Activation Energy For Hydrogen Desorption Research Articles

Improvement of dehydrogenation performance by adding CeO2 to α-AlH3

The α-AlH3 + nCeO2 (n = 0.5, 1 and 2, mol%) composite materials were prepared by ball milling, and the phase composition and dehydrogenation performance of the composites were investigated. The results revealed that α-AlH3 doped with CeO2 could effectively decrease the dehydrogenation temperature. The dehydrogenation onset temperature of α-AlH3 + 2 mol% CeO2 dropped to 106 °C, which was significantly reduced by 24.3% compared with pure α-AlH3. At 100 °C, the α-AlH3 + 2 mol% CeO2 composite released 3.8 wt% hydrogen within 100 min, while the pure α-AlH3 released only 0.12 wt% hydrogen in the same duration. Kissinger analysis indicated that apparent activation energy for hydrogen desorption of α-AlH3 was significantly decreased with CeO2 doping. The improvement of dehydrogenation performance was due to the finely dispersed CeH2+x, which was formed by the reaction between CeO2 and α-AlH3.

In-situ synthesis of Mg2Ni-Ce6O11 catalyst for improvement of hydrogen storage in magnesium

To decrease the hydrogen sorption temperature and increase the hydrogen sorption rate is important for the practical application of magnesium for hydrogen storage. The binary nano-catalysts Mg2Ni and Ce6O11 with oxygen vacancy defects was in-situ synthesized on Mg surface via the hydrogen activation of Mg-Ni-CeO2. The hydrogen storage material Mg-20Ni-CeO2 can release 4.19 wt% H2 in 5 min at 320 °C, which is significantly higher than that of Mg with 10 wt% Ni (3.44 wt% H2) or 10 wt% CeO2 (0.34 wt% H2). The peak temperature and apparent activation energy for hydrogen desorption of Mg-20Ni-CeO2 are reduced by 115.8 °C and 63.89 kJ/mol respectively comparing with that without catalyst. Structural analysis suggests that Mg2Ni and Ce6O11 can be in-situ synthesized on Mg surface, which shows a synergistic catalysis for hydrogen storage. During H2 absorption, the oxygen vacancy defects on Ce6O11 surface can trap H2 molecules, while the Mg2Ni on Mg/Mg2Ni interface can promote H2 dissociation. For H2 desorption, the Mg2Ni can weaken the Mg-H bond and act as “hydrogen pump” to transfer H for H2 formation with the assistance of Ce3+/Ce4+ transformation in Ce6O11. The results of this study provide a new horizon for a novel binary catalyst design for hydrogen storage.

Enhancement of Formic Acid Dehydrogenation Selectivity of Pd(111) Single Crystal Model Catalyst Surface via Brønsted Bases

The influence of ammonia (NH3) on the doubly deuterated formic acid (DCOOD, FA) dehydrogenation selectivity for a Pd(111) single crystal model catalyst surface was investigated under ultrahigh vacuum conditions using temperature-programmed desorption and temperature-programmed reaction spectroscopy techniques. NH3 adsorption on Pd(111) revealed reversible, molecular desorption without any significant decomposition products, while DCOOD adsorption on Pd(111) yielded D2, D2O, CO, and CO2 as a result of dehydration and dehydrogenation pathways. Functionalizing the Pd(111) surface with ammonia suppressed the FA dehydration and enhanced the dehydrogenation pathway. The boost in the FA dehydrogenation of Pd(111) in the presence of NH3 can be linked to the ease of FA deprotonation as well as the stabilization of the decomposition intermediate (i.e., formate) due to the presence of ammonium counterions on the surface. In addition, the presence of a H-bonded ammonia network on the Pd(111) surface increased the hydrogen atom mobility and decreased the activation energy for molecular hydrogen desorption. In the presence of NH3, catalytic FA decomposition on Pd(111) also yielded amidation reactions, which further suppressed CO liberation and prevented poisoning of the Pd(111) active sites due to strongly bound CO species.

