Fabrication of Cobaltosic Oxide Nanoparticle-Doped 3 D MXene/Graphene Hybrid Porous Aerogels for All-Solid-State Supercapacitors

MXenes from MAX phases: synthesis, hybridization, and advances in supercapacitor applications

MXenes, which are essentially 2D layered structures composed of transition metal carbides and nitrides obtained from MAX phases, have gained substantial interest in the field of energy storage, especially for their potential as electrodes in supercapacitors due to their unique properties such as high electrical conductivity, large surface area, and tunable surface chemistry that enable efficient charge storage. However, their practical implementation is hindered by challenges like self-restacking, oxidation, and restricted ion transport within the layered structure. This review focuses on the synthesis process of MXenes from MAX phases, highlighting the different etching techniques employed and how they significantly influence the resulting MXene structure and subsequent electrochemical performance. It further highlights the hybridization of MXenes with carbon-based materials, conducting polymers, and metal oxides to enhance charge storage capacity, cyclic stability, and ion diffusion. The influence of dimensional structuring (1D, 2D, and 3D architectures) on electrochemical performance is critically analyzed, showcasing their role in optimizing electrolyte accessibility and energy density. Additionally, the review highlights that while MXene-based supercapacitors have seen significant advancements in terms of energy storage efficiency through various material combinations and fabrication techniques, key challenges like large-scale production, long-term stability, and compatibility with electrolytes still need to be addressed. Future research should prioritize developing scalable synthesis methods, optimizing hybrid material interactions, and investigating new electrolyte systems to fully realize the potential of MXene-based supercapacitors for commercial applications. This comprehensive review provides a roadmap for researchers aiming to bridge the gap between laboratory research and commercial supercapacitor applications.

An Efficient Cathode Catalyst for Rechargeable Zinc-air Batteries based on the Derivatives of MXene@ZIFs

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial processes at the cathode of zinc-air batteries. Developing highly efficient and durable electrocatalysts at the air cathode is significant for the practical application of rechargeable zinc-air batteries. Herein, N-doped layered MX containing Co2P/Ni2P nanoparticles is synthesized by growing CoNi-ZIF on the surface and interlayers of the two-dimensional material MXene (Ti2C3) followed by phosphating calcination. The growth of CoNi-ZIF on the surface of MXene results in the attenuation of high-temperature structural damage of MXene, which in turn leads to the formation of Co2P/Ni2P@MX with a hierarchical configuration, higher electron conductivity, and abundant active sites. The optimized Co2P/Ni2P@MX achieves a half-wave potential of 0.85 V for the ORR and an overpotential of 345 mV for the OER. In addition, DFT calculations were adopted to investigate the mechanism at the atomic and molecular levels. The liquid zinc-air battery with Co2P/Ni2P@MX as the cathode exhibits a specific capacity of 783.7 mAh g-1 and exceeds 280 h (840 cycles) cycle stability, superior to zinc-air batteries constructed by the cathode of commercial Pt/C+RuO2 and other previous works. Furthermore, a solid-state battery synthesized with Co2P/Ni2P@MX as the cathode exhibits stable cycle performance (154 h/462 cycles).

Can Doped-MXene-Based Supercapacitors Be the Game-Changer for Future Energy Landscape? A Critical Perspective

MXene has attracted considerable attention for supercapacitor applications in the past decade owing to its exceptional electrochemical properties. Although major research interests are focused on composite-based MXene, doping engineering of MXene has recently emerged as a promising alternative. This work unveils the potential of doped MXene for supercapacitor applications with a critical perspective. Various doping engineering strategies and synthesis methods adopted are explicitly delineated. Detailed discussions on the optimization of lattice, functionalization, substitution, and interface modification are provided. Further, it sheds light on recent developments with the asssociated mechanism of doped MXene supercapacitors, followed by the associated challenges. Finally, a roadmap for further progress of doped MXene for the realization of advanced and high-performing energy storage systems has been described. We envision that this Perspective will open up new avenues for the further exploration of this domain.

