Responses to comments on the paper “two-dimensional Sc2C: A reversible and high capacity hydrogen storage material predicted by first-principles calculations”

A recent commentary by Santhosh and Ravindran on our paper (Int. J. Hydrogen Energy 2014, 39:10,606) demonstrated that the interaction between H₂ and MXene (Sc₂C and Ti₂C) phases are not Kubas-type and should be of weak physisorption, and thus made a conclusion that 2D Sc₂C and Ti₂C are not suitable for practical hydrogen storage applications. In this responses, we recalculated hydrogen adsorption on 2D Sc₂C and Ti₂C by using different exchange-correlation functionals. And based on the calculated results, bare MXenes (especially the Ti₂C) are suitable as hydrogen storage materials at temperatures of several tens degrees lower than room temperature. And the hydrogen adsorptions on the MXenes terminated with oxygen group were also investigated. Among the Ti₂C, Sc₂C and their oxygen-functional counterparts, the binding energy of H₂ on Sc₂CO₂ surface is the closest to the ideal range of 0.16–0.42 eV/H₂ at ambient conditions, and thus the Sc₂C with oxygen group is expected to be more suitable as hydrogen storage materials.     

A theoretical research of dihydrogen storage in ScxNy (x+y=4) compounds

The dihydrogen storage capacity of Sc_xN_y (x + y = 4) compounds have been theoretically investigated at different levels. At B3LYP-D3/6-311G(3df,3pd) level, ScN₃ has multiple isomers with similar energies, which is an interference of hydrogen storage research. Sc₂N₂ and Sc₃N has four and three isomers, respectively. For both systems, the lowest-lying isomers are planar Sc₂N₂ 01 and Sc₃N 01, which are energetically much low-lying by at least 20 kcal/mol than the other isomers, respectively. Sc₃N 01 can adsorb 8H₂ with gravimetric uptake capacity of 9.77 wt %. It satisfies the target specified by US DOE, however, some hydrogen molecules will dissociate and bond atomically on scandium atoms. The strong binding energy (0.66 eV/H₂) exceeds the reversible adsorption range (0.1–0.4 eV/H₂), which will cause high operating temperature to desorb hydrogen during the application process. Sc₂N₂ 01 can adsorb 9H₂ in the molecular form. The H₂ gravimetric uptake capacity of Sc₂N₂ 01 (9H₂) (13.33 wt %) exceeds the target set by US Department of Energy, moreover, its average adsorption energy (0.32 eV/H₂) is in the reversible adsorption range. The interaction of Sc₂N₂ 01 with H₂ molecules is considered by means of the bond critical points (bcp) in the quantum theory of atoms in molecules (QTAIM). The Gibbs free energy corrected adsorption energy points that the adsorption of Sc₂N₂ 01(9H₂) is energetically favorable below 240 K. Therefore, in Sc_xN_y (x + y = 4), the planar compound Sc₂N₂ 01 is more suitable to be a dihydrogen adsorption material.     

Catalytic effect of carbon nanostructures on the hydrogen storage properties of MgH2–NaAlH4 composite

The present investigation describes the hydrogen storage properties of 2:1 molar ratio of MgH₂–NaAlH₄ composite. De/rehydrogenation study reveals that MgH₂–NaAlH₄ composite offers beneficial hydrogen storage characteristics as compared to pristine NaAlH₄ and MgH₂. To investigate the effect of carbon nanostructures (CNS) on the de/rehydrogenation behavior of MgH₂–NaAlH₄ composite, we have employed 2 wt.% CNS namely, single wall carbon nanotubes (SWCNT) and graphene nano sheets (GNS). It is found that the hydrogen storage behavior of composite gets improved by the addition of 2 wt.% CNS. In particular, catalytic effect of GNS + SWCNT improves the hydrogen storage behavior and cyclability of the composite. De/rehydrogenation experiments performed up to six cycles show loss of 1.50 wt.% and 0.84 wt.% hydrogen capacity in MgH₂–NaAlH₄ catalyzed with 2 wt.% SWCNT and 2 wt.% GNS respectively. On the other hand, the loss of hydrogen capacity after six rehydrogenation cycles in GNS + SWCNT (1.5 + 0.5) wt.% catalyzed MgH₂–NaAlH₄ is diminished to 0.45 wt.%.     

