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.     

Template-free preparation of anthracite-based nitrogen-doped porous carbons for high-performance supercapacitors and efficient electrocatalysts for the oxygen reduction reaction

The conversion of coal into high-performance electrochemical energy materials, exemplified by electrodes and electrocatalysts for supercapacitors and fuel cells, is currently crucial to the advancement of high value-added, clean and non-fuel utilization of coal resources. In this work, anthracite-based nitrogen-doped porous carbon (ANPC) materials with well-defined pore architectures and adjustable nitrogen concentrations were prepared without any template: ANPC-1 by a one-step activation/doping process and ANPC-2 by a two-step process. The specific capacitance value of the ANPC-1 materials could attain a maximum of 346.0 F g⁻¹ at the current density of 0.5 A g⁻¹ in 6 M KOH. Supercapacitors composed of the ANPC-1 electrodes were able to achieve high energy densities up to 10.3 W h kg⁻¹ and 20.8 W h kg⁻¹, together with good charge/discharge stabilities of 95.4% and 91.3% after 5000 cycles, in KOH and Na₂SO₄ aqueous electrolytes, respectively. The ANPC-2 materials are more associated with the oxygen reduction reaction (ORR): one possessed a comparable ORR electrocatalytic activity to the commercial JM Pt/C (20% Pt) catalyst, and, moreover, its onset potential (0.96 V vs. RHE), half-wave potential (0.85 V vs. RHE), catalyst durability (95.9% activity retained after 40 000 s) and methanol tolerance were all superior to the benchmark electrocatalyst. This study provides a feasible route to rational design of coal-based multifunctional materials towards electrochemical energy storage and conversion.

Acting as supercapacitor electrodes and ORR electrocatalysts, anthracite-based nitrogen-doped porous carbons are prepared by one/two-step activation/doping processes.     

KOH activation of coal-derived microporous carbons for oxygen reduction and supercapacitors

Due to the dilemma of rapid consumption of fossil fuels and environmental pollution, development of clean, efficient and renewable energy conversion and storage technology has become an urgent need. Supercapacitors and hydrogen–oxygen fuel cells as typical representatives have become the focus of scientific research, in which the electrode materials are of much importance to their improved activity. In this work, a series of porous carbons (PCs) with high specific surface areas were prepared using natural coals as carbon precursors coupled with KOH activation. The effects of the mass ratio of coal and KOH as well as different activation temperatures on the microstructures of the PCs and electrochemical properties were studied in detail. The optimal PC4 (KOH: coal = 4) possessed a high specific surface area (SSA) of 2092 m² g⁻¹ and a well-developed microporous structure. As the electrocatalyst, it exhibited a positive onset potential of 0.88 V (vs. reversible hydrogen electrode (RHE)) and half-wave potential of 0.78 V (vs. RHE) towards the oxygen reduction reaction (ORR) in an alkaline solution. PC4 also showed the highest specific capacitance of 128 F g⁻¹ at a current density of 0.5 A g⁻¹ among all the samples in this work. The relatively good performance of PC4 resulted from its well-developed microporous structure and large SSA, enabling fast mass transfer of electrolytes.

Porous carbons (PCs) with high specific surface areas were prepared using natural coal as precursor coupled with KOH activation, showing great potential as electrode materials for oxygen reduction and supercapacitors.     

Flexible polypyrrolone-based microporous carbon nanofibers for high-performance supercapacitors

