Li-Mg-B-H system with the composition of 2LiBH 4 -MgH 2 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-C 3 N 4 , 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 2LiBH 4 -MgH 2 by reducing the activation energy for the dehydrogenation of LiBH 4 from 200±6 to 126±7 kJ/mol. Multiple postmortem analyses revealed that the in situ formed MgNi 3 B 2 NPs and the H + defects on g-C 3 N 4 play the key role in synergically enhancing the hydrogen reversible desorption/absorption kinetics at low temperature: 1) in situ formed MgNi 3 B 2 acts as nucleation center of MgB 2 and leads to an excellent catalytic effect to accelerate the dehydrogenation kinetics of 2LiBH 4 -MgH 2 ; 2) the H + defects on g-C 3 N 4 , will interact with the H – in LiBH 4 to trigger the release of H 2 at low temperature and destabilize LiBH 4 and MgH 2 to form Li 3 N and Mg 3 N 2 respectively for the high electronegativity of N: 3) these in situ formed intermediates of Li 3 N and Mg 3 N 2 will be readily rehydrogenated as highly reversible Mg(NH 2 ) 2 and LiH. This novel transition metal and g-C 3 N 4 catalytic composites could provide alternative insights into designing nanosized catalytic hydrogen storage system with low desorption temperature and high capacity for hydrogen-fueled applications.
In this paper, the unclear Ni atom coordination structure evolution in one-pot pyrolysis method synthesizing Ni single atom catalyst using graphitic carbon nitride as support (Ni-g-C 3 N 4 ) was deeply discussed. In-situ X-ray absorption fine structure spectra and other assisted methods were used to characterize the structural evolution. The results demonstrate that the pyrolysis process can be divided into three stages. In the first stage from room temperature to 183 °C (Stage I), the NiCl 2 ⋅6H 2 O was mixed with the supporting material (dicyandiamide) to form the precursor with Ni-(H 2 O) 6 octahedral coordination configuration. In the second stage from 183 to 350 °C (Stage II), the H 2 O molecules have been removed and the supporting material was converted to melamine. The melamine molecules were polycondensed to melem during 308–350 °C. The intermediates labeled Ni-melamine and Ni-melem was formed with Ni-N 4 planar quadrilateral coordination configuration. In the third stage from 350 to 600 °C (Stage III ), the supporting material (melem) was gradually converted to g-C 3 N 4 , and the Ni-N 4 coordination configurations were replaced by Ni-N 6 planar hexagon coordination configuration one by one. The three Ni coordination configurations were further confirmed by Reverse Monte Carlo simulations and ab initio calculations .
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