Hydrogen storage properties of nano-structured 0.65MgH2/0.35ScH2

The intermetallic compound Mg_0.65Sc_0.35 was found to form a nano-structured metal hydride composite system after a (de)hydrogenation cycle at temperatures up to 350 °C. Upon dehydrogenation phase separation occurred forming Mg-rich and Sc-rich hydride phases that were clearly observed by SEM and TEM with the Sc-rich hydride phase distributed within Mg/MgH₂-rich phase as nano-clusters ranging in size from 40 to 100 nm. The intermetallic compound Mg_0.65Sc_0.35 showed good hydrogen uptake, ca. 6.4 wt.%, in the first charging cycle at 150 °C and in the following (de)hydrogenation cycles, a reversible hydrogen capacity (up to 4.3 wt.%) was achieved. Compared to the as-received MgH₂, the composite had faster cycling kinetics with a significant reduction in activation energy E_a from 159 ± 1 kJ mol⁻¹ to 82 ± 1 kJ mol⁻¹ (as determined from a Kissinger plot). Two-dehydrogenation events were observed by DSC and pressure–composition-isotherm (PCI) measurements, with the main dehydrogenation event being attributed to the Mg-rich hydride phase. Furthermore, after the initial two cycles the hydrogen storage capacity remained unchanged over the next 55 (de)hydrogenation cycles.     

ZrO2@Nb2CTx composite as the efficient catalyst for Mg/MgH2 based reversible hydrogen storage material

While Mg/MgH₂ system has a high hydrogen storage capacity, its sluggish hydrogen desorption rate has hindered practical applications. Herein, we report that the hydrogen absorption and desorption kinetics of Mg/MgH₂ system can be significantly improved by using the synergetic effect between Nb₂CT_x MXene and ZrO₂. The catalyst of Nb₂CT_x MXene loading with ZrO₂ (ZrO₂@Nb₂CT_x) is successfully synthesized, and the dehydrogenation activation energy of MgH₂ becomes as low as 60.0 kJ/mol H₂ when ZrO₂@Nb₂CT_x is used as the catalyst, which is far smaller than the case of ZrO₂ (94.8 kJ/mol H₂) and Nb₂CT_x MXene (125.6 kJ/mol H₂). With the addition of ZrO₂@Nb₂CT_x catalyst, MgH₂ can release about 6.24 wt.% and 5.69 wt.% of hydrogen within 150 s at 300 °C and within 900 s even at 240 °C, respectively. Moreover, it realizes hydrogen absorption at room temperature, which can uptake 2.98 wt.% of hydrogen within 1800 s. The catalytic mechanism analysis demonstrates that the in-situ formed nanocomposites can weaken the Mg–H bonding and provide more hydrogen diffusion channels, enabling the dissociation and recombination of hydrogen under milder reaction conditions.     

Hydrogen storage in TiO2 functionalized (10, 10) single walled carbon nanotube (SWCNT) – First principles study

Hydrogen storage in titanium dioxide (TiO₂) functionalized (10, 10) armchair single walled carbon nanotube (SWCNT) is investigated through first principle calculations using density functional theory (DFT). This first principles study uses Vienna Ab-initio Simulation Package (VASP) with ultrasoft pseudopotentials and local density approximation (LDA). The necessary benchmark and other systematic calculations were carried out to project the hydrogen storage capability of the designed system. Interestingly, the TiO₂ molecules functionalized on the outer surface of SWCNT do not undergo any dimerization/clustering thus giving excellent stability and usable gravimetric hydrogen storage capacity of 5.7 wt.% and the value nearly fulfills the US DOE target (i.e. 6 wt.%). The band structure and density of states (DOS) plots suggest that the functionalization can lead a way to transform the nature (metallic → semiconducting) of the pristine SWCNT. The nominal values of H₂ storage capacity and binding energies give much hope for using CNT functionalized with TiO₂ as a practical and reversible hydrogen storage medium (HSM).     

