Deuterium absorption properties of V85Ni15 and evidence of isotope effect

The pressure-composition absorption isotherms of deuterium in an alloy with nominal composition V₈₅Ni₁₅ were measured between 87 and 400 °C for pressures up to ≈17 bar. For T ≤ 144 °C one observes the occurrence of a solid solution α phase for D/M < 0.15, the coexistence of α and β phase up to D/M ≈ 0.5 and a subsequent change to a different phase for higher deuterium concentrations. For temperatures higher than 200 °C only a solid solution with a deuteration enthalpy ΔH_deutr = 47 ± 2 kJ/mol is detected. The critical temperature of this V-Ni alloy (144 °C ≤ T*≤200 °C) is much higher than that of pure V (100 °C). Moreover, well above the critical temperature, one can observe a “normal” isotope effect, i.e. the equilibrium pressure of the V₈₅Ni₁₅-H system is higher than that of V₈₅Ni₁₅-D.     

Modeling the kinetic behavior of the Li-RHC system for energy-hydrogen storage: (I) absorption

The Lithium–Boron Reactive Hydride Composite System (Li-RHC) (2 LiH + MgB₂/2 LiBH₄ + MgH₂) is a high-temperature hydrogen storage material suitable for energy storage applications. Herein, a comprehensive gas-solid kinetic model for hydrogenation is developed. Based on thermodynamic measurements under absorption conditions, the system's enthalpy ΔH and entropy ΔS are determined to amount to −34 ± 2 kJ∙mol H₂⁻¹ and −70 ± 3 J∙K⁻¹∙mol H₂⁻¹, respectively. Based on the thermodynamic behavior assessment, the kinetic measurements' conditions are set in the range between 325 °C and 412 °C, as well as between 15 bar and 50 bar. The kinetic analysis shows that the hydrogenation rate-limiting-step is related to a one-dimensional interface-controlled reaction with a driving-force-corrected apparent activation energy of 146 ± 3 kJ∙mol H₂⁻¹. Applying the kinetic model, the dependence of the reaction rate constant as a function of pressure and temperature is calculated, allowing the design of optimized hydrogen/energy storage vessels via finite element method (FEM) simulations.     

A novel complex oxide TiVO3.5 as a highly active catalytic precursor for improving the hydrogen storage properties of MgH2

Herein, we demonstrate the successful preparation of a novel complex transition metal oxide (TiVO_3.5) by oxidizing a solid-solution MXene (Ti_0.5V_0.5)₃C₂ at 300 °C and its high activity as a catalyst precursor in the hydrogen storage reaction of MgH₂. The prepared TiVO_3.5 inherits the layered morphology of its MXene precursor, but the layer surface becomes very coarse because of the presence of numerous nanoparticles. Adding a minor amount of TiVO_3.5 remarkably reduces the dehydrogenation and hydrogenation temperatures of MgH₂ and enhances the reaction kinetics. The 10 wt% TiVO_3.5-containing sample exhibits optimal hydrogen storage properties, as it desorbs approximately 5.0 wt% H₂ in 10 min at 250 °C and re-absorbs 3.9 wt% H₂ in 5 s at 100 °C and under 50 bar of hydrogen pressure. The apparent activation energy is calculated to be approximately 62.4 kJ/mol for the MgH₂-10 wt% TiVO_3.5 sample, representing a 59% reduction in comparison with pristine MgH₂ (153.8 kJ/mol), which reasonably explains the remarkably reduced dehydrogenation operating temperature. Metallic Ti and V are detected after ball milling with MgH₂; they are uniformly dispersed on the MgH₂ matrix and act as actual catalytic species for the improvement of the hydrogen storage properties of MgH₂.     

