Conversion of CH4 and Hydrogen Storage via Reactions with MgH2-12Ni

The main key to the future transition to a hydrogen economy society is the development of hydrogen production and storage methods. Hydrogen energy is the energy produced via the reaction of hydrogen with oxygen, producing only water as a by-product. Hydrogen energy is considered one of the potential substitutes to overcome the growing global energy demand and global warming. A new study on CH4 conversion into hydrogen and hydrogen storage was performed using a magnesium-based alloy. MgH2-12Ni (with the composition of 88 wt% MgH2 + 12 wt% Ni) was prepared in a planetary ball mill by milling in a hydrogen atmosphere (reaction-involved milling). X-ray diffraction (XRD) analysis was performed on samples after reaction-involved milling and after reactions with CH4. The variation of adsorbed or desorbed gas over time was measured using a Sieverts'-type high-pressure apparatus. The microstructure of the powders was observed using a scanning transmission microscope (STEM) with energy-dispersive X-ray spectroscopy (EDS). The synthesized samples were also characterized using Fourier transform infrared (FT-IR) spectroscopy. The XRD pattern of MgH2-12Ni after the reaction with CH4 (12 bar pressure) at 773 K and decomposition under 1.0 bar at 773 K exhibited MgH2 and Mg2NiH4 phases. This shows that CH4 conversion took place, the hydrogen produced after CH4 conversion was then adsorbed onto the particles, and hydrides were formed during cooling to room temperature. Ni and Mg2Ni formed during heating to 773 K are believed to cause catalytic effects in CH4 conversion. The remaining CH4 after conversion is pumped out at room temperature.

Three Methods for Application of Data from a Volumetric Method to the Kissinger Equation to Obtain Activation Energy

Thermal analysis methods have been used in many reports to determine the activation energy for hydride decomposition (dehydrogenation). In our preceding work, we showed that the dehydrogenation rate of Mg-5Ni samples obeyed the first-order law, and the Kissinger equation could thus be used to determine the activation energy. In the present work, we obtained the activation energy for dehydrogenation by applying data from a volumetric method to the Kissinger equation. The quantity of hydrogen released from hydrogenated Mg-5Ni samples and the temperature of the reactor were measured as a function of time at different heating rates (Φ) in a Sieverts-type volumetric apparatus. The values of dH_d/dt, the dehydrogenation rate, were calculated as time elapsed and the temperature (T_m) with the highest dH_d/dt was obtained. The values of dH_d/dT, the rate of increase in released hydrogen quantity (H_d) to temperature (T) increase, were calculated according to time, and the temperature (T_m) with the highest dH_d/dT was also obtained. In addition, the values of dT/dt, the rate of increase in temperature to time (t) increase, were calculated according to time, and the temperature (T_m) with the highest dH_d/dt was obtained. Φ and T_m were then applied to the Kissinger equation to determine the activation energy for dehydrogenation of Mg-5Ni samples.

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 TiH_1.924. TiH_1.924 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.

Study on the Variation in Microstructure of a Ferritic Stainless Steel with Surface Roughness and Thermal Cycling in Air

A ferritic stainless steel, Crofer 22 APU, is one of candidates for metallic interconnects of solid oxide fuel cells. Ferritic stainless steel Crofer 22 APU specimens with different surface roughnesses were prepared by grinding with SiC powder papers of various grits and were then thermally cycled. Polished Crofer 22 APU specimens after one thermal cycle and five thermal cycles had relatively straight oxide layers with similar thicknesses of 30 μm, suggesting that after one cycle (total oxygen exposure time of 100 h at 1073 K), the oxidation does not progress. Micrographs of a trench made by milling with the FIB (focused ion beam) for a Crofer 22 APU specimen rubbed with grit 80 SiC powder paper after 8 thermal cycles (total oxygen exposure time of 200 h at 1073 K), captured by ESB, InLens, and SE2, showed that the surface of the sample was very coarse and its oxide layer was undulated. In the oxide layer, the phase of the sublayer was Cr₂O₃, and that of the top layer was (Cr, Mn)₃O₄ spinel. The sample ground with grit 80 SiC powder paper after 60 thermal cycles (total oxygen exposure time of 1500 h at 1073 K) was very coarse. Some ridges were quite straight and continuous. After 20 and 40 thermal cycles, ASR (area specific resistance) decreased as the number of grit of the SiC powder paper increased, suggesting that the polished Crofer 22 APU is better than those with rougher surfaces for application as an interconnect of SOFC.

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.

