Intricate Hollow Structures: Controlled Synthesis and Applications in Energy Storage and Conversion

Three-Dimensional Carbon-Coated LiFePO4 Cathode with Improved Li-Ion Battery Performance

LiFePO4 (LFPO)has great potential as the cathode material for lithium-ion batteries; it has a high theoretical capacity (170 m·A·h·g−1), high safety, low toxicity and good economic benefits. However, low conductivity and a low diffusion rate inhibit its future development. To overcome these weaknesses, three-dimensional carbon-coated LiFePO4 that incorporates a high capacity, superior conductivity and low volume expansion enables faster electron transport channels. The use of Cetyltrimethyl Ammonium Bromid (CTAB) modification only requires a simple water bath and sintering, without the need to add a carbon source in the LFPO synthesis process. In this way, the electrode shows excellent reversible capacity, as high as 159.8 m·A·h·g−1 at 2 C, superior rate capability with 97.3 m·A·h·g−1 at 5 C and good cycling ability, preserving ~84.2% capacity after 500 cycles. By increasing the ion transport rate and enhancing the structural stability of LFPO nanoparticles, the LFPO-positive electrode achieves excellent initial capacity and cycle life through cost-effective and easy-to-implement carbon coating. This simple three-dimensional carbon-coated LiFePO4 provides a new and simple idea for obtaining comprehensive and high-performance electrode materials in the field of lithium cathode materials.

Recent Advances on Pt-Free Electro-Catalysts for Dye-Sensitized Solar Cells

Since Prof. Grätzel and co-workers achieved breakthrough progress on dye-sensitized solar cells (DSSCs) in 1991, DSSCs have been extensively investigated and wildly developed as a potential renewable power source in the last two decades due to their low cost, low energy-intensive processing, and high roll-to-roll compatibility. During this period, the highest efficiency recorded for DSSC under ideal solar light (AM 1.5G, 100 mW cm-2) has increased from ~7% to ~14.3%. For the practical use of solar cells, the performance of photovoltaic devices in several conditions with weak light irradiation (e.g., indoor) or various light incident angles are also an important item. Accordingly, DSSCs exhibit high competitiveness in solar cell markets because their performances are less affected by the light intensity and are less sensitive to the light incident angle. However, the most used catalyst in the counter electrode (CE) of a typical DSSC is platinum (Pt), which is an expensive noble metal and is rare on earth. To further reduce the cost of the fabrication of DSSCs on the industrial scale, it is better to develop Pt-free electro-catalysts for the CEs of DSSCs, such as transition metallic compounds, conducting polymers, carbonaceous materials, and their composites. In this article, we will provide a short review on the Pt-free electro-catalyst CEs of DSSCs with superior cell compared to Pt CEs; additionally, those selected reports were published within the past 5 years.

Sustained CO2-photoreduction activity and high selectivity over Mn, C-codoped ZnO core-triple shell hollow spheres

Solar conversion of CO₂ into energy-rich products is one of the sustainable solutions to lessen the global energy shortage and environmental crisis. Pitifully, it is still challenging to attain reliable and affordable CO₂ conversion. Herein, we demonstrate a facile one-pot approach to design core-triple shell Mn, C-codoped ZnO hollow spheres as efficient photocatalysts for CO₂ reduction. The Mn ions, with switchable valence states, function as “ionized cocatalyst” to promote the CO₂ adsorption and light harvesting of the system. Besides, they can capture photogenerated electrons from the conduction band of ZnO and provide the electrons for CO₂ reduction. This process is continuous due to the switchable valence states of Mn ions. Benefiting from such unique features, the prepared photocatalysts demonstrated fairly good CO₂ conversion performance. This work is endeavoured to shed light on the role of ionized cocatalyst towards sustainable energy production.

Photoreduction of CO₂ into energy-rich products is a sustainable solution to lessen the global energy and environmental crisis. Here the authors show that Mn ions in Mn, C-codoped ZnO hollow spheres function as “ionized cocatalyst” to promote the CO₂ adsorption and light harvesting to boost the CO₂ photoreduction activity.

Controlled preparation of hollow Zn0.3Cd0.7S nanospheres modified by NiS1.97 nanosheets for superior photocatalytic hydrogen production

Developing durable and efficient photocatalysts for H₂ evolution is highly desirable to expedite current research on solar-chemical energy conversion. In this work, a novel photocatalytic H₂ evolution system based on Zn_0.3Cd_0.7S/NiS_1.97 nanocomposite was rationally designed for the first time. In this advanced composite structure, NiS_1.97 nanosheets as a co-catalyst were intimately coupled to the inner surface of the hollow spherical Zn_0.3Cd_0.7S. The construction of the hollow spherical shell shortened the distance of charge migration to the surface site and increased the multiple absorption of incident light. The introduction of NiS_1.97 nanosheets increased the light absorption capacity of the composite system and also greatly improved the separation and migration behavior of photo-generated carriers due to its narrower band gap and relatively low conduction band position, which had been confirmed by DRS, EIS and PL. As a result, the hollow Zn_0.3Cd_0.7S/NiS_1.97 composite material exhibited excellent photocatalytic activity. At the loading amount of NiS_1.97 up to 15 at.%, the hollow Zn_0.3Cd_0.7S/NiS_1.97 composite exhibited the best photocatalytic activity with a corresponding H₂ production rate of 22.637 mmol g^-1h^-1, which was 1.42 times and 1.85 times that of hollow Zn_0.3Cd_0.7S and solid Zn_0.3Cd_0.7S, respectively. Moreover, this novel catalyst also displayed a long-term stability without apparent debasement in H₂ evolution activity. It is expected that this work could provide new inspiration to the design and development of other highly active photocatalytic systems for water splitting.

