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.     

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.     

Spontaneous dissociation of H2 and high energy barrier of H invasion on W doped Al2O3(0001) surface

In view of the wide use of tungsten in fusion experimental devices and the importance of hydrogen isotopes permeation, here we studied the adsorption, dissociation, diffusion and invasion behavior of hydrogen on W doped α-Al₂O₃ (0001) surface. Based on the first-principle approaches, we found the W substitution for a top surface Al atom is the most energetically favorable. H₂ molecule prefers to be adsorbed on the surface W and spontaneously dissociates into two H anions. Near the W defects, H atoms favor to be adsorbed at the W and Al sites rather than O sites on the surface, and within the subsurface layer H can only bond to W stably. As a result, H migration to subsurface should occur around W with an energy barrier as large as 4.22 eV which is much larger than the 1.91 eV around the O atom on undoped α-Al₂O₃ (0001) surface. These findings suggest that W surface doping is beneficial to α-Al₂O₃ as tritium permeation barrier.     

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.     

Influence of the Fe-doping on hydrogen behavior on the ZrCo surface

Ab initio calculations have been carried out to investigate the adsorption, dissociation, and diffusion of atomic and molecular hydrogen on the Fe-doped ZrCo (110) surface. It is found that the adsorption of H₂ on doped surface seems thermodynamically more stable with more negative adsorption energy than that on the pure surface, and the dissociation energy of H₂ on doped surface is much bigger therefore. However, compared with the pure system, there are fewer adsorption sites for spontaneous dissociation. After dissociation, the higher hydrogen adsorption strength sites would promote the H atom diffusion towards them where they can permeate into the bulk further. Furthermore, the ZrCo (110) surface possesses much higher hydrogen permeability and lower hydrogen diffusivity than its corresponding ZrCo bulk. Moreover, further comparison of the present results to analogous calculations for pure surface reveals that the Fe dopant facilitates the H₂ molecule dissociation. Unfortunately, this does not improve the hydrogen storage performance of ZrCo alloy due to the H atom diffusion on the surface and into bulk are prevented with higher reaction energetic barriers by doping Fe. Consequently, ZrCo (110) surface modified with Fe atoms should not be preferred as a result of its terrible hydrogen permeability. A clear and deep comprehending of the inhibiting effect of Fe dopant on the hydrogen storage of ZrCo materials from the perspective of the surface adsorption of hydrogen are obtained from the present results.     

Mechanisms for adsorption,dissociation and diffusion of hydrogen in hydrogen permeation barrier of α-Al2O3: The role of crystal orientation

The mechanisms of adsorption of hydrogen on α-Al₂O₃(1-102) surface and of its diffusion in bulk are investigated, using first principles thermodynamics and kinetics, and compared with similar results obtained for the diffusion of hydrogen on α-Al₂O₃(0001) surface. Because of the different oxygen environments on both surfaces, the H binding energies on the (1-102) surface are 0.3–1.2 eV smaller than in the (0001) surface. The H₂ binding energies on (1-102) and (0001) surfaces are resembled. We have identified four main mechanisms, leading to dissociation of H₂, H migration on the surface, H diffusion into and inside the bulk. Equilibrium constant and activation barrier show that H₂ dissociation is the most favorable process and significant diffusion of H into the bulk can occur more readily from the (1-102) surface compared to the (0001) surface. Based on the hydrogen interaction with α-Al₂O₃(1-102) surface, a mechanism of α-Al₂O₃ suppressing H-permeation is identified.     

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.     

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.     

Frequency-domain analysis of hydrogen permeation across Pd77Ag23 metallic membranes

Hydrogen permeation across palladium-alloys membranes is an industrial process used for purification purposes. In state of the art systems, several tens of microns thick metallic membranes can be used and rate limitations generally come from atomic H diffusion. Cost considerations (for example for application in the automotive industry) require a reduction of the membrane thickness and operation at lower temperature. In the micron-thick range, surface contributions become increasingly rate-determining. To optimize permeation membranes, there is therefore a need to separately measure surface and bulk rate contributions to the overall permeation process. In this paper, pneumato-chemical impedance spectroscopy (PIS) is used to analyze the dynamics of hydrogen permeation across Pd₇₇Ag₂₃ membranes. Experimental pneumato-chemical transfer functions of the membrane are measured at different temperatures. Model impedances are calculated and fitted to the experimental ones, yielding microscopic rate parameter values such as surface resistance and hydrogen diffusion coefficient.     

