In-Situ Synergistic 2D/2D MXene/BCN Heterostructure for Superlative Energy Density Supercapacitor with Super-Long Life

A Comprehensive Review of Mxene-Based Emerging Materials for Energy Storage Applications and Future Perspectives

MXenes is a rapidly emerging class of two-dimensional (2D) materials. It exhibits unique properties that make it suitable for a wide range of applications. This review provides a comprehensive overview of the synthesis and processing techniques for MXenes including both bottom-up and top-down approaches. The synthesis of MXene-based composites is explored in detail focusing on Mxene-carbon composites, Mxene-metal oxides, Mxene-metal sulfides, Mxene-polymer composites and MXene-ceramic composites. Key properties of MXenes are examined including structural, electrical, morphological, optical, mechanical, chemical stability, electrical and thermal properties, conductivity, magnetic properties, dielectric charge and catalytic properties. Characterization techniques used to study these properties is also reviewed. Their 2D structure provides a high surface area and unique interlayer spacing, making MXenes ideal for applications in energy storage devices (like supercapacitors and batteries) where surface area and ion transport are critical for performance. The diverse applications of MXenes are presented emphasizing their use in batteries, catalysis, sensors, environmental remediation and supercapacitors. Special attention is given to the supercapacitor applications of MXenes of their potential in energy storage devices. Due to their high capacitance, fast charge/discharge rates, and excellent stability, MXenes are used in supercapacitors, lithium-ion batteries, and sodium-ion batteries. They can store energy more efficiently than many other materials, making them valuable in the quest for efficient, sustainable energy solutions. The progress in MXene supercapacitor devices is providing insights into the latest advancements and future prospects. MXenes are highlighted as versatile materials with significant potential in various technological fields particularly in energy storage. Future research directions and challenges are also outlined for ongoing and future studies in this dynamic area of materials science.

Multi-Interface Engineering of MXenes for Self-Powered Wearable Devices

Self-powered wearable devices with integrated energy supply module and sensitive sensors have significantly blossomed for continuous monitoring of human activity and the surrounding environment in healthcare sectors. The emerging of MXene-based materials has brought research upsurge in the fields of energy and electronics, owing to their excellent electrochemical performance, large surface area, superior mechanical performance, and tunable interfacial properties, where their performance can be further boosted via multi-interface engineering. Herein, a comprehensive review of recent progress in MXenes for self-powered wearable devices is discussed from the aspects of multi-interface engineering. The fundamental properties of MXenes including electronic, mechanical, optical, and thermal characteristics are discussed in detail. Different from previous review works on MXenes, multi-interface engineering of MXenes from termination regulation to surface modification and their impact on the performance of materials and energy storage/conversion devices are summarized. Based on the interfacial manipulation strategies, potential applications of MXene-based self-powered wearable devices are outlined. Finally, proposals and perspectives are provided on the current challenges and future directions in MXene-based self-powered wearable devices.

Borocarbonitride Catalyzed Ethylbenzene Oxidative Dehydrogenation: Activity Enhancement via Encapsulation of Mn Clusters inside the Tube

Borocarbonitride (BCN) catalysts, boasting multiple redox sites, have shown considerable potential in alkane oxidative dehydrogenation (ODH) to olefin molecules. However, their catalytic efficiency still lags behind that of leading commercial catalysts, primarily due to the limited reactivity of oxygen functional groups. In this study, a groundbreaking hybrid catalyst is developed, featuring BCN nanotubes (BCNNTs) encapsulated with manganese (Mn) clusters, crafted through a meticulous supramolecular self-assembly and postcalcination strategy. This novel catalyst demonstrates a remarkable enhancement in activity, achieving 30% conversion and ≈100% selectivity toward styrene in ethylbenzene ODH reactions. Notably, its performance surpasses both pure BCNNTs and those hosting Mn nanoparticles. Structural and kinetic analyses unveil a robust interaction between BCNNTs and the Mn component, substantially boosting the catalytic activity of BCNNTs. Furthermore, density functional theory (DFT) calculations elucidate that BCNNTs encapsulated with Mn clusters not only stabilize key intermediates (─B─O─O─B─) but also enhance the nucleophilicity of active sites through electron transfer from the Mn cluster to the BCNNTs. This electron transfer mechanism effectively lowers the energy barrier for ─C─H cleavage, resulting in a 13% improvement in catalytic activity compared to pure BCNNTs.

