Recent Progress on Two-Dimensional Nanoflake Ensembles for Energy Storage Applications

Tailored interface engineering of Co3Fe7/Fe3C heterojunctions for enhancing oxygen reduction reaction in zinc-air batteries

The rational construction of highly active and robust non-precious metal oxygen reduction electrocatalysts is a vital factor to facilitate commercial applications of Zn-air batteries. In this study, a precise and stable heterostructure, comprised of a coupling of Co3Fe7 and Fe3C, was constructed through an interface engineering-induced strategy. The coordination polymerization of the resin with the bimetallic components was meticulously regulated to control the interfacial characteristics of the heterostructure. The synergistic interfacial effects of the heterostructure successfully facilitated electron coupling and rapid charge transfer. Consequently, the optimized CST-FeCo displayed superb oxygen reduction catalytic activity with a positive half-wave potential of 0.855 V vs. RHE. Furthermore, the CST-FeCo air electrode of the liquid zinc-air battery revealed a large specific capacity of 805.6 mAh gZn-1, corresponding to a remarkable peak power density of 162.7 mW cm-2, and a long charge/discharge cycle stability of 220 h, surpassing that of the commercial Pt/C catalyst.

Preparation of Volborthite by a Facile Synthetic Chemical Solvent Extraction Method

In this work, the extraction of vanadium (V) ions from an alkaline solution using a commercial quaternary ammonium salt and the production of metal vanadates through precipitation stripping were carried out. The crystallization of copper vanadates from the extracts was performed using a solution containing a copper(II) source in concentrated chloride media as a stripping agent. In an attempt to control growth, a stabilizing polymer (polyvinylpyrrolidone, PVP) was added to the stripping solution. The structural characteristics of the crystallized products, mainly copper pyrovanadate (volborthite, Cu₃V₂O₇(OH)₂·(H₂O)₂) nanoflakes and nanoflowers and the experimental parameter influencing the efficiency of the stripping process were studied. From the results, the synthesis of nanostructured vanadates is a simple and versatile method for the fabrication of valuable three-dimensional structures providing abundant active zones for energy and catalytic applications.

Transition metal chalcogenides for next-generation energy storage

Transition-metal chalcogenide nanostructures provide a unique material platform to engineer next-generation energy storage devices such as lithium-ion, sodium-ion, and potassium-ion batteries and flexible supercapacitors. The transition-metal chalcogenide nanocrystals and thin films have enhanced electroactive sites for redox reactions and hierarchical flexibility of structure and electronic properties in the multinary compositions. They also consist of more earth-abundant elements. These properties make them attractive and more viable new electrode materials for energy storage devices compared to the traditional materials. This review highlights the recent advances in chalcogenide-based electrodes for batteries and flexible supercapacitors. The viability and structure-property relation of these materials are explored. The use of various chalcogenide nanocrystals supported on carbonaceous substrates, two-dimensional transition metal chalcogenides, and novel MXene-based chalcogenide heterostructures as electrode materials to improve the electrochemical performance of lithium-ion batteries is discussed. The sodium-ion and potassium-ion batteries offer a more viable alternative to lithium-ion technology as they consist of readily available source materials. Application of various transition metal chalcogenides such as MoS2, MoSe2, VS2, and SnSx, composite materials, and heterojunction bimetallic nanosheets composed of multi-metals as electrodes to enhance the long-term cycling stability, rate capability, and structural strength to counteract the large volume expansion during the ion intercalation/deintercalation processes is highlighted. The promising performances of layered chalcogenides and various chalcogenide nanowire compositions as electrodes for flexible supercapacitors are also discussed in detail. The review also details the progress made in new chalcogenide nanostructures and layered mesostructures for energy storage applications.