Flexible materials have drawn considerable attention due to the demand for wearable and flexible electronic products. Seeking new kinds of precursors for preparing carbon nanofibers with good flexibility for high-performance supercapacitors is a hot issue. In this work, a flexible polypyrrolone (BBB)/polyimide (PI) composite-based carbon nanofiber membrane (PBPICF) is prepared by a facile electrospinning and carbonization process. The PBPICF membranes exhibit a three-dimensional (3D) porous, fluffy and self-standing structure with good mechanical performance and flexibility, and can be arbitrarily bent and folded. PBPICF-65-35 (consisting of BBB (65 wt%) and PI (35 wt%)) exhibits a high specific capacitance of 172.44 F g⁻¹ in 6 M KOH aqueous solution, which is two-fold more than that of commercial polyacrylonitrile-based carbon nanofibers. In addition, PBPICF-65-35 also displays good power density (90 W kg⁻¹) and energy density (19.4 W h kg⁻¹), and the capacitance remains at 96% even after 10 000 cycles at 1.0 A g⁻¹. Therefore, the simple preparation and good capacitance performance of PBPICFs make them a promising binder-free electrode for wearable supercapacitors.

A flexible polypyrrolone (BBB)/polyimide (PI) composite-based carbon nanofiber membrane (PBPICF) has been prepared by a facile electrospinning and carbonization process.     

Three-dimensional tellurium and nitrogen Co-doped mesoporous carbons for high performance supercapacitors

Tellurium-doped mesoporous carbon composite materials (Te/NMC) have been prepared by a facile intercalation method in the presence of nitrogen-doped mesoporous carbon (NMC) with tellurium powder, for the first time. The effects of the co-doped N and Te in the mesoporous carbon matrix on the physical/chemical properties and capacitance performances were investigated via the use of various characterization methods and electrochemical studies. The as-prepared NMC and Te/NMC materials were found to mainly be composed of mesopores and maintained the 3D hierarchical graphite-like structure with lots of defect sites. By intercalation of Te atoms into the NMC materials, 2.12 at% (atom%) of Te was doped into NMC and the specific surface area of Te/NMC (261.07 m² g⁻¹) decreased by about 1.5 times compared to that of NMC (437.96 m² g⁻¹). In electrochemical measurements as a supercapacitor (SC) electrode, the Te/NMC based electrode, even with its lower porosity parameters, exhibited a higher capacitive performance compared to the NMC-based electrode. These results for Te/NMC arise due to the pseudo-capacitive effect of doped Te and the increase in the capacitive area available from the formation of interconnections in the mesoporous carbons through Te–O bonds. As a result, the synergetic effect of the Te and N atoms enables Te/NMC to exhibit the highest specific capacitance of 197 F g⁻¹ at a current density of 0.5 A g⁻¹. Moreover, remarkable long-term cycling stability with the retention of more than 95% of the initial capacitance is observed for Te/NMC at a current density of 5 A g⁻¹ and also for 1000 charge–discharge cycles.

The incorporation of the Te element into nitrogen-doped carbon-based nanomaterials is a good strategy to improve the capacitive performance of carbon materials and the incorporation of two types of atoms improves the overall capacitive performance of the materials due to a synergetic effect.     

Hierarchical hollow tubular fibrous brucite-templated carbons obtained by KOH activation for supercapacitors

Hierarchical hollow tubular porous carbons have been widely used in applications of supercapacitors, batteries, CO₂ capture and catalysis due to their hollow tubular morphology, large aspect ratio, abundant pore structure and superior conductivity. Herein, hierarchical hollow tubular fibrous brucite-templated carbons (AHTFBC_s) were prepared using natural mineral fiber brucite as the template and KOH as the chemical activator. The effects of different KOH additions on the pore structure and capacitive performance of AHTFBC_s were systematically studied. The specific surface area and micropore content of AHTFBC_s after KOH activation were higher than those of HTFBC. The specific surface area of the HTFBC is 400 m² g⁻¹, while the activated AHTFBC₅ has a specific surface area of up to 625 m² g⁻¹. In particular, compared with HTFBC (6.1%), a series of AHTFBC_s (22.1% for AHTFBC₂, 23.9% for AHTFBC₃, 26.8% for AHTFBC₄ and 22.9% for AHTFBC₅) with significantly increased micropore content were prepared by controlling the amount of KOH added. The AHTFBC₄ electrode displays a high capacitance of 197 F g⁻¹ at 1 A g⁻¹ and the capacitance retention of 100% after 10 000 cycles at 5 A g⁻¹ in the three-electrode system. And an AHTFBC₄//AHTFBC₄ symmetric supercapacitor exhibits the capacitance of 109 F g⁻¹ at 1 A g⁻¹ in 6 M KOH and an energy density of 5.8 W h kg⁻¹ at 199.0 W kg⁻¹ in 1 M Na₂SO₄ electrolyte. In addition, the capacity retention of AHTFBC₄ in the symmetric supercapacitor was maintained at 92% after 5000 cycles in both 6 M KOH and 1 M Na₂SO₄ electrolytes.

