Improvement in the Hydrogenation and Dehydrogenation Features of Mg by Milling in Hydrogen with Vanadium Chloride

VCl3 (vanadium (III) chloride) was selected as an additive to Mg to increase the hydrogenation and dehydrogenation rates and the hydrogen storage capacity of Mg. Instead of MgH2, Mg was used as a starting material since Mg is cheaper than MgH2. Samples with a composition of 95 wt% Mg + 5 wt% VCl3 (named Mg-5VCl3) were prepared by milling in hydrogen atmosphere (reactive milling). In the first cycle (n=1), Mg-5VCl3 absorbed 5.38 wt% H for 5 min and 5.95 wt% H for 60 min at 573 K in 12 bar hydrogen. The activation of Mg-5VCl3 was completed after three hydrogenation-dehydrogenation cycles. During milling in hydrogen, β-MgH2 and γ-MgH2 were produced. The formed β-MgH2 and γ-MgH2 are considered to have made the effects of reactive milling stronger as β-MgH2 and γ-MgH2 themselves were being pulverized. The introduced defects and the interfaces between the Mg and the phases formed during the reactive milling and during hydrogenation-dehydrogenation cycling are believed to serve as heterogeneous active nucleation sites for MgH2 and Mg-H solid solution. The phases generated during hydrogenation-dehydrognation cycling are also believed to prevent the particles from coalescing during hydrogenation-dehydrognation cycling.

Improvement in the Hydrogenation and Dehydrogenation Features of Mg by Milling in Hydrogen with Vanadium Chloride

VCl3 (vanadium (III) chloride) was selected as an additive to Mg to increase the hydrogenation and dehydrogenation rates and the hydrogen storage capacity of Mg. Instead of MgH2, Mg was used as a starting material since Mg is cheaper than MgH2. Samples with a composition of 95 wt% Mg + 5 wt% VCl3 (named Mg-5VCl3) were prepared by milling in hydrogen atmosphere (reactive milling). In the first cycle (n=1), Mg-5VCl3 absorbed 5.38 wt% H for 5 min and 5.95 wt% H for 60 min at 573 K in 12 bar hydrogen. The activation of Mg-5VCl3 was completed after three hydrogenation-dehydrogenation cycles. During milling in hydrogen, β-MgH2 and γ-MgH2 were produced. The formed β-MgH2 and γ-MgH2 are considered to have made the effects of reactive milling stronger as β-MgH2 and γ-MgH2 themselves were being pulverized. The introduced defects and the interfaces between the Mg and the phases formed during the reactive milling and during hydrogenation-dehydrogenation cycling are believed to serve as heterogeneous active nucleation sites for MgH2 and Mg-H solid solution. The phases generated during hydrogenation-dehydrognation cycling are also believed to prevent the particles from coalescing during hydrogenation-dehydrognation cycling.

Development of Magnesium-Based Material with Hydrogen-Storage Capacity of 7 wt%

TiCl3 was chosen as an additive to increase hydriding and dehydriding rates of Mg. In our previous works, we found that the optimum percentage of additives that improved the hydriding and dehydriding features of Mg was approximately ten. Specimens consisting of 90 wt% Mg and 10 wt% TiCl3 (named Mg–10TiCl3) were prepared by high-energy ball milling in hydrogen. The specimens’ hydriding and dehydriding properties were then studied. Mg–10TiCl3 had an effective hydrogenstorage capacity (the quantity of hydrogen absorbed in 60 min) of approximately 7.2 wt% at 593 K under 12 bar H2 at the second cycle. After high-energy ball milling in hydrogen, Mg–10TiCl3 contained Mg, β-MgH2, and small amounts of γ-MgH2 and TiH1.924. TiH1.924 remained undercomposed even after dehydriding at 623 K in a vacuum for 2 h. The hydriding and dehydriding properties of Mg–10TiCl3 were compared with those of other specimens such as Mg–10Fe2O3, Mg–10NbF5, and Mg–5Fe2O3–5Ni, for which the hydrogen-storage properties were previously reported.

Effects of Milling in Hydrogen on Magnesium Hydride with a Hydride-Forming Titanium Additive

A hydride-forming element titanium (Ti) was selected as an additive to improve the hydrogen uptake and release properties of MgH2. The hydrogen uptake and release properties of three Ti-added MgH2 alloys [named MgH2-xTi (x = 6, 12, and 15)] prepared by milling in hydrogen (reactive mechanical grinding) were investigated and those of MgH2-12Ti were studied in more detail because it had the highest initial hydrogen uptake and release rates and the largest quantities of hydrogen absorbed and released for 60 min. At the cycle number, n, of one (n = 1), MgH2-12Ti absorbed 4.01 wt.% H for 2.5 min and 6.39 wt.% H for 60 min at 573 K in 12 bar H2, having an effective hydrogen storage capacity of 6.39 wt.%. MgH2-12Ti released 0.44 wt.% H for 2.5 min and 1.86 wt.% H for 60 min at 593 K in 1.0 bar H2. γ-MgH2, TiH1.924, and MgO were formed during reactive mechanical grinding. We believe that the brute forces and tensile, compressive, or shear stresses, which are applied to the materials during reactive mechanical grinding, introduce imperfections, fabricate cracks, expose fresh and clean surfaces, decrease the particle size, and disperse the additive among the particles. The γ-MgH2, TiH1.924, and MgO formed during reactive mechanical grinding and their pulverization during reactive mechanical grinding are believed to make these effects stronger.

