Two-Dimensional Mn3O4 Nanowalls Grown on Carbon Fibers as Electrodes for Flexible Supercapacitors

Cf/SiC Ceramic Matrix Composites with Extraordinary Thermomechanical Properties up to 2000 °C

The thermomechanical properties of carbon fiber reinforced silicon carbide ceramic matrix composites (Cf/SiC CMCs) were studied up to 2000 °C using high-temperature in situ flexural testing in argon. The CMC specimens were fabricated using an ultrahigh concentration (66 vol%) aqueous slurry containing nano-sized silicon carbide powder. The SiC powder compacts were obtained by drying the slurry and were densified using the precursor impregnation and pyrolysis (PIP) method with field assisted sintering technology/spark plasma sintering (FAST/SPS). The high relative density of the SiC green body (77.6%) enabled densification within 2.5 days using four PIP cycles. In contrast, conventional PIP processes take over 7 days. The in situ flexural strength of the Cf/SiC CMC was 434 MPa at 1750 °C, which was 84% higher than the room temperature value. The value further increased to 542 MPa at 2000 °C. Possible mechanisms to explain the excellent strength of the CMC at elevated temperatures are discussed.

Elucidating the Role of Copper-Induced Mixed Phases on the Electrochemical Performance of Mn-Based Thin-Film Electrodes

Manganese oxide is a fascinating material for use as a thin-film electrode in supercapacitors. Herein, the consequences of copper incorporation on spray pyrolyzed manganese oxide thin films and their electrochemical performance were investigated. The Cu-incorporated manganese oxide thin films were deposited by spray pyrolysis, and their structural and electrochemical properties were thoroughly evaluated. The formation of the spinel Mn3O4 phase with effective Cu incorporation was confirmed by X-ray diffraction investigation. Through Raman studies, it was noticed that mixed phases of manganese oxide tend to form after Cu incorporation, and this result was also reflected in X-ray photoelectron spectroscopic studies. The surface morphology and roughness were also altered by the addition of copper. However, electrochemical measurements implied a reduction in the specific capacitance upon copper inclusion. The cyclic voltammetry test indicated a specific capacitance of 132 F/g for Mn3O4 electrodes, but a substantial drop for copper-incorporated samples due to the mixed manganese phase. The decremental tendency was further supported by galvanostatic charge-discharge studies and electrochemical impedance spectroscopic measurements. These results provide valuable insights into the effects of copper addition in manganese oxide thin-film-based electrodes for energy storage applications.

Road Map for In Situ Grown Binder-Free MOFs and Their Derivatives as Freestanding Electrodes for Supercapacitors

Among several electrocatalysts for energy storage purposes including supercapacitors, metal-organic frameworks (MOFs), and their derivatives have spurred wide spread interest owing to their structural merits, multifariousness with tailor-made functionalities and tunable pore sizes. The electrochemical performance of supercapacitors can be further enhanced using in situ grown MOFs and their derivatives, eliminating the role of insulating binders whose "dead mass" contribution hampers the device capability otherwise. The expulsion of binders not only ensures better adhesion of catalyst material with the current collector but also facilitates the transport of electron and electrolyte ions and remedy cycle performance deterioration with better chemical stability. This review systematically summarizes different kinds of metal-ligand combinations for in situ grown MOFs and derivatives, preparation techniques, modification strategies, properties, and charge transport mechanisms as freestanding electrode materials in determining the performance of supercapacitors. In the end, the review also highlights potential promises, challenges, and state-of-the-art advancement in the rational design of electrodes to overcome the bottlenecks and to improve the capability of MOFs in energy storage applications.

Energized Composites for Electric Vehicles: A Dual Function Energy-Storing Supercapacitor-Based Carbon Fiber Composite for the Body Panels

The current electric vehicles (EVs) face many challenges like limited charge capacity, low miles/charge, and long charging times. Herein, these issues are addressed by developing a dual-function supercapacitor-based energy-storing carbon fiber reinforced polymer (e-CFRP) that can store electrical energy and function as the structural component for the EV's body shell. This is achieved by developing a unique design, vertically aligned graphene sheets attached to carbon fiber electrodes on which different metal oxides are deposited to obtain high-energy density electrodes. A high-strength multilayer e-CFRP assembly is fabricated using an alternate layer patterning configuration of epoxy and polyacrylamide gel electrolyte. The e-CFRP so developed delivers a high areal energy density of 0.31 mWh cm^-2 at 0.3 mm thickness and a high tensile strength of 518 MPa, bending strength of 477 MPa, and impact strength of 2666 J m^-1 . To show its application in EVs, a toy car's body panel is fabricated with e-CFRP and the toy car is able to operate using the energy stored in its frame. Moreover, when integrated with a solar cell, this composite powers an Internet of Things device, showing its feasibility in communication satellites.

