Ultrathin Nanosheets Research Articles

High-efficiency freeze-dried ultrathin V2O5 nanosheet arrays on reduced graphene oxide as aqueous Zn-ion battery cathode

The aqueous rechargeable zinc-ion batteries (ZIBs) are a potential grid-scale energy storage device because of the high security, economy and high performance. There is a need for developing ZIBs cathode materials with high specific energies. Herein, novel ultrathin V2O5 nanosheet arrays deposited on reduced graphene oxide (V2O5/rGO) are prepared via freeze-drying technique in combination with a scalable in-situ thermal decomposition-reduction method for ZIBs cathode. The introduction of rGO not only enhances electrical conductivity and provides exceptional structural stability for V2O5/rGO, but also increase the electrode hydrophobicity, effectively inhibiting the dissolution of vanadium. As a result, the assembled ZIBs demonstrate a high capacity of 438.5 mAh/g at 0.1 A/g, a capacity of 278.2 mAh/g after 1000 cycles at 2 A/g, with an 85.6 % capacity retention. This work provides a simple and scalable strategy to prepare graphene-based nanocomposites.

Crystallinity and interfacial Mo–N–C bond engineered MoS2 embedded graphitic nitrogen doped carbon hollow sphere for enhanced HER activity

To restrain fossil fuel depletion, the need of the hour is the development of efficient, durable, and non-precious electrocatalysts for the hydrogen evolution reaction (HER). The hierarchical nanostructure consisting of transition metal dichalcogenides and graphitic heteroatom-doped carbon with an abundant interfacial M–N–C catalytic site is highly demanding for electrolytic applications. Herein, we report crystallinity-engineered ultrathin MoS2 nanosheets hierarchy over nitrogen-doped graphitic carbon (NC) hollow spheres as a promising material for HER. The well optimized NC@MoS2 demonstrates superior HER activity with a low onset overpotential of 9 mV and an overpotential of 145 mV at a current density of −10 mA cm−2. It exhibits low Tafel slope of 39 mV dec−1 and excellent chronoamperometric stability. Superior HER activity originates from interfacial Mo–N–C bonds. Density functional theory (DFT) calculations unveil that Mo–N–C bonds between MoS2 and NC matrix ease electronic transportation and further diminish Gibbs free energy for HER.

(1‐10) Facet‐Dominated TiB2 Nanosheets with High Exposure of Dual‐Atom‐Sites for Enhanced Polysulfide Conversion in Li‐S Batteries

AbstractElaborate modulation of the highly active crystal facet emerges as an efficient strategy for enhancing the nanocrystalline catalytic activity. Herein, ultrathin TiB2 nanosheets with preferentially exposed (1‐10) facets are developed as highly efficient catalyst with enriched bonding and electrocatalytic sites in Li‐S batteries. Attributed to the highly equivalent exposure of Ti and B active sites on the (1‐10) surface, the (1‐10) facet‐dominated TiB2 nanosheets maximize the binding effect via Ti and B dual‐atom‐sites adsorption through Ti─S and B─S bonds. More importantly, experiments and theoretical calculations confirm the superb catalytic activity of (1‐10)TiB2 in facilitating the polysulfide conversion and Li2S decomposition, thereby markedly suppressing the shuttling effect and improving the redox kinetics. Consequently, excellent electrochemical properties are achieved in Li‐S batteries, which demonstrate a high discharge capacity of 1469 mAh g−1 at 0.2 C and maintain high capacity reversibility at 1 C with a low capacity decay rate of 0.048% over 500 cycles. Even under a sulfur loading of 5 mg cm−2, a prominent areal capacity of 4.86 mAh cm−2 is still attained. It is proposed that the crystal surface engineering provides a new path for the structure optimization of sulfur catalysts in Li‐S batteries.