Fabrication of Sandwiched NiCo-Layered Double Hydroxides/Carbon Nanoballs for Sustainable Energy Storage

This study presents a promising method for creating high-performance supercapacitor electrodes. The approach involves crafting a unique composite material-nickel-cobalt-layered double hydroxides (NiCo-LDH) grown on carbon nanoballs (CNBs). This is achieved by first creating a special carbon material rich in oxygen and nitrogen from a polybenzoxazine source. At first, eugenol, ethylene diamine and paraformaldehyde undergo Mannich condensation to form the benzoxazine monomer, which undergoes self-polymerization in the presence of heat to produce polybenzoxazine. This was then carbonized and activated to produce CNBs containing heteroatoms. Then, through a hydrothermal technique, NiCo-LDH nanocages are directly deposited onto the CNBs, eliminating the need for complicated templates. The amount of CNBs used plays a crucial role in performance. By optimizing the CNB content to 50%, a remarkable specific capacitance of 1220 F g-1 was achieved, along with excellent rate capability and impressive cycling stability, retaining 86% of its capacitance after 5000 cycles. Furthermore, this NiCo-LDH/CNB composite, when combined with active carbon in a supercapacitor configuration, delivered outstanding overall performance. The exceptional properties of this composite, combined with its simple and scalable synthesis process, position it as a strong contender for next-generation sustainable energy storage devices. The ease of fabrication also opens doors for its practical application in advancing energy storage technologies.

A durable hydrophobic photothermal membrane based on a honeycomb structure MXene for stable and efficient solar desalination

Solar powered water evaporation is a green and environmentally friendly water treatment technology, which is a hot research topic for water purification at present. Advanced structural design and hydrophilic photothermal materials have achieved efficient solar evaporation of pure water, but the long-term stability of high salinity desalination has become a problem that cannot be ignored in practical applications. In order to solve this problem, a hydrophobic honeycomb structure MXene/AuNFs composite membrane was proposed in this paper, which used the three-dimensional highly porous microstructure of MXene and multibranched structure of gold nanoflowers particles to improve the light absorption and photothermal conversion efficiency of MXene/AuNFs. At the same time, the surface of the composite membrane was modified with hydrophobic fluorosilane 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFTE). The hydrophobic layer can prevent the accumulation of salt particles on the surface of the membrane, so that the composite film can continue to produce water vapor in a high salt environment. With high utilization rate of light energy, multiple-level geometrical structures of MXene for rapid water transport on the filter membrane and salt barrier on the membrane good stability, the hydrophobic MXene/AuNFs achieves solar evaporation rate of 1.59 kg m-2 h-1 and solar conversion efficiency is 97.8%, and stable operation under simulated sea water conditions under one sun irradiation over more than 10 cycles. The hydrophobic MXene/AuNFs membrane proved to be an efficient and stable photothermal material for solar desalination.

Polybenzoxazine-Based Nitrogen-Containing Porous Carbon and Their Composites with NiCo Bimetallic Oxides for Supercapacitor Applications

Supercapacitors (SCs) are considered as emerging energy storage devices that bridge the gap between electrolytic capacitors and rechargeable batteries. However, due to their low energy density, their real-time usage is restricted. Hence, to enhance the energy density of SCs, we prepared hetero-atom-doped carbon along with bimetallic oxides at different calcination temperatures, viz., HC/NiCo@600, HC/NiCo@700, HC/NiCo@800 and HC/NiCo@900. The material produced at 800 °C (HC/NiCo@800) exhibits a hierarchical 3D flower-like morphology. The electrochemical measurement of the prepared materials was performed in a three-electrode system showing an enhanced specific capacitance for HC/NiCo@600 (Cs = 1515 F g-1) in 1 M KOH, at a current density of 1 A g-1, among others. An asymmetric SC device was also fabricated using HC/NiCo@800 as anode and HC as cathode (HC/NiCo@600//HC). The fabricated device had the ability to operate at a high voltage window (~1.6 V), exhibiting a specific capacitance of 142 F g-1 at a current density of 1 A g-1; power density of 743.11 W kg-1 and energy density of 49.93 Wh kg-1. Altogether, a simple strategy of hetero-atom doping and bimetallic inclusion into the carbon framework enhances the energy density of SCs.

