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In this study, first-principles calculations were performed to investigate the catalytic effect of NiN4-G on the dehydrogenation of MgH 2. Side-on MgH 2 can be adsorbed stably on the NiN4-G monolayer and is preferentially adsorbed on the NiN4 site compared with the graphene site. The hydrogen desorption process, in which MgH 2 dissociated to the Mg atom on the NiN4 site or graphene site and an H 2 molecule in the vacuum, should overcome lower barriers than pure MgH 2. This is because the corresponding Mg–H bond is weakened owing to the electron transfer between the Mg atom and the substrate. The hydrogen desorption enthalpies of the (MgH 2) 5 cluster on the NiN4 active and graphene sites are significantly smaller (0.11 eV and 1.50 eV, respectively) when H2+H2 is released from the cluster compared with those of the undoped MgH 2 cluster (2.48 eV). Therefore, the NiN4-G monolayer can provide the double effect of the NiN4 active and graphene sites on improving the dehydrogenation performance of MgH 2. [Display omitted] • The hydrogen desorption paths of a single MgH 2 on NiN4-G were determined. • The hydrogen desorption enthalpies of (MgH 2) 5 cluster on NiN4-G were discussed. • NiN4-G exhibits a double catalytic effect to improve the dehydrogenation performance of MgH 2. [ABSTRACT FROM AUTHOR]

The effect of interstitial impurities (H and S) on the atomic, electronic structure, and mechanical properties of the γ-Fe Σ5 (021) grain boundary (GB) was investigated via first-principles calculations. H atoms act as an intergranular embrittler in the Σ5 GB due to a reduction in the charge density between the Fe atoms connected to the grains, whereas H and S co-segregation produces more pronounced embrittlement behavior, resulting in intergranular fracture. The S-induced embrittlement plays a crucial role in the H and S segregation, due to a combination of the structural and chemical effects. The fracturing of Σ5 GB due to S and H segregation is a two-step process. The first step is the breaking of Fe–Fe bonds in the GB, followed by the breaking of the remaining Fe–S bonds in the second step, resulting in the complete separation of the two grains. Moreover, the S atom can slightly compensate for the embrittlement induced by H, because some of the Fe atoms that obtain electrons from the S atoms can provide more electrons to the H atoms, and thus, they can compensate for the electrons that must be acquired from other Fe atoms. We call this "the electrons compensating effect" and this effect is helpful in the design and alloying of steels that are resistant to H embrittlement. • The co-segregation of S and H reduce the strength of Σ5 GB in γ-Fe more obviously. • The preferential breaking sites are discussed in clean, impurities segregated GB. • The fracture of H–S-segregated Σ5 GB is a two-step breakup process. • S can suppress GB embrittlement contributing to H due to electrons compensating. [ABSTRACT FROM AUTHOR]

The hydrogen induced intergranular crack is most common in hydrogen embrittlement phenomenon. Hydrogen atoms will preferentially diffuse into grain boundaries and then accumulate in them. Therefore, it is necessary to reveal the diffusion mechanism of hydrogen atoms at grain boundaries, which is helpful for deeply understanding hydrogen embrittlement. In this work, we performed first-principles calculations to investigate the trapping sites and diffusion behaviours of hydrogen atoms at Σ3 [ 1 ¯ 10] (111), Σ5 [ 1 ¯ 00] (021) and Σ9 [110] (2 2 ¯ 1) grain boundary structures of γ-Fe. Hydrogen atoms have no tendency to be segregated from the bulk to the Σ3 GB but hydrogen atoms tend to segregate into the Σ5 and Σ9 GBs. The solution energy of hydrogen in grain boundaries is mainly the consequence of the chemical bonding between Fe atoms in GBs and the hydrogen atom. In addition, according to the hydrogen diffusion behaviour in grain boundaries, Σ3 and Σ5 GBs act as three-dimensional barriers for hydrogen migration while the Σ9 GB acts as a one-dimensional barrier for H diffusion. Significantly, the difficulty of hydrogen diffusion along grain boundary mainly depends on the connectivity of low electron density region along grain boundary. This conclusion can provide a deep understanding of hydrogen distributions at different grain boundaries in metal materials, which is helpful for hydrogen embrittlement resistant materials design on the basis of grain boundary regulation. • The trapping mechanism of H atom in different GBs in γ-Fe was discussed. • The diffusion mechanism of H atom along GBs was revealed. • Σ3 and Σ5 GBs act as three-dimensional barriers for hydrogen migration inγ-Fe. • Σ9 GB acts as a one-dimensional barrier for hydrogen diffusion in γ-Fe. [ABSTRACT FROM AUTHOR]

