A high-performance hybrid supercapacitor with Li4Ti5O12-C nano-composite prepared by in situ and ex situ carbon modification

In this work, we report on the synthesis of in situ and ex situ carbon-modified Li₄Ti₅O₁₂-C (LTO-C) nano-composite and its application in a hybrid supercapacitor constructed using activated carbon (AC) and LTO-C nano-composite as positive and negative electrodes, respectively. The hybrid capacitors are characterized by galvanostatic charge–discharge, cycle life testing, and electrochemical impedance spectroscopy. The results reveal that the AC/LTO-C hybrid capacitors exhibit high rate capability and long cycle life. In the potential range of 1.5–3.0 V, the AC/LTO-C hybrid system can deliver a specific capacitance of 83 F?g^?1 based on the total mass of AC and LTO-C electrodes at a current density of 60 mA g^?1 (2 C rate). At a higher discharge rate of 980 mA g^?1 (32 C), the capacity is 68 F?g^?1, about 82?% of that at 2 C rate. After 9,000 deep cycles at 32 C, the hybrid capacitor still maintains 84?% of its initial capacitance. The specific energy of such hybrid system is 20 Wh kg^?1, which is at least twice that of an AC/AC system. Combining the high energy density with power capability, the AC/LTO-C hybrid supercapacitor has demonstrated high performance for applications needing high power output.     

Preparation,characterization, and evaluation of LiNi0.4Co0.6O2 nanofibers for supercapacitor applications

We prepared LiNi_0.4Co_0.6O₂ nanofibers by electrospinning at the calcination temperature of 450 °C for 6 h. The prepared LiNi_0.4Co_0.6O₂ nanofibers was characterized by thermal, X-ray diffraction, and Fourier transform infrared (FTIR) studies. The morphology of LiNi_0.4Co_0.6O₂ nanofibers was characterized by scanning electron microscopy studies. The asymmetric supercapacitor was fabricated using LiNi_0.4Co_0.6O₂ nanofibers as positive electrode and activated carbon (AC) as negative electrode and a porous polypropylene separator in 1 M LiPF₆–ethylene carbonate/dimethyl carbonate (LiPF₆–EC:DMC) (1:1?v/v) as electrolyte. Cyclic voltammetry studies were then carried out in the potential range of 0 to 3.0 V at different scan rates which exhibited the highest specific capacitance of 72.9 F g^?1. The electrochemical impedance measurements were carried out to find the charge transfer resistance and specific capacitance of the cell, and they were found to be 5.05 Ω and 67.4 F g^?1, respectively. Finally, the charge–discharge studies were carried out at a current density of 1 mA cm^?2 to find out the discharge-specific capacitance, energy density, and power density of the capacitor cell, and they were found to be 70.9 F g^?1, 180.2 Wh kg^?1, and 248.0 W kg^?1, respectively.     

Reed straw derived active carbon/graphene hybrids as sustainable high-performance electrodes for advanced supercapacitors

Reed straw-derived active carbon@graphene (AC@GR) hybrids were prepared by one-step carbonization/activation process using a mixture of reed straw and graphene oxide (GO) as raw materials and ZnCl₂ as activation agent. The as-prepared hybrids exhibit high specific surface area in a range of 1971–2497 m² g^?1, abundant porosity, as well as excellent energy storage capability. The symmetric C//C supercapacitor using the hybrid obtained at 700 °C as electrodes demonstrates superior cycling durability, ca. 90 % retention after 6000 cycles at 2 A g^?1, and a high energy density of 6.12 Wh kg^?1 at a power density of up to 4660 W kg^?1 in 6 M KOH aqueous electrolyte. The excellent capacitive performance is attributed to the synergistic effect of AC and GR.     

A high mass loading electrode based on ultrathin Co<Subscript>3</Subscript>S<Subscript>4</Subscript> nanosheets for high performance supercapacitor

There is a growing need for the electrode with high mass loading of active materials, where both high energy and high power densities are required, in current and near-future applications of supercapacitor. Here, an ultrathin Co₃S₄ nanosheet decorated electrode (denoted as Co₃S₄/NF) with mass loading of 6 mg cm^?2 is successfully fabricated by using highly dispersive Co₃O₄ nanowires on Ni foam (NF) as template. The nanosheets contained lots of about 3～5 nm micropores benefiting for the electrochemical reaction and assembled into a three-dimensional, honeycomb-like network with 0.5～1 μm mesopore structure for promoting specific surface area of electrode. The improved electrochemical performance was achieved, including an excellent cycliability of 10,000 cycles at 10 A g^?1 and large specific capacitances of 2415 and 1152 F g^?1 at 1 and 20 A g^?1, respectively. Impressively, the asymmetric supercapacitor assembled with the activated carbon (AC) and Co₃S₄/NF electrode exhibits a high energy density of 79 Wh kg^?1 at a power density of 151 W kg^?1, a high power density of 3000 W kg^?1 at energy density of 30 Wh kg^?1 and 73 % retention of the initial capacitance after 10,000 charge-discharge cycles at 2 A g^?1. More importantly, the formation process of the ultrathin Co₃S₄ nanosheets upon reaction time is investigated, which is benefited from the gradual infiltration of sulfide ions and the template function of ultrafine Co₃O₄ nanowires in the anion-exchange reaction.