Progress of 2D MXene as an Electrode Architecture for Advanced Supercapacitors: A Comprehensive Review

Supercapacitors, designed to store more energy and be proficient in accumulating more energy than conventional batteries with numerous charge-discharge cycles, have been developed in response to the growing demand for energy. Transition metal carbides/nitrides called MXenes have been the focus of researchers' cutting-edge research in energy storage. The 2D-layered MXenes are a hopeful contender for the electrode material due to their unique properties, such as high conductivity, hydrophilicity, tunable surface functional groups, better mechanical properties, and outstanding electrochemical performance. This newly developed pseudocapacitive substance benefits electrochemical energy storage because it is rich in interlayer ion diffusion pathways and ion storage sites. Making MXene involves etching the MAX phase precursor with suitable etchants, but different etching methods have distinct effects on the morphology and electrochemical properties. It is an overview of the recent progress of MXene and its structure, synthesis, and unique properties. There is a strong emphasis on the effects of shape, size, electrode design, electrolyte behavior, and other variables on the charge storage mechanism and electrochemical performance of MXene-based supercapacitors. The electrochemical application of MXene and the remarkable research achievements in MXene-based composites are an intense focus. Finally, in light of further research and potential applications, the challenges and future perspectives that MXenes face and the prospects that MXenes present have been highlighted.

Implementation of High-Capacity 3D Ti3C2TX MXene Supercapacitors with Terminal Group Modification

MXene is a highly latent capacity electrode material for supercapacitors, but its capacity limits its development. Herein, we have constructed an independently cross-linked three-dimensional (3D) Ti3C2TX MXene film (Zn-A-MXene) with a hydroxylation surface through a zinc ion (Zn2+) and NaOH. The alkalization of NaOH is used to replace the -F functional group that is not conducive to electrochemical reactions and cross-link the MXene nanosheets through the electrostatic interaction of zinc ions. The synergistic effect can greatly improve the effective area of the electrode, the accessibility of the electrolyte, and the specific capacitance. The 3D Zn-A-MXene films exhibit an extremely high capacity (465.1 F g-1 at 1 A g-1). The all-solid-state flexible supercapacitor assembled using a 3D Zn-A-MXene thin film also has a high energy density of 9.55 Wh kg at a power density of 603.16 W kg. After 5000 cycles, the flexible supercapacitor still has 81.25% of its initial capacity, demonstrating good cycling stability. This work furnishes the innovative idea for constructing high-capacity MXene flexible supercapacitors.

Fabrication, Performance, and Potential Applications of MXene Composite Aerogels

Aerogel, known as one of the remarkable materials in the 21st century, possesses exceptional characteristics such as high specific surface area, porosity, and elasticity, making it suitable for a diverse range of applications. In recent years, MXene-based aerogels and MXene composite aerogels as functional materials have solved some limitations of traditional aerogels, such as improving the electrical conductivity of biomass and silicon aerogels, further improving the energy storage capacity of carbon aerogels, enhancing polymer-based aerogels, etc. Consequently, extensive research efforts have been dedicated to investigating MXene-based aerogels, positioning them at the forefront of material science studies. This paper provides a comprehensive review of recent advancements in the preparation, properties, and applications of MXene-based composite aerogels. The primary construction strategies employed (including direct synthesis from MXene dispersions and incorporation of MXene within existing substrates) for fabricating MXene-based aerogels are summarized. Furthermore, the desirable properties (including their applications in electrochemistry, electromagnetic shielding, sensing, and adsorption) of MXene composite aerogels are highlighted. This paper delves into a detailed discussion on the fundamental properties of composite aerogel systems, elucidating the intricate structure-property relationships. Finally, an outlook is provided on the opportunities and challenges for the mass production and functional applications of MXene composite aerogels in the field of material engineering.

