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.     

Wearable High‐Performance Supercapacitors Based on Silver‐Sputtered Textiles with FeCo2S4–NiCo2S4 Composite Nanotube‐Built Multitripod Architectures as Advanced Flexible Electrodes

To achieve high‐performance wearable supercapacitors (SCs), a new class of flexible electrodes with favorable architectures allowing large porosity, high conductivity, and good mechanical stability is strongly needed. Here, this study reports the rational design and fabrication of a novel flexible electrode with nanotube‐built multitripod architectures of ternary metal sulfides' composites (FeCo₂S₄–NiCo₂S₄) on a silver‐sputtered textile cloth. Silver sputtering is applicable to almost all kinds of textiles, and S^2? concentration is optimized during sulfidation process to achieve such architectures and also a complete sulfidation assuring high conductivity. New insights into concentration‐dependent sulfidation mechanism are proposed. The additive‐free FeCo₂S₄–NiCo₂S₄ electrode shows a high specific capacitance of 1519 F g^?1 at 5 mA cm^?2 and superior rate capability (85.1% capacitance retention at 40 mA cm^?2). All‐solid‐state SCs employing these advanced electrodes deliver high energy density of 46 W h kg^?1 at 1070 W kg^?1 as well as achieve remarkable cycling stability retaining 92% of initial capacitance after 3000 cycles at 10 mA cm^?2, and outstanding reliability with no capacitance degradation under large twisting. These are attributed to the components' synergy assuring rich redox reactions, high conductivity as well as highly porous but robust architectures. An almost linear increase in capacitance with devices' area indicates possibility to meet various energy output requirements. This work provides a general, low‐cost route to wearable power sources.     

Hierarchical Multicomponent Electrode with Interlaced Ni(OH)2 Nanoflakes Wrapped Zinc Cobalt Sulfide Nanotube Arrays for Sustainable High‐Performance Supercapacitors

High energy density, fast recharging ability, and sustained cycle life are the primary requisite of supercapacitors (SCs); these necessities can be fulfilled by engineering a smart current collector with hierarchical combination of different active materials. This study reports a multicomponent design of hierarchical zinc cobalt sulfide (ZCS) hollow nanotube arrays wrapped with interlaced ultrathin Ni(OH)₂ nanoflakes for high‐performance electrodes. The ZCS exhibits a unique pentagonal cross‐section and a rough surface that facilitates the deposition of Ni(OH)₂ nanoflakes with a thickness of 7.5 nm. The ZCS/Ni(OH)₂ hierarchical electrode exhibits a high specific capacitance of 2156 F g^?1 and excellent cyclic stability with 94% retention over 3000 cycles. This is attributed to enhanced redox reactions, the direct growth of arrays on 3D porous foam acting as a “superhighway” for electron transport, and the increased availability of electrochemical active sites provided by the ultrathin Ni(OH)₂ flakes that also sustain the stability of the electrode by sacrificing themselves during long charge/discharge cycles. Symmetric SCs are assembled to achieve high energy density of 74.93 W h kg^?1 and exhibit superior cyclic stability of 78% retention with 81% coulombic efficiency over 10 000 cycles.     

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.     

Ultrathin and Porous Ni3S2/CoNi2S4 3D‐Network Structure for Superhigh Energy Density Asymmetric Supercapacitors

3D‐networked, ultrathin, and porous Ni₃S₂/CoNi₂S₄ on Ni foam (NF) is successfully designed and synthesized by a simple sulfidation process from 3D Ni–Co precursors. Interestingly, the edge site‐enriched Ni₃S₂/CoNi₂S₄/NF 3D‐network is realized by the etching‐like effect of S^2? ions, which made the surfaces of Ni₃S₂/CoNi₂S₄/NF with a ridge‐like feature. The intriguing structural/compositional/componental advantages endow 3D‐networked‐free‐standing Ni₃S₂/CoNi₂S₄/NF electrodes better electrochemical performance with specific capacitance of 2435 F g^?1 at a current density of 2 A g^?1 and an excellent rate capability of 80% at 20 A g^?1. The corresponding asymmetric supercapacitor achieves a high energy density of 40.0 W h kg^?1 at an superhigh power density of 17.3 kW kg^?1, excellent specific capacitance (175 F g^?1 at 1A g^?1), and electrochemical cycling stability (92.8% retention after 6000 cycles) with Ni₃S₂/CoNi₂S₄/NF as the positive electrode and activated carbon/NF as the negative electrode. Moreover, the temperature dependences of cyclic voltammetry curve polarization and specific capacitances are carefully investigated, and become more obvious and higher, respectively, with the increase of test temperature. These can be attributed to the components' synergetic effect assuring rich redox reactions, high conductivity as well as highly porous but robust architectures. This work provides a general, low‐cost route to produce high performance electrode materials for portable supercapacitor applications on a large scale.     

