Study on reversible hydrogen sorption behaviors of a 3NaBH4/HoF3 composite

In this work, the complex hydrogen sorption behaviors in a 3NaBH₄/HoF₃ composite prepared through mechanical milling were carefully investigated, including the reactions occurred during ball milling and de-/rehydrogenation processes. Different from other rear earth fluorides, the HoF₃ can react with NaBH₄ during ball milling, leading to the formations of Na–Ho–F and Na–Ho–BH₄ complex compounds. The first dehydriding of the 3NaBH₄/HoF₃ composite can be divided into 4 steps, including the ion exchange between H⁻ and F⁻, the formation of NaHo(BH₄)₄, the decomposition of NaHo(BH₄)₄ and reaction of NaBH₄ with Na–Ho–F compounds. The final products, HoB₄, HoH₃ and NaF, can be rehydrogenated to generate NaBH₄ and NaHoF₄ with an absorption capacity of 2.3 wt% obtained at 400 °C. Based on the Pressure–Composition–Temperature measurements, the de-/rehydrogenation enthalpies of the 3NaBH₄/HoF₃ composite are determined to be 88.3 kJ mol⁻¹ H₂ and −27.1 kJ mol⁻¹ H₂, respectively.     

Reversible hydrogen storage in LiBH4/Ca(AlH4)2 systems

To improve the hydrogen storage property of LiBH₄, the LiBH₄/Ca(AlH₄)₂ reactive systems with various ratios were constructed, and their de-/hydrogenation properties as well as the reaction mechanisms were investigated experimentally. It was found that the sample with the LiBH₄ to Ca(AlH₄)₂ molar ratio of 6:1 exhibits the best comprehensive hydrogen storage properties, desorbing hydrogen completely (8.2 wt.%) within 35 min at 450 °C and reversibly absorbing 4.5 wt.% of hydrogen at 450 °C under a hydrogen pressure as low as 4.0 MPa. During the first dehydrogenation process of the LiBH₄/Ca(AlH₄)₂ systems, the CaH₂ and Al particles were in situ precipitated via the self-decomposition of Ca(AlH₄)₂, and then reacted with LiBH₄ to form CaB₆, AlB₂ and LiH. Whereafter, the sample can cycle between LiBH₄ + Ca(BH₄)₂ + Al in the hydrogenated state and CaB₆ + AlB₂ + LiH in the dehydrogenated state.     

Devices for producing hydrogen via NaBH4 and LiH hydrolysis

Chemical hydrides have already been pointed out as great potential hydrogen storage materials. In this paper, the hydrolysis of two solid hydrides, namely sodium borohydride (NaBH₄) and lithium hydride (LiH) was studied to check their performance as hydrogen generators. The simplicity of the reactor design, the absence of high pressure or very high temperatures as well as the benignity of the spent fuel make this hydrogen storage approach conceptually feasible. Several devices have been developed and tested. The devices have been designed to generate hydrogen flows in the 0.5-1.0 L min⁻¹ range. Batches up to 500 g of sodium borohydride powder were hydrolyzed with liquid water. 10.0 wt. % nickel acetate was used as catalyst. Hydrogen flows in the desired range have been continuously produced for several hours (up to 30 h). Due to the high reactivity lithium hydride was hydrolyzed without any catalyst. In this case batches of about 50 g have been hydrolyzed with steam for 4 h.     

Improved hydrogen storage capacity through hydrolysis of solid NaBH4 catalyzed with cobalt boride

In this article the feasibility of the reaction of liquid water with a solid NaBH₄/catalyst mixture for improved hydrogen storage capacity and on-demand H₂ generation is reported. The synthesized low-cost nanosized catalyst consists of a Co₂B core surrounded by an oxide layer, presenting a relatively large specific surface area (70 m² g⁻¹). Calorimetric experiments coupled to simultaneous measurements of the evolved hydrogen volume have shown the positive effect of the locally heat release during reduction of the superficial oxidized layer. The synergetic effects of the exothermicity of both the oxidized layer reduction and the hydrolysis reaction coupled to the high efficiency of the cobalt boride catalyst led to an “enhanced regime” observed at room temperature. The “enhanced regime” corresponds to a global reaction stoichiometry of 1 mol of NaBH₄ reacting with 3 mol of water, conducting to a hydrogen yield of 8.7 wt.%. Effects of temperature and catalyst content were studied.     

