Redox-Active Metaphosphate-Like Terminals Enable High-Capacity MXene Anodes for Ultrafast Na-Ion Storage

Heterogeneous MoS2 Nanosheets on Porous TiO2 Nanofibers toward Fast and Reversible Sodium-Ion Storage

2D layered molybdenum disulfide (MoS2) has garnered considerable attention as an attractive electrode material in sodium-ion batteries (SIBs), but sluggish mass transfer kinetic and capacity fading make it suffer from inferior cycle capability. Herein, hierarchical MoS2 nanosheets decorated porous TiO2 nanofibers (MoS2 NSs@TiO2 NFs) with rich oxygen vacancies are engineered by microemulsion electrospinning method and subsequent hydrothermal/heat treatment. The MoS2 NSs@TiO2 NFs improves ion/electron transport kinetic and long-term cycling performance through distinctive porous structure and heterogeneous component. Consequently, the electrode exhibits excellent long-term Na storage capacity (298.4 mAh g-1 at 5 A g-1 over 1100 cycles and 235.6 mAh g-1 at 10 A g-1 over 7200 cycles). Employing Na3V2(PO4)3 as cathode, the full cell maintains a desirable capacity of 269.6 mAh g-1 over 700 cycles at 1.0 A g-1. The stepwise intercalation-conversion and insertion/extraction endows outstanding Na+ storage performance, which yields valuable insight into the advancement of fast-charging and long-cycle life SIBs anode materials.

An Engineering Porous Spherical Carbon Cathode for High-Energy Sodium-Ion Capacitor

Sodium-ion hybrid capacitors (SICs) are emerging as promising devices that can balance energy and power output. However, the lack of a high-capacity cathode that can match the anode has limited its further application. In this work, we develop an efficient method to prepare spherical porous carbons (SPCs) with great specific surface area and narrow pore size distribution from coal-based humic acid via spray drying and a subsequent chemical activation process. Thanks to this unique porous structure, the SPC cathode has a superb capacity of 223 F g-1 at 0.05 A g-1, as well as splendid rate performance and cycling stability. SICs constructed by an SPC cathode and hard carbon anode can exhibit a high energy density of 179.8 Wh kg-1 at 155 W kg-1 and achieved 89.4% capacity retention after 10 000 cycles at 0.5 A g-1. This outcome presents a viable approach to attaining high-capacity cathodes for constructing outstanding performance hybrid capacitors.

Microscopic-Level Insights into P-O-Induced Strong Electronic Coupling Over Nickel Phosphide with Efficient Benzyl Alcohol Electrooxidation

Electrooxidation of biomass into fine chemicals coupled with energy-saving hydrogen production for a zero-carbon economy holds great promise. Advanced anode catalysts determine the cell voltage and electrocatalytic efficiency greatly, further the rational design and optimization of their active site coordination remains a challenge. Herein, a phosphorus-oxygen terminals-rich species (Ni2P-O-300) via an anion-assisted pyrolysis strategy is reported to induce strong electronic coupling and high valence state of active nickel sites over nickel phosphide. This ultimately facilitates the rapid yet in-situ formation of high-valence nickel with a high reaction activity under electrochemical conditions, and exhibits a low potential of 1.33 V vs. RHE at 10 mA cm-2, exceeding most of reported transition metal-based catalysts. Advanced spectroscopy, theoretical calculations, and experiments reveal that the functional P-O species can induce the favorable local bonding configurations for electronic coupling, promoting the electron transfer from Ni to P and the adsorption of benzyl alcohol (BA). Finally, the hydrogen production efficiency and kinetic constant of BA electrooxidation by Ni2P-O-300 are increased by 9- and 2.8- fold compared with the phosphorus-oxygen terminals-deficient catalysts (Ni2P-O-500). This provides an anion-assisted pyrolysis strategy to modulate the electronic environment of the Ni site, enabling a guideline for Ni-based energy/catalysis systems.

