An asymmetric supercapacitor based on controllable WO3 nanorod bundle and alfalfa-derived porous carbon

A novel asymmetric supercapacitor (ASC) is assembled on the basis of an inerratic hexagonal-like WO₃ nanorod bundle as a negative electrode and graphene-like alfalfa-derived porous activated carbon (APAC) as the positive electrode in 1 M H₂SO₄ aqueous electrolyte. The WO₃ nanostructures prepared at pH of 1.6, 1.8, 2.0, 2.5 and 3.0 display hexagonal disc-like, nanorod bundle, inerratic hexagonal-like, sphere-like, and needle-shaped nanorod morphology. WO₃-2.0, which was prepared at a pH of 2.0, exhibits high specific capacitance (415.3 F g⁻¹ at 0.5 A g⁻¹). APAC-2, which had the mass ratios of dried alfalfa and ZnCl₂ as 1 : 2, showed a 3D porous structure, large surface area (1576.3 m² g⁻¹), high specific capacitance (262.1 F g⁻¹ at 0.5 A g⁻¹), good cycling stability with 96% of initial specific capacitance after 5000 consecutive cycles. The ASC assembled with WO₃-2.0 and APAC-2 exhibits high energy density (27.3 W h kg⁻¹ at a power density of 403.1 W kg⁻¹), as well as good electrochemical stability (82.6% capacitance retention after 5000 cycles). Such outstanding electrochemical behavior implies that the electrode materials are promising for practical energy-storage systems.

A asymmetric supercapacitor is assembled on the basis of an inerratic hexagonal-like WO₃ nanorod bundle as a negative electrode and graphene-like alfalfa-derived porous activated carbon as the positive electrode in 1 M H₂SO₄ aqueous 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.     

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.     

All-solid-state supercapacitors using a highly-conductive neutral gum electrolyte

Recently, the development of safe, stable, and long-life supercapacitors has attracted considerable interest driven by the fast-growth of flexible wearable devices. Herein, we report an MnO₂-based symmetric all-solid-state supercapacitor, using a neutral gum electrolyte that was prepared by embedding aqueous sodium sulfate solution in a biopolymer xanthan gum. Resulting from the high ion conductivity 1.12 S m⁻¹, good water retention, and high structure adaption of such gum electrolyte, the presently described supercapacitor showed high electrochemical performance with a specific capacitance of 347 F g⁻¹ at 1 A g⁻¹ and an energy density of 24 μW h cm⁻² The flexible supercapacitor possesses excellent reliability and achieves a retaining capacitance of 82% after 5000 cycles. In addition, the as-prepared supercapacitor demonstrated outstanding electrochemical stability at temperatures between −15 °C to 100 °C.

A highly-conductive neutral gum electrolyte was used for all-solid-state supercapacitors with outstanding electrochemical performance at temperatures between −15 °C to 100 °C.     

An ultrasound-assisted approach to bio-derived nanoporous carbons: disclosing a linear relationship between effective micropores and capacitance

Ultrasound irradiation is a technique that can induce acoustic cavitation in liquids, leading to a highly interactive mixture of reactants. In pursuit of high-performance and cost-effective supercapacitor electrodes, pore size distributions of carbonaceous materials should be carefully designed. Herein, fruit skins (mango, pitaya and watermelon) are employed as carbon precursors to prepare nanoporous carbons by the ultrasound-assisted method. Large BET specific surface areas of the as-prepared carbons (2700–3000 m² g⁻¹) are reproducible with pore diameters being concentrated at about 0.8 nm. Among a suite of the bio-derived nanoporous carbons, one reaches a maximum specific capacitance of up to 493 F g⁻¹ (at 0.5 A g⁻¹ in 6 M KOH) in the three-electrode system and achieves high energy densities of 27.5 W h kg⁻¹ (at 180 W kg⁻¹ in 1 M Na₂SO₄) and 10.9 W h kg⁻¹ (at 100 W kg⁻¹ in 6 M KOH) in the two-electrode system. After 5000 continuous charge/discharge cycles, the capacitances maintain 108% in 1 M Na₂SO₄ and 98% in 6 M KOH, exhibiting long working stability. Moreover, such high capacitive performance can be attributed to the optimization of surface areas and pore volumes of the effective micropores (referred to as 0.7–2 nm sized pores). Notably, specific capacitances have been found linearly correlated with surface areas and pore volumes of the effective micropores rather than those of any other sized pore (i.e., <0.7, 2–50 and 0.5–50 nm). Consequently, the fit of electrolyte ions into micropore frameworks should be an important consideration for the rational design of nanopore structures in terms of supercapacitor electrodes.

