MOF-derived Ni nanoparticles dispersed on monolayer MXene as catalyst for improved hydrogen storage kinetics of MgH

Abstract

We designed a state-of-art catalyst Ni@C-MXene, which provides ample Ni and Ti active sites to significantly promote the hydrogen storage performance of MgH 2 . Experimental methods and theoretical calculations reveal that the in-situ formed Ti 0 facilitates the mutual transformation of Mg 2 Ni/Mg 2 NiH 4 , which is called “facilitated hydrogen pump” effect in this work. • Monolayer MXene was used as the functional carrier to undertake Ni nanoparticles derived from Ni-MOFs. • Striking improvements in hydrogen storage properties were achieved for MgH 2 when the catalyst was added. • DFT calculations revealed the effect of in-situ formed Ti 0 on the Mg 2 Ni/Mg 2 NiH 4 mutual transformation. • A concept of “facilitated hydrogen pump” effect was proposed. A novel catalyst—MOFs derived Ni nanoparticles dispersed on exfoliated monolayered MXene functional carrier was prepared and introduced into MgH 2 in this work. Striking improvements in hydrogen storage properties were achieved for MgH 2 when the catalyst was added. Concisely, the hydrogen releasing peak temperature of the MgH 2 + 10 wt% Ni@C-MXene is 241.1 °C, about 120.2 °C lower than that of the additive-free MgH 2 . According to Kissinger method, the dehydrogenation activation energy of the MgH 2 + 10 wt% Ni@C-MXene composite is calculated to be 54.79 kJ mol −1 , over 60% lower when compared with the pristine MgH 2 (145.08 kJ mol −1 ). At 300 °C, the MgH 2 + 10 wt% Ni@C-MXene composite releases about 5.6 wt% hydrogen within 2 min, while only 2 wt% hydrogen is desorbed even the dehydriding time prolongs to 60 min for the pure MgH 2 . In addition, the completely dehydrogenated MgH 2 + 10 wt% Ni@C-MXene exhibits eminent hydrogen absorption performance, with approximate 5 wt% hydrogen uptaken within 2 min under 3.2 MPa hydrogen pressure at 150 °C. Moreover, an excellent hydrogen cycling stability is achieved in the MgH 2 + 10 wt% Ni@C-MXene without decay for both capacity and kinetics after 10 cycles. Beside the regular XRD, XPS and TEM techniques to clarify the evolution of Ni and Ti during hydrogen de/absorption cycling, more importantly we performed the first principle based calculations to reveal how the in-situ formed Ti 0 would affect the Mg 2 Ni/Mg 2 NiH 4 mutual transformation. Apart from the nano confinement effect of MXene for Ni/Mg 2 Ni/Mg 2 NiH 4 , the in-situ formed Ti 0 from MXene also favors the “hydrogen pump” effect of Mg 2 NiH 4 due to the decreased formation energy E f under the assistance of Ti 0 . Through such a synergistic “nano-confinement and facilitated hydrogen pump” effect, the hydrogen de-/absorption kinetics of MgH 2 is significantly accelerated.