Porous g-C3N4 and MXene Dual-Confined FeOOH Quantum Dots for Superior Energy Storage in an Ionic Liquid

A robust Fe-based heterogeneous photocatalyst for the visible-light-mediated selective reduction of an impure CO2 stream

The transformation of CO2 into value-added products from an impure CO2 stream, such as flue gas or exhaust gas, directly contributes to the principle of carbon capture and utilization (CCU). Thus, we have developed a robust iron-based heterogeneous photocatalyst that can convert the exhaust gas from the car into CO with an exceptional production rate of 145 μmol g-1 h-1. We characterized this photocatalyst by PXRD, XPS, ssNMR, EXAFS, XANES, HR-TEM, and further provided mechanistic experiments, and multi-scale/level computational studies. We have reached a clear understanding of its properties and performance that indicates that this highly robust photocatalyst could be used to design an efficient visible-light-mediated reduction strategy for the transformation of impure CO2 streams into value-added products.

Interfacial Engineering of Ti3C2Tx MXene Electrode Using g-C3N4 Nanosheets for High-Performance Supercapacitor in Neutral Electrolyte

The superior performance of the Ti3C2Tx (MXene)-based supercapacitor in acidic electrolytes has recently gained much interest in the energy storage community. Nevertheless, its performance in most neutral electrolytes is unfavorably low, plausibly due to limited ion diffusion between the MXene layers. Herein, protonated g-C3N4 (pg-C3N4) is incorporated into the Ti3C2Tx electrode by using a facile self-assembling process and annealing, which results in increased interlayer d-spacing and electrical conductivity of the composite electrode. As a result, the annealed Ti3C2Tx/pg-C3N4 film revealed an enhanced ion-accessibility and gravimetric capacitance of 140 F g-1 in 1 M aqueous MgSO4 electrolyte. The cyclic stability test also indicates excellent capacitance retention, with negligible loss of capacitance over 10000 cycles.

Boosting the photocatalytic activity of β-FeOOH catalyst for toluene oxidation by constructing internal electric field at 0D/1D homojunction interfaces

Photocatalytic degradation is considered as the most energy-efficient, environmentally benign, and effective method for treating low fraction organic contaminants. However, the photocatalysts still suffer from low utilization efficiency of visible-light and severe carrier recombination. Heterojunctions can resolve these two main problems in some extent but still be restrained by the low quality of hetero-interface. In this study, homojunction was constructed of β-FeOOH quantum dots and nanorods with the same lattice by a two-step precipitation method, to avoid the heterointerface with too many defects and possess good charge separation as a consequence. The catalysts were characterized by activity test, electron spin resonance, Mott-Schottky plots, photocurrent density tests and open-circuit potential measurements, etc. The results revealed that a strong internal electric fields (IEFs) was created at the interface of catalyst. Beneficently, the electron rearrangement leads to a more rational distribution of oxygen vacancies in the catalyst, resulting in more efficient dissociation of oxygen molecules and formation of active radicals, thus facilitating the efficient degradation of toluene. This study proposes a novel strategy to boosting the photocatalytic activity of low dimensional semiconductors via forming homojunction interfaces to improve their charge transfer.

High-Voltage MXene-Based Supercapacitors: Present Status and Future Perspectives

As an emerging class of 2D materials, MXene exhibits broad prospects in the field of supercapacitors (SCs). However, the working voltage of MXene-based SCs is relatively limited (typically ≤ 0.6 V) due to the oxidation of MXene electrode and the decomposition of electrolyte, ultimately leading to low energy density of the device. To solve this issue, high-voltage MXene-based electrodes and corresponding matchable electrolytes are developed urgently to extend the voltage window of MXene-based SCs. Herein, a comprehensive overview and systematic discussion regarding the effects of electrolytes (aqueous, organic, and ionic liquid electrolytes), asymmetric device configuration, and material modification on the operating voltage of MXene-based SCs, is presented. A deep dive is taken into the latest advances in electrolyte design, structure regulation, and high-voltage mechanism of MXene-based SCs. Last, the future perspectives on high-voltage MXene-based SCs and their possible development directions are outlined and discussed in depth, providing new insights for the rational design and realization of advanced next-generation MXene-based electrodes and high-voltage electrolytes.

