ZIF-67/rGO/NiPc composite electrode material for high-performance asymmetric supercapacitors

In this reported study, novel multiple dimensional ZIF-67/rGO/NiPc composite materials were prepared for supercapacitors. The electrochemical test showed that the ZIF-67/rGO/NiPc electrode achieved a remarkable specific capacitance of 860 F g^?1 at a current density of 1 A g^?1, which was superior to that of the rGO/NiPc and ZIF-67/rGO electrodes. An asymmetric supercapacitor based on ZIF-67/rGO/NiPc//activated carbon exhibited a high specific capacitance of 200.67 F g^?1 and an extraordinary energy density of 62.7 Wh kg^?1 at a corresponding power density of 750 W kg^?1. In addition, the device demonstrated 94.6% capacitance retention after 5000 cycles. The assembled asymmetric supercapacitors could easily powered a green light-emitting diode. This work revealed a promising research route for the rational construction of multiple dimensioned high-performance electrodes materials for use in new energy storage devices.

     

Development of 3D printing technology for the manufacture of flexible electric double-layer capacitors

This study presents a novel process and manufacturing system for the fabrication of Electric Double-Layer Capacitors (EDLCs) as energy storage devices. It shows an approach for printing multilayer EDLC components using 3D printing technology. This process allows layers of activated carbon (AC) slurry, gel electrolyte, and composite solid filaments to be printed with high precision. The study describes the detailed process of deposition of the AC and gel electrolyte using the dual nozzle system. The performance of the flexible EDLCs manufactured by 3D printing in a rectilinear infill pattern has been investigated. It describes the energy storage performance of the printed supercapacitors in relation to the differences in thickness of the AC printed layers and the differences in density of gel electrolyte. A supercapacitor based on printed AC and composite materials displays a specific capacitance of 38.5?mF?g^?1 when measured at a potential rate change of 20?mV?s^?1 and a current density of 0.136?A?g^?1. The highest energy density value for the flexible EDLC was 0.019?Wh?kg^?1 and power density of 165.0?W?kg^?1 in 1.6?M H₂SO₄/PVA gel electrolyte.     

Hierarchical porous carbon obtained from directly carbonizing carex meyeriana for high-performance supercapacitors

Hierarchical porous carbon materials with high surface area are facilely prepared by directly carbonizing carex meyeriana without any extra activation procedure. The as-prepared porous carbon samples possess high Brunauer–Emmett–Teller (BET) surface areas (in the?~?518–742 m² g^?1 range) and unique hierarchical porous structure containing macropore channels and mesopores and micropores developed in the wall of macropores. These intriguing characteristics make the as-prepared hierarchical porous carbon samples a promising electrode material for supercapacitors. The capacitive performance was measured in the three-electrode system with 6 M KOH electrolyte. The hierarchical porous carbon prepared at the carbonization temperature of 1000 °C presents a high specific capacitance of 178.6 F g^?1 at a current density of 0.5 A g^?1, a good rate performance ( about 65.2% retention ratio at the current density of 20 A g^?1), and an excellent cycling stability (no obvious performance fading after 10,000 cycles). In addition, the fabricated two-electrode device achieves an energy density of 4.33 Wh kg^?1 at a high power density of 5 kW kg^?1. These results provide a green and facile method to synthesize the electrode material from biomass for high-performance supercapacitors.

     

Development of redox deposition of birnessite-type MnO2 on activated carbon as high-performance electrode for hybrid supercapacitors

Birnessite-type MnO₂/activated carbon nanocomposites have been synthesized by directly reducing KMnO₄ with activated carbon in an aqueous solution. It is found that the morphologies of MnO₂ grown on activated carbon can be tailored by varying the reaction ratio of activated carbon and KMnO₄. An asymmetric supercapacitor with high energy density was fabricated by using MnO₂/activated carbon (MnO₂/AC) nanocomposite as positive electrode and activated carbon as negative electrode in 1 M Na₂SO₄ aqueous electrolyte. The asymmetric supercapacitor can be cycled reversibly in the cell voltage of 0–2 V, and delivers a specific capacitance of 50.6 F g⁻¹ and a maximum energy density of 28.1 Wh kg⁻¹ (based on the total mass of active electrode materials of 9.4 mg), which is much higher than that of MnO₂/AC symmetric supercapacitor (9.7 Wh kg⁻¹).     

