The electronic properties and structural stability of LaFeO3 oxide by niobium doping: A density functional theory study

The niobium doping perovskite has been used in anode materials of solid oxide fuel cell. The electronic properties and structural stability of LaFeO₃ (LFO) oxide by Nb-doping and the adsorption of H₂ molecule at the clean and Nb-doped LFO (001) surface are investigated by theoretical calculations. The calculated results reveal that the band gap of the orthorhombic LFO is 2.04 eV and the gap disappears after the Nb-doping, which improves the electrical conductivity. The Nb-doping increases the formation energy of oxygen vacancy in orthorhombic LFO. The calculated results of binding energy and formation enthalpy imply that the structural stability is strengthened after Nb-doping, which provides a theoretical explanation for recent experimental observations. This result can be attributed to the change of electronic structure after the Nb-doping. The bond mechanisms for LFO and Nb-doped LFO are obtained by analyzing density of states, Mulliken charges and bond population. Based on adsorption properties, it can be found that the adsorption of H₂ molecule is slightly enhanced after Nb-doping and the Nb-doping facilitates that H₂ molecule dissociates to H atoms. These results could provide powerful interpretations for the origin of experimental phenomenon.     

Is chitin a promising hydrogen storage material? A thorough quantum mechanical study

Due to their biodegradability and low toxicity, biopolymers have drawn much research interest in the field of material science. The stability of the biopolymers due to existence of strong intermolecular hydrogen bonding makes these materials more promising as suitable adsorbents. In the current study, chitin has been explored as potential hydrogen storage material using density functional theory calculations. The presence of several heteroatoms on the surface of adsorbent makes it suitable for hydrogen adsorption. Thirty hydrogen molecules are adsorbed at six available sites on monomeric chitin, making it a promising naturally available hydrogen storage material. Natural bond orbital analysis and surface analysis reveal that hydrogen is adsorbed on the substrate by charge transfer from heteroatom on the adsorbent to the adsorbed hydrogen molecule. Independent gradient model analysis and atoms-in-molecules analysis have revealed the weak dispersion forces between the adsorbate and the adsorbent. The adsorbent also exhibits high gravimetric density (∼27%) and the adsorption process is thermodynamically favorable.     

Density functional theory study on adsorption of Pt nanoparticle on graphene

The mechanism of CO oxidation was catalyzed by Pt nanoparticles on graphene through first-principle density functional theory (DFT) calculations. The simulation results show that the lowest-energy Pt₇ nanoparticle carries slightly negative charges which enhance the O₂ binding energy compared to the corresponding graphene surfaces. We placed the Pt nanoparticle on different adsorption sites, and the Pt₇ nanoparticle was found to preferentially absorb on Bond (B) site. To gain insight into the high-catalytic activity of the Pt nanoparticles, the interaction between the adsorbate and substrate was also analyzed by detailed electronic analysis such as activation barrier, adsorption energy and Mulliken charge analysis.     

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 penetration and diffusion on Mg17Al12 (110) surface: A density functional theory investigation

The adsorption, diffusion and penetration of H on the Mg₁₇Al₁₂ (110) surface are investigated systematically by means of the density functional theory calculations. Results indicate that H and H₂ prefer to adsorb on the Mg-Mg bridge sites of the Mg₁₇Al₁₂ (110) surface. The lowest barrier energy of molecular hydrogen dissociation on the (110) surface is ～0.87 eV. The penetration processes of atomic hydrogen incorporation into the Mg₁₇Al₁₂ (110) surface are discussed. It is obtained that the H penetrates from the Mg₁₇Al₁₂ (110) surface into the subsurface with the minimum barrier of ～0.63 eV, while the hydrogen atom spreads into the deeper Mg₁₇Al₁₂ (110) surface with lower barrier.     

