Metal oxide-based supercapacitors: progress and prospectives

"Unveiling marigold assembled micro flowers of tungsten oxide towards solid-state flexible pouch and coin cell supercapacitors"

Marigold analogues micro flowers of tungsten oxide (WO3) have been grown in thin film form through simple and cost-effective solution chemistry approach on stainless steel substrate. Aqueous precursor involving WO4-2 ions agglomerated as self-sacrificing template growing initially into the nano-petal, followed by self-assembly; leading to marigold analogues micro flower surface architecture. This enthralling morphology motivated us not only to fabricate supercapacitive electrode but also to design complete solid-state supercapacitor devices in dual configurations: flexible pouch cell and coin cell. Interestingly, both devices even in symmetric configuration yields remarkable potential window of 1.82 V when sandwiched by gel inclusive of Li+ ions dispersed in non-conducting polyvinyl alcohol matrix. Solid-state flexible pouch cell and coin cell delivered specific capacitances of 103.98 ± 3.59 and 30.09 ± 1.03 F/g respectively at a scan rate of 5 mV/s. Assembled electrode, coin-cell and flexible pouch-cells have been well assessed in-depth through specific capacitances using cyclic voltammetry and galvanostatic charge discharge, diffusive and capacitive contributions, mechanical bending tests, electrochemical active surface area, and electrochemical impedance analysis. Practical applicability has been demonstrated for designed flexible pouch cell to run small fan and light emitting diode panel whereas coin cell to run light emitting diode panel.

Multi-Layer PVA-PANI Conductive Hydrogel for Symmetrical Supercapacitors: Preparation and Characterization

This work describes a simple, inexpensive, and robust method to prepare a flexible "all in one" integrated hydrogel supercapacitors (HySCs). Preparing smart hydrogels with high electrical conductivity, ability to stretch significantly, and excellent mechanical properties is the last challenge for tailored wearable devices. In this paper, we employed a physical crosslinking process that involves consecutive freezing and thawing cycles to prepare a polyvinyl alcohol (PVA)-based hydrogel. Exploiting the self-healing properties of these materials, the assembly of the different layers of the HySCs has been performed. The ionic conductivity within the electrolyte layer arises from the inclusion of an H2SO4 solution in the hydrogel network. Instead, the electronic conductivity is facilitated by the addition of the conductive polymer PANI-PAMPSA into the hydrogel layers. Electrochemical measures have highlighted newsworthy properties related to our HySCs, opening their use in wearable electronic applications.

Asymmetric Supercapacitors Based on ZnCo2O4 Nanohexagons and Orange Peel Derived Activated Carbon Electrodes

Herein, the performance of asymmetric supercapacitors (ASC) fabricated using ZnCo2O4 (ZCO) nano-hexagons and orange peel-derived activated carbon (OPAC) as electrodes was studied. ZCO was prepared by a double hydroxide method and OPAC was prepared from orange peel followed by KOH activation. For ZCO, the calcination temperature was determined using TGA analysis. The XRD showed the presence of a cubic spinel structure. The chemical structure was analyzed using XPS, FTIR, and Raman spectroscopy respectively. For OPAC, the presence of an amorphous nature was inferred; FTIR and Raman studies indicate the presence of functional groups and defect structure in the material. The presence of ZCO nano-hexagons was observed from SEM and TEM respectively. For OPAC, an interconnected pore structure was observed from the SEM image. The specific capacitance for ZCO and OPAC was found to be 194 F.g-1 and 159 F.g-1 at a current density of 0.25 A.g-1. Further, an ASC was fabricated using ZCO as a positive and OPAC as a negative electrode in 2M KOH-soaked separator. A cell voltage of 1.2 V was achieved and the specific capacitance was calculated to be 64 F.g-1 at 0.25 A.g-1. Further, the cyclic stability and the changes at the electrode/electrolyte interface were studied.

High-performance electrode materials of heteroatom-doped lignin-based carbon materials for supercapacitor applications

Supercapacitors are the preferred option for supporting renewable energy sources owing to many benefits, including fast charging, long life, high energy and power density, and saving energy. While electrode materials with environmentally friendly preparation, high performance, and low cost are important research directions of supercapacitors. At present, the growing global population and the increasingly pressing issue of environmental pollution have drawn the focus of numerous researchers worldwide to the development and utilization of renewable biomass resources. Lignin, a renewable aromatic polymer, has reserves second only to cellulose in nature. Ten million tonnes of industrial lignin are produced in pulp and paper mills annually, most of which are disposed of as waste or burned for fuel, seriously depleting natural resources and polluting the environment. One practical strategy to accomplish sustainable development is to employ lignin resources to create high-value materials. Based on the high carbon content and rich functional groups of lignin, the lignin-based carbon materials generated after carbonization treatment display specific electrochemical properties as electrode materials. Nevertheless, low electrochemical activity of untreated lignin precludes it from achieving its full potential for application in energy storage. Heteroatom doping is a common modification method that aims to improve the electrochemical performance of the electrode materials by optimizing the structure of the lignin, improving its pore structure and increasing the number of active sites on its surface. This paper aims to establish theoretical foundations for design, preparation, and optimizing the performance of heteroatom-doped lignin-based carbon materials, as well as for developing high-value-added lignin materials. The most reported the mechanism of supercapacitors, the doping process involving various types of heteroatoms, and the analysis of how heteroatoms affect the performance of lignin-based carbon materials are also detailed in this review.

An aqueous symmetric supercapacitor with wide window and high energy density using redox electrode of Cu-Al-layered double hydroxides and λ-manganese dioxide

Manganese oxide is a potential agent in the field of energy storage owing to its changeable redox characteristics, high theoretical specific capacitance and valence shells for charge transfer. On the other hand, due to huge surface area, affordability, customisable composition, layered structure and high theoretical specific capacitance, layered double hydroxides, or LDHs, have drawn a lot of interest. This study employs a three-electrode setup to investigate the supercapacitive performance of λ-manganese dioxide/Cu-Al LDH composite at different compositional ratios. To enhance the adhesive and conductivity capabilities, 10% of CNT additive and PVDF binder are added for the composites. Out of all the composites, the one with the greatest weight percentage of λ-manganese dioxide shows the best electrode performance with a superior specific capacitance of 164 F/g at a scan rate of 10 mV/s. Additionally, using a symmetrical two-electrode setup, the best-performing electrode is examined. The result shows an exceptional potential window of 2.7 V in a basic electrolyte, a power density of 4.04 kW/kg at 3 A/g, an energy density of 20.32 Wh/kg at 1 A/g, and a specific capacitance of 37 F/g.

Recent progress in metal oxide-based electrode materials for safe and sustainable variants of supercapacitors

A significant amount of energy can be produced using renewable energy sources; however, storing massive amounts of energy poses a substantial obstacle to energy production. Economic crisis has led to rapid developments in electrochemical (EC) energy storage devices (EESDs), especially rechargeable batteries, fuel cells, and supercapacitors (SCs), which are effective for energy storage systems. Researchers have lately suggested that among the various EESDs, the SC is an effective alternate for energy storage due to the presence of the following characteristics: SCs offer high-power density (PD), improvable energy density (ED), fast charging/discharging, and good cyclic stability. This review highlighted and analyzed the concepts of supercapacitors and types of supercapacitors on the basis of electrode materials, highlighted the several feasible synthesis processes for preparation of metal oxide (MO) nanoparticles, and discussed the morphological effects of MOs on the electrochemical performance of the devices. In this review, we primarily focus on pseudo-capacitors for SCs, which mainly contain MOs and their composite materials, and also highlight their future possibilities as a useful application of MO-based materials in supercapacitors. The novelty of MO's electrode materials is primarily due to the presence of synergistic effects in the hybrid materials, rich redox activity, excellent conductivity, and chemical stability, making them excellent for SC applications.

