Understanding the Behavior of Supercapacitor Materials via Electrochemical Impedance Spectroscopy: A Review

Energy harvesting and energy storage are two critical aspects of supporting the energy transition and sustainability. Many studies have been conducted to achieve excellent performance devices for these two purposes. As energy-storing devices, supercapacitors (SCs) have tremendous potential to be applied in several sectors. Some electrochemical characterizations define the performance of SCs. Electrochemical impedance spectroscopy (EIS) is one of the most powerful analyses to determine the performance of SCs. Some parameters obtained from this analysis include bulk resistance, charge-transfer resistance, total resistance, specific capacitance, response frequency, and response time. This work provides a holistic and comprehensive review of utilizing EIS for SC characterization. Overall, researchers can benefit from this review by gaining a comprehensive understanding of the utilization of electrochemical impedance spectroscopy (EIS) for characterizing supercapacitors (SCs), enabling them to enhance SC performance and contribute to the advancement of energy harvesting and storage technologies.

Rational Design of High-Performance Electrodes Based on Ferric Oxide Nanosheets Deposited on Reduced Graphene Oxide for Advanced Hybrid Supercapacitors

The core strategy for constructing ultra-high-performance hybrid supercapacitors is the design of reasonable and effective electrode materials. Herein, a facile solvothermal-calcination strategy is developed to deposit the phosphate-functionalized Fe2O3 (P-Fe2O3) nanosheets on the reduced graphene oxide (rGO) framework. Benefiting from the superior conductivity of rGO and the high conductivity and fast charge storage dynamics of phosphate ions, the synthesized P-Fe2O3/rGO anode exhibits remarkable electrochemical performance with a high capacitance of 586.6 F g-1 at 1 A g-1 and only 4.0% capacitance loss within 10 000 cycles. In addition, the FeMoO4/Fe2O3/rGO nanosheets are fabricated by utilizing Fe2O3/rGO as the precursor. The introduction of molybdates successfully constructs open ion channels between rGO layers and provides abundant active sites, enabling the excellent electrochemical features of FeMoO4/Fe2O3/rGO cathode with a splendid capacity of 475.4 C g-1 at 1 A g-1. By matching P-Fe2O3/rGO with FeMoO4/Fe2O3/rGO, the constructed hybrid supercapacitor presents an admirable energy density of 82.0 Wh kg-1 and an extremely long working life of 95.0% after 20 000 cycles. Furthermore, the continuous operation of the red light-emitting diode for up to 30 min demonstrates the excellent energy storage properties of FeMoO4/Fe2O3/rGO//P-Fe2O3/rGO, which provides multiple possibilities for the follow-up energy storage applications of the iron-based composites.

Fabrication of Mesoporous Materials Based on Supramolecular Self-Assembly of Guanosine Monophosphonate-Nickel Chloride (GMP-Ni) for High-Performance Hybrid Supercapacitors

Supramolecular self-assembly of nickel chloride and guanosine mono-phosphonate (GMP) and nickel (Ni)-based GMP-Ni and their calcinated mesoporous electrode materials GMP-Ni-500 and GMP-Ni-700 at 500 and 700 °C, respectively, have been fabricated. GMP-Ni, GMP-Ni-500, and GMP-Ni-700 are examined for their supercapacitor performance in a three-electrode configuration. The electrochemical tests demonstrate the mesoporous battery-type nature of GMP-Ni-500 which exhibited a specific capacity (Cs) of about 289 C g-1 at 0.5 A g-1 current density. In addition, a cost-effective and simple asymmetric supercapacitor device has been fabricated with battery-type GMP-Ni-500 as a cathode material and capacitive-type activated carbon (AC) as an anodic material. In an operating voltage window of 0 to 1.5 V, hybrid supercapacitors (HSCs) based on GMP-Ni-500//AC exhibited a remarkable performance with a specific capacity (Cs) of 144 C g-1 at 0.5 A g-1. For the HSC device, the maximum of 66% capacity retention has been observed after 5000 charging/discharging cycles at 5 A g-1. Furthermore, the HSC device demonstrates a high energy density of 24 W h kg-1 at a power density of 297 W kg-1. The molecular transformation was established by employing theoretical calculations. These results suggest that our HSC has outstanding potential in technology development for next-generation commercial applications.

Regulating the Oxygen Vacancy and Electronic Structure of NiCo Layered Double Hydroxides by Molybdenum Doping for High-Power Hybrid Supercapacitors

Amelioration of nickel-cobalt layered double hydroxides (NiCo-LDH) with a high specific theoretical capacitance is of great desire for high-power supercapacitors. Herein, a molybdenum (Mo) doping strategy is proposed to improve the charge-storage performance of NiCo-LDH nanosheets growing on carbon cloth (CC) via a rapid microwave process. The regulation of the electronic structure and oxygen vacancy of the LDH is consolidated by the density functional theory (DFT) calculation, which demonstrates that Mo doping narrows the band gap, reduces the formation energy of hydroxyl vacancies, and promotes ionic and charge transfer as well as electrolyte adsorption on the electrode surface. The optimal Mo-doped NiCo-LDH electrode (MoNiCo-LDH-0.05/CC) has an amazing specific capacity of 471.1 mA h g-1 at 1 A g-1 , and excellent capacity retention of 84.8% at 32 A g-1 , far superior to NiCo-LDH/CC (258.3 mA h g-1 and 76.4%). The constructed hybrid supercapacitor delivers an energy density of 103.3 W h kg-1 at a power density of 750 W kg-1 and retains the cycle retention of 85.2% after 5000 cycles. Two assembled devices in series can drive thirty LED lamps, revealing a potential application prospect of the rationally synthesized MoNiCo-LDH/CC as an energy-storage electrode material.

CoOx @NiMoN/Ti3 C2 Tx Interface for Stable High-Performance Electrochemical Energy Storage Devices

Transition metal nitrides (TMNs) are promising electrode materials for use in high-performance electrochemical energy storage devices due to their unique properties, which include a high conductivity, pseudocapacitance, and energy density. However, structural instability during electrochemical reactions has limited their practical deployment for energy storage devices. In this context, the present study fabricated a CoOx @NiMoN/Ti3 C2 Tx electrode via in situ growth on Ni foam using hydrothermal treatment with post-nitrogenization. The effect of atomic layer deposition (ALD) of CoOx on the TMN/Ti3 C2 Tx interface and the consequent electrochemical charge storage mechanisms are investigated in detail. The proposed CoOx @NiMoN/Ti3 C2 Tx electrode delivers an impressive specific capacity in a 2 m potassium hydroxide (KOH) electrolyte and is then employed in both a hybrid solid-state supercapacitor (HSSC) with reduced graphene oxide and a symmetric SC in a 2 m KOH + polyvinyl alcohol (PVA) gel electrolyte. Outstanding charge storage and high capacity retention during cyclic testing are observed for both energy storage devices. The exceptional electrochemical performance of the fabricated electrode is a result of its high conductivity and high number of active sites. Here a feasible new strategy is demonstrated for the fabrication of stable energy storage devices with a high energy density using TMNs and MXenes.

