Supercapacitors: An Efficient Way for Energy Storage Application

To date, batteries are the most widely used energy storage devices, fulfilling the requirements of different industrial and consumer applications. However, the efficient use of renewable energy sources and the emergence of wearable electronics has created the need for new requirements such as high-speed energy delivery, faster charge-discharge speeds, longer lifetimes, and reusability. This leads to the need for supercapacitors, which can be a good complement to batteries. However, one of their drawbacks is their lower energy storage capability, which has triggered worldwide research efforts to increase their energy density. With the introduction of novel nanostructured materials, hierarchical pore structures, hybrid devices combining these materials, and unconventional electrolytes, significant developments have been reported in the literature. This paper reviews the short history of the evolution of supercapacitors and the fundamental aspects of supercapacitors, positioning them among other energy-storage systems. The main electrochemical measurement methods used to characterize their energy storage features are discussed with a focus on their specific characteristics and limitations. High importance is given to the integral components of the supercapacitor cell, particularly to the electrode materials and the different types of electrolytes that determine the performance of the supercapacitor device (e.g., storage capability, power output, cycling stability). Current directions in the development of electrode materials, including carbonaceous forms, transition metal-based compounds, conducting polymers, and novel materials are discussed. The synergy between the electrode material and the current collector is a key factor, as well as the fine-tuning of the electrode material and electrolyte.

Conducting Interface for Efficient Growth of Vertically Aligned Carbon Nanotubes: Towards Nano-Engineered Carbon Composite

Vertically aligned carbon nanotubes (VACNT) are manufactured nanomaterials with excellent properties and great potential for numerous applications. Recently, research has intensified toward achieving VACNT synthesis on different planar and non-planar substrates of various natures, mainly dependent on the user-defined application. Indeed, VACNT growth has to be adjusted and optimized according to the substrate nature and shape to reach the requirements for the application envisaged. To date, different substrates have been decorated with VACNT, involving the use of diffusion barrier layers (DBLs) that are often insulating, such as SiO₂ or Al₂O₃. These commonly used DBLs limit the conducting and other vital physico-chemical properties of the final nanomaterial composite. One interesting route to improve the contact resistance of VACNT on a substrate surface and the deficient composite properties is the development of semi-/conducting interlayers. The present review summarizes different methods and techniques for the deposition of suitable conducting interfaces and controlled growth of VACNT on diverse flat and 3-D fibrous substrates. Apart from exhibiting a catalytic efficiency, the DBL can generate a conducting and adhesive interface involving performance enhancements in VACNT composites. The abilities of different conducting interlayers are compared for VACNT growth and subsequent composite properties. A conducting interface is also emphasized for the synthesis of VACNT on carbonaceous substrates in order to produce cost-effective and high-performance nano-engineered carbon composites.

Vertically Aligned Graphene-Carbon Fiber Hybrid Electrodes with Superlong Cycling Stability for Flexible Supercapacitors

Graphene electrode-based supercapacitors are in high demand due to their superior electrochemical characteristics. A major bottleneck of using the supercapacitors for commercial applications lies in their inferior electrode cycle life. Herein, a simple and facile method to fabricate highly efficient supercapacitor electrodes using pristine graphene sheets vertically stacked and electrically connected to the carbon fibers which can result in vertically aligned graphene-carbon fiber nanostructure is developed. The vertically aligned graphene-carbon fiber electrode prepared by electrophoretic deposition possesses a mesoporous 3D architecture which enabled faster and efficient electrolyte-ion diffusion with a gravimetric capacitance of 333.3 F g^-1 and an areal capacitance of 166 mF cm^-2 . The electrodes displayed superlong electrochemical cycling stability of more than 100 000 cycles with 100% capacitance retention hence promising for long-lasting supercapacitors. Apart from the electrochemical double layer charge storage, the oxygen-containing surface moieties and α-Ni(OH)₂ present on the graphene sheets enhance the charge storage by faradaic reactions. This enables the assembled device to provide an excellent gravimetric energy density of 76 W h kg^-1 with a 100% capacitance retention even after 1000 bending cycles. This study opens the door for developing high-performing flexible graphene electrodes for wearable energy storage applications.