Engineering sulfur defective Bi2S3@C with remarkably enhanced electrochemical kinetics of lithium-ion batteries

Introducing the appropriate vacancies to augment the active sites and improve the electrochemical kinetics while maintaining high cyclability is a major challenge for its widespread application in electrochemical energy storage. Here, core-shell structured Bi2S3@C with sulfur vacancies was prepared by hydrothermal method and one-step carbonization/sulfuration process, which significantly improves the intrinsic electrical conductivity and ion transport efficiency of Bi2S3. Additionally, the uniform protective carbon layer around surface of composite maintains structural stability and effectively alleviates volume expansion during alloying/dealloying. As a result, the BSC-500 anode exhibits a brilliant reversible capacity of 636 mAh/g at 0.2 A/g and a long-term stable capacity of 524 mAh/g for 500 cycles at a high current density of 3 A/g in lithium-ion batteries. In addition, the assembled Bi2S3@C//LiCoO2 full cell delivered a capacity of 184 mAh/g at 1 A/g and excellent cyclability (125 mAh/g after 1000 cycles). The proposed strategy of combining sulfur vacancies with a core-shell structure to improve the electrochemical kinetics of Bi2S3 in lithium-ion batteries off the prospect for practical applications of transition metal sulfide anodes.

NbC Nanoparticles Decorated Carbon Nanofibers as Highly Active and Robust Heterostructural Electrocatalysts for Ammonia Synthesis

Transition-metal carbides with metallic properties have been extensively used as electrocatalysts due to their excellent conductivity and unique electronic structures. Herein, NbC nanoparticles decorated carbon nanofibers (NbC@CNFs) are proposed as an efficient and robust catalyst for electrochemical synthesis of ammonia from nitrate/nitrite reduction, which achieves a high Faradaic efficiency (FE) of 94.4 % and a large ammonia yield of 30.9 mg h-1 mg-1 cat.. In situ electrochemical tests reveal the nitrite reduction at the catalyst surface follows the *NO pathway and theoretical calculations reveal the formation of NbC@CNFs heterostructure significantly broadens density of states nearby the Fermi energy. Finite element simulations unveil that the current and electric field converge on the NbC nanoparticles along the fiber, suggesting the dispersed carbides are highly active for nitrite reduction.

Unlocking Ultrahigh Initial Coulombic Efficiency of MXene Anode via Presodiation and Electrolyte Optimization

The low initial Coulombic efficiency (ICE) greatly hinders the practical application of MXenes in sodium-ion batteries. Herein, theoretical calculations confirm that -F and -OH terminations as well as the tetramethylammonium ion (TMA+) intercalator in sediment Ti3C2Tx (s-Ti3C2Tx) MXene possess strong interaction with Na+, which impedes Na+ desorption during the charging process and results in low ICE. Consequently, Na+-intercalated sediment Ti3C2Tx (Na-s-Ti3C2Tx) is constructed through Na2S·9H2O treatment of s-Ti3C2Tx. Specifically, Na+ can first exchange with TMA+ of s-Ti3C2Tx and then combine with -F and -OH terminations, thus leading to the elimination of TMA+ and preshielding of -F and -OH. As expected, the resulting Na-s-Ti3C2Tx anode delivers considerably boosted ICE values of around 71% in carbonate-based electrolytes relative to s-Ti3C2Tx. Furthermore, electrolyte optimization is employed to improve ICE, and the results demonstrate that an ultrahigh ICE value of 94.0% is obtained for Na-s-Ti3C2Tx in the NaPF6-diglyme electrolyte. More importantly, Na-s-Ti3C2Tx exhibits a lower Na+ migration barrier and higher electronic conductivity compared with s-Ti3C2Tx based on theoretical calculations. In addition, the cyclic stability and rate performance of the Na-s-Ti3C2Tx anode in various electrolytes are comprehensively explored. The presented simple strategy in boosting ICE significantly enhances the commercialization prospect of MXenes in advanced batteries.