Rb and Cs doping effects in sodium borohydride: Density functional theory for hydrogen (H2) storage purpose

Fuel cell technology based on stationary and mobile applications is needing new hydrogen storage materials equipped with huge gravimetric and volumetric hydrogen densities. Examining the fundamental properties of hydrides is an important part of such process, mainly to understand the structure change's impact on the hydrogen storage. Herein, we applied ab-initio density functional theory using full potential linear augmented plane method to explore the effect of rubidium and cesium doping in sodium borohydride, NaBH₄. The electronic structure calculations exposed the semiconducting nature of NaBH₄ and derived doped structures NaRbBH₄ and NaCsBH₄. The hydrogen (H₂) storage capacity is found 10.66 wt %, 3.27 wt % and 2.36 wt % within a reasonable free energy of ?28.514 kJ/mol, ?29.709 kJ, ?28.51 kJ/mol for NaBH₄, NaRbBH₄ and NaCsBH₄ respectively from quasi-harmonic approximation. Also, we extracted the heat capacity and Debye temperature from vibrational analysis based on phonon calculation. The discovered features show the potential use of presented sodium borohydrides for practical H₂ storage devices.     

Kinetic properties of La2Mg17–x wt.% Ni (x = 0–200) hydrogen storage alloys prepared by ball milling

The as-cast La₂Mg₁₇ with different amount of Ni powders were mixed through ball milling to produce a new type of La₂Mg₁₇–x wt.% Ni (x = 50, 100, 150, 200) alloy. The microstructures of the alloys were characterized by XRD technique, the results show that the crystal structure transfers to amorphous one with the increasing amount of Ni powders. La₂Mg₁₇–50 wt.% Ni alloy reaches the highest hydrogen absorption capacity of 5.13 wt.% at 300 °C under 2 MPa hydrogen pressure due to its amorphous structure. Furthermore, La₂Mg₁₇–50 wt.% Ni alloy expresses fast hydriding kinetics and absorbs 4.99 wt.% hydrogen gas in 200 s. The hydrogen desorption ability described as discharge capacity during electrochemical reaction is fade next to La₂Mg₁₇–200 wt.% Ni alloy, attributed to the less Mg₂NiH₄ with lower enthalpies and easier to release H₂. The maximum discharge capacity of La₂Mg₁₇–200 wt.% Ni alloy reaches to exciting 980.90 mAh/g, while the La₂Mg₁₇ alloy is only 18.10 mAh/g with inconspicuous improvement of cycle stability. These dramatic difference in electrochemical performance reflect the consequence of sluggish dehydriding process of La₂Mg₁₇–50 and 100 wt.% Ni alloys again. Whereas La₂Mg₁₇–200 wt.% Ni alloy has lower resistance both on alloy surface and in the bulk.     

The effect of charge on the dihydrogen storage capacity of Sc2–C6H6

The effect of charge on the dihydrogen storage capacity of Sc₂–C₆H₆ has been investigated at B3LYP-D3/6-311G(d,p) level. The neutral system Sc₂–C₆H₆ can store 8H₂ with gravimetric density of 8.76 wt %, and one H₂ dissociates and bonds atomically on the scandium atom. The adsorption of 8H₂ on Sc₂–C₆H₆ is energetically favorable below 155 K. The atom-centered density matrix propagation (ADMP) molecular dynamics simulations show that Sc₂–C₆H₆ can adsorb 3H₂ within 1000 fs at 300K. Compared with Sc₂–C₆H₆, the charged systems can adsorb more hydrogen molecules with higher gravimetric density, and all the H₂ are adsorbed in the molecular form. The gravimetric densities of Sc₂–C₆H₆⁺ and Sc₂–C₆H₆²⁺ are 9.75 and 10.71 wt%. Moreover, the maximum adsorption of charged systems are favorable in wider temperature range. Most importantly, the ADMP-MD simulations indicate that Sc₂–C₆H₆²⁺ can adsorb 6 hydrogen molecules within 1000 fs at 300K. It can be found that the gravimetric density (6.72 wt%) of Sc₂–C₆H₆²⁺ still exceeds the target of US Department of Energy (DOE) under ambient conditions.     