Evolution of Stabilized 1T-MoS2 by Atomic-Interface Engineering of 2H-MoS2 /Fe-Nx towards Enhanced Sodium Ion Storage

Metallic conductive 1T phase molybdenum sulfide (MoS₂ ) has been identified as promising anode for sodium ion (Na⁺ ) batteries, but its metastable feature makes it difficult to obtain and its restacking during the charge/discharge processing result in part capacity reversibility. Herein, a synergetic effect of atomic-interface engineering is employed for constructing 2H-MoS₂ layers assembled on single atomically dispersed Fe-N-C (SA Fe-N-C) anode material that boosts its reversible capacity. The work-function-driven-electron transfer occurs from SA Fe-N-C to 2H-MoS₂ via the Fe-S bonds, which enhances the adsorption of Na⁺ by 2H-MoS₂ , and lays the foundation for the sodiation process. A phase transfer from 2H to 1T/2H MoS₂ with the ferromagnetic spin-polarization of SA Fe-N-C occurs during the sodiation/desodiation process, which significantly enhances the Na⁺ storage kinetics, and thus the 1T/2H MoS₂ /SA Fe-N-C display a high electronic conductivity and a fast Na⁺ diffusion rate.

Nanodesigns for Na3V2(PO4)3-based cathode in sodium-ion batteries: a topical review

With the rapid development of sodium-ion batteries (SIBs), it is urgent to exploit the cathode materials with good rate capability, attractive high energy density and considerable long cycle performance. Na₃V₂(PO₄)₃(NVP), as a NASICON-type electrode material, is one of the cathode materials with great potential for application because of its good thermal stability and stable. However, NVP has the inherent problem of low electronic conductivity, and various strategies are proposed to improve it, moreover, nanotechnology or nanostructure are involved in these strategies, the construction of nanostructured active particles and nanocomposites with conductive carbon networks have been shown to be effective in improving the electrical conductivity of NVP. Herein, we review the research progress of NVP performance improvement strategies from the perspective of nanostructures and classifies the prepared nanomaterials according to their different nano-dimension. In addition, NVP nanocomposites are reviewed in terms of both preparation methods and promotion effects, and examples of NVP nanocomposites at different nano-dimension are given. Finally, some personal views are presented to provide reasonable guidance for the research and design of high-performance polyanionic cathode materials of SIBs.

Two-Dimensional NH4V3O8 Nanoflakes as Efficient Energy Conversion and Storage Materials for the Hydrogen Evolution Reaction and Supercapacitors

Herein, for the first time, we present two-dimensional (2D) NH₄V₃O₈ nanoflakes as an excellent material for both energy conversion of the hydrogen evolution reaction and storage of supercapacitors by a simple and fast two-step synthesis, which exhibit a completely sheet-like morphology, high crystallinity, good specific surface area, and also stability, as determined by thermogravimetric analysis. The 2D-NH₄V₃O₈ flakes show an acceptable hydrogen evolution performance in 0.5 M H₂SO₄ on a glassy carbon electrode (GCE) coated with 2D-NH₄V₃O₈, which results in a low overpotential of 314 mV at -10 mA cm^-2 with an excellent Tafel slope as low as 90 mV dec^-1. So far, with the main focus on energy storage, 2D-NH₄V₃O₈ nanoflakes were found to be ideal for supercapacitor electrodes. The NH₄V₃O₈ working electrode in 1 M Na₂SO₄ shows an excellent electrochemical capability of 274 F g^-1 at 0.5 A g^-1 for a maximum energy density of 38 W h kg^-1 at a power density as high as 250 W kg^-1. Moreover, the crystal structure of 2D-NH₄V₃O₈ is demonstrated by density functional theory (DFT) computational simulation using three functionals, GGA, GGA + U, and HSE06. The simple preparation, low cost, and abundance of the NH₄V₃O₈ material provide a promising candidate for not only energy conversion but also energy-storage applications.