Flexible free-standing Ni-Mn oxide antenna decorated CNT/nanofiber membrane for high-volumetric capacitance supercapacitors

There is growing demand for lightweight flexible supercapacitors with high electrochemical performance for wearable and portable electronics. Here, we spun nanoparticles of nickel-manganese oxides along with carbon nanotubes into carbon nanofibers and engineered a 3D networked Ni-Mn oxides/CNT@CNF free-standing membrane for flexible supercapacitor applications. The electrospinning process controlled the nanoparticle aggregation while subsequent heat treatment generates nanochannels in the fibres, resulting in a very porous tubular nanocomposite structure. The preparation process also enabled good interfacial contact between the nanoparticles and the conductive carbon network. The resulting Ni-Mn oxides/CNT@CNF membrane displays high mass loading (Ni-Mn oxides) of 855 mg cm^-3 and low CNT incorporation of ∼0.4%. The outstanding porous structure, synergy of the carbon with Ni-Mn oxides, and fast and facile faradaic reactions on the electrode were responsible for the superior volumetric capacitance of 250 F cm^-3 at 1 A cm^-3, energy density as high as 22 mW h cm^-3 and an excellent power density of 12 W cm^-3. Despite the low CNT loading, the hybrid electrode exhibits excellent cycling performance with capacitance retention of 96.4% after 10 000 cycles evidencing a well-preserved Ni-manganese oxide nanostructure throughout the cycling. The resulting outstanding electrochemical performances of the Ni-Mn oxides/CNT@CNF synergic system offer new insights into effective utilization of transition metal oxides for establishing high-performance flexible supercapacitors within a confined volume.

MnOx-Electrodeposited Fabric-Based Stretchable Supercapacitors with Intrinsic Strain Sensing

The increasing number of devices needed by wearable systems to bring radical advances in healthcare, robotics, and human-machine interfaces is a threat to their growth if the integration and energy-related challenges are not managed. A natural solution is to reduce the number of devices while retaining the functionality or simply using multifunctional devices, as demonstrated here through a stretchable supercapacitor (SSC) with intrinsic strain sensing. The presented SSC was obtained by electrodeposition of nanoflower MnO_x on fabric (as a pseudocapacitive electrode) and three-dimensional conductive wrapping of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) to boost the performance. Among fabricated devices, the stretchable PEDOT:PSS/MnO_x/PEDOT:PSS supercapacitor (SPMP-SC) showed the best performance (specific capacitance of 580 mF·cm^-2 (108.1 F·g^-1); energy density of 51.4 μWh·cm^-2 at 0.5 mA). The stretchability (0-100%; 1000 cycles) analysis of SPMP-SC with Ecoflex encapsulation showed high capacitance retention (>90% for 40% stretch). The intrinsic strain sensing of the SSC was confirmed by the linear variation of capacitance (sensitivity -0.4%) during stretching. Finally, as a proof-of-concept, the application of SSC with intrinsic sensing was demonstrated for health monitoring through volumetric expansion of a manikin during ventilator operation and in robotics and by measuring the joint angle of a robotic hand.

Transition metal dichalcogenide (TMDs) electrodes for supercapacitors: a comprehensive review

As globally, the main focus of the researchers is to develop novel electrode materials that exhibit high energy and power density for efficient performance energy storage devices. This review covers the up-to-date progress achieved in transition metal dichalcogenides (TMDs) (e.g. MoS₂, WS₂, MoSe_2,and WSe₂) as electrode material for supercapacitors (SCs). The TMDs have remarkable properties like large surface area, high electrical conductivity with variable oxidation states. These properties enable the TMDs as the most promising candidates to store electrical energy via hybrid charge storage mechanisms. Consequently, this review article provides a detailed study of TMDs structure, properties, and evolution. The characteristics technique and electrochemical performances of all the efficient TMDs are highlighted meticulously. In brief, the present review article shines a light on the structural and electrochemical properties of TMD electrodes. Furthermore, the latest fabricated TMDs based symmetric/asymmetric SCs have also been reported.

