Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (6): 827-832

#### The article information

Han-hong Huang, Yi-hu Wu, Mo-zhen Wang, Xue-wu Ge

Polyaniline Nanotubes Prepared by One-Step Synergistic Polymerization of Aniline and Acrylic Acid

Chinese Journal of Chemical Physics, 2018, 31(6): 827-832

http://dx.doi.org/10.1063/1674-0068/31/cjcp1805095

### Article history

Accepted on: May 31, 2018
Polyaniline Nanotubes Prepared by One-Step Synergistic Polymerization of Aniline and Acrylic Acid
Han-hong Huang, Yi-hu Wu, Mo-zhen Wang, Xue-wu Ge
Dated: Received on May 9, 2018; Accepted on May 31, 2018
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China
*Author to whom correspondence should be addressed. Mo-zhen Wang, E-mail: pstwmz@ustc.edu.cn; Xue-wu Ge, E-mail: xwge@ustc.edu.cn; Tel: +86 551 63600843
Abstract: The electrochemical property of electrode materials greatly depends on their morphologies. This report introduces a novel and facile synthesis method for polyaniline (PANI) nanotubes from one-step synergistic polymerization of aniline and acrylic acid in an aqueous solution induced by the addition of ammonium persulfate (APS). The molar ratio of aniline to AA ($X_{\rm{ani/AA}}$) is found to have great influence on the morphology of the produced PANI. Hollow PANI nanotubes with an average inner diameter of 80 nm and outer diameter of 180 nm can be mainly produced when $X_{\rm{ani/AA}}$ is not higher than 1. The electrochemical properties of the prepared PANI nanotubes have been investigated using a three-electrode system. The specific capacitance of PANI nanotubes can reach 436 F/g at a current density of 0.5 A/g in 1 mol/L H$_2$SO$_4$ solution. Furthermore, the specific capacitance of the PANI nanotube maintains 89.2% after 500 charging/discharging cycles at a current density of 0.5 A/g, indicating a good cycling stability.
Key words: Polyaniline nanotubes    Synergistic polymerization    Acrylic acid    Supercapacitor
Ⅰ. INTRODUCTION

Supercapacitors have attracted keen attention due to their rapid charge-discharge rate, high power density and long cycle life [1]. Polyaniline (PANI) is recognized as one of the most promising electrode materials of supercapacitors due to its extremely high theoretical specific capacitance (720$-$1530 F/g), excellent conductivity, ease of synthesis, and low cost [2-5]. Since the electrochemical performance of the electrode materials depends strongly on their morphologies [6], PANI with various morphologies, such as spheres [7], nanorods [8], nanofibers [9], and nanotubes [10] has been prepared to explore the potential optimal performance of PANI-based electrodes. PANI nanotube is considered to have the highest specific capacitance currently since it can provide large electroactive regions and short diffusion lengths for both electron and ion transport [11]. Generally, PANI nanotubes are synthesized by sacrificial hard-template method [12-14]. In this method, the nanostructure and performance of the prepared PANI nanotubes will be easily affected by the operation of removing hard template. For example, Alshareef et al. [14] synthesized PANI nanotubes by in situ chemical polymerization of aniline on the surface of MnO$_2$ nanotubes, followed by etching MnO$_2$ with hydrochloric acid. The specific capacitance of the prepared PANI nanotubes can reach 528 F/g at a current density of 1 A/g. But the etching of MnO$_2$ hard templates possibly destroyed the final PANI nanostructures, resulting in a poor cycle performance. Thus, suitable templates should be developed in order to obtain high-performance PANI nanotubes electrode materials.