A novel hierarchically carbon foam templated carbon nanotubes/polyaniline electrode for efficient electrochemical supercapacitor

In this work, a high-performance electrode material has been fabricated by the incorporation of carbon nanotubes (CNTs) and polyaniline (PANI) on a carbon foams (CF) to improve its electrochemical performance. The microstructure and performance of as-prepared material was characterized in detail. Results showed that the resultant material exhibited a high gravimetric capacitance up to 467.1?F g^?1, higher energy density of 104. 2?Wh kg^?1 and power density of 3000?W kg^?1 at a current density 3?A g^?1 when the electrochemical doping time of PANI equals to 20?min. Furthermore, it appeared a good cycling stability with capacitance retention of 94.5% after 10000 cycles. The enhanced electrochemical performance can be attributed to the unique carbon nanostructure and synergistic effects of active materials CNTs and PANI. It indicates that this novel CF/CNTs/PANI-20 composite is a promising candidate for electrochemical capacitors.     

Activated carbons derived from sugarcane bagasse for high-capacitance electrical double layer capacitors

Activated carbon (AC) from sugarcane bagasse was prepared using a simple two-step method of carbonization and chemical activation with four different activating agents (HNO₃, H₂SO₄, NaOH, and KOH). Amorphous carbon structure as identified by X-ray diffraction was observed in all samples. Scanning electron microscopy revealed that the AC had more porosity than the non-activated carbon (non-AC). Specific capacitance of the non-AC electrode was 32.58 F g^?1 at the current density of 0.5 A g^?1, whereas the AC supercapacitor provided superior specific capacitances of 50.25, 69.59, 109.99, and 138.61 F g^?1 for the HNO₃ (AC-HNO₃), H₂SO₄ (AC-H₂SO₄), NaOH (AC-NaOH), and KOH (AC-KOH) activated carbon electrodes, respectively. The AC-KOH electrode delivered the highest specific capacitance (about 4 times of the non-AC electrode) because of its good surface wettability, the largest specific surface area (1058.53 m² g^?1), and the highest total specific pore volume (0.474 cm³ g^?1). The AC-KOH electrode also had a great capacitance retention of almost 100% after 1000 GCD cycles. These results demonstrate that our AC developed from sugarcane bagasse has a strong potential to be used as high stability supercapacitor electrode material.

     

Hierarchically porous carbon derived from renewable Chingma Abutilon Seeds for high-energy supercapacitors

The cost-effectively biomass-derived porous carbon is highly promising for usage in electrochemical energy storage as the electrode materials. Herein, a series of hierarchically porous carbons with biomass Chingma Abutilon Seeds as the renewable precursor were synthesized via KOH activation and high-temperature carbonization technique. The resulting carbon material possessed an interconnected structure, high specific surface area (120–3566 m² g^?1), hierarchical pores as well as the heteroatom-substituted functional groups. Based on the synergistic effect of the above-mentioned merits, the optimized material exhibited the remarkably electrochemical performance with high specific capacitance (389 F g^?1 at 0.5 A g^?1) and excellent rate stability (72% capacitance retention at 20 A g^?1) in the three-electrode configuration. More significantly, the symmetric two-electrode device assembled in 6 M KOH delivered a high energy density of 39.2 Wh kg^?1 and excellent chemical stability (90% capacitance retention after 10,000 cycles at 5 A g^?1). Such prominent results might provide a new perspective on the value-added application of the renewable biomass resources in the electrochemical field.     

A facile synthetic method and electrochemical performances of nickel oxide/carbon fibers composites

The uniform and completed nanofilms of nickel oxide (NiO) were electrodeposited on the carbon fibers (CFs) by a facile method of cyclic voltammetric. The as-prepared NiO/CFs composites can be used as a flexible electrode for electrochemical supercapacitors. Electrochemical measurements showed that 1.0-NiO/CFs had a good redox process and reversibility, and displayed the specific capacitances as high as 929 F g^?1 at a current density of 1 A g^?1. After 5000 cycles of charge and discharge, the 1.0-NiO/CFs composite materials could retain more than 88% of initial capacitance and show an excellent cyclability. Meanwhile, this supercapacitor exhibited a higher energy density of 20.8 Wh kg^?1 at a power density of 200 W kg^?1. The carbon fibers acting as active substrate for the composite electrode are a good conductor and have a larger capacitance of electrical double layer. The nanofilm structure of NiO could facilitate the contact of the electrolyte with the active materials, thus increasing the Faradaic pseudo-capacitance.     