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.     

A Novel Sustainable Flour Derived Hierarchical Nitrogen‐Doped Porous Carbon/Polyaniline Electrode for Advanced Asymmetric Supercapacitors

Hierarchically porous nitrogen‐doped carbon (HPC)/polyaniline (PANI) nanowire arrays nanocomposites are synthesized by a facile in situ polymerization. 3D interconnected honeycomb‐like HPC was prepared by a cost‐effective route via one‐step carbonization using urea and alkali‐treated wheat flour as carbon precursor with a high specific surface area (1294 m² g^?1). The specific capacitances of HPC and HPC/PANI (with a surface area of 923 m² g^?1) electrode are 383 and 1080 F g^?1 in 1 m H₂SO₄, respectively. Furthermore, an asymmetric supercapacitor based on HPC/PANI as positive electrode and HPC as negative electrode is successfully assembled with a voltage window of 0–1.8 V in 1 m Na₂SO₄ aqueous electrolyte, exhibiting high specific capacitance (134 F g^?1), high energy density (60.3 Wh kg^?1) and power density (18 kW kg^?1), and excellent cycling stability (91.6% capacitance retention after 5000 cycles).     

Antimonene: A Novel 2D Nanomaterial for Supercapacitor Applications

In pursuing higher energy density, without compromising the power density of supercapacitor platforms, the application of an advanced 2D nanomaterial is utilized to maximize performance. Antimonene, for the first time, is characterized as a material for applications in energy storage, being applied as an electrode material as the basis of a supercapacitor. Antimonene is shown to significantly improve the energy storage capabilities of a carbon electrode in both cyclic voltammetry and galvanostatic charging. Antimonene demonstrates remarkable performance with a capacitance of 1578 F g^?1, with a high charging current density of 14 A g^?1. Hence, antimonene is shown to be a highly promising material for energy storage applications. The system also demonstrates a highly competitive energy and power densities of 20 mW h kg^?1 and 4.8 kW kg^?1_, respectively. In addition to the excellent charge storing abilities, antimonene shows good cycling capabilities.     

Chemically Exfoliating Biomass into a Graphene‐like Porous Active Carbon with Rational Pore Structure,Good Conductivity,and Large Surface Area for High‐Performance Supercapacitors

Active carbons have unique physicochemical properties, but their conductivities and surface to weight ratios are much poorer than graphene. A unique and facile method is innovated to chemically process biomass by “drilling” holes with H₂O₂ and exfoliating into graphene‐like nanosheets with HAc, followed by carbonization at a high temperature for highly graphitized activated carbon with greatly enhanced porosity, unique pore structure, high conductivity, and large surface area. This graphene‐like carbon exhibits extremely high specific capacitance (340 F g^?1 at 0.5 A g^?1) and high specific energy density (23.33 to 16.67 W h kg^?1) with excellent rate capability and long cycling stability (remains 98% after 10 000 cycles), which is much superior to all reported carbons including graphene. Synthesis mechanism for deriving biomass into porous graphene‐like carbons is discussed in detail. The enhancement mechanism for the porous graphene‐like carbon electrode reveals that rationally designed meso‐ and macropores are very critical in porous electrode performance, which can network micropores for diffusion freeways, high conductivity, and high utilization. This work has universal significance in producing highly porous and conductive carbons from biomass including biowastes for various energy storage/conversion applications.     

High Energy and High Power Lithium‐Ion Capacitors Based on Boron and Nitrogen Dual‐Doped 3D Carbon Nanofibers as Both Cathode and Anode

High energy density at high power density is still a challenge for the current Li‐ion capacitors (LICs) due to the mismatch of charge‐storage capacity and electrode kinetics between capacitor‐type cathode and battery‐type anode. In this work, B and N dual‐doped 3D porous carbon nanofibers are prepared through a facile method as both capacitor‐type cathode and battery‐type anode for LICs. The B and N dual doping has profound effect in tuning the porosity, functional groups, and electrical conductivity for the porous carbon nanofibers. With rational design, the developed B and N dual‐doped carbon nanofibers (BNC) exhibit greatly improved electrochemical performance as both cathode and anode for LICs, which greatly alleviates the mismatch between the two electrodes. For the first time, a 4.5 V “dual carbon” BNC//BNC LIC device is constructed and demonstrated, exhibiting outstanding energy density and power capability compared to previously reported LICs with other configurations. In specific, the present BNC//BNC LIC device can deliver a large energy density of 220 W h kg^?1 and a high power density of 22.5 kW kg^?1 (at 104 W h kg^?1) with reasonably good cycling stability (≈81% retention after 5000 cycles).     