Hydrogen storage properties of core-shell structured Mg@TM (TM = Co,V) composites

Target improving the hydrogen sorption properties of Mg, core-shell structured Mg@TM (TM = Co, V) composites were synthesized via an approach combining arc plasma method and electroless plating. The core-shell structures with the MgH₂ core and V or Co containing hydride shells for hydrogenated Mg@TM particles were observed through HAADF-STEM and HRTEM techniques. The measured hydrogenation enthalpy (ΔH_abs = ?70.02 kJ/mol H₂) and activation energy (E_a = 67.66 kJ/mol H₂) of the ternary Mg@Co@V composite were lower than those of binary composites and the pure Mg powder. In addition, the onset dehydrogenation temperature for the hydrogenated ternary composite measured from DSC was 323 °C, about 60 °C lower than that of pure MgH₂. On one hand, these improved properties can be attributed to the core-shell structure which may introduce more contacts between catalysts and Mg, thus providing more nucleation sites for hydrogen sorption. On the other hand, the co-effect of MgCo hydrides (Mg₂CoH₅&Mg₃CoH₅) acting as “hydrogen pump” and V₂H accelerating the dissociation of H₂ might also contribute to the improved hydrogen sorption properties of Mg.     

Temperature-hydrogen pressure phase boundaries and corresponding thermodynamics for ZrCoH system

The thermodynamically and kinetically stable regions of the temperature–H₂ pressure phase boundaries for the ZrCoH system were established using the Temperature-Concentration-Isobar (TCI) method. Based on this, the enthalpy change and entropy change values of dehydrogenation and disproportionation reactions were successfully obtained. The average enthalpy change (ΔH) and entropy change (ΔS) estimated from the phase boundaries for dehydrogenation of ZrCoH₃ to ZrCo are respectively 103.07 kJ mol^?1H₂ and 148.85 J mol^?1 H₂ K^?1, which are well agreement with the data reported in literature. The average ΔH and ΔS were estimated to be ?120.91 kJ mol^?1H₂ and -149.32 J mol^?1 H₂ K^?1 for the disproportionation of ZrCoH₃, whereas the ΔH and ΔS were calculated to be ?84.6 kJ mol^?1H₂ and -92.29 J mol^?1 H₂ K^?1 for disproportionation of ZrCo. In addition, it was found from the established phase boundaries that the anti-disproportionation property of ZrCo alloy can be enhanced if the phase boundaries of hydrogenation/dehydrogenation are far away from the phase boundaries of disproportionation by adjusting the thermodynamics. Meanwhile, it is possible to keep ZrCo away from disproportionation even at high temperature of 650 °C under hydrogen atmosphere, if the temperature-H₂ pressure trajectory is carefully controlled without crossing the phase boundaries of disproportionation. Therefore, the established phase boundaries can be used as a guide to the eye avoiding disproportionation and improving the anti-disproportionation property of ZrCo alloy.     

Structure, morphology and hydrogen storage properties of a Ti0.97Zr0.019V0.439Fe0.097Cr0.045Al0.026Mn1.5 alloy

The Ti_0.97Zr_0.019V_0.439Fe_0.097Cr_0.045Al_0.026Mn_1.5 alloy is a hexagonal C14 Laves phase material that reversibly stores hydrogen under ambient temperatures. Structural changes are studied by XRD and SEM with regard to hydrogenation and dehydrogenation cycling at 25, 40 and 60 °C. The average particle size is reduced after hydrogenation and dehydrogenation cycling through decrepitation. The maximum hydrogen capacity at 25 °C is 1.71 ± 0.01 wt. % under 78 bar H₂, however the hydrogen sorption capacity decreases and the plateau pressure increases at higher temperatures. The enthalpy (ΔH) and entropy (ΔS) of hydrogen absorption and desorption have been calculated from a van’t Hoff plot as −21.7 ± 0.1 kJ/mol H₂ and −99.8 ± 0.2 J/mol H₂/K for absorption and 25.4 ± 0.1 kJ/mol H₂ and 108.5 ± 0.2 J/mol H₂/K for desorption, indicating the presence of a significant hysteresis effect.     

