Enhanced Hydrogen Storage Properties Of Magnesium Hydride By Coupling Nano-confinement And P-doping

Energy is one of the most important issues in the 21 st century with the rapid depletion of energy and increasingly environmental pollution. The development of clean energy sources becomes more urgent. Hydrogen（H2） is an ideal energy carrier with high energy density and generates energy in a completely clean way. The development of hydrogen storage materials with highly efficient is the most important process of the use of hydrogen. Due to the high hydrogen storage capacity, good cycle performance, low cost, magnesium has been extensively researched for more than 5 decades. Nevertheless, the practical application of Mg is still restricted by the sluggish kinetics and high desorption temperature. To get over this problem, nano-confinement strategy has been proposed, which means that MgH₂ nano particles are confined in porous scaffolds. Confining magnesium particles into mesoporous CMK-3 can overcome sintering and oxidation of Mg particles. In terms of the modification of carbon scaffold, doping with non- metal heteroatoms as a technique has been widely employed in the literatures. The basic idea is that heteroatoms incorporated into carbon network can result in local distortion and change the original electron distribution, and consequently activate a part of carbon atoms.In this work, the hydrogen desorption properties of magnesium particles confined into mesoporous CMK-3 doped nonmetallic element P were systematically investigated. By doping non-metallic elements P, the defects introduced result in the structural distortion of mesoporous carbon. The N2 sorption analysis of CMK-3 and P/CMK-3 shows a representative type IV isotherm, indicating a high uniformity of mesopores. After P-doping, the Brunauer–Emmett–Teller（BET） surface area declines slightly, from 1423 m2/g of CMK-3 to 1044 m2/g of P/CMK-3.After loading Mg/MgH₂ into ordered mesoporous carbons of CMK-3 and P/CMK-3, the BET surface area decreases remarkably, down to 600 m2/g and 359 m2/g for MgH₂/CMK-3 and MgH₂@P/CMK-3. Based on the theoratical capacity of MgH₂, the sample of MgH₂@P/CMK-3 released about 0.8 wt. % H2 below 150 °C, 1.5 wt.% H2 below 200 °C, and 2.8 wt.% H2 below 250 °C. Correspondently, the sample of MgH₂@CMK-3 released only about 0.1, 0.3, and 0.9 wt. % H2 at the same conditions. It is noteworthy that the sample of MgH₂@P/CMK-3 exhibits much better desorption behavior than MgH₂@CMK-3 at low temperature region. We proved that P-doping can remarkably enhance the hydrogen release properties of nanoconfined MgH₂ at low temperature.The results of theoratical cacalation shows that, for bulk MgH₂, desorption energy（DE） is as high as 0.75 eV per H2, as determined by the strong Mg-H bonding. For Mg76H152 clusters, DE is reduced to 0.62 eV with respect to the bulk case, whose improvement is made due to the size effect. When it is confined in CMK-3, DE can be further reduced to 0.55 eV, due to the confinement effect. Among four non- metal elements confined Mg76H152 clusters, the DE decreases in order of S, C, B, N and P. Especially, P-dopants can achieve an impressive DE = 0.2 eV per H2.With the increase of the P-doping content, the defects and distortions of the P/CMK-3 structure becomes more serious. When the P contents are 0.45 wt.%, 1.21 wt.% and 1.82 wt.%, the BET surface of CMK-3 decreased from 1440 m2/g to 1318 m2/g、1103 m2/g and1044 m2/g. But with increasing of P content, the MgH₂ shows better desorption properties. P contents of 1.21 wt.% and 1.82 wt.% released about 0.4 and 0.8 wt% H2 below 150 °C.The synegestic effect of P-doping and nano-confinement would enhance the hydrogen desorption properties of MgH₂. On the one hand, the increase of P-doping content enhances the hydrogen desorption behavior at low temperature zone; on the other hand, the increase of P-doping content decreases the BET surface of P/CMK-3, which restrains the nano-confinement effect and leads to a degradation of desorption behavior at high temperature zone.