Investigation of the effect of oxygen-containing groups on the hydrogen adsorption behavior of CSCNTs using density functional theory

The hydrogen adsorption behavior of cup-stacked carbon nanotubes (CSCNTs) decorated by –CO, –OH and –COOH at the edge of conical graphene layer (CGL) is investigated using density functional theory (DFT). The results reveal that the edge of pristine CGL hardly adsorbs hydrogen molecule due to the positive potential. When oxygen-containing group is decorated, the negative potential of O atom adsorbs hydrogen molecule strongly and the adsorption energy increases from 5.19 to 5.58, 6.25 and 6.53 kJ/mol following the order of –COOH > –OH > –CO. When two –COOH are decorated next to each other, the equilibrium position of hydrogen molecule deviates from the extended surface of CGL in an angle of 38°. When they are decorated next but one, the deviation angle of hydrogen molecule is reduced to 12°, so that the adsorption surface is expanded and the steric hindrance is avoided.     

Calcium-decorated graphdiyne as a high hydrogen storage medium: Evaluation of the structural and electronic properties

Graphdiyne (GDY) is a new member of carbon allotropes consisting of sp and sp² hybridized carbon atoms. In this work, the hydrogen adsorption on Calcium (Ca) decorated GDY and the influence of adatom on structural properties of GDY are investigated, using first principles plane wave calculations with Van der Waals corrections. The results show that similar to graphyne (GY) and unlike carbon nanotube (CNT), fullerene and graphene, clustering of Ca on GDY hinders due to the higher binding energy of the adatom to the carbon frame than the Ca cohesive energy. It can be seen that the Ca-decoration promotes hydrogen storage capacity of GDY, extremely (E_ads = ?0.266 and ?0.066 eV for Ca-decorated and pristine GDY, respectively). It is concluded that, the best site for the Ca trapping is 18-membered ring in which, Ca lies in-plane of GDY (E_ads = ?3.171 eV). Fourteen H₂ molecules (with the average adsorption energy of ～0.2 eV/H₂) can be adsorbed on the Ca atom from one side. The hydrogen storage capacity is estimated to be as high as 17.95 wt% for the both sides of GDY. So, the Ca-decorated GDY is offered as a promising candidate for hydrogen storage applications.     

Hydrogen storage performance of platinum supported carbon nanohorns: A DFT study of reaction mechanisms,thermodynamics, and kinetics

Platinum (Pt) is one of a robust hydrogen dissociative catalyst. However, the migration of dissociated hydrogens from Pt nanoparticles to carbon supports such as graphene and carbon nanotube are energetically unfavorable reactions. To enhance the hydrogen storage via migration mechanism, carbon nanohorn is applied as a support for Pt nanoparticles (Pt and Pt₄). The H₂ storage performance of Pt and Pt₄ supported on the mono-vacancy carbon nanohorn (vNH) has been investigated by using density functional theory calculations. The Pt and Pt₄ firmly deposit at the vacancy site through the three strong Pt–C bonds with binding energies about ?7.0 eV, which can prevent the metal desorption and migration. The mechanism of H₂ storage starts with H₂ adsorption followed by H₂ spillover reaction. The calculation results reveal that the supported Pt nanoparticles are the active sites for H₂ dissociative adsorption while the high curvature surface of carbon nanohorn is the active area for accommodating the migrated H atoms from the spillover reaction. Remarkably, the hydrogen spillover reactions over Pt– and Pt₄-supported on vNHs in this study are spontaneous at room temperature with highly exothermic reaction energy. The fundamental understanding obtained from this study is beneficial for further design and synthesis of high-performance materials for H₂ storage applications.     

Revealing the interfacial phenomenon of metal-hydride formation and spillover effect on C48H16 sheet by platinum metal catalyst (Pt (n=1-4)) for hydrogen storage enhancement

