Hierarchical Activated Mesoporous Phenolic‐Resin‐Based Carbons for Supercapacitors

A series of hierarchical activated mesoporous carbons (AMCs) were prepared by the activation of highly ordered, body‐centered cubic mesoporous phenolic‐resin‐based carbon with KOH. The effect of the KOH/carbon‐weight ratio on the textural properties and capacitive performance of the AMCs was investigated in detail. An AMC prepared with a KOH/carbon‐weight ratio of 6:1 possessed the largest specific surface area (1118 m² g^?1), with retention of the ordered mesoporous structure, and exhibited the highest specific capacitance of 260 F g^?1 at a current density of 0.1 A g^?1 in 1 M H₂SO₄ aqueous electrolyte. This material also showed excellent rate capability (163 F g^?1 retained at 20 A g^?1) and good long‐term electrochemical stability. This superior capacitive performance could be attributed to a large specific surface area and an optimized micro‐mesopore structure, which not only increased the effective specific surface area for charge storage but also provided a favorable pathway for efficient ion transport.     

Mesopore-dominant porous carbon derived from bio-tars as an electrode material for high-performance supercapacitors

For the first time, toxic bio-tars collected from the gasification of pine sawdust are used as the precursor for activated carbons. Various types of activation agents including KOH, K₂CO₃, H₃PO₄ and ZnCl₂ were screened for obtaining superior activated carbons. When KOH was used as an activation agent, the obtained activated carbons exhibited high specific surface area and large mesopore volume. The activated carbons were further employed to be the electrode material of supercapacitors, and its specific capacitance reached up to 260 F g^?1 at 0.25 A g^?1 current density. Also, it showed an excellent rate performance from preserving a relatively high specific capacitance of 151 F g^?1 at 50 A g^?1. The assembled device also exhibited the good electrochemical stability with the capacity retention of 90% after 5000 cycles. Furthermore, the maximum energy density of the activated carbons in organic electrolyte reached 17.8 Wh kg^?1.     

Preparation of activated carbon from waste Camellia oleifera shell for supercapacitor application

The cost-effective activated carbons derived from waste Camellia oleifera shell (COS) by ZnCl₂ activation method are investigated as the active electrode material in electric double-layer capacitors (EDLCs) for the first time. The activation temperature and ZnCl₂/COS impregnation ratio are two key factors affecting the surface area and pore structure of the prepared activated carbons, which accordingly affect their capacitive performances. Electrochemical investigations indicate that the activated carbon (AC-3-600) obtained at the activation temperature of 600 °C and impregnation ratio of 3 shows the maximum specific capacitance of 374 and 266 F?g^?1 in 1 mol L^?1 H₂SO₄ and 6 mol L^?1 KOH electrolytes at 0.2 A g^?1, respectively. The high capacitance of the AC-3-600 is attributed to its high surface area (1,935 m² g^?1), high total pore volume (1.02 cm³ g^?1), and especially the large percentage of micropores (735 m² g^?1). Meanwhile, the activated carbon presents good cycle stability in both acid and alkaline electrolytes during 5,000 cycles at a fair current density of 4 A g^?1. So, we had reasons to believe that the activated carbons from waste COS by ZnCl₂ activation might be one of the innovative carbon electrode materials for EDLCs application.     

Highly stable performance of supercapacitors using microporous carbon derived from phenol–melamine–formaldehyde resin

A series of microporous carbons were prepared by simple carbonization and activation of phenol–melamine–formaldehyde resin. The morphology, surface area, and elemental composition of the samples were investigated by scanning electron microscope, Brunauer–Emmett–Teller measurement, Raman spectra, and elemental analysis, respectively. Electrochemical characteristics were evaluated by cyclic voltammograms, galvanostatic charge/discharge, and electrochemical impedance spectroscopy measurements in 6.0?mol?L^?1 KOH. The microporous carbon activated by KOH presented a high specific capacitance of 202?F?g^?1 at a scan rate of 2?mV?s^?1. Furthermore, the KOH-activated microporous carbon electrode exhibited durable operation, the total loss of capacitance after 20,000 cycles is 2% at a current density of 500?mA?g^?1. The good electrochemical performance of the activated carbon was ascribed to well-developed micropores, high surface area, larger pore volume as well as oxygen groups.     