Highly sensitive electrochemical determination of rutin based on the synergistic effect of 3D porous carbon and cobalt tungstate nanosheets

Rutin, a flavonoid found in fruits and vegetables, is a potential anticancer compound with strong anticancer activity. Therefore, electrochemical sensor was developed for the detection of rutin. In this study, CoWO₄ nanosheets were synthesized via a hydrothermal method, and porous carbon (PC) was prepared via high-temperature pyrolysis. Successful preparation of the materials was confirmed, and characterization was performed by transmission electron microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. A mixture of PC and CoWO₄ nanosheets was used as an electrode modifier to fabricate the electrochemical sensor for the electrochemical determination of rutin. The 3D CoWO₄ nanosheets exhibited high electrocatalytic activity and good stability. PC has a high surface-to-volume ratio and superior conductivity. Moreover, the hydrophobicity of PC allows large amounts of rutin to be adsorbed, thereby increasing the concentration of rutin at the electrode surface. Owing to the synergistic effect of the 3D CoWO₄ nanosheets and PC, the developed electrochemical sensor was employed to quantitively determine rutin with high stability and sensitivity. The sensor showed a good linear range (5–5000 ng/mL) with a detection limit of 0.45 ng/mL. The developed sensor was successfully applied to the determination of rutin in crushed tablets and human serum samples.

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Highly sensitive electrochemical sensor based on 3D porous carbon and CoWO₄ nanosheets.

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Electrochemical signal of rutin is mainly based on its concentration at the electrode surface.

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The introduction of porous carbon improved the electrochemical performance of 3D CoWO₄.

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The sensor was successfully applied to determine rutin in human serum samples.

Molecular Coupling and Self-Assembly Strategy toward WSe2 /Carbon Micro-Nano Hierarchical Structure for Elevated Sodium-Ion Storage

Sodium (Na) ion-based dual-ion batteries (Na-DIBs) have attracted great attention, owing to their benefits of low cost, high working voltage, and environmental friendliness. However, the limited capacity and low tap density of currently reported anode materials restrict the further improvement of Na-DIBs. Herein, a micro-nano structure with vertically aligned WSe₂ nanoflakes anchored tightly on a micron-sized carbon sphere (WSe₂ /CS) is successfully constructed via combining the molecular coupling and self-assembly strategy. Within this hierarchical structure, the WSe₂ nanoflakes can shorten the diffusion path for Na⁺ ions and alleviate structural deformation during the charge/discharge process; meanwhile, the micron-sized carbon core provides conductive support and helps improve the total tap density of the anode electrode. As a result, this micron-sized WSe₂ /CS displays a high specific capacity of ≈252.8 mAh g^-1 and good cycling performance with ≈92% capacity retention after 1200 cycles. Moreover, by pairing this WSe₂ /CS anode with environmental friendly graphite as cathode, a proof-of-concept Na-DIB shows 85.6% capacity retention after 1000 cycles, which is among the best performances of previously reported Na-DIBs.

DFT-Guided Design and Fabrication of Carbon-Nitride-Based Materials for Energy Storage Devices: A Review

Carbon nitrides (including CN, C₂N, C₃N, C₃N₄, C₄N, and C₅N) are a unique family of nitrogen-rich carbon materials with multiple beneficial properties in crystalline structures, morphologies, and electronic configurations. In this review, we provide a comprehensive review on these materials properties, theoretical advantages, the synthesis and modification strategies of different carbon nitride-based materials (CNBMs) and their application in existing and emerging rechargeable battery systems, such as lithium-ion batteries, sodium and potassium-ion batteries, lithium sulfur batteries, lithium oxygen batteries, lithium metal batteries, zinc-ion batteries, and solid-state batteries. The central theme of this review is to apply the theoretical and computational design to guide the experimental synthesis of CNBMs for energy storage, i.e., facilitate the application of first-principle studies and density functional theory for electrode material design, synthesis, and characterization of different CNBMs for the aforementioned rechargeable batteries. At last, we conclude with the challenges, and prospects of CNBMs, and propose future perspectives and strategies for further advancement of CNBMs for rechargeable batteries.

