Double-Shelled Nanocages with Cobalt Hydroxide Inner Shell and Layered Double Hydroxides Outer Shell as High-Efficiency Polysulfide Mediator for Lithium-Sulfur Batteries

Hollow Defect-Rich Nanofibers as Sulfur Hosts for Lithium-Sulfur Batteries

The slow redox kinetics of lithium-sulfur batteries severely limit their application, and sulfur utilization can be effectively enhanced by designing different cathode sulfur host materials. Herein, we report the hollow porous nanofiber LaNi0.6Co0.4O3 as a bidirectional host material for lithium-sulfur batteries. After Co is substituted into LaNiO3, oxygen vacancies are generated to enhance the material conductivity and enrich the active sites of the material, and the electrochemical reaction rate can be further accelerated by the synergistic catalytic ability of Ni and Co elements in the B-site of the active site of LaNi0.6Co0.4O3. As illustrated by the kinetic test results, LaNi0.6Co0.4O3 effectively accelerated the interconversion of lithium polysulfides, and the nucleation of Li2S and the dissolution rate of Li2S were significantly enhanced, indicating that LaNi0.6Co0.4O3 accelerated the redox kinetics of the lithium-sulfur battery during the charging and discharging process. In the electrochemical performance test, the initial discharge specific capacity of S/LaNi0.6Co0.4O3 was 1140.4 mAh g-1 at 0.1 C, and it was able to release a discharge specific capacity of 584.2 mAh g-1 at a rate of 5 C. It also showed excellent cycling ability in the long cycle test, with a single-cycle capacity degradation rate of only 0.08%. Even under the harsh conditions of high loaded sulfur and low electrolyte dosage, S/LaNi0.6Co0.4O3 still delivers excellent specific capacity and excellent cycling capability. Therefore, this study provides an idea for the future development of bidirectional high-activity electrocatalysts for lithium-sulfur batteries.

Leveraging doping strategies and interface engineering to enhance catalytic transformation of lithium polysulfides for high-performance lithium-sulfur batteries

The commercialization of lithium-sulfur (Li-S) batteries has faced challenges due to the shuttle effect of soluble intermediate polysulfides and the sluggish kinetics of sulfur redox reactions. In this study, a synergistic catalyst medium was developed as a high-performance sulfur cathode material for Li-S batteries. Termed A/R-TiO2@ Ni-N-MXene, this sulfur cathode material features an in-situ derived anatase-rutile homojunction of TiO2 nanoparticles on Ni-N dual-atom-doped MXene nanosheets. Using in-situ transmission electron microscopy (TEM) technique, we observed the growth process of the homojunction for the first time confirming that homojunctions facilitated charge transfer, while dual-atom doping offered abundant active sites for anchoring and converting soluble polysulfides. Theoretical calculations and experiments showed that these synergistic effects effectively mitigated the shuttle effect, leading to improved cycling performance of Li-S batteries. After 500 cycles at a 1C rate, Li-S batteries using A/R-TiO2@Ni-N-MXene as cathode materials exhibited stable and highly reversible capacity with a capacity decay of only 0.056 % per cycle. Even after 150 cycles at a 0.1C rate, a high-capacity retention rate of 62.8 % was achieved. Additionally, efficient sulfur utilization was observed, with 1280.76 mA h/g at 0.1C, 694.24 mA h/g at 1C, alongside a sulfur loading of 1.5-2 mg/cm2. The effective strategy based on homojunctions showcases promise for designing high-performance Li-S batteries.

Rechargeable Metal-Sulfur Batteries: Key Materials to Mechanisms

Rechargeable metal-sulfur batteries are considered promising candidates for energy storage due to their high energy density along with high natural abundance and low cost of raw materials. However, they could not yet be practically implemented due to several key challenges: (i) poor conductivity of sulfur and the discharge product metal sulfide, causing sluggish redox kinetics, (ii) polysulfide shuttling, and (iii) parasitic side reactions between the electrolyte and the metal anode. To overcome these obstacles, numerous strategies have been explored, including modifications to the cathode, anode, electrolyte, and binder. In this review, the fundamental principles and challenges of metal-sulfur batteries are first discussed. Second, the latest research on metal-sulfur batteries is presented and discussed, covering their material design, synthesis methods, and electrochemical performances. Third, emerging advanced characterization techniques that reveal the working mechanisms of metal-sulfur batteries are highlighted. Finally, the possible future research directions for the practical applications of metal-sulfur batteries are discussed. This comprehensive review aims to provide experimental strategies and theoretical guidance for designing and understanding the intricacies of metal-sulfur batteries; thus, it can illuminate promising pathways for progressing high-energy-density metal-sulfur battery systems.

Dependence of Interlayer or Sulfur Host on Hollow Framework of Lithium-Sulfur Battery

Lithium-sulfur batteries are recognized as the next generation of high-specific energy secondary batteries owing to their satisfactory theoretical specific capacity and energy density. However, their commercial application is greatly limited by a series of problems, including disordered migration behavior, sluggish redox kinetics, and the serious shuttle effect of lithium polysulfides. One of the most efficient approaches to physically limit the shuttle effect is the rational design of a hollow framework as sulfur host. However, the influence of the hollow structure on the interlayers has not been clearly reported. In this study, the Mo2 C/C catalysts with hollow(H-Mo2 C/C) and solid(S-Mo2 C/C) frameworks are rationally designed to explore the dependence of the hollow structure on the interlayer or sulfur host. In contrast to the physical limitations of the hollow framework as host, the hollow structure of the interlayer inhibited lithium-ion diffusion, resulting in poor electrochemical properties at high current densities. Based on the superiority of the various frameworks, the H-Mo2 C/C@S | S-Mo2 C/C@PP | Li cells are assembled and displayed excellent electrochemical performance. This work re-examines the design requirements and principles of catalyst frameworks in different battery units.

