Nano-porous carbon materials derived from different biomasses for high performance supercapacitors

Nano-porous carbon materials derived from various natural plants are fabricated by a facile, cost-effective and efficient approach. The influence of well-dispersed intrinsic elements in different precursors and chemical activation process under different temperatures on the morphology, surface chemistry, textural structures and electrochemical performance have been studied and analysed in detail. These as-prepared nano-porous carbons possess high accessible surface area (685.75–3143.9 m² g⁻¹), well-developed microporosity and high content of naturally-derived heteroatom functionalities (16.43 wt%). When applied as electrode materials for supercapacitors in a three-electrode system with 6 M KOH, the obtained nano-porous carbons derived from lotus leaves at 700^oC possess a high specific capacitance of 343.1 F g⁻¹ at 0.5 A g⁻¹ and a capacitance retention of 96.2% after 10000 cycles at 5 A g⁻¹. The assembled symmetrical supercapacitor presents a high energy density of 24.4 Wh kg⁻¹ at a power density of 224.6 W kg⁻¹ in Na₂SO₄ gel electrolyte. This work provides guiding function for unified and large-scale utilization of agricultural biomass waste. The obtained sustainable activated carbon products can be used in diverse applications.     

Nitrogen-doped porous carbons through KOH activation with superior performance in supercapacitors

A series of nitrogen-doped porous carbons are prepared through KOH activation of a nonporous nitrogen-enriched carbon which is synthesized by pyrolysis of the polymerized ethylenediamine and carbon tetrachloride. The porosity and nitrogen content of the nitrogen-doped porous carbons depend strongly on the weight ratio of KOH/carbon. As the weight ratio of KOH/carbon increases from 0.5 to 2, the specific surface area increases from 521 to 1913 m² g⁻¹, while the nitrogen content decreases from 10.8 to 1.1 wt.%. The nitrogen-doped porous carbon prepared with a moderate KOH/carbon weight ratio of 1, which possesses a balanced specific surface area (1463 m² g⁻¹) and nitrogen content (3.3 wt.%), exhibits the largest specific capacitance of 363 F g⁻¹ at a current density of 0.1 A g⁻¹ in 1 M H₂SO₄ aqueous electrolyte, attributed to the co-contribution of double-layer capacitance and pseudocapacitance. Moreover, it shows excellent rate capability (182 F g⁻¹ remained at 20 A g⁻¹) and good cycling stability (97% capacitance retention over 5000 cycles), making it a promising electrode material for supercapacitors.     

Facile Preparation and Improved Electrochemical Performance of Oxygen-Enriched Porous Carbon Materials Based on Diacetal-Containing Polybenzoxazine

Porous oxygen-doped carbon materials based on the copolymers of a diacetal-type benzoxazine (ACE-a) and melamine with different ratios are prepared by a facile template-free method. The oxygen-rich diacetal structure in ACE-a allowed for a high oxygen content of 66.2% in surface in these porous carbon materials, and the degradation of the melamine at a high temperature gave an ideal pore structure with a specific surface area of 1383.9 m² g⁻¹ and a pore capacity of 0.748 cm³ g⁻¹. The electrochemical performances are analyzed by Cyclic voltammetry (CV) and Galvanostatic charge-discharge (GCD) tests, and the effects of melamine contents and electrolyte types are also investigated. The results suggested that this material possessed specific capacitance of 430 F g⁻¹ in 0.5 M H₂SO₄ and 194 F g⁻¹ in 6 M KOH (0.5 A g⁻¹), respectively, and showed excellent charge-discharge behavior and magnification characteristic. Moreover, the copolymerization reactions, crosslinking structures, and degradation process of the benzoxazine/melamine copolymers are studied by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis-infrared spectrometry (TGA-IR), respectively. This kind of oxygen-enriched porous carbon material has a good applicative prospect in the field of supercapacitor.     

