Adsorption Of H2 Molecules Research Articles

The enhancement of hydrogen storage capacity in Li, Na and Mg-decorated BC3 graphene by CLICH and RICH algorithms

New hydrogen adsorption states on Li, Na, and Mg-decorated graphene-type BC3 sheet have been investigated by first-principles calculations. The structural, electronic and binding properties, metal binding and nH2 (n = 1–10) adsorption states of these systems are studied in detail with the Mulliken analysis, charge density differences, and partial density of states. To enhance the number of the adsorbed H2 molecules per metal atom, and also to generate the better initial coordinates for reducing the simulation time, we present two masthematical algorithms (CLICH and RICH). The tested results on BC3 sheet and boron-doped graphene (C30B2) show that these algorithms can increase the number of adsorbed hydrogen molecules by minimizing the computational time. It is seen that nH2 (n = 1–10) adsorbed Li,/Na and/Mg-decorated BC3 single- and double-sided systems are industrial materials for hydrogen storage technology, their adsorption energies fall into the acceptable adsorption energy range (0.20–0.60 eV/H2). It is concluded from the optimized geometries and charge density differences for the higher number of H2 adsorbed systems that not only decorated metal atom but also the sheet plays an important role in hydrogen storage process, because the boron atoms in the sheet expand the induced electric field between the adatoms and BC3 sheet. From Mulliken analysis, there is a charge transfer mechanism between H2 molecules and metal atoms. Moreover, the resonant peaks for the sheet, metal atoms and H2 molecules in partial density of states curves indicate the possible hybridizations. Additionally, these adsorption processes are supported by charge density difference plots.

Ti-doped B/N substituted benzene complexes for hydrogen storage: A comparison

Structural, electronic and hydrogen storage properties of Ti-doped boron(B)/nitrogen(N) substituted benzene complexes are studied and compared with Ti-doped benzene (BTi) complex using ab initio calculations. The H2 uptake capacity is increased and decreased for B-substituted BTi (BBTi) and N-substituted BTi (BNTi) complexes respectively than that for BTi complex. However, H2 molecules interact more strongly with BNTi than BTi and BBTi complexes. The BBTi is an electron deficient structure and accepts an additional electron which is responsible for the adsorption of one extra H2 molecule than BTi and BNTi complexes. More number of states are introduced above the highest occupied molecular orbitals after B-substitution than N-substitution. The charge on Ti in BBTi is more positive than BTi and BNTi complexes. There is a Kubas type interaction between adsorbed H2 molecules and Ti atom for all the complexes.

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.

Ab-initio investigations on titanium (Ti) atom-doped divacancy monolayer h-BN system for hydrogen storage systems

This study demonstrates the hydrogen (H) storage process on titanium (Ti) atom doped divacancy (DV) hexagonal boron nitride (h-BN) system using first-principles density functional theory (DFT). Two configurations were adopted for H storage on the h-BN system. In first approach, H2 molecules were adsorbed on one side and in second approach H2 molecules were adsorbed on both sides of the Ti-atom substituted DV h-BN system. The binding energies of each system for both of these adsorption approaches were calculated, and it was revealed that the system having H2 molecules adsorbed on both sides of h-BN layer produce improved binding energy in comparison to single sided H2 adsorption on Ti decorated h-BN layer. These findings suggest that storing H2 molecules on double sides of Ti-atom substituted DV h-BN system is a preferred approach in comparison to single sided adsorption. The electronic and magnetic properties of H2 adsorbed Ti atom substituted DV BN complexes were also calculated. Further, spin-polarized band structure analysis suggested that the H2 adsorption along with Ti atom substitution at DV site converts insulating h-BN material to a narrow band semiconducting material. In addition, the difference in gap opening during positive and negative spin is substantial making it viable for spintronic applications. The obtained band gap values were in the range of 1–2.5 eV for different H2-adsorbed Ti atom substituted DV h-BN systems respectively. Also, the H2-molecule adsorption on Ti atom substituted DV h-BN system introduces finite magnetic moments ranging up to 2.00 μB, thus producing a magnetic h-BN material by nature. The density of states (DOS) plots were generated to observe the origin of magnetism and orbital occupation. It is observed that the d orbitals of Ti atom mainly attribute to the magnetic behavior of DV h-BN system. The results of this study establish that Ti atom-doped DV h-BN system is geometrically stable and it can provide higher hydrogen storage capability.