Boosted photocatalytic H2 production over p-n S-scheme heterojunction of 0D Ni3V2O8 quantum dots decorated on ultra-thin 2D g-C3N4 nanosheets

Novel 0D Ni3V2O8 quantum dots (QDs) were decorated on the ultra-thin 2D g-C3N4 nanosheet (UCN) using a hydrothermal approach. The embedded Ni3V2O8 QDs play a dual role by trapping charge carriers, promoting electron-hole separation, and forming a p-n S-scheme heterojunction, improving the redox potential of the electrons and holes for surface reactions. HR-TEM analysis confirmed that Ni3V2O8 QDs with a size varying between 3 to 5 nm have intimate contact with the UCN surface. XPS and ESR spin-trapping agents experiment confirmed the formation of S-scheme heterojunctions between Ni3V2O8 QDs and UCN. Photocurrent and PL spectra indicated the synthesized heterostructures have high charge carrier transportation and lower recombination rates. The optimum Ni3V2O8 QDs/UCN heterostructure showed the highest photocatalytic hydrogen evolution activity, about 14 times higher than that of pristine UCN. This remarkable improvement in activity can be ascribed to the synergy effect of the S-scheme charge pathway mechanism and the improved light absorption facilitated by the Ni3V2O8 quantum dots. As a result, these factors effectively accelerate the migration and transportation of photoinduced charge carriers, offering increased active sites for the H2 generation process. This study emphasizes the importance of a well-designed interfacial heterojunction to achieve enhanced photocatalytic performance.

‘MoS2-coated nitrogen/oxygen co-doped carbon nanocages composite as active material for supercapacitor electrodes

Nitrogen/oxygen co-doped carbon nanocages (NOCN) have been deemed as promising materials in renewable energy-storage applications. Herein, we report a facile one-pot hydrothermal approach for formation of three-dimensional (3D) flower-like MoS2 nanoflakes/NOCN composite (MoS2@NOCN) with excellent storage properties as anode for supercapacitor. As a result, ultrathin MoS2 nanosheets were vertically grown onto the surface of NOCN, which can prevent sheet aggregation and provide a rapid pathway for electron transfer. The as-prepared MoS2@NOCN composites were characterized by electrochemical measurements to demonstrate the supercapacitor performance. The results indicate that MoS2@NOCN-3 exhibited a high specific capacitance (240.8 F g−1 at 0.5 A g−1) and excellent rate performance (approximately 46.4 % from 1 A g−1 to 10 A g−1). The as-obtained composites demonstrated their potential applications in supercapacitor electrode materials.

Facile synthesis of ultrathin carbon nanosheets through NaCl-KCl templates coupled with ice-induced assembly strategy from carbon quantum dots as lithium-ion batteries anodes

Carbon nanosheets (CNSs) have drawn numerous attention as promising anodes in energy storage devices due to their large specific surface area and excellent electrical conductivity close to graphene materials. Restricted by the expensive raw materials, harsh conditions and cumbersome post-processing procedures, the large-scale production of CNSs is a major challenge. Herein, ultrathin CNSs were constructed through NaCl-KCl templates coupled with ice-induced assembly strategy from coal tar pitch-based carbon quantum dots (CQDs). The ice can manipulate the salt crystal growth direction and dominantly guide the distribution of CQDs on the surface of NaCl-KCl crystal templates, whereas the liquid salts not only act as the solvent to load and disperse CQDs but as molecular templates to facilitate the formation of ultrathin CNSs during the carbonization process. The prepared CNSs exhibit twisted and ultrathin lamellar structures with a thickness of ∼1.70 nm, abundant hierarchical nanopores, and a moderate specific surface area of 302.00 m2·g−1. Benefiting from these unique microstructure features, the optimized CNS-3 adopted as anode for lithium-ion batteries displays prominent electrochemical features with high reversible capacity (453 mAh·g−1 at 50 mA·g−1), outstanding rate capability (225 mAh·g−1 at 1000 mA·g−1), and superior cycle stability (94.3% capacitance retention after 500 cycles at 500 mA·g−1). Our work provides a novel and effective approach to fabricate ultrathin carbon nanosheets by employing low-cost carbon quantum dots derived from coal tar pitch as potential candidates applied for lithium-ion batteries and next-generation energy storage devices.

