Effect of pore characteristics on electrochemical capacitance of activated carbons

Activated carbons (ACs) for electric double layer capacitors (EDLCs) were fabricated from waste tea leaves, activated with the pore-forming substances ZnCl₂ then, carbonized at high-temperature in N₂ atmosphere. The surface texture and porosity of the ACs were determined using transmission electron micros-copy and N₂ adsorption/desorption studies. The surface area of the 20 wt % ZnCl₂ treated sample was found to be 1029 m²g^?1 and had a distribution of micropores and mesopores. The electrochemical properties of the ACs were evaluated by using cyclic voltammetry and galvanostatic charge-discharge studies. ACs from waste tea leaves exhibited excellent specific capacitance as high as 196 F g^?1 in the 0.1 M Na₂SO₄ neutral electrolyte, with rectangular-like cyclic voltammetry curves at a cell potential of 1.5 V and good cyclability with a capacitance retention of 95% at a high current density of 100 mA g^?1 for 2000 cycles. The results show that the pore texture properties and specific surface area of ACs are dominated by changing carbonization temperature and the amount of activating agent ZnCl₂. The electrochemical performance is influenced mainly by surface area, but the pore size distribution becomes a dominating factor for specific capacitance of a carbon electrode material when the pore structure is in range of micropores/mesopores.     

Graphitized carbon from wastepaper as electrodes for high-performance electric double-layer capacitors

Graphitized carbon electrode material was prepared from wastepaper by graphitization in molten sodium metal. X-ray diffraction and Raman spectroscopy were used to investigate the structural change of resulted carbons, both of which well proved the formation of graphite structure. Graphitized carbons have surface area that is nearly 26 times larger than initial carbonized paper and exhibit better electrochemical performances. The electrochemical performances of graphitized carbons were investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge methods. The cyclic voltammetry results show a maximum specific capacitance of 194 F g^?1. Therefore, wastepaper can be a promising electrode material for high-performance electric double-layer capacitors (EDLCs).     

Electrochemical behavior of activated carbon electrodes in electric double layer capacitors with tetrametylammonium bis(oxalato)borate electrolyte synthesized by microwave irradiation

The electrochemical behavior of electric double layer capacitors (EDLCs) with tetramethylammonium bis(oxalato)borate electrolyte and electrodes based on various activated carbons (ACs) was studied. Tetraalkylammonium bis(oxalate)borate salts were synthesized by means of microwave (MW) irradiation. The specific conductivities of salt solutions were determined. It was shown that the efficiency of electric double layer capacitors increases with an increase in specific surface area and a decrease in the purity of carbon materials.     

活性炭电极材料的表面改性和性能

以硝酸、双氧水、氨水三种化学试剂分别对活性炭进行表面改性, 用N2吸附法和FTIR表征炭材料改性前后孔结构和表面官能团的变化. 制备了以改性活性炭为电极材料, KOH溶液为电解质的模拟双电层电容器. 用恒流充放电、循环伏安、交流阻抗等方法考察了双电层电容器的电化学性能. 结果表明, 改性活性炭比表面积和平均孔径有所降低, 并且在炭材料表面引入了含氧或含氮官能团, 如—OH、>CO、—NH2等, 使炭材料的润湿性增强、电阻减小、电化学性能显著提高. 用65%硝酸改性后炭材料的比容量最高达到250 F·g-1, 比原样炭提高了72.4%; 实验电容器的漏电流急剧下降, 只有3-18 μA, 为原来电容器的漏电流(371 μA)的0.8%-4.9%.     

稻壳多孔碳材料的电容性能及模型计算

采用NaOH和H₃PO₄化学活化法, 由廉价的稻壳制备了2种高比表面积的介孔活性碳, 测试了其比表面积和平均孔径, 利用循环伏安、 恒流充放电和交流阻抗测试了其电化学性能, 其比电容分别达到145.8和168.5 F/g. 利用一种多分支的三级孔道模型对其进行交流阻抗谱拟合, 拟合结果符合度较高, 表明该模型对分析活性碳电极结构有一定指导意义.     