Hierarchical mesoporous carbon materials: preparation by direct tri-constituent co-assembly and the electrochemical performance

Hierarchical mesoporous carbon materials with large microporosity were prepared by direct tri-constituent co-assembly with the use of resols as the carbon precursor, tetraethyl orthosilicate as the inorganic precursor, and triblock copolymer F127 as the soft template. Bimodal pore size distributions in the range of 1.5–4 and 7.5–12 nm were obtained in the synthesized hierarchical mesoporous carbon materials after etching of silica by HF acid, showing a high surface area of 1,675 m²?g^?1 with a large pore volume of 2.06 cm³?g^?1. The electrochemical performance of the hierarchical mesoporous carbons was evaluated as an electrode material for electrochemical supercapacitor, showing a specific capacitance as high as 152 F?g^?1 at a scan rate of 5 mV?s^?1 in 6 M KOH aqueous solution and a good cycling stability with capacitance retention of 99 % over 500 cycles.     

The production of activated carbon from cation exchange resin for high-performance supercapacitor

High-performance activated carbon for electrochemical double-layer capacitors (EDLC) has been prepared from cation exchange resin by carbonization and subsequent activation with KOH. The activation temperature has a key role in the determination of porous carbon possessing high surface areas, and large pore structures. The porous carbon activated at 700 °C (carbon-700-1:4) has high surface area (2236 m²?g^?1) and large total pore volume (1.15 cm³?g^?1), which also displays best capacitive performances due to its well-balanced micro- or mesoporosity distribution. In details, specific capacitances of the carbon-700-1:4 sample are 336.5 F?g^?1 at a current density of 1 A?g^?1 and 331.8 F?g^?1 at 2 A?g^?1. At high current density as 20 A?g^?1, the retention of its specific capacitance is 68.4 %. The carbon-700-1:4 sample also exhibits high performance of energy density (46.7 Wh?kg^?1) and long cycle stability (～8.9 % loss after 3,000 cycles). More importantly, due to the amount of waste ion-exchange resins increasing all over the world, the present synthetic method might be adopted to dispose them, producing high-performance porous carbons for EDLC electrode materials.     

Activated nitrogen-doped carbons from polyvinyl chloride for high-performance electrochemical capacitors

Activated nitrogen-doped carbons (ANCs) were prepared by carbonization/activation approach using aminated polyvinyl chloride (PVC) as precursor. ANCs exhibit larger porosities and higher specific surface areas than those of their nitrogen-free counterparts for the same KOH/carbon ratio. The specific surface area of ANC-1 is up to 1,398 m² g^?1 even at a low KOH/carbon ratio of 1:1. Fourier transform infrared spectroscopy investigation of the nitrogen-enriched resin precursor indicates the efficient dehydrochlorination of PVC by ethylenediamine at a low temperature. The nitrogen content and the population of nitrogen functionalities strongly depend on the KOH/carbon ratios and decrease drastically after KOH activation as seen from the elemental and X-ray photoelectron spectroscopy analysis. The surface concentration of N-6 and N-Q almost disappears and the dominant nitrogen groups become N-5 after KOH activation. The highest specific capacitance of ANCs is up to 345 F g^?1 at a current density of 50 mA g^?1 in 6 M KOH electrolyte. ANCs also exhibit a good capacitive behavior at a high scan rate of 200 mV s^?1 and an excellent cyclability with a capacitance retention ratio as high as ～93 % at a current density of 2,000 mA g^?1 for 5,000 cycles.     

Preparation and performance of polyvinyl alcohol-based activated carbon as electrode material in both aqueous and organic electrolytes

Porous carbon materials with high surface area and different pore structure have been successfully prepared by phenolic resin combined with polyvinyl alcohol (PVA) and KOH as activation agents. The surface morphology, structure, and specific surface area of the carbon materials were studied by scanning electron microscopy, X-ray diffraction, and nitrogen sorption measurement, respectively. Furthermore, the effects of specific surface area, pore structure, and electrolyte on electrochemical properties were investigated by galvanostatic charge–discharge measurement. The results show that KOH–PVA-activated carbon materials display specific capacitance as high as 218 F?g^?1 in 30 wt.% KOH aqueous electrolyte, 147 F?g^?1 in 1 M LiPF₆/(ethylene carbonate (EC) + dimethyl carbonate) (1:1?v/v), and 115 F?g^?1 in 1 M Et₃MeNBF₄/propylene carbonate organic electrolyte, respectively. In addition, the carbon materials demonstrate long-term cycle stability, especially the AK3P-0.30 in aqueous electrolyte and the AK2P-0.30 with excellent rate capability in organic electrolyte. These reveal that the existence of a micro-mesoporous structure of activated carbon is beneficial to store energy in an aqueous supercapacitor and broad pore size distribution of activated carbon is favorable to energy storage in an organic supercapacitor. The carbon materials with pore size distribution in different ranges improve the electrochemical performance of supercapacitor in different electrolytes. A new pore-expand agent (PVA combining with KOH) was used to obtain porous carbons with enhanced properties for supercapacitor.     

