Preparation and electrochemical performance of a polyaniline-carbon microsphere hybrid as a supercapacitor electrode

LIU Wei-feng, YANG Yong-zhen, LIU Xu-guang, XU Bing-she

(1.Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology),Ministry of Education, Taiyuan030024, China;2.Research Center on Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan030024, China;3.College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan030024, China)

Preparation and electrochemical performance of a polyaniline-carbon microsphere hybrid as a supercapacitor electrode

LIU Wei-feng1,2, YANG Yong-zhen1,2, LIU Xu-guang1,3, XU Bing-she1,2

(1.KeyLaboratoryofInterfaceScienceandEngineeringinAdvancedMaterials(TaiyuanUniversityofTechnology),MinistryofEducation,Taiyuan030024,China;2.ResearchCenteronAdvancedMaterialsScienceandTechnology,TaiyuanUniversityofTechnology,Taiyuan030024,China;3.CollegeofChemistryandChemicalEngineering,TaiyuanUniversityofTechnology,Taiyuan030024,China)

A polyaniline-carbon microsphere (PANI-CMS) hybrid was prepared by an electrochemical deposition method and used as an electrode for supercapacitors. Field emission scanning electron microscopy and Fourier transform infra-red spectroscopy were used to characterize its morphology and structure. The supercapacitive performance of the hybrid was investigated by cyclic voltammetry, galvanostatic charge/discharge, electrochemical impedance spectroscopy and cycling tests. Results indicate that polyaniline is uniformly coated on the outer surfaces of the CMSs by the electrochemical deposition. The hybrid has a specific capacitance of 206 F·g-1at a current density of 1 A·g-1. It has a higher specific capacitance and more stable cycle performance than PANI, which is ascribed to a synergistic effect between the PANI and the CMSs.

Carbon microspheres; Polyaniline; Electrochemical co-deposition; Supercapacitor

1 Introduction

Supercapacitor as a new environment-friendly electrochemical energy storage device, has attracted growing attentions owing to its wide range of application in hybrid electric vehicles, mobile electronic devices, backup power sources for computer memory, etc.[1,2]. As electrode materials for supercapacitor, conducting polymers are recognized as typical representatives because of their unique properties, such as fast charge/discharge kinetics, low cost, mild synthesis condition and suitable morphology. In the series of the conducting polymers, polyaniline (PANI) has been considered as one of the most promising electrode materials for supercapacitors because of its easy synthesis, remarkable environmental stability, simplicity in doping, high electrochemical activity and low cost[3-6]. However, irregular granular or flake PANI films obtained by the conventional polymerization methods show poor cycle stability compared with carbon-based electrodes because the redox sites in its polymer backbone are not sufficiently stable and the backbone can be destroyed within a limited number of charge/discharge cycles. Recently, some researchers used carbons as substrate materials to prepare composites to improve the cycle life of PANI, and there is a number of literature on PANI/carbon composite electrodes such as PANI with activated carbon[7], carbon nanotubes[8], carbon fibers[9]and graphene[10]. For example, Zhu et al.[11]synthesized a PANI-MWCNT hybrid with a capacitance of 515 F·g-1compared to 273 F·g-1of pure PANI and a high cycling stability (below 10% capacity loss after 1 000 cycles). Feng et al.[12]reported a graphene-PANI hybrid prepared by the electrochemical reduction method with a high specific capacitance of 640 F·g-1with a capacitance retention of 90% after 1 000 charge/discharge cycles.

Although above mentioned PANI-carbon material hybrids have improved electrochemical properties[13,14], the difficulties in the preparation of carbon materials (graphene, carbon nanotubes, or carbon nanofibers) in large scale hinder their industrial applications. Among various carbon materials, carbon miscrospheres (CMSs), with fullerenes-like cage structures composed of fairly concentric graphitic shells, have great potential application in many fields such as reinforcing agents, lubrication, and the support of surface molecularly imprinted polymer[15,16]. What is more, CMSs can be prepared continuously by a simple chemical vapor deposition method. However, there are few reports about CMSs as a electrode material for supercapactors. The combination of conducting PANI with CMSs would be an effective way to improve the capacitance and cycling stability of PANI. Wu et al.[5]prepared a PANI-activated mesocarbon microsbead hybrid by an in situ chemical oxidation polymerization method. The hybrid possessed both high specific capacitance and excellent cycle stability. The specific capacitance stabilizes nearly at a fixed value (110.21 F·g-1) at a current density of 250 mA·g-1. Based on the PANI-activated mesocarbon microsbead hybrid, Wu et al.[7]synthesized the nitrogen-enriched carbon materials by carbonization and HNO3treatment, and the specific capacitance was 385 F·g-1at a current density of 1 A·g-1in 6 M KOH electrolyte.

