Effect of succinic anhydride as an electrolyte additive on electrochemical characteristics of silicon thin-film electrode

The effect of an electrolyte additive, succinic anhydride (SA), on the electrochemical performances of a silicon thin-film electrode, which is prepared by radio-frequency magnetron sputtering, is investigated. The introduction of SA into a liquid electrolyte consisting of ethylene carbonate/diethyl carbonate/1 M LiPF₆ significantly enhances the capacity retention and coulombic efficiency of the electrode. This improvement in the electrochemical performance of the electrode is attributed to modification of the solid/electrolyte interphase (SEI) layer by the introduction of SA. The differences in the characteristic properties of SEI layers, with or without SA, are explained by analysis with scanning electron microscopy, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy.     

The influence of LiH and TiH2 on hydrogen storage in MgB2 I: Promotion of bulk hydrogenation at reduced temperature

Mg(BH₄)₂ is an attractive hydrogen storage material, owing to its high gravimetric capacity of 14.9 wt %. However, the dehydrogenated material MgB₂ is very difficult to rehydrogenate, requiring excessive pressures and temperatures. Here we report the influence of LiH and TiH₂ on hydrogen storage reactions involving Bulk MgB₂ using XRD, XAS, FTIR and NMR. In ball-milled mixtures of LiH/MgB₂, the LiH loses crystallinity but remains undissociated, forming a weakly bound complex with MgB₂. The weak interactions produce minor variations in the local electronic structure at B and Mg, but do not markedly affect the underlying MgB₂ hexagonal crystal structure. No evidence is found for a mixed-metal boride Mg_1-xLi_xB₂ in the as-prepared LiH/MgB₂ materials. The presence of LiH dramatically improves the hydrogenation of MgB₂ at 700 bar, forming borohydride 100 °C below the minimum hydrogenation temperature of pure MgB₂ and without the formation of undesirable intermediates such as [B₃H₈]^-, [B₁₀H₁₀]^2- or [B₁₂H₁₂]^2-. Evidence is reported for a mixed-metal borohydride of the type Mg_(3-x)/2Li_x(BH₄)₃ produced by the hydrogenation. Subsequent desorption is also improved compared to pure Mg(BH₄)₂ and LiBH₄, showing single-step hydrogen release up to ～8 wt% by 380 °C, whereas Mg(BH₄)₂ and LiBH₄ still retain significant amounts of hydrogen at this temperature. The material produced by desorption contains both MgB₂ and Mg metal, revealing the original LiH/MgB₂ system is not fully reversible. In contrast to LiH, TiH₂ is essentially inert when ball-milled with MgB₂, and high-pressure hydrogenation leaves only unreacted TiH₂ and MgB₂. Thus, added TiH₂ provides no benefit to MgB₂ hydrogenation.     

The influence of LiH and TiH2 on hydrogen storage in MgB2 II. XPS study of surface and near-surface phenomena

Mg(BH₄)₂ is a promising solid-state hydrogen storage material, releasing 14.9 wt% hydrogen upon conversion to MgB₂. The rehydrogenation of MgB₂ is particularly challenging, requiring prolonged exposure to high pressures of hydrogen at high temperature. Here we report an XPS study probing the influence of LiH and TiH₂ on the hydrogen storage properties of MgB₂ in the surface and near-surface regions, as a complementary investigation to a preceding study of the bulk properties. Surface and near-surface properties are important considerations for nanoscale and bulk hydrogen storage materials. If there are reactions occurring at the surface that modify the chemical composition in the near-surface region, species diffusion can alter the chemical composition even deep into the bulk of the material. For LiH/MgB₂, metastable LiH–B and LiH–Mg species are produced that are more reactive than Bulk MgB₂. With prolonged glovebox storage, the LiH/MgB₂ material shows increased reactivity towards O and C and enriched levels of Li and B in the near-surface region. In addition, Li induces the growth of Li₂CO₃ in the surface and near surface regions. Exposing LiH/MgB₂ to hydrogen at 700 bar and 280 °C for 24 h produces borohydride at a temperature 100 °C below the threshold for bulk MgB₂ hydrogenation. In a specifically surface process with macroscopic implications, the hydrogenation conditions also cause Li₂CO₃ to react with boron hydroxide in the sample to form a Li-deficient glassy lithium borate melt at the interfaces of the particles, bonding them together. Subsequent heating to 380 °C dehydrogenates the borohydride and eliminates the Li-deficient glassy lithium borate. The LiH/MgB₂ material is not reversible because desorption does not lead back to LiH/MgB₂, but rather to elemental B and Mg metal in the near-surface region. In contrast to LiH, TiH₂ does not react with MgB₂, despite the favorable thermodynamics for destabilization via TiB₂ formation. Furthermore, high pressure hydrogenation yields only unreacted TiH₂ and MgB₂ in the surface and near-surface regions. Thus, added TiH₂ provides no benefit to MgB₂ hydrogenation, in agreement with the findings of the preceding bulk study.     

