Extremely Stable Polypyrrole Achieved via Molecular Ordering for

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Extremely Stable Polypyrrole Achieved from Molecular Ordering for Highly Flexible Supercapacitors Yan Huang, Minshen Zhu, Zengxia Pei, Yang Huang, Huiyuan Geng, and Chunyi Zhi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11815 • Publication Date (Web): 07 Jan 2016 Downloaded from http://pubs.acs.org on January 8, 2016

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ACS Applied Materials & Interfaces

Extremely Stable Polypyrrole Achieved from Molecular Ordering for Highly Flexible Supercapacitors Yan Huang,a Minshen Zhu,a Zengxia Pei,a Yang Huang,a Huiyuan Gengb and Chunyi Zhia,c,* a

Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee

Avenue, Hong Kong, China. Email address: [email protected] (Prof. Dr. C. Y. Zhi) b

State Key Laboratory of Advance Welding and Joining, Harbin Institute of Technology,

Harbin 150001, China. c

Shenzhen Research Institute, City University of Hong Kong, Shenzhen 518000, China.

KEYWORDS: polypyrrole, cycling stability, flexible supercapacitors, molecular ordering, electropolymerization

Abstract: The cycling stability of flexible supercapacitors with conducting polymers as electrodes is limited by the structural breakdown arising from repetitive counter ion flow during charging/discharging. Supercapacitors made of facilely electropolymerized polypyrrole (e-PPy) have ultrahigh capacitance retentions of over 97%, 91%, and 86% after 15000, 50000, and 100000

charging/discharging

cycles,

respectively,

and

can

sustain

over

230000

charging/discharging cycles with still about half of initial capacitance retained. To the best of our knowledge, such excellent long-term cycling stability was never reported. The fully controllable

ACS Paragon Plus Environment


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electropolymerization shows superiority in molecular ordering, favouring uniform stress distribution and charge transfer. Being left at ambient conditions for even 8 months, e-PPy supercapacitors keep the good electrochemical performance completely. The extremely stable supercapacitors with excellent flexibility and scalability hold considerable promise for the commerical application of flexible and wearable electronics.

1. INTRODUCTION Flexible supercapacitors with long-term stability are essential for the practical application of wearable devices, which have become a representative advance in personalized electronics.1-6 Among all kinds of electrocapacitive materials, conducting polymers such as polypyrrole (PPy) are intrinsically flexible which can maintain or even enhance performance under small strains,7-12 validating themselves as excellent candidates for flexible and wearable devices.13-14 However, the cycling stability of PPy is much frustrating, with over 10% loss of capacitance after no longer than 1000 charging/discharging cycles.15-17 This is limited by the structural breakdown resulting from repetitive counter ion flow during the charge/discharge process.18-19 On one hand, as a ptype conjugated polymer, undoped neutral nitrogen groups in the PPy turns to be positively charged attracting counter anions from the electrolyte during charge, leading to swell. In the reverse discharge process, the positively charged nitrogen groups turn back to be neutral repelling the counter anions into the electrolyte, leading to shrink.19-20 The structural breakdown due to such repeated volumetric swelling/shrinking inevitably results in the capacitance loss. On the other hand, being similar to the problem of Li ion batteries, the repetitive flow of counter ions makes more and more ion flow channels collapse to be a compact structure after cycles of


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charging/discharging. Thus, less and less counter anions can flow reversibly, jeopardizing the free charge transfer.21-22 Many efforts have been devoted to improve the poor cycling stability of conducting polymers.18, 23-27 It has been reported that special anion dopants such as Tiron23 and sulfanilic acid azochromotrop24 retained the capacitance over 90% after 1000 charging/discharging cycles due to better adhesion of PPy by the hydroxyl groups in the dopants behaving as chelating agents. However, no more cycles were performed in the references and such stability is far from being satisfactory. As part of efforts developing nanocomposite materials for the stability,18-19, 25-26, 28 Tang et al.25 increased the cycle number from the commonly-demonstrated 1000 to a higher value of 4000 and achieved a capacitance retention of 90% by depositing PPy on MoS2 monolayers. Liu et al.18 even increased the cycle number to 10000 and remarkably maintained the capacitance of 85% by deposition of a thin carbonaceous shell on PPy. It was believed that the deposited shell could serve as a physical buffering layer which suppresses the structural breakdown and meanwhile hold the electrode fragments during charge/discharge cycling.18, 26 With the incorporation of functionalized partial-exfoliated graphite and β-naphthalene sulfonate anions dopant, Song et al.19 further improved the cycling stability of nanocomposite PPy to be 97% capacitance retention after 10000 cycles. However, multi-step complicated synthesis is involved, making the improvement time-consuming and cost-expensive. In addition, so far the stability was demonstrated only below 10000 cycles. Recently PPy prepared by electropolymerization shows great high-rate performance and the sign of good cycling stability.29 Keeping in mind that electropolymerization is facile and low-cost, the PPy made by electropolymerization could be a very promising material for various stable electrochemical devices. However, some essential issues are still not clear. For example, the mechanism behind the stability keeps unknown, which



