Structure-Induced Catalytic Activity of Nickel- and Cobalt-Substituted Layered MoB₂ toward Hydrogen Evolution

Production of high-quality green hydrogen gas as a potential alternative to fossil fuels has been of vital importance in recent years. Among many functional materials, layered metal diborides (MDbs) have attracted great attention from the scientific community due to their outstanding catalytic activity toward hydrogen evolution. This study is dedicated to examine the water electrolysis of a series of metal-substituted MoB2 electrocatalysts (Mo(1‑x)TMxB2; TM = Ni and Co; x = 0, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5) synthesized by a facile molten salt technique. The electrocatalysis of materials was assessed in 1.0 M KOH by measuring the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting. Interestingly, as the substitution content of TM increases, α-MoB2, with graphene-like boron layers, transforms into β-MoB2 comprising both flat and puckered boron layers, where α-MoB2 has demonstrated enhanced electrocatalytic performance. Mo0.9Ni0.1B2 afforded 10 mA cm–2 at a low overpotential of 222 mV toward HER (located very close to the commercial Pt/C). The assembled Mo0.9Ni0.1B2 (cathode)∥Mo0.8Co0.2B2 (anode) couple demanded 1.75 V to produce 10 mA cm–2, which is near the potential of state-of-the-art Pt/C∥RuO2 pair. The Faradaic efficiency of generated H2 was determined to be approximately 80%

Structure-Induced Catalytic Activity of Nickel- and Cobalt-Substituted Layered MoB₂ toward Hydrogen Evolution

Production of high-quality green hydrogen gas as a potential alternative to fossil fuels has been of vital importance in recent years. Among many functional materials, layered metal diborides (MDbs) have attracted great attention from the scientific community due to their outstanding catalytic activity toward hydrogen evolution. This study is dedicated to examine the water electrolysis of a series of metal-substituted MoB2 electrocatalysts (Mo(1‑x)TMxB2; TM = Ni and Co; x = 0, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5) synthesized by a facile molten salt technique. The electrocatalysis of materials was assessed in 1.0 M KOH by measuring the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting. Interestingly, as the substitution content of TM increases, α-MoB2, with graphene-like boron layers, transforms into β-MoB2 comprising both flat and puckered boron layers, where α-MoB2 has demonstrated enhanced electrocatalytic performance. Mo0.9Ni0.1B2 afforded 10 mA cm–2 at a low overpotential of 222 mV toward HER (located very close to the commercial Pt/C). The assembled Mo0.9Ni0.1B2 (cathode)∥Mo0.8Co0.2B2 (anode) couple demanded 1.75 V to produce 10 mA cm–2, which is near the potential of state-of-the-art Pt/C∥RuO2 pair. The Faradaic efficiency of generated H2 was determined to be approximately 80%

Enhancing Thermoelectric and Mechanical Properties of <i>p</i>‑Type (Bi, Sb)₂Te₃ through Rickardite Mineral (Cu_2.9Te₂) Incorporation

Bi2Te3-based alloys are widely utilized in Peltier coolers owing to their highest thermoelectric performance at near-room-temperatures. However, their peak dimensionless thermoelectric figure of merit, zT, is limited to a narrow temperature window due to minority carrier excitation emerging upon heating at around 400 K. Here, we show how this issue can be overcome by incorporating a synthetic rickardite mineral, Cu3–xTe2, in p-type (Bi, Sb)2Te3. The significant enhancement of the electronic and thermal properties could be achieved due to small Cu incorporation into the crystal structure of (Bi, Sb)2Te3 and homogeneous precipitation of Cu3–xTe2 at the grain boundaries. This leads to a high average zT value (zTave) of 1.22 between 350 and 500 K for two compositions, Bi0.5Sb1.5Te3 (BST-5) and Bi0.3Sb1.7Te3 (BST-3), with peak zT values of 1.32 at 467 K and 1.30 at 400 K, respectively. These high zT values result in a considerably high maximum device ZT of ca. 1.15 and a theoretical efficiency of up to 7% between 325 and 525 K. Additionally, room-temperature micro-hardness is substantially improved, which is desirable for constructing reliable and durable thermoelectric modules

