The reactions in LiBH4–NaNH2 hydrogen storage system

Stepwise reactions were observed in the ball milling and heating process of the LiBH4–NaNH2 system by means of X-ray diffraction (XRD) and Fourier transform infrared spectrometry (FT-IR). During the ball milling process, two concurrent reactions take place in the mixture: 3LiBH4 + 4NaNH2 → Li3Na(NH2)4 + 3NaBH4 and LiBH4 + NaNH2 → LiNH2 + NaBH4. The heating process from 50 °C to 400 °C is mainly the concurrent reactions of Li3Na(NH2)4 + 3LiBH4 → 2Li3BN2 + NaBH4 + 8H2 and 2LiNH2 + LiBH4 → Li3BN2H8 → Li3BN2 + 4H2, where the intermediate phases Li3Na(NH2)4 and LiNH2 serve as the reagents to decompose LiBH4. The merged equations for the mechanochemical and the heating reactions below 400 °C can be denoted as 3LiBH4 + 2NaNH2 → Li3BN2 + 2NaBH4 + 4H2. The maximum dehydrogenation capacity in closed system below 400 °C is 5.1 wt.% H2, which agrees well with the theoretical capacity (5.5 wt.% H2) of the merged equation and thus demonstrates the suggested pathway. The subsequent step consists of the decompositions of NaBH4 and Li3Na(NH2)4 within the temperature range of 400 °C–600 °C. The apparent activation energies of the two steps are 114.8 and 123.5 kJ/mol, respectively. They are all lower than that of our previously obtained bulk LiBH4.

► Dehydrogenation in the LiBH4–NaNH2 starts from 50 °C. ► The intermediate phases Li3Na(NH2)4 and LiNH2 during heating process serve as the reagents to decompose LiBH4. ► The maximum dehydrogenation capacity in closed system below 400 °C is 5.1 wt.% H2. ► The apparent activation energies of the two steps (114.8 and 123.5 kJ/mol) are all lower than that of bulk LiBH4.