Holey graphene/MnO2 nanosheets with open ion channels for high-performance solid-state asymmetric supercapacitors

Holey graphene/MnO₂ (HG/MnO₂) composites with open ion channels are synthesized by an electrostatic self-assembly method. The HG with rich in-plane nanopores is prepared via a mild etching reaction first, followed by modified with poly-diallyldimethylammoniumchloride (PDDA) to transfer the surface charge of HG nanosheets from negative to positive, which are eventually assembled on the negatively charged MnO₂ nanosheets via electrostatic attraction. As a result, the HG allows ions to pass through the graphene sheet, improving the ion transport channels. Besides, the electrostatic self-assembly between HG and MnO₂ enables the composite a high conductivity, providing effective electron transport pathways. The HG and MnO₂ sheets are observed to be tightly bounded by the transmission electron microscope (TEM), and the HG content in the composites is determined to be 9.6% to 20% by the thermogravimetric (TG) test. The HG/MnO₂-2 electrode with the HG content of around 14.8% displays the large specific capacitance of 219.3 F g⁻¹ at 0.5 A g⁻¹ and the high rate capacity of 134.7 F g⁻¹ at 10 A g⁻¹. Furthermore, the as-prepared solid-state asymmetric supercapacitors (SSAS) achieve a wide stable operating voltage of 1.8 V, a high energy density of 16.8 Wh kg⁻¹ at the power density of 224.6 W kg⁻¹, and low capacitance degradation of only 6.3% after 5000 cycles.     

Constructing 3D honeycomb-like CoMn2O4 nanoarchitecture on nitrogen-doped graphene coating Ni foam as flexible battery-type electrodes for advanced supercapattery

Reasonable structural design is significant to enable the performance in advanced energy storage devices. Herein, a 3D honeycomb-like CoMn₂O₄ nanoarchitecture (CMO) on nitrogen-doped graphene (NG) coating Ni foam (denoted as Ni/NG/CMO) flexible battery-type electrode was prepared by a facile two-step hydrothermal strategy. The honeycomb-like CoMn₂O₄ arrays not only provide abundant active sites but can also be closely combined with the Ni foam/NG substrate, which enables high reversible capacity and good cycle stability during the long cycles. Benefiting from the compositional features and 3D honeycomb-like nanoarchitecture, the Ni/NG/CMO composite electrode displays improved electrochemical performance with remarkable specific capacity of 527.0C g⁻¹ at a current density of 1 A g⁻¹, outstanding rate capability (338.6C g⁻¹ even at 20 A g⁻¹). In addition, a flexible binder-free supercapattery device has been assembled with Ni/NG/CMO as positive electrode and 3D Ni/NG as negative electrode. Such a supercapattery delivers a high energy density of 44.1 Wh·kg⁻¹ at 992.6 W kg⁻¹, 20.3 Wh·kg⁻¹ at 12430.0 W kg⁻¹ as well as excellent cycling durability. The 3D honeycomb-like Ni/NG/CMO could be considered as an advanced flexible battery-type material for high capacity and energy density fields.     

Synthesis and characterisation of amorphous MnO2/CNT via solid-state microwave for high-performance supercapacitors

Amorphous MnO₂ was attached to carbon nanotubes (CNTs) in 60 seconds by solid-state microwave method. As electrodes in supercapacitors, the coactions of the MnO₂ and CNT augmented the actual value of capacitance to 1250 F g⁻¹, and cycle steadiness held 80% of the starting value after 7000 cycles. The equipped MnO₂/CNT//AC asymmetric supercapacitor (ASC) manifested preferable energy density with 33.5 Wh kg⁻¹ (at 400 W kg⁻¹). Its glorious electrochemical properties of MnO₂/CNT demonstrated that the microwave method has great potency to realise the composite of electrode materials.     

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.     