<i>In Situ</i> Design of a Nanostructured Interface between NiMo and CuO Derived from Metal–Organic Framework for Enhanced Hydrogen Evolution in Alkaline Solutions

Hydrogen shows great promise as a carbon-neutral energy carrier that can significantly mitigate global energy challenges, offering a sustainable solution. Exploring catalysts that are highly efficient, cost-effective, and stable for the hydrogen evolution reaction (HER) holds crucial importance. For this, metal–organic framework (MOF) materials have demonstrated extensive applicability as either a heterogeneous catalyst or catalyst precursor. Herein, a nanostructured interface between NiMo/CuO@C derived from Cu-MOF was designed and developed on nickel foam (NF) as a competent HER electrocatalyst in alkaline media. The catalyst exhibited a low overpotential of 85 mV at 10 mA cm–2 that rivals that of Pt/C (83 mV @ 10 mA cm–2). Moreover, the catalyst’s durability was measured through chronopotentiometry at a constant current density of −30, −100, and −200 mA cm–2 for 50 h each in 1.0 M KOH. Such enhanced electrocatalytic performance could be ascribed to the presence of highly conductive C and Cu species, the facilitated electron transfer between the components because of the nanostructured interface, and abundant active sites as a result of multiple oxidation states. The existence of an ionized oxygen vacancy (Ov) signal was confirmed in all heat-treated samples through electron paramagnetic resonance (EPR) analysis. This revelation sheds light on the entrapment of electrons in various environments, primarily associated with the underlying defect structures, particularly vacancies. These trapped electrons play a crucial role in augmenting electron conductivity, thereby contributing to an elevated HER performance

Les droits de l'homme et la Convention du 28 juillet 1951 relative au Statut des réfugiés.

Controlling extrinsic defects to tune the carrier concentration of electrons or holes is a crucial point with regard to the engineering application of thermoelectric semiconductors. To understand the defect-controlled electronic structure in thermoelectric materials, we apply density functional theory (DFT) to investigate the defect chemistry of dopants M (M = O, S, Se, or Te) in CoSb₃. DFT predicts that the breakage of Sb₄ rings induced by these dopants produces the unexpected (n- or p-type) conductivity behavior in CoSb₃. For example, energetically dominant O interstitials (O_i) chemically break Sb₄ rings and form O–4Sb five-membered rings, leading to the charge neutral behavior of O_i. While S interstitials (S_i) collapse Te–3Sb four-membered rings within Te doped CoSb₃ leading to p-type conduction behavior, Se substitution on Sb (Se_Sb) breaks the Se–Te–2Sb four-membered ring, resulting in a charge neutral behavior of the Se_Sb+Te_Sb complex defect. Furthermore, the solubility limits of M dopants (M = S, Se, or Te) are also calculated to provide essential information about single-phase material design. This study provides new insight into understanding the complicated chemical structure in doped CoSb₃, which is beneficial for devising effective doping strategies for the development of high-performance bulk thermoelectric materials

BaGe₆ and BaGe_6‑x: Incommensurately Ordered Vacancies as Electron Traps

We report the high-pressure high-temperature synthesis of the germanium-based framework compounds BaGe₆ (<i>P</i> = 15 GPa, <i>T</i> = 1073 K) and BaGe_6–<i>x</i> (<i>P</i> = 10 GPa, <i>T</i> = 1073 K) which are metastable at ambient conditions. In BaGe_6‑<i>x</i>, partial fragmentation of the BaGe₆ network involves incommensurate modulations of both atomic positions and site occupancy. Bonding analysis in direct space reveals that the defect formation in BaGe_6–<i>x</i> is associated with the establishment of free electron pairs around the defects. In accordance with the electron precise composition of BaGe_6‑<i>x</i> for <i>x</i> = 0.5, physical measurements evidence semiconducting electron transport properties which are combined with low thermal conductivity

