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Polypyrrole Films with Micro/ Nano Sphere Shapes for Electrodes of High Performance Supercapacitors JuKyung Lee, Hobin Jeong, Rodrigo Lavall, Ahmed Busnaina, Young Lae Kim, Yung Joon Jung, and HeaYeon Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11574 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 10, 2017

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ACS Applied Materials & Interfaces

Polypyrrole Films with Micro/ Nano Sphere Shapes for Electrodes of High Performance Supercapacitors JuKyung Leea,b, Hobin Jeonga, Rodrigo Lassarote Lavalla,c, Ahmed Busnainaa, , Younglae Kima, Yung Joon Junga*, HeaYeon Leed,e AUTHOR ADDRESS a

JuKyung Lee, Hobin Jeong, Ahmed Busnaina, Rodrigo Lassarote Lavall, Younglae Kim, Yung Joon Jung Department of Mechanical and Industrial Engineering, College of Engineering, Northeastern University, Boston, MA 02115, USA E-mail: [email protected] b

JuKyung Lee, Korea Institute of Toxicology, Jeongeup-Si 56212,Republic of Korea c

Rodrigo Lassarote Lavall Chemistry Department/ICEx/Federal University of Minas Gerais. Av. Antônio Carlos, 6627, Pampulha, Belo Horizonte, Minas Gerais, 31270-901, Brazil d

HeaYeon Lee Department of Pharmaceutical Sciences, School of Pharmacy, Bouve College of Health Science, Northeastern University, Boston, MA 02115, USA E-mail: [email protected] e

HeaYeon Lee Department of Nano-Integrated Cogno-Mechatronics Engineering, Pusan National University, Busan 46241, Republic of Korea

KEYWORDS supercapacitor, conductive Polymer, polypyrrole (PPy), electrochemical impedance spectroscopy (EIS), high cyclability

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ABSTRACT We demonstrate a simple and efficient one step procedure for synthesizing a solid state Polypyrrole (PPy) thin film for supercapacitor applications using AC impedance spectroscopy. By controlling the frequency and amplitude we were able to create unique PPy nano/micro structures with a particular morphology of loop. Our PPy micro-nano sphere shows extremely high capacitance of 568 F/g which is closed to the theoretical value of 620 F/g, and 20-100% higher than other reported PPy electrodes. Most of all, this material presents high capacitance and significantly improved electrochemical stability without pulverization of its structure demonstrating 77 % retention of the capacitance value even after 10,000 charge/discharge cycles. These results are consequence of the larger surface area and adequate porosity generated due to the balance between the nano/micro PPy loops. This created pore structure also allows the favored penetration of electrolyte and high ion mobility within the polymer, and prevents the mechanical failure of the physical structure during volume variation associated with the insertion/deinsertion of ions upon cycling.


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1. INTRODUCTION Supercapacitors are high-capacitance electrochemical capacitors, which are attractive for high performance energy storage devices due to their high power density, long cycle life, and low maintenance cost1-3. A supercapacitor deriving its performance from a double-layer capacitance is often referred to as an electrochemical double layer capacitor4-8. This type of capacitor is mainly based on carbon materials due to its electrochemical stability, high surface area and proper micropores/mesoporous balance leading to high power density, and excellent cycle life9,10. Redox supercapacitors or pseudocapacitors are devices that store charges by the faradaic process (redox reactions). Transition metal oxides (MnO2 and RuO2) and π conjugated conductive polymers (polyaniline (PAni), polypyrrole (PPy), polythiophene (PTh) derivatives) are mainly used for the electrode materials to enhance the capacity through faradaic pseudocapacitance effects11. Among them Polypyrrole (PPy) is one of the particular interest in the supercapacitor applications since it is relatively ease to be synthesized using both chemical and electrochemical processes at low cost, and also can be presented in low environmental toxicity12. It also has high conductivity (1-100 Scm-1),11, 13 wide potential window and high capacitance per unit volume (400-500 Fcm-3) resulting in high charge/discharge rate (doping/dedoping) and high charge density similar to other conjugated polymers14-16. In addition, the formation and growth of PPy conducting polymer on metal or transparent conducting nanomaterials such as carbon nanotube or NiCo2O4 nanowire array17 electrodes demonstrated significantly enhanced capacitances, up to 506 F/g by 3-electrode measurement14,18, higher than those of commercial activated carbon (< 200 F/g)19. However this value is still lower than those fabricated with expensive, rare transition metals such as ruthenium oxide (1170 F/g)20. Furthermore the pseudocapacitors prepared with PPy suffer from poor cyclability and reduced capacitance per gram of active material since