Achieving Rich Mixed-Valence Polysulfide/Carbon Nanotube Films toward Ultrahigh Volume Energy Density and Largely Deformable Pseudocapacitors

In this work, new insights into dependence of electrochemical performance enhancement on transition metals' rich mixed valence and their atomic ratio as well as redox active polysulfides are proposed. Especially, the influence of atomic ratio is further demonstrated by both experiments and density functional theoretical calculation where increasing Co/S leads to the enlargement of both interatom distance and hole diameter in a Mn_xCo_yS_z cell. We rationally designed and prepared novel flexible electrodes of a rich mixed-valence polysulfide Mn_xCo_yS_z/carbon nanotube film (CNTF) through acid activation of a dense CNTF into a hydrogel-like conductive matrix, growth of the Mn_xCo_y(CO₃)_0.5OH precursor on each CNT, and controlled sulfidation. Nanostructure control allows us to obtain fast electron/ion transfer and increased availability of active sites/interfaces. The optimal MnCo₉S₁₀/CNTF shows a specific capacitance reaching 450 F cm^-3 at 10 mA cm^-2, much higher than reported values for CNT-based electrodes. Also, it exhibits remarkable cycling stability with only 1.6% capacity loss after 10 000 cycles at a high current density of 80 mA cm^-2. An all-solid-state asymmetric supercapacitor (ASC) applying MnCo₉S₁₀/CNTF delivers an exceptionally high volumetric energy density of 67 mW h cm^-3 (at 10 W cm^-3). Particularly, integrated electric sources with adjustable output voltages can be obtained by connecting several ASCs in series, and there are no structural failure and capacity loss during repeated large-angle twisting and vigorous hammering. This work provides a general route to energy storage devices with ultrahigh volumetric energy density and outstanding reliability for wearable electronics.

Novel mesoporous electrode materials for symmetric, asymmetric and hybrid supercapacitors

Electrochemical capacitors or supercapacitors have achieved great interest in the recent past due to their potential applications ranging from microelectronic devices to hybrid electric vehicles. Supercapacitors can provide high power densities but their inherently low energy density remains a great challenge. The high-performance supercapacitors utilize large electrode surface area for electrochemical double-layer capacitance and/or pseudocapacitance. To enhance the performance of supercapacitors, various strategies have been adopted such as electrode nanostructuring, hybrid electrode designs using nanocomposite electrodes and hybrid supercapacitor (HSC) configurations. Nanoarchitecturing of electrode-active materials is an effective way of enhancing the performance of supercapacitors as it increases the effective electrode surface area for enhanced electrode/electrolyte interaction. In this review, we focus on the recent developments in the novel electrode materials and various hybrid designs used in supercapacitors for obtaining high specific capacitance and energy density. A family of electrode-active materials including carbon nanomaterials, transition metal-oxides, transition metal-nitrides, transition metal-hydroxides, electronically conducting polymers, and their nanocomposites are discussed in detail. The HSC configurations for attaining enhanced supercapacitor performance as well as strategies to integrate with other microelectronic devices/wearable fabrics are also included.

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

Emerging flexible and wearable electronic devices necessitates the development of fiber-type energy storage devices to power them. Supercapacitors received great attention for applications in flexible and wearable devices due to their scalability, safety, and miniature size. Herein, we report the fabrication of a flexible supercapacitor using manganese(II,III) oxide (Mn₃O₄) nanowalls (NWs) grown by electrochemical deposition on carbon fiber (CF) as electrode-active material. Here, CF serves as both a substrate for the growth of Mn₃O₄ NWs and a current collector for making a lightweight supercapacitor. Two-dimensional Mn₃O₄ NWs were uniformly grown on CF with high surface coverage. A three-dimensional nanostructured electrode is obtained using these individual two-dimensional Mn₃O₄ NWs. The Mn₃O₄ NWs grown on CF are characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy. A symmetric sandwich-type supercapacitor is fabricated using two-dimensional Mn₃O₄ NW electrodes in an aqueous 1 M Na₂SO₄ electrolyte. The Mn₃O₄ NW supercapacitor electrode exhibits a specific capacitance of 300.7 F g^-1 at a scan rate of 5 mV s^-1. The assembled symmetric sandwich-type supercapacitor displayed high flexibility even at a bending angle of 180° without altering its performance. The Mn₃O₄ NW supercapacitor also displayed a long cycle life of 7500 cycles with 100% capacitance retention.

