Carbon For Electric Double-layer Capacitors Research Articles

Tailoring Macro/Meso/Microporous Structures of Cellophane Noodle-Derived Activated Carbon for Electric Double-Layer Capacitors.

To address the bottleneck associated with the slow ion transport kinetics observed in the porosity of activated carbons (ACs), hierarchically structured pore sizes were introduced on ACs used for electric double-layer capacitors (EDLCs) to promote ion transport kinetics under fast-rate charge-discharge conditions. In this study, we synthesized cellophane noodle-derived activated carbon (CNAC) with tailored porous structures, including the pore volume fraction of macro/meso/micropores and the specific surface area. The porous structures were effectively modulated by adjusting the KOH concentration during chemical activation. In addition, optimized KOH activation in CNAC modulated the chemical bonding ratios of C=O, pyrrolic-N, and graphitic-N. Given the hierarchically designed porous structure and chemical bonding states, the CNAC fabricated with optimized KOH activation exhibited a superior ultrafast rate capability in EDLCs (132.0 F/g at 10 A/g).

A Study on Pre-Oxidation of Petroleum Pitch-Based Activated Carbons for Electric Double-Layer Capacitors.

Electric double-layer capacitors (EDLCs) are an excellent electrochemical energy storage system (ESS) because of their superior power density, faster charge–discharge ability, and longer cycle life compared to those of other EES systems. Activated carbons (ACs) have been mainly used as the electrode materials for EDLCs because of their high specific surface area, superior chemical stability, and low cost. Petroleum pitch (PP) is a graphitizable carbon that is a promising precursor for ACs because of its high carbon content, which is obtained as an abundant by-product during the distillation of petroleum. However, the processibility of PP is poor because of its stable structure. In this study, pre-oxidized PP-derived AC (OPP-AC) was prepared to investigate the effects of pre-oxidation on the electrochemical behaviors of PP. The specific surface area and pore size distribution of OPP-AC were lower and narrower, respectively, compared to the textural properties of untreated PP-derived AC (PP-AC). On the other hand, the specific capacitance of OPP-AC was 25% higher than that of PP-AC. These results revealed that pre-oxidation of PP induces a highly developed micropore structure of ACs, resulting in improved electrochemical performance.

Mesophase Pitch-Derived Carbons with High Electronic and Ionic Conductivity Levels for Electric Double-Layer Capacitors.

We develop a temperature-programmed pretreatment strategy for converting aliphatic-rich petroleum pitch into a mesophase framework, which can then be activated using KOH to produce high-performance carbons for electric double-layer capacitors (EDLCs). In the pretreatment of pitch at an optimal temperature, both the temperature ramp and holding time influence the mesophase structure, which governs the pore structure and crystallinity of the resulting activated carbon. High carbon microporosity is beneficial to capacitance maximization but detrimental to ion transport. To resolve this problem, we develop a multistep ramp incorporating aliphatic species into the aromatic framework during mesophase formation. This incorporation process produces a mesophase framework that can be activated to form carbons with high crystallinity, thereby enhancing electronic conductivity and hierarchical porosity, which improves ionic conductivity. The resulting carbon electrode is used to assemble a symmetric EDLC, which exhibits a capacitance of 160 F g–1 and excellent high-rate retention in a propylene carbonate solution of N,N-diethyl-N-methylethanaminium tetrafluoroborate. The EDLC delivers a superior specific energy of 40 Wh kg–1 (based on the total carbon mass) within a voltage range of 0–2.7 V and sustained a high energy of 24 Wh kg–1 at a high power of 50 kW kg–1. The findings of this study demonstrate that incorporating aliphatic species into aromatic mesophase frameworks plays a crucial role in regulating the crystallinity and pore structure of pitch-derived carbons for charge storage.

