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Two-step method for synthesizing polyaniline with bimodal nanostructures for high performance supercapacitors
Accepted Manuscript Two-step method for synthesizing polyaniline with bimodal nanostructures for high performance supercapacitors Yuan Yuan, Wenliang Zhu, Guo Du, Dengzhi Wang, Jiliang Zhu, Xiaohong Zhu, Giuseppe Pezzotti PII:
S0013-4686(18)31293-3
DOI:
10.1016/j.electacta.2018.06.006
Reference:
EA 31999
To appear in:
Electrochimica Acta
Received Date: 1 April 2018 Revised Date:
28 May 2018
Accepted Date: 1 June 2018
Please cite this article as: Y. Yuan, W. Zhu, G. Du, D. Wang, J. Zhu, X. Zhu, G. Pezzotti, Two-step method for synthesizing polyaniline with bimodal nanostructures for high performance supercapacitors, Electrochimica Acta (2018), doi: 10.1016/j.electacta.2018.06.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT
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The as-prepared polyaniline exhibits a bimodal morphology composed of a coral-like structure and a nanowire structure. A large specific capacitance of 689
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EP
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F/g at 1A/g and retention of 613 F/g at 10 A/g is obtained for net polyaniline.
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Two-step method for synthesizing polyaniline with bimodal nanostructures for high performance supercapacitors Yuan Yuana, Wenliang Zhub, Guo Dua, Dengzhi Wanga, Jiliang Zhua*, Xiaohong Zhua,
a
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Giuseppe Pezzottib*
College of Materials Science and Engineering, Sichuan University, Chengdu 610064, China
Ceramic Physics Laboratory, Kyoto Institute of Technology, Sakyo-ku, Matsugasaki,
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b
606-8585 Kyoto, Japan
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Abstract
Here we report a novel and simple two-step method to fabricate polyaniline, which consists of two successive low-temperature (-20 °C, frozen) and near-room temperature (30 °C) periods. Melting of the frozen part and solute migration mainly
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drives polymerization. It is proved that the two-step method is an efficient way to fabricate polyaniline with a bimodal morphology composed of coral-like structure
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with thorns and interconnected nanowire structure. The mechanism of formation of such bimodal morphology is discussed in detail. Optimized polyaniline prepared by
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this method showed an excellent electrochemical energy storage performance, with a specific capacitance of 689 F g-1 at 1 A g-1 and a good retention rate of 89% (613 F g-1) at 10 A g-1. Symmetric cells based on it show an energy density of 12.6 Wh kg-1 at a power density of 188.3 W kg-1. In summary, this work provides a new way to fabricate polyaniline for supercapacitor applications.
Keywords: polyaniline, two-step method, freezing, bimodal nanostructures, supercapacitors * Corresponding author. Tel.:+86 28 85432078; fax:+86 28 85432078.Email address: [email protected] (J. Zhu), [email protected] (G.Pezzotti)
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1. Introduction Nowadays, a large amount of energy consumption seems inevitable for modern life due to the fast development of the technologies utilized by humans. Therefore,
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both renewable energy harvesting and energy storage systems are of great need to be developed. Among the energy storage systems, supercapacitors play an important role because they can provide a high power density with a relatively high energy density
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[1-3].Recently, significant achievements have been realized in combining polyaniline
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with other materials like MnO2 [4, 5], NiCo2O4 [6], MoS2 [7] and especially graphene [8-13] for supercapacitor electrodes. Employing carbon-based materials only can hardly provide a large specific capacitance, usually less than 400 F g-1 [2, 3]. However, the combination of polyaniline and graphene, can often provide a specific capacitance
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lager than 700 F g-1 and good rate capability [9-17]. For example, Wu et al. reported a phase-separated polyaniline/graphene composite electrode, which possesses a specific capacitance of 783 F g-1 at a current density of 27.3 A g-1 [11]. On the one hand, the
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relatively large capacitance originates from the pseudocapacitance of polyaniline; on
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the other hand, the synergistic effect seems vitally important for such a composite. As the content of polyaniline increases, the specific capacitance of such a composite often shows an increasing and then a decreasing trend [11-13, 16-18]. Moreover, net polyaniline shows a much smaller specific capacitance than the composites, which is usually less than 600 F g-1 [9, 17]. Thus, there is still potential to promote the performance of a composite provided that the performance of polyaniline itself could also be promoted.
ACCEPTED MANUSCRIPT Polyaniline, as a conducting polymer, has been long researched due to its facile synthesis, multiple intrinsic redox states [19, 20], fibrillar morphology in nanoscale [21-28], and environmental stability. The nanofibrillar morphology enables
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polyaniline to be used for versatile applications, especially for supercapacitors, because it can provide a high surface area for charging [22, 23]. Basically, there are two methods to fabricate polyaniline: electrochemical polymerization and chemical
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oxidative polymerization. Nanowires could often be expected from electrochemical
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polymerization, and they can provide a high specific capacitance of 500~800 F g-1 [20, 29-31]. But large amounts of polyaniline can hardly be expected from electrochemical polymerization. Meanwhile, different simple methods of chemical oxidative polymerization have been invented to fabricate polyaniline nanofibers including
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interfacial polymerization [24], dilute polymerization [25] and rapidly mixed polymerization [26]. Net polyaniline nanofibers based on these methods have been fabricated for supercapacitor applications. However, a specific capacitance less than
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600 F g-1 is usually observed [32-34].
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It should be noted that the nanofibrillar morphology of polyaniline is intrinsic and the methods mentioned above simply suppress the secondary growth and agglomeration of polyaniline. Conditions like dopant acids [24], oxidant redox potential [35], steric stabilizers [36], ionic strength, concentrations and temperatures [27] can also significantly affect the morphology of polyaniline. Among which, temperature is a condition of great importance and one that can be easily controlled. A low temperature (in ice-water bath, for instance) is often recommended in a
ACCEPTED MANUSCRIPT conventional chemical polymerization because the reaction is exothermic. However, in methods like interfacial polymerization, the issue is that the polyaniline fabricated at room temperature owns distinguished nanofibrillar morphology [24]. Even at
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higher temperatures, nanofiber formations are more likely to emerge [27]. Thus in most literature, temperatures around 0 °C or room temperature are often applied in the fabrication of polyaniline [10, 12-18]. There are also reported works in which
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polyaniline was fabricated at much lower temperatures (-10 °C ~ -50 °C) [37-42]. Ice
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formation is commonly avoided through adding additives like ethanol and more dopant acid. However, it was also reported that polymerization of aniline proceeds well even in the frozen state [38, 39]. Lately, Wang et al. reported about a “hierarchical” polyaniline obtained with the help of NaCl, where the reaction system
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was firstly rapidly mixed at 4 °C and then frozen at -18 °C thus leading to a relatively good specific capacitance of 520 F g-1 [40]. In this work, we have conducted a novel and simple two-step process based on
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solute migrations to fabricate polyaniline. As shown in Fig. 1, the reaction system was
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firstly frozen at a low temperature of -20 °C and then melted at a higher temperature of 30 °C. Such fabricated polyaniline shows a novel bimodal morphology composed of a coral-like structure with thorns and an interconnected nanowire structure. More importantly, good energy storage properties can be expected from it. Electrodes based on it can deliver a specific capacitance of 689 F g-1 at 1 A g-1 and a good retention rate of 89% (613 F g-1) at 10 A g-1, which is outstanding for net polyaniline (see Table 1 for comparison). Furthermore, symmetric cells with 2M H2SO4 aqueous electrolyte
ACCEPTED MANUSCRIPT were assembled based on it, and they could deliver an energy density of 12.6 Wh kg-1 at a power density of 188.3 W kg-1. To our knowledge, all these results are among the best for net polyaniline by chemical oxidative polymerization.
polymerization methods in a three-electrode system.
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Table 1. Comparison of the specific capacitance of net polyaniline by different chemical oxidative
Specific
Discharge
Potential Electrolyte
rates
ranges /V
capacitance
Ref.
