Can Anions Be Inserted into MXene?

Interlayer Structure Manipulation of FeOCl/MXene with Soft/Hard Interface Design for Safe Water Production Using Dechlorination Battery Deionization

Suffering from the susceptibility to decomposition, the potential electrochemical application of FeOCl has greatly been hindered. The rational design of the soft-hard material interface can effectively address the challenge of stress concentration and thus decomposition that may occur in the electrodes during charging and discharging. Herein, interlayer structure manipulation of FeOCl/MXene using soft-hard interface design method were conducted for electrochemical dechlorination. FeOCl was encapsulated in Ti3C2Tx MXene nanosheets by electrostatic self-assembly layer by layer to form a soft-hard mechanical hierarchical structure, in which Ti3C2Tx was used as flexible buffer layers to relieve the huge volume change of FeOCl during Cl- intercalation/deintercalation and constructed a conductive network for fast charge transfer. The CDI dechlorination system of FeOCl/Ti3C2Tx delivered outstanding Cl- adsorption capacity (158.47 ± 6.98 mg g-1), rate (6.07 ± 0.35 mg g-1 min-1), and stability (over 94.49 % in 30 cycles), and achieved considerable energy recovery (21.14 ± 0.25 %). The superior dechlorination performance was proved to originate from the Fe2+/Fe3+ topochemical transformation and the deformation constraint effect of Ti3C2Tx on FeOCl. Our interfacial design strategy enables a hard-to-soft integration capacity, which can serve as a universal technology for solving the traditional problem of electrode volume expansion.

Super-Stretchable and High-Energy Micro-Pseudocapacitors Based on MXene Embedded Ag Nanoparticles

The advancement of aqueous micro-supercapacitors offers an enticing prospect for a broad spectrum of applications, spanning from wearable electronics to micro-robotics and sensors. Unfortunately, conventional micro-supercapacitors are characterized by low capacity and slopy voltage profiles, limiting their energy density capabilities. To enhance the performance of these devices, the use of 2D MXene-based compounds has recently been proposed. Apart from their capacitive contributions, these structures can be loaded with redox-active nanowires which increase their energy density and stabilize their operation voltage. However, introducing rigid nanowires into MXene films typically leads to a significant decline in their mechanical properties, particularly in terms of flexibility. To overcome this issue, super stretchable micro-pseudocapacitor electrodes composed of MXene nanosheets and in situ reconstructed Ag nanoparticles (Ag-NP-MXene) are herein demonstrated, delivering high energy density, stable operation voltage of ≈1 V, and fast charging capabilities. Careful experimental analysis and theoretical simulations of the charging mechanism of the Ag-NP-MXene electrodes reveal a dual nature charge storage mechanism involving ad(de)sorption of ions and conversion reaction of Ag nanoparticles. The superior mechanical properties of synthesized films obtained through in situ construction of Ag-NP-MXene structure show an ultra stretchability, allowing the devices to provide stable voltage and energy output even at 100% elongation.

Tuning MXene Properties through Cu Intercalation: Coupled Guest/Host Redox and Pseudocapacitance

MXenes are 2D transition metal carbides, nitrides, and/or carbonitrides that can be intercalated with cations through chemical or electrochemical pathways. While the insertion of alkali and alkaline earth cations into Ti3C2Tx MXenes is well studied, understanding of the intercalation of redox-active transition metal ions into MXenes and its impact on their electronic and electrochemical properties is lacking. In this work, we investigate the intercalation of Cu ions into Ti3C2Tx MXene and its effect on its electronic and electrochemical properties. Using X-ray absorption spectroscopy (XAS) and ab initio molecular dynamics (AIMD), we observe an unusual phenomenon whereby Cu2+ ions undergo partial reduction upon intercalation from the solution into the MXene. Furthermore, using in situ XAS, we reveal changes in the oxidation states of intercalated Cu ions and Ti atoms during charging. We show that the pseudocapacitive response of Cu-MXene originates from the redox of both the Cu intercalant and Ti3C2Tx host. Despite highly reducing potentials, Cu ions inside the MXene show an excellent stability against full reduction upon charging. Our findings demonstrate how electronic coupling between Cu ions and Ti3C2Tx modifies electrochemical and electronic properties of the latter, providing the framework for the rational design and utilization of transition metal intercalants for tuning the properties of MXenes for various electrochemical systems.