There is a linear relationship between the effective micropore volume (surface area) and the specific capacitance of bio-derived nanoporous carbons, regardless of biomass type and activation temperature employed.     

Onion-derived activated carbons with enhanced surface area for improved hydrogen storage and electrochemical energy application

High surface area activated carbons (ACs) were prepared from a hydrochar derived from waste onion peels. The resulting ACs had a unique graphene-like nanosheet morphology. The presence of N (0.7%) and O content (8.1%) in the OPAC-800 °C was indicative of in situ incorporation of nitrogen groups from the onion peels. The specific surface area and pore volume of the best OPAC sample was found to be 3150 m² g⁻¹ and 1.64 cm³ g⁻¹, respectively. The hydrogen uptake of all the OPAC samples was established to be above 3 wt% (at 77 K and 1 bar) with the highest being 3.67 wt% (800 °C). Additionally, the OPAC-800 °C achieved a specific capacitance of 169 F g⁻¹ at a specific current of 0.5 A g⁻¹ and retained a capacitance of 149 F g⁻¹ at 5 A g⁻¹ in a three electrode system using 3 M KNO₃. A symmetric supercapacitor based on the OPAC-800 °C//OPAC-800 °C electrode provided a capacitance of 158 F g⁻¹ at 0.5 A g⁻¹. The maximum specific energy and power was found to be 14 W h kg⁻¹ and 400 W kg⁻¹, respectively. Moreover, the device exhibited a high coulombic efficiency of 99.85% at 5 A g⁻¹ after 10 000 cycles. The results suggested that the high surface area graphene-like carbon nanostructures are excellent materials for enhanced hydrogen storage and supercapacitor applications.

Graphene-like activated carbons (ACs), with excellent properties for enhanced hydrogen storage and supercapacitor applications, were prepared from waste onion peels.     

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.     

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.     

Rapid microwave activation of waste palm into hierarchical porous carbons for supercapacitors using biochars from different carbonization temperatures as catalysts

A rapid, simple and cost-effective approach to prepare hierarchical porous carbons (PCs) for supercapacitors is reported by microwave activation of abundant and low-cost waste palm, biochar (BC) and KOH. BCs from waste palm at different carbonization temperatures (300–700 °C), as catalysts and microwave receptors, were used here for the first time to facilitate the conversion of waste palm into hierarchical PCs. As a result, the high-graphitization PC obtained at a BC carbonization temperature of 300 °C (PC-300) possessed a high surface area (1755 m² g⁻¹), a high pore volume (0.942 cm³ g⁻¹) and a moderate mesoporosity (37.79%). Besides their high-graphitization and hierarchical porous structure, the oxygen doping in PC-300 can also promote the rapid transport of electrolyte ions. The symmetric supercapacitor based on the PC-300 even in PVA/LiCl gel electrolyte exhibited a high specific capacitance of 164.8 F g⁻¹ at a current density of 0.5 A g⁻¹ and retained a specific capacitance of 121.3 F g⁻¹ at 10 A g⁻¹, demonstrating a superior rate capacity of 73.6%. Additionally, the PC-300 supercapacitor delivered a high energy density of 14.6 W h kg⁻¹ at a power density of 398.9 W kg⁻¹ and maintained an energy density of 10.8 W h kg⁻¹ at a high power density of 8016.5 W kg⁻¹, as well as an excellent cycling stability after 2000 cycles with a capacitance retention of 92.06%.

A rapid, simple and cost-effective approach to prepare hierarchical porous carbons (PCs) for supercapacitors is reported by microwave activation of abundant and low-cost waste palm, biochar (BC) and KOH.     

Facile synthesis of bio-based nitrogen- and oxygen-doped porous carbon derived from cotton for supercapacitors

Biomass-derived O- and N-doped porous carbon has become the most competitive supercapacitor electrode material because of its renewability and sustainability. We herein presented a facile approach to prepare O/N-doped porous carbon with cotton as the starting material. Absorbent cotton immersed in diammonium hydrogen phosphate (DAP) was activated at 800 °C (CDAP800s) and then was oxidized in a temperature range of 300–400 °C. The electrochemical capacitance of the impregnated cotton was significantly improved by doping with O and N, and the yield was improved from 13% to 38%. The sample oxidation at 350 °C (CDAP800-350) demonstrated superior electrical properties. CDAP800-350 showed the highest BET surface area (1022 m² g⁻¹) and a relatively high pore volume (0.53 cm³ g⁻¹). In a three-electrode system, the CDAP800-350 electrodes had high specific capacitances of 292 F g⁻¹ in 6 M KOH electrolyte at a current density of 0.5 A g⁻¹. In the two-electrode system, CDAP800-350 electrode displayed a specific capacitance of 270 F g⁻¹ at 0.5 A g⁻¹ and 212 F g⁻¹ at 10 A in KOH electrolyte. In addition, the CDAP800-350-based symmetric supercapacitor achieved a high stability with 87% of capacitance retained after 5000 cycles at 5 A g⁻¹, as well as a high volumetric energy density (18 W h kg⁻¹ at 250 W kg⁻¹).