Cellulose-based aerogel derived N, B-co-doped porous biochar for high-performance CO2 capture and supercapacitor

The remarkable characteristics of porous biochar have generated significant interest in various fields, such as CO2 capture and supercapacitors. The modification of aerogel-derived porous biochar through activation and heteroatomic doping can effectively enhance CO2 adsorption and improve supercapacitor performance. In this study, a novel N, B-co-doped porous biochar (NBCPB) was synthesized by carbonating and activating the N, B dual-doped cellulose aerogel. N and B atoms were doped in-situ using a modified alkali-urea method. The potassium citrate was served as both an activator and a salt template to facilitate the formation of a well-developed nanostructure. The optimized NBCPB-650-1 (where 650 corresponded to activation temperature and 1 represented mass ratio of potassium citrate activator to carbonized NBCPB-400 precursor) displayed the largest micropore volume of 0.40 cm3·g-1 and a high specific surface area of 891 m2·g-1, which contributed to an excellent CO2 adsorption capacity of 4.19 mmol·g-1 at 100 kPa and 25 °C, a high CO2/N2 selectivity, and exceptional reusability (retained >97.5 % after 10 adsorption-desorption cycles). Additionally, the NBCPB-650-1 electrode also delivered a high capacitance of 220.9 F·g-1 at 1 A·g-1. Notably, the symmetrical NBCPB-650-1 supercapacitor exhibited a high energy density of 9 Wh·kg-1 at the power density of 100 W·kg-1. This study not only presents the potential application of NBCPB-650-1 material in CO2 capture and electrochemical energy storage, but also offers a new insight into easy-to-scale production of heteroatomic-modified porous biochar.

Ultrarapid Gelation of Porous Ti3C2Tx MXene Monoliths Induced by Ionic Liquids

Gelation is a promising method to assemble 3D macroscopic structures from MXene sheets for various applications. However, the fine control and scalable manufacturing of 3D MXene monoliths remains a great challenge. Herein, the controllable gelation of Ti3C2Tx MXene initiated by various ionic liquids (ILs) is first proposed, where the IL serve as linkers to bond the nanosheets together through electrostatic and hydrogen bonding interactions, forming 3D monoliths with well-adjustable structure. Furthermore, density functional theory calculations and experiments further reveal the cross-linking effect of different ILs. Typically, 3D porous structure with high specific surface area, suitable pore size, and improved electrolyte affinity is designed through the cross-linking of Ti3C2Tx with 1-vinyl-3-ethylimidazole bromide ([C2VIm]Br-Ti3C2Tx). Due to the strong coupling, the as-synthesized monolith possesses excellent rate performance and high energy density. The methodology is quite flexible, controllable, and universal that provides a new perspective for promoting innovative applications of 2D materials.

Borocarbonitride-Based Emerging Materials for Supercapacitor Applications: Recent Advances, Challenges, and Future Perspectives

Supercapacitors have emerged as a promising energy storage technology due to their high-power density, fast charging/discharging capabilities, and long cycle life. Moreover, innovative electrode materials are extensively explored to enhance the performance, mainly the energy density of supercapacitors. Among the two-dimensional (2D) supercapacitor electrodes, borocarbonitride (BCN) has sparked widespread curiosity owing to its exceptional tunable properties concerning the change in concentration of the constituent elements, along with an excellent alternative to graphene-based electrodes. BCN, an advanced nanomaterial, possesses excellent electrical conductivity, chemical stability, and a large specific surface area. These factors contribute to supercapacitors' overall performance and reliability, making them a viable option to address the energy crisis. This review provides a detailed survey of BCN, its structural, electronic, chemical, magnetic, and mechanical properties, advanced synthesis methods, factors affecting the charge storage mechanism, and recent advances in BCN-based supercapacitor electrodes. The review embarks on the scrupulous elaboration of ways to enhance the electrochemical properties of BCN through various innovative strategies followed by critical challenges and future perspectives. BCN, as an eminent electrode material, holds great potential to revolutionize the energy landscape and support the growing energy demands of the future.

Electrostatic Self-Assembly of MXene on Ruthenium Dioxide-Modified Carbon Cloth for Electrochemical Detection of Kaempferol

A superior composite material consisting of MXene and ruthenium dioxide-modified carbon cloth is synthesized by pulsed laser deposition and electrostatic self-assembly, which is further utilized to construct a class of novel electrochemical (EC) sensors for kaempferol (KA) detection. The carbon-cloth-based electrodes modified by ruthenium dioxide and then MXene are characterized by X-ray diffraction, scanning electron microscope, and X-ray photoemission spectroscopy. The EC process on the modified electrodes is analyzed by cyclic voltammetry, EC impedance spectroscopy, and differential pulse voltammetry. It is found that positively charged RuO2 not only possesses the remarkable electrical conductivity and electrocatalysis activity but also hampers the restacking of MXene, which accordingly enhances the exposure of the active surface area and greatly boosts the electrocatalysis activity of the entire composite. Consequently, this newly developed composite-based EC sensor exhibits a high sensitivity, selectivity, and remarkable stability to detect KA with two linear ranges of 0.06-1 and 1-15 µM. The inferred limit of detection is 0.039 µM via differential pulse voltammetry. More importantly, this novel EC sensor is found to be applicable for detecting KA in practical traditional Chinese medicines.

