聚吡咯/硝酸活化碳气凝胶纳米复合材料的制备表征及其在超级电容器中的应用

李亚捷　倪星元　沈　军　刘　冬　刘念平　周小卫(同济大学物理科学与工程学院，上海市特殊人工微结构材料与技术重点实验室，上海200092)

聚吡咯/硝酸活化碳气凝胶纳米复合材料的制备表征及其在超级电容器中的应用

李亚捷倪星元*沈军刘冬刘念平周小卫
(同济大学物理科学与工程学院，上海市特殊人工微结构材料与技术重点实验室，上海200092)

通过化学氧化聚合法制备出不同比例的聚吡咯(PPY)/硝酸活化碳气凝胶(HCA)复合材料。采用傅里叶变换红外光谱(FT-IR)和扫描电子显微镜(SEM)表征材料的成分和形貌，结果表明，通过硝酸活化及与聚吡咯的复合，并未破坏碳气凝胶的多孔形貌，硝酸活化碳气凝胶及聚吡咯/硝酸活化碳气凝胶都仍然保持着原碳气凝胶的三维纳米多孔结构。采用对照实验的方法，设计并合成五组不同配比的复合材料，聚吡咯与硝酸活化碳气凝胶的质量比例分别为3:1、2:1、1:1、1:2、1:3，通过循环伏安法，恒流充放电，交流阻抗及循环性测试等考察材料的电化学性能。结果证明，当聚吡咯与硝酸活化碳气凝胶比例为1:1时，复合材料显示出最优电化学性能：比电容高达336 F∙g－1，是纯碳气凝胶(103 F∙g－1)的三倍有余，除此还显示出卓越的导电性与循环稳定性，2000次循环后仍保持初始电容的91%，具备优良的超级电容器电极材料性能。因此聚吡咯/硝酸活化碳气凝胶复合纳米材料是超级电容器的理想电极材料。

活化碳气凝胶；聚吡咯；超级电容器；化学氧化聚合法；复合电极材料；电化学性能

doi:10.3866/PKU.WHXB201511131

1　Introduction

Increasing attention has been paid to supercapacitors due to their high power and energy densities,long cycle life and wide range of operating temperature.Besides,they are safe and environmentally friendly1.Thus,they are widely used in power and energy applications such as hybrid electric vehicles(HEVs),burst power generation,and backup sources2.

According to the energy storage mechanism,supercapacitors can be classified into two categories:electrochemical double-layer capacitors(EDLCs)and pseudo-capacitors3.In the EDLCs,energy is stored by the accumulation of ionic and electronic charges at the interface between electrolytes and electrode materials4.Carbonaceous materials,such as carbon fibers5,porous carbons6,activated carbons7,and carbon aerogels8,are promising electrode materials for their high specific area,long cycle life,and relatively low cost.In particular,carbon aerogel(CA)is a unique class of three-dimensional nanoporous carbon materials that have high surface area,good electrical conductivity,and high porosity1,9. However,its specific capacitance is lower than expected due to poor wettability.Chemical surface modification in nitric acid solutions has been reported to improve the wettability of carbon materials,which results in a higher usable surface area,smaller internal resistance,and higher specific capacitance10.By comparison,pseudo-capacitors store energy through relatively fast and reversible faradaic processes in a redox-active material at characteristic potentials11.They are able to store charge not only in the electrical double layer,but also throughout the body of the electrodes by rapid faradaic charge transfer.The faradaic pseudocapacitance of pseudo-capacitors is almost 10－100 times higher than EDLCs,but the improvement of capacitance by pseudofaradaic reactions is not stable and the capacitance decays with cycling11,12.Transition metal oxides including ruthenium and tantalum oxides are ideal electrode materials for pseudo-capacitors because they have great specific capacitance.However,their high cost has limited the practical application13.Electrically conducting polymers(ECPs),such as polyaniline(PANI)14,poly(3-methylthiophene)(pMeT)15,and polypyrrole(PPY)16,are promising electrode materials due to their high conductivity and relatively low cost.Among these ECPs,PPY has good thermal and environmental stability,high storage stability,and relative ease of synthesis16.However,during the cyclic electrochemical oxidation and reduction process,continuously injection and rejection of solvated ions will lead to the framework swelling and contraction of the polymer chain,which influences the cycling performance of PPY17.