Ti3C2T x MXenes-based flexible materials for electrochemical energy storage and solar energy conversion

Over the past decade, two-dimensional (2D) Ti3C2T x MXenes demonstrated attractive characteristics such as high electrical conductivity, tunable layered structure, controllable interfacial chemical composition, high optical transparency, and excellent electromagnetic wave absorption, enabling Ti3C2T x MXenes as promising electrode materials in energy storage devices. Among these devices, flexible energy storage devices have attracted wide attention and developed rapidly due to the synchronously excellent electrochemical and mechanical properties. This review summarizes the recent progress of Ti3C2T x MXenes pertaining to novel material preparation and promising applications in energy storage and conversion including batteries, supercapacitors, solar cells, and solar steam generation. This work aims to provide an in-depth and reasonable understanding of the relationship between the unique nanostructure/chemical composition of Ti3C2T x MXenes and competitive electrochemical properties, which will facilitate the development of 2D Ti3C2T x MXenes for practical energy storage and solar energy conversion devices.

Recent Trends in Synthesis and Applications of porous MXene Assemblies: A Topical Review

MXene possesses high conductivity, excellent hydrophilicity, rich surface chemistry, hence holds great potential in various applications. However, MXene materials have low surface area utilization due to the agglomeration of ultrathin nanosheets. Assembling 2D MXene nanosheets into 3D multi-level architectures is an effective way to circumvent this issue. Incorporation of MXene with other nanomaterials during the assembly process could rationally tune and tailor the specific surface area, porosity and surface chemistry of the MXene assemblies. The complementary and synergistic effect between MXene and nanomaterials could expand their advantages and make up for their disadvantages, thus boost the performance of 3D porous MXene composites. Herein, we summarize the recent progress in fabrication of porous MXene architectures from 2D to 3D, and also discuss the potential applications of MXene nanostructures in energy harvesting systems, sensing, electromagnetic interference shielding, water purification and photocatalysis.

Mn3-x Fex O4 Hollow Nanostructures for High-Performance Asymmetric Supercapacitor Applications

Design of hollow nanostructure and controllable phase of mixed metal oxides for improving performance in supercapacitor applications is highly desirable. Here we demonstrate the rational design and synthesis of Mn_3-x Fe_x O₄ hollow nanostructures for supercapacitor applications. Owing to high porosity and the specific surface area that provides more active sites for electrochemical reactions, the electrochemical performance of Mn_3-x Fe_x O₄ hollow nanostructure substantially enhanced comparing with pristine Mn₃ O₄ . Particularly, in 1.0 M KOH electrolyte, Mn_0.16 Fe_2.84 O₄ with a typical diameter of 20 nm exhibits excellent specific capacitance of 2675, 2320, 1662, 987 F g^-1 at current densities of 1, 2, 5, 10 A g^-1 , respectively, which is significantly superior to those of other transition metal oxides. Besides, an asymmetric supercapacitor is assembled by using Mn_0.16 Fe_2.84 O₄ and activated carbon as a positive and a negative electrode, respectively. Electrochemical results indicate a high energy density of 42 Wh kg^-1 at a power density of 0.75 kW kg^-1 , which makes this hollow nanostructure a highly promising electrode for achieving high-performance next-generation supercapacitors.

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.

Supercapacitive brophene-graphene aerogel as elastic-electrochemical dielectric layer for sensitive pressure sensors

A sensitive pressure sensor based on ultralight and superelastic supercapacitive borophene-graphene aerogel as dielectric layer is reported. The borophene-graphene aerogel not only combines large specific surface area of reduced graphene oxide and high conductivity of borophene, but also exhibits rich porous structure. The strong synergy and intercalation between two different two-dimensional materials benefit electron transfer and electrolyte ion diffusion. On the one hand, the aerogel exhibits greater mass specific capacitance of 330 F g^-1 than pure graphene aerogel. More importantly, serving as dielectric layer for pressure sensors with a symmetrical structure, the sensor represents ultra-high sensitivity (0.90 KPa^-1) in the pressure range (<3 KPa), ultra-rapid response time (~110 ms), ultra-low detection limit as 8.7 Pa and excellent working stability after 1000 cycles. In practical application, the sensor demonstrates great performance in monitoring human physiological signals, and agricultural applications.

In situ synthesis of oxidized MXene-based metal cobalt spinel nanocomposites for an excellent promotion in thermal decomposition of ammonium perchlorate

Oxidized MXene-supported MCo2O4 (oxidized MXene/MCo2O4, M = Mn, Zn, Cu and Co) nanocomposites with an excellent catalytic performance for AP decomposition were successfully synthesized through a facile hydrothermal assisted calcination method.