Tuning the Microenvironment of Water Confined in Ti3C2Tx MXene by Cation Intercalation

The local microenvironment has recently been found to play a major role in the electrocatalytic activity of nanomaterials. Modulating the microenvironment by adding alkali metal cations into the electrolyte can be used to either suppress hydrogen or oxygen evolution, thereby extending the electrochemical window of energy storage systems, or to tune the selectivity of electrocatalysts. MXenes are a large family of two-dimensional transition metal carbides, nitrides, and carbonitrides that have shown potential for use in electrochemical energy storage applications. Due to their negatively charged surfaces, MXenes can accommodate cations and water molecules between the layers. Nevertheless, the nature of the aqueous microenvironment in the MXene interlayer space is poorly understood. Here, we apply Fourier transform infrared spectroscopy (FTIR) to probe the hydrogen bonding of intercalated water in Ti3C2Tx as a function of intercalated cation and relative humidity. Substantial changes in the FTIR spectra after cation exchange demonstrate that the hydrogen bonding of water molecules confined between the MXene layers is strongly cation-dependent. Furthermore, the IR absorbance of the confined water correlates with resistivity estimated by 4-point probe measurements and interlayer distance calculated from XRD patterns. This work demonstrates that cation intercalation strongly modulates the confined microenvironment, which can be used to tune the activity or selectivity of electrochemical reactions in the interlayer space of MXenes in the future.

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.

Light-Augmented Multi-ion Interaction in MXene Membrane for Simultaneous Water Treatment and Osmotic Power Generation

The mixing of wastewater and natural water releases abundant osmotic energy. Harvesting this energy could significantly reduce the energy and economic cost of water treatment, leading to sustainable wastewater treatment technology. Yet, such energy harvesting is highly challenging because it requires a material that is highly permeable to nontoxic ions while rejecting toxic ions in wastewater to reach high power density and prevent environmental pollution. In this work, we demonstrate that a light-augmented biomimetic multi-ion interaction in an MXene membrane can simultaneously realize high permeability of Na+ ions for enhanced osmotic power generation and high selectivity to heavy metal ions up to a ratio of 2050 for wastewater treatment. The Na+ permeability is enhanced by the photothermal effect of the MXene membrane. The transport of heavy metal ions, however, is suppressed because, under angstrom-confinement, heavy metal ions are strongly electrostatically repelled by the increased number of permeating Na+ ions. As a result, the membrane can stably generate osmotic power from simulated industrial wastewater, and the power density can be enhanced by 4 times under light illumination of approximate 1 sun intensity. This work highlights the importance of multi-ion interaction for the transport properties of ionic materials, which remains rarely investigated and poorly understood in previous studies.

Anion Storage of MXenes

Recently, anion storage materials have gained significant attention owing to the widened cell voltage and additional anion storing capacity for a large energy density. MXenes are considered as the emerging anion storing materials owing to their sufficient interlayer spacing, rich surface chemistries, tunable structures, remarkable electrochemical properties, and mechanical integrity. Herein, a comprehensive review on the anion storage of MXenes covering their anion storage mechanism and state-of-the-art chemical strategies for the improved anion storage performances is reported. The recent progress of MXenes on aluminum ion batteries, metal halogen batteries, halogen ion batteries, and electrochemical electrode deionization is addressed. The scientific and technical challenges and the research direction into the anion storage of MXenes are also addressed and finally the authors' perspective on anion storage of MXenes is provided. Therefore, this review offers an insight into the rational design of MXenes for anion storage materials and the correlation of surface chemistries and structural modifications with anion storage properties for the applications into electrochemical energy storage and water purification.

