Modified MXene/Holey Graphene Films for Advanced Supercapacitor Electrodes with Superior Energy Storage

Sandwich construction of chitosan/reduced graphene oxide composite as additive-free electrode material for high-performance supercapacitors

The sandwich construction of chitosan (CS)/reduced graphene oxide (rGO) composite was synthesized through microwave-assisted hydrothermal method without further carbonization or activation process (CRG). CS homogeneous attached between the rGO slice sheet and improve the dispersion of CRG effectively, which can increase its specific surface area with hierarchical porous structure. Dehydration condensation occurred between CS and rGO, forming NHCO groups that can promote the wettability and conductivity of the composites. CRG exhibited improved degree of order and reduced graphitization defect, N-5 and OI groups were the dominant nitrogen and oxygen-containing groups. When used as additive-free electrode, CRG exhibited a high specific capacitance of 274 F g^-1 at the current density of 0.5 A g^-1 with good rate performance in a three-electrode system using 1 M H₂SO₄ electrolyte. Solid-state supercapacitor device was assembled with CRG electrode and lignin hydrogel electrolytes, high gravimetric energy densities of 8.4 Wh kg^-1 at the power density of 50 W kg^-1 was achieved.

Nacre-inspired composite films with high mechanical strength constructed from MXenes and wood-inspired hydrothermal cellulose-based nanofibers for high performance flexible supercapacitors

Two dimensional MXenes with fascinating characteristics of high electrical conductivity, high density and electroactivity show promising applications in various fields. However, the direct applications of MXenes have been limited due to their inferior mechanical properties and easy restacking. Herein, a kind of nacre-like composite film constructed with Ti3C2Tx, cellulose nanofiber (HCNF) and sodium lignosulfonate (Lig) obtained through the hydrothermal process, named Ti3C2Tx/HCNF@Lig, has been successfully synthesized. The hydrothermal cellulose nanofiber (HCNF) film shows an enhanced mechanical strength (114 MPa) compared to that of the CNF film (95 MPa). Wood-inspired HCNF@Lig composite films present an enhanced mechanical tensile strength of up to 133 MPa. Nacre-like deformable Ti3C2Tx/HCNF@Lig(3@1) composite films exhibit high conductivity (up to 1.75 × 105 S m-1) and mechanical properties (up to 258 MPa). The electrodes of Ti3C2Tx/HCNF@Lig(3@1)97/3 composite film assembled flexible solid-state supercapacitors possess an excellent volumetric specific capacitance of 748.96 F cm-3. The corresponding deformable supercapacitors show an excellent energy density of 16.2 W h L-1 and outstanding electrochemical cycling stability. The as-prepared nacre-like Ti3C2Tx/HCNF@Lig composite films with high mechanical properties and electrochemical performance are expected to be practically applied in flexible/wearable energy storage devices.

Adhesion Between MXenes and Other 2D Materials

MXenes, a large family of two-dimensional (2D) early transition metal carbides and nitrides, have excellent electrical and electrochemical properties, which can also be explored in assemblies with other 2D materials, like graphene and transition metal dichalcogenides (TMDs), creating heterostructures with unique properties. Understanding the interaction mechanism between 2D materials is critical for the design and manipulation of these 2D heterostructures. Our previous work investigated the interaction between SiO₂ and two MXenes (Ti₃C₂T_x and Ti₂CT_x). However, no experimental research has been done on MXene interlayer interactions and interactions in MXene heterostructures. Here, we used atomic force microscopy (AFM) with SiO₂ tip and Ti₃C₂T_x and Ti₂CT_x MXene-coated tips, respectively, to measure the adhesion energies of graphene, MoSe₂, Ti₃C₂T_x, and Ti₂CT_x MXene with other 2D materials. The measured adhesion energies show that only the interfaces involving graphene demonstrate dependence on the number of material monolayers in a stack. Comparing 40 interacting pairs of 2D materials, the lowest adhesion energy (∼0.27 J/m²) was found for the interfaces involving MoSe₂ and the highest adhesion energy was observed for the interfaces involving Ti₃C₂T_x (∼1.23 J/m²). The obtained set of experimental data for 2D interfaces involving MXenes provides a basis for a future in-depth understanding of adhesive mechanisms at interfaces between 2D materials, which is an important topic for the design of 2D heterostructures with controlled interfacial strength and properties.