Hierarchical hollow tubular fibrous brucite-templated carbons (AHTFBCs) were prepared using natural mineral fiber brucite as the template and KOH as the chemical activator.     

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.     

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.     

Black titania nanotubes/spongy graphene nanocomposites for high-performance supercapacitors

A simple method is demonstrated to prepare functionalized spongy graphene/hydrogenated titanium dioxide (FG-HTiO₂) nanocomposites as interconnected, porous 3-dimensional (3D) network crinkly sheets. Such a 3D network structure provides better contact at the electrode/electrolyte interface and facilitates the charge transfer kinetics. The fabricated FG-HTiO₂ was characterized by X-ray diffraction (XRD), FTIR, scanning electron microscopy (FESEM), Raman spectroscopy, thermogravimetric analysis (TGA), UV-Vis absorption spectroscopy, and transmission electron microscopy (TEM). The synthesized materials have been evaluated as supercapacitor materials in 0.5 M H₂SO₄ using cyclic voltammetry (CV) at different potential scan rates, and galvanostatic charge/discharge tests at different current densities. The FG-HTiO₂ electrodes showed a maximum specific capacitance of 401 F g⁻¹ at a scan rate of 1 mV s⁻¹ and exhibited excellent cycling retention of 102% after 1000 cycles at 100 mV s⁻¹. The energy density was 78.66 W h kg⁻¹ with a power density of 466.9 W kg⁻¹ at 0.8 A g⁻¹. The improved supercapacitor performance could be attributed to the spongy graphene structure, adenine functionalization, and hydrogenated titanium dioxide.

A simple method is demonstrated to prepare functionalized spongy graphene/hydrogenated titanium dioxide (FG-HTiO₂) nanocomposites as interconnected, porous 3-dimensional (3D) network crinkly sheets.     

Nitrogen-doped hollow carbon spheres with tunable shell thickness for high-performance supercapacitors

Nitrogen-doped hollow carbon spheres (NHCSs) are well prepared by using Cu₂O microspheres as a hard template and 3-aminophenol formaldehyde resin polymer as carbon and nitrogen precursors. The thickness of the carbon shell can be easily controlled in the range of 15–84 nm by simply adjusting the weight ratios of the precursors to Cu₂O microspheres, and the Cu₂O templates can also be further reused. Physicochemical characterization demonstrates that the obtained NHCSs possess a well-developed hollow spherical structure, thin carbon shell and high nitrogen doping content. Due to these characteristics, when further utilized as electrodes for supercapacitors, the NHCSs with the carbon shell thickness of 15 nm show a high capacitance of 263.6 F g⁻¹ at 0.5 A g⁻¹, an outstanding rate performance of 122 F g⁻¹ at 20 A g⁻¹ and an excellent long-term cycling stability with only 9.8% loss after 1000 cycles at 5 A g⁻¹ in 6 M KOH aqueous electrolyte. This finding may push forward the development of carbon materials, exhibiting huge potential for electrochemical energy storage applications.

Nitrogen-doped hollow carbon spheres (NHCSs) are well prepared by using Cu₂O microspheres as a hard template and 3-aminophenol formaldehyde resin polymer as carbon and nitrogen precursors.     