Scalable solid-state synthesis of 2D transition metal oxide/graphene hybrid materials and their utilization for microsupercapacitors

Two-dimensional metal oxide (MO) nanostructures have unique properties compared with their bulk or 0D and 1D (nanoparticle and nanowire) counterparts. Their abundant surface area and atomically thin 2D structure are advantageous for their applications in catalysis and energy, as well as integration with 2D layered materials such as graphene and reduced graphene oxide (rGO). However, fast and scalable synthesis of 2D MOs and their nanocomposites remains challenging. Here, we developed a microwave-assisted solid-state synthesis method for the scalable generation of 2D MOs and 2D MO/rGO nanocomposites with tunable structure and composition. The structures and properties of 2D Fe2O3 and 2D ZnO as well as their nanocomposites with rGO were systematically investigated. The excellent electrochemical properties of such 2D MO/rGO nanocomposites also enable us to use them as electrode materials to fabricate microsupercapacitors. This work provides new insights into the scalable and solid-state synthesis of 2D nanocomposites and their potential applications in catalysis, energy conversion and storage.

3D net-like Co3O4@NiO nanostructures for high performance supercapacitors

Co3O4@NiO composite electrode materials were successfully synthesized by a two-step hydrothermal method followed by annealing treatment. Due to their three-dimensional network structure, these composite materials exhibited a large specific surface area, enhancing their electrochemical performance. Consequently, the Co3O4@NiO electrode demonstrated a specific capacitance of 1306 F g-1 at a current density of 1 A g-1, an excellent specific capacitance retention rate of 95.5% after 3000 cycles even at 8 A g-1 and a coulombic efficiency approaching 100%. These outstanding properties make the Co3O4@NiO composite materials promising electrode materials for high performance supercapacitors.

Phosphorus-Doped Nickel Oxide Micro-Supercapacitor: Unleashing the Power of Energy Storage for Miniaturized Electronic Devices

For an uninterrupted self-powered network, the requirement of miniaturized energy storage device is of utmost importance. This study explores the potential utilization of phosphorus-doped nickel oxide (P-NiO) to design highly efficient durable micro-supercapacitors. The introduction of P as a dopant serves to enhance the electrical conductivity of bare NiO, leading to 11-fold augmentation in volumetric capacitance to 841.92 Fcm-3 followed by significant enhancement of energy and power density from 6.71 to 42.096 mWhcm-3 and 0.47 to 1.046 Wcm-3, respectively. Theoretical calculations used to determine the adsorption energy of OH- ions, revealing higher in case of bare NiO (1.52 eV) as compared to phosphorus-doped NiO (0.64 eV) leading to high electrochemical energy storage performance. The as-designed micro-supercapacitor (MSC) device demonstrates a facile integration with the photovoltaic system for renewable energy storage and smooth transfer to external loads for enlightening the blue LED for ≈1 min. The choice of P-NiO/Ni not only contributes to cost reduction but also ensures minimal lattice mismatch at the interface facilitating high durability up to 15 K cycles along with capacitive retention of ≈100% and coulombic efficiency of 93%. Thus, the heterostructure unveils the possibilities of exploring miniaturized energy storage devices for portable electronics.

Switchable Charge Storage Mechanism via in Situ Activation of MXene Enables High Capacitance and Stability in Aqueous Electrolytes

The need for reliable renewable energy storage devices has become increasingly important. However, the performance of current electrochemical energy storage devices is limited by either low energy or power densities and short lifespans. Herein, we report the synthesis and characterization of multilayer Ti4N3Tx MXene in various aqueous electrolytes. We demonstrate that Ti4N3Tx can be electrochemically activated through continuous cation intercalation over a 10 day period using cyclic voltammetry. A wide operating window of 2 V is maintained throughout activation. After activation, capacitance at 2 mV s-1 increases by 300%, 140%, and 500% in 1 M H2SO4, 1 M MgSO4, and 1 M KOH, respectively, while maintaining ∼600 F g-1 at 2 mV s-1 after 50000 cycles in 1 M H2SO4. This activation process is possibly attributed to the unique morphology of the multilayered material, allowing cation intercalation to increase access to redox-active sites between layers. This work adds to the growing repository of electrochemically stable MXenes reported for aqueous energy storage applications. These findings offer a reliable option for reliable energy storage devices with potential applications in large-scale grid storage and electric vehicles.

Recent Developments in Electrodeposition of Transition Metal Chalcogenides-Based Electrode Materials for Advance Supercapacitor Applications: A Review

High-performance supercapacitive electrode materials have received significant attention from researchers worldwide, thus aiming for comparable performance similar to the extensively used rechargeable batteries. For emerging energy storage technologies like flexible supercapacitors, transition metal chalcogenides (TMCs) have been in the spotlight due to their promising electrochemical features compared to other electrode materials. Among the synthesis techniques, electrodeposition-mediated preparation of thin films of TMCs offered an affordable binder-free approach for electrode fabrication that effectively improved the supercapacitor performance. Hence, this review mainly focussed on the electrodeposition-based syntheses of single/ multinary chalcogenides and their composites for supercapacitors applications. Further, the effects of different deposition parameters were discussed for boosting the supercapacitor performance. Finally, this review outlined the existing challenges and future perspectives in this research domain, which will assist the upcoming exploration in the energy storage field.

Resent Development of Binder-Free Electrodes of Transition Metal Oxides and Nanohybrids for High Performance Supercapacitors - A Review

The entire world is aware of the serious issue of global warming and therefore utilizing renewable energy sources is the most encouraging steps toward solving energy crises, and as a result, energy storage solutions are necessary. The supercapacitors (SCs) have a high-power density and a long cycle life, they are promising as an electrochemical conversion and storage device. In order to achieve high electrochemical performance, electrode fabrication must be implemented properly. Electrochemically inactive and insulating binders are utilized in the conventional slurry coating method of making electrodes to provide adhesion between the electrode material and the substrate. This results in an undesirable "dead mass," which lowers the overall device performance. In this review, we focused on binder-free SCs electrodes based on transition metal oxides and composites. With the best examples providing the critical aspects, the benefits of binder-free electrodes over slurry-coated electrodes are addressed. Additionally, different metal-oxides used in the fabrication of binder-free electrodes are assessed, taking into account the various synthesis methods, giving an overall picture of the work done for binder-free electrodes. The future outlook is provided along with the benefits and drawbacks of binder-free electrodes based on transition metal oxides.