Reversible hydrogen storage for NLi4-Decorated honeycomb borophene oxide

The boron-based two-dimensional (2D) materials decorated with functional groups NLi₄ has been numerically investigated for hydrogen storage via first principles calculations method. Strain-energy analysis and molecular dynamics simulations shows the pristine planar honeycomb B₂O has strong mechanical and thermal stability. Crystal Orbital Hamiltonian Population analysis confirmed that there exist stronger B–B/B–O covalent bonds within B₂O monolayer. In functional material, a local electric field around each lithium atom can be formed and the overall electronic structure is favorably changed for gas adsorptions. Both electrostatic forces and the van der Waals interaction are the dominant hydrogen-attached mechanisms of lithium cation. An anchored functional group NLi₄ can adsorb at most 11 hydrogen molecules, and the average adsorption energy per hydrogen molecules is around ?0.20 eV, indicating high hydrogen storage capacity and reversible applicability. The highest hydrogen storage capacity can reach to 9.1 wt%. The study shows the investigated material is a good candidate for hydrogen storage.     

Enhanced hydrogen storage properties of LiBH4-MgH2 composite by the catalytic effect of MoCl3

Though LiBH₄-MgH₂ system exhibits an excellent hydrogen storage property, it still presents high decomposition temperature over 350 °C and sluggish hydrogen absorption/desorption kinetics. In order to improve the hydrogen storage properties, the influence of MoCl₃ as an additive on the hydrogenation and dehydrogenation properties of LiBH₄-MgH₂ system is investigated. The reversible hydrogen storage performance is significantly improved, which leads to a capacity of about 7 wt.% hydrogen at 300 °C. XRD analysis reveals that the metallic Mo is formed by the reaction between LiBH₄ and MoCl₃, which is highly dispersed in the sample and results in improved dehydrogenation and hydrogenation performance of LiBH₄-MgH₂ system. From Kissinger plot, the activation energy for hydrogen desorption of LiBH₄-MgH₂ system with additive MoCl₃ is estimated to be ∼43 kJ mol⁻¹ H₂, 10 kJ mol⁻¹ lower than that for the pure LiBH₄-MgH₂ system indicating that the kinetics of LiBH₄-MgH₂ composite is significantly improved by the introduction of Mo.     

Synthesis of solid-state NaBH4/Co-based catalyst composite for hydrogen storage through a high-energy ball-milling process

Solid-state composites of NaBH₄ and Co-based catalyst have been fabricated for hydrogen generation via a novel mechanochemical technique, i.e. the high-energy ball milling, in which the gravimetric storage capacity of hydrogen has reached 6.7 wt%, meeting the 2010 target of at least 0.06 kg H₂/kg set by the U.S. Department of Energy (DOE). The active catalysts used in the hydrolysis reaction of sodium borohydride for hydrogen generation are mainly cobalt oxides. Controlled addition of water, namely water used as a limiting agent, to the solid composites of NaBH₄ and Co-based catalyst greatly improves the H₂ storage capacity and resolved the issues of low gravimetric H₂ storage in conventional aqueous system of sodium borohydride. Factors influencing the performance of hydrogen production such as amounts of H₂O added, catalyst loadings and durations of ball-milling processes are investigated. Moreover the hydrolyzed products of NaBH₄ and spent catalysts are analyzed as well.     

Assessing the reactivity of TiCl3 and TiF3 with hydrogen

TiCl₃ and TiF₃ additives are known to facilitate hydrogenation and dehydrogenation in a variety of hydrogen storage materials, yet the associated mechanism remains under debate. Here, experimental and computational studies are reported for the reactivity with hydrogen gas of bulk and ball-milled TiCl₃ and TiF₃ at the temperatures and pressures for which these additives are observed to accelerate reactions when added to hydrogen storage materials. TiCl₃, in either the α or δ polymorphic forms and of varying crystallite size ranging from ～5 to 95 nm, shows no detectable reaction with prolonged exposure to hydrogen gas at elevated pressures (～120 bar) and temperatures (up to 200 °C). Similarly, TiF₃ with varying crystallite size from ～4 to 25 nm exhibits no detectable reaction with hydrogen gas. Post-exposure vibrational and electronic structure investigations using Fourier transform infrared spectroscopy and x-ray absorption spectroscopy confirm this analysis. Moreover, there is no significant promotion of H₂ dissociation at either interior or exterior surfaces, as demonstrated by H₂/D₂ exchange studies on pure TiF₃. The computed energy landscape confirms that dissociative adsorption of H₂ on TiF₃ surfaces is thermodynamically inhibited. However, Ti-based additives could potentially promote H₂ dissociation at interfaces where structural and compositional varieties are expected, or else by way of subsequent chemical transformations. At interfaces, metallic states could be formed intrinsically or extrinsically, possibly enabling hydrogen-coupled electronic transfer by donating electrons.     

