A collaboration including scientists from Lawrence Livermore National Laboratory (LLNL), Sandia National Laboratories, the Indian Institute of Technology Gandhinagar and Lawrence Berkeley National Laboratory has created 3-4 nanometer ultrathin nanosheets of a MgB₂metal hydride that increase hydrogen storage capacity significantly. The research appears in the journal Small.

Metal boride nanostructures have shown significant promise for hydrogen storage applications. However, the synthesis of nanoscale metal boride particles is challenging because of their high surface energy, strong inter- and intraplanar bonding, and difficult-to-control surface termination. Here, it is demonstrated that mechanochemical exfoliation of magnesium diboride in zirconia produces 3–4 nm ultrathin MgB₂ nanosheets (multilayers) in high yield. High-pressure hydrogenation of these multilayers at 70 MPa and 330 °C followed by dehydrogenation at 390 °C reveals a hydrogen capacity of 5.1 wt%, which is ≈50 times larger than the capacity of bulk MgB₂ under the same conditions.

This enhancement is attributed to the creation of defective sites by ball-milling and incomplete Mg surface coverage in MgB₂ multilayers, which disrupts the stable boron–boron ring structure. The density functional theory calculations indicate that the balance of Mg on the MgB₂ nanosheet surface changes as the material hydrogenates, as it is energetically favorable to trade a small number of Mg vacancies in Mg(BH₄)₂ for greater Mg coverage on the MgB₂ surface. The exfoliation and creation of ultrathin layers is a promising new direction for 2D metal boride/borohydride research with the potential to achieve high-capacity reversible hydrogen storage at more moderate pressures and temperatures.

—Gunda et al.

Hydrogen has the highest energy density of any fuel and is considered a viable solution for ground transportation, aircraft and marine vessels. However, hydrocarbon fuel sources outperform compressed hydrogen gas in terms of volumetric energy density, motivating the development of alternative, higher-density materials-based storage methods.

Complex metal hydrides are a class of hydrogen storage materials that, while having high absolute storage capacity, can require extreme pressures and temperatures to achieve that capacity. The team tackled this challenge by nano-sizing, which increases the surface area to react with hydrogen and decreases the required depth of hydrogenation.

Previous studies have analyzed nanoscale magnesium diboride (MgB₂), including work by LLNL, however, the material in that study was not as thin and wound up clustering together.

The material created in this most recent collaboration came from solvent-free mechanical exfoliation in zirconia, yielding material that is only 11-12 atomic layers thick and can hydrogenate to about 50 times the capacity of the bulk material.

This 50-fold increase in the hydrogenation neatly corresponds to a 50-fold increase in the surface-to-volume ratio, suggesting that both the bulk and nanosheet material hydrogenate approximately the first two layers, a universal behavior independent of particle size. For two layers on either side of the 11-12-layer nanomaterial, this represents a third of the maximum hydrogen capacity of MgB₂.

MgB2 consists of alternating magnesium and boron layers for which charge transfer from the magnesium layer to the boron layer drives the boron layer stability. LLNL calculations reveal that the incomplete Mg coverage on the surface of the material energetically favors a surface structure with islands of complete magnesium coverage and other areas of less stable disordered surface boron layers. Building from previous work on the disordering of surface boron layers, calculations show how magnesium coverage on MgB₂ evolves as it hydrogenates.

These results show how a reactive MgB₂ surface with exposed boron may become more stable as it hydrogenates because the magnesium coverage increases. By this mechanism the hydrogenation slows and halts for moderate hydrogenation conditions.

Further nano-sizing or a novel chemical modification to delay or disrupt the increase in surface magnesium may further increase MgB₂ performance as a hydrogen storage material.

—LLNL physicist and author Keith Ray

Resources

• Gunda, H., Ray, K. G., Klebanoff, L. E., Dun, C., Marple, M. A. T., Li, S., Sharma, P., Friddle, R. W., Sugar, J. D., Snider, J. L., Horton, R. D., Davis, B. C., Chames, J. M., Liu, Y.-S., Guo, J., Mason, H. E., Urban, J. J., Wood, B. C., Allendorf, M. D., Jasuja, K., Stavila, V. (2023) Hydrogen Storage in Partially Exfoliated Magnesium Diboride Multilayers. Small 19, 2205487.doi: 10.1002/smll.202205487