Micro-mechanism study on the effect of O2 poisoned ZrCo surface on hydrogen absorption performance

Hydrogen storage alloys are usually susceptible to poisoning by O₂, CO, CO₂, etc., which decreases the hydrogen storage property sharply. In this paper, the adsorption characteristics of oxygen on the ZrCo(110) surface were investigated, and the effect of oxygen occupying an active site on the surface on the hydrogen adsorption behavior was discussed. The results show that the dissociation barrier of H₂ is increased by more than 26% after O occupies the active sites on the ZrCo(110) surface, and the probability of H₂ adsorption and dissociation decreases significantly. The adsorption energy of H atoms on the O–ZrCo(110) surface decreased by 18–56%, and the adsorption stability of H decreased. In addition, H atom diffusion on the surface and into bulk are prevented with higher reaction energetic barriers by O occupying active sites. Eventually, the ability of the ZrCo surface to adsorb hydrogen is seriously reduced.     

Influence of niobium/tantalum doping on the hydrogen behavior of ZrCo(110) surface

In this paper, based on first-principles calculations, the effects of Nb/Ta modified ZrCo(110) surfaces on the adsorption, dissociation and diffusion of hydrogen are discussed. It is demonstrated that for the pure ZrCo(110) surface, H easily dissociates spontaneously, and is then captured by the holes on the surface. After entering the subsurface, H tends to fill the subsurface and continue to diffuse inward instead of escaping. After Nb/Ta doping, the maximum dissociation energy barrier of hydrogen decreases by 12.3%/37.0%, respectively. The diffusion energy barrier of H on the surface decreased by 15.8%/12.4%, and that on the subsurface decreased by 16.7%/24.1%. Charge transfer and density of states analysis showed that the bonding strength of H in the surface is improved after Nb/Ta doping. The results confirm that Nb/Ta doping can improve the hydrogen storage performance of ZrCo.     

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

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

DFT study of boron doped MgH2: Bonding mechanism,hydrogen diffusion and desorption

The impact of boron doping on MgH₂ bonding mechanism, hydrogen diffusion and desorption was calculated using density functional theory (DFT). Atomic interactions in doped and non-doped system and its influence on hydrogen and vacancy diffusion were studied in bulk hydride. Slab calculations were performed to study hydrogen desorption energies from (110) boron doped surface and its dependence on the surface configuration and depth position. To study kinetics of hydrogen diffusion in boron vicinity and hydrogen molecule desorption activation energies from boron doped and non-doped (110) MgH₂ surface Nudged Elastic Band (NEB) method was used. Results showed that boron forms stronger, covalent bonds with hydrogen causing the destabilization in its first and second coordination. This leads to lower hydrogen desorption energies and improved hydrogen diffusion, while the impact on the energy barriers for H₂ desorption from hydride (110) surface is less pronounced.     

Hydrogen diffusion and anti-disproportionation properties in ZrCo alloys: The effect of Sc,V, and Ni dopants

The use of ZrCo alloys as hydrogen isotope storage materials is limited by the significant reduction of storage capacity caused by disproportionation reaction. In this study, the effects of Sc/V/Ni substitution on the hydrogen diffusion and anti-disproportionation properties of ZrCo alloys and their hydrides were systematically investigated by theoretical calculations. The doping of V reduces the migration barrier of absorbed H at different octahedral sites, leading to improved hydrogen permeability and fast kinetics. For ZrCoH₃ hydrides, V/Ni dopant increases the formation energy of H in 8e site, while Sc has opposite effect. Additionally, the migration barrier for H away from 8e site to its nearby 4c₂ site is obviously lowered by V, but is increased by Sc/Ni. These results indicate that V substitution can effectively improve hydride anti-disproportionation capability, which deepens the understanding of experimental phenomena, and sheds valuable light on the design of ZrCo alloys with high performance.     

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 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.     