Nowadays,supercapacitors which combine the advantages of both EDLCs and pseudo-capacitors have become a promising subject.For example,some researchers combined carbonaceous materials with ECPs or with transition metal oxides to synthesize composite electrodes such as CNTs(carbon nanotubes)-PANI composite18,CNTs(carbon nanotubes)-PPY composite19,TiO2-activated carbon composite20,and manganese oxide/MWNTs (multiwalled carbon nanotubes)composite electrodes21.The results show that after recombination,the electrochemical performances of electrodes are improved.

In this work,PPY/nitric acid activated carbon aerogel(HCA) composites were synthesized via chemical oxidation polymerization with different PPY contents and we explored the optimum mass ratio of HCA to PPY.The method involved in this paper is simple,convenient and beneficial for commercial applications. The final results indicate that the PPY/HCA composite with the mass ratio of 1:1 has high specific capacitance,excellent conductivity,and long term stability.

2　Experimental

2.1Preparation of carbon aerogel

The carbon aerogel was prepared via a sol-gel process:resorcinol(R)(analytical reagent)and formaldehyde(F)(analytical reagent)were mixed in a 1:2 molar ratio with alkaline sodium carbonate(C)(analytical reagent)as catalyst and deionized water as solvent.The mass percentage of reactions in solution was 30%, the molar ratio of R to C was held at a constant value of 500. Stirring the above solution at room temperature for 30 min,then the hydrosols were sealed and heat-treated at 30°C for one day, 50°C for one day,and 90°C for three days,respectively.The resultant RF hydrogels were rinsed in an ethanol bath for a week, then CO2supercritical drying at 31.8°C and 7.3 MPa was carried out to get cylindrical organic aerogels.The obtained RF organic aerogels were carbonized at 1050°C for 3 h with the rising temperature rate of 5°C∙min－1in a tubular furnace under N2flow of 100 mL∙min－122,23.The obtained RF carbon aerogel sample was denoted as CA.

2.2Nitric acid activation of carbon aerogel

CAwas dispersed in concentrated nitric acid(65%)at 60°C for 12 h,filtered and washed with deionized water until the pH was about 6,then the product was dried at 90°C for 24 h,denoted as HCA.

2.3Synthesis of polypyrrole/nitric acid activated carbon aerogel

Firstly,0.1 g HCAwas dispersed in 200 mL of deionized water, then mixed with 0.02 g sodium dodecyl sulfate(SDS)(analytical reagent)as surfactant,stirred for 30 min and kept for ultrasound for 2 h.Secondly,pyrrole monomer(analytical reagent)was added into the above solution in five kinds of pyrrole:HCAmass ratios (1:1,2:1,3:1,1:2,1:3),stirred for 10 min and irradiated by ultrasonic wave for 30 min.Thirdly,0.2 g FeCl3(93%)was addedinto the solution as oxidant to motivate polymerization reaction, stirred for 30 min and kept for ultrasound for 2 h.The reaction was carried out under static condition for 24 h and then the PPY/HCA composite precipitate was filtered and washed with deionized water and ethanol.Lastly,the product was dried at 60°C for 24 h24.The name of PPY/HCAcomposite was abbreviated as PPYHCA-x,where x stands for the mass ratio of PPY to HCA.For example,PPY-HCA-21 means that the mass ratio of PPY to HCA was 2:1.

2.4Structural characterization

The components of samples were determined by Fourier transform infrared(FT-IR)spectroscopy(Bruker-TENSOR27). The field emission scanning electron microscopy(FESEM,Philips XL30FEG)was used to examine the structures and morphology of materials.N2adsorption isotherms were recorded with an AUTOSORB-1 Surface Area Analyzer(Quantachrome Instrument Corporation)at－196°C.Prior to measurements,the samples were degassed at 300°C for 2 h.The specific surface areas were determined on the basis of the Brunauer-Emmett-Teller(BET) method.The pore size distribution was obtained by employing density functional theory(DFT).The total pore volume was estimated from the amount of N2adsorbed at the relative pressure of p/p0=0.99.And t-plot method was used to determine the micropore surface area and micropore volume.