Self-activated ‘green’ carbon nanoparticles for symmetric solid-state supercapacitors

Tuning of porosity and surface properties of nanoparticles especially on carbon-based nanomaterials, adopting a ‘greener’ or self-activation synthesis technique for electrical charge storage, is progressing. Herein, we report the self-activation of Teak wood sawdust in a nitrogen atmosphere at different activation temperatures to synthesize carbon nanoparticles. The activated carbon nanoparticles synthesized at 900 °C exhibits a maximum?~?360 m² g^?1 surface area with?~?2 nm average pore size diameter. Five electrolytes viz. KOH, KCl, Na₂SO₄, NaCl, and H₃PO₄ are used for studying the supercapacitance nature of the activated carbon nanoparticles in a 3-electrode configuration. A maximum specific capacitance of?~?208 F g^?1 @ 0.25 A g^?1 is obtained in 1 M KOH as the electrolyte. Two symmetric supercapacitors, aqueous (1 M KOH) and solid-state (PVA/KOH), are fabricated, and their performance difference is compiled. The solid-state symmetric supercapacitor performs in a wider voltage window (1.7 V) with a superior energy density of 27.1 Wh kg^?1 at a power density of 178 W kg^?1.

Graphical abstract

     

Facial synthesis of MnO<Subscript>2</Subscript>/three dimensional graphene composite and its application in supercapacitors

MnO₂ nanoparticle/three dimensional graphene composite (MnO₂/3DG) was synthesized by a hydrothermal template-free method and subsequent ultrasonic treatment in KMnO₄ solution. The MnCO₃/3DG particles can be detected after the hydrothermal process, which may be produced through the reaction between Mn²⁺ and \({\text{C}}{{\text{O}}_{\text{3}}}^{{\text{2}} - }\) due to the decarboxylation of GO under the hydrothermal condition. The final product MnO₂/3DG displayed high specific capacitance (324 F g^??1 at 0.4 A g^?1) and good cycle stability (91.1% capacitance retention after 5000 cycles). Furthermore, the asymmetric supercapacitor assembled with MnO₂/3DG and activated carbon (AC) exhibits an energy density of 33.78 Wh kg^?1 at the powder density of 380 W kg^?1. The excellent supercapacitance of the MnO₂/3DG composite may be due to the high pseudocapacitance of the dispersed MnO₂ nanoparticles and the conductive graphene with three dimensional porous microstructure.     

Graphene-nickel cobaltite nanocomposite asymmetrical supercapacitor with commercial level mass loading

A high performance asymmetric electrochemical supercapacitor with a mass loading of 10 mg·cm^?2 on each planar electrode has been fabricated by using a graphene-nickel cobaltite nanocomposite (GNCC) as a positive electrode and commercial activated carbon (AC) as a negative electrode. Due to the rich number of faradaic reactions on the nickel cobaltite, the GNCC positive electrode shows significantly higher capacitance (618 F·g^?1) than graphene-Co₃O₄ (340 F·g^?1) and graphene-NiO (375 F·g^?1) nanocomposites synthesized under identical conditions. More importantly, graphene greatly enhances the conductivity of nickel cobaltite and allows the positive electrode to charge/discharge at scan rates similar to commercial AC negative electrodes. This improves both the energy density and power density of the asymmetric cell. The asymmetric cell composed of 10 mg GNCC and 30 mg AC displayed an energy density in the range of 19.5 Wh·kg^?1 with an operational voltage of 1.4 V. At high sweep rate, the system is capable of delivering an energy density of 7.6 Wh·kg^?1 at a power density of about 5600 W·kg^?1. Cycling results demonstrate that the capacitance of the cell increases to 116% of the original value after the first 1600 cycles due to a progressive activation of the electrode, and maintains 102% of the initial value after 10000 cycles.      

Activated carbon with high capacitance prepared by NaOH activation for supercapacitors

Activated carbons with high volumetric capacitance are prepared from apricot shell by optimizing the carbonization temperature prior to NaOH activation to balance the porosity and density. The carbonization temperature has a marked effect on both the pore structure and the electrochemical performances of the activated carbons. As the carbonization temperature increases, the specific surface area and gravimetric capacitance of the carbons decrease, while the apparent electrode density increases. Moderate carbonization at 500 °C results in not only high gravimetric capacitance (339 F g^?1) but also high apparent electrode density (0.504 g cm^?3), and hence a highest volumetric capacitance of 171 F cm^?3 in 6 mol L^?1 KOH aqueous electrolyte is obtained. The activated carbons also show good rate capability.     