Facile preparation of SnS2 nanoflowers and nanoplates for the application of high-performance hybrid supercapacitors

In this work, the SnS₂ nanoflowers (SnS₂ NFs) were solvothermally prepared in the solvent of ethanol, while SnS₂ nanoplates (SnS₂ NPs) were obtained through the identical conditions except for the solvent of water. The flowers were assembled with numerous nanosheets with very thin thickness, and the NPs exhibited hexagonal shape. When used as the battery-type electrode material for supercapacitors, the SnS₂ NFs delivered a specific capacity of as high as 264.4 C g^?1 at 1 A g^?1, which was higher than the 201.6 C g^?1 of SnS₂ NPs. Furthermore, a hybrid supercapacitor (HSC) was assembled with the SnS₂ as positive electrode and activated carbon (AC) as negative electrode, respectively. The SnS₂ NFs//AC HSC exhibited a high energy density of 28.1 Wh kg^?1 at 904.3 W kg^?1, which was higher than the 24.2 Wh kg^?1 at 844.3 W kg^?1 of SnS₂ NPs//AC HSC. Especially, when the power density was enhanced to the highest value of 8666.8 W kg^?1, the NFs-based device could still hold 20.4 Wh kg^?1. In addition, both HSC devices showed an excellent cycling stability after 5000 cycles at 5 A g^?1. The present method is simple and can be extended to the preparation of other transition metal sulfides (TMSs)-based electrode materials with brilliant electrochemical performance for supercapacitors.     

Facile fabrication of comb-like porous NiCo2O4 nanoneedles on Ni foam as an advanced electrode for high-performance supercapacitor

It is very desirable to develop the high-performance supercapacitors to meet the rapidly growing demands for energy-autonomous operation and miniaturization of devices. Herein, comb-like porous NiCo₂O₄ nanoneedles on the three-dimension (3D) nickel foam (NF) have been successfully synthesized through a facile pulsed laser ablation (PLA) approach without any post-treatments and surfactant (denoted as NiCo₂O₄-PLA). The influence of working solution during the fabricated process on the properties of NiCo₂O₄-PLA has been demonstrated in detail in terms of the crystalline structure, specific surface area, morphology, and electrochemical performance. Benefiting from the large specific surface (261.4 m² g⁻¹), abundant pores, and highly conductive scaffold, the NiCo₂O₄-PLA binder-free electrode exhibits an outstanding specific capacitance (1650 F g⁻¹ at a current density of 1 A g⁻¹) and eminent cycling performance (91.78% retention after a 12,000-cycle test at a current density of 10 A g⁻¹) compared with the control samples. The assembled asymmetric device (NiCo₂O₄-PLA//AC-ASCs) delivers the high specific capacitance of 126.9 F g⁻¹ at the current density of 1 A g⁻¹, the large energy density of 56.7 Wh kg⁻¹ at a power density of 756 W kg⁻¹, and the low internal resistance. The attractive results strongly prove that it is an ideal candidate for advanced supercapacitor application.     

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.     

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).     

One-step hydrothermal preparation of bi-functional NiS2/reduced graphene oxide composite electrode material for dye-sensitized solar cells and supercapacitors

To screen out suitable electrode materials and overcome the shortcomings of the existed electrode materials for the application in dye-sensitized solar cells and supercapacitors, NiS₂/reduced graphene oxide (NiS₂/rGO) composite material was prepared by a simple one-step hydrothermal method in this paper and applied in the field of both dye-sensitized solar cells and supercapacitors as electrode material. In an electrolyte of 6 M KOH, the NiS₂/rGO composite material with bilayer capacitance characteristics exhibited a high specific capacitance of 259.20 F g⁻¹ at the current density of 0.6 A g⁻¹, which was significantly higher than that of rGO (188.94 F g⁻¹). Moreover, at a current density of 2 A g⁻¹, the NiS₂/rGO composite material had 92.85% capacitance retention after 2000 cycles. When applied as counter electrode material for the dye-sensitized solar cells, the NiS₂/rGO composite material counter electrode exhibited a satisfactory photoelectric conversion efficiency (η) of 3.16% under standard simulated sunlight (AM 1.5 G), which was significantly higher than that of single rGO counter electrode (improved by 90.40%). The NiS₂/rGO composite electrode material prepared by a simple one-step hydrothermal method is a potential bi-functional composite electrode materials for both dye-sensitized solar cells and supercapacitors.     

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.     

