High-performance quasi-solid-state flexible supercapacitors based on a flower-like NiCo metal–organic framework

NiCo metal–organic framework (MOF) electrodes were prepared by a simple hydrothermal method. The flower-like NiCo MOF electrode exhibited an exciting potential window of 1.2 V and an excellent specific capacitance of 927.1 F g⁻¹ at 1 A g⁻¹. The flower-like NiCo MOF//activated carbon (AC) device delivered a high energy density of 28.5 W hkg⁻¹ at a power density of 400.5 W kg⁻¹ and good cycle stability (95.4% after 5000 cycles at 10 A g⁻¹). Based on the flower-like NiCo MOF electrode, the asymmetric quasi-solid-state flexible supercapacitor (AFSC) was prepared and exhibited good capacitance retention after bending (79% after 100 bends and 64.4% after 200 bends). Furthermore, two AFSCs in series successfully lit up ten parallel red LED lights, showing great application potential in flexible and wearable energy storage devices.

The flower-like NiCo MOF prepared by a hydrothermal has a specific capacitance of 927.1 F g⁻¹ at 1 A g⁻¹ and a capacitance retention of 69.7% from 1 A g⁻¹ to 10 A g⁻¹, showing excellent electrochemical performance.     

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 universal electrochemical lithiation–delithiation method to prepare low-crystalline metal oxides for high-performance hybrid supercapacitors

The electrochemical performance of transition metal oxides (TMOs) for hybrid supercapacitors has been optimized through various methods in previous reports. However, most previous research was mainly focused on well-crystalline TMOs. Herein, the electrochemical lithiation–delithiation method was performed to synthesise low-crystallinity TMOs for hybrid supercapacitors. It was found that the lithiation–delithiation process can significantly improve the electrochemical performance of “conversion-type” TMOs, such as CoO, NiO, etc. The as-prepared low-crystallinity CoO exhibits high specific capacitance of 2154.1 F g⁻¹ (299.2 mA h g⁻¹) at 0.8 A g⁻¹, outstanding rate capacitance retention of 63.9% even at 22.4 A g⁻¹ and excellent cycling stability with 90.5% retention even after 10 000 cycles. When assembled as hybrid supercapacitors using active carbon (AC) as the active material of the negative electrode, the devices show a high energy density of 50.9 W h kg⁻¹ at 0.73 kW kg⁻¹. Another low-crystallinity NiO prepared by the same method also possesses a much higher specific capacitance of 2317.6 F g⁻¹ (302.6 mA h g⁻¹) compared to that for pristine commercial NiO of 497.2 F g⁻¹ at 1 A g⁻¹. The improved energy storage performance of the low-crystallinity metal oxides can be ascribed to the disorder of as-prepared low-crystallinity metal oxides and interior 3D-connected channels originating from the lithiation–delithiation process. This method may open new opportunities for scalable and facile synthesis of low-crystallinity metal oxides for high-performance hybrid supercapacitors.

The electrochemical performance of transition metal oxides (TMOs) for hybrid supercapacitors has been optimized through various methods in previous reports.     

One-step preparation of N,O co-doped 3D hierarchically porous carbon derived from soybean dregs for high-performance supercapacitors

Hierarchically porous carbon (HPC) material based on environmental friendliness biomass has spurred much attention, due to its high surface area and porous structure. Herein, three-dimensional (3D) N,O co-doped HPC (N–O-HPC) was prepared by using a one-step fabrication process of simultaneously carbonizing and activating soybean dregs and used as an electrode for supercapacitors (SCs). The obtained N–O-HPC with 4.8 at% N and 6.1 at% O exhibits a pretty small charge transfer resistance (0.05 Ω) and a large specific capacitance (408 F g⁻¹ at 1 A g⁻¹), due to its 3D hierarchically porous framework structure with extremely large specific surface area (1688 m² g⁻¹). Moreover, a symmetrical SC assembled with the HPC electrode exhibits an amazingly high energy density (22 W h kg⁻¹ at 450 W kg⁻¹) and a stable long cycling life with only 6% capacitance loss after 5000 cycles in 1 M Na₂SO₄ solution. This work provides a facile, green, and low-cost way to prepare electrode materials for SCs.

N,O co-doped 3D HPC derived from soybean dregs was prepared by a one-step method and displays an amazingly high energy density of 22 W h kg⁻¹ (450 W kg⁻¹) using 1 M Na₂SO₄ solution.     

