Preparation Of Porous Carbon And The Research On Their Electrochemical Capacitance Performance

In this thesis, porous carbons with high specific surface areas and optimum pore size distributions were prepared from phenol-formaldehyde series polymer by different methods, including chemical activation method, polymer blends method, sol-gel polymerization method and self-assemble synthesis carbonization. The influences of the preparation method and conditions on structure and performance of porous carbons were investigated. The relationship between the specific capacitance of porous carbons and their specific surface areas and pore size distributions was discussed. Based on these, the technologic methods and conditions of controlling structure and properties of porous carbons used as electrode materials of electric double-layer capacitance （EDLC） were established, which would provide a theoretic and experimental support for further research on porous carbon with excellent performance.From phenol-formaldehyde resin, the two kinds of high specific surface areas porous carbons were derived by carbonization followed by KOH activation, and by one step of K₂CO₃ activation, respectively. The influences of the content of activated reagent, the curing temperature, carbonization temperature and activation temperature on yield, adsorption properties, pore structures and electrochemical performances of porous carbons were studied. The surface morphology images of the samples were observed by field emission scanning electron microscope. The BET specific surface area and pore structure of porous carbons were determined by adsorption method of low temperature nitrogen. Also, the relationship between the specific capacitance and specific surface area, pore size distribution was studied.Results show that the kinds of activating reagent have great effect on the pore structure of high specific surface area porous carbon. For KOH as activating reagent, the obtained carbons provide predominantly micropores, which have a narrow PSD with smaller than 2 nm. In contrast, for K₂CO₃ as activating reagent, the pore structure of carbons derived is composed of micropores and mesopores in 3⁴nm range. Furthermore, the mesopores volume ratios could be increased by increasing K₂CO₃ content and activation temperature. The specific capacitance of obtained carbon increases gradually with increasing of the specific surface area, but the relationship between them is not linear dependence. It is because high specific surface area porous carbon has more micropores and its surface is difficultly infused by electrolyte. The best technologic conditions of KOH activation method are the curing temperature, carbonization temperature, the ratio of KOH to carbon and activation temperature being 150℃, 700℃, 4 and 800℃respectively. The highest specific capacitance of 296F/g could be reached and its BET surface area, iodine adsorption value and tap density are 1824m²/g, 1722mg/g and 0.35 g/cm³, respectively. Meanwhile, For carbon prepared by K₂CO₃ activation method, the highest specific capacitance, BET surface area, iodine adsorption value and tap density are 202F/g, 1239m²/g, 1400mg/g and 0.16 g/cm³, when the curing temperature, the ratio of K₂CO₃ to phenol-formaldehyde resin and activation temperature being 120℃, 2 and 1000℃, respectively.The polymer blends technique was proposed as a method for designing porous carbon materials used as electrodes of EDLC. Phenol-formaldehyde （PF） resin and polyvinyl butyral （PVB） or Polyethylene glycol （PEG） were used as carbon precursor polymer （CPP） and thermally decomposable polymer （TDP）, which were with and without carbon residue after heating under an inert atmosphere, respectively. The influences of the kinds of TDP and its content, curing temperature and carbonization temperature on the surface morphology image, pore distribution and specific capacitance of porous carbon were studied. Furthermore, the pyrolytic behaviors of single PF resin and single PVB or PEG and their blends in course of carbonization were researched; the mechanism of pore formation was also discussed. The results indicate that there is no interaction occurred between PF and PVB or PEG. And the micro phase separation structure formed among them is the key factor affecting pore formation of resulted carbons. With PVB as TDP, the obtained carbons have narrow pore size distribution of 4nm and high mesopore volume ratio of 50%. While for the carbon prepared by PEG as TDP, the pore sizes distribute in the range of 2¹0 nm and the highest mesopore volume ratio of 85% could be reached.By polymer blends of PF and PVB, the pore structure and performances of prepared carbons are affected by the content of PVB, curing temperature and carbonization temperature. With the increase of PVB content, curing and carbonization temperature, the specific capacitance increases, and the maximum value （130F/g） results at the PF/PVB ratio of 3, curing temperature of 120℃and carbonization temperature of 900℃. The capacitance performance has correlation with the specific surface area, pore volume and pore size distribution.While by polymer blends of PF and PEG, the content and molecular weight of PEG are the key factors affecting the pore structure and performance of obtained carbons. As PEG content increases, pore sizes of obtained carbons decrease, while specific surface area, total pore volume and specific capacitance increase firstly and then decrease, and the maximum values result at PF/PEG mass ratio of 1. With the increase of PEG molecular weight, the specific surface area and specific