Advanced Insights on MXenes: Categories, Properties, Synthesis, and Applications in Alkali Metal Ion Batteries

The development and optimization of promising anode material for next-generation alkali metal ion batteries are significant for clean energy evolution. 2D MXenes have drawn extensive attention in electrochemical energy storage applications, due to their multiple advantages including excellent conductivity, robust mechanical properties, hydrophilicity of its functional terminations, and outstanding electrochemical storage capability. In this review, the categories, properties, and synthesis methods of MXenes are first outlined. Furthermore, the latest research and progress of MXenes and their composites in alkali metal ion storage are also summarized comprehensively. A special emphasis is placed on MXenes and their hybrids, ranging from material design and fabrication to fundamental understanding of the alkali ion storage mechanisms to battery performance optimization strategies. Lastly, the challenges and personal perspectives of the future research of MXenes and their composites for energy storage are presented.

Ion pair sites for efficient electrochemical extraction of uranium in real nuclear wastewater

Electrochemical uranium extraction from nuclear wastewater represents an emerging strategy for recycling uranium resources. However, in nuclear fuel production which generates the majority of uranium-containing nuclear wastewater, fluoride ion (F-) co-exists with uranyl (UO22+), resulting in the complex species of UO2Fx and thus decreasing extraction efficiency. Herein, we construct Tiδ+-PO43- ion pair extraction sites in Ti(OH)PO4 for efficient electrochemical uranium extraction in wastewater from nuclear fuel production. These sites selectively bind with UO2Fx through the combined Ti-F and multiple O-U-O bonds. In the uranium extraction, the uranium species undergo a crystalline transition from U3O7 to K3UO2F5. In real nuclear wastewater, the uranium is electrochemically extracted with a high efficiency of 99.6% and finally purified as uranium oxide powder, corresponding to an extraction capacity of 6829 mg g-1 without saturation. This work paves an efficient way for electrochemical uranium recycling in real wastewater of nuclear production.

Ultra-Efficient Synthesis of Nb4 C3 Tx MXene via H2 O-Assisted Supercritical Etching for Li-Ion Battery

Nb4 C3 Tx MXene has shown extraordinary promise for various applications owing to its unique physicochemical properties. However, it can only be synthesized by the traditional HF-based etching method, which uses large amounts of hazardous HF and requires a long etching time (> 96 h), thus limiting its practical application. Here, an ultra-efficient and environmental-friendly H2 O-assisted supercritical etching method is proposed for the preparation of Nb4 C3 Tx MXene. Benefiting from the synergetic effect between supercritical CO2 (SPC-CO2 ) and subcritical H2 O （SBC-H2 O）, the etching time for Nb4 C3 Tx MXene can be dramatically shortened to 1 h. The as-synthesized Nb4 C3 Tx MXene possesses uniform accordion-like morphology and large interlayer spacing. When used as anode for Li-ion battery, the Nb4 C3 Tx MXene delivers a high reversible specific capacity of 430 mAh g-1 at 0.1 A g-1 , which is among the highest values achieved in pure-MXene-based anodes. The superior lithium storage performance of the Nb4 C3 Tx MXene can be ascribed to its high conductivity, fast Li+ diffusion kinetics and good structural stability.

1D Insertion Chains Induced Small-Polaron Collapse in MoS2 2D Layers Toward Fast-Charging Sodium-Ion Batteries

