First Principles Insights into Amorphous Mg2Sn Alloy Anode for Mg-ion Batteries

<p>Rechargeable Mg-ion batteries (MIBs) are an advantageous alternative solution to Li-ion batteries in many ways. Mg is safer and abundant in the Earth, and has a high electrochemical capacity owing to its divalent nature. It is yet relatively less studied largely due to primal success of Li-base batteries and challenges associated with the design of MIBs including high performance electrode materials. Herein, using first principles calculation, we study the electrochemical and mechanical properties of the most viable alloy anode Mg₂Sn with special attention to its amorphous phase—unavoidable phase forming during cyclic Sn magnesiation in MIBs due to volume changes. We create amorphous Mg₂Sn via simulated annealing technique using <i>ab initio</i> molecular dynamics. We find while Mg₂Sn undergoes a substantial atomic-level structural changes during the crystal-to-amorphous transformation, its polycrystalline properties degrade slightly and become softer by only 20 % compared to the crystal phase. Moreover, we predict competitive electrochemical properties for the amorphous phase assuming it goes under similar reaction path as the average electronic charge on Mg ions almost remain unaffected. This work thus not only demonstrate that a-Mg₂Sn phase could be a bypass to combat the challenges associated with the crystal cracking during volume change, but also serves as first step to better understand the widely used Mg₂Sn alloy anode in MIBs.</p

Postcombustion CO₂ Capture in Functionalized Porous Coordination Networks

Motivated by recent experimental reports of zirconium porous coordination networks (PCNs) [<i>J. Am. Chem. Soc.</i> <b>2012</b>, <i>134</i>, 14690–14693], which have demonstrated a good stability and CO₂ adsorption capacity, we investigate the influence of flue gas impurities and functional groups on the performance of PCN frameworks in selective CO₂ capture. Using a combination of grand canonical Monte Carlo (GCMC) simulations and first-principles calculations, we find that O₂ and SO₂ impurities in flue gas have a negligible influence on CO₂ selectivity in all PCN frameworks. However, because of strong electrostatic interaction between H₂O molecules and the framework, CO₂ selectivity decreases in all PCN structures in the presence of water impurities in the flue gas. Our studies suggest that the PCN-59 framework can be a good candidate for selective CO₂ separation from a predehydrated flue gas mixture

Methane Adsorption and Separation in Slipped and Functionalized Covalent Organic Frameworks

Understanding atomic-level mechanisms of methane adsorption in nanoporous materials is of great importance to increase their methane storage capacity targeting energy sources with low carbon emission. In this work, we considered layered covalent organic frameworks (COFs) with low density and revealed the effect of slipping and chemical functionalization on their methane adsorption and separation properties. We performed grand canonical Monte Carlo simulations studies of methane (CH₄) adsorption and carbon-dioxide:methane (CO₂:CH₄) separation in various slipped structures of TpPa1, TpBD, PI-COFs, and functionalized TpPa1 and TpBD COFs as well. We observed that the slipping improves the total CH₄ uptake by 1.1–1.5 times, while functionalization does not have a significant effect on CH₄ uptake. We also observed improvement in CO₂:CH₄ selectivity due to slipping, whereas functionalization results in decrease in the selectivity. In all considered COFs, we found the highest CH₄ delivery capacity of 141 cm³ (STP) cm^–3 at 65 bar and selectivity of ∼25 at 1 bar in 60-AB slipped structure of TpBD COF. We analyzed the molecular details of CH₄ adsorption using binding energy, heat of adsorption, pore characteristics, and expectation energy landscape. Our results show that COFs with increasing profile of heat of adsorption with pressure have the higher CH₄ delivery capacity. In these COFs, we found proximity (∼4–6 Å) of CH₄ binding sites, resulting in higher CH₄–CH₄ interactions and hence the increasing profile of CH₄ heat of adsorption

Surface Charge Transfer Induced Ferromagnetism in Nanostructured ZnO/Al

The present study reports on the origins of room temperature ferromagnetism in zinc oxide (ZnO)-Al nanoparticles using a combination of X-ray absorption near edge structure (XANES) experiments and density functional theory (DFT) simulations. Our findings reveal that the spontaneous magnetization observed in these systems originates from the adsorption of Al on surfaces of ZnO nanoparticles. Our DFT simulations have identified unique configurations for Al adsorption on ZnO surfaces that lead to a spin-polarized charge transfer to O 2<i>p</i> states in surface and subsurface layers. XANES spectra of the magnetic ZnO/Al nanoparticles provide the necessary experimental evidence for the charge transfer to ZnO surfaces and confirm the origin of ferromagnetic behavior. Our results illustrate a complex interplay between the atomic level interfacial structure and the resulting ferromagnetic ordering in metal-coated semiconductor oxide nanostructures