Hydrogen storage in Li decorated and defective pentaoctite phosphorene: A density functional theory study

Hydrogen storage in 2D pentaoctite phosphorene was investigated by density functional theory (DFT) calculations. Defect engineering and Li decoration were adopted to evaluate their effects on the hydrogen storage. The formation energies for two types of point defects, single vacancy (SV) and double vacancy (DV) were calculated. The DFT results showed that pristine pentaoctite had a very weak binding with H₂ molecule. With the defect formation energies in the order of black phosphorene, the point defects marginally improved the binding energy of H₂ molecule. However, Li decoration over pristine and defective substrates enhanced the binding energy of H₂ molecule by 5–10 fold improving from around ?0.03 eV/H₂ to ?0.25 eV/H₂, thereby, resulting a better H₂ storage capacity. PDOS calculation evidenced the charge transfer from Li atom as its key attribute. In addition, multiple Li adatoms were decorated over the substrate at the favorable sites. In Li decorated pristine, SV, and DV defective substrates, up to 5, 6, and 3 H₂ molecules could be absorbed at each Li adatom. The diffusion energy barrier of Li from one favorable site to another was calculated to be an order of magnitude higher that its thermal energy causing an impedance to clustering.     

Hydrogen storage in lithium,sodium and magnesium-decorated on tetragonal silicon carbide

Motivated by the widespread application and fascinating properties of various silicon-carbon nanomaterials, we have extensively investigated the properties of tetragonal silicon carbides (t-SiC) monolayer as a novel 2D material for hydrogen storage.Using calculations of the density functional theory comprising van der Waals interactions of type vdW-DF2-C09_x, the structural stability, electronic properties and hydrogen molecules adsorption energies on the surface of pure t-SiC were investigated.The results show that adsorption energies of H₂ molecules in this system are stronger than that of graphene. We also found that the decoration with alkali (Li, Na) and alkaline-earth (Mg) metals atoms increases the stability of hydrogen compared to the pure system. The studied system decorated with 8 elements of Li/Na/Mg is able to adsorb up 24 molecules of hydrogen and reaches a gravimetric capacity of 6.50, 5.54 and 5.48 wt%, respectively. The desorption temperatures found are significantly high compared to other 2D systems.     

The influence of stacking faults on hydrogen storage in TiCx

The hydrogen storage ability of TiC_x with stacking faults (SFs) is studied in this work. It is found that the absorption of hydrogen atoms is possible in both TiC_x with and without SFs. And, besides the carbon vacancy sites, the hydrogen atoms can also occupy the tetrahedral sites in the SFs layers. More importantly, it is confirmed that the diffusion of hydrogen in TiC_x with SFs is much easier than that in TiC_x without SFs, especially the diffusion around the SFs layers. The energy barrier for diffusion of the hydrogen atom in the SFs layers and diffusion from the SF layer to the next layer is only 0.099 eV and 0.185 eV, respectively. Therefore, bringing in the SFs in TiC_x with ordered carbon vacancies will be an effective method to solve the diffusion problem during the processes of hydrogen storage.     

Ca and K decorated germanene as hydrogen storage: An ab initio study

The hydrogen storage capacity and performance of Ca and K decorated germanene were studied using density functional theory calculation. The Ca and K adatoms were found to be sufficiently bonded to the germanene without clustering at the hollow site. Further investigation has shown an ionic bonding is apparent based on the charge density difference and Bader charge analysis. Upon adsorption of H₂ on the decorated germanene, it was found that the Ca and K decorated systems could adsorb 8 and 9 H₂ molecules, respectively. The adsorption energies of H₂ molecules were within the Van der Waals energy (400–435 meV), suggesting weak physisorption. The charge density profile revealed that the electron of H₂ moved toward the adatom decoration without leaving the local region of H₂. This suggests that a dipole-dipole interaction was apparent and consistent with the energy range found. Finally, the gravimetric density obtained from the adsorption of H₂ on the decorated germanene shows that this material is a potential for H₂ storage media.     

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.     

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.     

High performance material for hydrogen storage: Graphenelike Si2BN solid

Recently, two dimensional graphenelike i.e. Si₂BN solid monolayer have attracted much attention for the use of hydrogen developments. The work is based on first principles calculations using density functional theory with long range van der Waal (vdW) interactions. The optimized structure is energetically more stable due to high formation energy 45.39 eV with PBE and 50.82 eV with HSE06 functionals, respectively. Our ab-initio studies show that Pd (palladium) adatoms secured graphenelike Si₂BN solid via two types of interactions; physisorption and chemisorptions reactions, which engrossing up to 3H₂ molecules signifying gravimetric limits of ≈6.95–10.21 wt %. The absorption energies vary from ?0.31 eV to ?1.93 eV with Pd-adatom and without Pd-adatom respectively, and it varies up to ?1.24 eV. The work function of pure Si₂BN is 5.36 eV while metal-adatom on monolayer Si₂BN with (1 to 6)H₂ molecules is 3.53 eV–4.99 eV and reaches up to 5.85 eV. The theoretical study suggests that the functionalized graphenelike Si₂BN is efficient for hydrogen storage and propose a possible improvement for advantageous storage of hydrogen at ambient conditions.     

