MXene Electrodes Research Articles

Work‐Function‐Tunable MXenes Electrodes to Optimize p‐CsCu2I3/n‐Ca2Nb3‐xTaxO10 Junction Photodetectors for Image Sensing and Logic Electronics

AbstractTunable work function has a high profile for the MXene‐based optoelectronic devices, and surface modification provides the huge potential to shift its Fermi level and modulate the work function. In this work, the window of MXene's work function is engineered from 4.55 to 5.25 eV by surface modification with LiF, Se, and polyethylenimine ethoxylated (PEIE). The vertical p‐CsCu2I3/n‐Ca2Nb3‐xTaxO10 junction photodetectors are constructed on the basis of the above surface‐modified MXenes, which changes the Schottky barrier between n‐Ca2Nb3‐xTaxO10 and the electrodes. In particular, the rectification effect is significantly enhanced by utilizing PEIE‐decorated MXene electrodes, resulting in a high rectification ratio of 16 136 and improved UV responsivity of 81.3 A W–1. Such high‐performance devices based on MXenes electrodes are compatible with the standard clean room fabrication process, realizing large‐scale flexible UV detectors that maintain 80% of the original current after 5000 times bending. Meanwhile, a photodetector array stimulated with UV of different wavelengths is constructed to reveal its potential for image sensing. Finally, functional “AND” and “OR” optoelectronic logic gates are developed for UV communication using Au/CsCu2I3/Ca2Nb3‐xTaxO10/MXene–PEIE photodetectors, enriching the application of MXene‐based optoelectronic devices. This work on tuning MXene work function via surface modification demonstrates that MXene is a promising candidate for future optoelectronics.

Synthesis of Nb2C MXene-based 2D layered structure electrode material for high-performance battery-type supercapacitors

The supercapacitor is a kind of electrochemical energy storage element, has attracted widespread attention, however, it is limited by a lack of ideal electrode materials with excellent cycle stability and high specific capacitance. To address this issue, we prepared Nb2CTx MXene-framework coated with Co3O4 to construct a robust 2D cross-linked structure. In this design, the self-assembly of Co3O4 between Nb2C MXene layers can effectively prevent the self-stacking of MXene sheets. The high conductivity and abundant active groups on the surface of MXenes provide a large number of active sites for the uniform distribution of Co3O4 on Nb2C MXene. The Co-MXene electrode achieves a specific capacitance of up to 1061 F g−1 at the current density of 2 A g−1, which is higher than the pure Co3O4 electrode and Nb2C MXene electrode. The asymmetric supercapacitors (ASCs) of Co-MXene//AC delivers specific energy densities (Es) of 60.3, 42.5, 31.0, 19.7 Wh kg−1 at specific power density (Ps) of 0.67, 2.0, 3.4, and 6.8 kW kg−1, respectively. The capacitance retention rate of 93% after 1000 cycles at 5 A g−1. The study provides a valuable guideline for fabricating satisfactory electrochemical energy storage devices by a feasible method.

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.

Kinetic regulation of MXene with water-in-LiCl electrolyte for high-voltage micro-supercapacitors.

MXenes are one of the key materials for micro-supercapacitors (MSCs), integrating miniaturized energy-storage components with microelectronics. However, the energy densities of MSCs are greatly hampered by MXenes’ narrow working potential window (typically ≤0.6 V) in aqueous electrolytes. Here, we report the fabrication of high-voltage MXene-MSCs through the efficient regulation of reaction kinetics in 2D Ti3C2Tx MXene microelectrodes using a water-in-LiCl (WIL, 20 m LiCl) salt gel electrolyte. Importantly, the intrinsic energy-storage mechanism of MXene microelectrodes in WIL, which is totally different from traditional electrolytes (1 m LiCl), was revealed through insitu and exsitu characterizations. We validated that the suppression of MXene oxidation at high anodic potential occurred due to the high content of WIL regulating anion intercalation in MXene electrodes, which effectively broadened the voltage window of MXene-MSCs. Remarkably, the symmetric planar MXene-MSCs presented a record operating voltage of 1.6 V, resulting in an exceptionally high volumetric energy density of 31.7 mWh cm−3. With the ultra-high ionic conductivity (69.5 mS cm−1) and ultralow freezing point (−57°C) of the WIL gel electrolyte, our MSCs could be operated in a wide temperature range of −40 to 60°C, and worked for a long duration even at −40°C, demonstrative of its practicality in extreme environments.