An ab initio study of dissociative adsorption of H2 on FeTi surfaces

Dissociative adsorption of H₂ on clean FeTi (001), (110) and (111) surfaces is investigated via ab initio pseudopotential-plane wave method. Adsorption energies of H atom and H₂ molecule on Fe and Ti terminated (001) and (111) and FeTi (110) surfaces are calculated on high symmetry adsorption sites. It is shown that, top site is the most stable site for horizontal H₂ molecule adsorption on (001) and (111) surfaces for both terminations. The most favorable site for H atom adsorption on these surfaces however, is the bridge site. In (110) surface, the 3-fold hollow site which is composed of a long Ti–Ti bridge and an Fe atom, (Ti–Ti)_L–Fe, and again a 3-fold hollow site this time composed of a short Ti–Ti bridge and an Fe atom, (Ti–Ti)_S–Fe, are the most stable sites for H₂ and H adsorption, respectively. With the analysis of the above favorable adsorption sites, probable dissociation paths for H₂ molecule over these surfaces are proposed. Activation energies of these dissociations are also determined with the use of the dynamics of the H₂ relaxation and climbing image nudged elastic band method. It is found that H₂ dissociation on (110) and Fe terminated (111) surfaces has no activation energy barrier. On other surfaces however, activation energies are calculated to be 0.178 and 0.190 eV per H₂ molecule for Fe and Ti terminated (001) surfaces respectively, and 1.164 eV for Ti terminated (111) surface.     

Fast hydrogen diffusion along Σ13 grain boundary of α-Al2O3 and its suppression by the dopant Cr: A first-principles study

To acquire knowledge of the effect of chromium dopant bringing on hydrogen diffusion along grain boundaries (GBs) of α-Al₂O₃, the energetics and mobility of hydrogen along Σ13 GB with and without Cr dopant have been studied via the first-principles calculations with the projector-augmented wave method. The energy barriers for hydrogen diffusion before and after Cr doped on Al-terminated Σ13 GB are determined to be 0.69 eV and 1.09 eV, respectively, which is smaller than or close to the hydrogen bulk diffusion with an activation energy of 1.10 eV. The results suggest that before doping Cr atoms, there is a rapid and easy diffusion channel for hydrogen along the Σ13 GB. However, the calculations show the segregation of Cr atoms to the GB is energetic favorable with a segregation energy of ?0.39 eV/atom, and the existence of Cr can suppress hydrogen diffusion along the Al-terminated Σ13 GB.     

Mechanism for adsorption,dissociation and diffusion of hydrogen in hydrogen permeation barrier of α-Al2O3: A density functional theory study

Toward understanding physical interaction of hydrogen isotopes with α-Al₂O₃ barrier, adsorption, dissociation and diffusion of hydrogen in α-Al₂O₃(0001) slab have been investigated by density functional theory (DFT) and rate theory. H₂ molecule, with parallel configuration, preferentially absorbs on a top Al atom site of first atomic layer on α-Al₂O₃(0001) surface, while H atom strongly bonds at a top O atom site of the second atomic layer, H atoms recombine into molecules on top Al atom sites of the third atomic layer. The barrier for H₂ exothermic dissociation on surface is 0.79 eV. The potential energy pathways of H diffusion in α-Al₂O₃ are studied, predicting that H atom diffusion preferentially occurs via surface path rather than bulk path involving elementary reorientation and hopping steps. The surface-to-subsurface diffusion is significantly endothermic except for the surface and subsurface-to-bulk path. Mechanism, in well agreement with experimental result, of α-Al₂O₃ resisting hydrogen permeation has proposed.     

Vanadium carbide coating as hydrogen permeation barrier: A DFT study

Density functional calculations are used to investigate hydrogen (H) behaviors in vanadium carbide (VC). Molecular H₂ dissociation, atomic H diffusion and penetration are analyzed using the transition state theory. H₂ prefers to be close to the surface as physical adsorption, providing an environment conducive for further dissociation, and dissociates into atomic H adsorbed at the top C atom sites with co-adsorption state. The dissociation rate on the surface is mainly limited by the temperature-controlled activation energy barrier. The adsorptivity of atomic H by the surface tends to decrease as increasing of H coverage. For atomic H penetration through the surface, a significantly endothermic energy barrier and the low diffusion prefactor suggest that the main resistant effect of H permeation takes place at the surface. Energetic, vibrational, electronic consequences, and quantum effects on the H behaviors are discussed thoroughly. Our theoretical investigation indicates VC is a promising hydrogen permeation barrier.     