Under-Coordinated CoFe Layered Double Hydroxide Nanocages Derived from Nanoconfined Hydrolysis of Bimetal Organic Compounds for Efficient Electrocatalytic Water Oxidation

Hierarchically structured bimetal hydroxides are promising for electrocatalytic oxygen evolution reaction (OER), yet synthetically challenging. Here, the nanoconfined hydrolysis of a hitherto unknown CoFe-bimetal-organic compound (b-MOC) is reported for the controllable synthesis of highly OER active nanostructures of CoFe layered double hydroxide (LDH). The nanoporous structures trigger the nanoconfined hydrolysis in the sacrificial b-MOC template, producing CoFe LDH core-shell octahedrons, nanoporous octahedrons, and hollow nanocages with abundant under-coordinated metal sites. The hollow nanocages of CoFe LDH demonstrate a remarkable turnover frequency (TOF) of 0.0505 s-1 for OER catalysis at an overpotential of 300 mV. It is durable in up to 50 h of electrolysis at step current densities of 10-100 mA cm-2 . Ex situ and in situ X-ray absorption spectroscopic analysis combined with theoretical calculations suggests that under-coordinated Co cations can bind with deprotonated Fe-OH motifs to form OER active Fe-O-Co dimmers in the electrochemical oxidation process, thereby contributing to the good catalytic activity. This work presents an efficient strategy for the synthesis of highly under-coordinated bimetal hydroxide nanostructures. The mechanistic understanding underscores the power of maximizing the amount of bimetal-dimer sites for efficient OER catalysis.

Research progress and future perspectives on electromagnetic wave absorption of fibrous materials

Electromagnetic waves have caused great harm to military safety, high-frequency electronic components, and precision instruments, and so forth, which urgently requires the development of lightweight, high-efficiency, broadband electromagnetic waves (EMW) absorbing materials for protection. As the basic fibrous materials, carbon fibers (CFs) and SiC fibers (SiCf) have been widely applied in EMW absorption due to their intrinsic characteristics of low density, high mechanical properties, high conductivity, and dielectric loss mechanism. Nevertheless, it has remained a great challenge to develop lightweight EMW-absorbing fibrous materials with strong absorption capability and broad frequency range. In this review, the fundamental electromagnetic attenuation mechanisms are firstly introduced. Furthermore, the preparation, structure, morphology, and absorbing performance of CFs and SiCf-based EMW absorbing composites are summarized. In addition, prospective research opportunities are highlighted toward the development of fibrous absorbing materials with the excellent absorption performance.

Atomization-Induced High Intrinsic Activity of a Biocompatible MgAl-LDH Supported Ru Single-Atom Nanozyme for Efficient Radicals Scavenging

Developing efficient nanozymes to mimic natural enzymes for scavenging reactive radicals remains a significant challenge owing to the insufficient activity of conventional nanozymes. Herein, we report a novel Ru single-atom nanozyme (SAE), featuring atomically dispersed Ru atoms on a biocompatible MgAl-layered double hydroxide (Ru1 /LDH). The prepared Ru1 /LDH SAE shows high intrinsic peroxidase (POD)-like catalytic activity, which outperforms the Ru nanoclusters (NCs) nanozyme by a factor of 20 and surpasses most SAEs. The density functional theory calculations reveal that the high intrinsic POD-like activity of Ru1 /LDH can be attributed to a heterolytic path of H2 O2 dissociation on the single Ru sites, which requires lower free energy (0.43 eV) compared to the homolytic path dissociation on Ru NC (0.63 eV). In addition, the Ru1 /LDH SAE shows excellent multiple free radicals scavenging ability, including superoxide anion radical (O2 ⋅- ), hydroxyl radical (⋅OH), nitric oxide radical (NO⋅) and 2, 2-diphenyl-1-picrylhydrazyl radical (DPPH⋅). Given the advantages of Ru1 /LDH with high enzymatic activities, biosafety, and ease to scale up, it paves the way for exploring SAEs in the practical biological immunity system.

Local Built-In Field at the Sub-nanometric Heterointerface Mediates Cascade Electrochemical Conversion of Lithium-sulfur Batteries

Heterogeneous catalytic mediators have been proposed to play a vital role in enhancing the multiorder reaction and nucleation kinetics in multielectron sulfur electrochemistry. However, the predictive design of heterogeneous catalysts is still challenging, owing to the lack of in-depth understanding of interfacial electronic states and electron transfer on cascade reaction in Li-S batteries. Here, a heterogeneous catalytic mediator based on monodispersed titanium carbide sub-nanoclusters embedded in titanium dioxide nanobelts is reported. The tunable catalytic and anchoring effects of the resulting catalyst are achieved by the redistribution of localized electrons caused by the abundant built-in fields in heterointerfaces. Subsequently, the resulting sulfur cathodes deliver an areal capacity of 5.6 mAh cm-2 and excellent stability at 1 C under sulfur loading of 8.0 mg cm-2 . The catalytic mechanism especially on enhancing the multiorder reaction kinetic of polysulfides is further demonstrated via operando time-resolved Raman spectroscopy during the reduction process in conjunction with theoretical analysis.

Ce-Doped Three-Dimensional Ni/Fe LDH Composite as a Sulfur Host for Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) have become the most promising choice in the new generation of energy storage/conversion equipment due to their high theoretical capacity of 1675 mAh g-1 and theoretical energy density of 2600 Wh kg-1. Nevertheless, the continuous shuttling of lithium polysulfides (LiPSs) restricts the commercial application of LSBs. The appearance of layered double hydroxides (LDH) plays a certain role in the anchoring of LiPSs, but its unsatisfactory electronic conductivity and poor active sites hinder its realization as a sulfur host for high-performance LSBs. In this paper, metal organic framework-derived and Ce ion-doped LDH (Ce-Ni/Fe LDH) with a hollow capsule configuration is designed rationally. The hollow structure of Ce-Ni/Fe LDH contains a sufficient amount of sulfur. Fe, Ni, and Ce metal ions effectively trap LiPSs; speed up the conversion of LiPSs; and firmly anchor LiPSs, thus effectively inhibiting the shuttle of LiPSs. The electrochemical testing results demonstrate that a lithium-sulfur battery with capsule-type S@Ce-Ni/Fe LDH delivers the initial discharge capacities of 1207 mAh g-1 at 0.1 C and 1056 mAh g-1 at 0.2 C, respectively. Even at 1 C, a lithium-sulfur battery with S@Ce-Ni/Fe LDH can also cycle 1000 times. This work provides new ideas to enhance the electrochemical properties of LSBs by constructing a hollow capsule configuration.

