酚醛基活性炭纤维孔结构及其电化学性能研究

利用水蒸汽活化法制备了酚醛基活性炭纤维(ACF-H2O), 对其比表面积、孔结构与在LiClO4/PC(聚碳酸丙烯酯)有机电解液中的电容性能之间的关系进行了探讨. 用N2(77 K)吸附法测定活性炭纤维的孔结构和比表面积, 用恒流充放电法和交流阻抗技术测量双电层电容器(EDLC)的电容量及内部阻抗. 研究表明, 在LiClO4/PC有机电解液中, ACF-H2O电极的可用孔径(d)应在0.7 nm以上. 随着活化时间的延长, ACF-H2O的孔容和比表面不断增大, 但微孔(0.7 nm ＜ d ＜ 2.0 nm)和中孔(d ＞ 2.0 nm)率变化很小, 活化过程中孔的延伸和拓宽同步进行, 但过度活化则造成孔壁塌陷, 孔容和比表面迅速下降. 因此, 除活化过度的样品外, 电容量随比表面积呈线性增长, 最高达到109. 6 F•g-1. 但中孔和微孔的孔表面对电容的贡献不同, 其单位面积电容分别为8.44 μF•cm-2和4.29 μF•cm-2, 中孔具有更高的表面利用率. ACF-H2O电极的电容量、阻抗特性和孔结构密切相关. 随着孔径的增大, 时间常数减小, 电解液离子更易于向孔内快速迁移, 阻抗降低, 电极具有更好的充放电倍率特性. 因此, 提高孔径和比表面积, 减少超微孔(d ＜ 0.7 nm), 是提高 EDLC能量密度和功率密度的重要途径. 然而仅采用水蒸汽活化, 只能在小中孔以下的孔径范围内进行调孔, ACF-H2O电极电容性能的提高受限.

Influence of Pore Structure of Phenolic Resin Based Activated Carbon Fibers on the Electrochemical Performance of Electrical Double-layer Capacitors

Phenolic resin based activated carbon fibers (ACF-H2O) prepared using steam activation at 800 ℃ were investigated by means of N2 adsorption, AC impedance and constant current discharge techniques. The relationship between the specific surface area and pore size distribution of ACF-H2O with their electrochemical performance as electrodes of electric double layer capacitors (EDLC) in 1 mol•L-1 LiClO4/PC was discussed in detail. It is found that the ultramicropores(d ＜ 0.7 nm＝ cannot be accessed by electrolyte solution. With increasing steam activation time, the specific surface area and pore volume increase gradually, whereas the micropore (2.0 nm＞ d ＞ 0.7 nm ) and meso-pore ratios change little due to the deepening and broadening of the micropore formed during carbonization. However, excessive activation destroyed the pore structure, causing a sharp decrease of both specific surface area and pore volume. For those samples with well-developed pore structure, there is a linear relationship between capacitance and specific surface area (C－S), and a maximum specific capacitance of 109.6 F•g-1 was achieved. The double layer capacitance per unit surface area is not the same for all porous surface, and the capacitances of 8.44 μF•cm-2 and 4.29 μF•cm-2 are obtained, for the micropore and mesopore surfaces respectively. The double layer capacitance and AC impedance data strongly depend on the porous structure, the bigger the pore, the easier and faster being accessed electrochemically, the less the time constant and resistance should be, and it can be discharged/charged at higher current density. ACF with larger pore surface and less ultramicropore is desirable for those applications either as energy devices or as power storage devices. Steam activation can only obtain carbon materials with small mesopore, thus the higher electrochemical behavior attainable in EDLC using non-aqueous electrolyte is restricted.