A high operating voltage micro-supercapacitor based on the interlamellar modulation type Ti3C2Tx MXene

The increasing demand for miniaturized, wearable, and flexible electronics has promoted the development of micro power sources such as microsupercapacitors (MSCs). This work reports a high-performance MSC based on Ti₃C₂T_x-layer/MnO₂-nanorod with an ionic liquid gel electrolyte, achieving a high areal capacitance of 24.7 mF cm^-2 within a wide voltage window of 2.5 V. The specific layer-rod interlaced structure of Ti₃C₂T_x/MnO₂ is designed to solve the inaccessibility of large-sized ions in ionic liquids into Ti₃C₂T_x layers. As a result, the structure modification provides an enhanced capacitance because the expanded interspace enables a sufficient number of large-sized ions to intercalate/deintercalate. This work provides insightful guidance for the interlaminar modification of Ti₃C₂T_x MXene to accommodate high operating voltage electrolyte with large-sized ions to obtain high-performance MSCs.

Unveiling the Relationship between the Surface Chemistry of Nanoparticles and Ion Transport Properties of the Resulting Composite Electrolytes

Fundamental understanding of the transport properties within nanoparticle composite electrolytes is necessary for the development of next-generation electrochemical devices. Herein, the effect of surface-modified silica nanoparticles with aminophenyl, amide, and sulfonate functional groups (AP-SiO₂, AM-SiO₂, and SU-SiO₂) on the ion transport properties of composite electrolytes is systematically investigated. The competition between surface repulsive and attractive interactions of nanoparticles is reflected in the nature of the morphology and particle network in electrolytes, further affecting the ionic conductivity of electrolytes and diffusion coefficient of ions. The obvious decrease is observed in the AP-SiO₂-based system because of the severe agglomeration of nanoparticles. By contrast, the AM-SiO₂ and SU-SiO₂ form the regular particle network structure and accelerate the salt dissociation in electrolytes, thereby providing an effective ion transport pathway and more mobile ions for conduction, respectively. Consequently, the composite systems with AM-SiO₂ and SU-SiO₂ deliver remarkable enhancement in the ion transport properties.

ZnO and MXenes as electrode materials for supercapacitor devices

Supercapacitor devices are interesting owing to their broad range of applicability from wearable electronics to energy storage in electric vehicles. One of the key parameters that affect the efficiency of supercapacitor devices is selecting the ideal electrode material for a specific application. Regarding this, recently developed metal oxides, specifically nanostructured ZnO, and MXenes with their defect structures, size effects, as well as optical and electronic properties have been presented as electrode material in supercapacitor devices. The discussion of MXenes along with ZnO, although different in chemistry, also highlights the differences in dimensionality when it comes to defect-driven effects, especially in carrier transport. The volume under the influence of the defect centers is expected to be different in bulk and 2D structures, regardless of composition. Hence, analysis and discussion of both materials provide a fundamental understanding regarding the manner in which 2D structures are impacted by defects compared to bulk. Such an approach would therefore serve the scientific community with the material design tools needed to fabricate the next generation of supercapacitor devices.

Hierarchical crumpled NiMn2O4@MXene composites for high rate ion transport electrochemical supercapacitors

MXenes have received great attention due to their excellent features such as metal-like electronic conductivity, hydrophilic surface groups, and high volumetric capacitance. However, many performances of MXenes are still unsatisfactory due to their low energy density and easy horizontal stacking. In this work, an NiMn2O4@MXene composite with a crumpled surface was fabricated by a hydrothermal method and a developed dip-coating method. The maximum specific capacitance of the electrode is about 1.52 times that of NiMn2O4. Besides, it delivers a retention rate of 93.3% after 4000 cycles due to the increased transport of ions and electrons by the crumpled surface. An asymmetrical device based on the crumpled NiMn2O4@MXene composite and AC was also assembled, which shows an ultra-high energy density. This work provides an effective strategy to solve the vertical stacking problem of MXenes, which can open new avenues for large-scale applications of MXenes in energy storage.