Activity of Ni/CeO2 catalyst for gasification of phenol in supercritical water

An effective Ni/CeO₂ catalyst prepared by the polyol reduction method for degrading phenol into CH₄, H₂ and CO₂ in supercritical water (SCW) was developed. About 80% carbon gasification efficiency can be achieved at 525 °C and 60 min with 5 wt% phenol, 0.098 kg/m³ water density and 0.5 g Ni/CeO₂/g phenol catalyst, forming CH₄ and H₂ as the main gaseous products. Comparison study indicated that the efficiency of present Ni/CeO₂ catalyst was about 20% higher than that of a commercial catalyst, i.e., Ni/SiO₂Al₂O₃ from Sigma-Aldrich with 65 wt%Ni, at a reaction conditions of 500 °C and 30 min. The characterization analyses of BET, TPR, XRD, XPS and TEM indicated that there was a NiCe alloy formed in Ni/CeO₂, which could be important to enhance the activities of the carbon gasification efficiencies and gas yields. A kinetic modelings were conducted and the results showed that the lnA and the activation energy (Ea) of gasification were 7.1 ± 0.5 and 58.1 ± 3.2 kJ/mol for the gaseous product, and were 2.6 ± 0.9 and Ea is 36.6 ± 5.6 kJ/mol for the char formation, respectively. The present Ni-based-metal Ni/CeO₂ catalyst is cheaper and has a potential application for the gasification to convert phenol into gases fuels in SCW process.     

7-ethylindole: A new efficient liquid organic hydrogen carrier with fast kinetics

We report a discovery of a new member of the liquid organic hydrogen carrier (LOHC) family, 7-ethylindole (7-EID), with a low melting point of ?14 °C and a decent hydrogen content of 5.23 wt%. Hydrogenation of the compound was carried out over a commercial 5 wt% Ru/Al₂O₃ catalyst in the H₂ pressure range of 5–8 MPa and a temperature range of 120–160 °C, respectively. It was found that the hydrogenation rate positively correlates with the reaction temperature. However, the rate was barely effected by the H₂ pressure if the pressure exceeds 6 MPa. The estimated apparent activation energy of 7-EID hydrogenation is 51.5 kJ/mol. The fully hydrogenated product, octahydro-7-ethylindole (8H-7-EID), was used as the reactant for the dehydrogenation reaction at 170–200 °C over a 5 wt% Pd/Al₂O₃ catalyst. Full dehydrogenation of 8H-7-EID to 7-EID can be achieved within 270 min at 190 °C. The apparent activation energy of 8H-7-EID dehydrogenation was calculated to be 101.9 kJ/mol at 170–200 °C. The liberated H₂ was found to be of high purity, which meets the requirement of proton exchange membrane fuel cells.     

Destabilization of LiBH4 by SrF2 for reversible hydrogen storage

The de-/rehydrogenation features of the 6LiBH₄/SrF₂ reactive hydride system have been systematically investigated. It was found that the thermal stability of LiBH₄ can be reduced markedly by combining it with SrF₂. Dehydrogenation of the 6LiBH₄/SrF₂ system proceeds via the 6LiBH₄ + SrF₂ → SrB₆ + 2LiF + 4LiH + 10H₂ reaction, which involves SrH₂ as the intermediate product. The dehydrogenation enthalpy change was experimentally determined to be 52 kJ/mol H₂ based on the P–C isotherm analysis. For rehydrogenation, LiBH₄ and SrF₂ were regenerated along with LiSrH₃ at 450 °C under ～8 MPa hydrogen pressure; thus, approximately 5.2 wt% of hydrogen can be released during the second dehydrogenation process.     

Microstructure and hydrogen absorption/desorption properties of Mg24Y3M (M = Ni,Co, Cu,Al) alloys

Ternary alloys with the nominal composition of Mg₂₄Y₃M (M = Ni, Co, Cu, Al) have been fabricated by using vacuum induction melting method. Their microstructure and phase composition are characterized by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The isothermal hydrogen absorption and desorption kinetics are measured by a Sievert's-type apparatus. The dehydrogenation behaviors of the full hydrogenated alloys are also analyzed by differential scanning calorimetry (DSC) method. Results show that each and every alloy has a distinct multiphase structure containing the main phase Mg₂₄Y₅ and some amount of Mg. Intermetallic compounds of YCo₂ and Al₂Y are detected in the M = Co and M = Al alloy, while long-period stacking ordered (LPSO) phase can be also observed in M = Ni and M = Cu alloy. The hydrogen absorption and desorption kinetics shows a decreased trend in the following order: (M = Ni) > (M = Al) > (M = Co) > (M = Cu). The M = Ni alloy has the best hydrogen storage performance among the investigated alloys. The dehydrogenation activation energy (E_a) of the M = Ni alloy decreases to 66 kJ/mol, and its decomposition peak temperature is also reduced to 313 °C. Moreover, the p–c–T (pressure-composition isotherms) curves of the studied alloys are also discussed.     