Hydrogen storage thermodynamics and dynamics of Mg–Y–Ni–Cu based alloys synthesized by melt spinning

Excessive heat stability, poor kinetics of hydrogen absorption and desorption reaction are viewed as the major setbacks for the Mg-based hydrogen storing materials application. The microstructures of samples Mg25-xYxNi9Cu (x = 0, 1, 3, 5, 7) alloys were observed by XRD, SEM, and TEM. Besides, aiming at studying the characteristics of hydrogenation/dehydrogenation, the Sievert instrument, DSC and TGA were applied in the experiments. Substituting Mg with Y can promote the formation of the second phase YNi3 but does not transform the Mg2Ni major phase. This substitution greatly prompts the amorphization. The increase of Y dosage triggers a mild decrease in the absolute values of ΔH and ΔS and remarkably boosts the improvement of dehydrogenation kinetics. Moreover, the increase in Y amount brings on a decrease in hydrogen absorption capability. The hydrogen desorption activation energy markedly lowers with increasing Y content (the Eade values of as-spun Y0 and Y3 samples are 71.38 and 66.13 kJ/mol, severally, and Ekde values of them are 69.03 and 65.53 kJ/mol, respectively), which brings about the promotion of dehydrogenation kinetics of the experimental samples.

A new strategy for remarkably improving anti-disproportionation performance and cycling stabilities of ZrCo-based hydrogen isotope storage alloys by Cu substitution and controlling cutoff desorption pressure

ZrCo1-xCux (x = 0–0.3) alloys for hydrogen isotope storage were prepared via induction levitation melting. The effects of partial substitution of Cu for Co on the microstructure, hydrogen storage properties including hydriding-dehydriding kinetics, thermodynamic characteristics, cycling stability and related mechanism have been systematically investigated. It is found that all synthesized alloy ingots consist of ZrCo main phase, the grain size is further refined but the segregation of Cu element at grain boundary is more serious with the increase of Cu content. The pressure-composition isotherms for dehydrogenation show that both ZrCoH3 and ZrCo0.9Cu0.1H3 hydrides undergo one-step desorption, while ZrCo0.8Cu0.2H3 and ZrCo0.7Cu0.3H3 hydrides experience two-step desorption since a new middle hydride phase of ZrCoH0.8 with CrB-type orthorhombic structure is discovered during their dehydrogenation. This result is proposed to be linked with the changed electronic structure and lattice distortion induced by Cu substitution. To compare their dehydriding kinetics, the apparent activation energies for hydrogen desorption of different hydride phases in the alloys are also calculated systematically, and the value of Ea for hydrogen desorption of ZrCoH3 phase is decreased from 118.02 kJ/mol to 95.90 kJ/mol, 77.98 kJ/mol and 78.65 kJ/mol, respectively. Prominent cycling stabilities of the Cu-substituted alloys are obtained and follow the trend: ZrCo0.8Cu0.2 > ZrCo0.7Cu0.3 > ZrCo0.9Cu0.1 > ZrCo. Specifically, the optimum cycling stable capacity of ZrCo-based alloy is increased from 0.4 wt % to 1.21 wt %. Furthermore, the ZrCo0.8Cu0.2 alloy exhibits further enhanced cycling stability during the cycle by controlling cutoff desorption pressure at 0.253 bar, where only the first-step dehydrogenation happens. Therefore, a new strategy for improving anti-disproportionation performance of ZrCo-based alloys by controlling cutoff desorption pressure is also proposed, which is in favor of the enhanced cycling stability and further application of Cu-substituted ZrCo-based alloys for hydrogen isotope storage and delivery in the International Thermonuclear Experimental Reactor (ITER).

Hydrogenation properties of MgH2- x wt% AC (x=0, 5, 10, 15) nanocomposites

Magnesium is considered as a promising candidate for hydrogen storage due to its high storage capacity (theoretical value ~ 7.6 wt%). Nanocomposites of Magnesium hydride and activated charcoal (AC) were prepared using ball milling method. These nanocomposites were characterized by XRD, TGA, DSC and SEM techniques. The TGA analysis show that the MgH2-5 wt% AC nanocomposite exhibits dehydrogenation capacity of 7.45 wt% (which is very close to the storage capacity of MgH2) and starts release of hydrogen at 140 °C temperature. The results from the Kissinger plot from DSC result showed that the activation energy for hydrogen desorption of MgH2 with 5 wt% AC was reduced compared to those of as-received.