Nitrogen and sulfur co-doped vanadium carbide MXene for highly reversible lithium-ion storage

As an emerging group of two-dimensional (2D) layered material, MXenes have received significant attention in the direction of energy storage. However, the restacking of MXene flakes severely hinders the ion transport within electrodes, which limits their application for lithium-ion batteries (LIBs). To address this issue, herein, we rationally designed and optimized the structure of N, S co-doped V₂CT_x MXene, which exhibits excellent electrochemical performance with a high reversible capacity of 590 mAh g^-1 after 100 cycles at 0.1 A g^-1 when used as anode of LIBs. Even at a high current density of 2 A g^-1, a reversible capacity of 298 mAh g^-1 is obtained after 300 cycles, which outperforms most of the V₂CT_x-based anode materials reported so far. The lithium-ion storage mechanism of N, S co-doped V₂CT_x MXene was studied by a series of characterizations. The results show that the significant improvement of electrochemical performance should be attributed to the facilitated charge transfer after N and S co-doping in V₂CT_x MXene, which can effectively improve the ion transfer kinetics during the lithiation-delithiation process. Furthermore, the expanded interlayer spacing of N, S co-doped V₂CT_x provides more active sites for the adsorption of lithium ions, promoting the insertion capacity of lithium ions. This work indicates that the N, S co-doped 2D V₂CT_x MXene should be a promising anode material for high-performance LIBs.

Arbitrary deformable and high-strength electroactive polymer/MXene anti-exfoliative composite films assembled into high performance, flexible all-solid-state supercapacitors

Flexible all-solid-state supercapacitors (ASSSs) are excellent energy storage devices for portable/wearable electronics, although the development of an excellent comprehensive performance film electrode for the extraordinary flexible ASSSs still faces a great challenge. Here, bendable, foldable and anti-exfoliative Ti3C2Tx MXene-based films utilized as supercapacitor electrodes are reported. Polyaniline/Ti3C2Tx composites (i-PANI@Ti3C2Tx) were prepared by in situ oxidant-free polymerization of aniline on Ti3C2Tx nanosheets with p-phenylenediamine (PPD) as an initiator. Lignosulfonate (Lig) and Ti3C2Tx were constructed into a compact composite (Lig@Ti3C2Tx) film based on the hydrogen bonds formed between Lig and Ti3C2Tx. The Lig@Ti3C2Tx/i-PANI@Ti3C2Tx(5/5) hybrid film was produced by vacuum-assisted filtration of the mixed two composite dispersions. The as-prepared films can be arbitrarily deformed (such as bending and folding). They show high tensile strength and vertical-plane (the plane of film) tensile strength with 33.2 and 0.28 MPa for the i-PANI@Ti3C2Tx film, 75.4 and 0.77 MPa for the Lig@Ti3C2Tx film, and 53.7 and 0.58 MPa for the Lig@Ti3C2Tx/i-PANI@Ti3C2Tx(5/5) film (those of Ti3C2Tx film are 17.4 and 0.21 MPa), respectively. The enhanced vertical-plane tensile strength of the as-prepared composite films indicates that the large binding force generated between the Ti3C2Tx nanosheets can effectively prevent the exfoliation of films. The electrodes of the as-prepared i-PANI@Ti3C2Tx, Lig@Ti3C2Tx and Lig@Ti3C2Tx/i-PANI@Ti3C2Tx(5/5) films assembled into symmetric flexible ASSSs can deliver excellent specific capacitances of 310 F g-1 (∼1001 F cm-3), 271 F g-1 (∼881 F cm-3) and 295 F g-1 (∼959 F cm-3), respectively. In addition, the corresponding supercapacitors exhibit ultrahigh energy densities of 34.8, 30.6 and 33.3 W h L-1, respectively. It is expected that the as-prepared MXene-based films can be applied in various fields, such as electromagnetic-interference shielding and batteries. Furthermore, the as-prepared flexible ASSSs can be practically used as a wearable energy storage device.