In this work, we report a novel and facile one-step strategy to synthesize PANI nanotubes through the synergistic polymerization of aniline and acrylic acid (AA) in an aqueous solution induced by the addition of ammonium persulfate (APS). The radicals produced by the reaction of aniline and APS can initiate the polymerization of AA, as well as the polymerization of aniline molecules which are fixed around the produced poly(acrylic acid) (PAA) chains through the strong electrostatic interaction between aniline and carboxyl groups. As a result, PANI is formed around the in situ PAA long chains. After the middle PAA chains are washed with hot water, hollow PANI nanotubes are left. The influence of the molar ratio of aniline to AA ($X_{\rm{ani/AA}}$) on the morphology of the synthesized PANI nanotubes is discussed. Further, the electrochemical properties of the prepared PANI nanotubes have been studied using a three-electrode system. The results show the prepared PANI nanotubes exhibit a high specific capacitance and excellent cycling stability, making them have a potential application as high-performance electrode material.

Ⅱ. EXPERIMENTS A. Materials

Aniline, acrylic acid (AA), ammonium persulfate (APS) were all purchased from Shanghai Chemical Reagents Co., Ltd. Aniline and AA were both distilled under vacuum before use. Deionized water was used throughout all the experiments.

B. Preparation of PANI nanotubes

Aniline (0.47 g) was added into 45 mL of deionized water, followed by the addition of a certain amount of AA. The molar ratio of aniline to AA ($X_{\rm{ani/AA}}$) was fixed to be 20, 5, 2, and 1, respectively. Half an hour later, 5 mL of an aqueous solution of APS was added according to a molar ratio of 1:1 of APS to aniline. After 12 h of polymerization at room temperature, the product was collected by centrifugation (5000 r/min, 5 min), followed by washing with hot water, and finally dried in a vacuum drying oven at 50 ℃ for 12 h.