Stacking fault energy is a fundamental material parameter in the discussion of plastic deformational mechanisms in metals. In this work, first-principles calculations were performed to investigate the effect of hydrogen atom and hydrogen filled vacancies on the stacking fault energy in fcc Fe, which is important for understanding the hydrogen embrittlement mechanism in austenite steel. In a perfect crystal, hydrogen atom can increase both the unstable and stable stacking fault energies, because of the bond between hydrogen atom and its nearest Fe atoms. The effect of a hydrogen atom on a stacking fault is short-ranged, covering only two atomic layers. It is suggested that the vacancies induced by hydrogen atoms are the main factor decreasing the stacking fault energy, instead of the hydrogen atoms in fcc Fe. Moreover, a hydrogen atom filled in a divacancy can help decrease the stacking fault energy barrier further, which in turn promotes stacking fault formation. Image 1 • The effect of H atom on the stable and unstable SFE in perfect fcc Fe were discussed. • The effect of H atom on shear deformation for perfect fcc Fe was simulated. • The effect of hydrogen filled vacancy and divacancy on the stable and unstable SFE was discussed. [ABSTRACT FROM AUTHOR]

The hydrogen diffusion process is a crucial issue related to hydrogen embrittlement. In this work, first-principle calculations were performed to investigate the diffusion behavior of hydrogen atoms in different Fe crystalline lattices. The hydrogen atom diffused in bcc Fe crystal through migrating from tetrahedral interstice to its nearest tetrahedral site; hydrogen atom diffused in fcc Fe along octahedral site-tetrahedral site-octahedral site; the diffusion path of the hydrogen atom in hcp Fe was from an octahedral site to its nearest octahedral site. The order of the diffusion coefficients of the hydrogen in the three crystal structures was D bcc = 1.379 × 10 −4 cm 2 /s exp (−1120/T) > D fcc = 3.22 × 10 −4 cm 2 /s exp (−8425/T) > D hcp = 6.161 × 10 −4 cm 2 /s exp (−8830/T). In addition, the diffusion behavior of a hydrogen atom near a vacancy was also investigated to determine the ability to trap a hydrogen atom in a vacancy defect in bcc-Fe and fcc-Fe, and the results indicated that hydrogen had higher mobility in the presence of a vacancy in the bcc Fe crystal. [ABSTRACT FROM AUTHOR]

The effects of Li 2 CO 3 and LiOH on the ionic conductivity of BaCe 0.9 Y 0.1 O 3 (BCY)-x%Li 2 CO 3 (x = 0, 5, 10 and 30) and BCY-y%LiOH (y = 0, 2, 5 and 8) composite electrolytes were investigated. Symmetrical cells with the structure of foam Ni–Ni 0.8 Co 0.15 Al 0.05 LiO 2 (NCAL)∖BCY-x%Li 2 CO 3 or y%LiOH∖foam Ni-NCAL and Pt∖BCY-x%Li 2 CO 3 or y%LiOH∖Pt were prepared with NCAL-coated foam Ni and porous Pt as symmetrical electrodes, respectively. The ionic conductivity test results of the cells with Pt as the symmetrical electrode showed that the addition of LiOH did not improve the ionic conductivity of the BCY electrolyte, while the addition of Li 2 CO 3 could significantly improve the ionic conductivity of the BCY electrolyte. The maximum power density of the cells with the structure of foam Ni-NCAL∖BCY-x%Li 2 CO 3 or y%LiOH∖foam Ni-NCAL decreases with the increase in the amount of Li 2 CO 3 or LiOH, which indicates that adding Li 2 CO 3 or LiOH to BCY via mechanical mixing did not improve the electrochemical performance of the cells with NCAL as the electrode, and the NCAL electrode has the greatest impact on the performance of the cells. The interface of Li 2 CO 3 and BCY should be a high-speed channel for ion conduction. Increasing the effective area of the interface between BCY and Li 2 CO 3 and optimizing its continuity in the electrolyte should be the main development direction for improving the ionic conductivity. • Adding Li 2 CO 3 could significantly improve the ionic conductivity of BCY electrolyte. • Adding LiOH cannot improve the ionic conductivity of BCY-LiOH composite electrolyte. • NCAL anode has the greatest impact on the performance of new structure SOFC. • Increase the effective interface area of Li 2 CO 3 /BCY is the key to enhance conductivity. [ABSTRACT FROM AUTHOR]

Precipitation strengthened austenitic stainless steels are widely used in hydrogen related applications. However, their applications may face hydrogen damage resulting in hydrogen-induced delayed failure. Results show that the weld is more sensitive to fracture and hydrogen-induced failure than the matrix. High density curved dislocations, abundant of large size precipitates and considerable γ′ precipitates coarsening are found in the weld. Large size precipitates are found to be major hydrogen traps and preferential microcrack nucleation sites. The γ′ precipitates coarsening make the weld more ductile than the matrix. With the decrease of the applied stress, hydrogen-induced cracking mechanism in the weld changes from brittle transgranular fracture to brittle intergranular fracture. [ABSTRACT FROM AUTHOR]