Relationship between Electric Double-Layer Structure of MXene Electrode and Its Surface Functional Groups

Relationship between Electric Double-Layer Structure of MXene Electrode and Its Surface Functional Groups

Scaled Deposition of Ti3C2Tx MXene on Complex Surfaces: Application Assessment as Rear Electrodes for Silicon Heterojunction Solar Cells.

Two-dimensional transition metal carbides (MXenes) are of great interest as electrode materials for a variety of applications, including solar cells, due to their tunable optoelectronic properties, high metallic conductivity, and attractive solution processability. However, thus far, MXene electrodes have only been exploited for lab-scale device applications. Here, to demonstrate the potential of MXene electrodes at an industry-relevant level, we implemented a scalable spray coating technique to deposit highly conductive (ca. 8000 S/cm, at a ca. 55 nm thickness) Ti3C2Tx films (Tx: surface functional groups, i.e., −OH, −O, −F) via an automated spray system. We employed these Ti3C2Tx films as rear electrodes for silicon heterojunction solar cells as a proof of concept. The spray-deposited MXene flakes have formed a conformal coating on top of the indium tin oxide (ITO)-coated random pyramidal textured silicon wafers, leading to >20% power conversion efficiency (PCE) over both medium-sized (4.2 cm2) and large (243 cm2, i.e., industry-sized 6 in. pseudosquare wafers) cell areas. Notably, the Ti3C2Tx-rear-contacted devices have retained around 99% of their initial PCE for more than 600 days of ambient air storage. Their performance is comparable with state-of-the-art solar cells contacted with sputtered silver electrodes. Our findings demonstrate the high-throughput potential of spray-coated MXene-based electrodes for solar cells in addition to a wider variety of electronic device applications.

A weldable MXene film assisted by water

<h2>Summary</h2> As an extremely important macroscopic architecture of 2D MXene nanosheets, MXene (Ti₃C₂T_x) film has shown great commercial value in many fields. However, it is difficult to design complex multi-scale structures directly by using MXene film as the base material. Herein, we have discovered a time-sensitive welding phenomenon of MXene film with the assistance of water. Specifically, MXene film can be connected during the time-sensitive period but would lose weldability outside the time-sensitive period, thus ensuring the stability and practicability of the connecting materials. Taking advantage of this special weldability, we have successfully fabricated a complex structure for humidity-responsive MXene actuators and vertically aligned compact MXene electrode with rapid electrolyte ion diffusion. This work demonstrates that MXene film has similar potential to that of metal and other engineering materials, which is a milestone in promoting such conductive 2D materials forward toward practical applications.

Nanoarchitectonics from 2D to 3D: MXenes-derived nitrogen-doped 3D nanofibrous architecture for extraordinarily-fast capacitive deionization

MXene-derived nitrogen-doped 3D nanofibrous architecture enables excellent fast deionization rate by breaking through the intrinsic structural limits of common 2D nanosheets. • Nitrogen-doped 3D nanofibrous architecture was successfully fabricated from MXene. • This architecture breaks through the intrinsic structural limits of common 2D nanosheets. • Superior deionization rate and remarkable cycling stability are achieved. Two-dimensional (2D) nanosheets are promising electrode materials for electrochemical desalination by capacitive deionization (CDI). Like most 2D nanosheets, the delicate design of MXene-based materials can achieve state-of-the-art desalination capacities, but the intrinsic low ion diffusion characteristic of 2D nanosheets limits the desalination rate. To address this problem, we synthesize a new-family of nitrogen-doped three-dimensional (3D) nanofibrous architectures from MXenes (denoted as N-TNF) via direct alkalization and subsequent nitrogenization of common MXene stacks. N-TNF has a unique nanofiber structure and plentiful nitrogen dopants, resulting in expanded interlayer spacing, high specific surface area and excellent electrochemical activity. As a result, the N-TNF shows an ultrahigh mean desalination rate of 5.6 mg g −1 min −1 , along with superior desalination capacity of 44.8 mg g −1 , as well as good long-term cycling stability, which is comparable to state-of-the-art MXene electrodes and better than most 2D materials. This work demonstrates the fabrication of MXene-derived 3D materials, and provides a new approach to overcome the limits of 2D nanosheets for efficient CDI.

