Highly synergetic catalytic mechanism of Ni@g-C3N4 on the superior hydrogen storage performance of Li-Mg-B-H system

Abstract

Li-Mg-B-H system with the composition of 2LiBH4-MgH2 is one of the most promising reversible hydrogen storage materials because of the extremely high hydrogen storage capacity (11.5 wt%). Yet, sluggish kinetics with high desorption temperature (~ 400 °C) hindered its practical applications. Herein, a highly efficient catalyst of nanoNi@g-C3N4, comprised of transition metal nanocatalyst and carbon-based nonmetal material, was designed and fabricated to dramatically reduce its initial hydrogen desorption temperature to as low as ~105 °C, and eliminate induction period between the two dehydrogenation steps of 2LiBH4-MgH2 by reducing the activation energy for the dehydrogenation of LiBH4 from 200±6 to 126±7 kJ/mol. Multiple postmortem analyses revealed that the in situ formed MgNi3B2 NPs and the H+ defects on g-C3N4 play the key role in synergically enhancing the hydrogen reversible desorption/absorption kinetics at low temperature: 1) in situ formed MgNi3B2 acts as nucleation center of MgB2 and leads to an excellent catalytic effect to accelerate the dehydrogenation kinetics of 2LiBH4-MgH2; 2) the H+ defects on g-C3N4, will interact with the H– in LiBH4 to trigger the release of H2 at low temperature and destabilize LiBH4 and MgH2 to form Li3N and Mg3N2 respectively for the high electronegativity of N: 3) these in situ formed intermediates of Li3N and Mg3N2 will be readily rehydrogenated as highly reversible Mg(NH2)2 and LiH. This novel transition metal and g-C3N4 catalytic composites could provide alternative insights into designing nanosized catalytic hydrogen storage system with low desorption temperature and high capacity for hydrogen-fueled applications.