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is of primarily importance to extend the concept to other material systems. Moreover, for practical applications, 10000 cycles might be not enough for a supercapacitor device considering their fast charging/discharging. Here we explore the cycling stability limit and investigate the stability mechanism of supercapacitors made of facile one-step electropolymerized pure polypyrrole (e-PPy). These ePPy supercapacitors are extremely stable, showing capacitance retentions of over 97% after 15000 charging/discharging cycles, 91% after 50000 cycles, and 86% after 100000 cycles as well as sustaining over 230000 charging/discharging cycles with still about half of initial capacitance retained at a fast charging/discharging rate of 3 A/g. We find that the fully controllable electropolymerization is superior in molecular ordering, favouring uniform stress distribution and charge transfer. Moreover, the e-PPy supercapacitors keep 100% of good electrochemical performances even after 8 months of being left at ambient conditions. To the best of our knowledge, such excellent long-term cycling and aging stability were never reported. We expect the same approach can be applied to improve the stability of other conducting polymers.

2. EXPERIMENTAL SECTION 2.1 Synthesis. Stainless steel meshes with a width of 0.5 cm and a length of 3 cm were washed in acetone, ethanol and deionized water, and then used as substrates. Two methods of electropolymerization (e-PPy) and chemical polymerization (c-PPy) were employed. For e-PPy, anodic electrodeposition of PPy was conducted for 5 min at a constant current density of 0.33 mA/cm2 in a solution of 0.1 M p-Toluenesulfonic acid, 0.3 M sodium toluenesulfate, and 0.5% pyrrole monomer (v : v) at 0 °C, resulting in a mass loading of 0.15 mg. For c-PPy, a mixed


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solution of 0.3 ml pyrrole monomer and 125 ml p-Toluenesulfonic acid (0.1 M) was stirred at room temperature, with 1.14 g ferric trichloride slowly added. Then the mixed solution was placed for 24 h at 0 °C. The resultant c-PPy was centrifuged, washed, filtered, and finally pressed onto the conductive substrates of stainless steel meshes as electrodes with a mass loading of 0.275 mg. 2.2 Fabrication and electrochemical characterization of supercapacitors. PVA/H3PO4 electrolyte was prepared with 6 g H3PO4, 6 g PVA and 60 ml deionized water at 90 °C. Two identical as-polymerized PPy electrodes were immersed in the cooled electrolyte solution for 5 min. Thereafter, the electrolyte wetted electrodes were placed in parallel and dried until gel solidification under ambient conditions. Finally the solid-state supercapacitor was obtained with the electrolyte also serving as a separator. The performance of all-solid-state supercapacitors was measured by CV and GCD in a two-electrode configuration using the potentiostat (CHI 760E). Electrochemical impedance spectra (EIS) were measured at frequencies ranging from 0.01 Hz to 10000 Hz with a potential amplitude of 5 mV. All measurements were carried out at room temperature. Capacitance with respect to the single electrode was calculated using the charge integrated from GCD and CV curves individually according to the formulas: ଶூ௧

‫ܥ‬௠ = ௎௠ ଵ

௎

‫ܥ‬௠ = ௎௩௠ ‫׬‬௎ శ ݅ሺܷሻܷ݀ ష

(1)

(2)

where I is the discharge current during GCD, t is the dicharge time during GCD, U is the voltage range (ܷ = ܷା − ܷି ሻ, m is the mass of PPy on one electrode, ‫ ݒ‬is the scan rate of the CV curve, and ݅ሺܷሻ is the current during CV.