Mo2C/C Hierarchical Double-Shelled Hollow Spheres as Sulfur Host for Advanced Li-S Batteries

One of the major challenges in the sulfur cathode of the Li-S batteries is to achieve high sulfur loading, fast Li ions transfer, and lithium polysulfides (LiPSs) shuttling suppressing simultaneously. This issue can be well solved by the development of molybdenum carbide decorated N-doped carbon hierarchical double-shelled hollow spheres (Mo2C/C HDS-HSs). The mesoporous thick inner shell and the central void of the HDS-HSs achieve the high sulfur loading, facilitate the ion/electrolyte penetration, and accelerate the charge transfer. The microporous thin outer shell suppresses the LiPSs shuttling and reduces the charge/mass diffusion distance. The double-shelled hollow structure accommodates the volume expansion during lithiation. Furthermore, Mo2C/C composition renders the HDS-HSs cathode with improved conductivity, enhanced affinity to LiPSs, and accelerated kinetics of LiPSs conversion. The structural and compositional advantages render the Mo2C/C/S HDS-HSs electrode with the high specific capacity, excellent rate capability, and ultra-long cycling stability in the composed Li-S batteries.

Yolk-Shell Nanocapsule Catalysts as Nanoreactors with Various Shell Structures and Their Diffusion Effect on the CO2 Reforming of Methane

Well-geometric-confined yolk-shell catalysts can act as nanoreactors that are of benefit for the antisintering of metals and resistance to coke formation in high-temperature reactions such as the CO₂ reforming of methane. Notwithstanding the credible advances of core/yolk-shell catalysts, the enlarged shell diffusion effects that occur under high space velocity can deactivate the catalysts and hence pose a hurdle for the potential application of these types of catalysts. Here, we demonstrated the importance of the shell thickness and porosity of small-sized Ni@SiO₂ nanoreactor catalysts, which can vary the diffusional paths/rates of the diffusants that directly affect the catalytic activity. The nanoreactor with an ∼4.5 nm shell thickness and rich pores performed the best in tolerating the shell diffusion effects, and importantly, no catalytic deactivation was observed. We further proposed a shell diffusion effect scheme by modifying the Weisz-Prater and blocker model and found that the "gas wall/hard blocker" formed on the openings of the shell pores can cause reversible/irreversible interruption of the shell mass transfer and thus temporarily/permanently deactivate the nanoreactor catalysts. This work highlights the shell diffusion effects, apart from the metal sintering and coke formation, as an important factor that are ascribed to the deactivation of a nanoreactor catalyst.

Advances in Lithium-Sulfur Batteries: From Academic Research to Commercial Viability

Lithium-ion batteries, which have revolutionized portable electronics over the past three decades, were eventually recognized with the 2019 Nobel Prize in chemistry. As the energy density of current lithium-ion batteries is approaching its limit, developing new battery technologies beyond lithium-ion chemistry is significant for next-generation high energy storage. Lithium-sulfur (Li-S) batteries, which rely on the reversible redox reactions between lithium and sulfur, appears to be a promising energy storage system to take over from the conventional lithium-ion batteries for next-generation energy storage owing to their overwhelming energy density compared to the existing lithium-ion batteries today. Over the past 60 years, especially the past decade, significant academic and commercial progress has been made on Li-S batteries. From the concept of the sulfur cathode first proposed in the 1960s to the current commercial Li-S batteries used in unmanned aircraft, the story of Li-S batteries is full of breakthroughs and back tracing steps. Herein, the development and advancement of Li-S batteries in terms of sulfur-based composite cathode design, separator modification, binder improvement, electrolyte optimization, and lithium metal protection is summarized. An outlook on the future directions and prospects for Li-S batteries is also offered.

Supercapattery electrode materials by Design: Plasma-induced defect engineering of bimetallic oxyphosphides for energy storage

Although transition metal hydroxides are promising candidates as advanced supercapattery materials, they suffer from poor electrical conductivity. In this regard, previous studies have typically analyzed separately the impacts of defect engineering at the atomic level and the conversion of hydroxides to phosphides on conductivity and the overall electrochemical performance. Meanwhile, this paper uniquely studies the aforementioned methodologies simultaneously inside an all-in-one simple plasma treatment for nickel cobalt carbonate hydroxide, examines the effect of altering the nickel-to-cobalt ratio in the binder-free defect-engineered bimetallic Ni-Co system, and estimates the respective quantum capacitance. Results show that the concurrent defect-engineering and phosphidation of nickel cobalt carbonate hydroxide boost the amount of effective redox and adsorption sites and increase the conductivity and the operating potential window. The electrodes exhibit ultra-high-capacity of 1462 C g^-1, which is among the highest reported for a nickel-cobalt phosphide/phosphate system. Besides, a hybrid supercapacitor device was fabricated that can deliver an energy density of 48 Wh kg^-1 at a power density of 800 W kg^-1, along with an outstanding cycling performance, using the best performing electrode as the positive electrode and graphene hydrogel as the negative electrode. These results outperform most Ni-Co-based materials, demonstrating that plasma-assisted defect-engineered Ni-Co-P/PO_x is a promising material for use to assemble efficient energy storage devices.