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Methods
Microemulsion 1 A g-1 polymerization 0.18 A g-1
Rapid mixing
25 mV s-1
Frozen system at a low temperature (-18 °C)
1M H2SO4
351
[13]
0-0.7
1M H2SO4
505
[32]
0.2-0.7
1M HCl
~490
[33]
0.5 A g-1
0-0.8
1M H2SO4
520
[40]
1 A g-1
0-0.8
2M H2SO4
570
This
0-0.8
2M H2SO4
689
work
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Rapid mixing
0-0.8
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Interfacial polymerization
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/F g-1
1 A g-1
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Two-step method
2. Experimental
2.1. Synthesis of polyaniline All the chemicals were analytical grade and used as received. Process of polyaniline synthesis in the two-step method is shown in Fig. 1. Typically, 2 mmol aniline was dissolved in 20 mL 1M HCl aqueous solution and then warmed in water bath at 30 °C for one hour, forming solution A. 2 mmol ammonium persulphate was
ACCEPTED MANUSCRIPT dissolved in 20 mL 1M HCl aqueous solution in a vial (40mL, ~2.7 cm in diameter) and then cooled in a refrigerator at -20 °C for 24 hours; then, the solution was frozen to become solid, forming solution B. Successively, we poured solution A into solution B, and kept the vial in the refrigerator at -20 °C for 24 hours. By doing so, the whole
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system was frozen. The vial was then transferred to water bath at 30 °C for the second step reaction. Different reaction times (from 3 minutes to 24 hours) of the second step were applied. According to such screening of reaction times, 30 minutes were found
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to be optimum (see Supplementary materials for a detailed discussion). Polyaniline obtained according to the above process is henceforth indicated with the abbreviation
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PANI-TS-HCl. Polyaniline using H2SO4 as dopant acid to substitute for HCl was also fabricated in this work (referred to as PANI-TS-H2SO4).
For comparison, a rapid mixing method was applied to fabricate polyaniline. The detailed process is as follows. Solution A and solution B were both warmed in water
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bath at 30°C for more than one hour. Then, solution A was poured into solution B and stirred vigorously for 60 seconds. Then, the reaction system was kept without any disturbance in water bath at 30 °C for 30 minutes. Polyaniline based on rapid mixing
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is henceforth referred to as PANI-RM-HCl or PANI-RM-H2SO4.
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Either vacuum filtration using deionized water and ethanol to flush or dialysis with deionized water under stirring was carried out for the purification of as-prepared polyaniline (see Supplementary materials for detailed discussion). The resultant was dried in an oven at 70 °C for more than 12 hours. 2.2. Fabrication of electrodes and symmetric cells The as-prepared polyaniline was milled gently into fine powder in an agate mortar before use for electrodes. And then polyaniline powder, acetylene black, and polytetrafluoroethylene (PTFE) were mixed sufficiently in a mass ratio of 85:10:5 in
ACCEPTED MANUSCRIPT ethanol to form slurry. The slurry was casted onto a graphite paper of 1 × 2 cm2 and dried in an oven at 70 °C. The electrodes were then pressed under a pressure of ~6 MPa for a solid conglutination. The mass of polyaniline on each electrode was controlled to be around 3-4 mg, thus a mass loading of 1.5-2 mg cm-2. Symmetric
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cells were simply assembled by conjugation of two electrodes (without any other pre-treatments) with similar mass loadings, with cellulose paper as the separator, and
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they were immersed in 2 M H2SO4 and sealed in a plastic bag. 2.3. Material characterization
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The morphologies of the resulted polyaniline were investigated by field emission scanning electron microscopy (FE-SEM, JSF-7500F, Japan) at an operating voltage of 15 kV and field emission transmission electron microscopy (FE-TEM, Tecnai G2 F20 S-TWIN, America) at an operating voltage of 200 kV. We dispersed polyaniline in deionized water through ultrasonic concussion, and then it was dropped on copper
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mesh and dried for FE-TEM tests. Fourier transform infrared (FT-IR) spectroscopy was applied on a Nicolet 6700 instrument. Ultraviolet-visible (UV-vis) spectra were
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measured by dissolving polyaniline in N-methylpyrrolidinone (NMP) on a UV-3600 instrument (Shimadzu Corporation, Japan). X-ray Photoelectron Spectroscopy (XPS)
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(XSAM 800, Kratos, UK) were carried out with an Al Kα X-ray source under a vacuum of < 10-9 torr.
2.4. Electrochemical measurements A three-electrode system was constructed with 2 M H2SO4 aqueous solution as the electrolyte, polyaniline electrodes as working electrodes, a Pt sheet as the counter electrode and Ag/AgCl as the reference electrode. Symmetric cells were also tested in 2 M H2SO4 aqueous electrolyte. Cyclic voltammetry (CV), galvanostatic
ACCEPTED MANUSCRIPT charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements were carried out on an electrochemical work station (CHI 660E, Chenhua, China). Values of the specific capacitance were calculated from the GCD curves in this work (see Supplementary materials for details). The capacitance
GCD cycles at 2 A g-1.
3. Results and discussions 3.1. Synthesis, morphology and growth mechanism
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retentions of symmetric cells were tested on a CT2001A (LANHE, Wuhan, China) by
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In the two-step polyaniline fabrication process carried out in this work, reactions at a low temperature (-20 °C) and a near-room temperature (30 °C) were combined. As shown in Fig. 1, in the first step, the frozen phase (oxidant solution) would be slightly melted as soon as the liquid phase (aniline solution) with a higher temperature was added. Then, polymerization was initiated at the interface and spread among the
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liquid phase as the melted oxidant solute migrated. The whole system was placed in a low temperature environment to be frozen in the first step. In the second step, the
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frozen system was placed in a higher temperature environment to thaw it. The polymerization continued as the oxidant solution and aniline solution melted and
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migrated. In substance, the polymerization of polyaniline is based on the solute migrations induced by the concentration difference, that is, migration of the oxidant only in the first step and migration of both oxidant and aniline in the second step. However, it is quite intriguing that the formation of polyaniline does not go to the very bottom of the vial in the second step. As we can see from the digital image of the vial in Fig. 1, there is a blank transparent section at the bottom of the vial after warming and melting. This phenomenon indicates two facts in the reactions: (i) aniline would be totally polymerized before it reached the bottom of vials, indicating
ACCEPTED MANUSCRIPT the fast reaction of the second step; (ii) the fabricated polyaniline has a light weight. Moreover, it is found that the polyaniline would gather and also float on the upper side of the solution when the reaction of the second step is prolonged. Generally, aggregations induced by shaking and stirring would cause sinking of polyaniline [43].
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This indicates that no aggregation like those induced by shaking or stirring exists in the two-step reactions.
SEM and TEM measurements were carried out to elucidate the morphology of
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the as-prepared polyaniline, as shown in Figs. 2 and 3. Fig. 2(a)-(c) show the SEM images of PANI-TS-HCl. It is intriguing and unexpected that PANI-TS-HCl owns a
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bimodal morphology composed of a coral-like structure and an interconnected nanowire structure. Fig. 2(b) shows the coral-like part in a higher magnification, and it is obvious that the “coral” has many short thorns. And this is even clearer in the TEM images shown in Figs. 3(a) and (b). The coral-like part in this work is similar to
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that shown in the work by Wang et al.[40], in which the polyaniline was claimed to be of a hierarchical structure, but the thorns in our work are slightly shorter. It should be noted that coral-like structure could also be found in other works [24, 27, 43, 44],
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where a low temperature or continuous stirring was applied. However, granular
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polyaniline or significant aggregation would commonly accompany such a structure. In contrast, when the frozen condition is applied to the reaction, the coral-like structure is more homogeneous, that is, granular structures or aggregations seldom emerge. Thus, it is reasonable to consider that the coral-like polyaniline mainly formed in the first frozen period. Meanwhile, more polyaniline in the two-step method shows a structure of interconnected nanowires as shown in Figs. 2(a) and (c). The nanowires are relatively homogeneous in their morphology with diameters in the range of 50~80 nm, as shown in Figs. 2(c) and 3(c)-(d). It should be noted that the
ACCEPTED MANUSCRIPT content of the resultant polyaniline can be very much limited if the time of the second step is too short. For instance, when the time is 3 minutes, the solution cannot totally melt and little polyaniline can be collected when vacuum filtration is applied. Thus, if coral-like polyaniline is assumed to form in the first step, it is reasonable to consider
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that the nanowires with a bigger proportion mainly form in the second step.
