Direct Activation Preparation And Performances Of Polyacrylonitrile-based Activated Carbon Fibers For Supercapacitors

The main commercial carbon-based materials with high specific capacitancehave been widely used as supercapacitor electrodes, such as activated carbon (AC),activated carbon fibers (ACFs). In comparison to AC，the ACFs as supercapacitorelectrode material have drawn more and more attention owing to their high specificcapacity, good conductivity, pores exposed to fiber surface, excellent characteristicsof strength, formability and chemical stability. However, the cost of the ACFs isvery high, the price of ACFs is about5~100times higher than that of AC, whichgreatly limits the application of the ACFs in the supercapacitor. Therefore, decliningthe cost of ACFs has important significance of practical application. In this study, inorder to reduce the cost and improve capacitance performance of ACFs, we havesynthesized the ACFs from polyacrylonitrile-based pre-oxidized fibers (PANOFs)and polyacrylonitrile-based carbon fiber waste from the perspective of simplifiedpreparation processes and using cheap raw materials. Furthermore, we have studiedthe preparation processes and capacitance performance of ACFs in detail. The mainresults of this research are as follows: (1) PANOFs used as precursor, the ACFs are prepared by CO2direct activation.The ACFs prepared by CO2direct activation have higher yield, greater specificsurface area, higher specific capacitance, higher specific capacitance retention andbetter cycle life performance than those prepared by traditional method whichproceedes by CO2activation after carbonization. Single factor researches haveshown that the optimal activation temperature and activation time are850°C and4h,respectively. The ACFs prepared by CO2direct activation at the optimal conditionshave the specific surface area of986m2/g and total pore volume of0.45cm3/g, andare microporous amorphous materials with the composition of91.65%C,0.96%Nand7.4%O. The specific surface area and total pore volume of the ACFs preparedby CO2direct activation increase first and then decrease with the rise of activationtemperature or the extension of activation time. In the symmetric capacitor based onthe ACFs prepared by CO2direct activation, quality specific capacitance of ACFs is158F/g at5mV/s, when the scanning speed increases to100mV/s, the retention rateof specific capacitance is82.3%. At0.5A/g in6M KOH solution, the decay ofspecific capacitance is9.7%after10000galvanostatic charge-discharge (GCD)cycles.The results shows that ACFs prepared by CO2direct activation have good ratecapability and cycle life performance in6M KOH solution.(2) PANOFs used as precursor, the ACFs are prepared by KOH directactivation. The ACFs prepared at800°C with impregnation ratios of1:2for1hexhibit the specific surface area of3029m2/g, mesopore percentage of84.2%andtotal pore volume of2.006cm3/g. In6M KOH solution, ACFs demonstrate goodperformance of electric double layer in the scanning speed range of5~100mV/s. The specific capacitance reaches288.6F/g under the GCD processes at0.5A/g. Theequivalent series resistance slightly increases from initial0.33ohm cm-2to0.38to0.42after10000GCD cycles.The surface structure properties of the ACFsprepared by KOH direct activation are controlled by activation conditions, theelectrochemical properties of the ACFs are effected by specific surface area and porestructure of materials. In this study, the bigger the specific surface area is, the higherthe mesoporosity is, the greater the specific capacitance is. Under insufficient activation conditions like low activation temperature, only the surface of fibers areetched into microporous; Appropriate conditions like proper impregnation ratio (theweight ratio of PANOFs to KOH), activation temperature, activation time, arebeneficial to the development of pore structure. The ACFs have developedmesopores structure and a certain amount of micropores structure. The presence ofmesopores will reduce the specific surface area of the materials, but at the same timeit can improve the capacitance properties; Under the condition of excessiveactivation, the formation micropores or mesopores of ACFs will collapse, thespecific surface area and specific capacitance decrease.(3) PANOFs used as precursor, the ACFs are prepared by NaOH directactivation. In the study of experimental parameters for ACFs preparation, theoptimal impregnation ratio, activation temperature and activation time are1:3,700°C, and60min, respectively. The ACFs prepared at the optimal conditions aremesoporous amorphous materials with the composition of93.08%C,3.01%N and3.91%O, and have the specific surface area of3134m2/g and total pore volume of2.124cm3/g. The specific surface area of ACFs present volcanic shape changes withthe increase of impregnation ratios, the rise of activation temperature or theextension of activation time, respectively. In6M KOH solution, the trends of ACFsspecific capacitance change with impregnation ratios, activation temperature oractivation time are in accordance with those of specific surface area in the scanningspeed range of5~100mV/s. In symmetric capacitors based on the ACFs prepared byNaOH direct activation,6M KOH and1M Na2SO4are used as electrolyte,respectively. For fitting the electrochemical impedance spectroscopy (EIS) ofcapacitors, the charge transfer resistance (Rct) is0.1859in the alkaline solutionand0.1687in neutral solution; the equivalent series resistance of the former is0.2801, the latter is0.5826, it can be seen the electrochemical capacitors basedon ACFs prepared by NaOH direct activation in the two kinds of electrolytes bothhave low resistances. In1M Na2SO4solution, the working voltage of theelectrochemical capacitors based on ACFs is1.8V; and in6M KOH solution itsworking voltage is1.0V due to the limitation of water decomposition voltage. The energy density of electrochemical capacitor in alkaline solution is4.67Wh/kg, andthat of electrochemical capacitor in neutral solution is11.83Wh/kg which is2.53times of that in the alkaline solution.(4) The polyacrylonitrile-based carbon fiber waste used as precursor, the ACFsare prepared by NaOH direct activation. The influences of activation temperature,activation time and activator dosage on electrochemical performance of ACFs arestudied. The optimal impregnation ratio, activation temperature and activation timeare1:3,750°C and1h, respectively. The ACFs prepared at optimal conditions havethe specific surface area of2360m2/g and total pore volume of1.564cm3/g. Underthe GCD process at0.5A/g, the capacitance of ACFs in6M KOH solution and1MNa2SO4solution are168F/g and124F/g, respectively. The decay of ACFs specificcapacitance in6M KOH solution and1M Na2SO4solution are6.1%and8.9%after10000th GCD cycles at0.5A/g. At1mV/s, the capacitance of ACFs in6M KOHsolution and1M Na2SO4solution are176.2F/g and131.3F/g, respectively; whenthe scanning speed increases to200mV/s, the capacitance of ACFs in6M KOHsolution and1M Na2SO4solution are105.6F/g and87.8F/g, the retention rate ofspecific capacitance is60%and67%, respectively. The results shows that ACFsprepared from polyacrylonitrile-based carbon fiber waste by NaOH direct activationhave good capacitive performance, rate capability and cycle life performance in6MKOH solution and1M Na2SO4solution. From the fitting results of EIS of capacitorsbased on the ACFs，Rct is0.1527in6M KOH solution and0.1793in1MNa2SO4solution; the equivalent series resistance of the former is0.2788, the latteris0.6006, it can be seen the electrochemical capacitors based on ACFs have lowerresistances in the alkaline solution than that of in neutral solution.