Multiple Ti and Li doped carbon nanoring for hydrogen storage

Multiple Ti and Li atom doped carbon nanorings are considered for hydrogen storage using density functional theory for the first time. There are five six membered carbon rings bonded through C–C bond in a carbon nanoring. Formation energy values show that both, Li as well as Ti atom doped carbon nanoring, are thermodynamically stable structures. Cohesive energy values indicate that Li and Ti atom doped carbon nanoring structures are more stable than undoped carbon nanoring. No clustering of metal atoms occurs in metal doped carbon nanorings which usually reduces the hydrogen storage capacity of a material. Li atom doped carbon nanoring is not suitable for hydrogen storage even at very low temperature at 1 atm pressure as well as at high pressure at room temperature. Ti atom doped carbon nanoring is suitable for hydrogen storage below 225 K and 1 atm pressure as well as at high pressure at room temperature. H₂ desorption temperature is found to be 113 and 450 K for Li and Ti atom doped carbon nanoring respectively. H₂ molecules interact strongly with Ti atom doped carbon nanoring than Li atom doped carbon nanoring that results in higher H₂ desorption temperature for the former than the latter.     

Study of the reversible hydrogen storage performance of Ti-decorated hexagonal B36 by DFT calculations with van der Waals corrections

In this work, we report on the study of the hydrogen storage capability of titanium (Ti) decorated B₃₆ nanosheets using density functional theory (DFT) calculations with van der Waals corrections. Ti atoms are strongly bonded to the surface of B₃₆ with a binding energy of 6.23 eV, which exceeds the bulk cohesive energy of crystalline Ti. Ti-decorated B₃₆ (2Ti@B₃₆) can reversibly adsorb up to 12 H₂ molecules with a hydrogen storage capacity of 4.75 wt % and average adsorption energy between 0.361 and 0.674 eV/H₂. The values of desorption temperature and the results of molecular dynamics simulations enable to conclude that 2Ti@B₃₆ is a perspective reversible material for hydrogen storage under real conditions.     

Hydrogen uptake of Ti-decorated multiwalled carbon nanotube composites

In this study, the hydrogen storage capacity of purified multiwalled carbon nanotubes (MWCNTs) was enhanced from 13- to 15-fold at a temperature of 298 K and pressure of 2.0 MPa, upon incorporation of 1.57–1.88 wt% of ultrafine Ti nanoparticles. The effect of a hydrogen spillover Ti catalyst on MWCNTs prepared using the sputtering method was investigated. A comparison between the hydrogen uptake by MWCNTs sputtered with Ti for 3000 s and that for 6000 s was also performed using the Sievert's volumetric apparatus. The significant enhancement in hydrogen uptake was attributed to the interfacial diffusion of hydrogen from Ti to the MWCNTs. The re-adsorption of hydrogen on the pristine MWCNTs and Ti-decorated MWCNTs dehydrogenated at 200 °C indicated that the samples did not compromise their reversible hydrogen uptake during the hydrogenation–dehydrogenation cycles. It was also found that longer exposure of Ti to the MWCNTs during sputtering improved the hydrogen storage capacity. This improvement could be attributed to the presence of a higher amount of Ti deposited on the MWCNTs, as indicated by thermogravimetric analysis study.     

Hydrogen storage in Ni-B nanoalloy-doped 2D graphene

Two-dimensional graphene material is doped with Ni-B nanoalloys via a chemical reduction method, and shows that the optimal graphene doped with Ni (0.14 wt.%) and B (0.63 wt.%) has a hydrogen capacity of 2.81 wt.% at 77 K and 106 kPa, which is more than twice of that of the pristine graphene. The measured adsorption isotherms of hydrogen and nitrogen suggest that the Ni-B nanoalloys function as catalytic centers to induce the dissociative adsorption of hydrogen (spillover) on the graphene. The Ni-B nanoalloys without using any noble metal may be a promising catalyst for hydrogen storage application.     