Water-Stabilized Vanadyl Phosphate Monohydrate Ultrathin Nanosheets toward High Voltage Al-Ion Batteries

Al ion batteries (AIBs) are attracting considerable attention owing to high volumetric capacity, low cost, and high safety. However, the strong electrostatic interaction between Al³⁺ and host lattice leads to discontented cycling life and inferior rate capability. Herein, a new strategy of employing water molecules contained VOPO₄ ·H₂ O to boost Al³⁺ migration via the charge shielding effect of water is reported. It is revealed that VOPO₄ ·H₂ O with water lubrication effect and smaller steric hindrance owns high capacity and fast Al³⁺ diffusion, while the loss of unstable water upon cycling leads to a rapid performance degradation. To address this problem, ultrathin VOPO₄ ·H₂ O@MXene nanosheets are fabricated via the formed TiOV bond between VOPO₄ ·H₂ O and MXene. The MXene aided exfoliation results in enhanced VO_water bond strength between H₂ O and VOPO₄ that endows the obtained composite with strong water holding ability, contributing to the extraordinary cycling stability. Consequently, the VOPO₄ ·H₂ O@MXene delivers a high discharge potential of 1.8 V and maintains discharge capacities of 410 and 374.8 mAh g^-1 after 420 and 2000 cycles at the current densities of 0.5 and 1.0 A g^-1 , respectively. This work provides a new understanding of water-contained AIBs cathodes and vital guidance for developing high-performance AIBs.

Safe Etching Route of Nb2SnC for the Synthesis of Two-Dimensional Nb2CTx MXene: An Electrode Material with Improved Electrochemical Performance

In this study, low-temperature synthesis of a Nb2SnC non-MAX phase was carried out via solid-state reaction, and a novel approach was introduced to synthesize 2D Nb2CTx MXenes through selective etching of Sn from Nb2SnC using mild phosphoric acid. Our work provides valuable insights into the field of 2D MXenes and their potential for energy storage applications. Various techniques, including XRD, SEM, TEM, EDS, and XPS, were used to characterize the samples and determine their crystal structures and chemical compositions. SEM images revealed a two-dimensional layered structure of Nb2CTx, which is consistent with the expected morphology of MXenes. The synthesized Nb2CTx showed a high specific capacitance of 502.97 Fg-1 at 1 Ag-1, demonstrating its potential for high-performance energy storage applications. The approach used in this study is low-cost and could lead to the development of new energy storage materials. Our study contributes to the field by introducing a unique method to synthesize 2D Nb2CTx MXenes and highlights its potential for practical applications.

Heterostructured and Mesoporous Nb2O5@TiO2 Core-Shell Spheres as the Negative Electrode in Li-Ion Batteries

Niobium pentoxides have received considerable attention and are promising anode materials for lithium-ion batteries (LIBs), due to their fast Li storage kinetics and high capacity. However, their cycling stability and rate performance are still limited owing to their intrinsic insulating properties and structural degradation during charging and discharging. Herein, a series of mesoporous Nb₂O₅@TiO₂ core-shell spherical heterostructures have been prepared for the first time by a sol-gel method and investigated as anode materials in LIBs. Mesoporosity can provide numerous open and short pathways for Li⁺ diffusion; meanwhile, heterostructures can simultaneously enhance the electronic conductivity and thus improve the rate capability. The TiO₂ coating layer shows robust crystalline skeletons during repeated lithium insertion and extraction processes, retaining high structural integrity and, thereby, enhancing cycling stability. The electrochemical behavior is strongly dependent on the thickness of the TiO₂ layer. After optimization, a mesoporous Nb₂O₅@TiO₂ core-shell structure with a ∼13 nm thick TiO₂ layer delivers a high specific capacity of 136 mA h g^-1 at 5 A g^-1 and exceptional cycling stability (88.3% retention over 1000 cycles at 0.5 A g^-1). This work provides a facile strategy to obtain mesoporous Nb₂O₅@TiO₂ core-shell spherical structures and underlines the importance of structural engineering for improving the performance of battery materials.

Multifunctional Integrated Interdigital Microsupercapacitors and Self-Powered Iontronic Tactile Pressure Sensor for Wearable Electronics

Multifunctionality and self-powering are key technologies for next-generation wearable electronics. Herein, an interdigitated MXene/TiS₂-based self-powered intelligent pseudocapacitive iontronic sensor system is designed, realizing integration of energy storage and pressure-sensitive sensing function into one device. The intercalation of TiS₂ nanosheet can effectively prevent self-stacking of MXene and results in mesoporous cross-linked framework, therefore exposing more active sites and broadening the electron/ion transport channels. The pressure sensing performance together with developed all-solid-state microsupercapacitor is explored systematically. When applied in a symmetrical microsupercapacitor, it presents a satisfactory energy density of 31.6 Wh/kg at 400 W/kg and 79.8% capacitance retention after 10 000 cycles. Meanwhile, with MXene/TiS₂//MXene/TiS₂ interdigitated structure as flexible self-powering pressure sensor, it illustrates outstanding pressure-sensing response toward external pressure, realizing accurate and continuous detection of human body motion signals. It is believed that this work proposes a feasible strategy by integrating pressure-sensing with a self-powering function for the next-generation self-powered E-skin electronics.