In Ukrainian

The features of the interaction of hydrogen molecules with graphene-like planes in which two carbon atoms are replaced by nitrogen or boron atoms are investigated by methods of quantum chemistry (DFT, B3LYP, 6-31G**). To take into account the dispersion contributions to the energy of the formation of intermolecular complexes that arise in the formation of adsorption supramolecular structures, the dispersion correction Grimme-D3 is used. To study the influence of the size of the graphene-like cluster on the molecular hydrogen chemisorption energy in the model of graphene nanoparticles, polyaromatic molecules (PAM) pyrene, coronene (Cor) and that of 54 carbon atoms, as well as their nitrogen and boron-containing analogues, in which the atoms of nitrogen and boron are placed in a para-position in relation to one another, in the so-called piperazine configuration of the atoms of nitrogen or boron. Equilibrium spatial structures of reagent molecules, formed complexes and products of dissociative chemisorption of hydrogen molecules were found by minimizing the norm of a gradient of total energy. An important stage in the transformation of physically sorbed H2 molecules on the surface of the most carbon materials is its decomposition into two hydrogen atoms that can bind to different carbon atoms of model molecules. In this case, a significant number of different reaction products of the same gross composition is formed. The lowest energy among them is related to one where the atoms of the hydrogen are bound to carbon atoms that are adjacent to the nitrogen or boron atoms. It has been found that, regardless of the size of the carbon PAM, the value of the chemisorption energy in all cases has a positive value and is greater than 100 kJ/mol. The chemisorption energy of the hydrogen molecule by PAM with heteroatoms depends on the size of the model, the position of the atoms of nitrogen and boron and, for the most part, has a small negative value (up to -35 kJ/mole), which indicates the spontaneity of the corresponding process. Calculations have shown that the lowest activation energy of the reaction of the H2 molecule with boron PAM, and the most - for pure carbon PAM, regardless of the size of the models. The nature of the heteroatom changes the structure of the transition state and the mechanism of chemisorption. Analysis of the results of quantum chemical calculations showed the highest exothermicity of dissociative adsorption of H2 molecule on boron-containing graphene-like molecules. For nitrogen-containing surfactants, the reaction is slightly less exothermic, as well as the possibility of desorption of atomic hydrogen from the surface of the latter with subsequent recombination in the gas phase. At the same time, for the models of pure graphene-like layer, these data indicate that chemosorption of molecular hydrogen is impossible. Without a total analysis of the results of all possible placements of a pair of hydrogen atoms (formed by the dissociation of the H2 molecule) when they are bound to nitrogen-containing polyaromatic molecules, it can be noted that the dissociation chemisorption of the H2 molecule, regardless of the nature of the heteroatom in the PAM, is thermodynamically more probable at the periphery of the model molecules than in their center.

A Single Hydrogen Molecule as an Intensity Chopper in an Electrically Driven Plasmonic Nanocavity.

Photon statistics is a powerful tool for characterizing the emission dynamics of nanoscopic systems and their photophysics. Recent advances that combine correlation spectroscopy with scanning tunneling microscopy induced luminescence (STML) have allowed the measurement of the emission dynamics from individual molecules and defects, demonstrating their nature as single-photon emitters. The application of correlation spectroscopy to the analysis of the dynamics of a well-characterized adsorbate system in an ultrahigh vacuum remained to be demonstrated. Here, we combine single-photon time correlations with STML to measure the dynamics of individual H2 molecules between a gold tip and an Au(111) surface. An adsorbed H2 molecule performs recurrent excursions below the tip apex. We use the fact that the presence of the H2 molecule in the junction modifies plasmon emission to study the adsorbate dynamics. Using the H2 molecule as a chopper for STM-induced optical emission intensity, we demonstrate bunching in the plasmonic photon train in a single measurement over 6 orders of magnitude in the time domain (from microseconds to seconds) that takes only a few seconds. Our findings illustrate the power of using photon statistics to measure the diffusion dynamics of adsorbates with STML.

