Facile preparation of flexible binder-free graphene electrodes for high-performance supercapacitors

A facile two-step strategy to prepare flexible graphene electrodes has been developed for supercapacitors using thermal reduction of graphene oxide (GO) and thermally reduced graphene oxide (TRGO) composite films. The tunable porous structure of the GO/TRGO film provided channels to release the high pressure generated by CO₂ gas. The graphene electrode obtained from reduced-GO/TRGO (1 : 1 in mass ratio) film showed great flexibility and high film density (0.52 g cm⁻³). Using the EMI-BF₄ electrolyte with a working voltage of 3.7 V, the as-fabricated free-standing reduced-GO/TRGO (1 : 1) film achieved a great gravimetric capacitance of 180 F g⁻¹ (delivering a gravimetric energy density of 85.6 W h kg⁻¹), a volumetric capacitance of 94 F cm⁻³ (delivering a volumetric energy density of 44.7 W h L⁻¹), and a 92% retention after 10 000 charge/discharge cycles. In addition, the solid state flexible supercapacitor with the free-standing reduced-GO/TRGO (1 : 1) film as the electrodes and the EMI-BF₄/poly (vinylidene fluoride hexafluopropylene) (PVDF-HFP) gel as the electrolyte also demonstrated a high gravimetric capacitance of 146 F g⁻¹ with excellent mechanical flexibility, bending stability, and electrochemical stability. The strategy developed in this study provides great potentials for the synthesis of flexible graphene electrodes for supercapacitors.

A supercapacitor electrode is developed with a free-standing graphene film by a facile two-step strategy. The graphene electrode achieved a gravimetric capacitance of 180 F g⁻¹ and a volumetric capacitance of 94 F cm⁻³.     

High-performance asymmetric supercapacitor based on CdCO3/CdO/Co3O4 composite supported on Ni foam

A CdCO₃/CdO/Co₃O₄ composite has been prepared on nickel foam through a combined hydrothermal-annealing method. An asymmetric hybrid supercapacitor (SC) device was assembled with this composite as the positive electrode and activated carbon was the negative electrode. The SC exhibited a high specific capacitance of 84 F g⁻¹ @ 1 mA cm⁻², a maximum energy density of 26.3 W h kg⁻¹, and a power density of 2290 W kg⁻¹, along with a wide potential window of 1.5 V and long cycle life (92% after 6000 cycles). SCs assembled in series powered various light-emitting diodes and moved an electrical mini-motor.

This work presents for the first time a CdCO₃/CdO/Co₃O₄@nickel foam based supercapacitor with high both specific capacitance and energy density, a widespread potential window and a long cycle life.     

A simple strategy to prepare a layer-by-layer assembled composite of Ni–Co LDHs on polypyrrole/rGO for a high specific capacitance supercapacitor

A facile and novel electrode material of nickel–cobalt layered double hydroxides (Ni–Co LDHs) layered on polypyrrole/reduced graphene oxide (PPy/rGO) is fabricated for a symmetrical supercapacitor via chemical polymerization, hydrothermal and vacuum filtration. This conscientiously layered composition is free from any binder or surfactants which is highly favorable for supercapacitors. The PPy/rGO serves as an ideal backbone for Ni–Co LDHs to form a free-standing electrode for a high-performance supercapacitor and enhanced the overall structural stability of the film. The well-designed layered nanostructures and high electrochemical activity from the hexagonal-flakes like Ni–Co LDHs provide large electroactive sites for the charge storage process. The specific capacitance (1018 F g⁻¹ at 10 mV s⁻¹) and specific energy (46.5 W h kg⁻¹ at 464.9 W kg⁻¹) obtained for the PPy/rGO|Ni–Co LDHs symmetrical electrode in the current study are the best reported for the two-electrode system for PPy- and LDHs-based composites. The outstanding performance in the prepared LBL film is a result of the LBL architecture of the film and the combined effect of redox reaction and electrical double layer capacitance.