Lattice strain induced d-band centre engineering enabled pseudocapacitive energy storage in 2D hypo-hyper electronic V-NiCo2O4 for asymmetric supercapacitors

Understanding the role of fundamental structural engineering of materials in unravelling the underlying rudimentary electronic structure-dependent charge storage mechanisms is crucial for developing new strategic approaches toward high-performance electrochemical energy storage devices. Here, we demonstrate the role of strain engineering by V doping-induced lattice contraction in NiCo2O4 for increasing the energy density and power density of aqueous asymmetric hybrid supercapacitors. For application in energy storage, we demonstrate the influence of electron-deficient V4+/5+ doping in electron-rich Ni2+ sites, which has been found to result in the formation of a hypo-hyper electronically coupled cation pair causing a shift in the d-band and O 2p band centres and distortion of CoO6 octahedra. Optimization of V doping to 3 mol%, achieved by a binder-free one-step hydrothermal method, has yielded a 96% increase in specific capacitance of up to 2316 F g-1 from 1193 F g-1 in pristine materials at 1 A g-1 in a three-electrode configuration with a coulombic efficiency (η%) of 94% and a 24% increase in rate capacity. A two-fold increase in specific capacitance in the pouch cell device, fabricated with a functionalized carbon nanosphere positive electrode, has been observed for the V-doped samples at 1 A g-1 with a η% of 97% and a maximum energy density of 96.3 W h g-1 and a maximum power density of 8733.6 W g-1 which are 41% and 24.3% higher than the pristine device, respectively. Excellent cycling stability of 95.4% capacitance retention has been observed after 6000 cycles. DFT calculations have been carried out to understand the previously unexplored effect of lattice strain on charge transport and quantum capacitance, and ultimately its effect on the transition state kinetics of energy storage faradaic reaction mechanisms. The aim of this work is to establish a fresh perspective on developing a deep understanding of the fundamental electronic and structural properties of materials by drawing in concepts from descriptor models in electrocatalysis to reveal the role of lattice strain and d-band centre tailoring in enabling pseudocapacitive energy storage.

Efficient pH-universal aqueous supercapacitors enabled by an azure C-decorated N-doped graphene aerogel

Current aqueous supercapacitors (SCs) possess the relative low energy density, and there is therefore widespread interest in cost-effective fabrication of capacitive materials with promoted specific capacitance and/or broadened voltage window. Here, a redox-active azure C-decorated N-doped graphene aerogel (AC - NGA) is fabricated using a simple hydrothermal self-assembly method through strong noncovalent π-π interaction. AC - NGA highlights an excellent charge storage performance (a high 591F g-1 gravimetric capacitance under a current density of 1.0 A g-1 and ultrahigh voltage window of 2.3 V) under pH-universal conditions. The capacitive contribution of charge storage is 91.7%, exceeding or comparable to those of the best pseudocapacitors known. Furthermore, a symmetric AC - NGA//AC - NGA device realizes high energy and power densities (15.2-60.2 Wh kg-1 at 650-23,000 W kg-1) and excellent cycling stability in acidic, neutral, and basic aqueous solutions. This work offers a cost-effective strategy to combine redox dye molecules with heteroatom-doped graphene aerogel for building green efficient pH-universal aqueous supercapacitors.

Application of green-synthesized cadmium oxide nanofibers and cadmium oxide/graphene nanosheet nanocomposites as alternative and efficient photocatalysts for methylene blue removal from aqueous matrix

For the first time, cadmium oxide (CdO) nanofibers (NFs) and graphene nanosheet (GNS)-doped CdO nanocomposites (NCs) have been synthesized by a simple green route using green tea (Camellia sinensis) extract, for subsequent application as photocatalysts for methylene blue (MB) removal from an aqueous matrix. In addition, the materials were tested as working electrodes for supercapacitors. The prepared samples were analyzed by FESEM, UV-Vis spectroscopy, FTIR, and X-ray diffraction (XRD). FESEM revealed that the obtained NPs and NCs show fiber-shaped nanostructure. FTIR confirmed the presence of biomolecules on CdO and carbon compounds on CdO/GNS, while XRD exhibited the cubic crystalline structure of obtained NPs and NCs. The Rietveld refinement using XRD data was performed to ascertain the crystallographic characteristics of the produced samples and look into lattice imperfections. UV-Vis spectroscopy evaluated the optical bandgap energies of CdO and CdO/GNS NCs. The CdO/GNS NCs demonstrated a fast cleavage of the dye molecule under UV irradiation, resulting in 97% removal in 120 min. In addition, CdO/GNS NCs showed remarkable chemical stability as an electrode material, with a high specific capacitance of 231 F g-1 at a scan rate of 25 mV s-1. These observed NCs characteristics are higher when compared to pristine CdO NPs. Finally, we found that the investigated NCs showed enhanced multifunctional properties, such as photocatalytic and supercapacitor characteristics, which can be useful in practical applications.

High-Performance 3D Nanostructured Silver Electrode for Micro-Supercapacitor Application

In the current energy crisis scenario, the development of renewable energy forms such as energy storage systems among the supercapacitors is an urgent need as a tool for environmental protection against increasing pollution. In this work, we have designed a novel 3D nanostructured silver electrode through an antireplica/replica template-assisted procedure. The chemical surface and electrochemical properties of this novel 3D electrode have been studied in a 5 M KOH electrolyte. Microstructural characterization and compositional analysis were studied by SEM, energy-dispersive X-ray spectroscopy, XRD technique, and Kripton adsorption at -198 °C, together with cyclic voltammetry and galvanostatic charge-discharge cycling measurements, Coulombic efficiency, cycle stability, and their leakage current drops, in addition to the self-discharge and electrochromoactive behavior, were performed to fully characterize the 3D nanostructured electrode. Large areal capacitance value of 0.5 F/cm2 and Coulombic efficiency of 97.5% are obtained at a current density of 6.4 mA/cm2 for a voltage window of 1.2 V (between -0.5 and 0.8 V). The 3D nanostructured silver electrode exhibits excellent capacitance retention (95%) during more than 2600 cycles, indicating a good cyclic stability. Additionally, the electrode delivers a high energy density of around 385.87 μWh/cm2 and a power density value of 3.82 μW/cm2 and also displays an electrochromoactive behavior. These experimental results strongly support that this versatile combined fabrication procedure is a suitable strategy for improving the electrochemical performances of 3D nanostructured silver electrodes for applications as micro-supercapacitors or in electrochemical devices.

Supercapacitor Performance of NiO, NiO-MWCNT, and NiO-Fe-MWCNT Composites

The NiO-CNT and NiO-Fe-CNT composites that have been prepared from waste high density polyethylene plastic and their carbon nanotube (CNT) quality-dependent supercapacitance tuning have been reported here. Multiwalled CNT (MWCNT) formation has been confirmed from TEM and Raman spectra with an ID/IG ratio of 0.77, which stands for high graphitization. The specific surface area (SSA) of MWCNTs in the NiO-Fe-CNT composite was 87.8 m2/g, while in the NiO-CNT composite, it was 25 m2/g. NiO-Fe-CNT displayed higher specific capacitance and energy density (1360 Fg-1 and 1180 W h kg-1) than NiO-CNT (1250 Fg-1 and 1000 W h kg-1), which may be due to the presence of higher-quality MWCNTs in the NiO-Fe-CNT composite. NiO-Fe-CNT displayed higher contributions of electric double-layer capacitor (59%) behavior compared to NiO-CNT (38%) and represented a hybrid supercapacitor. NiO-Fe-CNT also displayed a capacitive retention of 96% after 1000 charge-discharge cycles. Furthermore, studies in acidic electrolytes revealed higher performance of NiO-Fe-CNT than NiO-CNT, displaying specific capacitances of NiO-Fe-CNT to be 1147 Fg-1 in 2 M H2SO4 and 943 Fg-1 in 2 M HCl. It has been qualitatively explored that the quality of CNTs, SSA, and quantum confinement effects in the composites may be the factors responsible for the performance difference in NiO-Fe-CNT and NiO-CNT. The present work is geared toward the low-cost fabrication of high-quality CNT composites for supercapacitors and energy storage applications. The present work also contributes quantitatively to the understanding of CNT quality as an important parameter for the performance of CNT-composite-based supercapacitors.