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.

MXene Contact Engineering for Printed Electronics

MXenes emerging as an amazing class of 2D layered materials, have drawn great attention in the past decade. Recent progress suggest that MXene-based materials have been widely explored as conductive electrodes for printed electronics, including electronic and optoelectronic devices, sensors, and energy storage systems. Here, the critical factors impacting device performance are comprehensively interpreted from the viewpoint of contact engineering, thereby giving a deep understanding of surface microstructures, contact defects, and energy level matching as well as their interaction principles. This review also summarizes the existing challenges of MXene inks and the related printing techniques, aiming at inspiring researchers to develop novel large-area and high-resolution printing integration methods. Moreover, to effectually tune the states of contact interface and meet the urgent demands of printed electronics, the significance of MXene contact engineering in reducing defects, matching energy levels, and regulating performance is highlighted. Finally, the printed electronics constructed by the collaborative combination of the printing process and contact engineering are discussed.

Interlayer Structure and Chemistry Engineering of MXene-Based Anode for Effective Capture of Chloride Anions in Asymmetric Capacitive Deionization

Negatively charged surfaces and readily oxidizabile characteristics fundamentally restrict the use of MXene building blocks as anodes for anion intercalation. Herein, by embedding bacterial cellulose nanofibers with conformal polypyrrole coating (BC@PPy) and populating them between MXene (Ti₃C₂T_x) interlayers, we enable the fabricated MXene/BC@PPy (MBP) composite films to be highly efficient anodes for Cl^--capturing in asymmetric capacitive deionization (CDI) systems. Performance gains are realized due to the surface electronegativity of MXene nanosheets becoming compensated by positively charged BC@PPy nanofibers, alleviating electrostatic repulsion, thus realizing reversible Cl^- intercalation. More crucially, the anodization voltage of MBP is effectively enhanced as a result of the increase of the Ti valence state in MXene nanosheets with the addition of the BC@PPy spacer. Furthermore, BC@PPy nanopillars effectively enlarge the interlayer space for facile Cl^- de-/intercalation, improve the vertical electron transfer between loosely deposited MXene nanosheets, and perform as additional active materials for Cl^--capturing. Consequently, the MBP anode exhibits a promising desalination capacity of up to 17.56 mg g^-1 at 1.2 V with a high capacity retention of 94.6% after 30 cycles in an asymmetric CDI system. This work offers a simple and effective strategy to unlock the application potential of MXene building blocks as anodes for Cl^--capturing in electrochemical desalination.

Hydrogen-Bond Restructuring of Water-in-Salt Electrolyte Confined in Ti3C2Tx MXene Monitored by Operando Infrared Spectroscopy

Highly concentrated water-in-salt aqueous electrolytes exhibit a wider potential window compared to conventional, dilute aqueous electrolytes. Coupled with MXenes, a family of two-dimensional transition metal carbides and nitrides with impressive charge storage capabilities, water-in-salt electrolytes present a potential candidate to replace flammable and toxic organic solvents in electrochemical energy storage devices. A new charge storage mechanism was recently discovered during electrochemical cycling of Ti₃C₂T_x MXene electrodes in lithium-based water-in-salt electrolytes, attributed to intercalation and deintercalation of solvated Li⁺ ions at anodic potentials. Nevertheless, direct evidence of the state of Li⁺ solvation during cycling is still missing. Here, we investigate the hydrogen bonding of water intercalated between MXene layers during electrochemical cycling in a water-in-salt electrolyte with operando infrared spectroscopy. The hydrogen-bonding state of the confined water was found to change significantly as a function of potential and the concentration of Li⁺ ions in the interlayer space. This study provides fundamentally new insights into the electrolyte structural changes while intercalating Li⁺ in the MXene interlayer space.