Multilayer carbon materials prepared from Zanthoxylum schinifolium husk for high-performance supercapacitors

In this work, porous carbon mixed with nitrogen (NC) prepared using Zanthoxylum schinifolium husk as a precursor has been successfully applied in a supercapacitor (SC). The effects of KOH dosage on the structure, composition and capacitive properties of the carbon were investigated by a variety of techniques (SEM, HRTEM, XRD, Raman spectroscopy, XPS, BET, and electrochemical tests). The results of physical characterizations also confirmed that NC had a high specific surface area, abundant pores and a large number of heteroatomic functional groups. Meanwhile, the sample exhibits the best electrochemical performance in a 6 M KOH electrolyte, including high specific capacitance (333.7 F g⁻¹ at a current density of 0.5 A g⁻¹), desirable rate capability and superior cycling stability (97.9% capacitance retention after 5000 cycles). More importantly, the assembled symmetrical supercapacitor (NC-3//NC-3) holds superior energy density (16.7 W h kg⁻¹ at a power density of 300.6 W kg⁻¹) and good cycling stability (98.5% specific capacitance retention after 5000 cycles).

By choosing Zanthoxylum schinifolium husk as a new precursor, successfully prepared nitrogen self-doped layered porous carbon, which exhibited excellent electrochemical performance.     

Porous g-C3N4 covered MOF-derived nanocarbon materials for high-performance supercapacitors

Supercapacitors with high power density and long cycle life have shown great potential in energy storage supply for modern electronic devices. Among the component parts of supercapacitors, electrode materials with high electrical conductivity, large surface area and porosity are critical to the energy storage performances of devices. Here, we report a porous g-C₃N₄ covered MOF-derived nanocarbon material with large specific surface area and high nitrogen doping level as an electrode material for supercapacitors. The large surface area provides high capacity for ion accommodation during electrochemical processes, and the high nitrogen doping facilitates electron and ion transport with extra pseudocapacitance. The supercapacitor based on the as-synthesized material delivers a high specific capacity of 106 F g⁻¹ at current density of 1 A g⁻¹ as well as good rate capability. Furthermore, the device presents good cycling stability with capacitance retention of 91% even after 10 000 cycles at 1 A g⁻¹ under 0.8 V. This study presents a new insight into the design of nanocomposite materials with high energy storage capability and will accelerate the practical application of supercapacitors in future.

Here, we report a porous g-C₃N₄ covered MOF-derived nanocarbon material with large surface area and high nitrogen doping for supercapacitors.     

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.     

Insights into the estimation of capacitance for carbon-based supercapacitors

Carbon-based materials are broadly used as the active component of electric double layer capacitors (EDLCs) in energy storage systems with a high power density. Most of the reported computational studies have investigated the electrochemical properties under equilibrium conditions, limiting the direct and practical use of the results to design electrochemical energy systems. In the present study, for the first time, the experimental data from more than 300 published papers have been extracted and then analyzed through an optimized support vector machine (SVM) by a grey wolf optimization (GWO) algorithm to obtain a correlation between carbon-based structural features and EDLC performance. Several structural features, including calculated pore size, specific surface area, N-doping level, I_D/I_G ratio, and applied potential window were selected as the input variables to determine their impact on the respective capacitances. Sensitivity analysis, which has only been performed in this study for approximating the EDLC capacitance, indicated that the specific surface area of the carbon-based supercapacitors is of the greatest effect on the corresponding capacitance. The proposed SVM-GWO, with an R² value of 0.92, showed more accuracy than all the other proposed machine learning (ML) models employed for this purpose.

Carbon-based materials are broadly used as the active component of electric double layer capacitors (EDLCs) in energy storage systems with a high power density.     