Effect of nitrogen and sulfur co-doping on the performance of electrochemical hydrogen storage of graphene

In recent decades, finding a solution to replace metal catalysts with inexpensive and available elements has been investigated extensively. Carbon nanomaterials doped with heteroatom such as (N, B and S) which do not have any metal content can provide sustainable materials with a remarkable electrocatalytic activity that can compete with their metal counterparts. Doped graphene has been considered as an electrode material for oxygen reduction reaction, supercapacitor and Li-ion batteries. In this present account, co-doped graphene with nitrogen and sulfur was studied in order to investigate their electrochemical hydrogen storage performance. The dual doped sample was prepared via a simple hydrothermal method, using thiourea as a nitrogen and sulfur source. The nitrogen and sulfur co-doped graphene (NSG) showed excellent electrical conductivity and electrochemical performance compared with the nitrogen doped graphene (NG) and graphene oxide (GO). Doping graphene with foreign atoms is a method to create a semiconducting gap in it and can act as an n-type semiconductor, therefore the electrochemical performance is remarkable when used as an electrode. According to the results by increasing the electrical conductivity of graphene, the storage capacity of hydrogen was increased. The discharge capacity of GO after 20 cycles was increased from 653 mAh/g to 1663 mAh/g (5.88 wt% hydrogen) and 2418 mAh/g (8.55 wt% hydrogen) in single doped graphene (NG) and co-doped graphene (NSG), respectively. The prepared samples were characterized via X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Brunauer-Emmet-Teller analysis (BET), vibration sample magnetometer (VSM) and infrared spectrum (FT-IR).     

Density functional theory calculations of hydrogen adsorption on Ti-, Zn-, Zr-, Al-, and N-doped and intrinsic graphene sheets

The effect of different doped atoms on the interactions between graphene sheets and hydrogen molecules were investigated by density functional theory calculations. The interactions between graphene sheets and hydrogen molecules can be adjusted by doped atoms. The Ti-doped graphene sheet had the largest interaction energy with the hydrogen molecule (approximately −0.299 eV), followed by the Zn-doped graphene sheet (about −0.294 eV) and then the Al-doped graphene sheet (approximately −0.13 eV). The doped N atom did not improve the interactions between the N-doped graphene sheet and the hydrogen molecule. Our results may serve as a basis for the development of hydrogen storage materials.     

Hydrogen adsorption on Li metal in boron-substituted graphene: An ab initio approach

The characteristics of hydrogen adsorption on Li metal atoms dispersed on graphene with boron substitution is investigated including Li clustering, hydrogen bonding characteristics, and the open metal states of Li adatom using density functional theory calculations. It is found that Li atoms are well dispersed on boron-substituted graphene and can form the (2 × 2) pattern because clustering of Li atoms is hindered by the repulsive Coulomb interaction between Li atoms. One Li adatom dispersed on the double side of graphene can absorb up to 8 hydrogen molecules corresponding to a 13.2% hydrogen storage capacity. In addition, the adsorption behaviors of non-hydrogen atoms such as C and B are calculated to determine whether Li atoms can remain as the open metal state in boron-substituted graphene.     

Enhanced hydrogen storage performance in Pd3Co decorated nitrogen/boron doped graphene composites

In order to harness the potential of hydrogen as an alternative energy carrier, overcoming the barrier related to its storage is of utmost importance. In this direction, it has been shown that dissociation of hydrogen molecules into atoms followed by their adsorption onto high surface area nanomaterials like reduced graphene oxide is a promising pathway. In the present study, we have exploited this pathway, commonly known as the “spillover mechanism” and achieved a hydrogen storage capacity of ～4.6 wt% at 30 bar and 25 °C in Pd₃Co decorated boron doped graphene composite. We demonstrate that optimum loading of transition metal alloy nanoparticles coupled with heteroatom (nitrogen or boron) doped graphene support is an efficient, easy and cost effective avenue towards meeting the US department of energy (DOE) targets for gravimetric hydrogen storage capacity at room temperature and moderate pressures.     

