(226b) Computational Design of Materials for Hydrogen Storage

Synthesis of materials capable of storing hydrogen with large gravimetric and volumetric density and operating at near ambient thermodynamic conditions is critical to the success of a hydrogen economy. Carbon based materials due to their light weight have been regarded as possible candidates for hydrogen storage. Recent works suggested that carbon nanostructures such as fullerenes, organic molecules, and nanotubes suitably functionalized with transition metal atoms can meet the hydrogen storage requirements. These materials store hydrogen in quasi-molecular form with binding energies intermediate between physisorbed and chemisorbed states. However, the stability of these materials has been a problem to deal with since transition metal atoms have a tendency to cluster and hence adversely affect the hydrogen storage capability. Using density functional theory and generalized gradient approximation for exchange and correlation, we have considered two classes of materials where this limitation can be avoided. These include coating of carbon nanostructures with Li where the low cohesive energy of Li does not induce clustering. A second class of materials involves silsesquioxanes (SQ) nano complex [RSiO3/2]n. This nano complex has the following advantages: (1) Storage capacity in the fully grafted case is 5 wt% where hydrogen is bound quasi-molecularly with a binding energy of about 0.6 eV /H2 molecule. (2) The structure of SQ itself is stable, and the synthesis is very flexible. We hope that our prediction of the effectiveness of functionalized SQ complex as a hydrogen storage material will encourage experimental investigation.

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