Solvothermal synthesis of NiWO4 nanostructure and its application as a cathode material for asymmetric supercapacitors

This study proposes a facile solvothermal synthesis of nickel tungstate (NiWO₄) nanowires for application as a novel cathode material for supercapacitors. The structure, morphology, surface area and pore distribution were characterized and their capacitive performances were investigated. The results showed that the NiWO₄ nanowires synthesized in ethylene glycol solvent could offer a high specific capacitance of 1190 F g⁻¹ at a current density of 0.5 A g⁻¹ and a capacitance retaining ratio of 61.5% within 0.5–10 A g⁻¹. When used as a cathodic electrode of an asymmetric supercapacitor (ASC), the NiWO₄ nanowire based device can be cycled reversibly in a high-voltage region of 0–1.7 V with a high specific capacitance of 160 F g⁻¹ at 0.5 A g⁻¹, which therefore contributed to an energy density of 64.2 W h kg⁻¹ at a power density of 425 W kg⁻¹. Moreover, 92.8% of its initial specific capacitance can be maintained after 5000 consecutive cycles (5 A g⁻¹). These excellent capacitive properties make NiWO₄ a credible electrode material for high-performance supercapacitors.

This study proposes a facile solvothermal synthesis of nickel tungstate (NiWO₄) nanowires for application as a novel cathode material for supercapacitors.     

Scalable nanohybrids of graphitic carbon nitride and layered NiCo hydroxide for high supercapacitive performance

The limited number of edge nitrogen atoms and low intrinsic electrical conductivity hinder the supercapacitive energy storage applications of the nitrogen-rich graphitic carbon nitride (g-C₃N₄). In this study, a novel graphitic carbon nitride/NiCo-layered double hydroxide (CNLDH), a two-dimensional nanohybrid, is prepared by a simple hydrothermal synthesis. The homogeneous interpolation of g-C₃N₄ nanosheets into NiCo LDH stacked nanosheets effectively increases the overall performances of the g-C₃N₄/NiCo LDH nanohybrid. The improved morphology of the nanohybrid electrode upon the addition of g-C₃N₄ to the NiCo LDH yields a specific capacity of 183.43 mA h g⁻¹ in 6 M KOH at 1 A g⁻¹, higher than those of bare g-C₃N₄ (20.89 mA h g⁻¹) and NiCo LDH (95.92 mA h g⁻¹) electrodes. The excellent supercapacitive performance of the CNLDH nanohybrid is complemented by its low internal resistance, excellent rate capability, and large cycling lifetime. Furthermore, the hybrid supercapacitor is assembled using CNLDH 0.1 as a positive electrode and activated carbon (AC) as a negative electrode. The hybrid supercapacitor device of CNLDH 0.1//AC shows the maximum specific capacity of 37.44 mA h g⁻¹ at 1 A g⁻¹ with remarkable energy density, power density and good cycling performance. This confirms that the CNLDH 0.1 nanohybrid is an excellent electrode material for supercapacitor applications.

A two dimensional CNLDH 0.1 nanohybrid supercapacitor electrode prepared by simple hydrothermal hybridization of g-C₃N₄ and NiCO LDH shows the maximum specific capacity of 183.43 mA h g⁻¹ with remarkable electrochemical performance.     

Nitrogen and sulfur co-doped graphene aerogel for high performance supercapacitors

The development of high energy density and power density supercapacitors is very necessary in energy storage and application fields. A key factor of such devices is high-performance electrode materials. In this work, nitrogen and sulfur co-doped graphene aerogels (N/S-GA) were synthesized using graphene oxide as the precursor and 2-mercapto-1-methylimidazole as both the reducing agent and the N/S doping agent. The pore size distribution of the as-prepared N/S-GA was measured and the N/S-GA possesses a hierarchical porous structure. As an electrode material of supercapacitors, the N/S-GA could provide a suitable structure for charge accommodation and a short distance for ion transport. When 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF₄) ionic liquid was used as the electrolyte, the specific capacitance of the N/S-GA electrode material reached 212 F g⁻¹ and 162 F g⁻¹ at the current densities of 1 A g⁻¹ and 10 A g⁻¹, respectively. And the energy density and average power density of the N/S-GA based supercapacitor could reach 117 W h kg⁻¹ and 1.0 kW kg⁻¹ at 1 A g⁻¹, 82 W h kg⁻¹ and 9.5 kW kg⁻¹ at 10 A g⁻¹, respectively. It is believed that the N/S-GA material can be used in high-performance supercapacitors.