Adsorption and Diffusion of Hydrogen on Low-Index (110) and (111) Surfaces of Aluminum

By the density-functional method, the authors have considered the energetics of adsorption and diffusion of a hydrogen molecule into atomically thin aluminum plates with body-centered tetragonal and rhombohedral crystallite structures bounded by the (110) and (111) surfaces, and also geometric configurations of surface aluminum hydrides. It has been shown that physical adsorption of H2 molecules on the Al(110) and Al(111) surfaces is a weak exothermic process. Polarization and deformation due to the physical adsorption lead to a certain increase in the H–H interatomic spacing (0.734 A). The diffusion of the hydrogen atoms into body-centered tetragonal and rhombohedral plates tends to increase the system's energy. It has been found that hydrogen diffuses most easily through the Al(110) surface; in atomic diffusion through the subsurface layer, the energy grows up to 1.1 eV. A stable energy state is attained due to the formation of a tetrahedral hydrogen complex.

DFT study of single-walled carbon hollows as media for hydrogen storage

Understanding the interaction of hydrogen molecules with hollow materials is crucial in the fields of hydrogen storage, sensors, catalysis, and fuel cells. In this paper, we studied hydrogen adsorption on single-walled carbon hollows by means of DFT-D3 calculations. These structures adsorb H2 molecules significantly stronger (adsorption energy, Ea, of −12.55 to −20.56 kJ/mol) than coronene (−4.15 kJ/mol), the representative of graphene, and corannulene (−7.83 kJ/mol), a slightly bent polyaromatic molecule. The reduced density gradient analysis was involved to visualize interacting regions between hydrogen and a series of adsorbents. Energy decomposition analysis was used to divide the total interaction energy into constituents. It is founded that the dispersion term is dominating in all studied cases. The present results should broaden our understanding of the mechanisms of hydrogen storage using carbon hollows.

Interplay between Single and Cooperative H2 Adsorption in the Saturation of Defect Sites at MgO Nanocubes

The configurations associated with reversible and irreversible adsorption of hydrogen on MgO escape consensus. Here, we report the dissociation of H2 on MgO nanocubes, which was examined by combining Fourier transform infrared spectroscopy and density functional theory (DFT)-based simulations. We found that the use of ultrahigh vacuum is essential for identifying the very first adsorption stages. Hydrogen pressure was varied from 10–8 mbar to near ambient, resulting in IR spectra of richer complexity than in current state of the art. Models with oxygen at regular corners (O3C) and Mg at inverse corners (MgIC) were identified to be the most reactive and to split H2 irreversibly already in the lowest pressure regime (PH2 < 10–3 mbar). The continuous increase in intensity of the corresponding IR bands (3712/1140 cm–1) in the intermediate range of pressures (10–3–1 mbar), along with the appearance of bands at 3605/1225 cm–1, was demonstrated to stem from cooperative adsorption mechanisms, which could be therefore considered as the main origin of irreversible hydrogen adsorption. At PH2 > 1 mbar, fully reversible adsorption was shown to occur at O4C (either on mono- or diatomic steps) and Mg3C sites. Another OH/MgH couple (3697/1030 cm–1) that became reinforced at high PH2 but remained stable upon pumping was correlated to O3C and MgIC in multiatomic steps. The difference in adsorption and desorption sequences confirmed the proposed cooperative adsorption of H2 molecules. Our study provides new insights into the mechanisms that can be beneficial for understanding the chemistry of H2 and other hydrogen-containing molecules, such as CH4, on oxide surfaces, but also for the advancement of hydrogen-storage technologies.

