Quantum Monte Carlo Simulation of Nanoscale MgH₂ Cluster Thermodynamics

Abstract

We calculated the desorption energy of MgH(2) clusters using the highly accurate quantum Monte Carlo (QMC) approach, which can provide desorption energies with chemical accuracy (within approximately 1 kcal/mol) and therefore provides a valuable benchmark for such hydrogen-storage simulations. Compared with these QMC results, the most widely used density functional theory (DFT) computations (including a wide range of exchange-correlation functionals) cannot reach a consistent and suitable level of accuracy across the thermodynamically tunable range for MgH(2) clusters. Furthermore, our QMC calculations show that the DFT error depends substantially on cluster size. These results suggest that in simulating metal-hydride systems it is very important to apply accurate methods that go beyond traditional mean-field approaches as a benchmark of their performance for a given material, and QMC is an appealing method to provide such a benchmark due to its high level of accuracy and favorable scaling (N(3)) with the number of electrons.

MgH2 Dehydrogenation Thermodynamics: Nanostructuring and Transition Metal Doping

“…Simulation has also demonstrated that nanoscale (MgH 2 ) n clusters, cut from the bulk, also possess lower dehydrogenation enthalpies than the bulk. 17,18,19,20 Interestingly however, in these simulations only very small clusters (n = 1 to 4) possess lower dehydrogenation enthalpies than the bulk, larger clusters actually possess dehydrogenation enthalpies greater than that of the bulk, nanostructuring will only improve thermodynamic properties if it is possible to synthesize MgH 2 as molecular-size units, larger units are less stable than the bulk. At the nanoscale, it is well-known that cluster structures can be vastly different from the bulk, strongly modifying energetics.…”

Effect of Magnesium Fluoride on Hydrogenation Properties of Magnesium Hydride

“…Nano-structuring of MgH 2 is one of the most adopted methods to improve the hydrogenation performance [1]. However, this method has a limitation to achieve the nanocrystalline size (<5 nm) required for destabilization of MgH 2 [2].…”

“…Later, it was found that some transition metal halides, such as FeF 3 , CrCl 3 , NiF 2 , NbCl 5 and TiCl 3 possess better catalytic activity than pure metals or their oxides [6][7][8]. In the case of halides, Malka et al [9] showed that fluorides are better catalysts than chlorides for MgH 2 . Addition of transition metal fluorides during milling helps to lower the hydrogen release temperature and increase the rate of hydrogen uptake by MgH 2 .…”

“…In the case of halides, Malka et al [9] showed that fluorides are better catalysts than chlorides for MgH 2 . Addition of transition metal fluorides during milling helps to lower the hydrogen release temperature and increase the rate of hydrogen uptake by MgH 2 . It has been shown by different groups [7][8][9] that during milling of MgH 2 with transition metal fluorides, the formed MgF 2 phase replaces the original oxide layer and provides a reactive and protective fluorinated surface for hydrogen uptake.…”

“…Loss in absorption capacity from the second cycle onwards was also observed. Recently, Ma et al [11] investigated the catalytic effects of MgF 2 and TiH 2 to understand the kinetic improvements obtained when MgH 2 was ball milled with 4 mol % TiF 3 . They reported that sole addition of 6 mol % MgF 2 has negligible catalytic effect on MgH 2 at an operating temperature of 150˝C.…”