Synthesis of Noble Metal M@YSiO2 Yolk-Shell Nanoparticles with Thin Organic/Inorganic Hybrid Outer Shells via an Aqueous Medium Phase

A simple method is proposed for the synthesis of noble metal M@YSiO₂ (M = Au, Pd, Ag) yolk-shell nanoparticles. The effects of synthesis conditions on the preparation of yolk-shell nanoparticles were discussed in detail. According to the different corrosion resistances between inorganic silica and organosilicone in a selective etching solution, yolk-shell nanoparticles with large cavity and thin shell were prepared using the same aqueous medium in a step-by-step synthesis process. Different from traditional methods, this method is facile and efficient because the main synthesis process is carried out in an aqueous phase. This extended method may benefit the synthesis and application of other nanomaterials with a similar yolk-shell structure.

Nickel/Cobalt Molybdate Hollow Rods Induced by Structure and Defect Engineering as Exceptional Electrode Materials for Hybrid Supercapacitor

Oxygen defects and hollow structures positively impact pseudocapacitive properties of diffusion/surface-controlled processes, a component of critical importance when building high-performance supercapacitors. Hence, we fabricated hollow nickel/cobalt molybdate rods with O-defects (D-H-NiMoO₄ @CoMoO₄ ) through a soft-template and partial reduction method, enhancing D-H-NiMoO₄ @CoMoO₄ 's electrochemical performance, yielding a specific capacitance of 1329 F g^-1 , and demonstrating excellent durability with 95.8 % capacity retention after 3000 cycles. D-H-NiMoO₄ @CoMoO₄ was used as the positive electrode to construct an asymmetric supercapacitor, displaying an energy density of up to 34.13 Wh kg^-1 and demonstrating good predisposition towards practical applications. This work presents an effective approach to fabricate and use hollow nickel/cobalt molybdate rods with O-defects as pseudocapacitor material for high-performance capacitive energy storage devices.

Precisely Engineering Architectures of Co/C Sub-Microreactors for Selective Syngas Conversion

Fischer-Tropsch synthesis (FTS) is an effective route to produce olefins, gasoline, diesel, and oxygenates from syngas (CO + H₂ ). However, it still remains a challenge for regulating the product distribution of FTS. Here, a series of Co/C sub-microreactors with precise designed nanoarchitectures are synthesized for selective syngas conversion. Through a combination of surface protection-assisted etching and following carbonization process, Co/C sub-microreactors with solid cube, double-shelled hollow box, and hollow box architectures, namely, Co/C-Cube, Co/C-DBox, Co/C-Box can be obtained. In FTS, comparing with solid Co/C-Cube, double-shelled hollow structured Co/C-DBox is inclined to grow long-chain hydrocarbon products, whereas hollow structured Co/C-Box avails the formation of short-chain hydrocarbon chemicals. Therefore, shape selective catalysis and controlled product distribution of FTS are realized by tuning the architectures of Co/C sub-microreactors. It is expected to fundamentally unravel the heterogeneous catalytic process via upfront designing and precisely regulating the architectures of micro/nanoreactors.

Surface Polymerization and Controlled Pyrolysis: Tailorable Synthesis of Bumpy Hollow Carbon Spheres for Energy Storage

Architectural design of hollow carbon spheres (HCSs) plays a vital role in improving their performance and expanding applications. The tailorable synthesis of bumpy or asymmetric HCSs with a refined structure remains a challenge. Herein, bumpy HCSs (BHCSs) and bumpy concave HCSs (BCHCSs) have been engineered. The synthesis involves the formation of a core/shell precursor via the surface polymerization of pyrrole monomers on polystyrene nanoparticles, followed by the controlled pyrolysis process under different conditions. In comparison with HCSs, the concave hollow structure can reduce the excessive interior cavity and maintain prevalent merits of hollow structures; the bumpy shell can improve the surface area and number of active sites, thus improving the kinetics as energy storage devices. As a result, among BCHCSs, BHCSs, and HCSs, BCHCSs exhibit optimal electrochemical performance. The lithium-ion hybrid capacitors employing BCHCSs as an anode can deliver an energy density of 0.2182 kW h kg^-1 at a power density of 0.2235 kW kg^-1. Overall, this study provides an innovative design and strategy for constructing unique carbon nano-architectures for energy storage.