Hydrogen is a worldwide green energy carrier, however due its low storage capacity, it has yet to be widely used as an energy carrier. Therefore, the quantum chemical method is being employed in this investigation for better understand the hydrogen storage behaviour on Pt _(n = 1-4) cluster decorated C₄₈H₁₆ sheet. The Pt_(n = 1-4) clusters are strongly bonded on the surface of C₄₈H₁₆ sheet with binding energies of ?3.06, ?4.56, ?3.37, and ?4.03 eV respectively, while the charge transfer from Pt_(n = 1-4) to C₄₈H₁₆ leaves an empty orbital in Pt atom, which will be crucial for H₂ adsorption. Initially, the molecular hydrogen is adsorbed on Pt_(n = 1-4) decorated C₄₈H₁₆ sheet through the Kubas interaction with adsorption energies of ?0.85, ?0.66, ?0.72, and ?0.57 eV respectively, while H–H bond is elongated due to the transfer of electron from σ (HH) orbital to unfilled d orbital of the Pt atom, resulting in a Kubas metal-dihydrogen complexes. Furthermore, the dissociative hydrogen atoms adsorbed on Pt_(n = 1-4) decorated C₄₈H₁₆ sheet have adsorption energies of ?1.14 eV, ?1.02 eV, ?0.95 eV, and ?1.08 eV, which are greater than the molecular hydrogen adsorption on Pt_(n = 1-4) cluster supported C₄₈H₁₆ sheet with lower activation energy of 0.007, 0.109, 0.046, and 0.081 eV respectively. To enhance the dissociative hydrogen adsorption energy, positive and negative external electric fields are applied in the charge transfer direction. Increasing the positive electric field makes H–H bond elongation and good adsorption, whereas increasing the negative electric field results H–H bond contraction and poor adsorption. Thus, by applying a sufficient electric field, the H₂ adsorption and desorption processes are can be easily tailored.     

Li-decorated double vacancy graphene for hydrogen storage application: A first principles study

Lithium decoration is an effective strategy for improving the hydrogen adsorption binding energy and the storage capacity in carbon nanostructures. Here, it is shown that Li-decorated double carbon vacancy graphene (DVG) can be used as an efficient hydrogen storage medium by means of Density Functional Theory (DFT) based calculations. The Li binding energy in DVG is 4.04 eV, which is much higher than that of pristine graphene. A maximum of four hydrogen molecules adsorb on Li decorated on one side of DVG and this leads to a gravimetric storage capacity of 3.89 wt% with an average adsorption binding energy of 0.23 eV/H₂. When Li is decorated on both sides of DVG, the gravimetric storage capacity reaches 7.26 wt% with a binding energy of 0.26 eV/H₂ which shows that desorption would take place at ambient conditions.     

Modification of graphenylene nanostructure with transition metals (Fe,Sc and Ti) to promote hydrogen storage ability: A DFT-D3 study

Hydrogen, as a clean alternative to fossil fuels, has received much attention in recent years. But its utilizing requires to overcome storage problems. Here, we investigated the hydrogen adsorption behavior of graphenylene (GPY), a 2D carbon nanostructure, and Sc, Fe and Ti transition metal (TM) decorated GPY by spin-polarized DFT calculations. For TM-decoration of GPY, seven different sites and various distances from carbon sheet were investigated, carefully. Structural and electronic properties of the structures, adsorption energies, band gap values, and the most stable configurations were considered and discussed. Results showed that 6-membered ring (H2 site) is the best site for Sc, Fe, and Ti-decoration and corresponding E_ads was −3.95, −2.66, and −3.65 eV, respectively. Also, pristine GPY and Sc and Ti-decorated GPY have not magnetic character, unlike Fe-GPY. As well, entrance of Sc, Fe and Ti atoms in H2 site of the GPY structure causes its band gap increases from 0.033 eV to of 0.491, 0.080, and 0.372 eV, respectively. E_ads of the H₂ molecule onto pristine GPY is low (−0.160 eV), and must be improved for practical hydrogen storage applications. Sc, Fe, and Ti-decoration improves it about 2.23, 5.69 and 3.63 times. Because of this improvement, we could store up to 20H₂ molecules on TM-decorated GPY systems. These results indicate that TM-decorated GPY can be a suitable option for H₂ storage applications in the future.     

Ruthenium decorated single walled carbon nanotube for molecular hydrogen storage: A first-principle study

Molecular hydrogen storage on Ruthenium (Ru) decorated single-walled carbon nanotube (SWCNT) has been studied by using spin-polarized density functional theory (DFT). When a Ru atom is adsorbed on SWCNT, the Bader analysis reveals that Ru transfers a charge of 0.44e⁻ to SWCNT. Accordingly, Ru acts as adsorption center for H₂ molecules; thus, it can hold up to four H₂ molecules with an adsorption energy (E_ads) of −0.93 eV/H₂. A uniform addition of Ru atoms on SWCNT shows that this nanomaterial can adsorb up to five Ru without clustering. Each Ru atom of 5Ru-decorated SWCNT system can bind up to four H₂ molecules involving an E_ads of −0.83 eV/H₂. After H₂ molecules adsorption, Ru atoms shifted from a near hollow site to a bridge site. Moreover, Ru-decorated systems reduce their magnetic moment when the number of H₂ molecules increase from 2 μ_B to 0 μ_B.     