One possible mechanism for the formation of the coral-like structure and nanowire structure in different periods can be proposed, as shown in Fig. 1. In the first
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step, oxidant would migrate due to a concentration gradient as solution B melts. The reaction rate can be rather slow due to the low temperature applied in this period, thus
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heterogeneous nucleation happens and new polyaniline formation tend to emerge on the as-formed polyaniline [43, 45]. However, due to the low temperature (-20 °C), frozen of the upper solution (solution A with migrated oxidant) could also happen at the same time. The insoluble and solid as-formed polyaniline is an ideal substrate for
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nucleation of ice, as the energy needed is much lowered. The nucleation of ice would hinder further growth of polyaniline on the as-formed polyaniline substrate, thus the final produced polyaniline of the first step is coral-like, and relatively homogeneous
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with thorns on it. Because of the higher temperature (30 °C, around room temperature)
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in the second step, both the reactions and the migrations of the solutes are much faster. Considering that no disturbance is applied, heterogeneous nucleation is suppressed, and thus nanofibers can be expected [43, 45]. To further test if the two-step method discussed above is an effective way for
polyaniline fabrication of a bimodal morphology, a different dopant acid (H2SO4 in this case) was applied. As shown in Figs. 2(d)-(f), the structure of PANI-TS-H2SO4 was found to be of a bimodal morphology composed of a coral-like structure and an interconnected nanowire structure as well, but the size of the nanowires in diameter
ACCEPTED MANUSCRIPT and the whole of the "coral" were slightly bigger than those of PANI-TS-HCl, which could be a result of different properties of the anions (e.g. size and electro-negativity) in the reaction system. Similar differences due to the different dopant acid were also
only nanofibers were found for PANI-RM-HCl (see Fig. S2). 3.2. Molecular structures
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reported in previous literature [24]. When the rapid mixing method was carried out,
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FT-IR and UV-vis were carried out for the analysis of molecular structures of PANI-TS-HCl and PANI-RM-HCl, as shown in Fig. 4. Typical peaks for polyaniline
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could be found for both PANI-TS-HCl and PANI-RM-HCl as shown in Fig. 4(a) (see Table S1 for detailed peak values and assignments) [9, 19, 40, 46]. Commonly, peaks around 1580 cm-1 and 1490 cm-1 are assigned to C=C stretching of quinoid ring and benzenoid ring, repectively; peaks around 1305 cm-1, 1240 cm-1could be assigned to C-N stretching mode; broad peaks around 1200-900 cm-1 arose from overlapped
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vibrations of C-H in-plane bending. Compared with PANI-RM-HCl, the peak of PANI-TS-HCl around 1240 cm-1 is much weaker, indicating that more quinoid units
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exist in PANI-TS-HCl as compared to PANI-RM-HCl [46]. It should be noted that a distinct peak at 1375 cm-1 was found for PANI-TS-HCl, and for PANI-RM-HCl the
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peak at 1375 cm-1 was very weak. Early studies showed that polyaniline in the leucoemeraldine (LM) form with no quinoid imine units does not absorb at this frequency and the absorption band at this frequency disappears once emeraldine (EM) base is acid-doped to become EM salt [19, 46, 47]. Thus, the distinct peak at 1375 cm-1 of PANI-TS-HCl indicates two facts: (i) PANI-TS-HCl has a higher oxidation level with more quinoid units than PANI-RM-HCl [46]. (ii) PANI-TS-HCl became dopant-free after the process. It should be noted that obvious dark green was observed in the fabrication process of PANI-TS-HCl as shown in the digital pictures of Fig. 1,
ACCEPTED MANUSCRIPT indicating the emeraldine salt form. But, a relatively high drying temperature of 70 °C was applied in our later process, which could possibly cause PANI-TS-HCl being devoid of dopant. This result was further verified by the UV-vis spectra. For UV-vis test, NMP was used as the solvent to dissolve the polyaniline powder. As shown in the
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inset of Fig. 4(b), an apparent blue color could be observed for the solutions, certifying the formation of dopant-free EM base of the polyaniline after drying [20]. It is worth noting that dissolution of PANI-TS-HCl and PANI-RM-HCl in NMP showed
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apparent difference. The blue color is rather weak for PANI-RM-HCl at a concentration of 25 µg mL-1 and so are the absorption peaks in its UV-vis spectra.
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Thus, we have carried out the UV-vis spectra of PANI-RM-HCl at a concentration of 100 µg mL-1 for a better view. PANI-TS-HCl and PANI-RM-HCl show similar UV-vis spectra as shown in Fig. 4(b). Peaks around 330 nm correspond to the π-π transition; relatively intense peaks near 630 nm further certify the molecule structure of EM base;
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shoulder peaks could be seen around 280 nm, indicating existence of some pernigraniline (PNA) form [19].
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XPS was also carried out to further probe the molecular structures. As shown in Fig. 5, obvious difference could be observed in N1s spectra of PANI-TS-HCl and
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PANI-RM-HCl. For further analysis, three peaks, corresponding to quinoid imine (=N-, 398.6 eV), benzenoid amine (-NH-, 399.7 eV) and positively charged nitrogen atoms (> 400 eV), were applied to the curve-fit [13, 15, 19]. It is revealed that, the -NH- structure dominates in PANI-RM-HCl, and the =N- structure dominates in PANI-TS-HCl. This is in accordance with FT-IR results. Thus PANI-TS-HCl is further proved to own a higher oxidation level than PANI-RM-HCl. 3.3. Electrochemical characterization
ACCEPTED MANUSCRIPT To evaluate the electrochemical performance of the fabricated polyaniline by the two-step method, a three-electrode system using 2M H2SO4 as electrolyte was constructed. Fig. 6(a) shows CV curves of PANI-TS-HCl at scan rates ranging from 10 mV s-1 to 100 mV s-1. Two pairs of apparent peaks could be seen in these curves,
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which are commonly assigned to the LM/EM (peaks between -0.1-0.3 V) and EM/PNA (peaks between 0.4-0.7 V) transformations, respectively [9]. It has been shown in early literature that the potentials of anodic peaks tend to increase with
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increasing acidity [20]. Considering the high acidity (pH ca. -0.6) of the electrolyte in this work, the potential range is not high enough for the transition of EM/PNA [11].
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Thus it is reasonable to conclude that the peaks between 0.4-0.7 V actually belong to the redox reactions of by-products and hydroquinone/benzoquinone instead of EM/PNA [11, 31, 40]. Though shifts of peaks could be seen as the scan rate increases, peak current density values increased almost linearly along with the increase of the
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scan rate (as concluded in Fig. 6(b)), indicating that the electrode processes are not limited by the mass transfer and good rate capability is obtained [9, 11]. Electrochemical impedance spectra (EIS) with amplitude of 5 mV were also carried
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out for further understanding of the PANI-TS-HCl electrode. Fig. 6(c) shows the
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Nyquist plot of PANI-TS-HCl. No obvious semicircle was found in the high frequency area, indicating a negligible charge transfer resistance (Rct) [31, 40]. An area with a slope near 45°, known as the Warburg portion, could be seen as marked by a line in inset to Fig. 6(c). This region is relatively short, indicating good ion transport properties [13, 31]. Fig. 6(d) depicts the galvanostatic charge-discharge (GCD) curves of the PANI-TS-HCl with current densities ranging from 1 A g-1 to 10 A g-1. Shapes approximate triangles, but with slightly deformations. The deformations indicate
ACCEPTED MANUSCRIPT different contributions of transitions among the different states of polyaniline to the capacitance [28]. Specific capacitance values based on the discharge curves were calculated (see Supplementary materials for detailed calculation). PANI-TS-HCl exhibited a large specific capacitance of 689 F g-1 at 1 A g-1. Such a value is
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outstanding for net polyaniline by chemical polymerizations (see Table 1 for comparison), and even close to or bigger than some polyaniline materials prepared by electrochemical polymerization and composites [15, 16, 31, 41]. Moreover, a good
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rate capability was also found as shown in Fig. 6(f). A retention rate of 89% (613 F g-1) was obtained as we increased the current density to 10 A g-1, and this corresponds to
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above discussions of the CV curves and EIS tests.
For comparison, we have also measured the electrochemical performance of the as-prepared polyaniline by rapid mixing in this work in a three-electrode system (see Fig. 6(e) and Fig. S3). It is observed that PANI-RM-HCl owns similar CV behavior
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along with increasing scan rates and Nyquist plot to PANI-TS-HCl, indicating an also good rate capability. As shown in Fig. 6(f), PANI-RM-HCl exhibited a specific capacitance of 570 F g-1 at 1 A g-1, and a retention rate of 88% (501 F g-1) at 10 A g-1.