Advanced High-Energy All-Solid-State Hybrid Supercapacitor with Nickel-Cobalt-Layered Double Hydroxide Nanoflowers Supported on Jute Stick-Derived Activated Carbon Nanosheets

Developing efficient, lightweight, and durable all-solid-state supercapacitors is crucial for future energy storage systems. The study focuses on optimizing electrode materials to achieve high capacitance and stability. This study introduces a novel two-step pyrolysis process to synthesize activated carbon nanosheets from jute sticks (JAC), resulting in an optimized JAC-2 material with a high yield (≈24%) and specific surface area (≈2600 m2  g-1 ). Furthermore, an innovative in situ synthesis approach is employed to synthesize hybrid nanocomposites (NiCoLDH-1@JAC-2) by integrating JAC nanosheets with nickel-cobalt-layered double hydroxide nanoflowers (NiCoLDH). These nanocomposites serve as positive electrode materials and JAC-2 as the negative electrode material in all-solid-state asymmetric hybrid supercapacitors (HSCs), exhibiting remarkable performance metrics. The HSCs achieve a specific capacitance of 750 F g-1 , a specific capacity of 209 mAh g-1 (at 0.5 A g-1 ), and an energy density of 100 Wh kg-1 (at 250 W kg-1 ) using PVA/KOH solid electrolyte, while maintaining outstanding cyclic stability. Importantly, a density functional theory framework is utilized to validate the experimental findings, underscoring the potential of this novel approach for enhancing HSC performance and enabling the large-scale production of transition metal-based layered double hydroxides.

2D Conjugated Metal-Organic Frameworks Embedded with Iodine for High-Performance Ammonium-Ion Hybrid Supercapacitors

Ammonium ions (NH4 + ) are emerging non-metallic charge carriers for advanced electrochemical energy storage devices, due to their low cost, elemental abundance, and environmental benignity. However, finding suitable electrode materials to achieve rapid diffusion kinetics for NH4 + storage remains a great challenge. Herein, a 2D conjugated metal-organic framework (2D c-MOF) for immobilizing iodine, as a high-performance cathode material for NH4 + hybrid supercapacitors, is reported. Cu-HHB (HHB = hexahydroxybenzene) MOF embedded with iodine (Cu-HHB/I2 ) features excellent electrical conductivity, highly porous structure, and rich accessible active sites of copper-bis(dihydroxy) (Cu─O4 ) and iodide species, resulting in a remarkable areal capacitance of 111.7 mF cm-2 at 0.4 mA cm-2 . Experimental results and theoretical calculations indicate that the Cu─O4 species in Cu-HHB play a critical role in binding polyiodide and suppressing its dissolution, as well as contributing to a large pseudocapacitance with adsorbed iodide. In combination with a porous MXene anode, the full NH4 + hybrid supercapacitors deliver an excellent energy density of 31.5 mWh cm-2 and long-term cycling stability with 89.5% capacitance retention after 10 000 cycles, superior to those of the state-of-the-art NH4 + hybrid supercapacitors. This study sheds light on the material design for NH4 + storage, enabling the development of novel high-performance energy storage devices.

Electrode Materials, Structural Design, and Storage Mechanisms in Hybrid Supercapacitors

Currently, energy storage systems are of great importance in daily life due to our dependence on portable electronic devices and hybrid electric vehicles. Among these energy storage systems, hybrid supercapacitor devices, constructed from a battery-type positive electrode and a capacitor-type negative electrode, have attracted widespread interest due to their potential applications. In general, they have a high energy density, a long cycling life, high safety, and environmental friendliness. This review first addresses the recent developments in state-of-the-art electrode materials, the structural design of electrodes, and the optimization of electrode performance. Then we summarize the possible classification of hybrid supercapacitor devices, and their potential applications. Finally, the fundamental theoretical aspects, charge-storage mechanism, and future developing trends are discussed. This review is intended to provide future research directions for the next generation of high-performance energy storage devices.

Porous Carbon Template Decorated with MOF-Driven Bimetallic Phosphide: A Suitable Heterostructure for the Production of Uninterrupted Green Hydrogen via Renewable Energy Storage Device

Water splitting via an uninterrupted electrochemical process through hybrid energy storage devices generating continuous hydrogen is cost-effective and green approach to address the looming energy and environmental crisis toward constant supply of hydrogen fuel in fuel cell driven automobile sector. The high surface area metal-organic framework (MOF) driven bimetallic phosphides (ZnP2 @CoP) on top of CNT-carbon cloth matrix is utilized as positive and negative electrodes in energy storage devices and overall water splitting. The as-prepared positive electrode exhibits excellent specific capacitances/capacity of 1600 F g-1 /800 C g-1 @ 1A g-1 and the corresponding hybrid device reveals an energy density of 83.03 Wh kg-1 at power density of 749.9 W kg-1 . Simultaneously, the electrocatalytic performance of heterostructure shows overpotentials of 90 mV@HER and 204 mV@OER at current density of 10 and 20 mA cm-2 , respectively in alkaline electrocatalyzer. Undoubtedly, it shows overall water splitting with low cell voltage of 1.53 V@10 mA cm-2 having faradic and solar-to-hydrogen conversion efficiency of 98.81% and 9.94%, respectively. In addition, the real phase demonstration of the overall water-splitting is performed where the electrocatalyzer is connected with a series of hybrid supercapacitor devices powered up by the 6 V standard silicon solar panel to produce uninterrupted green H2 .

Impact of Iodine Electrodeposition on Nanoporous Carbon Electrode Determined by EQCM, XPS and In Situ Raman Spectroscopy

The charging of nanoporous carbon via electrodeposition of solid iodine from iodide-based electrolyte is an efficient and ecofriendly method to produce battery cathodes. Here, the interactions at the carbon/iodine interface from first contact with the aqueous electrolyte to the electrochemical polarization conditions in a hybrid cell are investigated by a combination of in situ and ex situ methods. EQCM investigations confirm the flushing out of water from the pores during iodine formation at the positive electrode. XPS of the carbon surface shows irreversible oxidation at the initial electrolyte immersion and to a larger extent during the first few charge/discharge cycles. This leads to the creation of functional groups at the surface while further reactive sites are consumed by iodine, causing a kind of passivation during a stable cycling regime. Two sources of carbon electrode structural modifications during iodine formation in the nanopores have been revealed by in situ Raman spectroscopy, (i) charge transfer and (ii) mechanical strain, both causing reversible changes and thus preventing performance deterioration during the long-term cycling of energy storage devices that use iodine-charged carbon electrodes.

Carbon-Shielded Selenium-Rich Trimetallic Selenides as Advanced Electrode Material for Durable Li-Ion Batteries and Supercapacitors

In order to achieve a sustainable future, researchers must continue to research improved electrode materials. Considering the high electronic conductivity, versatile redox activity, and enhanced energy storage performance, nanostructures have been employed as a novel electrode material for high-performance lithium-ion batteries (LIBs) and supercapacitors. Herein, carbon-coated selenium-rich trimetallic selenide (Cu₂ NiSnSe₄ @C) nanoparticles (NPs) as an efficient electrode material in energy storage devices are prepared. The prepared core-shell Cu₂ NiSnSe₄ @C NPs electrode is employed as an anode material for LIBs, which demonstrated a high reversible specific capacity of 988.46 mA h g^-1 over 100 cycles at 0.1 A g^-1 with good rate capability. Additionally, the core-shell Cu₂ NiSnSe₄ @C NPs electrode exhibited an outstanding capacity of 202.5 mA h g^-1 at 5 A g^-1 even after 10 000 cycles. Exploiting the synergistic characteristics, the core-shell Cu₂ NiSnSe₄ @C NPs material is also investigated as a battery-type electrode for hybrid supercapacitors. The assembled hybrid supercapacitor with Cu₂ NiSnSe₄ @C NPs and activated carbon showed excellent rate capability including high power (5597.77 W kg^-1 ) and energy (64.26 Wh kg^-1 ) densities. Considering the simple synthesis and enhanced energy storage properties, carbon-coated selenium-rich trimetallic selenide can be used as a durable electrode material for practical energy storage devices.