2.5Electrochemical measurements

Working electrodes were prepared by the following method:a mixture of active materials,conductive carbon black,and polytetrafluoroethylene(PTFE)at a mass ratio of 8:1:1 was pasted onto nickel foam,pressed at 8 MPa and dried at 80°C for 48 h.Electrochemical tests were carried out at room temperature using an electrochemical work station(CHI660C,Chenhua, China).A three-electrode cell was set up using nickel foam as counter electrode,Hg/HgO electrode as reference electrode,PPY/ HCA composite as working electrode,and KOH(6 mol∙L－1) aqueous solution as electrolyte.

Cyclic voltammetry(CV)measurements were conducted over a potential window from－0.8 to－0.1 V at different scan rates ranging from 5 to 100 mV∙s－1.Galvanostatic charge-discharge measurements were ranged from－0.8 to－0.1 V at different current densities ranging from 0.5 to 5 A∙g－1.Electrochemical impedance spectroscopy(EIS)measurements were recorded from 0.01 Hz to 100 kHz with 5 mV amplitude ofAC signal.

3　Results and discussion

3.1Structural characterization

Fig.1(a)shows the FT-IR spectra of CA,HCA,PPY-HCA-11, PPY-HCA-21,and PPY-HCA-31.As shown in this image,the spectrum of HCA is similar to that of CA,but a few new peaks such as 1690 and 1725 cm－1appear,which represent the deformation vibration of―COOH and＝C＝O,respectively.Because of the strong oxidation of concentrated nitric acid,oxidationreduction reaction may take place on the surface of CA,engendering a great deal of oxygen containing groups such as―COOH and＝C＝O with hydrophilic property,which could improve the wettability of CA12.The spectra of three composites are similar but they are quite different from CA and HCA,there emerge a series of new peaks:794 cm－1is attributed to the C＝C deformation of PPY,the broad band at 1320 cm－1demonstrates the C―H and C―N in-plane deformation vibration,966 and 1047 cm－1are attributed to the C―H deformation of PPY,the peak near 930 cm－1is attributed to the characteristic absorption of pyrrole ring,the peak near 1213 cm－1is attributed to the C―N stretching vibration of PPY,the peaks near 1462 and 1546 cm－1are attributed to C―N and C―C asymmetric and symmetric pyrrole ring stretching, respectively.Additionally,2850 and 2920 cm－1present the C―H asymmetric and symmetric stretching vibration of SDS which acted as surfactant25,26.The above results illustrate that PPY/HCA composites are successfully synthesized.

Fig.1(b)is the standard infrared spectrum of PPY27,where we can see characteristic peaks:794,930,966,1047,1213,1320, 1462,and 1546 cm－1,they are allattributed to PPY and are all conformed to former analysis.Therefore,PPY/HCA composites are successfully prepared via chemical oxidative polymerization.

Fig.1　(a)FT-IR spectra of CA,HCA,PPY-HCA-11,PPY-HCA-21,PPY-HCA-31 and(b)standard infrared spectrum of PPY

The SEM images of CA,HCA and PPY/HCA composites are shown in Fig.2.From Fig.2(a,b),it can be seen that CAand HCA consist of interconnected sphere nanoparticles with diameters of 30 to 40 nm.Both of them have porous structure,which suggests that nitric acid activation did not destroy the overall skeleton andmaintains the three-dimensional nanoporous structure of carbon aerogel.Fig.2(c,d,e)shows the SEM images of PPY-HCA-11, PPY-HCA-21,and PPY-HCA-31,respectively.They are similar to CA,and appear no obvious clusters,which indicate that PPY is uniformly coated on the carbon aerogel.The composites still maintain three-dimensional nanoporous structure,which facilitates the ion transfer in charging and discharging processes.

