MoS2 nanosheet as a promising nanostructure membrane for gas separation

Helium (He) is widely used in many scientific and industrial fields, and the shortage of He resources and the growing demand make He separation extremely important. In this work, the He separation performances of a series of graphanes containing crown ether nanopores (crown ether graphane, CG-<i>n</i>, <i>n</i> = 3, 4, 5, 6) are studied by first-principles calculations. At first, the minimum energy paths of He and other 10 gas molecules (Ne, Ar, H<sub>2</sub>, CO, NO, NO<sub>2</sub>, N<sub>2</sub>, CO<sub>2</sub>, SO<sub>2</sub> and CH<sub>4</sub>) passing through CG-<i>n</i> membranes are calculated, and the factors affecting the energy barriers are also investigated. The calculated results show that He is the easiest to pass through all the four CG-<i>n</i> membranes with energy barriers of 4.55, 1.05, 0.53 and 0.01 eV, respectively. He can be separated by CG-5 and CG-6 with very low energy barriers, and the energy barrier of He passing through CG-6 is the lowest, so far as we know. Moreover, all gas molecules can pass through CG-6 with low energy barriers, including many molecules with large kinetic diameters, such as CO (0.13 eV) and N<sub>2</sub> (0.16 eV). Therefore, CG-6 is also expected to be used in the screening field of other gas molecules. In addition, it is found that the energy barriers of gas molecules passing through CG-<i>n</i> are synergistically affected by the size of the crown ether nanopore, the kinetic diameter and the type of the gas molecules. Secondly, the diffusion rates of gas molecules passing through CG-5 and CG-6 and the He selectivity towards other 10 gases of CG-5 and CG-6 at different temperatures are calculated. It is found that CG-5 exhibits extremely high He selectivity in a wide temperature range (0–600 K). In summary, the crown ether graphanes CG-5 and CG-6 can serve as excellent He separation membranes with high He selectivity. This work is expected to inspire one to develop other graphene-based two-dimensional separation membranes for separating He and other gas molecules.

Because inorganic membranes have an excellent chemical and thermal stability, they can be used in water desalination, ultrafiltration process in food industries, waste water treatment and separation of gas mixtures.1 Inorganic membrane can be classified into two types; namely, non-porous(dense) membranes and porous membranes.2 The important factors in the membrane separation process are both the permeability and the selectivity. However, the selectivity of a membrane decreases as the permeability increases. This trend is confirmed by the corelation of selectivity versus permeability.3 Since the polymer membrane is non-porous, the fluxes and permeabilities are very small. However, as the separation occurs via the so-called solution-diffusion mechanism, the selectivity of polymer membrane is much high.4 The mechanism of gas separation by a porous alumina membrane or Vycor glass, is closely related to the membrane pore size relative to the permeate molecule size as well as other physical and chemical properties of the membrane and the permeate species.5,6

