H10.
The development was achieved by a team from the Universities of Birmingham and Oxford and the Rutherford Appleton Laboratory in Oxfordshire, under the auspices of the UK Sustainable Hydrogen Energy Consortium (UK-SHEC). UK-SHEC is funded by the SUPERGEN (Sustainable Power Generation and Supply) initiative managed and led by the Engineering and Physical Sciences Research Council (EPSRC).
Reversible complex metal hydrides are one of the promising areas under exploration for hydrogen storage for mobile applications. While there are copious numbers of possible solid state compounds that could store hydrogen, none has as yet provided the optimum balance of properties that would make it the material of choice.
In an article in Chemical Reviews in 2004, Peter Edwards, leader of the research group at Oxford, detailed the characteristics of a viable hydrogen storage material:
An ideal solid hydrogen-storage material (HSM), therefore, for practical applications should, for both economic and environmental reasons, obey the five main commandments of hydrogen storage.
(i) High storage capacity: minimum 6.5 wt % abundance of hydrogen and at least 65 g/L of hydrogen available from the material.
(ii) Tdec = 60-120 °C.
(iii) Reversibility of the thermal absorption/desorption cycle: low temperature of hydrogen desorption and low pressure of hydrogen absorption (a plateau pressure of the order of a few bars at room temperature), or ease of nonthermal transformation between substrates and products of decomposition.
(iv) Low cost.
(v) Low-toxicity of a nonexplosive and possibly inert (to water and oxygen) storage medium.
...Simple atomic-mass-based calculations reveal that only the light (i.e., low atomic number) chemical elements can be strictly entertained if criterion (i) is to be met. Thus, the main backbone of any efficient HSM can only be built from targeted chemical elements from an unforgiving and tantalizingly short list: Li, Be, B, C, N, O, F,4 Na, Mg, Al, Si, and P...Due to the toxicity and/or unfavorable chemical properties of H’s connections with Be, F, Si, and P, the effective list of chemical cog-wheels constituting HSM now consists of only eight elements. Heavier ones may enter the multiple component system only as a low-abundant additive, presumably for fine-tuning of properties or as a catalyst.
|Comparison of HSM properties. Click to enlarge.|
The UK-SHEC researchers tested thousands of solid-state compounds in search of a light, cheap, readily available material which would enable the absorption/desorption process to take place rapidly and safely at typical fuel cell operating temperatures. The lithium hydride could offer the right blend of properties; further development work is needed to investigate its potential.
UK-SHEC is led by the Universities of Oxford and Bath. UK-SHEC partners are as follows: University of Bath, University of Birmingham, University of Glamorgan, Greater London Authority, University of Nottingham, University of Oxford, Queen Mary, University of London, Policy Studies Institute and University of Salford. Collaborators include: BOC Group, BP, STFC Rutherford Appleton Laboratory, Corus UK Ltd, DSTL, Johnson Matthey, Ilika Technologies Ltd, QinetiQ, Shell Global Solution UK and Tetronics Ltd.
Launched in 2003, SUPERGEN is a multidisciplinary research initiative that aims to help the UK meet its environmental emissions targets through a radical improvement in the sustainability of power generation and supply. SUPERGEN is managed and led by EPSRC in partnership with the Biotechnology and Biological Sciences Research Council (BBSRC), the Economic and Social Research Council (ESRC), the Natural Environmental Research Council (NERC) and the Carbon Trust. A total of 13 research consortia are now at work or have been announced; hydrogen energy is one.
(Hat-tips to Raj and Martin!)
“Synthesis and crystal structure of Li4BH4(NH2)3”; Philip A. Chater, William I. F. David, Simon R. Johnson, Peter P. Edward and Paul A. Anderson; Chem. Commun., 2006, 2439–2441 DOI: 10.1039/b518243c
“Thermal Decomposition of the Non-Interstitial Hydrides for the Storage and Production of Hydrogen”; Grochala, W.; Edwards, P. P.; Chem. Rev.; (Review); 2004; 104(3); 1283-1316. DOI: 10.1021/cr030691s