Iron phosphide anchored nanoporous carbon as an efficient electrode for supercapacitors and the oxygen reduction reaction

Inspired by their distinctive properties, transition metal phosphides have gained immense attention as promising electrode materials for energy storage and conversion applications. The introduction of a safe and large-scale method of synthesizing a composite of these materials with carbon is of great significance in the fields of electrochemical and materials sciences. In the current effort, we successfully synthesize an iron phosphide/carbon (FeP/C) with a high specific surface area by the pyrolysis of the gel resulting from the hydrothermal treatment of an iron nitrate–phytic acid mixed solution. In comparison with the blank (P/C), the as-synthesized FeP/C appears to be an efficient electrode material for supercapacitor as well as oxygen reduction reaction (ORR) applications in an alkaline medium in a three-electrode system. In the study of supercapacitors, FeP/C shows areal capacitance of 313 mF cm⁻² at 1.2 mA cm⁻² while retaining 95% of its initial capacitance value after 10 000 cycles, while in the ORR, the synthesized material exhibits high electrocatalytic activity with an onset potential of ca. 0.86 V vs. RHE through the preferred four-electron pathway and less than 6% H₂O₂ production calculated in the potential range of 0.0–0.7 V vs. RHE. The stability is found to be better than those of the benchmark Pt/C (20 wt%) catalyst.

Synthesis of a nanoporous FeP/C material through a two-step method involving hydrothermal and carbonization processes for supercapacitors and the oxygen reduction reaction.     

Promising activated carbon derived from sugarcane tip as electrode material for high-performance supercapacitors

We present a simple, low-cost method for producing activated-carbon materials from sugarcane tips (ST) via two-step pre-carbonization and KOH activation treatment. After optimizing the amount of KOH, the resulting ST-derived activated carbon prepared with a KOH to PC-ST mass ratio of 2 (ACST-2) contained 17.04 wt% oxygen and had a large surface area of 1206.85 m² g⁻¹, which could be attributed to the large number of micropores in ACST-2. In a three-electrode system, the ACST-2 electrode exhibited a high specific capacitance of 259 F g⁻¹ at 0.5 A g⁻¹ and good rate capability with 82.66% retention from 0.5 to 10 A g⁻¹. In addition, it displayed a high capacitance retention of 89.6% after 5000 cycles at a current density of 3 A g⁻¹, demonstrating excellent cycling stability. Furthermore, the ACST-2//ACST-2 symmetric supercapacitor could realize a high specific energy density of 7.93 W h kg⁻¹ at a specific power density of 100 W kg⁻¹ in 6 M KOH electrolyte. These results demonstrate that sugarcane tips, which are inexpensive and easily accessible agricultural waste, can be used to create a novel biomass precursor for the production of low-cost activated carbon materials for high-performance supercapacitors.

The aim of this study is to produce activated-carbon materials from sugarcane tips (ST) via two-step pre-carbonization and evaluate the electrochemical performance.     

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.     

N/O co-enriched graphene hydrogels as high-performance electrodes for aqueous symmetric supercapacitors

In this study, an easy one-pot hydrothermal strategy was used to prepare N/O co-enriched graphene hydrogels (NOGHs) using graphene oxide (GO) solution and n-propylamine as a reactant. The n-propylamine can be used not only as a reductant, nitrogen dopant and structure regulator, but also as a spacer to inhibit the agglomeration of graphene sheets. Benefiting from the synergistic effect between the heteroatoms (N, O), 3D porous structures and high specific surface area, the as-prepared NOGH samples present excellent electrochemical properties. Remarkably, the NOGH-140 based binder-free symmetric supercapacitor shows a high specific capacitance of 268.1 F g⁻¹ at the current density of 0.3 A g⁻¹ and retains 222.5 F g⁻¹ (82.9% of its initial value) at 10.0 A g⁻¹ in 6 M KOH electrolyte. Furthermore, the assembled device also displays a notable energy density (9.3 W h kg⁻¹) and outstanding cycling performance (1.8% increase of its initial specific capacitance after 10 000 cycles at 10 A g⁻¹). The simple preparation method and excellent electrochemical properties indicate that NOGHs can be used as electrode materials for commercial supercapacitors.

N/O co-enriched graphene hydrogel based supercapacitors were assembled. Owing to the synergistic effect between the heteroatoms (N, O), 3D porous structures and high specific surface area, they present excellent electrochemical properties.