Reactivity of Ni,Cr and Zr doped ceria in CO2 splitting for CO production via two-step thermochemical cycle

This work focuses on modification and screening of ceria-based oxides for solar H₂O/CO₂ splitting via two-step thermochemical cycle. Ce_1-xM_xO_2-δ (M = Zr, Ni, Cr; x = 0, 0.05, 0.10, 0.15, 0.20) were synthesized via sol-gel method and tested for CO₂-splitting via two-step thermochemical cycles. Reduction was conducted at 1500 °C through a ramp rate of 10 °C/min and oxidation was performed at 1000 °C isothermally. Both Ni and Cr showed low solubility in ceria and no or very limited promoting effect on CO productivity. Cr could be reduced in the first reduction step but cannot be oxidized by CO₂ in the following oxidation step. Zr doped sample showed advantages in both CO productivity and lattice stability. 15% Zr doped exhibited the best performance with the CO productivity of 315.40 μmol/g. However, the oxidation rate of Zr doped samples was much lower than that of pure ceria. Compromise between fuel productivity and fast kinetics should be made in practical application.     

Synthesis of BaFe12O19 by solid state method and its effect on hydrogen storage properties of MgH2

Previous studies have shown that ferrites give a positive effect in improving the hydrogen sorption properties of magnesium hydride (MgH₂). In this study, another ferrite, i.e., BaFe₁₂O₁₉, has been successfully synthesised via the solid state method, and it was milled with MgH₂ to enhance the sorption kinetics. The result showed that the MgH₂ + 10 wt% BaFe₁₂O₁₉ sample started to release hydrogen at about 270 °C which is about 70 °C lower than the as-milled MgH₂. The doped sample was able to absorb hydrogen for 4.3 wt% in 10 min at 150 °C, while as-milled MgH₂ only absorbed 3.5 wt% of hydrogen under similar conditions. The desorption kinetic results showed that the doped sample released about 3.5 wt% of hydrogen in 15 min at 320 °C, while the as-milled MgH₂ only released about 1.5 wt% of hydrogen. From the Kissinger plot, the apparent activation energy of the BaFe₁₂O₁₉-doped MgH₂ sample was 115 kJ/mol which was lower than the milled MgH₂ (141 kJ/mol)_. Further analyses demonstrated that MgO, Fe and Ba or Ba-containing contribute to the improvement by serving as active species, thus enhancing the MgH₂ for hydrogen storage.     

Study of the NaBH4–NaBr system and the behaviour of its low temperature phase transition

A mechano-chemical method was used to synthesize solid solution Na(BH₄)_1-xBr_x with 0 ≤ x ≤ 1. Samples with compositions of x ≤ 0.333 were annealed, in order to form a single phase material. Bromide substitution leads to smaller unit cell size and lower temperature and enthalpy of the order-disorder phase transition of NaBH₄. There is a linear relation between the amount of substitution, the temperature, the enthalpy and the kinetics of the phase transition. This linear relation between enthalpy and amount of substitution can be expressed by the function ΔH = ?6.268x + 1.206 where x is the amount of substitution and ΔH is the enthalpy.     

