The Research Development of Quantum Dots in Electrochemical Energy Storage

Application of Inorganic Quantum Dots in Advanced Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have emerged as one of the most attractive alternatives for post-lithium-ion battery energy storage systems, owing to their ultrahigh theoretical energy density. However, the large-scale application of Li-S batteries remains enormously problematic because of the poor cycling life and safety problems, induced by the low conductivity , severe shuttling effect, poor reaction kinetics, and lithium dendrite formation. In recent studies, catalytic techniques are reported to promote the commercial application of Li-S batteries. Compared with the conventional catalytic sites on host materials, quantum dots (QDs) with ultrafine particle size (<10 nm) can provide large accessible surface area and strong polarity to restrict the shuttling effect, excellent catalytic effect to enhance the kinetics of redox reactions, as well as abundant lithiophilic nucleation sites to regulate Li deposition. In this review, the intrinsic hurdles of S conversion and Li stripping/plating reactions are first summarized. More importantly, a comprehensive overview is provided of inorganic QDs, in improving the efficiency and stability of Li-S batteries, with the strategies including composition optimization, defect and morphological engineering, design of heterostructures, and so forth. Finally, the prospects and challenges of QDs in Li-S batteries are discussed.

Interlayer Sodium Plating/Stripping in Van der Waals-Layered Quantum Dot Superstructure

Assembling quantum dots (QDs) into van der Waals (vdW)-layered superstructure holds great promise for the development of high-energy-density metal anode. However, designing such a superstructure remains to be challenging. Here, a chemical-vapor Oriented Attachment (OA) growth strategy is proposed to achieve the synthesis of vdW-layered carbon/QDs hybrid superlattice nanosheets (Fe₇ S₈ @CNS) with a large vdW gap of 3 nm. The Fe₇ S₈ @CNS superstructure is assembled by carbon-coated Fe₇ S₈ (Fe₇ S₈ @C) QDs as building blocks. Interestingly, the Fe₇ S₈ @CNS exhibits two kinds of edge dislocations similar to traditional atom-layered materials, suggesting that Fe₇ S₈ @C QDs exhibit quasi-atomic growth behavior during the OA process. More interestingly, when used as host materials for sodium metal anodes, the Fe₇ S₈ @CNS shows the interlayer sodium plating/stripping behavior, which well suppresses Na dendrite growth. As a result, the cell with Fe₇ S₈ @CNS anode can keep stable cycling for 1000 h with a high Coulombic efficiency (CE) of ≈99.5% at 3.0 mA cm^-2 and 3.0 mAh cm^-2 . Noticeably, the Na@Fe₇ S₈ @CNS||Na₃ V₂ (PO₄ )₃ full cells can attain a capacity of 88.8 mAh g^-1 with a retention of 97% after 1000 cycles at 1.0 A g^-1 (≈8 C), showing excellent cycle stability for practical applications. This work enriches the vdW-layered QDs superstructure family and their application toward energy storage.

Advances in Electrostatic Spinning of Polymer Fibers Functionalized with Metal-Based Nanocrystals and Biomedical Applications

Considering the metal-based nanocrystal (NC) hierarchical structure requirements in many real applications, starting from basic synthesis principles of electrostatic spinning technology, the formation of functionalized fibrous materials with inorganic metallic and semiconductor nanocrystalline materials by electrostatic spinning synthesis technology in recent years was reviewed. Several typical electrostatic spinning synthesis methods for nanocrystalline materials in polymers are presented. Finally, the specific applications and perspectives of such electrostatic spun nanofibers in the biomedical field are reviewed in terms of antimicrobial fibers, biosensing and so on.