Mesoporous Cobalt Oxide (CoOx) Nanowires with Different Aspect Ratios for High Performance Hybrid Supercapacitors

Cobalt oxide (CoOx) nanowires have been broadly explored as advanced pseudocapacitive materials owing to their impressive theoretical gravimetric capacity. However, the traditional method of compositing with conductive nanoparticles to improve their poor conductivity will unpredictably lead to a decrease in actual capacity. The amelioration of the aspect ratio of the CoOx nanowires may affect the pathway of electron conduction and ion diffusion, thereby improving the electrochemical performances. Here, CoOx nanowires with various aspect ratios were synthesized by controlling hydrothermal temperature, and the CoOx electrodes achieve a high gravimetric specific capacity (1424.8 C g-1) and rate performance (38% retention at 100 A g-1 compared to 1 A g-1). Hybrid supercapacitors (HSCs) based on activated carbon anode reach an exceptional specific energy of 61.8 Wh kg-1 and excellent cyclic performance (92.72% retention, 5000 cycles at 5 A g-1). The CoOx nanowires exhibit great promise as a favorable cathode material in the field of high-performance supercapacitors (SCs).

Zn-Metal-Organic Framework Derived Ordered Mesoporous Carbon-Based Nanostructure for High-Performance and Universal Multivalent Metal Ion Storage

Metal-organic framework (MOF) derivatives promise great potential in energy storage and conversion because of their excellent tunability in both the active metal sites, organic links, and the overall structures down to atomic and up to mesoscale. Nevertheless, a big challenge is to precisely control and thoroughly understand the actual MOF-to-derivative conversion process to realize the template-free synthesis of the MOF-derived ordered mesoporous materials. Here, a class of ordered mesoporous N-doped carbon nanoflakes is presented with slit-shaped pores synthesized by one-step pyrolysis of Zn₁ Cu_x -MOF, where the Cu doping plays a critically important direction-inducing function on the dissociation of organic ligands during the pyrolysis. Benefiting from the uniquely ordered mesoporous structure and large specific surface area (910 m² g^-1 ), the Zn₁ Cu_x -MOF-derived ordered mesoporous carbon nanoflakes present outstanding electrochemical storage performance for multivalent metal ions, such as Mg²⁺ , Ca²⁺ , Co²⁺ , Ni²⁺ , Al³⁺ , and Zn²⁺ , demonstrating the universal nature of the slit-shaped pores in enabling the multivalent metal ions for energy storage. Moreover, the assembled flexible Zn-ion hybrid supercapacitor (ZHSC) delivers a high specific capacity of 134 mAh g^-1 at 0.5 A g^-1 , excellent cycling and mechanical stability, showing great application potential in the new generation energy storage devices.

Hierarchical Nanocages Assembled by NiCo-Layered Double Hydroxide Nanosheets for a High-Performance Hybrid Supercapacitor

Layered double hydroxides (LDHs) have attracted broad attention as cathode materials for hybrid supercapacitors (HSCs) because of their ultrahigh theoretical specific capacitance, high compositional flexibility, and adjustable interlayer spacing. However, as reported, specific capacitance of LDHs is still far below the theoretical value, inspiring countless efforts to these ongoing challenges. Herein, a hierarchical nanocage structure assembled by NiCo-LDH nanosheet arrays was rationally designed and fabricated via a facile solvothermal method assisted by the ZIF-67 template. The transformation from the ZIF-67 template to this hollow structure is achieved by a synergistic effect involving the Kirkendall effect and the Ostwald ripening process. The enlarged specific surface area co-occurred with broadened interlayer spacing of LDH nanosheets by finely increasing the Ni concentration, leading to synchronous improvement of electron/ion transfer kinetics. The optimized NiCo-LDH-210 electrode displays a maximum specific capacitance of 2203.6 F g^-1 at 2 A g^-1, excellent rate capability, and satisfactory cycling stability because of the highly exposed active sites and shortened ion transport paths provided by vertically aligned LDH nanosheets together with the cavity. Furthermore, the assembled HSC device achieves a superior energy density of 57.3 Wh kg^-1 with prominent cycling stability. Impressively, the design concept of complex construction derived from metal-organic frameworks (MOF) derivatives shows tremendous potential for use in energy storage systems.

Zinc-Ion Hybrid Supercapacitors Employing Acetate-Based Water-in-Salt Electrolytes

Halide-free, water-in-salt electrolytes (WiSEs) composed of potassium acetate (KAc) and zinc acetate (ZnAc₂ ) are investigated as electrolytes in zinc-ion hybrid supercapacitors (ZHSs). Molecular dynamics simulations demonstrate that water molecules are mostly non-interacting with each other in the highly concentrated WiSEs, while "bulk-like water" regions are present in the dilute electrolyte. Among the various concentrated electrolytes investigated, the 30 m KAc and 1 m ZnAc₂ electrolyte (30K1Zn) grants the best performance in terms of reversibility and stability of Zn plating/stripping while the less concentrated electrolyte cannot suppress corrosion of Zn and hydrogen evolution. The ZHSs utilizing 30K1Zn, in combination with a commercial activated carbon (AC) positive electrode and Zn as the negative electrode, deliver a capacity of 65 mAh g^-1 (based on the AC weight) at a current density of 5 A g^-1 . They also offer an excellent capacity retention over 10 000 cycles and an impressive coulombic efficiency (≈100%).

All Transition Metal Selenide Composed High-Energy Solid-State Hybrid Supercapacitor

Transition metal selenides (TMSs) have enthused snowballing research and industrial attention due to their exclusive conductivity and redox activity features, holding them as great candidates for emerging electrochemical devices. However, the real-life utility of TMSs remains challenging owing to their convoluted synthesis process. Herein, a versatile in situ approach to design nanostructured TMSs for high-energy solid-state hybrid supercapacitors (HSCs) is demonstrated. Initially, the rose-nanopetal-like NiSe@Cu₂ Se (NiCuSe) positive electrode and FeSe nanoparticles negative electrode are directly anchored on Cu foam via in situ conversion reactions. The complementary potential windows of NiCuSe and FeSe electrodes in aqueous electrolytes associated with the excellent electrical conductivity results in superior electrochemical features. The solid-state HSCs cell manages to work in a high voltage range of 0-1.6 V, delivers a high specific energy density of 87.6 Wh kg^-1 at a specific power density of 914.3 W kg^-1 and excellent cycle lifetime (91.3% over 10 000 cycles). The innovative insights and electrode design for high conductivity holds great pledge in inspiring material synthesis strategies. This work offers a feasible route to develop high-energy battery-type electrodes for next-generation hybrid energy storage systems.

Toward Enhanced Electrochemical Performance by Investigation of the Electrochemical Reconstruction Mechanism in Co2V2O7 Hexagonal Nanosheets for Hybrid Supercapacitors

As for hybrid supercapacitors, it is important to enhance the long cycling performance and high specific capacitance. In this paper, cobalt vanadate (Co₂V₂O₇) hexagonal nanosheets on nickel foam are manufactured by a facile hydrothermal method and then transformed into numerous smaller size interconnected hierarchical nanosheets without any shape change via electrochemical reconstruction. Benefiting from the favorable architecture of hierarchical nanosheets via electrochemical reconstruction, the Co₂V₂O₇ hexagonal nanosheet electrode exhibits a remarkable long cycling performance with 272% specific capacitance retention after 100,000 cycles at a current density of 5 A g^-1 and then displays an increasing specific capacitance of 1834 F g^-1 (tested at 1 A g^-1). Furthermore, an aqueous hybrid supercapacitor device based on the Co₂V₂O₇ hexagonal nanosheet electrode exhibits a high energy density of 35.2 Wh kg^-1 at a power density of 1.01 kW kg^-1 and an excellent cyclic stability with 71.4% capacitance retention after 10,000 cycles at 5 A g^-1. These results offer a practicable pathway for enhancing the electrochemical properties of other metal oxides through electrochemical reconstruction.

