Hydrogen storage in platinum loaded single-walled carbon nanotubes

To study the hydrogen storage capacity, platinum (Pt) nanoparticles were deposited on single-walled carbon nanotubes (SWNT) using hexachloroplatinic acid (H₂PtCl₆·6H₂O) as a precursor. To verify Pt deposition on the surface of the SWNT, a Transmission Electron Microscope (TEM) was used to obtain surface morphology. The TEM images show that Pt nanoparticles were homogeneously distributed on the surface of SWNT. Commercial SWNT were also used to compare the results. Thermal Gravimetric Analysis at heating rate of 5 °C/min is measured for pure SWNT and Pt loaded SWNT. Before hydrogen storage measurements these samples were reduced in 10% of H₂ in Ar, flowing at 900 °C in a tubular furnace for 1 hour. Hydrogen storage capacity of these SWNT was investigated under 25 bar pressure and room temperature as well as liquid nitrogen temperature.     

Spillover enhancement for hydrogen storage by Pt doped hypercrosslinked polystyrene

High surface area physisorption materials are of interest for room temperature (RT) hydrogen storage enhancement by spillover. In this study, six different commercially available hypercrosslinked polystyrenes were screened considering the specific surface area, average pore size, pore volume, and adsorption enthalpy. MN270 was selected mainly due to its high surface area and narrow pores for investigation of the spillover enhancement at RT. Two different platinum (Pt) doped MN270 samples were prepared by wet impregnation (MN270-6wt%Pt) and bridge building technique (MN270-Bridged) with an average Pt particle size of 3.9 and 9.9 nm, respectively, as obtained from X-ray diffraction analysis. Pt doping altered the surface property of MN270, and reduced the nitrogen and hydrogen uptake at 77 K and 1 atm due to pore blocking. The RT hydrogen uptake at 100 atm demonstrated a 10% enhancement (0.36 wt. %) for MN270-Bridged compared to pristine MN270, but did not show any enhancement for MN270-6wt%Pt under the same conditions. The hydrogen uptake of MN270-Bridged has little value for practical applications yet showed the effectiveness of the bridge building technique.     

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.     

Hydrogen storage behaviors of platinum-supported multi-walled carbon nanotubes

In this work, the hydrogen storage behaviors of multi-walled carbon nanotubes (MWNTs) loaded by crystalline platinum (Pt) particles were studied. The microstructure of the Pt/MWNTs was characterized by X-ray diffraction and transmission electron microscopy. The pore structure and total pore volumes of the Pt/MWNTs were analyzed by N₂/77 K adsorption isotherms. The hydrogen storage capacity of the Pt/MWNTs was evaluated at 298 K and 100 bar. From the experimental results, it was found that Pt particles were homogeneously distributed on the MWNT surfaces. The amount of hydrogen storage capacity increased in proportion to the Pt content, with Pt-5/MWNTs exhibiting the largest hydrogen storage capacity. The superior amount of hydrogen storage was linked to an increase in the number of active sites and the optimum-controlled micropore volume for hydrogen adsorption due to the well-dispersed Pt particles. Therefore, it can be concluded that Pt particles play an important role in hydrogen storage characteristics due to the hydrogen spillover effect.     

Mechanochemical reactions and hydrogen storage capacities in MBH4–SiS2 systems (MLi or Na)