Light metal decorated graphdiyne nanosheets for reversible hydrogen storage

The sensitive nature of molecular hydrogen (H2) interaction with the surfaces of pristine and functionalized nanostructures, especially two-dimensional materials, has been a subject of debate for a while now. An accurate approximation of the H2 adsorption mechanism has vital significance for fields such as H2 storage applications. Owing to the importance of this issue, we have performed a comprehensive density functional theory (DFT) study by means of several different approximations to investigate the structural, electronic, charge transfer and energy storage properties of pristine and functionalized graphdiyne (GDY) nanosheets. The dopants considered here include the light metals Li, Na, K, Ca, Sc and Ti, which have a uniform distribution over GDY even at high doping concentration due to their strong binding and charge transfer mechanism. Upon 11% of metal functionalization, GDY changes into a metallic state from being a small band-gap semiconductor. Such situations turn the dopants to a partial positive state, which is favorable for adsorption of H2 molecules. The adsorption mechanism of H2 on GDY has been studied and compared by different methods like generalized gradient approximation, van der Waals density functional and DFT-D3 functionals. It has been established that each functionalized system anchors multiple H2 molecules with adsorption energies that fall into a suitable range regardless of the functional used for approximations. A significantly high H2 storage capacity would guarantee that light metal-doped GDY nanosheets could serve as efficient and reversible H2 storage materials.

Adsorption of molecular hydrogen on inorganometallic complexes B2H4M (M=Li, Be, Sc, Ti, V)

Molecular interaction between hydrogen molecules and B2H4M (M=Li, Be, Sc, Ti, V) complexes has been studied using the DFT method (M06 functional) and 6-311++G** basis set. The hydrogen uptake capacity of the complexes considered is higher than the target set by the US Department of Energy (5.5 wt% by 2020). The metal atom bound strongly to the B2H4 substrate. Adsorption of molecular hydrogen on Be-, Ti-, and V-decorated complexes is thermodynamically possible for all the pressures and temperatures considered whereas it is unfavorable for Li-decorated complexes for all the pressure and temperatures. For the Sc-doped complexes, adsorption of molecular hydrogen is favorable below 330 K and entire pressure range considered. All the H2 adsorbed complexes are kinetically stable. For all the complexes, the interaction between the inorganometallic complexes and the H2 molecules adsorbed is attractive whereas that between adsorbed H2 molecules is repulsive. We have also performed molecular dynamics simulations to confirm the same number of H2 molecule adsorption from the simulations and DFT calculations.

Li adsorption on monolayer and bilayer MoS2 as an ideal substrate for hydrogen storage**Project supported by the National Key Basic Research Program of China (Grant No. 2012CB932304), the National Natural Science Foundation of China (Grant No. 21763007), the Innovation Team Foundation of the Education Department of Guizhou Province, China (Grant No. [2014]35), and the Key Laboratory of Low Dimensional Condensed

Based on the first-principles plane wave calculations, we show that Li adsorbed on monolayer and bilayer MoS2 forming a uniform and stable coverage can serve as a high-capacity hydrogen storage medium, and Li-coated MoS2 can be recycled by operations at room temperature due to Li having strength binding, big separation and is stable against clustering. The full Li coverage MoS2 system (2*2 hexagonal MoS2 supercell) can reach up to eight H2 molecules on every side, corresponding to the gravimetric density of hydrogen storage up to 4.8 wt% and 2.5 wt% in monolayer and bilayer MoS2, respectively. The adsorption energies of hydrogen molecules are in the range of 0.10eV/H2–0.25 eV/H2, which are acceptable for reversible H2 adsorption/desorption near ambient temperature. In addition, compared with light metals decorated low dimension carbon-based materials, the sandwiched structure of MoS2 exhibits the greatly enhanced binding stability of Li atoms as well as slightly decreased Li-Li interaction and thus avoids the problem of metal clustering. It is interesting to note that the Li atom apart from the electrostatic interaction, acts as a bridge of hybridization between the S atoms of MoS2 and adsorbed H2 molecules. The encouraging results show that such light metals decorated with MoS2 have great potential in developing high performance hydrogen storage materials.

