Theoretical study of the synthesis, characterization and hydrogen storage properties of a high-density hydrogen storage material: (CH3NH3)BH4

Abstract Inspired by both alkaline metal borohydrides and organic-inorganic hybrid perovskite, we predict a pair of complex structures of (CH3NH3)BH4 with tremendous high hydrogen capacity (21.27 wt.%). Through comparison and analysis of the electronic structures of alkali metal atoms, CH3NH3, NH4, and NH3BH3 molecule, it is concluded that similar spatial and electronic structures show the feasibility of synthesizing (CH3NH3)BH4 by a substitution reaction. Firstly, theoretical structures (S1 and S2 in P1) with stable configurations have been reconstructed by cation substitution followed by a series of restrictive structural optimizations, and both the lattice parameters and the position coordinate information of (CH3NH3)BH4 are obtained. Ignoring the relatively mobile hydrogen, the structural symmetries of S1 and S2 are I4mm and P4/nmm, respectively. X-ray diffraction characterizations of S1 and S2 are consistent with the experimental results. Secondly, the calculated elastic constants of (CH3NH3)BH4 (S1 and S2) with P1 symmetry indicate that angles α, β and γ oscillate at right angles due to the influence of the cation orientation. The calculated spatial dependence of bulk (B), Young's (E), and shear (G) modulus obviously show that the two P1 phases all have strong elastic anisotropy. Thirdly, the calculated electronic properties show that the protonic amine-H, hydridic borane-H, and neutral methane-H are widely distributed in (CH3NH3)BH4, which allow for weaving in a planar dihydrogen bonding network, which in turn influences the dehydrogenation reaction. Last and most important, we propose the following dehydrogenation process of (CH3NH3)BH4 via the intermediate compounds: 2(CH3NH3)BH4 → CH3NH2BH2NHCH3BH3+3H2. For each dehydrogenation step, the free energy change is negative, which means (CH3NH3)BH4 can decompose spontaneously, similar to ammonium borohydride, which is strongly related to the planar dihydrogen bonding network.