Significant Thermodynamic Destabilization and Superior Hydrogen Storage Properties of Nanocrystalline Mg-20 wt % Ti–Cr–V_x (x = 0.4, 0.6, 0.8; Ti/Cr = 2:3) Composites Synthesized by Reactive Ball Milling

Abstract

Nanocrystalline Mg-20 wt % Ti−Cr−V x (x = 0.4, 0.6, 0.8; Ti/Cr = 2:3) composites were synthesized by reactive ball milling of Mg and Ti−Cr−V x powders for 8 h under 5.0 MPa H 2 at room temperature. The morphology, crystal structure, and isothermal hydrogen ab-/desorption properties of the materials were investigated. The as-synthesized nanocomposites comprise β-MgH 2 , γ-MgH 2 , and Ti−Cr−V x −H y (1.88 < y < 1.92) as confirmed by X-ray diffraction analysis. High-resolution transmission electron microscopy images show that both β-MgH 2 and γ-MgH 2 co-existed in a single nanocrystalline. Differential scanning calorimetry results reveal that Mg-20 wt % Ti 0.16 Cr 0.24 V 0.6 powders possess the lowest hydrogen desorption temperature at 224 °C with the apparent activation energy of 76.32 kJ/mol for dehydrogenation. The nanocrystalline Mg-20 wt % Ti 0.16 Cr 0.24 V 0.6 powders exhibit excellent dehydrogenation kinetics. They can desorb 5.67 wt % H 2 in 20 min at 270 °C under 0.01 MPa H 2 pressure, compared with only 0.10 wt % H 2 for a pure Mg sample under the same preparation and measurement conditions. More remarkably, the presence of γ-MgH 2 is found to lead to a significant reduction of reaction enthalpy from 74.8 to 54.16 kJ/(mol•H 2 ) for the Mg-20 wt % Ti 0.16 Cr 0.24 V 0.6 powders. The thermodynamic destabilization and superior hydrogen storage properties of the nanocomposite can be attributed to the synergistic desorption effect co-existing in the nanocomposite γ-MgH 2 /β-MgH 2 /Ti 0.16 Cr 0.24 V 0.6 H y phases as well as the high content of γ-MgH 2 (35.4 wt %). Furthermore, cycling results show that the metastable γ-MgH 2 phase still exists after several cycles at selected temperatures ranging from 225 to 255 °C, which is important for improving the thermodynamic properties of the composite, whereas the content of γ-MgH 2 decreased with the increasing de-/absorption cycles. However, this sheds light on the ways to further improve the thermodynamic properties of magnesium-based composites.

Review on Hydrogen Storage Performance of MgH2: Development and Trends

“…In contrast, the pure Mg sample could only release 0.10 wt % H 2 under the same preparation and measurement conditions. The reaction enthalpy of the composite decreased significantly from 74.8 kJ/mol to 54.16 kJ/mol due to the coexistence of Ti 0.16 Cr 0.24 V 0.6 H y and more γ‐MgH 2 phase (35.4 %) [21f] . C. Zhou et al [21g] .…”

“…The kinetic performance of MgH 2 had been continuously improved through the addition of catalysts, but it had little impact on the thermodynamic performance of MgH 2 [21g] . In some reports, the enthalpy of dehydrogenation of MgH 2 was reduced from 77 kJ/mol to 70.6 kJ/mol by adding catalyst, but the added amount of catalyst reached 20 wt % [21f] . But if the size of MgH 2 itself is reduced to the nanoscale, the nanoscale effect can greatly promote the Mg−H bond interaction process, destroy the stability of Mg−H bond, and improve the thermodynamic performance of MgH 2 .…”

Improvement of Mg‐Based Hydrogen Storage Materials by Metal Catalysts: Review and Summary

“…Meanwhile, it can desorb about 5 wt % H 2 at 300°C. [45] Zhang et al [46] synthesized TiCrV x composites by reactive ball milling under hydrogen pressure of 5Mpa at room temperature. The prepared Mg-20 wt %Ti 0.16 Cr 0.24 V 0.6 powder showed excellent dehydrogenation kinetics and had the lowest hydrogen desorption temperature at 224°C.…”

Nanoparticulate ZrNi: In Situ Disproportionation Effectively Enhances Hydrogen Cycling of MgH2

“…34 The hydride of nanocrystalline Mg with 20 wt % Ti 0.16 Cr 0.24 V 0.6 liberated 5.67 wt % H 2 under 0.1 bar H 2 when operating at 270 °C for 20 min. 35 By directly hydriding Mg 80 Ce 18 Ni 2 , a CeH 2.73 -MgH 2 -Ni nanocomposite with remarkable hydrogen storage properties was fabricated in situ. 36 This nanocomposite featured numerous in situ generated Ni nanoparticles, high-density grain boundaries in nanocrystalline MgH 2 , and interfaces between CeH 2.73 and MgH 2 functioned as nucleation sites of hydrides and H diffusion channels.…”

“…The measured H 2 capacity was 6 wt % . The hydride of nanocrystalline Mg with 20 wt % Ti 0.16 Cr 0.24 V 0.6 liberated 5.67 wt % H 2 under 0.1 bar H 2 when operating at 270 °C for 20 min . By directly hydriding Mg 80 Ce 18 Ni 2 , a CeH 2.73 -MgH 2 -Ni nanocomposite with remarkable hydrogen storage properties was fabricated in situ .…”