Octahedral/Tetrahedral Vacancies in Fe3 O4 as K-Storage Sites: A Case of Anti-Spinel Structure Material Serving as High-Performance Anodes for PIBs

Potassium-ion batteries (PIBs) have attracted more and more attention as viable alternatives to lithium-ion batteries (LIBs) due to the deficiency and uneven distribution of lithium resources. However, it is shown that potassium storage in some compounds through reaction or intercalation mechanisms cannot effectively improve the capacity and stability of anodes for PIBs. The unique anti-spinel structure of magnetite (Fe3 O4 ) is densely packed with thirty-two O atoms to form a face-centered cubic (fcc) unit cell with tetrahedral/octahedral vacancies in the O-closed packing structure, which can serve as K+ storage sites according to the density functional theory (DFT) calculation results. In this work, carbon-coated Fe3 O4 @C nanoparticles are prepared as high-performance anodes for PIBs, which exhibit high reversible capacity (638 mAh g-1 at 0.05 A g-1 ) and hyper stable cycling performance at ultrahigh current density (150 mAh g-1 after 9000 cycles at 10 A g-1 ). In situ XRD, ex-situ Fe K-edge XAFS, and DFT calculations confirm the storage of K+ in tetrahedral/octahedral vacancies.

Iron Selenide Particles for High-Performance Supercapacitors

Nowadays, iron (II) selenide (FeSe), which has been widely studied for years to unveil the high-temperature superconductivity in iron-based superconductors, is drawing increasing attention in the electrical energy storage (EES) field as a supercapacitor electrode because of its many advantages. In this study, very small FeSe particles were synthesized via a simple, low-cost, easily scalable, and reproducible solvothermal method. The FeSe particles were characterized using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) measurements, and electrochemical impedance spectroscopy (EIS), revealing enhanced electrochemical properties: a high capacitance of 280 F/g at 0.5 A/g, a rather high energy density of 39 Wh/kg and a corresponding power density of 306 W/kg at 0.5 A/g, an extremely high cycling stability (capacitance retention of 92% after 30,000 cycles at 1 A/g), and a rather low equivalent series resistance (RESR) of ~2 Ω.

Superior cyclability of high surface area vanadium nitride in salt electrolytes

High surface area vanadium nitrides (VNs) have been extensively studied as materials for aqueous supercapacitors due to the high initial capacitance in alkaline media at low scan rates. However, low capacitance retention and safety limit their implementation. The use of neutral aqueous salt solutions has the potential to mitigate both of these concerns, but is limited in analysis. Hence, we report on the synthesis and characterization of high surface area VN as a supercapacitor material in a wide variety of aqueous chlorides and sulfates using Mg²⁺, Ca²⁺, Na⁺, K⁺, and Li⁺ ions. We observe the following trend in the salt electrolytes: Mg²⁺ > Li⁺ > K⁺ > Na⁺ > Ca²⁺. Mg²⁺ systems provide the best performance at higher scan rates with areal capacitances of 294 μF cm^-2 in 1 M MgSO₄ over a 1.35 V operating window at 2000 mV s^-1. Furthermore, VN in 1 M MgSO₄ maintained a 36% capacitance retention from 2 to 2000 mV s^-1 compared to 7% in 1 M KOH. Capacitance in 1 M MgSO₄ and 1 M MgCl₂ increased to 121% and 110% of their original values after 500 cycles and maintained capacitances of 589 and 508 μF cm^-2 at 50 mV s^-1 after 1000 cycles, respectively. In contrast, in 1 M KOH the capacitance decreases to 37% of its original value, reaching only 29 F g^-1 at 50 mV s^-1 after 1000 cycles. The superior performance of the Mg system is attributed to a reversible surface 2 e^- transfer pseudocapacitive mechanism between Mg²⁺ and VN_xO_y. These findings can be used to further the field of aqueous supercapacitors to build safer and more stable energy storage systems that can charge quicker compared to KOH systems.

Transition metal dichalcogenide electrodes with interface engineering for high-performance hybrid supercapacitors

Transition metal dichalcogenides (TMDCs) have been explored in recent years to utilize in electronics due to their remarkable properties. This study reports the enhanced energy storage performance of tungsten disulfide (WS₂) by introducing the conductive interfacial layer of Ag between the substrate and active material (WS₂). The interfacial layers and WS₂ were deposited through a binder free method of magnetron sputtering and three different prepared samples (WS₂ and Ag-WS₂) were scrutinize via electrochemical measurements. A hybrid supercapacitor was fabricated using Ag-WS₂ and activated carbon (AC) since Ag-WS₂ was observed to be the most proficient of all three samples. The Ag-WS₂//AC devices have attained a specific capacity (Q_s) of 224 C g^-1, while delivering the maximum specific energy (E_s) and specific power (P_s) of 50 W h kg^-1 and 4003 W kg^-1, respectively. The device was found to be stable enough as it retains 89% capacity and 97% coulombic efficiency after 1000 cycles. Additionally, the capacitive and diffusive currents were obtained through Dunn's model to observe the underlying charging phenomenon at each scan rate.

Nanoarchitectonics of Three-Dimensional Carbon Nanofiber-Supported Hollow Copper Sulfide Spheres for Asymmetric Supercapacitor Applications

Three-dimensional carbon nanofiber (3D-CNF)-supported hollow copper sulfide (HCuS) spheres were synthesized by the facile hydrothermal method. The morphology of the as-synthesized HCuS@3D-CNF composite clearly revealed that the 3D-CNFs act as a basement for HCuS spheres. The electrochemical performance of as-synthesized HCuS@3D-CNFs was evaluated by cyclic voltammetry (CV) tests, gravimetric charge-discharge (GCD) tests, and Nyquist plots. The obtained results revealed that the HCuS@3D-CNFs exhibited greater areal capacitance (4.6 F/cm²) compared to bare HCuS (0.64 F/cm²) at a current density of 2 mA/cm². Furthermore, HCuS@3D-CNFs retained excellent cyclic stability of 83.2% after 5000 cycles. The assembled asymmetric device (HCuS@3D-CNFs//BAC) exhibits an energy density of 0.15 mWh/cm² with a working potential window of 1.5 V in KOH electrolyte. The obtained results demonstrate that HZnS@3D-CNF nanoarchitectonics is a potential electrode material for supercapacitor applications.

