Metal–organic frameworks (MOFs), also known as porous coordination polymers (PCPs), synthesized by assembling metal ions with organic ligands have recently emerged as a new class of crystalline porous materials. The amenability to design as well as fine-tunable and uniform pore structures makes them promising materials for a variety of applications. Controllable integration of MOFs and functional materials is leading to the creation of new multifunctional composites/hybrids, which exhibit new properties that are superior to those of the individual components through the collective behavior of the functional units. This is a rapidly developing interdisciplinary research area. This review provides an overview of the significant advances in the development of diverse MOF composites reported till now with special emphases on the synergistic effects and applications of the composites. The most widely used and successful strategies for composite synthesis are also presented.

Metal–organic framework composites

Owing to high specific energy, low cost, and environmental friendliness, lithium–sulfur (Li–S) batteries hold great promise to meet the increasing demand for advanced energy storage beyond portable electronics, and to mitigate environmental problems. However, the application of Li–S batteries is challenged by several obstacles, including their short life and low sulfur utilization, which become more serious when sulfur loading is increased to the practically accepted level above 3–5 mg cm−2. More and more efforts have been made recently to overcome the barriers toward commercially viable Li–S batteries with a high sulfur loading. This review highlights the recent progress in high-sulfur-loading Li–S batteries enabled by hierarchical design principles at multiscale. Particularly, basic insights into the interfacial reactions, strategies for mesoscale assembly, unique architectures, and configurational innovation in the cathode, anode, and separator are under specific concerns. Hierarchy in the multiscale design is proposed to guide the future development of high-sulfur-loading Li–S batteries.

Review on High-Loading and High-Energy Lithium–Sulfur Batteries

Liquid electrolyte lithium/sulfur battery: Fundamental chemistry, problems, and solutions

Transition metal carbides and nitrides (MXenes), a family of two-dimensional (2D) inorganic compounds, are materials composed of a few atomic layers of transition metal carbides, nitrides, or carbonitrides. Ti3C2, the first 2D layered MXene, was isolated in 2011. This material, which is a layered bulk material analogous to graphite, was derived from its 3D phase, Ti3AlC2 MAX. Since then, material scientists have either determined or predicted the stable phases of >200 different MXenes based on combinations of various transition metals such as Ti, Mo, V, Cr, and their alloys with C and N. Extensive experimental and theoretical studies have shown their exciting potential for energy conversion and electrochemical storage. To this end, we comprehensively summarize the current advances in MXene research. We begin by reviewing the structure types and morphologies and their fabrication routes. The review then discusses the mechanical, electrical, optical, and electrochemical properties of MXenes. The focus then turns to their exciting potential in energy storage and conversion. Energy storage applications include electrodes in rechargeable lithium- and sodium-ion batteries, lithium-sulfur batteries, and supercapacitors. In terms of energy conversion, photocatalytic fuel production, such as hydrogen evolution from water splitting, and carbon dioxide reduction are presented. The potential of MXenes for the photocatalytic degradation of organic pollutants in water, such as dye waste, is also addressed, along with their promise as catalysts for ammonium synthesis from nitrogen. Finally, their application potential is summarized.

Applications of 2D MXenes in energy conversion and storage systems

Over the last decade, significant effort has been devoted to the applications of hierarchically structured porous materials owing to their outstanding properties such as high surface area, excellent accessibility to active sites, and enhanced mass transport and diffusion. The hierarchy of porosity, structural, morphological and component levels in these materials is key for their high performance in all kinds of applications. The introduction of hierarchical porosity into materials has led to a significant improvement in the performance of materials. Herein, recent progress in the applications of hierarchically structured porous materials from energy conversion and storage, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine is reviewed. Their potential future applications are also highlighted. We particularly dwell on the relationship between hierarchically porous structures and properties, with examples of each type of hierarchically structured porous material according to its chemical composition and physical characteristics. The present review aims to open up a new avenue to guide the readers to quickly obtain in-depth knowledge of applications of hierarchically porous materials and to have a good idea about selecting and designing suitable hierarchically porous materials for a specific application. In addition to focusing on the applications of hierarchically porous materials, this comprehensive review could stimulate researchers to synthesize new advanced hierarchically porous solids.

