Selective decoration of nitrogenated holey graphene (C2N) with titanium clusters for enhanced hydrogen storage application

For an envisioned hydrogen (H₂) economy, the design of new multifunctional two-dimensional (2D) materials have been a subject of intense research for the last several decades. Here, we report the thriving H₂ storage capacity of 2D nitrogenated holey graphene (C₂N), functionalized with Ti_n (n = 1–5) clusters. By using spin polarized density functional theory (DFT) calculations implemented with the van der Waals corrections, the most favourable adsorption site for the Ti_n clusters on C₂N has been revealed. With the monomer Ti, the functionalization was evenly covered on C₂N having 5% doping concentration (C₂N–Ti). For C₂N–Ti sheet, Ti binds to C₂N with a strong binding energy of ~6 eV per Ti which is robust enough to hinder any Ti–Ti clustering. Bader charge analysis reveals that the Ti_n clusters donate significant charges to C₂N sheet and become cationic to polarize the H₂ molecules, thus act as efficient anchoring agents to adhere multiple H₂ molecules. Each Ti in C₂N–Ti could adsorb a maximum of 10H₂ molecules, with the adsorption energies in the range of ?0.2 to ?0.4 eV per H₂ molecule, resulting into a high H₂ storage capacity of 6.8 wt%, which is promising for practical H₂ storage applications at room temperature. Furthermore, Ti_m (m = 2, 3, 4, 5) clusters have been selectively decorated over C₂N. However, with Ti_m functionalization H₂ storage capacities fall short of the desirable range due to large molecular weights of the systems. In addition, the H₂ desorption mechanism at different conditions of pressure and temperature were also studied by means of thermodynamic analysis that further reinforce the potential of C₂N–Ti as an efficient H₂ storage material.     

Novel sandwich-type dimetallocenes: Toward promising candidate media for high-capacity hydrogen storage

With respect to first-principles calculations, the sandwich-type dinuclear organometallic compounds as (C₅H₅)₂TM₂ (M = Sc and Ti) can adsorb up to eight hydrogen molecules. The corresponding gravimetric hydrogen-storage capacity is 6.7 wt% for (C₅H₅)₂Ti₂ and 6.8 wt% for (C₅H₅)₂Sc₂. The multimetallocenes (e.g., CpTi₃Cp and CpTi₄Cp) complexes can further increase the H₂ adsorption capacity to 8.7 wt% and 10.4 wt%, respectively. These sandwich-type organometallocenes proposed in this work are favorable for reversible adsorption and desorption of hydrogen under ambient conditions. These predictions will likely provide a new route for developing novel high-capacity hydrogen-storage materials.     

C2H2M (M = Ti,Li) complex: A possible hydrogen storage material

The hydrogen storage capacity of Ti-acetylene (C₂H₂Ti) and Li-acetylene (C₂H₂Li) complex has been tested using second order Møller Plesset method with different basis sets. Single Ti(Li) decorated acetylene complex can adsorb maximum of five(four) hydrogen molecules, which corresponds to the gravimetric hydrogen storage capacity of 12(19.65) wt % and it meets the target of 9 wt % by 2015 specified by US Department of Energy. The hydrogen adsorption energies with zero point energy and Gibbs free energy correction show that hydrogen adsorption on C₂H₂Ti is energetically favourable for a wide range of temperature and that is unfavourable on C₂H₂Li complex even at a very low temperature. Atom centered density matrix propagation molecular dynamics simulations reveal that four H₂ molecules remain adsorbed on C₂H₂Ti complex at 300 K. Though H₂ uptake capacity of C₂H₂Li complex is higher than that of C₂H₂Ti complex, the thermochemistry results favour to C₂H₂Ti complex over C₂H₂Li complex as a possible hydrogen storage media.     