Ultrathin CuNi2Al-LDH nanosheets with enhanced electron transfer for visible-light-driven photo-fenton-like water decontamination

The Cu-based photo-Fenton-like process stands out as a promising technology for water decontamination, but its catalytic efficiency for refractory organic pollutants removal still suffers from shortage of Cu(I) supply in the activation of H2O2. Herein, we, for the first time, developed ultrathin nanosheets of Cu(I)-enrich CuNi2Al layered double hydroxides (CuNi2Al-LDH) with fast electron transfer rate to achieve superior visible-light-driven photo-Fenton catalytic performance. In the brucite-like sheets of LDH, the electron donation effect of Ni enabled the electron transfer from Ni to Cu, resulting in generation of 62.9 % Cu(I) content, which provided abundant Cu(I) active sites for Fenton-like reaction. Notably, the coexistence of Ni and Cu ions in the nanosheets significantly enhanced the visible-light harvesting ability by d-d transitions, thereby boosting the production of photogenerated carriers. More importantly, the Ni sites with excellent electroactivity served as electron-transfer mediators to remarkably promote the photogenerated carriers charge separation and photogenerated electrons transfer, leading to nearly 20 times improvement in the photocurrent density of CuNi2Al-LDH in comparison to the Ni-free LDH, which greatly expedited the Cu(II)/Cu(I) cycle and promoted H2O2 activation. Benefiting from these advantages, CuNi2Al-LDH could degrade 100 % of phenol, tetracycline (TC) and dyes within 30 min at room temperature through the synergy effect of photocatalysis and Fenton oxidation, and the reactive oxygen species •OH, •O2− and 1O2 contributed 44.2 %, 12.9 % and 37.7 % to the degradation of pollutants, respectively. This study provides an ingenious strategy to the rational design of on-demand multifunctional catalysts for advanced water remediation.

Revealing The Role of Electronic Asymmetricity on Supported Ru Nanoclusters for Alkaline Hydrogen Evolution Reaction

AbstractModulating the electronic asymmetricity of catalysts is an effective method for optimizating the elementary steps of water dissociation and hydrogen adsorption/desorption process for the alkaline hydrogen evolution reaction (HER). Herein, uniform Ru nanoclusters anchored on N doped ultrathin carbon nanosheets (Ru/NC) are synthesized to optimize the asymmetricity electronic properties of supported Ru for efficient HER. It is found that Ru and NC with a large work function difference (ΔΦ) leading to the formation of stronger asymmetrical charge distributions of Ru that electron‐deficient high‐valence Ru (Run+) coupling with low‐valence Ru (Ru0). Experimental and theoretical studies indicate the Run+ sites lowered the energy barrier for water dissociation and provided enough hydrogen proton to promote the hydrogen spillover from the Run+ to Ru0 sites, and Ru0 sites can enhance H desorption process, thus synergistically enhancing the hydrogen evolution activity. Notably, the Ru/NC catalyst exhibits a high alkaline HER activity (21.9 mV@10 mA cm−2, 29.03 mV dec−1). The role of electronic asymmetricity on supported Ru nanoclusters for the alkaline HER are demonstrated, which will provide guidelines for the rational design of high‐efficiency alkaline HER catalysts.

Fabrication of Carboxylate-Functionalized 2D MOF Nanosheet with Caged Cavity for Efficient and Selective Extraction of Uranium from Aqueous Solution.

The efficient removal of radioactive uranium from aqueous solution is of great significance for the safe and sustainable development of nuclear power. An ultrathin 2D metal-organic framework (MOF) nanosheet with cavity structures was elaborately fabricated based on a calix[4]arene ligand. Incorporating the permanent cavity structures on MOF nanosheet can fully utilize its structural characteristics of largely exposed surface area and accessible adsorption sites in pollutant removal, achieving ultrafast adsorption kinetics, and the functionalized cavity structure would endow the MOF nanosheets with the ability to achieve preconcentration and extraction of uranium from aqueous solution, affording ultrahigh removal efficiency even in ultra-low concentrations. Thus, more than 97% uranium can be removed from the concentration range of 50-500µgL-1 within 5min. Moreover, the 2D nano-material exhibits ultra-high anti-interference ability, which can efficiently remove uranium from groundwater and seawater. The adsorption mechanism was investigated by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) analysis, and density functional theory (DFT) calculations, which revealed that the cavity structure plays an important role in uranium capture. This study not only realizes highly efficient uranium removal from aqueous solution but also opens the door to achieving ultrathin MOF nanosheets with cavity structures, which will greatly expand the applications of MOF nanosheets.

Manipulation of Electron Spins with Oxygen Vacancy on Amorphous/Crystalline Composite-Type Catalyst.