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Though similar rate capabilities were found for both PANI-TS-HCl and
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PANI-RM-HCl, a larger specific capacitance was obtained for PANI-TS-HCl, and the possible reasons for this could be explained, as follows: (i) the bimodal microstructures provide more effective surface area for contact of polyaniline and the electrolyte. Especially the coral-like structure with small thorns, which is hierarchical, could provide a high specific surface and possibly facilitate the diffusion of the electrolyte into the inner region [40]; (ii) From the FT-IR spectra and XPS discussed above, PANI-TS-HCl had a higher oxidation level than PANI-RM-HCl. Previous literature has shown that polyaniline with higher oxidation level owns higher areal
ACCEPTED MANUSCRIPT capacitance [36], thus higher specific capacitance could be expected for polyaniline with higher oxidation levels, the result of our work further proving this concept. To further test the performance of PANI-TS-HCl for practical applications of supercapacitors, symmetric cells based on PANI-TS-HCl were constructed as shown
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in Fig. 7(a) and measured in 2M H2SO4. Fig. 7(b) shows the GCD curves of a typical cell with current densities ranging from 0.5 A g-1 to 10 A g-1. The IR drop is a relatively large value of ~0.045 V at the discharge current of 0.5 A g-1. On the one
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hand, the separator (cellulose paper) may constrain the migration of the ions in the electrolyte, thus increasing the resistance. On the other hand, our simple construction
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of simple symmetric cells (without considering open circuit potential, doped states and appropriate potential for the devices) may also lead to a large IR drop. However, such symmetric cells still own good energy storage properties. Accordingly, we have calculated the specific capacitance of the devices based on the total mass of
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PANI-TS-HCl on both electrodes. A relatively large value of 160 F g-1 at a current density of 0.5 A g-1 was obtained. The cell also possessed a good rate capability, with a retention rate of 84.4% (135 F g-1) when the current density was increased to 10 A
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g-1. In addition, the energy density and power density of the cell were also calculated based on the GCD curves, as shown in Fig. 7(c). The cell exhibited a high energy
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density of 12.6 Wh kg-1 at a power density of 188.2W kg-1, which is even comparable to cells based on polyaniline-based composites [11, 13, 48, 49]. Fig. 7(d) shows a plot of the capacitance retention of a cell in 10000 GCD cycles
at a current density of 2 A g-1. Sharp drops were found in the first 500 cycles, after which the trend remained relatively stable with a slight growth (about 60% was retained after 10000 cycles). Such a result can be a little bit unsatisfactory but yet understandable. Polyaniline endures swelling and shrinking in the charge/discharge
ACCEPTED MANUSCRIPT process, thus leading to the degradation of the performance [13, 17, 41]. It is thus very common for net polyaniline without attachments to any supporting matter in micro scales [8, 9, 13, 17, 34, 40, 41]. Fig. 7(e) and (f) show a comparison of the CV curves at 10 mV s-1 and Nyquist plots before and after the cycles, respectively. Less intense
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peaks were found for the CV curve after the cycles. Nyquist plots both before and after cycles showed small semicircles (~0.2 Ω in diameter) in the high frequency area and nearly vertical trends in the low frequency area, indicating small Rct and good
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capacitor behavior. The high-frequency intercept with the X-axis (Res) apparently increased (from 2.7 Ω to 5.3 Ω) after the cycles. Res, namely, the equivalent series
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resistance, is associated with the sum of the electrolyte solution resistance, the intrinsic resistance of active material and the contact resistance at the electrode/electrolyte interface [9]. Thus, we propose that the swelling and shrinking induced by the cycles may cause the detachment of polyaniline with conductive agent
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and current collector, resulting in the observed increase of Res. The unsatisfactory cycling stability could also be a result of inappropriate potentials applied. In the potential range of -0.2~0.8 V (vs Ag/AgCl, in a three-electrode system), relatively
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unstable initial CV curves could be observed (see Fig. S5). To further elucidate this,
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we have carried out five consecutive CVs at 5 mV s-1 on a PANI-TS-HCl electrode in different intervals (-0.2~0.5 V, -0.2~0.6 V, -0.2~0.7 V and -0.2~0.8 V), as shown in Figs. 8(a)-(d). It could be seen that variations of CV curves appear as the potential reaches larger than 0.7 V, and variations become obvious as the potential reaches 0.8 V. This could be attributed to irreversible degradations of the polymer [20]. Capacitance retention rates were calculated from these CV curves and plotted in Fig. 8(e). Capacitance loss becomes large as the potential outreaches 0.7 V. This could partially explain the poor cycling stability of the symmetric cells, but situations of
ACCEPTED MANUSCRIPT potential state can be more complicated in a cell compared to those in a three-electrode system. Thus more explorations are necessary for further improvement of relevant devices.
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4. Conclusion In summary, we have developed a simple two-step method composed of a low temperature period (-20 °C) freezing and a higher temperature (30 °C) thawing
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periods. A bimodal morphology composed of a coral-like structure with thorns and an interconnected nanowire structure was found for polyaniline prepared by such a
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method. The formation of these structures is mainly based on solute migration, temperature adjustment and freezing, as discussed. FT-IR spectra and XPS indicated a higher oxidation level for PANI-TS-HCl than for PANI-RM-HCl. Both hierarchical microstructure and high oxidation level are supposed to improve the electrochemical
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storage behavior. In three-electrode measurements, PANI-TS-HCl shows a relatively high specific capacitance of 689 F g-1 at 1 A g-1, and retention of 89% (613 F g-1) at 10 A g-1. Moreover, for symmetric cells, values of 160 F g-1 at 0.5 A g-1 and 12.6 Wh kg-1
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at 188.2W kg-1 were obtained. All these electrochemical properties for energy storage were among the best for net polyaniline through chemical oxidative polymerization.
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Thus, the two-step method proposed in this work may provide a better choice to fabricate polyaniline and relevant composites for supercapacitor applications.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51472172). The authors acknowledge the help of Dr. Shanling Wang and Ms. Hui
ACCEPTED MANUSCRIPT Wang, and the support by the Analytical and Testing Center of Sichuan University for TEM analysis.
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Figure captions Fig. 1. Schematic illustration of bimodal structure formation of PANI-TS-HCl and digital photos of the vials in different reaction periods.
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Fig. 2. SEM images in different magnifications of (a)-(c) PANI-TS-HCl and (d)–(f) PANI-TS-H2SO4 with bimodal morphologies.
with thorns and (c)-(d) an interconnected nanowire structure.
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Fig. 3. TEM images in different magnifications of PANI-TS-HCl with (a)-(b) a coral-like structure
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Fig. 4. (a) FT-IR and (b) UV-vis spectra of PANI-TS-HCl and PANI-RM-HCl, inset: digital photos of PANI-TS-HCl and PANI-RM-HCl dissolved in NMP with different concentrations and pure NMP.
Fig. 5. N1s spectra of (a) PANI-TS-HCl and (b) PANI-RM-HCl.
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Fig. 6. (a) CV curves of the PANI-TS-HCl electrode at varying scan rates ranging from 10 mV s-1 to 100 mV s-1. (b) Peak current density values vs. scan rates of the PANI-TS-HCl electrode derived
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from CV curves. (c) Nyquist plot of PANI-TS-HCl in the frequency range from 100 kHz to 0.01 Hz, the inset shows an enlarged view of the higher frequency range. (d) GCD curves of the
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PANI-TS-HCl electrode with current densities ranging from 1 A g-1 to 10 A g-1. (e) CV curves of PANI-TS-HCl and PANI-RM-HCl electrodes at a scan rate of 5 mV s-1. (f) Specific capacitance of the PANI-TS-HCl and PANI-RM-HCl electrodes derived from GCD curves at different current densities. Fig. 7. (a) Illustration of the symmetric cell structure. Electrochemical performance of the symmetric cell based on PANI-TS-HCl: (b) GCD curves with current densities ranging from 0.5 A g-1 to 10 A g-1. (c) Ragone plots of gravimetric energy density vs. gravimetric power density and polyaniline-based composites in literature [11, 13, 48, 49]. (d) Capacitance retention in the process
ACCEPTED MANUSCRIPT of 10000 GCD cycles at a current density of 2 A g-1. (e) Comparison of CV curves before and after 10000 GCD cycles at a scan rate of 10 mV s-1. (f) Comparison of Nyquist plots in the frequency range from 100 kHz to 0.01 Hz before and after 10000 GCD cycles. The inset shows an enlarged view of the high frequency range before cycles.
