Electrochemical reduction of nitrate to ammonia (nitrate reduction reaction, NO3-RR) under ambient conditions, which overcomes the drawbacks of energy-intensive Haber−Bosch reaction and low-efficient N2 electroreduction, is one of the alternatives...

It is crucial to understand hydrogen interactions with intrinsic point defects in the hydrogen permeation barrier (HPB) of α-Al2O3, finding underlying reasons for the not-so-low H-permeability of the barrier, and thereby produce samples with tailored defects for optimal performance. Using density functional theory (DFT), the formation energies of intrinsic point defects in an α-Al2O3 lattice, including extrinsic H-related defects (H(i), V(Al)-H complex, HO(i) and H(O)), in all possible charged states, are first calculated under HPB working conditions, to determine the dominant basic defect species for hydrogen. We find that the stable forms of H-related defects in α-Al2O3 are charged H interstitials (H(i)(q), where q is the charge state of the defect) and hydrogenation of the bulk V(Al)(3-) ([V(Al)(3-)-H(+)](q)), under hydrogen-rich conditions. As the system reaches equilibrium, H in α-Al2O3 is mainly present in the H(i)(+) state, and preferentially exists in the form of [V(Al)(3-)-H(+)] and H(O)(+). Migration processes of the dominant defects are further investigated, predicting that H(i)(+) is the predominant diffusion species in α-Al2O3. [V(Al)(3-)-H(+)](2-) and H(O)(+) can release trapped hydrogen during high temperature annealing, contributing to the H-transport in α-Al2O3. The formation energy is much higher than the migration energy for H(i)(+), suggesting that the migration of H(i)(+) is the bottleneck for creating low enough H-permeation in α-Al2O3, and corresponding strategies for optimum H-suppressing performance for an α-Al2O3 HPB are proposed.

Metallic MoS2 (i.e., 1T‐MoS2) is considered as the most promising precious‐metal‐free electrocatalyst with outstanding hydrogen evolution reaction (HER) performance in acidic media comparable to Pt. However, sluggish kinematics of HER in alkaline media and its inability for the oxygen evolution reaction (OER), hamper its development as bifunctional catalysts. The instability of 1T‐MoS2 further impedes its applications for scaling up, calling an urgent need for simple synthesis to produce stable 1T‐MoS2. In this work, the challenge of 1T‐MoS2 synthesis is first addressed using a direct one‐step hydrothermal method by adopting ascorbic acid. 1T‐MoS2 with flower‐like morphology is obtained, and transition metals (Ni, Co, Fe) are simultaneously doped into 1T‐MoS2. Ni‐1T‐MoS2 achieves an enhanced bifunctional catalytic activity for both HER and OER in alkaline media, where the key role of Ni doping as single atom is proved to be essential for boosting HER/OER activity. Finally, a Ni‐1T‐MoS2||Ni‐1T‐MoS2 electrolyzer is fabricated, reaching a current density of 10 mA cm−2 at an applied cell voltage of only 1.54 V for overall water splitting.

Hydrogen transport in the outer Al2O3 scale is the rate-limiting step for H-permeation resistance of FeAl/Al2O3-type aluminide tritium permeation barriers (TPBs) in fusion reactors. With first-principle calculations, the effects of Cr, a main impurity element in the Al2O3 scale, on intrinsic point defect formation, hydrogen interactions with intrinsic point defects, and hydrogen diffusion in α-Al2O3 have been investigated under the working conditions of aluminide TPBs. It is found that Cr addition is favorable for the formation of VO, yet unfavorable for VAl formation in α-Al2O3. Compared with the α-Al2O3–H case, Cr is beneficial for the formation of H-related defects in α-Al2O3, whereas it is unfavorable for the Hi trapping ability of VAl and VO. HO – will dominate among Hi –, VAl 3–, VO 0, and [VAl 3––H+]2–, and only one step of Hi reorientation will be involved for the Hi diffusion in Cr-doped α-Al2O3. Hi is the dominant diffusion species in both pure and Cr-doped α-Al2O3, whereas the activation energy of H diffusion in Cr-doped α-Al2O3 is sharply reduced, which is unfavorable for H-permeation resistance of aluminide TPBs. The Cr effect on hydrogen behaviors in α-Al2O3 can be attributed to the chemically unstable electron structure of Cr3+ and a relatively stronger bonding interaction between H and Cr than that between H and Al or O atoms.

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First-principles plane-wave pseudopotential calculations have been performed to study the charge states and energetics of intrinsic point defects as vacancies, interstitials and antisite atoms in α-Al2O3, and thus a new perspective on the process of intrinsic point defects has been proposed. Considering the various charge states for each intrinsic point defects, V(Al)(3-), V(O)(0), Al(i)(3+), O(i)(2-), Al(O)(3+), and O(Al)(3-), not all in their fully ionized states are found to be most stable and in pure Al2O3. From the formation energies of individual point defects, the antisite atom O(Al) will be readily formed in α-Al2O3 in an O-rich environment. By combination of charge states and formation energies, the defect types of Schottky, Al Frenkel and antisite pairs formed are found to be dependent on the O condition, and the most stable Schottky defect type is not the commonly considered {3V(O)(2+):2V(Al)(3-)}. There are two types of possible O Frenkel defects under both O conditions, yet the most stable defect is {O(i)(1+):V(O)(1-)} rather than the commonly believed {O(i)(2+):V(O)(2-)}. The bizarre configuration and the charge state of Schottky and Frenkel defects predicated in this work provide a new perspective on the process of intrinsic point defects in α-Al2O3.

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