Co-precipitation synthesis of nickel cobalt hexacyanoferrate for binder-free high-performance supercapacitor electrodes

Prussian blue analogue with a typical metal-organic framework has been widely used as an electrode material in supercapacitor. In this work, nickel cobalt hexacyanoferrate (Ni₂CoHCF) was grown on nickel foam directly using a simple co-precipitation method. The as-prepared Ni₂CoHCF was tested by transmission electron microscope, scanning electron microscope, X-ray diffraction and X-ray electron energy spectrum. The results showed that Ni₂CoHCF has a unique open face-centered cubic structure. The Ni₂CoHCF was used to set an asymmetric supercapacitor directly. A series of electrochemical tests showed that Ni₂CoHCF had an excellent electrochemical performance. The specific capacitance of the supercapacitor was 585 C g⁻¹ (1300.0 F g⁻¹, 162.5 mAh g⁻¹) at the current density of 0.5 A g⁻¹. After 2000 cycles, it still maintained 85.57% of its initial specific capacitance at the current density of 10 A g⁻¹. The energy density was 30.59 Wh kg⁻¹ at the power density of 378.7 W kg⁻¹. The results show that the supercapacitor constructed by Ni₂CoHCF as an electrode material has high-current charge-discharge capacity, high energy density and long cycle life.     

Design of high-performance flexible symmetric supercapacitors energized by redox-mediated hydrogels including metal-doped acidic polyelectrolyte

The fabrication of flexible supercapacitors was achieved by employing the novel redox-activated polymer electrolytes comprising poly(vinylphosphonic acid) (PVPA) and nickel nitrate Ni(NO₃)₂, Ni. The hydrogels, PVPA/NiX, were produced in various contents, in which X denotes the doping fraction of Ni in PVPA. The structure, thermal, and morphology of the materials were characterized, and then they were applied for construction of supercapacitors. The performance evaluations of the fabricated devices were carried out by electrochemical impedance spectroscopy, galvanostatic charge-discharge, and cyclic voltammetry experiments. Flexible supercapacitor devices assembled with activated carbon (AC) electrodes and PVPA/NiX hydrogels produced 793 F g⁻¹ specific capacitance with 30 times enhanced capacitance compared with Ni-free system. The energy density of 103.1 Wh kg⁻¹ was yielded from the device at a power density of 500 W kg⁻¹. The supercapacitor demonstrated an excellent performance during 5.000 charge-discharge cycles, while preserving 84% of its initial capacitance. The supercapacitor constructed of 1 × 5 cm dimension, successfully operates the LED after charging at 3 V.     

A comparative study of various polyelectrolyte/nanocomposite electrode combinations in symmetric supercapacitors

A more practical, nontoxic and cheaper electrolyte, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) was used to construct supercapacitors with different nanocomposite electrodes. The flexible devices were fabricated including active carbon (AC) electrode and nanocomposites electrodes of AC/nano-silica (nano-SiO₂) and AC/multiwalled carbon nanotubes (MWCNTs) at various weight percentages. The symmetrical cell made from AC electrodes generated a maximum specific capacitance (Cs) of 315 F g⁻¹ at 0.5 A g⁻¹. The energy density of this device was 55.5 Wh kg⁻¹ at a power density of 690 W kg⁻¹. Excellent performance was achieved after 5000 charge-discharge cycles where the supercapacitor maintains 92% of its activity. The energy storage capability of the supercapacitors was also investigated with the addition of nano-SiO₂ and MWCNTs. The Cs of the supercapacitors made with the electrodes AC/nano-SiO₂ (5%, 10%, 25% and 50%) were 172, 228, 247 and 55 F g⁻¹, respectively. Similarly, the capacity of the device including the electrodes of AC/MWCNTs (5%, 10%, 25% and 50%) varied as 191, 244, 93 and 20 F g⁻¹ at 0.5 A g⁻¹. The maximum energy density of the devices having nano-SiO₂ and MWCNT were 44.4 Wh kg⁻¹ and 43.8 Wh kg⁻¹, respectively at a power density of 520 W kg⁻¹. A supercapacitor with certain dimension successfully operated a light-emitting diode (LED).     