Hierarchically porous carbon by activation of shiitake mushroom for capacitive energy storage

We present a facile yet effective two-step activation method to prepare a hierarchically porous carbon with natural shiitake mushroom as the starting materials. The first step involves the activation of shiitake mushroom with H₃PO₄, while the second step is to further activate the product with KOH. The resulting carbon is comprised of abundant micro-, mesopores and interconnected macropores that has a specific surface area up to 2988 m² g⁻¹ and pore volume of 1.76 cm³ g⁻¹. With the unique porous nature, the carbon exhibited a specific capacitance of 306 and 149 F g⁻¹ in aqueous and organic electrolyte, respectively. Moreover, this carbon also shows a high capacitance retention of 77% at large current density of 30 A g⁻¹ and exhibited an outstanding cycling stability with 95.7% capacitance preservation after 15,000 cycles in 6 M KOH electrolyte. The far superior performance as compared with those of the commercially most used activated carbon RP20 in both aqueous and non-aqueous electrolyte demonstrates its great potential as high-performance supercapacitor electrode. The two-step method developed herein also represents a very attractive approach for scalable production of various functional carbon materials using diverse biomasses as starting materials.     

Decorating carbon nanosheets with copper oxide nanoparticles for boosting the electrochemical performance of symmetric coin cell supercapacitor with different electrolytes

The synergetic combination of double-layer capacitor carbon nanosheets and pseudocapacitive CuO particles with enhanced electrochemical properties had been proposed. Herein, CuO/carbon nanosheets electrode material with outstanding electrochemistry performance was successfully synthesized via a low-cost and controllable strategy. Such rational architecture integrates high-conductivity carbon nanosheets with rich-chemical-activity CuO particles. The surface-functional carbon nanosheets serve as a conductive substrate, provide an efficient pathway and accelerate the fast diffusion of electrons. This electrode material depicts high specific capacitance up to 183.9 and 371.1 F g⁻¹ at 1 A g⁻¹ in Na₂SO₄ and KOH electrolyte using three-electrode tests, respectively. Moreover, two symmetric devices using this CuO/carbon nanosheets electrode material were assembled with different electrolytes. The as-fabricated device with KOH electrolyte delivers remarkable energy density of 19.36 W h kg⁻¹ at power density of 355.6 W kg⁻¹ and still maintains 12.06 W h kg⁻¹ at 1750.7 W kg⁻¹. The as-fabricated device with Na₂SO₄ electrolyte achieves the maximum energy density of 12.46 W h kg⁻¹ at 355.6 W kg⁻¹. The capacitance retention rate is maintained at 94.4% after 2000 cycles in the as-fabricated coin cell supercapacitor with Na₂SO₄ electrolyte, showing outstanding long-cycling life. Herein, the strategic integration of CuO particles with two-dimensional functional carbon nanosheets as the electrode material provides superior electrochemical performance for supercapacitors.     

Facile synthesis of a novel Fe3O4-rGO-MoO3 ternary nano-composite for high-performance hybrid energy storage applications

Supercapacitors (SCs) have been considered as inspiring energy storage devices due to the long cycle lifetime and high power densities. However, their energy density is limited due to the low capacitance of cathode materials and inferior cycling stability at practically useable potential windows >1.2 V. In this paper, we demonstrate the synthesis of a novel ternary Fe₃O₄-rGO-MoO₃ nano-composite (FGM) with nanoparticles-like morphology (NPs) by utilizing the fast and facile microwave hydrothermal process. The optimized composition of FGM nanocomposite is characterized by the XPS, EDS, Raman, SEM, TEM and HRTEM techniques. The FGM-NPs supported on the carbon cloth (FGM@CC) electrode is used to investigate the electrochemical charge storage properties in basic potassium hydroxide (KOH) electrolyte. The charge-storage properties of the FGM@CC electrode were studied by the CV, GCD and EIS techniques. The obtained results of FGM@CC electrode in aqueous electrolyte showed excellent electrochemical performance as compared with single metal oxides: maximum specific capacitance of 1666.50 F g⁻¹ (FGM@CC), 1075.26 F g⁻¹ (Fe₃O₄ NPs) and 952.38 F g⁻¹ (MoO₃ NPs) at a current density of 2.5 A g⁻¹. The capacitance retention was 95.01% (FGM@CC), 94.1% (Fe₃O₄ NPs) and 92.5% (MoO₃ NPs) after 5000 cycles. Further, the charge storage mechanism is analyzed in the light of power's law and systematical investigated the capacitive and diffusion controlled based stored charge in FGM@CC electrode. Thus FGM nano-composite showed best performance as the cathode material for the next generation flexible supercapacitors.     