C. Characterization

The morphologies of PANI samples were investigated by transmission electron microscopy (TEM, H-7650, 100 kV), and field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F, 5 kV). The samples were dispersed in ethanol, then dropped on copper grids. As for the SEM observation, the samples were first treated by spray-gold. Fourier transform infrared (FTIR) spectra of the samples were measured in a range of 4000$-$400 cm$^{-1}$ on a Bruker VECTOR-22 IR spectrometer using KBr pellets. X-ray diffraction (XRD) patterns of PANI samples were conducted on a Rigaku SmartLab high resolution X-ray diffraction system with Cu K$\alpha$ radiation ($\lambda$=0.1541 nm) at room temperature. The electrochemical performance of PANI was evaluated in CHI 660D electrochemical workstation (Chenhua Instruments Co. Shanghai, China) at room temperature using a three-electrode system consisting of the sample modified GCE (glassy carbon electrode) as the working electrode, platinum electrode as the counter electrode, and calomel electrode as the reference electrode. The working electrode was prepared as follows: 5 mg of the sample was ultrasonically dispersed in 45 mg of the $N$-methyl pyrrolidone solution of PVDF (5%) for 20 min. Then, the dispersion was dropped onto GCE and dried in an oven at 50 ℃ for 2 h. The electrolyte is 1 mol/L H$_2$SO$_4$ for all electrochemical tests including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge. CV curves were measured at different scan rates between $-$0.2 and 1.0 V. Galvanostatic charge/discharge tests were carried out at a current density of 0.5 A/g. EIS was recorded in a frequency range of 10$^5$$-0.01 Hz at an amplitude of 5 mV referring to the open circuit potential. Ⅲ. RESULTS AND DISCUSSION A. Synthesis and formation mechanism of PANI nanotubes PANI can be prepared conventionally by the polymerization of aniline induced by APS in aqueous solutions only containing inorganic acid for proton doping [15-17]. But only PANI nanoparticles and nanosheets are obtained by this kind of method. In this work, the morphology of PANI synthesized in an aqueous solution containing polymerizable organic acid, i.e., acrylic acid (AA), is studied. FIG. 1 shows the morphologies of PANI prepared at different molar ratios of aniline to AA (X_{\rm{ani/AA}}). When the content of AA is very low (0.04wt%), i.e., X_{\rm{ani/AA}} is 20, the product is the aggregates of nanosheets with a thickness of 60 nm, as shown in FIG. 1 (a) and (b). The FTIR spectrum of the product is displayed in FIG. 1(c), which shows the characteristic absorption of PANI. The peaks around 1500 cm^{-1} and 1582 cm^{-1} are due to the C-C stretching of the benzenoid rings and quinoid ring, respectively. The 1300 cm^{-1} peak is attributed to C=N stretching vibration with aromatic conjugation, while that at 1150 cm^{-1} is assigned to the quinoid ring stretching mode [18], but both the characteristic absorption for the stretch vibration of C-H of methylene groups on PAA and that for the stretch vibration of C=O in carboxyl groups cannot be distinguished.  FIG. 1 SEM, TEM images and FTIR spectra of PANI prepared at different X_{\rm{ani/AA}}: (a-c) X_{\rm{ani/AA}}=20, (d-f) X_{\rm{ani/AA}}=5, (g-i) X_{\rm{ani/AA}}=2, (j-l) X_{\rm{ani/AA}}=1. The red line in (c) is the FTIR spectrum of PANI product without being washed by hot water. In order to confirm whether AA monomers have been induced to polymerization into PAA, the FTIR spectrum of the product collected by direct evaporation of water, i.e., without being washed by hot water, was also shown in FIG. 1(c) (red line) for a comparison. It can be clearly seen that the peak at 2850 cm^{-1} which is assigned to the absorption of the stretch vibration of C-H in -CH_2- groups, which should be produced only by the formation of PAA macromolecular chains in this reaction system. The result indicates that both of aniline and AA monomers have been polymerized after the addition of APS, and the formed PAA chains can be washed by hot water so as to be separated from the synthesized PANI. With the increase of the AA content, i.e., X_{\rm{ani/AA}} becomes 5, the obtained products are the mixture of nanosheets and nanorods as shown in FIG. 1 (d) and (e). The FTIR spectrum of the product in FIG. 1(f) also confirms that the structure of PANI, and no characteristic absorptions of PAA can be detected. If the AA content continuously rises to 0.4wt%, i.e., X_{\rm{ani/AA}} decreases to 2, fine voids can be found in the middle of some PANI nanorods (FIG. 1 (g) and (h)). At the same time, FTIR spectrum of the corresponding product (FIG. 1(g)) shows the characteristic