Effect of boron substitution on hydrogen storage capacity of Li and Ti decorated naphthalene

Interaction of molecular hydrogen with Li and Ti doped boron substituted naphthalene viz. C₆B₄H₈Ti₂ and C₆B₄H₈Li₂ has been studied using density functional theory (DFT) method. The C₆B₄H₈Li₂ complex can interact with maximum of four hydrogen molecules, whereas three H₂ molecules are adsorbed on C₁₀H₈Li₂ complex. The C₆B₄H₈Ti₂ complex can interact with maximum of eight hydrogen molecules. The gravimetric hydrogen uptake capacity of C₆B₄H₈Ti₂ and C₆B₄H₈Li₂ complex is found to be 6.85 and 5.55 wt % respectively, which is higher than that of unsubstituted C₁₀H₈Ti₂ and C₁₀H₈Li₂ complexes. The boron substitution has significantly affected the hydrogen adsorption energies. The H₂ adsorption energy and Gibb's free energy corrected H₂ adsorption energy are found to be more prominent after boron substitution. The C₆B₄H₈Ti₂ and C₆B₄H₈Li₂ complexes are more stable than the respective unsubstituted C₁₀H₈Ti₂ and C₁₀H₈Li₂ complexes due to their higher binding energies. According to the atom-centered density matrix propagation (ADMP) molecular dynamics simulations C₆B₄H₈Li₂ complex retain not a single adsorbed hydrogen molecule during the simulation at room temperature, whereas five hydrogen molecules at 300 K and eight at 100 K are remain absorbed on C₆B₄H₈Ti₂ complex. The C₆B₄H₈Ti₂ complex is found to be more promising material for hydrogen storage than C₁₀B₄H₈Li₂.     

The effect of functional groups (O,F, or OH) on reversible hydrogen storage properties of Ti2X (X=C or N) monolayer

The effect of functional groups (O, F, or OH) on the hydrogen storage properties of Ti₂X (X = C or N) monolayer was systematically investigated by first-principles calculations. The results show that the reversible hydrogen storage capacity of Ti₂X(OH)₂ monolayer is approximately 2.7 wt%, which is larger than that of Ti₂XO₂ and Ti₂XF₂ monolayers. The binding energy of the OH group at the F site is stronger than H atom. Thus, H₂ molecules will not be dissociated on Ti₂X(OH)₂ monolayer. At this time, the loss of 1.8 wt% hydrogen storage capacity is not produced in Ti₂X(OH)₂ monolayer. Furthermore, the PDOS, the population analysis, and the electron density difference explore that electron transfer appears between Ti and the second layer H₂ molecules on Ti₂X(OH)₂ monolayer, and a Dewar-Kubas interaction lies between second layer H₂ molecules and Ti₂X(OH)₂ monolayer. For Ti₂X(OH)₂ monolayer, the molecular dynamic simulation indicates that the H₂ molecules by Dewar-Kubas interaction sable adsorption at 300 K, and desorption at 400 K. Therefore, Ti₂X(OH)₂ is appropriate for reversible hydrogen sorbent storage materials under ambient conditions.     

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.     

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.     

Functionalized tetrahedral silsesquioxane cages for hydrogen storage

The hydrogen storage capacity of functionalized Tetrahedral Silsesquioxane (H₄Si₄O₆) cages is obtained using density functional theory (M062X) and second order Møller-Plesset (MP2) method with 6-311++G7 basis set. We labelled Tetrahedral Silsesquioxane (H₄Si₄O₆) as ‘TS’. We replaced four hydrogen in TS one by one with C₂HBe or C₂HTi group and labelled as TSR₁M_1, TSR₂M₂ TSR₃M₃ and TSR₄M₄ where RM can be either C₂HBe or C₂HTi. In TSRM when one hydrogen in a cage is replaced by C₂HBe or C₂HTi maximum of two and five hydrogen molecules, get adsorbed per Be and Ti atom respectively with respective H₂ capacity of 1.61 and 3.42 wt %. H₂ uptake capacity of TSR_mM_m (m = 1, 2, 3 and 4) has increased extensively when all the hydrogen in cage are replaced either C₂HBe or C₂HTi. TSR₄M₄ with RM = C₂HTi can adsorbs maximum of 20H₂ molecules with highest H₂ uptake of 7.46 wt % among all the studied complexes. Calculated Gibbs free energy corrected H₂ adsorption energies show that adsorption of H₂ molecules on all the complexes is thermodynamically favourable. The desorption temperature for the complexes were calculated by using the van't Hoff equation. Calculated interaction energies show that H₂ molecules interact strongly with Be atom than Ti atom. The molecular dynamics (MD) simulations have also been performed using atom centered density matrix propagation (ADMP) at ambient conditions. Interaction of hydrogen molecules and the metal atom is confirmed through the density of states (DOS) plot.     