Investigation on platinum loaded multi-walled carbon nanotubes for hydrogen storage applications

Molecular hydrogen uptake of modified carbon nanotubes is a prospect for efficient hydrogen storage in fuel cell vehicles. In this study, a simple and efficient method to decorate the surface of multi-walled carbon nanotubes (MWNT) with platinum nanoparticles is presented. To load the Pt nanoparticles, hexachloroplatinic acid (H₂PtCl₆·6H₂O) is used as a precursor. Surface morphology of these Pt loaded MWNT is observed using Scanning and Transmission Electron Microscopy. Both samples are also characterized by X-Ray Diffraction. Thermal Gravimetric Analysis results indicate that both as purchased MWNT and Pt loaded MWNT have decomposition temperature higher than 500 °C in air. N₂ adsorption experiments yields a BET area of the sample close to 500 m²/g. This MWNT/Pt sample was reduced in 10% of H₂ in Ar, flowing at 900 °C in a tubular furnace for 1 h before hydrogen adsorption measurements. Hydrogen uptake of MWNT/Pt was measured at 2.5 MPa and 77 K. This hydrogen uptake isotherm is also compared with measurements at ambient temperature.     

Bonding in PdH2 and Pd2H2 systems adsorbed on carbon nanotubes: Implications for hydrogen storage

This work presents a bonding study of hydrogen adsorption processes on palladium decorated carbon nanotubes by using the density functional theory (DFT). First, we considered simple decoration models involving single palladium atoms or palladium dimers, and then we analyzed the adsorption of several molecular and dissociated hydrogen coordination structures, including Kubas-type complexes. In all cases we computed the energy, bonding and electronic structure for the different nanotube-supported hydrogen–palladium systems. Our results show that Pd(H₂) and Pd₂(H₂) complexes with relaxed but not dissociated H–H bonds are the most stable adsorbed systems. The role of s, p and d orbitals on the bonding mechanism for all adsorbates and substrates was also addressed. We found intermolecular donor–acceptor C–Pd and Pd–H delocalizations after adsorption. We also studied the palladium clustering effect on the hydrogen uptake based on Kubas-type bonding.     

Engineering bimetallic PtX (X=Sn,Cu) immobilized on graphene as efficient catalysts: Boosted charge migration and hydrogen evolution

In this work, bimetallic PtX (X = Sn, Cu) decorated graphene nanohybrids (PtX/G) were developed, which showed enhanced photocatalytic hydrogen evolution performance than that of Pt/G in Eosin Y sensitized H₂ production system. The presence of Sn or Cu in PtX/G nanohybrids can remarkably improve the photogenerated charge separation efficiency and contribute to promoting the reduction of protons to molecular hydrogen in comparison to with noble metal Pt. Meanwhile, graphene acted as a more suitable electronic support to accelerate the migration of electrons from sensitizer to catalysts, owing to its higher electron mobility and larger surface area than other supports (such as carbon sphere, Al₂O₃ and SiO₂). The optimal H₂ evolution rate PtSn/G and PtCu/G was about 2.2 and 2.0 times higher than that of Pt/G. The apparent quantum efficiency (AQE) of PtSn/G and PtCu/G reached up to 12.46% and 11.06% under visible light irradiation (λ ≥ 420 nm), respectively.     

Effect of transition metal (Cu and Pt) doping/ co-doping on hydrogen gas sensing capability of graphene: A DFT study

Carbon nano-materials are found to demonstrate good hydrogen gas sensing capability and researchers are trying their modified derivatives for enhanced sensitivity. Studies have confirmed improvement in sensing performance of graphene when doped with N or Si or Sb. However, effect of the doping of graphene with transition metals of comparable size on its hydrogen sensing properties has not yet been studied. In the present study, we investigated the sensitivity of pristine graphene, Pt-doped graphene; Cu-doped graphene and Pt–Cu co-doped graphene surface towards hydrogen molecule adsorption utilizing density functional theory (DFT) by ab initio method. The adsorption energies for the optimized geometries have been calculated to probe the suitability and effectiveness of the modified graphene structures for Hydrogen sensing. In addition, the electronic properties for instance charge transfer analysis, band gap and density of states have also been taken into consideration. The reactivity of graphene surface for hydrogen adsorption was found to be greatly enhanced with Pt–Cu co-doped graphene surface as demonstrated by the adsorption energies and electronic properties.     