双电层电容器用新型无灰煤(HyperCoal)基活性炭的制备

以无灰煤(HyperCoal)为原料,KOH和CaCO3为活化剂制备了煤基活性炭,采用低温N2吸附法表征了活性炭的比表面积和孔结构,测定了活性炭用作双电层电容器(EDLC)电极材料的电化学性能。考察了炭化温度、活化温度、活化时间和活化剂对活性炭电容特性的影响。研究结果表明,比表面积和比电容随着炭化温度的升高而降低,活化温度过高或活化时间太长对比电容有不利影响。此外,CaCO3影响活化过程中孔的开发,显著降低所制备活性炭的比表面积和比电容。在炭化温度为500℃、活化温度为800℃、KOH与焦的质量比为4∶1和活化时间2 h下所得活性炭的比表面积和总孔容分别达到2 540 m2/g和1.65 cm3/g,该活性炭电极在0.5 mol/L TEABF4/PC电解液中的比电容达到最大值46.0 F/g。     

Capacitance response of carbons in solvent-free ionic liquid electrolytes

Ionic liquids (IL) are very promising “solvent-free” electrolytes for high-voltage double-layer supercapacitors (EDLCs) and to this purpose they are generally selected on the basis of their bulk properties, such as electrochemical stability and ion conductivity, without taking into account those of the electrified electrode-IL interface. This interface, which has yet to be well characterized, has features that notably affect electrode capacitance, and our paper for the first time highlights the importance of the molecular chemistry and structure of the ions for the double-layer capacitive response of carbonaceous electrodes in IL. The double-layer capacitive responses of negatively charged electrodes based on activated carbons and aero/cryo/xerogel carbons in two ILs featuring the same anion and different cations of almost the same size, i.e. the N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) and 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI) are reported. The porosity, structure and surface chemistry of the carbons are compared to their capacitive response to evince the role played by these carbon properties and by the chemistry and structure of the IL ions in the electric double-layer.     

Surface Modification and Performance of Activated Carbon Electrode Material

A commercial activated carbon was modified by surface treatment using three chemicals, nitric acid, hydrogen peroxide, and ammonia, respectively. The modified carbons were characterized by N₂ adsorption-desorption isotherms and FTIR spectroscopy. The resultant carbon electrode-based electric double-layer capacitors (EDLCs) were assembled with 6 mol·L⁻¹ KOH as the electrolyte. The influence of surface modification on the performance of EDLCs was studied by galvanostatic charge-discharge, cyclic voltammetry, and alternating current impedance. The surface modification resulted in no big decrease in specific surface area and little decrease in average pore size, and introduced functional groups, such as hydroxyl, carbonyl, and amidogen, on the carbon surface. These functional groups significantly improved the wettability and reduced the resistance of the activated carbon. As a result, the specific capacitance of the carbon modified with 65% HNO₃ reached 250 F·g⁻¹, 72.4% higher than that of original carbon. The leakage current of testing EDLCs decreased unexpectedly to 3-18 μA, only 0.8%-4.9% that of the original carbon electrode-based EDLC (371 μA).     

Charge storage performance of doped carbons prepared from polyaniline for supercapacitors

Doped carbons have been prepared from polyaniline for supercapacitors. The morphology of samples has been characterized by scanning electron microscope, the surface chemical composition of samples has been investigated by X-ray photoelectron spectroscopy, and the surface area of samples has been calculated by Brunauer–Emmett–Teller measurement. Electrochemical properties have been studied by cyclic voltammograms, galvanostatic charge/discharge, and electrochemical impedance spectroscopy measurements in 6 mol L⁻¹ potassium hydroxide. Their charge storage performance has been evaluated, and the effect of nitrogen atomic functionalities on the pseudocapacitive property has been studied. The experimental results have proved two mechanisms of energy storage in doped carbons: double-layer formation and pseudocapacitance. The overall specific capacitance of doped unactivated carbon is mainly attributed to pseudocapacitance, that of doped activated carbon prepared by physical activation is attributed to the synergic effect of pseudocapacitance and double-layer capacitance, but that of doped activated carbon prepared by chemical activation is mainly attributed to double-layer capacitance.     