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.     

Lithium metal hydrides (Li2CaH4 and Li2SrH4) for hydrogen storage; mechanical,electronic and optical properties

Hydrogen storage is a critical step for commercialisation of hydrogen consumed energy production. Among other storage methods, solid state storage of hydrogen attracts much attention and requires extensive research. This study rationally and systematically designs novel solid state hydrides; Li₂CaH₄ (GHD is obtained as −6.95 wt %) and Li₂SrH₄ (GHD is obtained as −3.83 wt %) using computational method. As a first step, we suggest and predict crystal structures of solid state Li₂CaH₄ and Li₂SrH₄ hydrides and look for synthesizability. Then, the mechanical stabilities of hydrides are identified using elastic constants. Both hydrides fulfil the well-known Born stability criteria, indicating that both Li₂CaH₄ and Li₂SrH₄ are mechanically stable materials. Several critical parameters, bulk modulus, shear modulus, Cauchy pressures, anisotropy factors of hydrides and bonding characteristics are obtained and evaluated. Furthermore, electronic and optical band structures of hydrides are computed. Both Li₂CaH₄ and Li₂SrH₄ have indirect bands gaps as 0.96 eV (Г-U) and 1.10 eV (Г-R). Thus, both materials are electronically semiconducting. Also, Bader charge analysis of hydrides have been carried out. Charge density distribution suggests an ionic-like (or polarized covalent) bonding interaction between the atoms.     

Study of kinetics and thermal decomposition of ammonia borane in presence of silicon nanoparticles

Ammonia borane (AB, NH₃BH₃) is a promising material by virtue of its high gravimetric hydrogen storage capacity of 19.6 wt%. Hydrogen release from AB initiates at around 100 °C and as such is compatible to meet the present-day requirements of a PEM fuel cell. The thermal decomposition of AB is a complex process involving several reactions. Major issues include poor reaction kinetics, leading to delayed commencement of hydrogen generation i.e. long induction period, and the small amount of hydrogen released at optimal temperature. In the current paper the thermal decomposition of AB is studied at different temperatures. Further the effect of Si nanoparticles on the induction period and kinetics as well as the gas release reaction is studied in detail using different characterization techniques. It was found that the induction period reduced and the amount of gas released increased as a result of Si nanoparticle addition. This was facilitated by a reduction in the activation energy of decomposition and improved kinetics with the addition of silicon nanoparticles.     

Effects of CeO2 additive on redox characteristics of Fe-based mixed oxide mediums for storage and production of hydrogen

Effects of CeO₂ additive to Fe-based mixed oxide mediums with Rh and ZrO₂ for chemical hydrogen storage were investigated in terms of stability and reactivity of the mediums in water splitting oxidation with repeated redox cycles. The mediums with CeO₂ content ranging from 0 to 30 wt% were prepared by co-precipitation method using urea solution as a precipitant. The hydrogen storage and release properties were investigated during repeated isothermal redox cycles at 823 K for reduction with hydrogen and 623 K for oxidation with water vapor under atmospheric pressure. The amount of hydrogen produced by the mediums, both with and without CeO₂, was maintained at an almost constant level over ten repeated redox cycles. However, the oxidation rates of the mediums without CeO₂ were decreased during repeated redox cycles while that increased with increasing CeO₂ contents. Especially, the mediums added with 30 wt% of CeO₂ (FRZC-30) showed high activity and stability for ten redox cycles, the degree of hydrogen storage was almost maintained ca. 1.9 wt% on the basis of total amount of the medium.     

