Constructing Reversible Li Deposition Interfaces by Tailoring Lithiophilic Functionalities of a Heteroatom-Doped Graphene Interlayer for Highly Stable Li Metal Anodes.

Lithium (Li) metal has been regarded as the ideal anode for rechargeable batteries due to its low reduction potential and high theoretical capacity. However, the formation of fatal Li dendrites during repeated cycling shortens the battery life and causes serious safety concerns. Functionalized separators with electrically conductive and lithiophilic coating layers potentially inhibit dendrite formation and growth on Li metal anodes by providing nucleation sites for reversible Li deposition/stripping. In this work, we propose functionalized separators incorporating heteroatom-doped (N or B) graphene interlayers to modulate the Li nucleation behavior. The electronegative N-doping and electropositive B-doping were investigated to understand their regulation of the Li deposition behavior. With the heteroatom-doped graphene-coated separators, we observe significantly improved cycling stability along with enhanced charge transfer kinetics and low Li nucleation overpotential. This is attributed to the heteroatom-doped graphene interlayer expanding the surface area of the Li metal anode while providing additional space for uniform Li deposition/stripping, thus preventing undesirable side reactions. As a result, the formation of dendrites and pits on the Li metal anode surface is suppressed, demonstrating the protective effect of the Li metal anode. Interestingly, N-doped graphene-coated separators exhibit lower Li nucleation overpotentials than B-doped graphene-coated separators but rather lower average Coulombic efficiencies and reduced cycling stability. This implies that adequate adsorption on B-based sites, as opposed to the strong adsorption on N-based sites, improves the reversibility. Notably, the Li adsorption strength of the lithiophilic functional groups critically affects the reversibility, as observed by Li nucleation barrier measurements and atomistic simulations. This work suggests that interface engineering using conductive and lithiophilic materials can be a promising strategy for controlling Li deposition in advanced Li metal batteries.

Horizontally Asymmetric Nanochannels of Graphene Oxide Membranes for Efficient Osmotic Energy Harvesting.

Reverse electrodialysis (RED) directly harvests renewable energy from salinity gradients, and the achievable potential power heavily relies on the ion exchange membranes. Graphene oxides (GOs) are considered a solid candidate for the RED membrane because the laminated GO nanochannels with charged functional groups provide an excellent ionic selectivity and conductivity. Yet, a high internal resistance and poor stability in aqueous solutions limit the RED performance. Here, we develop a RED membrane that concurrently achieves high ion permeability and stable operation based on epoxy-confined GO nanochannels with asymmetric structures. The membrane is fabricated by reacting epoxy-wrapped GO membranes with ethylene diamine via vapor diffusion, overcoming the swelling properties in aqueous solutions. More importantly, the resultant membrane exhibits asymmetric GO nanochannels in terms of both channel geometry and electrostatic surface charges, leading to the rectified ion transport behavior. The demonstrated GO membrane exhibits the RED performance up to 5.32 W·m-2 with >40% energy conversion efficiency across a 50-fold salinity gradient and 20.3 W·m-2 across a 500-fold salinity gradient. Planck-Nernst continuum models coupled to molecular dynamics simulations rationalize the improved RED performance in terms of the asymmetric ionic concentration gradient within the GO nanochannel and the ionic resistance. The multiscale model also provides the design guidelines for ionic diode-type membranes configuring the optimum surface charge density and ionic diffusivity for efficient osmotic energy harvesting. The synthesized asymmetric nanochannels and their RED performance demonstrate the nanoscale tailoring of the membrane properties, establishing the potentials for 2D material-based asymmetric membranes.

Super Proton Conductivity Through Control of Hydrogen-Bonding Networks in Flexible Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) have received much attention as a solid-state electrolyte in proton exchange membrane fuel cells. The introduction of proton carriers and functional groups into MOFs can improve the proton conductivity attributed to the formation of hydrogen-bonding networks, while the underlying synergistic mechanism is still unclear. Here, a series of flexible MOFs (MIL-88B, [Fe3 O(OH)(H2 O)2 (O2 C-C6 H4 -CO2 )3 ] with imidazole) is designed to modify the hydrogen-bonding networks and investigate the resulting proton-conducting characteristics by controlling the breathing behaviors. The breathing behavior is tuned by varying the amount of adsorbed imidazole into pore (small breathing (SB) and large breathing (LB)) and introducing functional groups onto ligands (-NH2 , -SO3 H), resulting in four kinds of imidazole-loaded MOFs-Im@MIL-88B-SB, Im@MIL-88B-LB, Im@MIL-88B-NH2 , and Im@MIL-88B-SO3 H. Im@MIL-88B-LB without functional groups exhibits the highest proton conductivity of 8.93 × 10-2  S cm-1 at 60 °C and 95% relative humidity among imidazole-loaded proton conductors despite the mild condition, indicating that functional groups may not be always required to enhance proton conductivity. The elaborately controlled pore size and host-guest interaction in flexible MOFs through imidazole-dependent structural transformation are translated into the high proton concentration without the limitation of proton mobility, contributing to the formation of effective hydrogen-bonding networks in imidazole conducting media.