Fig.2　SEM images of CA(a),HCA(b),PPY-HCA-11(c),PPY-HCA-21(d),and PPY-HCA-31(e)

Fig.3 shows theN2adsorption-desorptionisothermsofCA,HCA, PPY-HCA-11,PPY-HCA-21,and PPY-HCA-31,respectively, which indicates a similar structure of 5 samples.As can be seen from the left of the curves,it reaches single layer absorption very fast,which demonstrates rich pore structures with diameter under 2 nm.The hysteresis loops on the right indicate abundant number of pores between 2 and 50 nm.All 5 samples show abundant structure of mesopore and micropore,which also proves the result of the FESEM that the pore structure remains after activation and composition.

Fig.3　N2adsorption-desorption isotherms of CA,HCA,PPY-HCA-11,PPY-HCA-21,and PPY-HCA-31

Fig.4 shows the comprehensive N2adsorption-desorption isotherms of CA,HCA,PPY-HCA-11,PPY-HCA-21,and PPY-HCA-31,which shows the pore volume of 5 samples.As can be seen from the left of the curves,CA has the highest volume platform on the left,following with HCA,PPY-HCA-11,PPY-HCA-21,andPPY-HCA-31.According to the adsorption theory,higher volume indicates more micropores,makes CA with the most micropores. And same order of curve height can be observed on the right. Higher volume indicates more pore structure and huger pore volume.Therefore,CAhas the most quantity of pore with hugest pore volume and followed with HCA,PPY-HCA-11,PPY-HCA-21,and PPY-HCA-31.

Fig.4　Comprehensive N2adsorption-desorption isotherms of CA, HCA,PPY-HCA-11,PPY-HCA-21,and PPY-HCA-31

Fig.5 shows the pore size distribution of 5 samples.It can be seen that most of the pores are mesopore between 10 and 50 nm. But there is a 4 nm peak on the curve of HCA,indicates the present of the micropore which may be generated during the activation.After polymerization,micropores are filled by PPY while has little effect on mesopore.This result demonstrates that all 5 samples have rich mesopore structure consistent with FESEM and N2adsorption-desorption isotherms results.

Fig.5　Pore size distribution of CA,HCA,PPY-HCA-11,PPY-HCA-21,and PPY-HCA-31

The schematic of PPY/HCA composite preparation process is shown in Fig.6.CAand SDS dispersed in the deionized water,the hydrophobic groups-long chain alkyl of SDS makes it attached to CAquickly.Simultaneously,the hydrophilic groups-sulfate anions of SDS form a negative charge layer on the surface of CA.Then pyrrole monomer,which was added into the above solution,will be attracted by the negative charge layer due to electrostatic interaction.Upon adding FeCl3as oxidant,pyrrole monomer on the surface of carbon aerogel will polymerize to PPY via chemical oxidative polymerization28.Finally,PPY/HCAcomposite materials are prepared.

Fig.6　Schematic diagrams of the PPY/HCAcomposite preparation process

3.2Electrochemical performance

The cyclic voltammogram(CV)curves of samples at the scan rate of 50 mV∙s－1are shown in Fig.7.It can be found that the CV curves of CA and HCA are rectangular and symmetrical,indicating a typical electric double layer(EDL)behavior and good stability.The energy storage mechanism of carbon aerogel in EDLCs has been described in previous literature29.HCA exhibitslarger CV area than that of CA,revealing that it has larger specific capacitance than CA.The CV curves of PPY-HCA-11,PPY-HCA-21,PPY-HCA-31,PPY-HCA-12,and PPY-HCA-13 are similar: compared with the CV curves of HCA,the oxidation and reduction peaks can be obviously seen,which are owing to the redox reaction of PPY18.

Fig.7　Cyclic voltammogram curves of samples at a scan rate of 50 mV∙s－1

ThisredoxreactionofPPYisrelatedwithsequentialLewisbaseor Lewis acid-producing steps:in the discharging process,a reductionprocesswithreleaseofhydroxylionsisinvolved.Withthe involvementofionsoftheelectrolyte,thisLewisionizationprocess quotesaquasi-linear,one-dimensionalcylindrical Helmholtz-like double layer developed;in the charging process,due to a Lewis ionization process which involves oxidation with electron transfer, positive charges are introduced on the PPYchain by p-doping30,31.