By substituting the oxygen evolution reaction (OER) with the anodic urea oxidation reaction (UOR), it not only reduces energy consumption for green hydrogen generation but also allows purification of urea-rich wastewater. Spin engineering of the d orbital and oxygen-containing adsorbates has been recognized as an effective pathway for enhancing the performance of electrocatalysts. In this work, we report the fabrication of a bifunctional electrocatalyst composed of amorphous RuO2-coated NiO ultrathin nanosheets (a-RuO2/NiO) with abundant amorphous/crystalline interfaces for hydrogen evolution reaction (HER) and UOR. Impressively, only 1.372 V of voltage is required to attain a current density of 10 mA cm-2 over a urea electrolyzer. The increased oxygen vacancies in a-RuO2/NiO by incorporation of amorphous RuO2 enhance the total magnetization and entail numerous spin-polarized electrons during the reaction, which speeds up the UOR reaction kinetics. The density functional theory study reveals that the amorphous/crystalline interfaces promote charge-carrier transfer, and the tailored d-band center endows the optimized adsorption of oxygen-generated intermediates. This kind of oxygen vacancy induced spin-polarized electrons toward boosting HER and UOR kinetics and provides a reliable reference for exploration of advanced electrocatalysts.

Single-atom sites regulation by the second-shell doping for efficient electrochemical CO2 reduction

Nitrogen-doped carbon loaded single-atom catalysts (SACs) are promising candidates for electrocatalytic conversion of CO2 into high-valuable chemicals, and the modification of catalysts by heteroatom-doping strategy is an effective approach to enhance the CO2 reduction performance. However, the large difference exists in atomic radius between nitrogen atoms and the doped heteroatoms may lead to the poor stability of active sites. In this study, we have synthesized a Ni single atom catalyst with S doping at the second-shell on the ultrathin carbon nanosheets support (Ni-N4-SC) by solid-phase pyrolysis. The S atom in the second-shell contributes to the higher efficiency of CO2 conversion at lower potentials while the Ni-N4-SC can be more stable. The experimental results and theoretical calculations indicate that the S atom in second-shell breaks the uniform charge distribution and reduces the free energy of hydrogenation, which can increase the adsorption of CO2, accelerate charge transfer, and reduce the reaction energy barrier. This work reveals the close relationship between the second-shell and the electrocatalytic activity of single atom sites, which also provides a new perspective to design efficient single atom catalysts.

Dual‐Plasmonic Ti3C2Tx/CuSe 2D/2D Solar Absorber and a Hydrophilic Device for Efficient Solar‐Driven Water Collection

Solar water evaporation is an emerging technology for drinkable water generation, while it remains a challenge to develop materials and device for efficient water evaporation and collection. Herein, dual‐plasmonic Ti3C2Tx/CuSe two‐dimensional (2D)/2D hybrids are prepared for high‐efficiency solar water evaporation and a hydrophilic device is designed for efficient water collection. The Ti3C2Tx/CuSe hybrids, which monocrystalline CuSe ultrathin nanosheets are chemically bonded with Ti3C2Tx nanosheets, show efficient photothermal conversion owing to the plasmon‐coupling‐induced strong light absorption and fast charge transfer in the 2D/2D interface. By transferring the hybrids on a cotton piece, the Ti3C2Tx/CuSe membrane displays over 95% of solar light absorption, a stable evaporation rate of 1.893 kg m−2 h−1, and solar‐to‐vapor efficiency of 99.13% under one‐sun irradiation. The membrane can also treat water with more than 20 wt% salinity due to the rich microchannel for ion diffusion. Furthermore, a water evaporation device, which the cambered roof is painted with hydrophilic SiO2, is designed for efficient water collection. The treated roof can efficiently reduce the optical loss and transfer the vapor condensation, leading to a high average freshwater generation of 16.4 kg m−2 in a daily (10 h) natural light irradiation, much higher than other reported devices.

Photodeposition of highly dispersed MoS2 nanosheets on CdS nanorods with enhanced photocatalytic activity for reduction of p-nitroaniline to p-phenylenediamine