Facile Fabrication of Defect-Controlled Graphene Oxide Membrane through Shear-Induced Alignment for Regulating Ion Transport.

Two-dimensional (2D) membranes enable ion-sieving through well-defined subnanoscale channels. In particular, graphene oxide (GO), a representative 2D material with a flexible structure, can be manufactured into various types of membranes, while defects such as pores and wrinkles are readily formed through self-aggregation and self-folding during membrane fabrication. Such defects provide a path for small ionic or molecular species to be easily penetrated between the layers, which deteriorates membrane performance. Here, we demonstrate the effect of shear-induced alignment with continuous agitation on GO membrane structure during pressure-assisted filtration. The shear stress exerted on the GO layers during deposition is controlled by varying the agitation rate and solution viscosity. The well-stacked 2D membrane is obtained via the facile shear-controlled process, leading to an improved salt rejection performance without additional physicochemical modifications. This simple approach can be extensively utilized to prepare the well-ordered structure of other 2D materials in various fields where the defect control is required.

Heteroatom-Doped Graphenes as Actively Interacting 2D Encapsulation Media for Mg-Based Hydrogen Storage.

Nanoencapsulation using graphene derivatives enables the facile fabrication of two-dimensional (2D) nanocomposites with unique microstructures and has been generally applied to many fields of energy materials. Particularly, metal hydrides such as MgH2 encapsulated by graphene derivatives have emerged as a promising hybrid material for overcoming the disadvantageous properties of Mg-based hydrogen storage. Although the behavior of the graphene-Mg nanoencapsulation interface has been studied for many composite materials, the direct modification of graphene with nonmetal foreign elements for changing the interfacial behavior has been limitedly reported. In this regard, using B-doped graphene and N-doped graphene as nanoencapsulation media for tuning the interfacial behavior of graphene derivative-Mg nanoparticles, we present altered hydrogen storage kinetics of heteroatom-doped (B and N) graphene-Mg composites. The effect of heteroatom doping is studied in terms of bonding configurations and heteroatom doping concentrations. The enhancement in hydrogen uptake was observed for all of the heteroatom-doped graphene-Mg nanocomposites. On the other hand, a few samples exhibit significantly low activation energy at the early stage of desorption, which can be related to the facilitated nucleus formation. Density functional theory calculation indicates that B-doping and N-doping accelerate hydrogen absorption kinetics in different ways, aiding charge transfer and inducing surface deformation of Mg nanoparticles, respectively. Their effects can be augmented in the presence of structural defects on graphene, such as vacancies, pores, or graphene edges. These results demonstrate that hydrogen storage kinetics of Mg-based systems can be altered by utilizing heteroatom-doped graphene oxide derivatives as 2D nanoencapsulation media, suggesting that the addition of a nonmetal doping element can also be applied to Mg-based hydrogen storage by modifying the nanoencapsulation interface without forming Mg alloy phases.

Highly unstable heterogeneous representations in VIP interneurons of the anterior cingulate cortex.

A hallmark of the anterior cingulate cortex (ACC) is its functional heterogeneity. Functional and imaging studies revealed its importance in the encoding of anxiety-related and social stimuli, but it is unknown how microcircuits within the ACC encode these distinct stimuli. One type of inhibitory interneuron, which is positive for vasoactive intestinal peptide (VIP), is known to modulate the activity of pyramidal cells in local microcircuits, but it is unknown whether VIP cells in the ACC (VIPACC) are engaged by particular contexts or stimuli. Additionally, recent studies demonstrated that neuronal representations in other cortical areas can change over time at the level of the individual neuron. However, it is not known whether stimulus representations in the ACC remain stable over time. Using in vivo Ca2+ imaging and miniscopes in freely behaving mice to monitor neuronal activity with cellular resolution, we identified individual VIPACC that preferentially activated to distinct stimuli across diverse tasks. Importantly, although the population-level activity of the VIPACC remained stable across trials, the stimulus-selectivity of individual interneurons changed rapidly. These findings demonstrate marked functional heterogeneity and instability within interneuron populations in the ACC. This work contributes to our understanding of how the cortex encodes information across diverse contexts and provides insight into the complexity of neural processes involved in anxiety and social behavior.