Sustainable Cellulose Nanofibers-Mediated Synthesis of Uniform Spinel Zn-Ferrites Nanocorals for High Performances in Supercapacitors

Spinel ferrites are versatile, low-cost, and abundant metal oxides with remarkable electronic and magnetic properties, which find several applications. Among them, they have been considered part of the next generation of electrochemical energy storage materials due to their variable oxidation states, low environmental toxicity, and possible synthesis through simple green chemical processing. However, most traditional procedures lead to the formation of poorly controlled materials (in terms of size, shape, composition, and/or crystalline structure). Thus, we report herein a cellulose nanofibers-mediated green procedure to prepare controlled highly porous nanocorals comprised of spinel Zn-ferrites. Then, they presented remarkable applications as electrodes in supercapacitors, which were thoroughly and critically discussed. The spinel Zn-ferrites nanocorals supercapacitor showed a much higher maximum specific capacitance (2031.81 F g^-1 at a current density of 1 A g^-1) than Fe₂O₃ and ZnO counterparts prepared by a similar approach (189.74 and 24.39 F g^-1 at a current density of 1 A g^-1). Its cyclic stability was also scrutinized via galvanostatic charging/discharging and electrochemical impedance spectroscopy, indicating excellent long-term stability. In addition, we manufactured an asymmetric supercapacitor device, which offered a high energy density value of 18.1 Wh kg^-1 at a power density of 2609.2 W kg^-1 (at 1 A g^-1 in 2.0 mol L^-1 KOH electrolyte). Based on our findings, we believe that higher performances observed for spinel Zn-ferrites nanocorals could be explained by their unique crystal structure and electronic configuration based on crystal field stabilization energy, which provides an electrostatic repulsion between the d electrons and the p orbitals of the surrounding oxygen anions, creating a level of energy that determines their final supercapacitance then evidenced, which is a very interesting property that could be explored for the production of clean energy storage devices.

Bifunctional electrode of bismuth tungsten for electrochemical sensing applications

A co-precipitation technique has been used to prepare Bismuth tungstate nanoparticles (Bi₂WO₆) for electrochemical capacitors and electrochemical sensing of Ascorbic acid (AA). Using a scanning rate of 10 mV s ^-1, the electrode was performed as the pseudocapacitance behavior and the specific capacitance to be up to 677 Fg ^-1 at 1 A/g. Bi₂WO₆ versus Glassy carbon electrode (GCE) was also used to study the behavior of the Bi₂WO₆ modified electrodes in detecting ascorbic acid. This electrochemical sensor shows excellent electrocatalytic performance when ascorbic acid is present, as determined by differential pulse voltammetry. In solution, ascorbic acid diffuses to an electrode surface and controls its surface properties. Based on the results from the investigation, the sensor showed a detection sensitivity of 0.26 mM/mA, and a limit of detection (LOD) of 77.85 mM. It is clear from these results that Bi₂WO₆ may find application as an electrode material for supercapacitors and glucose sensors.

Composites containing resins and carbon nano-onions as efficient porous carbon materials for supercapacitors

Herein, we report the functionalization of carbon nano-onions (CNOs) with the hydroxyaryl group and subsequent modifications with resins: resorcinol-formaldehyde using porogenic Pluronic F-127, resorcinol-formaldehyde-melamine, benzoxazine made of bisphenol A and triethylenetetramine, and calix[4]resorcinarene-derived using F-127. Following the direct carbonization, extensive physicochemical analysis was carried out, including Fourier transform infrared, Raman and X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, and adsorption-desorption of N₂. The addition of CNO to the materials significantly increases the total pore volume (up to 0.932 cm³ g^-1 for carbonized resorcinol-formaldehyde resin and CNO (RF-CNO-C) and 1.242 cm³ g^-1 for carbonized resorcinol-formaldehyde-melamine resin and CNO (RFM-CNO-C)), with mesopores dominating. However, the synthesized materials have poorly ordered domains with some structural disturbance; the RFM-CNO-C composite shows a more ordered structure with amorphous and semi-crystalline regions. Subsequently, cyclic voltammetry and galvanostatic charge-discharge method studied the electrochemical properties of all materials. The influence of resins' compositions, CNO content, and amount of N atoms in carbonaceous skeleton on the electrochemical performance was studied. In all cases, adding CNO to the material improves its electrochemical properties. The carbon material derived from CNO, resorcinol and melamine (RFM-CNO-C) showed the highest specific capacitance of 160 F g^-1 at a current density of 2 A g^-1, which is stable after 3000 cycles. The RFM-CNO-C electrode retains approximately 97% of its initial capacitive efficiency. The electrochemical performance of the RFM-CNO-C electrode results from the hierarchical porosity's stability and the presence of nitrogen atoms in the skeleton. This material is an optimal solution for supercapacitor devices.

Atypical performance of CoO-accelerated interface tweaking in hierarchical cobalt phosphide/oxide@P-doped rGO heterostructures for hybrid supercapacitors

Designing two-dimensional (2D) heterostructures based on suitable energy materials is a promising strategy to achieve high-performance supercapacitors with hybridized transition metal and carbonaceous-based electrodes. The influence of each component and its content on the capacitor performance necessitates deeper insights. In this study, a 2D/2D heterostructure made of hierarchical pseudocapacitive cobalt phosphide/oxide and P-doped reduced graphene oxide (PrGO) nanosheets (CoP/CoO@PrGO) was fabricated using porous zeolitic-imidazolate framework precursor. The decoration of 2D leaf-like CoP/CoO hybrid onto PrGO could create a unique interface with a large number of active sites, CoO-driven creation of pseudocapacitive surface PO_x species, and high P content (∼3 at.%) in PrGO, thus promoting the Faradaic reaction, electrical conductivity, and overall charge storage. This framework yields a high specific capacitance of 405 F g^-1 at 5 A g^-1 and excellent cycling stability (over 100 % after 10,000 cycles), superior to those of pristine CoP@PrGO (300 F g^-1 at 5 A g^-1). Furthermore, the fabricated asymmetric supercapacitor delivers reasonable energy density of 4.2 Wh kg^-1 at a power density of 785 W kg^-1 and cycling stability of ∼100 % after 10,000 cycles. Therefore, CoP/CoO@PrGO with its unique interfacial properties can promote the development of heterostructure electrode for high-performance supercapacitors.

In Situ Synthesis of Ni-BTC Metal-Organic Framework@Graphene Oxide Composites for High-Performance Supercapacitor Electrodes

In response to serious ecological and environmental problems worldwide, a novel graphene oxide (GO) induction method for the in situ synthesis of GO/metal organic framework (MOF) composites (Ni-BTC@GO) for supercapacitors with excellent performance is presented in this study. For the synthesis of the composites, 1,3,5-benzenetricarboxylic acid (BTC) is used as an organic ligand due to its economic advantages. The optimum amount of GO is determined by a comprehensive analysis of morphological characteristics and electrochemical tests. 3D Ni-BTC@GO composites show a similar spatial structure to that of Ni-BTC, revealing that Ni-BTC could provide an effective framework and avoid GO aggregation. The Ni-BTC@GO composites have a more stable electrolyte-electrode interface and an improved electron transfer route than pristine GO and Ni-BTC. The synergistic effects of GO dispersion and Ni-BTC framework on electrochemical behavior are determined, where Ni-BTC@GO 2 achieves the best performance in energy storage performance. Based on the results, the maximum specific capacitance is 1199 F/g at 1 A/g. Ni-BTC@GO 2 has an excellent cycling stability of 84.47% after 5000 cycles at 10 A/g. Moreover, the assembled asymmetric capacitor exhibits an energy density of 40.89 Wh/kg at 800 W/kg, and it still remains at 24.44 Wh/kg at 7998 W/kg. This material is expected to contribute to the design of excellent GO-based supercapacitor electrodes.

