Hydrogen storage in destabilized borohydride materials

Several new destabilized borohydride systems were prepared by mechanochemical synthesis and characterized to determine their suitability for hydrogen storage. The mixtures included: Mg(BH₄)₂/Ca(BH₄)₂; Mg(BH₄)₂/CaH₂/3NaH; and Mg(BH₄)₂/CaH₂; systems as well as a double cation hydride MnLi(BH₄)₃. Temperature programmed desorption, TPD, analyses showed that the desorption temperature of Mg(BH₄)₂ can be lowered by ball milling it with Ca(BH₄)₂. The resulting mixture absorbed and released hydrogen with the pressure composition temperature, PCT, isotherm displaying a well-defined plateau region. The other two systems; Mg(BH₄)₂/CaH₂ and Mg(BH₄)₂/CaH₂/NaH, can also absorb and release hydrogen. The desorption enthalpies are all in the 84–88 kJ/mol range. These systems, however, are only partially reversible and lose some of their hydrogen-holding capacity after the initial desorption. A plausible explanation for this is that the mechanisms involve the formation of a (B₁₂H₁₂)⁻²-containing intermediate which has a high kinetic barrier to re-hydrogenation. TPD analysis also showed that the double cation material, MnLi(BH₄)₃ can release hydrogen in the range of 130 °C but the process is irreversible. A Kissinger analysis of the first decomposition step in the differential thermal analysis, DTA, data showed that the activation energies for all the Mg(BH₄)₂-based borohydrides range from 115 to 167 kJ/mol.     

Effect of nitride additives on Li-N-H hydrogen storage system

Solid state reaction between LiNH₂ and LiH potentially offers a practical pathway for hydrogen supply to fuel cell powered vehicles, particularly if the reaction kinetics can be further improved. Here we performed a comparative study of the effects of selected micron and nano-sized nitrides using temperature programmed desorption, mass spectrometry, X-ray diffraction and infrared spectroscopy. It was found that both micron and nano-sized BN and TiN act as effective catalysts within the system. While an increase in the concentration of TiN reduces dehydrogenation temperature, the opposite was observed for BN catalyst. Employment of both nano and micron-sized BN catalysts resulted in an almost similar dehydrogenation temperature; but dehydrogenation temperature was decreased about 20 °C by switching from micron to nano-sized TiN. The catalytic effects of the additives were proposed to be an improvement of surface reactivity and diffusion enhancement across the interface of the reactants. However, the role of BN and TiN are different in the way that TiN is likely to improve the surface reactivity of LiNH_2, while BN mainly enhances diffusion across the interface of the reactants. Our findings also indicate that TiCl₃ behaves like TiN, as a catalyst in Li-N-H system.     

Hydrogen production from solid reactions between MAlH4 and NH4Cl

Solid reactions between alkali aluminum hydrides (MAlH₄, M = Li or Na) and NH₄Cl (at mole ratio 1:1) at 170 °C were investigated quantitatively using temperature programmed reaction (TPR), thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC) and x-ray diffraction (XRD). The release of 3 mol of H₂ from per mole of MAlH₄ was measured, corresponding to 5.6 wt.% H₂ capacity for the NaAlH₄/NH₄Cl system and 6.6 wt.% for LiAlH₄/NH₄Cl, respectively. By ball milling of the precursor compounds prior to the mixing, the reaction proceeded fast and NH₃ production as the by-product could be avoided. The quick solid reactions may be attributed to the low melting temperatures of MAlH₄ and the exothermic nature of the reactions. The reaction mechanism was also discussed.     

Hydrogen storage in lithium-decorated benzene complexes

Based on first-principles calculations, we find Li-decorated benzene complexes are promising materials for high-capacity hydrogen storage. Lithium atoms in the complexes are always positively charged, and each one can bind at most four H₂ molecules by a polarization mechanism. Therefore, a hydrogen uptake of 8.6 wt% and 14.8 wt% can be achieved in isolated C₆H₆–Li and Li–C₆H₆–Li complexes, respectively. The binding energy in the two cases is 0.22 eV/H₂ and 0.29 eV/H₂, respectively, suitable for reversible hydrogen storage. Various dimers may form, but the hydrogen storage capacity remains high or uninfluenced. This study provides not only a promising hydrogen storage medium but also comprehensions to other hydrogen storage materials containing six-carbon rings.     