Effect of catalysts on microstructure, hydrogen storage thermodynamics, and kinetics performance of La5Mg85Ni10 alloy

The effects of the type and amount of transition metal catalyst on the microstructure and hydrogen storage performance of La5Mg85Ni10 + x wt.% M (x = 1, 3, 5, 7; M = TiF3, NbF5, Cr2O3) alloys milled for 10 h have been investigated. The evolution of microstructure and phase of catalyzed alloys in the absorption/desorption process have been characterized by XRD and HRTEM. The results show that the hydrogen storage capacity of the alloy decreases as the catalyst increases. On the one hand, the catalytic effects of different amount of catalyst TiF3 were studied. TiF3 exists in form of MgF2 and TiH2 phases and Ea decreases firstly and then increases as the amount of TiF3 increases. When 5 wt.% TiF3 is added, the hydrogen desorption activation energy shows the lowest Ea = 45.2 kJ/mol. On the other hand, the catalytic effects of TiF3, Cr2O3 and NbF5 are compared in detail. It was found that TiF3 has better catalytic effect than Cr2O3 and NbF5 due to TiF3 nanoparticles can refine the grains better, provide hydrogen diffusion channels and reduce the nucleation driving force of the alloys.

Effects of nano-composites (FeB, FeB/CNTs) on hydrogen storage properties of MgH2

The carbon nanotubes (CNTs), the as-prepared FeB nanoparticles and the CNTs decorated with FeB nanoparticles (denoted as FeB/CNTs) are introduced respectively into MgH2 system by wet milling with cyclohexane (CYH), and results show that wet milling with CYH and adding dopants (CNTs, FeB or FeB/CNTs) both improve the hydrogen storage properties of MgH2. The MgH2−10 wt% FeB/CNTs (CYH) composite shows the best properties. Its onset dehydrogenation temperature is 196 °C, which is 114 °C lower than that of the MgH2 (without CYH). Furthermore, the hydrogen desorption activation energy of MgH2−10 wt% FeB/CNTs (CYH) is reduced to 89.7 kJ/mol. The composite starts to absorb hydrogen at temperature as low as 39 °C, and can take up 6.2 wt% of H2 at 150 °C and 5 MPa H2 within 10 min. The hydrogen sorption cycling measurements indicate that the kinetics of the MgH2−10 wt% FeB/CNTs (CYH) composite keeps almost stable within 20 cycles, and the hydrogen storage capacity remains at 6.02 wt% (93% of the initial capacity) after 20 cycles. It is believed that the in-situ formed Fe and B as the active species and the good thermal conductivity of CNTs help enhance the hydrogen storage properties of the MgH2.

Enhanced dehydrogenation properties of LiAlH4–Mg2NiH4 nanocomposites via doping Ti-based catalysts

Metal alanate and magnesium-based hydride complex has potential to promote the application of hydrogen storage materials in energy storage systems. In this work, the hydrogen storage properties of LiAlH4–Mg2NiH4 destabilized system with Ti-based additives were investigated. Both the dehydrogenation temperature and kinetics of LiAlH4–Mg2NiH4 were significantly improved by doping TiF3, surpassing those of the Ti- and TiO2-doped LiAlH4–Mg2NiH4 composites. Particularly, the onset dehydrogenation temperature of the TiF3 doped LiAlH4–Mg2NiH4 composite was reduced to as low as 50 °C, far superior to that of the undoped sample and the Ti- and TiO2-doped samples. The hydrogen desorption activation energy of TiF3-doped LiAlH4–Mg2NiH4 composite was 81.56 kJ mol−1 fitted by Johnson–Mehl–Avrami model, which was lower than that of the undoped composite (85.30 kJ mol−1). The XPS results indicated that Al3Ti (Ti0) phase could be in situ formed during dehydrogenation process, exerting positive catalytic effect on the dehydrogenation of the TiF3-doped LiAlH4–Mg2NiH4 composite. Moreover, the TiF3-doped LiAlH4–Mg2NiH4 composite also exhibited desirable re-hydrogenation behavior.

