A novel MnO2/Ti3C2Tx MXene nanocomposite as high performance electrode materials for flexible supercapacitors

MXene: A Roadmap to Sustainable Energy Management, Synthesis Routes, Stabilization, and Economic Assessment

MXenes with their wide range of tunability and good surface chemistry provide unique and distinctive characteristics offering potential employment in various aspects of energy management applications. These high-performance materials have attracted considerable attention in recent decades due to their outstanding characteristics. In the literature, most of the work is related to specific methods for the preparation of MXenes. In this Review, we present a detailed discussion on the synthesis of MXenes through different etching routes involving acids, such as hydrochloric acid, hydrofluoric acid, and lithium fluoride, and non-acidic alkaline solution, electrochemical, and molten salt methods. Furthermore, a concise overview of the different structural, optical, electronic, and magnetic properties of MXenes is provided corresponding to their role in supporting high thermal, chemical, mechanical, environmental, and electrochemical stability. Additionally, the role of MXenes in maintaining the thermal management performance of photovoltaic thermal systems (PV/T), wearable light heaters, solar water desalination, batteries, and supercapacitors is also briefly discussed. A techno-economic and life cycle analysis of MXenes is provided to analyze their sustainability, scalability, and commercialization to facilitate a comprehensive array of energy management systems. Lastly, the technology readiness level of MXenes is defined, and future recommendations for MXenes are provided for their further utilization in niche applications. The present work strives to link the chemistry of MXenes to process economics for energy management applications.

Unlocking Enhanced Electrochemical Performance of MBene-MoB Through Controlled Aluminum Dissipation from MoAlB

2D transition metal borides, known as MBenes, have attracted considerable attention due to their exceptional properties. This study explores the feasibility of aluminum (Al) etching from MoAlB using environmentally friendly and sustainable fluoride-free dilute acidic/alkaline solutions at room temperature, revealing its thermodynamic and kinetic viability. Furthermore, it is found that complete removal of Al can be achieved in dilute alkaline reagent under hydrothermal conditions, yielding pristine single/few-layered MBene-MoB for the first time, while acidic solutions result in ≈33% etching rates. XRD refinement, which tracks aluminum removal from 0% to 100%, reveals transient metastable phases of MoAl1-xB (x < 0.5) in the initial etching stages, evolving into relatively stable pure Mo2AlB2 structures with 50% Al deficiency, serving as a precursor to MBenes. The subsequent loss of Al results in a 2D MBene-MoB structure. DFT calculations confirm excellent conductivity for MoAlB, MoAl1-xB (x = 0-1), and MBene-MoB. Remarkably, MBene-MoB exhibits superior supercapacitor performance with a 4025.60 mF cm-2/201.28 F g-1 capacitance. Simulations validate rapid electrolyte diffusion in layered MBene-MoB, contributing significantly to enhanced capacitance. Additionally, in the hydrogen evolution reaction (HER), MBene-MoB demonstrates superior catalytic activity compared to the precursor MoAlB and commercial MoB. Calculations suggest the potential for enhancing HER through surface modulation, considering its suboptimal hydrogen adsorption energy.

Boron and Nitrogen Co-Doped Porous Graphene Nanostructures for the Electrochemical Detection of Poisonous Heavy Metal Ions

Heavy metal poisoning has a life-threatening impact on the human body to aquatic ecosystems. This necessitates designing a convenient green methodology for the fabrication of an electrochemical sensor that can detect heavy metal ions efficiently. In this study, boron (B) and nitrogen (N) co-doped laser-induced porous graphene (LIGBN) nanostructured electrodes were fabricated using a direct laser writing technique. The fabricated electrodes were utilised for the individual and simultaneous electrochemical detection of lead (Pb2+) and cadmium (Cd2+) ions using a square wave voltammetry technique (SWV). The synergistic effect of B and N co-doping results in an improved sensing performance of the electrode with better sensitivity of 0.725 µA/µM for Pb2+ and 0.661 µA/µM for Cd2+ ions, respectively. Moreover, the sensing electrode shows a low limit of detection of 0.21 µM and 0.25 µM for Pb2+ and Cd2+ ions, with wide linear ranges from 8.0 to 80 µM for Pb2+ and Cd2+ ions and high linearity of R2 = 0.99 in case of simultaneous detection. This rapid and facile method of fabricating heteroatom-doped porous graphene opens a new avenue in electrochemical sensing studies to detect various hazardous metal ions.