Structural, Optical, Magnetic and Electrochemical Properties of CeXO2 (X: Fe, and Mn) Nanoparticles

CeXO₂ (X: Fe, Mn) nanoparticles, synthesized using the coprecipitation route, were investigated for their structural, morphological, magnetic, and electrochemical properties using X-ray diffraction (XRD), field emission transmission electron microscopy (FE-TEM), dc magnetization, and cyclic voltammetry methods. The single-phase formation of CeO₂ nanoparticles with FCC fluorite structure was confirmed by the Rietveld refinement, indicating the successful incorporation of Fe and Mn in the CeO₂ matrix with the reduced dimensions and band gap values. The Raman analysis supported the lowest band gap of Fe-doped CeO₂ on account of oxygen non-stoichiometry. The samples exhibited weak room temperature ferromagnetism, which was found to be enhanced in the Fe doped CeO₂. The NEXAFS analysis supported the results by revealing the oxidation state of Fe to be Fe²⁺/Fe³⁺ in Fe-doped CeO₂ nanoparticles. Further, the room temperature electrochemical performance of CeXO₂ (X: Fe, Mn) nanoparticles was measured with a scan rate of 10 mV s^-1 using 1 M KCL electrolyte, which showed that the Ce_0.95Fe_0.05O₂ electrode revealed excellent performance with a specific capacitance of 945 Fּ·g^-1 for the application in energy storage devices.

Organic Supercapacitors as the Next Generation Energy Storage Device: Emergence, Opportunity, and Challenges

Harnessing new materials for developing high-energy storage devices set off research in the field of organic supercapacitors. Various attractive properties like high energy density, lower device weight, excellent cycling stability, and impressive pseudocapacitive nature make organic supercapacitors suitable candidates for high-end storage device applications. This review highlights the overall progress and future of organic supercapacitors. Sustainable energy production and storage depend on low cost, large supercapacitor packs with high energy density. Organic supercapacitors with high pseudocapacitance, lightweight form factor, and higher device potential are alternatives to other energy storage devices. There are many recent ongoing research works that focus on organic electrolytes along with the material aspect of organic supercapacitors. This review summarizes the current research status and the chemistry behind the storage mechanism in organic supercapacitors to overcome the challenges and achieve superior performance for future opportunities.

Nanostructured mixed transition metal oxide spinels for supercapacitor applications

There have been numerous applications of supercapacitors in day-to-day life. Along with batteries and fuel cells, supercapacitors play an essential role in supplementary electrochemical energy storage technologies. They are used as power sources in portable electronics, automobiles, power backup, medical equipment, etc. Among various working electrode materials explored for supercapacitors, nanostructured transition metal oxides containing mixed metals are highly specific and special, because of their stability, variable oxidation states of the constituted metal ions, possibility to tune the mixed metal combinations, and existence of new battery types and extrinsic pseudocapacitance. This review presents the key features and recent developments in the direction of synthesis and electrochemical energy storage behavior of some of the recent morphology-oriented transition metal oxide and mixed transition metal oxide nanoparticles. We also targeted the studies on a few of the recently developed flexible and bendable supercapacitor devices based on these mixed transition metal oxides.

Square-Facet Nanobar MOF-Derived Co3O4@Co/N-doped CNT Core-Shell-based Nanocomposites as Cathode Materials for High-Performance Supercapacitor Studies

The binary as well as ternary nanocomposites of the square-facet nanobar Co-MOF-derived Co₃O₄@Co/N-CNTs (N-CNTs: nitrogen-doped carbon nanotubes) with Ag NPs and rGO have been synthesized via an easy wet chemical route, and their supercapacitor behavior was then studied. At a controlled pH of the precursor solution, square-facet nanobars of Co-MOF were first synthesized by the solvothermal method and then pyrolyzed under a controlled nitrogen atmosphere to get a core-shell system of Co₃O₄@Co/N-CNTs. In the second step, different compositions of Co₃O₄@Co/N-CNT core-shell structures were formed by an ex-situ method with Ag NPs and rGO moieties. Among several bare, binary, and ternary compositions tested in 6 M aqueous KOH electrolyte, a ternary nanocomposite having a 7.0:1.5:1.5 stoichiometric ratio of Co₃O₄@Co/N-CNT, Ag NPs, and rGO, respectively, reported the highest specific capacitance (3393.8 F g^-1 at 5 mV s^-1). The optimized nanocomposite showed the energy density, power density, and Coulombic efficiency of 74.1 W h.kg^-1, 443.7 W.kg^-1, and 101.3%, respectively, with excellent electrochemical stability. After testing an asymmetrical supercapacitor with a Co₃O₄@Co/N-CNT/Ag NPs/rGO/nickel foam cathode and an activated carbon/nickel foam anode, it showed 4.9 W h.kg^-1 of energy density and 5000.0 W.kg^-1 of power density.

In Situ Growth of MnO2 Nanosheets on a Graphite Flake as an Effective Binder-Free Electrode for High-Performance Supercapacitors

In this work, manganese dioxide (MnO₂) nanosheets in situ loaded on a high-purity graphite flake (GF) were prepared by one-step hydrothermal deposition. It was found that the specific capacitance value of a single MnO₂/GF electrode was 882 F/g at a current density of 1.0 A/g in a KOH electrolyte, and the specific capacitance retention of the MnO₂/GF electrode can reach about 90.1% after 5000 charge-discharge cycles at a current density of 10 A/g. Furthermore, a MnO₂/GF∥MnO₂/GF symmetric supercapacitor device was fabricated with two pieces of MnO₂/GF electrodes and ordinary filter paper with a 1 M KOH/PVA gel electrolyte as a separator. The single symmetric device displayed a high energy density of 64.2 Wh/kg at a power density of 400 W/kg within an applied voltage of 1.6 V, and this value was superior to those of previously reported MnO₂-based systems. A tandem device consisting of a five-series tandem device (the applied voltage of a single device was 0.7 V) and a three-series tandem device (the applied voltage of a single device was 1.6 V) was prepared to drive a red light-emitting diode (LED). These findings open up application prospects for MnO₂-based composite electrode materials for high-performance supercapacitors.