Two-Dimensional rGO-MoS2 Hybrid Additives for High-Performance Magnetorheological Fluid

Magnetorheological fluids (MRF) that undergo a change in their viscoelastic properties under the magnetic fields have been considered as one of most important smart functional materials for vibration dampers and shock absorbers in several engineering applications. However, the use of magnetorheological fluids in practical applications has been limited by poor sedimentation ratio and on-state yield stress. Herein, we report hybrid rGO-MoS₂ additives for a high-performance magnetorheological fluid. Two different kinds of hybrid additives, which are called non-magnetic rGO-MoS₂ and magnetic Fe-rGO-MoS₂, were synthesized by using a hydrothermal method. The rGO-MoS₂ added suspensions remained stable for the first 90 min whereas the CIP MRFs settled down quickly (65%) in the first 10 minutes. The Fe-rGO-MoS₂ additives showed a 24% higher on-state shear stress as compared to CIP MRFs. On the other hand, an increase of 60% in the on-state yield stress for Fe-rGO-MoS₂ MRF can be attributed to the gap-filling by the hybrid additives during columnar-structure formation. Among two-dimensional (2D) materials, Molybdenum Disulphide (MoS₂) is a member of transition metal dichalcogenides (TMDCs), traditionally used as solid lubricant, while reduced graphene-oxide (rGO) is a well-known 2D material with supreme mechanical properties. We believe that this study will blaze the new way for developing a high-performance magnetorheological fluids based on various 2D material hybrids.

Hierarchical Carbon Nanotube-Coated Carbon Fiber: Ultra Lightweight, Thin, and Highly Efficient Microwave Absorber

Strong EM wave absorption and lightweight are the foremost important factors that drive the real-world applications of the modern microwave absorbers. This work mainly deals with the design of highly efficient microwave absorbers, where a hierarchical carbon nanotube (CNT) forest is first grown on the carbon fiber (CF) through the catalytic chemical vapor deposition method. The hierarchical carbon nanotube grown on the carbon fiber (CNTCF) is then embedded in the epoxy matrix to synthesize lightweight nanocomposites for their use as efficient microwave absorbers. The morphological study shows that carbon nanotubes (CNTs) self-assemble to form a trapping center on the carbon fiber. The electromagnetic characteristics of resultant nanocomposites are investigated exclusively in the X-band (8.2-12.4 GHz) using the network analyzer. The synthesized nanocomposites, containing 0.35 and 0.50 wt % CNTCFs, exhibit excellent microwave absorption properties, which could be attributed to the better impedance matching conditions and high dielectric losses. The reflection loss (RL) of -42.0 dB (99.99% absorption) with -10 dB (90% absorption) and -20 dB (99% absorption) bandwidths of 2.7 and 1.16 GHz, respectively, is achieved for 0.35 wt % CNTCF loading at 2.5 mm thickness. The composite with 0.50 wt % CNTCF loading illustrates substantial absorption efficiency with the RL reaching -24.5 dB (99.65% absorption) at 9.8 GHz and -10 dB bandwidth comprising 84.5% of the entire X band. The excellent microwave properties obtained here are primarily due to the electric dipole polarization, interfacial polarization, and unique trapping center. These trapping centers basically induce multiple reflections and scatterings, which attenuate more microwave energy. This investigation opens a new approach for the development of extremely lightweight, small-thickness, and highly efficient microwave absorbers for X-band applications.

A combustion synthesis route for magnetically separable graphene oxide–CuFe2O4–ZnO nanocomposites with enhanced solar light-mediated photocatalytic activity

A novel GO–CuFe₂O₄–ZnO ternary nanocomposite has been designed as an efficient photocatalyst for the degradation of four toxic organic pollutants.