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Fig. 8. (a)-(d) five consecutive CVs of a PANI-TS-HCl electrode at 5 mV s-1 in potential ranges of -0.2~0.5 V, -0.2~0.6 V, -0.2~0.7 V and -0.2~0.8 V, respectively. (e) Capacitance retention
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calculated from the CV curves of (a)-(d).
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S0013-4686(18)31293-3
DOI:
10.1016/j.electacta.2018.06.006
Reference:
EA 31999
To appear in:
Electrochimica Acta
Received Date: 1 April 2018 Revised Date:
28 May 2018
Accepted Date: 1 June 2018
Please cite this article as: Y. Yuan, W. Zhu, G. Du, D. Wang, J. Zhu, X. Zhu, G. Pezzotti, Two-step method for synthesizing polyaniline with bimodal nanostructures for high performance supercapacitors, Electrochimica Acta (2018), doi: 10.1016/j.electacta.2018.06.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT
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The as-prepared polyaniline exhibits a bimodal morphology composed of a coral-like structure and a nanowire structure. A large specific capacitance of 689
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F/g at 1A/g and retention of 613 F/g at 10 A/g is obtained for net polyaniline.
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Two-step method for synthesizing polyaniline with bimodal nanostructures for high performance supercapacitors Yuan Yuana, Wenliang Zhub, Guo Dua, Dengzhi Wanga, Jiliang Zhua*, Xiaohong Zhua,
a
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Giuseppe Pezzottib*
College of Materials Science and Engineering, Sichuan University, Chengdu 610064, China
Ceramic Physics Laboratory, Kyoto Institute of Technology, Sakyo-ku, Matsugasaki,
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b
606-8585 Kyoto, Japan
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Abstract
Here we report a novel and simple two-step method to fabricate polyaniline, which consists of two successive low-temperature (-20 °C, frozen) and near-room temperature (30 °C) periods. Melting of the frozen part and solute migration mainly
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drives polymerization. It is proved that the two-step method is an efficient way to fabricate polyaniline with a bimodal morphology composed of coral-like structure
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with thorns and interconnected nanowire structure. The mechanism of formation of such bimodal morphology is discussed in detail. Optimized polyaniline prepared by
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this method showed an excellent electrochemical energy storage performance, with a specific capacitance of 689 F g-1 at 1 A g-1 and a good retention rate of 89% (613 F g-1) at 10 A g-1. Symmetric cells based on it show an energy density of 12.6 Wh kg-1 at a power density of 188.3 W kg-1. In summary, this work provides a new way to fabricate polyaniline for supercapacitor applications.
Keywords: polyaniline, two-step method, freezing, bimodal nanostructures, supercapacitors * Corresponding author. Tel.:+86 28 85432078; fax:+86 28 85432078.Email address: [email protected] (J. Zhu), [email protected] (G.Pezzotti)
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1. Introduction Nowadays, a large amount of energy consumption seems inevitable for modern life due to the fast development of the technologies utilized by humans. Therefore,
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both renewable energy harvesting and energy storage systems are of great need to be developed. Among the energy storage systems, supercapacitors play an important role because they can provide a high power density with a relatively high energy density
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[1-3].Recently, significant achievements have been realized in combining polyaniline
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with other materials like MnO2 [4, 5], NiCo2O4 [6], MoS2 [7] and especially graphene [8-13] for supercapacitor electrodes. Employing carbon-based materials only can hardly provide a large specific capacitance, usually less than 400 F g-1 [2, 3]. However, the combination of polyaniline and graphene, can often provide a specific capacitance
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lager than 700 F g-1 and good rate capability [9-17]. For example, Wu et al. reported a phase-separated polyaniline/graphene composite electrode, which possesses a specific capacitance of 783 F g-1 at a current density of 27.3 A g-1 [11]. On the one hand, the
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relatively large capacitance originates from the pseudocapacitance of polyaniline; on
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the other hand, the synergistic effect seems vitally important for such a composite. As the content of polyaniline increases, the specific capacitance of such a composite often shows an increasing and then a decreasing trend [11-13, 16-18]. Moreover, net polyaniline shows a much smaller specific capacitance than the composites, which is usually less than 600 F g-1 [9, 17]. Thus, there is still potential to promote the performance of a composite provided that the performance of polyaniline itself could also be promoted.
ACCEPTED MANUSCRIPT Polyaniline, as a conducting polymer, has been long researched due to its facile synthesis, multiple intrinsic redox states [19, 20], fibrillar morphology in nanoscale [21-28], and environmental stability. The nanofibrillar morphology enables
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polyaniline to be used for versatile applications, especially for supercapacitors, because it can provide a high surface area for charging [22, 23]. Basically, there are two methods to fabricate polyaniline: electrochemical polymerization and chemical
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oxidative polymerization. Nanowires could often be expected from electrochemical
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polymerization, and they can provide a high specific capacitance of 500~800 F g-1 [20, 29-31]. But large amounts of polyaniline can hardly be expected from electrochemical polymerization. Meanwhile, different simple methods of chemical oxidative polymerization have been invented to fabricate polyaniline nanofibers including
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interfacial polymerization [24], dilute polymerization [25] and rapidly mixed polymerization [26]. Net polyaniline nanofibers based on these methods have been fabricated for supercapacitor applications. However, a specific capacitance less than
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600 F g-1 is usually observed [32-34].
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It should be noted that the nanofibrillar morphology of polyaniline is intrinsic and the methods mentioned above simply suppress the secondary growth and agglomeration of polyaniline. Conditions like dopant acids [24], oxidant redox potential [35], steric stabilizers [36], ionic strength, concentrations and temperatures [27] can also significantly affect the morphology of polyaniline. Among which, temperature is a condition of great importance and one that can be easily controlled. A low temperature (in ice-water bath, for instance) is often recommended in a
ACCEPTED MANUSCRIPT conventional chemical polymerization because the reaction is exothermic. However, in methods like interfacial polymerization, the issue is that the polyaniline fabricated at room temperature owns distinguished nanofibrillar morphology [24]. Even at
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higher temperatures, nanofiber formations are more likely to emerge [27]. Thus in most literature, temperatures around 0 °C or room temperature are often applied in the fabrication of polyaniline [10, 12-18]. There are also reported works in which
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polyaniline was fabricated at much lower temperatures (-10 °C ~ -50 °C) [37-42]. Ice
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formation is commonly avoided through adding additives like ethanol and more dopant acid. However, it was also reported that polymerization of aniline proceeds well even in the frozen state [38, 39]. Lately, Wang et al. reported about a “hierarchical” polyaniline obtained with the help of NaCl, where the reaction system
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was firstly rapidly mixed at 4 °C and then frozen at -18 °C thus leading to a relatively good specific capacitance of 520 F g-1 [40]. In this work, we have conducted a novel and simple two-step process based on
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solute migrations to fabricate polyaniline. As shown in Fig. 1, the reaction system was
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firstly frozen at a low temperature of -20 °C and then melted at a higher temperature of 30 °C. Such fabricated polyaniline shows a novel bimodal morphology composed of a coral-like structure with thorns and an interconnected nanowire structure. More importantly, good energy storage properties can be expected from it. Electrodes based on it can deliver a specific capacitance of 689 F g-1 at 1 A g-1 and a good retention rate of 89% (613 F g-1) at 10 A g-1, which is outstanding for net polyaniline (see Table 1 for comparison). Furthermore, symmetric cells with 2M H2SO4 aqueous electrolyte
ACCEPTED MANUSCRIPT were assembled based on it, and they could deliver an energy density of 12.6 Wh kg-1 at a power density of 188.3 W kg-1. To our knowledge, all these results are among the best for net polyaniline by chemical oxidative polymerization.
polymerization methods in a three-electrode system.
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Table 1. Comparison of the specific capacitance of net polyaniline by different chemical oxidative
Specific
Discharge
Potential Electrolyte
rates
ranges /V
capacitance
Ref.