Herein, the water-soluble CMSs with a high specific surface area were obtained by a combination of acid-oxidation and heat-treatment[15,17]. PANI-CMS hybrid was synthesized through a one-step electrochemical deposition method in H2SO4solution. The physical and electrochemical properties of the PANI-CMS hybrid were studied.

2 Experimental

2.1 Instruments and Materials

All chemicals were of analytical grade and all solutions were prepared using deionized water. CMSs (～350 nm in diameter) were synthesized by chemical vapor deposition. Aniline was distilled under reduced pressure before use and all other chemical reagents were used as received. Electrochemical experiments were conducted at 25 ℃ on a VMP3 Potentiostat (Princeton, USA) controlled with an EC-Lab software. A standard three-electrode system was used for preparation and characterization of the PANI-CMS hybrid. The hybrid film and platinum plate (10 mm × 10 mm × 0.2 mm) served as the working electrode and the counter electrode, respectively. A saturated calomel electrode (SCE) was used as the reference electrode and all potentials reported herein are referenced to SCE. The morphologies and structures of the products were characterized by field emission scanning electron microscopy (FESEM; JSM-6700F, operated at 10 kV) and Fourier transformation infrared spectroscopy (FTIR; FTS-165).

2.2 Preparation of the PANI-CMS hybrid

CMSs (0.5 g) were dispersed in an acid mixture (120 mL, 96 wt% H2SO4and 65 wt% HNO3in volume ratio 3∶1) in a flask under ultrasonication for 20 min. To increase the specific surface area, heat-treatment was conducted on the acid-treated CMSs in temperature ranging from 25 ℃ to 800 ℃ at a heating rate of 20 ℃/min in Ar atmosphere. The specific surface area increased to 179 m2·g-1from 9 m2·g-1[17]. Then, the obtained CMSs (20 mg) was added into a mixed solution (20 mL, 0.1 M aniline and 0.5 M H2SO4), and the mixture was sonicated for another 10 min. The PANI-CMS hybrid was electrochemically prepared in the mixed aqueous solution using potentiost at method at 0.9 V for 10 min. The deposition of PANI on CMSs was performed at 25 ℃ under static conditions.

2.3 Electrochemical measurement

Electrochemical performance was determined mainly by the cyclic voltammetry (CV) and galvanostatic charge/discharge in a 0.5 M H2SO4aqueous solution, where the three electrode system was equipped with a platinum plate as a counter electrode and a saturated calomel electrode (SCE) as a reference electrode. The PANI-CMS hybrid on the platinum plate was used as the working electrode. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range from 105to 0.01 Hz at open circuit potential with an alternating perturbation of 5 mV. Galvanostatic charge/discharge curves were measured between 0 and 0.6 V at different current densities (1, 5, 10 and 20 A·g-1). Galvanostatic cycling was performed between 0 and 0.6 V at a current density of 5 A·g-1for 2 000 times.

3 Results and discussion

3.1 Electrosynthesis of PANI-CMS hybrid

The formation of the PANI-CMS hybrid is summarized in Fig. 1. There were oxygen-enriched (e.g. carboxyl) functionalities on the surfaces of CMSs as a result of the acid-oxidation. These functional groups acted as anchor sites and enabled the subsequent electrochemical polymerization of PANI on the surfaces of CMSs. Meanwhile, the π-π electron interaction between the CMSs and the aniline was beneficial to the polymerization of aniline on the surfaces of CMSs. Then, the PANI would gradually grow along the initial nuclei of PANI and extend along CMSs to form a network structure.

Fig. 1 A schematic representation of the formation of the PANI-CMS hybrid.

3.2 Structural characterization

Fig. 2 shows the FESEM images of PANI, the PANI-CMS hybrid and CMSs.

Fig. 2 FESEM images of (a, b) PANI, (c, d, e) PANI-CMS hybrid and (f) CMSs.