Effect of removing silica in rice husk for the preparation of activated carbon for supercapacitor applications

The significant influence of silica inside rice husk in the preparation and electrochemical performances of activated carbon are investigated. The removing of silica results in high mesoporous ratio and good rate capability.     

Partially graphitized ordered mesoporous carbons for high-rate supercapacitors

Partially graphitized ordered mesoporous carbons have been prepared with a soft template method using low-molecular-weight phenolic resol as a carbon source, triblock copolymer F127 as a template, and ferric citrate as a graphitization catalyst. N₂ sorption and transmission electron microscopy analysis show that the ordered mesoporous carbons have been partially graphitized when the carbonization temperature is above 700 °C. The graphitic ordered mesoporous carbons exhibit better rate performance than amorphous ordered mesoporous carbons. The specific capacitance of the graphitic ordered mesoporous carbons (GOMCs) prepared at 700 °C reaches to 112 F g^?1 at a scan rate of up to 1,000 mV s^?1. Its capacitance retention ratio is 64 %, which is much higher than that of the amorphous ordered mesoporous carbons prepared at 600 °C (33 %). High electronic conductivity and ordered mesoporous structure lead to the high electrochemical performance of the partially graphitized ordered mesoporous carbons.     

Supercapacitive behaviors of the nitrogen-enriched activated mesocarbon microbead in aqueous electrolytes

The activated nitrogen-enriched novel carbons (a-NENCs) have been prepared by direct carbonization of polyaniline/activated mesocarbon microbead composites and further activated by 16 M?HNO₃. The electrochemical performances and supercapacitive behaviors of the a-NENCs in 6 M KOH, 1 M?H₂SO₄, and 0.5 M?K₂SO₄ solutions are evaluated by cyclic voltammetry, galvanostatic charge/discharge, electrochemical impedance spectroscopy, cyclic life, leakage current, and self-discharge measurements. The results demonstrate that the supercapacitors perform definitely supercapacitive behaviors; especially in 6 M KOH electrolyte, the supercapacitor represents much better electrochemical performance with more excellent reversibility, shorter relaxation time of 1.11 s, and nearly ideal polarizability. The maximum specific capacitance of the supercapacitors using a-NENCs as active electrode material is 85.1 F?g^?1 at a rate of 500 mA?g^?1 in 6 M?KOH. These outcomes indicate that the 6 M?KOH aqueous solution is a promising electrolyte for the supercapacitor with a-NENCs as electrode material.     

Nitric acid oxidation of ordered mesoporous carbons for use in electrochemical supercapacitors

Ordered mesoporous carbon materials with high microporosity were synthesized by a low temperature autoclaving of citric acid-catalyzed polymerized resorcinol/formaldehyde in the presence of the triblock copolymer F127 and were activated by nitric acid oxidation. The materials were used as electrode materials in electrochemical supercapacitors. A bimodal pore size distribution of 2.1–2.3 and 5.3 nm with a surface area of 465–578 m² g^?1 and pore volume of 0.44–0.54 cm³ g^?1 was obtained with the retention of an ordered mesoporous structure after nitric acid (2 M) treatment. The introduced functional groups produced a pseudocapacitance, which resulted in an increase in the specific capacitance. The electrochemical capacitance of the resulting mesoporous carbons showed a marked increase after 3 h of nitric acid activation, exhibiting a high value of 295 F g^?1 at the scan rate of 10 mV s^?1 in 6 M KOH aqueous solution and good cycling stability with specific capacitance retention over 500 cycles.     

Facile synthesis of biomass-derived hierarchical porous carbon microbeads for supercapacitors