Layer-Stacking Sequence Governs Ion-Storage in Layered Double Hydroxides

In layered materials, the layer-stacking sequence allows the tuning of ion transport and storage properties by modulating the host-ion interactions. However, unlike in the case of cations, the relationship between the stacking sequence and anion transport and storage properties is less clearly understood. Herein, we demonstrate that the stacking sequence governs the nitrate-storage properties of layered double hydroxides (LDHs); the 2H₁ polytype enhances the nitrate-storage capacity to 400% of that of the 3R₁ polytype. A quartz crystal microbalance with dissipation monitoring combined with multimodal ex situ experiments indicated that the high ion-storage capacity of the 2H₁ polytype originates from the soft nature of LDHs lattices, which facilitates nitrate with minimal lattice changes. In contrast, the rigid lattice of the 3R₁ sequence requires a notably large lattice expansion, which is detrimental to ion storage. Our findings can aid the rational design of anion-host interaction-derived functionalities.

Critical role of water structure around interlayer ions for ion storage in layered double hydroxides

Water-containing layered materials have found various applications such as water purification and energy storage. The highly structured water molecules around ions under the confinement between the layers determine the ion storage ability. Yet, the relationship between the configuration of interlayer ions and water structure in high ion storage layered materials is elusive. Herein, using layered double hydroxides, we demonstrate that the water structure is sensitive to the filling density of ions in the interlayer space and governs the ion storage. For ion storage of dilute nitrate ions, a 24% decrease in the filling density increases the nitrate storage capacity by 300%. Quartz crystal microbalance with dissipation monitoring studies, combined with multimodal ex situ experiments and theoretical calculations, reveal that the decreasing filling density effectively facilitates the 2D hydrogen-bond networking structure in water around interlayer nitrate ions along with minimal change in the layered structure, leading to the high storage capacity.

Polyoxometalate intercalated MXene with enhanced electrochemical stability

MXene/polyoxometalate (POM) hybrids are useful target materials for a variety of applications. Yet, the goal of preparing simple binary hybrids by intercalation of POMs into MXene has not been achieved. We propose and demonstrate here a method to intercalate POMs (phosphotungstate, PW12) into Ti₃C₂T_x MXene through the interaction between POM anions and pre-intercalated surfactant cations. A variety of quaternary ammonium cations have been used to expand Ti₃C₂T_x interlayer spacing. Cetyltrimethylammonium cations (CTA⁺) lead to an expansion of 2 nm while allowing intercalation of a considerable load (10 wt%) thanks to their tadpole-like shape and size. CTAPW12 has a layered structure compatible with Ti₃C₂T_x. The CTA⁺-delaminated Ti₃C₂T_x keeps the large interlayer spacing after being coupled with PW12. The PW12 clusters are dispersed and kept isolated thanks to CTA surfactant and the confinement into Ti₃C₂T_x layers. The redox reactions in CTA⁺-delaminated Ti₃C₂T_x/PW12 are diffusion-controlled, which proves the well-dispersed PW12 clusters are not adsorbed on the surface of Ti₃C₂T_x particles but within Ti₃C₂T_x layers. The CTA⁺- delaminated Ti₃C₂T_x/PW12 shows superior electrochemical stability (remaining redox active after 5000 cycles) over the other MXene/POM hybrids prepared in this work (inactive after 500 cycles). We associate this improved stability to the effective intercalation of PW12 within Ti₃C₂T_x layers helped by the CTA cations, as opposed to the external aggregation of PW12 clusters into micro or nanocrystals taking place for the other cations. The results provide a solid guide to help develop high-performance MXene/POM hybrid materials for a variety of applications.