Fe2O3 Nanoparticles Anchored on the Ti3C2Tx MXene Paper for Flexible Supercapacitors with Ultrahigh Volumetric Capacitance

Ti₃C₂T_x MXene, with high conductivity and flexibility, has drawn great attention in the wearable energy storage devices. However, the easy nanoflake-restacking phenomenon greatly restricts the achievable electrochemical performance of Ti₃C₂T_x-based supercapacitors, in particular volumetric capacitance. Herein, we report a flexible hybrid paper consisting of Fe₂O₃ nanoparticles (NPs) anchored on Ti₃C₂T_x (Fe₂O₃ NPs@MX) via electrostatic self-assembly and annealing treatments. The interlayer spacing of Ti₃C₂T_x nanoflakes is effectively enlarged through the incorporation of Fe₂O₃ NPs, allowing more electrochemical active sites to store charge. Meanwhile, Ti₃C₂T_x nanoflakes form a continuous metallic skeleton and inhibit the volume expansion of Fe₂O₃ NPs during the charging/discharging process, enhancing the cycling stability. The flexible, ultrathin (4.1 μm) Fe₂O₃ NPs@MX hybrid paper shows considerably improved electrochemical performances compared to those of pure Ti₃C₂T_x and Fe₂O₃, including a wide potential window of 1 V, an ultrahigh volumetric capacitance of ∼2607 F cm^-3 (584 F g^-1), and excellent capacitance retention after 13,000 cycles. Besides, the as-assembled symmetric solid-state supercapacitor exhibits an energy density of 29.7 Wh L^-1 and excellent mechanical flexibility. We believe that the present nanostructure design, decorating NPs within a two-dimensional metallic network, has general applicability and could be used to fabricate highly efficient composites for advanced energy storage devices.

True Meaning of Pseudocapacitors and Their Performance Metrics: Asymmetric versus Hybrid Supercapacitors

The development of pseudocapacitive materials for energy-oriented applications has stimulated considerable interest in recent years due to their high energy-storing capacity with high power outputs. Nevertheless, the utilization of nanosized active materials in batteries leads to fast redox kinetics due to the improved surface area and short diffusion pathways, which shifts their electrochemical signatures from battery-like to the pseudocapacitive-like behavior. As a result, it becomes challenging to distinguish "pseudocapacitive" and "battery" materials. Such misconceptions have further impacted on the final device configurations. This Review is an earnest effort to clarify the confusion between the battery and pseudocapacitive materials by providing their true meanings and correct performance metrics. A method to distinguish battery-type and pseudocapacitive materials using the electrochemical signatures and quantitative kinetics analysis is outlined. Taking solid-state supercapacitors (SSCs, only polymer gel electrolytes) as an example, the distinction between asymmetric and hybrid supercapacitors is discussed. The state-of-the-art progress in the engineering of active materials is summarized, which will guide for the development of real-pseudocapacitive energy storage systems.

Intercalating Ultrathin MoO3 Nanobelts into MXene Film with Ultrahigh Volumetric Capacitance and Excellent Deformation for High-Energy-Density Devices

The restacking hindrance of MXene films restricts their development for high volumetric energy density of flexible supercapacitors toward applications in miniature, portable, wearable or implantable electronic devices. A valid solution is construction of rational heterojunction to achieve a synergistic property enhancement. The introduction of spacers such as graphene, CNTs, cellulose and the like demonstrates limited enhancement in rate capability. The combination of currently reported pseudocapacitive materials and MXene tends to express the potential capacitance of pseudocapacitive materials rather than MXene, leading to low volumetric capacitance. Therefore, it is necessary to exploit more ideal candidate materials to couple with MXene for fully expressing both potentials. Herein, for the first time, high electrochemically active materials of ultrathin MoO3 nanobelts are intercalated into MXene films. In the composites, MoO3 nanobelts not only act as pillaring components to prevent restacking of MXene nanosheets for fully expressing the MXene pseudocapacitance in acidic environment but also provide considerable pseudocapacitive contribution. As a result, the optimal M/MoO3 electrode not only achieves a breakthrough in volumetric capacitance (1817 F cm-3 and 545 F g-1), but also maintains good rate capability and excellent flexibility. Moreover, the corresponding symmetric supercapacitor likewise shows a remarkable energy density of 44.6 Wh L-1 (13.4 Wh kg-1), rendering the flexible electrode a promising candidate for application in high-energy-density energy storage devices.

