Nature of FeSe2/N‐C Anode for High Performance Potassium Ion Hybrid Capacitor

Potassium ion hybrid capacitors have great potential for large‐scale energy devices, because of the high power density and low cost. However, their practical applications are hindered by their low energy density, as well as electrolyte decomposition and collector corrosion at high potential in potassium bis(fluoro‐sulfonyl)imide‐based electrolyte. Therefore, anode materials with high capacity, a suitable voltage platform, and stability become a key factor. Here, N‐doping carbon‐coated FeSe₂ clusters are demonstrated as the anode material for a hybrid capacitor, delivering a reversible capacity of 295 mAh g^?1 at 100 mA g^?1 over 100 cycles and a high rate capability of 158 mAh g^?1 at 2000 mA g^?1 over 2000 cycles. Meanwhile, through density functional theory calculations, in situ X‐ray diffraction, and ex situ transmission electron microscopy, the evolution of FeSe₂ to Fe₃Se₄ for the electrochemical reaction mechanism is successfully revealed. The battery‐supercapacitor hybrid using commercial activated carbon as the cathode and FeSe₂/N‐C as the anode is obtained. It delivers a high energy density of 230 Wh kg^?1 and a power density of 920 W kg^?1 (the energy density and power density are calculated based on the total mass of active materials in the anode and cathode).     

Metal–Organic Framework Derived Honeycomb Co9S8@C Composites for High‐Performance Supercapacitors

Unique nanostructures always lead to extraordinary electrochemical energy storage performance. Here, the authors report a new strategy for using Metal‐organic frameworks (MOFs) derived cobalt sulfide in a carbon matrix with a 3D honeycombed porous structure, resulting in a high‐performance supercapacitor with unrivalled capacity of ≈1887 F g^‐1 at the current density of 1 A g^‐1. The honeycomb‐like structure of Co₉S₈@C composite is loosely adsorbed, with plentiful surface area and high conductivity, leading to improved Faradaic processes across the interface and enhanced redox reactions at active Co₉S₈ sites. Therefore, the heterostructure‐fabricated hybrid supercapacitor, using activated carbon as the counter electrode, demonstrates a high energy density of 58 Wh kg^‐1 at the power density of 1000 W kg^‐1. Even under an ultrahigh power density of 17 200 W kg^‐1, its energy density maintains ≈38 Wh kg^‐1. The hybrid supercapacitor also exhibits suitable cycling stability, with ≈90% capacity retention after 10 000 continuous cycles at the current density of 5 A g^‐1. This work presents a practical method for using MOFs as sacrificial templates to synthesize metal‐sulfides for highly efficient electrochemical energy storage.     

An Aqueous Zn‐Ion Hybrid Supercapacitor with High Energy Density and Ultrastability up to 80 000 Cycles

Integrating a battery‐type electrode to build a hybrid supercapacitor is a promising approach to improve the overall energy density of a supercapacitor‐type energy storage device without sacrificing its power output. However, this strategy is usually achieved at the expense of cycling lifespan. In this work, a hybrid supercapacitor comprising Zn foil and porous carbon derived from chemical activated graphene (aMEGO) is developed, and the trade‐off between energy density and cycling life is well‐balanced by the utilization of 3 m Zn(CF₃SO₃)₂ electrolyte with high Zn stripping/plating efficiency. Such a hybrid supercapacitor demonstrates an energy density of 106.3 Wh kg^?1 and a power density of 31.4 kW kg^?1, and significantly a wide operation voltage of 1.9 V is achieved in aqueous electrolyte. Benefitting from the high Zn stripping/plating efficiency, the Zn‐aMEGO hybrid‐supercapacitor also exhibits an ultralong cycling life up to 80 000 cycles with capacity retention of 93%, which is comparable to that of conventional electrochemical double‐layer capacitors.     

