Electrochemical capacitors based on highly porous carbons prepared by KOH activation

Various coal and pitch-derived carbonaceous materials were activated for 5 h at 800 °C using potassium hydroxide and 1:4 component ratio. Porosity development of the resultant activated carbons (ACs) was assessed by N₂ sorption at 77 K and their capability of the charge accumulation in electric double layer was determined using galvanostatic, voltammetric and impedance spectroscopy techniques. ACs produced from different precursors are all microporous in character but differ in terms of the total pore volume (from 1.05 to 1.61 cm³ g⁻¹), BET surface area (from 1900 to 3200 m² g⁻¹) and pore size distribution. Very promising capacitance values, ranging from 200 to 320 F g⁻¹, have been found for these materials operating in acidic 1 mol l⁻¹ H₂SO₄ electrolytic solution. The variations in the electrochemical behavior (charge propagation, self-discharge, frequency response) are considered in relation to the porous texture characteristics, elemental composition but also possible effect of structural ordering due to various precursor materials used. Cycling investigation of all the capacitors has been also performed to compare ability of the charge accumulation for different carbon materials during subsequent cycles.     

An investigation of the porosity of carbons prepared by constant rate activation in air

Nutshell carbon was activated in air/N₂ mixtures using controlled rate (CR) methods and the porosity characteristics compared with carbons activated conventionally in CO₂ at 800 °C to the same degree of burn off. The advantages of CR activation in air include the use of lower temperatures and the avoidance of thermal runaway. It was also possible to prepare activated carbons with significant microporosity, showing that excessive external burn off was prevented. In the CR experiments, the rate of evolution of CO₂ was controlled and constrained at a set level, either by altering the furnace temperature or the concentration of air in the activating gas. Although the highest micropore volumes (0.4 cm³ g⁻¹) were obtained at 40% burn off with the conventional method, at 20% burn off, the CR method using air concentration to control CO₂ evolution yielded carbons with similar micropore volumes (0.2 cm³ g⁻¹) to those activated conventionally.     

Catalytic performance of porous carbons obtained by chemical activation

Porous carbons were prepared by the pyrolysis of pinewood in nitrogen at 773 K, followed by chemical activation using different concentrations of KOH in nitrogen at 973 K. Both water-washed and acid-washed samples exhibited much higher specific surface areas than unactivated carbon. The water-washed sample showed strong basicity and a high catalytic performance in the decomposition of isopropanol, even higher than superbase 26%KNO₃/γ-Al₂O₃. Moreover, these porous carbons can act as water-resistant solid bases. The formation of the insoluble basic sites is most possibly related to the intercalation of potassium in the carbon during the chemical activation.     

Preparation of porous carbons from petroleum coke by different activation methods

Porous carbons were prepared from Shengli petroleum coke (SPC) and Minxi petroleum coke (MPC) by different activation methods with H₂O, KOH and/or KOH+H₂O as active agents. The porous carbons were characterized by nitrogen adsorption at 77 K. It has been found that activation method and component of petroleum coke, of which different kinds of transitional metals on petroleum coke are crucial for preparing high quality porous carbons. Under the identical experimental conditions, the co-activation with KOH and H₂O as active agents in the same activation process, which has been rarely reported in literature, is the easiest method for the preparation of porous carbons with high surface area. The sequence of active agents in terms of difficulty in the preparation of porous carbons with high surface area is as follows: KOH+H₂O>KOH>H₂O. A drawback of KOH+H₂O activation in the preparation of porous carbon in this work is found to be its low carbon yield in comparison to KOH activation. Compared with the SPC coke, the MPC coke with higher contents of transitional metal and carbon and lower content of nitrogen is more suitable for making high surface area porous carbons, which is believed to be mainly due to the difference in the contents of transitional metals. Porous carbon with surface area around 2500–3000 m²/g and carbon yield about 25–30% has been obtained from MPC coke by KOH+H₂O activation with less KOH and shorter activation time in comparison to the traditional methods.     

Porous graphitic carbons prepared by combining chemical activation with catalytic graphitization

Porous graphitic carbons were prepared by combining chemical activation (ZnCl₂) with catalytic graphitization (metal Fe and Ni). The activating agent ZnCl₂ increased the surface area of porous carbon, and the catalyst (Fe, Ni) accelerated the graphitization. With increasing heat treatment temperature (600–900 °C), metal Fe and Ni promoted the degree of graphitization which was characterized by powder X-ray diffraction, Raman spectroscopy and high-resolution transmission electron microscopy. The surface area of samples prepared using Fe and Ni catalysts varied from 275 to 787 m² g⁻¹ and from 83 to 121 m² g⁻¹, respectively.     