A facile and novel electrode material of nickel–cobalt layered double hydroxides (Ni–Co LDHs) layered on polypyrrole/reduced graphene oxide (PPy/rGO) is fabricated for a symmetrical supercapacitor via chemical polymerization, hydrothermal and vacuum filtration.     

Hydrothermal synthesis of flower-like molybdenum disulfide microspheres and their application in electrochemical supercapacitors

Three-dimensional flower-like molybdenum disulfide microspheres composed of nanosheets were prepared by a hydrothermal method using ammonium molybdate as the molybdenum source and thiourea as the sulfur source. Structural and morphological characterizations were performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS). The electrochemical properties of MoS₂ electrode were studied by performing cyclic voltammetry (CV), galvanostatic charge–discharge analysis and electrochemical impedance spectroscopy (EIS). When used as an electrode material for supercapacitor, the hybrid MoS₂ showed a high specific capacity of 518.7 F g⁻¹ at a current density of 1 A g⁻¹ and 275 F g⁻¹ at a high discharge current density of 10 A g⁻¹. In addition, a symmetric supercapacitor composed of MoS₂ as positive and negative electrodes was prepared, which exhibited a high energy density of 12.46 W h kg⁻¹ at a power density of 70 W kg⁻¹ and still maintains an impressive energy density of 6.42 W h kg⁻¹ at a large power density of 7000 W kg⁻¹. The outstanding performance of the MoS₂ electrode material indicates its great potential for applications in high-performance energy storage systems.

Three-dimensional flower-like molybdenum disulfide microspheres composed of nanosheets were prepared by a hydrothermal method using ammonium molybdate as the molybdenum source and thiourea as the sulfur source.     

Lavender-like cobalt hydroxide nanoflakes deposited on nickel nanowire arrays for high-performance supercapacitors

Hierarchical nanostructured electrodes with excellent electronic properties and high specific surface areas have promising applications in high-performance supercapacitors. However, high active mass loading and uniform structure are still crucial in fabricating such architectures. Herein, Co(OH)₂ nanoflakes were homogeneously deposited on nickel nanowire arrays (NNA) through a hydrothermal approach to form an NNA@Co(OH)₂ (NNACOH) composite electrode. The as-synthesized one dimensional (1D) system had a lavender-like structure with a high mass loading of 5.42 mg cm⁻² and a high specific surface area of 74.5 m² g⁻¹. Due to the unique electrode structure characteristics, the electrode could deliver a high specific capacitance of 891.2 F g⁻¹ at the current density of 1 A g⁻¹ (corresponding to an areal capacitance of 4.83 F cm⁻² at 5.42 mA cm⁻²). The capacitance could still maintain a high value of 721 F g⁻¹ when the current density is increased to 50 A g⁻¹. In addition, the electrode showed superior cycle stability with a capacitance retention of 89.3% after charging/discharging at the current density of 10 A g⁻¹ for 20 000 cycles. A flexible asymmetric supercapacitor (ASC) was assembled by employing NNACOH as the positive electrode and activated carbon (AC) as the negative electrode. It delivered a maximum energy density of 23.1 W h kg⁻¹ at the power density of 712 W kg⁻¹ and an energy density of 13.5 W h kg⁻¹ at the maximum power density of 14.7 kW kg⁻¹ (based on the total mass of the electrodes), showing the state-of-the-art energy storage ability of the Co(OH)₂ cathode material at device level.

Co(OH)₂ nanoflakes deposited on nickel nanowire arrays with a lavender-like architecture are prepared. Using it as cathode and activated carbon as anode, the flexible supercapacitor shows superior energy storage ability.     