Effect of LiCe(BH4)3Cl with a high Li ion conductivity on the hydrogen storage properties of Li[sbnd]Mg[sbnd]N[sbnd]H system

The effect of LiCe(BH4)3Cl on the hydrogen storage properties of Mg(NH2)22LiH system was studied systematically, which has a high Li ion conductivity. The hydrogen desorption temperatures for LiMgNH system shift to lower temperatures by 0.05LiCe(BH4)3Cl doping with the onset temperature of dehydrogenation decreasing by 40 °C and the peak temperature decreasing by 30 °C. The Mg(NH2)22LiH–0.03LiCe(BH4)3Cl composite exhibits an improved comprehensive hydrogen storage properties, which can reversibly store about 5.0 wt% hydrogen at 160 °C, and released hydrogen as much as 8.6 times faster than that of the Mg(NH2)22LiH composite at 160 °C. The results indicated that the LiCe(BH4)3Cl-containing sample exhibited much better cycling properties than that of Mg(NH2)22LiH sample. XRD and FTIR results show that the structure of LiCe(BH4)3Cl does not change before and after hydrogen absorption/desorption, indicating it plays the catalytic effect. The hydrogen desorption activation energy of Mg(NH2)22LiH doped with 0.03LiCe(BH4)3Cl was reduced by 37.5%. The rate-controlling step of desorption shifted from the diffusion to the chemical reaction by the addition of LiCe(BH4)3Cl, indicating that the diffusion rates of small ions like H+, Li+ and Mg2+ in LiMgNH system are significantly enhanced, which could be well explained by the improved ionic conductivity of LiCe(BH4)3Cl doped sample.

Microstructure and hydrogen storage properties of V48Fe12Ti15-xCr25Alx (x=0, 1) alloys

The microstructure and hydrogen storage characteristics of V48Fe12Ti15-xCr25Alx (x = 0, 1) alloys prepared by vacuum arc melting were studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and pressure–composition isotherm measurements. It was confirmed that all of the alloys comprise a BCC phase, a Ti-rich phase, and a TiFe phase. Al as a substitute for part of the Ti content caused an increase of lattice parameters of the BCC phase and of the equilibrium pressures of hydrogen desorption, but decrease of the hydrogen storage capacities. The kinetic mechanism of the hydrogenation and dehydrogenation of the alloys was investigated by the classical Johnson–Mehl–Avrami equation. The reaction enthalpies (ΔH) for the dehydrogenation of alloys without and with Al were calculated by the Van't Hoff equation based on the PCI measurement data, which are 30.12 ± 0.14 kJ/mol and 28.02 ± 0.46 kJ/mol, respectively. The thermal stability of the metal hydride was measured by differential scanning calorimetry. The hydrogen desorption activation energies were calculated using the Kissinger method as 79.41 kJ/mol and 83.56 kJ/mol for x = 0 and 1, respectively. The results suggest that the substitution of titanium with aluminum improves the thermodynamic properties of hydrogen storage and reduces the kinetic performance of hydrogen desorption.

Effect of rGO supported NiCu derived from layered double hydroxide on hydrogen sorption kinetics of MgH2