Surface and chemical characteristics of platinum modified activated carbon electrodes and their electrochemical performance

Platinum (Pt) loaded activated carbons (ACs) were synthesized by the thermal decomposition of platinum (II) acetylacetonate (Pt(acac)₂) over chemically activated glucose-based biochar. The effect of Pt loading on surface area, pore characteristics, surface chemistry, chemical structure, and surface morphology were determined by various techniques. XPS studies proved the presence of metallic Pt⁰ on the AC surface. The graphitization degree of Pt loaded ACs were increased with the loaded Pt⁰ amount. The electrochemical performance of the Pt-loaded ACs (Pt@AC) was determined not only by the conventional three-electrode system but also by packaged supercapacitors in CR2032 casings. The capacitive performance of Pt@AC electrodes was investigated via cyclic voltammetry (CV), galvanostatic charge-discharge curves (GCD), and impedance spectroscopy (EIS). It was found that the Pt loading increased the specific capacitance from 51 F/g to 100 F/g. The ESR drop of the packaged cell decreased with the Pt loading due to the fast flow of charge through the conductive pathways. The results showed that the surface chemistry is more dominant than the surface area for determining the capacitive performance of Pt loaded AC-based packaged supercapacitors.     

电化学双电层电容器电极材料——豌豆荚基活性炭的制备与表征

以豌豆荚为碳源、ZnCl₂或KOH为活化剂制备了活性炭, 并用作双电层电容器的电极材料. 采用比表面及孔隙度分析仪表征了豌豆荚基活性炭的孔结构. 通过KOH或ZnCl₂活化后, 活性炭比表面积从1.69 m²·g^-1增大到2237或621 m²·g^-1. 采用循环伏安法和恒流充放电测试技术表征了豌豆荚基活性炭的电化学特性. 结果表明: 在6 mol·L^-1 KOH溶液中经KOH活化处理的活性炭的质量比电容高达297.5 F·g^-1, 并具有良好的充放电稳定性, 在5 A·g^-1的高电流密度下循环充放电500次后, 质量比电容仅衰减8.6%.     

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.     

A review on the development of a porous carbon-based as modeling materials for electric double layer capacitors

Carbon electrodes are a key factor for electric double layer capacitors (EDLCs). Carbon gels have high porosity with a controllable pore structure by changing synthesis conditions and modifying preparation processing to improve the electrochemical performance of EDLCs. This review summarizes the preparation of carbon gels and their derivatives, the criteria to synthesize high surface area in each process, the development by some carbon forms, and EDLC applications. Porous carbons are also prepared as model materials by concentrating on how pore structure increases electrochemical capacitance, such as electronic and ion resistance, the tortuosity of pore channel, suitable micropore and mesopore sizes, and mesopore size distribution. This review emphasizes the significance of pore structures as the key factor to allow for the design of suitable pore structures that are suitable as the carbon electrode for EDLCs.     

Influence of KMnO4 oxidation on the electrochemical performance of pitch-based activated carbons

In this work, activated carbons (ACs) are obtained from petroleum pitch by the combination of a chemical treatment with different potassium permanganate (KMnO₄) amounts, i.e., 0, 0.5, 1.0, and 2.0 g, and a chemical activation with KOH at a constant KOH/pitch ratio of 3/1. The effects of the chemical activating agent on the surface morphology and porosity are evaluated with scanning electron microscopy and N₂ adsorption isotherms at 77 K, respectively. The specific surface area of the pitch-based ACs is increased with increasing the amount of KMnO₄ pre-treatment and showed the highest value of 2,334 m² g^?1 at 2 g of KMnO₄ amount. The electrochemical performance of AC electrodes is examined by cyclic voltammetry and galvanostatic charge/discharge characteristics in 6 M KOH electrolyte. Among the prepared ACs, 2.0 K-ACs possesses a specific capacitance as high as 237 F g^?1 and showed excellent electrochemical performance due to its suitable porous structure and low interface resistance.     

A study of ion charge transfer on electrochemical behaviors of poly(vinylidene fluoride)-derived carbon electrodes

In this work, porous carbon with a high specific surface area as electrode materials for supercapacitors are obtained by a carbonization process at various temperatures from 700 °C to 1000 °C without activation process using poly(vinylidene fluoride) (PVDF) as a carbon precursor. The electrochemical performance is characterized by cyclic voltammetry and galvanostatic charge–discharge cycling performance using two-electrode system in 6.0 M KOH as an aqueous electrolyte. The results indicate that carbonization temperature significantly affected the specific surface area and pore volume of the PVDF-derived carbons and their capacitive behavior. In particular, the electrochemical performance of the prepared PVDF-derived carbon is determined by both the electric double-layer capacitance and the pseudo-capacitance resulting from the residual surface functional groups on PVDF-derived carbons.     

