Boosted Oxygen Kinetics of Hierarchically Mesoporous Mo2C/C for High-current-density Zn-air Battery.

The high-current-density Zn-air battery shows big prospects in next-generation energy technologies, while sluggish O2 reaction and diffusion kinetics barricade the applications. Herein, the sequential assembly is innovatively demonstrated for hierarchically mesoporous molybdenum carbides/carbon microspheres with a tunable thickness of mesoporous carbon layers (Meso-Mo2C/C-x, where x represents the thickness). The optimum Meso-Mo2C/C-14 composites (≈2 µm in diameter) are composed of mesoporous nanosheets (≈38 nm in thickness), which possess bilateral mesoporous carbon layers (≈14 nm in thickness), inner Mo2C/C layers (≈8 nm in thickness) with orthorhombic Mo2C nanoparticles (≈2 nm in diameter), a high surface area of ≈426 m2 g-1, and open mesopores (≈6.9 nm in size). Experiments and calculations corroborate the hierarchically mesoporous Mo2C/C can enhance hydrophilicity for supplying sufficient O2, accelerate oxygen reduction kinetics by highly-active Mo2C and N-doped carbon sites, and facilitate O2 diffusion kinetics over hierarchically mesopores. Therefore, Meso-Mo2C/C-14 outputs a high half-wave potential (0.88 V vs RHE) with a low Tafel slope (51 mV dec-1) for oxygen reduction. More significantly, the Zn-air battery delivers an ultrahigh power density (272 mW cm-2), and an unprecedented 100 h stability at a high-current-density condition (100 mA cm-2), which is one of the best performances.

Implanting Colloidal Nanoparticles into Single-Crystalline Zeolites for Catalytic Dehydration.

The encapsulation of functional colloidal nanoparticles (100 nm) into single-crystalline ZSM-5 zeolites, aiming to create uniform core-shell structures, is a highly sought-after yet formidable objective due to significant lattice mismatch and distinct crystallization properties. In this study, we demonstrate the fabrication of a core-shell structured single-crystal zeolite encompassing an Fe3O4 colloidal core via a novel confinement stepwise crystallization methodology. By engineering a confined nanocavity, anchoring nucleation sites, and executing stepwise crystallization, we have successfully encapsulated colloidal nanoparticles (CN) within single-crystal zeolites. These grafted sites, alongside the controlled crystallization process, compel the zeolite seed to nucleate and expand along the Fe3O4 colloidal nanoparticle surface, within a meticulously defined volume (1.5×107≤V≤1.3×108 nm3). Our strategy exhibits versatility and adaptability to an array of zeolites, including but not restricted to ZSM-5, NaA, ZSM-11, and TS-1 with polycrystalline zeolite shell. We highlight the uniformly structured magnetic-nucleus single-crystalline zeolite, which displays pronounced superparamagnetism (14 emu/g) and robust acidity (~0.83 mmol/g). This innovative material has been effectively utilized in a magnetically stabilized bed (MSB) reactor for the dehydration of ethanol, delivering an exceptional conversion rate (98 %), supreme ethylene selectivity (98 %), and superior catalytic endurance (in excess of 100 hours).

Solvent Effect on the Nanotextural Formation of Reduced Graphene Oxide Membranes.

Solvent is involved in many wet-chemical synthesis and bottom-up assembly processes. Understanding its influence on the nanotextural formation of the resultant assemblies is essential for the design and control of the properties for targeted applications. With wet chemically reduced graphene oxide (rGO) membranes as a materials platform, this study investigates the solvent effect on nanotexture formation in 2D nanomaterial-based membranes through light scattering and electrochemical characterization. Our finding indicates that the nanotexture of the resultant rGO membrane is largely correlated to the dielectric constant of the solvent. Specifically, solvents with higher dielectric constants yield rGO membranes with more wrinkled, loosely stacked, and less graphitized structures. In contrast, solvents with a lower dielectric constant tend to yield densely stacked structures with larger graphitized domains. Our finding underscores the important role of solvents in wet processing and nanoengineering of 2D nanomaterial-based membranes and provides valuable insights for their controlled synthesis and application.