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Graphical abstract The ultrathin 2D Co₃S₄ nanosheets fabricated on 3D Ni foam and the formation process of the ultrathin Co₃S₄ nanosheets upon reaction times has been investigated. At the same time, the Co₃S₄/NF electrode displays an outstanding specific capacitance of 2420 F g^?1 at 1 A g^?1 with high mass loading of 6 mg cm^?2.

     

A High-Performance Asymmetric Supercapacitor Based on Tungsten Oxide Nanoplates and Highly Reduced Graphene Oxide Electrodes

Tungsten oxide/graphene hybrid materials are attractive semiconductors for energy-related applications. Herein, we report an asymmetric supercapacitor (ASC, HRG//m-WO₃ ASC), fabricated from monoclinic tungsten oxide (m-WO₃) nanoplates as a negative electrode and highly reduced graphene oxide (HRG) as a positive electrode material. The supercapacitor performance of the prepared electrodes was evaluated in an aqueous electrolyte (1 m H₂SO₄) using three- and two-electrode systems. The HRG//m-WO₃ ASC exhibits a maximum specific capacitance of 389 F g⁻¹ at a current density of 0.5 A g⁻¹, with an associated high energy density of 93 Wh kg⁻¹ at a power density of 500 W kg⁻¹ in a wide 1.6 V operating potential window. In addition, the HRG//m-WO₃ ASC displays long-term cycling stability, maintaining 92 % of the original specific capacitance after 5000 galvanostatic charge–discharge cycles. The m-WO₃ nanoplates were prepared hydrothermally while HRG was synthesized by a modified Hummers method.     

Asymmetric supercapacitors based on stabilized α-Ni(OH)2 and activated carbon

In this work, stabilized Al-substituted α-Ni(OH)₂ materials were successfully synthesized by a chemical coprecipitation method. The experimental results showed that the 7.5% Al-substituted α-Ni(OH)₂ materials exhibited high specific capacitance (2.08?×?10³ F/g) and excellent rate capability due to the high stability of Al-substituted α-Ni(OH)₂ structures in alkaline media, suggesting its potential application in electrode material for supercapacitors. To enhance energy density, an asymmetric type pseudo/electric double-layer capacitor was considered where α-Ni(OH)₂ materials and activated carbon act as the positive and negative electrodes, respectively. Values for the maximum specific capacitance of 127 F/g and specific energy of 42 W·h/kg were demonstrated for a cell voltage between 0.4 and 1.6 V. By using the α-Ni(OH)₂ electrode, the asymmetric supercapacitor exhibited high energy density and stable power characteristics. The hybrid supercapacitor also exhibited a good electrochemical stability with 82% of the initial capacitance over consecutive 1,000 cycle numbers.     

In situ synthesis of crosslinked-polyaniline nano-pillar arrays/reduced graphene oxide nanocomposites for supercapacitors

Crosslinked-polyaniline (CPA) nano-pillar arrays adsorbed on the surface of reduced graphene oxide (RGO) sheets were synthesized by in situ solution polymerization through two steps of reduction. The electrochemical analyses demonstrated that the befittingly reduced CPA/RGO composite exhibited high performance as electrode materials for supercapacitors. The CPA/RGO composite showed very high specific capacitance of 1532 F g^?1 at a scan rate of 10 mV s^?1 or 694 F g^?1 at a current density of 2 A g^?1 in 1 M H₂SO₄ electrolyte, as well as great energy density of 61.4 W h kg^?1 at a current density of 2 A g^?1. The electrode material also had decent power density of 4 kW kg^?1 at a current density of 10 A g^?1, and good cycling stability of 92.5 % capacitance retained after 500 cycles of cyclic voltammetry at 500 mV s^?1. The neat microstructures and super electrochemical properties suggest the potential use of the composites in supercapacitors.     