A reduced graphene oxide-supported NiCu nanocatalyst (NiCu/rGO) was prepared via the reduction of self-assembled layered double hydroxide (LDH) and graphene oxide (GO) composite precursor. The effect of NiCu/rGO on the hydrogen sorption kinetics of MgH2 was investigated systematically. TEM observations reveal that the ultrafine NiCu (6 nm) particles highly disperse on the rGO surface. Hydrogen storage measurements reveal that MgH2-5 wt.% NiCu/rGO has superior hydrogen sorption kinetics over as-milled MgH2 and MgH2-5 wt.% NiCu without rGO. MgH2-5 wt.% NiCu/rGO absorbs 5.0 wt% hydrogen within 100 s at 200 °C, while MgH2-5 wt.% NiCu only absorbs 2.0 wt% hydrogen and as-milled MgH2 hardly absorbs hydrogen under the same conditions. Moreover, MgH2-5 wt.% NiCu/rGO starts to desorb hydrogen at 185 °C, which is 115 °C lower than that of as-milled MgH2, and desorbs 5.8 wt% hydrogen within 1200 s at 300 °C. The apparent activation energy for hydrogen desorption of MgH2-5 wt.% NiCu/rGO is 71.7 kJ/mol H2, significantly lower than 107.3 kJ/mol H2 for MgH2-5 wt.% NiCu. The enhanced hydrogen sorption kinetics of MgH2-5 wt.% NiCu/rGO mainly attributes to the synergistic effect between rGO and NiCu nanoparticles. This research extends the knowledge of designing efficient catalyst via layered double hydroxides precursor in the hydrogen storage materials.

Effects of doping with yttrium on the hydrogen storage performances of the La2Mg17 alloy surface

A simple method of the magnetron sputtering is applied on the surface modification process of the hydrogen storage alloys. Surface doping of La2Mg17 alloy with yttrium-based coatings is illustrated in the research. Some new intermetallics can be formed on the alloys surface via three steps of magnetron sputtering, surface absorption and heat treatment molding. For the hydrogen storage properties, the reversible capacity and cycle stability of the doped sample are all improved, and the hydrogen desorption activation energy is decreased. On the other hand, the surface conditions during the hydrogen absorption and desorption process are also discussed through the SEM, XRD and XPS analyses.

Highly ameliorated gaseous and electrochemical hydrogen storage kinetics of nanocrystalline and amorphous CeMg12-type alloys by mechanical milling

An attempt that effectively advances the quantity of hydrogen storage for CeMg12-type alloys was adopting a method of partial substitution of Mg in alloy by Ni. It is also employing mechanical ball-milling technology to prepare the nanocrystalline and amorphous CeMg11Ni + x wt.% Ni (x = 100, 200) alloys. And these two factors, Ni content after replacement as well as ball grinding time, affecting the structures and hydrogen storage dynamics belonging experimental alloys were studied systematically. Thereinto, characterization analysis on structures was using XRD, SEM and HRTEM. Then, the test of gaseous hydrogen storage performance was carried out through Sievert device as well as differential scanning calorimeter (DSC) that connected with H2 detector. Besides, Arrhenius and Kissinger equations were served as a convenient method for calculating the hydrogen desorption activation energy of alloys. Finally, it can adopt the automatic galvanostatic system to measure the electrochemical hydrogen storage performance. It is shown that, for experimental alloys, the more Ni content shows beneficial on the formation of glass, meanwhile, it also strikingly ameliorates both gaseous and electrochemical hydrogen storage dynamics properties. In addition, milling time varying brings a very large impact upon hydrogen storage properties of alloys. When the ball-grinding time is constantly changing, the gaseous hydrogen absorption capacity, gaseous hydriding rate as well as high-rate discharge ability (HRD) of alloys are able to appear the maximum values respectively. However, following the prolongation of milling time, both electrochemical discharge capacity and gaseous dehydrogenating rate raise all the time. The increase in Ni content and the prolongation of milling time can both enhance the gaseous hydrogen storage dynamics of alloys, inferring that their essence all can be attributed to the reduction of the activation energy of hydrogen desorption.

Synergistic catalysis in monodispersed transition metal oxide nanoparticles anchored on amorphous carbon for excellent low-temperature dehydrogenation of magnesium hydride