Enhanced low temperature hydrogen desorption properties and mechanism of Mg(BH4)2 composited with 2D MXene

Mg(BH₄)₂ occupies a large hydrogen storage capacity of 14.7 wt%, and has been widely recognized to be one of the potential candidates for hydrogen storage. In this work, 2D MXene Ti₃C₂ was introduced into Mg(BH₄)₂ by a facile ball-milling method in order to improve its dehydrogenation properties. After milling with Ti₃C₂, Mg(BH₄)₂–Ti₃C₂ composites exhibit a novel “layered cake” structure. Mg(BH₄)₂ with greatly reduced particle sizes are found to disperse uniformly on Ti₃C₂ layered structure. The initial dehydrogenation temperature of Mg(BH₄)₂ has been decreased to 124.6 °C with Ti₃C₂ additive and the hydrogen liberation process can be fully accomplished below 400 °C. Besides, more than 10.8 wt% H₂ is able to be liberated from Mg(BH₄)₂–40Ti₃C₂ composite at 330 °C within 15 min, while pristine Mg(BH₄)₂ merely releases 5.3 wt% hydrogen. Moreover, the improved dehydrogenation kinetics can be retained during the subsequent second and third cycles. Detailed investigations reveal that not only Ti₃C₂ keeps Mg(BH₄)₂ particles from aggregation during de/rehydrogenation, but also the metallic Ti formed in-situ serves as the active sites to catalyze the decomposition of Mg(BH₄)₂ by destabilizing the B–H covalent bonds. This synergistic effect of size reduction and catalysis actually contributes to the greatly advanced hydrogen storage characteristics of Mg(BH₄)₂.     

Sc/Ti decorated novel C24N24 cage: Promising hydrogen storage materials

Inspired by the TM?N₄ coordination environment and the promising hydrogen adsorption property of the Ti–C₄ unit, four novel TM₆C₂₄N₂₄ (TM = Sc, Ti) cages with six TM?N₄/TM?C₄ units are designed simultaneously for the first time in this paper. Their stabilities are studied by using DFT calculations and confirmed by MD simulations. Sc₆C₂₄N₂₄(N₄) and Ti₆C₂₄N₂₄(N₄) can absorb 30 hydrogen molecules and keep the original structures intact. The average hydrogen adsorption energy is 0.13–0.26 eV for Sc₆C₂₄N₂₄(N₄)-6nH₂ (n = 1–5), and 0.09–0.23 eV for Ti₆C₂₄N₂₄(N₄)-6nH₂ (n = 1–5). The hydrogen storage capacities are 6.30 wt % and 6.20 wt %, respectively. Besides, the 30H₂ can be readily adsorbed on Sc₆C₂₄N₂₄(N₄) under 160 K and 100% desorbed at 270 K under 0.1 MPa. The desorption temperature could increase if the pressure becomes higher. Sc₆C₂₄N₂₄(C₄)/Ti₆C₂₄N₂₄(C₄) complexes are higher in energy than corresponding Sc₆C₂₄N₂₄(N₄)/Ti₆C₂₄N₂₄(N₄). They are not suitable as room-temperature hydrogen storage materials due to the structural deformation in the hydrogen storage process.     

Vibrational spectra of Ti:C2H4(nH2) and Ti:C2H4(nD2) (n = 1-5) complexes and the equilibrium isotope effect: Calculations and experiment

Calculations of the ability of titanium-ethylene complexes of the type, Ti:C₂H₄, to absorb molecular hydrogen have been performed using density functional theory. A maximum of 5H₂ molecules can be adsorbed on Ti:C₂H₄ thereby giving an uptake capacity of 11.72 wt%, in excellent agreement with previous experimental results reported by two of us (Phys. Rev. Lett., 100, 105505, 2008). Calculations of the vibrational frequencies in such complexes with both H₂ and D₂, Ti:C₂H₄(nH₂) and Ti:C₂H₄(nD₂), n = 1-5, have also been performed and the values obtained used to find the Equilibrium Isotope Effect (EIE). Measurements of the EIE are also reported and these are in excellent agreement with the EIE calculated for 5H₂ molecules adsorbed in the complex.     