In situ observation of H2 dissociation on the ZnO (0001) surface under high pressure of hydrogen using ambient-pressure XPS

The interaction of H2 molecules with a ZnO (0001) single crystal surface has been studied over a wide pressure (10−6–0.25 Torr) and temperature (300–600 K) range using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). ZnO is well-known for interstitial hydrogen and hydrogen atoms in ZnO are believed to be incorporated by the dissociative adsorption of H2 molecules in the atmosphere and their subsequent diffusion into the bulk. The dissociative adsorption of H2 has been investigated at elevated pressures because H2 molecules are not dissociated on the ZnO single crystal surface under ultrahigh vacuum (UHV) conditions. When the pressure is increased to several mTorr, the dissociative adsorption of H2 takes place to form OH bonds on the surface. At 0.25 Torr, the ZnO surface is saturated with H atoms and the coverage is estimated to be 1.1 × 1015 atoms/cm2 at 300 K. At higher surface temperatures, the equilibrium between the dissociative adsorption of gas-phase H2 molecules and the associative desorption of surface H atoms is established. While maintaining the equilibrium, the surface has been monitored successfully in situ by utilizing AP-XPS.

Boron substitution effect on adsorption of H2 molecules on organometallic complexes

Abstract Hydrogen adsorption properties of Be/Sc doped pentalene complexes are investigated using second ordered Moller-Plesset method (MP2). In order to study the boron substitution effect, pentalene is further modified by substituting two and four boron atoms for carbon atoms at different positions and named as TBP1 and TBP2 for two boron atom substituted structures and FBP1 and FBP2 for four boron atom substituted structures. Two H2 molecules get adsorbed on each Be doped complex and having 3.25, 3.31, 3.31, 3.38 and 3.38 wt% H2 uptake capacity for C8H6Be2, TBP1Be2, TBP2Be2, FBP1Be2 and FBP2Be2 complexes respectively. All Sc doped pentalene and boron substituted pentalene complexes can interact with nine H2 molecules except TBP2Sc2 complex. The TBP2Sc2 complex can adsorb eight H2 molecules. The H2 uptake capacity is found to be 8.63, 8.73, 7.84, 8.83 and 8.83 wt% for C8H6Sc2, TBP1Sc2, TBP2Sc2, FBP1Sc2 and FBP2Sc2 complexes respectively. Gibbs free energy corrected adsorption energy plots show that the H2 adsorption on all Be doped complexes is possible at all temperatures and pressures considered here. The TBP1Sc2 complex seems to be more promising hydrogen storage material among all Sc doped complexes over a wide range of temperature and pressure. The H2 desorption temperatures obtained for the Be doped complexes are higher than the Sc doped complexes. Stability of the complexes is predicted with the help of the gap between the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals.

Large entropy derived from low-frequency vibrations and its implications for hydrogen storage

Adsorption and desorption are driven by the energy and entropy competition, but the entropy effect is often ignored in hydrogen storage and the optimal adsorption strength for the ambient storage is controversial in the literature. This letter investigated the adsorption states of the H2 molecule on M-B12C6N6 (M = Li, Na, Mg, Ca, and Sc) and analyzed the correlation among the zero point energy (ZPE), the entropy change, and the adsorption energy and their effects on the delivery capacities. The ZPE has large correction to the adsorption energy due to the light mass of hydrogen. The computations show that the potential energies along the spherical surface centered at the alkali metals are very flat and it leads to large entropy (∼70 J/mol·K) of the adsorbed H2 molecules. The entropy change can compensate the enthalpy change effectively, and the ambient storage can be realized with relatively weak adsorption of ΔH = −12 kJ/mol. The results are encouraging and instructive for the design of hydrogen storage materials.

Hydrogen Storage in Sc and Li Decorated Metal–Inorganic Framework

Hydrogen is a versatile, clean, and efficient energy carrier considered as an ideal substitute for a future energy source in the automobile industry. A metal–inorganic framework with borazocine (BN) linker resulting in a metal–BN framework (MBF) has been studied for hydrogen storage. Borazocine (B4N4H8) is decorated with metals, M (Sc, Li), and studied the stability and hydrogen storage capacity. Density functional theory with generalized gradient approximation and Perdew–Burke–Ernzerhof functional with double numeric polarized basis set augmented with p-function are used to explore the structural stability, and hydrogen sorption kinetics of metal decorated MBF. It is observed that each Sc and Li physisorbed 4 and 3 H2 molecules, respectively. The BN ring binds with metals (Sc and Li) by Dewar coordination while the metal atoms adsorb H2 molecules by the Kubas–Niu–Rao–Jena mechanism. Molecular dynamics simulations show that the Sc decorated MBF system is stable and the adsorbed hydrogen is reversible at a...