Nanostructured Titanium Nitride and Its Composites as High-Performance Supercapacitor Electrode Material

Electrochemical supercapacitors as an energy storage device have become trademark in current electronic, medical and industrial applications, as they are sources of impressive power output. Supercapacitors supply fast power output, suitable to cover the energy demand of future electronic devices. Electrode material design is a subject of intense research in the area of energy development and advancement, due to its essential role in the electrochemical process of charge storage and the cost of capacitors. The nano-dimensions allow for more electroactive sites, different pore size distributions, and a large specific surface area, making nanostructured electrode materials more promising. Electrode materials based on metal oxides, metal nitrides, and metal carbides are considered ideal for highly efficient electrochemical supercapacitors. Recently, much effort has been devoted to metal nitride-based electrodes and their diverse compositions as they possess higher electrical conductivity and better corrosion resistance, electrochemical stability, and chemical reactivity. Among these, titanium nitride (TiN), possesses high electrochemical stability, outstanding electrical conductivity, and a unique electronic structure. Nanocomposites based on titanium nitrides are known to deliver higher electrochemical performance than pristine nanostructured TiNs due to potential synergetic effects from both the materials. In this paper, recent advancements made in the field of nanostructural TiN electrode materials for SCs are reviewed along with their challenges and future opportunities. Additionally, some of the major techniques involved in the synthesis process are discussed, along with some basic concepts.

Supercapacitor Properties of rGO-TiO2 Nanocomposite in Two-component Acidic Electrolyte

The electrochemical properties of the highly porous reduced graphene oxide/titanium dioxide (rGO/TiO₂) nanocomposite were studied to estimate the possibility of using it as a supercapacitor electrode. Granular aerogel rGO/TiO₂ was used as an initial material for the first time of manufacturing the electrode. For the aerogel synthesis, industrial TiO₂ Hombikat UV100 with a high specific surface area and anatase structure was used, and the aerogel was carried out with hydrazine vapor. Porous structure and hydrophilic-hydrophobic properties of the nanocomposite were studied with a method of standard contact porosimetry. This is important for a supercapacitor containing an aqueous electrolyte. It was found that the hydrophilic specific surface area of the nanocomposite was approximately half of the total surface area. As a result of electrochemical hydrogenation in the region of zero potential according to the scale of a standard hydrogen electrode, a reversible Faraday reaction with high recharge rate (exchange currents) was observed. The characteristic charging time of the indicated Faraday reaction does not exceed several tens of seconds, which makes it possible to consider the use of this pseudocapacitance in the systems of fast energy storage such as hybrid supercapacitors. Sufficiently high limiting pseudo-capacitance (about 1200 C/g TiO₂) of the reaction was obtained.

Smart Electronic Textile-Based Wearable Supercapacitors

Electronic textiles (e-textiles) have drawn significant attention from the scientific and engineering community as lightweight and comfortable next-generation wearable devices due to their ability to interface with the human body, and continuously monitor, collect, and communicate various physiological parameters. However, one of the major challenges for the commercialization and further growth of e-textiles is the lack of compatible power supply units. Thin and flexible supercapacitors (SCs), among various energy storage systems, are gaining consideration due to their salient features including excellent lifetime, lightweight, and high-power density. Textile-based SCs are thus an exciting energy storage solution to power smart gadgets integrated into clothing. Here, materials, fabrications, and characterization strategies for textile-based SCs are reviewed. The recent progress of textile-based SCs is then summarized in terms of their electrochemical performances, followed by the discussion on key parameters for their wearable electronics applications, including washability, flexibility, and scalability. Finally, the perspectives on their research and technological prospects to facilitate an essential step towards moving from laboratory-based flexible and wearable SCs to industrial-scale mass production are presented.

Layer-by-Layer Heterostructure of MnO2@Reduced Graphene Oxide Composites as High-Performance Electrodes for Supercapacitors

In this paper, δ-MnO₂ with layered structure was prepared by a facile liquid phase method, and exfoliated MnO₂ nanosheet (e-MnO₂) was obtained by ultrasonic exfoliation, whose surface was negatively charged. Then, positive charges were grafted on the surface of MnO₂ nanosheets with a polycation electrolyte of polydiallyl dimethylammonium chloride (PDDA) in different concentrations. A series of e-MnO₂@reduced graphene oxide (rGO) composites were obtained by electrostatic self-assembly combined with hydrothermal chemical reduction. When PDDA was adjusted to 0.75 g/L, the thickness of e-MnO₂ was ~1.2 nm, and the nanosheets were uniformly adsorbed on the surface of graphene, which shows layer-by-layer morphology with a specific surface area of ~154 m²/g. On account of the unique heterostructure, the composite exhibits good electrochemical performance as supercapacitor electrodes. The specific capacitance of e-MnO₂-0.75@rGO can reach 456 F/g at a current density of 1 A/g in KOH electrolyte, which still remains 201 F/g at 10 A/g. In addition, the capacitance retention is 98.7% after 10000 charge-discharge cycles at 20 A/g. Furthermore, an asymmetric supercapacitor (ASC) device of e-MnO₂-0.75@rGO//graphene hydrogel (GH) was assembled, of which the specific capacitance achieves 94 F/g (1 A/g) and the cycle stability is excellent, with a retention rate of 99.3% over 10000 cycles (20 A/g).

Metal-Free Homocoupling of Pyrene inside the Pores of Mesoporous Carbons via Electrochemical Oxidation: Application for Electrochemical Capacitors

A pyrene dimer (PYD) is synthesized by electrochemical oxidation via homocoupling of pyrene (PY) inside the pores of MgO-templated mesoporous carbons without any metal catalysts or organic solvents. The resulting MgO-C/PYD hybrids can be used as high-performance aqueous electrochemical capacitor electrodes due to the reversible redox property of PYD and large contact area between the hybridized PYD and conductive carbon surfaces, which enable rapid charge transfer at the large contact interface. In our previous study, PY was considered to polymerize through electrochemical oxidation, and activated carbon with the pore sizes of ∼4 nm was used as a porous carbon substrate. In this study, the MgO-templated carbons have the average pore sizes of 5, 10, and 30 nm, and their large mesopore volumes can accommodate a large amount of PYD for enhancing the capacitance. To develop high-performance electrochemical capacitors, the dependence of the capacitance enhancement and the capacitance retention on the amount of PY and the pore sizes of MgO-templated carbons are studied. It is found that mesopores are necessary for fast charging/discharging, but the capacitance retention and capacitance enhancement decrease with increasing the mesopore sizes and the amount of PY due to the decreased utilization ratio of PY.

Carbon-α-Fe2O3 Composite Active Material for High-Capacity Electrodes with High Mass Loading and Flat Current Collector for Quasi-Symmetric Supercapacitors

In this work, we report the synthesis of an active material for supercapacitors (SCs), namely α-Fe2O3/carbon composite (C-Fe2O3) made of elongated nanoparticles linearly connected into a worm-like morphology, by means of electrospinning followed by a calcination/carbonization process. The resulting active material powder can be directly processed in the form of slurry to produce SC electrodes with mass loadings higher than 1 mg cm−2 on practical flat current collectors, avoiding the need for bulky porous substrate, as often reported in the literature. In aqueous electrolyte (6 M KOH), the so-produced C-Fe2O3 electrodes display capacity as high as ~140 mAh g−1 at a scan rate of 2 mV s−1, while showing an optimal rate capability (capacity of 32.4 mAh g−1 at a scan rate of 400 mV s−1). Thanks to their poor catalytic activity towards water splitting reactions, the electrode can operate in a wide potential range (−1.6 V–0.3 V vs. Hg/HgO), enabling the realization of performant quasi-symmetric SCs based on electrodes with the same chemical composition (but different active material mass loadings), achieving energy density approaching 10 Wh kg−1 in aqueous electrolytes.