Self-Assembly of Ir-Based Nanosheets with Ordered Interlayer Space for Enhanced Electrocatalytic Water Oxidation.

Iridium (Ir)-based electrocatalysts are widely explored as benchmarks for acidic oxygen evolution reactions (OERs). However, further enhancing their catalytic activity remains challenging due to the difficulty in identifying active species and unfavorable architectures. In this work, we synthesized ultrathin Ir-IrOx/C nanosheets with ordered interlayer space for enhanced OER by a nanoconfined self-assembly strategy, employing block copolymer formed stable end-merged lamellar micelles. The interlayer distance of the prepared Ir-IrOx/C nanosheets was well controlled at ∼20 nm and Ir-IrOx nanoparticles (∼2 nm) were uniformly distributed within the nanosheets. Importantly, the fabricated Ir-IrOx/C electrocatalysts display one of the lowest overpotential (η) of 198 mV at 10 mA cm-2geo during OER in an acid medium, benefiting from their features of mixed-valence states, rich electrophilic oxygen species (O(II-δ)-), and favorable mesostructured architectures. Both experimental and computational results reveal that the mixed valence and O(II-δ)- moieties of the 2D mesoporous Ir-IrOx/C catalysts with a shortened Ir-O(II-δ)- bond (1.91 Å) is the key active species for the enhancement of OER by balancing the adsorption free energy of oxygen-containing intermediates. This strategy thus opens an avenue for designing high performance 2D ordered mesoporous electrocatalysts through a nanoconfined self-assembly strategy for water oxidation and beyond.

Constructing Unique Mesoporous Carbon Superstructures via Monomicelle Interface Confined Assembly.

Constructing hierarchical three-dimensional (3D) mesostructures with unique pore structure, controllable morphology, highly accessible surface area, and appealing functionality remains a great challenge in materials science. Here, we report a monomicelle interface confined assembly approach to fabricate an unprecedented type of 3D mesoporous N-doped carbon superstructure for the first time. In this hierarchical structure, a large hollow locates in the center (∼300 nm in diameter), and an ultrathin monolayer of spherical mesopores (∼22 nm) uniformly distributes on the hollow shells. Meanwhile, a small hole (4.0-4.5 nm) is also created on the interior surface of each small spherical mesopore, enabling the superstructure to be totally interconnected. Vitally, such interconnected porous supraparticles exhibit ultrahigh accessible surface area (685 m2 g-1) and good underwater aerophilicity due to the abundant spherical mesopores. Additionally, the number (70-150) of spherical mesopores, particle size (22 and 42 nm), and shell thickness (4.0-26 nm) of the supraparticles can all be accurately manipulated. Besides this spherical morphology, other configurations involving 3D hollow nanovesicles and 2D nanosheets were also obtained. Finally, we manifest the mesoporous carbon superstructure as an advanced electrocatalytic material with a half-wave potential of 0.82 V (vs RHE), equivalent to the value of the commercial Pt/C electrode, and notable durability for oxygen reduction reaction (ORR).

Precisely Designed Mesoscopic Titania for High-Volumetric-Density Pseudocapacitance.

Surface redox pseudocapacitance, which enables short charging times and high power delivery, is very attractive in a wide range of sites. To achieve maximized specific capacity, nanostructuring of active materials with high surface area is indispensable. However, one key limitation for capacitive materials is their low volumetric capacity due to the low tap density of nanomaterials. Here, we present a promising mesoscale TiO2 structure with precisely controlled mesoporous frameworks as a high-density pseudocapacitive model system. The dense-packed mesoscopic TiO2 in micrometer size offers a high accessible surface area (124 m2 g-1) and radially aligned mesopore channels, but high tap density (1.7 g cm-3) that is much higher than TiO2 nanoparticles (0.47 g cm-3). As a pseudocapacitive sodium-ion storage anode, the precisely designed mesoscopic TiO2 model achieved maximized gravimetric capacity (240 mAh g-1) and volumetric capacity (350 mAh cm-3) at 0.025 A g-1. Such a designed pseudocapacitive mesostructure further realized a commercially comparable areal capacity (2.1 mAh cm-2) at a high mass loading of 9.47 mg cm-2. This mesostructured electrode that enables fast sodiation in dense nanostructures has implications for high-power applications, fast-charging devices, and pseudocapacitive electrode design.