Rate-Controlling Steps for the Hydriding Reaction of the Intermetallic Compound Mg₂Ni

Mg₂Ni samples were prepared by sintering a pelletized mixture under an argon atmosphere in a stainless steel crucible at 823 K. The XRD pattern of the prepared Mg₂Ni sample showed a well crystallized Mg₂Ni phase. The hydriding and dehydriding properties of the prepared samples were examined at 518-593 K under relatively low hydrogen pressures of 3-7 bar H₂. At 573 K under 7 bar H₂, the activation of the Mg₂Ni sample was completed at the number of cycles of six (n = 6). At n = 7, the hydrided fractions of the sample were 0.53 (1.99 wt% H) at 4.97 min, 0.72 (2.71 wt% H) at 9.52 min, 0.81 (3.05 wt% H) at 31.15 min, and 0.81 (3.05 wt% H) at 60.07 min. The particle sizes of the prepared Mg₂Ni were not homogeneous and the particles had irregular shapes. We analyzed the rate-controlling steps for the hydriding reaction of the intermetallic compound Mg₂Ni by examining the dependences of hydriding rates on hydrogen pressure and temperature in the same reacted fraction ranges. The analyses in the same reacted fraction ranges were done in order to eliminate the influence of the interfacial area on the hydriding rate. When the driving force, which is the difference between the applied hydrogen pressure P_o and equilibrium plateau pressure P_eq at a given temperature, was low, the nucleation of Mg₂Ni hydride controlled the hydriding rate of Mg₂Ni. After the nucleation of the Mg₂Ni hydride, the rate-controlling step of the hydriding reaction of Mg₂Ni was analyzed to be the forced flow of hydrogen molecules through pores, inter-particle channels, or cracks.

Increasing the Hydrogenation and Dehydrogenation Rates of Magnesium by Incorporating CMC(Na) (Carboxymethylcellulose-Sodium Salt) and Nickel

Samples with compositions of 95 wt% Mg + 5 wt% CMC(Na) [carboxymethylcellulose, sodium salt, {C₆H₇O₂(OH)_x(C₂H₂O₃Na)_y}_n] [named Mg-5CMC(Na)] and 90 wt% Mg + 10 wt% CMC(Na) [named Mg-10CMC(Na)] were prepared via milling in hydrogen (hydride-forming milling). Mg-5CMC(Na) and Mg-10CMC(Na) had very high hydrogenation rates but low dehydrogenation rates. Adding Ni to Mg is known to increase the hydrogenation and dehydrogenation rates of Mg. We chose Ni as an additive to increase dehydrogenation rates of Mg-5CMC(Na) and Mg-10CMC(Na). A sample with a composition of 90 wt% Mg + 5 wt% CMC + 5 wt% Ni [named Mg-5Ni-5CMC(Na)] was prepared via hydride-forming milling. The activation of Mg-5Ni-5CMC(Na) was completed at the third hydrogenation-dehydrogenation cycle (N ═ 3). Mg-5Ni-5CMC(Na) had an effective hydrogen-storage capacity (the quantity of hydrogen absorbed for 60 min) of 5.83 wt% at 593 K in 12 bar hydrogen at N ═ 3. Mg-5Ni-5CMC(Na) released 2.73 wt% H for 10 min and 4.61 wt% H for 60 min at 593 K in 1.0 bar hydrogen at N ═ 3. Mg-5Ni-5CMC(Na) dehydrogenated at N ═ 4 contained Mg and small amounts of MgO, β-MgH₂, Mg₂Ni, and Ni. Hydride-forming milling of Mg with CMC and Ni and Mg₂Ni formed during hydrogenation-dehydrogenation cycling are believed to have increased the dehydrogenation rates of Mg-5CMC(Na) and Mg-10CMC(Na). As far as we know, this study is the first in which a polymer CMC(Na) and Ni were added to Mg via hydride-forming milling to improve the hydrogenation and dehydrogenation rates of Mg.

Hydrogenation and Dehydrogenation Behaviors of Mg₂Ni Synthesized by Sintering Pelletized Mixtures Under an Ar Atmosphere

A Mg₂Ni intermetallic compound was synthesized by sintering under an argon atmosphere in a stainless steel crucible at 823 K. The hydrogenation and dehydrogenation features of the synthesized samples were investigated. Hydrogenation and dehydrogenation behaviors of Mg₂Ni were plotted using the Johnson-Mehl equation for the nucleation and growth mechanism. In addition, we analyzed the dependences of hydrogenation rates on hydrogen pressure and temperature and a rate-controlling step for the hydrogenation of Mg₂Ni after the nucleation of Mg₂Ni hydride. The XRD pattern of the Mg₂Ni sample synthesized by sintering pelletized mixtures under an Ar atmosphere in a stainless-steel crucible showed a well crystallized Mg₂Ni phase, revealing hardly any impurities. At 571 K under 30 bar H₂, the Mg₂Ni sample was activated completely at the number of cycles, n, of three. As the temperature increased form 474 K to 522 K, the initial hydrogenation rate and the quantity of hydrogen absorbed for 10 min increased. The rate-controlling step of the hydrogenation of Mg₂Ni after the nucleation of Mg₂Ni hydride was found to be the forced flow of hydrogen molecules through pores, interparticle channels, or cracks. The dehydrogenation after activation proceeded by a nucleation and growth mechanism and could be expressed by a Johnson-Mehl equation.