Structural study of a new B-rich phase obtained by partial hydrogenation of 2NaH + MgB2

The structure of an unknown crystalline phase observed during the hydrogen absorption reaction of the powder mixtures 2NaH + MgB₂ at high pressure has been studied by ab-initio structure determination from powder diffraction. The sequence of un-overlapped peaks extracted from the X-ray powder diffraction pattern could be indexed with a primitive cubic cell with lattice parameter a = 7.319 Å. The diffraction patterns of the peaks are matched with the Pa-3 space group. The stoichiometry of the hydrogen absorption reaction suggests the presence of a high-boron content phase in the compound under investigation. Assuming this phase to be composed only by boron atoms and therefore having a density similar to that found for boron polymorphs, the solution with a space group of Pa-3 leads to reasonable B–B interatomic distances.     

Hydrogen storage by Mg-based nanocomposites

Hydrogen storage materials research is entered to a new and exciting period with the advance of the nanocrystalline alloys, which show substantially enhanced absorption/desorption kinetics, even at room temperatures. In this work, hydrogen storage capacities and the electrochemical discharge capacities of the Mg₂(Ni, Cu)-, LaNi₅-, ZrV₂-type nanocrystalline alloys and Mg₂Ni/LaNi₅-, Mg₂Ni/ZrV₂-type nanocomposites have been measured. The electronic properties of the Mg₂Ni_1-xCu_x, LaNi₅ and ZrV₂ alloys were calculated. The nanocomposite structure reduced hydriding temperature and enhanced hydrogen storage capacity of Mg-based materials. The nanocomposites (Mg,Mn)₂Ni (50 wt%)-La(Ni,Mn,Al,Co)₅ (50 wt%) and (Mg,Mn)₂Ni (75 wt%)-(Zr,Ti)(V,Cr,Ni)_2.4 (25 wt%) materials releases 1.65 wt% and 1.38 wt% hydrogen at 25 °C, respectively. The strong modifications of the electronic structure of the nanocrystalline alloys could significantly influence hydrogenation properties of Mg-based nanocomposities.     

Hydrogen storage properties of Mg1−xCaxNi2−y defect solid solutions

Ternary Mg_1−xCa_xNi_2−y solid solutions were synthesized by powder sintering. The phase structures and hydrogen storage properties of the sintered samples were investigated. In a certain range of x and y values, the samples are a single C15 Laves phase with various types of defects. The reduction of Ni content leads to the formation of omission solid solution with vacancies on the sites of Ni. These vacancies increase the hydrogen storage capacity, but decrease the reversibility of hydrogen absorption and desorption.     

Hydrogen storage based on polymeric material

In this work a functionalised polymer was studied and a polymeric matrix was chosen as a base with the aim of producing both a low cost and low weight hydrogen storage material. A poly-ether-ether-ketone (PEEK) was chosen as a base polymeric matrix and functionalised in situ by manganese oxide formation. The functionalisation process and the preliminary results on hydrogen storage capability of the synthesized polymer are reported. The polymer was characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, BET method surface and gravimetric hydrogen adsorption measurements. The metallic compound introduction modifies the morphology of the material and supplies an increased surface area for hydrogen chemisorptions, revealing a 1.2 wt% hydrogen adsorption capability at 77 K. Preliminary results from gravimetric measurements showed that by increasing the temperature hydrogen storage capability was reduced but not eliminated; for example, a 0.24 wt% at 50 °C and 60 bar was obtained. Moreover, reversibility of hydrogen adsorption and desorption in a wide range of both temperatures and pressures was confirmed. For this reason this approach is considered promising and deeper studies are in progress.     

Hydrogen storage properties of Zr2Co crystalline and amorphous alloys

To further explore the application feasibility of Zr₂Co alloy in tritium-related fields, hydrogenation/dehydrogenation properties of this material of crystalline or amorphous structure, prepared by arc melting or melt spinning, were studied by pressure-composition temperature measurement, X-ray diffraction, differential scanning calorimeter, thermal desorption spectroscopy. It was found that the two kinds of Zr₂Co alloys can absorb hydrogen in a close full concentration of ~9 mmol/g, and may have similar equilibrium hydrogen pressure in the order of 10^?6 Pa at room temperature. In their hydrogenated samples various hydrides were observed to form, including ZrH₂, Zr₂CoH₅, ZrCoH₃ and an amorphous one with gradually decreasing general thermostability. The amorphous alloy exhibited easier hydrogen induced disproportionation caused by highly stable ZrH₂ and much slower hydrogen absorption kinetics. This disproportionation behavior of the crystalline alloy was found to be entirely suppressed by changing heating process. The results firmly indicate that crystalline Zr₂Co alloy could be more favorable for tritium treatment due to very low equilibrium pressure and the feasibility of eliminating the disproportionation.     