Comparison of theoretical methods of the hydrogen storage capacities of nanoporous carbons

The hydrogen storage capacities of nanoporous carbons, simulated as graphene slit-shaped pores, have been calculated using simple theoretical methods that do not involve computationally expensive calculations. The theoretical methods calculate the storage of hydrogen molecules on a solid porous material by using the Equation Of State, EOS, of the hydrogen gas and the interaction potential energy of H₂ with the surfaces of the pores of the material. Calculations have been carried out using the same interaction potential energy and empirical EOS. The interaction potential energy is obtained from calculations of H₂ on graphene, using a DFT-based method that includes the dispersion interactions. The storage capacities have been calculated as a function of pressure in the range 0.1–25 MPa, of pore width in the range 4.7–20 Å and at 80.15 and 298.15 K. The storage capacities obtained with the methods are compared and the advantages and limitations of the methods are discussed, as well as the storage capacities predicted by the methods for wide pores. These simple theoretical methods are useful to design novel materials for hydrogen storage.     

Improved hydrogen storage properties of MgH2 by the addition of KOH and graphene

Mg hydride is a competitive candidates for hydrogen storage based on its high gravimetric hydrogen capacity and accessibility. In this study, a small amount of KOH and graphene were added into MgH₂ by high energy ball milling. MgH₂ doped with both KOH and graphene has a greatly improved hydrogen storage performance. The existence of graphene and the in-situ formed KMgH₃ and MgO decreased activation energy of MgH₂ to 109.89 ± 6.03 kJ/mol. The both KOH and graphene doped sample has a reversibly capacity of 5.43 wt % H₂ and can released H₂ as much as 6.36 times and 1.84 times faster than those of undoped sample and only KOH doped sample at 300 °C, respectively. The addition of graphene not only can provide more “H diffusion channels”, but also can disperse the catalyst.     

Ti decorated heterocyclic rings for hydrogen storage

This study uses first-principles calculations to investigate and compare the hydrogen storage properties of Ti doped benzene (C₆H₆Ti) and Ti doped borazine (B₃N₃H₆Ti) complexes. C₆H₆Ti and B₃N₃H₆Ti complex each can adsorb four H₂ molecules, but the former has a 0.11 wt% higher H₂ uptake capacity than the latter. Ti atoms bind to C₆H₆ more strongly than B₃N₃H₆. The hydrogen adsorption energies with Gibbs free energy correction for C₆H₆Ti and B₃N₃H₆Ti complexes are 0.17 and 0.45 eV, respectively, indicating reversible hydrogen adsorption. The hydrogen adsorption properties of C₆H₆Ti have also been studied after boron (B) and nitrogen (N) atom substitutions. Several B and N substituted structures between C₆H₆Ti and B₃N₃H₆Ti with different boron and nitrogen concentration and at different positions were considered. Initially, one boron and one nitrogen atom is substituted for two carbon atoms of benzene at three different positions and three different structures are obtained. Seven structures are possible when four carbon atoms of benzene are replaced by two boron and two nitrogen atoms at different positions. The hydrogen storage capacity of the C₆H₆Ti complex increases as boron and nitrogen atom concentrations increases. The positions of substituted boron and nitrogen atoms have less impact on H₂ uptake capacity for the same B and N concentration. The position and concentration of B and N affects the H₂ adsorption energy as well as the temperature and pressure range for thermodynamically favorable H₂ adsorption. The H₂ desorption temperature for all the complexes is found to be higher than 250 K indicates the stronger binding of H₂ molecules with these complexes.     

Fabrication of Ni-MOF74 derived Ni-carbon material for the highly efficient H2 gas adsorption at mild operating condition

This study fabricates nickel nanoparticle-carbon hybrid (Ni-MOF74) composite via solvothermal technique, and carbonized at different temperature under inert atmosphere, which was then used for the hydrogen storage. Morphology, crystallinity, electronic distribution and other surface properties of the synthesized material were extensively checked via different analytical techniques. Results showed that the Ni-MOF74 sample carbonized at 500 °C displays an improved hydrogen storage capacity of～400% compared to the parent Ni-MOF74 material (0.088 vs 0.002 wt %), at 298 K and 1 atm. We believe that these Ni-carbon hybrid materials hold promising results for hydrogen storage as they show potential for storage at room temperature, and it is believed to be cost effective compared to traditionally used hydrogen spillover catalysts.     