Using the facile method of solvent evaporation, the leonardite fulvic acids (LFA)-based porous carbon microbeads (PCM) have been successfully prepared at ambient pressure, followed by carbonization and KOH activation (a low mass ratio alkali/LFA = 1.5) in an inert atmosphere. The effects of KOH treatment on pore structures and the formation mechanism of the PCM were discussed. The results showed that the sample exhibited remarkable improvement in textural properties. The activated carbon microbeads had high surface area (2269 m² g^?1), large pore volume (1.97 cm³ g^?1), and displayed excellent capacitive performances, compared with carbon powder. The porous carbon material electrodes with the “porous core structure” behaved superiorly at a specific capacitance of 320 F g^?1 at a current density of 0.05 A g^?1 in 6 M KOH electrolyte, which could still remain 193 F g^?1 when the current density increased to 100 A g^?1. Remarkably, in the 1 M TEABF₄/PC electrolyte, the PCM samples could reach 156 F g^?1 at 0.05 A g^?1, possess an outstanding energy density of 39.50 Wh kg^?1, and maintain at 22.05 Wh kg^?1 even when the power density rose up to 5880 W kg^?1. The balance of structural characteristic and high performance makes the porous carbon microbeads a competitive and promising supercapacitor electrode material.     

The effect of activation technology on the electrochemical performance of calcium carbide skeleton carbon

Porous CaC₂-derived carbon (CCDC) was synthesized by one-step route from CaC₂ in a freshly prepared chlorine environment at lower temperature. As-prepared CCDC was activated by H₃PO₄, ZnCl₂, and KOH, respectively. The effects of the activation technology on the structure and morphology of CCDC were studied by X-ray diffraction, physical N₂ adsorption/desorption, and transmission electron microscopy. It has been found that the pore structure and specific surface area of CCDC are apparently improved after activation; the CCDC activated by KOH especially showed an excellent specific surface area of 1,100?m²?g^?1. The electrochemical performance of supercapacitors using activated CCDC as electrode active material was studied by cyclic voltammery, galvanostatic charge/discharge, and cycle life measurements. The results indicated that the CCDCs activated by H₃PO₄, ZnCl₂, and KOH revealed enhanced capacitances of 172.6, 198.1, and 250.1?F?g^?1 in 6?M KOH electrolyte, which were increased by 11.4, 27.8, and 61.2?% compared with the pristine CCDC (155?F?g^?1), respectively. Furthermore, the supercapacitors using all activated CCDCs as electrode active material exhibited excellent cycle stability, and the specific capacitance for all activated CCDC samples had nearly no change after 5,000 cycles.     

Improving the electrochemical performances of active carbon-based supercapacitors through the combination of introducing functional groups and using redox additive electrolyte

High-performance and low-cost electrochemical capacitors (ECs) are essential for large-scale applications in energy storage. In this work, the specific capacitance of active carbon (AC) electrode was significantly improved through the combination of introducing functional groups on the surface of AC and adding redox-active molecules (K₃Fe(CN)₆) into 2?M KOH aqueous electrolytes. The surface-oxygen functionalized AC (FAC) was synthesized using HNO₃ echoed as the electrode and 2?M KOH with 0.1?M K₃Fe(CN)₆ as the electrolyte. The surface functional groups of the AC not only contribute to the pseudocapacitance but also increase the active sites of the electrode/electrolyte interface, which enhances the electrochemical activity of the Fe(CN)₆^3?/Fe(CN)₆^4? redox pair, thus leading to high capacitance. In the redox electrolyte, the specific capacitance was much higher in 229.17?F?g^?1 (1?A?g^?1) achieved for those FAC than in raw AC (only 147.06?F?g^?1). Similarly, the FAC electrode suggested high energy density and extended cycling stability in the KOH?+?K₃Fe(CN)₆ electrolyte.     

Activated carbon prepared from polyaniline base by K2CO3 activation for application in supercapacitor electrodes

An activated carbon was prepared from a polyaniline base using K₂CO₃ as an activating agent. The morphology, surface chemical composition, and surface area of the as-prepared carbon materials were investigated by scanning electron microscope, X-ray photoelectron spectroscopy, and Brunauer?CEmmett?CTeller measurement, respectively. Electrochemical properties of the as-prepared sample were studied by cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy measurements in 6?mol?L^?1 KOH aqueous solution. Compared with the non-activated carbon, activated carbon showed superior capacitive performance. The activation carbon presented a high specific gravimetric capacitance of 210?F?g^?1. The good electrochemical performance of the activated carbon was ascribed to well-developed micropores, high surface area, the presence of nitrogen and oxygen functional groups, and larger pore volume.     