Assessment of hydrogen storage property of CaMgBH system using NMR and thermal analysis techniques

Thermal dehydrogenation of Ca(BH₄)₂ and Ca(BH₄)₂2MgH₂ composite has been investigated, and the results were compared. The Ca((BH₄)₂ dehydrogenated in two steps between 325 °C and 500 °C as per the reactions Ca(BH₄)₂ = CaH₂ + 2B + 3H₂, and Ca(BH₄)₂ = 1/3CaB₆ + 2/3CaH₂ + 10/3H₂. The partial dehydrogenation of CaH₂ also takes place during the second step dehydrogenation according to the reaction CaH₂ + 6B = CaB₆ + H₂. The completion of second step dehydrogenation requires a temperature of higher than 500 °C. The activation energies corresponding to these steps were found to be 149 ± 8 kJ/mol and 162 ± 10 kJ/mol, respectively. The Ca(BH₄)₂2MgH₂ composite dehydrogenates in a single step as per the reaction: Ca(BH₄)₂ + 3MgH₂ = CaMg₂ + 2B + 7H₂ + Mg. The dehydrogenation of Ca(BH₄)₂2MgH₂ started at 340 °C and completed before 450 °C. The activation energy of Ca(BH₄)₂2MgH₂ dehydrogenation was found to be 180 ± 8 kJ/mol.     

Reversible hydrogen storage in a 3NaBH4/YF3 composite

A 3NaBH₄/YF₃ hydrogen storage composite was prepared through ball milling and its hydrogen sorption properties were investigated. It is shown that NaBH₄ does not react with YF₃ during ball milling. The dehydrogenation of the composite starts at 423 °C, which is about 100 °C lower than the dehydrogenation temperature of pure NaBH₄, with a mass loss of 4.12 wt%. Pressure–Composition–Temperature tests reveal that the composite has reversible hydrogen sorption performance in the temperature range from 350 °C to 413 °C and under quite low hydrogenation plateau pressures (<1 MPa). Its maximum hydrogen storage capacity can reach up to 3.52 wt%. The dehydrogenated composite can absorb 3.2 wt% of hydrogen within 5 min at 400 °C. Based on the Pressure–Composition–Temperature analyses, the hydrogenation enthalpy of the composite is determined to be −46.05 kJ/mol H₂, while the dehydrogenation enthalpy is 176.76 kJ/mol H₂. The mechanism of reversible hydrogen sorption in the composite involves the decomposition and regeneration of NaBH₄ through the reaction with YF₃. Therefore, the addition of the YF₃ to NaBH₄ as a reagent forms a reversible hydrogen storage composite.     

Improved hydrogen storage properties of Mg/MgH2 thanks to the addition of nickel hydride complex precursors

In order to improve the hydrogenation/dehydrogenation properties of the Mg/MgH₂ system, the nickel hydride complex NiHCl(P(C₆H₁₁)₃)₂ has been added in different amounts to MgH₂ by planetary ball milling. The hydrogen storage properties of the formed composites were studied by different thermal analyses methods (temperature programmed desorption, calorimetric and pressure-composition-temperature analyses). The optimal amount of the nickel complex precursor was found to be of 20 wt%. It allows to homogeneously disperse 1.8 wt% of nickel active species at the surface of the Mg/MgH₂ particles. After the decomposition of the complex during MgH₂ dehydrogenation, the formed composite is stable upon cycling at low temperature. It can release hydrogen at 200 °C and absorb 6.3 wt% of H₂ at 100 °C in less than 1 h. The significantly enhanced H₂ storage properties are due to the impact of the highly dispersed nickel on both the kinetics and thermodynamics of the Mg/MgH₂ system. The hydrogenation and dehydrogenation enthalpies were found to be of −65 and 63 kJ/mol H₂ respectively (±75 kJ/mol H₂ for pure Mg/MgH₂) and the calculated apparent activation energies of the hydrogen uptake and release processes are of 22 and 127 kJ/mol H₂ respectively (88 and 176 kJ/mol H₂ for pure Mg/MgH₂). The change in the thermodynamics observed in the formed composite is likely to be due to the formation of a Mg_0.992Ni_0.008 phase during dehydrogenation/hydrogenation cycling. The impact of another hydride nickel precursor in which chloride has been replaced by a borohydride ligand, namely NiH(BH₄)(P(C₆H₁₁)₃)₂, is also reported.     