Challenges and Solutions for Low-Temperature Lithium-Sulfur Batteries: A Review

The lithium-sulfur (Li-S) battery is considered to be one of the attractive candidates for breaking the limit of specific energy of lithium-ion batteries and has the potential to conquer the related energy storage market due to its advantages of low-cost, high-energy density, high theoretical specific energy, and environmental friendliness issues. However, the substantial decrease in the performance of Li-S batteries at low temperatures has presented a major barrier to extensive application. To this end, we have introduced the underlying mechanism of Li-S batteries in detail, and further concentrated on the challenges and progress of Li-S batteries working at low temperatures in this review. Additionally, the strategies to improve the low-temperature performance of Li-S batteries have also been summarized from the four perspectives, such as electrolyte, cathode, anode, and diaphragm. This review will provide a critical insight into enhancing the feasibility of Li-S batteries in low-temperature environments and facilitating their commercialization.

Improving the intrinsic activity of ultrathin 2D-2D heterostructures by bridge-bonded Ni-O-Ti ligands for efficient oxygen evolution

The integration of ultrathin two-dimensional (2D) semiconductors with other conductive 2D materials to form hybrid electrocatalysts with abundant heterointerfaces can enhance the electrocatalytic activity by facilitating interfacial charge transfer. However, the hybrid electrocatalysts with weak interfacial bonding have limited effect on the electrocatalytic performance because the intrinsic activity of interfacial sites cannot be altered by weak interfacial interactions. As a proof-of-concept, we design ultrathin 2D-2D heterostructures with bridge-bonded Ni-O-Ti ligands based on single-layered Ti₃C₂T_xMXene and metal hydroxides, and further reveal the structure-activity correlation between interfacial bonding and electrocatalytic oxygen evolution reaction by combining theoretical and experimental studies. Density functional theory calculations reveal the modulation of the electronic structure of interfacial metal sites after the formation of bridged interfacial Ni-O-Ti bonding. Compared with the hydrogen-bond-linked heterostructure, the ultrathin 2D-2D heterostructure with bridge-bonded Ni-O-Ti ligands shows enhanced intrinsic activity and stability towards electrocatalytic oxygen evolution with a very low overpotential of 205 mV at 10 mA cm^-2and the long-term durability. This work provides a new understanding and approach for the design and development of 2D hybrid catalysts with highly efficient electrocatalytic activity.

Carbon-Hybridized Hydroxides for Energy Conversion and Storage: Interface Chemistry and Manufacturing

Carbon-hybridized hydroxides (CHHs) have been intensively investigated for uses in the energy conversion/storage fields. Nevertheless, the intrinsic structure-activity relationships between carbon and hydroxides within CHHs are still blurry, which hinders the fine modulation of CHHs in terms of practical applications to some degree. This review aims to figure out the intrinsic role of carbon materials in CHHs with a focus on the interface chemistry and the engineering strategy in-between two components. The fundamental effects of the carbon materials in enhancing the charge/mass transfer kinetics are first analyzed, particularly the extra electron pathways for fast charge transfer and the anchoring sites for boosting the mass transfer. Subsequently, the surface-guided/confined effects of carbon materials in CHHs to modify the morphology and tailor the hydroxides, and functional heterojunction for regulating the inner electronic structure are decoupled. The methods to efficiently construct a stable yet robust solid-solid heterointerface are summarized, including oxygen functional groups engrafting, topological defective sites construction and heteroatom incorporation to activate the inert carbon surface. The smart CHHs in some typical energy applications are demonstrated. Additionally, the methodologies that can reveal the hybridization electron configuration between two components are summed up. At last, the perspective and challenges faced by the CHHs for energy-related applications are outlined.

Nitrogen and titanium-codoped porous carbon nanocomposites derived from metal-organic framework as cathode to address polysulfides shuttle effects by Ti-assisted N-inhibiting strategy

To address the problem of shutting effect of Li-S batteries, we used Ti-based MOF as precursor to obtain a conductive matrix with dual inhibitors. The target material, namely NTiPC, shown remarkable discharge capacity with 1178 mA h g^-1, and maintained at 732 mA h g^-1 after 100 cycles. The results indicated the N- and Ti-active sites synergistic acted with conductive framework can facilitate binding reaction between matrix and polysulfides.

Composites Filled with Metal Organic Frameworks and Their Derivatives: Recent Developments in Flame Retardants

Polymer matrix is vulnerable to fire hazards and needs to add flame retardants to enhance its performance and make its application scenarios more extensive. At this stage, it is more necessary to add multiple flame-retardant elements and build a multi-component synergistic system. Metal organic frameworks (MOFs) have been studied for nearly three decades since their introduction. MOFs are known for their structural advantages but have only been applied to flame-retardant polymers for a relatively short period of time. In this paper, we review the development of MOFs utilized as flame retardants and analyze the flame-retardant mechanisms in the gas phase and condensed phase from the original MOF materials, modified MOF composites, and MOF-derived composites as flame retardants, respectively. The effects of carbon-based materials, phosphorus-based materials, nitrogen-based materials, and biomass on the flame-retardant properties of polymers are discussed in the context of MOFs. The construction of MOF multi-structured flame retardants is also introduced, and a variety of MOF-based flame retardants with different morphologies are shown to broaden the ideas for subsequent research.