A nitrogen and sulfur co-doped graphene aerogel (N/S-GA) was synthesised in one step, and N/S-GA based supercapacitors exhibited high performance.     

Performance enhancement of asymmetric supercapacitors with bud-like Cu-doped Mn3O4 hollow and porous structures on nickel foam as positive electrodes

Cu-doped Mn₃O₄ hollow nanostructures supported on Ni foams as high-performance electrode materials for supercapacitors were successfully synthesized through a facile hydrothermal method and subsequent calcination. The morphology, structure, and electrochemical performance of the as-prepared Mn₃O₄ nanostructures can be tuned just by varying the Cu doping content. Benefiting from the unique bud-like hollow structure, the 1.5 at% Cu-doped Mn₃O₄ sample has a high specific capacitance of 257.6 F g⁻¹ at 1 A g⁻¹ and remarkable stability (about 90.6% retention of its initial capacitance after 6000 electrochemical cycles). Besides, an asymmetric supercapacitor (ASC) cell based on the 1.5 at% Cu-doped Mn₃O₄ exhibits a high specific capacitance of 305.6 F g⁻¹ at 1 A g⁻¹ and an energy density of 108.6 W h kg⁻¹ at a power density of 799.9 W kg⁻¹. More importantly, the ASC shows good long-term stability with 86.9% capacity retention after charging/discharging for 6000 cycles at a high current density of 5 A g⁻¹.

The effect of Cu doping on the electrochemical performance of bud-like Mn₃O₄ nanostructures for supercapacitor application was comparatively investigated.     

B,N-dual doped sisal-based multiscale porous carbon for high-rate supercapacitors

B, N dual-doped sisal-based activated carbon (BN-SAC) with a multiscale porous structure for high-rate supercapacitor electrode was prepared through a novel and facile strategy. With the inherent cellular channels serving as primary macropores, secondary mesopores and micropores are generated on the fiber surface and tracheid walls through low-pressure rapid carbonization of (NH₄)₂B₄O₇-containing sisal fibers and successive KOH activation. In addition to introducing B, N atoms into the BN-SAC, the additive also facilitates the formation of mesopores due to the rapid gas evaporation during its decomposition, leading to significantly increased specific surface area (2017 m² g⁻¹) and mesoporosity (68.6%). As a result, the BN-SAC-3 shows highly enhanced electrochemical performance including a high specific capacitance of 304 F g⁻¹, excellent rate capability (with 72.6% retention at 60 A g⁻¹) and superior cycling stability (4.6% capacitance loss after 3000 cycles). After assembling the BN-SAC-3 into symmetric supercapacitor, it shows a specific capacitance of 258 F g⁻¹ at 1 A g⁻¹ with 76.4% retention at 40 A g⁻¹ in 6 M KOH electrolyte, and delivers a maximum energy density of 24.3 W h kg⁻¹ at a power density of 612.8 W kg⁻¹ in 1 M TEABF₄/AN electrolyte. This work provides a new strategy for the synthesis of multiscale porous ACs for high-performance supercapacitors or other energy storage and conversion devices and is expected to be applied on other biomasses for large-scale production.

B, N dual-doped sisal-based activated carbon (BN-SAC) with a multiscale porous structure for high-rate supercapacitor electrode was prepared through a novel and facile strategy.     

Facile synthesis of a two-dimensional layered Ni-MOF electrode material for high performance supercapacitors

Recently, various metal–organic framework (MOF)-based supercapacitors (SCs) have received much attention due to their porosity and well-defined structures. Yet poor conductivity and low capacitance in most MOF-based devices limit their wide application. As an electrode material, 2D MOFs exhibit a rapid electron transfer rate and high specific surface area due to their unique structures. In this work, a 2D layered Ni-MOF is synthesized through a simple solvothermal method and serves as an electrode material for SCs. Electrochemical studies show that the Ni-MOF exhibits low charge transfer resistance, excellent specific capacitance of 1668.7 F g⁻¹ at 2 A g⁻¹ and capacitance retention of 90.3% after 5000 cycles at 5 A g⁻¹. Moreover, Ni-MOF//AC asymmetric SCs are assembled. The device exhibits high specific capacitance of 161 F g⁻¹ at 0.2 A g⁻¹ and the energy density reached 57.29 W h kg⁻¹ at a power density of 160 W kg⁻¹. The high electrochemical performance can be ascribed to the inherent porosity of MOFs and the 2D layered structure.