So far, the significant effect of different assembly methods on the microstructures and photocatalytic properties of MoS2/CdS nanocomposite photocatalysts has rarely been reported. In the study, single-crystalline CdS nanorods (NRs) were prepared by a solvothermal method. MoS2 cocatalyst was decorated on the CdS NRs to obtain MoS2/CdS-PD and MoS2/CdS-HT photocatalysts by an in situ photodeposition (PD) technique and a hydrothermal (HT) method, respectively. Under visible light irradiation, the photocatalytic activity of MoS2/CdS-PD and MoS2/CdS-HT for reduction of p-nitroaniline (p-NA) to p-phenylenediamine (p-PDA) was comparatively studied. Results indicate: i) When MoS2 was attached on CdS NRs by the in situ PD, ultrathin MoS2 nanosheets could be uniformly and tightly tethered to CdS NRs to form core-shell MoS2/CdS-PD composite, whereas MoS2 nanosheets were aggregated to form stacked multilayered structure and loosely coupled to CdS NRs to produce non-core-shell MoS2/CdS-HT by the HT method. ii) MoS2/CdS-PD exhibits higher photocatalytic efficiency for p-NA reduction than MoS2/CdS-HT, which can be ascribed to the fact that the interfacial contact between MoS2 and CdS is more intimate and the electron transfer from CdS to MoS2 is more efficient in MoS2/CdS-PD than that in MoS2/CdS-HT.

Nickel selenide/g-C3N4 heterojunction photocatalyst promotes C[sbnd]C coupling for photocatalytic CO2 reduction to ethane

Photocatalytic CO2 reduction to generate high value-added and renewable chemicals is of great potential in facilitating the realization of closed-loop and carbon–neutral hydrogen economy. Stabilizing and accelerating the formation of COCO* intermediate is crucial to achieve high-selectivity ethane production. Herein, a novel 3D/2D NiSe2/g-C3N4 heterostructure that mesoscale hedgehog nickel selenide (NiSe2) grown on the ultrathin g-C3N4 nanosheets were synthesized via a successively high temperature calcination process and in-situ thermal injection method for the first time. The optimum 2.7 % NiSe2/g-C3N4 heterostructure achieved moderate C2H6 generation rate of 46.1 μmol·g−1·h−1 and selectivity of 97.5 % without any additional photosensitizers and sacrificial agents under light illumination. Based on the results of the theoretical calculations and experiments, the improvement of photocatalytic CO2 to C2H6 production and selectivity should be ascribed to the increased visible light absorption ability, unique 3D/2D heterostructures with promoted adsorption of CO2 molecules on the Ni active sites, the type II heterojunction with improved charge transfer dynamics and lowered interfacial transfer resistance, as well as the formation of COCO* key intermediate. This work provides an inspiration to construct efficient photocatalysts for the direct transformation of CO2 to multicarbon products (C2+).

(Poster Award - 3rd Place) Photocatalysis of TiO2 Nanosheets Prepared by the Cellulose Nanofiber Template

Two-dimensional material inspired tremendous research interest by its unique structure and specific property different from bulk material, which could benefit various applications such as optics, electronics, and chemistry. However, most documented processes for 2D materials are complicated with a lengthy process. We introduced the cellulose nanofiber (CNF) extracted from agricultural waste rice straws, which inherently possesses many functional groups, e.g., hydroxyl group, and carboxylate group, making it a terrific template method candidate. Besides, aerogel composed of cellulose nanofiber provides many surface and layered structures that facilitate the adhesion of titanium isopropoxide to form a uniform coating layer, which hydrolyzed to TiO2. The cellulose nanofiber serves as a template and could be removed entirely in an easy thermal process by thermogravimetric analysis; meanwhile, we can transform the original amorphous TiO2 nanosheet into an anatase phase which is flavored in the photocatalyst field. Atomic force microscopy (AFM) results spotted a stacked ultrathin TiO2 nanosheet with a thickness of less than 5 nm for each layer, and the lateral size could reach micron size. For the RhB dye degradation test, TiO2 nanosheets could decompose the 10 ppm RhB solution within 30 mins with a reaction rate of 0.065 min-1. In conclusion, we combine the CNF template and hydrolysis process, proposing a simple 2D TiO2 nanosheets

(Invited) Polymeric Carbon Nitride for Photocatalytic Overall Water Splitting

Photocatalytic overall water splitting can be achieved using Z-scheme systems that mimic natural photosynthesis by combining dissimilar semiconductors in series. However, coupling well-suited H2- and O2-evolving components remains challenging. Here, we fabricate a Z-scheme system for photocatalytic overall water splitting based on boron-doped, nitrogen-deficient carbon nitride two-dimensional (2D) nanosheets. We prepare ultrathin carbon nitride nanosheets with varying levels of boron dopants and nitrogen defects, which leads to nanosheets that can act as either H2- or O2-evolving photocatalysts. Using an electrostatic self-assembly strategy, the nanosheets are coupled to obtain a 2D/2D polymeric heterostructure. Owing to their ultrathin nanostructures, strong interfacial interaction and staggered band alignment, a Z-scheme route for efficient charge-carrier separation and transfer is realized. The obtained heterostructure achieves stoichiometric H2 and O2 evolution in the presence of Pt and Co(OH)2 co-catalysts, and the solar-to-hydrogen efficiency reaches 1.16% under one-sun illumination.

