Ammonia as Hydrogen Storage Media
The hydrogen H2 storage capacity of Ammonia NH3 is 18 mass%, which is above 8 times of hydrogen-absorbing alloys, and the standard enthalpy change (heat of formation) of -31 kJ/molH2 is similar to those of the hydrogen-absorbing alloys (Ti-Cr-V: -34 kJ/molH2, LaNi5: -31 kJ/molH2). However, the high H2 decomposition temperature (873 K) due to the slow reaction kinetics (high activation energy) limits the practical application of NH3 as a hydrogen storage material. To realize a usable H2 storage system by NH3, H2 storage/ generation at ambient temperature is necessary.
In this study, lithium hydride LiH was mechanically milled for activation. The activated LiH was reacted with NH3 at room temperature by the exothermic reaction (ΔH: -43.1 kJ/molH2), generating H2 and lithium amide LiNH2. X-ray diffraction and gas chromatography indicated that LiNH2 and H2 were formed by the reaction of the activated LiH in the NH3 atmosphere of 0.5 MPa at room temperature. After H2 generation, LiNH2 was able to store H2 at 573 K to form LiH and NH3 under 0.5 MPa H2 flow. In contrast, LiNH2 could not store H2 in a closed vessel at the same pressure and the temperature, because the low NH3 partial pressure prevents the decomposition of LiNH2. Thus, we found that the H2 storage and the generation of the LiH-NH3 system took the following reaction path, LiH + NH3<-->LiNH2 + H2. H2 of 8.1 mass% [H2/LiH+NH3] can be reversibly stored in this reaction.