Biomimetic Construction of Ferrite Quantum Dot/Graphene Heterostructure for Enhancing Ion/Charge Transfer in Supercapacitors

Spinel ferrites are regarded as promising electrode materials for supercapacitors (SCs) in virtue of their low cost and high theoretical specific capacitances. However, bulk ferrites suffer from limited electrical conductivity, sluggish ion transport, and inadequate active sites. Therefore, rational structural design and composition regulation of the ferrites are approaches to overcome these limitations. Herein, a general biomimetic mineralization synthetic strategy is proposed to synthesize ferrite (XFe₂ O₄ , X = Ni, Co, Mn) quantum dot/graphene (QD/G) heterostructures. Anchoring ferrite QD on the graphene sheets not only strengthens the structural stability, but also forms the electrical conductivity network needed to boost the ion diffusion and charge transfer. The optimized NiFe₂ O₄ QD/G heterostructure exhibits specific capacitances of 697.5 F g^-1 at 1 A g^-1 , and exceptional cycling performance. Furthermore, the fabricated symmetrical SCs deliver energy densities of 24.4 and 17.4 Wh kg^-1 at power densities of 499.3 and 4304.2 W kg^-1 , respectively. Density functional theory calculations indicate the combination of NiFe₂ O₄ QD and graphene facilitates the adsorption of potassium atoms, ensuring rapid ion/charge transfer. This work enriches the application of the biomimetic mineralization synthesis and provides effective strategies for boosting ion/charge transfer, which may offer a new way to develop advanced electrodes for SCs.

Electron-ion conjugation sites co-constructed by defects and heteroatoms assisted carbon electrodes for high-performance aqueous energy storage

Rapid preparation strategies of carbon-based materials with a high power density and energy density are crucial for the large-scale application of carbon materials in energy storage. However, achieving these goals quickly and efficiently remains challenging. Herein, the rapid redox reaction of concentrated H₂SO₄ and sucrose was employed as a means to destroy the perfect carbon lattice to form defects and insert large numbers of heteroatoms into the defects to rapidly form electron-ion conjugated sites of carbon materials at room temperature. Among prepared samples, CS-800-2 showed an excellent electrochemical performance (377.7 F g^-1, 1 A g^-1) and high energy density in 1 M H₂SO₄ electrolyte owing to its large specific surface area and a significant number of electron-ion conjugated sites. Additionally, CS-800-2 exhibited desirable energy storage performance in other aqueous electrolytes containing various metal ions. The theoretical calculation results revealed increased charge density near the carbon lattice defects, and the presence of heteroatoms effectively reduced the adsorption energy of carbon materials toward cations. Accordingly, the constructed "electron-ion" conjugated sites comprising defects and heteroatoms on the super-large surface of carbon-based materials accelerated the pseudo-capacitance reactions on the material surface, thereby greatly enhancing the energy density of carbon-based materials without sacrificing power density. In sum, a fresh theoretical perspective for constructing new carbon-based energy storage materials was provided, promising for future development of high-performance energy storage materials and devices.

3D Porous Compact 1D/2D Fe2 O3 /MXene Composite Aerogel Film Electrodes for All-Solid-State Supercapacitors

2D MXene nanoflakes usually undergo serious restacking, that easily aggravates during the traditional vacuum-assisted filtration process; and thus, hinders the electrochemical performance of the corresponding film electrodes. Herein, 3D porous compact 1D/2D Fe₂ O₃ /MXene aerogel film electrode with an enhanced electrochemical performance is fabricated by freeze-drying assisted mechanical pressing. An introduction of 1D α-Fe₂ O₃ nanorods can not only alleviate the restacking of 2D MXene but also provide additional pseudocapacitance for the composite film system. Thus, the resulting Fe₂ O₃ /MXene aerogel film electrode shows an enhanced specific capacitance of 182 F g^-1 (691 mF cm^-2 ) at a current density of 1 A g^-1 in 3 m H₂ SO₄ electrolyte as well as with 81.74% capacitance retention after 10 000 charge-discharge cycles. Besides, the addition of 1D α-Fe₂ O₃ nanorods has a significant contribution in the volumetric capacitance of the composite aerogel film (150 F cm^-3 ), which is 2.68 times that of the pure MXene aerogel film (56 F cm^-3 ). Moreover, the fabricated all-solid-state symmetric supercapacitor (SSSC) delivers a superior areal energy density of 3.61 µWh cm^-2 at a power density of 119.04 µW cm^-2 . This rapid-forming 3D porous, binder-free, and freestanding aerogel film provides a progressive strategy for the fabrication of MXene-based electrode for supercapacitors.