Design of high-performance core-shell hollow carbon nanofiber@nickel-cobalt double hydroxide composites for supercapacitive energy storage

The supercapacitive performances of supercapacitor mainly depend on the physical nanostructure and micro-morphology of electrode materials. Here, we demonstrated the design, synthesis and electrochemical performances of core-shell hollow carbon nanofiber@nickel-cobalt-layered double hydroxide (HCNF@ Ni_0.67Co_0.33-LDH) nanocomposites with an optimized Ni/Co molar ratio of 2:1. The HCNF was used as superiorly conductive core to sustain the nanoporous silky Ni_0.67Co_0.33-LDH shell, which can efficiently provide fast transport pathways for electrons and electrolyte ions. The outstanding specific capacitance of 2486 F g^?1 at 1 A g^?1 based on galvanostatic charge-discharge curves were acquired for the highly electroactive HCNF@Ni_0.67Co_0.33-LDH. Furthermore, the HCNF@Ni_0.67Co_0.33-LDH electrode delivered a distinguished rate capability with a specific capacitance of 1890 F g^?1 even at 15 A g^?1. Notably, an asymmetric supercapacitor with HCNF@Ni_0.67Co_0.33-LDH as cathode and HCNF as anode was devised, which presented a prominent specific capacitance of 228 F g^?1, good energy density of 62.1 Wh kg^?1, and impressive cycling stability (90.6% capacitance retention after 10,000 cycles).     

KOH direct treatment of kombucha and in situ activation to prepare hierarchical porous carbon for high-performance supercapacitor electrodes

Kombucha, a renewable biomass, has been successfully utilized as an accessible carbon source to fabricate kombucha-derived hierarchical porous carbon (KHPC) by KOH direct treatment and in situ activation. The prepared KHPC shows an interconnected hierarchical porous structure, a pore volume of 0.41 cm³ g^?1, and a specific surface area of 917 m² g^?1. Due to the multiple synergistic effects of these advantages, the KHPC-3 exhibits a high specific capacitance of 326 F g^?1 at a current density of 1 A g^?1 in 6 M KOH, good rate capability of 82% retention from 1 to 20 A g^?1, and cycling performance with 91.3% retention over 5000 cycles. Moreover, the KHPC-3 symmetric supercapacitor reveals a good energy density of 20.97 Wh kg^?1 at a power density of 871.2 W kg^?1 and retains 8.08 Wh kg^?1 at 6330 W kg^?1 in 1 M Na₂SO₄ electrolyte. Therefore, the KHPC obtained via the simple synthesis process shows great promise as an electrode material in energy storage devices.     

Supercapacitor based on graphene and ionic liquid electrolyte

A new kind of supercapacitor by using chemical reduced graphene (CRG) as electrode material and ionic liquid with addition of acetonitrile as electrolyte is assembled and investigated. CRG materials with high surface area are prepared by chemical reduction of graphene oxide. The capacitive properties of the supercapacitor composed of the CRG and ionic liquid electrolyte are studied by electrical impedance spectroscopy, cyclic voltammetry and galvanostatic charge–discharge. With the combined advantages of graphene and ionic liquid, the supercapacitor shows perfect performance. The supercapacitor possesses wide cell voltage and good stability. The specific capacitance, energy density, and specific power density of the present supercapacitor are 132 Fg⁻¹, 143.7 Wh kg⁻¹, and 2.8 kW kg⁻¹, respectively. The results demonstrate the potential application of electrical energy storage devices with high performance based on this new kind of supercapacitor.

     

Electrospun porous MnMoO<Subscript>4</Subscript> nanotubes as high-performance electrodes for asymmetric supercapacitors

MnMoO₄ nanotubes of diameter about 120 nm were successfully synthesized by a single-spinneret electrospinning technique followed by calcination in air, and their structural, morphological, and electrochemical properties were studied with the aim to fabricate high-performance supercapacitor devices. The obtained MnMoO₄ nanotubes display a 1D architecture with a porous structure and hollow interiors. Benefiting from intriguing structural features, the unique MnMoO₄ nanotube electrodes exhibit a high specific capacitance, excellent rate capability, and cycling stability. As an example, the tube-like MnMoO₄ delivers a specific capacitance of 620 F g^?1 at a current density of 1 A g^?1, and 460 F g^?1 even at a very high current density of 60 A g^?1. Remarkably, almost no decay in specific capacitance is found after continuous charge/discharge cycling for 10,000 cycles at 1 A g^?1. An asymmetric supercapacitor fabricated from this MnMoO₄ nanotubes and activated carbon displayed a maximum high energy density of 31.7 Wh kg^?1 and a power density of 797 W kg^?1, demonstrating a good prospect for practical applications in energy storage electronics.     