Unique Mechanisms of Ion Storage in Polyaniline Electrodes for Pseudocapacitive Energy Storage Devices Unraveled by EQCM-D Analysis

The optimal performance of organic electrodes for aqueous batteries requires their full compatibility with selected electrolyte solutions. Electrode materials having 1-3-dimensional structures of variable rigidity possess a confined space in their structure filled with water and electrolyte solutions. Depending on the rigidity and confined space geometry, insertion and extraction of ions into electrode structures are often coupled with incorporation/withdrawal of water molecules. Aside from the scientific interest in understanding the charging mechanism of such systems, co-insertion of solvent molecules affects strongly the charge storage capability of the electrodes for energy storage devices. We present herein in situ electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) investigations of polyaniline (PANI) electrodes operating in various aqueous Na⁺-containing electrolytes, namely, Na₂SO₄, NaClO₄, NaBF₄, and NaPF₆. Careful analysis of the EQCM-D results provides a dynamic snapshot of the mixed anionic/protonic fluxes and the accompanying water molecules' insertion/extraction to/from the PANI electrodes. Based on our observations, it was found that the charging mechanism, as well as the capacity values, strictly depends on the electrolyte pH, the chaotropic/kosmotropic character of the anionic dopants, and the amount of the extracted water molecules. This study demonstrates the effectiveness of analysis by EQCM-D in selecting electrolytes for batteries comprising organic electrodes.

Programmable dual-electric-field immunosensor using MXene-Au-based competitive signal probe for natural parathion-methyl detection

Immunosensor is a promising tool for natural parathion-methyl (PTM) detection, and its analytical advantages can be magnified by introducing flexibly-fabricating technique. Herein, we present a dual-electric-field PTM immunosensor on highly-compatible screen-printed electrode (SPE). MXene-Au, the product of in-situ gold nanoparticle growth on MXene, provides considerable binding sites for PTM antigen (ATG) and methylene blue (MB). During sensing, the MXene-Au-MB-ATG probe competitively binds antibody against PTM, composing a ratiometric immune-system. With DC-biased sine excitations from complementary waveforms, on-chip electric field couple improves immunoreactions among PTM, probe, and antibody. Electric field distribution is programmed by trimming bypass resistors to pursue optimal performance. Probe synthesis is solidly proven with morphological examinations, and competition mechanism between the probe and target PTM is clarified in electrochemical analyses. Remarkably, this method brings less consumption of immune time than electric-field-free or solo-electric-field setup (50 s vs. 900 or 70 s), and simultaneously provides more powerful ratiometric signal than the rivals. Log-linear relationship, between PTM level and sensor readout, is established in 0.02-38 ng/mL, and limit of detection is found as 0.01 ng/mL. This method is applied in laboratorial and natural PTM analyses, and the readouts are consistent with high performance liquid chromatography and recovery test.

Evidence of Formation of Monolayer Hydrated Salts in Nanopores

Despite much effort being devoted to the study of ionic aqueous solutions at the nanoscale, our fundamental understanding of the microscopic kinetic and thermodynamic behaviors in these systems remains largely incomplete. Herein, we reported the first 10 μs molecular dynamics simulation, providing evidence of the spontaneous formation of monolayer hexagonal honeycomb hydrated salts of XCl₂·6H₂O (X = Ba, Sr, Ca, and Mg) from electrolyte aqueous solutions confined in an angstrom-scale slit under ambient conditions. By using both the classical molecular dynamics simulations and the first-principles Born-Oppenheimer molecular dynamics simulations, we further demonstrated that the hydrated salts were stable not only at ambient temperature but also at elevated temperatures. This phenomenon of formation of hydrated salt in water is contrary to the conventional view. The free energy calculations and dehydration analyses indicated that the spontaneous formation of hydrated salts can be attributed to the interplay between ion hydration and Coulombic attractions in the highly confined water. In addition to providing molecular-level insights into the novel behavior of ionic aqueous solutions at the nanoscale, our findings may have implications for the future exploration of potential existence of water molecules in the saline deposits on hot planets.