Synthesis and Characterization of Self-Standing and Highly Flexible δ-MnO2@CNTs/CNTs Composite Films for Direct Use of Supercapacitor Electrodes

Self-standing and flexible films worked as pseudocapacitor electrodes have been fabricated via a simple vacuum-filtration procedure to stack δ-MnO2@carbon nanotubes (CNTs) composite layer and pure CNT layer one by one with CNT layers ended. The lightweight CNTs layers served as both current collector and supporter, while the MnO2@CNTs composite layers with birnessite-type MnO2 worked as active layer and made the main contribution to the capacitance. At a low discharge current of 0.2 A g(-1), the layered films displayed a high areal capacitance of 0.293 F cm(-2) with a mass of 1.97 mg cm(-2) (specific capacitance of 149 F g(-1)) and thickness of only 16.5 μm, and hence an volumetric capacitance of about 177.5 F cm(-3). Moreover, the films also exhibited a good rate capability (only about 15% fading for the capacitance when the discharge current increased to 5 A g(-1) from 0.2 A g(-1)), outstanding cycling stability (about 90% of the initial capacitance was remained after 5,000 cycles) and high flexibility (almost no performance change when bended to different angles). In addition, the capacitance of the films increased proportionally with the stacked layers and the geometry area. E.g., when the stacked layers were three times many with a mass of 6.18 mg cm(-2), the areal capacitance of the films was increased to 0.764 F cm(-2) at 0.5 A g(-1), indicating a high electronic conductivity. It is not overstated to say that the flexible and lightweight layered films emerged high potential for future practical applications as supercapacitor electrodes.

Polypyrrole-decorated 2D carbon nanosheet electrodes for supercapacitors with high areal capacitance

Supercapacitor with polypyrrole-decorated 2D carbon nanosheet electrodes exhibits area specific capacitance of 376.9 mF cm⁻².

Hierarchical carbon nanopetal/polypyrrole nanocomposite electrodes with brush-like architecture for supercapacitors

Hierarchical 3D nanocomposite electrodes with tube brush-like morphology are synthesized by electrochemically depositing polypyrrole on carbon nanopetal-coated carbon fibers.

An investigation into the solar light-driven enhanced photocatalytic properties of a graphene oxide–SnO2–TiO2ternary nanocomposite

Highly efficient graphene oxide–SnO₂–TiO₂ternary nanocomposite was designedviaone step solvothermal method considering the step-wise band edge energy levels of each component and the corresponding photocatalytic performance was investigated.

Layer-by-layer assembled (high-energy carbon nanotube/conductive carbon nanotube)n nanocomposites for high volumetric capacitance supercapacitor electrodes

We report supercapacitor electrodes with high volumetric capacitance and remarkable operational stability using a ligand exchange layer-by-layer (LbL) assembly of high-energy multiwall carbon nanotube (MWCNT) hybrids and conductive MWCNTs.

Core–shell nanospherical polypyrrole/graphene oxide composites for high performance supercapacitors

Novel core–shell polypyrrole/graphene oxide (PPy–GO) nanomaterials of uniform PPy nanospheres and GO have been synthesized by an in situ surface-initiated polymerization method.

Hierarchically structured catalyst layer for the oxygen reduction reaction fabricated by electrodeposition of platinum on carbon nanotube coated carbon fiber

Hierarchically structured fuel cell cathode catalysts consisting of Pt-nanoparticle clusters coated on a CNT-based, ORR active catalyst support were synthesized.

Enhanced photocatalytic degradation of methylene blue and adsorption of arsenic(iii) by reduced graphene oxide (rGO)–metal oxide (TiO2/Fe3O4) based nanocomposites

Hazardous methylene blue dye and As(iii) ions from wastewater are removed by the rGO and TiO₂/Fe₃O₄ based binary and ternary nanocomposites, where ternary rGO–Fe₃O₄–TiO₂ nanocomposite provides maximum degradation and adsorption of the pollutants.