High-performance flexible asymmetric supercapacitor based on rGO anode and WO3/WS2 core/shell nanowire cathode

Flexible smart electronics require their energy storage device to be flexible in nature. Developing high-performance flexible energy storage devices require direct integration of electrode active materials on current collectors to satisfy the high electronic/ionic conductivity and long-term durability requirements. Herein, we develop a flexible all-solid-state asymmetric supercapacitor comprised of reduced graphene oxide (rGO) and core/shell tungsten trioxide/tungsten disulfide (WO₃/WS₂) nanowire based electrodes. The electrodes synthesized via electrochemical deposition and chemical vapor deposition avoided the necessity to use non-conductive binders and offered excellent cyclic stability. The structural integrity provided by the rGO and WO₃/WS₂ electrodes facilitated excellent electrochemical stability with capacitance retention of 90% and 100% after 10 000 charge-discharge cycles, respectively. An all-solid-state device provides a voltage window of 1.5 V and more than 70% capacitance retention after 10 000 charge-discharge cycles. Providing 97% capacitance retention upon mechanical bending reveals its potential to be used as an energy storage devices in flexible electronics.

Flexible supercapacitor electrodes using metal-organic frameworks

Advancements in the field of flexible and wearable devices require flexible energy storage devices to cater their power demands. Metal-ion batteries (such as lithium-ion batteries, sodium-ion batteries, etc.) and electrochemical capacitors (also called supercapacitors or ultracapacitors) have achieved great interest in the recent past due to their superior energy storage characteristics like high power density and long cycle life. A major bottleneck of using metal-ion batteries in wearable devices is their lack of flexibility. Low power density, toxicity and flammability due to organic electrolytes inhibit them from safe on-body device applications. On the other hand, supercapacitors can be made with aqueous electrolytes, making them a safer alternative for wearable applications. Metal-organic frameworks (MOFs) are novel candidates as electrode materials due to their salient features such as large surface area, three-dimensional porous architecture, permeability to foreign entities, structural tailorability, etc. Though pristine MOFs suffer from poor intrinsic conductivity, this can be rectified by preparing composites with other electronically conducting materials. MOF-based electrodes are highly promising for flexible and wearable supercapacitors since they exhibit good energy and power densities. This review focuses on the new developments in the field of MOF-based composite electrodes for developing flexible supercapacitors.

Electrode Materials for Supercapacitors: A Review of Recent Advances

The advanced electrochemical properties, such as high energy density, fast charge–discharge rates, excellent cyclic stability, and specific capacitance, make supercapacitor a fascinating electronic device. During recent decades, a significant amount of research has been dedicated to enhancing the electrochemical performance of the supercapacitors through the development of novel electrode materials. In addition to highlighting the charge storage mechanism of the three main categories of supercapacitors, including the electric double-layer capacitors (EDLCs), pseudocapacitors, and the hybrid supercapacitors, this review describes the insights of the recent electrode materials (including, carbon-based materials, metal oxide/hydroxide-based materials, and conducting polymer-based materials, 2D materials). The nanocomposites offer larger SSA, shorter ion/electron diffusion paths, thus improving the specific capacitance of supercapacitors (SCs). Besides, the incorporation of the redox-active small molecules and bio-derived functional groups displayed a significant effect on the electrochemical properties of electrode materials. These advanced properties provide a vast range of potential for the electrode materials to be utilized in different applications such as in wearable/portable/electronic devices such as all-solid-state supercapacitors, transparent/flexible supercapacitors, and asymmetric hybrid supercapacitors.