CO₂ Adsorption in Azobenzene Functionalized Stimuli Responsive Metal–Organic Frameworks

Recent reports of externally triggered, controlled adsorption of carbon dioxide (CO₂) have raised the prospects of using stimuli responsive metal–organic frameworks (MOFs) for energy efficient gas storage and release. Motivated by these reports, here we investigate CO₂ adsorption mechanisms in photoresponsive PCN-123 and azo-IRMOF-10 frameworks. Using a combination of grand canonical Monte Carlo and first-principles quantum mechanical simulations, we find that the CO₂ adsorption in both frameworks is substantially reduced upon light-induced isomerization of azobenzene, which is in agreement with the experimental measurements. We show that the observed behavior originates from inherently weaker interactions of CO₂ molecules with the frameworks when azobenzene groups are in cis state rather than due to any steric effects that dramatically alter the adsorption configurations. Our studies suggest that even small changes in local environment triggered by external stimuli can provide a control over the stimuli responsive gas adsorption and release in MOFs

Electrochemical Stability of Magnesium Surfaces in an Aqueous Environment

An insight into the electrochemical stability of Mg surfaces is of practical importance for improving the corrosion resistance of Mg as well as its performance as a battery electrode. The present paper employs first-principles density functional theory simulations to study the electrochemical stability of magnesium surfaces in aqueous environments. A number of electrochemical reactions that describe the interactions between the Mg(0001) surface and water were analyzed. It was verified that water dissociation is favored upon the Mg surface, in agreement with recent works; however, it is also shown that the previously unstudied Heyrovsky reaction may play an important role in controlling the surface stability. Furthermore, it was found that the surface stability also crucially depends on the concentration of adsorbed hydroxyl groups. Specifically, the surface work function was determined to vary as the function of hydroxyl coverage, which has ramifications for the catalytic behavior of the Mg surface. The influences of surface doping with Ca (a reactive element) and Fe (a comparatively noble element) were also studied to provide an atomic-level understanding of how the dopants influence surface properties and subsequent electrochemical reactions. With a keen recent empirical interest in Mg surface stability given the industrial relevance of Mg, the present study provides key new insights into the physical processes related to the enhanced catalytic activity of Mg and its alloys

Electric Field Control of Molecular Charge State in a Single-Component 2D Organic Nanoarray

Quantum dots (QD) with electric-field-controlled charge state are promising for electronics applications, e.g., digital information storage, single-electron transistors, and quantum computing. Inorganic QDs consisting of semiconductor nanostructures or heterostructures often offer limited control on size and composition distribution as well as low potential for scalability and/or nanoscale miniaturization. Owing to their tunability and self-assembly capability, using organic molecules as building nanounits can allow for bottom-up synthesis of two-dimensional (2D) nanoarrays of QDs. However, 2D molecular self-assembly protocols are often applicable on metals surfaces, where electronic hybridization and Fermi level pinning can hinder electric-field control of the QD charge state. Here, we demonstrate the synthesis of a single-component self-assembled 2D array of molecules [9,10-dicyanoanthracene (DCA)] that exhibit electric-field-controlled spatially periodic charging on a noble metal surface, Ag(111). The charge state of DCA can be altered (between neutral and negative), depending on its adsorption site, by the local electric field induced by a scanning tunneling microscope tip. Limited metal–molecule interactions result in an effective tunneling barrier between DCA and Ag(111) that enables electric-field-induced electron population of the lowest unoccupied molecular orbital (LUMO) and, hence, charging of the molecule. Subtle site-dependent variation of the molecular adsorption height translates into a significant spatial modulation of the molecular polarizability, dielectric constant, and LUMO energy level alignment, giving rise to a spatially dependent effective molecule–surface tunneling barrier and likelihood of charging. This work offers potential for high-density 2D self-assembled nanoarrays of identical QDs whose charge states can be addressed individually with an electric field