Manipulating the crack path through the surface functional groups of MXenes

As newly developed 2D materials, due to their advanced physical/chemical properties and potential applications in energy storage, electromagnetic interference shielding, electronics and photon-detectors, MXenes have attracted extensive attention in recent years. MXenes are synthesized by selectively etching the A element from MAX phases using hydrochloric acid and lithium fluoride, so hardly any pure structures of MXenes (e.g. Ti₂C, T₃C₂) exist. Usually, the surfaces of MXenes are terminated by -OH, -F, -O or other surface terminations. However, fundamental knowledge of the mechanical properties of MXenes, especially the effect of different functional groups, is limited due to the difficulty in mechanical testing. In this work, we studied the mechanical properties of a series of MXenes with emphasis on the effect of the functional groups. We took the most common MXene Ti_n+1C_nO₂ and Ti_n+1C_n(OH)₂ as examples using molecular dynamics simulation. We evaluated the moduli of various MXenes and the influence of the functional groups. Besides, we studied the fracture behavior of MXenes with different functional groups. Based on our simulations, we found that the functional group did affect the mechanical and fracture properties. And we also proposed a possible approach to controlling the crack propagation path which would benefit the further design of nanodevices and nanocomposites.

Ionic Liquid-Based Polymer Nanocomposites for Sensors, Energy, Biomedicine, and Environmental Applications: Roadmap to the Future

Current interest toward ionic liquids (ILs) stems from some of their novel characteristics, like low vapor pressure, thermal stability, and nonflammability, integrated through high ionic conductivity and broad range of electrochemical strength. Nowadays, ionic liquids represent a new category of chemical-based compounds for developing superior and multifunctional substances with potential in several fields. ILs can be used in solvents such as salt electrolyte and additional materials. By adding functional physiochemical characteristics, a variety of IL-based electrolytes can also be used for energy storage purposes. It is hoped that the present review will supply guidance for future research focused on IL-based polymer nanocomposites electrolytes for sensors, high performance, biomedicine, and environmental applications. Additionally, a comprehensive overview about the polymer-based composites' ILs components, including a classification of the types of polymer matrix available is provided in this review. More focus is placed upon ILs-based polymeric nanocomposites used in multiple applications such as electrochemical biosensors, energy-related materials, biomedicine, actuators, environmental, and the aviation and aerospace industries. At last, existing challenges and prospects in this field are discussed and concluding remarks are provided.

A Novel Hybrid Point Defect of Oxygen Vacancy and Phosphorus Doping in TiO2 Anode for High-Performance Sodium Ion Capacitor

Although sodium ion capacitors (SICs) are considered as one of the most promising electrochemical energy storage devices (organic electrolyte batteries, aqueous batteries and supercapacitor, etc.) due to the combined merits of battery and capacitor, the slow reaction kinetics and low specific capacity of anode materials are the main challenges. Point defects including vacancies and heteroatoms doping have been widely used to improve the kinetics behavior and capacity of anode materials. However, the interaction between vacancies and heteroatoms doping have been seldomly investigated. In this study, a hybrid point defects (HPD) engineering has been proposed to synthesize TiO₂ with both oxygen vacancies (OVs) and P-dopants (TiO₂/C-HPD). In comparison with sole OVs or P-doping treatments, the synergistic effects of HPD on its electrical conductivity and sodium storage performance have been clarified through the density functional theory calculation and sodium storage characterization. As expected, the kinetics and electronic conductivity of TiO₂/C-HPD3 are significantly improved, resulting in excellent rate performance and outstanding cycle stability. Moreover, the SICs assembled from TiO₂/C-HPD3 anode and nitrogen-doped porous carbon cathode show outstanding power/energy density, ultra-long life with good capacity retention. This work provides a novel point defect engineering perspective for the development of high-performance SICs electrode materials.

Flexible MXene-Based Composite Films: Synthesis, Modification, and Applications as Electrodes of Supercapacitors

MXenes, as a 2D planar structure nanomaterial, were first reported in 2011. Due to their large specific surface area, high ductility, high electrical conductivity, strong hydrophilic surface, and high mechanical flexibility, MXenes have been extensively explored in the development of various functional materials with desired performances. This review is aimed to summarize the current progress in synthesis, modification, and applications of MXene-based composite films as electrode materials of flexible energy storage devices. In the synthesis of MXenes, the evolution and exploration of etchants are emphasized. Furthermore, in order to develop MXene-based composite films, the components used to modify the MXene nanoflakes, including 0D, 1D, and 2D nanomaterials, are summarized, and the perspectives and research direction of such materials are also discussed.

Roles of Metal Ions in MXene Synthesis, Processing and Applications: A Perspective

With a decade of effort, significant progress has been achieved in the synthesis, processing, and applications of MXenes. Metal ions play many crucial roles, such as in MXene delamination, structure regulation, surface modification, MXene composite construction, and even some unique applications. The different roles of metal ions are attributed to their many interactions with MXenes and the unique nature of MXenes, including their layered structure, surface chemistry, and the existence of multi-valent transition metals. Interactions with metal ions are crucial for the energy storage of MXene electrodes, especially in metal ion batteries and supercapacitors with neutral electrolytes. This review aims to provide a good understanding of the interactions between metal ions and MXenes, including the classification and fundamental chemistry of their interactions, in order to achieve their more effective utilization and rational design. It also provides new perspectives on MXene evolution and exfoliation, which may suggest optimized synthesis strategies. In this respect, the different effects of metal ions on MXene synthesis and processing are clarified, and the corresponding mechanisms are elaborated. Research progress on the roles metal ions have in MXene applications is also introduced.