Activated carbons prepared from rice hull by one-step phosphoric acid activation

Activated carbons were prepared from rice hull by one-step phosphoric acid activation in this work. The evolution of pore structure and surface chemistry in the activation temperature range of 170–450 °C was investigated through various characterization techniques. The results showed that the development of porosity (extent of activation) was negligible at activation temperature below 300 °C, and rapid evolution occurred in 300–400 °C. Porous activated carbon with bimodel pore structure (pore < 1 nm and pore > 1 nm) and BET surface area as high as 1295 m²/g was obtained at 450 °C. The ash contents of samples prepared in this study were in the range of 5–21%. The ash contents of carbons prepared in this study initially decreased from 21.03% to 4.89% with the change of temperature from 170 to 300 °C, then increased to 8.72% at 450 °C. Boehm titration results suggested that low activation temperature (300 °C) benefits the formation of acidic surface groups. With the increase of activation temperature from 300 to 350 °C, the concentrations of strong, intermediate and weak acidic surface groups decreased from 2.23, 1.87, and 2.73 to 1.66, 1.32, and 2.16 mmol H⁺/g, respectively. Over 350 °C, the change of these groups were insignificant. FTIR results revealed the existence of carbonyl-containing, phosphorus-containing groups, and groups containing Si–O bond. The relative concentration of carbonyl-containing groups decreases with an increase in activation temperature, while that of phosphorus-containing groups follows the reverse trend. The content of Si–O decreased first, then slowly increased with the increase of activation temperature. Boehm titration and FTIR (Fourier transform infrared spectroscopy) results indicated that the surfaces of these carbons contain both temperature-sensitive and temperature-insensitive groups. The temperature-sensitive part consists mainly of carbonyl-containing groups, such as carboxylic groups, while the temperature-insensitive part is primarily phosphorus-containing groups and groups containing Si–O bond. This study demonstrated that carbon products with relative low ash content and high activation degree can be prepared from rice hull by H₃PO₄ activation at suitable temperature.     

HRTEM study of activated carbons prepared by alkali hydroxide activation of anthracite

A high-resolution transmission electron microscopy study has been made of the structure and porosity of activated carbons prepared by activation of anthracites by hydroxides. The results show the structural changes occurring during chemical activation and present several experimental parameters that can be observed to determine the activation degree of the materials.     

Effect of activation time on the properties of activated carbons prepared by microwave-assisted activation for electric double layer capacitors

Activated carbons (ACs) were prepared by microwave-assisted heat treatment of petroleum coke with KOH as activation agent, and characterized by infrared spectroscopy and nitrogen adsorption technique with the aim of studying the effect of activation time on the properties of ACs for electrodes in electric double layer capacitors (EDLCs). The electrochemical properties of AC electrodes in EDLCs were studied by cyclic voltammetry, constant current charge-discharge and electrochemical impedance spectroscopy. The results show that the specific surface area (S_BET) and total pore volume of ACs goes through a maximum as the activation time increases. At 35 min of the activation time, the as-made AC (denoted as AC-35) has a S_BET of 2312 m²/g. With AC-35 as the electrode, its specific capacitance in EDLC at a current density of 50 mA/g can reach 342.8 F/g, and remains at 245.6 F/g even after 800 cycles while the energy density of the capacitor remains at 8.0 Wh/kg. The results have demonstrated that the microwave-assisted heat treatment is an efficient approach to the preparation of ACs with high performance for EDLCs.     

化学活化法制备玉米芯基多孔炭材料

以玉米芯为原料,采用化学活化法可制备多孔炭材料。分别考察了活化剂、碱/炭质量比对多孔炭比表面积以及孔隙结构的影响。结果表明:由Na2CO3活化所得活性炭的中孔较多,比表面积小;而KOH因其强碱性,适合制备微孔发达的高比表面积活性炭,在碱炭比为31时能够制备总孔容和比表面积分别高达1.339cm3/g和2342m2/g的样品;用混合碱(Na2CO3:KOH:C=1:2:1)活化样,其特殊之处在于其微孔所占比例达到93.38%,且中孔分布更窄(2～4.5nm),说明混合碱的作用更易于制备微孔发达的活性炭。     

Preparation and characterization of porous carbons from PAN-based preoxidized cloth by KOH activation

Porous carbons (PCs) were prepared from PAN-based preoxidized cloth with potassium hydroxide (KOH) as active reagent by the chemical activation method. The PCs have been systematically studied by the adsorptions of nitrogen, benzene and iodine. It has been found that the process parameters such as weight ratio of KOH to the starting material, activation temperature and activation time are crucial for preparing high quality PCs. A series of PCs with high BET surface area and well-developed porous structure in which micropores are dominant were obtained with less KOH and shorter activation time in comparison to the traditional methods. The optimum conditions for preparing PCs with high BET surface area from PAN-based preoxidized cloth were given, and the relationships between pore structure and adsorption property of PCs were explored.     

