The current generation of fuel cell light duty vehicles use high-pressure (700 bar) “composite over-wrapped pressure vessels” (i.e. composite tanks) to store the hydrogen fuel. While enabling the roll-out of light duty fuel cell vehicles, 700-bar composite tanks are not optimal and alternative hydrogen storage systems/materials/approaches can, in principle, provide significantly improved gravimetric and volumetric storage densities.

Metal hydrides and other solid-state materials offer potential advantages for hydrogen storage, such as higher gravimetric and volumetric storage densities, filling from lower pressure hydrogen sources (thus avoiding compression energy losses), and added safety stemming from a typically order of magnitude lower overall hydrogen storage pressure. However, metals hydrides that posses the high gravimetric and volumetric storage densities have not demonstrated adequate cyclability and fast refueling under practical conditions To identify causes of these limitations and strategies to mitigate them, a better microscopic characterization of these materials during hydrogenation and dehydrogenation must be obtained.

—Ray et al.

Complex metal hydrides are often characterized by poor kinetics and multi-step hydrogenation pathways that are not well understood. In the new study, the team took an important step toward understanding and improving these shortcomings.

They found that in the initial stages of hydrogen exposure, MgB₂ can hydrogenate to Mg(BH₄)₂ without the formation of intermediate compounds. Since these intermediates are known to inhibit the speed at which a hydrogen vehicle can be refueled, the possibility of avoiding them is an important development toward making MgB₂ practically viable.

A hydrogenation mechanism that directly forms magnesium borohydride avoids issues known to inhibit the speed at which a hydrogen vehicle can be refueled. Hydrogen molecules (gray) dissociate on exposed magnesium (blue) layers of magnesium diboride and migrate to boron (green) edge sites to form borohydride units (BH4, center, light green and light gray). Ray et al. Click to enlarge.

We showed that if you can combine spectroscopy, first principles calculations and kinetic modeling, it’s possible to understand the reaction pathway and specific chemical mechanism in a way that hasn’t been done before.

—Tae Wook Heo, LLNL materials scientist and co-author

The research team also discovered that MgB₂ hydrogenation occurs in two separate reaction stages as hydrogen molecules split and migrate to exposed edges in the material.

Brandon Wood, the LLNL materials scientist who led the project, said this research provides a roadmap for integrating experiments and theory toward a more comprehensive understanding of complex reactions in solid-state hydrogen storage materials. The research is part of a broader study of complex metal hydrides being conducted through the Department of Energy’s Hydrogen Storage Materials—Advanced Research Consortium (HyMARC).

Resources

• Keith G. Ray, Leonard E. Klebanoff, Jonathan R. I. Lee, Vitalie Stavila, Tae Wook Heo, Patrick Shea, Alexander A. Baker, Shinyoung Kang, Michael Bagge-Hansen, Yi-Sheng Liu, James L. White and Brandon C. Wood (2017) “Elucidating the mechanism of MgB₂ initial hydrogenation via a combined experimental–theoretical study” Phys. Chem. Chem. Phys. 19, 22646-22658 doi: 10.1039/C7CP03709K