Space-Confined Atomic Clusters Catalyze Superassembly of Silicon Nanodots within Carbon Frameworks for Use in Lithium-Ion Batteries.

Incorporating nanoscale Si into a carbon matrix with high dispersity is desirable for the preparation of lithium-ion batteries (LIBs) but remains challenging. A space-confined catalytic strategy is proposed for direct superassembly of Si nanodots within a carbon (Si NDs⊂C) framework by copyrolysis of triphenyltin hydride (TPT) and diphenylsilane (DPS), where Sn atomic clusters created from TPT pyrolysis serve as the catalyst for DPS pyrolysis and Si catalytic growth. The use of Sn atomic cluster catalysts alters the reaction pathway to avoid SiC generation and enable formation of Si NDs with reduced dimensions. A typical Si NDs⊂C framework demonstrates a remarkable comprehensive performance comparable to other Si-based high-performance half LIBs, and higher energy densities compared to commercial full LIBs, as a consequence of the high dispersity of Si NDs with low lithiation stress. Supported by mechanic simulations, this study paves the way for construction of Si/C composites suitable for applications in future energy technologies.

Covalent Assembly of MoS2 Nanosheets with SnS Nanodots as Linkages for Lithium/Sodium-Ion Batteries.

Weak van der Waals interactions between interlayers of two-dimensional layered materials result in disabled across-interlayer electron transfer and poor layered structural stability, seriously deteriorating their performance in energy applications. Herein, we propose a novel covalent assembly strategy for MoS2 nanosheets to realize unique MoS2 /SnS hollow superassemblies (HSs) by using SnS nanodots as covalent linkages. The covalent assembly based on all-inorganic and carbon-free concept enables effective across-interlayer electron transfer, facilitated ion diffusion kinetics, and outstanding mechanical stability, which are evidenced by experimental characterization, DFT calculations, and mechanical simulations. Consequently, the MoS2 /SnS HSs exhibit superb rate performance and long cycling stability in lithium-ion batteries, representing the best comprehensive performance in carbon-free MoS2 -based anodes to date. Moreover, the MoS2 /SnS HSs also show excellent sodium storage performance in sodium-ion batteries.

Direct Superassemblies of Freestanding Metal-Carbon Frameworks Featuring Reversible Crystalline-Phase Transformation for Electrochemical Sodium Storage.

High-power sodium-ion batteries (SIBs) with long-term cycling attract increasing attention for large-scale energy storage. However, traditional SIBs toward practical applications still suffer from low rate capability and poor cycle induced by pulverization and amorphorization of anodes at high rate (over 5 C) during the fast ion insertion/extraction process. The present work demonstrates a robust strategy for a variety of (Sb-C, Bi-C, Sn-C, Ge-C, Sb-Bi-C) freestanding metal-carbon framework thin films via a space-confined superassembly (SCSA) strategy. The sodium-ion battery employing the Sb-C framework exhibits an unprecedented performance with a high specific capacity of 246 mAh g-1, long life cycle (5000 cycles), and superb capacity retention (almost 100%) at a high rate of 7.5 C (3.51A g-1). Further investigation indicates that the unique framework structure enables unusual reversible crystalline-phase transformation, guaranteeing the fast and long-cyclability sodium storage. This study may open an avenue to developing long-cycle-life and high-power SIBs for practical energy applications.

Subnanometric Stacking of Two-Dimensional Nanomaterials: Insights from the Nanotexture Evolution of Dense Reduced Graphene Oxide Membranes.

