One-step synthesis of nanoblocks@nanoballs NiMnO3/Ni6MnO8 nanocomposites as electrode material for supercapacitors

A novel nanoblocks@nanoballs NiMnO₃/Ni₆MnO₈ electrode material was synthesized by one-step solvothermal–hydrothermal method, followed by thermal annealing. At the same time, electrode materials with different nanostructure were prepared by changing the volume ratio of deionied water and ethylene glycol. The results show that different structure has been gained including nanospheres, nanosheets and nanoblocks. When the deionied water: ethylene glycol = 1:1 (nanoblocks@nanoballs NiMnO₃/Ni₆MnO₈ composite structure), the electrode material has a maximum specific surface area of 55.3 m² g⁻¹. The electrode material exhibited outstanding electrochemical performance with specific capacitance reached 494.4 F g⁻¹ at a current density of 1 A g⁻¹ as well as superior cycling performance of 88.0% capacitance retention after 5000 cycles at 3 A g⁻¹. Such excellent performance was due to the synergistic effective between the Ni₆MnO₈ nanoballs and NiMnO₃ nanoblocks. Nanoballs structure will increase in specific surface area and redox reaction active sites, and the blocks structure acts as a holder to improved the cycle performance. The NiMnO₃/Ni₆MnO₈ become a promising candidate as next-generation electrode material for high-performance supercapacitors.     

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.     

Controlled growth of hierarchical FeCo2O4 ultrathin nanosheets and Co3O4 nanowires on nickle foam for supercapacitors

In recent years, the tenable design and synthesis of the core/shell heterostructure as electrode for the supercapacitor, have attained a huge attention and concerns. In this article, the three-dimensional heterostructure consisting of FeCo₂O₄ ultrathin nanosheets grown on the space of vertical Co₃O₄ nanowires has been designed and synthesized onto nickel foam (NF) for pseudocapacitive electrode applications. According to previous research, the NF@ FeCo₂O₄ electrodes can only exhibit specific capacity of 1172 F g⁻¹ at a current density of 1 A g⁻¹. In addition, although the capacity of the NF@Co₃O₄ electrodes can reach to 1482 F g⁻¹ and it has the disadvantage of agglomeration, which restricts the diffusion of ions and has a negative effect on the progress of electrochemical reactions. Therefore, a core-shell nanostructure is fabricated by an improved two-step hydrothermal process, which improves the probability of ion reaction with more efficient charge transfer. Furthermore, in as-prepared unique core/shell heterostructure, the resultant electrode possesses the merits of large capacitance of 1680 F g⁻¹ at a current density of 1 A g⁻¹, an excellent rate capability of 70.1% at 20 A g⁻¹ and only 9.8% loss of initial capacitance at a high charge/discharge current density after 2000 cycles. These results demonstrate that this kind of distinct electrode has potential utilization for 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.     

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.     

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.     

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.     

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.     

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.     

3D carboxyl and hydroxyl co-enriched graphene hydrogels as binder-free electrodes for symmetric supercapacitors

At present, amino acids are often used as the source of heteroatom functional groups for the preparation of doped graphene materials. However, a large amount of amino acids will be used as reaction precursors in the preparation process, which will lead to increased cost, reduced efficiency and waste of resources. Herein, a very small amount of neutral l-alanine is employed to synthesize 3D carboxyl and hydroxyl co-enriched graphene hydrogels (CHGHs) by a one-pot hydrothermal method. The CHGHs contain copious carboxyl and hydroxyl groups, and a small amount of nitrogen-containing functional groups. In addition, the CHGHs also present large specific surface areas and 3D porous structures. Therefore, the CHGH-20 binder-free electrode displays a high specific capacitance of 262.8 F g⁻¹ at 0.3 A g⁻¹, and this value still maintains 84.3% (221.6 F g⁻¹) at 10 A g⁻¹ in a two-electrode system in 6 M KOH. Furthermore, the CHGH-20 electrode also displays outstanding cycle stability with 103.6% of its initial capacitance after 10,000 cycles at 10 Ag⁻¹. Therefore, the CHGHs samples prepared by a very small amount of neutral l-alanine have great significance for the practical application of supercapacitors.     

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.     