Preparation and hydrogen penetration performance of TiO2/TiCx composite coatings

Titanium carbide is a good candidate for tritium permeation barrier in a fusion reactor. However, its oxidation susceptibility and the mismatch between the ceramic coating and substrate are still a challenge. In this study, a promising candidate as a hydrogen permeation barrier, comprising a titanium-based ceramic TiO₂/TiC_x composite coating, was proposed. The preparation process of this TiO₂/TiC_x composite coating involves two steps of carbon ion implantation and oxidation under ultra-low oxygen partial pressure. According to the results, the optimal oxidation temperature for TiO₂ coating is 550 °C, with the increase of the oxidation temperature, the particles on the surface of the oxide layer become coarse and loosely arranged, and the protective performance of the oxide layer is greatly reduced. The hydrogen barrier permeation behavior of the composite coating in a fusion reactor was simulated via hydrogen plasma discharge environment, the results show that the hydrogen barrier permeation performance of the composite is significantly better than that of a single TiO₂ coating. In addition, the coatings treated with hydrogen plasma showed a certain self-repairing performance through the diffusion growth of the TiC_x layer. These findings illustrate a novel method for preparing composite coatings to restrain hydrogen permeation, providing insight into the development of hydrogen permeation barrier materials.     

Atom doping in α-Fe2O3 thin films to prevent hydrogen permeation

Prevention of hydrogen (H) penetration into passive films and steels plays a vital role in lowering hydrogen damage. This work reports effects of atom (Al, Cr, or Ni) doping on hydrogen adsorption on the α-Fe₂O₃ (001) thin films and permeation into the films based on density functional theory. We found that the H₂ molecule prefers to dissociate on the surface of pure α-Fe₂O₃ thin film with adsorption energy of −1.18 eV. Doping Al or Cr atoms in the subsurface of α-Fe₂O₃ (001) films can reduce the adsorption energy by 0.03 eV (Al) or 0.09 eV (Cr) for H surface adsorption. In contrast, Ni doping substantially enhances the H adsorption energy by 1.08 eV. As H permeates into the subsurface of the film, H occupies the octahedral interstitial site and forms chemical bond with an O atom. Comparing with H subsurface absorption in the pure film, the absorption energy decreases by 0.01–0.22 eV for the Al- and Cr-doped films, whereas increases by 0.82–0.96 eV for the Ni-doped film. These results suggest that doping Al or Cr prevents H adsorption on the surface or permeation into the passive film, which effectively reduces the possibility of hydrogen embrittlement of the underlying steel.     

Essential effect of proton coupled electron sequent transfer on photocatalytic water complete dissociation: A DFT study

Photocatalytic water splitting for hydrogen energy is one of the most promising ways to solve the energy crises. The mechanism is unclear on the sequence of proton coupled electron transfer (PCET) in photocatalytic water dissociation catalyzed by organic material. Here, the water splitting catalyzed by zinc porphyrin with boron dipyrrin (ZnPP-BDP) is systematically investigated by density functional theory (DFT) calculations. The H₂O/ZnPP in valence state based on the electron transfer pulls the trigger on the water splitting process. For the oxygen evolution reaction (OER) step on ZnPP, the processes for the two H ions dissociate from water are exothermic with the −1.06 and −0.95 eV energies respectively, and the O/ZnPP system is formed. The second H₂O molecule on O/ZnPP system can provide the extra oxygen atom to produce the free O₂ with 1.22 eV energy barrier. For the hydrogen evolution reaction (HER) step on BDP, the H ions dissociated from OER process can be captured by BDP to form free H₂ with 2.24 eV/molecule energy released. In this attractive clean cyclic process, the ZnPP-BDP can continuously catalyze the H₂O dissociated into free H₂ and O₂ by the shifting potential barrier. Besides, the proton coupled electron sequent transfer possess an essential role in photocatalytic water dissociation. It is very instructive to design sustainable photodecomposition catalysts for both OER and HER, and to design extraordinary biomimetic photosynthesizers for clean energy.     

Transportation of hydrogen atom and molecule through X12Y12 nano-cages

Hydrogen technology provides efficient, clean and environment friendly alternative to fossil fuel. A major challenge in use of hydrogen fuel is effective storage and release of hydrogen. Therefore, information regarding the barrier for encapsulation and decapsulation are very vital for understanding the phenomenon. A number of reports describe exo(endo)hedral binding of H₂ to inorganic X₁₂Y₁₂ fullerenes; however, the information regarding the barrier for en(de)capsulation are very scarce. In this study, the barriers for encapsulation and release of hydrogen atom and hydrogen molecule through X₁₂Y₁₂ nano-cages (where X = Al, B, Y = N, P) are studied. The translation of H/H₂ through the surface of nano-cages (permeability) is studied through density functional theory calculations with M05-2X method. The kinetic barriers for en(de)capsulation are obtained through scanning potential energy surface along the motion through hexagon of the nano-cage. The size of the nano-cage plays significant role in determining the barrier for en(de)capsulation. The relative stability of exohedral and endohedral complexes of H₂/H with X₁₂Y₁₂ nano-cages is obtained through binding energy calculations. Distortion energies are also calculated and the results show that encapsulation of H₂/H does not distort the nano-cage. Moreover, important minima along PES are also fully characterized. Electronic structures of nano-cages including HOMO–LUMO gap, TDOS, PDOS and excitation energies are analyzed. The H–L gap analysis shows that exohedral complexes have minimal effect on the electronic nature of the nano-cage whereas the endohedral complexes have marked effect on the H–L gap.     