Harmonizing Energy and Power Density toward 2.7 V Asymmetric Aqueous Supercapacitor

Tremendous research efforts are devoted to developing wide potential window aqueous supercapacitors to resolve their low energy density concern. While the operational potential window is dictated by the intrinsic electrochemical stability of water (1.23 V), such a bottleneck may be surpassed by leveraging the additional overpotential of the oxygen evolution reaction and the hydrogen evolution reaction (HER). Herein, by employing an electroreduction technique, Na⁺ is adsorbed onto the carbon negative electrode which effectively acts as a physical barrier to hinder intermediate HER product formation, thereby reducing HER activity. To complement the wide potential carbon electrode, Na_0.25MnO₂ is employed as the positive electrode to take advantage of the extra energy (i.e., increased overpotential) required for Na⁺ insertion process into the structure. The asymmetric supercapacitor exhibits high energy density of 61.1 W h kg^?1 at a power density of 982 W kg^?1, and even at an ultrahigh power density of 42.9 kW kg^?1, a respectable energy density of 16.3 W h kg^?1 is attained. In addition, 93.7% capacitance retention is recorded after cycling for 10 000 cycles which further demonstrates its suitability as supercapacitor. The present success in fabricating a 2.7 V asymmetric supercapacitor will open a promising research route toward achieving high energy density and high power density.     

Inverted Design for High‐Performance Supercapacitor Via Co(OH)2‐Derived Highly Oriented MOF Electrodes

Metal organic frameworks (MOFs) are considered as promising candidates for supercapacitors because of high specific area and potential redox sites. However, their shuffled orientations and low conductivity nature lead to severely‐degraded performance. Designing an accessibly‐manipulated and efficient method to address those issues is of outmost significance for MOF application in supercapacitors. It is the common way that MOFs scarify themselves as templates or precursors to prepare target products. But to reversely think it, using target products to prepare MOF could be the way to unlock the bottleneck of MOFs' performance in supercapacitors. Herein, a novel strategy using Co(OH)₂ as both the template and precursor to fabricate vertically‐oriented MOF electrode is proposed. The electrode shows a double high specific capacitance of 1044 Fg^?1 and excellent rate capability compared to MOF in powder form. An asymmetric supercapacitor was also fabricated, which delivers a maximum energy density of 28.5 W h kg^?1 at a power density of 1500 W kg^?1, and the maximum of 24000 W kg^?1 can be obtained with a remaining energy density of 13.3 W h kg^?1. Therefore, the proposed strategy paves the way to unlock the inherent advantages of MOFs and also inspires for advanced MOF synthesis with optimum performance.     

Molecularly Stacking Manganese Dioxide/Titanium Carbide Sheets to Produce Highly Flexible and Conductive Film Electrodes with Improved Pseudocapacitive Performances

2D nanostructures with high surface area and flexibility are regarded as a promising building platform for flexible supercapacitors that are attracting tremendous attention due to their potential applications in various wearable technologies. Notably, although pseudocapacitive metal oxides are widely accepted as a very important class of electrochemically active materials, the utilization of 2D metal oxide sheets in the preparation of flexible supercapacitors is very rare. The scarcity of a suitable filler with the integrated properties of both high conductivity and excellent hydrophilicity is probably to blame. In this work, by introducing a recently discovered intriguing material, Ti₃C₂ sheets, a novel MnO₂/Ti₃C₂ hybrid with a molecularly stacked structure is developed using a simple and scalable mixing and filtration method. Their individual advantages are combined in the hybrid, thus delivering excellent electrochemical performances. A highly flexible and symmetric supercapacitor based on the novel hybrid electrode manifests top‐class electrochemical performance with maximum energy and power densities of 8.3 W h kg^?1 (at 221.33 W kg^?1) and 2376 W kg^?1 (at 3.3 W h kg^?1), respectively, regardless of the various bending states, suggesting enormous possibilities for applications in future flexible and portable micropower systems.     

High Energy Density Aqueous Li‐Ion Flow Capacitor

High energy density Li‐ion hybrid flow capacitors are demonstrated by employing LiMn₂O₄ and activated carbon slurry electrodes. Compared to the existing aqueous flow electrochemical capacitors, the hybrid one exhibits much higher energy densities due to the introduction of high capacity Li‐insertion materials (e.g., LiMn₂O₄ in the present work) as the flowable electrode with asymmetrical cell configuration. A record energy density, i.e., 23.4 W h kg^?1 at a power of 50.0 W kg^?1 has been achieved for aqueous flow capacitors tested at static condition reported to date. A full operational Li‐ion flow capacitor tested in an intermittent‐flow mode has also been demonstrated. The Li‐ion hybrid flow capacitor shows great promise for high‐rate grid applications.     

