The fraction of edge- and corner-sharing tetrahedra in liquid ZnCl is quantified as a function of temperature using Raman spectroscopy and ab initio molecular dynamic simulations. Two distinct regimes are found in the temperature dependence of the change in these structural units. This behavior is consistent with the existence of a fragile-to-strong transition in liquid ZnCl as suggested by calorimetric and viscosity measurements. The structural origin of this transition is rationalized in terms of a constraint counting formalism. It is suggested that the ratio of edge- to corner-sharing tetrahedra controls the configurational entropy and in turn the viscosity of the melt. The temperature dependence of this ratio above the melting point is also found to be qualitatively consistent with neutron diffraction data. The observation of a similar fragile-to-strong transition in the isostructural GeSe melt indicates that it may be a common feature of tetrahedral liquids.

The structure of molten ZnCl2 is investigated using a combination of computer simulation and experimental methods. Ab initio molecular dynamics (AIMD) is used to model the structure of ZnCl2 at 600 K. The structure factors and pair distribution functions derived from AIMD show a good match with those previously measured by neutron diffraction (ND). In addition, Raman spectroscopy is used to investigate the structure of liquid ZnCl2 and identify the relative fractions of constituent structural units. To ascertain the assignment of each Raman mode, a series of ZnCl2 crystalline prototypes are modeled and the corresponding Raman modes are derived by first-principles calculations. Curve fitting of experimental Raman spectra using these mode assignments shows excellent agreement with both AIMD and ND. These results confirm the presence of significant fractions of edge-sharing tetrahedra in liquid ZnCl2. The presence of these structural motifs has significant impact on the fragility of this tetrahedral glass-forming liquid. The assignment of Raman bands present in molten ZnCl2 is revised and discussed in view of these results.

Hydrogen has a potential to be a clean energy carrier that emits only water after combustion and can be produced from diverse feedstocks. Hydrogen has much better combustion characteristics in conventional combustion systems and higher energy efficiency when used with fuel cells. More than 75 million tons of hydrogen are currently produced primarily using fossil fuels as feedstock via steam methane reforming processes. Steam methane reforming is the mature technology for producing hydrogen and when coupled with CO2 capture can help address climate challenges. Inorganic palladium (Pd) membranes have demonstrated great potential to separate hydrogen due to their stability and high selectivity for hydrogen. In this review, several methods of fabricating Pd-alloy membranes are discussed and compared in terms of membrane stability and selectivity of hydrogen. Such methods include electroless plating (ELP), chemical vapor deposition (CVD), physical vapor deposition (PVD), and electroplating deposition (EPD). The permeability of hydrogen in different Pd-based alloy membranes are presented and compared. Focus has been made, in this review, on Pd–Ag, Pd–Cu, Pd–Au, and Pd–Ru alloys. The effects of impurities (H2S, CO, O2, and CO2) on performance of different Pd-based alloy membranes are also investigated. Moreover, the subject of using Pd-membrane reactors for fuel reforming and H2 production is investigated in detail based on numerous experimental and numerical studies in the literature, considering different membrane reactor designs: axial-flow tubular, radial-flow tubular, axial-flow spherical, packed-bed, fluidized bed, and slurry bubble column. The performance of Pd-membranes in such reactors for hydrogen production is compared, and the effects of temperature, pressure, H2O/CH4 ratio, and residence time on reformer performance are also investigated. Finally, the use of computational methods, particularly, density functional theory (DFT), to complement well-established experimental methods for studying the diffusion of H and its isotopes in different metals is reviewed. The review concludes with some insights into future work to bring Pd-membrane reactors to the level required for hydrogen production at the commercial level.

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Graphene twistronics have recently gained significant attention due their superconductive behavior as a consequence of their tunable electronic properties. Although the electronic properties of twisted graphene have been extensively studied, the mechanical properties and integrity of twisted trilayer graphene (tTLG) under loading is still elusive. We investigated the fracture mechanics of tTLG with a twist angle of ±1.53° utilizing molecular dynamics simulation. This twist angle was chosen because it is known to exhibit highly superconductive behavior. The results indicate that tTLG does not preserve the excellent mechanical properties typically associated with graphene, with toughness and fracture strain values much lower in comparison. The Young’s modulus was an exception with values relatively close to pristine graphene, whereas the tensile strength was found to be roughly half of the intrinsic strength of graphene. The fracture toughness, fracture strain and strength converge as the crack length increases, reaching 0.26 J/m3, 0.0217 and 39.9 GPa at a crack length of 8 nm, respectively. The Griffth critical strain energy is 19.98 J/m2 and the critical stress intensity factor Kc is 4.47 MPa M1/2, in good agreement with that of monolayer graphene in the experiment. Our atomic insights might be helpful in the material design of twisted trilayer graphene-based electronics.

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