Assembling two-dimensional (2D) nanomaterials into laminar membranes with a subnanometer (subnm) interlayer spacing provides a material platform for studying a range of nanoconfinement effects and exploring the technological applications related to the transport of electrons, ions and molecules. However, the strong tendency for 2D nanomaterials to restack to their bulk crystalline-like structure makes it challenging to control their spacing at the subnm scale. It is thus necessary to understand what nanotextures can be formed at the subnm scale and how they can be engineered experimentally. In this work, with dense reduced graphene oxide membranes as a model system, we combine synchrotron-based X-ray scattering and ionic electrosorption analysis to reveal that their subnanometric stacking can result in a hybrid nanostructure of subnm channels and graphitized clusters. We demonstrate that the ratio of these two structural units, their sizes and connectivity can be engineered by stacking kinetics through the reduction temperature to allow the realization of high-performance compact capacitive energy storage. This work highlights the great complexity of subnm stacking of 2D nanomaterials and provides potential methods to engineer their nanotextures at will.

Mechanistic Principles for Engineering Hierarchical Porous Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) have generated tremendous research interest in the past two decades, due to their high surface areas, tailorable active sites, and tunable structures. Hierarchical porous MOFs (HP-MOFs) with two or more pore systems are particularly attractive, benefiting from improved active site accessibility and enhanced mass diffusivity in applications involving bulk molecules. This review outlines the mechanistic principles used for the rational design of HP-MOFs, current techniques used to measure their hierarchical porosities, as well as their emerging applications. We then critically summarize the current challenges in this field and provide a contemporary perspective on the technological innovations that would address current synthetic challenges in the field of HP-MOFs. The aim of this review is to provide an in-depth understanding of the formation mechanisms, materials chemistry, and structural and chemical properties of HP-MOFs while exploring ways to enhance the performance of current MOF materials in a range of fields.

Zero-Strain High-Capacity Silicon/Carbon Anode Enabled by a MOF-Derived Space-Confined Single-Atom Catalytic Strategy for Lithium-Ion Batteries.

Developing zero-strain electrode materials with high capacity is crucial for lithium-ion batteries (LIBs). Here, a new zero-strain composite material made of ultrasmall Si nanodots (NDs) within metal organic framework-derived nanoreactors (Si NDs⊂MDN) through a novel space-confined catalytic strategy is reported. The unique Si NDs⊂MDN anode features a low strain (<3%) and a high theoretical lithium storage capacity (1524 mAh g-1 ) which far surpasses the traditional single-crystal counterparts that suffer from a low capacity delivery. The zero-strain property is evidenced by substantial characterizations including ex/in situ transmission electron microscopy and mechanical simulations. The Si NDs⊂MDN exhibits superior cycling stability and high reversible capacity (1327 mAh g-1 at 0.1 A g-1 after 100 cycles) in half-cells and high energy density (366 Wh kg-1 after 300 cycles) in a full cell. This study reports a new catalog of zero-strain electrode material with significantly improved capacity beyond the traditional single-crystal zero-strain materials.

CoPSe: A New Ternary Anode Material for Stable and High-Rate Sodium/Potassium-Ion Batteries.

The exploration of ideal electrode materials overcoming the critical problems of large electrode volume changes and sluggish redox kinetics induced by large ionic radius of Na+ /K+ ions is highly desirable for sodium/potassium-ion batteries (SIBs/PIBs) toward large-scale applications. The present work demonstrates that single-phase ternary cobalt phosphoselenide (CoPSe) in the form of nanoparticles embedded in a layered metal-organic framework (MOF)-derived N-doped carbon matrix (CoPSe/NC) represents an ultrastable and high-rate anode material for SIBs/PIBs. The CoPSe/NC is fabricated by using the MOF as both a template and precursor, coupled with in situ synchronous phosphorization/selenization reactions. The CoPSe anode holds a set of intrinsic merits such as lower mechanical stress, enhanced reaction kinetics, as well as higher theoretical capacity and lower discharge voltage relative to its counterpart of CoSe2 , and suppressed shuttle effect with higher intrinsic electrical conductivity relative to CoPS. The involved mechanisms are evidenced by substantial characterizations and density functional theory (DFT) calculations. Consequently, the CoPSe/NC anode shows an outstanding long-cycle stability and rate performance for SIBs and PIBs. Moreover, the CoPSe/NC-based Na-ion full cell can achieve a higher energy density of 274 Wh kg-1 , surpassing that based on CoSe2 /NC and most state-of-the-art Na-ion full cells based on P-, Se-, or S-containing binary/ternary anodes to date.