Fast Sodium Storage in TiO2@CNT@C Nanorods for High‐Performance Na‐Ion Capacitors

Na‐ion capacitors have attracted extensive interest due to the combination of the merits of high energy density of batteries and high power density as well as long cycle life of capacitors. Here, a novel Na‐ion capacitor, utilizing TiO₂@CNT@C nanorods as an intercalation‐type anode and biomass‐derived carbon with high surface area as an ion adsorption cathode in an organic electrolyte, is reported. The advanced architecture of TiO₂@CNT@C nanorods, prepared by electrospinning method, demonstrates excellent cyclic stability and outstanding rate capability in half cells. The contribution of extrinsic pseudocapacitance affects the rate capability to a large extent, which is identified by kinetics analysis. A key finding is that ion/electron transfer dynamics of TiO₂@CNT@C could be effectively enhanced due to the addition of multiwalled carbon nanotubes. Also, the biomass‐derived carbon with high surface area displays high specific capacity and excellent rate capability. Owing to the merits of structures and excellent performances of both anode and cathode materials, the assembled Na‐ion capacitors provide an exceptionally high energy density (81.2 W h kg^?1) and high power density (12 400 W kg^?1) within 1.0–4.0 V. Meanwhile, the Na‐ion capacitors achieve 85.3% capacity retention after 5000 cycles tested at 1 A g^?1.     

A High‐Energy Density Asymmetric Supercapacitor Based on Fe2O3 Nanoneedle Arrays and NiCo2O4/Ni(OH)2 Hybrid Nanosheet Arrays Grown on SiC Nanowire Networks as Free‐Standing Advanced Electrodes

In this paper, a novel freestanding core‐branch negative and positive electrode material through integrating trim aligned Fe₂O₃ nanoneedle arrays (Fe₂O₃ NNAs) is first proposed with typical mesoporous structures and NiCo₂O₄/Ni(OH)₂ hybrid nanosheet arrays (NiCo₂O₄/Ni(OH)₂ HNAs) on SiC nanowire (SiC NW) skeletons with outstanding resistance to oxidation and corrosion, good conductivity, and large‐specific surface area. The original built SiC NWs@Fe₂O₃ NNAs is validated to be a highly capacitive negative electrode (721 F g^?1 at 2 A g^?1, i.e., 1 F cm^?2 at 2.8 mA cm^?2), matching well with the similarly constructed SiC NWs@NiCo₂O₄/Ni(OH)₂ HNAs positive electrode (2580 F g^?1 at 4 A g^?1, i.e., 3.12 F cm^?2 at 4.8 mA cm^?2). Contributed by the uniquely engineered electrodes, a high‐performance asymmetric supercapacitor (ASC) is developed, which can exhibit a maximum energy density of 103 W h kg^?1 at a power density of 3.5 kW kg^?1, even when charging the device within 6.5 s, the energy density can still maintain as high as 45 W h kg^?1 at 26.1 kW kg^?1, and the ASC manifests long cycling lifespan with 86.6% capacitance retention even after 5000 cycles. This pioneering work not only offers an attractive strategy for rational construction of high‐performance SiC NW‐based nanostructured electrodes materials, but also provides a fresh route for manufacturing next‐generation high‐energy storage and conversion systems.     

High Energy Density Supercapacitor Based on a Hybrid Carbon Nanotube–Reduced Graphite Oxide Architecture

A high energy density supercapacitor device is reported that utilizes hybrid carbon electrodes and the ionic liquid, 1‐butyl‐3‐methylimidazolium tetrafluoroborate (BMIMBF₄) as an electrolyte. The hybrid electrodes are prepared from reduced graphite oxide (rGO) and purified single‐walled carbon nanotubes (SWCNTs). A simple casting technique gives the hybrid structure with optimum porosity and functionality that provides high energy and power densities. The combination of SWCNTs and rGO in a weight ratio of 1:1 is found to afford a specific capacitance of 222 F g^?1 and an energy density of 94 Wh kg^?1 at room temperature.     

