Indirect, Reversible High-Density Hydrogen Storage in Compact Metal Ammine Salts

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Abstract
The indirect hydrogen storage capabilities of Mg(NH3)6Cl2, Ca(NH3)8Cl2, Mn(NH3)6Cl2, and Ni(NH3)6Cl2 are investigated. All four metal ammine chlorides can be compacted to solid tablets with densities of at least 95% of the crystal density. This gives very high indirect hydrogen densities both gravimetrically and volumetrically. Upon heating, NH3 is released from the salts, and by employing an appropriate catalyst, H2 can be released corresponding to up to 9.78 wt % H and 0.116 kg H/L for the Ca(NH3)8Cl2 salt. The NH3 release from all four salts is investigated using temperature-programmed desorption employing different heating rates. The desorption is found mainly to be limited by heat transfer, indicating that the desorption kinetics are extremely fast for all steps. During desorption from solid tablets of Mg(NH3)6Cl2, Mn(NH3)6Cl2, and Ni(NH3)6Cl2, nanoporous structures develop, which facilitates desorption from the interior of large, compact tablets. Density functional theory calculations reproduce trends in desorption enthalpies for the systems studied, and a mechanism in which individual chains of the ammines are released from the surface of the crystal is proposed to explain the fast absorption/desorption processes.

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