High Energy Density in Combination with High Cycling Stability in Hybrid Supercapacitors

Hybrid supercapacitors are considered the next-generation energy storage equipment due to their superior performance. In hybrid supercapacitors, battery electrodes need to have large absolute capacities while displaying high cycling stability. However, enhancing areal capacity via decreasing the size of electrode materials results in reductions in cycling stability. To balance the capacity-stability trade-off, rationally designed proper electrode structures are in urgent need and still of great challenge. Here we report a high-capacity and high cycling stability electrode material by developing a nickel phosphate lamination structure with ultrathin nanosheets as building blocks. The nickel phosphate lamination electrode material exhibits a large specific capacity of 473.9 C g^-1 (131.6 mAh g^-1, 1053 F g^-1) at 2.0 A g^-1 and only about 21% capacity loss at 15 A g^-1 (375 C g^-1, 104.2 mAh g^-1, 833.3 F g^-1) in 6.0 M KOH. Furthermore, hybrid supercapacitors are constructed with nickel phosphate lamination and activated carbon (AC), possessing high energy density (42.1 Wh kg^-1 at 160 W kg^-1) as well as long cycle life (almost 100% capacity retention after 1000 cycles and 94% retention after 8000 cycles). The electrochemical performance of the nickel phosphate lamination structure not only is commensurate with the nanostructure or ultrathin materials carefully designed in supercapacitors but also has a longer cycling lifespan than them. The encouraging results show the great potential of this material for energy storage device applications.

Nickel Cobaltite: A Positive Electrode Material for Hybrid Supercapacitors

The increased demand of energy due to the recent technological advances in diverse fields such as portable electronics and electric vehicles is often hindered by the poor capability of energy-storage systems. Although supercapacitors (SCs) exhibit higher power density than state-of-the art batteries, their insufficient energy density remains a major challenge. An emerging concept of hybrid supercapacitors (HSCs) with the combination of one capacitive and one battery electrode in a single cell holds a great promise to deliver high energy density without sacrificing power density and cycling stability. This Minireview elaborates the recent advances of use of nickel cobaltite (NiCo₂ O₄ ) as a potential positive electrode (battery-like) for HSCs. A brief introduction on the structural benefits and charge storage mechanisms of NiCo₂ O₄ was provided. It further shed a light on composites of NiCo₂ O₄ with different materials like carbon, polymers, metal oxides, and others, which altogether helps in increasing the electrochemical performance of HSCs. Finally, the key scientific challenges and perspectives on building high-performance HSCs for future-generation applications were reviewed.

CuCo LDHs Coated CuCoTe Honeycomb-Like Nanosheets as a Novel Anode Material for Hybrid Supercapacitors

Metal-organic frameworks derived metal chalcogenides as a new class of active materials can abolish the existing challenges in supercapacitors with their large electroactive sites and enhanced electrochemical conductivities. With its adequate conductivity and electrochemical properties, tellurium based metal chalcogenide electrodes can deliver better electrochemical performances than other chalcogenides. Herein, CuCoTe honeycomb-like nanosheets are grown on nickel foam (CuCoTe HNSs/NF) and then CuCo layered double hydroxides are successively coated on them (CTC HLSs/NF). The CTC HLSs/NF electrode exhibits tremendous performance with its high specific capacity of 399 mAh g^-1 at 7 A g^-1 of current density and good capacity retention (81.3%) after 3000 cycles. Finally, CTC HLSs/NF electrode is utilized for the hybrid supercapacitor (HSC) assembly along with activated carbon coated nickel foam in an aqueous electrolyte. The fabricated HSC shows high energy density (214.7 Wh kg^-1 ) and power density (40 kW kg^-1 ). Moreover, the device retains 96.3% of its capacitance at the end of the 5000th cycle, showing its high stability. Owing to their unique morphology and superior electrochemical properties, the present method of fabrication and selected materials can address the issues faced by electrochemical capacitors.

Site-Selective Transformation for Preparing Tripod-like NiCo-Sulfides@Carbon Boosts Enhanced Areal Capacity and Cycling Reliability

Flexible power supply systems for future wearable electronics desperately require high areal capacity (C_a) and robust cycling reliability due to the limited surface area of the human body. Transition metal sulfides are preferred as cathode materials for their improved conductivity and rich redox centers, yet their practical applications are severely hindered by the sluggish charge transport kinetics and unavoidable capacity decay due to the phase transformation during charge/discharge processes. Herein, we develop a site-selective transformation strategy for preparing tripod-like NiCo-sulfides@carbon (T-NCS@C) arrays on carbon cloth. The mass loading of active materials is balanced with charge (electron and ion) transport efficiency. The optimized T-NCS@C delivers a superior C_a of 494 μA h/cm² (corresponding to 235 mA h/g) at 3 mA/cm². Due to the protection of the carbon layer that is derived from transformed metal-organic framework (MOF) sheath, the T-NCS@C displays excellent stability with 92% retention over 5000 charge/discharge cycles. The flexible full cell adopting Fe₂O₃ as the anode and T-NCS@C as the cathode exhibits an improved E_a (areal energy density) of 389 μW h/cm² at a P_a (areal power density) of 4.22 mW/cm² together with robust cycling reliability.

Design and Fabrication of Hierarchical NiCoP-MOF Heterostructure with Enhanced Pseudocapacitive Properties

Metal-organic framework (MOF)-derived heterostructures possessing the merits of each component are thought to display the enhanced energy storage performance due to their synergistic effect. Herein, a functional heterostructure (NiCoP-MOF) composed of nickel/cobalt-MOF (NiCo-MOF) and phosphide (NiCoP) is designed and fabricated via the localized phosphorization of unusual lamellar brick-stacked NiCo-MOF assemblies obtained by a hydrothermal method. The experimental and computational analyses reveal that such-fabricated heterostructures possess the modulated electronic structure, abundant active sites, and hybrid crystalline feature, which is kinetically beneficial for fast electron/ion transport to enhance the charge storage capability. Examined as the supercapacitor electrode, the obtained NiCoP-MOF compared to the NiCo-MOF delivers a high capacity of 728 C g^-1 (1.82 C cm^-2 ) at 1 A g^-1 with a high capacity retention of 430 C g^-1 (1.08 C cm^-2 ) when increasing the current density to 20 A g^-1 . Importantly, the assembled solid-state NiCoP-MOF-based hybrid supercapacitor displays superior properties regarding the capacity (226.3 C g^-1 ), energy density (50.3 Wh kg^-1 ), and durability (≈100% capacity retention over 10 000 cycles). This in situ heterogenization approach sheds light on the electronic structure modulation while maintaining the well-defined porosity and morphology, holding promise for designing MOF-based derivatives for high performance energy storage devices.

Hollow Biphase Cobalt Nickel Perselenide Spheres Derived from Metal Glycerol Alkoxides for High-Performance Hybrid Supercapacitors

Transition-metal selenides (TMSe) incorporate reversible multielectron Faradaic reactions that can deliver high specific capacitance. Unfortunately, they usually exhibit actual capacitance lower than their theoretical value and suffer from sluggish kinetics, which do not satisfy the demands of hybrid supercapacitors (HSCs), due to poor electron-transmission capability and inferior ion-transport rate. Herein, a kind of hollow biphase and bimetal cobalt nickel perselenide composed of metastable marcasite-type CoSe₂ (m-CoSe₂) and stable pyrite-type NiCoSe₄ (p-NiCoSe₄) is synthesized with metal glycerol alkoxide as precursors by regulating the Ni/Co ratios. This unique hollow biphase structure and bimetallic synergistic effect serves to boost electron-transmission capability and accelerate the ion/electron transfer rate, delivering an excellent specific capacitance of 1008 F g^-1 at 0.5 A g^-1 and a high discharge rate capability of 859 F g^-1 at 20 A g^-1. The capacitance remains around 80% of the initial capacitance after 5000 cycles. Consequently, a HSC based on the cobalt nickel perselenide cathode and a hierarchical porous carbon anode reveals a maximum energy density of 34.8 W h kg^-1 and a maximum power density of 7272 W kg^-1. This polymorphic bimetallic phase engineering provides an advanced and effective guidance for TMSe with high electrochemical properties.