Amorphous Tin‐Based Composite Oxide: A High‐Rate and Ultralong‐Life Sodium‐Ion‐Storage Material

Energy‐storage technology is moving beyond lithium batteries to sodium as a result of its high abundance and low cost. However, this sensible transition requires the discovery of high‐rate and long‐lifespan anode materials, which remains a significant challenge. Here, the facile synthesis of an amorphous Sn₂P₂O₇/reduced graphene oxide nanocomposite and its sodium storage performance between 0.01 and 3.0 V are reported for the first time. This hybrid electrode delivers a high specific capacity of 480 mA h g^?1 at a current density of 50 mA g^?1 and superior rate performance of 250 and 165 mA h g^?1 at 2 and 10 A g^?1, respectively. Strikingly, this anode can sustain 15 000 cycles while retaining over 70% of the initial capacity. Quantitative kinetic analysis reveals that the sodium storage is governed by pseudocapacitance, particularly at high current rates. A full cell with sodium super ionic conductor (NASICON)‐structured Na₃V₂(PO₄)₂F₃ and Na₃V₂(PO₄)₃ as cathodes exhibits a high energy density of over 140 W h kg^?1 and a power density of nearly 9000 W kg^?1 as well as stability over 1000 cycles. This exceptional performance suggests that the present system is a promising power source for promoting the substantial use of low‐cost energy storage systems.     

Going Beyond Lithium Hybrid Capacitors: Proposing a New High‐Performing Sodium Hybrid Capacitor System for Next‐Generation Hybrid Vehicles Made with Bio‐Inspired Activated Carbon

A novel sodium hybrid capacitor (NHC) is constructed with an intercalation‐type sodium material [carbon coated‐Na₃V₂(PO₄)₃, C‐NVP] and high surface area‐activated carbon derived from an eco‐friendly resource cinnamon sticks (CDCs) in an organic electrolyte. This novel NHC possesses a combination of high energy and high power density, along with remarkable electrochemical stability. In addition, the C‐NVP/CDC system outperforms present, well‐established lithium hybrid capacitor systems in all areas, and can thus be added to the list of candidates for future electric vehicles. A careful optimization of mass balance between electrode materials enables the C‐NVP/CDC cell to exhibit extraordinary capacitance performance. This novel NHC produces an energy density of 118 Wh kg^?1 at a specific power of 95 W kg^?1 and retains an energy density of 60 Wh kg^?1 with high specific power of 850 W kg^?1. Furthermore, a discharge capacitance of 53 F g^?1 is obtained from the C‐NVP/CDC cell at a 1 mA cm^?2 current density, along with 95% capacitance retention, even after 10 000 cycles. The sluggish kinetics of the Na ion battery system is successfully overcome by developing a stable, high‐performing NHC system.     

One‐Step Fabrication of Ultrathin Porous Nickel Hydroxide‐Manganese Dioxide Hybrid Nanosheets for Supercapacitor Electrodes with Excellent Capacitive Performance

A facile one‐step hydrothermal co‐deposition method for growth of ultrathin Ni(OH)₂‐MnO₂ hybrid nanosheet arrays on three dimensional (3D) macroporous nickel foam is presented. Due to the highly hydrophilic and ultrathin nature of hybrid nanosheets, as well as the synergetic effects of Ni(OH)₂ and MnO₂, the as‐fabricated Ni(OH)₂‐MnO₂ hybrid electrode exhibits an ultrahigh specific capacitance of 2628 F g^?1. Moreover, the asymmetric supercapacitor with the as‐obtained Ni(OH)₂‐MnO₂ hybrid film as the positive electrode and the reduced graphene oxide as the negative electrode has a high energy density (186 Wh kg^?1 at 778 W kg^?1), based on the total mass of active materials.     