Influence of operation conditions on direct NaBH4/H2O2 fuel cell performance

Although hydrogen fuel cells have attracted so much attentions in these years because of the application prospect in electric vehicles, some obstacles have not been solved yet, among which hydrogen storage is one of the biggest. Direct borohydride fuel cell (DBFC) is another choice without hydrogen storage problem because borohydride is used as reactant directly in the fuel cell. In this paper, DBFC performance under different operation conditions was studied including electrolyte membrane type, operation temperature, borohydride concentration, supporting electrolyte and oxidant. Results showed that, with Pt/C and MnO₂ as anode and cathode electrocatalyst, respectively, Nafion^® 117 membrane as electrolyte, 1.0 M, 3.0 M and 6.0 M NaBH₄ and H₂O₂ solution in NaOH as reactant solution, 80 °C operation, the peak power density could reach 130 mW/cm².     

Characterizations of composite NaNH2-NaBH4 hydrogen storage materials synthesized via ball milling

Composite NaNH₂-NaBH₄ (molar ratio of 2/1) hydrogen storage materials are prepared by a ball milling method with various ball milling times. The compositions and hydrogen generation characteristics are investigated by means of X-ray diffraction (XRD) and thermo gravimetric-differential thermal analysis (TG-DTA). The structural characteristics imply that ball milling produces a new phase of Na₃(NH₂)₂BH₄, and mechanical energy accumulated in the ball milling process may be responsible for the phase change of Na₃(NH₂)₂BH₄. TG-DTA demonstrates that the phase change temperature of the composite NaNH₂-NaBH₄ (2/1) ball milled for 16 h is 141.8 °C, and the melting point is 197.3 °C; below 400 °C, composite hydrogen storage material is mainly decomposed to give hydrogen and Na₃BN₂; while above 400 °C, the previous by-product Na₃BN₂ continues to decompose so as to give metal Na gradually.     

Hydrogen storage properties of various carbon supported NaBH4 prepared via metathesis

Sodium borohydride nanoparticles prepared via the metathesis reaction between LiBH₄ and NaCl were successfully deposited on various carbon supporting materials such as graphite, graphene oxide and carbon nanotubes. The X-ray diffraction analyses were conducted to identify the phase of NaBH₄ deposited on various carbon supporting materials. The transmittance electron micrograph analyses were also conducted to investigate the particle size and dispersion of NaBH₄ within carbon supporting materials. The particle size and size distribution of NaBH₄ on graphite were observed to be larger and broader than of other two supporting materials, graphene oxide and CNT due to the lower surface energy as compared to GO and CNT. The bonding state of NaBH₄ was confirmed by the Fourier-transformed infrared spectroscopy analysis. The TG and PCT results show that the hydrogen desorption of the NaBH₄ deposited on carbon supports takes place at temperature (130 °C～) significantly lower than that of pure NaBH₄ (above 500 °C) and the amount of desorption was in the order of graphene oxide (12.3 mass %) > CNT (9.8 mass %) > graphite (5.7 mass %). The reversibility of hydrogen adsorption after five cycles of adsorption-desorption showed that NaBH₄/GO and NaBH₄/CNT were much better than that of pure NaBH₄ due to excellent structural stability.     

Novel fabrication of solid-state NaBH4/Ru-based catalyst composites for hydrogen evolution using a high-energy ball-milling process

Solid-state NaBH₄/Ru-based catalyst composites have been fabricated for hydrogen generation through a high-energy ball-milling process, providing uniform dispersion of resin-supported Ru³⁺ catalysts among pulverized NaBH₄ (SBH) particles, so as to increase the contacts of SBH with active catalytic sites. Consequently, the gravimetric hydrogen storage capacity as high as 7.3 wt% could be achieved by utilizing water as a limiting reagent to overcome the issue of deactivated catalysts whose active sites are often blocked by precipitates caused by limited NaBO₂ solubility occurring in conventional aqueous SBH systems for hydrogen productions. Products of hydrolyzed SBH that greatly influence the gravimetric H₂ storage capacity are found to be most likely NaBO₂·2H₂O and NaBO₂·4H₂O from SBH/H₂O reacting systems with initial weight ratios, SBH/H₂O = 1/2 and 1/10, respectively, according to the TGA and XRD analyses.     