Catalytic activity of Co–Ag nanoalloys to dissociate molecular hydrogen. New insights on the chemical environment

Adsorption of molecular hydrogen on the surface of catalytic metal nanoparticles and its dissociation in atomic hydrogen are processes of interest in many chemical technologies. As in other chemical reactions, alloying can improve the efficiency of the catalysts. By focusing on Co₆, Co₅Ag, Co₃Ag₃ and CoAg₅, we explore the effect of changing the relative concentration of the two components in small Co_mAg_n clusters, a peculiar nanoalloy because Co and Ag do not form bulk solid alloys. Molecular hydrogen adsorbs preferentially on the Co atoms, and the binding is mainly due to the electrical polarization of the charges of adsorbate and host. The preference for Co sites and the trend in the strength of the H₂-cluster binding are explained by the combination of two effects characterizing the host environment. One of these is geometric, arising from the degree of exposure of the host atom: the lower the atomic coordination of the host atom, the stronger its bonding with H₂. The second effect, newly identified, reveals the importance of the chemical nature of the host atom environment: host Co atoms having both Co and Ag neighbors maintain their capacity to bind hydrogen more intact than those with only Co neighbors. The alloy nanoclusters catalyze the dissociation of adsorbed H₂ by building up quite small activation barriers. After dissociation, the H atoms occupy bridge positions between Co atoms (between Co and Ag in CoAg₅). H₂ adsorption and dissociation may trigger structural transformations of the cluster. The work shows that the adsorption and dissociation properties of H₂ can be tuned by varying the relative composition of the two atomic species in the nanoalloy.     

The d band center as an indicator for the hydrogen solution and diffusion behaviors in transition metals

Understanding the hydrogen solution and diffusion behaviors are crucial to design the hydrogen embrittlement resistant structural materials in industry applications. Whether there exists an indicator for prediction of the hydrogen solution and diffusion behaviors is an important but unclarified question. In this study, the hydrogen solution and diffusion behaviors in transition metals and alloys were investigated by density functional theory (DFT) and nudged elastic band (NEB) method calculations. The calculation results reveal that the d band center is directly correlated with hydrogen solution ability in structural materials. The strain and alloying effects on the variations of hydrogen solution energy and d band center have also been considered and the well variation consistency between them is established. With respect to the hydrogen diffusion in transition metals, the transition state has a lower d band center than the initial state. Especially, the hydrogen diffusion energy barrier is proportional to the d band center difference between the initial state and transition state in alloying Fe. Thus the d band center can be regarded as an indicator for hydrogen solution and diffusion in structural materials, which can enlighten us to design the structural materials with high hydrogen embrittlement resistance.     

Storage and permeation of hydrogen molecule,atom and ions (H+ and H−) through silicon carbide nanotube; a DFT approach

The barriers for the encapsulation and decapsulation of hydrogen ions (cationic hydrogen and hydride), atom, and molecule through silicon carbide nanotube are thoroughly studied. DFT method is selected to measure the kinetic barriers for the passage of hydrogen atom, ions and molecule through nanotube via scanning potential energy surface. The kinetic barriers for the passage (encapsulation and decapsulation) of hydrogen are very important to understand the mechanism of hydrogen storage and release. The barriers for the permeation of H, H⁺ and H^? across SiC nanosheet are lower compared to hydrogen molecule (H₂). The exohedral and endohedral adsorption of hydrogen ions (cation and anion), atom and exohedral hydrogen molecule on silicon carbide are exothermic in nature. Whereas the encapsulation of hydrogen molecule in silicon carbide is endothermic. Electronic properties are analyzed through measurement of energy gap between highest occupied and lowest unoccupied molecular orbitals gap (G_H-L) and the density of state (DOS) spectra. The G_H-L analysis reveals that endohedral complexes have more pronounced effect on electronic properties compared to exohedral complexes. The SiC nanotube has highly favorable properties for storage and release of hydrogen ions, and atom.     