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dopant anions have a limited access to interior sites of the high density PPy particularly when thicker electrodes are used15,16 This resulted in more than 50 % capacitance decrease after 1000 cycles of charge/discharge in aqueous electrolytes due to irreversible structural changes in the polymer matrix caused by volume change during ion doping and dedoping process16,21. Here, we report the fabrication of high-density PPy films with micro/nano sphere shape to be used as electrode material for high-performance and further reliable supercapacitor cells. By employing highly controlled electrochemical process, alternative current polymerization (ACep) of PPy, the size, density and morphology of PPy micro-nano sphere could be controlled for the use as electrodes in supercapacitor cells (Scheme 1). Our PPy micro-nano sphere materals shows 568 F/g which is closed to the theoretical value of 620 F/g, and 20-100 % higher than other reported PPy electrodes. Most of all unlike previous reports, this material presents high capacitance and significantly improved electrochemical stability without pulverization of its structure demonstrating 77 % retention of the capacitance value even after 10,000 charge/discharge cycles.


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Scheme 1. Electropolymerization of pyrrole by applying DC and AC power.

2. EXPERIMENTAL SECTION 2.1. Reagents and Chemicals Chemicals were obtained from the following source: pyrrole monomer, sodium perchlorate (NaClO4), sulfuric acid (H2SO4), potassium chloride (KCl), PBS tablets (Sigma-Aldrich, St. Louis, USA). A glass Ag/AgCl reference electrode (diameter 6 mm, length 5 cm) and Pt counter electrode were purchased from BAS analytical instruments (West Lafayette, USA).

2.2. Sample Cleaning All electro polymerization and electrochemical analysis were performed with an electrochemical analyzer (ALS 660A; CH instruments, USA) at room temperature. 4 x 2 mm2 size gold electrode


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was fabricated as the working electrode on Si wafer by sputtering method followed by a photolithography. Gold surfaces were cleaned in 10 mM H2SO4 solution with two scans of cyclic voltammetry (CV) with a scan range from 0 V to 1.8 V. This sequence was for removing dust by electrochemical cleaning. Next, electrochemical etching was performed by a CV method in 50 mM K3Fe(CN)6 solution. The CV was performed for more than 3 cycles with a sweep speed of 100 mV/s and a sweep range of -1 V to 1 V (vs Ag/AgCl) for the formation of a flat gold surface.

2.3. Sample Preparation PPy structures were fabricated and shaped by an electrochemical impedance spectroscopy (EIS) with a controlled AC wave (ACeP: Alternative current electropolymerization). EIS was performed in 0.1 M monomer pyrrole/ 0.05 M NaClO4 electrolyte solutions at a room temperature by electrochemical analyzer. The impedance spectra were recorded from 1000 Hz to 100 Hz, 10 Hz, 1 Hz, and 0.1 Hz with changing amplitude 10 mV, 30 mV, 50 mV, and 100 mV, 200 mV based on 640 mV respectively. DCeP (Direct current electropolymerization) was performed by simple chronoamperometry. DC potential was fixed at 640 mV because pyrrole is oxidized and polymerized nearby this potential as previously observed22.

