First-principles prediction of high-capacity, thermodynamically reversible hydrogen storage reactions based on (NH${}_4$)${}_2$B${}_{12}$H${}_{12}$

We use a combination of first-principles density functional calculations along with the recently developed grand canonical linear programing method to predict a novel, high-capacity hydrogen storage reaction with thermodynamics suitable for near-ambient reversible storage. Unlike the vast majority of previously proposed complex hydrides, which typically rely on a hydrogen-containing anionic unit, our reaction is based on an ammonium-containing hydride, (NH${}_4$)${}_2$B${}_{12}$H${}_{12}$, which contains increased storage capacity due to both anionic and cationic hydrogen-containing complexes. The predicted decomposition of this hydride is a two-step reaction sequence: (NH${}_4$)${}_2$B${}_{12}$H${}_{12}$ → 2BN $+$ ½B${}_{20}$H${}_{16}$ $+$ 6H${}_2$ →2BN $+$ 10B $+$ 10H${}_2$, which possesses a theoretical gravimetric capacity of 11.3 wt% ${\mathrm{H}}_2$, a single-crystal volumetric density of 52 g ${\mathrm{H}}_2$/L, and $T=300$ K reaction enthalpies of 17 and 33 kJ/mol ${\mathrm{H}}_2$, respectively, which are well-suited for near-ambient reversible storage. The combination of these three attributes in a single material makes this decomposition reaction sequence highly promising.

Figure 1

Crystal structure of the (NH${}_4$)${}_2$B${}_{12}$H${}_{12}$ compound.

Figure 2

Calculated van't Hoff plot for reactions listed in Table . The region within the rectangular box corresponds to desirable temperatures and pressures for onboard hydrogen storage: pressures from 1 to 700 bar and temperatures from −40 to $+$80 °C. We also show the calculated van't Hoff lines for $X_n$B${}_{12}$H${}_{12}$ compounds with simple metal cations.[3]