absorptions of C=O at 1720 cm^{-1} and C-H at 2850 cm^{-1} of PAA besides the absorptions of PANI, indicating PAA cannot be totally washed away from the obtained PANI at a high weight content. When the content of AA is up to 0.8wt%, i.e., X_{\rm{ani/AA}} is 1, hollow PANI nanotubes with an average inner diameter of 80 nm and outer diameter of 180 nm were obtained, which can be observed from FIG. 1 (j) and (k) obviously. The FTIR spectrum of the corresponding product in FIG. 1(l) also shows the characteristic absorption of PAA and PANI. All the above obtained PANI at different X_{\rm{ani/AA}} show a similar XRD patterns, as shown in FIG. 2. The characteristic peaks at around 2\theta=19.4^\circ and 24.5^\circ correspond to (020) and (200) crystal planes of PANI in its emeraldine salt form [19], respectively.  FIG. 2 XRD spectra of PANI prepared at different X_{\rm{ani/AA}} of 20 (PANI-1), 5 (PANI-2), 2 (PANI-3), 1 (PANI-4). Based on the above results, it can be concluded that the morphology of PANI prepared from an aqueous solution of AA depends greatly on X_{\rm{ani/AA}}. The increase of the AA content favors to the formation of tubular PANI. However, the crystal structure of the obtained PANI will not change with the content of AA. Therefore, we think a synergistic polymerization of aniline and AA occurs in the system, as illustrated in Scheme 1.  Scheme 1 Formation of PANI nanotubes via a synergistic polymerization of AA and aniline. According to the classic initiation mechanism of APS on the polymerization of vinyl monomer and aniline (Scheme S1 in supplementary materials), the radicals produced by the reaction of aniline and APS can initiate the polymerization of AA, as well as the polymerization of aniline molecules. When the content of AA is high enough, most of aniline molecules are fixed around the produced poly(acrylic acid) (PAA) chains through the strong electrostatic interaction between aniline and carboxyl groups, which also makes the PAA molecular chains keep a stretch conformation. As a result, PANI is formed around the in situ PAA long chains. After the middle PAA chains are dissolved in hot water and washed away, hollow PANI nanotubes are left. To prove the synergistic polymerization of AA and aniline caused by the strong electrostatic interaction between -COOH and aniline, the aqueous solution of AA and aniline was heated to 50 ℃ to destroy the intermolecular electrostatic interaction [20]. In this case, after APS was added in the system, only disk-like PANI nanoparticles can be fabricated as shown in FIG. 3, which is in accord with the case of very low AA content at room temperature. Thus, the synergistic polymerization of AA and aniline caused by strong electrostatic interaction between -COOH and aniline is the key process resulting in the formation of PANI nanotubes.  FIG. 3 SEM (a) and TEM (b) images of PANI prepared with aniline and AA at X_{\rm{ani/AA}}=1 at 50 ℃. B. Electrochemical behavior of PANI nanotubes The electrochemical properties of PANI nanomaterials were examined by cyclic voltammetry (CV) and galvanostatic charge/discharge tests, as shown in FIG. 4. The CV curves in FIG. 4(a) were obtained at a scan rate of 100 mV/s. It is obvious that the CV curves of all PANI samples exhibit redox peaks, which correspond to redox transitions of semiconducting state (leucoemeraldine form) to a conducting state (emeraldine form) [21]. Both the intensities of the redox peaks and the CV curves area of PANI-4 nanotubes are apparently larger than PANI of other morphologies. The CV curves of bare GCE are shown in FIG. S1 in supplementary materials. It can be seen that the current intensity of PANI nanotube is about 1000 times that of bare GCE at the same potential. Thus, the capacitive contribution of GCE substrate can be neglected in this case. These results imply that PANI nanotubes have a remarkable better electrochemical performance than PANI of other morphologies.  FIG. 4 (a) CV curves (scan rate of 100 mV/s), (b) galvanostatic charge-discharge curves (0.5 A/g), (c) the specific capacitance measured at different current densities, and (d) cycling stability of PANI-1, PANI-2, PANI-3, and PANI-4 at a current density of 0.5 A/g. FIG. 4(b) shows the galvanostatic charge-discharge curves for all PANI nanomaterials at a current density of 0.5 A/g. All curves exhibit approximate saw-toothed, but partially deviate from linearity, owing to the contribution of pseudocapacitance [22]. The specific capacitance of PANI-1, PANI-2, PANI-3, and PANI-4 can be determined as 60 F/g, 197 F/g, 365 F/g and 436 F/g, respectively, which is calculated according to the charge-discharge