Lichen-like anchoring of MoSe2 on functionalized multiwalled carbon nanotubes: an efficient electrode for asymmetric supercapacitors

In the present study, we have developed a composite electrode of MSNT using a simple and scalable two-step scheme to synthesize a composite electrode material comprising MoSe₂/multiwalled carbon nanotubes (MoSe₂/MWCNTs) for supercapacitor applications. First, a MWCNT thin film was deposited on a stainless steel substrate by using a “dip and dry” coating technique. Subsequently, MoSe₂ was deposited onto the MWCNT thin film using the successive ionic layer adsorption and reaction method. The lichen-like growth of MoSe₂ on the MWCNT network provided dual charge storage and an effective ion transfer path. The composite electrode of MSNT has been studied systematically with different electrolytes and concentrations of electrolyte. As a result, the MoSe₂/MWCNT (MSNT) electrode exhibited excellent electrochemical properties such as a specific capacity of 192 mA h g⁻¹ and a capacitance retention of 88% after 2000 cycles in 1 M LiCl electrolyte. The results demonstrated the huge potential of the MSNT composite electrode for practical application in supercapacitors. The aqueous symmetric cell fabricated using the MSNT composite as both the anode and cathode showed an energy density of 17.9 W h kg⁻¹. Additionally, the energy density improved by designing an asymmetric device of MSNT//MnO₂ and notably, it reveals two-fold improvement in the energy density compared to a symmetric MSNT cell. The MSNT//MnO₂-based asymmetric cell exhibited a maximum specific capacitance of 112 F g⁻¹ with a high energy density of 35.6 W h kg⁻¹.

Simple and scalable chemical synthesis approach to develop a MoSe₂/MWCNTs composite thin film electrode for a highly efficient asymmetric supercapacitor cell.     

Flexible and high energy density solid-state asymmetric supercapacitor based on polythiophene nanocomposites and charcoal

An asymmetric supercapacitor (ASC) was constructed using a polythiophene/aluminium oxide (PTHA) nanocomposite as an anode electrode and charcoal as a cathode electrode. The highest specific capacitance (C_sp) of the PTHA electrode was found to be 554.03 F g⁻¹ at a current density (CD) of 1 A g⁻¹ and that of the charcoal electrode was 374.71 F g⁻¹ at 1.4 A g⁻¹, measured using a three electrode system. The maximum C_sp obtained for the assembled PTHA//charcoal asymmetric supercapacitor (ASC) was 265.14 F g⁻¹ at 2 A g⁻¹. It also showed a high energy density of 42.0 W h kg⁻¹ at a power density of 735.86 W kg⁻¹ and capacitance retention of 94.61% even after 2000 cycles. Moreover, it is worth mentioning that the asymmetric device was used to illuminate a light emitting diode (LED) for more than 15 minutes. This PTHA//charcoal ASC also possesses stable electrochemical properties in different bending positions and hence finds a promising application in flexible, wearable and portable energy storage electronic devices.

A high energy density flexible solid-state asymmetric supercapacitor is fabricated using polythiophene nanocomposites and charcoal which exhibits stable electrochemical properties in different bending position.     

Aqueous supercapacitors based on carbonized silk electrodes

Graphitic nitrogen-doped hierarchical porous carbon nanosheets for supercapacitor application were derived from an easily obtained and green silk by simultaneous ZnCl₂ activation and FeCl₃ graphitization at different heating temperatures. By increasing the heating temperature from 700 to 850 °C, the degree of graphitization and BET surface area rose to their highest levels, while the nitrogen doping content was maintained at 2.24 wt%. Carbonized silk at 850 °C displays a nanosheet morphology and a considerable specific surface area (1285.31 m² g⁻¹), and it was fabricated into a supercapacitor as an electrode material, exhibiting superior electrochemical performance with a high specific capacitance of 178 F g⁻¹ at 0.5 A g⁻¹ and an excellent rate capability (81% capacitance retention ratio even at 20 A g⁻¹) in 1 mol L⁻¹ H₂SO₄ electrolyte. A symmetric supercapacitor using carbonized silk at 850 °C as the electrodes has an excellent specific energy of 14.33 W h kg⁻¹ at a power density of 251 W kg⁻¹ operated over a wide voltage range of 2.0 V in aqueous neutral Na₂SO₄ electrolyte.