KOH‐Activated Porous Carbons Derived from Chestnut Shell with Superior Capacitive Performance

Porous carbons (PC) were prepared from a waste biomass named chestnut shell via a two‐step method involving carbonization and KOH activation. The morphology, pore structure and surface chemical properties were investigated by scanning electron microscopy (SEM), N₂ sorption, Raman spectroscopy, X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS). The carbons have been evaluated as the electrode materials for supercapacitors by a two‐electrode system in 6 mol/L KOH electrolyte. Benefiting from the porous texture, high surface area and high oxygen content, the PCs derived from chestnut shell have exhibited high specific capacitance of 198.2 (PC‐1), 217.2 (PC‐2) and 238.2 F·g^?1 (PC‐3) at a current density of 0.1 A·g^?1, good rate capability of 55.7%, 56.6% and 54.9% in a range of 0.1–20 A·g^?1 and high energy density of 5.6, 6.1 and 6.7 Wh·kg^?1, respectively. This is believed to be due to electric double layer capacitance induced by the abundant micropores and extra pseudo‐capacitance generated by oxygen‐containing groups. At a power density of 9000 Wh·kg^?1, the energy density is 3.1, 3.5 and 3.7 Wh·kg^?1 for PC‐1, PC‐2 and PC‐3, respectively, demonstrating the potential of the carbons derived from chestnut shells in energy storage devices.     

Investigation of porous carbon fabricated by polymer blending of phenolic resin and suberic acid

Porous carbons used for electric double-layer capacitors were fabricated by chemical blending and carbonization of phenolic resin (PF) and suberic acid (SA). The reaction of PF with diacid was confirmed by longer wavelength shift of carbonyl stretching peak of diacid in FTIR spectra and higher decomposition temperature of suberic acid in TG curves. The decomposition of suberic acid at high temperature contributed to the formation of micropores in PF resin during carbonization process. The influence of the ratio of PF to SA on pore structure, adsorption behavior and capacity performance was investigated. The specific surface area and total pore volume increase with decrease of ratio of PF to SA at first and then decrease. The maximum specific surface area and total pore volume were obtained at ratio 3, which corresponds to 511 m² g^?1and 0.26?cm³?g^?1, respectively. Electrochemical investigation indicates that a satisfactory specific capacitance of 145?Farad?g^?1 in 30 wt% KOH aqueous electrolytes is obtained. The capacitance maintenance achieves 72% as the current density increases 50 times. Porous carbon with reasonable pore structure was produced by chemical blending of diacid and phenolic resin, which offers a new development direction for preparation of porous carbon materials.     

Preparation of activated carbon from cotton stalk and its application in supercapacitor

High specific capacitance and low cost are the critical requirements for a practical supercapacitor. In this paper, a new activated carbon with high specific capacitance and low cost was prepared, employing cotton stalk as the raw material, by using the phosphoric acid (H₃PO₄) chemical activation method. The optimized conditions were as follows: the cotton stalk and activating agent with a mass ratio of 1:4 at an activation temperature of 800 °C for 2 h. The samples were characterized by nitrogen adsorption isotherms at 77 K. The specific surface area and pore volume of activated carbon were calculated by Brunauer–Emmett–Teller (BET) and t-plot methods. With these experimental conditions, an activated carbon with a BET surface area of 1,481 cm²?g^?1 and micropore volume of 0.0377 cm³?g^?1 was obtained. The capacitance of the prepared activated carbon was as high as 114 F?g^?1.The results indicate that cotton stalk can produce activated carbon electrode materials with low cost and high performance for electric double-layer capacitor.     

Mesoporous Carbons Templated by PEO-PCL Block Copolymers as Electrode Materials for Supercapacitors

In this study, samples of activated mesoporous carbon are fabricated with pore structures with cylinder and gyroid nanostructures through the templating effect of amphiphilic poly(ethylene oxide-block-caprolactone) (PEO-PCL) and by using specific resol/PEO-PCL weight ratios (e.g., 60:40 for cylinders; 55:45 for gyroids). After carbonization and KOH activation, the activated mesoporous carbons were tested as electrode materials for electric double-layer capacitor (EDLC) supercapacitors. The electrochemical properties were examined by using three-electrode (6 m KOH_(aq) as electrolyte) and CR2032 coin-cell (1 m tetraethylammonium tetrafluoroborate (TEABF₄)/CN as the electrolyte) systems. The gyroid carbon samples provided specific capacitances higher than those of the cylinder carbon samples in both aqueous and organic systems: 155 F g⁻¹ compared with 135 F g⁻¹ in 6 m KOH_(aq), and 105.6 compared with 96 F g⁻¹ in 1 m TEABF₄/MeCN, after 100 charge/discharge cycles. It is suspected that the bi-continuous mesochannels of the gyroid-type activated mesoporous carbons provided a relatively higher effective adsorption surface area; in other words, the greater surface area for energy storage originated from a moderate pore size and an interconnected pore structure.