Manganese-based coordination framework derived manganese sulfide nanoparticles integrated with carbon sheets for application in supercapacitor

Manganese sulfide (MnS) with high specific capacitance and low-cost merits, has been investigated as a potential electroactive material for supercapacitor. However, in practical application, MnS has been suffering from some disadvantageous issues such as insufficient electrical conductivity, serious particle agglomeration as well as huge volume change during continuous charges and discharges, which resulted in a limited specific capacitance, shortened working life and inferior rate performance. Engineering electrode materials with controlled nanostructure and composition is pivotal to improve electrichemical performance of supercapacitors. This paper introduces a facile in situ sulfuration method to fabricate MnS/NSC composite with Mn-hexamethylene tetramine coordination framework as precursor. The results indicated that MnS nanoparticles were highly dispersed and incorporated into nitrogen, sulfur-doped carbon microsheets in MnS/NSC composite. Carbon matrix effectively dispersed and confined the MnS nanoparticles, thus inhibiting aggregation, relieving volume change and retaining structural integrity. Moreover, the 2D conductive carbon matrix reduced the diffusion distance for ions and ensured fast electron delivery. As a result, MnS/NSC electrode delivered a tremendously boosted electrochemical performance for supercapacitor. A large capacitance value about 1881.8F/g was achieved at 1A/g. Even cycling for 3000 loops at 40 A/g, MnS/NSC electrode retained a large capacitance of 404.3F/g. Furthermore, an asymmetric capacitor based on assembly of MnS/NSC composite cathode and activated carbon anode was fabricated. As tested under a current density of 0.1 A/g, it delivered a capacitance of ～ 110.1F/g and achieved an energy density of 12.4 Wh kg^?1 along with a power density of 3.03 kW kg^?1. These results demonstrate the potential utilization of MnS/NSC composite as electrodes for energy conversion and storage devices and open up a route for material design for future energy storage devices.     

MOF‐Derived Hollow Cage NixCo3−xO4 and Their Synergy with Graphene for Outstanding Supercapacitors

Highly optimized nickel cobalt mixed oxide has been derived from zeolite imidazole frameworks. While the pure cobalt oxide gives only 178.7 F g^?1 as the specific capacitance at a current density of 1 A g^?1, the optimized Ni:Co 1:1 has given an extremely high and unprecedented specific capacitance of 1931 F g^?1 at a current density of 1 A g^?1, with a capacitance retention of 69.5% after 5000 cycles in a three electrode test. This optimized Ni:Co 1:1 mixed oxide is further used to make a composite of nickel cobalt mixed oxide/graphene 3D hydrogel for enhancing the electrochemical performance by virtue of a continuous and porous graphene conductive network. The electrode made from GNi:Co 1:1 successfully achieves an even higher specific capacitance of 2870.8 F g^?1 at 1 A g^?1 and also shows a significant improvement in the cyclic stability with 81% capacitance retention after 5000 cycles. An asymmetric supercapacitor is also assembled using a pure graphene 3D hydrogel as the negative electrode and the GNi:Co 1:1 as the positive electrode. With a potential window of 1.5 V and binder free electrodes, the capacitor gives a high specific energy density of 50.2 Wh kg^?1 at a high power density of 750 W kg^?1.     

Co2SnO4/activated carbon composite electrode for supercapacitor

A new kind of Co₂SnO₄-based electrode materials for supercapacitor was synthesized by co-precipitation method. The microstructure and surface morphology of Co₂SnO₄ were characterized by X-ray diffraction and scanning electron microscopy, respectively. Cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy were employed for the determination of specific capacitance and the equivalent series resistance of Co₂SnO₄/activated carbon composite electrode in KCl solution. It was shown that the composite electrode with 25 wt% Co₂SnO₄ had excellent specific capacitance up to 285.3 F g⁻¹ at the current density of 5 mA cm⁻². In addition, the composite electrode exhibited excellent long-term stability and, after 1000 cycles, 70.6% of initial capacitance was retained. Regarding the low cost, easy preparation, steady performance and environment friendliness, Co₂SnO₄/activated carbon composite electrode could have potentially promising application for supercapacitor.     

Activated carbon from agave wastes (agave tequilana) for supercapacitors via potentiostatic floating test

To prepare an efficient supercapacitor, an activated carbon from agave wastes was prepared and their electrochemical performance was evaluated as a novel electrode for supercapacitor. The carbon was prepared by two thermal pyrolysis processes under nitrogen atmosphere. The first pyrolysis was achieved at 500 °C until the charring of the bagasse; in the second pyrolysis step, the char was impregnated with different mass ratios of KOH (1:2–1:4) and thermally treated at 800 or 900 °C, for 1 h under N₂ flow. The textural analysis showed that the activated carbon had a specific surface area of 1462 m² g^?1 and depicted a type I isotherm (IUPAC) characteristic of a microporous carbon. Raman spectroscopy and XRD measurements confirm that the activated carbon contains a small graphitization degree and a disordered structure. The electrochemical study of the symmetric carbon supercapacitor was carried out in 1 M Li₂SO₄ solution as the electrolyte. The electrochemical performance of the coin cell supercapacitor was evaluated under an accelerated aging floating test consisting of potentiostatic steps at different voltages (1.5, 1.6 and 1.8 V) for 10 h followed by galvanostatic charge/discharge sequences, and the overall procedure summarized a floating time up to 200 h. The highest capacitance was observed at a floating voltage of 1.5 V, with a large initial specific capacitance of 297 F g^?1.