Fig.8　Cyclic voltammogram curves of CA(a),HCA(b),PPY-HCA-11(c),PPY-HCA-21(d),PPY-HCA-31(e), PPY-HCA-12(f),and PPY-HCA-13(g)at different scan rates

In order to estimate the detail electrochemical properties,cyclic voltammetry was carried out for each electrode at various scan rates of 5,10,50,and 100 mV∙s－1,as shown in Fig.8.The specific capacitance of the electrode can be calculated by the following equation:

whereIistheaveragecurrent,mistheeffectivemassofelectrode materials,andvisthescanrate32.TheeffectivemassesofCA,HCA, PPY-HCA-11,PPY-HCA-21,PPY-HCA-31,PPY-HCA-12,and PPY-HCA-13 are 0.012,0.011,0.008,0.012,0.012,0.012,and 0.012g,respectively.TheresultsobtainedarelistedinTable1.

By comparing the specific capacitance of CAwith that of HCA, we can find that after nitric acid activation,the capacitance is 50% higher than CA,which is owing to the improvement of wettability. Thus the utilization rate of electrode materials in aqueous electrolytes increases.PPY/HCA composite electrode materials havemuch higher specific capacitance,almost 1－2 times higher than that of CA.On the whole,PPY-HCA-11 has the highest specific capacitance,followed by PPY-HCA-31,PPY-HCA-21,PPY-HCA-11 and then PPY-HCA-13,their specific capacitances are all much higher than those of HCA and CA.The specific capacitance of PPY-HCA-11 reaches 336 F∙g－1at a scan rate of 5 mV∙s－1,while the capacitance of CA electrode is only 103 F∙g－1.According to the above results and related theories,we can come to the following conclusions:composite materials combine the double-layer capacitances of HCAand Faradic capacitances of PPY.Therefore, the composites have a substantial increase in specific capacitance18.

Table1　Specific capacitances of materials at different scan rates

In general,the specific capacitance of each sample increases with the decreasing of scan rates,the reason is that electrostatic adsorption-desorption reaction and oxidation-reduction reaction occur not only in the surface of electrode,but also inside the materials33.Hence at lower scan rate,the electrolyte can penetrate well into the electrode materials,increasing the utilization rate of materials,so the specific capacitance is improved.

The charge-discharge curves of samples measured in 6 mol∙L－1KOH aqueous electrolyte at a current density of 1A∙g－1are shown in Fig.9.The curves of CAand HCAare almost linear and present typical symmetrical triangle shape,indicating that they have double-layer capacitive behavior,while the curves of composite electrodes are not linear due to the existence of the Faradic reaction of PPY.

Fig.9　Charge-discharge curves of samples at a current density of 1A∙g－1

The specific capacitance of samples can be calculated according to equation(2):

where,t is the discharge time,and ΔV is the voltage34.From Fig.9 we can find that the discharge time sequence of samples is PPYHCA-11>PPY-HCA-31>PPY-HCA-21>PPY-HCA-12>PPYHCA-13>HCA>CA,so does the order of specific capacitance. This sequence is resistent with the results acquired by cyclic voltammetry,illustrates that composite electrodes have larger specific capacitance attributed to the existence of pseudo-capacitance and confirms that PPY-HCA-11 has the highest specific capacitance.

At the beginning of the discharge,a sharp voltage change, which can be used to estimate the resistance of materials,is observed in each curves,the larger the voltage dip,the bigger the equivalent series resistance(ESR).The resistance of samples can be calculated by the following equation:

where ΔU is the voltage dips and I is the charge-discharge current32.The resistances of CA,HCA,PPY-HCA-11,PPY-HCA-21, PPY-HCA-31,PPY-HCA-12,and PPY-HCA-13 are 0.026,0.018, 0.014,0.013,0.011,0.013,and 0.017 Ω,respectively.PPY-HCA composites have lower resistance than CA and HCA,indicating that the addition of PPY improve the conductivity of CA.For supercapacitors that utilize pseudo-capacitance,there are three types of electrochemical processes in the energy storage:(1) surface adsorption of ions from the electrolyte;(2)redox reactions involving ions from the electrolyte;(3)the doping and undoping of ECPs in the electrodes.In the above processes,the electrodes must have high electronic conductivity to distribute and collect the electron current3.Hence the composites with higher conductivity are ideal electrode materials for supercapacitors.