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Methods
Microemulsion 1 A g-1 polymerization 0.18 A g-1
Rapid mixing
25 mV s-1
Frozen system at a low temperature (-18 °C)
1M H2SO4
351
[13]
0-0.7
1M H2SO4
505
[32]
0.2-0.7
1M HCl
~490
[33]
0.5 A g-1
0-0.8
1M H2SO4
520
[40]
1 A g-1
0-0.8
2M H2SO4
570
This
0-0.8
2M H2SO4
689
work
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Rapid mixing
0-0.8
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Interfacial polymerization
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/F g-1
1 A g-1
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Two-step method
2. Experimental
2.1. Synthesis of polyaniline All the chemicals were analytical grade and used as received. Process of polyaniline synthesis in the two-step method is shown in Fig. 1. Typically, 2 mmol aniline was dissolved in 20 mL 1M HCl aqueous solution and then warmed in water bath at 30 °C for one hour, forming solution A. 2 mmol ammonium persulphate was
ACCEPTED MANUSCRIPT dissolved in 20 mL 1M HCl aqueous solution in a vial (40mL, ~2.7 cm in diameter) and then cooled in a refrigerator at -20 °C for 24 hours; then, the solution was frozen to become solid, forming solution B. Successively, we poured solution A into solution B, and kept the vial in the refrigerator at -20 °C for 24 hours. By doing so, the whole
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system was frozen. The vial was then transferred to water bath at 30 °C for the second step reaction. Different reaction times (from 3 minutes to 24 hours) of the second step were applied. According to such screening of reaction times, 30 minutes were found
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to be optimum (see Supplementary materials for a detailed discussion). Polyaniline obtained according to the above process is henceforth indicated with the abbreviation
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PANI-TS-HCl. Polyaniline using H2SO4 as dopant acid to substitute for HCl was also fabricated in this work (referred to as PANI-TS-H2SO4).
For comparison, a rapid mixing method was applied to fabricate polyaniline. The detailed process is as follows. Solution A and solution B were both warmed in water
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bath at 30°C for more than one hour. Then, solution A was poured into solution B and stirred vigorously for 60 seconds. Then, the reaction system was kept without any disturbance in water bath at 30 °C for 30 minutes. Polyaniline based on rapid mixing
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is henceforth referred to as PANI-RM-HCl or PANI-RM-H2SO4.
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Either vacuum filtration using deionized water and ethanol to flush or dialysis with deionized water under stirring was carried out for the purification of as-prepared polyaniline (see Supplementary materials for detailed discussion). The resultant was dried in an oven at 70 °C for more than 12 hours. 2.2. Fabrication of electrodes and symmetric cells The as-prepared polyaniline was milled gently into fine powder in an agate mortar before use for electrodes. And then polyaniline powder, acetylene black, and polytetrafluoroethylene (PTFE) were mixed sufficiently in a mass ratio of 85:10:5 in
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cells were simply assembled by conjugation of two electrodes (without any other pre-treatments) with similar mass loadings, with cellulose paper as the separator, and
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they were immersed in 2 M H2SO4 and sealed in a plastic bag. 2.3. Material characterization
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The morphologies of the resulted polyaniline were investigated by field emission scanning electron microscopy (FE-SEM, JSF-7500F, Japan) at an operating voltage of 15 kV and field emission transmission electron microscopy (FE-TEM, Tecnai G2 F20 S-TWIN, America) at an operating voltage of 200 kV. We dispersed polyaniline in deionized water through ultrasonic concussion, and then it was dropped on copper
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mesh and dried for FE-TEM tests. Fourier transform infrared (FT-IR) spectroscopy was applied on a Nicolet 6700 instrument. Ultraviolet-visible (UV-vis) spectra were
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measured by dissolving polyaniline in N-methylpyrrolidinone (NMP) on a UV-3600 instrument (Shimadzu Corporation, Japan). X-ray Photoelectron Spectroscopy (XPS)
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(XSAM 800, Kratos, UK) were carried out with an Al Kα X-ray source under a vacuum of < 10-9 torr.
2.4. Electrochemical measurements A three-electrode system was constructed with 2 M H2SO4 aqueous solution as the electrolyte, polyaniline electrodes as working electrodes, a Pt sheet as the counter electrode and Ag/AgCl as the reference electrode. Symmetric cells were also tested in 2 M H2SO4 aqueous electrolyte. Cyclic voltammetry (CV), galvanostatic
ACCEPTED MANUSCRIPT charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements were carried out on an electrochemical work station (CHI 660E, Chenhua, China). Values of the specific capacitance were calculated from the GCD curves in this work (see Supplementary materials for details). The capacitance
GCD cycles at 2 A g-1.
3. Results and discussions 3.1. Synthesis, morphology and growth mechanism
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retentions of symmetric cells were tested on a CT2001A (LANHE, Wuhan, China) by
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In the two-step polyaniline fabrication process carried out in this work, reactions at a low temperature (-20 °C) and a near-room temperature (30 °C) were combined. As shown in Fig. 1, in the first step, the frozen phase (oxidant solution) would be slightly melted as soon as the liquid phase (aniline solution) with a higher temperature was added. Then, polymerization was initiated at the interface and spread among the
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liquid phase as the melted oxidant solute migrated. The whole system was placed in a low temperature environment to be frozen in the first step. In the second step, the
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frozen system was placed in a higher temperature environment to thaw it. The polymerization continued as the oxidant solution and aniline solution melted and
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migrated. In substance, the polymerization of polyaniline is based on the solute migrations induced by the concentration difference, that is, migration of the oxidant only in the first step and migration of both oxidant and aniline in the second step. However, it is quite intriguing that the formation of polyaniline does not go to the very bottom of the vial in the second step. As we can see from the digital image of the vial in Fig. 1, there is a blank transparent section at the bottom of the vial after warming and melting. This phenomenon indicates two facts in the reactions: (i) aniline would be totally polymerized before it reached the bottom of vials, indicating
ACCEPTED MANUSCRIPT the fast reaction of the second step; (ii) the fabricated polyaniline has a light weight. Moreover, it is found that the polyaniline would gather and also float on the upper side of the solution when the reaction of the second step is prolonged. Generally, aggregations induced by shaking and stirring would cause sinking of polyaniline [43].
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This indicates that no aggregation like those induced by shaking or stirring exists in the two-step reactions.
SEM and TEM measurements were carried out to elucidate the morphology of
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the as-prepared polyaniline, as shown in Figs. 2 and 3. Fig. 2(a)-(c) show the SEM images of PANI-TS-HCl. It is intriguing and unexpected that PANI-TS-HCl owns a
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bimodal morphology composed of a coral-like structure and an interconnected nanowire structure. Fig. 2(b) shows the coral-like part in a higher magnification, and it is obvious that the “coral” has many short thorns. And this is even clearer in the TEM images shown in Figs. 3(a) and (b). The coral-like part in this work is similar to
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that shown in the work by Wang et al.[40], in which the polyaniline was claimed to be of a hierarchical structure, but the thorns in our work are slightly shorter. It should be noted that coral-like structure could also be found in other works [24, 27, 43, 44],
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where a low temperature or continuous stirring was applied. However, granular
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polyaniline or significant aggregation would commonly accompany such a structure. In contrast, when the frozen condition is applied to the reaction, the coral-like structure is more homogeneous, that is, granular structures or aggregations seldom emerge. Thus, it is reasonable to consider that the coral-like polyaniline mainly formed in the first frozen period. Meanwhile, more polyaniline in the two-step method shows a structure of interconnected nanowires as shown in Figs. 2(a) and (c). The nanowires are relatively homogeneous in their morphology with diameters in the range of 50~80 nm, as shown in Figs. 2(c) and 3(c)-(d). It should be noted that the
ACCEPTED MANUSCRIPT content of the resultant polyaniline can be very much limited if the time of the second step is too short. For instance, when the time is 3 minutes, the solution cannot totally melt and little polyaniline can be collected when vacuum filtration is applied. Thus, if coral-like polyaniline is assumed to form in the first step, it is reasonable to consider
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that the nanowires with a bigger proportion mainly form in the second step.