An aqueous symmetrical supercapacitor was achieved by assembling SC-850 electrodes, which possess a specific energy of 14.33 W h kg⁻¹ at a power density of 251 W kg⁻¹ operated over the wide voltage range of 2.0 V in aqueous neutral Na₂SO₄ electrolyte.     

N,O and P co-doped honeycomb-like hierarchical porous carbon derived from Sophora japonica for high performance supercapacitors

Novel N, O and P co-doped honeycomb-like hierarchically porous carbon (N-O-P-HHPC) materials with a large specific surface area from Sophora japonica were prepared via a one-step activation and carbonization method and used as an electrode for supercapacitors. The results indicate that as-prepared N-P-HHPC with a large specific surface area (2068.9 m² g⁻¹) and N (1.5 atomic%), O (8.4 atomic%) and P (0.4 atomic%) co-doping has a high specific capacitance of 386 F g⁻¹ at 1 A g⁻¹. Moreover, a 1.8 V symmetrical SC was assembled from the N-O-P-HHPC-3 electrode using 1 M Na₂SO₄ gel electrolyte, presenting a high energy density (28.4 W h kg⁻¹ at 449.9 W kg⁻¹) and a long life cycling stability with only 7.3% capacitance loss after 10 000 cycles. Furthermore, the coin-type symmetrical SC using EMIMBF4 as electrolyte presents an ultrahigh energy density (80.8 W h kg⁻¹ at 1500 W kg⁻¹). When the two coin-type symmetrical SCs are connected in series, eight red light-emitting diodes (LEDs) and a small display screen can be powered. These results demonstrate as-prepared N, O and P co-doped HHPC is a considerable candidate as a carbon electrode for energy storage devices.

N, O and P co-doped honeycomb-like hierarchical porous carbon (N-P-HHPC-3) derived from Sophora japonica displays an ultrahigh energy density (80.8 W h kg⁻¹ at 1500 W kg⁻¹) and outstanding long-term stability.     

A SnO2QDs/GO/PPY ternary composite film as positive and graphene oxide/charcoal as negative electrodes assembled solid state asymmetric supercapacitor for high energy storage applications

The work demonstrates tin oxide quantum dots/graphene oxide/polypyrrole (SnO₂QDs/GO/PPY) ternary composite deposited on titanium foil as a positive electrode and graphene oxide (GO)/charcoal on titanium foil as negative electrode separated by polyvinyl alcohol/potassium hydroxide (PVA/KOH) gel-electrolyte as a solid-state asymmetric supercapacitor for high energy storage applications. Here, tin oxide quantum dots (SnO₂QDs) were successfully synthesized by a hydrothermal technique, and SnO₂QDs/GO/PPY ternary composite was synthesized by an in situ method with pyrrole monomer, SnO₂, and GO. A pH value controlled, which maintained the uniform size of SnO₂QDs dispersed on PPY, through GO ternary composite was used for fabricating the asymmetric supercapacitor electrode with the configuration (SnO₂QDs/GO/PPY)/GO/charcoal (85 : 10 : 5). The device achieved the highest specific capacitance of 1296 F g⁻¹, exhibited an energy density of 29.6 W h kg⁻¹ and the highest power density of 5310.26 W kg⁻¹ in the operating voltage from 0 to 1.2 V. The device also possessed excellent reliability and retained the capacitance of 90% after 11 000 GCD cycles. This ternary composite is a prominent material for potential applications in next-generation energy storage and portable electronic devices.

Representation of the synthesis steps of SnO₂QDs/GO/PPY ternary composites and SnO₂QDs/GO/PPY//GO/charcoal asymmetric supercapacitor device.     