Energetic Stability and Interfacial Complexity of Ti3C2Tx MXenes Synthesized with HF/HCl and CoF2/HCl as Etching Agents

MXenes are ultra-thin two-dimensional layered early transition-metal carbides and nitrides with potential applications in various emerging technologies, such as energy storage, water purification, and catalysis. MXenes are synthesized from the parent MAX phases with different etching agents [hydrofluoric acid (HF) or fluoride salts with a strong acid] by selectively removing a more weakly bound crystalline layer of Al or Ga replaced by surface groups (-O, -F, -OH, etc.). Ti₃C₂T_x MXene synthesized by CoF₂/HCl etching has layered heterogeneity due to intercalated Al³⁺ and Co²⁺ that act as pillars for interlayer spacings. This study investigates the impacts of etching environments on the compositional, interfacial, structural, and thermodynamic properties of Ti₃C₂T_x MXenes. Specifically, compared with HF/HCl etching, CoF₂/HCl treatment leads to a Ti₃C₂T_x MXene with a broader distribution of interlayer distances, increased number of intercalated cations, and decreased degree of hydration. Moreover, we determine the enthalpies of formation at 25 °C (ΔH_f,25°C) of Ti₃C₂T_x MXenes etched with CoF₂/HCl, ΔH_f,25°C = -1891.7 ± 35.7 kJ/mol Ti₃C₂, and etched with HF/HCl, ΔH_f,25°C = -1978.2 ± 35.7 kJ/mol Ti₃C₂, using high-temperature oxidation drop calorimetry. These energetic data are discussed and compared with experimentally derived and computationally predicted values to elucidate the effects of intercalants and surface groups of MXenes. We find that MXenes with intercalated metal cations have a less exothermic ΔH_f,25°C from an increase in the interlayer space and dimension heterogeneity and a decrease in the degree of hydration leading to reduced layer-layer van der Waals interactions and weakened hydration effects applied on the MXene layers. The outcomes of this study further our understanding of MXene's energetic-structural-interfacial property relationships.

Enhanced ion transport in nanochannels of MXenes by Mg2+ pre-intercalation

How to enhance the ion transport between MXene layers is a critical topic in the fields of electrochemical storage (especially supercapacitors) and water treatment. Vertical structure design of MXene nanosheets and single-molecule organic pre-intercalation are proposed, but the methods to enhance the ion transport through MXene nanochannels by modulating MXene's surface state have not been investigated yet. The interaction mechanism between Mg²⁺ and MXene 2D nanochannels during the transport process has not been thoroughly explored. In our work, we used a facile infiltration method to immerse the Ti₃C₂T_x membranes in MgCl₂ solution for ion pre-intercalation. We found that the pre-intercalation of Mg²⁺ has a significant effect on the increase of the ion transport rate of Ti₃C₂T_x membranes, especially for Li⁺ which reached 268.49% compared with those of non-intercalation membranes. Through multiple characterization methods, we discovered that the enhancement of ion transport rate by pre-intercalation of Mg²⁺ mainly originated from the fact that the pre-intercalation of Mg²⁺ increased the layer spacing of MXene films as the channel support between layers while Mg²⁺ increased the work function (WF) of 2D nanochannels thereby reducing the interaction of other ions with the channel surface. The acceleration phenomenon of ion transport by surface state modulation proposed in our work will provide new strategies for the design of structure and regulation of surface states, revealing the mechanism of capacity improvement.

Spontaneous and Selective Potassium Transport through a Suspended Tailor-Cut Ti3C2Tx MXene Film

Biological ion pumps selectively transport target ions against the concentration gradient, a process that is crucial to maintaining the out-of-equilibrium states of cells. Building an ion pump with ion selectivity has been challenging. Here we show that a Ti₃C₂T_x MXene film suspended in air with a trapezoidal shape spontaneously pumps K⁺ ions from the base end to the tip end and exhibits a K⁺/Na⁺ selectivity of 4. Such a phenomenon is attributed to a range of properties of MXene. Thanks to the high stability of MXene in water and the dynamic equilibrium between evaporation and swelling, the film keeps a narrow interlayer spacing of ∼0.3 nm when its two ends are connected to reservoirs. Because of the polar electrical structure and hydrophilicity of the MXene nanosheet, K⁺ ions experience a low energy barrier of ∼4.6 k_BT when entering these narrow interlayer spacings. Through quantitative simulations and consistent experimental results, we further show that the narrow spacings exhibit a higher energy barrier to Na⁺, resulting in K⁺/Na⁺ selectivity. Finally, we show that the spontaneous ion transport is driven by the asymmetric evaporation of the interlayer water across the film, a mechanism that is similar to pressure driven streaming current. This work shows how ion transport properties can be facilely manipulated by tuning the macroscopic shape of nanofluidic materials, which may attract interest in the interface of kirigami technologies and nanofluidics and show potential in energy and separation applications.