Li decorated heteroborospherene C4B32 as high capacity and reversible hydrogen storage media: A DFT study

In this work, adsorption of H₂ molecules on heteroborospherene C_2v C₄B₃₂ decorated by alkali atoms (Li) is studied by density functional theory calculations. The interaction between Li atoms and C₄B₃₂ is found to be strong, so that it prevents agglomeration of the former. An introduced hydrogen molecule tilts toward the Li atoms and is stably adsorbed on C₄B₃₂. It is obtained that Li₄C₄B₃₂ can store up to 12H₂ molecules with hydrogen uptake capacity of 5.425 wt% and average adsorption energy of ?0.240 eV per H₂. Dynamics simulation results show that 6H₂ molecules can be successfully released at 300 K. Obtained results demonstrate that Li decorated C₄B₃₂ is a promising material for reversible hydrogen storage.     

Improved hydrogen storage properties of Ti2CrV alloy by Mo substitutional doping

Controllable hydrogen release is of great importance to the practical application of hydrogen storage materials. Ti₂CrV alloy possesses the maximum hydrogen absorption capacity in the Ti–Cr–V series alloys, however, can hardly meet the reversible storage capacity of practical applications due to its stable dihydride. Here we report an advancement in hydrogen storage property of the Ti₂CrV alloy by Mo partial substitution for Ti. Although the hydrogen absorption kinetics slightly decreased with the increase of Mo content, the Mo substitution alloy achieves an effective hydrogen capacity of 2.23 wt% cutting-off at 0.1 MPa, much higher than Ti₂CrV alloy (0.8 wt%). It is ascribed that Mo partial substitution for Ti significantly decreased the dihydride stability as well as the enthalpy change value. The cyclic property of Ti₂CrV alloy drastically decreased, while Mo substitution alloy with smaller FWHM value can maintain 90% storage capacity after 20 cycles. Because lattice strain and distortion of the Ti₂CrV alloy were decreased by Mo doping.     

Heterofullerene C48B12-impregnated MOF-5 and IRMOF-10 for hydrogen storage: A combined DFT and GCMC simulations study

The H₂ storage properties of isoreticular metal-organic framework materials (IRMOFs), MOF-5 and IRMOF-10, impregnated with different numbers and types of heterogeneous C₄₈B₁₂ molecules were investigated using density functional theory and grand canonical Monte Carlo (GCMC) calculations. The excess hydrogen adsorption isotherms of IRMOFs at 77 K within 20 bar indicate that suitable number and type of C₄₈B₁₂ molecules play a crucial role in improving the H₂ storage properties of IRMOFs. Among the studied pure and nC₄₈B₁₂ (n = 1, 2, 4, 8) in Ci symmetry impregnating into MOF-5, at 77 K under 6 bar, MOF-5-4C₄₈B₁₂ with a 3.5 wt% and 29.9 g/L hydrogen storage density, and at 77 K under 12 bar, the pure MOF-5 with a 4.9 wt% and 31.0 g/L hydrogen storage density has the best hydrogen storage properties. Whereas, among the studied pure and nC₄₈B₁₂ (n = 1, 2, 4, 8) in S6 symmetry impregnating into IRMOF-10, IRMOF-10-8C₄₈B₁₂ always shows the best hydrogen storage properties among the pure and C₄₈B₁₂-impregnated IRMOF-10 at 77 K within 20 bar. IRMOF-10-8C₄₈B₁₂ has a 6.0 wt% and 34.6 g/L hydrogen storage density at 77 K under 6 bar, and has a 7.1 wt% and 41.4 g/L hydrogen storage density at 77 K under 12 bar. The confinement effect of IRMOFs on C₄₈B₁₂ molecules, and steric hindrance effect of C₄₈B₁₂ molecules on IRMOFs mainly affects the H₂ uptake capacity by comparing the absolute H₂ molecules in individual IRMOFs units, C₄₈B₁₂ molecules, and IRMOFs-nC₄₈B₁₂ compounds. The absolute hydrogen adsorption profiles show that eight C₄₈B₁₂ molecules impregnating into MOF-5 can exert obvious steric effects for H₂ adsorption. The saturated gravimetric and volumetric H₂ densities of IRMOF-10-8C₄₈B₁₂ higher than those of MOF-5-8C₄₈B₁₂ due to with larger free volume.     