Ternary 3D reduced graphene oxide/Ni0.5Zn0.5Fe2O4/polyindole nanocomposite for supercapacitor electrode application

A facile two-step strategy has been reported for the preparation of a ternary 3D reduced graphene oxide/Ni_0.5Zn_0.5Fe₂O₄/polyindole nanocomposite (GNP) and this composite is applied as an electrode material for supercapacitor applications. Remarkably, Ni_0.5Zn_0.5Fe₂O₄ nanoparticles (NZF) decorated on reduced graphene oxide (GN2) are achieved by a facile hydrothermal method followed by coating with polyindole (PIN) through an in situ emulsion polymerization process. The structure, porosity, morphology, and thermal stability of the resulting ternary GNP hybrid material were characterized via X-ray diffraction (XRD), Raman spectroscopy, Brunauer–Emmett–Teller (BET) surface area measurements, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). This combination of hybrid material has a favorable mesoporous structure that enables high exposure of active sites for fast electron transport for supercapacitor applications. We demonstrate here that the ternary GNP hybrid electrode material is capable of delivering a favorable specific capacitance of ∼320 F g⁻¹ at 0.3 A g⁻¹ within the potential range from −0.1 to 1 V, with desirable rate stability and excellent cycling stability in the three-electrode system. Furthermore, an asymmetric supercapacitor (ASC) of a two-electrode configuration was fabricated using 3D RGO and GNP as the negative and positive electrodes, respectively. Such a device manifests a favourable C_sp of 48.9 F g⁻¹ at 0.5 A g⁻¹ and retains stability of 84% even after 2000 cycles. This ASC device exhibits a significant energy density of 16.38 W h kg⁻¹ at a power density of 1784 W kg⁻¹. The synergistic effects of pseudo and double layer capacitive contributions from PIN and GN2 make this ternary GNP hybrid electrode material of great promise in supercapacitor applications.

A facile two-step strategy has been reported for the preparation of a ternary 3D reduced graphene oxide/Ni_0.5Zn_0.5Fe₂O₄/polyindole nanocomposite (GNP) and this composite is applied as an electrode material for supercapacitor applications.     

Hydrothermal synthesis of cobalt telluride nanorods for a high performance hybrid asymmetric supercapacitor

Cobalt telluride nanostructured materials have demonstrated various applications, particularly in energy generation and storage. A high temperature and reducing atmosphere are required for the preparation of cobalt telluride-based materials, which makes this a difficult and expensive process. The development of a facile route for producing the desirable nanostructure of cobalt telluride remains a great challenge. We demonstrated a simple hydrothermal method for preparing cobalt telluride nanorods (CoTe NRs) and telluride nanorods (Te NRs) for supercapacitor applications. The morphology of CoTe NRs and Te NRs was analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The prepared CoTe NR electrode material exhibited a high specific capacity of 170 C g⁻¹ at a current density of 0.5 A g⁻¹ with an exceptional cyclic stability. The asymmetric supercapacitor was assembled using CoTe NRs and orange peel-derived activated carbon (OPAA-700) as a positive and negative electrode, respectively. The fabricated device delivered a high energy density of 40.7 W h kg⁻¹ with a power density of 800 W kg⁻¹ at 1 A g⁻¹ current density. When the current density was increased to 30 A g⁻¹, the fabricated device delivered a high power density of 22.5 kW kg⁻¹ with an energy density of 16.3 W h kg⁻¹. The fabricated asymmetric supercapacitor displayed a good cyclic stability performance for 10 000 cycles at a high current density of 30 A g⁻¹ and retained 85% of its initial capacity for after 10 000 cycles. The prepared materials indicate their applicability for high performance energy storage devices.

A one-step hydrothermal derived cobalt telluride nanorods and activated carbon-based hybrid asymmetric supercapacitor delivered a high energy (40.7 W h kg⁻¹) and power density (22.5 kW kg⁻¹) with an electrochemical stability of 85% for 10000 cycles.     