Thermo-economic analysis of a hydrogen production system by sodium borohydride (NaBH4)

In the spectrum of current energy possibilities, hydrogen represents a solution of great interest toward a future sustainable energy system. No single technology can sustain the energy needs of the whole society, but integration and hybridization are two key strategic features for viable energy production based in hydrogen economy.In the present work, a hydrogen energy model is analyzed. In this model hydrogen is produced through the electrolysis of water, taking advantage of the electrical energy produced by a renewable generator (photovoltaic panels). The produced hydrogen is chemically stored by the synthesis of sodium borohydride (NaBH₄). NaBH₄ promising features in terms of safety and high volumetric density are exploited for transportation to a remote site where hydrogen is released from NaBH₄ hydrolysis and used for energy production.This model is compared from an economic standpoint with the traditional hydrogen storage and transportation technology (compressed hydrogen in tanks).This paper presents a thermodynamic and economic analysis of the process in order to determine its economic feasibility. Data employed for the realization of the model have been gathered from recent important progresses made on the subject.The innovative plant including NaBH₄ synthesis and transportation is compared from an economic standpoint with the traditional hydrogen storage and transportation technology (compressed hydrogen in tanks). As a final point, the best technology and the components' optimal sizes are evaluated for both cases in order to minimize production costs.     

Dehydriding and rehydriding properties of yttrium borohydride Y(BH4)3 prepared by liquid-phase synthesis

Y(BH₄)₃ was prepared by liquid-phase synthesis, and its dehydriding and rehydriding properties were systematically investigated by performing thermogravimetry and differential thermal analysis (TG-DTA) and powder X-ray diffraction (XRD) measurement. The dehydriding reaction of Y(BH₄)₃ starts at appropriately 460 K, and a total of 7.8 wt% of hydrogen is released up to 773 K. Phase transformation and melting are observed in Y(BH₄)₃ at approximately 474 K and 499 K, respectively. Both DTA and XRD measurement results indicate that the decomposition of Y(BH₄)₃ proceeds via multistep dehydriding reactions accompanied with the formation of an intermediate phase. Furthermore, Y(BH₄)₃ is proved to be partially rehydrided.     

Enhanced hydrogen storage performance of LiAlH4-MgH2-TiF3 composite

The mutual destabilization of LiAlH₄ and MgH₂ in the reactive hydride composite LiAlH₄-MgH₂ is attributed to the formation of intermediate compounds, including Li-Mg and Mg-Al alloys, upon dehydrogenation. TiF₃ was doped into the composite for promoting this interaction and thus enhancing the hydrogen sorption properties. Experimental analysis on the LiAlH₄-MgH₂-TiF₃ composite was performed via temperature-programmed desorption (TPD), differential scanning calorimetry (DSC), isothermal sorption, pressure-composition isotherms (PCI), and powder X-ray diffraction (XRD). For LiAlH₄-MgH₂-TiF₃ composite (mole ratio 1:1:0.05), the dehydrogenation temperature range starts from about 60 °C, which is 100 °C lower than for LiAlH₄-MgH₂. At 300 °C, the LiAlH₄-MgH₂-TiF₃ composite can desorb 2.48 wt% hydrogen in 10 min during its second stage dehydrogenation, corresponding to the decomposition of MgH₂. In contrast, 20 min was required for the LiAlH₄-MgH₂ sample to release so much hydrogen capacity under the same conditions. The hydrogen absorption properties of the LiAlH₄-MgH₂-TiF₃ composite were also improved significantly as compared to the LiAlH₄-MgH₂ composite. A hydrogen absorption capacity of 2.68 wt% under 300 °C and 20 atm H₂ pressure was reached after 5 min in the LiAlH₄-MgH₂-TiF₃ composite, which is larger than that of LiAlH₄-MgH₂ (1.75 wt%). XRD results show that the MgH₂ and LiH were reformed after rehydrogenation.     

Effects of alkaline treatment of hydrogen storage alloy on electrocatalytic activity for NaBH4 oxidation

Activation of the MmNi_4.03Co_0.42Mn_0.31Al_0.24 hydrogen storage alloy electrode is performed by immersing the electrode in a solution containing 6.0 mol dm⁻³ NaOH and 0.1 mol dm⁻³ NaBH_4. The effects of activation on the electrocatalytic activity of the electrode for NaBH₄ oxidation are investigated by cyclic voltammetry and chronoamperometry. Immersion activation greatly improves the electrocatalytic activity of the alloy electrode. Hydrogen was absorbed in the alloy during the immersion activation treatment and its electrooxidation is responsible for the high initial oxidation current. The stabilized current mainly results from the direct oxidation starting from the borohydride species. The effects of activation on structure and surface chemistry of the alloy are also discussed.     