Chemical Functionalisation of 2D Materials by Batch and Continuous Hydrothermal Flow Synthesis

2D materials are single or few layered materials consisting of one or several elements with a thickness of a few nanometres. Their unique, tuneable physical and chemical properties including ease of chemical functionalisation makes this class of materials useful in a variety of technological applications. The feasibility of 2D materials strongly depends on better synthetic approaches to improve properties, increase performance, durability and reduce costs. As such, in the synthesis of nanomaterials, hydrothermal processes are widely adopted through a precursor-product synthesis route. This method includes batch or continuous flow systems, both employing water at elevated temperatures (above boiling point) and pressures to fine-tune the physical, chemical, optical and electronic properties of the nanomaterial. Both techniques yield particles with different morphology, size and surface area due to different mechanisms of particle formation. In this Minireview, we present batch and continuous hydrothermal flow synthesis of a selection of 2D derivatives (graphene, MXene and molybdenum disulfide), their chemical functionalisation as an advantageous approach in exploring properties of these materials as well as the benefits and challenges of employing these processes, and an outlook for further research.

Enhancing Capacitance Performance of Ti3C2Tx MXene as Electrode Materials of Supercapacitor: From Controlled Preparation to Composite Structure Construction

Ti3C2Tx, a novel two-dimensional layer material, is widely used as electrode materials of supercapacitor due to its good metal conductivity, redox reaction active surface, and so on. However, there are many challenges to be addressed which impede Ti3C2Tx obtaining the ideal specific capacitance, such as restacking, re-crushing, and oxidation of titanium. Recently, many advances have been proposed to enhance capacitance performance of Ti3C2Tx. In this review, recent strategies for improving specific capacitance are summarized and compared, for example, film formation, surface modification, and composite method. Furthermore, in order to comprehend the mechanism of those efforts, this review analyzes the energy storage performance in different electrolytes and influencing factors. This review is expected to predict redouble research direction of Ti3C2Tx materials in supercapacitors.

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.

Layer-by-Layer Assembly of Polyaniline Nanofibers and MXene Thin-Film Electrodes for Electrochemical Energy Storage

The growing demand for compact energy storage devices may be met through the use of thin-film microbatteries, which generally rely on charge storage in thin or conformal layers. A promising technique for creating thin-film electrodes is layer-by-layer (LbL) assembly, based on the alternating adsorption of oppositely charged species to a surface to form a nanostructured electrode. Thin-film energy storage devices must have a high energy density within a limited space, so new electrode structures, materials, and assembly methods are important. To this end, both two-dimensional MXenes and polyaniline nanofibers (PNFs) have shown promising energy storage properties. Here, we report on the LbL assembly of positively charged PNFs and negatively charged Ti₃C₂T_x MXenes into hybrid electrodes for thin-film energy storage devices. The successful assembly is demonstrated in which MXenes and PNFs are deposited in films of 49 nm/layer pair thickness. The resulting composition was 77 wt % PNFs and 23 wt % MXenes. The charge storage process was deconvoluted into faradaic/non-faradaic contributions and separated into contributions from PNFs and MXenes. A sandwich cell showed a maximum areal capacity, energy, and power of 17.6 μA h cm^-2, 22.1 μW h cm^-2, and 1.5 mW cm^-2, respectively, for PNF/MXene multilayers of about 2 μm thickness. This work suggests the possibility of using LbL PNF/MXene thin films as electrode materials for thin-film energy storage devices used in next-generation small electronics.