A 2D Ni-MOF was synthesized by a hydrothermal method and used as an electrode for SCs.     

Diaminopyrene modified reduced graphene oxide as a novel electrode material for excellent performance supercapacitors

In this paper, novel reduced graphene oxide (rGO) composites (DAPrGOs) modified by diaminopyrene (DAP) were successfully synthesized via a facile solvothermal reaction method and used for supercapacitors. Compared with the pristine rGO, the DAPrGO1 electrode showed distinctly better performance (397.63 F g⁻¹vs. 80.29 F g⁻¹ of pristine rGO at 0.5 A g⁻¹) with small charge transfer resistance. When a symmetric device was fabricated using DAPrGO1 as the active material, it also exhibited a high capacitance of 82.70 F g⁻¹ at 0.5 A g⁻¹ with an energy density of 25.84 W h kg⁻¹ at a power density of 375 W kg⁻¹, and even offered a high power density of 7500 W kg⁻¹ (18.71 W h kg⁻¹) at 10 A g⁻¹. Moreover, the device possessed good electrochemical stability up to 20 000 cycles, implying promising applications in energy storage fields.

Schematic illustration of the facile synthesis process of DAPrGOs nanocomposites, Ragone plots and the superior cyclic stability of the assembled DAPrGO1//DAPrGO1 SSS.     

One-pot synthesis of CoFe2O4/rGO hybrid hydrogels with 3D networks for high capacity electrochemical energy storage devices

CoFe₂O₄/reduced graphene oxide (CoFe₂O₄/rGO) hydrogel was synthesized in situ via a facile one-pot solvothermal approach. The three-dimensional (3D) network structure consists of well-dispersed CoFe₂O₄ nanoparticles on the surfaces of graphene sheets. As a binder-free electrode material for supercapacitors, the electrochemical properties of the CoFe₂O₄/rGO hybrid hydrogel can be easily adjusted by changing the concentration of the graphene oxide (GO) precursor solution. The results indicate that the hybrid material made using 3.5 mg mL⁻¹ GO solution exhibits an outstanding specific capacitance of 356 F g⁻¹ at 0.5 A g⁻¹, 68% higher than the pure CoFe₂O₄ counterpart (111 F g⁻¹ at 0.5 A g⁻¹), owing to the large specific surface area and good electric conductivity. Additionally, an electrochemical energy storage device based on CoFe₂O₄/rGO and rGO was assembled, which exhibits a high energy density of 17.84 W h kg⁻¹ at a power density of 650 W kg⁻¹ and an excellent cycling stability with 87% capacitance retention at 5 A g⁻¹ after 4000 cycles. This work takes one step further towards the development of 3D hybrid hydrogel supercapacitors and highlights their potential application in energy storage devices.

CoFe₂O₄/reduced graphene oxide (CoFe₂O₄/rGO) hydrogel was synthesized in situ via a facile one-pot solvothermal approach.     

Anchoring carbon layers and oxygen vacancies endow WO3−x/C electrode with high specific capacity and rate performance for supercapacitors

Herein, novel hierarchical carbon layer-anchored WO_3−x/C ultra-long nanowires were developed via a facile solvent-thermal treatment and a subsequent rapid carbonization process. The inner anchored carbon layers and abundant oxygen vacancies endowed the WO_3−x/C nanowire electrode with high conductivity, as measured with a single nanowire, which greatly enhanced the redox reaction active sites and rate performance. Surprisingly, the WO_3−x/C electrode exhibited outstanding specific capacitance of 1032.16 F g⁻¹ at the current density of 1 A g⁻¹ in a 2 M H₂SO₄ electrolyte and maintained the specific capacitance of 660 F g⁻¹ when the current density increased to 50 A g⁻¹. Significantly, the constructed WO_3−x/C//WO_3−x/C symmetric supercapacitors achieved specific capacitance of 243.84 F g⁻¹ at the current density of 0.5 A g⁻¹ and maintained the capacitance retention of 94.29% after 5000 charging/discharging cycles at the current density of 4 A g⁻¹. These excellent electrochemical performances resulted from the fascinating structure of the WO_3−x/C nanowires, showing a great potential for future energy storage applications.