Molybdenum disulfide (MoS2 ) with high theoretical capacity is viewed as a promising anode for sodium-ion batteries but suffers from inferior rate capability owing to the polaron-induced slow charge transfer. Herein, a polaron collapse strategy induced by electron-rich insertions is proposed to effectively solve the above issue. Specifically, 1D [MoS] chains are inserted into MoS2 to break the symmetry states of 2D layers and induce small-polaron collapse to gain fast charge transfer so that the as-obtained thermodynamically stable Mo2 S3 shows metallic behavior with 107 times larger electrical conductivity than that of MoS2 . Theoretical calculations demonstrate that Mo2 S3 owns highly delocalized anions, which substantially reduce the interactions of Na-S to efficiently accelerate Na+ diffusion, endowing Mo2 S3 lower energy barrier (0.38 vs 0.65 eV of MoS2 ). The novel Mo2 S3 anode exhibits a high capacity of 510 mAh g-1 at 0.5 C and a superior high-rate stability of 217 mAh g-1 at 40 C over 15 000 cycles. Further in situ and ex situ characterizations reveal the in-depth reversible redox chemistry in Mo2 S3 . The proposed polaron collapse strategy for intrinsically facilitating charge transfer can be conducive to electrode design for fast-charging batteries.

Functional MXenes: Progress and Perspectives on Synthetic Strategies and Structure-Property Interplay for Next-Generation Technologies

MXenes are a class of 2D materials that include layered transition metal carbides, nitrides, and carbonitrides. Since their inception in 2011, they have garnered significant attention due to their diverse compositions, unique structures, and extraordinary properties, such as high specific surface areas and excellent electrical conductivity. This versatility has opened up immense potential in various fields, catalyzing a surge in MXene research and leading to note worthy advancements. This review offers an in-depth overview of the evolution of MXenes over the past 5 years, with an emphasis on synthetic strategies, structure-property relationships, and technological prospects. A classification scheme for MXene structures based on entropy is presented and an updated summary of the elemental constituents of the MXene family is provided, as documented in recent literature. Delving into the microscopic structure and synthesis routes, the intricate structure-property relationships are explored at the nano/micro level that dictate the macroscopic applications of MXenes. Through an extensive review of the latest representative works, the utilization of MXenes in energy, environmental, electronic, and biomedical fields is showcased, offering a glimpse into the current technological bottlenecks, such asstability, scalability, and device integration. Moreover, potential pathways for advancing MXenes toward next-generation technologies are highlighted.

High-field and fast-spinning 1H MAS NMR spectroscopy for the characterization of two-dimensional covalent organic frameworks

Two-dimensional (2D) materials, like 2D covalent organic frameworks (COFs), have been attracting increasing research interest. They are usually obtained as polycrystalline powders. Solid-state NMR spectroscopy is capable of delivering structural information about such materials. Previous studies have applied, for example, 13C cross-polarization magic angle spinning (CP MAS) NMR experiments to characterize 2D COFs. Herein, we demonstrate the usefulness of high-field and fast-spinning 1H MAS NMR spectroscopy to resolve and quantify the signals of different 1H species within 2D COFs, including the edge sites and/or defects. Moreover, 1H-13C heteronuclear correlation (HETCOR) spectroscopy has also been applied and can provide improved resolution to obtain further information about stacking effects as well as edge sites/defects.

2D Amorphous Iron Selenide Sulfide Nanosheets for Stable and Rapid Sodium-Ion Storage

Sodium ion batteries (SIBs) suffer from large electrode volume change and sluggish redox kinetics for the relatively large ionic radius of sodium ions, raising a significant challenge to improve their long-term cyclability and rate capacity. Here, it is proposed to apply 2D amorphous iron selenide sulfide nanosheets (a-FeSeS NSs) as an anode material for SIBs and demonstrate that they exhibit remarkable rate capability of 528.7 mAh g-1 at 1 A g-1 and long-life cycle (10 000 cycles) performance (300.4 mAh g-1 ). This performance is much more superior to that of the previously reported Fe-based anode materials, which is attributed to their amorphous structure that alleviates volume expansion of electrode, 2D nature that facilitates electrons/ions transfer, and the S/Se double anions that offer more reaction sites and stabilize the amorphous structure. Such a 2D amorphous strategy provides a fertile platform for structural engineering of other electrode materials, making a more secure energy prospect closer to a reality.