Hierarchitecture Co2(OH)3Cl@FeCo2O4 composite as a novel and high-performance electrode material applied in supercapacitor

One-step hydrothermal reaction has successfully been used to prepared three-dimensional hierarchitecture Co₂(OH)₃Cl@FeCo₂O₄ composite without any annealing treatment. The samples are investigated to confirm the crystal structure, elemental composition, morphology structure and electrochemical performance. The results show the sample has a three-dimensional hierarchitecture that nanoblocks are assembled with nanoparticles. And the specific surface area is 87.5 m² g⁻¹ and the total pore volume is 0.17 cm³ g⁻¹. Meanwhile, the composite shows a high specific capacitance of 1110.0 F·g⁻¹ at 1 A·g⁻¹ and great cycling stability with 98.8% capacitance retention after 3000 cycles. To evaluate the electrochemical performances, the results are used to compare with the Co₂(OH)₃Cl and FeCo₂O₄ nanomaterials, indicating a higher capacitance and longer cycle stability shown by the as-synthesized sample. The as-synthesized Co₂(OH)₃Cl@FeCo₂O₄ composite has an outstanding electrochemical performance, predicting an enormous potential and promising future as a novel electrode material applied in supercapacitor.     

Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance

Engineering multicomponent active materials as an advanced electrode with the rational designed core-shell structure is an effective way to enhance the electrochemical performances for supercapacitors. Herein, three-dimensional self-supported hierarchical CoMoO₄@CoS core-shell heterostructures supported on reduced graphene oxide/Ni foam have been rationally designed and prepared via a facile approach. The unique structure and the synergistic effects between two different materials, as well as excellent electronic conductivity of the reduced graphene oxide, contribute to the increased electrochemically active site and enhanced capacitance. The core-shell CoMoO₄@CoS composite displays the superior specific capacitance of 3380.3 F g⁻¹ (1 A g⁻¹) in the three-electrode system and 81.1% retention of the initial capacitance even after 6000 cycles. Moreover, an asymmetric device was successfully prepared using CoMoO₄@CoS and activated carbon as positive/negative electrodes. It is worth mentioning that the device delivered the high energy density of 59.2 W h kg⁻¹ at the power density of 799.8 W kg⁻¹ and the excellent cycle performance (about 91.5% capacitance retention over 6000 cycles). These results indicate that the core-shell CoMoO₄@CoS composites offers the novelty strategy for preparation of electrodes for energy conversion and storage devices.     

Three-dimensional nitrogen rich bubbled porous carbon sponge for supercapacitor & pressure sensing applications

Three-dimensional (3D) porous carbonaceous materials offer numerous merits such as light-weight, high surface area, flexibility, and thus hold immense potential in energy storage applications. In this work, we report preparation of nitrogen-rich free-standing compressible porous neuron-like carbon sponge using commercially available kitchen sponge by a facile, cost-effective, and scalable synthetic strategy. The unique neuron-like bubbled interconnected carbon structure with enhanced N/O functionalities improves the electrochemical performance by providing sufficient space for ion transport and large accessible surface-active sites. This material also delivers high current response under compressive stress acting as a pressure sensor. This bubbled carbon material achieves an improved specific capacitance of 268.5 F g⁻¹ at 0.5 A g⁻¹. As a self-supporting electrode in a symmetrical supercapacitor cell, it still delivers a good specific capacitance of 167 F g⁻¹ at 0.35 A g⁻¹, retaining 92.5% of capacitance over 7000 charge/discharge cycles. Furthermore, the device delivers a maximum energy density of 14.8 Wh Kg⁻¹, demonstrating its immense potential for multi-functional applications owing to its unique features.     