Investigation of Heteroatom Doped Graphene Anode for High-Capacity Alkali Metal-Ion Batteries

The drastic growth of Li-ion batteries (LIBs) in global electronic market and low crustal abundance of Li demands for more abundant alternatives such as sodium and potassium having similar electrochemical properties. Sodium-ion battery (SIB) and potassium-ion battery (PIB) can be used “side-by-side” to LIB which can help battery technology progress efficiently. 1Graphite has been a commercial anode in LIB due to its low cost, high abundance, high electrical conductivity, with a theoretical storage capacity (LiC6) of 372 mAh g-1. In contrast the utilization of graphite in SIBs and PIBs is challenging. This is due to the large radius and unstable graphite-intercalation compounds (NaC64) giving very low theoretical capacity of 30 mAh g-1. In PIBs, graphite exhibits a theoretical capacity of 279 mAh g-1 but suffers from poor stability due to huge volume expansion (60%) in fully potassiated state (KC8). Thus, focus is now shifted on to class of amorphous carbon materials such as hard carbon, soft carbon, graphene etc. with variations in micro-structures, larger interlayer spacing, and defect rich sites promoting enhanced capacity and stability even at high current densities. 2 In my presentation I will be discussing about the synthesis of heteroatom doped graphene with high charge storage and stability for LIB, SIB and PIB compared to graphite. The heteroatom doped graphene exhibits nearly 1000 mAh g-1 (LIB), 380 mAh g-1 (SIB) and 379 mAh g-1 (PIB) at a low current density of 25 mAg-1. Additionally, the elastic nature of ultrathin graphene nanosheets aids in attaining long term cyclability at 1 A g-1 with high-capacity retention of 83% for 300 cycles (LIB), 73% for 400 cycles (SIB) and 91% for 600 cycles (PIB). The enhanced stability in case of PIB, is attributed to optimizing solid electrolyte interphase (SEI) components and improving interfacial reaction kinetics at high current densities. Choice of high concentrated electrolyte (HCE) over conventional KPF6 in EC:DEC determines the nature of SEI and thereby helps in achieving cycling stability. The stability in SEI is characterized via voltage profile and understanding chemical composition by ex-situ X-ray photoelectron spectroscopy. Robust SEI in HCE is due to difference in anion structure studied by Raman spectroscopy.Furthermore, the charge-discharge mechanism, alkali ion diffusion kinetics and structure-activity relationship of electrode materials was investigated using various techniques like cyclic voltammetry profiles at different scan rates and Galvanostatic intermittent titration technique (GITT). Additionally, deeper insights into the fundamental understanding of interfacial and bulk behavior were provided with the help of in-situ Raman spectroscopy and in-situ potentiostatic electrochemical impedance spectroscopy (PEIS) analysis. The investigation of peak area and shift in D (defect) and G (graphitic) band analyzed from in-situ Raman studies helped in understanding the evolution and reversibility of alkali ion storage behavior during charging and discharging. PEIS analysis gave us a thorough understanding of the evolution of stable SEI on charging-discharging and after cycling, which helps us to understand the long-term stability in LIB, SIB and PIB.