Magnesium hydride (MgH2) has been considered to be one of the most promising solid-state hydrogen storage materials owing to its high hydrogen capacities, excellent reversibility and abundant source. However, the high dehydrogenation energy barrier and poor kinetics embarrass the practical application of MgH2 in fuel cell. Doping nano-catalyst is deemed to be the most effective method to improve kinetics property of hydrogen storage materials, but the nanoparticles generally suffer from agglomeration and inactivation during the cycling hydrogen storage. Here we present a promising strategy to facilely prepare a high-efficiency transition metal oxide nano-catalyst, TiO2 nanoparticles, in which monodispersed single-crystal-like TiO2 nanoparticles are wrapped with amorphous carbon. The in-situ synthesized TiO2 nanoparticles/amorphous carbon catalyst exhibit superior catalytic effect on the dehydrogenation properties of MgH2. A significant reductions of hydrogen desorption temperature (163.5 °C) and activation energy (69.2 kJ mol−1) have been obtained for the TiO2 nanoparticles/amorphous carbon catalyzed MgH2, which can be fully rehydrogenated with a reversible capacity of about 6.5 wt% at 200 °C within 5 min, and then completely dehydrogenated at 275 °C within 10 min. It is demonstrated that such significantly improved hydrogen desorption properties can be attributed to the in-situ formation of TiO2 nanoparticles, amorphous carbon and multi-valance Ti species, which play the synergistically catalytic roles in the nano-catalyst. In particular, the presence of amorphous carbon in the catalyst can not only prevent the aggregation and growth of catalyst nanoparticles, but also dramatically reduce the desorption energy value of H in MgH2, according to the density functional theory calculation. This finding opens a new venue for the synthesis of monodispersed single-crystal-like TiO2 nanoparticles/amorphous carbon catalyst with high-activity, safety, low cost, and its practical application in MgH2 and other hydrogen storage systems.

Gaseous hydrogen storage properties of Mg-Y-Ni-Cu alloys prepared by melt spinning

For purpose of promoting the hydrogen absorption and desorption thermodynamics and kinetics properties of MgNi-based alloys, partially substituting Y and Cu for Mg and Ni respectively and melt spinning technique were applied for getting Mg25–xYxNi9Cu (x = 0–7) alloys. Their microstructures and phases were characterized with the help of X-ray diffraction and transmission electron microscopy. Their hydrogen absorbing and desorbing properties were tested by a Sievert apparatus, DSC, and TGA, which were connected with a H2 detector. In order to estimate the dehydrogenation activation energy of alloy hydride, both Arrhenius and Kissinger methods were applied for calculation. It is found that their hydriding kinetics notably declines, however, their hydrogen desorption kinetics conspicuously improves, with spinning rate and Y content increasing. Their hydrogen desorption activation energy markedly decreases under the same constraint, and it is found that melt spinning and Y substituting Mg improve the real driving force for dehydrogenation. As for the tendency of hydrogen absorption capacity, it presents an elevation firstly and soon after a decline with the rising of spinning rate, however, it always lowers with Y content growing. With Y content and spinning rate increasing, their thermodynamic parameters (ΔH and ΔS absolute values) visibly decrease, and the starting hydrogen desorption temperatures of alloy hydrides obviously lower.

Mg-based Nanocomposites for hydrogen storage containing La23Nd8.5Ti1.1Ni33.9Co32.9Al0.65 alloys as additives

Abstract Mg based nanocomposites containing different concentrations of La23Nd8.5Ti1.1Ni33.9Co32.9Al0.65 alloy has been undertaken at present work. Doped alloy was prepared by arc melting furnace and ball milled to get its nano structured. Hydrogen sorption properties of MgH2+ x wt% (x=10, 25 and 50) La23Nd8.5Ti1.1Ni33.9Co32.9Al0.65 alloy was characterized for their thermal, morphological and structural properties. Result shows that the alloy significantly reduced onset temperature of desorption. The TGA analysis showed higher desorption temperature for milled MgH2 compared to x wt% alloy added MgH2. Furthermore, the estimated activation energy for hydrogen desorption was found to be -177.90 , -204.65, -79.15 and -185.29 kJ/mol for milled ,10 wt%, 25 wt% and 50 wt % MgH2 respectively and also shows the reduction of activation energy by 98 kJ/mol due to the addition of 25 wt% alloy additives resulted the improved hydrogen storage capacity. The phase and purity of MgH2 during milling was studied by XRD and it is revealed that maximum stable and improved phase alteration for Hydrogen sorption was obtained during 25 wt % alloy milling. Morphological studies by SEM were undertaken to investigate the effect of hydrogenation of nanostructured alloy. DSC investigations were carried out to investigate the effect of alloy on hydrogen absorption behaviour of MgH2.