Effect of different sized CeO2 nano particles on decomposition and hydrogen absorption kinetics of magnesium hydride

In present paper, different sizes of CeO₂ nanoparticles were synthesized by ball milling and their effect on the absorption kinetics and decomposition temperature of MgH₂ was studied. It was found that a small amount of admixing of the above said catalysts with MgH₂ exhibits improved hydrogen storage properties. Among these different sizes of CeO₂ nanoparticles, 2 weight % admixed CeO₂ with a particle size of ∼10–15 nm led to decrease in desorption temperature by ∼50 K. Moreover, it also shows 1.5 times better absorption kinetics with respect to pure MgH₂. The samples were characterized using SEM, TEM and XRD techniques. The hydrogenation/dehydrogenation properties were measured by gas reaction controller.     

The role of Li and Ni metals in the adsorbate complex and their effect on the hydrogen storage capacity of single walled carbon nanotubes coated with metal hydrides,LiH and NiH2

In this first principles study based on density functional theory, we report the hydrogen storage capability of (5, 5) single walled carbon nanotubes coated with Lithium hydride and Nickel hydride. The paper brings out the role of lightweight Li atom and heavy Ni atom in binding the respective hydrides and hydrogen molecules with the single walled carbon nanotubes. The investigation is carried out for half and full coverage of the adsorbates (metal hydrides) on the sidewalls of the carbon nanotubes. The clustering of the adsorbates is observed in full coverage case of both the systems and its effect on hydrogen storage capacity and binding energy is reported. The clustering patterns are different in each of the systems and dependent on the nature of the metal atom in the metal hydride. The storage capacity of single walled carbon nanotubes coated with heavy transition metal hydride is around 3 wt.% whereas it is around 6 wt.% in their counterparts coated with lightweight metal hydride.     

The effects of Ti-based additives on the kinetics and reactions in LiH/MgB2 hydrogen storage system

In this work we investigated the effect of Ti, TiH₂, TiB₂, TiCl₃, and TiF₃ additives on the hydrogen de/re-sorption kinetics and reaction pathways of LiH/MgB₂ mixture. From high pressure differential scanning calorimeter (HP-DSC) measurements it was found that these additives all effectively decrease the onset temperature of hydrogenation. The isothermal hydrogenation/dehydrogenation measurements suggest that Ti, TiH₂, and TiB₂ can significantly improve the hydrogen sorption kinetics of LiH/MgB₂ mixture. The absorption kinetics of TiF₃ and TiCl₃ doped samples are slower than the baseline (2LiH-MgB₂ without additive), but their desorption kinetics are faster than the baseline and other additives doped systems. X-ray diffraction (XRD) analysis reveals that the additive Ti in LiH/MgB₂ actively participates in both hydrogenation and dehydrogenation process, which can be regarded as an effective additive of this system.     

Small hydrogen storage tank filled with 2LiBH4MgH2 nanoconfined in activated carbon: Reaction mechanisms and performances

De/rehydrogenation performances and reaction pathways of nanoconfined 2LiBH₄MgH₂ into activated carbon (AC) packed in small hydrogen storage tank are proposed for the first time. Total and material storage capacities upon five hydrogen release and uptake cycles are 3.56–4.55 and 2.03–3.28 wt % H₂, respectively. Inferior hydrogen content to theoretical capacity (material capacity of 5.7 wt % H₂) is due to partial dehydrogenation during sample preparation and incomplete decomposition of LiBH₄ as well as the formation of thermally stable Li₂B₁₂H₁₂ upon cycling. Two-step dehydrogenation of MgH₂ and LiBH₄ to produce Mg and MgB₂+LiH, respectively is found at all positions in the tank. For rehydrogenation, reversibility of MgH₂ and LiBH₄ proceeds via different reaction mechanisms. Although isothermal condition (T_set = 350 °C) and controlled pressure range (e.g., 30–40 bar H₂ for hydrogenation) are applied, temperature gradient inside the tank and poor hydrogen diffusion through hydride bed, especially in the sample bulk are detected. This results in alteration of de/rehydrogenation pathways of hydrides at different positions in the tank. Thus, further development of hydrogen storage tank based 2LiBH₄MgH₂ nanoconfined in AC includes the improvement of thermal conductivity of materials and temperature control system as well as hydrogen permeability.     

DFT study of boron doped MgH2: Bonding mechanism,hydrogen diffusion and desorption

The impact of boron doping on MgH₂ bonding mechanism, hydrogen diffusion and desorption was calculated using density functional theory (DFT). Atomic interactions in doped and non-doped system and its influence on hydrogen and vacancy diffusion were studied in bulk hydride. Slab calculations were performed to study hydrogen desorption energies from (110) boron doped surface and its dependence on the surface configuration and depth position. To study kinetics of hydrogen diffusion in boron vicinity and hydrogen molecule desorption activation energies from boron doped and non-doped (110) MgH₂ surface Nudged Elastic Band (NEB) method was used. Results showed that boron forms stronger, covalent bonds with hydrogen causing the destabilization in its first and second coordination. This leads to lower hydrogen desorption energies and improved hydrogen diffusion, while the impact on the energy barriers for H₂ desorption from hydride (110) surface is less pronounced.     