Catalytic effects of V and V2O5 on hydrogen storage property of Mg17Al12 alloy

A Mg₁₇Al₁₂ alloy was synthesized via sintering, and the catalytic effects of V and V₂O₅ on the hydrogen (H₂)-storage properties of this alloy were investigated. The results revealed that the hydrogenation/dehydrogenation temperature of Mg₁₇Al₁₂ decreased markedly and the reversible hydrogen storage properties improved with the addition of V or V₂O₅. For example, at 250 °C, the Mg₁₇Al₁₂ alloy underwent hydrogenation only and a hydrogen absorption capacity of 2.22 wt.% was realized. However, with the addition of V and V₂O₅, (i) reversible hydrogen absorption/desorption occurred, (ii) the hydrogen absorption capacity increased to 2.95 wt.% and 3.35 wt.%, and (iii) the hydrogenation/dehydrogenation enthalpy of the Mg₁₇Al₁₂alloy decreased from 65.7/83.1 kJ·mol^?1 to 62.6/69.3 kJ·mol^?1 and 59.9/68.1 kJ·mol^?1, respectively.     

Hydrogen storage properties of nanocrystalline Mg2Ni prepared from compressed 2MgH2Ni powder

In the present work, nanocrystalline Mg₂Ni with an average size of 20–50 nm was prepared via ball milling of a 2MgH₂Ni powder followed by compression under a pressure of 280 MPa. The phase component, microstructure, and hydrogen sorption properties were characterized by using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), pressure-composition-temperature (PCT) and synchronous thermal analyses (DSC/TG). Compared to the non-compressed 2MgH₂Ni powder, the compressed 2MgH₂Ni pellet shows lower dehydrogenation temperature (290 °C) and a single-phase Mg₂Ni is obtained after hydrogen desorption. PCT measurements show that the nanocrystalline Mg₂Ni obtained from dehydrogenated 2MgH₂Ni pellet has a single step hydrogen absorption and desorption with fairly low absorption (?57.47 kJ/mol H₂) and desorption (61.26 kJ/mol H₂) enthalpies. It has very fast hydrogen absorption kinetics at 375 °C with about 3.44 wt% hydrogen absorbed in less than 5 min. The results gathered in this study show that ball milling followed by compression is an efficient method to produce Mg-based ternary hydrides.     

The enhanced de/re‐hydrogenation performance of MgH2 with TiH2 additive

Catalytic effects of TiH₂ on hydrogenation/dehydrogenation kinetics of MgH₂ were investigated in this study. The TG analysis showed that the addition of the x wt% TiH₂ exhibited lower onset temperature of 160°C which is 100°C and 190°C lower than as‐milled and as‐received MgH₂. The dehydrogenation and hydrogenation kinetics were significantly improved compared with the pure MgH₂. The activation energy for the hydrogen desorption of MgH₂ was reduced from ?137.13 to ?77.58 kJ/mol by the addition of TiH₂. XRD and XPS results showed that the phase of TiH₂ remained same during the dehydrogenation without any intermediate formation confirming its role as catalyst.     

Effect of graphene templated fluorides of Ce and La on the de/rehydrogenation behavior of MgH2

The present investigation describes the hydrogen storage properties of MgH₂ ball milled with different additives i.e. graphene templated rare earth metal (La and Ce) fluorides, CeF₄ and LaF₃. MgH₂ ball milled with graphene templated CeF₄ (MgH₂:CeF₄@Gr) has onset desorption temperature of 245 °C, which is 50 °C, 52 °C and 75 °C lower than MgH₂ ball milled with LaF₃ templated graphene, CeF₄ and LaF₃ respectively. CeF₄@Gr also shows the superior effect amongst all additives during rehydrogenation where MgH₂:CeF₄@Gr absorbs 5.50 wt% within 2.50 min at 300 °C under 15 atm H₂ pressure. Dual tuning effect, i.e. lowering of thermodynamic (62.77 kJ/mol H₂: lower from 74 kJ/mol for pristine MgH₂) and kinetics barrier (93.01 kJ/mol) has been observed for MgH₂:CeF₄@Gr. Additionally, MgH₂ ball milled with CeF₄@Gr shows good reversibility up to 24 cycles of de/rehydrogenation. The feasible working mechanism of CeF₄@Gr as additive for MgH₂ has been studied in detail with the help of Transmission Electron Microscope (TEM), Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Diffraction characterizations (XRD).