Applications of hierarchically structured porous materials from energy storage and conversion, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine

Facile Synthesis of Crumpled Nitrogen-Doped MXene Nanosheets as a New Sulfur Host for Lithium–Sulfur Batteries

Porous Cryo-Dried MXene for Efficient Capacitive Deionization

Lithium-sulfur batteries have attracted increasing attention as one of the most promising candidates for next-generation energy storage systems. However, the poor cycling performance and the low utilization of sulfur greatly hinder its practical applications. Here we report the improved performance of lithium-sulfur batteries by coating Ti3C2Tx MXene nanosheets (where T stands for the surface termination, such as -O, -OH, and/or -F) on commercial "Celgard" membrane. In favor of the ultrathin two-dimensional structure, the Ti3C2Tx MXene can form a uniform coating layer with a minimum mass loading of 0.1 mg cm-2 and a thickness of only 522 nm. Owing to the improved electric conductivity and the effective trapping of polysulfides, the lithium-sulfur batteries with MXene-functionalized separators exhibit superior performance including high specific capacities and cycling stability.

Immobilizing Polysulfides with MXene-Functionalized Separators for Stable Lithium-Sulfur Batteries.

3D metal carbide@mesoporous carbon hybrid architecture (Ti3C2Tx@Meso-C, TX ≈ FxOy) is synthesised and applied as cathode material hosts for lithium-sulfur batteries Exfoliated-metal carbide (Ti3C2Tx) nanosheets have high electronic conductivity and contain rich functional groups for effective trapping of polysulfides Mesoporous carbon with a robust porous structure provides sufficient spaces for loading sulfur and effectively cushion the volumetric expansion of sulfur cathodes Theoretical calculations have confirmed that metal carbide can absorb sulfur and polysulfides, therefore extending the cycling performance The Ti3C2Tx@Meso-C/S cathodes have achieved a high capacity of 12258 mAh g−1 and more than 300 cycles at the C/2 current rate The Ti3C2Tx@Meso-C hybrid architecture is a promising cathode host material for lithium-sulfur batteries

3D Metal Carbide@Mesoporous Carbon Hybrid Architecture as a New Polysulfide Reservoir for Lithium-Sulfur Batteries

Sodium-sulfur batteries using abundant elements offer an attractive alternative to currently used batteries, but they need better sulfur host materials to compete with lithium-ion batteries in capacity and cyclability. We report an in situ sulfur-doping strategy to functionalize MXene nanosheets by introducing heteroatomic sulfur into the MXene structure form the MAX phase precursor. By employing the vacuum freeze-drying method, a three-dimensional (3D) wrinkled MXene nanoarchitecture with the high specific surface area was prepared. The tailor-made wrinkled sulfur-doped MXene (S-Ti3C2Tx) nanosheets were applied as an electrode host material in room temperature sodium-sulfur batteries. The S-Ti3C2Tx matrix shows high polarity with sodium polysulfides, restricting the diffusion of sodium polysulfides. The MXene/sulfur electrode can achieve high areal sulfur loading up to 4.5 mg cm-2 as well as good electrochemical performance (reversible capacity of 577 mAh g-1 at 2 C after 500 cycles).

Weizhai Bao

Papers

Facile Synthesis of Crumpled Nitrogen-Doped MXene Nanosheets as a New Sulfur Host for Lithium–Sulfur Batteries

Porous Cryo-Dried MXene for Efficient Capacitive Deionization

Immobilizing Polysulfides with MXene-Functionalized Separators for Stable Lithium-Sulfur Batteries.

3D Metal Carbide@Mesoporous Carbon Hybrid Architecture as a New Polysulfide Reservoir for Lithium-Sulfur Batteries

Boosting Performance of Na-S Batteries Using Sulfur-Doped Ti3C2Tx MXene Nanosheets with a Strong Affinity to Sodium Polysulfides.