Synthesis,properties and thermal decomposition particularities of magnesium borohydride ammoniates

An elemental analysis, as well as XPD, IR, XPS and TGA-DSC/TPD-MS methods are used to compare composition and properties of Mg(BH₄)₂(NH₃)_n (n = 1, 2, 3) complexes obtained by following methods: (1) mechanochemical treatment and (2) heating of magnesium borohydride hexaammoniate and unsolvated magnesium borohydride mixtures. A systematic study of the thermal decomposition features and gases composition released during the thermolysis of the synthesized compounds is carried out. The differences found are discussed.     

In operando study of TiVCr additive in MgH2 composites

We report an in operando study of the hydrogenation and dehydrogenation of MgH₂–TiVCr composites. The experiment was performed by means of in situ synchrotron XRD in order to get insights on the influence of the TiVCr additive on the sorption properties of the MgH₂ based composite. Sequential Rietveld refinement analysis was performed to investigate the structural changes of MgH₂ and of the additive during hydrogenation and dehydrogenation processes. Significant non-monotonic changes in the lattice volume of the TiVCrH_x solid solution were observed concomitantly to the MgH₂ formation or decomposition. These volume changes are assigned to the variation of the hydrogen content in TiVCrH_x. These results provide evidence of cooperative effects between the H₂ storage material and the additive.     

Achieving both high hydrogen capacity and low decomposition temperature of the metastable AlH3 by proper ball milling with TiB2

AlH₃ is a metastable hydride with a high hydrogen density of 10.1 wt% and it can release hydrogen at a low temperature of 150–200 °C. Many additives (e.g., NbF₅, TiF₃, etc.) introduced by ball milling can significantly reduce the decomposition temperature of AlH₃, but often simultaneously decrease the available hydrogen capacity. In this work, TiB₂ was introduced by ball milling to improve the decomposition performance of AlH₃. AlH₃ + x wt% TiB₂ (x = 2.5, 5, 7.5, 10) composites were prepared by ball milling, and the milling conditions were optimized. It was shown that the decomposition performance of the AlH₃ + 2.5 wt% TiB₂ ball milled at 225 rpm for 108 min is the best. The onset decomposition temperature is 78 °C, which is 60 °C lower than that of pure AlH₃. The decomposition is terminated at 130 °C with 8.5 wt% of hydrogen is obtained. In addition, 5.3 wt% of hydrogen can be released within 200 min at constantly 80 °C. Under the same conditions, ball-milled AlH₃ can hardly release any hydrogen. The activation energy calculated by the Kissinger's method is 86 kJ mol^?1, which was 28 kJ mol^?1 lower than that of ball-milled AlH₃. Catalytic mechanism study reveals that the Al₂O₃ layers on the surface of AlH₃ will interact with TiB₂ to form Al–Ti–B solid solution, resulting in lattice distortion. Through lattice activation, the decomposition kinetics of AlH₃ is improved. This work provides an efficient strategy to achieve both high hydrogen capacity and low decomposition temperature of metastable AlH₃ by proper ball milling with metal borides.     

Application of hydrides in hydrogen storage and compression: Achievements,outlook and perspectives

Metal hydrides are known as a potential efficient, low-risk option for high-density hydrogen storage since the late 1970s. In this paper, the present status and the future perspectives of the use of metal hydrides for hydrogen storage are discussed. Since the early 1990s, interstitial metal hydrides are known as base materials for Ni – metal hydride rechargeable batteries. For hydrogen storage, metal hydride systems have been developed in the 2010s [1] for use in emergency or backup power units, i. e. for stationary applications.With the development and completion of the first submarines of the U212 A series by HDW (now Thyssen Krupp Marine Systems) in 2003 and its export class U214 in 2004, the use of metal hydrides for hydrogen storage in mobile applications has been established, with new application fields coming into focus.In the last decades, a huge number of new intermetallic and partially covalent hydrogen absorbing compounds has been identified and partly more, partly less extensively characterized.In addition, based on the thermodynamic properties of metal hydrides, this class of materials gives the opportunity to develop a new hydrogen compression technology. They allow the direct conversion from thermal energy into the compression of hydrogen gas without the need of any moving parts. Such compressors have been developed and are nowadays commercially available for pressures up to 200 bar. Metal hydride based compressors for higher pressures are under development. Moreover, storage systems consisting of the combination of metal hydrides and high-pressure vessels have been proposed as a realistic solution for on-board hydrogen storage on fuel cell vehicles.In the frame of the “Hydrogen Storage Systems for Mobile and Stationary Applications” Group in the International Energy Agency (IEA) Hydrogen Task 32 “Hydrogen-based energy storage”, different compounds have been and will be scaled-up in the near future and tested in the range of 500 g to several hundred kg for use in hydrogen storage applications.     