2.4. Characterization After the polymerization, the electrode was thoroughly washed using distilled water. Each electrode was transferred to a pyrrole-free solution to block further polymerization. The mass of the deposit of PPy was measured after drying in air for 24 h using precision analytical balance (XPE206DR, Mettler Toledo, USA). Electrochemical measurements were conducted in a threeelectrode electrochemical reactor used by PPy as working electrode and Pt coil as the counter


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and Ag/AgCl as the reference electrode. The best performance PPy material was used to prepare a symmetrical supercapacitor using a two cell configuration: PPy│electrolyte│PPy. Cyclic voltammetry, galvanostatic charge/ discharge data were collected in two different electrolytes as 3 M KCl and 0.5 M H2SO4. Also electrochemical cycling stability of the electrode was tested by galvanostatic charge/ discharge cycling measurements for 10,000 cycles in a same electrolyte.

2.5. Calculations of Specific Capacitance, Power& Energy Density The specific capacitance is calculated based on galvanostatic charge/ discharge profile with applying 3, 5, 10, 15, 20 A/g current densities by following Equation 1 where Cs is the specific capacitance (F/g) or (F/cm2), I represents the discharge current (A), m is the mass of PPy (g), S is the electrode area (cm2), z is the thickness of polymer film (cm), ∆t is the discharge time (s) and ∆U is the potential window . (Equation 1)

‫ܥ‬௦ =

ூ∆௧ ௠(∆௎)

=

ூ∆௧ ௌ·௭(∆௎)

We also calculated power density (P, kW/kg) and Energy density (E, kWh/kg) of the device (considering mass of one electrode) using the following Equation (2-3) where Cs is the specific capacitance (F/g), ∆t is the discharge time (s) and ∆U is the potential window (V).

(Equation 2)

ܲ=

(Equation 3)

‫=ܧ‬

ா ∆௧ ஼ೞ (∆௎ మ ) ଶ


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The mass of PPy was determined by the difference of the mass of the Au substrate before and after electrodeposition. The mass loading of the PPy films are 0.626 mg/cm2 in DCeP, 1.511 mg/cm2 in ACeP 10 mV, 1.879 mg/cm2 in ACeP 30 mV, 3.004 mg/cm2 in ACeP 50 mV, 6.254 mg/cm2 in ACeP 100 mV, 21.129 mg/cm2 in ACeP 200 mV, respectively.

2.6. AFM & SEM Morphology The width, height, and pore size (diameter) of PPy films deposited on the gold surface were investigated using Atomic Force Microscopy (AFM) (Model: XE-10, Park Systems, Korea). A normal tapping mode of the silicon cantilever with the oscillation frequency of 365 kHz and spring constant of 47 N/m (NCH-10 V; Digital Instruments) was used for AFM imaging. No destruction of the sample surface was noticed during imaging. All images are presented in the height mode, where the higher parts appear brighter. SEM micrographs of were recorded with a Supra 25 FESEM (Carl Zeiss Ag, Germany). The samples were sputtered with Au prior to the microscopy analyses.

3. RESULTS AND DISCUSIION 3.1. Control Factor of AC Polymerization ACeP was controlled by electrochemical impedance spectroscopy (EIS). Impedance could be carried out over a range of frequencies at specific amplitude. In the previous research we reported that amplitude is one of the key factors for PPy DCeP22. When ACeP process is used, frequency is another important factor in order to fabricate the supercapacitance system since the size and density of PPy deposits can be controlled by frequency. To optimize electrode performance using AC method, firstly we controlled the frequency from high to low values.


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Based on our preliminary results, when the applied frequency is higher than 1000 Hz the magnitude of impedance had been saturated without any change and no morphological change of the polymer was observed. So the superior limit was chosen to be 1000 Hz. When the applied frequency was lower than 0.1 Hz, the polymerization takes more than one hour with increased film thickness. In experimental conditions, the optimal range of frequency (from 0.1 to 1000 Hz) was found for the fast polymerization reaction and it took only 3~5 min.

Figure 1. (a) Bode plot of electrochemical polymerization with pyrrole and (b-e) their SEM image (x 500). Applied potential was set to be 640 mV with 10 mV amplitude. Frequency range was (b) 1000 to 100 Hz, (c) 1000 to 10 Hz, (d) 1000 to 1 Hz, (e) 1000 to 0.1 Hz.