Preparation and electrochemical capacitance performances of super-hydrophilic conducting polyaniline

Super-hydrophilic conducting polyaniline was prepared by surface modification of polyaniline using tetraethyl orthosilicate in water/ethanol solution, whereas its conductivity was 4.16 S cm⁻¹ at 25 °C. And its electrochemical capacitance performances as an electrode material were evaluated by the cyclic voltammetry and galvanostatic charge/discharge test in 0.1 M H₂SO₄ aqueous solution. Its initial specific capacitance was 500 F g⁻¹ at a constant current density of 1.5 A g⁻¹, and the capacitance still reached about 400 F g⁻¹ after 5000 consecutive cycles. Moreover, its capacitance retention ratio was circa 70% with the growth of current densities from 1.5 to 20 A g⁻¹, indicating excellent rate capability. It would be a promising electrode material for aqueous redox supercapacitors.     

Metal phthalocyanine-linked conjugated microporous polymer hybridized with carbon nanotubes as a high-performance flexible electrode for supercapacitors

Metal phthalocyanine-linked conjugated microporous polymers (MPc-CMPs) have huge potential applications in energy conversion and storage systems. However, the inherent low conductivity limits their practical application. Herein, the MPc-CMPs are hybridized with highly conductive carbon nanotubes (CNTs) via the easy vacuum filtration method. Interestingly, the composite (denoted as CoPc-CMP/CNTs) shows the flexible feature, which can be served as the flexible binder-free electrode for supercapacitors (SCs). As expected, the flexible CoPc-CMP/CNTs exhibits a high specific capacitance of 289.1 F g⁻¹ at a current density of 1 A g⁻¹ and good capacity retention of 82.4% over 1350 cycles at a high current density of 10 A g⁻¹. Furthermore, First-principle calculations are used to elucidate the superiority of CoPc-CMP to other analogues. The good electrochemical performance could be attributed to the synergistic effect from the high pseudocapacitance and good conductivity of CoPc-CMP as well as the capacitive contribution and good conductivity of CNTs. Our strategy provides a new avenue to develop the high-performance SCs via rational integration of MPc-CMPs with highly conductive CNTs.     

High-density oxygen-enriched graphene hydrogels for symmetric supercapacitors with ultrahigh gravimetric and volumetric performance

The contradiction between the porous structure and density of graphene materials makes it unable to meet the dual requirements of the next generation supercapacitors for gravimetric capacitance and volumetric capacitance. Herein, we successfully synthesized high-density oxygen-enriched graphene hydrogels (HOGHs) by a one-step hydrothermal method using high concentration graphene oxide (GO) solution and trometamol as precursors. The as-prepared HOGHs samples present a dense 3D network structure and moderate specific surface areas, which leads to a high packing density. In addition, the HOGHs samples also contain abundant oxygen-containing functional groups and some nitrogen-containing functional groups. These heteroatomic functional groups can provide pseudocapacitance for the electrode materials. Therefore, the HOGH-140 based symmetric supercapacitor shows ultrahigh gravimetric and volumetric specific capacitance (325.7 F g⁻¹, 377.8 F cm⁻³), excellent rate performance and cycling stability. Simultaneously, the symmetric binder-free supercapacitor exhibits high gravimetric specific energy density (11.3 Wh kg⁻¹) and volumetric specific energy density (13.1 Wh L⁻¹) in 6 M KOH, respectively. These outstanding properties make the material have a good application prospect in the field of compact energy storage devices.     

Effect of Nafion on the preparation and capacitance performance of polyaniline

With manganese dioxide (MnO₂) as the oxidant, perfluorinated sulfonic acid ion exchange resin (Nafion) as the doping agent and emulsifier, Nafion doped polyaniline (PANI-Nafion) was prepared by emulsion polymerization method. Scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were carried out to characterize the structure and morphology of PANI-Nafion. Symmetric redox supercapacitor was assembled with PANI-Nafion as active electrode material and 1.0 mol L⁻¹ H₂SO₄ aqueous solution as electrolyte. The electrochemical characteristics of these supercapacitors were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge tests. These results show that the diameter of PANI-Nafion nanofiber is about 30 ∼ 40 nm and the pores between PANI-Nafion composite materials are distributed uniformly. The specific capacitance of PANI-Nafion electrode is about 385.3 F g⁻¹, which is higher than that of undoped PANI (235.8 F g⁻¹). After 1000 charge/discharge cycles the specific capacitance of PANI-Nafion electrode is 272.4 F g⁻¹, its capacity retention is 70.7%, which is significantly better than that of PANI electrode materials.     

