Formation of hierarchical 3D cross-linked porous carbon with small addition of graphene for supercapacitors

In the present paper, starch was used as raw material to prepare carbon material with low-temperature hydrothermal route and hierarchical three-dimensional cross-linked porous carbon was successfully synthesized with the help of a small amount of graphene for high-performance supercapacitors. It's found that presence of graphene is a crucial condition for the formation of 3D porous carbon and graphene acts as a skeleton in the porous carbon. This kind of carbon material exhibited very high surface area of 1887.8 m² g⁻¹ and delivered excellent electrochemical performance. Its specific capacitance can reach 141 F g⁻¹ at 0.5 A g⁻¹ and more importantly, after 10,000 cycles 98.6% of initial specific capacitance can be maintained. To explore the practical application of the 3D porous carbon, an asymmetric supercapacitor coin-type device was assembled with 3D porous carbon and graphene as electrode materials in organic electrolyte. The constructed device exhibited high energy density of 48.5 Wh·kg⁻¹ at a power density of 1.5 kW kg⁻¹ and still maintains 39.625 Wh·kg⁻¹ under the high power density (15 kW kg⁻¹). These results will promote the rapid development of 3D porous carbon prepared by low-temperature route and the application in supercapacitors.     

Interconnected NiS-nanosheets@porous carbon derived from Zeolitic-imidazolate frameworks (ZIFs) as electrode materials for high-performance hybrid supercapacitors

Nickel sulfide-based materials have shown great potential for electrode fabrication owing to their high theoretical specific capacitance but poor conductivity and morphological aggregation. A feasible strategy is to design hybrid structure by introducing highly-conductive porous carbon as the supporting matrix. Herein, we synthesized hybrid composites consisting of interconnected NiS-nanosheets and porous carbon (NiS@C) derived from Zeolitic-imidazolate frameworks (ZIFs) using a facile low-temperature water-bath method. When employed as electrode materials, the as-prepared NiS@C nanocomposites present remarkable electrochemical performance owing to the complex effect that is the combined advantages of double-layer capacitor-type porous carbon and pseudocapacitor-type interconnected-NiS nanosheets. Specifically, the NiS@C nanocomposites exhibit a high specific capacitance of 1827 F g⁻¹ at 1 A g⁻¹, and excellent cyclic stability with a capacity retention of 72% at a very high current density of 20 A g⁻¹ after 5000 cycles. Moreover, the fabricated hybrid supercapacitor delivers 21.6 Wh kg⁻¹ at 400 W kg⁻¹ with coulombic efficiency of 93.9%, and reaches 10.8 Wh kg⁻¹ at a high power density of 8000 W kg⁻¹, along with excellent cyclic stability of 84% at 5 A g⁻¹ after 5000 cycles. All results suggest that NiS@C nanocomposites are applicable to high-performance electrodes in hybrid supercapacitors and other energy-storage device applications.     

Facile one pot sonochemical synthesis of CoFe2O4/MWCNTs hybrids with well-dispersed MWCNTs for asymmetric hybrid supercapacitor applications

In this study, a facile sonochemical strategy is used for the fabrication of CoFe₂O₄/MWCNTs hybrids as an electrode material for supercapacitor applications. FE-SEM image demonstrates the uniformly well-distributed MWCNTs as well as porous structures in the prepared CoFe₂O₄/MWCNTs hybrids, suggesting 3D network formation of conductive pathway, which can enhance the charge and mass transport properties between the electrodes and electrolytes during the faradic redox reactions. The as-fabricated CoFe₂O₄/MWCNTs hybrids with the MWCNTs concentration of 15 mg (CFC15) delivers maximum specific capacitance of 390 F g⁻¹ at a current density of 1 mA cm⁻², excellent rate capability (275 F g⁻¹ at 10 mA cm⁻²), and outstanding cycling stability (86.9% capacitance retention after 2000 cycles at 3 mA cm⁻²). Furthermore, the electrochemical performance of the CFC15 is superior to those of pure CoFe₂O₄ and other CoFe₂O₄/MWCNTs hybrids (CFC5, CFC10 and CFC20), indicating well-dispersion MWCNTs and uniform porous structures. Also, as-fabricated asymmetric supercapacitor device using the CoFe₂O₄/MWCNTs hybrids as the positive electrode and activated carbon as the negative electrode materials shows the outstanding supercapacitive performance (high specific capacitance, superior cycling stability and good rate capability) for energy storage devices. It delivers a capacitance value of 81 F g⁻¹ at 3 mA cm⁻², ca. 92% retention of its initial capacitance value after 2000 charge-discharge cycles and excellent energy density (26.67 W h kg⁻¹) at high power density (~319 W kg⁻¹).     

