Mechanism and kinetics of early transition metal hydrides, oxides, and chlorides to enhance hydrogen release and uptake properties of MgH₂

Abstract

Selected hydrides (TiH 2 , ZrH 2 ), chlorides (VCl 3 , ScCl 3 ) or oxides (V 2 O 5 ) utilized as additives facilitating hydrogen release and uptake for magnesium hydride were investigated using mechanochemical treatment and in-situ synchrotron radiation powder X-ray diffraction studies. The fastest hydrogen desorption and absorption kinetics for MgH 2 was observed for a sample with 5 mol% V 2 O 5 at 320°C. Additional activation of the system (2 cycles, vacuum/p(H 2 ) ∼150 bar, 450°C) leads to significant improvement of the kinetics even at lower temperatures, 270°C. The observed prolific effect is achieved through the full reduction of vanadium oxides and formation of an efficient vanadium catalyst as nanoparticles and possibly interfacial effects in the MgO/Mg/MgH 2 /V system introduced during cycling hydrogen release and uptake in hydrogen/dynamic vacuum at 450°C. Nanostructuring as well as hydrogen permeability via vanadium nanoparticles may improve kinetics and reduce the apparent activation energy for hydrogen release. Thus, the enhancement of hydrogen release/uptake in the MgH 2 owe to "in situ" formation of vanadium nanoparticles by reduction of V 2 O 5 .

Fast hydrogen sorption from MgH2–VO2(B) composite materials

“…As shown in Table 1, the quantity of MgO in cycled samples is about 46%. This finding suggests that oxidation arises from both, high-temperature reaction with oxygen impurities physisorbed on the samples during handling [6,7] and from the reactions: MgH2→Mg+H2 2Mg+VO2→ 2MgO + V An increase of MgO quantity due to reaction with vanadium oxides has been already reported by several authors [20,21]. The interaction of V with H2 may lead to formation of an interstitial solid solution, of VH~0.8, or of VH2 depending on the operating temperature and pressures At the conditions of our H-sorption cycles, an interstitial solid solution of H in bcc V is expected according to V-H thermodynamics [5].…”

“…After activation, one can see that the hydrogen desorption capacity attains a higher value of 5.4 wt.%, which remains constant during subsequent cycles. Korablov et al spotted similar behaviour after cycling of MgH2-V2O5 system and correlate it to the in situ formation of VH2/V in the sample [20]. In fact, in the system MgH2-VO2 ,Raman and XRD analysis shows formation of VH2 upon cycling and therefore the existence of VO2/VH2 (V 4+ /V 2+ ) system is probably responsible for strongly improved (de)sorption kinetics.…”

Remarkable hydrogen storage properties of MgH2doped with VNbO5

“…Similar microstructure properties were also satisfied in vanadium-doped Mg compounds with enhanced hydrogen storage properties. [32][33][34] However, despite the several numbers of additives tested, no data are reported in the current literature, to our knowledge, concerning a synergic effect of the V-Nb oxide phase characterized by octahedral clusters. A restricted number of stoichiometric and non-stoichiometric V-Nb-O compounds with different compositions have been studied sporadically, and their crystal structures have not always been determined unquestionably.…”

Dehydrogenation Properties of Magnesium Hydride Loaded with Fe, Fe−C, and Fe−Mg Additives

“…Moreover, due to kinetic constrains, a temperature of 350–400 °C is required for the fast release of hydrogen . Reports in the literature suggest that the hydrogen de/ab‐sorption performance can be improved by means of a small amount of an additive, either a metal, metal oxide, metal halide, or carbon nanostructure …”

“…Moreover,due to kinetic constrains, at emperature of 350-400 8Ci sr equired for the fast release of hydrogen. [2] Reports in the literature suggest that the hydrogen de/ab-sorption performance can be improved by meanso fas mall amount of an additive,e ither am etal, [3][4][5] metal oxide, [6][7][8] metal halide, [9,10] or carbon nanostructure. [11][12][13] Considering the immense importance of Fe in catalysis, [14] our interesti nt he presenti nvestigation is to explore the effect of iron-containing additives for MgH 2 .I nt he literature contradictory opinions have been reported regarding the chemical role of Fe, although it is generally agreedt hat Fe can improve the hydrogen de-and absorption performance when added as an additive to MgH 2, [15][16][17] .W hereas most of the experimental studies do not confirm the substitution of Fe in the lattice of MgH 2 ,v arioust heoretical studies suggest that Fe substituted Mg/MgH 2 surfaces actively influence the Mg-H interaction.…”