Ideal Strength and Deformation Mechanism in High-Efficiency Thermoelectric SnSe

The widespread use of thermoelectric conversion technology requires thermoelectric materials of high thermoelectric efficiency and high fracture strength. Single crystal SnSe shows an extremely high <i>zT</i> value in the moderate temperature range, but its mechanical properties have rarely been studied so far. Here we use density functional theory to determine the ideal strength and deformation mechanism of perfect SnSe single crystals for shear deformations. The lowest ideal strength of SnSe is found to be 0.59 GPa under the (100)/⟨001⟩ shear load, which agrees with experimental evidence that single crystals cleave along the (100) slip plane. The van der Waals-like Se–Sn bond, which couples the different Se–Sn layered substructures, is much softer than the covalent Se–Sn bond which constructs the Se–Sn layered substructure. This creates pathways of easy slip between Se–Sn layered substructures, which can release shear stress and lead to structural failure. Meanwhile, the layered substructures themselves can resist shearing within the (100)/⟨001⟩ slip system. These results provide a plausible atomic explanation for understanding the intrinsic mechanics of SnSe

Switch of Thermal Expansions Triggered by Itinerant Electrons in Isostructural Metal Trifluorides

Manageable thermal expansion (MTE) of metal trifluorides can be achieved by introducing local structure distortion (LSD) in the negative thermal expansion ScF3. However, an open issue is why isostructural TiF3, free of LSD, exhibits positive thermal expansion. Herein, a combined analysis of synchrotron X-ray diffraction, X-ray pair distribution function, and rigorous first-principles calculations was performed to reveal the important role of itinerant electrons in mediating soft phonons and lattice dynamics. Metallic TiF3 demonstrates itinerant electrons and a suppressed Grüneisen parameter γ ≈ −20, while insulating ScF3 absence of itinerant electrons has a considerable γ ≈ −120. With increasing electron doping concentrations in ScF3, soft phonons become hardened and the γ is repressed significantly, identical to TiF3. The presented results update the thermal expansion transition mechanism in framework structure analogues and provide a practical approach to obtaining MTE without inducing sizable structure distortion

BaGe₆ and BaGe_6‑x: Incommensurately Ordered Vacancies as Electron Traps

We report the high-pressure high-temperature synthesis of the germanium-based framework compounds BaGe₆ (<i>P</i> = 15 GPa, <i>T</i> = 1073 K) and BaGe_6–<i>x</i> (<i>P</i> = 10 GPa, <i>T</i> = 1073 K) which are metastable at ambient conditions. In BaGe_6‑<i>x</i>, partial fragmentation of the BaGe₆ network involves incommensurate modulations of both atomic positions and site occupancy. Bonding analysis in direct space reveals that the defect formation in BaGe_6–<i>x</i> is associated with the establishment of free electron pairs around the defects. In accordance with the electron precise composition of BaGe_6‑<i>x</i> for <i>x</i> = 0.5, physical measurements evidence semiconducting electron transport properties which are combined with low thermal conductivity

Micro- and Macromechanical Properties of Thermoelectric Lead Chalcogenides

Both n- and p-type lead telluride (PbTe)-based thermoelectric (TE) materials display high TE efficiency, but the low fracture strength may limit their commercial applications. To find ways to improve these macroscopic mechanical properties, we report here the ideal strength and deformation mechanism of PbTe using density functional theory calculations. This provides structure–property relationships at the atomic scale that can be applied to estimate macroscopic mechanical properties such as fracture toughness. Among all the shear and tensile paths that are examined here, we find that the lowest ideal strength of PbTe is 3.46 GPa along the (001)/⟨100⟩ slip system. This leads to an estimated fracture toughness of 0.28 MPa m^1/2 based on its ideal stress–strain relation, which is in good agreement with our experimental measurement of 0.59 MPa m^1/2. We find that softening and breaking of the ionic Pb–Te bond leads to the structural collapse. To improve the mechanical strength of PbTe, we suggest strengthening the structural stiffness of the ionic Pb–Te framework through an alloying strategy, such as alloying PbTe with isotypic PbSe or PbS. This point defect strategy has a great potential to develop high-performance PbTe-based materials with robust mechanical properties, which may also be applied to other materials and applications