Formation of V6O11@Ni(OH)2/NiOOH hollow double-shell nanoflowers for the excellent cycle stability of supercapacitors

The rational design of multi-shelled hollow structured electrode materials is of great importance and has met with fundamental challenges in recent years. Herein, we demonstrate a combination approach of self-templating and sacrificial templating method for synthesizing double-shelled hollow nanoflower-structured V₆O₁₁@Ni(OH)₂/NiOOH. Firstly, highly uniform vanadium-glycerate (VG) solid nanospheres are controllably synthesized and employed as the template, then Ni(OH)₂/NiOOH nanosheets grow vertically on it, following with VG solid nanospheres changing to the V₆O₁₁ hollow structure. By controlling the amount of Ni(OH)₂/NiOOH nanosheets, the optimized V₆O₁₁@Ni(OH)₂/NiOOH-6 (VN-6) delivers high performance for supercapacitors. Specifically, the specific capacitance of VN-6 is 1018.2 F g^-1 at the current density of 1 A g^-1 and the energy density is 24.3 W h kg^-1 at the power density of 850 W kg^-1. Impressively, an outstanding cycling stability of over 120% specific capacitance retention can be obtained after 5000 cycles in the three-electrode and two-electrode systems. The excellent performance can be ascribed to the compositional and structural advantages.

Intricately structured mesoporous organosilica nanoparticles: synthesis strategies and biomedical applications

Intricately structured mesoporous organosilica nanoparticles (IMONs) are being increasingly studied from their synthesis strategies to their use in biomedical applications, because of their distinctive hierarchical structures, excellent physicochemical features and satisfactory biological properties. This minireview is the first to summarize recently developed IMONs, including yolk-shell-structured nanoparticles, multi-shelled hollow spheres, deformable nanocapsules, Janus nanostructures and virus-like bionic-structured nanocarriers, and describe the corresponding formation mechanisms and recent evolution of the strategies used to synthesize these kinds of IMONs. Structure-dependent biomedical applications, such as multidrug delivery, bioimaging, synergistic therapy and biocatalysis, are also discussed. Finally, we provide an outlook for IMONs ranging from their structural control to synthesis strategies and ending with their use in biomedical applications.

Titanium dioxide and graphitic carbon nitride-based nanocomposites and nanofibres for the degradation of organic pollutants in water: a review

The paper reviews graphitic carbon nitride-based nanostructured photocatalytic materials and nanofibres for applications in water purification. Titanium dioxide has shown unique features that continue to attract research and development (R&D) due to its unique properties such as availability, ultraviolet absorptivity, photocatalysis, adsorption of pollutants and solar cell engineering. Graphitic carbon nitride is an attractive photocatalyst due to its non-toxicity characteristics, good visible light absorption and good thermal and chemical stabilities. In water purification, nanofibres are currently noticed due to their distinctive properties of effective separation and sometimes elimination of organic pollutants in water. In this review, synthesis and utility of doped titanium dioxide and carbon nitride with metal nanoparticles and polymeric nanofibres from nanocomposites as effective materials for the degradation of organic contaminations from water are discussed. The history, current trends and future perspectives are highlighted.

General Programmable Growth of Hybrid Core-Shell Nanostructures with Liquid Metal Nanodroplets

Core-shell and hollow nanostructures have been receiving significant interest due to their potential in wide scientific and technological fields. Given such large scope, however, they still lag far behind in terms of the ambition toward controllably, or even programmatically, synthesizing libraries of core-shell structures on a large scale. Here, a general route for the programmable preparation of complex core-shell nanostructures by using liquid metal (LM) droplets as reformable templates is presented, and the triggering of a localized galvanic replacement reaction in one ultrasonication system is demonstrated. Benefiting from the activity and mobility of the metal components in LM templates, high-level compositional diversity control and quantitative regulation of both the core and the shell layers of the heterogeneous products are achieved, which cannot be realized with a solid-template synthetic route.

Engineering porous Pd-Cu nanocrystals with tailored three-dimensional catalytic facets for highly efficient formic acid oxidation

Rational synthesis of bi- or multi-metallic nanomaterials with both dendritic and porous features is appealing yet challenging. Herein, with the cubic Cu₂O nanoparticles composed of ultrafine Cu₂O nanocrystals as a self-template, a series of Pd-Cu nanocrystals with different morphologies (e.g., aggregates, porous nanodendrites, meshy nanochains and porous nanoboxes) are synthesized through simply regulating the molar ratio of the Pd precursor to the cubic Cu₂O, indicating that the galvanic replacement and Kirkendall effect across the alloying process are well controlled. Among the as-developed various Pd-Cu nanocrystals, the porous nanodendrites with both dendritic and hollow features show superior electrocatalytic activity toward formic acid oxidation. Comprehensive characterizations including three-dimensional simulated reconstruction of a single particle and high-resolution transmission electron microscopy reveal that the surface steps, defects, three-dimensional architecture, and the electronic/strain effects between Cu and Pd are responsible for the outstanding catalytic activity and excellent stability of the Pd-Cu porous nanodendrites.