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3. RESULTS AND DISCUSSION 3.1 Electrochemical Performance of Supercapacitors Made of e-PPy and c-PPy Electrochemical oxidation and chemical oxidation of the monomer are two typical synthetic methods of conducting polymers. During the oxidation, dopant ions are inserted to the polymer backbone.20 For PPy, the maximum number of dopant anions is one for three monomer units.30 Compared with chemical polymerization, electropolymerization affords several advantages of one-step formation directly on the electrode surface, controllability and tunability over a wide range of parameters, minimization on side reactions such as the introduction of free radicals by over-oxidation30-31 etc.

Figure 1. Electrochemical performance of supercapacitors made of e-PPy and c-PPy. (a) CV curves at scan rates of 1 mV/s and 5 mV/s. (b) GCD curves at different charging/discharging specific currents of 1 A/g and 0.16 A/g. (c) Capacitances as functions of scan rates. (d) Capacitances as functions of charging/discharging specific currents.


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As demonstrated in Fig. 1, performances of our electropolymerized PPy (e-PPy) and chemical polymerized PPy (c-PPy) are of remarkable difference. e-PPy shows rectangular CVs and CDs of ideal isosceles triangles. The specific capacitances of e-PPy are comparable to, or even higher than many reported results with PPy tested in liquid electrolytes.10, 15-16, 18, 27, 32-34 In significant contrast, c-PPy exhibits distorted CVs and CDs, low specific capacitances, and much inferior rate capability. These indicate the e-PPy can endure ultrafast voltage/current change rates, which is believed to be a result of effective electrochemical dynamic processes in our e-PPy. As CVs are distorted at higher potential windows (Supplementary Fig. S1), the potential window of 0.6 V is appropriate for PPy.

3.2 Cycling Stability Performance of Supercapacitors Made of e-PPy and c-PPy The cycling performances at a fast charging/discharging rate of 3 A/g for e-PPy and at 0.27 A/g for c-PPy are studied as shown in Fig. 2. Lower charging/discharging rate favors the cycling result because the rate of deterioration depends strongly on how frequently the device undergoes a volumetric change (Supplementary Fig. S2). The capacitance of e-PPy keeps over 97% after 15000 charging/discharging cycles. Capacitance retentions after 50000, 100000, and 130000 cycles are over 91%, 86%, and 80%, respectively. Even when suffered more than 230000 cycles, e-PPy still maintains about 50% capacitance. To the best of our knowledge, such long-term cycling stability has not been performed and achieved. These results are summarized and compared with previously reported results in Supplementary Table S1. Regarding the c-PPy, the decent capacitance retention of over 80% in the first 10000 cycles is better than most reported PPy-based electrodes which lose over 50% of the initial capacitance after merely 1000 cycles.



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However, it degrades rapidly with less than 20% of initial capacitance left after 95000 cycles, suggesting the necessity to test the long cycle lifetime in studies of cycling stability. Capacitance fluctuations during cycling are observed in many literatures and could attributed to small disturbances from temperature etc.

18-19, 25, 35

As the thickness of these PPy electrodes is in the

magnitude of micrometer (~ 1 µm), these remarkable capacitance retentions of PPy are believed not to arise from the ultrathin electrodes. Electrodeposition, as a facile approach to the synthesis of conducting polymers, was employed widely and similar results were obtained up to 10000 cycles in some works.19 Unfortunately, no more cycles were tested and the mechanism for enhanced cycling performance solely from electrodeposition was not studied.

Figure 2. Cycling stability performance of supercapacitors made of e-PPy and c-PPy: capacitance retention at a specific current of 3 A/g (e-PPy) and of 0.27 A/g (c-PPy). (Insets are GCD curves during various cycle numbers.)