Inorganic-Organic Hybrid Membrane Based on Pillararene-Intercalated MXene Nanosheets for Efficient Water Purification

Discharge of antibiotic-containing wastewater causes environmental pollution and threatens biological and human health. An efficient treatment method for this wastewater is urgently required. We prepared inorganic-organic hybrid MXene-pillararene nanosheets with a large lateral size (5-8 μm). The hybrid nanosheets were stacked on supports via vacuum-assisted filtration to prepare membranes with regular parallel slits and an interlayer spacing of 1.36 nm, which were used to purify antibiotic-containing water. Permeance through the membrane increased 100-fold compared with most polymeric and other two-dimensional nanofiltration membranes with similar rejection. This high permeance and rejection was attributed to the large lateral size of the nanosheets, regular interlayer spacing, and electrostatic interaction between the membrane and antibiotics. These membranes will broaden the applications of lamellar materials for the separation of high-value-added drugs in academia and industry.

Ion Dynamics at the Carbon Electrode/Electrolyte Interface: Influence of Carbon Nanotubes Types

Electrochemical quartz crystal microbalance (EQCM) and AC-electrogravimetry methods were employed to study ion dynamics in carbon nanotube base electrodes in NaCl aqueous electrolyte. Two types of carbon nanotubes, Double Wall Carbon Nanotube (DWCNT) and Multi Wall Carbon Nanotube (MWCNT), were chosen due to their variable morphology of pores and structure properties. The effect of pore morphology/structure on the capacitive charge storage mechanisms demonstrated that DWCNT base electrodes are the best candidates for energy storage applications in terms of current variation and specific surface area. Furthermore, the mass change obtained via EQCM showed that DWCNT films is 1.5 times greater than MWCNT films in the same potential range. In this way, the permselectivity of DWCNT films showed cation exchange preference at cathode potentials while MWCNT films showed anion exchange preference at anode potentials. The relative concentration obtained from AC-electrogravimetry confirm that DWCNT base electrodes are the best candidates for charge storage capacity electrodes, since they can accommodate higher concentration of charged species than MWCNT base electrodes.

Compositing MXenes with hierarchical ZIF-67/cobalt hydroxide via controllable in situ etching for a high-performance supercapacitor

The accumulation and self-aggregation of nanosheets have been effectively inhibited. The interlamellar cobalt hydroxide nanostructures ensure efficient electron transfer. MXene as a conductive substrate improves electron transfer significantly.

Titanium Carbide MXene Shows an Electrochemical Anomaly in Water-in-Salt Electrolytes

Identifying and understanding charge storage mechanisms is important for advancing energy storage. Well-separated peaks in cyclic voltammograms (CVs) are considered key indicators of diffusion-controlled electrochemical processes with distinct Faradaic charge transfer. Herein, we report on an electrochemical system with separated CV peaks, accompanied by surface-controlled partial charge transfer, in 2D Ti₃C₂T_x MXene in water-in-salt electrolytes. The process involves the insertion/desertion of desolvation-free cations, leading to an abrupt change of the interlayer spacing between MXene sheets. This unusual behavior increases charge storage at positive potentials, thereby increasing the amount of energy stored. This also demonstrates opportunities for the development of high-rate aqueous energy storage devices and electrochemical actuators using safe and inexpensive aqueous electrolytes.