It can be seen that the PANI film (Fig. 2a and b) was flat and smooth. Besides, there were some holes evenly distributed on the surface of PANI film. Unlike the dense PANI film, it is obviously observed that the PANI-CMS hybrid (Fig. 2c, d and e) had a uniform network structure, and CMSs were with a good spherical shape. Compared with the acid-treated CMSs (Fig. 2f), the surfaces of the PANI-CMS hybrid became rough, indicating that the CMSs had been coated with PANI. The network structure caused by the CMSs is favorable to improve the electrochemical properties of the hybrid.

Fig. 3 FT-IR spectra of (a) CMSs, (b) PANI and (c) PANI-CMS hybrid.

3.3 Electrochemical characterization

To evaluate the electrochemical characteristics of the PANI-CMS hybrid, the CV curves in 0.5 M H2SO4electrolyte at different scan rates were recorded at the potential window of -0.2- 0.6 V versus SCE (Fig. 4).

Fig. 4 Cyclic voltammograms of (a) PANI and (b) PANI-CMS hybrid.

Notably, it can be seen that because of the existence of polarization, a positive shift of oxidation peaks and a negative shift of reduction peaks were observed with the increase of the scan rate. Also, the curve shape is steady, indicating the good electrochemical stability of the electrode material. The two couples (at ca. 0/0.2 and 0.4/0.5 V) of apparent redox peaks were attributed to the redox transition of PANI between a semiconducting state (leucoemeraldine form) and a conducting state (polaronic emeraldine form) and the emeraldine-pernigraniline transformation. In addition, the PANI-CMS hybrid electrode exhibited a higher current value and more obvious redox peaks compared with PANI under the same conditions. The results reveal that the electroactivity of PANI was effectively improved by the introduction of CMSs during the quick charge/discharge process.

For further understanding electrochemical behavior of the PANI-CMS hybrid, the galvanostatic charge/discharge measurements at different current densities within a potential window (-0.2- 0.6 V vs. SCE) were carried out, and the results are shown in Fig. 5. As can be seen, the charge/discharge curves of PANI and the PANI-CMS hybrid were almost linear and presented a typical symmetrical triangle shape, indicating that the hybrid had a good double-layer capacitive behavior[18,19]. Besides, it can be noted that the discharge time increased distinctly with decreasing current density, the reason is that the electrolyte ions could not penetrate well into the inner of active materials as a result of low diffusion at large current density. Although the charge/discharge curves of PANI are similar to those of the PANI-CMS hybrid, but the latter would have much longer charge/discharge duration and larger charge storage capacity than the former.

Fig. 5 Galvanostatic charge/discharge curves of (a) PANI and (b) PANI-CMS hybrid.

The specific capacitance (Cs) values may be calculated from the charging and discharging curves according toCs=(I·Δt)/(ΔV·m), whereIis the discharge current,Δtis the discharge time,ΔVis the potential drop in the discharge process (in our experimentsΔV=0.6 V), and m is the mass of active material. Specific capacitances increased from 146 to 206 F·g-1with current densities from 1 to 20 A·g-1for the PANI-CMS hybrid, which were higher than those of PANI (88-135 F·g-1). The highCsof the PANI-CMS hybrid may be attributed to the uniform coating of PANI around CMSs, which could help to provide a large electrolyte-accessible surface area to improve utilization of PANI for redox reactions. Besides, electrical conductivity was increased with the introduction of CMSs, resulting in a increased specific capacitance. In addition, with the increase of current density, the PANI-CMS hybrid has only a 29%Csreduction from 1 to 20 A·g-1, which is less than that of PANI (35%). It suggests that the hybrid exhibited a better electrochemical stability. The capacitances of the PANI-CMS hybrid are even higher than those of previously reported graphene/PANI composite[20]. The reason is that a stable structure was formed by the chemical linking of PANI and CMSs, and CMSs provide a good framework for the hybrid. These results support the formation mechanism of the PANI-CMS hybrid proposed in Fig. 1, and are consistent with the structural characterization (Fig. 2).

The Nyquist plots of PANI and the PANI-CMS hybrid are demonstrated in Fig. 6. The electrochemical resistances of PANI and the PANI-CMS hybrid electrodes were small, whereas the electrochemical resistance of the pure PANI was larger than that of the hybrid, which may result in the excellent capacitive behaviors of the hybrid. In addition, these plots did not show semicircle regions, probably due to the low faradaic resistances of these films.

Fig. 6 Nyquist diagrams for the PANI-CMS hybrid and PANI.