Walnut shell-derived hierarchical porous carbon with high performances for electrocatalytic hydrogen evolution and symmetry supercapacitors

Walnut Shell-derived hierarchical porous carbon has been successfully synthesized by the efficient KOH activation process. The hierarchical porous carbon material activated at 600 °C, has the specific micropore area of 1037.31 m² g⁻¹ and micropore volume of 0.51 cm³ g⁻¹, which leads to have electrochemical performances of the hydrogen evolution reaction (HER) and supercapacitors. Specifically, as the hydrogen evolution reaction electrocatalyst, the walnut shell-derived carbon material activated at 600 °C exhibits a lower onset potential of 6.00 mV, a smaller Tafel slope of 69.76 mV dec⁻¹ and outstanding stability above long-term cycling. As a supercapacitor electrode material, the sample possesses specific capacitance of 262.74 F g⁻¹ at 0.5 A g⁻¹, the remarkable rate capability of 224.60 F g⁻¹ at even 10 A g⁻¹ and good long-term stability. A symmetric supercapacitor shows the highly energy density of 7.97 Wh kg⁻¹ at a power density of 180.80 W kg⁻¹. This novel and low-cost biomass material is very promising for the electrocatalytic water splitting and supercapacitors.     

A free-standing electrode based on 2D SnS2 nanoplates@3D carbon foam for high performance supercapacitors

A reasonable formation of an electrode material with three-dimensional (3D) microstructure for supercapacitors was proposed. Two-dimensional (2D) SnS₂ nanoplates were uniformly in situ grown on 3D carbon foam (CF) through a controllable strategy. The composite displayed excellent electrochemical performance due to the synergistic effect of SnS₂ and CF. The SnS₂@CF-2 composite containing 23.92 wt% of SnS₂ has a superior specific capacitance of 283.6 F g⁻¹ at the current density of 1 A g⁻¹. Moreover, a symmetric supercapacitor based on SnS₂@CF-2 composite has a capacitance of 82.5 F g⁻¹ at 1 A g⁻¹ and a high energy density of 13.9 Wh kg⁻¹ at the power density of 551.7 W kg⁻¹.     

A new Co-ZIF derived nanoporous cobalt-rich carbons with high-potential-window as high-performance electrodes for supercapacitors

In this work, a Co-ZIF material and the derived nanoporous cobalt-rich carbons by direct carbonization of this Co-ZIF material were synthesized and used as electrode materials for supercapacitors. This ZIF material exhibited a high specific capacitance of 160.3 F g⁻¹ at 0.5 A g⁻¹, an excellent rate capability (73.72 F g⁻¹ at 10 A g⁻¹), and a good cycling stability with 100% of its initials specific capacitance after 8000 cycles. In addition, the obtained derived nanoporous carbons displayed ideal capacitor behaviors and were promising electroactive materials for supercapacitors at low current density. The nanoporous carbon obtained at 650 °C possessed a highest specific capacitance of 393 F g⁻¹ at 0.5 A g⁻¹ and a wide potential application range of −1.0–0.33 V. In addition, a symmetric supercapacitor device consisting of Z-C-650 and activated carbon exhibited a maximum energy density of 61.23 Wh Kg⁻¹ at a power density of 700 W kg⁻¹ and predicted that Z-C-650 could be used as a potential energy storage material.     