3D Printed Template-Assisted Assembly of Additive-Free Ti3C2Tx MXene Microlattices with Customized Structures toward High Areal Capacitance.

Ti3C2Tx MXene is a promising material for electrodes in microsupercapacitors. Recent efforts have been made to fabricate MXene electrodes with designed structures using 3D printing to promote electrolyte permeation and ion diffusion. However, challenges remain in structural design diversity due to the strict ink rheology requirement and limited structure choices caused by existing extrusion-based 3D printing. Herein, additive-free 3D architected MXene aerogels are fabricated via a 3D printed template-assisted method that combines 3D printed hollow template and cation-induced gelation process. This method allows the use of MXene ink with a wide range of concentrations (5 to 150 mg mL-1) to produce MXene aerogels with high structural freedom, fine feature size (>50 μm), and controllable density (3 to 140 mg cm-3). Through structure optimization, the 3D MXene aerogel shows high areal capacitance of 7.5 F cm-2 at 0.5 mA cm-2 with a high mass loading of 54.1 mg cm-2. It also exhibits an ultrahigh areal energy density of 0.38 mWh cm-2 at a power density of 0.66 mW cm-2.

Theoretical design of high-performance halogen anion batteries with MXene electrodes: influence of functional groups, metals, and anions

13 stable models were screened from 36 halogen anion batteries with MXene electrodes, exhibiting remarkable electrochemical performance.

Making MXenes more energetic in aqueous battery

MXenes, featuring outstanding electronic conductivity, abundant surface functional groups, tunable interlayer structure, and interfacial chemistry, have aroused increasing attention in safe aqueous batteries (ABs). However, current MXene-based ABs suffer from the issues of inadequate energy density due to their low specific capacity and limited output voltage, which hinders the wide applications of MXenes. Herein, we comprehensively appraise the four charge storage mechanisms in constructing high-capacity/voltage MXene-based electrodes and the state-of-the-art strategies toward energetic ABs. Finally, we render the insights properly for the future development of next-generation MXene-based ABs. MXenes, featuring outstanding electronic conductivity, abundant surface functional groups, tunable interlayer structure, and interfacial chemistry, have aroused increasing attention in safe aqueous batteries (ABs). However, current MXene-based ABs suffer from the issues of inadequate energy density due to their low specific capacity and limited output voltage, which hinders the wide applications of MXenes. Herein, we comprehensively appraise the four charge storage mechanisms in constructing high-capacity/voltage MXene-based electrodes and the state-of-the-art strategies toward energetic ABs. Finally, we render the insights properly for the future development of next-generation MXene-based ABs. Intrinsic voltage plateau of a Nb2CTx MXene cathode in an aqueous electrolyte induced by high-voltage scanningLi et al.JouleOctober 11, 2021In BriefIn the past decade, two-dimensional MXenes have been a rising star in the energy storage field, featured by fast surficial redox and high-rate energy storage. However, typically, MXene electrodes exhibit capacitive behavior with a rapidly descending output voltage during discharge. Moreover, the absence of distinct voltage plateaus deteriorates their capacities and energy densities. Our work outlines an efficient route toward achieving high-energy-density MXene electrodes with distinct discharge voltage plateau through a high-voltage-scanning approach. Full-Text PDF

2D MXenes: Synthesis, properties, and electrochemical energy storage for supercapacitors – A review

• A detailed view of the synthesis, properties, and applications of MXenes. • The recent trends in MXene synthesis. • The in-depth understanding of the various types of etchants and intercalants that can be used in the formation of MXene. • Understanding the charge storage mechanism of MXenes. • A detailed discussion on the use of MXene composites as supercapacitor electrodes. Supercapacitors are one of the most frequently explored devices for energy storage applications. In comparison with conventional dielectric capacitors, supercapacitors have energy storage capacities several orders of magnitude higher, however much lower than those of secondary batteries. Their long-life cycles, high power densities, and relatively less carbon footprint over their counterparts have encouraged industries to explore and build reliable energy systems for the future. One family of materials that have garnered attention for supercapacitor applications since their discovery in 2011 are 2D transition metal carbides and nitrides (MXenes). Unlike 3D carbon, which possesses complex ion diffusion pathways, 2D nanosheets directly offer large active sites to the electrolyte. With a high surface area, shorter ion diffusion pathways, and high conductivity, MXenes enhance the energy storage characteristics of a supercapacitor. The key to high rate pseudocapacitive energy storage in MXene electrodes is the hydrophilicity of MXenes combined with their metallic conductivity and surface redox reactions. In this review, we have explored different types of supercapacitors, charge storage mechanisms, and modified synthesis methods of MXene and its properties. Finally, we discuss the advancement in this field while evaluating future challenges and prospects of MXene composites, which will provide a guide for developing high-performance MXene-based energy storage devices with high throughput and sustainable derivatives.

