Thermodynamics of spontaneous dissociation and dissociative adsorption of hydrogen molecules and hydrogen atom adsorption and absorption on steel under pipelining conditions

While hydrogen pipelines have attracted increased attention, safety of the pipelines has been a concern in terms of hydrogen embrittlement (HE) occurring upon hydrogen atom (H) generation and permeation in the steels. In this work, thermodynamic analyses regarding H generation and adsorption on pipeline steels by two potential mechanisms, i.e., spontaneous dissociation and dissociative adsorption, were conducted through theoretical calculations based on Gibbs free energy change of the H generation reactions. Moreover, H adsorption free energy and configurations were determined based on density functional theory (DFT) calculations. Effects of H adsorption site, H coverage and hydrostatic stress on H adsorption and absorption were discussed. Spontaneous dissociation of hydrogen gas molecules to generate hydrogen atoms is thermodynamically impossible. Dissociative adsorption is thermodynamically feasible at wide temperature and pressure ranges. Particularly, an increased hydrogen gas partial pressure and elevated temperature favor the dissociative adsorption of hydrogen. Hydrogen atoms generated by dissociative adsorption mechanism can adsorb stably at On-Top (OT) and 2-fold (2F) Cross-Bridge sites of Fe (100), while hydrogen adsorption at 2F site is more stable due to a higher electron density and a stronger electronic hybridization between Fe and H. The influence of H atom coverage on dissociative adsorption occurs at low coverages only, i.e., 0.25–1.00 ML as determined in this work. External stresses make dissociative adsorption more difficult to occur compared with a fully relaxed steel. Both tetrahedral sites (TS) and octahedral sites (OS) can potentially host absorbed H atoms at subsurface of the steel. Absorbed H atoms will be predominantly trapped at TS due to a low energy path and exothermic feature. Diffusion of H atoms from steel surface to the subsurface is more difficult compared with the dissociative adsorption.     

Electronic structure and nanoindentation properties of electrochemical hydrogen-charged Zr-based metallic glasses

Zr₅₅Cu₃₀Ni₅Al₁₀ metallic glasses were treated by an electrochemical hydrogen- (H-)charging method. Samples with different H content were obtained by changing the H-charging current density and charging time. X-ray diffraction, nanoindentation, X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy, and positron annihilation experiments were used to investigate amorphous structure, nanomechanical properties, electronic structure, and positron annihilation behavior of Zr₅₅Cu₃₀Ni₅Al₁₀ metallic glasses after electrochemical hydrogenation treatment. The results showed that the diffraction angle corresponding to the diffuse scattering peak gradually moved to a low angle with increased H content. At the same time, the hardness and elastic modulus of the MG were significantly increased with increased H-charging content. When the H content was high, the sawtooth rheological phenomenon disappeared in the load displacement curve of the MG during nanoindentation. XPS narrow spectrum analysis showed that the Zr-3d peak in samples shifted to higher binding energies, while the other elements shifted toward lower binding energy, indicating that H addition led to the transfer of valence electrons from Zr-3d to the Zr–H bond state, resulting in hardening. Three lifetime components are observed in the uncharged and charged sample, indicating the presence of three size ranges of open volume sites. After electrochemical hydrogen charging it causes a significant decrease in the size (lifetime) of the three open volume defects, indicating that the hydrogen occupies those sites. With the increase of hydrogen content, the concentration (intensity) of the first two open volume defects gradually decreases, while the third open volume defect gradually enhances, indicating that hydrogen atom mainly occupies the first two open volume defects. Positron annihilation experiments showed that H addition reduced the average annihilation lifetime of positron vacancies in these MGs, but no new defects were produced.     

The study of amorphous La@Mg catalyst for high efficiency hydrogen storage

Amorphous catalysts have a large number of catalytic active sites. Here, we report a magnesium composite trace lanthanum catalyst (La@Mg), in which La and Mg layers form amorphous Mg–La compound on the surface of layered Mg. The test shows this La@Mg has hydrogen storage capacity of about 7.6 wt% and hydrogen desorption of 7.2 wt%, higher than that of crystalline La@Mg and sole Mg, rapid absorption/desorption kinetic and stable reversible absorption/desorption cycles. La@Mg exhibits an optimistic hydrogen storage performance than Mg-based materials previously reported in the literature. Combined with theoretical calculations, it is shown that the amorphous Mg–La has an catalysis on hydrogen storage performance of La@Mg system, which contributing to the dispersion of Mg and providing channels for hydrogen diffusion, facilitating hydrogenation by accelerating H atoms diffuse between the subsurface and the surface. This work provides experiment and mechanism guidance for the development of efficient hydrogen storage materials.     

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 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.     

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.     