Effect of the presence of chlorides on the synthesis and decomposition of Ca(BH4)2

Calcium borohydride is one of the most interesting compounds for solid-state hydrogen storage, in particular because of its high hydrogen capacity. In this paper, the synthesis of Ca(BH₄)₂ by metathesis reaction via ball milling of a mixture of LiBH₄ and CaCl₂ is described. The effectiveness of this synthesis technique and the possible substitution of Cl ions in the borohydride phases is analysed depending on the back-pressure used for milling. When performed by ball milling under Ar, the metathesis reaction is not successful. A large quantity of a solid solution Li(BH₄)_1−xCl_x remains in the sample and CaHCl is formed rather than Ca(BH₄)₂. In contrast, the use of H₂ back-pressure during milling favours the borohydride phases rather than CaHCl and leads to the formation of a solid solution Ca(BH₄)_2-yCl_y where [BH₄]⁻ groups are partially substituted by Cl ions. This compound has a similar structure as β-Ca(BH₄)₂ but with smaller lattice parameters. It is present in the as-milled sample together with LiCl and Li(BH₄)_1−xCl_x. The decomposition of the mixture occurs at lower temperature than for pure LiBH₄ but higher than for pure Ca(BH₄)₂. The presence of chlorides in the structure of borohydride compounds changes dramatically the thermal properties of the material prepared and should be considered each time a metathesis reaction is used for synthesis.     

Importance of pyrolysis and catalytic decomposition for the direct utilization of methanol in solid oxide fuel cells

There is interest in developing solid oxide fuel cells (SOFC) operated directly with liquid fuels such as methanol. This mode of operation increases the complexity of the anodic processes, since thermal and catalytic decomposition reactions are relevant. In this study, the pyrolysis and catalytic decomposition of methanol are investigated experimentally for conditions typical of SOFC. The results are compared to the thermodynamic equilibrium values and also to the predictions of a kinetics model. The main species of the thermal decomposition of methanol are H₂, CO, and HCHO; soot formation is relevant below 973 K. The presence of a catalyst allows the gas-phase composition to reach equilibrium. However, the catalysts tested – Ni/YSZ, Ni/CeO₂, Cu/CeO₂ and Cu–Co/CeO₂ – deactivate by coking so that the gas-phase composition reverts to that of pyrolysis alone. The results presented reveal part of the complex dynamics occurring within the anode compartment during the direct utilization of methanol.     

Hydrogenation of LiH/Al catalyzed with TiN,TiMn2 and LaNi5

In order to investigate the catalytic effect of TiN, TiMn₂ and LaNi₅ on the hydrogen storage capacity of LiAlH₄, 2 mol% of the catalyst was milled with LiH/Al and then hydrogenated in Me₂O. Doping with TiN, TiMn₂ or LaNi₅ led to substantial hydrogenation of LiH/Al in accordance with the formation of LiAlH₄. In each case the amount of hydrogen absorbed was dependent on the catalyst and the ball-to-powder ratio used during milling. A high ball-to-powder ratio results in an improvement in the hydrogen storage capacity of LiAlH₄. For each ball-to-powder ratio the highest hydrogen storage capacity was recorded for the TiN-catalyzed sample; hydrogen storage capacity increased from 3.2 to 4.8 to 6.0 wt.% H as the ball to-powder ratio increased from 10:1 to 20:1 to 40:1. The high levels of hydrogenation of LiH/Al catalyzed with TiN, TiMn₂ and LaNi₅ are remarkable because for the LiAlH₄ system only a TiCl₃ catalyst has previously been shown to result in rehydrogenation of the dehydrogenated products to LiAlH₄.