Solid/Quasi-Solid Phase Conversion of Sulfur in Lithium-Sulfur Battery

The lithium-sulfur (Li-S) battery is considered as one of the most promising options because the redox couple has almost the highest theoretical specific energy (2600 Wh kg^-1 ) among all solid anode-cathode candidates for rechargeable batteries. The "solid-liquid-solid" mechanism has become a dominating phase transformation process since it was first reported, although this cathode mode suffers from a tough "shuttle" phenomenon due to the dissolution of the soluble intermediate polysulfides generated during the charging-discharging process, which causes rapid loss of energy-bearing material and shortened lifespan. For decades, tremendous efforts have been made to restrict the shuttle effect. Changing sulfur conversion to "solid-solid" mode or "quasi-solid" mode, which successfully exceed the limit of the dissolution of the intermediates, and may address the root of the problem. In this review, the main focus is on the fundamental chemistry of the "solid-solid" and "quasi-solid" phase transformation of the sulfur cathode. First, the strategies of sulfur immobilization in "solid-liquid-solid" multi-phase conversions as well as the pivotal influence factors for the electrochemical conversion process are briefly introduced. Then, the different routes are summarized to realize the "solid-solid" and "quasi-solid" redox mechanisms. Finally, a perspectives on building high-energy-density Li-S batteries are provided.

Mesoporous LDH Metastructure from Multiscale Assembly of Defective Nanodomains by Laser Shock for Oxygen Evolution Reaction

Laser is a powerful tool for the synthesis of nanomaterials. The intensive laser pulses delivered to materials within nanoseconds allow the formation of novel structures that are inaccessible for conventional methods. Layered double hydroxide (LDH) nanostructures with high porosity, suitable dopants, and rich defects are desirable for catalysts, however, tremendously difficult in a one-pot synthesis. Here it is found that confined laser shock in solvent leads to the formation of nanoreactors which guide the assembly of multiscale LDH building units, larger nanosheets as frame and smaller nanodomains as building blocks. These nanodomains have rich vacancy defects and are interlocked in a high packed density of 10¹³  cm^-2 , leaving rich mesopores across the nanosheets and coral-like morphology. Like the natural coral reef that has multiscale structure to accommodate different marine organisms, the coral-like LDH metastructure provides large surface area and rich active sites for the interaction with guest molecules. Benefiting from the multiscale porous structure and rational dopant, this LDH catalyst exhibits a low overpotential of 220 mV at 10 mA cm^-2 for oxygen evolution reaction (OER), standing as one of the best LDH catalysts to date.

Metal-organic frameworks as a good platform for the fabrication of multi-metal nanomaterials: design strategies, electrocatalytic applications and prospective

MOF-derived multi-metal nanomaterials are attracting numerous attentions in widespread applications such as catalysis, sensors, energy storage and conversion, and environmental remediation. Compared to the monometallic counterparts, the presence of foreign metal is expected to bring new physicochemical properties, thus exhibiting synergistic effect for enhanced performance. MOFs have been proved as a good platform for the fabrication of polymetallic nanomaterials with requisite features. Herein, various design strategies related to constructing multi-metallic nanomaterials from MOFs are summarized for the first time, involving metal nodal substitution, seed epitaxial growth, ion-exchange strategy, guest species encapsulation, solution impregnation and combination with extraneous substrate. Afterwards, the recent advances of multi-metallic nanomaterials for electrocatalytic applications, including oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), are systematically discussed. Finally, a personal outlook on the future trends and challenges are also presented with hope to enlighten deeper understanding and new thoughts for the development of multi-metal nanomaterials from MOFs.

Adsorption-Catalysis-Conversion of Polysulfides in Sandwiched Ultrathin Ni(OH)2 -PANI for Stable Lithium-Sulfur Batteries

Strong adsorption and catalysis for lithium polysulfides (LiPSs) are critical toward the electrochemical stability of Li-S batteries. Herein, a hollow sandwiched nanoparticle is put forward to enhance the adsorption-catalysis-conversion dynamic of sulfur species. The outer ultrathin Ni(OH)₂ nanosheets not only confine LiPSs via both physical encapsulation and chemical adsorption, but also promote redox kinetics and accelerate the conversion of sulfur species, which is revealed by experiments and theoretical calculations. Meanwhile, the inner hollow polyaniline soft core provides a strong chemical bonding to LiPSs after vulcanization, which can chemically adsorpt LiPSs, and synergistically confine the shuttle effect. Moreover, the Ni(OH)₂ nanosheets with a large specific area can enhance the wettability of electrolyte, and the flexible hollow sandwiched structure can accommodate the volume expansion, promoting sulfur utilization and structural stability. The obtained cathode exhibits excellent electrochemical performance with an initial discharge capacity of 1173 mAh g^-1 and a small capacity decay of 0.08% per cycle even after 500 cycles at 0.2 C, among the best results of Ni(OH)₂ -based materials for Li-S batteries. It is believed that the combination of adsorption-catalysis-conversion will shed a light on the development of cathode materials for stable Li-S batteries.

Layered double hydroxides functionalized by carbonaceous materials: from preparation to energy and environmental applications

Along with the exponential demand for energy and pollution-free-environment, layered double hydroxides (LDHs) have gained extensive explorations because of their diverse nanostructures and tunable elemental compositions. However, the applications of LDHs are hindered by their poor activity, sluggish mass transfer, and aggregation. LDHs functionalized by carbonaceous materials (CMs) (LDH-CM) are expected to overcome the above disadvantages and even generate more excellent performance. This review first analyzes the research evolvement of LDH-CM composites during the past 25 years. Next, the advantages of LDH-CM composites are highlighted, such as morphology optimization, high electrical conductivity, more stable, good heat, and mass transfer performance. Following the synthetic strategies, including chemical assembly of LDHs and CMs, direct growth of LDH on CMs (two-step nucleation and growth and surface-confined growth) and direct CM formation on LDHs are fully discussed. Then, the recent progress achieved in LDH-CM composites for the application of energy storage and environmental protection is summarized in detail. In particular, the review illustrates the reasons why these constructing strategies can improve the performance of LDH-CM composites. Finally, challenges and future research prospects of LDH-CM composites are highlighted.