One possible mechanism for the formation of the coral-like structure and nanowire structure in different periods can be proposed, as shown in Fig. 1. In the first
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step, oxidant would migrate due to a concentration gradient as solution B melts. The reaction rate can be rather slow due to the low temperature applied in this period, thus
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heterogeneous nucleation happens and new polyaniline formation tend to emerge on the as-formed polyaniline [43, 45]. However, due to the low temperature (-20 °C), frozen of the upper solution (solution A with migrated oxidant) could also happen at the same time. The insoluble and solid as-formed polyaniline is an ideal substrate for
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nucleation of ice, as the energy needed is much lowered. The nucleation of ice would hinder further growth of polyaniline on the as-formed polyaniline substrate, thus the final produced polyaniline of the first step is coral-like, and relatively homogeneous
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with thorns on it. Because of the higher temperature (30 °C, around room temperature)
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in the second step, both the reactions and the migrations of the solutes are much faster. Considering that no disturbance is applied, heterogeneous nucleation is suppressed, and thus nanofibers can be expected [43, 45]. To further test if the two-step method discussed above is an effective way for
polyaniline fabrication of a bimodal morphology, a different dopant acid (H2SO4 in this case) was applied. As shown in Figs. 2(d)-(f), the structure of PANI-TS-H2SO4 was found to be of a bimodal morphology composed of a coral-like structure and an interconnected nanowire structure as well, but the size of the nanowires in diameter
ACCEPTED MANUSCRIPT and the whole of the "coral" were slightly bigger than those of PANI-TS-HCl, which could be a result of different properties of the anions (e.g. size and electro-negativity) in the reaction system. Similar differences due to the different dopant acid were also
only nanofibers were found for PANI-RM-HCl (see Fig. S2). 3.2. Molecular structures
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reported in previous literature [24]. When the rapid mixing method was carried out,
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FT-IR and UV-vis were carried out for the analysis of molecular structures of PANI-TS-HCl and PANI-RM-HCl, as shown in Fig. 4. Typical peaks for polyaniline
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could be found for both PANI-TS-HCl and PANI-RM-HCl as shown in Fig. 4(a) (see Table S1 for detailed peak values and assignments) [9, 19, 40, 46]. Commonly, peaks around 1580 cm-1 and 1490 cm-1 are assigned to C=C stretching of quinoid ring and benzenoid ring, repectively; peaks around 1305 cm-1, 1240 cm-1could be assigned to C-N stretching mode; broad peaks around 1200-900 cm-1 arose from overlapped
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vibrations of C-H in-plane bending. Compared with PANI-RM-HCl, the peak of PANI-TS-HCl around 1240 cm-1 is much weaker, indicating that more quinoid units
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exist in PANI-TS-HCl as compared to PANI-RM-HCl [46]. It should be noted that a distinct peak at 1375 cm-1 was found for PANI-TS-HCl, and for PANI-RM-HCl the
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peak at 1375 cm-1 was very weak. Early studies showed that polyaniline in the leucoemeraldine (LM) form with no quinoid imine units does not absorb at this frequency and the absorption band at this frequency disappears once emeraldine (EM) base is acid-doped to become EM salt [19, 46, 47]. Thus, the distinct peak at 1375 cm-1 of PANI-TS-HCl indicates two facts: (i) PANI-TS-HCl has a higher oxidation level with more quinoid units than PANI-RM-HCl [46]. (ii) PANI-TS-HCl became dopant-free after the process. It should be noted that obvious dark green was observed in the fabrication process of PANI-TS-HCl as shown in the digital pictures of Fig. 1,
ACCEPTED MANUSCRIPT indicating the emeraldine salt form. But, a relatively high drying temperature of 70 °C was applied in our later process, which could possibly cause PANI-TS-HCl being devoid of dopant. This result was further verified by the UV-vis spectra. For UV-vis test, NMP was used as the solvent to dissolve the polyaniline powder. As shown in the
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inset of Fig. 4(b), an apparent blue color could be observed for the solutions, certifying the formation of dopant-free EM base of the polyaniline after drying [20]. It is worth noting that dissolution of PANI-TS-HCl and PANI-RM-HCl in NMP showed
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apparent difference. The blue color is rather weak for PANI-RM-HCl at a concentration of 25 µg mL-1 and so are the absorption peaks in its UV-vis spectra.
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Thus, we have carried out the UV-vis spectra of PANI-RM-HCl at a concentration of 100 µg mL-1 for a better view. PANI-TS-HCl and PANI-RM-HCl show similar UV-vis spectra as shown in Fig. 4(b). Peaks around 330 nm correspond to the π-π transition; relatively intense peaks near 630 nm further certify the molecule structure of EM base;
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shoulder peaks could be seen around 280 nm, indicating existence of some pernigraniline (PNA) form [19].
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XPS was also carried out to further probe the molecular structures. As shown in Fig. 5, obvious difference could be observed in N1s spectra of PANI-TS-HCl and
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PANI-RM-HCl. For further analysis, three peaks, corresponding to quinoid imine (=N-, 398.6 eV), benzenoid amine (-NH-, 399.7 eV) and positively charged nitrogen atoms (> 400 eV), were applied to the curve-fit [13, 15, 19]. It is revealed that, the -NH- structure dominates in PANI-RM-HCl, and the =N- structure dominates in PANI-TS-HCl. This is in accordance with FT-IR results. Thus PANI-TS-HCl is further proved to own a higher oxidation level than PANI-RM-HCl. 3.3. Electrochemical characterization
ACCEPTED MANUSCRIPT To evaluate the electrochemical performance of the fabricated polyaniline by the two-step method, a three-electrode system using 2M H2SO4 as electrolyte was constructed. Fig. 6(a) shows CV curves of PANI-TS-HCl at scan rates ranging from 10 mV s-1 to 100 mV s-1. Two pairs of apparent peaks could be seen in these curves,
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which are commonly assigned to the LM/EM (peaks between -0.1-0.3 V) and EM/PNA (peaks between 0.4-0.7 V) transformations, respectively [9]. It has been shown in early literature that the potentials of anodic peaks tend to increase with
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increasing acidity [20]. Considering the high acidity (pH ca. -0.6) of the electrolyte in this work, the potential range is not high enough for the transition of EM/PNA [11].
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Thus it is reasonable to conclude that the peaks between 0.4-0.7 V actually belong to the redox reactions of by-products and hydroquinone/benzoquinone instead of EM/PNA [11, 31, 40]. Though shifts of peaks could be seen as the scan rate increases, peak current density values increased almost linearly along with the increase of the
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scan rate (as concluded in Fig. 6(b)), indicating that the electrode processes are not limited by the mass transfer and good rate capability is obtained [9, 11]. Electrochemical impedance spectra (EIS) with amplitude of 5 mV were also carried
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out for further understanding of the PANI-TS-HCl electrode. Fig. 6(c) shows the
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Nyquist plot of PANI-TS-HCl. No obvious semicircle was found in the high frequency area, indicating a negligible charge transfer resistance (Rct) [31, 40]. An area with a slope near 45°, known as the Warburg portion, could be seen as marked by a line in inset to Fig. 6(c). This region is relatively short, indicating good ion transport properties [13, 31]. Fig. 6(d) depicts the galvanostatic charge-discharge (GCD) curves of the PANI-TS-HCl with current densities ranging from 1 A g-1 to 10 A g-1. Shapes approximate triangles, but with slightly deformations. The deformations indicate
ACCEPTED MANUSCRIPT different contributions of transitions among the different states of polyaniline to the capacitance [28]. Specific capacitance values based on the discharge curves were calculated (see Supplementary materials for detailed calculation). PANI-TS-HCl exhibited a large specific capacitance of 689 F g-1 at 1 A g-1. Such a value is
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outstanding for net polyaniline by chemical polymerizations (see Table 1 for comparison), and even close to or bigger than some polyaniline materials prepared by electrochemical polymerization and composites [15, 16, 31, 41]. Moreover, a good
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rate capability was also found as shown in Fig. 6(f). A retention rate of 89% (613 F g-1) was obtained as we increased the current density to 10 A g-1, and this corresponds to
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above discussions of the CV curves and EIS tests.
For comparison, we have also measured the electrochemical performance of the as-prepared polyaniline by rapid mixing in this work in a three-electrode system (see Fig. 6(e) and Fig. S3). It is observed that PANI-RM-HCl owns similar CV behavior
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along with increasing scan rates and Nyquist plot to PANI-TS-HCl, indicating an also good rate capability. As shown in Fig. 6(f), PANI-RM-HCl exhibited a specific capacitance of 570 F g-1 at 1 A g-1, and a retention rate of 88% (501 F g-1) at 10 A g-1.