The hydrogen storage properties, and phase compositions of mechanochemically prepared mixtures of xMBH₄-SiS₂ (x = 2–8), where M = Li or Na, were investigated using gas sorption analysis, powder X-ray diffraction, and infrared and solid-state NMR spectroscopic methods. The 2LiBH₄:1SiS₂ system forms an amorphous product that releases ca. 4.3 wt % of H₂ below 385 °C with a T_onset of 88 °C without detectable diborane emission. The dehydrogenated sample reversibly absorbs 1.5 wt % of H₂ at 385 °C under 160 bar pressure. The H₂ release from materials with varying LiBH₄:SiS₂ ratios peaks at 8.2 wt % for the 6LiBH₄:1SiS₂ composition, with a reversible hydrogen storage capacity of 2.4 wt %. The H₂ desorption capacities of the Li-containing systems surpass those of Na-containing systems. Solid-state NMR studies indicate that products of mechanochemical reactions in the LiBH₄SiS₂ system consist of one-dimensional chains of edge-sharing SiS_4/2 tetrahedra in which the non-bridging S-ends are terminated with Li⁺, which are further coordinated to the [BH₄]⁻ anions. A variety of possible polymorphs in the LiSiS-(BH₄) composition space have been identified using first principles and thermodynamic modeling that supports the likelihood of formation of such novel complexes.     

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.     

Hydrogen storage in liquid organic hydride: Selectivity of MCH dehydrogenation over monometallic and bimetallic Pt catalysts

Hydrogen storage for mobile and stationary applications is an expanding research topic. One of the more promising storage techniques relies on the reversibility, high selectivity, and high hydrogen density of liquid organic hydrides, in particular methylcyclohexane (MCH). Catalyst evaluation for MCH dehydrogenation to toluene is based on three catalytic parameters: activity, selectivity, and stability. Current catalysts, optimized for catalytic reforming, do not meet the targeted aromatic selectivity (+99%) for MCH dehydrogenation. Therefore, a range of Pt catalysts was prepared and compared with commercially available catalysts in a fixed-bed reactor under operating conditions suitable for mobile and stationary applications. The best overall performance was realized by a particular monometallic Pt catalyst. This catalyst showed superior activity, selectivity, and stability compared with other prepared and commercial catalysts. As an effort to further enhance the aromatic selectivity, this study identified the main side-reactions associated with MCH dehydrogenation, the effect of operating parameters on by-product yields, and the effect of catalyst deactivation on long-term selectivity.     

Hydrogen storage by decalin dehydrogenation/naphthalene hydrogenation pair over platinum catalysts supported on activated carbon

Different platinum catalysts supported on activated carbon have been investigated for decalin dehydrogenation under several reaction conditions. Organic hydride as hydrogen storage media is an efficient source for fuel cells when the catalytic dehydrogenation reaction occurs rapidly under mild conditions. Among hydrides, the choice of decalin in this study is owed to the simplicity of analyses, as well as comparison with works reported in literature.     

Hydrogen storage properties of the novel crosslinked UiO-66-(OH)2

Metal organic frameworks (MOF) are a type of nanoporous materials with large specific surface area, which are especially suitable for gas separation and storage. In this work, we report a new approach of crosslinking UiO-66-(OH)₂ to enhance its hydrogen storage capacity. UiO-66-(OH)₂ was synthesized using hafnium tetrachloride (HfCl₄) and 2, 5-dihydroxyterephthalic acid (DTPA) through a canonical modulated hydrothermal method (MHT), followed by a post-synthesis modification, which is to form a crosslinking structure inside the porous structure of UiO-66-(OH)₂. During the modification process, the phenolic hydroxyl groups on the UiO-66-(OH)₂ reacted with methanal, and HCl aqueous solution and triethylamine served as catalyst (the products denoted as UiO-66-H and UiO-66-T, respectively). Powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR), ¹³C nuclear magnetic resonance spectroscopy (¹³C NMR) proved that the crosslinking was formed. The BET specific surface area and the average adsorption pore size of UiO-66-H and UiO-66-T significantly increased after modification. The hydrogen storage capacity of UiO-66-H reached a maximum of 3.37 wt% (16.87 mmol/g) at 77 K, 2 MPa. Hydrogen adsorption enthalpy of UiO-66-T was 0.986 kJ/mol, which was higher than that of UiO-66-(OH)₂ (0.695 kJ/mol). This work shows that UiO-66-(OH)₂ is a promising candidate for potential application in high-performance hydrogen storage.     