The decomposition of aromatic hydrocarbons during coal pyrolysis in hydrogen plasma: A density functional theory study

Coal pyrolysis in hydrogen plasma has been proposed to undergo two steps. Volatiles such as aromatic hydrocarbons vaporize from coal and subsequently decompose to produce acetylene and hydrogen. We employed a density functional theory (DFT) to investigate the decomposition pathway of benzene, a model aromatic hydrocarbon, for understanding the coal pyrolysis in hydrogen plasma. The results indicate that there are two low-energy decomposition channels. Active hydrogen atoms in the plasma play an important role in the initiation of benzene decomposition, which leads to the formation of c-C₆H₅ particle and hydrogen molecule. The c-C₆H₅ could further decompose to yield acetylene, hydrogen and carbon soot, which is more favorable based on its lower activation energies. The decomposition makes the primary contribution to acetylene formation, and the dehydrogenation results in the additional hydrogen gas and serious coking. The active hydrogen atoms in plasma can remarkably lessen the energy barriers required for the reactions.     

Density functional study of the chemisorption of O2 on the zig-zag surface of graphite

The reaction between molecular oxygen and the zig-zag surface of two model graphites has been studied using density functional theory at the B3LYP/6-31G(d) level of theory. Chemisorption, desorption, rearrangement, and surface migration pathways were characterized and kinetic parameters computed in order to provide a mechanistic understanding of the processes occurring during carbon gasification. The chemisorption reaction is barrierless and highly exothermic, releasing . Surface migration reactions were found to occur with a barrier of ∼170 kJ mol⁻¹, while two desorption processes were found to have barriers of 420 and 340 kJ mol⁻¹, with the possibility of a stable intermediate forming in the latter pathway. Loss of CO from this stable intermediate was found to occur with a barrier of ∼170 kJ mol⁻¹. The initial chemisorption products are highly activated due to the exothermicity of their formation and may proceed directly to CO desorption without stabilization, especially at higher temperatures. Once one molecule of CO is lost, surface migration, rearrangement, and desorption reactions of the remaining oxide were found to have barriers from 195, 470, and 400 kJ mol⁻¹, respectively.     

Hydrogen adsorption and dissociation on the TM-doped (TM=Ti,Nb) Mg55 nanoclusters: A DFT study

The slow hydrogenation kinetics and high reaction temperature of Mg primarily limit its application for mobile hydrogen storage. H₂ adsorption and dissociation on the pure and TM-doped (TM = Ti, Nb) Mg₅₅ nanoclusters are systematically studied by using density functional theory (DFT) calculations. It is found that the introduction of Ti and Nb atoms into Mg₅₅ nanocluster can greatly modify the electronic structure of Mg₅₅ nanocluster and enhance the stability of system. Through the analyses of results from the climbing image nudged elastic band (CI-NEB) and reaction rate constant, we also find that the energy barriers of H₂ dissociation on TM-doped Mg₅₅ nanoclusters can be significantly decreased due to the addition of Ti and Nb. Adding Ti and Nb atoms can dramatically improve the rate constant of H₂ dissociation, especially for H₂ dissociation on Mg₅₄TM2 (TM atom replacing the inner shell position), Mg₅₄TM3 (TM atom replacing the outermost vertex) and Mg₅₄TM4 (TM atom replacing the outermost edge position) nanoclusters. Moreover, compared with the Ti dopant, the Nb will generate a lower activation barrier for H₂ dissociation on TM-doped Mg₅₅ nanoclusters. We also suggest that the subsurface and surface positions (Mg₅₄TM2, Mg₅₄TM3, Mg₅₄TM4) are the ideal substitutional sites for TMs.     