Furthermore,to understand more about the electrochemical behavior of PPY-HCA-11,galvanostatic charge-discharge tests were carried out at various current densities of 0.5,1,2,3,4,and 5A∙g－1,as shown in Fig.10.With the increasing of current density, the curves still present typical symmetrical triangle shape. However,the specific capacitance decreases due to the relatively low rate of ions diffusion within micropores at large current density.

Fig.10　Charge-discharge curves of PPY-HCA-11 at different current densities of 0.5,1,2,3,4,and 5A∙g－1

Electrochemical impedance spectroscopy(EIS)is a useful technique to characterize the electrochemical properties of elec-trodes.The Nyquist plots of electrodes are shown in Fig.11(a).All of them exhibit three connected parts:a semicircle in the high frequency region which corresponds to the charge transfer reaction at the interface of electrode and electrolyte,a 45°line in the intermediate frequency region associated with Warburg impedance of ion diffusion inside the electrode materials and a straight line in the low frequency region responding to the capacitive performance35.

The specific capacitance can be derived from the imaginary part of the impedance spectrum and frequency according to the following equation:

where,f is the frequency,and Zʺis the imaginary impedance36. Fig.11(b)shows the specific capacitance of samples on frequency derived from impedance spectroscopy.The specific capacitance of electrodes increased with the decreasing of frequency.This is because the electrolyte ions can reach the inner surface sites of the electrode materials under lower frequency37.In the lower frequency region,under the same frequency,the order of the specific capacitances of electrodes is PPY-HCA-11>PPY-HCA-31>PPYHCA-21>PPY-HCA-12>PPY-HCA-13>HCA>CA,which is in accordance with the results obtained from cyclic voltammetry and galvanostatic charge-discharge test.It confirmed that PPYHCA-11 has the best capacitance-frequency response and the highest specific capacitance.

Fig.11　AC impedance spectra of materials

The cycle life is a significant factor for supercapacitors.As shown in Fig.12,the specific capacitance of CAand HCAis nearly 100%after 2000 times.The specific capacitance of HCA is 50% higher than CA,indicating that nitric acid activation does not influence the cyclic stability of carbon aerogel and improve the specific capacitance.Composite electrode materials have higher capacitance,but their stability is inferior to CA and HCA.After 2000times,thecapacitydeteriorationsofPPY-HCA-11,PPY-HCA-21,PPY-HCA-31,PPY-HCA-12,andPPY-HCA-13are9%,15%, 21%,12%,and20%,respectively.Thecapacitydeteriorationrises withthechangingofthecontentofPPY,PPY-HCA-11showingthe best stability among composite materials:the loss of capacitance mainly happens at first 500 cycles,after 1000 times the specific capacitance stabilizes at a fixed high value;after 2000 times,the specificcapacitancestillremains91%oftheinitialvalue,whichis stillmuchhigherthanothersamples.Theorderofthespecificcapacitances of electrodes is PPY-HCA-11>PPY-HCA-31>PPYHCA-21>PPY-HCA-12>PPY-HCA-13>HCA>CA,whichis consistent with the results obtained from cyclic voltammetry,galvanostaticcharge-dischargetest,andelectrochemicalimpedance spectroscopy.ThisresultalsocorroboratesthatPPY-HCA-11has thehighestspecificcapacitance.

Fig.12　Cyclic life of materials CA(a),HCA(b),PPY-HCA-11(c), PPY-HCA-21(d),PPY-HCA-31(e),PPY-HCA-12(f), and PPY-HCA-13(g)

In the composite materials,PPY can enhance the capacitance remarkably;meanwhile,as a conductive framework,HCA can increase the cycling and physical stability of PPY.The above results confirmed that the optimum ratio of PPYto HCAis 1:1.

The reasons are as follows:SEM shows that the sample has rich pore structure and appears no obvious clusters,which indicates that PPY is uniformly coated on the carbon aerogel,and maintains the three-dimensional nanoporous structure after activation and composition.The pore structure facilitates the ion transfer in charging and discharging processes,thus propitious to the improvement of electrochemical properties.