Research on Preparation Methods of Carbon Nanomaterials Based on Self-Assembly of Carbon Quantum Dots

Here, based on self-assembly of carbon quantum dots (CDs), an innovative method to prepare nanomaterials under the action of a metal catalyst was presented. CDs were synthesized by a one-step hydrothermal method with citric acid (CA) as the carbon source, ethylenediamine (EDA) as the passivator and FeSO₄•7H₂O as the pre-catalyst. In the experiment, it was found that the nano-carbon films with a graphene-like structure were formed on the surface of the solution. The structure of the films was studied by high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FT-IR), etc. The results demonstrated that the films were formed by the self-assembly of CDs under the action of the gas–liquid interface template and the metal catalyst. Meanwhile, the electrochemical performance of the films was evaluated by linear cyclic voltammetry (CV) and galvanostatic charge discharge (GOD) tests. In addition, the bulk solution could be further reacted and self-assembled by reflux to form a bifunctional magnetic–fluorescent composite material. Characterizations such as X-ray diffractometer (XRD), fluorescence spectra (FL), vibrating sample magnetometer (VSM), etc. revealed that it was a composite of superparamagnetic γ-Fe₂O₃ and CDs. The results showed that self-assembly of CDs is a novel and effective method for preparing new carbon nanomaterials.

Recent Developments of Transition Metal Compounds-Carbon Hybrid Electrodes for High Energy/Power Supercapacitors

Due to their rapid power delivery, fast charging, and long cycle life, supercapacitors have become an important energy storage technology recently. However, to meet the continuously increasing demands in the fields of portable electronics, transportation, and future robotic technologies, supercapacitors with higher energy densities without sacrificing high power densities and cycle stabilities are still challenged. Transition metal compounds (TMCs) possessing high theoretical capacitance are always used as electrode materials to improve the energy densities of supercapacitors. However, the power densities and cycle lives of such TMCs-based electrodes are still inferior due to their low intrinsic conductivity and large volume expansion during the charge/discharge process, which greatly impede their large-scale applications. Most recently, the ideal integrating of TMCs and conductive carbon skeletons is considered as an effective solution to solve the above challenges. Herein, we summarize the recent developments of TMCs/carbon hybrid electrodes which exhibit both high energy/power densities from the aspects of structural design strategies, including conductive carbon skeleton, interface engineering, and electronic structure. Furthermore, the remaining challenges and future perspectives are also highlighted so as to provide strategies for the high energy/power TMCs/carbon-based supercapacitors.

Multi-Yolk-Shell MnO@Carbon Nanopomegranates with Internal Buffer Space as a Lithium Ion Battery Anode

Multi-yolk-shell MnO@mesoporous carbon (MnO@m-carbon) nanopomegranates, featuring MnO nanoparticles within cavities of m-carbon with internal space between the MnO nanoparticle and a cavity carbon shell, were subtly constructed. Moreover, the buffer space was well controlled by means of regulating the size of the cavity in m-carbon or the content of MnO. The results of electrochemical measurements demonstrated that MnO(10)@m-carbon(22) nanopomegranates (MnO nanoparticle, 15 nm; cavity size, 22 nm) had the best cycling and rate performance for lithium ion storage. The pomegranate-like MnO@m-carbon nanostructures have shown several advantages for their excellent performance: the nanocavity in m-carbon can restrict the growth and agglomeration of MnO nanoparticles; the well-interconnected mesoporous carbon matrix provides a "highway" for electrons and lithium ion transport; the voids between the MnO nanoparticle and cavity shell can alleviate the volume expansion.

The study of chiral recognition on ibuprofen enantiomers by a fluorescent probe based on β-cyclodextrin modified ZnS:Mn quantum dots