Heterostructures of MXenes and transition metal oxides for supercapacitors: an overview

MXenes are a large family of two dimensional (2D) materials with high conductivity, redox activity and compositional diversity that have become front-runners in the materials world for a diverse range of energy storage applications. High-performing supercapacitors require electrode materials with high charge storage capabilities, excellent electrical conductivity for fast electron transfer, and the ability of fast charging/discharging with good cyclability. While MXenes show many of these properties, their energy storage capability is limited by a narrow electrochemically stable potential window due to irreversible oxidation under anodic potentials. Although transition metal oxides (TMOs) are often high-capacity materials with high redox activity, their cyclability and poor rate performance are persistent challenges because of their dissolution in aqueous electrolytes and mediocre conductivity. Forming heterostructures of MXenes with TMOs and using hybrid electrodes is a feasible approach to simultaneously increase the charge storage capacity of MXenes and improve the cyclability and rate performance of oxides. MXenes could also act as conductive substrates for the growth of oxides, which could perform as spacers to stop the aggregation of MXene sheets during charging/discharging and help in improving the supercapacitor performance. Moreover, TMOs could increase the interfacial contact between MXene sheets and help in providing short-diffusion ion channels. Hence, MXene/TMO heterostructures are promising for energy storage. This review summarizes the most recent developments in MXene/oxide heterostructures for supercapacitors and highlights the roles of individual components.

Ag2S QDs integration with MnO2 nanosheets for the sensitive detection of Cr (VI) via the redox reaction induced photoelectrochemical variation

The heavy metal Cr (VI) will remain, accumulate, and migrate after entering the environment or ecosystem, causing serious harm to the environment. Here, a photoelectrochemical sensor was developed for Cr (VI), utilizing the Ag₂S quantum dots (QDs) and MnO₂ nanosheets as photoactive components. By introducing Ag₂S QDs with a narrow gap, a staggered energy level match is created which effectively prevents the carrier recombination in MnO₂ nanosheets, resulting in an enhanced photocurrent response. In the presence of the electron donor, l-ascorbic acid (AA), the photocurrent of the Ag₂S QDs and MnO₂ nanosheets modified photoelectrode is further enhanced. As AA has the ability to convert Cr (VI) to Cr (Ⅲ), the photocurrent may decline due to the decrease in the electron donors when Cr (VI) is added. This phenomenon can be utilized for the sensitive detection of Cr (VI) over a wider linear range (100 pM-30 μM) with a lower detection limit of 6.46 pM (S/N = 3). This work using the strategy that the targets induced the variations of the electron donor shows the advantages of good sensitivity and nice selectivity. The sensor holds many advantages such as simple fabrication process, economical material expense, and consistent photocurrent signals. It also holds significant potential for environmental monitoring and serves as a practical photoelectric sensing approach for detecting Cr (VI).

Synthesis and applications of MXene-based composites: a review

Recently, there has been considerable interest in a new family of transition metal carbides, carbonitrides, and nitrides referred to as MXenes (Ti₃C₂T_x) due to the variety of their elemental compositions and surface terminations that exhibit many fascinating physical and chemical properties. As a result of their easy formability, MXenes may be combined with other materials, such as polymers, oxides, and carbon nanotubes, which can be used to tune their properties for various applications. As is widely known, MXenes and MXene-based composites have gained considerable prominence as electrode materials in the energy storage field. In addition to their high conductivity, reducibility, and biocompatibility, they have also demonstrated outstanding potential for applications related to the environment, including electro/photocatalytic water splitting, photocatalytic carbon dioxide reduction, water purification, and sensors. This review discusses MXene-based composite used in anode materials, while the electrochemical performance of MXene-based anodes for Li-based batteries (LiBs) is discussed in addition to key findings, operating processes, and factors influencing electrochemical performance.