In Situ Construction of MIL-100@NiMn-LDH Hierarchical Architectures for Highly Selective Photoreduction of CO2 to CH4

Layered double hydroxides (LDHs) are considered a promising catalyst for photocatalytic CO₂ reduction due to their broad photoresponse, facile channels for electron transfer, and the presence of abundant defects. Herein, we reported for the first time the fabrication of a novel photocatalyst MIL-100@NiMn-LDH with a hierarchical architecture by selecting MIL-100 (Mn) as a template to provide Mn³⁺ for the in situ growth of ultrathin NiMn-LDH nanosheets. Moreover, the in situ growth strategy exhibited excellent universality toward constructing MIL-100@LDH hierarchical architectures. When applied in the photocatalytic CO₂ reduction reaction, the as-prepared MIL-100@NiMn-LDH exhibited excellent CH₄ selectivity of 88.8% (2.84 μmol h^-1), while the selectivity of H₂ was reduced to 1.8% under visible light irradiation (λ > 500 nm). Such excellent catalytic performance can be attributed to the fact that (a) the MIL-100@NiMn-LDH hierarchical architectures with exposed catalytic active sites helped to enhance the CO₂ adsorption and activation and (b) the presence of rich oxygen vacancies and coordinately unsaturated metal sites in MIL-100@NiMn-LDH that optimized the band gap and accelerated the separation/transport of photoinduced charges.

An Ultrathin Functional Layer Based on Porous Organic Cages for Selective Ion Sieving and Lithium-Sulfur Batteries

Thin films with effective ion sieving ability are highly desired in energy storage and conversion devices, including batteries and fuel cells. However, it remains challenging to design and fabricate cost-effective and easy-to-process ultrathin films for this purpose. Here, we report a 300 nm-thick functional layer based on porous organic cages (POCs), a new class of porous molecular materials, for fast and selective ion transport. This solution processable material allows for the design of thin films with controllable thickness and tunable porosity by tailoring cage chemistry for selective ion separation. In the prototype, the functional layer assembled by CC3 can selectively sieve Li⁺ ions and efficiently suppress undesired polysulfides with minimal sacrifice for the system's total energy density. Separators modified with POC thin films enable batteries with good cycle performance and rate capability and offer an attractive path toward the development of future high-energy-density energy storage devices.

Magneto-Electrodeposition of 3D Cross-Linked NiCo-LDH for Flexible High-Performance Supercapacitors

Layered double hydroxides (LDHs) with outstanding redox activity on flexible current collectors can serve as ideal cathode materials for flexible hybrid supercapacitors in wearable energy storage devices. Electrodeposition is a facile, time-saving, and economical technique to fabricate LDHs. The limited loading mass induced by insufficient mass transport and finite exposure of active sites, however, greatly hinders the improvement of areal capacity. Herein, magneto-electrodeposition (MED) under high magnetic fields up to 9 T is developed to fabricate NiCo-LDH on flexible carbon cloth (CC) as well as Ti₃ C₂ T_x functionalized CC. Owing to the magneto-hydrodynamic effect induced by magnetic-electric field coupling, the loading mass and exposure of active sites are significantly increased. Moreover, a 3D cross-linked nest-like microstructure is constructed. The MED-derived NiCo-LDH delivers an ultrahigh areal capacity of 3.12 C cm^-2 at 1 mA cm^-2 and as-fabricated flexible hybrid supercapacitors show an excellent energy density with an outstanding cycling stability. This work provides a novel route to improve electrochemical performances of layered materials through MED technique.

Critical Role of Phosphorus in Hollow Structures Cobalt-Based Phosphides as Bifunctional Catalysts for Water Splitting

Cobalt phosphides electrocatalysts have great potential for water splitting, but the unclear active sides hinder the further development of cobalt phosphides. Wherein, three different cobalt phosphides with the same hollow structure morphology (CoP-HS, CoP₂ -HS, CoP₃ -HS) based on the same sacrificial template of ZIF-67 are prepared. Surprisingly, these cobalt phosphides exhibit similar OER performances but quite different HER performances. The identical OER performance of these CoP_x -HS in alkaline solution is attributed to the similar surface reconstruction to CoOOH. CoP-HS exhibits the best catalytic activity for HER among these CoP_x -HS in both acidic and alkaline media, originating from the adjusted electronic density of phosphorus to affect absorption-desorption process on H. Moreover, the calculated ΔG_H* based on P-sites of CoP-HS follows a quite similar trend with the normalized overpotential and Tafel slope, indicating the important role of P-sites for the HER process. Moreover, CoP-HS displays good performance (cell voltage of 1.67 V at a current density of 50 mA cm^-2 ) and high stability in 1 M KOH. For the first time, this work detailly presents the critical role of phosphorus in cobalt-based phosphides for water splitting, which provides the guidance for future investigations on transition metal phosphides from material design to mechanism understanding.

MgCo layered double hydroxide-based yolk shell polyhedrons as multifunctional sulfur mediator for lithium-sulfur batteries

The development of efficient sulfur host materials to address the shuttle effect issues of lithium polysulfides (LiPSs) is crucial in the lithium-sulfur (Li-S) batteries but still challenging. In the present study, a novel yolk shell structured MgCo-LDH/ZIF-67 composite is designed as Li-S battery cathode. In this composite, the shell layer is MgCo layered double hydroxide constructed by partially etching ZIF-67 nanoparticle by Mg²⁺, and the core is the unreacted ZIF-67 particle. The unique yolk shell structure not only provides abundant pores for sulfur accommodation, but also facilitates the electrolyte penetration and ion transport. The ZIF-67 core exhibits strong polar adsorption to LiPSs through the Lewis acid-base interactions, and the micropores/mesoporous can further trap LiPSs. Meanwhile, the MgCo-LDH shell exposes enough sulfur-philic sites for enhancing chemisorption and catalyzes LiPSs conversion. As a result, when MgCo-LDH/ZIF-67 is used as sulfur host in the cathode, the cell achieves a high discharge capacity of 1121 mAh g^-1at 0.2 C, and an areal capacity of 5.0 mAh cm^-2under high sulfur loading of 5.8 mg cm^-2. The S/MgCo-LDH/ZIF-67 electrode holds a promising potential for the development of Li-S batteries.