In this paper, a fluorescence method for chiral detection of ibuprofen and its enantiomer was developed. The L-cystenine-capped ZnS:Mn quantum dots were synthesized and functionalized with β-cyclodextrin (β-CD-QDs). The β-CD-QDs exhibited different quenching effect to the S-(+)-ibuprofen and the R-(-)-ibuprofen based on the advantage of the inclusion complex of cyclodextrin. It was found that the quenching of β-CD-QDs by S-(+)-ibuprofen was due to the formation of inclusion complex through both static quenching and photoinduced electron transfer, but only slight quenching with the R-(-)-ibuprofen. The stability constants derived from Hildebrand-Benesi method and absorption titration experiments were applied to determine the stability constants of the formed complexes, the double reciprocal plots suggest that a conclusion complex with a ratio of 1:1 was formed between β-CD-QDs and S-(+)-ibuprofen, but did not with the R-(-)-ibuprofen. The fluorescence intensity of the β-CD-QDs was linearly dependent on the concentration of the S-(+)-IBP in the range of 0-0.5 nmol/L with an limit of detection of 0.29 nmol/L.

Compressible, anisotropic lamellar cellulose-based carbon aerogels enhanced by carbon dots for superior energy storage and water deionization

Heteroatom-doped carbon materials have received great attention for applications in electrode materials. However, conventional heteroatom-doping methods sacrifice conductivity, stability, and specific surface area (SSA). Here, the carbon quantum dots (CDs) are used as carriers of N, P, O to form electron-rich regions promoting electron transport without decreasing stability and SSA. The CDs promote the formation of graphitic nitrogen in the composite, which effectively reduces their internal resistance by increasing the dielectric constant. Moreover, the orderly growth of ice crystals generates a unique bridged layer structure under bidirectional freeze-casting in a mixture of GO/CDs/microfibrillated cellulose, which gives the composite super-compressibility. Notably, the optimal sample has a 117% increase in specific capacitance. The CDs also improve wettability and thus reduce the charge transfer resistance giving a large desalination capacity of 32.59 mg g^-1 (504 mg L^-1 NaCl). This work illustrates the unique role of CDs in improving the electrochemical performance of composites.

Electrochemistry in Carbon-based Quantum Dots

Electrochemistry belongs to an important branch of chemistry that deals with the chemical changes produced by electricity and the production of electricity by chemical changes. Therefore, it can not only act a powerful tool for materials synthesis, but also offer an effective platform for sensing and catalysis. As extraordinary zero-dimensional materials, carbon-based quantum dots (CQDs) have been attracting tremendous attention due to their excellent properties such as good chemical stability, environmental friendliness, nontoxicity and abundant resources. Compared with the traditional methods for the preparation of CQDs, electrochemical (EC) methods offer advantages of simple instrumentation, mild reaction conditions, low cost and mass production. In return, CQDs could provide cost-effective, environmentally friendly, biocompatible, stable and easily-functionalizable probes, modifiers and catalysts for EC sensing. However, no specific review has been presented to systematically summarize both aspects until now. In this review, the EC preparation methods of CQDs are critically discussed focusing on CQDs. We further emphasize the applications of CQDs in EC sensors, electrocatalysis, biofuel cells and EC flexible devices. This review will further the experimental and theoretical understanding of the challenges and future prospective in this field, open new directions on exploring new advanced CQDs in EC to meet the high demands in diverse applications.

Ti3C2Tx Nanosheets/Ti3C2Tx Quantum Dots/RGO (Reduced Graphene Oxide) Fibers for an All-Solid-State Asymmetric Supercapacitor with High Volume Energy Density and Good Flexibility

By using Ti₃C₂T_x quantum dots as interlayer spacers, Ti₃C₂T_x nanosheets/Ti₃C₂T_x quantum dots/RGO (reduced graphene oxide) fiber (M₆M₃RG₁) is prepared by a wet-spinning method; it shows good capacitance and excellent flexibility. The M₆M₃RG₁ fiber electrode possesses a novel network structure and a maximum volumetric capacitance of 542 F cm^-3, and its capacitance and flexibility are affected by the amount of Ti₃C₂T_x quantum dots. Also, the Ti₃C₂T_x/PEDOT:PSS [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)] fiber (M₇P₃) is prepared by injecting a homogeneous suspension of Ti₃C₂T_x nanosheets and PEDOT:PSS into a bath of 98 wt % H₂SO₄. The M₆M₃RG₁ fiber is used as the positive electrode, and the M₇P₃ fiber is used as the negative electrode; a M₆M₃RG₁//M₇P₃ asymmetric, flexible, solid-state supercapacitor is assembled in a PVA-H₂SO₄ gel electrolyte. The assembled device exhibits a volumetric capacitance of 53.1 F cm^-3 and a good cycle stability of 96.6% after 5000 cycles. It also shows outstanding flexibility and mechanical properties; for example, the volumetric capacitance has no obvious change after the device is bent at 90° for 500 times. Moreover, its voltage window can be expanded to 1.5 V, and a maximum volumetric energy density of 16.6 mWh cm^-3 can be achieved. This work will open up a new application area for new wearable energy storage devices based on the Ti₃C₂T_x fibers.