Exceptional supercapacitive performance of bicontinuous carbon/MnO2 composite electrodes

A new type of porous carbon/MnO₂ composites, having bicontinuous structures, i.e., continuous channels and carbon skeletons, was prepared using a phase separation method, followed by a carbonization procedure and a subsequent redox reaction. In this work, such composite electrodes show a high specific capacitance of ca. 260?F?g^?1 at 0.5?A?g^?1 in 1?M Na₂SO₄ aqueous solution, a superior cycling stability (~80% retention after 2000 cycles) and a distinctive high-rate performance. Especially, unique bicontinuous structures endow such composites with a great specific capacitance of the constituent MnO₂ (~1100?F?g^?1), very close to the theoretical value. These excellent electrochemical behaviors may render this material a promising candidate as high-performance electrodes in supercapacitors. Therefore, our findings suggest that the strategy for constructing bicontinuous hybrid electrodes represents an exciting direction for designing next-generation supercapacitors.     

Facile synthesis of functionalized porous carbon with three-dimensional interconnected pore structure for high volumetric performance supercapacitors

Functionalized porous carbon with three-dimensional (3D) interconnected pore structure has been successfully synthesized through direct heat-treatment of KOH-soaked soybeans. Benefiting from heteroatoms (N, O) doping, interconnected porous carbon framework with high surface area as well as high packing density (up to 1.1 g cm⁻³), the as-obtained porous carbon material exhibits high volumetric capacitance of 468 F cm⁻³, good rate capability and excellent cycling stability (91% of capacitance retention after 10,000 cycles) in 6 M KOH electolyte. More importantly, the as-assembled symmetric supercapacitor delivers high volumetric energy density of 28.6 Wh L⁻¹ in 1 M Na₂SO₄ aqueous solution.     

Impregnation assisted synthesis of 3D nitrogen-doped porous carbon with high capacitance

Three-dimensional (3D) porous carbons with controlled mesopore and micropore structures were prepared through a simple and low-cost ultrasonic and impregnation assisted method from waste air-laid paper. The ammonia management was used to dope the 3D porous carbons with different types of nitrogen heteroatoms in a way that replaced carbon atoms. The N₂ adsorption–desorption characterization suggested that the nitrogen-doped carbons have a high surface area of 1470 m² g⁻¹ with the average pore diameter of 4.2 nm, which are conducive to form electric double layer under high current density. The resulting 3D carbon exhibited a higher capacitance at 296 F g⁻¹ in comparison with the nitrogen-free one at 252 F g⁻¹ in 6 M KOH electrolyte. Moreover, a high power density ca. 0.313 kW kg⁻¹ and energy density ca. 34.3 Wh kg⁻¹ were achieved in the ionic liquid ([EMIm]BF₄). The findings will open a new avenue to use waste materials for high-performance energy-storage devices.     

Constructing ZnCo2O4@LDH Core–Shell hierarchical structure for high performance supercapacitor electrodes

In this article, ZnCo₂O₄ nanowires deposited with two kinds of typical layered double hydroxides (Ni–Al,Co–Al LDH) are developed via scalable method. Two materials all display core–shell hierarchical structure. High specific capacitance of 2041 F g⁻¹ and 1586 F g⁻¹ are obtained for ZnCo₂O₄@Co–Al LDH and ZnCo₂O₄@Ni–Al LDH,respectively. Combining with active surface and synergistic effect, the role of hierarchical structure in enhancing performance is revealed. Moreover, two all solid state supercapacitors based on fabricated materials as positive electrodes, activated carbon as negative electrodes and PVA–KOH as polymer electrolyte are assembled. The maximum power and energy densities of ZnCo₂O₄@Co–Al LDH//AC are 6200 W kg⁻¹ and 50.1 Wh kg⁻¹, respectively. While the power and energy densities of ZnCo₂O₄@Ni–Al LDH//AC are 3400 W kg⁻¹ and 27.8 Wh kg⁻¹. At last, an energy storage–conversion system, based on solar cell as input device and assembled asymmetric supercapacitor ZnCo₂O₄@Co–Al LDH//AC as output device, is integrated as a self–sustaining power device, indicating its potential in practical applications.     