A Comparative Review of Lead-Acid, Lithium-Ion and Ultra-Capacitor Technologies and Their Degradation Mechanisms

As renewable energy sources, such as solar systems, are becoming more popular, the focus is moving into more effective utilization of these energy sources and harvesting more energy for intermittency reduction in this renewable source. This is opening up a market for methods of energy storage and increasing interest in batteries, as they are, as it stands, the foremost energy storage device available to suit a wide range of requirements. This interest has brought to light the downfalls of batteries and resultantly made room for the investigation of ultra-capacitors as a solution to these downfalls. One of these downfalls is related to the decrease in capacity, and temperamentality thereof, of a battery when not used precisely as stated by the supplier. The usable capacity is reliant on the complete discharge/charge cycles the battery can undergo before a 20% degradation in its specified capacity is observed. This article aims to investigate what causes this degradation, what aggravates it and how the degradation affects the usage of the battery. This investigation will lead to the identification of a gap in which this degradation can be decreased, prolonging the usage and increasing the feasibility of the energy storage devices.

Conjugated Polyelectrolytes: Underexplored Materials for Pseudocapacitive Energy Storage

Conjugated polyelectrolytes (CPEs) are characterized by an electronically delocalized backbone bearing ionic functionalities. These features lead to properties relevant for use in energy-storing pseudocapacitor devices, including ionic conductivity, water processability, gel-formation, and formation of polaronic species stabilized by electrostatic interactions. In this Perspective, the basis for evaluating the figures of merit for pseudocapacitors is provided, together with the techniques used for their evaluation. The general utility and challenges encountered with neutral conjugated polymers are then discussed. Finally, recent advances on the use of CPEs in pseudocapacitor devices are reviewed. The article is concluded by discussing how their miscibility in aqueous media permits the incorporation of CPEs in living materials that are capable of switching function from extraction of energy from bacterial metabolic pathways to pseudocapacitor energy storage.

Effective regeneration of mixed composition of spent lithium-ion batteries electrodes towards building supercapacitor

Recycling of different manufacturers of spent lithium-ion batteries cathode and anode via a simple regeneration process has an opportunity to fabricate new energy devices. In this study, the different manufacturers of spent LIB cathode pieces were subjected to lixiviation process and found the best-optimized conditions such as tartaric acid concentration (2.5 M), H₂O₂ concentration (7.5 vol%), solid-liquid ratio (80 g/L), temperature (80 °C), and lixiviation time (80 min) for maximum ~ 99% extraction efficiency of metals. Further, 3D-MnCo₂O₄ (MCO) spheres were regenerated from the cathode lixivium containing metal ions via hydrothermal technique. Besides, anode graphite and Al foils after cathode lixiviation were exploited to prepare reduced graphene oxide (RGO) at room temperature in a simple method. The electrochemical performance of both regenerated electrodes from spent LIBs was explored in the half-cell configuration using the 1 M Na₂SO₄ electrolyte. Additionally, the constructed MCO//RGO asymmetric supercapacitor device offers an operational voltage of 1.8 V and displays a high energy density of ~ 23.9 Wh kg^-1 at 450 W kg^-1 with 8000 cycles. This alternative recycling method proposes the possibility to construct high-energy storage devices from different compositions of spent LIBs.

Synthesis of Needle-like Nanostructure Composite Electrode of Co3O4/rGO/NF for High-Performance Symmetric Supercapacitor

In this work, a hierarchical electrode structure of cobaltosic oxide (Co3O4) growing on a reduced graphene oxide (rGO)-covered nickel foam (NF) substrate (named Co3O4/rGO/NF) is fabricated by a facile hydrothermal and subsequent annealing process. Thousands of nanoneedle units uniformly arranged on the surface of the rGO sheet stimulate the evident increase in the specific surface area and thus produce more active sites. Because of the special hierarchical structure, the Co3O4/rGO/NF electrode shows a high specific capacitance of 1400 F g−1 at 1 A g−1 and retains 58% capacitance even when the current density increases to 30 A g−1. In addition, a symmetric supercapacitor based on the Co3O4/rGO/NF electrode is assembled, exhibiting high specific capacitance of 311 F g−1 at 1 A g−1, as well as remarkable power density and energy density (40.67 Wh kg−1 at 12 kW kg−1). The device also demonstrates a great cycling performance after 10,000 cycles under the current density of 10 A g−1, acquiring 89.69% capacitance retention of the initial state. The accessible synthetic method and superior electrochemical performance of the Co3O4/rGO/NF composite electrode implicate its extensive application prospects in terms of new energy storage.

Unveiling the Redox Electrochemistry of MOF-Derived fcc-NiCo@GC Polyhedron as an Advanced Electrode Material for Boosting Specific Energy of the Supercapattery

Metal organic frameworks (MOFs), which constitute a new class of porous organic-inorganic hybrid materials, have gained considerable attention in the fields of electrochemical energy storage and conversion devices owing to their open topological structures, large surface areas, tunable morphologies, and extreme redox activity. A synthesis protocol that comprises coprecipitation followed by controlled calcination processes to design a battery-type electrode is used. This electrode consists of three-dimensional (3D), ant cave-like polyhedrons of nickel-cobalt alloy on graphitic carbon (GC; NiCo@GC) nanostructures; trimesic acid is used as a potential MOF-linker. The developed NiCo@GC sample exhibits mesoporous characteristics with the maximum surface area of 94.08 m² g^-1 at 77 K. In addition, the redox activity at different sweep rates reveals the battery-type charge storage behavior of the NiCo@GC electrode; its three-electrode assembly provides 444 C g^-1 specific capacity at 2 A g^-1 with long-term capacity retention. The constructed supercapattery (SC) devices (i.e., AC//NiCo@GC) achieved capacity, specific energy, and specific power are 74.3 mAh g^-1 , 39.5 Wh kg^-1 , and 665 W kg^-1 , respectively. Owing to its reasonable electrochemical characteristics, the prepared NiCo@GC material is a promising candidate for supercapattery electrodes for portable electronic devices.

Computation and Investigation of Two-Dimensional WO3·H2O Nanoflowers for Electrochemical Studies of Energy Conversion and Storage Applications

The aim of this study is to prepare a two-dimensional (2D) WO3·H2O nanostructure assembly into a flower shape with good chemical stability for electrochemical studies of catalyst and energy storage applications. The 2D-WO3·H2O nanoflowers structure is created by a fast and simple process at room condition. This cost-effective and scalable technique to obtain 2D-WO3·H2O nanoflowers illustrates two attractive applications of electrochemical capacitor with an excellent energy density value of 25.33 W h kg-1 for high power density value of 1600 W kg-1 and good hydrogen evolution reaction results (low overpotential of 290 mV at a current density of 10 mA cm-2 with a low Tafel slope of 131 mV dec-1). A hydrogen evolution reaction (HER) study of WO3 in acidic media of 0.5 M H2SO4 and electrochemical capacitor (supercapacitors) in 1 M Na2SO4 aqueous electrolyte (three electrode system measurements) demonstrates highly desirable characteristics for practical applications. Our design for highly uniform 2D-WO3·H2O as catalyst material for HER and active material for electrochemical capacitor studies offers an excellent foundation for design and improvement of electrochemical catalyst based on 2D-transition metal oxide materials.