Adsorption and dissociation of H2 molecule over first-row transition metal doped C24 nanocage as remarkable SACs: A comparative study

Transition metal doped carbon materials such as carbon nanotubes and fullerenes have been extensively investigated for hydrogen adsorption and storage applications. However, the strength of these hydrogen storage material is mainly dependent on the metal-support and hydrogen-metal interaction energies. In this work, we have designed, and explored transition metal doped C24 (TM@C24) complexes as single atom catalysts for hydrogen dissociation. Adsorption energy results for all studied TM@C24 complexes reveal the high thermodynamic stability of designed TM@C24 catalysts. Among all considered TMs@C24 catalysts, the highest adsorption energy (−6.22 eV) is calculated for Mn@C24 catalyst. Moreover, H₂ dissociation mechanism is evaluated for both gas phase and in aqueous media to get insight into the solvent effect. In both gas phase and in aqueous media, the best catalytic activity for hydrogen dissociation is computed for Mn@C24 catalyst with the lowest energy barrier of 0.04 eV and 0.30 eV, respectively. NCI analysis is carried out to confirm the shared shell interactions (covalent interactions) between adsorbed hydrogen and TM@C24 complexes. Furthermore, natural bond orbital and electron density differences analysis are also performed to explore the activation and dissociation of H₂ molecule. Overall results reveal that Mn@C24 complex can at as a promising low cost, highly abundant and noble metal free single atom catalyst to effectively catalyze hydrogen dissociation reaction.     

Hydrogen dissociation and diffusion on transition metal (= Ti,Zr, V,Fe, Ru,Co, Rh,Ni, Pd,Cu, Ag)-doped Mg(0001) surfaces

The kinetics of hydrogen absorption by magnesium bulk is affected by two main activated processes: the dissociation of the H₂ molecule and the diffusion of atomic H into the bulk. In order to have fast absorption kinetics both activated processed need to have a low barrier. Here we report a systematic ab initio density functional theory investigation of H₂ dissociation and subsequent atomic H diffusion on TM (= Ti, V, Zr, Fe, Ru, Co, Rh, Ni, Pd, Cu, Ag)-doped Mg(0001) surfaces. The calculations show that doping the surface with TMs on the left of the periodic table eliminates the barrier for the dissociation of the molecule, but the H atoms bind very strongly to the TM, therefore hindering diffusion. Conversely, TMs on the right of the periodic table do not bind H, however, they do not reduce the barrier to dissociate H₂ significantly. Our results show that Fe, Ni and Rh, and to some extent Co and Pd, are all exceptions, combining low activation barriers for both processes, with Ni being the best possible choice.     

Hydrogen flux of BCC and FCC PdCuAg membranes

First-principles calculations have been used to study the effects of Ag addition on adsorption and dissociation of H₂ on BCC and FCC PdCu surfaces as well as hydrogen diffusion and recombinative hydrogen desorption through the PdCu membranes. It is found that the Ag addition makes it energetically difficult for the adsorption of H₂ on PdCu surfaces and hydrogen diffusion through PdCu, while could help the recombinative desorption of H atoms from both BCC (110) and FCC (111) surfaces of PdCu. Moreover, substitution of Ag for Pd or Cu would impede or improve the dissociation of H₂ on PdCu surface. Calculations also reveal that the overall hydrogen flux of BCC Pd₈Cu₈ (Pd₈Cu₇Ag) membranes is determined by the recombinative desorption and diffusion when the membrane thickness is smaller and bigger than 10.31 (4.73) μm, respectively. In addition, hydrogen diffusion is the dominant step of hydrogen permeation of FCC PdCu and PdCuAg as well as BCC Pd₇Cu₈Ag membranes. The present results not only agree well with experimental observations in the literature, but also deepen the understanding of the effect of Ag alloying on hydrogen permeation through PdCu membranes.     

Hydrogen adsorption studies of TiFe surfaces via 3-d transition metal substitution