Defect-Free Few-Layer M4 C3 Tx (M = V, Nb, Ta) MXene Nanosheets: Synthesis, Characterization, and Physicochemical Properties

High-quality few-layer M4 C3 Tx (M = V, Nb, Ta) MXenes are very important for applications and are necessary for clarifying their physicochemical properties. However, the difficulty in etching for themselves and the existence of MC/MC1-δ and M-Al alloy impurities in their M4 AlC3 precursors seriously hinder the achievement of defect-free few-layer M4 C3 Tx (M = V, Nb, Ta) MXenes nanosheets. Herein, three different defect-free few-layer M4 C3 Tx (M = V, Nb, Ta) nanosheets are obtained by using a universal synthesis strategy of calcination, selective etching, intercalation, and exfoliation. Comprehensive characterizations confirm their defect-free few-layer structure feature, large interlayer spacing (1.702-1.955 nm), types of functional groups (-OH, -F, -O), and abundant valance states (M5+ , M4+ , M3+ , M2+ , M0 ). M4 C3 Tx (M = V, Nb, Ta) free-standing films obtained by vacuum filtration of few-layer M4 C3 Tx inks show good hydrophilia, high thermostability, and conductivity. A roadmap on synthesis of defect-free few-layer M4 C3 Tx (M = V, Nb, Ta) nanosheets are proposed and three key points are summarized. This work provides detailed guidelines for the synthesis of other defect-free few-layer MXenes nanosheets, but also will stimulate extensive functional explorations for M4 C3 Tx (M = V, Nb, Ta) MXenes nanosheets in the future.

MXene Derivatives for Energy Storage and Conversions

Associated with the rapid development of 2D transition metal carbides, nitrides, and carbonitrides (MXenes), MXene derivatives have been recently exploited and exhibited unique physical/chemical properties, holding promising applications in the areas of energy storage and conversions. This review provides a comprehensive summarization of the latest research and progress on MXene derivatives, including termination-tailored MXenes, single-atom implanted MXenes, intercalated MXenes, van der Waals atomic layers, and non-van der Waals heterostructures. The intrinsic relationship between structure, properties, and corresponding applications for MXene derivatives are then emphasized. Finally, the essential challenges are addressed and perspectives for the MXene derivatives are also discussed.

Vanadium and Niobium MXenes-Bilayered V2 O5 Asymmetric Supercapacitors

MXenes offer high metallic conductivity and redox capacitance that are attractive for high-power, high-energy storage devices. However, they operate limitedly under high anodic potentials due to irreversible oxidation. Pairing them with oxides to design asymmetric supercapacitors may expand the voltage window and increase the energy storage capabilities. Hydrated lithium preintercalated bilayered V2 O5 ( δ-Lix V2 O5 ·nH2 O) is attractive for aqueous energy storage due to its high Li capacity at high potentials; however, its poor cyclability remains a challenge. To overcome its limitations and achieve a wide voltage window and excellent cyclability, it is combined with V2 C and Nb4 C3 MXenes. Asymmetric supercapacitors employing lithium intercalated V2 C (Li-V2 C) or tetramethylammonium intercalated Nb4 C3 (TMA-Nb4 C3 ) MXenes as the negative electrode, and a δ-Lix V2 O5 ·nH2 O composite with carbon nanotubes as the positive electrode in 5 m LiCl electrolyte operate over wide voltage windows of 2 and 1.6 V, respectively. The latter shows remarkably high cyclability-capacitance retention of ≈95% after 10 000 cycles. This work highlights the importance of selecting appropriate MXenes to achieve a wide voltage window and a long cycle life in combination with oxide anodes to demonstrate the potential of MXenes beyond Ti3 C2 in energy storage.