Pentacyclic and hexacyclic cucurbitacins from Elaeocarpuspetiolatus.

Four undescribed cucurbitacins, designated as petiolaticins A-D, and four known cucurbitacins were isolated from the bark and leaves of Elaeocarpus petiolatus (Jack) Wall. Their chemical structures were elucidated based on detailed analyses of the NMR and MS data. The absolute configuration of petiolaticin A was also determined by X-ray diffraction analysis. Petiolaticin A represents a cucurbitacin derivative incorporating a 3,4-epoxyfuranyl-bearing side chain, while petiolaticin B possesses a furopyranyl unit fused to the tetracyclic cucurbitane core structure. Petiolaticins A, B, and D were evaluated in vitro against a panel of human breast, pancreatic, and colorectal cancer cell lines. Petiolaticin A exhibited the greatest cytotoxicity against the MDA-MB-468, MDA-MB-231, MCF-7, and SW48 cell lines (IC50 7.4, 9.2, 9.3, and 4.6 µM, respectively). Additionally, petiolaticin D, 16α,23α-epoxy-3ß,20ß-dihydroxy-10αH,23ßH-cucurbit-5,24-dien-11-one, and 16α,23α-epoxy-3ß,20ß-dihydroxy-10αH,23ßH-cucurbit-5,24-dien-11-one 3-O-ß-D-glucopyranoside were tested for their ability to inhibit cell entry of a pseudotyped virus bearing the hemagglutinin envelope protein of a highly pathogenic avian influenza virus. Petiolaticin D showed the highest inhibition (44.3%), followed by 16α,23α-epoxy-3ß,20ß-dihydroxy-10αH,23ßH-cucurbit-5,24-dien-11-one (21.0%), and 16α,23α-epoxy-3ß,20ß-dihydroxy-10αH,23ßH-cucurbit-5,24-dien-11-one 3-O-ß-D-glucopyranoside showed limited inhibition (9.0%). These preliminary biological assays have demonstrated that petiolaticins A and D possess anticancer and antiviral properties, respectively, which warrant for further investigations.

Uncovering the encapsulation effect of reduced graphene oxide sheets on the hydrogen storage properties of palladium nanocubes.

Decades of research on solute-induced phase transformation of metal hydride systems have shown the possibility to enhance hydrogen storage properties through novel material design such as nanoconfinement engineering. Nevertheless, the fundamentals of mechanical stress effect on confined Pd nanoparticles remain yet to be elucidated due to the difficulty in linking with hydrogen sorption thermodynamics. Here, a thermodynamic tuning of Pd nanocubes associated with hydrogen sorption as a result of encapsulation by reduced graphene oxide (rGO) layers is demonstrated. Pd nanocubes are constrained by rGO to such a degree that the chemical potential and the pressure hysteresis of the system during hydrogen sorption drastically change while showing a size dependence. A thorough thermodynamic analysis elucidates the role of constraints on hydrogen uptake and release; despite the nanoscale regime, the thermodynamic parameters (enthalpy and entropy) during phase transition considerably increase, a phenomenon not seen before in unconstrained Pd nanoparticle systems.

Rapid Access to Ordered Mesoporous Carbons for Chemical Hydrogen Storage.

Ordered mesoporous carbon materials offer robust network of organized pores for energy storage and catalysis applications, but suffer from time-consuming and intricate preparations hindering their widespread use. Here we report a new and rapid synthetic route for a N-doped ordered mesoporous carbon structure through a preferential heating of iron oxide nanoparticles by microwaves. A nanoporous covalent organic polymer is first formed in situ covering the hard templates of assembled nanoparticles, paving the way for a long-range order in a carbonaceous nanocomposite precursor. Upon removal of the template, a well-defined cubic mesoporous carbon structure was revealed. The ordered mesoporous carbon was used in solid state hydrogen storage as a host scaffold for NaAlH4 , where remarkable improvement in hydrogen desorption kinetics was observed. The state-of-the-art lowest activation energy of dehydrogenation as a single step was attributed to their ordered pore structure and N-doping effect.

Reversing the Irreversible: Thermodynamic Stabilization of LiAlH4 Nanoconfined Within a Nitrogen-Doped Carbon Host.