In situ green synthesis of highly fluorescent Fe2 O3 @CQD/graphene oxide using hard pistachio shells via the hydrothermal-assisted ball milling method

In this study, highly photoluminescent and photocatalytic Fe₂ O₃ @carbon quantum dots/graphene oxide nanostructures were synthesized using ball milling-assisted hydrothermal synthesis with hard pistachio shells. Different analyses, such as X-ray diffraction, energy dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy were used to study the product structure. Scanning electron microscopy and transmission electron microscopy images were used to study product size and morphology. Optical properties of the as-synthesized nanomaterials were investigated using ultraviolet-visible light and photoluminescence analyses. To increase photoluminescence intensity, ethylene diamine tetraacetic acid, polyethylene glycol, polyvinylpyrrolidone, and acetylacetonate anions were used to modify the product surface. Thermal stability of the product was studied using thermal gravimetric analysis. Finally, photocatalytic activity and surface adsorption of the product were investigated; the produce was found to be highly photoluminescent with high photocatalytic and surface activities.

Superior performance of flexible solid-state supercapacitors enabled by ultrafine graphene quantum dot-decorated porous carbon spheres

Graphene quantum dot-decorated porous carbon spheres were synthesized via a facile and green route in this study.

Ultrafast Zinc-Ion Diffusion Ability Observed in 6.0-Nanometer Spinel Nanodots

Rechargeable aqueous Zn-ion batteries (ZIBs) have recently attracted much attention due to their low cost and superior safety. Unfortunately, their low capacity and poor cycle life still hinder their practical application. Here, we have developed a general synthesis strategy for ultrasmall spinel oxide nanodots (Mn₃O₄, CoMn₂O₄, MnCo₂O_4.5, Co₃O₄, and ZnMn₂O₄) with abundant oxygen vacancies and highly active surface. Among them, 6.0-nanometer-sized Mn₃O₄ nanodots deliver the best Zn-ion storage ability with a high reversible capacity of 386.7 mA h g^-1 at 0.1 A g^-1, excellent rate performance, and a long-term stability of 500 cycles at 0.5 A g^-1. Taking advantage of the highly activated surficial atoms, shortened transfer pathway, and introduction of numerous oxygen vacancies, an ultrahigh Zn²⁺ diffusion coefficient of 2.4 × 10^-10 cm² s^-1 has been detected during the discharge process. This value is more than 2 orders of magnitude higher than that of other spinel oxide nanostructures in previous reports and also the highest one in all of the as-reported ZIB cathode materials to date. Our finding offers promising opportunities for the development of ZIB cathode materials with high energy density, long-term cycling stability, excellent flexibility, and wearability.

Nanomaterials-based Electrochemical Immunosensors

With the development of nanomaterials and sensor technology, nanomaterials-based electrochemical immunosensors have been widely employed in various fields. Nanomaterials for electrode modification are emerging one after another in order to improve the performance of electrochemical immunosensors. When compared with traditional detection methods, electrochemical immunosensors have the advantages of simplicity, real-time analysis, high sensitivity, miniaturization, rapid detection time, and low cost. Here, we summarize recent developments in electrochemical immunosensors based on nanomaterials, including carbon nanomaterials, metal nanomaterials, and quantum dots. Additionally, we discuss research challenges and future prospects for this field of study.

