Sodium borohydride hydrazinates: synthesis, crystal structures, and thermal decomposition behavior

Abstract

Metallic borohydride hydrazinates are novel boron- and nitrogen-based materials that appear to be promising candidates for chemical hydrogen storage. Herein, sodium borohydride hydrazinates, including NaBH₄·N₂H₄ and NaBH₄·2N₂H₄, were synthesized via a facile solution synthesis approach based on the solid–liquid reaction between NaBH₄ and hydrazine in tetrahydrofuran (THF) solution. The crystal structure of NaBH₄·2N₂H₄ was solved to a monoclinic structure with unit cell parameters a = 8.4592(2) Å, b = 11.7131(2) Å, c = 6.4584(1) Å, and β = 100.0777(14)°, with space group A1a1 (9). Each Na⁺ ion interacts with four neighboring N₂H₄ molecules, leading to the formation of Na₄[N₂H₄]₄⁴⁺ cationic chains along the a direction. Such cationic chains are surrounded by BH₄⁻ anions. The NaBH₄·xN₂H₄ (x = 1, 2) was characterized by Powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and mass spectroscopy (MS) to obtain a full picture of the relationship of the structure to the decomposition, which will be useful for future work on borohydride hydrazinates or the study of other B–N materials. Furthermore, magnesium borohydride hydrazinates were synthesized by ball milling the sodium borohydride hydrazinates with magnesium borohydride, and their thermal decomposition was investigated as well.