Silica-assisted urea-formaldehyde-based carbon microspheres with enhanced supercapacitor performances as electrode

Novel silica (SiO₂)-assisted urea-formaldehyde-based microspheres (SUFMs) have been successfully prepared by condensation polymerization with SiO₂ nanoparticles as template. The support effect of SiO₂ and the improved thermal stability of SUFM have been investigated. The SUFM is pyrolyzed at high temperature under Ar atmosphere to achieve its carbonized product (SUFCM), which retains its spherical morphology by alkali etching after carbonization. The SiO₂ acts as a stabilizer that limits the flowing domain of the melt resin around the SiO₂. When used as the electrode material for supercapacitors, the SUFCM exhibits excellent electrochemical performance with a high specific capacitance of 218 F g⁻¹ at 1 A g⁻¹ in 2 M H₂SO₄ aqueous electrolyte and a good rate capability of 138 F g⁻¹ (63% capacitance retention) at 10 A g⁻¹. More importantly, the SUFCM electrode demonstrates a robust long-term stability without capacitance fading after 10,000 cycles. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48388.     

Perovskite oxide and polyazulene–based heterostructure for high–performance supercapacitors

Several types of electrode materials have been developed for high–performance supercapacitors. Most of the relevant studies have focused on the discovery of new atomic structures and paid limited attention to the effect of heterostructures in supercapacitor electrodes, which has long hindered the fundamental understanding of the use of hybrid materials in supercapacitors. In this study, a novel heterostructure based on perovskite oxide (LaNiO₃) nanosheets and polyazulene was synthesized. The as–prepared heterostructure–based supercapacitor exhibited a specific capacitance of up to 464 F g⁻¹ at a high current density of 2 A g⁻¹ in 1–ethyl–3–methylimidazolium tetrafluoroborate. In a symmetric supercapacitor, this heterostructure delivered an energy density of up to 56.4 Wh kg⁻¹ at a power density of 1100 W kg⁻¹. Both LaNiO₃ and polyazulene contributed pseudocapacitance and dominated the performance. Unexpectedly, electric double–layer capacitance was found to contribute in this system. Density functional theory calculations indicated that the advantage of the high electrical conductivity of the heterostructure benefited the supercapacitor operation. Electrochemical quartz crystal microbalance analysis revealed that the fast ion flux and adsorption boosted performance. The high intrinsic electrical conductivity and improved stability make this heterostructure a promising electrode material candidate for supercapacitors.     

Ultra-high specific capacitance of self-doped 3D hierarchical porous turtle shell-derived activated carbon for high-performance supercapacitors

The turtle shell of biomass waste is used as raw material, and the natural inorganic salt contained in it is used as a salt template in combination with a chemical activation method to successfully prepare a high-performance activated carbon with hierarchical porous structure. The role of hydroxyapatite (HAP) and KOH in different stages of preparation was investigated. The prepared turtle shell-derived activated carbon (TSHC-5) has a well-developed honeycomb pore structure, which gives it a high specific surface area (SSA) of 2828 m² g^?1 with a pore volume of 1.91 cm³ g^?1. The excellent hierarchical porous structure and high heteroatom content (O 6.88%, N 5.64%) allow it to have an ultra-high specific capacitance of 727.9 F g^?1 at 0.5 A g^?1 with 92.27% of capacitance retention even after 10,000 cycles. Excitingly, the symmetric supercapacitor assembled from TSHC-5 activated carbon exhibits excellent energy density and cycling stability in a 1 M Na₂SO₄ aqueous solution. The energy density is 45.1 Wh·kg^?1 at a power density of 450 W kg^?1, with 92.05% capacitance retention after 10,000 cycles. Therefore, turtle shell-derived activated carbon is extremely competitive in sustainable new green supercapacitor electrode materials.     