curves. It is obviously that PANI nanotubes (PANI-4) have the highest specific capacitance, which nearly doubles that of PANI nanorods (PANI-2) and is seven times that of PANI nanosheets (PANI-1). PANI-3 contains both nanorods and nanotubes (FIG. 1 (g) and (h)) so that it also has a relative high specific capacitance, only a little smaller than that of PANI-4. As we know, the nanotubes structure of PANI with regular hollow structure possesses higher available specific surface area than that of the PANI products with other morphologies, which can enhance the electroactive regions, reduce the diffusion path, and further improve the electrochemical properties, resulting in excellent electrochemical performance. The rate performance of PANI nanomaterials has been evaluated by charge-discharge curves at different current densities as shown in FIG. 4(c). It is obvious that the capacitance decreases with the increase of the current density. The PANI-4 nanotubes also show the best rate performance. When the current density changes from 0.5 A/g to 10 A/g, the capacitance remains about 69%, while the capacitances of PANI-1, PANI-2, and PANI-3 only remains 32%, 37%, and 59%, respectively. The excellent rate performance of PANI-4 nanotubes is of great importance for supercapacitor electrode materials to provide high power density in practical applications. The electrochemical cycling performance of electrodes has great influence on their practical service life for supercapacitors. Here, 500 charge-discharge cycles of all PANI nanomaterials were performed at a current density of 0.5 A/g, as shown in FIG. 4(d). Similarly, the cycling stability of PANI-4 nanotubes is far superior to those of all other samples. 89.2% of the initial capacitance was maintained after 500 cycles. The above results manifest that the PANI-4 nanotubes exhibit the best electrochemical performance when compared with PANI-1 nanosheets, PANI-2 nanorods, and PANI-3 with the composites of PANI nanorods and nanotubes. Furthermore, the CV curves of PANI-4 nanotubes at different scan rates are showed in FIG. 5. It can be seen that the redox peaks of PANI nanotubes have little change at a wide range of scan rates, but the peak current density decrease with the decreases of the scan rates, indicating a good rate ability of PANI-4 nanotubes.  FIG. 5 CV curves of PANI nanotubes (PANI-4) at different scan rates. Ⅳ. CONCLUSION A novel and facile one-step strategy to synthesize PANI nanotubes through the synergistic polymerization of aniline and acrylic acid (AA) in an aqueous solution induced by the addition of ammonium persulfate (APS) is reported in this work. The radicals produced by the reaction of aniline and APS initiate the polymerization of AA, as well as the polymerization of aniline molecules which are fixed around the produced PAA chains through the strong electrostatic interaction between aniline and carboxyl groups. As a result, hollow PANI nanotubes are fabricated. The electrochemical properties of the obtained PANI nanotubes have been investigated using a three-electrode system. Compared with other morphologies, the prepared PANI nanotube exhibits very high specific capacitance, i.e., 436 F/g at the current density of 0.5 A/g in 1 mol/L H_2SO_4 solution, and excellent cycling stability, i.e., the capacitance of the PANI nanotube maintains 89.2% after 500 charging/discharging cycles at a current density of 0.5 A/g. The excellent electrochemical performance of PANI nanotubes makes them have a potential application as high-performance electrode materials for pseudocapacitors. Supplementary materials: The polymerization mechanism of AA and aniline and the CV curves of bare glassy carbon electrode. Ⅴ. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (No.51473152, No.51573174, and No.51773189), and the Fundamental Research Funds for the Central Universities (No.WK3450000001 and No.WK3450000004). Supplementary Information Part 1 The CV curve of bare glassy carbon electrode(GCE). 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DOI:10.1016/j.jpowsour.2011.08.105 [22] J. H. Chen, X. Y. Feng, W. F. Chen, Y. Q. Song, and L. F. Yan, Chin. J. Chem. Phys. 30 , 112 (2017). DOI:10.1063/1674-0068/30/cjcp1604092 苯胺和丙烯酸协同聚合一步法制备聚苯胺纳米管 黄汉弘, 吴义虎, 汪谟贞, 葛学武 中国科学技术大学高分子材料科学与工程系，中国科学院软物质化学重点实验室，合肥 230026 摘要: 本文通过加入过硫酸铵(APS)引发苯胺和丙烯酸(AA)在水溶液中的协同聚合反应，一步合成出具有中空结构的聚苯胺纳米管.聚苯胺纳米管的外径平均为180 nm，内径约80 nm，其形成取决于苯胺与丙烯酸的摩尔比(X$$_\text{ani/AA}$).当$X$$_\text{ani/AA}$不超过1时，主要得到聚苯胺纳米管.在三电极体系中测定了所制备的聚苯胺纳米管的电化学性能.在1 mol/L H$_2$SO$_4$为电解液，电流密度为0.5 A/g的条件下，聚苯胺纳米管比电容高达436 F/g，是通常聚苯胺纳米片的七倍.此外，在0.5 A/g的电流密度下进行500次充放电循环后，其比电容能够保持最初始的89.2%，表现出优异的充放电循环稳定性.