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Though similar rate capabilities were found for both PANI-TS-HCl and
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PANI-RM-HCl, a larger specific capacitance was obtained for PANI-TS-HCl, and the possible reasons for this could be explained, as follows: (i) the bimodal microstructures provide more effective surface area for contact of polyaniline and the electrolyte. Especially the coral-like structure with small thorns, which is hierarchical, could provide a high specific surface and possibly facilitate the diffusion of the electrolyte into the inner region [40]; (ii) From the FT-IR spectra and XPS discussed above, PANI-TS-HCl had a higher oxidation level than PANI-RM-HCl. Previous literature has shown that polyaniline with higher oxidation level owns higher areal
ACCEPTED MANUSCRIPT capacitance [36], thus higher specific capacitance could be expected for polyaniline with higher oxidation levels, the result of our work further proving this concept. To further test the performance of PANI-TS-HCl for practical applications of supercapacitors, symmetric cells based on PANI-TS-HCl were constructed as shown
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in Fig. 7(a) and measured in 2M H2SO4. Fig. 7(b) shows the GCD curves of a typical cell with current densities ranging from 0.5 A g-1 to 10 A g-1. The IR drop is a relatively large value of ~0.045 V at the discharge current of 0.5 A g-1. On the one
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hand, the separator (cellulose paper) may constrain the migration of the ions in the electrolyte, thus increasing the resistance. On the other hand, our simple construction
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of simple symmetric cells (without considering open circuit potential, doped states and appropriate potential for the devices) may also lead to a large IR drop. However, such symmetric cells still own good energy storage properties. Accordingly, we have calculated the specific capacitance of the devices based on the total mass of
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PANI-TS-HCl on both electrodes. A relatively large value of 160 F g-1 at a current density of 0.5 A g-1 was obtained. The cell also possessed a good rate capability, with a retention rate of 84.4% (135 F g-1) when the current density was increased to 10 A
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g-1. In addition, the energy density and power density of the cell were also calculated based on the GCD curves, as shown in Fig. 7(c). The cell exhibited a high energy
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density of 12.6 Wh kg-1 at a power density of 188.2W kg-1, which is even comparable to cells based on polyaniline-based composites [11, 13, 48, 49]. Fig. 7(d) shows a plot of the capacitance retention of a cell in 10000 GCD cycles
at a current density of 2 A g-1. Sharp drops were found in the first 500 cycles, after which the trend remained relatively stable with a slight growth (about 60% was retained after 10000 cycles). Such a result can be a little bit unsatisfactory but yet understandable. Polyaniline endures swelling and shrinking in the charge/discharge
ACCEPTED MANUSCRIPT process, thus leading to the degradation of the performance [13, 17, 41]. It is thus very common for net polyaniline without attachments to any supporting matter in micro scales [8, 9, 13, 17, 34, 40, 41]. Fig. 7(e) and (f) show a comparison of the CV curves at 10 mV s-1 and Nyquist plots before and after the cycles, respectively. Less intense
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peaks were found for the CV curve after the cycles. Nyquist plots both before and after cycles showed small semicircles (~0.2 Ω in diameter) in the high frequency area and nearly vertical trends in the low frequency area, indicating small Rct and good
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capacitor behavior. The high-frequency intercept with the X-axis (Res) apparently increased (from 2.7 Ω to 5.3 Ω) after the cycles. Res, namely, the equivalent series
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resistance, is associated with the sum of the electrolyte solution resistance, the intrinsic resistance of active material and the contact resistance at the electrode/electrolyte interface [9]. Thus, we propose that the swelling and shrinking induced by the cycles may cause the detachment of polyaniline with conductive agent
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and current collector, resulting in the observed increase of Res. The unsatisfactory cycling stability could also be a result of inappropriate potentials applied. In the potential range of -0.2~0.8 V (vs Ag/AgCl, in a three-electrode system), relatively
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unstable initial CV curves could be observed (see Fig. S5). To further elucidate this,
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we have carried out five consecutive CVs at 5 mV s-1 on a PANI-TS-HCl electrode in different intervals (-0.2~0.5 V, -0.2~0.6 V, -0.2~0.7 V and -0.2~0.8 V), as shown in Figs. 8(a)-(d). It could be seen that variations of CV curves appear as the potential reaches larger than 0.7 V, and variations become obvious as the potential reaches 0.8 V. This could be attributed to irreversible degradations of the polymer [20]. Capacitance retention rates were calculated from these CV curves and plotted in Fig. 8(e). Capacitance loss becomes large as the potential outreaches 0.7 V. This could partially explain the poor cycling stability of the symmetric cells, but situations of
ACCEPTED MANUSCRIPT potential state can be more complicated in a cell compared to those in a three-electrode system. Thus more explorations are necessary for further improvement of relevant devices.
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4. Conclusion In summary, we have developed a simple two-step method composed of a low temperature period (-20 °C) freezing and a higher temperature (30 °C) thawing
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periods. A bimodal morphology composed of a coral-like structure with thorns and an interconnected nanowire structure was found for polyaniline prepared by such a
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method. The formation of these structures is mainly based on solute migration, temperature adjustment and freezing, as discussed. FT-IR spectra and XPS indicated a higher oxidation level for PANI-TS-HCl than for PANI-RM-HCl. Both hierarchical microstructure and high oxidation level are supposed to improve the electrochemical
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storage behavior. In three-electrode measurements, PANI-TS-HCl shows a relatively high specific capacitance of 689 F g-1 at 1 A g-1, and retention of 89% (613 F g-1) at 10 A g-1. Moreover, for symmetric cells, values of 160 F g-1 at 0.5 A g-1 and 12.6 Wh kg-1
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at 188.2W kg-1 were obtained. All these electrochemical properties for energy storage were among the best for net polyaniline through chemical oxidative polymerization.
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Thus, the two-step method proposed in this work may provide a better choice to fabricate polyaniline and relevant composites for supercapacitor applications.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51472172). The authors acknowledge the help of Dr. Shanling Wang and Ms. Hui
ACCEPTED MANUSCRIPT Wang, and the support by the Analytical and Testing Center of Sichuan University for TEM analysis.
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Figure captions Fig. 1. Schematic illustration of bimodal structure formation of PANI-TS-HCl and digital photos of the vials in different reaction periods.
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Fig. 2. SEM images in different magnifications of (a)-(c) PANI-TS-HCl and (d)–(f) PANI-TS-H2SO4 with bimodal morphologies.
with thorns and (c)-(d) an interconnected nanowire structure.
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Fig. 3. TEM images in different magnifications of PANI-TS-HCl with (a)-(b) a coral-like structure
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Fig. 4. (a) FT-IR and (b) UV-vis spectra of PANI-TS-HCl and PANI-RM-HCl, inset: digital photos of PANI-TS-HCl and PANI-RM-HCl dissolved in NMP with different concentrations and pure NMP.
Fig. 5. N1s spectra of (a) PANI-TS-HCl and (b) PANI-RM-HCl.
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Fig. 6. (a) CV curves of the PANI-TS-HCl electrode at varying scan rates ranging from 10 mV s-1 to 100 mV s-1. (b) Peak current density values vs. scan rates of the PANI-TS-HCl electrode derived
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from CV curves. (c) Nyquist plot of PANI-TS-HCl in the frequency range from 100 kHz to 0.01 Hz, the inset shows an enlarged view of the higher frequency range. (d) GCD curves of the
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PANI-TS-HCl electrode with current densities ranging from 1 A g-1 to 10 A g-1. (e) CV curves of PANI-TS-HCl and PANI-RM-HCl electrodes at a scan rate of 5 mV s-1. (f) Specific capacitance of the PANI-TS-HCl and PANI-RM-HCl electrodes derived from GCD curves at different current densities. Fig. 7. (a) Illustration of the symmetric cell structure. Electrochemical performance of the symmetric cell based on PANI-TS-HCl: (b) GCD curves with current densities ranging from 0.5 A g-1 to 10 A g-1. (c) Ragone plots of gravimetric energy density vs. gravimetric power density and polyaniline-based composites in literature [11, 13, 48, 49]. (d) Capacitance retention in the process
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Fig. 8. (a)-(d) five consecutive CVs of a PANI-TS-HCl electrode at 5 mV s-1 in potential ranges of -0.2~0.5 V, -0.2~0.6 V, -0.2~0.7 V and -0.2~0.8 V, respectively. (e) Capacitance retention
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calculated from the CV curves of (a)-(d).
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