Hydrogen storage performance in palladium-doped graphene/carbon composites

In this study a two-dimensional graphene sheet (GS) doped with palladium (Pd) nanoparticles was physically mixed with a superactivated carbon (AC) receptor and used as a hydrogen adsorbent. The hydrogen adsorption/desorption isotherm of the Pd-doped GS catalyst/AC composite (Pd-GS/AC) is determined using a static volumetric measurement at room temperature (RT) and pressure up to 8 MPa. The experiments show that the H₂ uptake capacity of 0.82wt.% for Pd-GS/AC is obviously enhanced, measuring 49% more than the 0.55wt.% for Pd-free GS/AC at RT and 8 MPa. Highly reversible behavior of Pd-GS/AC is also observed. Moreover, the isosteric heat of adsorption for Pd-GS/AC (−14 to −10 kJ/mol) is higher than that for pristine AC (−8 kJ/mol). An increase in H₂ uptake in the Pd-GS/AC suggests the occurrence of a relatively strong interaction between the spilt-over H and the receptor sites due to the spillover effect.     

Hydrogen storage for off-grid power supply

The use of intermittent renewable energy sources for power supply to off-grid electricity consumers depends on energy storage technology to guarantee continuous supply. Potential applications of storage-guaranteed systems range from small installations for remote telecoms, water-pumping and single dwellings, to farms and whole communities for whom grid connection is too expensive or otherwise infeasible, to industrial, military and humanitarian uses. In this paper we explore some of the technical issues surrounding the use of hydrogen storage, in conjunction with a PEM electrolyser and PEM fuel cell, to guarantee electricity supply when the energy source is intermittent, most typically solar photovoltaic. We advocate metal-hydride storage and compare its energy density to that of Li-ion battery storage, concluding that a significantly smaller package is possible with metal-hydride storage. A simple approach to match the output of a photovoltaic array to an electrolyser is presented. The properties required for the metal-hydride storage material to interface the electrolyser to the fuel cell are discussed in detail. It is concluded that relatively conventional Mischmetal-based AB₅ alloys are suitable for this application.     

Hydrogen storage materials for hydrogen and energy carriers

Hydrogen storage technology is essentially necessary to promote renewable energy. Many kinds of hydrogen storage materials, which are hydrogen storage alloys, inorganic chemical hydrides, carbon materials and liquid hydrides have been studied. In those materials, ammonia (NH₃) is easily liquefied by compression at 1 MPa and 298 K, and has a highest volumetric hydrogen density of 10.7 kg H₂/100 L. It also has a high gravimetric hydrogen density of 17.8 wt%. The theoretical hydrogen conversion efficiency is about 90%. NH₃ is burnable without emission of CO₂ and has advantages as hydrogen and energy carriers.     

Hydrogen storage in dinuclear Pt(II) metallacycles

Hydrogen gas physisorption in two distinct dinuclear Pt(II) metallacycles was investigated with ²H nuclear magnetic resonance (NMR) spectroscopy. Hydrogen adsorption isotherm data and ²H NMR spectroscopy indicated that hydrogen storage occurs via condensation within the cavities of one of the macrocycles and at the interstitial sites in the other. In addition, this study further supports the notion that heat of adsorption and pore size play critical roles in hydrogen storage.     

Hydrogen storage in coiled carbon nanotubes

We report a density functional calculation of the adsorption of molecular hydrogen on the external surface of coiled carbon nanotube (CCNT). Binding energies of single molecule have been studied as a function of three different orientations and at three different sites like hexagon, pentagon and heptagon. The binding energy values are larger than linear (5,5) armchair nanotube, which has approximately same diameter as that of coiled carbon nanotube. The curvature and topology of CCNT are responsible for this considerable enhancement. The system with full coverage is also studied. When the nanotube surface is fully covered with one molecule per graphitic hexagon, pentagon and heptagon gives the 6.8 wt% storage capacity. The binding energy per molecule decreases due to repulsive interactions between neighbor molecules. It gives good storage medium for hydrogen. Almost it meets the DOE target.     