Rational design of flower-like MnO2/Ti3C2Txcomposite electrode for high performance supercapacitors

Combining the new two-dimensional conductive MXene with transition metal oxide to build composite structure is a promising path to improve the conductivity of metal oxide. However, a critical challenge still remains in how to achieve a good combination of MXene and metal oxide. Herein, we develop a facile hydrothermal route to synthesize the MnO₂/Ti₃C₂T_xcomposite electrode for supercapacitors by synergistically coupling MnO₂nanowires with Ti₃C₂T_xMXene nanoflakes. Compared with the pure MnO₂electrode, the morphology of the MnO₂/Ti₃C₂T_xcomposite electrode changes from nanowires to nanoflowers. Moreover, the overall conductivity and electrochemical performance of the composite electrode are greatly improved due to an addition of Ti₃C₂T_xMXene. The specific capacitance of the MnO₂/Ti₃C₂T_xcomposite electrode achieves 210.8 F·g^-1at a scan rate of 2 mV·s^-1, while that of the pure MnO₂electrode is only 55.2 F·g^-1. Furthermore, the specific capacitance of the MnO₂/Ti₃C₂T_xcomposite electrode still can remain at 97.2% even after 10 000 charge-discharge cycles, revealing an excellent cycle stability. The synthesis strategy of this work can pave the way for the research and practical application of the electrode materials for supercapacitors.

Scalable Manufacturing of Environmentally Stable All-Solid-State Plant Protein-Based Supercapacitors with Optimal Balance of Capacitive Performance and Mechanically Robust

The development of advanced biomaterial with mechanically robust and high energy density is critical for flexible electronics, such as batteries and supercapacitors. Plant proteins are ideal candidates for making flexible electronics due to their renewable and eco-friendly natures. However, due to the weak intermolecular interactions and abundant hydrophilic groups of protein chains, the mechanical properties of protein-based materials, especially in bulk materials, are largely constrained, which hinders their performance in practical applications. Here, a green and scalable method is shown for the fabrication of advanced film biomaterials with high mechanical strength (36.3 MPa), toughness (21.25 MJ m^-3 ), and extraordinary fatigue-resistance (213 000 times) by incorporating tailor-made core-double-shell structured nanoparticles. Subsequently, the film biomaterials combine to construct an ordered, dense bulk material by stacking-up and hot-pressing techniques. Surprisingly, the solid-state supercapacitor based on compacted bulk material shows an ultrahigh energy density of 25.8 Wh kg^-1 , which is much higher than those previously reported advanced materials. Notably, the bulk material also demonstrates long-term cycling stability, which can be maintained under ambient condition or immersed in H₂ SO₄ electrolyte for more than 120 days. Thus, this research improves the competitiveness of protein-based materials for real-world applications such as flexible electronics and solid-state supercapacitors.

Recent Advances in Two-Dimensional MXene for Supercapacitor Applications: Progress, Challenges, and Perspectives

MXene is a type of two-dimensional (2D) transition metal carbide and nitride, and its promising energy storage materials highlight its characteristics of high density, high metal-like conductivity, tunable terminals, and charge storage mechanisms known as pseudo-alternative capacitance. MXenes are a class of 2D materials synthesized by chemical etching of the A element in MAX phases. Since they were first discovered more than 10 years ago, the number of distinct MXenes has grown substantially to include numerous M_nX_n-1 (n = 1, 2, 3, 4, or 5), solid solutions (ordered and disordered), and vacancy solids. To date, MXenes used in energy storage system applications have been broadly synthesized, and this paper summarizes the current developments, successes, and challenges of using MXenes in supercapacitors. This paper also reports the synthesis approaches, various compositional issues, material and electrode topology, chemistry, and hybridization of MXene with other active materials. The present study also summarizes MXene's electrochemical properties, applicability in pliant-structured electrodes, and energy storage capabilities when using aqueous/non-aqueous electrolytes. Finally, we conclude by discussing how to reshape the face of the latest MXene and what to consider when designing the next generation of MXene-based capacitors and supercapacitors.

MXene: fundamentals to applications in electrochemical energy storage

A new, sizable family of 2D transition metal carbonitrides, carbides, and nitrides known as MXenes has attracted a lot of attention in recent years. This is because MXenes exhibit a variety of intriguing physical, chemical, mechanical, and electrochemical characteristics that are closely linked to the wide variety of their surface terminations and elemental compositions. Particularly, MXenes are readily converted into composites with materials including oxides, polymers, and CNTs, which makes it possible to modify their characteristics for a variety of uses. MXenes and MXene-based composites have demonstrated tremendous promise in environmental applications due to their excellent reducibility, conductivity, and biocompatibility, in addition to their well-known rise to prominence as electrode materials in the energy storage sector. The remarkable characteristics of 2D MXene, including high conductivity, high specific surface area, and enhanced hydrophilicity, account for the increasing prominence of its use in storage devices. In this review, we highlight the most recent developments in the use of MXenes and MXene-based composites for electrochemical energy storage while summarizing their synthesis and characteristics. Key attention is paid to applications in supercapacitors, batteries, and their flexible components. Future research challenges and perspectives are also described.