Preheated self-aligned graphene oxide for enhanced room temperature hydrogen storage

This study explored the hydrogen adsorption capacity of self-assembled aligned graphene oxide at room temperature. The characteristics of as-prepared graphene oxide were determined by scanning electron microscopy, Raman spectroscopy, and X-ray diffractometry techniques. Three different temperatures were taken for preheating, i.e., 25, 250, and 400 °C. The maximum adsorption pressure was given to 20 bar, and we evaluated the hydrogen adsorption competency at room temperature (25 ± 2 °C). The maximum hydrogen storage capacity was achieved ~2.5 wt%, which was found for the graphene oxide sample preheated at 400 °C. This hydrogen storage capacity was 67% and 40% more than the graphene oxide samples preheated at 25 and 250 °C, respectively. Such an enhancement of hydrogen storage capacity in the self-aligned graphene oxide samples at room temperature is attributed to reduced interlayer spacing and increased topological defects in preheated graphene oxide samples at 400 °C.     

Role of Mn-substitution towards the enhanced hydrogen storage performance in FeTi

The role of Mn substitution in FeTi towards the hydrogenation kinetics and hydrogen storage capacity was investigated using a combination of experimental and theoretical tools. Pristine and Mn-substituted FeTi was produced by electro-deoxidation of oxide precursors, such as natural ilmenite, titania and manganese dioxide. The produced materials were evaluated for hydrogen storage capacity. Ab-initio density functional theory (DFT) calculations were performed to understand the thermodynamics and kinetics of hydrogen absorption in pristine and doped FeTi. DFT calculations demonstrate that although thermodynamic and kinetic parameters of hydrogen absorption with Mn substitution is similar to those in the pristine FeTi, oxygen affinity of Mn at the surface is higher than Fe or Ti. We conclude that Mn acts as a sacrificial oxidizing element and oxidizes more readily at the surface over Fe or Ti, resulting in easy activation of the FeTi alloy. We show superior cyclic-hydrogen absorption behavior in bulk in FeTi with Mn substitution. After 20 charge-discharge cycles, the measured hydrogen storage capacity of FeTi with Mn substitution was steady ～120 mA h/g (0.8 wt %), which is noticeably higher than that of pristine FeTi. The experimental and theoretical results shows that in case of a practical hydrogen storage scenario, Mn substitution will benefit in reducing absorption fatigue in FeTi. Further, most likely it may not be possible to use pristine FeTi phase.     

The performance of green carbon as a backbone for hydrogen storage materials

We investigate the use of carbonized bamboo, which has an organic porous structure, as a hydrogen storage material. Bamboo samples were thermally treated at 800, 900, 1000, and 1100 °C for 24 h. The pore size and hydrogen storage capacity of each sample were measured by N₂ and H₂ gas sorption up to 1.13 bar at 77 K. The maximum hydrogen storage was exhibited by the sample treated at 900 °C, which reached 1.35 wt% at 1.13 bar/77 K. The results showed that the bamboo, one of the green carbons, has the potential to be used as an environmental-friendly carbon backbone for hybrid hydrogen storage materials.     

Cerium hydride generated during ball milling and enhanced by graphene for tailoring hydrogen sorption properties of sodium alanate

In this study, NaAlH₄?based hydrogen storage materials with dopants were prepared by a two-steps in-situ ball milling method. The dopants adopted included Ce, few layer graphene (FLG), Ce + FLG, and CeH_2.51. The hydrogen storage materials were studied by non-isothermal and isothermal hydrogen desorption measurements, X-ray diffractions analysis, cycling sorption tests, and morphology analysis. The hydrogen storage performance of the as-prepared NaAlH₄ with Ce addition is much better than that with CeH_2.51 addition. This is due to that the impact of Ce occurs from the body to the surface of the materials. The addition of FLG further enhances the impact of Ce on the hydrogen storage performance of the materials. The hydrogen storage capacity, hydrogen sorption kinetics, and cycle performance of NaAlH₄ with Ce + FLG additions are all better than NaAlH₄ materials with the addition of either Ce or FLG alone. The NaAlH₄ with Ce and FLG addition starts to release hydrogen at 85 °C and achieves a capacity of 5.06 wt% after heated to 200 °C. The capacity maintains at 4.91 wt% (94.7% of the theoretical value) for up to 8 cycles. At 110 °C, the material can release isothermally a hydrogen capacity of 2.8 wt% within 2 h. The activation energies for the two hydrogen desorption steps of NaAlH₄ with Ce and FLG addition are estimated to be 106.99 and 125.91 kJ mol^?1 H₂, respectively. The related mechanisms were studied with first-principle and experimental methods.     