Effects of ultra-high pressure on phase compositions,phase configurations and hydrogen storage properties of LaMg4Ni alloys

The influences of ultrahigh pressure (UHP, under 5 GPa) on phase compositions, phase morphologies and hydrogen storage properties of LaMg₄Ni alloys were studied. The X-ray diffraction patterns show that the as-cast alloy consists of La₂Mg₁₇, LaMg₂Ni and Mg₂Ni phases, whereas a new LaMg₃ phase is observed in the UHP samples in addition to LaMg₂Ni and Mg₂Ni phases. The scanning electron microscopy graphs indicate that the phase distribution is more homogenous in the UHP alloys than in the as-cast one. Additionally, the microstructure of the UHP alloy heat-treated at 973 K is finer than that at 823 K. Both the reversible hydrogen storage capacity and the plateau of hydrogen pressure of the UHP alloys are close to those of the as-cast one. Of particular interest is that both UHP alloys exhibit better activation properties compared with the as-cast alloy. Moreover, the dehydriding onset temperature of the UHP alloys (5 GPa at 973 K) is about 490 K, which is obviously lower than that of the as-cast alloy. The amount of hydrogen desorption in the UHP alloy (5 GPa at 973 K) is 2.69 wt.% at 573 K, which corresponds to 89.6% of the saturated capacity. However, the corresponding values change to 1.75 wt.% and 58.3% in the as-cast alloy, respectively. It is confirmed the UHP treatment is one of effective approaches to tune the hydrogen storage performances of those rare earth–magnesium–nickel alloys.     

Hydrogen storage in Ca-decorated carbyne C10-ring on either Dnh or D(n/2)h symmetry. DFT study

We computationally investigate the hydrogen storage properties of carbyne C₁₀-ring structure on either D_nh or D_(n/2)h symmetry decorated with calcium (Ca) atoms adsorbed on its outer surface. The calculations are carried out on DFT-GGA-PW91 and DFT-GGA-PBE levels of theory as implemented in Biovia Materials Studio modeling and simulation software. To account for van der Waals interactions we also carried out calculations using DFT-D method of Grimme. Dmol³ is used to calculate total energies, HOMO-LUMO electronic charge density, Mulliken population analysis, and electrostatic potential fitting charges (ESP). Based on these results: i) the average binding energy of Ca atom doping to C₁₀-ring is ~2.3 eV (PW91) and ~2.1 eV (PBE). ii) Up to seven H₂ molecules per Ca atom can be physically adsorbed with an average energy of ~0.2 eV per H₂ molecule. iii) This physisorption leads to 8.09 wt percentage (wt. %) for the gravimetric storage capacity. According to these results, calcium-decorated carbyne C₁₀-ring structure is excellent candidate for hydrogen storage at ambient conditions with application to fuel cells.     

Theoretical studies in the stability of vacancies in zeolite templated carbon for hydrogen storage

In this work, we report DFT calculations of the energy formation and stability of multi-vacancies in a unit of Zeolite Template Carbon (C₃₉H₉). We label as V_n the respective vacancy where n carbon atoms have been removed from the pristine C₃₉H₉ structure. The results show that V₂, V₄, V₆ and V₉ are the most stable vacancies on the ZTC structure. This result agrees with many other studies. Besides, the most stable vacancy of ZTC structure is when nine carbon atoms are removed (V₉) from the ZTC structure. The formation of pentagon rings in the reconstruction of the ZTC vacancy give drastic effect on the energetics stability. Therefore, the formation of pentagon rings eliminates the dangling bonds thus lowering the energy formation. It is also carried out the decoration of ZTC vacancy with Lithium and Calcium atoms, this is the way to use de ZTC vacancy decorated as a medium for hydrogen storage. The results show that the ZTC vacancy decorated with 3 Lithium atoms can adsorb a maximum of nine hydrogen molecules (3 hydrogen molecules per Lithium atom). This gives a gravimetric storage capacity of 4.44 wt percent (wt. %), which is not enough for meeting DOE gravimetric target. On the other hand, to reach DOE gravimetric target, the study of ZTC vacancy decorated with 3 Calcium atoms is carried out, which can adsorb maximum of fifteen hydrogen molecules (5 hydrogen molecules per Calcium atom), this gives gravimetric storage capacity of 5.81 wt %, which meet DOE gravimetric targets, besides the binding energy of hydrogen molecules on ZTC vacancy decorated with 3 Calcium is calculated. These energies are in the range 0.2453–0.2053 eV/H₂, which are desirable energies for hydrogen adsorption. This is demonstrated by building isotherm adsorption path. The results show that forming vacancies on ZTC structure decorated with three Calcium atoms (3CaC₃₀H₉) is good candidate as medium for hydrogen storage.     