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.

High Capacitive Antimonene/CNT/PANI Free-Standing Electrodes for Flexible Supercapacitor Engaged with Self-Healing Function

In virtue of the high electrochemical activity and inherent flexibility, polyaniline (PANI) is an ideal electrode material for flexible supercapacitors (SCs). However, in practical applications, the inevitable agglomeration of PANI leads to low capacitance, poor rate performance, and cycling stability. Here, antimonene (Sb) nanosheets with ultrathin thickness, excellent mechanical strength, and flexibility are introduced into the carbon nanotube (CNT) framework for PANI electrodeposition via simple vacuum filtration, which enables the continuous and uniform growth of PANI. The resultant free-standing Sb/CNT/PANI electrode can thus exhibit a high specific capacitance of 578.57 F g^-1 together with a high rate capability. Besides, thanks to the introduction of Sb nanosheets, the agglomeration of PANI during the electrodeposition is improved, which correspondingly alleviates the structural deterioration of PANI during repeated charge/discharge. Thus, the flexible SC assembled by Sb/CNT/PANI electrodes demonstrates both an impressive specific capacitance of 416 F g^-1 and outstanding cycling stability over 12 000 cycles. Moreover, this SC device can have a practical self-healing function by employing self-healable polyurethane. The facile strategy reported herein sheds light on the design of high-performance flexible SCs, catering to the needs of portable and wearable electronics.

Ultrasound-assisted fabrication of Ti3C2Tx MXene toward enhanced energy storage performance

Ti₃C₂Tx MXenes are normally fabricated by removal of main group element from the corresponding transition metal carbides, and they have been actively studied due to their superior energy storage performance. However, the low efficiency in removal of main group element (named as chemical etching) has significantly limited the application of MXene or MXene-related materials. Herein, we demonstrated an ultrasound-assisted approach to synthesize Ti₃C₂Tx MXene material by using Ti₃AlC₂ as the precursor. The experimental results indicate that the efficiency of chemical etching of Ti₃AlC₂ was dramatically promoted by ultrasound. The etching time was greatly shortened to 8 h while typically 24 h is sufficient in dilute hydrofluoric acid. Particularly, the high etching efficiency was achieved by using 2% hydrofluoric acid under the aid of ultrasound, which is lower in concentration than those reported in the previous literature. The specific capacitance of the 8 h sonicated sample is 155F/g, which is much higher than that of the un-sonicated sample prepared under the same experimental conditions. Additionally, the specific capacitance retention of the prepared 8 h sonicated sample was 97.5% after 20,000 cycles of charging/discharging, exhibiting an outstanding energy storage stability compared with the materials reported in previous literatures. It was proposed that removal of AlF₃ from the surface of the etched particles was significantly promoted and the hydrogen bonds between the terminations of two different adjacent layers were broken by the acoustic cavitation effect of ultrasound.

Crosslinking Nanoarchitectonics of Nitrogen-doped Carbon/MoS2 Nanosheets/Ti3 C2 Tx MXene Hybrids for Highly Reversible Sodium Storage