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.     

Nanostructured Ternary Electrodes for Energy‐Storage Applications

A three‐component, flexible electrode is developed for supercapacitors over graphitized carbon fabric, utilizing γ‐MnO₂ nanoflowers anchored onto carbon nanotubes (γ‐MnO₂/CNT) as spacers for graphene nanosheets (GNs). The three‐component, composite electrode doubles the specific capacitance with respect to GN‐only electrodes, giving the highest‐reported specific capacitance (308 F g^?1) for symmetric supercapacitors containing MnO₂ and GNs using a two‐electrode configuration, at a scan rate of 20 mV s^?1. A maximum energy density of 43 W h kg^?1 is obtained for our symmetric supercapacitors at a constant discharge‐current density of 2.5 A g^?1 using GN–(γ‐MnO₂/CNT)‐nanocomposite electrodes. The fabricated supercapacitor device exhibits an excellent cycle life by retaining ≈90% of the initial specific capacitance after 5000 cycles.     

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.     

High‐Energy Density Li‐Ion Capacitor with Layered SnS2/Reduced Graphene Oxide Anode and BCN Nanosheet Cathode

Lithium‐ion capacitors (LICs) with capacitor‐type cathodes and battery‐type anodes are considered a promising next‐generation advanced energy storages system that meet the requirements of high energy density and power density. However, the mismatch of charge‐storage capacity and electrode kinetics between positive and negative electrodes remains a challenge. Herein, layered SnS₂/reduced graphene oxide (RGO) nanocomposites are developed for negative electrodes and a 2D B/N codoped carbon (BCN) nanosheet is designed for the positive electrode. The SnS₂/RGO derived from SnS₂‐bonded RGO of high conductivity exhibits a capacity of 1198 mA h g^?1 at 100 mA g^?1. Boron and nitrogen atoms in BCN are found to promote adsorption of anions, which enhance the pseudocapacitive contribution as well as expanding the voltage of LICs. A quantitative kinetics analysis indicates that the SnS₂/RGO electrodes with a dominating capacitive mechanism and a diminished intercalation process, benefit the kinetic balance between the two electrodes. With this particular structure, the LIC is able to operate at the highest operating voltage for these devices recorded to date (4.5 V), exhibiting an energy density of 149.5 W h kg^?1, a power density of 35 kW kg^?1, and a capacity retention ratio of 90% after 10 000 cycles.     

High‐Performance Fiber‐Shaped All‐Solid‐State Asymmetric Supercapacitors Based on Ultrathin MnO2 Nanosheet/Carbon Fiber Cathodes for Wearable Electronics

Flexible fiber‐shaped supercapacitors have shown great potential in portable and wearable electronics. However, small specific capacitance and low operating voltage limit the practical application of fiber‐shaped supercapacitors in high energy density devices. Herein, direct growth of ultrathin MnO₂ nanosheet arrays on conductive carbon fibers with robust adhesion is exhibited, which exhibit a high specific capacitance of 634.5 F g^?1 at a current density of 2.5 A g^?1 and possess superior cycle stability. When MnO₂ nanosheet arrays on carbon fibers and graphene on carbon fibers are used as a positive electrode and a negative electrode, respectively, in an all‐solid‐state asymmetric supercapacitor (ASC), the ASC displays a high specific capacitance of 87.1 F g^?1 and an exceptional energy density of 27.2 Wh kg^?1. In addition, its capacitance retention reaches 95.2% over 3000 cycles, representing the excellent cyclic ability. The flexibility and mechanical stability of these ASCs are highlighted by the negligible degradation of their electrochemical performance even under severely bending states. Impressively, as‐prepared fiber‐shaped ASCs could successfully power a photodetector based on CdS nanowires without applying any external bias voltage. The excellent performance of all‐solid‐state ASCs opens up new opportunity for development of wearable and self‐powered nanodevices in near future.     