A general problem when designing functional nanomaterials for energy storage is the lack of control over the stability and reactivity of metastable phases. Using the high-capacity hydrogen storage candidate LiAlH4 as an exemplar, we demonstrate an alternative approach to the thermodynamic stabilization of metastable metal hydrides by coordination to nitrogen binding sites within the nanopores of N-doped CMK-3 carbon (NCMK-3). The resulting LiAlH4@NCMK-3 material releases H2 at temperatures as low as 126 °C with full decomposition below 240 °C, bypassing the usual Li3AlH6 intermediate observed in bulk. Moreover, >80% of LiAlH4 can be regenerated under 100 MPa H2, a feat previously thought to be impossible. Nitrogen sites are critical to these improvements, as no reversibility is observed with undoped CMK-3. Density functional theory predicts a drastically reduced Al-H bond dissociation energy and supports the observed change in the reaction pathway. The calculations also provide a rationale for the solid-state reversibility, which derives from the combined effects of nanoconfinement, Li adatom formation, and charge redistribution between the metal hydride and the host.

Computational Screening of Trillions of Metal-Organic Frameworks for High-Performance Methane Storage.

In the past decade, there has been an increasing number of computational screening works to facilitate finding optimal materials for a variety of different applications. Unfortunately, most of these screening studies are limited to their initial set of materials and result in a brute-force type of screening approach. In this work, we present a systematic strategy that can find metal-organic frameworks (MOFs) with the desired properties from an extremely diverse and large set of over 100 trillion possible MOFs using machine learning and evolutionary algorithm. It is demonstrated that our algorithm can discover 964 MOFs with methane working capacity over 200 cm3 cm-3 and 96 MOFs with methane working capacity over the current world record of 208 cm3 cm-3. We believe that this methodology can take advantage of the modular nature of MOFs and can readily be extended to other important applications as well.

Enhanced hydrogen storage kinetics and air stability of nanoconfined NaAlH4 in graphene oxide framework.

With a growing concern over climate change, hydrogen offers a wide range of opportunities for decarbonization and provides a flexibility in overall energy systems. While hydrogen energy is already plugged into industrial sectors, a physical hydrogen storage system poses a formidable challenge, giving momentum for safe and efficient solid-state hydrogen storage. Accommodating such demands, sodium alanate (NaAlH4) has been considered one of the candidate materials due to its high storage capacity. However, it requires a high temperature for hydrogen desorption and becomes inactive irreversibly upon air-exposure. To enhance sluggish reaction kinetics and reduce the hydrogen desorption temperature, NaAlH4 can be confined into a porous nanoscaffold; however, nanoconfined NaAlH4 with sufficient hydrogen storage performance and competent stability has not been demonstrated so far. In this work, we demonstrate a simultaneously enhanced hydrogen storage performance and air-stability for NaAlH4 particles confined in a nanoporous graphene oxide framework (GOF). The structure of the GOF was elaborately optimized as a nanoscaffold, and NaAlH4 was infiltrated into the pores of the GOF via incipient wetness impregnation. As a result of the nanoconfinement, both the onset temperature and activation energy for hydrogen desorption of NaAlH4 are significantly decreased without transition metal catalysts, while simultaneously achieving the stability under ambient conditions.

Nanostructured Metal Hydrides for Hydrogen Storage.

Knowledge and foundational understanding of phenomena associated with the behavior of materials at the nanoscale is one of the key scientific challenges toward a sustainable energy future. Size reduction from bulk to the nanoscale leads to a variety of exciting and anomalous phenomena due to enhanced surface-to-volume ratio, reduced transport length, and tunable nanointerfaces. Nanostructured metal hydrides are an important class of materials with significant potential for energy storage applications. Hydrogen storage in nanoscale metal hydrides has been recognized as a potentially transformative technology, and the field is now growing steadily due to the ability to tune the material properties more independently and drastically compared to those of their bulk counterparts. The numerous advantages of nanostructured metal hydrides compared to bulk include improved reversibility, altered heats of hydrogen absorption/desorption, nanointerfacial reaction pathways with faster rates, and new surface states capable of activating chemical bonds. This review aims to summarize the progress to date in the area of nanostructured metal hydrides and intends to understand and explain the underpinnings of the innovative concepts and strategies developed over the past decade to tune the thermodynamics and kinetics of hydrogen storage reactions. These recent achievements have the potential to propel further the prospects of tuning the hydride properties at nanoscale, with several promising directions and strategies that could lead to the next generation of solid-state materials for hydrogen storage applications.

Atomically Thin Interfacial Suboxide Key to Hydrogen Storage Performance Enhancements of Magnesium Nanoparticles Encapsulated in Reduced Graphene Oxide.