InOOH as an efficient bidirectional catalyst for accelerated polysulfides conversion to enable high-performance lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries with the prominent advantages are greatly expected to be the attractive alternatives in the next-generation energy-storage systems. However, the practical success of Li-S batteries suffers from the shuttle effect and depressed redox kinetics of polysulfides. Herein, for the first time, InOOH nanoparticles are employed as a potent catalytic additive in sulfur electrode to overcome these issues. As demonstrated by the theoretical and experimental results, the strong interactions between the InOOH nanoparticles and sulfur species enable the effective adsorption of polysulfides. More significantly, InOOH nanoparticles not only effectively expedite the reduction of sulfur during the discharge process, but also dramatically accelerate the oxidation of Li₂S during the charge process, presenting the marvelous bidirectional catalytic effects. Benefited from these distinctive superiorities, the cells with InOOH nanoparticles harvest an excellent capacity retention of 69.5% over 500 cycles at 2C and a commendable discharge capacity of 891 mAh g^-1 under a high-sulfur loading of 5.0 mg cm^-2. The detailed investigations in this work provide a novel insight to ameliorate the Li-S electrochemistry by the bidirectional catalyst for high-performance Li-S batteries.

Maximizing Metal-Support Interactions in Pt/Co3O4 Nanocages to Simultaneously Boost Hydrogen Production Activity and Durability

Catalytic hydrolysis of ammonia borane (AB) provides an effective way to generate pure H₂ at ambient temperature for fuel cells. Pt-based catalysts usually exhibit great initial activity toward this reaction but deactivate quickly. Here, we report that the metal-support interactions in Pt/Co₃O₄ nanocages can simultaneously accelerate the H₂ generation and enhance the catalyst's stability. The Pt/Co₃O₄ catalyst is made for the first time by embedding Pt clusters (∼1.2 nm) in a high-surface-area Co₃O₄ nanocage to maximize the metal-support interface. The turnover frequency of the Pt/Co₃O₄ catalyst is about nine times higher than that of commercial Pt/C and outperforms almost all other Pt-based catalysts. X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, in situ spectroscopy, and density functional theory calculations suggest that the Co₃O₄ nanocages with rich oxygen vacancies facilitate the adsorption and dissociation of H₂O to give electropositive H (H^δ+), while the in situ embedded Pt clusters can accelerate the formation of electronegative H (H^δ-) from AB. Subsequently, the H^δ+ and H^δ- spill over to the abundant interfacial sites and bond into H₂. In addition to this dual-function synergy effect, the strong metal-support electronic interactions between Co₃O₄ and Pt benefit the desorption of poisonous B-containing byproducts from Pt sites. This effect together with cluster anchoring leads to a fivefold enhancement in durability compared to commercial Pt/C. The metal-support interactions revealed in this study provide more options for catalyst design toward facile H₂ production from chemical hydrogen storage materials.

Templating Synthesis of Metal-Organic Framework Nanofiber Aerogels and Their Derived Hollow Porous Carbon Nanofibers for Energy Storage and Conversion

A kind of metal-organic framework (MOF) aerogels are synthesized by the self-assembly of uniform and monodisperse MOF nanofibers. Such MOF nanofiber aerogels as carbon precursors can effectively avoid the aggregation of nanofibers during calcination, resulting in the formation of well-dispersed hollow porous carbon nanofibers (HPCNs). Moreover, HPCNs with well-dispersion are investigated as sulfur host materials for Li-S batteries and electrocatalysts for cathode oxygen reduction reaction (ORR). On the one hand, HPCNs act as hosts for the encapsulation of sulfur into their hierarchical micro- and mesopores as well as hollow nanostructures. The obtained sulfur cathode exhibits excellent electrochemical features, good cycling stability and high coulombic efficiency. On the other hand, HPCNs exhibit better electrocatalytic activity than aggregated counterparts for ORR. Furthermore, a highly active single atom electrocatalyst can be prepared by the carbonization of bimetallic MOF nanofiber aerogels. The results indicate that well-dispersed HPCNs show enhanced electrochemical properties in contrast to their aggregated counterparts, suggesting that the dispersion situation of nanomaterials significantly influence their final performance. The present concept of employing MOF nanofiber aerogels as precursors will provide a new strategy to the design of MOF-derived nanomaterials with well-dispersion for their applications in energy storage and conversion.

Lithium-Sulfur Battery Cathode Design: Tailoring Metal-Based Nanostructures for Robust Polysulfide Adsorption and Catalytic Conversion

Lithium-sulfur (Li-S) batteries have a high specific energy capacity and density of 1675 mAh g^-1 and 2670 Wh kg^-1 , respectively, rendering them among the most promising successors for lithium-ion batteries. However, there are myriads of obstacles in the practical application and commercialization of Li-S batteries, including the low conductivity of sulfur and its discharge products (Li₂ S/Li₂ S₂ ), volume expansion of sulfur electrode, and the polysulfide shuttle effect. Hence, immense attention has been devoted to rectifying these issues, of which the application of metal-based compounds (i.e., transition metal, metal phosphides, sulfides, oxides, carbides, nitrides, phosphosulfides, MXenes, hydroxides, and metal-organic frameworks) as sulfur hosts is profiled as a fascinating strategy to hinder the polysulfide shuttle effect stemming from the polar-polar interactions between the metal compounds and polysulfides. This review encompasses the fundamental electrochemical principles of Li-S batteries and insights into the interactions between the metal-based compounds and the polysulfides, with emphasis on the intimate structure-activity relationship corroborated with theoretical calculations. Additionally, the integration of conductive carbon-based materials to ameliorate the existing adsorptive abilities of the metal-based compound is systematically discussed. Lastly, the challenges and prospects toward the smart design of catalysts for the future development of practical Li-S batteries are presented.