Tunable Synthesis of Hierarchical Yolk/Double-Shelled SiOx @TiO2 @C Nanospheres for High-Performance Lithium-Ion Batteries

This work reports the preparation of unique hierarchical yolk/double-shelled SiO_x @TiO₂ @C nanospheres with different voids by a facile sol-gel method combined with carbon coating. In the preparation process, SiO_x nanosphere is used as a hard template. Etch time of SiO_x yolk affects the morphology and electrochemical performance of SiO_x @TiO₂ @C. With the increase in etch time, the yolk/double-shelled SiO_x @TiO₂ @C with 15 and 30 nm voids and the TiO₂ @C hollow nanospheres are obtained. The yolk/double-shelled SiO_x @TiO₂ @C nanospheres exhibit remarkable lithium-ion battery performance as anodes, including high lithium storage capacity, outstanding rate capability, good reversibility, and stable long-term cycle life. The unique structure can accommodate the large volume change of the SiO_x yolk, provide a unique buffering space for the discharge/charge processes, improve the structural stability of the electrode material during repeated Li⁺ intercalation/deintercalation processes, and enhance the cycling stability. The SiO_x @TiO₂ @C with 30 nm void space exhibits a high discharge specific capacity of ≈1195.4 mA h g^-1 at the current density of 0.1 A g^-1 after 300 cycles and ≈701.1 mA h g^-1 at 1 A g^-1 for over 800 cycles. These results suggest that the proposed particle architecture is promising and may have potential applications in improving various high performance anode materials.

Mesoporous Nanostructures Encapsulated with Metallic Nanodots for Smart SERS Sensing

In virtue of uniform mesopores and core-shell nanoarchitectures, metallic nanodot-encapsulated hollow mesoporous nanostructures have shown promising potential in various applications. However, their fabrication with versatile tunability of the encapsulated metallic content has been a challenge. Herein, we have prepared metallic nanodot-encapsulated hollow mesoporous silica nanoparticles (M-HMSNPs) with adjustable inner metallic components. The sacrificial template of polystyrene (PS) nanoparticles precoated with metals (Au/Ag/Pt) is fully wrapped with mesoporous silica (mSiO₂). The metallic nanodots are formed during the template removal process by calcination. The type and content of the encapsulated nanodots can be readily and precisely controlled by the initially deposited metallic layers. We demonstrate the application of the gold (Au) nanodot-loaded HMSNPs (denoted Au-HMSNPs) as smart surface-enhanced Raman spectroscopy (SERS) probes, which can screen between big molecules and small analytes. With the aid of a Raman reporter, the SERS probe can successfully quantify H₂O₂, which is used to distinguish cancer cells in vitro. Further integrated with enzymes, the SERS chips of specificity are prepared and used to detect corresponding substrates of glucose and uric acid, responsively. Besides SERS sensing, the current strategy can inspire future development of many other M-HMSNPs for various applications such as catalysis, energy storage, theranostics, etc.

Size-controllable synthesis of Zn2GeO4 hollow rods supported on reduced graphene oxide as high-capacity anode for lithium-ion batteries

Germanium-based ternary oxides have aroused wide attention as an anode for high-performance lithium-ion batteries (LIBs). Nevertheless, they usually suffer a large volume expansion and rapid capacity fading during lithiation/delithiation cycles. To address this issue, herein, Zn₂GeO₄/RGO composites are synthesized with Zn₂GeO₄ hollow rods in-situ grown on reduced graphene oxide (RGO) sheets. The Zn₂GeO₄ hollow rods can be facilely adjusted from nano- to micro-size. The lithium storage performances of the composites strongly depend on the size of Zn₂GeO₄ hollow rods and the content of RGO. The optimized Zn₂GeO₄/RGO composite exhibits a pseudocapacitance-dominated Li⁺ storage performance, with a large reversible capacity of 1005 mAh g^-1 after 100 cycles at 0.5 A g^-1, an excellent rate capability (515 mAh g^-1 at a high rate of 5 A g^-1) and a good long cycling stability of 500 cycles with a low capacity loss of 0.05% per cycle at 1 A g^-1. The outstanding electrochemical performance can be attributed to the unique composition and microstructure of the material as well as the synergistic effect of the conductive RGO sheets and the hollow Zn₂GeO₄ nanostructure. This work provides a promising anode for high-performance LIBs and a useful inspiration for further improving the Ge-based ternary oxide anodes.

Facile fabrication of hollow CuO nanocubes for enhanced lithium/sodium storage performance

Facile and low-toxicity fabrication of CuO hollow nanocubes towards high-performance alkali ion batteries.

TiO2/g-C3N4 heterojunction hollow porous nanofibers as superior visible-light photocatalysts for H2 evolution and dye degradation

Hollow porous TiO2/g-C3N4 exhibited high photocatalytic performance ascribed to multiple light reflections and fast photoelectron transport in heterojunctions.

Synthesis, structural evolution, and optical properties of SnO2 hollow microspheres with manageable shell thickness

SnO2 hollow (H-SnO2) microspheres were successfully prepared via a facile one-step synthesis using SiO2 microspheres as templates and NaOH as a reactive etching agent.