Integrated electronic skin (e-skin) for harvesting of TENG energy through push-pull ionic electrets and ion-ion hopping mechanism

The development of highly durable, stretchable, and steady triboelectric nanogenerators (TENGs) is highly desirable to satisfy the tight requirement of energy demand. Here, we presented a novel integrated polymeric membrane that is designed by PEDOT: PSSa-naphthalene sulfonated polyimide (PPNSP)-EMI.BF₄ Electronic skin (e-skin) for potential TENG applications. The proposed TENG e-skin is fabricated by an interconnected architecture with push-pull ionic electrets that can threshold the transfer of charges through an ion-hopping mechanism for the generation of a higher output voltage (Voc) and currents (Jsc) against an electronegative PTFE film. PPNSP was synthesized from the condensation of naphthalene-tetracarboxylic dianhydride, 2,2'-benzidine sulfonic acid, and 4,4'diaminodiphenyl ether through an addition copolymerization protocol, and PEDOT: PSSa was subsequently deposited using the dip-coating method. Porous networked PPNSP e-skin with continuous ion transport nano-channels is synthesized by introducing simple and strong molecular push-pull interactions via intrinsic ions. In addition, EMI.BF₄ ionic liquid (IL) is doped inside the PPNSP skin to interexchange ions to enhance the potential window for higher output Voc and Iscs. In this article, we investigated the push-pull dynamic interactions between PPNSP-EMI.BF₄ e-skin and PTFE and tolerable output performance. The novel PPNSP- EMI.BF₄ e-skin TENG produced upto 49.1 V and 1.03 µA at 1 Hz, 74 V and 1.45 µA at 2 Hz, 122.3 V and 2.21 µA at 3 Hz and 171 V and 3.6 µA at 4 Hz, and 195 V and 4.43 µA at 5 Hz, respectively. The proposed novel TENG device was shown to be highly flexible, highly durable, commercially viable, and a prospective candidate to produce higher electrical charge outputs at various applied frequencies.

In-situ construction of heterostructure (Ni, Co)Se2 nanoarrays derived from cone-like ZIF-L for high-performance hybrid supercapacitors

The construction of heterostructure could enhance the electron transfer efficiency and increase the number of active sites, which can further develop high-performance electrode materials of supercapacitors. Herein, (Ni, Co)Se₂ nanorod arrays were prepared based on the NiCo-LDH derived from a conical ZIF-L. Significantly, the single nanorod is composed of interconnected NiSe₂ and CoSe₂ nanoparticles, the heterostructure can expose higher conductivity, more sufficient redox reaction active sites and larger specific surface area. The as-obtained CF@(Ni, Co)Se₂ achieved a high specific capacity of 188.8 mAh g^-1 at the current density of 1.0 A g^-1 and an outstanding cycling stability with a high capacity retention of 90% after 8000 cycles. Finally, an hybrid supercapacitor device composed of activated carbon (AC) as negative electrode and CF@(Ni, Co)Se₂ as positive electrode was designed, which revealed an ideal voltage window of 0-1.6 V and exhibited a great energy density of 36.02 Wh kg^-1 at the power density of 800 W kg^-1, such surpassing energy storage characteristics evidently testify that (Ni, Co)Se₂ nanorod arrays can be as the potential electrode material to promote the development of high-performance supercapacitors.

Synthesis of Co/CeO2 hetero-particles with abundant oxygen-vacancies supported by carbon aerogels for ORR and OER

It is highly significant for the fabrication of rechargeable metal-air batteries to develop cost-efficient and high-performance electrocatalysts of bifunctionality for both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, we demonstrate a hybrid composed of CeO₂-decorated Co nanoparticles supported on three-dimensionally porous carbon aerogels (Co-CeO₂/C aerogels) as a superior bifunctional electrocatalyst. The preparation of Co-CeO₂/C aerogels depends on the formation of a novel CeCl₃/K₃Co(CN)₆-chitosan (CS) hydrogel, during which the cyanide groups of K₃Co(CN)₆ combines the hydroxyls in CS by hydrogen bridges, accompanying with the substitution of chloride groups in CeCl₃ by cyanide groups in K₃Co(CN)₆. The electron spin resonance offers a convincing proof that numerous oxygen vacancies were found in Co-CeO₂/C aerogels after the introduction of CeO₂. The developed Co-CeO₂/C aerogels showed an outstanding electrochemical performance for both OER and ORR in comparsion with RuO₂ and Pt/C catalysts in 0.1 M KOH solution. A small overpotential (380 mV) and a low Tafel slope (99 mV dec^-1) were observed for OER, while the half-wave potential (0.75 V) and the onset potential (0.92 V) were high for ORR. The superior performance could be put down to the multihole heterostructure, multiple components and abundant oxygen vacancies. It was very helpful for the adsorption and the catalyzation of the reactants and the efficient mass transport of reagent/product. This work paves a neoteric method to synthesize a bifunctional hybrid catalyst with a highly efficient performance of energy conversion and storage.