Boron‐ and nitrogen‐doped penta‐graphene as a promising material for hydrogen storage: A computational study

We fulfill a comprehensive study based on density functional theory (DFT) computations to cast insight into the dissociation mechanism of hydrogen molecule on pristine, B‐, and N‐doped penta‐graphene. The doping effect has been also illustrated by varying the concentration of dopant from 4.2 at% (one doping atom in 24 host atoms) to 8.3 at% (two doping atoms in 24 host atoms) and by contemplating different doping sites. Our theoretical investigation shows that the adsorption energy of H₂ molecule and H atom on the substrate can be substantially enhanced by incorporating boron or nitrogen into penta‐graphene sheet. The B‐ and N‐doped penta‐graphene can effectively decompose H₂ molecule into two H atoms. Our results demonstrate that activation energies for H₂ dissociation and H diffusion on the B‐ and N‐doped penta‐graphene are much smaller than the pristine penta‐graphene. Further investigation of increasing concentration dopants of the penta‐graphene sheet gives sufficiently low activation barrier for H₂ dissociation process. This investigation reveals that the boron and nitrogen dopants can act as effective active site for H₂ dissociation and storage.     

Study on the hydrogen storage properties of the dual active metals Ni and Al doped graphene composites

The graphene nanosheets are synthesized by modified Hummer's method, based on which the dual active metals Ni and Al doped graphene composites are prepared through in-suit reaction and self-assembly with high-temperature reduction process. The molecular structure, morphology and specific surface area of graphene nanosheets are characterized systematically. The phase composition, surface morphology and hydrogen storage properties of dual active metals Ni and Al doped graphene composites are further investigated by X-ray diffraction, scanning electron microscopy and gas reaction controller. Results show that the graphene nanosheets have typical graphene feature, whose transparent graphene edges can be observed clearly, and the specific surface area is as high as 604.2 m² g⁻¹. The Ni and Al doped graphene composites are composed with Ni, Al and C phases, which have high hydrogen storage capacity and excellent hydriding/dehydriding stabilities. The maximum hydrogen storage uptake of such composites is up to 5.7 wt% at 473 K, and the dehydriding efficiency is high as 96%∼97% at the dehydriding temperature of 380 K. The hydrogen adsorption and desorption rate control step of the Ni and Al doped graphene composites is complied to the nucleation and two-dimensional growth mechanism.     

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.     

Analysis of hydrogen storage mechanism in bilayer double-vacancy defective graphene modified using transition metals: Insights from Ti-BDVG(Ti)-Ti

In this study, using the first principles calculation and analysis, we found that the B-doping in double-vacancy defective graphene could effectively increase the binding energy of Ti atoms in each adsorption site, especially in the H2 adsorption site with a maximum binding energy of 8.3 eV. However, N-doped bilayer graphene (N-BLG) reduced the binding energy of Ti atoms by 88% of the adsorption sites. Given these two findings, a B- and N-doped bilayer double-vacancy-defective graphene (Ti-BDVG(Ti)-Ti) was constructed. Our findings also showed that the Ti-BDVG(Ti)-Ti outer surface and inner surface could adsorb 32 and 12H₂ molecules, respectively, of which 22, 20 and 2H₂ molecules are adsorbed by Kubas, electrostatic interactions and chemisorption, respectively. The hydrogen storage mechanism of Ti-BDVG(Ti)-Ti involves multiple adsorption modes, and this hydrogen storage mechanism provides a theoretical basis for the rational design of hydrogen storage materials with maximum effective hydrogen storage capacity.