Although it is a promising sodium storage material due to its excellent electrochemical activity, small bandgap, and large interlayer spacing, layered molybdenum disulfide (MoS₂ ) suffers from poor rate capability and degraded cycling life, resulting from its serious aggregation upon preparation, sluggish reaction kinetics, and structure expansion during cycling. To address these issues, a polyethyleneimine (PEI)-assisted fabrication approach was developed for the rational synthesis of an interconnected framework with nitrogen-doped carbon-confined MoS₂ nanosheets/Ti₃ C₂ T_x MXene (MoS₂ /Ti₃ C₂ T_x @NC), where the PEI could guide the uniform growth of MoS₂ on Ti₃ C₂ T_x and the self-generated NC simultaneously enhanced its synergistic coupling with MoS₂ /Ti₃ C₂ T_x , thus contributing to the improvement of charge transfer, diffusion kinetics, and structural integrity of the hybrid electrode. Consequently, the desired MoS₂ /Ti₃ C₂ T_x @NC delivered impressive sodium storage performance, demonstrating high reversible capacities of 397.3 and 206.8 mAh g^-1 at 0.1 A g^-1 after 100 cycles and 0.5 A g^-1 after 500 cycles, respectively. Moreover, electrochemical kinetics analysis and charge storage mechanism manifested that high capacitive contribution, facilitated Na⁺ transport pathways, and synergistic electronic coupling between MoS₂ /Ti₃ C₂ T_x and NC contributed to the superior sodium storage performance.

Fabrication of Cobaltous Sulfide Nanoparticle-Modified 3D MXene/Carbon Foam Hybrid Aerogels for All-Solid-State Supercapacitors

MXene is a neoteric type of bidimensional (2D) transition metal carbide/nitride with broad application prospects, in particular with electrochemical energy storage. The electrochemical performance of MXene is unsatisfactory because it is easy to stack resulting in the difficulty of electrolyte penetration and ion transport. In this study, the cobaltous sulfide-modified 3D MXene/N-doped carbon foam (CoS@MXene/CF) hybrid aerogel is projected and manufactured via simple in situ growth and thermal annealing strategies. The capacitance of the as-fabricated 300-CMC-31:1 electrode material reaches 250 F g^-1 (1 A g^-1), which is obviously higher than those of MXene, CoS@CF, 400-CMC-31:1, 300-CMC-10:1, 300-CMC-50:1, CF, and MXene/CF electrode materials. Moreover, it can hold 97.5% of the original capacitance after 10,000 cycles and the internal resistance (R_s) is only 0.50 Ω. A green bulb can be lit by two all-solid asymmetric supercapacitors installed in series. The prepared CoS@MXene/CF hybrid aerogel exhibits promising potential for practical application in energy storage areas.

Cationic intermediates assisted self-assembly two-dimensional Ti3C2Tx/rGO hybrid nanoflakes for advanced lithium-ion capacitors

Two-dimensional (2D) material MXenes have been intensively concerned in energy-storage field due to these unique properties of metallic-like conductivity, good hydrophilicity and high volumetric capacity. However, the self-restocking of ultra-thin 2D materials seriously hinders these performances, which significantly inhibits the full exploitation of MXenes in the field of energy storage. To solve this issue, a strategy to prepare delaminated Ti₃C₂T_x (MXene) nanoflakes/reduced graphene oxide (rGO) composites is proposed using the electrostatic self-assembly between positively charged Ti₃C₂T_x with tetrabutylammonium ion (TBA⁺) modification and negatively charged graphene. The nanoflakes of Ti₃C₂T_x/rGO are well dispersed and arranged in a face-to-face structure to effectively alleviate the self-restacking and provide more electroactive sites for accessible of electrolyte ions. The prepared delaminated Ti₃C₂T_x/rGO anode shows a high reversible capacity up to 1394 mAh g^-1 at a current density of 50 mA g^-1. Moreover, a lithium-ion capacitor (LIC) was assembled with delaminated Ti₃C₂T_x/rGO anode and activated carbon (AC) cathode which can exhibit a specific capacity of 70.7 F g^-1 at a current density of 0.1 A g^-1 and deliver an ultrahigh energy density of 114 Wh kg^-1 at a relatively high power density of 3125 W kg^-1. These good electrochemical performances demonstrate the potential of delaminated Ti₃C₂T_x/rGO as an anode material for lithium-ion capacitors.

Efficient BiVO4 photoanode decorated with Ti3C2TX MXene for enhanced photoelectrochemical sensing of Hg(II) ion

A highly sensitive photoelectrochemical (PEC) sensing platform was constructed for Hg²⁺ determination based on the Schottky heterojunction between an emerging 2D material Ti₃C₂T_X MXene and a promising semiconductor material BiVO₄. Through simply spin-coating the single-layer Ti₃C₂T_X onto the surface of BiVO₄ film, the modified electrode exhibited significantly enhanced PEC activity. However, the boost in photocurrent could be noticeably suppressed due to the consumption of hole-scavenging agents (reduced glutathione) by the added Hg²⁺. Owing to the selective decrease in the photocurrent with the addition of Hg²⁺, the PEC sensor based on BiVO₄/Ti₃C₂T_X displayed a wide linear range from 1 pM to 2 nM with the limit of detection down to 1 pM. Moreover, the PEC sensor also exhibited satisfactory accuracy and repeatability in practical sample water, the Yangtze River water, demonstrating the great potential for monitoring heavy metal ions in natural water resources.