高性能NiCoP基超级电容器电化学性能

采用水热和低温磷化反应两步法,在无添加沉淀剂条件下成功在泡沫镍上合成纳米花状镍钴磷化物(NiCoP/NF).研究结果表明,镍/钴元素物质的量之比为1∶1时,在1A·g-1电流密度下,Ni1/2Co1/2P/NF的比容量高达1276.36 F·g-1,在10A·g-1电流密度下充放电循环3000次后,比容量保持率为78....     

Synthesis of Phosphorus‐Doped Graphene and its Wide Potential Window in Aqueous Supercapacitors

Phosphorus‐doped (P‐doped) graphene with the P doping level of 1.30 at % was synthesized by annealing the mixture of graphene and phosphoric acid. The presence of P was confirmed by elemental mapping and X‐ray photoelectron spectroscopy, while the morphology of P‐doped graphene was revealed by using scanning electron microscopy and transmission electron microscopy. To investigate the effect of P doping, the electrochemical properties of P‐doped graphene were tested as a supercapacitor electrode in an aqueous electrolyte of 1 M H₂SO₄. The results showed that doping of P in graphene exhibited significant improvement in terms of specific capacitance and cycling stability, compared with undoped graphene electrode. More interestingly, the P‐doped graphene electrode can survive at a wide voltage window of 1.7 V with only 3 % performance degradation after 5000 cycles at a current density of 5 A g^?1, providing a high energy density of 11.64 Wh kg^?1 and a high power density of 831 W kg^?1.     

A high performance solid-state asymmetric supercapacitor based on Anderson-type polyoxometalate-doped graphene aerogel

The manufacture of single-atom transition metal-doping carbon nanocomposites as electrode materials is crucial for electrochemical energy storage with high energy and power density. However, the simple strategy for preparation of such active materials with controlled structure remains a great challenge. Here, cobalt-doped carbon nanocomposites (Co-POM/rGO) were synthesized successfully by deposition of Anderson-type polyoxometalate (POM) on the surface of reduced graphene oxide (rGO) aerogel via one-pot hydrothermal treatment. The resulting Co-POM/rGO possesses three-dimensional graphene-based frameworks with hierarchical porous structure, high surface area and uniform single-atom metal doping. These intriguing features render Co-POM/rGO to be a promising electrode for applications in electrochemical energy storage. As an electrode material of a supercapacitor, Co-POM/rGO shows high-performance electrochemical energy storage (211.3 F g^?1 at 0.5 A g^?1). Furthermore, the solid-state asymmetric supercapacitor (ASC) device, using Co-POM/rGO as a positive electrode, exhibits the outstanding energy density of 37.6 Wh kg^?1 at a power density of 500 W kg^?1, and high capacitance retention of 95.2% after 5000 charge–discharge cycles. These results indicate that the proposed strategy for rational design of single-atom-metal doped carbon nanocomposites for flexible ASC devices with excellent capacitive properties.

     

Ultrathin Mn Doped Ni-MOF Nanosheet Array for Highly Capacitive and Stable Asymmetric Supercapacitor

In this study, we demonstrate that an Mn-doped ultrathin Ni-MOF nanosheet array on nickel foam (Mn_0.1-Ni-MOF/NF) serves as a highly capacitive and stable supercapacitor positive electrode. The Mn_0.1-Ni-MOF/NF shows an areal capacity of 6.48 C cm⁻² (specific capacity C: 1178 C g⁻¹) at 2 mA cm⁻² in 6.0 m KOH, outperforming most reported MOF-based materials. More importantly, it possesses excellent cycle stability to maintain 80.6 % capacity after 5000 cycles. An asymmetric supercapacitor device utilizing Mn_0.1-Ni-MOF/NF as the positive electrode and activated carbon as the negative electrode attains a high energy density of 39.6 Wh kg⁻¹ at 143.8 Wkg⁻¹ power density with a capacitance retention of 83.6 % after 5000 cycles.     

Hierarchical Mn3O4 Anchored on 3D Graphene Aerogels via C−O−Mn Linkage with Superior Electrochemical Performance for Flexible Asymmetric Supercapacitor

Flexible asymmetric supercapacitors are more appealing in flexible electronics because of high power density, wide cell voltage, and higher energy density than symmetric supercapacitors in aqueous electrolyte. In virtues of excellent conductivity, rich porous structure and interconnected honeycomb structure, three dimensional graphene aerogels show great potential as electrode in asymmetric supercapacitors. However, graphene aerogels are rarely used in flexible asymmetric supercapacitors because of easily re-stacking of graphene sheets, resulting in low electrochemical activity. Herein, flower-like hierarchical Mn₃O₄ and carbon nanohorns are incorporated into three dimensional graphene aerogels to restrain the stack of graphene sheets, and are applied as the positive and negative electrode for asymmetric supercapacitors devices, respectively. Besides, a strong chemical coupling between Mn₃O₄ and graphene via the C-O-Mn linkage is constructed and can provide a good electron-transport pathway during cycles. Consequently, the asymmetric supercapacitor device shows high rate cycle stability (87.8 % after 5000 cycles) and achieves a high energy density of 17.4 μWh cm⁻² with power density of 14.1 mW cm⁻² (156.7 mW cm⁻³) at 1.4 V.     