Iron oxide-based nanomaterials for supercapacitors

As highly efficient and clean electrochemical energy storage devices, supercapacitors (SCs) have drawn widespread attention as promising alternatives to batteries in recent years. Among various electrode materials, iron oxide materials have been widely studied as negative SC electrode materials due to their broad working window in negative potential, ideal theoretical specific capacitance, good redox activity, abundant availability, and eco-friendliness. However, iron oxides still suffer from the problems of low stability and poor conductivity. In this review, recent progress in iron oxide-based nanomaterials, including Fe₂O₃, Fe₃O₄, Fe_xO_y, and FeOOH, as electrode materials of SCs, is discussed. The nanostructure design and various synergistic effects of nanocomposites for improving the electrochemical performance of iron oxides are emphasized. Research on iron oxide-based symmetric/asymmetric SCs is also discussed. Future outlooks regarding iron oxides for SCs are likewise proposed.

Silver-Quantum-Dot-Modified MoO3 and MnO2 Paper-Like Freestanding Films for Flexible Solid-State Asymmetric Supercapacitors

Free-standing paper-like thin-film electrodes have great potential to boost next-generation power sources with highly flexible, ultrathin, and lightweight requirements. In this work, silver-quantum-dot- (2-5 nm) modified transition metal oxide (including MoO₃ and MnO₂ ) paper-like electrodes are developed for energy storage applications. Benefitting from the ohmic contact at the interfaces between silver quantum dots and MoO₃ nanobelts (or MnO₂ nanowires) and the binder-free nature and 0D/1D/2D nanostructured 3D network of the fabricated electrodes, substantial improvements on the electrical conductivity, efficient ionic diffusion, and areal capacitances of the hybrid nanostructure electrodes are observed. With this proposed strategy, the constructed asymmetric supercapacitors, with Ag quantum dots/MoO₃ "paper" as anode, Ag quantum dots/MnO₂ "paper" as cathode, and neutral Na₂ SO₄ /polyvinyl alcohol hydrogel as electrolyte, exhibit significantly enhanced energy and power densities in comparison with those of the supercapacitors without modification of Ag quantum dots on electrodes; present excellent cycling stability at different current densities and good flexibility under various bending states; offer possibilities as high-performance power sources with low cost, high safety, and environmental friendly properties.

Low-cost and environmentally friendly synthesis of an Al3+ and Mn4+ co-doped Li4Ti5O12 composite with carbon quantum dots as an anode for lithium-ion batteries

To increase the specific capacity and conductivity of lithium titanate (LTO), low-cost and environmentally friendly carbon quantum dots (CQDs) were used to composite with Al³⁺ and Mn⁴⁺ co-doped Li₄Ti₅O₁₂ (LTO-Al/Mn) to improve its electrical properties. The Al³⁺ and Mn⁴⁺ were successfully substituted for Ti located at (16d) sites in the LTO and the CQDs formed a composite with LTO-Al/Mn. The specific capacity of the first cycle at 0.1C increased to 296.5 mA h g⁻¹, and the impedance decreased to 16.8 Ω. The specific capacity maintained 236.0 mA h g⁻¹ after 100 cycles.

To increase the specific capacity and conductivity of lithium titanate, low-cost and environmentally friendly carbon quantum dots (CQDs) were used to composite with Al³⁺ and Mn⁴⁺ co-doped Li₄Ti₅O₁₂ (LTO-Al/Mn) to improve its electrical properties.

Core–shell-type ZIF-8@ZIF-67@POM hybrids as efficient electrocatalysts for the oxygen evolution reaction

ZIF-8@ZIF-67@POM hybrids were synthesized using a simple coprecipitation method, and they exhibit remarkable performance in OER, with the synergistic effect between POM and ZIF species, their regular architecture and their high surface area.