capacitance increase, when the molecular weitht of PEG is 10K, the highest specific surface area of 484 m2/g and specific capacitance values of 90F/g could be reached.Carbon aerogels with high specific surface area of 731m2/g and developed mesopore structure were produced successfully by sol-gel polymerization of resorcinol-formaldehyde （RF） aerogels followed by ambient drying and pyrolysis. The effects of R/C ratio, mass ratio, pH value and carbonization temperature on the pore structure and capacitance performance were studied systematically. And the technologic conditions of preparing carbon aerogels were established.The results show that the pore structure of carbon aerogel could be controlled by changing synthetic conditions such as R/C ratio, mass ratio and pH value. The pore sizes increase with the R/C ratio, while increase firstly and then decrease with the increase of mass ratio and pH, the maximum value results at mass ratio of 40% and pH of 6.5. With increase of specific surface area and mesopore volume ratio, the specific capacitance of carbon aerogel increases. Howerver, it decreases instead when mesopore volume ratio and pore size is over large. In conclusion, only when the pore size is closer to the ionic size, the best capacitance performance of 160F/g could be achieved. The optima technologic conditions are that R/C ratio is 300, mass ratio is 30%, pH value is 6.0 and carbonization temperature is 900℃.A novel RF resin was self-assembly synthesized firstly, through polymerization of resorcinol and formaldehyde by employing Hexamethylenetetramine （HMTA） as both a catalyst and a curing agent. From the novel RF resin, the porous carbon was produced only by heat treatment without any additional activation process, and further used as electrode materials of EDLC. This RF polymer-based carbon shows attractive characteristics such as high specific surface area （650¹000m2/g）, optimum pore size distribution （1¹0nm）, high carbonization yield （⁵0 %） and large specific capacitance （up to 200 F/g）. The effect of F/R ratio, R/H ratio, R/S ratio and carbonization temperature on the specific surface area, pore size distribution, chemical structure and electrochemical capacitance was studied, respectively. The results show that HMTA plays an important role; it works not only a catalyst but also cross-linking reagent. On one hand, it promotes the formation of RF nano-cluster by polymerization of resorcinol with formaldehyde. On the other hand, it provides methylene group to connect the RF nano-cluster to form porous resin while not gel. Compared with RF gel, the obtained porous RF resin does not need long time to dry, which would shorten the preparation period greatly. RF resin-based carbons show an interconnected pore structure and the pore sizes are generally 2⁵ nm in diameter, which is beneficial to the penetration and mobility of ion in micropores of the porous carbon. The micropores result from porosity in the nano-cluster while mesopores correlate with prolysis of HMTA. The main factors affecting the structure and properties of RF resin-based porous carbon are the R/H ratio, R/S ratio, F/R ratio and carbonization temperature. With the R/H ratio increases, pore sizes increase and polydispersed pore structure is obtained. For low R/H ratio, the resulting polymer is composed of interconnected spherical particles with diameters ofμm. In contrast, for higher R/H ratio, the polymer obtained is irregular in shape but with porous structure. The specific surface area and pore volume increase with R/S ratio, while increase firstly and then decrease with the increase of F/R ratio and carbonization temperature, and the maximum values result at F/R ratio of 1.6 and carbonization temperature of 900℃. The highest specific capacitance of 200F/g could be reached, at R/H ratio of 50, R/S ratio of 0.1 g/ml, F/R ratio of 2.0 and carbonization temperature of 900℃.Baed on the research above, the correlation between electrochemical properties such as electric doule-layer capacitance, charging/discharging performance, frequency response characteristic and pore structure and surface area was investigated by means of galvanostatic charging/discharging, cycle voltammetry and impedance spectroscopy. The power density properties and formation of electric double-layer capacitance were discussed. The pore size distributions of carbons were analyzed by electrochemical in-situ method. Several kinds of porous carbons prepared by different methods were compared on the pore structure and electrochemical performance. The results show that the EDLC with carbons electrodes prepared by above different methods has an excellent stability at large discharge current and long cycle life. After 5000 cycles, no decrease of the capacitance was observed. For porous carbon prepared by KOH activation, the dc specific capacitance is the largest, while for the RF resin-based carbon, the AC specific capacitance is the largest. The RF carbon aerogel prepared by sol-gel polymerization shows an excellent charging and discharging performance at large discharge current. With the increase of discharge current density in experimental range, the capacitance of RF carbon aerogel has almost unchanged, which is attributed to the high meso-porosity of electrode material. The specific capacitance not only correlates with surface area, but also depends on the pore size distribution of carbons. The power characteristics and resistance are determined by the pore structure of porous carbons. Increasing the pore size has a positive effect on the charging resistance and charging/discharging response.