Mushroom-like cobalt nickle metaphosphate@nickel diselenide core-shell nanorods for asymmetric supercapacitors

Although transition metal metaphosphates (TMPOs) display special physical/chemical features and high theoretical capacities, their applications for supercapacitors (SCs) are still restricted by their low energy densities and inferior cycling stability. Herein, a novel strategy has been proposed to address these issues through in situ construction of cobalt nickle metaphosphate (Co_0.2Ni_0.8(PO₃)₂)@nickel diselenide (NiSe₂) core-shell heterostructure on carbon paper (CP) as a self-supporting flexible electrode for SCs. Particularly, this unique mushroom-like porous nanoarchitecture assembled by one-dimensional (1D) Co_0.2Ni_0.8(PO₃)₂ nanorods and zero-dimensional (0D) NiSe₂ nanospheres can expose abundant active sites and afford multi-dimensional channels, which favors rapid electron ions/electron transfer, accelerates the reaction kinetics, and alleviates volume changes during charging/discharging processes. Profiting from its well-aligned 1D/0D nanostructure and strong synergistic effect between Co_0.2Ni_0.8(PO₃)₂ and NiSe₂, the Co_0.2Ni_0.8(PO₃)₂@NiSe₂/CP electrode delivers a specific capacity of 219.4 mAh/g/0.414 mAh cm^-2 at 1 A/g and good cycling stability with capacity retention of 90.7% after 5000 cycles, outperforming many previously reported TMPO-based electrodes in literature. Impressively, an asymmetric supercapacitor (ASC) device assembled with Co_0.2Ni_0.8(PO₃)₂@NiSe₂ as cathode and porous carbon as anode achieves an energy density of 69.2 Wh kg^-1 at 736.0 W kg^-1 and maintains a capacity retention of 97.6% after 20,000 charge-discharge cycles. This work provides an efficient approach to design multi-dimensional hybrid nanomaterials for high-performance SCs.

Self-supporting network-structured MoS2/heteroatom-doped graphene as superior anode materials for sodium storage

Layered graphene and molybdenum disulfide have outstanding sodium ion storage properties that make them suitable for sodium-ion batteries (SIBs). However, the easy and large-scale preparation of graphene and molybdenum disulfide composites with structural stability and excellent performance face enormous challenges. In this study, a self-supporting network-structured MoS₂/heteroatom-doped graphene (MoS₂/NSGs-G) composite is prepared by a simple and exercisable electrochemical exfoliation followed by a hydrothermal route. In the composite, layered MoS₂ nanosheets and heteroatom-doped graphene nanosheets are intertwined with each other into self-supporting network architecture, which could hold back the aggregation of MoS₂ and graphene effectively. Moreover, the composite possesses enlarged interlayer spacing of graphene and MoS₂, which could contribute to an increase in the reaction sites and ion transport of the composite. Owing to these advantageous structural characteristics and the heteroatomic co-doping of nitrogen and sulfur, MoS₂/NSGs-G demonstrates greatly reversible sodium storage capacity. The measurements revealed that the reversible cycle capacity was 443.9 mA h g^-1 after 250 cycles at 0.5 A g^-1, and the rate capacity was 491.5, 490.5, 453.9, 418.1, 383.8, 333.1, and 294.4 mA h g^-1 at 0.1, 0.2, 0.5, 1, 2, 5 and 10 A g^-1, respectively. Furthermore, the MoS₂/NSGs-G sample displayed lower resistance, dominant pseudocapacitive contribution, and faster sodium ion interface kinetics characteristic. Therefore, this study provides an operable strategy to obtain high-performance anode materials, and MoS₂/NSGs-G with favorable structure and excellent cycle stability has great application potential for SIBs.