Adsorption of H2S on carbonaceous materials of different dimensionality

The effect of the dimension of carbonaceous systems, from two to zero, on the adsorption strength of H₂S is investigated by density functional theory based methods. To this end, a carbon nanocone (CNC), a (3, 3) carbon nanotube ((3, 3)-CNT), and graphene (G) are chosen as models for zero-, one- and two-dimensional systems, respectively. Pristine G and CNC have low tendency to adsorb H₂S but on the (3, 3)-CNT the molecule adsorbs dissociatively and deforms the surface. The effect of doping the surface of these materials with Ti has also been investigated. The presence of Ti modifies H₂S adsorption capability to the point that it is chemically adsorbed on the three decorated surfaces although H₂S adsorption on Ti decorated graphene appears to be different from two other doped surfaces. Only in this case, the H₂S molecule dissociates and releases hydrogen atoms which form H₂ molecule. The resulting H₂ molecule is physisorbed on the Ti-decorated graphene surface and the S atom adsorbs directly on the Ti atom. The density of states of pristine, Ti decorated and H₂S adsorbed nanostructures demonstrate that the systems change their conductivity and magnetic properties.     

An ab-initio study of hydrogen storage performance of Si2BN nanotubes decorated with group 8B transition metals

Binding of group 8B transition metals (TMs) on Si₂BN nanotubes and the adsorption of H₂ molecules on TM-decorated Si₂BN nanotubes are investigated in the framework of first principles-based density functional theory (DFT). Our results show the adsorption energy of H₂ molecule on TM-decorated Si₂BN nanotube (−2.57 eV) to be greater than those of the other reported adsorbents. The enhancement of the adsorption property is attributed to the structural deformation and the induced charge transfer between TM–decorated Si₂BN nanotube and the hydrogen molecule. Our findings reveal the TM–decorated Si₂BN nanotubes to be highly sensitive to the presence of H₂ molecules. Additionally, we also investigated the adsorption process of multiple H₂ molecules on TM-decorated Si₂BN nanotubes. Our observations lead us to surmise that the maximum storage number of H₂ molecules adsorbed over the TM-decorated Si₂BN nanotubes is four. These results suggest useful potential for the TM-decorated Si₂BN nanotube to be considered as an appropriate medium for hydrogen storage.     

Remarkable improvement in hydrogen storage capabilities of graphitic carbon nitride nanosheets under selected transition metal embedding: A DFT study

The adsorption performance of hydrogen molecules over the transition metals (TM = Os, Ru, and Fe)-embedded graphitic carbon nitride (gCN) and also the binding energy of these TM elements over the gCN are investigated using DFT computations. The obtained results showthat the interaction energy between Os-embedded gCN and H₂ molecule (with E_ads of −2.452 eV) is superior than those of the other reported adsorbents. Based on these results, it is inferred that the maximum storage number of H₂ molecules adsorbed over the TM–embedded gCN are 6 hydrogen molecules. The results reveal that with adsorption of H₂ molecules over the gCN, conduction band and valence band energy levels have crossed each other close to the Fermi level E_F, thus the semi-conductive behavior of these systems is converted to a conductive state. Finally, it is concluded that the Os–modified gCN is suitable for storaging of H₂ molecules.     

Influence of 2-(4-chlorophenyl)-2-oxoethyl benzoate on the hydrogen evolution and corrosion inhibition of 18 Ni 250 grade weld aged maraging steel in 1.0 M sulfuric acid medium

Electrochemical corrosion behavior and hydrogen evolution reaction of weld aged maraging steel have been investigated, in 1.0 M sulfuric acid solution containing different concentrations of 2-(4-chlorophenyl)-2-oxoethyl benzoate (CPOB). The data obtained from polarization technique showed that the corrosion current density (i_corr) and the hydrogen evolution rate decrease, indicating a decrease in the corrosion rate of weld aged maraging steel as well as an increase in the inhibition efficiency (η%) with the increase in inhibitor concentration. Changes in impedance parameters were indicative of adsorption of CPOB on the metal surface, leading to the formation of protective film. Both activation (E_a) and thermodynamic parameters (ΔG_ads⁰, ΔH_ads⁰ and ΔS_ads⁰) were calculated and discussed. The adsorption of CPOB on the weld aged maraging steel surface obeyed the Langmuir adsorption isotherm model. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) study confirmed the formation of an adsorbed protective film on the metal surface.     

