Synthesis and characterization of hexaammine nickel halides intended for solid-state hydrogen

Hydrogen is a versatile energy carrier but its storing is challenging. For that reason, alternative hydrogen storage approaches are being intensively researched. Solid state storing of hydrogen in the form of ammines offers certain advantages, such as high storage capacity and relatively low toxicity. Therefore, in this work hexaammine complexes of nickel chloride and nickel bromide were prepared and investigated. Ni(NH3)6Cl2 and Ni(NH3)6Br2, were obtained by precipitation synthesis. The prepared complexes were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The prepared particles are polydisperse, agglomerated and show a characteristic octahedral morphology. The samples consist solely of pure, well-crystalline, corresponding hexaamine complexes. Ni(NH3)6Br2 shows somewhat weaker nitrogen bonds with the central cation and weaker hydrogen bonds with halogen compared to Ni(NH3)6Cl2. Thermal analyses have shown that under the experimental conditions used, ammonia is released from Ni(NH3)6Cl2 in three stages, while in the case of Ni(NH3)6Br2, decomposition occurs in two stages. This difference is a consequence of the kinetic parameters of the desorption process. The maximum of the first stage of desorption occurs for the Ni(NH3)6Cl2 sample at a slightly lower temperature than for the Ni(NH3)6Br2 sample, which is attributed to thermodynamic factors.