First-principle modeling of hydrogen site solubility and diffusion in disordered Ti–V–Cr alloys

Hydrogen diffusion and solubility in disordered alloys are of paramount importance to a variety of practical applications from hydrogen storage materials to separation membranes and protection against hydrogen embrittlement. By employing density functional theory calculations we unveil the atomic-level understanding of hydrogen diffusion in disordered Ti–V–Cr alloys used for hydrogen storage. Hydrogen distribution over interstitial sites of the bcc and fcc lattices of TiV_0.8Cr_1.2 has been simulated using a supercell approach. Taking into account both structural and energy factors we identify tetrahedral sites coordinated by three different metal atoms as the most favorable for hydrogen. The calculations carried out within the nudged elastic band method show that hydrogen diffusion between two tetrahedral site in fcc TiV_0.8.Cr_1.2H_5.25 occurs nearby an intermediate octahedral site with the activation barrier of 0.158 eV for the most probable diffusion pathway. An estimation of the hydrogen diffusion coefficient in fcc TiV_0.8.Cr_1.2H_5.25 at 294 K provides the value of 2.6 × 10^?11 m²/s that is in fair agreement with experiment data. Despite the modeling was done for a hydride of a definite composition we anticipate that the present results could be extended to Ti–V–Cr hydrides with various compositions.     

First-principles study on the dissolution and diffusion behavior of hydrogen in carbide precipitates

To understand the hydrogen (H) behavior in the carbide precipitates, the dissolution and diffusion properties of interstitial H in the transition metal carbide (TMC; TM = Hf, Nb, Ta, Ti, V, and Zr) were studied by first-principles calculations. In these carbides, it can be seen that H tends to occupy the trigonal site (tri2-site) surrounded by three transition metal atoms and one carbon atom rather than the face center (fc-site) and the body center (bc-site) which with the larger space. We found that the bonding interaction between H atom and the nearest-neighbor (1NN) carbon atom is the dominant influence on the stability of H dissolution. Besides, we obtained the temperature-dependent solubility and diffusion coefficients of H in TMC and pure vanadium through Sievert's law and transition state theory. Compared with pure vanadium, H shows the worse solubility in TMC, and it is more difficult for hydrogen to migrate in TMC, but segregate toward the interface. Furthermore, it is interesting to note that, the diffusion barrier and the H solution energy show a linear relationship for transition metal carbides in the same period. These results can help us deepen the understanding of H behavior in vanadium alloys strengthened by carbide precipitates, and furtherly providing the theoretical guidance for the design of alloys with excellent performance.     

Structure and hydrogen absorption properties of Ti53Zr27Ni20(Pd,V) quasicrystals

The structure and hydrogen absorption properties of Pd and V doped TiZrNi quasicrystals were investigated in terms of the equilibrium vapor pressure of hydrogen, and the results were compared with those of undoped samples. Rapidly quenched Ti₅₃Zr₂₇Ni₂₀ alloys formed quasicrystals and absorbed hydrogen H/M (hydrogen to host metal atom ratio) value of 1.79 at room temperature. This was attributed to their structure, which contains mostly tetrahedral interstitial sites that are chemically formed by atoms having a high affinity with hydrogen. However, the relatively low equilibrium vapor pressure of hydrogen, 0.14 Torr at 300 °C, prevents TiZrNi quasicrystals for the practical application on energy storage materials. To overcome this limitation, we replaced Ti with Pd and V to increase the vapor pressure of hydrogen and investigated the properties of hydrogen absorption behaviors. Results of XRD measurements revealed that the quasicrystal structure was maintained by the replacement of Ti with a maximum of 8 at. % of Pd and V. Total amounts of the absorbed hydrogen decreased from 1.33 to 1.06 and to 1.12 of the H/M values when the Ti was replaced by 8 at. % of Pd and V, respectively, at 300 °C. The pressure-composition-temperature data measured using an automatic gas-handling system revealed that the equilibrium vapor pressure increased from 0.14 to 0.21 and to 0.56 Torr at H/M value of 0.5 when Ti atoms were replaced by 8 at. % Pd, and V, respectively, without the appearance of an impurity phase. Our results demonstrate that the replacement of Ti with Pd and V is an effective method to increase the equilibrium vapor pressure of hydrogen without a phase transformation in a TiZrNi quasicrystal system.     