Bioinspired Assembly of Double Honeycomb-Like Hierarchical Capsule Confined Encapsulation with Functional Micro/Nanocrystals

Inspired by "micro/nanoreactor" effect of cellular organelle on specific biochemical reactions, a double honeycomb-like hierarchical capsule confined encapsulation with functional micro/nanocrystals is designed. The bioinspired hierarchical capsules derived from polymeric composite microspheres are successfully fabricated through a combination of selective chemical etching and pyrolysis. In situ introduction of functional guests (including organometallic molecules, tetraethoxysilane, or metal-organic frameworks (MOFs)) into internal cellular structure of microspheres is first put forward by phase inversion method. The development of selective etching creates honeycomb-like structure on the outside surface of capsule and allows sulfur to homogeneously distribute into matrix. With the novel approach, the hierarchical channels (micro-meso-macropore) of composite capsule enhance transportation of reactants and dispersion of active sites, and thus exhibit superior photocatalytic oxidation and electromagnetic absorbing. The promising strategy will be applied more generally to encapsulate different species into hierarchical capsule with tailored properties and functionalities.

Towards High Performance Chemical Vapour Deposition V2O5 Cathodes for Batteries Employing Aqueous Media

The need for clean and efficient energy storage has become the center of attention due to the eminent global energy crisis and growing ecological concerns. A key component in this effort is the ultra-high performance battery, which will play a major role in the energy industry. To meet the demands in portable electronic devices, electric vehicles, and large-scale energy storage systems, it is necessary to prepare advanced batteries with high safety, fast charge ratios, and discharge capabilities at a low cost. Cathode materials play a significant role in determining the performance of batteries. Among the possible electrode materials is vanadium pentoxide, which will be discussed in this review, due to its low cost and high theoretical capacity. Additionally, aqueous electrolytes, which are environmentally safe, provide an alternative approach compared to organic media for safe, cost-effective, and scalable energy storage. In this review, we will reveal the industrial potential of competitive methods to grow cathodes with excellent stability and enhanced electrochemical performance in aqueous media and lay the foundation for the large-scale production of electrode materials.

Steering Hollow Multishelled Structures in Photocatalysis: Optimizing Surface and Mass Transport

Hollow multishelled structures (HoMSs) provide a promising platform for fabricating photocatalysts, because the unique structure optimizes the effective surface and mass transport, showing enhanced light absorption, optimized mass transport and highly effective active sites exposed. Subsequently, the rational design on HoMS photocatalytsts is elaborated to boost the photocatalytic activity with efforts in all dimensions, from nanoscale to microscale. Breakthroughs in synthetic methodology of HoMSs have greatly evoked the prosperous photocatalytic researches for HoMSs since the developing of sequential templating approach in 2009. The dawn of HoMS photocatalyst is coming after revealing the temporal-spatial ordering property, which is also discussed in this paper with pioneer works demonstrating the greatly enhanced energy/mass transfer processes. Some insights into the key challenges and perspectives of HoMSs photocatalysts are also discussed. With the reviewed fate and future of HoMSs photocatalysts, hopefully new concepts and innovative works can be inspired to flourish this sun-rise field.

Efficient sequential harvesting of solar light by heterogeneous hollow shells with hierarchical pores

In nature, sequential harvesting of light widely exists in the old life entity, i.e. cyanobacteria, to maximize the light absorption and enhance the photosynthesis efficiency. Inspired by nature, we propose a brand new concept of temporally-spatially sequential harvesting of light in one single particle, which has purpose-designed heterogeneous hollow multi-shelled structures (HoMSs) with porous shells composed of nanoparticle subunits. Structurally, HoMSs consist of different band-gap materials outside-in, thus realizing the efficient harvesting of light with different wavelengths. Moreover, introducing oxygen vacancies into each nanoparticle subunit can also enhance the light absorption. With the benefit of sequential harvesting of light in HoMSs, the quantum efficiency at wavelength of 400 nm is enhanced by six times compared with the corresponding nanoparticles. Impressively, using these aforementioned materials as photocatalysts, highly efficient photocatalytic water splitting is realized, which cannot be achieved by using the nanoparticle counterparts. This new concept of temporally-spatially sequential harvesting of solar light paves the way for solving the ever-growing energy demand.

Fabrication of Magnetic Superstructure NiFe2O4@MOF-74 and Its Derivative for Electrocatalytic Hydrogen Evolution with AC Magnetic Field

As an ideal hydrogen production route, electrolyzed water still faces the challenges of high cost of noble-metal electrocatalysts and low performance of non-noble-metal catalysts in scalable applications. Recently, introduction of external fields (such as magnetic fields, light fields, etc.) to improve the electrocatalytic water splitting performance of non-noble-metal catalysts has attracted great attention due to their simplicity. Here, a simple method for preparing magnetic superstructure (NiFe₂O₄@MOF-74) is described, and the hydrogen evolution reaction (HER) behavior of its carbonized derivative, a ferromagnetic superstructure, is revealed in a wide range of applied voltage under an AC magnetic field. The overpotential (@10 mA cm^-2) required for the HER of the obtained ferromagnetic superstructure in 1 M KOH was reduced by 31 mV (7.7%) when a much small AC magnetic field (only 2.3 mT) is applied. Surprisingly, the promotion effect of the AC magnetic field is not monotonically increasing with the increase of the applied voltage or the strength of AC magnetic field, but increasing first, then weakening. This unusual behavior is believed to be mainly caused by the enhanced induced electromotive force and the additional energy by the applied AC magnetic field. This discovery provides a new idea for adjusting the performance of electrocatalytic reactions.