Rational design of Ti3C2Cl2 MXenes nanodots-interspersed MXene@NiAl-layered double hydroxides for enhanced pseudocapacitor storage

Although electrodes based on two dimensional hybrids with interstratification-assemble have been widely studied for supercapacitors, the performance enhancement still remains challenge mainly due to the random dispersion of surface passivated two dimensional nanosheets. Herein, a new covalent surface functionalization of MXene-based Ti₃C₂Cl₂ nanodots-interspersed MXene@NiAl-layered double hydroxides (QD-Ti₃C₂Cl₂@NiAl-LDHs) hybrid electrode with superior pseudocapacitor storage performance has been elaborately designed by electrostatic-assembled. As a result, the QD-Ti₃C₂Cl₂@NiAl-LDHs electrode exhibits a super specific capacitance of 2010.8F g^-1 at 1.0 A g^-1 and high energy density of 100.5 Wh kg^-1 at a power density of 299.8 W kg^-1. In addition, 94.1% capacitance retention is achieved after cycling for 10,000 cycles at 1.0 A g^-1, outperforming previously reported of two dimensional hybrids electrode for supercapacitor. Furthermore, density functional theory (DFT) calculations show that the superior pseudocapacitor storage performance of the QD-Ti₃C₂Cl₂@NiAl-LDHs may be attributed to the creation of numerous electrochemical active sites and the enhancement of electrical conductivity by the QD-Ti₃C₂Cl₂ MXene. This work provides new strategy for developing excellent pseudocapacitor supercapacitor based on two dimensional hybrid electrode.

Nitrogen-rich carbon nitride as solid-phase microextraction fiber coating for high-efficient pretreatment of polychlorinated biphenyls from environmental samples

A two-dimensional nitrogen-rich carbon nitrogen (C₃N₅) material was prepared via a facile high temperature thermal polymerization. For the first time, the C₃N₅ was used as fiber coating of solid-phase microextraction (SPME) to extract and preconcentrate polychlorinated biphenyls (PCBs) before gas chromatography (GC) analysis. The X-ray diffraction, N₂ adsorption-desorption, Fourier transform-infrared spectroscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy were performed to investigate structure, functional groups, thermal stability, bonding type, element composition, and atomic ratio of C₃N₅. The two-dimensional planar stacking structure was further verified by scanning electron microscopy and transmission electron microscopy. Five PCBs including PCB-4, PCB-12, PCB-29, PCB-52 and PCB-101 were selected as targets to evaluate performance of SPME fiber. Under the optimal conditions, the method showed a good linear range from 0.01 to 1000 ng/mL with the correlation coefficients (R²) higher than 0.9990. Enrichment factors of the method were obtained from 2045 to 3080. The limits of detection (LODs, S/N = 3) and limits of quantification (LOQs, S/N = 10) were calculated as 0.0031-0.0111 ng/mL and 0.01-0.05 ng/mL, respectively. The precisions of intra-day and inter-day were obtained with the relative standard deviations (RSDs) at 1.5-6.6% and 0.8-6.9%, respectively. The fiber-to-fiber producibility was achieved with RSDs ranged from 3.5% to 11.4%. The method was applied to detect PCBs in river water and soil samples. The contents were calculated at 0.040-0.147 ng/mL in water and 0.520-3.218 ng/g in soil. The C₃N₅ as SPME fiber coating material may be applied to extract and preconcentrate other environmental pollutants which have similar chemical structures with PCBs.

A Brief Review of the Role of 2D Mxene Nanosheets toward Solar Cells Efficiency Improvement

This article discusses the application of two-dimensional metal MXenes in solar cells (SCs), which has attracted a lot of interest due to their outstanding transparency, metallic electrical conductivity, and mechanical characteristics. In addition, some application examples of MXenes as an electrode, additive, and electron/hole transport layer in perovskite solar cells are described individually, with essential research issues highlighted. Firstly, it is imperative to comprehend the conversion efficiency of solar cells and the difficulties of effectively incorporating metal MXenes into the building blocks of solar cells to improve stability and operational performance. Based on the analysis of new articles, several ideas have been generated to advance the exploration of the potential of MXene in SCs. In addition, research into other relevant MXene suitable in perovskite solar cells (PSCs) is required to enhance the relevant work. Therefore, we identify new perspectives to achieve solar cell power conversion efficiency with an excellent quality-cost ratio.

