Institut Laue-Langevin

New metal-hydride clusters provide insights into hydrogen storage

A study published in Nature Chemistry by researchers at the RIKEN Advanced Science Institute, Dalian University of Technology, University of Southern California in Los Angeles, and the ILL has shed first-ever light on a class of heterometallic molecular structures whose unique features point the way to breakthroughs in the development of lightweight fuel-cell technology. The structures contain a previously-unexplored combination of rare-earth and d-transition metals ideally suited to the compact storage of hydrogen.

The most abundant element in the universe, hydrogen, holds great promise as a source of clean, renewable energy, producing nothing but water as a by-product and thus avoiding the environmental dangers associated with existing mainstream energy sources. Broad adoption of hydrogen, however, has stalled because, in its natural gaseous state, the element simply takes up too much space to store and transport efficiently.

One way to solve this problem is to use metal hydrides, metallic compounds that incorporate hydrogen atoms, as a storage medium for hydrogen. In this technique, the metal hydrides can bind additional hydrogen atoms to produce a solid one thousand times or more smaller than the original volume of hydrogen gas. The hydrogen can then later be released from the solid by heating it to a given temperature.

The new heterometallic hydride clusters synthesized by the RIKEN researchers use rare-earth and d-transition metals as building blocks and exploit the hydrogen-binding advantages of both. Even though rare-earth metal hydrides can accommodate several hydrogen atoms around each metal atom, on their own they do not undergo reversible hydrogen addition and release, the cornerstone of hydrogen storage. This becomes possible through the addition of a d-transition metal, in this case tungsten or molybdenum.

While rare-earth / d-transition-metal hydride complexes have been studied in the past, the current research is the first to explore complexes of the form Ln₄MH_n with multiple rare-earth atoms and well-defined structures (Ln = a rare-earth metal such as yttrium, M = a d-transition metal, either tungsten or molybdenum, and H = hydrogen). In the paper, the researchers show that these complexes exhibit unique reactivity properties, pointing the way to new hydrogen-storage techniques (Figure 1) and promising environmentally-friendly solutions to today's pressing energy needs.

The RIKEN researchers were quite confident of the location of the hydrogen atoms and the mechanism from their extensive X-ray studies of the complexes before and after the addition of hydrogen, reinforced by an obvious colour change in a remarkable real-time study of the hydrogenation process in a single crystal, and by DFT calculations at Dalian University of Technology. Neutron diffraction on D19 and VIVALDI of two of the hydrogen-loaded complexes allowed more detailed discussion of the structure of the metal-hydride frameworks, thanks to the considerably greater sensitivity of neutrons to hydrogen in the neighbourhood of heavy metal atoms. For the first yttrium-tungsten complex (Figure 2a) accurate Y-H bond lengths and angles were obtained on the monochromatic D19 instrument (Figure 2a), thanks to its new very large detector. The small volume of the single crystal of the second yttrium-tungsten complex dictated the use of neutron Laue diffraction. This complex is the first example of a well-defined hydride cluster that contains a trigonal bipyramidal five-coordinate hydrogen atom, and its unit cell is the largest that has been successfully studied on VIVALDI to date (Figure 2b).

Re.: Takanori Shima, Yi Luo, Timothy Stewart, Robert Bau, Garry J. McIntyre, Sax A. Mason and Zhaomin Hou. "Molecular heterometallic hydride clusters composed of rare-earth and d-transition metals." Nature Chemistry, 2011, DOI: 10.1038/NCHEM.1147

Figure 2: Neutron structures of the cores of two of the heterometallic polyhydrides:

(left) the Y₄W(PMe₃)H₁₁ core of [{Cp'Y)₄(μ-H)₇}(μ-H)₄WCp*(PMe₃)] studied on D19;

(right) the Y₄WH₁₁ core of [{Cp'Y)₄(μ-H)₆}(μ-H)₅WCp*] studied on VIVALDI.