Efficient coupling of MnO2/TiN on carbon cloth positive electrode and Fe2O3/TiN on carbon cloth negative electrode for flexible ultra-fast hybrid supercapacitors

Recent research and development of energy storage devices has focused on new electrode materials because of the critical effects on the electrochemical properties of supercapacitors. In particular, MnO₂ and Fe₂O₃ have drawn extensive attention because of their low cost, high theoretical specific capacity, environmental friendliness, and natural abundance. In this study, MnO₂ ultrathin nanosheet arrays and Fe₂O₃ nanoparticles are fabricated on TiN nanowires to produce binder-free core–shell positive and negative electrodes for a flexible and ultra-fast hybrid supercapacitor. The MnO₂/TiN/CC electrode shows larger pseudocapacitance contributions than MnO₂/CC. For example, at a scanning rate of 2 mV s⁻¹, the pseudocapacitance contribution of MnO₂/TiN/CC is 87.81% which is nearly 25% bigger than that of MnO₂/CC (71.26%). The supercapacitor can withstand a high scanning rate of 5000 mV s⁻¹ in the 2 V window and exhibits a maximum energy density of 71.19 W h kg⁻¹ at a power density of 499.79 W kg⁻¹. Even at 5999.99 W kg⁻¹, it still shows an energy density of 31.3 W h kg⁻¹ and after 10 000 cycles, the device retains 81.16% of the initial specific capacitance. The activation mechanism is explored and explained.

MnO₂ ultrathin nanosheet arrays and Fe₂O₃ nanoparticles are fabricated on carbon based TiN nanowires to produce binder-free and core–shell positive and negative electrodes for a flexible and ultra-fast hybrid supercapacitor.     

One-pot mechanochemical exfoliation of graphite and in situ polymerization of aniline for the production of graphene/polyaniline composites for high-performance supercapacitors

Graphene/polyaniline composites have attracted considerable attention as high-performance supercapacitor electrode materials; however, there are still numerous challenges for their practical applications, such as the complex preparation process, high cost, and disequilibrium between energy density and power density. Herein, we report an efficient method to produce graphene/polyaniline composites via a one-pot ball-milling process, in which aniline molecules act as both the intercalator for the exfoliation of graphite and the monomer for mechanochemical polymerization into polyaniline clusters on the in situ exfoliated graphene sheets. The graphene/polyaniline composite electrode delivered a large specific capacitance of 886 F g⁻¹ at 5 mV s⁻¹ with a high retention of 73.4% at 100 mV s⁻¹. The high capacitance and rate capability of the graphene/polyaniline composite can contribute to the fast electron/ion transfer and dominantly capacitive contribution because of the synergistic effects between the conductive graphene and pseudocapacitive polyaniline. In addition, a high energy density of 40.9 W h kg⁻¹ was achieved by the graphene/polyaniline-based symmetric supercapacitor at a power density of 0.25 kW kg⁻¹, and the supercapacitor also maintained 89.1% of the initial capacitance over 10 000 cycles.

Efficient ball-milling production of graphene/polyaniline composites as supercapacitor electrodes with enhanced capacitive contribution, rate capability, and specific capacitance.     

Multidimensional structure of CoNi2S4 materials: structural regulation promoted electrochemical performance in a supercapacitor