Li decorated penta-silicene as a high capacity hydrogen storage material: A density functional theory study

The H₂ adsorption characteristics of Li decorated single-sided and double-sided penta-silicene are predicted via density functional theory (DFT). The orbital hybridization results in Li atom strongly bind onto the surface of the penta-silicene with a large binding energy and it keeps the decorated Li atoms from aggregation. Moreover, Li decorated double-sided penta-silicene can store up to 12H₂ molecules with the average hydrogen adsorption energy of ?0.220 eV/H₂ and hydrogen uptake capacity of 6.42 wt%, respectively. The ab initio molecular dynamics (AIMD) simulations demonstrate the H₂ molecules are released gradually from the substrate material with the increasing simulation time and the calculated desorption temperature T_D is 281 K in the suitable operating temperature range. Our explorations confirm that Li decorated penta-silicene can be regarded as a promising hydrogen storage candidate for hydrogen storage applications.     

Electronic properties and hydrogen storage application of designed porous nanotubes from a polyphenylene network

Based on a polyphenylene network, a series of porous graphene nanotubes (PGNTs) are created and optimised via density functional theory calculations. The calculated band dispersion of the two-dimensional porous graphene can be tuned by rolling it into nanotube form. To explore the energy application of PGNTs, we studied H₂ adsorptions on metal (Li, Ca, and Na) decorated structures of PGNTs as well as B-substituted PGNTs. The results indicate that both the curvature effect and B substitution can strengthen the metal binding and prevent the metal atoms from clustering. Particularly for H₂ adsorption, modification of the electronic property by the curvature effect is beneficial to provide more accessible space, leading to much higher adsorption energies of H₂ on PGNTs than that on planar porous graphene, which is promising for the practical application of hydrogen storage.     

The structural and electronic properties of Li-doped fluorinated graphene and its application to hydrogen storage

By using first-principles density functional theory, a theoretical investigation of Li-doped fluorinated graphene and its application as a hydrogen storage media is performed. It is found that a mixture between sp³ and a higher degree of sp² of the carbon orbitals after doping with Li would restore the distorted fluorinated graphene, and a fluorinated graphene layer with Li adsorbed on single or double-sides could store hydrogen up to 9 or 16.2 wt%. Regarding the H₂ adsorption mechanism, it has been demonstrated that the enhanced electrostatic field around the Li atom originates from the increased charge transfer from Li to graphene and F atoms with more electronegativity. Hybridization interaction between Li and graphene is also responsible for the adsorption of H₂ molecules.     

Theoretical study of hydrogen adsorption on nitrogen doped graphene decorated with palladium clusters

Density functional theory was used to study the adsorption of hydrogen on small palladium clusters (Pd_n, n = 1–4) supported on pyridine-like nitrogen doped graphene. Charge transfer and strong binding (up to four times higher than binding energy of Pd cluster supported on graphene) between graphene–nitrogen and palladium clusters prevent detachment of clusters and leads to three types of adsorption states of hydrogen. The first type is a molecular hydrogen physisorbed, the second one is an activated state of H₂ without adsorption barriers where H–H bond is relaxed and the third type is dissociated state. In dissociated cases, we found barriers below 0.56 eV. This means the process might occur spontaneously at room temperature. We also show that metal–N–vacancy complexes are very stable and involve nitrogen sp² and p_z, carbon p_z and palladium d orbitals. Besides, these systems exhibit an interesting magnetic behavior.     

Superalkali NLi4 decorated graphene: A promising hydrogen storage material with high reversible capacity at ambient temperature

This paper investigates the decoration of superalkali NLi₄ on graphene and the hydrogen storage properties by using first principles calculations. The results show that the NLi₄ units can be stably anchored on graphene while the Li atoms are strongly bound together in the superalkali clusters. Decoration using the superalkali clusters not only solve the aggregation of metal atoms, it also provide more adsorption sites for hydrogen. Each NLi₄ unit can adsorb up to 10 H₂ molecules, and the NLi₄ decorated graphene can reach a hydrogen storage capacity 10.75 wt% with an average adsorption energy ?0.21 eV/H₂. We also compute the zero-point energies and the entropy change upon adsorption based on the harmonic frequencies. After considering the entropy effect, the adsorption strengths fall in the ideal window for reversible hydrogen storage at ambient temperatures. So NLi₄ decorated graphene can be promising hydrogen storage material with high reversible storage capacities.