N2adsorption-desorption isotherms also show abundant structure of mesopore and micropore in samples.The composite contains carbon aerogel and PPY,which combine the double-layer capacitances of HCA and Faradic capacitances of PPY.If PPY is over 50%,the pseudo-capacitance will be too huge for cycle performance as well as huge impedance of PPY,and poor cycling stability.If PPY is less than 50%,the pseudo-capacitance will be too low to improve the specific capacitance.

According to the above results and related theories,we can come to the following conclusions:composite materials combine the double-layer capacitances of HCA and Faradic capacitances of PPY.Therefore,the composites have a substantial increase in specific capacitance.The above results confirmed that the optimum ratio of PPY to HCAis 1:1,the composite material shows the best electrochemical properties.It is a promising electrode material for supercapacitors.

4　Conclusions

Wettability of CAcan be improved by surface modification with nitric acid.After activation,the specific capacitance of CA increases by 50%.Besides,the modified CA maintains excellent cycle stability.The PPY/HCA composites have three-dimensional nanoporous structure as CA.Because they possess not only doublelayer capacitance but also pseudo-capacitance,their specific capacitances are 1－2 times higher than that of CA.With the increasing of PPY content in composites,the conductivity increases, but the long term stability becomes worse due to the existence of Faradic pseudo-capacitance.

PPY-HCA-11 has the highest specific capacitance among samples,its capacitance reaches 336 F∙g－1at a scan rate of 5 mV∙s－1,while the capacitance of CAis only 103 F∙g－1,it also has good cycling stability and retains 91%of initial capacitance over 2000 times,which is still much higher than that of CA.Besides,its conductivity is excellent and it is more cost-saving than other composites.Consequently,the PPY/HCAcomposite with the ratio of 1:1 is an ideal electrode material for supercapacitors.

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Preparation and Performance of Polypyrrole/Nitric Acid Activated Carbon Aerogel Nanocomposite Materials for Supercapacitors

LI Ya-JieNI Xing-Yuan*SHEN JunLIU DongLIU Nian-PingZHOU Xiao-Wei
(Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology,Institute of Physical Science and Engineering,Tongji University,Shanghai 200092,P.R.China)

Polypyrrole(PPY)/nitric acid(HNO3)activated carbon aerogel(HCA)composites are prepared through chemical oxidative polymerization with different PPY/HCAmass ratios.Fourier transform infrared(FTIR)spectroscopy and scanning electron microscope(SEM)were employed to investigate the components and morphology of the samples.The results demonstrate that the synthesized materials maintain the threedimensional nanoporous structure of the carbon aerogel(CA);the activation by nitric acid and composition with PPY do not destroy the porous structure of the carbon aerogel and the complex still has the original threedimensional nanoporous structure.Composites with different mass ratios(3:1,2:1,1:1,1:2,1:3)of PPY/ HCA were prepared and the electrochemical properties were measured by cyclic voltammetry,galvanostatic charge-discharge test,and electrochemical impedance spectroscopy.The results confirm that the PPY/HCA composite with a ratio of 1:1 exhibits the best electrochemical performances;it has a high specific capacitance of 336 F∙g－1,which is more than two times higher than that of CA(103 F∙g－1);it also exhibits outstanding conductivity and cycling stability,retaining 91%of its initial capacitance after 2000 cycles.Therefore,thiscomposite is quite a promising electrode material for supercapacitors.

June 25,2015;Revised:November 10,2015;Published on Web:November 13,2015.

Activated carbon aerogel;Polypyrrole;Supercapacitor;Chemical oxidative polymerization; Composite electrode material;Capacitive property

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*Corresponding author.Email:nixingyuan@tongji.edu.cn;Tel/Fax:+86-21-65986071.

The project was supported by the National Natural Science Foundation of China(51072137,50802064,11074189),Key Projects in the National

Science&Technology Pillar Program,China(2009BAC62B02),and Shanghai Committee of Science and Technology,China(11nm0501600).

国家自然科学基金(51072137,50802064,11074189),国家科技支撑计划重点项目(2009BAC62B02)及上海科学技术委员会项目(11nm0501600)资助

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