CsPbBr3 Perovskite Nanocrystal Grown on MXene Nanosheets for Enhanced Photoelectric Detection and Photocatalytic CO2 Reduction

All-inorganic CsPbX₃ (X = Cl, Br or I) perovskite nanocrystals have attracted extensive interest recently due to their exceptional optoelectronic properties. In an effort to improve the charge separation and transfer following efficient exciton generation in such nanocrystals, novel functional nanocomposites were synthesized by the in situ growth of CsPbBr₃ perovskite nanocrystals on two-dimensional MXene nanosheets. Efficient excited state charge transfer occurs between CsPbBr₃ NCs and MXene nanosheets, as indicated by significant photoluminescence (PL) quenching and much shorter PL decay lifetimes compared with pure CsPbBr₃ NCs. The as-obtained CsPbBr₃/MXene nanocomposites demonstrated increased photocurrent generation in response to visible light and X-ray illumination, attesting to the potential application of these heterostructure nanocomposites for photoelectric detection. The efficient charge transfer also renders the CsPbBr₃/MXene nanocomposite an active photocatalyst for the reduction of CO₂ to CO and CH₄. This work provides a guide for exploration of perovskite materials in next-generation optoelectronics, such as photoelectric detectors or photocatalyst.

Room-Temperature Liquid Metal Confined in MXene Paper as a Flexible, Freestanding, and Binder-Free Anode for Next-Generation Lithium-Ion Batteries

Exploring flexible lithium-ion batteries is required with the ever-increasing demand for wearable and portable electronic devices. Selecting a flexible conductive substrate accompanying with closely coupled active materials is the key point. Here, a lightweight, flexible, and freestanding MXene/liquid metal paper is fabricated by confining 3 °C GaInSnZn liquid metal in the matrix of MXene paper without any binder or conductive additive. When used as anode for lithium-ion cells, it can deliver a high discharge capacity of 638.79 mAh g^-1 at 20 mA g^-1 . It also exhibits satisfactory rate capacities, with discharge capacities of 507.42, 483.33, 480.22, 452.30, and 404.47 mAh g^-1 at 50, 100, 200, 500, and 1000 mA g^-1 , respectively. The cycling performance is obviously improved by slightly reducing the charge-discharge voltage range. The composite paper also has better electrochemical performance than liquid metal coated Cu foil. This study proposes a novel flexible anode by a clever combination of MXene paper and low-melting point liquid metal, paving the way for next-generation lithium-ion batteries.

Two-dimensional hierarchically porous carbon nanosheets for flexible aqueous supercapacitors with high volumetric capacitance

Two-dimensional carbon nanomaterials can be assembled into a dense film and used for flexible supercapacitors, but the high packing density leads to restacking problems and poor volumetric capacitance. To address this challenge, we report two-dimensional hierarchically porous carbon (2D-HPC) consisting of micro-, meso-, and macro-pores on the 2D sheet, which shows superior capacitance and rate capability compared with a 2D carbon nanosheet consisting predominantly of micro-pores. A flexible supercapacitor fabricated with the 2D-HPC presents a high volumetric capacitance of 412 F cm^-3, a high volumetric energy density of 9.2 mW h cm^-3 and a power density of 120 mW cm^-3.

All Hierarchical Core-Shell Heterostructures as Novel Binder-Free Electrode Materials for Ultrahigh-Energy-Density Wearable Asymmetric Supercapacitors

High-performance fiber-shaped energy-storage devices are indispensable for the development of portable and wearable electronics. Composite pseudocapacitance materials with hierarchical core-shell heterostructures hold great potential for the fabrication of high-performance asymmetric supercapacitors (ASCs). However, few reports concerning the assembly of fiber-shaped ASCs (FASCs) using cathode/anode materials with all hierarchical core-shell heterostructures are available. Here, cobalt-nickel-oxide@nickel hydroxide nanowire arrays (NWAs) and titanium nitride@vanadium nitride NWAs are constructed skillfully with all hierarchical core-shell heterostructures directly grown on carbon nanotube fibers and are shown to exhibit ultrahigh capacity and specific capacitance, respectively. The specific features and outstanding electrochemical performances of the electrode materials are exploited to fabricate an FASC device with a maximum working voltage of 1.6 V, and this device exhibits a high specific capacitance of 109.4 F cm-3 (328.3 mF cm-2) and excellent energy density of 36.0 mWh cm-3 (108.1 µWh cm-2). This work therefore provides a strategy for constructing all hierarchical core-shell heterostructured cathode and anode materials with ultrahigh capacity for the fabrication of next-generation wearable energy-storage devices.

Applications of 2D MXenes in energy conversion and storage systems

This article provides a comprehensive review of MXene materials and their energy-related applications.