Yuba-like porous carbon microrods derived from celosia cristata for high-performance supercapacitors and efficient oxygen reduction electrocatalysts

Here, a novel yuba-like porous carbon microrod is prepared via a simple and facile strategy by using the fluffy fibers of celosia cristata petals (FCCP) as the raw material. The optimized carbon microrod (FCCP-CM-900) possesses unique yuba-like structure, high specific surface area (1680 m² g⁻¹) and large pore volume (0.98 cm³ g⁻¹), and effective nitrogen (∼4.52 at.%) and oxygen (∼5.49 at.%) doping, which can enhance the wettability and conductivity (7.9 S cm⁻¹). As the electrode material for supercapacitor, FCCP-CM-900-based supercapacitor presents high specific capacitance (314.5 F g⁻¹ at 0.5 A g⁻¹) in 6.0 M KOH aqueous electrolyte. The FCCP-CM-900-based symmetrical supercapacitor displays high energy density (18.6 Wh kg⁻¹ at 233.4 W kg⁻¹) and outstanding cycling stability (98% capacitance retention after 10,000 cycles) in 1.0 M Na₂SO₄ electrolyte. In addition, served as oxygen reduction electrocatalyst, the FCCP-CM-900 also exhibits excellent catalytic activity, good durability, together with high methanol tolerance in alkaline electrolyte, which makes it a highly efficient air cathode material toward zinc–air cell.     

Porous activated carbon monolith with nanosheet/nanofiber structure derived from the green stem of cassava for supercapacitor application

Carbonization and activation have been exploited as an economic and efficient approach toward the production of porous activated carbon monolith derived from green stem of cassava (GSC). In addition, ZnCl₂ was used as a chemical activator agent at various concentrations, therefore serving as a key factor in the development of porous carbon. The carbonization process (N₂) was integrated with physical activation (CO₂), and then N₂ sorption, scanning electron microscopy, X-ray diffraction, energy dispersive X-ray were examined to evaluate the specific surface area, pore structure characteristic, morphology structure, crystallinity, and the surface element, respectively. Furthermore, cyclic voltammetry was used to measure the electrochemical performance, through a two-electrode system in 1M H₂SO₄. Therefore, the synthesized porous activated carbon exhibits a micropores-mesopores combination, and the optimized sample demonstrated nanosheet and nanofiber structures. The results show a high electrochemical behavior in 1M H₂SO₄ electrolytes, by the electrodes, with specific capacitance, energy, and power densities of 164.58 F g⁻¹, 22.86 Wh kg⁻¹, and 82.38 W kg⁻¹, respectively. This route confirms the opportunity of using novel GSC in the production of porous carbon monolith with nanosheet/nanofiber structure for supercapacitor applications.     

Transforming waste sugar solution into N-doped hierarchical porous carbon for high performance supercapacitors in aqueous electrolytes and ionic liquid

Waste sugar solution is a by-product in the process of manufacturing vitamin C. Nowadays, the unused industrial waste residues are transformed into high efficient energy storage devices, such as supercapacitors electrodes, which are worth exploring because they are consistent with the concept of green and sustainable development. In this paper, a nitrogen-doped hierarchical porous carbon are obtained via pre-carbonization and KOH activation. The as-prepared material, possessed proper pore size distribution, large specific surface area and nitrogen-doping, exhibits good electrochemical performance, such as a high specific capacitance of 342 F g⁻¹ (0.1 A g⁻¹), good stability with 95% capacitance retention after 15,000 cycles in 6 M KOH. Moreover, the supercapacitors deliver a high energy density of 25.6 and 65.9 W h kg⁻¹ in the 1 M Na₂SO₄ and EMIMBF₄, respectively. The good electrochemical performance illustrates that the nitrogen-doped hierarchically porous carbon derived from the waste sugar solution is a potential candidate for energy storage.     

Divulging the electrochemical hydrogen storage of ternary BNP-doped carbon derived from biomass scaled to a pouch cell supercapacitor