Metal Organic Framework Derived Α, Γ–Mns-S@Co3s4, S–N Co–Doped Carbon Structure with Lattice Distortion and Layer–By–Layer Deposited Multi-Walled Carbon Nanotube/Ti3c2tx–Mxene Electrodes for High–Energy Quasi–Solid–State Supercapacitor

Metal Organic Framework Derived Α, Γ–Mns-S@Co3s4, S–N Co–Doped Carbon Structure with Lattice Distortion and Layer–By–Layer Deposited Multi-Walled Carbon Nanotube/Ti3c2tx–Mxene Electrodes for High–Energy Quasi–Solid–State Supercapacitor

Diameter-optimized PVA@PPy nanofibers: MXene interlayer space expansion without sacrificing electron transport

PVA@PPy nanofibers are proposed as a novel spacer for MXene electrode interlayer space expansion in Zn-ion micro-supercapacitors. The PPy shell mitigates the well-known challenge of electron transport degradation due to the expansion.

Flexible Broadband Photodetectors Enabled by MXene/PbS Quantum Dots Hybrid Structure

Flexible photodetectors have attracted much attention due to their great potentials in wearable electronics, bendable image sensors, and environmental monitoring, which requires functional components with good machinal properties. Herein, we introduced a novel metallic two-dimensional (2D) material, MXene, to form the electrodes by a spray-coating method and zero-dimensional (0D) PbS quantum dots (QDs) to work as photoactive layer. All solution-processable fabrication technique contributes to a simplified, low-cost and large-scale route to develop flexible photodetectors. Benefiting from the excellent conductivity of MXene electrodes and broadband photoresponse of PbS QDs, our devices realize a high detectivity up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.4\times10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> Jones and exhibit good stability even after 1000 bending cycles. This work provides a new device configuration with 2D electrodes and 0D photoactive layers for designing high-performance flexible electronics.

Synthesis of new two‐dimensional titanium carbonitride Ti2C0.5N0.5TxMXene and its performance as an electrode material for sodium‐ion battery

AbstractTwo‐dimensional (2D) layered transition metal carbides/nitrides, called MXenes, are attractive alternative electrode materials for electrochemical energy storage. Owing to their metallic electrical conductivity and low ion diffusion barrier, MXenes are promising anode materials for sodium‐ion batteries (SIBs). Herein, we report on a new 2D carbonitride MXene, viz., Ti2C0.5N0.5Tx (Tx stands for surface terminations), and the only second carbonitride after Ti3CNTx so far. A new type of in situ HF (HCl/KF) etching condition was employed to synthesize multilayer Ti2C0.5N0.5Tx powders from Ti2AlC0.5N0.5. Spontaneous intercalation of tetramethylammonium followed by sonication in water allowed for large‐scale delamination of this new titanium carbonitride into 2D sheets. Multilayer Ti2C0.5N0.5Tx powders showed higher specific capacities and larger electroactive surface area than those of Ti2CTx powders. Multilayer Ti2C0.5N0.5Tx powders show a specific capacity of 182 mAh g−1 at 20 mA g−1, the highest among all reported MXene electrodes as SIBs with excellent cycling stability.image

Micro‐Structural and Flexible Reduced Graphene Oxide/Ti3C2Tx Composite Film Electrode with Long Cycle Life for Supercapacitor