OPE molecular junction as a hydrogen gas sensor

Oligo(phenylene ethynylene) (OPE) molecular junction has been suggested as a H2 molecule sensor based on calculations using the first principles of density–functional theory and non-equilibrium Green's function. The electronic transport properties of the OPE molecule between two Au electrodes with or without adsorbed H2 molecules are investigated. Results show that the adsorbed H2 molecule significantly changes the characteristics of the current–voltage curve of the OPE molecular junction. The pure OPE molecular junction exhibits a significant negative differential resistance, but this kind of phenomenon will disappear or weaken after hydrogen molecules are adsorbed. The conductance of the junction also obviously decreases in the bias range of [−0.4, 0.4] V after adsorbing H2 molecules. These effects can be used to design a H2 molecule sensor.

Exfoliation, point defects and hydrogen storage properties of monolayer TiS3: an ab initio study.

The possibility of H2 molecule adsorption on the basal plane of monolayer TiS3 at various sites has been studied. Among the studied adsorption sites, few sites were found to be suitable for physisorption with binding energy up to 0.10 eV per H2. To increase the activity of hydrogen sorption, the possibility of generating S-vacancies, by removing sulfur atoms from the basal plane of monolayer TiS3, was investigated. Despite the fact that the structures containing vacancies were found to be stable enough, there was no increase in the activity towards hydrogen adsorption. The same effect was obtained with the use of common methods of increasing of the H2 adsorption energy: the decoration of the two-dimensional material with alkali metals (Li, Na). This might be caused by the negatively charged surfaces of single layer TiS3, which hinder the increase in binding by alkali metals through a weak electrostatic interaction.

Optimized hydrogen sensing characteristic of Pd/ZnO nanoparticles based Schottky diode on glass substrate

The present work deals with the development of the Pd/ZnO naoparticles based sensor for detection of hydrogen (H2) gas at relatively low temperature (75–110 °C). Pd/ZnO Schottky diode was fabricated by ZnO nanoparticles based thin film on glass substrate using sol-gel spin coating technique. These ZnO nanoparticles have been characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), energy dispersive x-ray spectroscope (EDS), and field emission scanning electron microscope (FE-SEM) which reveals the ZnO film having particles size in the range of ~25 to ~110 nm with ~52.73 nm surface roughness. Gas dependent diode parameters such as barrier height and ideality factor have been evaluated upon exposure of H2 gas concentration in the range from 200–2000 ppm over the temperature range from 75 to 110 °C. The sensitivity of the Pd/ZnO sensor has been studied in terms of change in diode forward current upon exposure to H2 gas. Experimental result shows the optimized sensitivity ~246.22% for H2 concentration of 2000 ppm at temperature 90 °C. The hydrogen sensing mechanism has been explained by surface and subsurface adsorption of H2 molecules on Pd surface; subsequently, dissociation of H2 molecules into H + H atoms and diffusion to trap sites (oxygen ions) available on ZnO surface, resulting in formation of dipole moments at Pd/ZnO interface. The variation in the sensitivity, response and recovery time with temperature of Pd/ZnO sensor has also been studied.

Sc-Decorated Porous Graphene for High-Capacity Hydrogen Storage: First-Principles Calculations

The generalized gradient approximation (GGA) function based on density functional theory is adopted to investigate the optimized geometrical structure, electron structure and hydrogen storage performance of Sc modified porous graphene (PG). It is found that the carbon ring center is the most stable adsorbed position for a single Sc atom on PG, and the maximum number of adsorbed H2 molecules is four with the average adsorption energy of −0.429 eV/H2. By adding a second Sc atom on the other side of the system, the hydrogen storage capacity of the system can be improved effectively. Two Sc atoms located on opposite sides of the PG carbon ring center hole is the most suitable hydrogen storage structure, and the hydrogen storage capacity reach a maximum 9.09 wt % at the average adsorption energy of −0.296 eV/H2. The adsorption of H2 molecules in the PG system is mainly attributed to orbital hybridization among H, Sc, and C atoms, and Coulomb attraction between negatively charged H2 molecules and positively charged Sc atoms.