Hydrogen storage and thermal conductivity properties of Mg-based materials with different structures

Mg-based hydrogen storage materials can be very promising candidates for stationary energy storage application due to the high energy density and low cost of Mg. Hydrogen storage kinetics and thermal conductivity are two important factors for the material development for this kind of application. Here we studied several types of Mg-based materials with different structure-micrometer scale Mg powders, Mg nanoparticles, single crystal Mg, nanocrystalline Mg₅₀Co₅₀ BCC alloy and Mg thin film samples. It seems the Mg materials with good kinetics usually are the ones with nanostructure and tend to show poor thermal conductivity due to electron/phonon scattering resulting from more interfaces and boundaries in nanomaterials. Based on this work, good crystallinity Mg phase incorporated in carbon nano framework could be one promising option for energy storage.     

Hydrogen production and storage analysis of a system by using TRNSYS

In this study, hydrogen production and storage were investigated. The Transient System Simulation Program (TRNSYS) and Generic Optimization Program (GenOpt) packages were combined for the design and optimization of a system that produces hydrogen from water and stores the hydrogen it produced in the compressed gas tank. The system design is based on the electricity grid. Electrical energy produced in photovoltaic (PV) panels was used to electrolyze water. The systems for Izmir, Istanbul and Ankara provinces which are in different climate zones of Turkey were optimized and the annual system performances based on the optimum angles were analyzed. For the mentioned provinces, the PV tilt angles which minimize electricity drawn from the grid at the electrolyzer are also investigated. The electrical energy produced in the photovoltaic panels, the hydrogen and oxygen amounts produced, the efficiency of the electrolyzer, the gas and pressure levels in the hydrogen tank were compared. According to the results of the analysis, the annual total power produced in photovoltaic panels is 42803.66 kW in İzmir, 42573.74 kW in Istanbul and 44613.95 kW in Ankara. Hydrogen levels produced in the system are calculated as 10488.39 m³ year⁻¹ in Izmir, 9824.70 m³ year⁻¹ in Istanbul, and 10368.65 m³ year⁻¹ in Ankara.     

Hydrogen storage on cross stacking nanocones

Hydrogen is one of promising energy sources with virtually non-polluting. In this paper, the hydrogenation mechanism and hydrogen storage weight percentage of new forms of CNCs, BNNCs and SiCNCs with an apex angle of 112.9° are investigated for first time using density functional theory (DFT) and applying B3LYP level at 6–31 g(d,p) basis. The calculations underscore that for all nanocones; CNCs, BNNCs and SiCNCs the convex surface is always more energetic favorite for hydrogenation comparing with the concave surface of nanocones. Also, the hydrogen storage weight percentage is always enhanced via cross stacking nanocones. Noticeably, it is found that the electron density is widely distributed up the next neighbor atoms of pentagon ring via cross stacking, however for single nanocones is mostly concentrated on the atoms of the conical part (pentagon ring). Finally, the results show that the best candidate nanocone for hydrogen storage is the cross stacking nanocones.     

Hydrogen storage of dual-Ti-doped single-walled carbon nanotubes

The hydrogen adsorption capacity of dual-Ti-doped (7, 7) single-walled carbon nanotube (Ti-SWCNTs) has been studied by the first principles calculations. Ti atoms show different characters at different locations due to local doping environment and patterns. The dual-Ti-doped SWCNTs can stably adsorb up to six H₂ molecules through Kubas interaction at the Ti₂ active center. The intrinsic curvature and the different doping pattern of Ti-SWCNTs induce charge discrepancy between these two Ti atoms, and result in different hydrogen adsorption capacity. Particularly, eight H₂ molecules can be adsorbed on both sides of the dual-Ti decorated SWCNT with ideal adsorption energy of 0.198 eV/H₂, and the physisorption H₂ on the inside Ti atom has desirable adsorption energy of 0.107 eV/H₂, ideal for efficient reversible storage of hydrogen. The synergistic effect of Ti atoms with different doping patterns enhances the hydrogen adsorption capacity 4.5H₂s/Ti of the Ti-doped SWCNT (VIII), and this awaits experimental trial.