A high-performance supercapacitor electrode comprising hierarchical carbon layer-anchored WO_3−x/C nanowires with inner abundant redox reaction active sites and numerous oxygen vacancies is presented.     

Heteroatom-doped porous carbons derived from moxa floss of different storage years for supercapacitors

Two novel carbons (MCs) derived from moxa floss of different storage years have been prepared by two low-cost and facile approaches, which are hydrothermal carbonization at a low temperature (200 °C) and direct pyrolysis at a moderate temperature (500 °C) followed by potassium hydroxide (KOH) activation strategy at a high temperature (800 °C), respectively. The physicochemical properties of MCs are investigated by Raman spectra, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and nitrogen adsorption–desorption isotherms. Results show that MCs derived from moxa floss of different storage years by two facile approaches possess different morphologies: MCs by hydrothermal carbonization (denoted as MC-1, MC-2 and MC-3) exhibit porous nanosheet structures, the highest specific surface area is about 1788.6 m² g⁻¹, and the largest total pore volumes is around 0.8170 cm³ g⁻¹, while MCs by direct pyrolysis (denoted as MC-4, MC-5 and MC-6) have basically blocky and rod-like morphologies, the highest specific surface area is about 1628.0 m² g⁻¹, and the largest total pore volume is around 0.7058 cm³ g⁻¹. However, despite the different morphologies, all MCs possess a similar hierarchical porous structure, numerous heteroatom groups and good electrical conductivity. Therefore, these low-cost, biomass-derived porous carbons with promising capacitive performance are used for supercapacitors application with high performance, for example, the as-assembled supercapacitor based on MC-5 exhibits a high specific capacitance of 288.3 F g⁻¹ at 0.25 A g⁻¹, an excellent rate performance of 243.5 F g⁻¹ even at 30 A g⁻¹ with 84.5% capacitance retention of its initial specific capacitance, and an outstanding long-term cycling stability with 98.7% capacitance retention after 10 000 cycles at 5 A g⁻¹. Furthermore, the maximum energy density for these supercapacitors with an aqueous electrolyte in a two-electrode system is about 10.0 W h kg⁻¹ at a power density of 70.3 W kg⁻¹. Therefore, this work opens up a whole new field for the applications of moxa floss and this novel concept of moxa floss use is an extremely promising strategy for developing high-performance carbons with porous structures and heteroatom-doping from renewable sources.

Two novel carbons (MCs) derived from moxa floss of different storage years have been prepared by two low-cost and facile approaches, which are hydrothermal carbonization and direct pyrolysis followed by KOH activation strategy, respectively.     

Preparation of N/O co-doped porous carbon by a one-step activation method for supercapacitor electrode materials

Heteroatom-doped carbon materials used in supercapacitors are low in cost and demonstrate extraordinary performance. Here, ethylenediamine tetraacetic acid (EDTA) with intrinsic N and O elements is selected as a raw material for the preparation of heteroatom self-doped porous carbon. Furthermore, N/O self-doped porous carbon with a large surface area has been successfully prepared using K₂CO₃ as the activator. The derived sample with a 1 : 2 molar ratio of EDTA to K₂CO₃ (EK-2) demonstrates a porous structure, rich defects, a large surface area of 2057 m² g⁻¹ and a micropore volume of 0.25 cm³ g⁻¹. Benefiting from high N content (2.89 at%) and O content (10.75 at%), EK-2 exhibits superior performance, including high capacitance of 325 F g⁻¹ at 1 A g⁻¹ and outstanding cycling stability with 96.8% retention after 8000 cycles at 10 A g⁻¹, which strongly confirms its immense potential toward many applications. Additionally, the maximum energy density of EK-2 reaches was 17.01 W h kg⁻¹ at a power density of 350 W kg⁻¹ in a two-electrode system. This facile and versatile strategy provides a scalable approach for the batch synthesis of N/O co-doped carbonaceous electrode materials for energy storage.

Heteroatom-doped carbon materials used in supercapacitors are low in cost and demonstrate extraordinary performance.     