多孔碳材料的化学气相后处理及其超级电容性能

以廉价的椰壳为原料制备了高比表面积的多孔碳材料,然后在密闭的反应釜中以硝酸蒸汽对多孔碳材料进行了后处理,制备了亲水性更好的多孔碳材料。采用扫描透射电子显微镜(TEM)、物理吸附、X射线粉末衍射(XRD)、拉曼光谱(Raman)和接触角测试对材料的微观形貌、孔道结构、组成和亲水性进行了表征,探究了不同温度下硝酸蒸汽对多孔碳材料的形貌、结构的影响,并采用循环伏安法、恒电流充放电法和交流阻抗法考察了多孔碳材料的超级电容性能。结果表明,经过硝酸蒸汽处理后的多孔碳材料的比表面积和孔体积均有所降低,且随着处理温度的升高,降低得更加明显,而亲水性却越来越好。电化学测试结果表明,经过100℃硝酸蒸汽处理的多孔碳材料(CSC-100)具有最佳的超级电容性能。在以6 mol·L^-1 KOH为电解液的三电极体系中,当电流密度为0.5 A·g^-1时CSC-100的比电容可达452.9 F·g^-1,而未经硝酸蒸汽处理的多孔碳材料(CSC)的比电容仅为350.4 F·g^-1。电容贡献分析表明CSC-100良好的亲水性和表面官能团不仅提高了双电层电容,也提高了赝电容。     

电解液离子与炭电极双电层电容的关系

以酚醛树脂基纳米孔玻态炭(NPGC)为电极, 通过微分电容伏安曲线的测试, 研究了水相体系电解液离子与多孔炭电极双电层电容的关系. 结果表明, 稀溶液中, 多孔炭电极的微分电容曲线在零电荷点(PZC)处呈现凹点, 电容降低, 双电层电容受扩散层的影响显著；若孔径小, 离子内扩散阻力大, 电容下降更为迅速, 扩散层对双电层电容的影响增大. 而增大炭材料的孔径或电解液浓度, 可明显减弱甚至消除扩散层对电容的影响. 炭电极的单位面积微分电容高, 仅表明孔表面利用率高, 如欲获得高的电容量, 还要有大的比表面积. 离子水化对炭电极的电容产生不利影响, 选用大离子和增大炭材料的孔径, 可有效降低离子水化对炭电极电容性能的影响.     

Pore structure and surface properties of chemically modified activated carbons for adsorption mechanism and rate of Cr(VI)

Effects of hydrochloric acid and sodium hydroxide treatments of activated carbons (ACs) on chromium(VI) reduction were studied. The surface properties were determined by pH, acid-base values, FT-IR, and X-ray photoelectron spectrometer (XPS). And the porous structure of the activated carbons was characterized by adsorption of N(2)/77 K. The Cr(VI) adsorption experiments were carried out to analyze the influence of porous texture and surface properties changed by the chemical surface treatments of ACs on adsorption rate with carbon-solution contact time. From the experimental results, it was observed that the extent of adsorption and reduction processes depends on both microporous structure and functional groups. And the adsorption of Cr(VI) ion was more effective in the case of acidic treatment on activated carbons, resulting from the increases of acid value (or acidic functional group) of activated carbon surfaces. However, basic treatment on activated carbons was not significantly effective on the adsorption of Cr(VI) ion, probably due to the effects of the decrease of specific surface area and basic Cr(VI) in nature.     

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.     

Effect of ozone treatment on ammonia removal of activated carbons

In this work, activated carbons (ACs) were modified by ozone treatment to enhance the efficiency of removal of ammonia gas over the ACs. Surface properties of the ACs were confirmed by X-ray photoelectron spectroscopy (XPS) analysis and N2 adsorption isotherms at 77 K were investigated by BET and D-A methods to characterize the specific surface area, total pore volume, and micropore volume. The ammonia removal efficiency was confirmed by the gas-detecting tube technique. The results showed that the specific surface area and micropore volume of ACs were slightly destroyed as the ozone treatment time increased. However, the ozone treatment led to an increase in ammonia removal efficiency of ACs, mainly due to an increase of acid functional groups, such as carbonyl and ether groups, on carbon surfaces. It was revealed that the improvement of ammonia removal efficiency of ACs was greatly affected by the interfacial acid-base interactions between modified ACs and basic ammonia adsorbate.