Pseudocapacitive Charge Storage in Thick Composite MoS2 Nanocrystal‐Based Electrodes

A synthesis methodology is demonstrated to produce MoS₂ nanoparticles with an expanded atomic lamellar structure that are ideal for Faradaic‐based capacitive charge storage. While much of the work on MoS₂ focuses on the high capacity conversion reaction, that process is prone to poor reversibility. The pseudocapacitive intercalation‐based charge storage reaction of MoS₂ is investigated, which is extremely fast and highly reversible. A major challenge in the field of pseudocapacitive‐based energy storage is the development of thick electrodes from nanostructured materials that can sustain the fast inherent kinetics of the active nanocrystalline material. Here a composite electrode comprised of a poly(acrylic acid) binder, carbon fibers, and carbon black additives is utilized. These electrodes deliver a specific capacity of 90 mAh g^?1 in less than 20 s and can be cycled 3000 times while retaining over 80% of the original capacity. Quantitative kinetic analysis indicates that over 80% of the charge storage in these MoS₂ nanocrystals is pseudocapacitive. Asymmetric full cell devices utilizing a MoS₂ nanocrystal‐based electrode and an activated carbon electrode achieve a maximum power density of 5.3 kW kg^?1 (with 6 Wh kg^?1 energy density) and a maximum energy density of 37 Wh kg^?1 (with 74 W kg^?1power density).     

All‐Solid‐State On‐Chip Supercapacitors Based on Free‐Standing 4H‐SiC Nanowire Arrays

All‐solid‐state on‐chip SiC supercapacitors (SCs) based on free‐standing SiC nanowire arrays (NWAs) are reported. In comparison to the widely used technique based on the interdigitated fingers, the present strategy can be much more facile for constructing on‐chip SCs devices, which is directly sandwiched with a solid electrolyte layer between two pieces of SiC NWAs film without any substrate. The mass loading of active materials of on‐chip SiC SCs can be up to ≈5.6 mg cm^?2, and the total device thickness is limited in ≈40 µm. The specific area energy and power densities of the SCs device reach 5.24 µWh cm^?2 and 11.2 mW cm^?2, and their specific volume energy and power densities run up to 1.31 mWh cm^–3 and 2.8 W cm^?3, respectively, which are two orders of magnitude higher than those of state‐of‐the‐art SiC‐based SCs, and also much higher than those of other solid‐state carbon‐based SCs ever reported. Furthermore, such on‐chip SCs exhibit superior rate capability and robust stability with over 94% capacitance retention after 10 000 cycles at a scan rate of 100 mV s^?1, representing their high performance in all merits.     

A Flexible and Ultrahigh Energy Density Capacitor via Enhancing Surface/Interface of Carbon Cloth Supported Colloids

Pseudocapacitors are now reaching the energy density limits set by the surface redox reaction of their electrode materials, requiring new cation paradigms for a fast cation Faradaic reaction with high capacitance. In this work, a flexible and ultrahigh energy density capacitor is reported via enhancing surface/interface of active colloids and supported carbon cloth. A flexible asymmetrical capacitor assembled with Ni²⁺ colloidal cathode and Fe³⁺ colloidal anode displays a high energy density of 353 W h kg^?1 at the power density of 2250 W kg^?1, outperforming recent reported pseudocapacitors, and shows superior cycling stability after 10 000 charge–discharge cycles at current density of 30 A g^?1. This work demonstrates that the optimized surface/interface of carbon cloth and colloids can lead to the enhancement of both stability and activity of colloidal electrode.     

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.     

Enhanced Li‐Ion Accessibility in MXene Titanium Carbide by Steric Chloride Termination

Pseudocapacitance is a key charge storage mechanism to advanced electrochemical energy storage devices distinguished by the simultaneous achievement of high capacitance and a high charge/discharge rate by using surface redox chemistries. MXene, a family of layered compounds, is a pseudocapacitor‐like electrode material which exhibits charge storage through exceptionally fast ion accessibility to redox sites. Here, the authors demonstrate steric chloride termination in MXene Ti₂CT_x (T_x : surface termination groups) to open the interlayer space between the individual 2D Ti₂CT_x units. The open interlayer space significantly enhances Li‐ion accessibility, leading to high gravimetric and volumetric capacitances (300 F g^?1 and 130 F cm^?3) with less diffusion limitation. A Li‐ion hybrid capacitor consisting of the Ti₂CT_x negative electrode and the LiNi_1/3Co_1/3Mn_1/3O₂ positive electrode displays an unprecedented specific energy density of 160 W h kg^?1 at 220 W kg^?1 based on the total weight of positive and negative active materials.