Tripodal Pd metallenes mediated by Nb2C MXenes for boosting alkynes semihydrogenation

2D metallene nanomaterials have spurred considerable attention in heterogeneous catalysis by virtue of sufficient unsaturated metal atoms, high specific surface area and surface strain. Nevertheless, the strong metallic bonding in nanoparticles aggravates the difficulty in the controllable regulation of the geometry of metallenes. Here we propose an efficient galvanic replacement strategy to construct Pd metallenes loaded on Nb₂C MXenes at room temperature, which is triggered by strong metal-support interaction based on MD simulations. The Pd metallenes feature a chair structure of six-membered ring with the coordination number of Pd as low as 3. Coverage-dependent kinetic analysis based on first-principles calculations reveals that the tripodal Pd metallenes promote the diffusion of alkene and inhibit its overhydrogenation. As a consequence, Pd/Nb₂C delivers an outstanding turnover frequency of 10372 h^-1 and a high selectivity of 96% at 25 ^oC in the semihydrogenation of alkynes without compromising the stability. This strategy is general and scalable considering the plentiful members of the MXene family, which can set a foundation for the design of novel supported-metallene catalysts for demanding transformations.

Electron-Extraction Engineering Induced 1T''-1T' Phase Transition of Re0.75 V0.25 Se2 for Ultrafast Sodium Ion Storage

Inducing new phases of transition metal dichalcogenides by controlling the d-electron-count has attracted much interest due to their novel structures and physicochemical properties. 1T'' ReSe₂ is a promising candidate for sodium storage, but the low electronic conductivity and limited active sites hinder its electrochemical capacity. Herein, new-phase 1T' Re_0.75 V_0.25 Se₂ crystals (P2/m) with zig-zag chains are successfully synthesized. The 1T''-1T' phase transition results from the electronic reorganization of 5d orbitals via electron extraction after V-atom doping. The electrical conductivity of 1T' Re_0.75 V_0.25 Se₂ is 2.7 × 10⁵ times higher than that of 1T'' ReSe₂ . Moreover, density functional theory (DFT) calculations reveal that 1T' Re_0.75 V_0.25 Se₂ has a larger interlayer spacing, lower bonding energy, and migration energy barrier for Na⁺ ions than 1T'' ReSe₂ . As a result, 1T' Re_0.75 V_0.25 Se₂ electrode shows an excellent rate capability of 203 mAh g^-1 at 50 C with no capacity fading over 5000 cycles for sodium storage, which is superior to most reported sodium-ion anode materials. This 1T' Re_0.75 V_0.25 Se₂ provides a new platform for various applications such as electronics, catalysis, and energy storage.

Exploring 2D Energy Storage Materials: Advances in Structure, Synthesis, Optimization Strategies, and Applications for Monovalent and Multivalent Metal-Ion Hybrid Capacitors

The design and development of advanced energy storage devices with good energy/power densities and remarkable cycle life has long been a research hotspot. Metal-ion hybrid capacitors (MHCs) are considered as emerging and highly prospective candidates deriving from the integrated merits of metal-ion batteries with high energy density and supercapacitors with excellent power output and cycling stability. The realization of high-performance MHCs needs to conquer the inevitable imbalance in reaction kinetics between anode and cathode with different energy storage mechanisms. Featured by large specific surface area, short ion diffusion distance, ameliorated in-plane charge transport kinetics, and tunable surface and/or interlayer structures, 2D nanomaterials provide a promising platform for manufacturing battery-type electrodes with improved rate capability and capacitor-type electrodes with high capacity. In this article, the fundamental science of 2D nanomaterials and MHCs is first presented in detail, and then the performance optimization strategies from electrodes and electrolytes of MHCs are summarized. Next, the most recent progress in the application of 2D nanomaterials in monovalent and multivalent MHCs is dealt with. Furthermore, the energy storage mechanism of 2D electrode materials is deeply explored by advanced characterization techniques. Finally, the opportunities and challenges of 2D nanomaterials-based MHCs are prospected.