Constructing electrostatic self-assembled ultrathin porous red 2D g-C3N4/Fe2N Schottky catalyst for high-efficiency tetracycline removal in photo-Fenton-like processes

The traditional heterogeneous photo-Fenton reaction was mainly restricted by the fewer surface-active sites, low Fe³⁺/Fe²⁺ transformation and H₂O₂ activation efficiency of catalyst. This work designed and fabricated the efficient photo-Fenton Schottky catalysts via a facile electrostatic self-assembly of metallic Fe₂N nanoparticles scattering on the surface of red g-C₃N₄ (ultrathin porous oxygen-doped 2D g-C₃N₄ nanosheets). The porous morphology and exceptional electrical structure of red g-C₃N₄ endowed more active sites and facilitated the photoexcited charge separation. Benefitting from the Schottky effect and unique dimensional coupling structure, the strong visible light absorption and fast spatial charge transfer were realized in the Schottky junction system. More strikingly, Fe₂N as an efficient co-catalyst was in favor of the trap and export of e^-, leading to the Fe³⁺/Fe²⁺ transformation and H₂O₂ activation during the photo-Fenton process. Accordingly, the as-prepared catalysts revealed outstanding activity in photo-Fenton like degradation of tetracycline (TC) although under 5 W white LED light irradiation. Furthermore, the reasonable degradation pathway of TC and corresponding toxicity of the intermediates, as well as the photo-Fenton catalytic mechanism were interpreted and discussed in detail. This study would be a great aid in the development of various Schottky catalysts for heterogeneous photo-Fenton-based environmental remediation systems.

Reduced Intercalation Energy Barrier by Rich Structural Water in Spinel ZnMn2O4 for High-Rate Zinc-Ion Batteries

Aqueous zinc-ion batteries are considered promising next-generation systems for large-scale energy storage due to low cost, environmental friendliness, and high reversibility of the Zn anode. However, the interfacial charge-transfer resistance for the insertion of divalent Zn²⁺ into cathode materials is normally high, which limits the kinetics of Zn²⁺ transfer at the cathode/electrolyte interface. This study reveals the presence of rich structural water in spinel ZnMn₂O₄ (ZnMn₂O₄·0.94H₂O, denoted as ZMO), synthesized by a scalable and low-temperature process, significantly overcoming the great interfacial charge-transfer resistance. ZMO exhibits excellent electrochemical performance toward Zn storage, that is, high capacity (230 and 101 mA h g^-1 at 0.5 and 8 A g^-1), high specific energy/specific power (329 W h kg^-1/706 W kg^-1 and 134 W h kg^-1/11,160 W kg^-1), and stable cycle retention (75% after 2000 cycles at 4 A g^-1) can be achieved. On the contrary, the controlled sample ZMO-450 with deficient structural water, prepared by post-heat treatment of ZMO at 450 °C, demonstrates limited discharge capacity (45 and 15 mA h g^-1 at 0.5 and 8 A g^-1). As examined by electrochemical impedance spectroscopy, rich structural water in ZMO effectively reduces the activation energy barrier upon Zn²⁺ insertion, rendering fast interfacial kinetics for Zn storage. Benefiting from rich structural water in ZMO, the involvement of Zn²⁺ during the charge/discharge process exhibits good reversibility, as characterized by X-ray diffraction and X-ray photoelectron spectroscopy.

Kinetically Controlled, Scalable Synthesis of γ-FeOOH Nanosheet Arrays on Nickel Foam toward Efficient Oxygen Evolution: The Key Role of In-Situ-Generated γ-NiOOH

Layered γ-type iron oxyhydroxide (γ-FeOOH) is a promising material for various applications; however, its sheet-shaped structure often suffers from instability that results in aggregation and leads to inferior performance. Herein, a kinetically controlled hydrolysis strategy is proposed for the scalable synthesis of γ-FeOOH nanosheets arrays (NAs) with enhanced structural stability on diverse substrates at ambient conditions. The underlying mechanisms for the growth of γ-FeOOH NAs associated with their structural evolution are systematically elucidated by alkalinity-controlled synthesis and time-dependent experiments. As a proof-of-concept application, γ-FeOOH NAs are developed as electrocatalysts for the oxygen evolution reaction (OER), where the sample grown on nickel foam (NF) exhibits superior performance of high catalytic current density, small Tafel slope, and exceptional durability, which is among the top level of FeOOH-based electrocatalysts. Density functional theory calculations suggest that γ-NiOOH in situ generated from the electrooxidation of NF would induce charge accumulation on the Fe sites of γ-FeOOH NAs, leading to enhanced OER intermediates adsorption for water splitting. This work affords a new technique to rationally design and synthesize γ-FeOOH NAs for various applications.