Excellent performance MWCNTs-GONRs/Ni(OH)2 electrode for outstanding supercapacitors

Due to the unconventional properties of MWCNTs-GONRs (multiwalled carbon nanotubes-graphene oxide nanoribbons), we have tried to use it as a carbon resource for supercapacitors. MWCNTs-GONRs/Ni(OH)₂ electrode was obtained by hydrothermal method. Velvet α-Ni(OH)₂ was prepared above NF (nickel-foam) loaded with MWCNTs-GONRs. This layered design can effectively promote the diffusion of ions and increase the active site for MWCNTs-GONRs/Ni(OH)₂ electrode, thus enhancing the electrochemical performance. The electrode exhibits extraordinary electrochemical performances in electrochemical testing, such as supernal specific capacitance (1713.2 F g⁻¹) and prominent working time. In addition, supercapacitors was assembled with MWCNTs-GONRs/Ni(OH)₂ and active carbon as materials. Which represents a prominent energy density (41.23 Wh kg⁻¹), high power (6.80 kW kg⁻¹) and prominent cycling stability property (95.18%, 3000 times). The electrode prepared in this work provides a clue to enlighten people for energy storage.     

Hybrid pseudocapacitor materials from polyaniline@multi-walled carbon nanotube with ultrafine nanofiber-assembled network shell

Porous carbon-based hybrids consisting of nanoscale building units are highlighted as supercapacitive materials for outstanding rate capability and cyclability. Herein reported are new hierarchically porous core–shell hybrids of polyaniline@multi-walled carbon nanotube (PANI@MWCNT) with ultrafine nanofiber-assembled network shell for supercapacitors via a facile reverse microemulsion polymerization route. Among various as-prepared samples, PANI@MWCNT2 hybrids (72 wt.% PANI loading) show the maximum specific capacitance of 663 F g⁻¹ at 0.5 A g⁻¹ with good capacitance retention of about 87% at 2 A g⁻¹ in contrast to pure PANI (526 F g⁻¹ with 76% retention) and other hybrids (613 and 538 F g⁻¹ for the hybrids with 84 wt.% and 48% wt.% PANI, respectively). Moreover, 82% of initial capacitance still remains over 1000 cycles at 5 A g⁻¹ for PANI@MWCNT2. The data clearly reveal a dramatic improvement of the hybrids in performance containing capacitance and rate capability meanwhile lifetime, thanks to unique structure features with highly surface-porous PANI nanofiber networks intimately wrapped around conductive MWCNTs helping to greatly promote faradic redox process and cycling behavior.     

Enhanced supercapacitive behavior by CuO@MnO2/carboxymethyl cellulose composites

The exploration of biocompatible materials has received greater significance in the research area of energy storage tools. In the present work, a composite material consisting of carboxymethyl cellulose (CMC) with CuO@MnO₂ is synthesized via thermal reduction protocol. The resulting composite material exhibited unique morphology and excellent electrochemical properties. The electrochemical properties were premeditated by CV, GCD, and spectral impedance analysis. Electrochemical analyses of the composite materials indicated the extraordinary specific capacitance in a three-electrode configuration. The composite displayed the value of ~414 F/g at a current density of 0.5 A g⁻¹ and the electrodes retaining 96.2% capacitance after 5000 cycles. Therefore, our study demonstrated the synergistic effect of CuO@MnO₂ nanoparticles with porous CMC network structures show enhanced electrochemical properties in the presence of 3 M KOH as an electrolyte.     