Hydrogen adsorption over TiFe surface and doped TiFe surface is investigated within density functional theory. Surface energy calculations confirm that TiFe (111) surface has the minimum value among three low index crystallographic surfaces, (100), (111) and (110). The (111) TiFe surface has two different terminations one with Fe and the other with Ti. Here both the (111) surfaces with different terminations are considered for doping with all the 3-d transition metal atoms from Sc to Zn. Furthermore, the molecular hydrogen adsorption over all the doped surfaces is investigated. V was found to be the most suitable element for doping in Fe terminated (111) surface. V doping in Fe terminated surface enhanced E_ads by 0.6 eV from ?3.30 eV (undoped) to ?3.90 eV after doping. Whereas in case of Ti terminated surface Co was found to be the best element for doping as it enhanced E_ads by ～0.5 eV from ?2.64 eV (undoped) to ?3.15 eV after doping. A significant decrease in d-band width from 1.95 eV to 1.22 eV in case of Co substitution in Ti terminated surface and from 2.42 eV to 1.33 eV in case of V substitution in Fe terminated surface enhances the hydrogen adsorption in TiFe (111) surface. Thus, even using a very small amount of dopant can influence the hydrogen adsorption properties of TiFe alloy.     

Hydrogen adsorption and dissociation on nickel-adsorbed and -substituted Mg17Al12 (100) surface: A density functional theory study

Adsorption and dissociation properties of hydrogen on Ni-adsorbed and -substituted Mg₁₇Al₁₂ (100) surface are investigated systematically by means of the density functional theory calculations. Results show that one Ni atom prefers to adsorb on MgMg bridge site of the surface with adsorption energy −4.90 eV. For substitution systems, the Mg₁₇Al₁₂ (100) surface doped with 3.94 wt% and 7.69 wt% of Ni are considered. It is obtained that Ni atoms tend to replace Mg atoms occupied at the subsurfaces. With the addition of Ni, the energy of atomic (molecular) hydrogen adsorption (dissociation) on the Mg₁₇Al₁₂ (100) surface are significantly improved. The dissociation of H₂ on the Ni-adsorbed surface is spontaneous. The mechanisms analyses based on the density functional theory are in line with the experimental results.     

The effect of Ti atom on hydrogenation of Al(111) surface: First-principles studies

First-principles calculations were performed to investigate hydrogen dissociation and subsequent diffusion over both clean and Ti-doped Al(111) surfaces. The calculations show that it is energetically favorable to dope the surface or subsurface layer of Al(111) with Ti atom. Through calculations on the detailed process associated with hydrogen dissociation and diffusion, we found that Ti doping will decrease the hydrogen dissociation barrier by about 0.6 eV. Additionally, the mobility of hydrogen atoms on surface will be easier if Ti atom is placed in subsurface layer instead of top surface layer. The present results further contribute towards understanding the improved kinetics observed in recycling of hydrogen in Ti-doped NaAlH₄.     

The enhanced hydrogen-sensing performance of the Fe-doped MoO3 monolayer: A DFT study

A pure monolayer of orthorhombic MoO₃ and Fe-doped MoO₃ were constructed to study their hydrogen sensing properties through first-principle density functional theory (DFT) calculations. The results show that Fe can be stably doped into the MoO₃ monolayer with a high binding energy of −8.09 eV. Further calculations revealed that pure MoO₃ is insensitive to molecular oxygen or hydrogen. However, oxygen can be chemisorbed onto the doped Fe in the modified MoO₃ with a high adsorption energy of −0.807 eV, capturing approximately 0.2 e from the sensing material. The introduced hydrogen molecules tended to interact strongly with the pre-adsorbed O₂ molecule to form two H₂O, releasing 1.01 e back to the sensing material. There were 1.92 e released back to the MoO₃ doped with two Fe atoms during the sensing process which significantly enhancing the hydrogen sensing performance of the modified material. Our study indicates that doping MoO₃ with Fe atoms improved its hydrogen sensing performance and is a reasonable way to design effective gas sensing materials.     

Can H2S poison the surface of yttria-stabilized zirconia?

The adsorption and dissociation of H₂S on the yttria-stabilized zirconia (YSZ) (111) surface are studied using the first-principles methods. It is found that H₂S and SH species are bound weakly on the YSZ(111) surface. Sulfur atom is essentially immobile both into the YSZ bulk and along the surface. Instead, it is stably anchored on the O atop of the YSZ surface with the formation of the SO²⁻ fragment. The nudged elastic band (NEB) calculations show that the formation of SH from H₂S (H₂S → SH + H) is very easy, while the presence of a co-adsorbed H would inhibit the further dissociation of SH. In contrast, the hydrogenation of the adsorbed sulfur is rather easy. It is concluded that H could inhibit the formation of sulfur, thus the sulfur poisoning of the YSZ surface would be prevented by hydrogen.