The safety of precipitation strengthened austenitic stainless steels used for hydrogen storage tanks is of great interest. However, their application may face hydrogen damage resulting in hydrogen-induced delayed failure. Results show that over-loading and hydrogen-induced failure always occur at the weld part of the alloy. Hydrogen damage such as microcracks could be observed on the surface of the matrix and the weld during charging even without any applied stress. Hydrogen-induced failure occurred during charging under constant load and the normalized threshold stress decreased exponentially with increasing defined time t c . It is shown that the threshold stress with no hydrogen-induced failure occurring for expected service life, i.e. forty years, was 713 MPa. Therefore under the service stress, which is less than the threshold stress, 713 MPa, the safety factor of the hydrogen storage tank for hydrogen-induced fracture is great enough to indicate the tank to last for the entire designed service time. [ABSTRACT FROM AUTHOR]

The electrochemical performance of the Ni 0.8 Co 0.15 Al 0.05 LiO 2 (NCAL) cathode was investigated by comparing it with the traditional La 0.4 Sr 0.6 Co 0.2 Fe 0.8 O 3-δ (LSCF) and LSCF/Ce 0.9 Gd 0.1 O 2-δ (GDC) cathodes with a GDC electrolyte-supported solid oxide fuel cell (SOFC). It is found that the electrochemical performance of the cells with the NCAL and NCAL/GDC cathode is better than that of the cells with the LSCF and LSCF/GDC cathode at 550 °C. The results of the electrochemical performance tests of the cells with different NCAL/GDC mass ratios (10/0, 9/1, 8/2, 7/3 and 6/4) show that the NCAL/GDC composite cathode with the mass ratio of 8/2 has the best electrochemical performance. XRD results show that when the sintering temperature is higher than 700 °C, the NCAL/GDC composite will undergo chemical reactions and generate new phases, reducing the performance of the composite cathode. XPS results show that a small amount of Li 2 CO 3 was formed on the surface of NCAL during cathode preparation, forming a special interface between NCAL, Li 2 CO 3 and GDC. At the NCAL-Li 2 CO 3 /GDC interfaces, due to the migration and aggregation of Li+ to the interface, a space charge region may be formed in which the Li+ enrichment may lead to the formation of the region with a high oxygen vacancy concentration. A very high oxygen vacancy concentration at the NCAL-Li 2 CO 3 /GDC interfaces will provide sufficient oxygen ion conductivity for oxygen reduction reaction (ORR) and reduce the activation energy of the reaction. NCAL will be a potential cathode material that can reduce the operating temperature of the traditional SOFC to 550 °C or lower. • Electrochemical performance of Ni 0.8 Co 0.15 Al 0.05 LiO 2 cathode was investigated. • NCAL cathode show very good catalytic activity for ORR. • NCAL/GDC composite cathode with mass ratio of 8/2 has the best performance. • NCAL react with GDC above 700 °C, resulting in poor performance of composite cathode. [ABSTRACT FROM AUTHOR]

A solid oxide fuel cell with 1.2 mm thick nanocrystalline BCY (BaCe 0.9 Y 0.1 O 3) electrolyte was prepared by the copressing method, and its electrochemical performance was tested in H 2 /air conditions. The maximum power density of the nanocrystalline electrolyte cell prepared with BCY powders calcined at 1000 °C and operated at 550 °C in H 2 is 405.2 mW cm−2. The carriers of nanocrystalline BCY electrolyte were studied by using double-layer electrolyte cells with pure oxygen ion conductor GDC (Gd 0.1 Ce 0.9 O 2) and proton conductor SCY (SrCe 0.95 Y 0.05 O 3), sintered at high temperatures as proton and oxygen ion filters. It is found that nanocrystalline BCY is a mixed conductor of oxygen ions and protons. The results of hydrogen and oxygen concentration cells show that the transference number of oxygen ions in the nanocrystalline BCY prepared in this study is higher than that of protons. XPS and HRTEM results of BCY particles before and after performance testing showed that an amorphous layer containing a high concentration of oxygen vacancies and suspected carbonate was formed on the surface of BCY particles after fuel cell performance testing. We believe that the amorphous layer should be a high-speed ion conduction channel for nanocrystalline BCY electrolyte during performance testing. • The ionic conduction mechanism of nanocrystalline BZY electrolyte was studied. • It is found that nanocrystalline BCY is a mixed conductor of oxygen ion and proton. • The transference number of oxygen ions in BCY is higher than that of proton. • An amorphous layer on the surface of BCY is considered to be ion conduction channel. [ABSTRACT FROM AUTHOR]