Metal-Organic Framework-Derived Co3 V2 O8 @CuV2 O6 Hybrid Architecture as a Multifunctional Binder-Free Electrode for Li-Ion Batteries and Hybrid Supercapacitors

Metal-organic frameworks (MOFs) are promising materials in diverse fields because of their constructive traits of varied structural topologies, high porosity, and high surface area. MOFs are also an ideal precursor/template to derive porous and functional morphologies. Herein, Co₃ V₂ O₈ nanohexagonal prisms are grafted on CuV₂ O₆ nanorod arrays (CuV-CoV)-grown copper foam (CF) using solution-processing methods, followed by thermal treatment. Direct preparation of active material on CF can potentially eliminate electrochemically inactive and non-conductive binders, leading to improved charge-transfer rate. Furthermore, solution-processing methods are simple and cost-effective. Owing to versatile valence states and good redox activity, the vanadium-incorporated mixed metal oxides (CuV-CoV) exhibited superior electrochemical performance in lithium (Li)-ion battery and supercapacitor (SC) studies. Furthermore, hollow carbon particles (HCPs) derived from MOF particles (MOF-HCPs) are used as the anode material in SCs. A hybrid SC (HSC) fabricated with CuV-CoV and MOF-HCP materials exhibited noteworthy electrochemical properties. Moreover, a solid-state HSC (SSHSC) is constructed and its real-time feasibility is investigated by harvesting the dynamic energy of a bicycle with the help of a direct current generator. The charged SSHSCs potentially powered various electronic components.

Ultra-High Energy Density Hybrid Supercapacitors Using MnO2/Reduced Graphene Oxide Hybrid Nanoscrolls

Manganese oxide (MnO2) is a promising material for supercapacitor applications, with a theoretical ultra-high energy density of 308 Wh/kg. However, such ultra-high energy density has not been achieved experimentally in MnO2-based supercapacitors because of several practical issues, such as low electrical conductivity of MnO2, incomplete utilization of MnO2, and dissolution of MnO2. The present study investigates the potential of MnO2/reduced graphene oxide (rGO) hybrid nanoscroll (GMS) structures as electrode material for overcoming the difficulties and for developing ultra-high-energy storage systems. A hybrid supercapacitor, comprising MnO2/rGO nanoscrolls as anode material and activated carbon (AC) as a cathode, is fabricated. The GMS/AC hybrid supercapacitor exhibited enhanced energy density, superior rate performance, and promising Li storage capability that bridged the energy-density gap between conventional Li-ion batteries (LIBs) and supercapacitors. The fabricated GMS/AC hybrid supercapacitor demonstrates an ultra-high lithium discharge capacity of 2040 mAh/g. The GMS/AC cell delivered a maximum energy density of 105.3 Wh/kg and a corresponding power density of 308.1 W/kg. It also delivered an energy density of 42.77 Wh/kg at a power density as high as 30,800 W/kg. Our GMS/AC cell's energy density values are very high compared with those of other reported values of graphene-based hybrid structures. The GMS structures offer significant potential as an electrode material for energy-storage systems and can also enhance the performance of the other electrode materials for LIBs and hybrid supercapacitors.

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

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

A High-Energy-Density Hybrid Supercapacitor with P-Ni(OH)2 @Co(OH)2 Core-Shell Heterostructure and Fe2 O3 Nanoneedle Arrays as Advanced Integrated Electrodes

Transition metal hydro/oxides (TMH/Os) are treated as the most promising alternative supercapacitor electrodes thanks to their high theoretical capacitance due to the various oxidation states and abundant cheap resources of TMH/Os. However, the poor conductivity and logy reaction kinetics of TMH/Os severely restrict their practical application. Herein, hierarchical core-shell P-Ni(OH)₂ @Co(OH)₂ micro/nanostructures are in situ grown on conductive Ni foam (P-Ni(OH)₂ @Co(OH)₂ /NF) through a facile stepwise hydrothermal process. The unique heterostructure composed of P-Ni(OH)₂ rods and Co(OH)₂ nanoflakes boost the charge transportation and provide abundant active sites when used as the intergrated cathode for supercapacitors. It delivers an ultrahigh areal specific capacitance of 4.4 C cm^-2 at 1 mA cm^-2 and the capacitance can maintain 91% after 10 000 cycles, showing an ultralong cycle life. Additionally, a hybrid supercapacitor composed with P-Ni(OH)₂ @Co(OH)₂ /NF cathode and Fe₂ O₃ /CC anode shows a wider voltage window of 1.6 V, a remarkable energy density of 0.21 mWh cm^-2 at the power density of 0.8 mW cm^-2 , and outstanding cycling stability with about 81% capacitance retention after 5000 cycles. This innovative study not only supplies a newfashioned electronic apparatus with high-energy density and cycling stability but offers a fresh reference and enlightenment for synthesizing advanced integrated electrodes for high-performance hybrid supercapacitors.

Rigid-Flexible Coupling Carbon Skeleton and Potassium-Carbonate-Dominated Solid Electrolyte Interface Achieving Superior Potassium-Ion Storage

Potassium-ion energy-storage devices are highly attractive in the large-scale energy storage field, but the intercalation of large K ions greatly worsens the stability of electrode structures and solid electrolyte interphase (SEI) films, causing slow reaction dynamics and poor durability. In this Article, inspired by bubble wraps in our life, a bubble-wrap-like carbon sheet (BPCS) with a rigid-flexible coupling porous architecture is fabricated on the microscale, exhibiting strong structural stability and good accommodation for volume expansion. In the meantime, a K₂CO₃·1.5H₂O-dominated SEI is created by an interfacial transfer behavior of carbonate groups. These K₂CO₃·1.5H₂O nanograins not only enhance the stability of the SEI by constructing a stable scaffold but also create more diffusion routes for K ions. On the basis of the above, using the BPCS as the anode of potassium-ion batteries delivers reversible capacities of 463 mAh g^-1 at 50 mA g^-1 and 195 mAh g^-1 at 10 A g^-1 with a long cycling life. The assembled BPCS//NPC potassium-ion hybrid capacitor exhibits a high energy density of 167 Wh kg^-1 and a superior cycling capability with 80.8% capacity retention over 10 000 cycles with nearly 100% Coulombic efficiency. Even at the higher current density of 10 A g^-1, the device could deliver an energy density of 92.9 Wh kg^-1 over 5000 cycles at a power density of 9200 W kg^-1 with only 0.002% fading per cycle, which can rival lithium-ion hybrid supercapacitors.