Niobium Tungsten Oxide in a Green Water-in-Salt Electrolyte Enables Ultra-Stable Aqueous Lithium-Ion Capacitors

Aqueous hybrid supercapacitors are attracting increasing attention due to their potential low cost, high safety and eco-friendliness. However, the narrow operating potential window of aqueous electrolyte and the lack of suitable negative electrode materials seriously hinder its future applications. Here, we explore high concentrated lithium acetate with high ionic conductivity of 65.5 mS cm-1 as a green "water-in-salt" electrolyte, providing wide voltage window up to 2.8 V. It facilitates the reversible function of niobium tungsten oxide, Nb18W16O93, that otherwise only operations in organic electrolytes previously. The Nb18W16O93 with lithium-ion intercalation pseudocapacitive behavior exhibits excellent rate performance, high areal capacity, and ultra-long cycling stability. An aqueous lithium-ion hybrid capacitor is developed by using Nb18W16O93 as negative electrode combined with graphene as positive electrode in lithium acetate-based "water-in-salt" electrolyte, delivering a high energy density of 41.9 W kg-1, high power density of 20,000 W kg-1 and unexceptionable stability of 50,000 cycles.

Two-Dimensional Materials to Address the Lithium Battery Challenges

Despite the ever-growing demand in safe and high power/energy density of Li⁺ ion and Li metal rechargeable batteries (LIBs), materials-related challenges are responsible for the majority of performance degradation in such batteries. These challenges include electrochemically induced phase transformations, repeated volume expansion and stress concentrations at interfaces, poor electrical and mechanical properties, low ionic conductivity, dendritic growth of Li, oxygen release and transition metal dissolution of cathodes, polysulfide shuttling in Li-sulfur batteries, and poor reversibility of lithium peroxide/superoxide products in Li-O₂ batteries. Owing to compelling physicochemical and structural properties, in recent years two-dimensional (2D) materials have emerged as promising candidates to address the challenges in LIBs. This Review highlights the cutting-edge advances of LIBs by using 2D materials as cathodes, anodes, separators, catalysts, current collectors, and electrolytes. It is shown that 2D materials can protect the electrode materials from pulverization, improve the synergy of Li⁺ ion deposition, facilitate Li⁺ ion flux through electrolyte and electrode/electrolyte interfaces, enhance thermal stability, block the lithium polysulfide species, and facilitate the formation/decomposition of Li-O₂ discharge products. This work facilitates the design of safe Li batteries with high energy and power density by using 2D materials.

"On-off" ratiometric fluorescent detection of Hg2+ based on N-doped carbon dots-rhodamine B@TAPT-DHTA-COF

Covalent-organic frameworks (COFs) are new porous crystalline materials owning outstanding stability, adsorbability and hypotoxicity. The assembly of fluorescence probes into porous COF provides a good method for ratiometric fluorescence detection avoiding the toxic effects of fluorescence probes to the samples. Herein, a two-dimensional COF (TAPT-DHTA-COF) was employed as a host to encapsulate N-doped carbon dots (NCDs) and Rhodamine B (RhB) (NCDs-RhB@COF). NCDs and RhB were uniformly assembled into the pores of TAPT-DHTA- COF based on the hydrogen bond. The as-prepared NCDs-RhB@COF nanocomposites exhibited blue emission of NCDs at 440 nm and red emission of RhB at 570 nm at excitation of 340 nm. After the addition of Hg²⁺, the blue emission became weaker while the red emission was enhanced due to the strong coordination between NCDs-RhB@COF and Hg²⁺. This "on-off" fluorescence probe was applied in detection of trace Hg²⁺ with linear range of 0.048-10 μM and detection limit of 15.9 nM together with appropriate selectivity, acceptable sensitivity and stability. The work shreds some light for COF as platform to construct ratiometric fluorescent sensor for industrial and biological application.