Artificial neural network prediction indicators of density functional theory metal hydride models

The metal hydride is a capable candidate for mobile storage for hydrogen-powered vehicles. An artificial neural network (ANN) has proved useful for many applications, and capable of much more in discovery of new materials. Because of its ability to generalize from examples presented to it, an ANN is a powerful tool for discovering new metal hydride combinations. An ANN can deduce quantitative structure property relationships for metal hydrides. The ANN found correlations between fundamental electronic and energy values modeled ab initio and several experimental parameters. Some of the properties successfully predicted with good correlation are: entropy, enthalpy, temperature at 1 atm of pressure, pressure at 25 °C, and the percent weight of hydrogen stored. The marriage of ANN to computational modeling produces good predictions for many important properties of metal hydrides.     

Hydrogen adsorption and diffusion in amorphous,metal-decorated nanoporous silica

Amorphous, nanoporous adsorbents composed of spherosilicate building blocks and incorporating isolated metal sites were investigated for their ability to adsorb and desorb hydrogen. This novel adsorbent contains cubic silicate building blocks (spherosilicate units: Si₈O₂₀), which are cross-linked by SiCl₂O₂ bridges and decorated with either –OTiCl₃ or –OSiMe₃ groups. The models for the structures were generated to describe experimentally synthesized materials, based on physical properties including density, surface area, and accessible volume. Adsorption isotherms and energies at 77 K and 300 K for pressures up to 100 bar were generated via molecular simulation describing physisorption only. The maximum gravimetric capacity of these materials is 5.8 wt% H₂, occurring at 77 K and 89.8 bar. A low density (high accessible volume) material with no –OTiCl₃ groups proved to be the best performing adsorbent. The presence of –OTiCl₃ did not enhance physisorption even on a volumetric basis, while the high molecular weight of Ti provided a strong penalty on a gravimetric basis. Pair correlation functions illustrate that the most favorable adsorption sites for hydrogen are located in front of the faces of the spherosilicate cubes. The self-diffusivity of hydrogen was reported and found to be highly correlated with accessible volume.     

A density functional theory study of the adsorption of Hg and HgCl2 on a CaO(001) surface

The adsorption of mercury and mercury chloride on a CaO(001) surface was investigated by the density functional theory (DFT) by using Ca₉O₉ cluster embedded in an electrostatic field represented by 178 point charges at the crystal CaO lattice positions. For the mercury molecular axis normal to the surface, the mercury can only coordinate to the O²⁻ anion and has a very weak binding energy of 19.649 kJ/mol. When the mercury chloride molecular axis is vertical to the surface, the Cl atom coordinates to the Ca²⁻ cation and has a binding energy of 23.699 kJ/mol. When the mercury chloride molecular axis is parallel to the surface, the Hg atom coordinates to the O²⁻ anion and has a binding energy of 87.829 kJ/mol, which means that the parallel geometry is more stable than the vertical one. The present calculations show that CaO injection could substantially reduce gaseous mercury chloride, but have no apparent effect on the mercury, which is compatible with the available experimental results. This research will provide valuable information for optimizing and selecting a sorbent for the trace element in flue gas. Translated from Proceeding of the CSEE, 2005, 25(13): 101–104 [译自: 中国电机工程学报]     

Density functional theory study of reversible hydrogen storage in monolayer beryllium hydride by decoration with boron and lithium

The hydrogen storage capacity of M-decorated (M = Li and B) 2D beryllium hydride is investigated using first-principles calculations based on density functional theory. The Li and B atoms were calculated to be successfully and chemically decorated on the Surface of the α-BeH₂ monolayer with a large binding energy of 2.41 and 4.45eV/atom. The absolute value was higher than the cohesive energy of Li and B bulk (1.68, 5.81eV/atom). Hence, the Li and B atoms are strongly bound on the beryllium hydride monolayer without clustering. Our findings show that the hydrogen molecule interacted weakly with B/α-BeH₂(B-decorated beryllium hydride monolayer) with a low adsorption energy of only 0.0226 eV/H₂ but was strongly adsorbed on the introduced active site of the Li atom in the decorated BeH₂ with an improved adsorption energy of 0.472 eV/H₂. Based on density functional theory, the gravimetric density of 28H₂/8li/α-BeH₂) could reach 14.5 wt.% higher than DOE's target of 6.5 wt. % (the criteria of the United States Department of Energy). Therefore, our research indicates that the Li-decorated beryllium hydride monolayer could be a candidate for further investigation as an alternative material for hydrogen storage.