Biomass-derived O- and N-doped porous carbon has become one of the most competitive supercapacitor electrode material because of its renewability and sustainability.     

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.     

All-solid-state flexible supercapacitor based on nanotube-reinforced polypyrrole hollowed structures

Supercapacitors are strong future candidates for energy storage devices owing to their high power density, fast charge–discharge rate, and long cycle stability. Here, a flexible supercapacitor with a large specific capacitance of 443 F g⁻¹ at a scan rate of 2 mV s⁻¹ is demonstrated using nanotube-reinforced polypyrrole nanowires with hollowed cavities grown vertically on a nanotube/graphene based film. Using these electrodes, we obtain improved capacitance, rate capability, and cycle stability for over 3000 cycles. The assembled all-solid-state supercapacitor exhibits excellent mechanical flexibility, with the capacity to endure a 180° bending angle along with a maximum specific and volumetric energy density of 7 W h kg⁻¹ (8.2 mW h cm⁻³) at a power density of 75 W kg⁻¹ (0.087 W cm⁻³), and it showed an energy density of 4.13 W h kg⁻¹ (4.82 mW h cm⁻³) even at a high power density of 3.8 kW kg⁻¹ (4.4 W cm⁻³). Also, it demonstrates a high cycling stability of 94.3% after 10 000 charge/discharge cycles at a current density of 10 A g⁻¹. Finally, a foldable all-solid-state supercapacitor is demonstrated, which confirms the applicability of the reported supercapacitor for use in energy storage devices for future portable, foldable, or wearable electronics.

Nanotube-reinforced polypyrrole nanowires with hollowed cavities allow the fabrication of a flexible supercapacitor with a large specific capacitance.     

“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.     

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.     

Facile preparation of 3D porous agar-based heteroatom-doped carbon aerogels for high-energy density supercapacitors

The fabrication of heteroatom-doped porous carbon materials with high electrical conductivity and large specific surface area via an environmentally friendly route is critical and challenging. Herein, nitrogen and oxygen co-doped agar porous carbon (APC) was developed for supercapacitors via a one-step carbonization method with agar as the raw material and ammonia as the activator and nitrogen source. APC outperformed pectin porous carbon, tamarind porous carbon, and the previously reported carbon-based supercapacitors with a high capacitance retention of 72% even from 0.5 A g⁻¹ to 20 A g⁻¹ and excellent cycling stability in 6 M KOH solution (retained after 10 000 cycles) with a rate of over 98.5%. Furthermore, the APC electrode-based symmetric device exhibited an impressive energy density of 20.4 W h kg⁻¹ and an ultra-high power density of 449 W kg⁻¹ in 1 M Na₂SO₄ electrolyte together with excellent cycling stability (103.2% primary capacitance retentivity after 10 000 cycles). This study offers a novel method for the synthesis of nitrogen heteroatom-doped hierarchical porous carbon materials for performance-enhanced energy storage devices.

N and O co-doped agar porous carbon (APC) as electrode materials exhibit excellent performance. A respectable energy density of 20.4 W h kg⁻¹ and an ultra-high power density of 449 W kg⁻¹, as well as excellent cycle stability in 1 M Na₂SO₄ electrolyte.     

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.     

Synthesis,characterizations and electrochemical performances of anhydrous CoC2O4 nanorods for pseudocapacitive energy storage applications