Improvement in the Hydrogen-Storage Characteristics of Magnesium Hydride by Grinding with Sodium Alanate and Transition Metals in a Hydrogen Atmosphere

In this work, MgH2 was used as a starting material instead of Mg. The sample was prepared by grinding MgH2 with sodium alanate and transition metals in a hydrogen atmosphere. Its hydriding and dehydriding properties were measured followed by X-ray diffraction (XRD) analyses and observations of its microstructure. Activation was not required for the 86MgH2 + 10Ni + 2NaAlH4 + 2Ti sample. At the first cycle (n = 1), the sample absorbed 4.96, 5.28 and 5.36 wt% H for 10, 15 and 60 min, respectively, at 593 K in 12 bar H2, showing that the sample absorbed quite a large amount of hydrogen for 60 min (nearly 5.5 wt% H). The initial hydriding rate increased as the temperature increased from 423 K to 553 K and decreased from 553 K to 593 K. The sample showed quite high hydriding rates at relatively low temperatures 423 K (at n = 1) and 473 K (at n = 2) in 12 bar H2, compared with those of other metallic element(s) or compound(s)-added Mg or MgH2 alloys, absorbing 2.89 wt% H for 5 min, 2.97 wt% H for 10 min, and 3.31 wt% H for 60 min at 473 K.

Development of an Mg-Based Alloy with High Hydriding and Dehydriding Rates and Large Hydrogen Storage Capacity by Adding TaF5

A sample with a composition of 95 wt% Mg + 5 wt% TaF5 (named Mg-5TaF5) was prepared by reactive mechanical grinding. The activation of Mg-5TaF5 was not necessary, and Mg-5TaF5 had an effective hydrogen storage capacity (the quantity of hydrogen absorbed for 60 min) larger than 5 wt%. At the first cycle (n = 1), the sample absorbed 4.50 wt% H for 10 min and 5.06 wt% H for 60 min at 593 K under 12 bar H2. At n = 1, the sample desorbed 1.58 wt% H for 10 min and 4.93 wt% H for 60 min at 593 K under 1.0 bar H2. The Mg-5TaF5 sample dehydrided at n = 3 contained MgF2 and Ta2H. The hydriding-dehydriding cycling of the sample, which forms MgF2 and Ta2H by reaction with hydrogen, is considered to produce defects on the surface of and inside the Mg particles, to create clean surfaces, and to reduce the particle size of Mg, due to the repetition of expansion with hydrogen absorption and contraction with hydrogen release. Mg-5TaF5 had a higher hydriding rate and a higher dehydriding rate after an incubation period and greater quantities of hydrogen absorbed and desorbed for 60 min than Mg-10TaF5, Mg-10MnO, or Mg-10Fe2O3.

Enhancement of the Hydrogen-Storage Properties of MgH2 by the Addition of Ni, NaAlH4, Ti, and CNT via Reactive Mechanical Grinding

MgH2 was used as the starting material in this study. A sample with the composition of 84 wt% MgH2 + 10 wt% Ni + 2 wt% NaAlH4 + 2 wt% Ti + 2 wt% CNT (named MgH2-10Ni-2NaAIH4-2Ti-2CNT) was prepared by the reactive mechanical grinding. Hydriding and dehydriding property measurements, X-ray diffraction (XRD) analyses, and microstructural observations were then performed. The activation of the sample was not required. At the first cycle (n = 1), the sample absorbed 2.84 wt% H for 5 min, 3.75 wt% H for 10 min, 4.09 wt% H for 15 min, and 4.17 wt% H for 60 min at 593 K under 12 bar H2. The MgH2-10Ni-2NaAlH4-2Ti-2CNT sample showed quite a high hydriding rate at a relatively low temperature of 423 K under 12 bar H2 (at n = 4), absorbing 2.81 wt% H for 5 min, 3.23 wt% H for 10 min, and 3.56 wt% H for 60 min. The reactive mechanical grinding of MgH2 with Ni, NaAlH4, Ti, and CNT is considered to create defects on the surface and in the interior of MgH2 as well as decrease the particle size of MgH2-

Hydrogen-Storage Properties of Ni and LiBH4-Added MgH2

In this work, MgH2 was employed as a starting material instead of Mg used in our previous work. Ni and LiBH4, which can absorb 18.4 wt% of hydrogen, were added. A sample with a composition of 86 wt% MgH2 + 10 wt% Ni + 4 wt% LiBH4 (named MgH2-10Ni-4LiBH4) was prepared by milling under hydrogen (reaction-involved milling) and its hydrogen-storage properties were examined. In addition, the rate-limiting step for the dehydriding reaction of the sample at the first cycle was analyzed. The activation of MgH2-10Ni-4LiBH4 for hydriding and dehydriding reactions was not required. The as-milled sample absorbed and released nearly 5 wt% H at 623 K for 60 min; it absorbed 4.90 wt% H under 12 bar H2 for 20 min and released 4.94 wt% H under 1.0 bar H2 for 60 min. The hydriding rate exhibited an inverse dependence on temperature. This is due to a decrease in the driving force for the hydriding reaction (the difference between the applied hydrogen pressure and the equilibrium plateau pressure) with the increase in temperature.