Double shelled hollow CoS2@MoS2@NiS2 polyhedron as advanced trifunctional electrocatalyst for zinc-air battery and self-powered overall water splitting

Electrocatalysts play important role in various energy conversion and storage devices. The catalytic performance of electrocatalysts can be enhanced through the increasement of intrinsic catalytic activity by optimizing electronic structure and the improvement of exposed active sites by designing proper nanostructures. In this work, CoS₂@MoS₂@NiS₂ nano polyhedron with double-shelled structure was prepared using metal organic framework as a precursor. Due to the rational integration of multifunctional active center, the strong electronic interaction of the various component, the high electrochemical surface area and shortened mass transport induced by the special structure, CoS₂@MoS₂@NiS₂ exhibits high catalytic activity for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Specifically, low overpotentials of 156 and 200 mV was achieved to deliver a current density of 10 mA cm^-2 for HER and OER, and a high half-wave potential of 0.80 V was observed for ORR. More importantly, the Zn-air battery assembled by CoS₂@MoS₂@NiS₂ exhibits a high-power density of 80.28 mW cm^-2 and could effectively drive overall water splitting. This work provides a new platform for designing multifunctional catalysts with high activity for energy conversion and storage.

Application of layered double hydroxide-biochar composites in wastewater treatment: Recent trends, modification strategies, and outlook

In recent years, layered double hydroxide-biochar (LDH-BC) composites as adsorbents and catalysts for contaminants removal (inorganic anions, heavy metals, and organics) have received increasing attention and became a new research point. It is because of the good chemical stability, abundant surface functional groups, excellent anion exchange ability, and good electronic properties of LDH-BC composites. Hence, we offer an overall review on the developments and processes in the synthesis of LDH-BC composites as adsorbents and catalysts. Special attention is devoted to the strategies for enhancing the properties of LDH-BC composites, including (1) magnetic treatment, (2) acid treatment, (3) alkali treatment, (4) controlling metal ion ratios, (5) LDHs intercalation, and (6) calcination. In addition, further studies are called for LDH-BC composites and potential areas for future application of LDH-BC composites are also proposed.

A facile synthesis of Ni0.85Se@Cu2-xSe nanorods as high-performance supercapacitor electrode materials

Transition-metal selenides are regarded as promising electrode materials due to their superior electrochemical performances for supercapacitors. In this study, a nanorod-like hybrid of Ni_0.85Se@Cu_2-xSe on a Ni-foam substrate is successfully synthesized via a facile one-step route. The Ni_0.85Se@Cu_2-xSe nanorods are found to be deposited uniformly on the Ni-form substrates. When used as a battery-type electrode in a supercapacitor, the as-deposited Ni_0.85Se@Cu_2-xSe electrode exhibits a high specific capacity of 1831 F g^-1 at 1 A g^-1 and 78.4% of capacitance retention after 8000 cycles at 10 A g^-1. Moreover, the assembled Ni_0.85Se@Cu_2-xSe//AC asymmetric supercapacitor (ASC) exhibits an energy density of 63.2 W h kg^-1 at a power density of 800.1 W kg^-1, as well as good cycling stability (92.1% capacitance retention after 5000 cycles).

Selective Catalysis Remedies Polysulfide Shuttling in Lithium-Sulfur Batteries

The shuttling of soluble lithium polysulfides between the electrodes leads to serious capacity fading and excess use of electrolyte, which severely bottlenecks practical use of Li-S batteries. Here, selective catalysis is proposed as a fundamental remedy for the consecutive solid-liquid-solid sulfur redox reactions. The proof-of-concept Indium (In)-based catalyst targetedly decelerates the solid-liquid conversion, dissolution of elemental sulfur to polysulfides, while accelerates the liquid-solid conversion, deposition of polysulfides into insoluble Li₂ S, which basically reduces accumulation of polysulfides in electrolyte, finally inhibiting the shuttle effect. The selective catalysis is revealed, experimentally and theoretically, by changes of activation energies and kinetic currents, modified reaction pathway together with the probed dynamically changing catalyst (LiInS₂ catalyst), and gradual deactivation of the In-based catalyst. The In-based battery works steadily over 1000 cycles at 4.0 C and yields an initial areal capacity up to 9.4 mAh cm^-2 with a sulfur loading of ≈9.0 mg cm^-2 .

Metal-Organic-Framework-Derived Nanostructures as Multifaceted Electrodes in Metal-Sulfur Batteries

Metal-sulfur batteries (MSBs) are considered up-and-coming future-generation energy storage systems because of their prominent theoretical energy density. However, the practical applications of MSBs are still hampered by several critical challenges, i.e., the shuttle effects, sluggish redox kinetics, and low conductivity of sulfur species. Recently, benefiting from the high surface area, regulated networks, molecular/atomic-level reactive sites, the metal-organic frameworks (MOFs)-derived nanostructures have emerged as efficient and durable multifaceted electrodes in MSBs. Herein, a timely review is presented on recent advancements in designing MOF-derived electrodes, including fabricating strategies, composition management, topography control, and electrochemical performance assessment. Particularly, the inherent charge transfer, intrinsic polysulfide immobilization, and catalytic conversion on designing and engineering of MOF nanostructures for efficient MSBs are systematically discussed. In the end, the essence of how MOFs' nanostructures influence their electrochemical properties in MSBs and conclude the future tendencies regarding the construction of MOF-derived electrodes in MSBs is exposed. It is believed that this progress review will provide significant experimental/theoretical guidance in designing and understanding the MOF-derived nanostructures as multifaceted electrodes, thus offering promising orientations for the future development of fast-kinetic and robust MSBs in broad energy fields.

Rational Design and Engineering of One-Dimensional Hollow Nanostructures for Efficient Electrochemical Energy Storage

The unique structural characteristics of one-dimensional (1D) hollow nanostructures result in intriguing physicochemical properties and wide applications, especially for electrochemical energy storage applications. In this Minireview, we give an overview of recent developments in the rational design and engineering of various kinds of 1D hollow nanostructures with well-designed architectures, structural/compositional complexity, controllable morphologies, and enhanced electrochemical properties for different kinds of electrochemical energy storage applications (i.e. lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, lithium-selenium sulfur batteries, lithium metal anodes, metal-air batteries, supercapacitors). We conclude with prospects on some critical challenges and possible future research directions in this field. It is anticipated that further innovative studies on the structural and compositional design of functional 1D nanostructured electrodes for energy storage applications will be stimulated.