A model study of effect of M = Li,Na, Be,Mg, and Al ion decoration on hydrogen adsorption of metal-organic framework-5

The effect of light metal ion decoration of the organic linker in metal-organic framework MOF-5 on its hydrogen adsorption with respect to its hydrogen binding energy (ΔB.E.) and gravimetric storage capacity is examined theoretically by employing models of the form MC₆H₆:nH₂ where M = Li⁺, Na⁺, Be²⁺, Mg²⁺, and Al³⁺. A systematic investigation of the suitability of DFT functionals for studying such systems is also carried out. Our results show that the interaction energy (ΔE) of the metal ion M with the benzene ring, ΔB.E., and charge transfer (Q_trans) from the metal to benzene ring exhibit the same increasing order: Na⁺ < Li⁺ < Mg²⁺ < Be²⁺ < Al³⁺. Organic linker decoration with the above metal ions strengthened H₂-MOF-5 interactions relative to its pure state. However, amongst these ions only Mg²⁺ ion resulted in ΔB.E. magnitudes that were optimal for allowing room temperature hydrogen storage applications of MOF-5. A much higher gravimetric storage capacity (6.15 wt.% H₂) is also predicted for Mg²⁺-decorated MOF-5 as compared to both pure MOF-5 and Li⁺-decorated MOF-5.     

Stability and hydrogen storage properties of Sc6O8 and Y6O8 cage-like complexes

To study the dihydrogen storage capacity of Sc₆O₈ and Y₆O₈ complexes, the stability and hydrogen adsorption behavior have been investigated by using density functional theoretical calculations. The lowest-lying isomers are cage-like complexes Sc₆O₈ 01 and Y₆O₈ 01, which are energetically much low-lying by at least 40.43 kcal/mol than the other isomers, respectively. Sc₆O₈ 01 can adsorb 26H₂ with gravimetric uptake capacity of 11.64 wt%. The average adsorption energy (ΔE_ave) is 0.12 eV/H₂, which is in the reversible adsorption range. The Y₆O₈ 01 seem have little ability to adsorb hydrogen molecules, because the ΔE_ave Y₆O₈ 01 (1H₂) is just only 0.065 eV. However, the binding capacity increases with the number of adsorbed H₂ increasing. Y₆O₈ 01 can adsorb 32H₂ with ΔE_ave of 0.11 eV/H₂, and the gravimetric uptake capacity is 8.89 wt%. Various characterization methods indicate that both transition metals and nonmetals in Sc₆O₈ 01 and Y₆O₈ 01 can effectively adsorb hydrogen molecules, and these two compounds can be regarded as candidate materials of dihydrogen adsorption under suitable condition.     

Catalytic hydrolysis of hydrazine borane for chemical hydrogen storage: Highly efficient and fast hydrogen generation system at room temperature