Void-bearing electrodes with microporous activated carbon for electric double-layer capacitors

Void-bearing electrodes composed of commercial microporous activated carbon were prepared for electric double-layer capacitors by using ammonium hydrogen carbonate to form void spaces between the carbon particles. The N2 adsorption isotherm showed that the introduction of void spaces increases the BET surface area and decreases the external surface area of the film electrodes. Additionally, although their volumetric capacitance was reduced as a result of lower electrode density, the void-bearing electrodes demonstrated superior rate capability owing to their lower diffusion resistance of ions. However, their lifetime performance was slightly worse than that of standard activated carbon electrodes. Moreover, gas evolution was mildly suppressed, but micropore blockage was promoted. The results indicate that solid deposition occurred around the micropores, which thus led to pore blockage, and that gaseous products were mainly generated on the external surface of the electrodes.

Symmetric supercapacitors based on porous 3D interconnected carbon framework

The construction and design of novel porous carbons for electric double-layer capacitors (EDLCs) application to meet the increasing demand and supply of energy is eminent. This is important because the pore volume (PV)/micropore volume (MV) in the porous network architecture of the carbon is mostly responsible for the ion traps in energy storage. Three dimensional carbon materials based on graphene materials with relatively high specific surface area (SSA) represents a promising material candidate for EDLCs applications. In this work, we synthesized highly porous carbon from graphene foam (GF) and polyvinyl alcohol PVA as a sacrificial template, and investigate their performance as electrodes for EDLCs applications. The as-produced carbons present a fairly large surface area (502m2g−1), and a highly porous interconnected framework with mesopore walls and micropore texture which are suitable as electrode for energy storage. As electrode material in a symmetric configuration the activated graphene foam (AGF) showed a specific capacitance of 65Fg−1, energy density of 12Whkg−1, power density of 0.4kWkg−1, good rate performance and excellent long term stability in 1MNa2SO4 aqueous with no capacitance loss after 3000 cycles.

Preparation of N-doped activated carbons for electric double-layer capacitors from waste fiberboard by K2CO3 activation

N-doped activated carbons for electric double-layer capacitors were prepared from waste medium density fiberboard using K 2 CO 3 activation. The effects of carbonization temperature, activation temperature, activation time and K 2 CO 3 /coke mass ratios on surface chemical compositions, pore structure and electrochemical performance of the resulting activated carbons were investigated. Results indicated that the activated carbons had nitrogen contents from 0.93 to 2.86%, Brunauer–Emmett–Teller specific surface areas from 569 to 1027 m 2 /g and specific capacitances from 147 to 223 F/g, depending on carbonization and activation conditions. The maximum specific capacitance was attributed to a high surface area, optimum pore size, large pore volume and high N-5 (the pyrrolic nitrogen and pyridinic nitrogen in association with oxygen functionality) content. [New Carbon Materials 2014, 29(2): 89–95]

Preparation of N-doped activated carbons for electric double-layer capacitors from waste fiberboard by K2CO3 activation

N-doped activated carbons for electric double-layer capacitors were prepared from waste medium density fiberboard using K2CO3 activation. The effects of carbonization temperature, activation temperature, activation time and K2CO3/coke mass ratios on surface chemical compositions, pore structure and electrochemical performance of the resulting activated carbons were investigated. Results indicated that the activated carbons had nitrogen contents from 0.93-2.86%, Brunauer-Emmett-Teller specific surface areas from 569 to 1 027 m2/g and specific capacitances from 147 to 223 F/g, depending on carbonization and activation conditions. The maximum specific capacitance was attributed to a high surface area, optimum pore size, large pore volume and high N-5 (the pyrrolic nitrogen and pyridinic nitrogen in association with oxygen functionality) content.

KOH activation of a HyperCoal to develop activated carbons for electric double-layer capacitors

A series of activated carbons (ACs) was prepared from HyperCoal by conventional and direct KOH activation methods, and was used as electrodes in electric double-layer capacitors (EDLCs) using 0.5M TEABF4/PC as the electrolytic solution. The effects of carbonization temperature (CT), activation temperature (AT) and KOH/sample ratio on BET surface area, average pore size and capacitance of ACs were investigated in this study. Nitrogen adsorption isotherms of all ACs indicated that the pores in ACs were mainly micropores. The BET surface area and capacitance decreased with raising CT from 500 to 900°C, while the highest BET surface area of 2540m2g−1 and maximum capacitance of 46.0Fg−1 were achieved at CT of 500°C and AT of 800°C in conventional activation. On the contrary, in the direct activation, the BET surface area and the capacitance increased with the increase of AT and KOH/sample ratio, getting the maximum value of 2440m2g−1 and 44.4Fg−1, which is comparable with that of conventional method, at AT of 800°C and KOH/sample ratio of 4.0. Such a procedure provides a cost-effective approach to preparation of high performance activated carbons from HyperCoal for EDLC.