Pt4, Pd4, Ni4, and Ti4 catalyzed hydrogen spillover on penta-graphene for hydrogen storage: The first-principles and kinetic Monte Carlo study

Penta-graphene is a new 2D allotrope of carbon exclusively consists of pentagons in a planar sheet geometry. In this work, we explored that if it can be a substrate for hydrogen spillover. The density-functional theory (DFT) studies show that the H atom can stably adsorb on sp² carbons. The saturation hydrogen storage density of penta-graphene is estimated to be 5.3 wt%. The Pt₄, Pd₄ Ni₄, and Ti₄ clusters are used as the catalyst for hydrogen spillover, and the migration barriers are 1.25, 1.07, 1.03 and 1.35 eV, respectively. The kinetic Monte Carlo simulations are performed to study the migration process for massive H atoms. The results show that the optimal reaction temperatures are 467, 405, 390, and 504 K for Pt₄, Pd₄, Ni₄, and Ti₄ catalyst, respectively. For Pd₄ and Ni₄ catalysts, the spillover reaction can occur at the appropriate temperature (355 and 340 K, respectively) for onboard hydrogen storage systems applied to light-duty vehicles.     

Microstructural study of hydrogen desorption in 2NaBH4 + MgH2 reactive hydride composite

The desorption mechanism of as-milled 2NaBH₄ + MgH₂ was investigated by volumetric analysis, X-ray diffraction and electron microscopy. Hydrogen desorption was carried out in 0.1 bar hydrogen pressure from room temperature up to 450 °C at a heating rate of 3 °C min⁻¹. Complete dehydrogenation was achieved in two steps releasing 7.84 wt.% hydrogen. Desorption reaction in this system is kinetically restricted and limited by the growth of MgB₂ at the Mg/Na₂B₁₂H₁₂ interface where the intermediate product phases form a barrier to diffusion. During desorption, MgB₂ particles are observed to grow as plates around NaH particles.     

Porous MgH2/C composite with fast hydrogen storage kinetics

A porous MgH₂/C composite can be synthesized through decomposition of an organo-magnesium precursor under hydrogen pressure. XRD patterns of the porous MgH₂/C composite exhibit a pure MgH₂ phase with a tetragonal structure. The morphology of the resulted samples is significantly dependent on the synthesis temperature and hydrogen pressure. The samples exhibit a rod-like structure and composed of nano-crystallites of MgH₂ with a size of less than 5 nm. TPD spectra of a sample synthesized at 220 °C for 4 h show a remarkable decrease of the onset hydrogen release temperature. Further, this sample also exhibits fast hydrogen adsorption kinetics adsorbing 6 wt % of hydrogen in 3 min at 250 °C. The same amount of hydrogen is adsorbed in 11 min at 200 °C and 40 min at 150 °C, respectively. N₂ physisorption measurements of this sample indicate meso-porosity with a BET surface area of 40.9 m² g⁻¹ and an average pore diameter of 20 nm.     

Enhanced hydrogen storage properties of 4MgH2 + LiAlH4 composite system by doping with Fe2O3 nanopowder

In this paper, the hydrogen storage properties and reaction mechanism of the 4MgH₂ + LiAlH₄ composite system with the addition of Fe₂O₃ nanopowder were investigated. Temperature-programmed-desorption results show that the addition of 5 wt.% Fe₂O₃ to the 4MgH₂ + LiAlH₄ composite system improves the onset desorption temperature to 95 °C and 270 °C for the first two dehydrogenation stage, which is lower 40 °C and 10 °C than the undoped composite. The dehydrogenation and rehydrogenation kinetics of 5 wt.% Fe₂O₃-doped 4MgH₂ + LiAlH₄ composite were also improved significantly as compared to the undoped composite. Differential scanning calorimetry measurements indicate that the enthalpy change in the 4MgH₂–LiAlH₄ composite system was unaffected by the addition of Fe₂O₃ nanopowder. The Kissinger analysis demonstrated that the apparent activation energy of the 4MgH₂ + LiAlH₄ composite (125.6 kJ/mol) was reduced to 117.1 kJ/mol after doping with 5 wt.% Fe₂O₃. Meanwhile, the X-ray diffraction analysis shows the formation of a new phase of Li₂Fe₃O₄ in the doped composite after the dehydrogenation and rehydrogenation process. It is believed that Li₂Fe₃O₄ acts as an actual catalyst in the 4MgH₂ + LiAlH₄ + 5 wt.% Fe₂O₃ composite which may promote the interaction of MgH₂ and LiAlH₄ and thus accelerate the hydrogen sorption performance of the MgH₂ + LiAlH₄ composite system.