Synergistic Effects of Cobalt Molybdate@Phosphate Core-Shell Architectures with Ultrahigh Capacity for Rechargeable Hybrid Supercapacitors

Designing binder-free and core-shell-like electrode materials with synergistic effects has attracted widespread attention for the development of high energy density hybrid supercapacitors (HSCs). Herein, binder-free cobalt molybdate nanosheet-laminated cobalt phosphate micropetals on nickel foam (CoM NS@CoP/NF) were facilely prepared for use as an effective battery-type electrode in HSCs. With the multifunctional features, the rationally combined core-shell-like CoM NS@CoP/NF electrode exhibited a maximum capacity of 886.8 μA h/cm² at a current density of 5 mA/cm² with a good rate capability of 64.2% and cycling stability of 87.4% (after 10 000 cycles). The high electrochemical performance of the hybrid composite could be attributed to the synergistic effects of hierarchical architectures and large accessible electroactive area, which facilitates the fast electron/transportation within the active material and accelerates the redox chemistry process. Utilizing the superior energy-storage properties, a pouch-type HSC was fabricated with core-shell-like CoM NS@CoP-6 h architectures as a battery-type electrode and activated carbon as a capacitive-type electrode in an aqueous alkaline electrolyte. The miniature hybrid device exhibited maximum energy and power densities of 0.44 mW h/cm² and 40.35 mW/cm², respectively, with good cycling stability. Moreover, the HSCs can energize various portable electronic equipments, which demonstrates their suitability for real-time applications.

Niobium-Based Oxides Toward Advanced Electrochemical Energy Storage: Recent Advances and Challenges

Niobium-based oxides including Nb₂ O₅ , TiNb_x O_2+2.5x compounds, M-Nb-O (M = Cr, Ga, Fe, Zr, Mg, etc.) family, etc., as the unique structural merit (e.g., quasi-2D network for Li-ion incorporation, open and stable Wadsley- Roth shear crystal structure), are of great interest for applications in energy storage systems such as Li/Na-ion batteries and hybrid supercapacitors. Most of these Nb-based oxides show high operating voltage (>1.0 V vs Li⁺ /Li) that can suppress the formation of solid electrolyte interface film and lithium dendrites, ensuring the safety of working batteries. Outstanding rate capability is impressive, which can be derived from their fast intercalation pseudocapacitive kinetics. However, the intrinsic poor electrical conductivity hinders their energy storage applications. Various strategies including structure optimization, surface engineering, and carbon modification are effectively used to overcome the issues. This review provides a comprehensive summary on the latest progress of Nb-based oxides for advanced electrochemical energy storage applications. Major impactful work is outlined, promising research directions, and various performance-optimizing strategies, as well as the energy storage mechanisms investigated by combining theoretical calculations and various electrochemical characterization techniques. In addition, challenges and perspectives for future research and commercial applications are also presented.

An Integrated Approach Toward Renewable Energy Storage Using Rechargeable Ag@Ni0.67 Co0.33 S-Based Hybrid Supercapacitors

Self-powered charging systems in conjunction with renewable energy conversion and storage devices have attracted promising attention in recent years. In this work, a prolific approach to design a wind/solar-powered rechargeable high-energy density pouch-type hybrid supercapacitor (HSC) is proposed. The pouch-type HSC is fabricated by engineering nature-inspired nanosliver (nano-Ag) decorated Ni_0.67 Co_0.33 S forest-like nanostructures on Ni foam (nano-Ag@NCS FNs/Ni foam) as a battery-type electrode and porous activated carbon as a capacitive-type electrode. Initially, the core-shell-like NCS FNs/Ni foam is prepared via a single-step wet-chemical method, followed by a light-induced growth of nano-Ag onto it for enhancing the conductivity of the composite. Utilizing the synergistic effects of forest-like nano-Ag@NCS FNs/Ni foam as a composite electrode, the fabricated device shows a maximum capacitance of 1104.14 mF cm^-2 at a current density of 5 mA cm^-2 and it stores superior energy and power densities of 0.36 mWh cm^-2 and 27.22 mW cm^-2 , respectively along with good cycling stability, which are higher than most of previous reports. The high-energy storage capability of HSCs is further connected to wind fans and solar cells to harvest renewable energy. The wind/solar charged HSCs can effectively operate various electronic devices for a long time, enlightening its potency for the development of sustainable energy systems.

Robust Negative Electrode Materials Derived from Carbon Dots and Porous Hydrogels for High-Performance Hybrid Supercapacitors

Hybrid supercapacitors generally show high power and long life spans but inferior energy densities, which are mainly caused by carbon negative electrodes with low specific capacitances. To improve the energy densities, the traditional methods include optimizing pore structures and modifying pseudocapacitive groups on the carbon materials. Here, another promising way is suggested, which has no adverse effects to the carbon materials, that is, constructing electron-rich regions on the electrode surfaces for absorbing cations as much as possible. For this aim, a series of hierarchical porous carbon materials are produced by calcinating carbon dots-hydrogel composites, which have controllable surface states including electron-rich regions. The optimal sample is employed as the negative electrode to fabricate hybrid supercapacitors, which show remarkable specific energy densities (up to 62.8-90.1 Wh kg^-1 ) in different systems.

Ionic Liquid-Controlled Growth of NiCo2 S4 3D Hierarchical Hollow Nanoarrow Arrays on Ni Foam for Superior Performance Binder Free Hybrid Supercapacitors

A significant development in the design of a NiCo₂ S₄ 3D hierarchical hollow nanoarrow arrays (HNA)-based supercapacitor binder free electrode assembled by 1D hollow nanoneedles and 2D nanosheets on a Ni foam collector through controlling ionic liquid 1-octyl-3-methylimidazolium chloride ([OMIm]Cl) concentration is reported. The unique NiCo₂ S₄ -HNA electrode acquires high specific capacity (1297 C g^-1 at 1 A g^-1 , 2.59 C cm^-2 at 2 mA cm^-2 ), excellent rate capability (maintaining 73.0% at 20 A g^-1 ), and long operational life (maintaining 92.4% after 10 000 cycles at 5 A g^-1 ), which are superior to those for 1D hollow nanoneedle arrays (HNN) and 2D porous nanoflake arrays (PNF). The outstanding electrochemical performance is attributed to the novel 3D structure with large specific surface, hollow cores, high porosity as well as stable architecture. In addition, a hybrid supercapacitor applying 3D NiCo₂ S₄ -HNA as the positive electrode and active carbon as the negative electrode exhibits a high energy density of 42.5 Wh kg^-1 at a power density of 2684.2 W kg^-1 in an operating voltage of 1.6 V. Robust cycling stability is also expressed with 84.9% retention after repeating 10 000 cycles at 5 A g^-1 , implying their great potential in superior-performance supercapacitors.

Phosphate Species up to 70% Mass Ratio for Enhanced Pseudocapacitive Properties

The emerging phosphate species on the surface or near-surface of electrode materials are versatile and have an intriguing ability for dramatically enhanced electrochemical performance. Unfortunately, the distribution/dispersion of phosphate species still keeps at levels on the exterior not within the interior surface of materials, and the micro-/nanoscale tuning is commonly rarely concerned and its function remains poorly understood. Herein, for the first time, well-dispersed phosphate species up to 70% mass ratio implanted within Ni-doped CoP nanowire matrix are presented via an efficient low-temperature phosphorization strategy. The resultant nanohybrids possess kinetics-favorable open frameworks with abundant mesopores and a high degree covalency in the chemical bonds, thus leading to rapid mass transport/charge transfer and enhanced redox reaction kinetics. Remarkably, the phosphate species feature superwettability toward water and strong affinity for OH^- in the electrolyte, evidenced by the shortened distance and reduced adsorption energy between the OH^- and the nuclear Co atoms on the nanohybrids as revealed by density functional theory calculations. As such, the nanohybrids exhibit an ultrahigh specific capacity of 250 mAh g^-1 even at 50 A g^-1 . This work presents a deeper understanding of the dispersion and role of phosphate species for supercapacitors and other energy-related storage/conversion devices.