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 with hetero porphyrin aggregates

Hydrogen interaction of porphyrin hetero-pairs has been studied by differential scanning calorimetry (DSC). Aggregates were prepared by spontaneous aggregation of water soluble anionic porphyrins meso-tetra(4-carboxyphenyl)porphine (TCPP) and meso-tetra(4-sulfonatophenyl)porphine (TPPS) with cationic meso-tetra-p-trimethylaminophenyl porphine tetrachloride (TAP). Aggregate formation in solution was investigated via UV-visible spectroscopy at different pH ranges. At neutral to basic conditions, porphyrin pairs formed partially soluble hetero H-aggregates and at low pH, soluble homo J-aggregates was formed. H-aggregates were isolated by centrifuging and freeze-drying. The obtained solids were studied by X-ray powder diffraction. Thermal stability of dry aggregates was determined by thermogravimetric analysis. The TCPP-TAP hetero aggregates exhibited hydrogen uptake between 80 °C and 250 °C. The amount of hydrogen absorbed by the sample corresponds to 0.29% by weight, indicating a potential use of such materials as a solid state hydrogen storage medium.     

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 in a Li–Al–N ternary system

Hydrogen-storage properties and mechanisms of a novel Li–Al–N ternary system were systematically investigated by a series of performance evaluation and structural examinations. It is found that ca. 5.2 wt% of hydrogen is reversibly stored in a Li₃N–AlN (1:1) system, and the hydrogenated product is composed of LiNH₂, LiH, and AlN. A stepwise reaction is ascertained for the dehydrogenation of the hydrogenated Li₃N–AlN sample, and AlN is found reacting only in the second step to form the final product Li₃AlN₂. The calculation of the reaction enthalpy change indicates that the two-step dehydrogenation reaction is more thermodynamically favorable than any one-step reaction. Further investigations exhibit that the presence of AlN in the LiNH₂–2LiH system enhances the kinetics of its first-step dehydrogenation with a 10% reduction in the activation energy due likely to the higher diffusivity of lithium and hydrogen within AlN.     

Hydrogen storage in bulk Mg-Ti and Mg-stainless steel multilayer composites synthesized via accumulative roll-bonding (ARB)

We have tested the hydrogen storage cycling behavior of bulk centimeter-scale magnesium - titanium and magnesium - stainless steel multilayer composites synthesized by accumulative roll-bonding (ARB). Roll-bonding of either titanium or stainless steel with magnesium allows the reversible hydrogen sorption of the resulting composite at 350 °C. Identically roll-bonded pure magnesium can hardly be absorbed at this temperature. In the composites, the kinetics of the first cycle of absorption (also called “activation”) improves with increased number of fold and roll (FR) operations. With increasing FR operations the distribution of the Ti phase is progressively refined, and the shape of the absorption curve no longer remains sigmoidal. Increasing the loading amount of the second phase also accelerates the kinetics. This holds true up to a threshold limit. Microscopy analysis performed on 1-2 wt.% hydrogen absorbed composites demonstrates that MgH₂ formed exclusively on various heterogeneous nucleation sites. During activation, MgH₂ nucleation occurred at the Mg-hard phase interfaces. During the subsequent absorption cycles, heterogeneous nucleation primarily occurred in the vicinity of “internal” free surfaces such as cracks.     

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 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 in Li-doped charged single-walled carbon nanotubes

Density functional calculations have been carried out to investigate the interaction of hydrogen molecules with Li-doped charged single-walled carbon nanotubes (SWNTs). The results show that binding of H₂ on positively charged Li-doped SWNTs is enhanced compared with that on uncharged systems. As the charge of the Li-doped SWNTs increases, the binding energy of H₂ increases with the largest binding energy of 0.26 eV. The reason for the increase in H₂ binding energy is that the positively charged tube improves the charge transfer from Li atom to the tube and make the Li atom more ionized, which strengthens the polarization interaction between H₂ and Li atom.