Preparations of NiFe2O4 Yolk-Shell@C Nanospheres and Their Performances as Anode Materials for Lithium-Ion Batteries

At present, lithium-ion batteries (LIBs) have received widespread attention as substantial energy storage devices; thus, their electrochemical performances must be continuously researched and improved. In this paper, we demonstrate a simple self-template solvothermal method combined with annealing for the synthesis of NiFe2O4 yolk-shell (NFO-YS) and NiFe2O4 solid (NFO-S) nanospheres by controlling the heating rate and coating them with a carbon layer on the surface via high-temperature carbonization of resorcinol and formaldehyde resin. Among them, NFO-YS@C has an obvious yolk-shell structure, with a core-shell spacing of about 60 nm, and the thicknesses of the NiFe2O4 shell and carbon shell are approximately 15 and 30 nm, respectively. The yolk-shell structure can alleviate volume changes and shorten the ion/electron diffusion path, while the carbon shell can improve conductivity. Therefore, NFO-YS@C nanospheres as the anode materials of LIBs show a high initial capacity of 1087.1 mA h g-1 at 100 mA g-1, and the capacity of NFO-YS@C nanospheres impressively remains at 1023.5 mA h g-1 after 200 cycles at 200 mA g-1. The electrochemical performance of NFO-YS@C is significantly beyond NFO-S@C, which proves that the carbon coating and yolk-shell structure have good stability and excellent electron transport ability.

Ternary MOF-on-MOF heterostructures with controllable architectural and compositional complexity via multiple selective assembly

Assembly of different metal-organic framework (MOF) building blocks into hybrid MOF-on-MOF heterostructures is promising in chemistry and materials science, however the development of ternary MOF-on-MOF heterostructures with controllable architectural and compositional complexity is challenging. Here we report the synthesis of three types of ternary MOF-on-MOF heterostructures via a multiple selective assembly strategy. This strategy relies on the choice of one host MOF with more than one facet that can arrange the growth of a guest MOF, where the arrangement is site-selective without homogenous growth of guest MOF or homogenous coating of guest on host MOF. The growth of guest MOF on a selected site of host MOF in each step provides the opportunity to further vary the combinations of arrangements in multiple steps, leading to ternary MOF-on-MOF heterostructures with tunable complexity. The developed strategy paves the way towards the rational design of intricate and unprecedented MOF-based superstructures for various applications.

Shaping Nanoparticles for Interface Catalysis: Concave Hollow Spheres via Deflation-Inflation Asymmetric Growth

Hollow spheres are charming objects in nature. In this work, an unexpected deflation-inflation asymmetric growth (DIAG) strategy is reported, generating hollow nanoparticles with tailored concave geometry for interface catalysis. Starting from aminophenol-formaldehyde (APF) nanospheres where the interior crosslinking degree is low, fully deflated nanobowls are obtained after etching by acetone. Due to APF etching and repolymerization reactions occuring asymmetrically within a single particle, an autonomous inflation process is observed similar to a deflated basketball that inflates back to a "normal" ball, which is rare at the nanoscale. A nucleophilic addition reaction between acetone and APF is elucidated to explain the chemistry origin of the DIAG process. Interestingly, the deflated APF hollow spheres enable preferential immobilization of lipase in the concave domain, which facilitates the stabilization of Pickering emulsion droplets for enhanced enzymatic catalysis at the oil-water interface. The study provides new understandings in the designable synthesis of hollow nanoparticles and paves the way toward a wide range of applications of asymmetric architectures.

Lithium-Sulfur Batteries: Advances and Trends

A review with 132 references. Societal and regulatory pressures are pushing industry towards more sustainable energy sources, such as solar and wind power, while the growing popularity of portable cordless electronic devices continues. These trends necessitate the ability to store large amounts of power efficiently in rechargeable batteries that should also be affordable and long-lasting. Lithium-sulfur (Li-S) batteries have recently gained renewed interest for their potential low cost and high energy density, potentially over 2600 Wh kg−1. The current review will detail the most recent advances in early 2020. The focus will be on reports published since the last review on Li-S batteries. This review is meant to be helpful for beginners as well as useful for those doing research in the field, and will delineate some of the cutting-edge adaptations of many avenues that are being pursued to improve the performance and safety of Li-S batteries.

Boosting the energy density of supercapacitors by encapsulating a multi-shelled zinc-cobalt-selenide hollow nanosphere cathode and a yolk-double shell cobalt-iron-selenide hollow nanosphere anode in a graphene network

The practical exploration of electrode materials with complex hollow structures is of considerable significance in energy storage applications. Mixed-metal selenides (MMSs) with favorable architectures emerge as new electrode materials for supercapacitor (SC) applications owing to their excellent conductivity. Herein, a facile and effective metal-organic framework (MOF)-derived strategy is introduced to encapsulate multi-shelled zinc-cobalt-selenide hollow nanosphere positive and yolk-double shell cobalt-iron-selenide hollow nanosphere negative electrode materials with controlled shell numbers in a graphene network (denoted as G/MSZCS-HS and G/YDSCFS-HS, respectively) for SC applications. Due to the considerable electrical conductivity and unique structures of both electrodes, the G/MSZCS-HS positive and G/YDSCFS-HS negative electrodes exhibit remarkable capacities (∼376.75 mA h g^-1 and 293.1 mA h g^-1, respectively, at 2 A g^-1), superior rate performances (83.4% and 74%, respectively), and an excellent cyclability (96.8% and 92.9%, respectively). Furthermore, an asymmetric device (G/MSZCS-HS//G/YDSCFS-HS) has been fabricated with the ability to deliver an exceptional energy density (126.3 W h kg^-1 at 902.15 W kg^-1), high robustness of 91.7%, and a reasonable capacity of 140.3 mA h g^-1.