Battery‐like Supercapacitors from Vertically Aligned Carbon Nanofiber Coated Diamond: Design and Demonstrator

To fabricate battery‐like supercapacitors with high power and energy densities, big capacitances, as well as long‐term capacitance retention, vertically aligned carbon nanofibers (CNFs) grown on boron doped diamond (BDD) films are employed as the capacitor electrodes. They possess large surface areas, high conductivity, high stability, and importantly are free of binder. The large surface areas result from their porous structures. The containment of graphene layers and copper metal catalysts inside CNFs leads to their high conductivity. Both electrical double layer capacitors (EDLCs) in inert solutions and pseudocapacitors (PCs) using Fe(CN)₆^3?/4? redox‐active electrolytes are constructed with three‐ and two‐electrode systems. The assembled two‐electrode symmetrical supercapacitor devices exhibit capacitances of 30 and 48 mF cm^?2 at 10 mV s^?1 for EDLC and PC devices, respectively. They remain constant even after 10 000 charging/discharging cycles. The power densities are 27.3 and 25.3 kW kg^?1 for EDLC and PC devices, together with their energy densities of 22.9 and 44.1 W h kg^?1, respectively. The performance of these devices is superior to most of the reported supercapacitors and batteries. Vertically aligned CNF/BDD hybrid films are thus useful to construct high‐performance battery‐like and industry‐orientated supercapacitors for future power devices.     

Design and Performance of Rechargeable Sodium Ion Batteries,and Symmetrical Li‐Ion Batteries with Supercapacitor‐Like Power Density Based upon Polyoxovanadates

The polyanion Li₇V₁₅O₃₆(CO₃) is a nanosized molecular cluster (≈1 nm in size), that has the potential to form an open host framework with a higher surface‐to‐bulk ratio than conventional transition metal oxide electrode materials. Herein, practical rechargeable Na‐ion batteries and symmetric Li‐ion batteries are demonstrated based on the polyoxovanadate Li₇V₁₅O₃₆(CO₃). The vanadium centers in {V₁₅O₃₆(CO₃)} do not all have the same V^IV/V redox potentials, which permits symmetric devices to be created from this material that exhibit battery‐like energy density and supercapacitor‐like power density. An ultrahigh specific power of 51.5 kW kg^?1 at 100 A g^?1 and a specific energy of 125 W h kg^?1 can be achieved, along with a long cycling life (>500 cycles). Moreover, electrochemical and theoretical studies reveal that {V₁₅O₃₆(CO₃)} also allows the transport of large cations, like Na⁺, and that it can serve as the cathode material for rechargeable Na‐ion batteries with a high specific capacity of 240 mA h g^?1 and a specific energy of 390 W h kg^?1 for the full Na‐ion battery. Finally, the polyoxometalate material from these electrochemical energy storage devices can be easily extracted from spent electrodes by simple treatment with water, providing a potential route to recycling of the redox active material.     

2D Metal Zn Nanostructure Electrodes for High‐Performance Zn Ion Supercapacitors

Recent supercapacitors show a high power density with long‐term cycle life time in energy‐powering applications. A supercapacitor based on a single metal electrode accompanying multivalent cations, multiple charging/discharging kinetics, and high electrical conductivity is a promising energy‐storing system that replaces conventionally used oxide and sulfide materials. Here, a hierarchically nanostructured 2D‐Zn metal electrode‐ion supercapacitor (ZIC) is reported which significantly enhances the ion diffusion ability and overall energy storage performance. Those nanostructures can also be successfully plated on various flat‐type and fiber‐type current collectors by a controlled electroplating method. The ZIC exhibits excellent pseudocapacitive performance with a high energy density of 208 W h kg^?1 and a power density from 500 W kg^?1, which are significantly higher than those of previously reported supercapacitors with oxide and sulfide materials. Furthermore, the fiber‐type ZIC also shows high energy‐storing performance, outstanding mechanical flexibility, and waterproof characteristics, without any significant capacitance degradation during bending tests. These results highlight the promising possibility of nanostructured 2D Zn metal electrodes with the controlled electroplating method for future energy storage applications.     

Flexible Asymmetric Micro‐Supercapacitors Based on Bi2O3 and MnO2 Nanoflowers: Larger Areal Mass Promises Higher Energy Density

A flexible asymmetric supercapacitor (ASC) with high energy density is designed and fabricated using flower‐like Bi₂O₃ and MnO₂ grown on carbon nanofiber (CNF) paper as the negative and positive electrodes, respectively. The lightweight (1.6 mg cm^?2), porous, conductive, and flexible features make the CNF paper an ideal support for guest active materials, which permit a large areal mass of 9 mg cm^?2 for Bi₂O₃ (≈85 wt% of the entire electrode). Thus, the optimal device with an operation voltage of 1.8 V can deliver a high energy density of 43.4 μWh cm^?2 (11.3 W h kg^?1, based on the total electrodes) and a maximum power density of 12.9 mW cm^?2 (3370 W kg^?1). This work provides an example of large areal mass and flexible electrode for ASCs with high areal capacitance and high energy density, holding great promise for future flexible electronic devices.     