Preparation of CoS2 supported flower-like NiFe layered double hydroxides nanospheres for high-performance supercapacitors

Layered double hydroxides (LDHs) are a kind of classic pseudocapacitive materials with lamellar structure and large specific surface area, which have attracted swinging attention in the electrochemical energy storage area. The CoS₂@Ni is synthesized through a hydrothermal process, followed by surface generation of the flower-like nickel-iron layered double hydroxide (NiFe-LDH) nanospheres through a hydrothermal process, which is directly used to design a binder-free electrode with a splendid capacitance capability. The as-synthesized NiFe-LDH@CoS₂@Ni electrode presents an outstanding specific capacitance of 11.28 F cm^-2 (3880 F g^-1) at 2 mA cm^-2 (1.17 A g^-1) in a three electrodes system. Also, the all-solid-state asymmetric supercapacitor (ASC) is combined utilizing the NiFe-LDH@CoS₂@Ni hybrid as the positive electrodes and active carbon covered Ni foam as negative electrodes, respectively. The as-fabricated ASC exhibits a high energy density of 15.84 Wh kg^-1 at the power density of 375.16 W kg^-1 and can be able to lighten a blue LED indicator for more than 30 min, revealing that the prepared NiFe-LDH@CoS₂@Ni owns great potential in the aspect of practical applications. Therefore, the prepared NiFe-LDH@CoS₂@Ni with outstanding electrochemical properties could be applied for high-performance supercapacitors.

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.

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.

Hollow 3D Frame Structure Modified with NiCo2 S4 Nanosheets and Spinous Fe2 O3 Nanowires as Electrode Materials for High-Performance All-Solid-State Asymmetric Supercapacitors

Supercapacitors have attracted tremendous research interest, since they are expected to achieve battery-level energy density, while having a long calendar life and short charging time. Herein, a novel asymmetric supercapacitor has been successfully assembled from NiCo₂ S₄ nanosheets and spinous Fe₂ O₃ nanowire modified hollow melamine foam decorated with polypyrrole as positive and negative electrodes, respectively. Owing to the well-designed nanostructure and suitable matching of electrode materials, the assembled asymmetric supercapacitor (ASC) exhibits an extended operation voltage window of 1.6 V with an energy density of 20.1 Wh kg^-1 at a power density of 159.4 kW kg^-1 . Moreover, the ASC shows stable cycling stability, with 81.3 % retention after 4000 cycles and a low internal resistance of 1.03 Ω. Additionally, a 2.5 V light-emitting diode indicator can be lit up by three ASCs connected in series; this provides evidence of the practical application potential of the assembled energy-storage system. The excellent electrochemical performances should be credited to the significant enhancement of the specific surface area, charge transport, and mechanical stability resulting from the unique 3D morphology.

Self-Healable Polyelectrolytes with Mechanical Enhancement for Flexible and Durable Supercapacitors

The practical application of advanced personalized electronics is inseparable from flexible, durable, and even self-healable energy storage devices. However, the mechanical and self-healing performance of supercapacitors is still limited at present. Herein, highly transparent, stretchable, and self-healable poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA)/poly(vinyl alcohol) (PVA)/LiCl polyelectrolytes were facilely prepared by one-step radical polymerization. The cooperation of PAMPSA and PVA significantly increased the mechanical and self-healing capacity of the polyelectrolyte, which exhibited superior stretchability of 938 %, stress of 112.68 kPa, good electrical performance (ionic conductivity up to 20.6 mS cm^-1 ), and high healing efficiency of 92.68 % after 24 h. After assembly with polypyrrole-coated single-walled carbon nanotubes, the resulting as-prepared supercapacitor had excellent electrochemical properties with high areal capacitance of 297 mF cm^-2 at 0.5 mA cm^-2 and good rate capability (218 mF cm^-2 at 5 mA cm^-2 ). Besides, after cutting in two the supercapacitor recovered 99.2 % of its original specific capacitance after healing for 24 h at room temperature. The results also showed negligible change in the interior contact resistance of the supercapacitor after ten cutting/healing cycles. The present work provides a possible solution for the development of smart and durable energy storage devices with low cost for next-generation intelligent electronics.