MXenes for Solar Cells

Application of two-dimensional MXene materials in photovoltaics has attracted increasing attention since the first report in 2018 due to their metallic electrical conductivity, high carrier mobility, excellent transparency, tunable work function and superior mechanical property. In this review, all developments and applications of the Ti₃C₂T_x MXene (here, it is noteworthy that there are still no reports on other MXenes' application in photovoltaics by far) as additive, electrode and hole/electron transport layer in solar cells are detailedly summarized, and meanwhile, the problems existing in the related studies are also discussed. In view of these problems, some suggestions are given for pushing exploration of the MXenes' application in solar cells. It is believed that this review can provide a comprehensive and deep understanding into the research status and, moreover, helps widen a new situation for the study of MXenes in photovoltaics.

Hexa-cata-hexabenzocoronene nanographene as a promising anode material for Mg-ion batteries

We investigated the possible use of a hexa-cata-hexabenzocoronene nanographene (HCHN) as an anode material for Mg-ion batteries (MIBs) implementing the B3LYP-gCP-D3/6-31G* scheme. The Mg cation or atom is adsorbed on the HCHN with the adsorption energy of - 200.3 or - 4.7 kcal/mol. The energy barrier related to transferring Mg cation on the HCHN surface was calculated to be 7.5 kcal/mol, producing the diffusion coefficient of 1.90 × 10^-8 cm²/s. It shows that the ion mobility is high and the rate of charge or discharge is fast. The calculated specific storage capacity of HCHN is 589.4 mAh/g and the great cell voltage is 4.23 V that is generated by the interaction of cation-π between Mg²⁺ and HCHN, which is strong. The HCHN is considered an ideal candidate to be used as an anode material in MIBs since its storage capacity and ion mobility are high, and it has a large cell voltage.

A high operating voltage micro-supercapacitor based on the interlamellar modulation type Ti3C2Tx MXene

The increasing demand for miniaturized, wearable, and flexible electronics has promoted the development of micro power sources such as microsupercapacitors (MSCs). This work reports a high-performance MSC based on Ti₃C₂T_x-layer/MnO₂-nanorod with an ionic liquid gel electrolyte, achieving a high areal capacitance of 24.7 mF cm^-2 within a wide voltage window of 2.5 V. The specific layer-rod interlaced structure of Ti₃C₂T_x/MnO₂ is designed to solve the inaccessibility of large-sized ions in ionic liquids into Ti₃C₂T_x layers. As a result, the structure modification provides an enhanced capacitance because the expanded interspace enables a sufficient number of large-sized ions to intercalate/deintercalate. This work provides insightful guidance for the interlaminar modification of Ti₃C₂T_x MXene to accommodate high operating voltage electrolyte with large-sized ions to obtain high-performance MSCs.

Recent progress on strategies for the preparation of 2D/2D MXene/g-C3N4 nanocomposites for photocatalytic energy and environmental applications

This review summarizes the possible synthetic routes, optical and morphological features to explore the 2D/2D interface and mechanism path in 2D/2D MXene/g-C₃N₄ nanocomposites for photocatalytic applications.

In situ synthesis of oxidized MXene-based metal cobalt spinel nanocomposites for an excellent promotion in thermal decomposition of ammonium perchlorate

Oxidized MXene-supported MCo2O4 (oxidized MXene/MCo2O4, M = Mn, Zn, Cu and Co) nanocomposites with an excellent catalytic performance for AP decomposition were successfully synthesized through a facile hydrothermal assisted calcination method.

Ni(OH)2 nanodot-decorated Co-Co LDH/C hollow nanocages for a high performance supercapacitor

A novel Co-Co LDH/C/Ni(OH)2 nanostructure was constructed by loading Ni(OH)2 nanodots on hollow Co-Co LDH/C nanocages derived from MOFs. The Co-Co LDH/C/Ni(OH)2 nanostructure revealed a high specific capacitance of up to 1426 F g-1 at 1 A g-1 and an outstanding rate capability with 90.2% retention at 10 A g-1 owing to the cooperative effect of the Ni(OH)2 nanodots and hollow Co-Co LDH/C nanocages. The electrochemical kinetic analysis showed that the Co-Co LDH/C/Ni(OH)2 electrode was dominated by surface capacitance control, demonstrating the origins of performance improvement. This work may provide an effective strategy by combining nanodots with hollow porous structures for low-cost and efficient energy storage materials.