Interfacial Charge-Modulated Multifunctional MoS2/Ti3C2Tx Penetrating Electrode for High-Efficiency Freshwater Production

Freshwater production is critical in terms of solving the global water shortage. Aiming at improving freshwater production capability and ensuring its quality, an interfacial charge-modulated MoS₂/Ti₃C₂T_x-modified carbon fiber (CF/MoS₂/Ti₃C₂T_x) penetrating electrode is designed. To maximize the desalination and degradation efficiencies of CF/MoS₂/Ti₃C₂T_x, a photocatalytic component is introduced into the membrane capacitive deionization (PMCDI) device. High desalination capability is derived from the lamellar architecture structure of MoS₂/Ti₃C₂T_x. Meanwhile, excellent degradation performance is due to the formation of two photoelctrocatalytic activity centers, directionally generating singlet oxygen (¹O₂) and hydroxyl radical (^•OH). The intercalated Cl^- (desalination) as the electron transfer bridge optimizes the charge distribution of MoS₂/Ti₃C₂T_x, reinforcing the photoelectrocatalytic activity (degradation). The formation of the electron-deficient (desalination) and electron-rich (regeneration) regions at the terminated O atom of Ti₃C₂T_x accelerate the generations of ^•OH and ¹O₂, respectively. In perspective, a mutual promotion process of desalination and degradation is achieved for high-efficiency production of high-quality freshwater.

3D Sodiophilic Ti3C2 MXene@g-C3N4 Hetero-Interphase Raises the Stability of Sodium Metal Anodes

Owing to several advantages of metallic sodium (Na), such as a relatively high theoretical capacity, low redox potential, wide availability, and low cost, Na metal batteries are being extensively studied, which are expected to play a major role in the fields of electric vehicles and grid-scale energy storage. Although considerable efforts have been devoted to utilizing MXene-based materials for suppressing Na dendrites, achieving a stable cycling of Na metal anodes remains extremely challenging due to, for example, the low Coulombic efficiency (CE) caused by the severe side reactions. Herein, a g-C₃N₄ layer was attached in situ on the Ti₃C₂ MXene surface, inducing a surface state reconstruction and thus forming a stable hetero-interphase with excellent sodiophilicity between the MXene and g-C₃N₄ to inhibit side reactions and guide uniform Na ion flux. The 3D construction can not only lower the local current density to facilitate uniform Na plating/stripping but also mitigate volume change to stabilize the electrolyte/electrode interphase. Thus, the 3D Ti₃C₂ MXene@g-C₃N₄ nanocomposite enables much enhanced average CEs (99.9% at 1 mA h cm^-2, 0.5 mA cm^-2) in asymmetric half cells, long-term stability (up to 700 h) for symmetric cells, and stable cycling (up to 800 cycles at 2 C), together with outstanding rate capability (up to 20 C), of full cells. The present study demonstrates an approach in developing practically high performance for Na metal anodes.

Hierarchical MXene/transition metal oxide heterostructures for rechargeable batteries, capacitors, and capacitive deionization

2D MXenes have attracted considerable attention due to their high electronic conductivity, tunable metal compositions, functional termination groups, low ion diffusion barriers, and abundant active sites. However, MXenes suffer from sheet stacking and partial surface oxidation, limiting their energy storage and water treatment development. To solve these problems and enhance the performance of MXenes in practical applications, various hierarchical MXene/transition metal oxide (MXene/TMO) heterostructures are rationally designed and constructed. The hierarchical MXene/TMO heterostructures can not only prevent the stacking of MXene sheets and improve the electronic conductivity and buffer the volume change of TMOs during the electrochemical reaction process. The synergistic effect of conductive MXenes and active TMOs also makes MXene/TMO heterostructures promising electrode materials for energy storage and seawater desalination. This review mainly introduces and discusses the recent research progress in MXene/TMO heterostructures, focusing on their synthetic strategies, heterointerface engineering, and applications in rechargeable batteries, capacitors, and capacitive deionization (CDI). Finally, the key challenges and prospects for the future development of the MXene/TMO heterostructures in rechargeable batteries, capacitors, and CDI are proposed.

An efficient vanadium/cobalt metaphosphate electrocatalyst for hydrogen and oxygen evolution in alkaline water splitting

Hydrogen, which meets the goal of carbon neutrality, is expected to replace conventional fossil fuels in the near future. Electrochemical water splitting is a clean hydrogen production technology, but its...