POMCPs with Novel Two Water-Assisted Proton Channels Accommodated by MXenes for Asymmetric Supercapacitors

To develop high-performance supercapacitors, the negative electrode is at present viewed as one of the most challenging tasks for obtaining the next-generation of energy storage devices. Therefore, in this study, a polyoxometalate-based coordination polymer [Zn(itmb)₃ H₂ O][H₂ SiW₁₂ O₄₀ ]·5H₂ O (1) is designed and prepared by a simple hydrothermal method for constructing a high-capacity negative electrode. Polymer 1 has two water-assisted proton channels, which are conducive to enhancing the electrical conductivity and storage capacity. Then, MXene Ti₃ C₂ T_x is chosen to accommodate coordination polymer 1 as the interlayer spacers to improve the conductivity and cycling stability of 1, while preventing the restacking of MXene. Expectedly, the produced composite electrode 1@Ti₃ C₂ T_x shows an excellent specific capacitance (1480.1 F g^-1 at 5 A g^-1 ) and high rate performance (a capacity retention of 71.5% from 5 to 20 A g^-1 ). Consequently, an asymmetric supercapacitor device is fabricated using 1@Ti₃ C₂ T_x as the negative electrode and celtuce leaves-derived carbon paper as the positive electrode, which demonstrates ultrahigh energy density of 32.2 Wh kg^-1 , and power density 2397.5 W kg^-1 , respectively. In addition, the ability to illuminate a red light-emitting diode for several minutes validates its feasibility for practical application.

MXene/Silver Nanowire-Based Spring Frameworks for Highly Flexible Waterproof Supercapacitors and Piezoelectrochemical-Type Pressure-Sensitive Sensor Devices

With widespread application of flexible electronic devices, the multifunction for supercapacitors has attracted tremendous attention. Here, developed is a novel multifunctional MXene-based pizeoelectrochemical-type pressure sensor based on highly compressible antiwater supercapacitor. This novel design realizes energy storage and pressure sensing functions simultaneously. The outstanding rate performance is realized by the reasonable design of electron and ion transport channels, originating from strong synergistic bridging interactions between silver nanowires (AgNWs) and MXene. Serving as the electrochemical storage device, even at large 500 mV s^-1, the cyclic voltammetry curve of AgNWs/MXene aerogel still maintains nearly rectangular characteristics. For the assembled antiwater symmetric supercapacitor, it records a high specific capacitance of 210.5 F g^-1 at 0.5 A g^-1, a maximum energy density of 74.7 W h Kg^-1 at 400 W Kg^-1, and outstanding waterproof cyclic stability of 86.51% in water. Based on elastic AgNWs/MXene aerogel, an antiwater pizeoelectrochemical-type strain sensor is designed, and the device presents stable and sensitive current response while facing external pressure. This study clearly demonstrates that our work promises a new research direction toward the design of next-generation wearable devices that could be used in wirelessly powered wearable devices.

2D/2D Nanoarchitectured Nb2C/Ti3C2 MXene Heterointerface for High-Energy Supercapacitors with Sustainable Life Cycle

Layered 2D/2D heterointerface composites experience interesting properties that greatly stimulate the recent surge in the attention as robust supercapacitor electrode materials, especially the MXene-based 2D/2D heterointerface for its robust energy storage compatibility. This report unveils a synergistically in situ prepared 2D/2D Nb₂C/Ti₃C₂ MXene (NCTC) heterointerface nanoarchitecture by facile one-pot chemical etching. The methodology adopted enables the interconnected and simultaneous growth of MXenes exposing and retaining their active surfaces for enhanced ion diffusion pathways, charge storage dynamics, microstructural stability, and a noticeable potential window. Henceforth, the in situ developed NCTC heterointerface electrode delivered an excellent specific capacitance of 584 F/g at 2 A/g with a commendable energy density of 38.5 W h/kg in MXene supercapacitors owing to the augmented surface- and redox-based charge storage at the interface. Finally, the developed all-solid-state system demonstrated a superior cycling retention of 98% capacitance after 50,000 cycles. These superlative results encourage the exploration of such prospective 2D/2D heterointerfaces with intriguing charge storage and microstructural attributes for designing next-generation energy storage systems.