Achieving Ultrahigh Energy Densities of Supercapacitors with Porous Titanium Carbide/Boron‐Doped Diamond Composite Electrodes

The energy densities of most supercapacitors (SCs) are low, hindering their practical applications. To construct SCs with ultrahigh energy densities, a porous titanium carbide (TiC)/boron‐doped diamond (BDD) composite electrode is synthesized on a titanium plate that is pretreated using a plasma electrolytic oxidation (PEO) technique. The porous and nanometer‐thick TiO₂ layer formed during PEO process prevents the formation of brittle titanium hydride and enhances the BDD growth during chemical vapor deposition processes. Meanwhile, the in situ conversion of TiO₂ into TiC is achieved. Combination of this capacitor electrode with soluble redox electrolytes leads to the fabrication of high‐performance SCs in both aqueous and organic solutions. In 0.05 m Fe(CN)₆^3?/4? + 1 m Na₂SO₄ aqueous solution, the capacitance is as high as 46.3 mF cm^?2 at a current density of 1 mA cm^?2; this capacitance remains 92% of its initial value even after 10 000 charge/discharge cycles; the energy density is up to 47.4 Wh kg^?1 at a power density of 2236 W kg^?1. The performance of constructed SCs is superior to most available SCs and some electrochemical energy storage devices like batteries. Such a porous capacitor electrode is thus promising for the construction of high‐performance SCs for practical applications.     

Rational Design of Nickel Hydroxide‐Based Nanocrystals on Graphene for Ultrafast Energy Storage

Compact, light, and powerful energy storage devices are urgently needed for many emerging applications; however, the development of advanced power sources relies heavily on advances in materials innovation. Here, the findings in rational design, one‐pot synthesis, and characterization of a series of Ni hydroxide‐based electrode materials in alkaline media for fast energy storage are reported. Under the guidance of density functional theory calculations and experimental investigations, a composite electrode composed of Co‐/Mn‐substituted Ni hydroxides grown on reduced graphene oxide (rGO) is designed and prepared, demonstrating capacities of 665 and 427 C g^?1 at current densities of 2 and 20 A g^?1, respectively. The superior performance is attributed mainly to the low deprotonation energy and the facile electron transport, as elaborated by theoretical calculations. When coupled with an electrode based on organic molecular‐modified rGO, the resulting hybrid device demonstrates an energy density of 74.7 W h kg^?1 at a power density of 1.68 kW kg^?1 while maintaining capacity retention of 91% after 10,000 cycles (20 A g^?1). The findings not only provide a promising electrode material for high‐performance hybrid capacitors but also open a new avenue toward knowledge‐based design of efficient electrode materials for other energy storage applications.     

Dial the Mechanism Switch of VN from Conversion to Intercalation toward Long Cycling Sodium‐Ion Battery

Transition metal nitrides are promising energy storage materials in regard to good metallic conductivity and high theoretical specific capacity, but their cycling stability is impeded by the huge volume change caused by the conversion reaction mechanism. Here, a simple strategy to produce an intercalation pseudocapacitive‐type vanadium nitride (VN) by one‐step ammonification of V₂C MXene for sodium‐ion batteries is reported. Profiting from a distinctive layered structure pillared by Al atoms in the layer spacing, it delivers a high capacity of 372 mA h g^?1 at 50 mA g^?1 and a desirable rate performance. More importantly, it shows remarkably long cycling stability over 7500 cycles without capacity attenuation at 500 mA g^?1. As expected, it is found that the intercalation pseudocapacitance plays an important role in the excellent performance, by using in situ X‐ray diffraction and ex situ X‐ray absorption structure characterization. Even more remarkable, are the high energy and power density of the sodium‐ion capacitor after coupling with a carbon‐based cathode. The hybrid device possesses an energy density of 78.43 Wh kg^?1 at power density of 260 W kg^?1. The results clearly show that such a unique‐layered VN with outstanding Na storage capability is an excellent new material for energy storage systems.