To overcome the environmental challenges caused by utilization of fossil fuel based energy technologies and to utilize the full potential of renewable energy sources such as solar, wind and tidal, high power and high energy density containing large scale electrochemical energy storage devices are a matter of concern and a need of the hour. Pseudocapacitors with accessibility to multiple oxidation states for redox charge transfer can achieve a higher degree of energy storage density compared to electric double layer capacitors (EDLC) and the hybrid supercapacitor is one of the prominent electrochemical capacitors that can resolve the low energy density issues associated with EDLCs. Due to its open pore framework structure with superior structural stability and accessibility of Co^2+/3+/4 redox states, porous anhydrous CoC₂O₄ nanorods are envisaged here as a potential energy storage electrode in a pseudo-capacitive mode. Superior specific capacitance equivalent to 2116 F g⁻¹ at 1 A g⁻¹ in the potential window of 0.3 V was observed for anhydrous CoC₂O₄ nanorods in aqueous 2 M KOH electrolyte. A predominant pseudo-capacitive mechanism seems to be operative behind the high charge storage at electrodes as intercalative (Inner) and surface (outer) charge storage contributions were found to be 75% and 25% respectively. Further, in full cell asymmetric supercapacitor (ASC) mode in which porous anhydrous CoC₂O₄ nanorods were used as positive electrodes and activated carbon (AC) was utilised as negative electrodes within an operating potential window of 1.3 V, a highest specific energy of W h kg⁻¹ and specific power of ∼647 W kg⁻¹ at 0.5 A g⁻¹ current density were obtained with superior cycling stability. High cycling stability coupled with superior electrochemical storage properties make anhydrous CoC₂O₄ nanorods potential pseudo-capacitive electrodes for large scale energy storage applications.

With active participation of Co^2+/3+ redox couples in an oxalate framework, Anhydrous CoC₂O₄ nanorods display a capacitance equivalent to 2116 F g⁻¹ at 1 A g⁻¹ current rate in the potential window of 0.3 V in aqueous 2 M KOH electrolyte.     

Fe3O4 hard templating to assemble highly wrinkled graphene sheets into hierarchical porous film for compact capacitive energy storage

Highly wrinkled graphene film (HWGF) with high packing density was synthesized by combining an electrostatically self-assembling process, a vacuum filtration-induced film assembling process and capillary compression. Fe₃O₄ nanoparticles were used as a low-cost and environment-friendly hard template. Hierarchical porosity and high packing density were achieved with the aid of capillary compression in the presence of Fe₃O₄ nanoparticles. This strategy enables integration of highly wrinkled graphene sheets to form highly compact carbon electrodes with a continuous ion transport network. The generated HWGF exhibited a high packing density of 1.53 g cm⁻³, a high specific surface area of 383 m² g⁻¹ and a hierarchically porous structure. The HWGF delivered a high capacitance of 242 F g⁻¹ and 370 F cm⁻³ at 0.2 A g⁻¹ in 6 M KOH aqueous electrolyte system with excellent rate capability (202 F g⁻¹ and 309 F cm⁻³ retained at 20 A g⁻¹). The capacity retention rate reached 97% after 10 000 cycles at 1 A g⁻¹. The HWGF-based supercapacitor exhibited a high energy density of 17 W h kg⁻¹ at the power density of 49 W kg⁻¹. Such high capacitive performances could be attributed to the highly dense but porous graphene assemblies composed of highly wrinkled graphene sheets.

A facile strategy was reported to synthesize highly wrinkled graphene film, exhibiting high packing density and excellent capacitive performance.     

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.     

Dual-doped hierarchical porous carbon derived from biomass for advanced supercapacitors and lithium ion batteries

Nowadays, designing heteroatom-doped porous carbons from inexpensive biomass raw materials is a very attractive topic. Herein, we propose a simple approach to prepare heteroatom-doped porous carbons by using nettle leaves as the precursor and KOH as the activating agent. The nettle leaf derived porous carbons possess high specific surface area (up to 1951 m² g⁻¹), large total pore volume (up to 1.374 cm³ g⁻¹), and high content of nitrogen and oxygen heteroatom doping (up to 17.85 at% combined). The obtained carbon as an electrode for symmetric supercapacitors with an ionic liquid electrolyte can offer a superior specific capacitance of 163 F g⁻¹ at 0.5 A g⁻¹ with a capacitance retention ratio as high as 67.5% at 100 A g⁻¹, and a low capacitance loss of 8% after 10 000 cycles. Besides, the as-built supercapacitor demonstrates a high specific energy of 50 W h kg⁻¹ at a specific power of 372 W kg⁻¹, and maintains 21 W h kg⁻¹ at the high power of 40 kW kg⁻¹. Moreover, the resultant carbon as a Li-ion battery anode delivers a high reversible capacity of 1262 mA h g⁻¹ at 0.1 A g⁻¹ and 730 mA h g⁻¹ at 0.5 A g⁻¹, and maintains a high capacity of 439 mA h g⁻¹ after 500 cycles at 1 A g⁻¹. These results demonstrate that the nettle leaf derived porous carbons offer great potential as electrodes for advanced supercapacitors and lithium ion batteries.

Nettle leaf derived nitrogen and oxygen dual-doped porous carbons exhibit great potential as anodes for high performance supercapacitors and lithium ion batteries.