There has been rapidly growing interest for materials suitable to store hydrogen in solid state for transportation of hydrogen that requires materials with high volumetric and gravimetric storage capacity. B-N compounds such as ammonia-triborane, ammonia-borane and amine-borane adducts are well suited for this purpose due to their light weight, high gravimetric hydrogen storage capacity and inclination for bearing protic (N-H) and hydridic (B-H) hydrogens. In addition to them, more recent study [26] has showed that hydrazine borane with a gravimetric hydrogen storage capacity of 15.4% wt needs to be considered as another B-N compound that can be used for the storage of hydrogen. Herein we report for the first time, metal catalyzed hydrolysis of hydrazine borane (N₂H₄BH₃, HB) under air at room temperature. Among the catalyst systems tested, rhodium(III) chloride was found to provide the highest catalytic activity in this reaction. In the presence of rhodium(III) chloride, the aqueous solution of hydrazine borane undergoes fast hydrolysis to release nearly 3.0 equivalent of H₂ at room temperature with previously unprecedented H₂ generation rate TOF = 12000 h⁻¹. More importantly, it was found that in the catalytic hydrolysis of hydrazine borane the reaction between hydrazine borane and water proceeds almost in stoichiometric proportion indicating that the efficient hydrogen generation can be achieved even from the highly concentrated solution of hydrazine borane or in the solid state when water added to the solid hydrazine borane. This finding is crucial especially for on-board application of the existing system. The work reported here also includes (i) finding the solubility of hydrazine borane plus its stability against self-hydrolysis in water, (ii) the definition of reaction stoichiometry and the identification of reaction products for the catalytic hydrolysis of hydrazine borane, (iii) the collection of wealthy kinetic data to demonstrate the effect of substrate and catalyst concentrations on the hydrogen generation rate and to determine the rate law for the catalytic hydrolysis of hydrazine borane, (iv) the investigation of the effect of temperature on the rate of hydrogen generation and determination of activation parameters (E_a, ΔH^#, and ΔS^#) for the catalytic hydrolysis of hydrazine borane.     

200 NL H2 hydrogen storage tank using MgH2–TiH2–C nanocomposite as H storage material

MgH₂-based hydrogen storage materials are promising candidates for solid-state hydrogen storage allowing efficient thermal management in energy systems integrating metal hydride hydrogen store with a solid oxide fuel cell (SOFC) providing dissipated heat at temperatures between 400 and 600 °C. Recently, we have shown that graphite-modified composite of TiH₂ and MgH₂ prepared by high-energy reactive ball milling in hydrogen (HRBM), demonstrates a high reversible gravimetric H storage capacity exceeding 5 wt % H, fast hydrogenation/dehydrogenation kinetics and excellent cycle stability. In present study, 0.9 MgH₂ + 0.1 TiH₂ +5 wt %C nanocomposite with a maximum hydrogen storage capacity of 6.3 wt% H was prepared by HRBM preceded by a short homogenizing pre-milling in inert gas. 300 g of the composite was loaded into a storage tank accommodating an air-heated stainless steel metal hydride (MH) container equipped with transversal internal (copper) and external (aluminium) fins. Tests of the tank were carried out in a temperature range from 150 °C (H₂ absorption) to 370 °C (H₂ desorption) and showed its ability to deliver up to 185 NL H₂ corresponding to a reversible H storage capacity of the MH material of appr. 5 wt% H. No significant deterioration of the reversible H storage capacity was observed during 20 heating/cooling H₂ discharge/charge cycles. It was found that H₂ desorption performance can be tailored by selecting appropriate thermal management conditions and an optimal operational regime has been proposed.     

High capacity reversible hydrogen storage in titanium doped 2D carbon allotrope Ψ-graphene: Density Functional Theory investigations

Using the state-of-the art Density Functional Theory simulations, here we report the hydrogen storage capability in titanium decorated ?- Graphene, an advanced 2D allotrope of carbon which is made of hexagonal, pentagonal and heptagonal ring of carbon and metallic in nature. Titanium is strongly bonded on the surface of ?- Graphene and each Ti can bind maximum of 9H₂ having average adsorption energy of ?0.30 eV and average desorption temperature of 387 K yielding gravimetric H₂ uptake of 13.14 wt%, much higher than the prescribed limit of 6.5 wt % by DoE's. The interaction of Ti on ?- Graphene have been presented by electronic density of states analysis, charge transfer and plot for spatial distribution of charge. There is orbital interaction between Ti 3d and C 2p of ?- Graphene involving transfer of charge whereas bonding of hydrogen molecules is through Kubas type of interactions involving charge donation from σ orbitals of hydrogen molecules to the vacant 3d orbital of Ti and the subsequent back donation to σ1 orbital of hydrogen from filled 3d orbital of Ti. The structural stability of the system at temperatures corresponding to the highest temperature at which H₂ desorbs was verified using ab-initio Molecular Dynamics calculations and presence of sufficient energy barrier for diffusion which prevents clustering between metal atoms assures the practical viability of the system as high capacity H₂ adsorbing material. Overall, found that Ti doped Ψ-Graphene is stable, 100% recyclable and has high hydrogen storage capacity with suitable desorption temperature. As a result of our findings, we are confident that Ti doped Ψ-Graphene may be used as a potential hydrogen adsorbing material in the upcoming clean, green, hydrogen economy.     