2D transitional-metal nickel compounds monolayer: Highly efficient multifunctional electrocatalysts for the HER,OER and ORR

The atomically dispersed Ni metal on two-dimensional (2D) monolayer species have exhibited impressive catalytic properties towards oxygen evolution and oxygen reduction reactions (OER/ORR). In this work, nickel boride and nickel carbide monolayers, which is inspired by the more different active sites of monolayer surface, such as Ni atom, that efficiently catalyze the reduction of OOH to O₂ to some extent, while B/C atom can work in the reduction of H1 to H₂, are theoretically reported. Remarkably, the density functional theory (DFT) calculations showed that the random combination of Ni atom to two free-metal atoms such as B and C to form catalytically active double sites lead to a remarkable reduction of the first or last hydrogenation free energy barrier step. The resulting Ni₂B₅ and NiC₃ exhibited ultra-low Gibbs free energy (ΔG_H1) of only 0.096 V and 0.018 V for HER and lower onset potentials of only 0.39 V (0.62 V) and 0.60 V (0.31 V) for OER (ORR), respectively, while the NiB₆ exhibited appropriate OER and ORR electrocatalytic activity. The superior bifunctional even multifunctional catalytic performance in the overall water splitting is mainly attributed to both the electron donation from the Ni metal atoms to the key intermediates, which significantly polarizes and weakens the O–H bond, and to the synergistic effect of the B/C atoms that moderates the binding strength of terminal top of H atoms. This work constitutes the DFT study of the water electroreduction processes on diverse nickel compounds monolayer catalytic sites and, consequently, paves the way towards the rational design of highly efficient bifunctional even multifunctional electrocatalysts.     

First-principles study of hydrogen storage on Si atoms decorated C60

Hydrogen storage capacity of Si_nC₆₀ is studied via first-principles theory based on DFT and Canonical Monte Carlo Simulation (CMCS). It is shown that Si atoms strongly prefer D-site rather than other sites and in these structures maximum number of hydrogen molecule onto any Si atom is one. Each Si atom adsorbs one hydrogen molecule in molecular form and with proper binding energies when Si atom is placed in any D-site of C₆₀. Si atoms enhance remarkably hydrogen storage capability in fullerene.     

Improved design of metal-organic frameworks for efficient hydrogen storage at ambient temperature: A multiscale theoretical investigation

A multiscale theoretical technique is used to examine the combination of different approaches for hydrogen storage enhancement in metal-organic frameworks at room temperature and high pressure by implementation lithium atoms in linkers. Accurate MP2 calculations are performed to obtain the hydrogen binding sites and parameters for the following grand canonical Monte Carlo (GCMC) simulations. GCMC calculations are employed to obtain the hydrogen uptake at different thermodynamic conditions. The results obtained demonstrate that the combination of different approaches can improve the hydrogen uptake significantly. The hydrogen content reaches 6.6 wt% at 300 K and 100 bar satisfying DOE storage targets (5.5 wt%).     

Aluminum-silicon hydride clusters for prospective hydrogen storage

The structures and bonding properties of Al₄Si₂H_2n (n = 0–10) clusters are systematically studied by using the evolutionary algorithm combined with ab initio computations. While the H atoms are bond on the terminal sites of the clusters at low H contents, the Al atoms are combined together by double H-bridges and the Al/Si atoms are tetrahedrally coordinated at high H contents. The Al₄Si₂H_2n clusters break into a few fragments for n = 9,10. Analysis on the bonding natures shows that the Al–H bonds are strongly polarized and the Si atoms balance the charge states of the Al/H atoms according to the hydrogen concentrations. The hydrogen storage capacity in Al₄Si₂H₁₆ cluster reaches 8.9 wt%, and the estimated strength of the hydrogen bonding is about −0.55 eV per H₂, which falls in the ideal window for reversible hydrogen storage at ambient temperatures. The high hydrogen capacity and moderate bonding strength suggest that Al–Si hydrides can be promising candidates for hydrogen storage.     

Study on the hydrogen storage performance of graphene(N)–Sc–graphene(N) structure

The electronic properties of a sandwich graphene(N)–Sc–graphene(N) structure and its average adsorption energies after the adsorption of 1, 3, 5, 7, 10, and 14H₂ molecules were investigated by first principles. The average binding energies and adsorption distances of Sc atoms at different adsorption sites in N-doped bilayer graphene (N–BLG) were calculated. It was found that Sc atoms located at different adsorption sites of BLG generated metal clusters. The binding energy of the Sc atom located at the TT position of N–BLG (5.19 eV) was higher than the experimental cohesion energy (3.90 eV), and eliminated the impact of metal clusters on adsorption properties. It was found that the G(N)–Sc–G(N) system could stably adsorb 10 hydrogen molecules with an average adsorption energy of 0.24 eV. Therefore, it can be speculated that G(N)–Sc–G(N) is an excellent hydrogen storage material.     