Facile Preparation of MnO2 Quantum Dots with Enhanced Fluorescence via Microenvironment Engineering with the Assistance of Some Reductive Biomolecules

MnO₂ nanomaterials have aroused widespread attention because of their nanozyme activity, redox properties, good biocompatibility, and therapy-related activities. However, not many reports on self-luminescent MnO₂ materials have been concerned to date, which greatly hampered their further development in various fields. In this paper, luminescent MnO₂ quantum dots (MnO₂ QDs) have been first prepared via a facile one-step ultrasonic method. With the assistance of bovine serum albumin (BSA) or cysteine (Cys), the synthesized MnO₂ QDs (BSA-MnO₂ QDs or Cys-MnO₂ QDs) display strongly enhanced fluorescence (FL). The prepared BSA-MnO₂ QDs with a particle size of about 1 to 2 nm show the maximum excitation and emission peaks at 320 and 410 nm with excellent salt stability, anti-photobleaching ability, and time stability. It is confirmed that BSA plays a dual function as the exfoliating agent to promote the exfoliation of bulk MnO₂ nanosheets and as the capping agent to provide a friendly microenvironment for MnO₂ QDs. Ag ions can destroy the microenvironment of BSA-MnO₂ QDs owing to the in situ formation of Ag nanoparticles (Ag NPs) mediated by BSA on the surface of the QDs. Then, these Ag NPs can quench the FL intensity of the QDs by fluorescence resonance energy transfer. However, the FL strength of the BSA-MnO₂ QDs is recovered after adding H₂O₂ and NaHS since they may react with Ag NPs to produce Ag⁺ and Ag₂S, which further confirmed the role of BSA. This work not only opens up a facile and universal avenue to synthesize luminescent MnO₂ QDs with enhanced FL but also provides a possible sensing platform through tuning the microenvironment of the MnO₂ QDs. The MnO₂ QDs with outstanding performance may show great potential as fluorescent probes in the fields of biological imaging, optical sensing, drug delivery, and therapy.

Progress of Two-Dimensional Ti3 C2 Tx in Supercapacitors

Exploring stable cycling electrode materials with high energy and power density is the key to accelerating the development and application of supercapacitors. Ti₃ C₂ T_x , which is the most investigated member of the family of two-dimensional layered transition-metal carbides, has attracted considerable attention, owing to its unique two-dimensional morphology, large interlayer spacing, outstanding metallic conductivity, abundant chemical surface, and ultrahigh volumetric capacitance. However, the inherent restacking tendency of ultrathin Ti₃ C₂ T_x sheets hinder its practical application. In this review, the synthetic methods and charge-storage mechanisms of Ti₃ C₂ T_x are stressed to provide clues for improving its electrochemical performance. Functionalization, including architectural construction, hybridization, and surface modification of the Ti₃ C₂ T_x sheets, to circumvent difficulties and application in supercapacitors is then summarized. Accordingly, the aim is to highlight the opportunities and challenges for Ti₃ C₂ T_x -based materials in practical applications in supercapacitors.

Recent Advances of Two-Dimensional Nanomaterials for Electrochemical Capacitors

Two-dimensional (2D) nanomaterials have drawn a wide range of research interests because of their unique ultrathin layered structures and attractive properties. In particular, the electrochemical properties and great variety of 2D nanomaterials make them highly attractive candidates for electrochemical capacitors, such as supercapacitors, lithium-ion capacitors, and sodium-ion capacitors. Herein, a comprehensive review of recent progress towards the application of 2D nanomaterials for electrochemical capacitors is provided. Several typical types of 2D nanomaterials are first briefly introduced, followed by detailed descriptions of their electrochemical capacitor applications. Finally, research perspectives and future research directions of these interesting areas are also provided.