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3.3 Characterizations of e-PPy and c-PPy Electrodes before and after the Cycling Test To reveal the mechanism behind, we initially utilized in-situ transmission electron microscope (in-situ TEM) to observe the volumetric change during charging/discharging, being similar to the lithiation of silicon anodes for Li ion batteries.36 However, no volumetric change was observed for c-PPy (Supplementary Fig. S3). This is reasonable as c-PPy did not lose any capacitance in the first thousands of cycles, while hundreds of charging/discharging cycles have already been the limit for most Li ion batteries. Thereafter, we carried out scanning electron microscopy (SEM) observations to see the morphology change after the long cycling test. It is observed that the continuous film of e-PPy retains well-integrated with only some small cracks and slightly expanded bumps (Fig. 3a and b). By contrast, the aggregated nanoparticles of c-PPy totally collapse after charging/discharging cycles (Fig. 3c and d). From the viewpoint of surface energy, these nanoparticles have higher surface energy than nanofilms, tending to be more unstable. Changes in structure are also observed. Before the cycling tests, both have identical Raman spectrum (Fig. 3e) and very similar featured bands in FTIR (Supplementary Fig. S4) which confirm the species of as-synthesized PPy.11 After the cycling tests, the peak at 1573 cm-1 of C-C in-ring and C-C inter-ring stretching is still remarkable in the spectrum of e-PPy. This suggests that the e-PPy backbone structure is not destroyed by the repetitive charging/discharging (being consistent with Fig. 3b) and therefore ions can still transfer smoothly. As conductivities of conducting polymers are mainly determined by conjugation length and doping level,37-38 those disappeared in-ring bands of e-PPy after cycles may indicate a decrease of conductivity. For cPPy, there are no featured peaks after cycling, suggesting both a conductivity decrease and a



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structural collapse (being consistent with Fig. 3d) which should increase the resistance of charge transfer.

Figure 3. Characterizations of e-PPy and c-PPy electrodes before and after the cycling test. (a) SEM image of e-PPy before the cycling tests. (b) SEM image of e-PPy after the cycling tests. (c) SEM image of c-PPy before the cycling tests. (d) SEM image of c-PPy after the cycling tests. (e) Raman spectra. (f) Nyquist plots. Scale bars represent 500 nm.

Electrochemical impedance (Fig. 3f) confirms the afore-mentioned speculations from Raman spectra. Before the cycling tests, the charge transfer resistance of e-PPy (about 1.6 Ω) is similar


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to that of c-PPy (about 1.8 Ω), revealing their similar electronic transportation and ionic diffusion in the initial state. The system resistances of e-PPy and c-PPy are 8 Ω and 15 Ω, respectively. This shows a difference on the electric conductivity of synthesized polymers and/or adhesion to the substrate. However, after the cycling tests, the system resistances of e-PPy and cPPy increase to 12 Ω and 20 Ω, respectively. The similar increase of system resistance rules out the possibility that the c-PPy film is more likely to detach from the substrate, compared to e-PPy which was directly grown on the substrate. Being great different, the charge transfer resistance of e-PPy slightly increases to be 2 Ω, while that of c-PPy is over 40 Ω. This also substantiates the observation from afore-mentioned SEMs that the substantial structural breakdown resulting from repetitive counter ions flow in c-PPy impedes the ionic diffusion.

3.4 Mechanisms for the Cycling Stability Difference of e-PPy and c-PPy To further reveal the mechanism underlying their differences on the cycling stability, TEM and electron diffraction investigations are performed and the results are shown in Fig. 4a-d. ePPy shows a clear diffraction ring, while there is no observable diffraction ring for c-PPy. These imply a more ordered structure of e-PPy. X-ray diffraction (XRD) profile further confirms their significant difference on the molecular ordering (Fig. 4e). There is a broad and low bump at small diffraction angles for c-PPy, while a sharp diffraction peak at 26° appears in e-PPy which is attributed to be the interplanar d spacing of 3.45 Å.39 This reveals that molecular chains of ePPy are much more regularly aligned and oriented than those of c-PPy. The high ordering of ePPy should arise from oligomers oxidized preferentially along the imposed electric field with the assistance of p-toluenesulfonate anions. It has been proposed that the template synthesis (e.g., ePPy) favors deprotonation at the α-positons of pyrrole units so that the subsequent α-α couplings



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lead to a more planar configuration and longer conjugation length. Whereas, the bulk chemical oxidation (c-PPy) generates a highly oxidized environment in the whole homogeneous singlephase system, favoring α-β couplings and resulting in the branching and growth of molecular chains in various directions.40-42


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Figure 4. Revealing the mechanism of molecular ordering for the cycling stability. (a) TEM image of e-PPy. (b) Electron diffraction of e-PPy. (c) TEM image of c-PPy. (d) Electron diffraction of c-PPy. (e) XRD spectra. (f) Capacitance as a function of v-1/2. Scale bars represent 500 nm.