Ultrapermeable Composite Membranes Enhanced Via Doping with Amorphous MOF Nanosheets.

Thin-film composite (TFC) polymeric membranes have attracted increasing interest to meet the demands of industrial gas separation. However, the development of high-performance TFC membranes within their current configuration faces two key challenges: (i) the thickness-dependent gas permeability of polymeric materials (mainly poly(dimethylsiloxane) (PDMS)) and (ii) the geometric restriction effect due to the limited pore accessibility of the underlying porous substrate. Here we demonstrate that the incorporation of trace amounts (∼1.8 wt %) of amorphous metal-organic framework (MOF) nanosheets into the gutter layer of TFC assemblies can simultaneously address these two limitations by the creation of rapid, transmembrane gas diffusion pathways. The resultant PDMS&MOF membrane displayed excellent CO2 permeance of 10450 GPU and CO2/N2 selectivity of 9.1. Leveraging this strategy, we successfully fabricate a novel TFC membrane, consisting of a PDMS&MOF gutter and an ultrathin (∼54 nm) poly(ethylene glycol) top selective layer via surface-initiated atom transfer radical polymerization. The complete TFC membrane exhibits excellent processability and remarkable CO2/N2 separation performance (1990 GPU with a CO2/N2 ideal selectivity of 39). This study reveals a strategy for the design and fabrication of a new TFC membrane system with unprecedented gas-separation performance.

Ti3C2Tx MXene Nanosheets as a Robust and Conductive Tight on Si Anodes Significantly Enhance Electrochemical Lithium Storage Performance.

Exploring Si-based anode materials with high electrical conductivity and electrode stability is crucial for high-performance lithium-ion batteries (LIBs). Herein, we propose the fabrication of a Si-based composite where Si porous nanospheres (Si p-NSs) are tightly wrapped by Ti3C2Tx (Tx stands for the surface groups such as -OH, -F) MXene nanosheets (TNSs) through an interfacial assembly strategy. The TNSs as a conductive and robust tight of the Si p-NSs can effectively improve electron transport and electrode stability, as revealed by substantial characterizations and mechanical simulations. Moreover, the TNSs with rich surface groups enable strong interfacial interactions with the Si p-NS component and a pseudocapacitive behavior, beneficial for fast and stable lithium storage. Consequently, the Si p-NS@TNSs electrode with a high Si content of 85.6% exhibits significantly enhanced battery performance compared with the Si p-NSs electrode such as high reversible capacity (1154 mAh g-1 at 0.2 A g-1), long cycling stability (up to 2000 cycles with a 0.026% capacity decay rate per cycle), and excellent rate performances. Notably, the Si p-NS@TNSs electrode-based LIB full cell delivers a high energy uptake of 405 Wh kg-1, many-times higher than that of the Si p-NSs full cell. This work offers a strategy to develop advanced Si-based anode materials with desirable properties for high-performance LIBs.

Three-Dimensional Hierarchical Porous Nanotubes Derived from Metal-Organic Frameworks for Highly Efficient Overall Water Splitting.

Effective design of bifunctional catalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is important but remains challenging. Herein, we report a three-dimensional (3D) hierarchical structure composed of homogeneously distributed Ni-Fe-P nanoparticles embedded in N-doped carbons on nickel foams (denoted as Ni-Fe-P@NC/NF) as an excellent bifunctional catalyst. This catalyst was fabricated by an anion exchange method and a low-temperature phosphidation of nanotubular Prussian blue analogue (PBA). The Ni-Fe-P@NC/NF displayed exceptional catalytic activity toward both HER and OER and delivered an ultralow cell voltage of 1.47 V to obtain 10 mA cm-2 with extremely excellent durability for 100 h when assembled as a practical electrolyser. The extraordinary performance of Ni-Fe-P@NC/NF is attributed to the abundance of unsaturated active sites, the well-defined hierarchical porous structure, and the synergistic effect between multiple components. Our work will inspire more rational designs of highly active non-noble electrocatalysts for industrial energy applications.