Emerging 2D MXenes for supercapacitors: status, challenges and prospects

This review provides a comprehensive understanding of the emerging 2D MXene electrode materials for supercapacitor application.

Self-assembled Ti3C2 /MWCNTs nanocomposites modified glassy carbon electrode for electrochemical simultaneous detection of hydroquinone and catechol

A versatile electrochemical sensor based on titanium carbide (Ti₃C₂) and multi-walled carbon nanotubes (MWCNTs) nanocomposite was constructed to detection catechol (CT) and hydroquinone (HQ). To prepare this novel nanocomposite, a self-assembled process was conducted by blending two-dimensional (2D) hierarchical Ti₃C₂ and MWCNTs under ultrasonic-assisted. X-ray diffraction (XRD), High resolution transmission electron microscopy (HR-TEM) and Scanning electron microscopy (SEM) methods as well as electrochemical technique, such as Electrochemical impedance spectroscopy (EIS), Cyclic voltammetry (CV) and Differential pulse voltammetry (DPV) were performed to characterize the Ti₃C₂-MWCNTs nanocomposite and illuminate the electrochemical oxidation process. Under the optimum conditions, wide linear range from 2 μM to 150 μM for both HQ and CT and low detection limit of 6.6 nM for HQ and 3.9 nM (S/N = 3) for CT have been achieved. Impressively, the sensor possesses superior selectivity, ultra-stability, and good repeatability, which was successfully applied for detecting CT and HQ in real industrial waste water sample with recovery of 96.9%-104.7% and 93.1%-109.9% for HQ and CT, respectively. Hence, Ti₃C₂ nanosheeets were proved to be a promising platform to construct electrochemical oxidation sensor in environmental analyses and phenolic isomers detection.

Ultrathin Ti3C2Tx (MXene) Nanosheet-Wrapped NiSe2 Octahedral Crystal for Enhanced Supercapacitor Performance and Synergetic Electrocatalytic Water Splitting

Metal selenides, such as NiSe₂, have exhibited great potentials as multifunctional materials for energy storage and conversation. However, the utilization of pure NiSe₂ as electrode materials is limited by its poor cycling stability, low electrical conductivity, and insufficient electrochemically active sites. To remedy these defects, herein, a novel NiSe₂/Ti₃C₂T_x hybrid with strong interfacial interaction and electrical properties is fabricated, by wrapping NiSe₂ octahedral crystal with ultrathin Ti₃C₂T_x MXene nanosheet. The NiSe₂/Ti₃C₂T_x hybrid exhibits excellent electrochemical performance, with a high specific capacitance of 531.2 F g^-1 at 1 A g^-1 for supercapacitor, low overpotential of 200 mV at 10 mA g^-1, and small Tafel slope of 37.7 mV dec^-1 for hydrogen evolution reaction (HER). Furthermore, greater cycling stabilities for NiSe₂/Ti₃C₂T_x hybrid in both supercapacitor and HER have also been achieved. These significant improvements compared with unmodified NiSe₂ should be owing to the strong interfacial interaction between NiSe₂ octahedral crystal and Ti₃C₂T_x MXene, which provides enhanced conductivity, fast charge transfer as well as abundant active sites, and highlight the promising potentials in combinations of MXene with metal selenides for multifunctional applications such as energy storage and conversion.

Reduced graphene oxide wrap buffering volume expansion of Mn2SnO4 anodes for enhanced stability in lithium-ion batteries

MSnO4 (M = Mn, Zn, Co, Mg, etc.) has been widely investigated as an anode material for lithium-ion batteries in recent years, but its practical applications are limited by serious capacity loss caused by severe volume expansion during Li+ insertion/extraction. So far, hollow structures, carbon coating, and encapsulation by reduced graphene oxide have been introduced to improve the electrochemical properties of MSnO4. In this study, Mn2SnO4 nanoparticles@reduced graphene oxide (Mn2SnO4@rGO) composites were prepared using simple steps and applied as anode materials for lithium-ion batteries. The rGO sheet encapsulated Mn2SnO4 nanoparticles show improved electrochemical properties. The first discharge capacity of Mn2SnO4@rGO reaches 1223.5 mAh g-1 and remains at 542.0 mAh g-1 after 100 cycles at a current density of 0.1 A g-1. The electrochemical properties were significantly improved compared to those of pure Mn2SnO4 nanoparticles.