The lack of stability of the capacitors based on conducting polymer films (especially PANI) during long-term charge/discharge cycling is one of their most fatal deficiencies. As shown in Fig. 7, the pure PANI lost 28% (from 129 to 93 F·g-1) of its capacitance after 2 000 charging/discharging cycles at a current density of 5 A·g-1. However, under the same conditions, the capacitance of the PANI-CMS hybrid decreased only 21% (from 192 to 151 F·g-1). In addition, the capacitance of the PANI-CMS hybrid maintained a good stability after 500 cycles while the capacitance for PANI electrode showed a decreasing trend. The enhanced specific capacitance is due to the synergistic effect between PANI and CMSs. On one hand, CMSs undertake some mechanical deformation in the redox process of the PANI-CMS hybrid, which avoids destroying the electrode material and thus benefits a better stability. On the other hand, the pseudocapacitance of PANI in the hybrid film is enhanced by the highly conductive CMSs. The results indicate that the high stability of the PANI-CMS hybrid film and its potential prospect as an electrode active material for long-term supercapacitor applications.

Fig. 7 Variations of the capacitance with cycle number for PAN and PANI/CMSs.

4 Conclusions

A novel PANI-CMS hybrid was prepared by electrochemical deposition method and its supercapacitive performance was systematically investigated. The PANI was grown on the external surfaces of CMSs. And the network structure was formed for the PANI-CMS hybrid. A drastically enhanced gravimetric capacitance of the PANI-CMS hybrid compared with PANI was detected in H2SO4aqueous solution, which could be ascribed to the synergistic effect between CMSs and PANI. A maximum specific capacitance of 206 F·g-1was achieved at a current density of 1 A·g-1, which was much higher than that of PANI at the same current density. Compared with PANI, the PANI-CMS hybrid possessed both a high specific capacitance and excellent cycle stability. The PANI-CMS hybrid is promising for supercapacitor applications.

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1007-8827(2016)06-0594-06

聚苯胺-炭微球复合材料的制备及其电化学性能

刘伟峰1,2， 杨永珍1,2， 刘旭光1,3， 许并社1,2

(1.新材料界面科学与工程教育部重点实验室(太原理工大学)，山西 太原030024；2.太原理工大学 新材料工程技术研究中心，山西 太原030024；3.太原理工大学 化学化工学院，山西 太原030024)

通过电化学沉积法制备得到聚苯胺/炭微球(PANI/CMS)复合电极材料，通过场发射扫描电子显微镜和红外光谱对PANI/CMS复合材料进行形貌和结构表征。并采用循环伏安、恒电流充放电、电化学阻抗谱及循环寿命测试等技术考察其电化学行为。结果表明：PANI均匀包覆于CMSs表面；在电流密度为1 A·g-1时，复合材料的比电容达到206 F·g-1；PANI/CMS复合材料表现出优异的电化学稳定性。说明PANI/CMS复合材料有望作为电极材料用于超级电容器。

炭微球； 聚苯胺； 电化学聚合； 超级电容器

TB332

A

国家自然科学基金(21176169，51152001);国家国际科技合作专项项目(2012DFR50460);山西省科技创新重点团队(2015013002-10);山西省自然科学青年基金(201601D021043);太原理工大学校基金(2014TD015).

刘旭光，教授，博士生导师. E-mail: liuxuguang@tyut.edu.cn; 杨永珍，教授，博士生导师. E-mail: yyztyut@126.com

刘伟峰，讲师. E-mail: lwf061586@yeah.net

Foundationitems: National Natural Science Foundation of China (21176169, 51152001); International Science & Technology Cooperation Program of China (2012DFR50460); Shanxi Provincial Key Innovative Research Team in Science and Technology (2015013002-10); Natural Science Foundation of Shanxi Province (201601D021043); Special/Youth Foundation of Taiyuan University of Technology (2014TD015).

LIU Xu-guang, Ph. D., Professor. E-mail: liuxuguang@tyut.edu.cn; YANG Yong-zhen, Ph. D., Professor. E-mail: yyztyut@126.com.

Authorintroduction: LIU Wei-feng, Lecturer. E-mail: lwf061586@yeah.net

10.1016/S1872-5805(16)60035-5

Receiveddate: 2016-07-26;Reviseddate: 2016-10-29

English edition available online ScienceDirect ( http:www.sciencedirect.comsciencejournal18725805 ).