Nitrogen-doped porous carbons through KOH activation with superior performance in supercapacitors

A series of nitrogen-doped porous carbons are prepared through KOH activation of a nonporous nitrogen-enriched carbon which is synthesized by pyrolysis of the polymerized ethylenediamine and carbon tetrachloride. The porosity and nitrogen content of the nitrogen-doped porous carbons depend strongly on the weight ratio of KOH/carbon. As the weight ratio of KOH/carbon increases from 0.5 to 2, the specific surface area increases from 521 to 1913 m² g⁻¹, while the nitrogen content decreases from 10.8 to 1.1 wt.%. The nitrogen-doped porous carbon prepared with a moderate KOH/carbon weight ratio of 1, which possesses a balanced specific surface area (1463 m² g⁻¹) and nitrogen content (3.3 wt.%), exhibits the largest specific capacitance of 363 F g⁻¹ at a current density of 0.1 A g⁻¹ in 1 M H₂SO₄ aqueous electrolyte, attributed to the co-contribution of double-layer capacitance and pseudocapacitance. Moreover, it shows excellent rate capability (182 F g⁻¹ remained at 20 A g⁻¹) and good cycling stability (97% capacitance retention over 5000 cycles), making it a promising electrode material for supercapacitors.     

CO2-pressure swing activation for efficient production of highly porous carbons

In this work, we describe a new type of activation method of carbon materials using pressure swing of CO₂. The porosity development markedly depends on the pressure swinging frequency. The porous carbon obtained from pressure-swing activation shows an additional porosity development without pitting corrosion on the surface, which occurs on CO₂ activation without pressure-swing. This phenomenon is ascribed to the enhancement of Knudsen diffusion and/or configurational diffusion of CO₂ which is caused by an exterior stimulus from the pressure swing.     

Adsorption of nitrogen by porous and non-porous carbons

Nitrogen isotherms have been determined at 77 K on a number of carbon blacks and microporous carbons. Application of the Dubinin-Radushkevich (DR) method has shown that linear plots are given by both nonporous and microporous samples. The range of linearity is considerably reduced by increasing the micropore size, while graphitisation of nonporous carbon leads to the formation of two distinct linear regions. Application of the α_s method provides strong evidence for two stages of micropore filling:

• 1.(a) a primary process involving enhanced adsorbate-adsorbent interactions
• 2.(b) a secondary process which is the result of cooperative effects associated with the filling of wider micropores.

     

KOH activation of ordered mesoporous carbons prepared by a soft-templating method and their enhanced electrochemical properties

Ordered mesoporous carbon (COU-2) was synthesized by a soft-templating method. The COU-2 mesoporous carbon was activated by using KOH to improve its porosity. The mesopore size of COU-2 was 5.5 nm and did not change by the KOH activation. But, the BET surface area of COU-2 largely increased from 694 to 1685 m²/g and total pore volume was increased from 0.54 to 0.94 cm³/g after the KOH activation. The large increase of micropore volume is due to the increase of the surface area. Electrochemical cyclic voltammetry measurements were conducted in aqueous (1 M sulfuric acid) and organic (1 M tetraethyl ammonium tetrafluoroborate/polypropylene carbonate) electrolyte solutions. The KOH-activated COU-2 carbon shows superior capacitances over the COU-2 carbon and a commercial microporous carbon both in aqueous and organic electrolyte solutions. These results suggest that the carbons having regularly-interconnected uniform mesopores and micropores in thin pore walls are desirable for the electrodes in electrochemical double-layer capacitors.     

Pore structure of activated carbons prepared by carbon dioxide and steam activation at different temperatures from extracted rockrose

The influence of the activation temperature on the pore structure of granular activated carbons prepared from rockrose (Cistus ladaniferus L.), extracted previously into petroleum ether, is comparatively studied. The preparation was carried out by pyrolysis of a char in nitrogen and its subsequent activation by carbon dioxide and steam (flow of water controlled to generate the same mol number per minute of water as well as carbon dioxide/nitrogen) at 700-950°C to 40% burn-off. The techniques applied to study the pore structure were: pycnometry (mercury, helium), adsorption (carbon dioxide, 298 K; nitrogen, 77 K), mercury porosimetry and scanning electron microscopy. The preparation by steam activation, especially at 700°C, yields activated carbons showing a total pore volume larger than those prepared by carbon dioxide activation. The pore structures present the greatest differences when the activations are carried out between 700 and 850°C and closer at higher temperatures. At high temperatures, the decrease of differences in pore development caused by carbon dioxide or steam is attributed to an external burn-off. The micropore structure of each activated carbon is mainly formed by wide micropores. At the lowest activation temperatures, especially at 700°C, steam develops the mesoporosity much more than carbon dioxide. At 950°C, a similar reduction of pore volume in the macropore range occurs.     

KOH activation of mesoporous carbons obtained by soft-templating

Mesoporous phenolic resin-based carbons were prepared by soft-templating synthesis and activated by KOH. It is shown that the KOH activation of these mesoporous carbons results in a substantial increase of microporosity with simultaneous preservation of mesoporous structure. The resulting micro- and mesoporous carbons possess high surface area and large volume of mesopores.