As a model system for hydrogen storage, magnesium hydride exhibits high hydrogen storage density, yet its practical usage is hindered by necessarily high temperatures and slow kinetics for hydrogenation-dehydrogenation cycling. Decreasing particle size has been proposed to simultaneously improve the kinetics and decrease the sorption enthalpies. However, the associated increase in surface reactivity due to increased active surface area makes the material more susceptible to surface oxidation or other side reactions, which would hinder the overall hydrogenation-dehydrogenation process and diminish the capacity. Previous work has shown that the chemical stability of Mg nanoparticles can be greatly enhanced by using reduced graphene oxide as a protecting agent. Although no bulklike crystalline MgO layer has been clearly identified in this graphene-encapsulated/Mg nanocomposite, we propose that an atomically thin layer of honeycomb suboxide exists, based on first-principles interpretation of Mg K-edge X-ray absorption spectra. Density functional theory calculations reveal that in contrast to conventional expectations for thick oxides this interfacial oxidation layer permits H2 dissociation to the same degree as pristine Mg metal with the added benefit of enhancing the binding between reduced graphene oxide and the Mg nanoparticle, contributing to improved mechanical and chemical stability of the functioning nanocomposite.

Tailoring Polymer Conformation for Nanocrystal Growth: The Role of Chain Length and Solvent.

The role of polymer dynamics on nanocrystal growth in the synthesis of composites is studied using a well-controlled Mg nanocrystal-poly(methyl methacrylate) (PMMA) system. The in situ formation of Mg nanocrystals in PMMA is examined under different conditions, varying polymer conformation in solution. The change of hydrogen sorption properties of Mg is investigated to assess functional impacts of morphological changes.

Transparent Metal-Organic Framework/Polymer Mixed Matrix Membranes as Water Vapor Barriers.

Preventing the permeation of reactive molecules into electronic devices or photovoltaic modules is of great importance to ensure their life span and reliability. This work is focused on the formation of highly functioning barrier films based on nanocrystals (NCs) of a water-scavenging metal-organic framework (MOF) and a hydrophobic cyclic olefin copolymer (COC) to overcome the current limitations. Water vapor transmission rates (WVTR) of the films reveal a 10-fold enhancement in the WVTR compared to the substrate while maintaining outstanding transparency over most of the visible and solar spectrum, a necessary condition for integration with optoelectronic devices.

Graphene oxide/metal nanocrystal multilaminates as the atomic limit for safe and selective hydrogen storage.

Interest in hydrogen fuel is growing for automotive applications; however, safe, dense, solid-state hydrogen storage remains a formidable scientific challenge. Metal hydrides offer ample storage capacity and do not require cryogens or exceedingly high pressures for operation. However, hydrides have largely been abandoned because of oxidative instability and sluggish kinetics. We report a new, environmentally stable hydrogen storage material constructed of Mg nanocrystals encapsulated by atomically thin and gas-selective reduced graphene oxide (rGO) sheets. This material, protected from oxygen and moisture by the rGO layers, exhibits exceptionally dense hydrogen storage (6.5 wt% and 0.105 kg H2 per litre in the total composite). As rGO is atomically thin, this approach minimizes inactive mass in the composite, while also providing a kinetic enhancement to hydrogen sorption performance. These multilaminates of rGO-Mg are able to deliver exceptionally dense hydrogen storage and provide a material platform for harnessing the attributes of sensitive nanomaterials in demanding environments.

Engineering Synergy: Energy and Mass Transport in Hybrid Nanomaterials.

An emerging class of materials that are hybrid in nature is propelling a technological revolution in energy, touching many fundamental aspects of energy-generation, storage, and conservation. Hybrid materials combine classical inorganic and organic components to yield materials that manifest new functionalities unattainable in traditional composites or other related multicomponent materials, which have additive function only. This Research News article highlights the exciting materials design innovations that hybrid materials enable, with an eye toward energy-relevant applications involving charge, heat, and mass transport.

Enhanced Water Vapor Blocking in Transparent Hybrid Polymer-Nanocrystal Films.

Highly transparent and effective encapsulating materials have become increasingly important for photovoltaic (PV) modules to prevent water vapor molecules from permeating PV cells. The composite consists of block copolymer (PS-b-P2VP), comprised of hydrophobic and hydrophilic parts, and hygroscopic nanocrystals (Magnesium Oxide, MgO) incorporated to enhance water vapor blocking by both presenting obstacles for mass transport and also scavenging water molecules. The water vapor transmission rate (WVTR) values were reduced ∼3000 times, compared to homopolymer (PS), for both polymer and composite samples. Achieving both high transparency and low WVTR, it is expected that the composite materials can function as an excellent water vapor blocking layer for PV modules.