Suppressed shuttling effect of polysulfides using three-dimensional nickel hydroxide polyhedrons for advanced lithium-sulfur batteries

In this work, controlled-size hollow polyhedron assembled by crumpled nickel hydroxide (Ni(OH)₂) nanosheets from silicon dioxide (SiO₂)-covered zeolitic imidazole framework-67 (ZIF-67@SiO₂) is prepared via a template-sacrificed method. It is found that SiO₂ plays an essential role in keeping intact polyhedrons and suppressing particle growth. Benefiting from structural and compositional advantages, the Ni(OH)₂@S electrode exhibits high specific capacity, excellent rate performance, and stable cycle life at 1C with a small capacity decay of 0.067% per cycle. The Ni(OH)₂ hollow polyhedrons can accommodate the volume expansion to maintain the integrity of the electrode and suppress the shuttling effect of polysulfides via abundant hydroxyl groups. Hence, this strategy is beneficial to anticipate the material for large-scale applications.

Metal-Organic Frameworks Derived Functional Materials for Electrochemical Energy Storage and Conversion: A Mini Review

With many apparent advantages including high surface area, tunable pore sizes and topologies, and diverse periodic organic-inorganic ingredients, metal-organic frameworks (MOFs) have been identified as versatile precursors or sacrificial templates for preparing functional materials as advanced electrodes or high-efficiency catalysts for electrochemical energy storage and conversion (EESC). In this Mini Review, we first briefly summarize the material design strategies to show the rich possibilities of the chemical compositions and physical structures of MOFs derivatives. We next highlight the latest advances focusing on the composition/structure/performance relationship and discuss their practical applications in various EESC systems, such as supercapacitors, rechargeable batteries, fuel cells, water electrolyzers, and carbon dioxide/nitrogen reduction reactions. Finally, we provide some of our own insights into the major challenges and prospective solutions of MOF-derived functional materials for EESC, hoping to shed some light on the future development of this highly exciting field.

Nanoscale melamine-based porous organic frameworks as host material for efficient polysulfides chemisorption in lithium-sulfur batteries

In order to improve the electrochemical capacity of lithium-sulfur batteries (LiSBs), it is necessary to introduce the porous organic frameworks with well-defined hetero atom species in cathode. In this work, porous nanomaterials with ultra-high nitrogen containing and adjustable porosity named Schiff-based networks (SNWs) were selected as potential candidate for sulfur host in LiSBs. Two SNW samples have been constructed by reacting melamine with phenyl or biphenyl dialdehydes through microwave-assisted method, respectively. The high BET surface area provided sufficient room to impregnate sulfur and mitigated volume changes during the cycling performance. Besides, the high density and homogeneous distribution of pyridinic-N and aminic-N in SNW nanoparticles can cooperatively form lithium polysulfides (LiPSs) chemisorption via enhanced Li⁺-N interactions to effectively suppressed the 'shuttle effect'. Attributed to its structural superiorities, SNW/S cathode demonstrates excellent electrochemical performance in LiSBs. In particular, SNW-2/S cathode delivers an excellent cyclability with a specific capacity of 620 mAh · g^-1 after 500 cycles at 0.5 C, counting with a low capacity fading of 0.0508% per cycle. This work highlights the importance of rational design for effective LiPSs chemisorption and pioneers a facile strategy for developing suitable sulfur host materials towards high-performance LiSBs.

Investigation on Fabrication of Reduced Graphene Oxide-Sulfur Composite Cathodes for Li-S Battery via Hydrothermal and Thermal Reduction Methods

Lithium-sulfur (Li-S) battery is considered one of the possible alternatives for next-generation high energy batteries. However, its practical applications are still facing great challenges because of poor electronic conductivity, large volume change, and polysulfides dissolution inducing “shuttle reaction” for the S cathode. Many strategies have been explored to alleviate the aforementioned concerns. The most common approach is to embed S into carbonaceous matrix for constructing C-S composite cathodes. Herein, we fabricate the C-S cathode reduced graphene oxide-S (rGO-S) composites via one step hydrothermal and in-situ thermal reduction methods. The structural features and electrochemical properties in Li-S cells of the two type rGO-S composites are studied systematically. The rGO-S composites prepared by one step hydrothermal method (rGO-S-HT) show relatively better comprehensive performance as compared with the ones by in-situ thermal reduction method (rGO-S-T). For instance, with a current density of 100 mA g⁻¹, the rGO-S-HT composite cathodes possess an initial capacity of 1290 mAh g⁻¹ and simultaneously exhibit stable cycling capability. In particular, as increasing the current density to 1.0 A g⁻¹, the rGO-S-HT cathode retains a reversible capacity of 582 mAh g⁻¹ even after 200 cycles. The enhanced electrochemical properties can be attributed to small S particles uniformly distributed on rGO sheets enabling to significantly improve the conductivity of S and effectively buffer large volume change during lithiation/delithiation.

Strategy of Enhancing the Volumetric Energy Density for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries hold the promise of the next generation energy storage system beyond state-of-the-art lithium-ion batteries. Despite the attractive gravimetric energy density (W_G ), the volumetric energy density (W_V ) still remains a great challenge for the practical application, based on the primary requirement of Small and Light for Li-S batteries. This review highlights the importance of cathode density, sulfur content, electroactivity in achieving high energy densities. In the first part, key factors are analyzed in a model on negative/positive ratio, cathode design, and electrolyte/sulfur ratio, orientated toward energy densities of 700 Wh L^-1 /500 Wh kg^-1 . Subsequently, recent progresses on enhancing W_V for coin/pouch cells are reviewed primarily on cathode. Especially, the "Three High One Low" (THOL) (high sulfur fraction, high sulfur loading, high density host, and low electrolyte quantity) is proposed as a feasible strategy for achieving high W_V , taking high W_G into consideration simultaneously. Meanwhile, host materials with desired catalytic activity should be paid more attention for fabricating high performance cathode. In the last part, key engineering technologies on manipulating the cathode porosity/density are discussed, including calendering and dry electrode coating. Finally, a future outlook is provided for enhancing both W_V and W_G of the Li-S batteries.