A polymer-direct-intercalation strategy for MoS2/carbon-derived heteroaerogels with ultrahigh pseudocapacitance.

The intercalation strategy has become crucial for 2D layered materials to achieve desirable properties, however, the intercalated guests are often limited to metal ions or small molecules. Here, we develop a simple, mild and efficient polymer-direct-intercalation strategy that different polymers (polyethyleneimine and polyethylene glycol) can directly intercalate into the MoS2 interlayers, forming MoS2-polymer composites and interlayer-expanded MoS2/carbon heteroaerogels after carbonization. The polymer-direct-intercalation behavior has been investigated by substantial characterizations and molecular dynamic calculations. The resulting composite heteroaerogels possess 3D conductive MoS2/C frameworks, expanded MoS2 interlayers (0.98 nm), high MoS2 contents (up to 74%) and high Mo valence (+6), beneficial to fast and stable charge transport and enhanced pseudocapacitive energy storage. Consequently, the typical MoS2/N-doped carbon heteroaerogels exhibit outstanding supercapacitor performance, such as ultrahigh capacitance, remarkable rate capability and excellent cycling stability. This study offers a new intercalation strategy which may be generally applicable to 2D materials for promising energy applications.

Hybrid Anatase/Rutile Nanodots-Embedded Covalent Organic Frameworks with Complementary Polysulfide Adsorption for High-Performance Lithium-Sulfur Batteries.

The shuttling effect of polysulfides species seriously deteriorates the performance of Li-S batteries, representing the major obstacle for their practical use. However, the exploration of ideal cathodes that can suppress the shuttling of all polysulfides species is challenging. Herein, we propose an ingenious and effective strategy for constructing hybrid-crystal-phase TiO2/covalent organic framework (HCPT@COF) composites where hybrid anatase/rutile TiO2 nanodots (10 nm) are uniformly embedded in the interlayers of porous COFs. The synthesis was realized via a multiple-step reaction relay accompanying by a pseudo-topotactic transformation of three-dimensional layered structures from 1,4-dicyanobenzene monomer-embedded Ti-intermediate networks to HCPT nanodots-embedded COF frameworks. The HCPT@COF/S cathodes show superior comprehensive performance such as high specific capacity, long cycling stability, and remarkable rate capability for Li-S batteries, owing to the complementary anchoring effect of hybrid anatase/rutile TiO2 in the HCPT@COF composite, which is evidenced by substantial characterizations including X-ray photoelectron spectroscopy and density functional theory calculations.

Mesoporous TiO2/TiC@C Composite Membranes with Stable TiO2-C Interface for Robust Lithium Storage.

Transition metal oxides/carbon (TMOs/C) composites are important for high-performance lithium-ion batteries (LIBs), but the development of interface-stable TMOs/C composite anodes for robust lithium storage is still a challenge. Herein, mesoporous TiO2/TiC@C composite membranes were synthesized by an in situ carbothermic reduction method. TiC nanodots with high conductivity and electrochemical inactivity at the TiO2-C interface can significantly enhance the electrical conductivity and structural stability of the membranes. Finite element simulations demonstrate that the TiO2/TiC@C membranes can effectively alleviate tensile and compression stress effects upon lithiation, which is beneficial for robust lithium storage. When used as additives and binder-free electrodes, the TiO2/TiC@C membranes show excellent cycling capability and rate performance. Moreover, a flexible full battery can be assembled by employing the TiO2/TiC@C membranes and shows good performance, highlighting the potential of these membranes in flexible electronics. This work opens an avenue to constructing interface-stable composite structures for the next-generation high-performance LIBs.

A titanium-based photo-Fenton bifunctional catalyst of mp-MXene/TiO2-x nanodots for dramatic enhancement of catalytic efficiency in advanced oxidation processes.

mp-MXene/TiO2-x nanodots (NDs) structurally composed of microporous MXene monolayers embedded with Ti3+-doped TiO2 nanodots were developed for the first time. The drastically enhanced catalytic efficiency (as much as 13 times higher than that of P25) in degrading dye molecules over mp-MXene/TiO2-x NDs is due to a synergistic effect of the pseudo-Fenton reaction and photocatalysis.