A flexible polyelectrolyte-based gel polymer electrolyte for high-performance all-solid-state supercapacitor application

A simple polymerization process assisted with UV light for preparing a novel flexible polyelectrolyte-based gel polymer electrolyte (PGPE) is reported. Due to the existence of charged groups in the polyelectrolyte matrix, the PGPE exhibits favorable mechanical strength and excellent ionic conductivity (66.8 mS cm⁻¹ at 25 °C). In addition, the all-solid-state supercapacitor fabricated with a PGPE membrane and activated carbon electrodes shows outstanding electrochemical performance. The specific capacitance of the PGPE supercapacitor is 64.92 F g⁻¹ at 1 A g⁻¹, and the device shows a maximum energy density of 13.26 W h kg⁻¹ and a maximum power density of 2.26 kW kg⁻¹. After 10 000 cycles at a current density of 2 A g⁻¹, the all-solid-state supercapacitor with PGPE reveals a capacitance retention of 94.63%. Furthermore, the specific capacitance and charge–discharge behaviors of the flexible PGPE device hardly change with the bending states.

A simple polymerization process assisted with UV light for preparing a novel flexible polyelectrolyte-based gel polymer electrolyte (PGPE) is reported.     

Mo-doped ZnO nanoflakes on Ni-foam for asymmetric supercapacitor applications

A single-step hydrothermal route for synthesizing molybdenum doped zinc oxide nanoflakes was employed to accomplish superior electrochemical characteristics, such as a specific capacitance of 2296 F g⁻¹ at current density of 1 A g⁻¹ and negligible loss in specific capacitance of 0.01025 F g⁻¹ after each charge–discharge cycle (up to 8000 cycles). An assembled asymmetric supercapacitor (Mo:ZnO@NF//AC@NF) also exhibited a maximum energy density and power density of 39.06 W h/kg and 7425 W kg⁻¹, respectively. Furthermore, it demonstrated a specific capacitance of 123 F g⁻¹ at 1 A g⁻¹ and retained about 75.6% of its initial capacitance after 8000 cycles. These superior electrochemical characteristics indicate the potential of this supercapacitor for next-generation energy storage devices.

Mo:ZnO nanoflakes were synthesized by single-step hydrothermal route to achieve superior electrochemical performance.     

Ultra-thin NiS nanosheets as advanced electrode for high energy density supercapacitors

Low energy density of supercapacitors is one of the major downsides for their practical applications. Here, a simple hydrothermal method was developed to synthesize NiS nanosheets on Ni foam. NiS nanosheets with a rough surface promise large electroactive surface area for energy storage, and show an ultra-high capacitance of 2587 F g⁻¹ at a scan rate of 0.2 A g⁻¹ (corresponding to the discharge time of 5563 s). The NiS nanosheets also present an outstanding cycling stability of 95.8% after 4000 cycles. As a positive electrode material for hybrid supercapacitors (HSC), NiS nanostructures provide a broad voltage window of 1.7 V. Our device also shows a high energy density of 38 W h kg⁻¹ at a power density of 1.5 kW kg⁻¹.

The NiS electrodes show an ultra-high capacitance of 2587.4 F g⁻¹ and a high energy density of 38.2 W h kg⁻¹.     

“Water-in-salt” electrolyte enhanced high voltage aqueous supercapacitor with carbon electrodes derived from biomass waste-ground grain hulls

To design high specific surface area and optimize the pore size distribution of materials, we employ a combination of carbonization and KOH activation to prepare activated carbon derived from ground grain hulls. The resulting carbon material at lower temperature (800, BSAC-A-800) exhibits a porous structure with a high specific surface area of 1037.6 m² g⁻¹ and a pore volume of 0.57 m³ g⁻¹. Due to the synergistic structural characteristics, BSAC-A-800 reveals preferable capacitive performance, showing a specific capacitance as high as 313.3 F g⁻¹ at 0.5 A g⁻¹, good rate performance (above 73%), and particularly stable cycling performance (99.1% capacitance retention after 10 000 cycles at a current density of 10 A g⁻¹). More importantly, the assembled symmetric supercapacitor using a water-in-salt electrolyte (17 m NaClO₄) with high discharge specific capacitance (59 F g⁻¹ at 0.5 A g⁻¹), high energy density (47.2 W h kg⁻¹) and high voltage (2.4 V) represents significant progress towards performance comparable to that of commercial salt-in-water electrolyte supercapacitors (with discharge specific capacitance of 50 F g⁻¹, energy densities of ∼28.1 W h kg⁻¹ and voltages of 2.0 V).

To design high specific surface area and optimize the pore size distribution of materials, we employ a combination of carbonization and KOH activation to prepare activated carbon derived from ground grain hulls.     

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.