β‐Co(OH)2 Nanosheets: A Superior Pseudocapacitive Electrode for High‐Energy Supercapacitors

In this work, β‐Co(OH)₂ nanosheets are explored as efficient pseudocapacitive materials for the fabrication of 1.6 V class high‐energy supercapacitors in asymmetric fashion. The as‐synthesized β‐Co(OH)₂ nanosheets displayed an excellent electrochemical performance owing to their unique structure, morphology, and reversible reaction kinetics (fast faradic reaction) in both the three‐electrode and asymmetric configuration (with activated carbon, AC). For example, in the three‐electrode set‐up, β‐Co(OH)₂ exhibits a high specific capacitance of ∼675 F g⁻¹ at a scan rate of 1 mV s⁻¹. In the asymmetric supercapacitor, the β‐Co(OH)₂∥AC cell delivers a maximum energy density of 37.3 Wh kg⁻¹ at a power density of 800 W kg⁻¹. Even at harsh conditions (8 kW kg⁻¹), an energy density of 15.64 Wh kg⁻¹ is registered for the β‐Co(OH)₂∥AC assembly. Such an impressive performance of β‐Co(OH)₂ nanosheets in the asymmetric configuration reveals the emergence of pseudocapacitive electrodes towards the fabrication of high‐energy electrochemical charge storage systems.     

Flaky Structured V2O5: Morphology,Formation Scheme and Supercapactive Performance

Vanadium pentoxide (V₂O₅) based electrodes for energy storage devices have captured sizeable attention in the past decade owing to their attractive physiochemical features. In the present work, flaky structured V₂O₅ was prepared using a single step hydrothermal route. The results from analytical investigations hold up well with the formation scheme proposed. The flaky morphology of V₂O₅ facilitates additional pathways for electron transport and effective ion access. When employed as a supercapacitor electrode in a neutral electrolyte, this flaky V₂O₅ electrode demonstrates a specific capacitance of 472 F g^?1. Besides, it retains maximum capacitance at higher current density confirming its good rate performance. An asymmetric type supercapacitor using flaky V₂O₅ as positive electrode and activated carbon as negative electrode exhibits specific capacitance of 69 F g^?1. This device shows energy density of 10 W h kg^?1 within the operational window of 1 V.

     

Ultrahigh Energy Density Realized by a Single‐Layer β‐Co(OH)2 All‐Solid‐State Asymmetric Supercapacitor

A conceptually new all‐solid‐state asymmetric supercapacitor based on atomically thin sheets is presented which offers the opportunity to optimize supercapacitor properties on an atomic level. As a prototype, β‐Co(OH)₂ single layers with five‐atoms layer thickness were synthesized through an oriented‐attachment strategy. The increased density‐of‐states and 100 % exposed hydrogen atoms endow the β‐Co(OH)₂ single‐layers‐based electrode with a large capacitance of 2028 F g^?1. The corresponding all‐solid‐state asymmetric supercapacitor achieves a high cell voltage of 1.8 V and an exceptional energy density of 98.9 Wh kg^?1 at an ultrahigh power density of 17 981 W kg^?1. Also, this integrated nanodevice exhibits excellent cyclability with 93.2 % capacitance retention after 10 000 cycles, holding great promise for constructing high‐energy storage nanodevices.     

Functionalized graphene foam as electrode for improved electrochemical storage

We report on a non-covalent functionalization of graphene foam (GF) synthesized via chemical vapour deposition (CVD). The GF was treated with pyrene carboxylic acid (PCA) which acted as a source of oxygen and/or hydroxyl groups attached to the surface of the graphene foam for its electrochemical performance improvement. The modified graphene surface enabled a high pseudocapacitive effect on the GF. A specific capacitance of 133.3 F g^?1, power density ～ 145.3 kW kg^?1 and energy density ～ 4.7 W h kg^?1 were achieved based on the functionalized foam in 6 M KOH aqueous electrolyte. The results suggest that non-covalent functionalization might be an effective approach to overcome the restacking problem associated with graphene electrodes and also signify the importance of surface functionalities in graphene-based electrode materials.