Vanadium based carbide-oxide heterogeneous V2O5@V2C nanotube arrays for high-rate and long-life lithium-sulfur batteries

Due to their ultra-high theoretical energy density, low cost, and environmental friendliness, lithium-sulfur batteries have become a potentially strong competitor for next-generation energy storage devices. The search for a host material that can effectively anchor sulfur to a cathode to solve the adverse effects of the shuttle effect on batteries has become a research hotspot in the academic world. Here, we propose a three-dimensional heterostructure of V₂O₅ nanotube arrays vertically grown on V₂C-MXenes as a sulfur-supporting host material for the cathode of lithium-sulfur batteries. Through first-principles calculations, we found that V₂O₅@V₂C exhibits an extreme adsorption capacity for polysulfides. Besides, thanks to the excellent catalytic performance of V₂O₅ for oxidation reactions, the conversion reaction potential of polysulfides to Li₂S and Li₂S₂ is significantly reduced, and the shuttle effect of lithium-sulfur batteries is effectively suppressed. Also, the larger specific surface area and tubular structure of the composite host material can increase the sulfur loading of the cathode while ensuring the stability of the electrode structure. The V₂O₅@V₂C/S electrode exhibits higher initial capacity (1173 mA h g^-1 at 0.2C), longer cycle life (1000 cycles with 0.047% decay per period), and higher sulfur loading (8.4 mg cm^-2). We believe that this work can provide a reference for the design of cathode host materials for lithium-sulfur batteries with long cycle life.

Textile-based supercapacitors for flexible and wearable electronic applications

Electronic textiles have garnered significant attention as smart technology for next-generation wearable electronic devices. The existing power sources lack compatibility with wearable devices due to their limited flexibility, high cost, and environment unfriendliness. In this work, we demonstrate bamboo fabric as a sustainable substrate for developing supercapacitor devices which can easily integrate to wearable electronics. The work demonstrates a replicable printing process wherein different metal oxide inks are directly printed over bamboo fabric substrates. The MnO₂–NiCo₂O₄ is used as a positive electrode, rGO as a negative electrode, and LiCl/PVA gel as a solid-state electrolyte over the bamboo fabrics for the development of battery-supercapacitor hybrid device. The textile-based MnO₂–NiCo₂O₄//rGO asymmetric supercapacitor displays excellent electrochemical performance with an overall high areal capacitance of 2.12 F/cm² (1,766 F/g) at a current density of 2 mA/cm², the excellent energy density of 37.8 mW/cm³, a maximum power density of 2,678.4 mW/cm³ and good cycle life. Notably, the supercapacitor maintains its electrochemical performance under different mechanical deformation conditions, demonstrating its excellent flexibility and high mechanical strength. The proposed strategy is beneficial for the development of sustainable electronic textiles for wearable electronic applications.

Turning Trash into Treasure: Additive Free MXene Sediment Inks for Screen-Printed Micro-Supercapacitors

Printed functional conductive inks have triggered scalable production of smart electronics such as energy-storage devices, antennas, wearable electronics, etc. Of particular interest are highly conductive-additive-free inks devoid of costly postdeposition treatments to eliminate sacrificial components. Due to the high filler concentration required, formulation of such waste-free inks has proven quite challenging. Here, additive-free, 2D titanium carbide MXene aqueous inks with appropriate rheological properties for scalable screen printing are demonstrated. Importantly, the inks consist essentially of the sediments of unetched precursor and multilayered MXene, which are usually discarded after delamination. Screen-printed structures are presented on paper with high resolution and spatial uniformity, including micro-supercapacitors, conductive tracks, integrated circuit paths, and others. It is revealed that the delaminated nanosheets among the layered particles function as efficient conductive binders, maintaining the mechanical integrity and thus the metallic conductive network. The areal capacitance (158 mF cm^-2 ) and energy density (1.64 µWh cm^-2 ) of the printed micro-supercapacitors are much superior to other devices based on MXene or graphene. The ink formulation strategy of "turning trash into treasure" for screen printing highlights the potential of waste-free MXene sediment printing for scalable and sustainable production of next-generation wearable smart electronics.

Morphology restrained growth of V2O5 by the oxidation of V-MXenes as a fast diffusion controlled cathode material for aqueous zinc ion batteries

Restrained morphology and structural engineering of layered V₂O₅ by the oxidation of V-MXene to form a unique V₂O₅@V₂C nanohybrids at different temperature and used as a cathode material for aqueous Zn-ion batteries (ZIBs).