Multidimensional architectures of CoNi₂S₄ electrode materials are rationally designed by engineering the surface structure toward that of high-performance supercapacitors. The fabrication of a special morphology is highly dependent on the synergistic effect between the guidance of Co–Ni precursor arrays and a subsequent sulfidation process. The unparalleled CoNi₂S₄ electrode materials (NS-3) deliver a significantly enhanced specific capacitance (3784.6 F g⁻¹ at 3 A g⁻¹), accompanied by an extraordinary rate capability (2932.3 F g⁻¹ at 20 A g⁻¹) and excellent cycling life. The outstanding supercapacitor performance stated above stems from the advantages of a multidimensional structure generated by crosslinking 2D microsheets/1D nanowires/2D ultrathin nanosheets; this structure supplies additional efficient active sites and a large contact area at the electrode–electrolyte interface, providing faster transport kinetics for electrons and ions. For practical applications, asymmetric devices based on an NS-3 positive electrode and active carbon negative electrode exhibit a high energy density of 38.5 W h kg⁻¹ accompanied by a power density of 374.9 W kg⁻¹ (22 W h kg⁻¹ at 7615.4 W kg⁻¹). The above results indicate that the design of multidimensional Co–Ni–S materials is an effective strategy to achieve a high-performance supercapacitor.

Multidimensional architectures of CoNi₂S₄ electrode materials are rationally designed by engineering the surface structure toward that of high-performance supercapacitors.     

Electrochemical properties of kenaf-based activated carbon monolith for supercapacitor electrode applications

Activated carbon monoliths of kenaf (ACMKs) were prepared by moulding kenaf fibers into a column-shape monolith and then carrying out pyrolysis at 500, 600, 700 or 800 °C, followed by activation with KOH at 700 °C. Then, the sample was characterized using thermogravimetric analyzer (TGA), field-emission scanning electron microscopy (FE-SEM), field-emission transmission electron microscopy (FE-TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD) and N₂ sorption instruments. The prepared ACMK was subjected to electrochemical property evaluation via cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS). The GCD study using a three-electrode system showed that the specific capacitance decreased with higher pyrolysis temperature (PYT) with the ACMK pyrolyzed at 500 °C (ACMK-500) exhibiting the highest specific capacitance of 217 F g⁻¹. A two-electrode system provided 95.9% retention upon a 5000 cycle test as well as the specific capacitance of 212 F g⁻¹, being converted to an energy density of 6 W h kg⁻¹ at a power density of 215 W kg⁻¹.

Monolithic carbon from kenaf-based fiber for supercapacitor electrode application provided a specific capacitance of 212 F g⁻¹via GCD at 1 A g⁻¹, converting to an energy density of 6 W h kg⁻¹ at the power density of 215 W kg⁻¹ as well as 95.9% retention upon 5000 cycling test.     

Enhanced electrochemical performance of nanoplate nickel cobaltite (NiCo2O4) supercapacitor applications

Well-ordered, unique interconnected nanostructured binary metal oxides with lightweight, free-standing, and highly flexible nickel foam substrate electrodes have attracted tremendous research attention for high performance supercapacitor applications owing to the combination of the improved electrical conductivity and highly efficient electron and ion transport channels. In this study, a unique interconnected nanoplate-like nickel cobaltite (NiCo₂O₄) nanostructure was synthesized on highly conductive nickel foam and its use as a binder-free material in energy storage applications was assessed. The nanoplate-like NiCo₂O₄ nanostructure electrode was prepared by a simple chemical bath deposition method under optimized conditions. The NiCo₂O₄ electrode delivered an outstanding specific capacitance of 2791 F g⁻¹ at a current density of 5 A g⁻¹ in a KOH electrolyte in a three-electrode system as well as outstanding cycling stability with 99.1% retention after 3000 cycles at a current density of 7 A g⁻¹. The as-synthesized NiCo₂O₄ electrode had a maximum energy density of 63.8 W h kg⁻¹ and exhibited an outstanding high power density of approximately 654 W h kg⁻¹. This paper reports a simple and cost-effective process for the synthesis of flexible high performance devices that may inspire new ideas for energy storage applications.

Schematic representation of the synthesis of nanoplate-like NiCo₂O₄ structure on nickel foam (NF).     