New Method for the Synthesis of 2D Vanadium Nitride (MXene) and Its Application as a Supercapacitor Electrode

MXenes are the class of two-dimensional transition metal carbides and nitrides that exhibit unique properties and are used in a multitude of applications such as biosensors, water purification, electromagnetic interference shielding, electrocatalysis, supercapacitors, and so forth. Carbide-based MXenes are being widely explored, whereas investigations on nitride-based ones are seldom. Among the nitride-based MXenes obtained from their MAX phases, only Ti₄N₃ and Ti₂N are reported so far. Herein, we report a novel synthesis of V₂NT _x (T _x is the surface termination) obtained by the selective removal of "Al" from V₂AlN by immersing powders of V₂AlN in the LiF-HCl mixture (salt-acid etching) followed by sonication to obtain V₂NT _x (T _x = -F, -O) MXene which is then delaminated using the dimethyl sulfoxide solvent. The V₂NT _x MXene is characterized by X-ray diffraction studies, field emission scanning electron microscope imaging, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscope imaging. Supercapacitor electrodes are prepared using V₂NT _x MXenes and their electrochemical performances are examined by cyclic voltammetry, galvanostatic charge/discharge measurement, and electrochemical impedance spectroscopy. The V₂NT _x MXene electrode exhibits a specific capacitance of 112.8 F/g at a current density of 1.85 mA/cm² with an energy and power density of 15.66 W h/kg and 3748.4 W/kg, respectively, in 3.5 M KOH aqueous electrolyte. The electrode exhibits an excellent capacitance retention of 96% even after 10,000 charge/discharge cycles. An asymmetric supercapacitor fabricated with V₂NT _x as a negative electrode and Mn₃O₄ nanowalls as a positive electrode helps obtain a cell voltage of 1.8 V in aqueous KOH electrolyte.

Formation of Ordered Honeycomb-like Structures of Manganese Oxide 2D Nanocrystals with the Birnessite-like Structure and Their Electrocatalytic Properties during Oxygen Evolution Reaction upon Water Splitting in an Alkaline Medium

In this work, a chemical reaction between gaseous ozone and aqueous solution of Mn(CH₃COO)₂ in drops has been researched. It has been shown that the formation of H _x MnO₂·nH₂O nanocrystals with a morphology of nanosheets and a birnessite-like crystal structure with a thickness of 5-8 nm is observed on the surface of drops. These nanocrystals are oriented spontaneously to the solution-gas interface and constitute peculiar ribbons with a width of 1-2 μm, some of which form ordered honeycomb structures (OHS) with a 5-20 μm cell size. To explain the observed effect, the scheme of chemical reactions that take place at the interface between the surface of a drop and ozone has been modeled, and it can be described using a diffusion pattern model taking into account the action of "force fields" on the surface of a drop, which arise due to its curvature. After the drop is dried, these structures practically retain their morphology and form a fractal structure with a geometric area equal to the area of the drop base on the surface of the substrate. The study of the electrocatalytic properties of these structures revealed that they are active electrocatalysts in the oxygen evolution reaction (OER) during water electrolysis in alkaline medium. The most efficient of the obtained electrocatalysts are characterized by an overpotential value of 284 mV at a current of 10 mA/cm² and the Tafel coefficient of 37.7 mV/dec and are currently one of the best among pure manganese oxides. Finally, it has also been assumed that this effect is explained by the morphological features of the structures obtained, which contribute to the removal of oxygen bubbles from the electrode surface during electrolysis.

Agar Hydrogel Template Synthesis of Mn₃O₄ Nanoparticles through an Ion Diffusion Method Controlled by Ion Exchange Membrane and Electrochemical Performance

A novel strategy, ion diffusion method controlled by ion exchange membrane combining with agar hydrogel template, was reported for the synthesis of Mn₃O₄ nanoparticles without any oxidizing agents. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Brunauere-Emmette-Teller (BET) isotherm were carried out to characterize the structure, morphology, pore size and distribution and specific surface area of the as-prepared nanomaterials. It is shown that the morphology and size of Mn₃O₄ nanoparticles can be controlled by the concentration of agar hydrogel. All the specific capacitances of the Mn₃O₄ samples prepared with agar hydrogel template are much higher than that of Mn₃O₄ prepared without any template agent. The Mn₃O₄ sample prepared at 1.5 g L-1 of agar hydrogel solution exhibits a highest specific capacitance of 183.0 F g-1 at the current density of 0.5 A g-1, which is increased by 293% compared with that of Mn₃O₄ synthesized without any template agent. The results indicate that the ion diffusion method controlled by ion exchange membrane combining with agar hydrogel template is a convenient and effective approach for preparing inorganic nanomaterials.