Probing an intermediate state by X-ray absorption near edge structure in nickel doped 2LiBH4-MgH2 reactive hydride composite at moderate temperature

Abstract Transition metal nickel (Ni) efficiently provoke an enhanced hydrogen desorption kinetics of 2LiBH4-MgH2 reactive hydride composite (RHC) system by in-situ formed MgNi3B2. However, the detailed information about the complete evolution process of Ni in 2LiBH4-MgH2 system is still insufficient and deserves further research. Herein, an interesting phenomenon that inconspicuous exothermic process coupled with unimpressive hydrogen releasing behavior at a relatively moderate temperature range (150-300 °C) was found in the typical 2LiBH4-MgH2 RHC system when doped with nanosized Ni/C derived from the pyrolysis of MOFs-74-Ni. The X-ray absorption near edge structure (XANES) spectra of 2LiBH4-MgH2-15%Ni/C sample under various temperatures confirm that chemical valence of Ni presents a distinct intermediate state. After that, phase analysis indicates that Ni most likely reacts with LiBH4 to form a temporary Ni4B3, and a series of meticulous supplementary data were calculated to identify the authenticity of the proposed assumption. More precisely, the experimental calorific value (15.43 mJ) and hydrogen release capacity (0.47 wt.%) are highly consistent with the theoretical calculation value (15.66∼15.99 mJ and 0.48 wt.%) of the proposed reaction. It is revealed that the Ni4B3 intermediate phase will further transform into catalytic active MgNi3B2 species, which provokes a significantly enhanced kinetic behavior and also high hydrogen storage capacity (above 9.0 wt.%) of 2LiBH4-MgH2 RHC system. Finally, the heterogeneous nucleation mechanism of the MgNi3B2 catalytic active substance is also collaboratively verified through carefully scanning electron microscope (SEM) and high resolution transmission electron microscopy (HRTEM) observations. This work ideally reveals the complete evolution process of Ni in 2LiBH4-MgH2 RHC system by the detection of nanosized intermediate state. More importantly, it provides ideas and inspirations for the other nanosized catalytic complex hydride systems.