Hierarchically Designed Ag@Ce6Mo10O39 Marigold Flower-Like Architectures: An Efficient Electrode Material for Hybrid Supercapacitors

We facilely prepared silver nanoparticle-decorated Ce₆Mo₁₀O₃₉ marigold flower-like structures (Ag NPs@CM MFs) for use as an effective positive material in hybrid supercapacitors (HSCs). With the aid of ethylenediaminetetraacetic acid (EDTA) as a chelating agent, self-assembled CM MFs were synthesized by a single-step hydrothermal method. When the electrochemical properties were tested in an aqueous alkaline electrolyte, the synthesized CM MFs with 0.15 g of EDTA exhibited a relatively high charge storage property (55.3 μA h/cm² at 2 mA/cm²) with a battery-type redox behavior. The high capacity performance is mainly because of the large surface area of the CM MFs, and the hierarchically connected nanoflakes provide wide open wells for rapid accessibility of electrolyte ions and enable fast transportation of electrons. A further improvement in electrochemical performance was achieved (62 μA h/cm² at 2 mA/cm²) by decorating Ag NPs on the surface of the CM MFs (i.e., Ag NPs@CM MFs), which is attributed to the increased electric conductivity. Considering the synergistic effect and the high electrochemical activity, Ag NPs@CM MFs were further employed as an effective positive electrode for the fabrication of pouch-type HSC with porous carbon (negative electrode) in an alkaline electrolyte. The HSC exhibited a high cell potential (1.5 V) with maximum energy and power densities of 0.0183 mW h/cm² and 10.237 mW/cm², respectively. The potency of HSC in practical applications was also demonstrated by energizing red and yellow light-emitting diodes as well as a three-point pattern torch light.

Self-Templating Synthesis of Cobalt Hexacyanoferrate Hollow Structures with Superior Performance for Na-Ion Hybrid Supercapacitors

Prussian blue (PB) and its analogues (PBA), especially with hollow structures, have attracted growing attention from the researchers of energy storage field. Herein, we have developed a facile self-templating method to synthesize hollow-structured cobalt hexacyanoferrate (CoHCF) with controllable morphologies by using water-soluble precursors as templates. The method is versatile and can be extended to synthesize various PB/PBA hollow structures with tunable composition and morphology. Profiting from the unique hollow structure, the CoHCF hollow prisms manifest exceptional electrochemical performance in the Na₂SO₄ aqueous electrolyte, including a high specific capacitance (284 F g^-1 at 1 A g^-1), a high rate capability, and an excellent cycling stability (92% retention after 5000 cycles). A hybrid supercapacitor device assembled with the CoHCF hollow prisms and activated carbon shows a high specific density of 47 W h kg^-1 at a specific power of 1000 W kg^-1 and stable cycling performance.

Fabrication of a 3D Hierarchical Sandwich Co9 S8 /α-MnS@N-C@MoS2 Nanowire Architectures as Advanced Electrode Material for High Performance Hybrid Supercapacitors

Supercapacitors suffer from lack of energy density and impulse the energy density limit, so a new class of hybrid electrode materials with promising architectures is strongly desirable. Here, the rational design of a 3D hierarchical sandwich Co₉ S₈ /α-MnS@N-C@MoS₂ nanowire architecture is achieved during the hydrothermal sulphurization reaction by the conversion of binary mesoporous metal oxide core to corresponding individual metal sulphides core along with the formation of outer metal sulphide shell at the same time. Benefiting from the 3D hierarchical sandwich architecture, Co₉ S₈ /α-MnS@N-C@MoS₂ electrode exhibits enhanced electrochemical performance with high specific capacity/capacitance of 306 mA h g^-1 /1938 F g^-1 at 1 A g^-1 , and excellent cycling stability with a specific capacity retention of 86.9% after 10 000 cycles at 10 A g^-1 . Moreover, the fabricated asymmetric supercapacitor device using Co₉ S₈ /α-MnS@N-C@MoS₂ as the positive electrode and nitrogen doped graphene as the negative electrode demonstrates high energy density of 64.2 Wh kg^-1 at 729.2 W kg^-1 , and a promising energy density of 23.5 Wh kg^-1 is still attained at a high power density of 11 300 W kg^-1 . The hybrid electrode with 3D hierarchical sandwich architecture promotes enhanced energy density with excellent cyclic stability for energy storage.

"One-for-All" Strategy in Fast Energy Storage: Production of Pillared MOF Nanorod-Templated Positive/Negative Electrodes for the Application of High-Performance Hybrid Supercapacitor

Currently, metal-organic frameworks (MOFs) are intensively studied as active materials for electrochemical energy storage applications due to their tunable structure and exceptional porosities. Among them, water stable pillared MOFs with dual ligands have been reported to exhibit high supercapacitor (SC) performance. Herein, the "One-for-All" strategy is applied to synthesize both positive and negative electrodes of a hybrid SC (HSC) from a single pillared MOF. Specifically, Ni-DMOF-TM ([Ni(TMBDC)(DABCO)_0.5 ], TMBDC: 2,3,5,6-tetramethyl-1,4-benzenedicarboxylic acid, DABCO: 1,4-diazabicyclo[2.2.2]-octane) nanorods are directly grown on carbon fiber paper (CFP) (denoted as CFP@TM-nanorods) with the help of triethylamine and function as the positive electrode of HSC under alkaline electrolyte. Meanwhile, calcinated N-doped hierarchical porous carbon nanorods (CFP@TM-NPCs) are produced and utilized as the negative counter-electrode from a one-step heat treatment of CFP@TM-nanorods. After assembling these two electrodes together to make a hybrid device, the TM-nanorods//TM-NPCs exhibit a wide voltage window of 1.5 V with a high sloping discharge plateau between 1-1.2 V, indicating its great potential for practical applications. This as-described "One-for-All" strategy is widely applicable and highly reproducible in producing MOF-based electrode materials for HSC applications, which shortens the gap between experimental synthesis and practical application of MOFs in fast energy storage.

Nano-Sized Structurally Disordered Metal Oxide Composite Aerogels as High-Power Anodes in Hybrid Supercapacitors

A general method for preparing nano-sized metal oxide nanoparticles with highly disordered crystal structure and their processing into stable aqueous dispersions is presented. With these nanoparticles as building blocks, a series of nanoparticles@reduced graphene oxide (rGO) composite aerogels are fabricated and directly used as high-power anodes for lithium-ion hybrid supercapacitors (Li-HSCs). To clarify the effect of the degree of disorder, control samples of crystalline nanoparticles with similar particle size are prepared. The results indicate that the structurally disordered samples show a significantly enhanced electrochemical performance compared to the crystalline counterparts. In particular, structurally disordered Ni _xFe _yO _z@rGO delivers a capacity of 388 mAh g^-1 at 5 A g^-1, which is 6 times that of the crystalline sample. Disordered Ni _xFe _yO _z@rGO is taken as an example to study the reasons for the enhanced performance. Compared with the crystalline sample, density functional theory calculations reveal a smaller volume expansion during Li⁺ insertion for the structurally disordered Ni _xFe _yO _z nanoparticles, and they are found to exhibit larger pseudocapacitive effects. Combined with an activated carbon (AC) cathode, full-cell tests of the lithium-ion hybrid supercapacitors are performed, demonstrating that the structurally disordered metal oxide nanoparticles@rGO||AC hybrid systems deliver high energy and power densities within the voltage range of 1.0-4.0 V. These results indicate that structurally disordered nanomaterials might be interesting candidates for exploring high-power anodes for Li-HSCs.