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).     

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.     

A bi-functional Co–Ni layered double hydroxide three-dimensional porous array electrode derived from ZIF-L(Co)@ZIF-L(Co,Ni) for oxygen evolution reaction and supercapacitors

Developing high-efficiency bifunctional materials for electro-catalysis and supercapacitors are urgently needed but challenging. Herein, we develop a self-supporting Co–Ni LDH electrode prepared by in-situ growing ZIF-L(Co)@ZIF-L(Co, Ni) on carbon paper followed by a pseudomorphic transformation. The optimized Co–Ni LDH/carbon paper electrodes (CN-2/CP) exhibit excellent electrochemical activity and stability in oxygen evolution reactions (OER) and supercapacitors. The CN-2/CP electrode displays a low overpotential of 230 mV at 10 mA cm^?2 and superior stability at 40-h chronopotentiometry for OER. For supercapacitor, the CN-2/CP electrode delivers a high specific capacitance of 1346 F g^?1 at 1 A g^?1 and maintains a capacitance of 88.5% after 7000 charge/discharge cycles at 20 A g^?1. Based on the physical and chemical characterization results, the high performance originates from the in-situ electrochemical conversion of metal hydroxide, improved conductivity, fast charge transfer at the interface and unique layered cross morphology providing more active sites.     

Construction of MnO2 nanosheets@graphenated carbon nanotube networks core-shell heterostructure on 316L stainless steel as binder-free supercapacitor electrodes

Carbon nanotubes are regarded as typical and promising electrode materials in supercapacitors. However, small specific capacitance of carbon nanotubes restricts the practical application in high energy density devices. Herein, MnO₂ nanosheets@graphenated carbon nanotube networks are synthesized directly on 316L stainless steel as binder-free electrodes for high-performance supercapacitors. Graphenated carbon nanotube networks are grown in-situ on stainless steel by chemical vapor deposition method followed by annealing treatment. Subsequently, MnO₂ nanosheets are uniformly deposited on graphenated carbon nanotube networks to construct core-shell heterostructure based on the facile hydrothermal reaction using KMnO₄ as the precursor. Core carbon nanotube networks can offer a stable structural backbone and shell MnO₂ nanosheets can shorten diffusion paths of ions. The MnO₂ nanosheets@graphenated carbon nanotube networks exhibit a high specific capacitance of 575.4 F g⁻¹ (areal capacitance of 274 mF cm⁻²) at the current density of 0.5 mA cm⁻² and good cycling stability (93% of capacity retention after 6000 cycles), due to the synergistic effects between pseudocapacitive MnO₂ nanosheets and conductive carbon nanotube networks. The developed synthetic strategy offers design guidelines for the construction of advanced binder-free electrodes for high-performance supercapacitors.     

Enhanced performance of carbon-based supercapacitors by constructing protonic and electric dual-channels in electrodes

Powdery carbonaceous materials have to use binder materials when they are integrated into electrodes for supercapacitors, which will results in high interfacial charge transfer resistances and reduced specific capacitance. To resolve the problem, protonic and electric dual-channels are constructed in electrodes by in situ synthesis of cesium hydrogen salt of phosphotungstic acid on the surface of carbonaceous materials. The cesium hydrogen salt particles are confirmed by a Fourier transform infrared spectroscopy, X-ray diffractometer and an energy dispersive X-ray spectroscopy. The electrochemical properties of as-fabricated electrodes are measured by cyclic voltammetry, galvanostatic charging-discharging, and impedance analysis with an electrochemical workstation. At a current density of 1 A g⁻¹, the electrode shows a specific capacitance of 152 F g⁻¹. Compared to the electrode without the cesium hydrogen salt, the value increases 25% at least. Furthermore, the specific capacitance retention of the electrode reaches 104% of its original capacitance after 5000 charge-discharge cycles, suggesting excellent cycling stability.