STRUCTURAL STUDIES ON VARIOUS HYDROGEN STORAGE MATERIALS:

MIL-53, K2Zn3[Fe(CN)6]2 and Li2B12H12

 

Jae-Hyuk Her1,2, Yun Liu1,2, Craig M. Brown1, Muhammed Yousufuddin1,2, Terrence J. Udovic1, and Dan A. Neumann1

1 NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

2 Department of Materials Science and Engineering, University of Maryland,

College Park, MD 20742, USA

 

The biggest obstacle to realizing a hydrogen-energized vehicle is the on-board hydrogen storage aspect.  There are several different approaches to practically store/retrieve hydrogen in solid state systems.  Amongst them, physisorption provides reversible and fast kinetics but does not show enough gravimetric/volumetric density and works best at cryogenic temperatures due to weak interactions between H2 molecules and the host materials.  Metal hydrides offer another method of storing hydrogen, whose strong and weak points are inverted from the former.  In order to optimize their properties as storage materials, detailed structural information is crucial since the system thermodynamics and kinetics are mainly determined by from the atomic level structures.  However, many of candidate materialsí structure were not known due to the lack of single crystal specimens.  Modern powder diffraction instruments combined with ab-initio structure determination methods allow us to solve the crystal structure using powder patterns only.  With this technique, we have studied various hydrogen storage materials from physisorbents to complex metal borohydrides.

For example, the porous framework material MIL-53 {AlIII(OH)(O2C-C6H4-CO2)} is a physisorption-based H2 storage material distinguished by its unique breathing phenomenon.  We found that MIL-53 undergoes a hysteretic phase transition between open- and closed-pore structures by only changing temperature.  The details of the hysteresis are strongly affected by the adsorbate gas and applied pressure.  A structural clamping mechanism is suggested to bind H2 molecules in the framework and to effectively increase the working temperature.  The ferrocyanide K2Zn3[Fe(CN)6]2 is another physisorption material studied.  Though first synthesized decades ago, it was recently re-visited and presents relatively high adsorption enthalpies attributed to the coordinately-unsaturated alkali-metal cations presumably located in the middle of pores after a dehydration/activation process.  Neutron powder diffraction experiments revealed that it undergoes a structural change upon dehydration and proved that adsorbed D2 molecules are indeed strongly bonded to the polarizing K+.

 

 

 

 

 

 

 

 

 

CATEGORY: Materials

Mentors Name: Craig M. Brown, Terrence J. Udovic

Division, Laboratory: NIST Center for Neutron Research (610), Neutron Condensed Matter Science

Room, Building Mail stop: Rm A117, Bldg 235, MS 6102

Tel: 301-975-8364

Fax: 301-921-9847

Email: jhher@nist.gov

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