Asymmetric Supercapacitor Electrodes and Devices

The world is recently witnessing an explosive development of novel electronic and optoelectronic devices that demand more-reliable power sources that combine higher energy density and longer-term durability. Supercapacitors have become one of the most promising energy-storage systems, as they present multifold advantages of high power density, fast charging-discharging, and long cyclic stability. However, the intrinsically low energy density inherent to traditional supercapacitors severely limits their widespread applications, triggering researchers to explore new types of supercapacitors with improved performance. Asymmetric supercapacitors (ASCs) assembled using two dissimilar electrode materials offer a distinct advantage of wide operational voltage window, and thereby significantly enhance the energy density. Recent progress made in the field of ASCs is critically reviewed, with the main focus on an extensive survey of the materials developed for ASC electrodes, as well as covering the progress made in the fabrication of ASC devices over the last few decades. Current challenges and a future outlook of the field of ASCs are also discussed.

Formation of Onion-Like NiCo2 S4 Particles via Sequential Ion-Exchange for Hybrid Supercapacitors

Onion-like NiCo₂ S₄ particles with unique hollow structured shells are synthesized by a sequential ion-exchange strategy. With the structural and compositional advantages, these unique onion-like NiCo₂ S₄ particles exhibit enhanced electrochemical performance as an electrode material for hybrid supercapacitors.

Neutral pH Gel Electrolytes for V2O5·0.5H2O-Based Energy Storage Devices

Gel electrolytes are considered to be promising candidates for the use in supercapacitors. It is worthy to systematically evaluate the internal electrochemical mechanisms with a variety of cations (poly(vinyl alcohol) (PVA)-based Li⁺, Na⁺, and K⁺) toward redox-type electrode. Herein, we describe a quasi-solid-state PVA-KCl gel electrolyte for V₂O₅·0.5H₂O-based redox-type capacitors, effectively avoiding electrochemical oxidation and structural breakdown of layered V₂O₅·0.5H₂O during 10 000 charge-discharge cycles (98% capacitance retention at 400 mV s^-1). With the gel electrolyte, symmetric V₂O₅·0.5H₂O-reduced graphene oxide (V₂O₅·0.5H₂O-rGO) devices exhibited a volumetric capacitance of 136 mF cm^-3, which was much higher than that of 68 mF cm^-3 for PVA-NaCl and 45 mF cm^-3 for PVA-LiCl. Additionally, hybrid full cells of activated carbon cloth//V₂O₅·0.5H₂O-rGO delivered an energy density of 102 μWh cm^-3 and a power density of 73.38 mW cm^-3 over a wide potential window of 2 V. The present study provides direct experimental evidence for the contribution of PVA-KCl gel electrolytes toward quick redox reactions for redox-type capacitors, which is also helpful for the development of neutral pH gel electrolytes for energy storage devices.

Graphene-Based Materials for Lithium-Ion Hybrid Supercapacitors

Lithium-ion hybrid supercapacitors (LIHSs), also called Li-ion capacitors, have attracted much attention due to the combination of the rapid charge-discharge and long cycle life of supercapacitors and the high energy-storage capacity of lithium-ion batteries. Thus, LIHSs are expected to become the ultimate power source for hybrid and all-electric vehicles in the near future. As an electrode material, graphene has many advantages, including high surface area and porous structure, high electric conductivity, and high chemical and thermal stability, etc. Compared with other electrode materials, such as activated carbon, graphite, and metal oxides, graphene-based materials with 3D open frameworks show higher effective specific surface area, better control of channels, and higher conductivity, which make them better candidates for LIHS applications. Here, the latest advances in electrode materials for LIHSs are briefly summarized, with an emphasis on graphene-based electrode materials (including 3D graphene networks) for LIHS applications. An outlook is also presented to highlight some future directions.

Facile Synthesis of Nb2O5@Carbon Core-Shell Nanocrystals with Controlled Crystalline Structure for High-Power Anodes in Hybrid Supercapacitors

Hybrid supercapacitors (battery-supercapacitor hybrid devices, HSCs) deliver high energy within seconds (excellent rate capability) with stable cyclability. One of the key limitations in developing high-performance HSCs is imbalance in power capability between the sluggish Faradaic lithium-intercalation anode and rapid non-Faradaic capacitive cathode. To solve this problem, we synthesize Nb2O5@carbon core-shell nanocyrstals (Nb2O5@C NCs) as high-power anode materials with controlled crystalline phases (orthorhombic (T) and pseudohexagonal (TT)) via a facile one-pot synthesis method based on a water-in-oil microemulsion system. The synthesis of ideal T-Nb2O5 for fast Li(+) diffusion is simply achieved by controlling the microemulsion parameter (e.g., pH control). The T-Nb2O5@C NCs shows a reversible specific capacity of ∼180 mA h g(-1) at 0.05 A g(-1) (1.1-3.0 V vs Li/Li(+)) with rapid rate capability compared to that of TT-Nb2O5@C and carbon shell-free Nb2O5 NCs, mainly due to synergistic effects of (i) the structural merit of T-Nb2O5 and (ii) the conductive carbon shell for high electron mobility. The highest energy (∼63 W h kg(-1)) and power (16 528 W kg(-1) achieved at ∼5 W h kg(-1)) densities within the voltage range of 1.0-3.5 V of the HSC using T-Nb2O5@C anode and MSP-20 cathode are remarkable.

A high energy and power Li-ion capacitor based on a TiO2 nanobelt array anode and a graphene hydrogel cathode

A novel hybrid Li-ion capacitor (LIC) with high energy and power densities is constructed by combining an electrochemical double layer capacitor type cathode (graphene hydrogels) with a Li-ion battery type anode (TiO(2) nanobelt arrays). The high power source is provided by the graphene hydrogel cathode, which has a 3D porous network structure and high electrical conductivity, and the counter anode is made of free-standing TiO(2) nanobelt arrays (NBA) grown directly on Ti foil without any ancillary materials. Such a subtle designed hybrid Li-ion capacitor allows rapid electron and ion transport in the non-aqueous electrolyte. Within a voltage range of 0.0-3.8 V, a high energy of 82 Wh kg(-1) is achieved at a power density of 570 W kg(-1). Even at an 8.4 s charge/discharge rate, an energy density as high as 21 Wh kg(-1) can be retained. These results demonstrate that the TiO(2) NBA//graphene hydrogel LIC exhibits higher energy density than supercapacitors and better power density than Li-ion batteries, which makes it a promising electrochemical power source.

Advanced hybrid supercapacitor based on a mesoporous niobium pentoxide/carbon as high-performance anode

Recently, hybrid supercapacitors (HSCs), which combine the use of battery and supercapacitor, have been extensively studied in order to satisfy increasing demands for large energy density and high power capability in energy-storage devices. For this purpose, the requirement for anode materials that provide enhanced charge storage sites (high capacity) and accommodate fast charge transport (high rate capability) has increased. Herein, therefore, a preparation of nanocomposite as anode material is presented and an advanced HSC using it is thoroughly analyzed. The HSC comprises a mesoporous Nb2O5/carbon (m-Nb2O5-C) nanocomposite anode synthesized by a simple one-pot method using a block copolymer assisted self-assembly and commercial activated carbon (MSP-20) cathode under organic electrolyte. The m-Nb2O5-C anode provides high specific capacity with outstanding rate performance and cyclability, mainly stemming from its enhanced pseudocapacitive behavior through introduction of a carbon-coated mesostructure within a voltage range from 3.0 to 1.1 V (vs Li/Li(+)). The HSC using the m-Nb2O5-C anode and MSP-20 cathode exhibits excellent energy and power densities (74 W h kg(-1) and 18,510 W kg(-1)), with advanced cycle life (capacity retention: ∼90% at 1000 mA g(-1) after 1000 cycles) within potential range from 1.0 to 3.5 V. In particular, we note that the highest power density (18,510 W kg(-1)) of HSC is achieved at 15 W h kg(-1), which is the highest level among similar HSC systems previously reported. With further study, the HSCs developed in this work could be a next-generation energy-storage device, bridging the performance gap between conventional batteries and supercapacitors.