3D Hierarchically Structured CoS Nanosheets: Li+ Storage Mechanism and Application of the High-Performance Lithium-Ion Capacitors

Lithium-ion capacitors possess excellent power and energy densities, and they can combine both of those advantages from supercapacitors and lithium-ion batteries, leading to novel generation hybrid devices for storing energy. This study synthesized one three-dimensional (3D) hierarchical structure, self-assembled from CoS nanosheets, according to a simple and efficient manner, and can be used as an anode for lithium ion capacitors. This CoS anode, based on a conversion-type Li⁺ storage mechanism dominated by diffusion control, showed a large reversible capacity, together with excellent stability for cycling. The CoS shows a discharge capacity ≈434 mA h/g at 0.1 A/g. The hybrid lithium-ion capacitor, which had the CoS anode as well as the biochar cathode, exhibits excellent electrochemical performance with ultrahigh energy and power densities of 125.2 Wh/kg and 6400 W/kg, respectively, and an extended cycling life of 81.75% retention after 40 000 cycles. The CoS with self-assembled 3D hierarchical structure in combination with a carbon cathode offers a versatile device for future applications in energy storage.

Large-scale surfactant-free synthesis of WS2 nanosheets: an investigation into the detailed reaction chemistry of colloidal precipitation and their application as an anode material for lithium-ion and sodium-ion batteries

Novel detailed chemistry of WS₂ synthesis.

Recent advances in MXenes and their composites in lithium/sodium batteries from the viewpoints of components and interlayer engineering

An up-to-date review about MXenes based on their distinguishing properties, namely, large interlayer spacing and rich surface chemistry.

Direct Inkjet Printing of Aqueous Inks to Flexible All-Solid-State Graphene Hybrid Micro-Supercapacitors

In this article, the inkjet printing technique is demonstrated for the stacking of reduced graphene oxide (RGO) and molybdenum trioxide (MoO₃) nanosheets for flexible all-solid-state micro-supercapacitors. The ammonium molybdate tetrahydrate/graphene oxide ((NH₄)₆Mo₇O₂₄·4H₂O/GO) aqueous inks are facilely printed on polymide (PI) film and transformed to RGO/MoO₃ hybrids via thermal treatments at air atmosphere. The compound inks are water-based, inkjet-printable, and nontoxic for inkjet printing to form two-dimensional crystal materials. The physical properties of aqueous inks are optimized within a printable range characterized by the Ohnesorge number of 1 < Z < 14. The inkjet-printed symmetric micro-supercapacitors (MSCs) with poly(vinyl alcohol) (PVA)-H₂SO₄ gel electrolyte possess a wide voltage window of 0-0.8 V, excellent flexibility, a high volumetric specific capacitance of 22.5 F cm^-3 at 0.044 A cm^-3, as well as good cyclic stability due to the synergistic effect of RGO and MoO₃. Furthermore, the inkjet-printed composite MSCs delivered a maximum energy density of 2 mWh cm^-3 and a power density of 0.018 W cm^-3, and the capacity retention rate of inkjet-printed MSCs is still retained 82% even after 10 000 charge-discharge cycles, indicating good electrochemical properties. Above all, the as-designed inkjet printing technique shows potential for flexible and wearable energy storage electronics.

Interfacial engineering of Ag nanodots/MoSe2 nanoflakes/Cu(OH)2 hybrid-electrode for lithium-ion battery

Although the Lithium ion batteries (LIBs) have attracted remarkable attentions, their practical development is hindered by the low rate performance and poor unit area capacity, which is significantly caused by the low conductivity of the active electrode materials. Herein, a three-dimensional (3D) architecture consisting of Ag nanodots embedded MoSe₂ sheets wrapping Cu(OH)₂ nanorods (Cu(OH)₂/MoSe₂/Ag) hybrids were in-situ synthesized on self-standing Cu- foam collector for LIBs application. The 2D MoSe₂ nanoflakes supported on 1D highly conductive Cu nanowires provides efficient pathways for both electrons and ions. The embedded Ag nanodots in the MoSe₂ as the internal-plane active sites not only improves the intrinsic conductivity but also allows the reversible formation and decompose of Ag-Li alloy, and thus leading to the promotion of Li⁺ ion storage. As a result, the Cu(OH)₂/MoSe₂/Ag electrode exhibits a high reversible discharge capacity of 1285.5 mAh g^-1 (current density of 0.2 C), good rate performance (discharge-specific capacity remained 544.8 mAh g^-1 at 5.0C), and excellent cycling stability (with almost no decay after 500 cycles). Significantly, the 3D Cu(OH)₂/MoSe₂/Ag electrode exhibits a high areal capacity of 2.50 mAh cm^-2 at a high current density of 1.82 mA cm^-2. This work provides the new insight into interfaces engineering for 3D architecture toward advanced self-standing LIB electrodes.