Preparation and molten salt-assisted KOH activation of porous carbon nanofibers for use as supercapacitor electrodes

Carbon nanofiber paper was prepared by electrospinning from thermosetting phenolic resin, followed by activation via KOH-containing molten salt at high temperature. By adding a small dosage of KOH in the molten salt the porous volume and specific surface area could be greatly improved. The obtained porous carbon nanofibers had a specific surface area of 1007 m² g^?1, total pore volume of 0.363 cm³ g^?1, micropore volume of 0.247 cm³ g^?1. The electrochemical measurements in 6 M KOH aqueous solution showed that the porous carbon nanofibers possessed high specific capacitance and considerable rate performance. The maximal specific capacitance of 288 F g^?1 was achieved at 0.2 A g^?1 and the specific capacitance could still remain 204 F g^??1 at 20 A g^?1 with the retention of 71%. In the molten salt system, the reaction between activating agent and carbon could be more efficient, hence, such molten salt-assisted activation method was considered as a general activation method for the high-specific-surface-areaed carbons.     

Novel binder-free electrode of NiCo2O4@NiMn2O4 core-shell arrays modified carbon fabric for enhanced electrochemical properties

There is still a great challenge to develop new-style battery-type electrode materials with low resistance, large surface area, and stable microstructures on carbon fabric, which limited the development of flexible devices. In this work, NiCo₂O₄ nanoneedle@NiMn₂O₄ nanosheet core-shell arrays are constructed on the carbon fabric as a high-capacitance and long-life supercapacitor electrode for the first time. Benefiting from this kind of binder-free core-shell microstructure, the CF@NiCo₂O₄@NiMn₂O₄ electrode displays extraordinary specific-capacitance of 539.2 F g⁻¹ at a current density of 2 A g⁻¹, and nearly 93.0% retention of total capacitance even after discharging 5000 cycles. The outstanding properties of the hybrid electrode demonstrate that it is of great potential for flexible supercapacitors and batteries the application.     

Preparation and one-step activation of microporous carbon nanofibers for use as supercapacitor electrodes

Microporous carbon nanofibers were prepared by electrospinning from resole-type phenolic resin, followed by one-step activation. KOH was utilized to tune the fiber diameter and improve porous texture. By adjusting KOH content in the spinning solution, the fiber diameter could be controlled in the range of 252–666 nm and the microporous volume and specific surface area could be greatly improved. The electrochemical measurements in 6 M KOH aqueous solution showed that the microporous carbon nanofibers possessed high specific capacitance, considerable rate performance, and superior specific surface capacitance to conventional microporous carbons. The maximal specific capacitance of 256 F g⁻¹ and high specific surface capacitance of 0.51 F m⁻² were achieved at 0.2 A g⁻¹. Furthermore, the specific capacitance could still remain 170 F g⁻¹ at 20 A g⁻¹ with the retention of 67%. Analysis showed that the high specific surface capacitance of the resultant carbons was mainly attributed to optimized pore size (0.7–1.2 nm) and the excellent rate performance should be principally due to the reduced ion transportation distance derived from the nanometer-scaled fibers.     

N/S-Dual doped MnO modified spore-based ellipsoidal porous carbons for supercapacitors

Owing to the significance and requirement of renewable energy resources, in this study, ellipsoidal porous carbons with yolk-shell structures assembled using MnO and Ganoderma lucidum spores are fabricated for application prospects in energy storage systems; they exhibit excellent ion transfer capability. However, the surface of carbon nanomaterials is naturally hydrophobic, resulting in a lower energy density. Herein, heteroatom doping and O₂/Ar plasma surface treatment are utilized to obtain high specific capacitance and fast charging. Surface functionalization increases the surface roughness and oxygen-containing functional groups of the material. The specific capacitance of the best sample MnO/GSC-O–NS–10 was 568.9 F g⁻¹ when the current density was 0.5 A g⁻¹. The performance test was carried out for 10000 cycles at a current density of 10 A g⁻¹ and the capacitance retention rate was 75.11%. The assembled two-electrode capacitor exhibited a specific capacitance of 240.4 F g⁻¹ and an energy density of 33.4 Wh kg⁻¹ at a power density of 407 W kg ⁻¹. These findings provide sufficient theoretical guidance for the development of high-performance supercapacitors.