Integrated Design of Hierarchical CoSnO3@NC@MnO@NC Nanobox as Anode Material for Enhanced Lithium Storage Performance

Transition-metal oxides (TMOs) are potential candidates for anode materials of lithium-ion batteries (LIBs) due to their high theoretical capacity (∼1000 mA h/g) and enhanced safety from suppressing the formation of lithium dendrites. However, the poor electron conductivity and the large volume expansion during lithiation/delithiation processes are still the main hurdles for the practical usage of TMOs as anode materials. In this work, the CoSnO3@NC@MnO@NC hierarchical nanobox (CNMN) is then proposed and fabricated to solve those issues. The as-prepared nanobox contains hollow cubic CoSnO3 as a core and dual N-doped carbon-"sandwiched" MnO particles as a shell. As anode materials of LIBs, the hollow and carbon interlayer structures effectively accommodate the volume expansion while dual active TMOs of CoSnO3 and MnO efficiently increase the specific capacity. Notably, the dual-layer structure of N-doped carbons plays a critical functional role in the incorporated composites, where the inner layer serves as a reaction substrate and a spatial barrier and the outer layer offers electron conductivity, enabling more effective involvement of active anode materials in lithium storage, as well as maintaining their high activity during lithium cycling. Subsequently, the as-prepared CNMN exhibits a high specific capacity of 1195 mA h/g after the 200th cycle at 0.1C and an excellent stable reversible capacity of about 876 mA h/g after the 300th cycle at 0.5C with only 0.07 mA h/g fade per cycle after 300 cycles. Even after a 250 times fast charging/discharging cycle both at 5C, it still retains a reversible capacity of 422.6 mA h/g. We ascribe the enhanced lithium storage performances to the novel hierarchical architectures achieved from the rational design.

3D structured materials and devices for artificial photosynthesis

Artificial photosynthesis is an effective way to convert solar energy into fuels, which is of great significance to energy production and reduction of atmospheric CO₂ content. In recent years, 3D structured artificial photosynthetic system has made great progress as an effective design strategy. This review first highlights several typical mechanisms for improved artificial photosynthesis with 3D structures: improved light harvesting, mass transfer and charge separation. Then, we summarize typical examples of 3D structured artificial photosynthetic systems, including bioinspired structures, photonic crystals (PC), designed photonic structures (PC coupling structure, plasmon resonance structure, optical resonance structure, metamaterials), 3D-printed systems, nanowire integrated systems and hierarchical 3D structures. Finally, we discuss the problems and challenges to the application and development of 3D artificial photosynthetic system and the possible trends of future development. We hope this review can inspire more progress in the field of artificial photosynthesis.

3.3 nm-sized TiO2/carbon hybrid spheres endowed with pseudocapacitance-dominated superhigh-rate Li-ion and Na-ion storage

Decreasing the particle size of nanoscaled battery materials will induce amazing enhancement effects on their charging rates, which holds a promise to overcome the common bottleneck of the low charging rates of batteries. However, the fabrication of ultrafine-sized battery materials remains a great challenge. Herein, 3.3 nm-sized anatase TiO2 particles embedded in electrically and ionically conductive carbon spheres have been designed and fabricated via the suppression of Ostwald ripening with the aim to obtain insight into the electrochemical behaviors of ultrafine-sized materials. The pseudocapacitive and diffusion-controlled intercalative characteristics of the 3.3 nm-sized TiO2/carbon hybrid spheres for Li-ion and Na-ion storage have been systematically investigated via a cyclic voltammetry (CV) method combined with a differential capacitance method that is introduced here for the first time to analyze battery materials. CV and galvanostatic voltage profiles demonstrate that pseudocapacitance dominates the charge storage and increases with cycling for both Li-ion and Na-ion storage. Capacitance accounts for >83% of the Li-ion storage. A specific pseudocapacitance of 558 F g-1 with a window voltage of ∼2 V in carbonate electrolyte has been achieved. The reversible capacity is higher than the theoretical capacity of TiO2 after 600 discharge/charge cycles at 2 C and maintains ∼60% of that of TiO2 even at 80 C (45 s for full discharge or charge). For Na-ion storage, a high cycliability of 2500 discharge/charge cycles has been obtained at 2 C. Capacitance accounts for ∼79% of the Na-ion storage with cycling. Ultrafine-sized materials are very promising electrode candidates for constructing pseudocapacitive batteries possessing both high energy and power densities.