Single-step hydrothermal synthesis of WO3-MnO2 composite as an active material for all-solid-state flexible asymmetric supercapacitor

In present study, new strategy is employed to build composite nanostructure and asymmetric configuration to enhance the capacitive performance of supercapacitor device. The WO₃-MnO₂ composite with mesoporous structure is prepared by single-step hydrothermal method and used to gain superior electrochemical performance in asymmetric configuration. A binder-free and additive-less WO₃-MnO₂ composite electrode exhibits high specific capacitance of 609 F g^?1 at a scan rate of 5 mV s^?1. The flexible asymmetric supercapacitor device with WO₃-MnO₂ as a positive electrode and WO₃ as a negative electrode demonstrates stable operating potential window of 1.4 V with specific capacitance of 103 F g^?1 at a scan rate of 5 mV s^?1 and energy density of 24.13 Wh kg^?1 at power density of 915 W kg^?1. Furthermore, WO₃-MnO₂//WO₃ device exhibits good cycle life with capacity retention of 95% after 2500 cycles and excellent mechanical flexibility. These results reveal the potential of WO₃-MnO₂ composite electrode for fabrication of high-performance supercapacitors.     

Construction of symmetric supercapacitors using anhydrous electrolytes containing heterocyclic oligomeric structures

The anhydrous electrolytes have become an important part of supercapacitors, which provide temperature-tolerant applications in various electronic devices. This work reports on the fabrication of a wide-temperature-range supercapacitor using 3-amino-1H-1,2,4-triazole (Atri)/1,4-butanediol diglycidyl ether (BG) and imidazole (Imi)/BG–based electrolytes in active carbon-based electrodes. The triazole-terminated BG (BG(Atri)₂) and Imi-terminated BG (BG(Imi)₂) were initially synthesized, and then anhydrous electrolytes were produced by doping BG(Atri)₂ and BG(Imi)₂ with phosphoric acid (H₃PO₄) and ionic liquid (IL) at different molar fractions. The supercapacitors constructed with the anhydrous BG(Atri)₂/H₃PO₄/0.1IL and BG(Imi)₂/H₃PO₄/0.1IL electrolytes provided maximum specific capacitances (Cs) of 114 and 191 F g⁻¹ at 1 A g⁻¹, respectively. The corresponding electrolytes yielded the highest energy densities of 15.8 and 26.7 Wh kg⁻¹ at the power densities of 1150 and 1225 W kg⁻¹, respectively. The Imi-terminated electrolyte-based supercapacitor indicated superior performance and efficiency even after 2300 charge-discharge cycles by holding 20% of its original capacitance. The temperature dependence of the supercapacitors' capacitances was studied, and they increased from 191 to 266 F g⁻¹ for BG(Imi)₂/H₃PO₄/0.1IL and from 114 to 148 F g⁻¹ for BG(Tri)₂/H₃PO₄/0.1IL as the temperature increased from 25°C to 75°C.     

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.     

Formation of ultra-small Mn3O4 nanoparticles trapped in nanochannels of hollow carbon spheres by nanoconfinement with excellent supercapacitor performance

Ultra-small Mn₃O₄ nanoparticles are formed in the shell of hollow carbon spheres (HCSMn₃O₄) via nanoconfinement growth. The HCS with special cylindrical channels (about 2 nm) provides the limited space for growth of Mn₃O₄ nanoparticles and serves as a conductive substrate in electrochemical applications. The unique hollow structure shortens the distance for electrolytes to access Mn₃O₄ during the charging/discharging process. In the hybrid HCS-Mn₃O₄ structure, the interaction between Mn₃O₄ nanoparticles and HCS was enhanced resulting in good conductivity and electrochemical properties. The two-electrode supercapacitor cell composed of HCS-Mn₃O₄ shows high capacitance and energy density. The highest specific capacitance of 430 F g⁻¹ is achieved at a scanning rate of 1 mV s⁻¹ and the largest energy density of 13.5 Wh kg⁻¹ is observed at a power density of 0.3 kW kg⁻¹ by HCS-Mn₃O₄-5 with 93.15% retention after 10,000 cycles in 5 M LiCl. Symmetric HCS-Mn₃O₄-5//HCS-Mn₃O₄-5 shows higher energy density of 22.6 Wh kg⁻¹ at a power density of 0.4 kW kg⁻¹ and power density of 3.3 kW kg⁻¹ at an energy density of 16 Wh kg⁻¹ with 1 M Li₂SO₄. The technique to prepare the HCS-Mn₃O₄ hybrid materials is simple and readily scalable to satisfy industrial demand.