V2O5 Nanospheres with Mixed Vanadium Valences as High Electrochemically Active Aqueous Zinc-Ion Battery Cathode

A V4+-V2O5 cathode with mixed vanadium valences was prepared via a novel synthetic method using VOOH as the precursor, and its zinc-ion storage performance was evaluated. The products are hollow spheres consisting of nanoflakes. The V4+-V2O5 cathode exhibits a prominent cycling performance, with a specific capacity of 140 mAh g-1 after 1000 cycles at 10 A g-1, and an excellent rate capability. The good electrochemical performance is attributed to the presence of V4+, which leads to higher electrochemical activity, lower polarization, faster ion diffusion, and higher electrical conductivity than V2O5 without V4+. This engineering strategy of valence state manipulation may pave the way for designing high-performance cathodes for elucidating advanced battery chemistry.

Facile Formation of Bi2O2CO3/Bi2MoO6 Nanosheets for Visible Light-Driven Photocatalysis

Bi2O2CO3/Bi2MoO6 heterojunction catalysts were prepared by treating Bi2MoO6 sheets with aqueous NaHCO3 solutions at room temperature. All the Bi2O2CO3/Bi2MoO6 heterojunctions exhibited higher activities than pristine Bi2MoO6 in the photocatalytic degradation of rhodamine B (RhB), methyl orange, and ciprofloxacin under visible-light irradiation, and the most active photocatalyst was found to be the one with a C/Bi molar ratio of ∼1/2.3. Relevant samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, N2 adsorption-desorption, Fourier transform infrared spectroscopy, and UV-vis spectroscopy. The higher activity of Bi2O2CO3/Bi2MoO6 than pristine Bi2MoO6 is explained by the enhanced separation and transfer of photogenerated electron/hole pairs, as verified by transient photocurrent densities, photoluminescence spectroscopy, and electrochemical impedance spectroscopy. Photogenerated holes (h+) and superoxide radical anions (•O2 -) were found to be the main active species. The good reusability of Bi2O2CO3/Bi2MoO6 was testified by cycling degradation of RhB and tetracycline hydrochloride.

Enhanced Roles of Carbon Architectures in High-Performance Lithium-Ion Batteries

Lithium-ion batteries (LIBs), which are high-energy-density and low-safety-risk secondary batteries, are underpinned to the rise in electrochemical energy storage devices that satisfy the urgent demands of the global energy storage market. With the aim of achieving high energy density and fast-charging performance, the exploitation of simple and low-cost approaches for the production of high capacity, high density, high mass loading, and kinetically ion-accessible electrodes that maximize charge storage and transport in LIBs, is a critical need. Toward the construction of high-performance electrodes, carbons are promisingly used in the enhanced roles of active materials, electrochemical reaction frameworks for high-capacity noncarbons, and lightweight current collectors. Here, we review recent advances in the carbon engineering of electrodes for excellent electrochemical performance and structural stability, which is enabled by assembled carbon architectures that guarantee sufficient charge delivery and volume fluctuation buffering inside the electrode during cycling. Some specific feasible assembly methods, synergism between structural design components of carbon assemblies, and electrochemical performance enhancement are highlighted. The precise design of carbon cages by the assembly of graphene units is potentially useful for the controlled preparation of high-capacity carbon-caged noncarbon anodes with volumetric capacities over 2100 mAh cm^-3. Finally, insights are given on the prospects and challenges for designing carbon architectures for practical LIBs that simultaneously provide high energy densities (both gravimetric and volumetric) and high rate performance.