Activated carbon materials have been studied extensively as electrode materials for supercapacitors (SCs), but their poor capacitance and energy density have hampered their growth. We present a one-step synthesis of a ternary boron-nitrogen-phosphorous-doped carbon (BNPC) from biomass hemp fibre to determine its electrochemical hydrogen storage ability using SC applications. FESEM micrographs reveal mixed morphologies like square, diamond and cylindrical-shaped nanosheets, confirming the hetero-atom doping into the carbon skeleton. The optimized BNPC electrode delivers a half-cell specific capacitance and hydrogen-storage capacity of 520 Fg^-1 (1 Ag^-1) and 360 mAhg⁻¹ (10 mVs⁻¹), respectively. To demonstrate the practicability of the as-prepared BNPC electrode, a symmetric pouch-cell supercapacitor device was assembled which exhibits a full-cell specific capacitance of 262.56 Fg^-1 at 1 Ag^-1 and a specific energy of ~118 Wh kg⁻¹ at a specific power of ~5759 Wkg^-1 with an operating potential window of 1.8 V and 99.7% capacitance retention over 10,000 cycles. This excellent electrochemical performance can be ascribed to the synergetic properties of fast-electrolyte-ion diffusion due to the doping of heteroatoms into the carbon matrix, high conductivity and high specific surface area and effective microporosity of BNPC (1555.5 m²g^-1). Also, the chemical stability of the BNPC materials, was investigated with density functional theory (DFT)-single point calculations, where the least molecular orbital energy gap was obtained by the BNPC, which confirms its structural stability. Thus, the prepared ternary BNP-doped carbon derived from biomass has provided a new direction to enhance the electrochemical energy storage potential.     

Dual carbon source method to fabricate hierarchical porous carbon with three-dimensional interconnected network structure toward advanced energy storage device

Selective fabrication of carbon materials with developed specific surface area and hierarchical porous structure is essential for high-performance carbon-based supercapacitors. Direct carbonization of organic acid salts represents a strategy that can produce porous carbon with high specific surface area, but it is still hindered by low carbon yield, impeding its large-scale application. Herein, a biomass-derived hierarchical porous carbon with large specific surface area is prepared via a facile one-pot calcination method. The optimal SCPC-4 sample presents three-dimensional interconnected network structure and plentiful heteroatom content. Hence, it delivers a large specific capacitance of 321 F g^?1 at a current density of 1 A g^?1, and negligible capacitance loss after 10,000 cycles at 10 A g^?1. In addition, the assembled SCPC-4 based symmetric supercapacitor exhibits an energy density of 21.2 Wh kg^?1 at a power density of 900 W kg^?1. This cost-effective binary biomass carbon source route provides a great possibility for the mass production of high-yield porous carbon materials.     

From basil seed to flexible supercapacitors: Green synthesis of heteroatom-enriched porous carbon by self-gelation strategy

Basil seed-derived multi-heteroatom–doped porous carbons (BHPCs) are successfully synthesized by a facile gelation, followed by a moderate gel water/KOH coactivation process. The BHPC-700 prepared at a relatively low KOH loading and activation temperature possesses large specific surface area (1178.3 m² g⁻¹), well-defined hierarchical micro/meso porosity, and rich self-doping heteroatom functionalities (13.08 at% of oxygen, nitrogen, phosphorous, and sulfur). Electrochemical tests demonstrate that the BHPC-700–based electrode achieves an ultrahigh specific capacitance (464 F g⁻¹ at 0.5 A g⁻¹), outstanding rate performance (retaining 73.3% capacitance at 50 A g⁻¹), and superior cyclic stability (96.8% capacitance retention over 5000 cycles). Furthermore, the BHPC-700 electrodes are assembled into all-solid-state symmetrical supercapacitors. The as-assembled device gives a high energy density of 15.0 Wh kg⁻¹ at a power density of 500 W kg⁻¹ and remarkable flexibility, demonstrating great application prospects in the area of sustainable portable electronics.     

Green and facile synthesis of porous carbon spheres from waste solution for high performance all-solid-state symmetric supercapacitors

Porous carbon spheres materials display huge potential for energy storage, but their general synthesis need chemical activation agent with highly corrosive to create pores. In this work, a simple, environment-friendly and less time-demanding method is used to prepared porous carbon spheres using K₂FeO₄ as activation agent and waste solution as the precursor. The K₂FeO₄ employ in this work acts both as an activating agent and a catalyst. In addition, replacing KOH with K₂FeO₄ does not only reduce the corrosion of equipment but also increases the content of oxygen. The optimized porous carbon spheres with high specific surface area, hierarchical pore structure and surface heteroatom can deliver a high specific capacitance of 260 F g⁻¹ at 0.1 A g⁻¹ and good cycling stability (90% retention after 15000 cycles at 5 A g⁻¹). Furthermore, the all-solid-state symmetric supercapacitors fabricated based on as-prepared samples exhibit good electrochemical performance in the PVA/KOH electrolyte. This work offers a green route to convert waste solution into porous carbon spheres, which are promising candidate material for supercapacitors to energy storage.     