An investigation of Li-decorated N-doped penta-graphene for hydrogen storage

By making use of first principles calculations, lithium-decorated (Li-decorated) and nitrogen-doped (N-doped) penta-graphene (PG) was investigated as a potential material for hydrogen storage. The geometric and electronic structures of two types of N-doped PG were studied, and the band gaps were 1.86 eV and 2.06 eV, respectively, depending on the positions of the substitution. The probable adsorption sites for Li atoms on topside and downside were calculated. Hydrogen molecules were added one by one to Li-decorated N-doped PG to research the maximum hydrogen gravimetric density. It is found that up to 5 hydrogen molecules on topside and 8 hydrogen molecules on downside can be adsorbed around a Li atom, and the average adsorption energies are in the range of physical adsorption processes (0.1–0.4 eV). The gravimetric densities can reach 7.88 wt% for N-doped PG with Li decoration. Our results suggest that Li-decorated N-doped PG is a significantly promising material for hydrogen storage.     

Activating basal plane of Janus VSSe for efficient hydrogen evolution reaction by non-noble metal element doping: A first-principal study

Searching electrocatalysts with excellent hydrogen evolution reaction (HER) performance is very important for developing clean hydrogen energy. Two-dimensional (2D) materials have been widely studied as HER electrocatalysts, however, the basal planes of 2D materials, which dominate the surface area, are usually with poor activity. In this work, we theoretically studied the HER activity of Janus 2H–VSSe with or without non-noble metal element doping. Density functional theory (DFT) calculations suggest that doping As and Si atoms in the S or Se sites of VSSe and the C and Ge atoms in the Se site of VSSe greatly promote the HER performance of the basal plane of VSSe, resulting in hydrogen adsorption free energy close to zero (i.e. ?0.022, ?0.040, 0.066, 0.065, ?0.030, 0.058 eV, respectively), which are better than the Pt catalyst (?0.09 eV). The doped atoms strengthen the interaction between their pz-orbital and the hydrogen s-orbital, resulting in a lower bonding state in energy and higher bind strength for the hydrogen atom. This work opens up a new way to design highly efficient and low-cost catalysts for HER.     

MgScH15: A highly stable cluster for hydrogen storage

Mg-Sc-H systems exhibit superior stability and high hydrogen storage capacity among the large class of magnesium-based hydrogen storage materials, but the underlying mechanisms for their outstanding hydrogen storage properties remain largely unexplored and require further investigation. Here, we have performed a comprehensive investigation on the structural evolution of MgScH_n (n = 10–20) clusters by unbiased Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) method combined with density functional theory (DFT) optimizations at the B3PW91/6-311 + G(d) level. Our results show that the MgScH₁₅ cluster with C_s symmetry is found to be the most stable cluster with good hydrogen storage capacity of 17.8 wt% due to the strong interaction among the 1s orbitals of the H atoms, the 2p orbitals of the Mg atom and the 3d orbitals of the Sc atom. The present findings are promising for further exploring novel hydrogen storage nanomaterials.     

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.     

Mechanism of hydrogen-induced defects and cracking in Ti and Ti–Mo alloy

Mechanism of Hydrogen-induced defects and cracking in Ti and Ti–Mo alloy was systematically analyzed from the perspective of microstructure. Results show that more hydrogen atoms can be dissolved in Ti–Mo alloy due to the existence of β phase, and the precipitation amount of hydride is reduced under the same hydrogen charging conditions. The analysis of positron annihilation results show that the solid solution of Mo element reduces the hydrogen induced defect concentration in the alloy and inhibit the combination of hydrogen and defects. At the same time, Mo atom can affect the electronic structure of Titanium hydrides, and then reduce the bonding between titanium atoms and hydrogen atoms, thereby reducing the formation of brittle titanium hydride. The reduction of hydride and hydrogen induced defect concentration significantly decrease the hardening rate of Ti–Mo alloy, thereby effectively reducing the hydrogen embrittlement sensitivity of titanium and inhibiting its hydrogen absorption cracking tendency.     

Freeze-dried ammonia borane-polyethylene oxide composites: Phase behaviour and hydrogen release

A solid-state hydrogen storage material comprising ammonia borane (AB) and polyethylene oxide (PEO) has been produced by freeze-drying from aqueous solutions from 0% to 100% AB by mass. The phase mixing behaviour of AB and PEO has been investigated using X-ray diffraction which shows that a new ‘intermediate’ crystalline phase exists, different from both AB and PEO, as observed in our previous work (Nathanson et al., 2015). It is suggested that hydrogen bonding interactions between the ethereal oxygen atom (–O–) in the PEO backbone and the protic hydrogen atoms attached to the nitrogen atom (N–H) of AB molecules promote the formation of a reaction intermediate, leading to lowered hydrogen release temperatures in the composites, compared to neat AB. PEO also acts to significantly reduce the foaming of AB during hydrogen release. A temperature-composition phase diagram has been produced for the AB-PEO system to show the relationship between phase mixing and hydrogen release.