Insight into the mechanism of hydrogen permeation inhibition by cerium during electroplating: Enhanced kinetics of hydrogen desorption

In our previous work, we found that hydrogen permeation can be noticeably reduced during Ni–Cu electroplating by the addition of Ce salt to the plating solution. The mechanism of hydrogen permeation inhibition via Ce salt was further studied in the present work. Through the Iver–Pickering–Zamenzadeh (IPZ) model fitting of the kinetic of hydrogen evolution reaction, we found that the trace Ce salt that precipitated during electroplating could improve Tafel reaction kinetic parameters and reduce the strength of the Ni–H and Cu–H bonds due to its abundant d/f electrons and enough d/f orbitals. Meanwhile, Ce can provide electrons for the Heyrovsky reaction. These effects promoted surface electron migration and thus led to the desorption of adsorbed hydrogen atoms (H_ads) and the decreased diffusion of H_ads into the Ni–Cu coatings. The accuracy of the IPZ model fitting results was verified by hydrogen evolution rate experiments during the electroplating process. Hence, Ce salt can effectively inhibit hydrogen permeation and reduce the dehydrogenation annealing time, thereby showing great potential for energy saving and emission reduction in the electroplating industry.     

First-principles study of hydrogen vacancies in lithium amide doped with titanium and niobium

The crystal and electronic structures, the formation energy of H vacancies, and the diffusion path of the H atom (i.e., diffusion path of H vacancy) in unsubstituted and substituted LiNH₂ crystal were investigated by periodic first-principles calculations. The bonding characters between atoms were studied by topological analysis of electron density. Our calculations reveal that substitution of the Li atom with Ti or Nb favors the formation of hydrogen vacancies adjacent to substitution, and the existence of an H vacancy and Ti or Nb substitution can cause weakening of nearby N–H bonds, which facilitates N–H bond dissociation. The minimum energy paths of H diffusion show that the substitution can reduce the energy barrier and thus favor H diffusion in the bulk phase of LiNH₂.     

Preparation of Cr2O3/Al2O3 bipolar oxides as hydrogen permeation barriers by selective oxide removal on SS and atomic layer deposition

Iron-nickel based stainless steel (SS) applied in nuclear plants as a substrate material barely suppresses the permeation of hydrogen plasmas, which are mainly composed of positive and negative hydrogen ions with trace amounts of non-ionized hydrogen atoms. In this work, a new Cr₂O₃/Al₂O₃ bipolar oxide barrier was prepared using atomic layer deposition (ALD) of Al₂O₃ on a Cr₂O₃ layer that was generated by removing partial oxides with cyclic voltammetry (CV) of SS that had been pre-oxidized at 550 °C in air. We found that a small layer of α-Al₂O₃ was formed by the template effect of Cr₂O₃ at the interface of this composite film. The hydrogen permeation behavior of this bipolar oxide barrier in a fusion reactor was simulated with hydrogen-discharging plasma treatment. The results demonstrated that the hydrogen permeation resistance of this bipolar oxide was superior to the original oxide or a Cr₂O₃ film. Impressively, hydrogen plasma treatment repaired the bipolar oxide via reduction of the defective CrO₃, resulting in an improvement in the hydrogen permeation resistance. These findings demonstrate a novel method of hydrogen permeation barrier preparation on SS, providing insight into hydrogen barrier construction for future nuclear energy applications.     

High-capacity hydrogen storage in zirconium decorated zeolite templated carbon: Predictions from DFT simulations

The hydrogen (H₂) storage capacity of Zirconium (Zr) decorated zeolite templated carbon (ZTC) has been investigated using sophisticated density functional theory (DFT) simulations. The analysis shows that the Zr atom gets bonded with ZTC strongly with binding energy (BE) of ?3.92 eV due to electron transfer from Zr 4d orbital to C 2p orbital of ZTC. Each Zr atom on ZTC can attach 7H₂ molecules with average binding energy of ?0.433 eV/H₂ providing gravimetric wt% of 9.24, substantially above the limit of 6.5 wt% set by the DoE of the United States of America. The H₂ molecules are involved via Kubas interaction with Zr atom, which involves the charge transfer between Zr 4d orbital and H 1s orbital with interaction energy higher than physisorption but lower than chemisorption. The structural integrity of the system is confirmed via molecular dynamics (MD) simulations at room temperature and at highest desorption temperature of 500 K. We have investigated the chances of metal clustering by computing diffusion energy (E_D) barrier for the movement of Zr atom, and we obtained via calculation, we can infer that the presence of E_D barrier of ～2.36 eV may prevent the possibility. As the system ZTC has been synthesized, Zr doped ZTC is stable, existence of sufficient diffusion barrier prevents the clustering and adsorption energy and wt% of H₂ are within the range prescribed by DoE, we feel that Zr decorated ZTC can be fabricated as promising hydrogen storage material for fuel cell applications.