Synthesis of spherical mesoporous carbon for electric double-layer capacitors

Resorcinol and formaldehyde were used as carbon precursors, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer was employed as a soft template, and tetraethylorthosilicate-generated silica was used as hard templates to synthesize spherical mesoporous carbon. The resulting spherical mesoporous carbons were characterized by nitrogen adsorption–desorption isotherms and electron microscopy (SEM and TEM) and used as electrode materials for aqueous electric double-layer capacitors. The average diameters of spherical particles ranged from 2 to 7 μm and the mesopore was ca 2 nm. The highest specific surface area of 1,000 m2/g and mesopore volume of 0.86 cm3/g was obtained. The specific capacitance of 130 F/g was obtained by means of galvanostatic charging/discharging and cycle voltammetry.

Study on the Capacitance Performance of Activated Carbon Material for Supercapacitor

Activated carbon for electric double-layer capacitors was prepared from bamboos by activation with KOH solution through heating by microwave radiation. The influence of the mass ratio of KOH to bamboo, power and radiation time of microwave was studied. The behavior of charge/discharge, cyclic voltammetry and AC impedence of bamboo-based activated carbon electric double-layer capacitor was investigated. The results indicated that the specific capacitance of bamboo-based activated carbon supercapacitors can reach 277.46F/g while KOH to bamboo is 6:1 and the power and radiation time of microwave are 720W and 12 minutes respectively. Moreover, the electric double-layer capacitor using the activated carbon as electrode materials has good charge/discharge properties and cycling performance.

Structure and electrochemical properties of resorcinol–formaldehyde polymer-based carbon for electric double-layer capacitors

A nano-porous carbon was prepared by carbonization of a novel synthetic resorcinol–formaldehyde (RF) polymer without any additional activation process, and used as electrode materials for aqueous electric double-layer capacitors (EDLCs). This novel RF polymer-based carbon shows high specific surface area with large carbonization yield (∼50%), and excellent specific dc capacitance over 200 F/g. The effect of R/CA ratio (i.e. molar ratio of resorcinol to curing agent) on the specific surface area, pore size distribution, nanostructure and electrochemical capacitance was studied, respectively. The results showed that a higher R/CA ratio yielded carbon with higher specific surface area, larger specific capacitance, and broader pore size distribution. The highest specific surface area of 825 m 2/g and specific capacitance exceeding 200 F/g were found to occur at R/CA ratio of 50. The electrochemical behaviors were characterized by means of galvanostatic charging/discharging, cycle voltammetry and impedance spectroscopy. The correlation between electrochemical properties and pore structure was investigated. Due to the excellent capacitance properties, low cost and simple process, this RF polymer-derived carbon would be a promising material for EDLCs applications.

Pore characteristics and electrochemical performance of ordered mesoporous carbons for electric double-layer capacitors

Three types of ordered mesoporous carbon materials with different pore characteristics have been synthesized via different routes. Whatever the synthesis route was, triblock copolymer was employed as both a carbon precursor and a structure-directing agent. The relationship between the capacitances of carbon electrodes and their pore characteristics was elucidated in detail. The material C–P exhibits the lowest resistance and highest specific capacitance value of exceeding 170 F/g among these carbon materials, which can be due to not only high surface area but also its appropriate pore size distribution. In addition, the noteworthy is that the maintenance of specific capacitance with increasing current load for each sample is better than that for general activated carbons, where larger mesopores and high mesopore fraction play important roles in the rate capability.