Rapid synthesis of chitin-based porous carbons with high yield,high nitrogen retention,and low cost for high-rate supercapacitors

It is a great challenge to simultaneously achieve high yield, high nitrogen retention, and low cost for chitin-based porous carbons (PCs) while obtaining highly porous structure. Herein, copper (II) chloride dihydrate (CuCl₂ 2H₂O) is innovatively used as the microwave absorber and also porogen for direct synthesis of PCs from chitin via microwave heating. A very short duration of 10 minutes is achieved for this synthesis, because microwave irradiation renders a rapid heating rate of 126°C min⁻¹ during the initial 5 minutes. In addition, the melted CuCl₂ wraps the chitin to preserve its overall structure during synthesis, thus obtaining a yield as high as 36% and a nitrogen retention up to 5.2%. Furthermore, a low temperature of about 600°C that triggers the redox reactions of CuCl₂ into Cu to achieve well-developed porous structure (specific surface area up to 1535 m² g⁻¹) is observed, suggesting a merit of low cost for this synthesis. The PC-based electrode exhibits not only a high specific capacitance of 227 F g⁻¹ at 0.5 A g⁻¹ in 6 mol L⁻¹ KOH electrolyte but also a good rate capability with a high capacitance retention of 67.7% even at an ultra-high current density of 50 A g⁻¹. Owing to these promising capacitive performances of PCs, the fabricated supercapacitor can remain 70.1% of the energy density when the power density was dramatically increased by 20 times.     

A hybrid composite of rGO/TiO2 as a double layer electrode with improved capacitance performance

Reduced graphene oxide (rGO) has unique properties that can revolutionize the performance of the functional devices. rGO hybrids can be designed with transition metal oxides for improved energy storage applications. Herein, a hybrid composite of conductive rGO with titanium dioxide, designed by a simple hydrothermal method, is reported to demonstrate a high double layer capacitance in aqueous electrolyte systems. The mesoporous structure of the composite provides short ion diffusion pathways and the resultant capacitance of the material is 334 F g⁻¹ with ~77% capacitance retention after 7000 charge-discharge cycles.     

Solvothermal synthesis of novel pod-like MnCo2O4.5 microstructures as high-performance electrode materials for supercapacitors

MnCo₂O_4.5 pod-like microstructures were successfully prepared through an initial solvothermal reaction in a mixed solvent containing water and ethanol, and combined with a subsequent calcinations treatment of the precursors in air. The total synthetic process was accomplished without any surfactant or template participation. The MnCo₂O_4.5 pods possessed a specific surface area as high as 73.7 m²/g and a mean pore size of 12.3 nm. The electrochemical performances were evaluated in a typical three-electrode system using 2 M of KOH aqueous electrolyte. The results demonstrated that such MnCo₂O_4.5 pods delivered a specific capacitance of 321 F/g at 1 A/g with a rate capability of 69.5% at 10 A/g. Moreover, the capacitance retention could reach 87% after 4000 cycles at 3 A/g, suggesting the excellent long-term cycling stability. Furthermore, the asymmetric device was fabricated by using MnCo₂O_4.5 porous pods as anode and active carbon as cathode. It could deliver a specific capacitance of 55.3 F g⁻¹ at 1 A g⁻¹ and an energy density of 19.65 W h kg⁻¹ at a power density of 810.64 W kg⁻¹. Such superior electrochemical behaviors indicate that the MnCo₂O_4.5 pods may be served as a promising electrode material for the practical applications of high-performance supercapacitors. The current synthesis is simple and cost-effective, and can be extended to the preparation of other binary metal oxides with excellent electrochemical properties.     

Flexible all-solid-state asymmetric supercapacitor based on three-dimensional MoS2/Ketjen black nanoflower arrays