Amelioration of Hydrogen Uptake and Release Features of Magnesium Adding a Polymer Polyvinylidene Fluoride via Milling in Hydrogen in a Planetary Ball Mill

In the present study, a polymer polyvinylidene fluoride (PVDF) was chosen as an adding material to ameliorate hydrogen uptake and release features of Mg. Samples with a composition of 95 wt.% Mg+5 wt.% PVDF (called 95Mg + 5PVDF) were made via milling in hydrogen atmosphere in a planetary ball mill (reactive planetary ball milling). The hydrogen release reaction of magnesium hydride formed in the as-prepared 95Mg+5PVDF during reactive planetary ball milling started at 681 K. In the third cycle (CN = 3), the amount of hydrogen absorbed for 60 min, A (60 min), was 3.44 wt.% hydrogen at 573 K in 12 bar hydrogen. The PVDF is believed to have melted during reactive planetary ball milling, and the sliding or lubrication between Mg particles and hardened steel balls was avoided, leading to a good contact between them and a highly effective milling. The milling in hydrogen atmosphere in a planetary ball mill of Mg with PVDF is believed to have generated defects and cracks. The Mg2C3 produced from PVDF during hydrogen uptake-release cycling is believed to have been spread among particles and to have kept particles from coalescing. To the best of our knowledge, this is the first study to use a polymer PVDF as an additive material for the amelioration of hydrogen uptake and release features of Mg.

Hydriding and Dehydriding Features of a Titanium-Added Magnesium Hydride Composite

Magnesium has excellent hydrogen-storage properties except low hydriding and dehydriding rates. In the present work, titanium (Ti) was chosen as an additive to increase the hydriding rate of Mg and the dehydriding rate of MgH2. 15 wt.% Ti was added to MgH2 by milling in hydrogen (reactive mechanical grinding). The hydriding and dehydriding features of the Ti-added MgH2 composite (named 85 MgH2 + 15 Ti) were investigated. At the first cycle (n = 1), 85 MgH2 + 15 Ti absorbed 2.96 wt.% H for 2.5 min and 5.51 wt.% H for 60 min at 593 K in 12 bar H2, having an effective hydrogen-storage capacity of 5.51 wt.%. β-MgH2, γ-MgH2, TiH1.924, MgO, and MgTi2O4 were formed during reactive mechanical grinding. Reactive mechanical grinding of MgH2 with Ti is believed to create imperfections, produce cracks and clean surfaces, and decrease particle sizes. The phases formed during reactive mechanical grinding and their pulverization during reactive mechanical grinding are believed to make these effects stronger. Since the γ-MgH2 phase is believed to be decomposed at n = 1, the existence of the γ-MgH2 phase in the milled sample does not contribute to the improvement of the sorption behavior of Mg.

Milling Processes and Hydrogen Storage Properties of Mg-Graphene Composites

Graphene was chosen as an additive to improve the hydrogen uptake and release properties of magnesium (Mg). Five weight percent of graphene was added to Mg or pre-milled Mg by milling in hydrogen (reactive milling). The milling processes and hydrogen uptake and release properties of the graphene-added Mg were investigated. Adding graphene to Mg and then milling the mixture of Mg and graphene in hydrogen for 6 h [named M5G (6 h)] had little effects on the improvement of hydrogen uptake and release properties of Mg. Pre-milling of Mg (for 24 h) and then adding 5 wt.% of graphene by milling in hydrogen (for 30 min) (named M5G) significantly increased the hydrogen uptake and release rates and the quantities of hydrogen absorbed and released for 60 min of Mg. The activation of M5G was completed after cycle number, CN, of two (CN = 2). M5G had a high effective hydrogen-storage capacity of 6.21 wt.% at 623 K in 12 bar H2 at CN = 3. M5G released 0.25 wt.% hydrogen for 2.5 min and 5.28 wt.% hydrogen for 60 min H2 in 1.0 bar H2 at 623 K at CN = 3. Pre-milling of Mg and then adding graphene by milling in hydrogen and hydrogen uptake-release cycling are believed to create defects, produce cracks and clean surfaces, and decrease particle sizes. DOI: http://dx.doi.org/10.5755/j01.ms.25.3.20567