Tuning the de/hydriding thermodynamics and kinetics of Mg by mechanical alloying with Sn and Zn

Mg95Sn3Zn2 alloy was prepared by mechanical alloying. The phase constituents and phase transition were analyzed by X-ray diffraction (XRD) method. The microstructure was characterized by scanning electron microscope (SEM). The hydrogen storage properties were evaluated in detail by the measurements of isothermal hydrogen absorption and desorption, and pressure-composition isotherms (PCI) using the Sieverts method. The addition of Zn benefits to extend the solubility of Sn in the Mg lattice, as a result supersaturated Mg(Sn, Zn) ternary solid solution was synthesized by mechanical alloying, which decomposed to MgH2, Sn and MgZn2 in the hydrogenating process. The in situ formed nanostructure Mg2Sn and MgZn2 have positive effects on the hydrogen absorption and desorption of Mg. Mg95Sn3Zn2 alloy showed significantly improved kinetics with lowered hydrogen absorption and desorption activation energies of 38.1 kJ/mol and 86.6 kJ/mol respectively, and exhibited a reduced dehydriding enthalpy of 67.0 ± 1.9 kJ/(mol·H2).

Dehydrogenation Properties and Catalytic Mechanism of the K2NiF6-Doped NaAlH4 System.

The K2NiF6 catalytic effect on the NaAlH4 dehydrogenation properties was studied in this work. The desorption temperature was studied using temperature-programmed desorption and exhibited a lower onset hydrogen release after doped with different wt % of K2NiF6 (5, 10, 15 and 20 wt %). It was found that the NaAlH4 doped with 5 wt % K2NiF6 showed the optimal value that can reduce the onset desorption temperature of about 160 °C compared to 190 °C for the milled NaAlH4. The NaAlH4 + 5 wt % K2NiF6 sample showed faster desorption kinetics where 1.5 wt % of hydrogen was released in 30 min at 150 °C. In contrast, the milled NaAlH4 only released about 0.2 wt % within the same time and temperature. From the Kissinger analysis, the apparent activation energy was 114.7 kJ/mol for the milled NaAlH4 and 89.9 kJ/mol for the NaAlH4-doped 5 wt % K2NiF6, indicating that the addition of K2NiF6 reduced the activation energy for hydrogen desorption of NaAlH4. It is deduced that the new phases of AlNi, NaF, and KH that were formed in situ during the dehydrogenation process are the key factors for the improvement of dehydrogenation properties of NaAlH4.

Improved hydrogen storage performance of as-milled Sm–Mg–Ni alloy by adding CeO2

To investigate the influence of adding CeO2 on the hydrogen storage characteristics of Sm–Mg–Ni-based SmMg11Ni-type alloy, mechanical milling was utilized to synthesize SmMg11Ni and SmMg11Ni + 5 wt.% CeO2 (named SmMg11Ni–5CeO2) alloys. The microstructure of as-cast and as-milled samples was measured via X-ray diffractometer and transmission electron microscope. Sieverts device was utilized to measure the isothermal hydriding and dehydriding kinetics. The non-isothermal dehydrogenation performance was explored by thermogravimetry and differential scanning calorimetry. The hydrogen desorption activation energy of the compound metal hydride can be computed by both Arrhenius and Kissinger methods. The related data show that adding CeO2 can engender a very slight influence on the hydrogen storage thermodynamics, but it can result in an obvious reduction in hydrogen absorption and desorption capacities. Furthermore, the hydrogen desorption performance of experimental alloys is conspicuously ameliorated by the addition of CeO2, viz. lowering the initial hydrogen desorption temperature and enhancing hydrogen desorption rate. The hydrogen desorption activation energies with and without CeO2 addition are 84.28 and 100.31 kJ/mol, respectively, with an obvious decrease of 16.03 kJ/mol. This is thought to be responsible for the ameliorated hydrogen desorption kinetics by adding CeO2.