AbstractPoor rate capability due to the sheet self‐stacking of conventional MXene electrode limits their electrochemical application to some extent. Herein, incorporating reduced graphene oxide into Ti3C2Tx MXene is reported to improve the electrochemical performance, cycle lifetime, and mechanical flexibility significantly. Graphene oxide is reduced by thermal heating, by which it can release gas locally to induce micro‐surface structure. The resulting film with an introduction of 20 wt% graphene oxide exhibits an expansion of the interlayer space to multiply the active sites and thereby lead to a specific capacitance of up to 322 F g−1 at 1 A g−1 in 3 m H2SO4 electrolyte. In addition, the fabricated composite electrode also exhibits an excellent cycle stability and mechanical flexibility even after 32 000 charge/discharge cycles. This work provides a progressive strategy to synthesize micro‐structural and flexible MXene‐based electrode for the future application in flexible energy storage devices.

Hollow Ti3C2 MXene/Carbon Nanofibers as an Advanced Anode Material for Lithium‐Ion Batteries

AbstractLithium‐ion batteries (LIBs) are rechargeable batteries that have attracted great interest as next‐generation energy storage devices that will lead future energy technologies because of their various excellent advantages. Two‐dimensional (2D) MXene‐based LIBs have been actively investigated because of their high energy/power density and good performance at high charge/discharge rates. However, three major limitations of 2D MXene electrodes – self‐stacking, low specific surface area, and disturbance of Li+ diffusion by surface terminations – have hindered the commercialization of MXene‐based LIBs. Herein, we fabricate 1D hollow Ti3C2Tx MXene/carbon (MX/C) nanofibers via an electrospinning process and use them as anode materials in LIBs. Compared with the pristine 2D MXene (MX) paste electrode and MXene/carbon (MX/C) paste electrode, the hollow MX/C nanofibers electrode exhibits a greater specific surface area, less self‐stacking of flakes, and surface functional groups tailored for LIBs. The LIBs based on the hollow electrode exhibit a higher energy density (306.5 mA h g−1 at 40 mA g−1) than those with the MX paste electrode (81.08 mA h g−1 at 40 mA g−1) and MX/C paste electrode (196.9 mA h g−1 at 40 mA g−1). In addition, the hollow MX/C nanofiber electrode shows a high reversible capacity, proving that it is a promising anode material for LIBs.

High supercapacitance performance of nitrogen-doped Ti3C2Tx prepared by molten salt thermal treatment

Ti3C2Tx with –F surface terminations were treated in LiCl-KCl-Li3N molten salt at atmospheric pressure to introduce N and simultaneously remove –F surface terminations, in which LiCl-KCl salt was used as the solvent and Li3N as the nitrogen source. By optimizing the mass ratio of Li3N:Ti3C2Tx to be 1:2, N-doped Ti3C2Tx (N-Ti3C2Tx-0.5) was obtained with 19.6 at% nitrogen element and 0.69 at% –F surface terminations, leading to the substitution of C by N atoms and the d-spacing expansion from 0.96 to 1.15 nm. Owing to its good electrical conductivity and high electrochemical activity area, the N-Ti3C2Tx-0.5 electrode showed a higher specific capacitance (332 F g−1 at 0.5 A g−1 in 1 M H2SO4 solution), good rate performance (83.1% at 20 A g−1), and excellent capacitance retention (87.5% after 10 000 charging-discharging cycles). This paves a new way to achieve MXene electrodes with high specific capacitances and excellent retention performance.

Intrinsic voltage plateau of a Nb2CTx MXene cathode in an aqueous electrolyte induced by high-voltage scanning

<h2>Summary</h2> Although MXene has been widely used in many energy storage systems, the absence of a high intrinsic voltage plateau results in its poor discharging feature. Herein, we report a Nb₂CTx MXene cathode with an apparent discharge voltage plateau for high-performance aqueous zinc ion batteries. Triggered by a repeated high-voltage scanning up to 2.4 V, the Nb₂CTx cathode gradually displays a stable Zn²⁺ storage dominated by a battery behavior. A high discharge voltage plateau of 1.55 V is presented, which is distinct from all reported MXene electrodes. Consequently, a record-level energy density among all aqueous MXenes electrodes of 146.7 Wh kg⁻¹ with a 63% contribution from the plateau region is obtained, which is 292% that of the capacitive Nb₂CTx electrode. The developed high-voltage scanning strategy is also effective for Ti₃C₂T_x MXene to achieve a remarkable voltage plateau.