Structural and hydrogenation studies of ZnO and Mg doped ZnO nanowires

In this work, Mg doped zinc oxide (Mg_xZn_1−xO, x = 5, 10 and 20 at. %) nanowires were successfully prepared by two step process. Initially, ZnO nanowires were grown by thermal evaporation of Zn powder under oxygen atmosphere. Mg powder was doped in as grown ZnO through solid state diffusion at low temperature. Energy dispersive x-ray spectroscopy (EDAX), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV–Visible absorption spectra analysis reveals that the Mg doping on ZnO nanowires induces lattice strain in ZnO. Rietveld analysis of XRD data confirms the wurtzite structure and a continuous compaction of the lattice (in particular, the c-axis parameter) as x increases. The hydrogenation properties of ZnO nanowires and Mg doped ZnO (Mg_xZn_1−xO, x = 0, 5, 10 and 20 at. %) nanowires were studied. The hydrogenated samples were further investigated through XRD and Fourier transform infrared spectroscopy (FTIR). The hydrogen storage capacity of as grown ZnO nanowires has been estimated to be 0.57 wt. % H₂ at room temperature. However, the hydrogen storage capacity gets increased to ∼1 wt. % upon doping ZnO with 10 at. % Mg. Further increase in Mg concentration decreases the hydrogen storage capacity of ZnO nanowires. Thus for 20 at. % Mg doped ZnO; the hydrogen absorption capacity gets decreased from ∼1 wt. % to 0.74 wt. %. The mechanism of hydrogen storage in ZnO nanowires and Mg doped samples of ZnO has been discussed.     

A first principles study of hydrogen storage in lithium decorated defective phosphorene

In this study, we studied defect-engineering and lithium decoration of 2D phosphorene for effective hydrogen storage using density functional theory. Contrary to graphene, it is found that the presence of point-defects is not preferable for anchoring of H₂ molecules over defective phosphorene. According to previous research, strategies such as defect engineering, metal decoration, and doping enhance the hydrogen storage capacity of several 2D materials. Our DFT simulations show that point defects in phosphorene do not improve the hydrogen storage capacity compared to pristine phosphorene. However, selective lithium decoration over the defective site significantly improves the hydrogen adsorption capacity yielding a binding energy of as high as ?0.48 eV/H₂ in Li-decorated single vacancy phosphorene. Differential charge densities and projected density of states have been computed to understand the interactions and charge transfer among the constituent atoms. Strong polarization of the H₂ molecule is evidenced by the charge accumulation and depletion. The PDOS shows that the presence of Li leads to enhanced charge transfer. The maximum gravimetric density was investigated by sequentially adding H₂ molecules to the Li-decorated single vacancy defective phosphorene. The Li-decorated single vacancy phosphorene is found to possess a gravimetric density of around 5.3% for hydrogen storage.     

NMR spectroscopic and thermodynamic studies of the etherate and the α, α′, and γ phases of AlH3

Aluminum hydride (alane; AlH₃) has been identified as a leading hydrogen storage material by the US Department of Energy. With a high gravimetric hydrogen capacity of 10.1 wt.%, and a hydrogen density of 1.48 g/cm³, AlH₃ decomposes cleanly to its elements above 60 °C with no side reactions. This study explores in detail the thermodynamic and spectroscopic properties of AlH₃; in particular the α, α′ and γ polymorphs, of which α′-AlH₃ is reported for the first time, free from traces of other polymorphs or side products. Thermal analysis of α-, α′-, and γ-AlH₃ has been conducted, using DSC and TGA methods, and the results obtained compared with each other and with literature data. All three polymorphs were investigated by ¹H MAS-NMR spectroscopy for the first time, and their ²⁷Al MAS-NMR spectra were also measured and compared with literature values. AlH₃·nEt₂O has also been studied by ¹H and ²⁷Al MAS-NMR spectroscopy and DSC and TGA methods, and an accurate decomposition pathway has been established for this adduct.