High-performance energy storage systems with reliable and sustainable electrochemical properties are urgently needed to satisfy the continuously surging demand in consumer electronics, electric vehicles (EVs), and grid-scale energy storage systems (ESSs). Among the enormours energy storage systems reported to date, lithium-ion rechargeable batteries (LIBs) have still garnered a great deal of attention. The rational design and fabrication of major battery components such as anodes, cathodes, electrolytes, and separator membranes are imperative prerequisites for the development of advanced batteries. Most research activities on battery components have been devoted to the electrochemically active materials, with a especially focus on electrode materials and electrolytes. Representative results include those related to high-voltage spinel nickel manganese oxides, overlithiated layer oxides, silicon- or metal alloys, functional electrolyte additives, and solid-state electrolytes. Taking into consider the fact that electrochemical performance of the batteries is basically governed by electron/ion transport phenomena, particular attention should be paid to battery separators as well as electrodes/electrolytes, because (i) all ions particiapted in Faradaic reaction of batteries should pass through electrolyte-filled porous separator membranes and (ii) Internal short-circuit failure occurring between electrodes (which is considered as a primary cause to trigger cell fire or explosion) is basically prevented by separator membranes. Currently, commercially available separators in LIBs are manufactured using polyolefin materials. These polyolefin separators have some advantageous attributes that render them suitable for practical use in LIBs. However, their intrinsic limitations (speci fically, sluggish/nonuniform ionic flow and poor thermal stability) often raise concerns regarding ion transport and electrical isolation between the electrodes. A large number of approaches to overcome these drawbacks have been undertaken, which include ceramic-coated separators, nonwoven separators, and electrospun nano fiber separators. Unfortunately, little attention has been devoted to controlling the ion transport phenomena in a separator, despite its important role in activating the electrochemical performance of cells. Here, as a new strategy to address these challenging issues, we demonstrate new surface energy-tailored amphiphilic polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer (BCP) membranes with hierarchical multiscale hyperporous structures and chemical functionality (Figure 1), as a revolutionary membrane to enable remarkable advances in cell performance far beyond those achievable with conventional separators. Among the various building blocks for use in porous membranes, BCPs have been extensively investigated because of their self-assembly-enabled nanodomains. Through spinodal decomposition, breath figures, solvent swelling, and nonsolvent-induced phase separation (NIPS), BCPs can produce precisely defined porous structures in versatile shapes. Owing to facile and effective control of membrane morphologies, NIPS process, which is also known as immersion precipitation, has attracted great attention as a promising way to fabricate large-scale membranes. In the NIPS method, final morphologies (e.g., dense or porous, symmetric or asymmetric) of the membranes can be adjusted by combining the involved mass transport phenomena with corresponding phase separation. For membrane-driven purification and separation processes, asymmetric porous structure is preferable because it endows the membranes with both permeability and selectivity. Asymmetric structures with a well-organized skin layer were explored using self-assembled BCP films. Time-controlled evaporation of volatile solvent produces a skin layer with vertically aligned cylindrical structure supported by a graded porous layer, when the BCP solution was immersed into a nonsolvent. Meanwhile, porous membranes with symmetric structures are suitable for allowing two-way ionic transport, which is essentially required for separation membranes of rechargeable power sources. The macropore size of NIPS-based BCP membranes is basically determined by polymer concentrations and solvent-nonsolvent exchange kinetics, while their nanopore size strongly depends on the compatibility between pore-forming block and nonsolvent. In particular, construction of well-defined nanoporous morphology is highly required for advanced BCP membranes. However, even though BCPs, good solvent and nonsolvent are appropriately chosen, conventional NIPS-based BCP membranes normally generate poorly-developed nanopores due to a limited compatibility between pore-forming block and nonsolvent. To date, the most common way to tune nanoporous structure is limited to variation of molecular weight of pore-forming blocks. Our strategy for tailoring high porous BCP separator membrane is based on the introduction of a surface-energy-modifying agent, which allows fine-tuning of nanoscale phase separation between the surface-modified BCPs and nonsolvent. The surface-energy-modifying agent can be easily introduced into a pore-forming block of BCPs via nucleophilic substitution reaction. Depending on the degree of substitution, the surface-energy-modifying agent contributes to tuning dual (macro-/nano-) phase separation of the BCPs, which eventually enables the fabrication of hierarchical multiscale porous block copolymer membranes. The novel block copolymer separator membrane exhibited superior rate capability and cycle performance, owing to its precisely tuned, highly developed porous structure far beyond those achievable with conventional separator membrane technologies. Figure 1

Utilizing molecular dynamics simulations, we examined how varying pore sizes affect the desalination capabilities of MoS2 membranes while keeping the total pore area constant. The total pore area within a MoS2 nanosheet was maintained at 200 Å2, and the single-pore areas were varied, approximately 20, 30, 40, 50, and 60 Å2. By comparing the water flux and ion rejection rates, we identified the optimal single-pore area for MoS2 membrane desalination. Our simulation results revealed that as the single-pore area expanded, the water flux increased, the velocity of water molecules passing the pores accelerated, the energy barrier decreased, and the number of water molecules within the pores rose, particularly between 30 and 40 Å2. Balancing water flux and rejection rates, we found that a MoS2 membrane with a single-pore area of 40 Å2 offered the most effective water treatment performance. Furthermore, the ion rejection rate of MoS2 membranes was lower for ions with lower valences. This was attributed to the fact that higher-valence ions possess greater masses and radii, leading to slower transmembrane rates and higher transmembrane energy barriers. These insights may serve as theoretical guidance for future applications of MoS2 membranes in water treatment.

Scalable synthesis of ultrathin MoS2 membranes for dye desalination

We investigated the removal of heavy metals from water by two-dimensional MoS2 nanosheets suspended in aqueous solution, and restacked as thin film membranes, respectively. From these studies we elucidated a new heavy metal ion removal mechanism that involves a reduction-oxidation (redox) reaction between heavy metal ions and MoS2 nanosheets. Ag+ was used as a model species and MoS2 nanosheets were prepared via chemical exfoliation of bulk powder. We found that the Ag+ removal capacity of suspended MoS2 nanosheets was as high as ∼4000 mg/g and adsorption accounted for less than 20% of removal, suggesting the reduction of Ag+ to metallic silver as a dominant removal mechanism. Furthermore, we demonstrated that MoS2 membranes were able to retain a similar high removal capacity, and attribute this capability to the formation of a conductive, permeable multilayer MoS2 structure, which enables a corrosion-type reaction involving electron transfer from a MoS2 site inside the membrane (anode) to another site on membrane surface (cathode) where heavy metal ions are reduced to metallic particles. The membrane surface remains active to efficiently recover metallic particles, because the primary oxidation products are soluble, nontoxic molybdate and sulfur species, which do not form an insulating oxide layer to passivate the membrane surface. Therefore, MoS2 membranes can be used effectively to remove and recover precious heavy metals from wastewater.