Advances in 2D Ultrathin Nanosheets of a2FeSiO4 (A= Li, Na, K) for Next-Generation Alkali-Ion Batteries

Although lithium-ion batteries (LIBs) have become ubiquitous in various applications, the limited resource of Li and increasing cost hinder their scalability for grid-scale energy storage applications. Thus, alternative rechargeable batteries that use more earth-abundant elements such as sodium and potassium have received significant attention. However, the distinct different properties of Li+, Na+, and K+ ions hinder the extensive development of universal host materials for alkali metal ions. Polyanion-type electrode materials, which contain polyanionic groups (XO4)n −, (XO4F)n −, and (XmO2m+1)n − (X = P, Si, S, etc.), have garnered significant interest owing to their remarkable structural stability and adjustable electrode potential; this properties make them excellent candidates as cathode materials in sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs). Among these, A2FeSiO4 (AFS) (A= Li, Na, K) is highlighted as a promising cathode material due to its abundant Fe resources, low cost, and non-toxicity. The crystal structure, transport pathways, and synthesis techniques of Li2FeSiO4 (LFS) have been extensively explored whereas research on Na2FeSiO4 (NFS) as a cathode material is still in the process of maturing and research on K2FeSiO4 (KFS) is very rare. The large ionic radius of sodium and potassium presents a significant challenge to effective ion-intercalation reactions for energy storage in electrodes, destabilizing the crystal structures of host materials. In addition, orthosilicates exhibit poor electronic conductivity, greatly limiting its practical application. To address this issue, various techniques have been explored, which include nanostructuring, coating with conductive polymers, and metal doping. In this study, a facile solvothermal technique for the fabrication of ultrathin 2D AFS nanosheets by hydrothermal transformation and propose the reaction mechanism for this chemical transformation. The as-obtained AFS nanosheets are highly exfoliated using oxalic acid as the exfoliating agent to improve their electrochemical performance. These nanosheets are designed to mitigate the diffusion limitations encountered by alkali ions when traversing the densely packed structure of AFS. The highly exfoliated AFS 2D nanosheets greatly improve the electrochemical performance and cyclability of the cathode. The nanosheets exhibit a specific discharge capacity of 149 mAh g-1 for Li+, 99 mAh g-1 for Na+, and 88 mAh g-1 for K+ when operated at a current density of 100 mA g-1. Moreover, the nanosheets display exceptional capacity retention, with a retention rate of 75% for Li+, 99% for Na+, and 65% for K+ after 100 charge/discharge cycles for each of the cathodes. This study demonstrates the potential of 2D nanosheets for stabilizing the silicate-based cathode materials against structural and volume changes, and for improving their electrochemical performance in SIBs and KIBs. Figure 1

Synthesis of amorphous cobalt hydroxide nanosheets and electrochemical performance according to phase transition into crystalline cobalt oxides

Ultrathin two-dimensional nanosheets have attracted attention as nanostructures that maximize the electrochemical performance in energy storage and conversion because of their unique advantages of large specific surface areas and large exposure to active sites. Herein, we synthesized cobalt hydroxide nanosheets with an amorphous phase and a thickness of <1.5 nm. Amorphous cobalt hydroxide nanosheets grown according to the surfactant concentration were converted into crystalline cobalt oxides through a simple heat treatment. The change in the electrochemical oxygen evolution reaction performance according to this process was investigated in detail by ex- and in-situ Raman spectroscopy, and the supercapacitor performance was confirmed. This study provides promising guidelines for developing phase-dependent catalysts from cobalt hydroxide and cobalt oxides and highlights the critical role of the crystal structure in facilitating electrocatalytic processes.

NiSe2-CoSe2 hollow polyhedron as counter electrode catalyst for high-efficiency dye-sensitized solar cells

The design of efficient counter electrode (CE) materials for dye sensitized solar cells (DSSCs) strongly depend on the microstructure construction and the effective combination of different active components. In this study, a hollow polyhedral structured NiSe2-CoSe2 composite electrode was synthesized through the direct selenization of the hollow nanocage morphology nickel-cobalt layered double hydroxide (NiCo-LDH) ultrathin nanosheets, which was derived from ZIF-67 nanocrystals. Notably, owing to the unique hollow structure and synergistic effect of the Ni-Co bimetallic selenides, the hollow polyhedral NiSe2-CoSe2 CE exhibited the expected electrochemical catalytic activity in the reduction of triiodide, as revealed by electrochemical impedance spectroscopy, cyclic voltammetry and Tafel polarization measurements. The photovoltaic results revealed that the PCE of the NiSe2-CoSe2 CE reached 8.21 %, which was higher than those of the Pt and CoSe2 CEs (7.62 % and 6.78 %, respectively). Moreover, the hollow NiSe2-CoSe2 CE exhibited a favourable photocurrent response upon light on-and-off cycling and improved stability over multiple start. This study provides a meaningful reference for the construction of hollow polymetallic transition-metal selenides for DSSC devices.