Hydrogen capability of bimetallic boron cycles: A DFT and ab initio MD study

Bimetallic boron cycle, B₆C₂TM₂ (TM = scandium, titanium), was recently predicted to have high stability and aromaticity. The hydrogen capabilities of these clusters were studied in the present work. Our computational results indicate that the gravimetric hydrogen uptake capacity of B₆C₂Sc₂ and B₆C₂Ti₂ and clusters are 11.7% and 11.4%, respectively. The adsorption energies of H₂ molecules on B₆C₂Sc₂ and B₆C₂Ti₂ clusters are predicted with different calculational schemes to meet the criteria of reversible hydrogen storage. The interaction of H₂ with B₆C₂Ti₂ cluster is a little stronger than that with B₆C₂Sc₂. Ab initio molecular dynamics simulations indicate that H₂ molecules can be efficiently released from the metal sites of B₆C₂TM₂ clusters at room temperature. The bulk-like B₆C₂Sc₂ and B₆C₂Ti₂ tetramer can also efficiently adsorb H₂ molecules.     

C7N6 monolayer as high capacity and reversible hydrogen storage media: A DFT study

Searching advanced materials with high capacity and efficient reversibility for hydrogen storage is a key issue for the development of hydrogen energy. In this work, we studied systematically the hydrogen storage properties of the pure C₇N₆ monolayer using density functional theory methods. Our results demonstrate that H₂ molecules are spontaneously adsorbed on the C₇N₆ monolayer with the average adsorption energy in the range of 0.187–0.202 eV. The interactions between H₂ molecules and C₇N₆ monolayer are of electrostatic nature. The gravimetric and volumetric hydrogen storage capacities of the C₇N₆ monolayer are found to be 11.1 wt% and 169 g/L, respectively. High hardness and low electrophilicity provides the stabilities of H₂–C₇N₆ systems. The hydrogenation/dehydrogenation (desorption) temperature is predicted to be 239 K. The desorption temperatures and desorption capacity of H₂ under practical conditions further reveal that the C₇N₆ monolayer could operate as reversible hydrogen storage media. Our results thus indicate that the C₇N₆ monolayer is a promising material with efficient, reversible, and high capacity for H₂ storage under realistic conditions.     

Sc/Ti-decorated and B-substituted defective C60 as efficient materials for hydrogen storage

Doping heteroatoms and producing defects are perfect methods to improve the hydrogen storage property of TM-decorated carbon materials. In this view, four novel Sc/Ti-decorated and B- substituted defective C₆₀ fullerenes (B₂₄C₂₄) are explored. The special stability, large specific surface, uniform distribution of the metal and positively charged states make these four fullerenes have high hydrogen storage capacities. Especially, each Sc atom in Sc₆B₂₄C₂₄(B₄) can adsorb up to five H₂ molecules with a storage capacity of 6.80 wt %. The adsorbed H₂ molecules in Sc₆B₂₄C₂₄(B₄)–30H₂ begin to relax at 190 K and are 100% released at 290 K. Moreover, a comparative study is carried out for hydrogen storage properties of Sc-decorated B₄, C₄, or N₄ coordination environments. These results provide a new focus on the nature of B-, and N-substituted defective carbon nanomaterials.     

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.     

VC3H3 organometallic compound: A possible hydrogen storage material

This work reports the hydrogen uptake capacity of V-capped and V-inserted VC₃H₃ organometallic complexes using density functional theory (DFT) with different exchange and correlation functionals. Maximum of five and three H₂ molecules are adsorbed on V-capped and V-inserted VC₃H₃ structures, respectively. This corresponds to the hydrogen uptake capacity of 10.07 and 6.66 wt% for the former and the latter, respectively. The first added hydrogen molecule is adsorbed in dihydride form on V-capped as well as V-inserted VC₃H₃ complex. A complex with a dissociated hydrogen molecule adsorbed has higher binding energy than that of molecular hydrogen adsorbed. The nature of interactions between H₂ molecules and organometallic complex is studied using many-body analysis approach. Thermo-chemistry calculations are performed to see whether H₂ adsorption on V-capped complex is energetically favorable or not for room temperature hydrogen storage.