The more ordered e-PPy with interplanar spacing faciliates ion diffusion. This is further supported by Fig. 4f. It is well known that electrochemical energy storage is controlled by two mechanisms: capacitive process and semi-infinite diffusion.43 In the first mechanism, capacitance is independent of scan rate v. In the second mechanism, capacitance can be plotted as a function of scan rate v-1/2. Fig. 4f shows that at scan rates no higher than 25 mV s-1, e-PPy is dominated by the capacitive process as demonstrated by the nearly flat fitting line, and it is not dominated by diffusion until at scan rates higher than 100 mV s-1 as by the fitting line with a large slope. By contrast, c-PPy is dominated by diffusion since an ultralow scan rate of 2 mV s-1. These arise from the layered ordering of e-PPy, which provides many interplanar channels for the long-term repetitive ionic diffusion so that the energy storage is not limited by diffusion until at high scan rates. By contrast, small voids in the orderless c-PPy are those channels for slow ionic diffusion.

3.5 Proposed Molecular Models of e-PPy and c-PPy Based on the analyses above, it is thus inferred that e-PPy α-α coupled chains are stacked layer by layer with a d spacing of 3.45 Å (Fig. 5a), while c-PPy α-β coupled chains are crossed orderlessly (Fig. 5b). The increased ordering of e-PPy effectively improves the reversible ionic transportation and thereby elevates the charging/discharging rate. Moreover, the ordered structure of e-PPy facilitates a homogeneous stress distribution in the molecular chain matrix



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during charging/discharging cycles, thereby improving the cycle stability significantly. Conversely, the orderless c-PPy inhibits ionic diffusion and homogeneous stress distribution, resulting in a fast decay of capacitance.

Figure 5. Models proposing possible molecular structures. (a) e-PPy with a long-distance layered ordering guaranteeing effective ion diffusion. (b) c-PPy with a disordered structure. Colors for different elements: C-gray, N-blue, H-white, O-red, and P-pink (O and P represent counter ions such as PO43-).

3.6 Aging Stability of Supercapacitors Made of e-PPy Not only the excellent cycyling stability, e-PPy with the ordering structure exhibits remarkable long-time aging performance of supercapacitors as well, which meets the goal for practical applications as flexible and wearable electronics. As seen in Fig. 6a and b, the good electrochemical performances are completely maintained even after 8 months of storage at ambient conditions. In addition, they suffer arbitrary deformations (Supplementary Fig. S5 and S6) and scale-ups by in parallel/series (Supplementary Fig. S7) without losing any capacitance, revealing a stable structure of our e-PPy.


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Figure 6. Aging stability tests of supercapacitors made of e-PPy at ambient condition. (a) CVs after various time from 0 to 8 months. (Inset is a photo of the supercapacitor.) (b) GCDs after various time from 0 to 8 months.

4. CONCLUSIONS In conclusion, we found PPy fabricated by a facile one-step and low-cost approach of electropolymerization can be extremely electrochemically stable. The supercapacitors based on the PPy can sustain over 230000 charging/discharging cycles at a high rate of 3 A/g meanwhile still maintain half of initial capacitance. The great stability achieved is fundamentally attributed to molecular ordering formed by the assistance of electric field, which favours uniform stress distribution and charge transfer. We expect to extend the concept to other conducting polymers. The combination of excellent cycling stability, aging stability, flexibility as well as scalable assembly opens up considerable opportunities for commerical applications such as flexible and wearable electronics.

ASSOCIATED CONTENT



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Supporting Information Available: Supplementary information detailing Figures S1-S7 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] Tel.: +852-34427891. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Early Career Scheme of the Research Grants Council of Hong Kong SAR, China (CityU 109213), and the Science Technology and Innovation Committee of Shenzhen Municipality (JCYJ20140419115507579). The authors thank J. Wang, T. F. Hung, D. Tang, K. Huo and X. Huang for experimental support.

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Table of Contents.

Flexible PPy supercapacitors with extremely cycling stability over 230000 cycles, are prepared by a facile one-step and low-cost approach of electropolymerization, which contributes to a high molecular ordering and thereby uniform stress distribution and fast charge transfer. They also keep 100% capacitance at ambient conditions for even 8 months and under complex nonplanar deformations as well as in parallel/series.


Extremely Stable Polypyrrole Achieved via Molecular Ordering for

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