High electrochemical properties of negative electrode materials are highly desirable for flexible asymmetric supercapacitors (ASCs). Although benefiting from the unique structure and broad operation potential, molybdenum disulfide (MoS₂) has caused concern as a negative electrode material because its low electrochemical stability and poor conductivity hinder the exploitation of its application in flexible ASCs. Here we investigated a facile two-step hydrothermal approach to fabricate MoS₂/Ketjen black (KB) composites on flexible carbon cloth. Following the construction of flower-like MoS₂ on carbon cloth, KB nanospheres were embedded in MoS₂ via a secondary hydrothermal route. The as-prepared MoS₂/KB electrode presents a high capacitance of 429 F g⁻¹ at a current specific of 1 A g¹. In addition, the hybrid ASC device of NiCo₂O₄//MoS₂/KB was built, which delivers a high energy density of 25.7 Wh kg⁻¹ and power density of 16 kW kg⁻¹. These results are ascribed to the favorable structure of MoS₂ and inherently superior conductivity of KB, which improves wettability, structural stability and electronic conductivity. In brief, the proposed all-solid-state ASC device offers potential application in future portable electronics and flexible energy storage devices.     

Lignin‐derived heteroatom‐doped porous carbons for supercapacitor and CO2 capture applications

The present study reports the economic and sustainable syntheses of functional porous carbons for supercapacitor and CO₂ capture applications. Lignin, a byproduct of pulp and paper industry, was successfully converted into a series of heteroatom‐doped porous carbons (LHPCs) through a hydrothermal carbonization followed by a chemical activating treatment. The prepared carbons include in the range of 2.5 to 5.6 wt% nitrogen and 54 wt% oxygen in its structure. All the prepared carbons exhibit micro‐ and mesoporous structures with a high surface area in the range of 1788 to 2957 m² g^?1. As‐prepared LHPCs as an active electrode material and CO₂ adsorbents were investigated for supercapacitor and CO₂ capture applications. Lignin‐derived heteroatom‐doped porous carbon 850 shows an outstanding gravimetric specific capacitance of 372 F g^?1 and excellent cyclic stability over 30,000 cycles in 1 M KOH. Lignin‐derived heteroatom‐doped porous carbon 700 displays a remarkable CO₂ capture capacity of up to 4.8 mmol g^?1 (1 bar and 298 K). This study illustrates the effective transformation of a sustainable waste product into a highly functional carbon material for energy storage and CO₂ separation applications.     

Binder free and high performance of sputtered tungsten nitride thin film electrode for supercapacitor device

Here, we demonstrates the fabrication of binder free and very efficient supercapacitor electrode based on tungsten nitride (W₂N) thin film on stainless steel (SS) substrate using reactive sputtering technique. W₂N thin film as a working electrode exhibits high specific capacitance (163 F g⁻¹ at 0.5 mA cm⁻² in 1 M H₂SO₄) along with excellent cycling stability. The binder free symmetric supercapacitor (W₂N||W₂N) device delivers a high specific capacitance (80 Fg^-1) and long life span (90.46% capacitance retention after 10,000 cycles) along with high energy (12.92 Whkg⁻¹) and power (∼674 kWkg⁻¹ at 9.36 Whkg⁻¹) densities. These observed excellent electrochemical performances of the present W₂N thin film based supercapacitor device, recommend it as a potential candidate for energy storage applications.     

Core-shell structured ZIF-7@ZIF-67 with high electrochemical performance for all-solid-state asymmetric supercapacitor

Zeolitic imidazolate frameworks (ZIFs) are considered as a promising material for energy storage in recent years. Here, core-shell structured ZIF-7@ZIF-67 is synthesized in this work. The core-shell structured material can promote electron transfer of inner-outer metals ions of ZIF-7@ZIF-67, quicken diffusion of electrolyte ions and improve the capacitance performance compared to the ZIF-7 and ZIF-67. ZIF-7@ZIF-67 delivers good energy storage ability with a specific capacitance of 518.9 F g⁻¹ at a current density of 1 A g⁻¹ and remarkable stability with a retention of 99.6% after 4000 cycles in the three-electrode system. Furthermore, an all-solid-state asymmetric supercapacitor (ASC) device is assembled based on core-shell structured ZIF-7@ZIF-67 as positive electrode. Impressively, the ASC device displays an energy density of 31 Wh kg⁻¹ at a power density of 400 W kg⁻¹ and an excellent cyclic stability with 99.5% retention after 10,000 cycles at a current density of 10 A g⁻¹. Finally, two all-solid-state ASCs are contacted to power various lighting-emitting diodes (LED). The red LED can be kept glowing for over 10 min. These electrochemical characteristics suggest that core-shell structured ZIF-7@ZIF-67 is a potential material for energy storage device with long-life cyclic stability.