Engineering NiCo2S4 nanoparticles anchored on carbon nanotubes as superior energy-storage materials for supercapacitors

Fabricating high-capacity electrode materials toward supercapacitors has attracted increasing attention. Here we report a three-dimensional CNTs/NiCo₂S₄ nanocomposite material synthesized successfully by a facile one-step hydrothermal technique. As expected, a CNTs/NiCo₂S₄ electrode shows remarkable capacitive properties with a high specific capacitance of 890 C g⁻¹ at 1 A g⁻¹. It also demonstrates excellent cycle stability with an 83.5% capacitance retention rate after 5000 cycles at 10 A g⁻¹. Importantly, when assembled into a asymmetric supercapacitor, it exhibits a high energy density (43.3 W h kg⁻¹) and power density (800 W kg⁻¹). The exceptional electrochemical capacity is attributed to the structural features, refined grains, and enhanced conductivity. The above results indicate that CNTs/NiCo₂S₄ composite electrode materials have great potential application in energy-storage devices.

A one-step hydrothermal method was used to successfully synthesize NiCo₂S₄ nanocomposites anchored on carbon nanotubes as excellent energy storage materials for supercapacitors.     

Improved performance of a CoTe//AC asymmetric supercapacitor using a redox additive aqueous electrolyte

Cobalt telluride (CoTe) nanosheets as supercapacitor electrode materials are grown on carbon fiber paper (CFP) by a facile hydrothermal process. The CoTe electrode exhibits significant pseudo-capacitive properties with a highest C_m of 622.8 F g⁻¹ at 1 A g⁻¹ and remarkable cycle stability. A new asymmetric supercapacitor (ASC) is assembled based on CoTe (positive electrode) and activated carbon (negative electrode), which can expand the operating voltage to as high as 1.6 V, and has a specific capacitance of 67.3 F g⁻¹ with an energy density of 23.5 W h kg⁻¹ at 1 A g⁻¹. The performance of the ASC can be improved by introducing redox additive K₄Fe(CN)₆ into alkaline electrolyte (KOH). The results indicate that the ASC with K₄Fe(CN)₆ exhibits an ultrahigh specific capacitance of 192.1 F g⁻¹ and an energy density of 67.0 W h kg⁻¹, which is nearly a threefold increase over the ASC with pristine electrolyte.

The performance of an ASC with CoTe as the positive electrode can be enhanced by introducing redox additive K₄Fe(CN)₆ into the alkaline electrolyte.     

Nitrogen and oxygen Co-doped porous carbon derived from yam waste for high-performance supercapacitors

It is a considerable challenge to produce a supercapacitor with inexpensive raw materials and employ a simple process to obtain carbon materials with a high specific surface area, rich pore structure, and appropriate doping of heterogeneous elements. In the current study, yam waste-derived porous carbon was synthesized for the first time by a two-step carbonization and KOH chemical activation process. An ultra-high specific surface area of 2382 m² g⁻¹ with a pore volume of 1.11 cm³ g⁻¹ and simultaneous co-doping of O–N was achieved for the optimized sample. Because of these distinct features, the optimized material exhibits a high gravimetric capacitance of 423.23 F g⁻¹ at 0.5 A g⁻¹ with an impressive rate capability at 10 A g⁻¹, and prominent cycling durability with a capacity retention of 96.4% at a high current density of 10 A g⁻¹ after 10 000 cycles in 6 M KOH in a three-electrode system. Moreover, in 6 M KOH electrolyte, the assembled symmetrical supercapacitor provides a large C of 387.3 F g⁻¹ at 0.5 A g⁻¹. It also presents high specific energy of 34.6 W h kg⁻¹ when the specific power is 200.1 W kg⁻¹ and a praiseworthy specific energy of 8.3 W h kg⁻¹ when the specific power is 4000.0 W kg⁻¹ in 1 M Na₂SO₄ electrolyte. Thus, this study provides reference and guidance for developing high-performance electrode materials for supercapacitors.

3D porous carbon with ultra-high specific surface area and excellent electrochemical performance is synthesized by a simple activation and carbonization process through adopting biomass yam waste as raw material.