Perovskite oxide is widely used in many important technological fields due to the excellent characteristics. In this work, the performances of the LaFeO3 as oxygen carrier in biomass chemical looping gasification were investigated. It is found that there is no significant decrease in CO and H2 yield with the increase in oxygen carrier cycle number. No obvious change of syngas yield with the increase in LaFeO3 mass is observed, indicating the stability of LaFeO3. The volume fraction of water vapor should be controlled as 39.9%. The DFT calculation results show that CO desorption is identified to be the rate-limiting step with an activation barrier of 0.648 eV for CO molecule formation. For CO oxidation, formation of COO* complex is the rate-limiting step with energy barrier of 0.368 eV. During the process of H2 formation, the pathway of H2 production over surface Fe site is more favorable with the calculated activation energy of 0.428 eV. Formations of H2O and H2 are competing reactions during chemical looping process. However, the relatively low activation energy barriers of H2O dissociation (0.141 eV or 0.081 eV) and H2 formation (0.428 eV) suggest that H2 formation is more competitive. For comparison, performances of Fe2O3 as oxygen carrier were also investigated. The calculation results show a relatively high energy barrier (1.314 eV) of CO formation and relatively low energy barrier (0.434 eV) of CO oxidation.

Precise ångström controlling the interlayer channel of MoS2 membranes by cation intercalation

Fe-doped Mo2C for boosting electrocatalytic N2 reduction

Abstract

Mechanism study on the high-performance BaFe2O4 during chemical looping gasification

Triiodide perovskites $${\rm CsPbI}_{3}$$ , $${\rm CsSnI}_{3}$$ , and $${\rm FAPbI}_{3}$$ (where FA is formamidinium) are highly promising materials for a range of optoelectronic applications in energy conversion. However, they are thermodynamically unstable at room temperature, preferring to form low-temperature (low-T) non-perovskite phases with one-dimensional anisotropic crystal structures. While such thermodynamic behavior represents a major obstacle toward realizing high-performance devices based on their high-temperature (high-T) perovskite phases, the underlying phase transition dynamics are still not well understood. Here we use in situ optical micro-spectroscopy to quantitatively study the transition from the low-T to high-T phases in individual $${\rm CsSnI}_{3}$$ and $${\rm FAPbI}_{3}$$ nanowires. We reveal a large blueshift in the photoluminescence (PL) peak (~38 meV) at the low-T/high-T two-phase interface of partially transitioned $${\rm FAPbI}_{3}$$ wire, which may result from the lattice distortion at the phase boundary. Compared to the experimentally derived activation energy of CsSnI3 (~1.93 eV), the activation energy of $${\rm FAPbI}_{3}$$ is relatively small (~0.84 eV), indicating a lower kinetic energy barrier when transitioning from a face-sharing octahedral configuration to a corner-sharing one. Further, the phase propagation rate in CsSnI3 is observed to be relatively high, which may be attributed to a high concentration of Sn vacancies. Our results could not only facilitate a deeper understanding of phase transition dynamics in halide perovskites with anisotropic crystal structures, but also enable controllable manipulation of optoelectronic properties via local phase engineering. Metal halide perovskites are a new class of semiconductors with great promise for a variety of optoelectronic applications. Owing to their soft ionic lattice, halide perovskites often exhibit rich phase transitions between different crystal structures, frequently from the “active” perovskite phases to undesirable “inactive” non-perovskite phases. Understanding and controlling this transition is vital for developing stable, high-performance devices. However, there is limited understanding on how different symmetry, crystal structure, and defect would impact such a phase transition process. In this report, in situ optical micro-spectroscopy is used to systematically investigate the phase transitions from non-perovskite to perovskite phases in individual CsSnI3 and FAPbI3 wires. Compared to the transition from an edge-sharing octahedral non-perovskite structure to a corner-sharing perovskite structure in CsSnI3, the activation energy for the FAPbI3 phase transition is relatively small, indicating a lower energy barrier when starting from a face-sharing octahedral structure in FAPbI3. The high concentration of Sn vacancies is probably responsible for the much higher phase propagation rate in CsSnI3 when compared to a CsPbBrxI3-x system with the same crystal structure but different halide vacancies. Our experimental results expand the knowledge of phase transition in halide perovskites and offer important guidance toward rationally designing more stable and efficient perovskite devices.

Determination of low energy barriers in metal-insulator-metal tunneling junctions

The nonequilibrium molecular dynamics simulations of transport and separation of a binary gas mixture through a porous membrane with interconnected pores of distributed sizes are reported. The membrane is modeled by a three-dimensional disordered molecular network of interconnected pores consisting of tens of thousands of atoms, based on a Voronoi tessellation of space. Results are presented for transport and adsorption of the gases, including the existence of an optimal pore structure for maximum separation of the gases.

Performance of cobalt silica membranes in gas mixture separation