     

Co(OH)2 nanosheet-decorated graphene–CNT composite for supercapacitors of high energy density

A composite of graphene and carbon nanotubes has been synthesized and characterized for application as supercapacitor electrodes. By coating the nanostructured active material of Co(OH)₂ onto one electrode, the asymmetric supercapacitor has exhibited a high specific capacitance of 310 F g⁻¹, energy density of 172 Wh kg⁻¹ and maximum power density of 198 kW kg⁻¹ in ionic liquid electrolyte EMI-TFSI.     

Making a wearable supercapacitor based on highly porous 3D nitrogen-doped graphene

In this paper, based on the hydrothermal method and using a non-toxic organic molecule, as a spacer (and nitrogen source), we synthesized a highly conductive and porous 3D graphene. Then, graphene is used as an electrode material to make a supercapacitor on the surface of activated carbon cloth electrode. The graphene is characterized by different methods, such as Fourier-transform infrared spectroscopy, thermogravimetric analysis, Raman spectroscopy, X-ray diffraction, energy-dispersive and transmission electron microscopy, energy-dispersive X-Ray spectroscopy, emission scanning electron microscopy, Barrett–Joyner–Halenda, and Brunauer–Emmett–Teller methods. The supercapacitor (2 and 3 electrodes) is studied by different electrochemical techniques, such as cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge. The 3-electrode system shows a specific capacity 101 F g^? 1 at the current density of 1.7 A g^? 1 (or 0.5 mA cm^? 2). The 2-electrode system (symmetric capacitor) has a power density of about 8000 W kg^? 1 and a maximum energy density of 12.85 Wh kg^? 1.

     

Bacterial cellulose membranes coated by polypyrrole/copper oxide as flexible supercapacitor electrodes

The developments of flexible supercapacitors are of great importance to the growing demand of portable electronic products. In the present work, we have successfully prepared bacterial cellulose (BC) membranes coated by polypyrrole (PPy) and copper oxide (CuO) as flexible composite electrodes for supercapacitor applications. The highest electrical conductivity value of 7.4 S cm^?1 was achieved using copper acetate aqueous solution with concentration of 1 wt%. Electrochemical measurements proved that the supercapacitors using the PPy/CuO/BC electrodes had a specific capacitance of 601 F g^?1 with an energy density of 48.2 Wh kg^?1 and a power density of 85.8 W kg^?1 at a current density of 0.8 mA cm^?2. The specific capacitance was kept at 385 F g^?1 after 300 cycles. The introduction of the CuO nanoparticles gave rise to the improved capacitance.     

Production of AgCu:NiO/Ni foam electrode with high charge accumulation and long cycling stability

Nickel oxide is a promising material for electrochemical energy storage devices due to its high specific surface area, rapid redox reactions, and short diffusion path in the solid electrode. It has been known that the loading of metallic elements into the NiO matrix enhances these superior properties. NiO material is electrochemically deposited on Ni foam, and then, Ag and Cu thin layers are coated on NiO by thermal evaporation. The produced NiO/Ni foam and AgCu:NiO/Ni foam electrodes are annealed at 400 °C for 1 h. Those are utilized as anode for high-performance energy storage electrode in an alkaline solution. The former has an energy density of 56.9 Wh kg^?1 at 3155.5 W kg^?1, while the latter has a high energy density of 107.6 Wh kg^?1 at the corresponding power density of 2957.7 W kg^?1. Although specific capacitance of the former decreases to 46.2% of its original capacitance at 10 A g^?1 after 5000 cycles, the latter exhibits higher cycling stability with 71.0% retention after 5000 charge–discharge cycles owing to the loading of Ag and Cu into NiO matrix. Charge transfer resistance of NiO/Ni foam, which is inversely proportional to electroactive surface area, reduces from 19.4 to 0.28 Ω after the incorporation of Ag and Cu. Compared to NiO/Ni foam, AgCu:NiO/Ni foam with a higher electroactive surface area is more appropriate for charge accumulation. As mention above, the features of AgCu:NiO/Ni foam indicate that it is a promising material as an effective start-of-art energy storage device.