Figure 1a shows the bode plot of impedance for the formation of PPy structure. We chose four polymerization frequency ranges for electropolymerization reaction and observed their PPy structures: I) 1000 to 100 Hz; II) 1000 to 10 Hz; III) 1000 to 1 Hz, and IV) 1000 to 0.1 Hz, respectively (Figure 1b-e). In the ranges between 1000 to 100 Hz and 1000 to 10 Hz (Figure 1b-c), Scanning electron microscope (SEM) images of PPy film showed no distinguishing changes in PPy morphology. Structure of gold grain was observed at the high magnitude of SEM


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images, showing that there is no polymerization at these frequency ranges. However, in the range III (1000 to 1 Hz), ACeP started and created the small number of PPy structures (Figure 1d). We further analyzed this stage with respect to the phase angle plot since no structural change of polymerization was observed in the |Z| growth tendency. Decreasing phase angle indicates that the balance between capacitance and resistance is started to change polymerization due to the generation of PPy structures. The complete Au coverage with PPy structures was observed in the Range IV (1000 to 0.1 Hz) in which the PPy growth rate and density was significantly increased at extremely low frequency region, (Figure 1e). In this range, we could observe the magnitude of impedance (|Z|) was reduced quickly below 1 Hz due to the generation of PPy structures. These results indicate that the optimal frequency region for the formation of PPy structures is below 1 Hz. We also did the same measurement at the lower frequency region from 1 to 0.1 Hz (Figure S1) in order to further observe the nucleation and growth behavior of PPy. At 1-0.1 Hz (Figure S1b), PPy was formed on the electrode but not completely covered with whole surface compared to 1000 - 0.1 Hz range (Figure S1a). These experimental results show that starting at high frequency is the optimal condition of perfect polymerization for a wide area of electrode.


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Figure 2. SEM Images of PPy. (a) Polymerization of DCeP and ACeP was expressed by amplitude potential graph. (b-g) SEM images for electrode coated by PPy with polymerization. DCeP and ACeP with 10 mV, 30 mV, 50 mV, 100 mV, 200 mV amplitude, sequentially.

After optimizing the frequency range, pyrrole was polymerized by controlling the potential amplitude. It was mainly required that the pyrrole monomer should have an oxidation potential which can be oxidized in a suitable solvent system. As shown in Figure 2a, AC sine wave with a minimum potential of 10 mV (DC 640 mV ± AC 10 mV amplitude) and a maximum potential of 100 mV (DC 640 mV ± AC 100 mV amplitude) were applied21. When the potential amplitude below 10 mV applied, the formation of PPy structures was not observed. When the potential amplitude higher than 100 mV used, the aggregation of PPy (Figure 2f-g) was mostly observed due to the fast growth rate of PPy. PPy films polymerized by DC showed the low growth density of micro sized PPy micro/nano structures (Figure 2b). However, in ACeP, particularly between 10-100mV ranges, more PPy micro/nano array


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structures were covered on the electrode than DCeP as shown in Figure 2c-e. It is possible to verify an increase in the film thickness (Figure S2). When the polymerization amplitude increased from 10 mV to 50 mV, the number of micro/nano structures was gradually increased and vacant space was packaged by smaller size ones. After large size micro PPy structures were generated, smaller size nano PPy spheres were packaged in the remaining area on the electrode. This process was continued until 50 mV case with filing up different size of PPy micro/nano structures on entire surface of the electrode. We assume it makes the difference of electrode’s electrical conductivity because of extension of conductive pathways. In fact, we performed impedance study after film growth where it is possible to observe low resistance values for the sample obtained at 50 mV (Figure S3). We also found that PPy entered to aggregation state when high AC amplitude over 50 mV was applied. PPy micro/nano structures cannot be existed at 100 mV and it changed to globular structure without pores (Figure 2f-g). In this state, PPy deposition weight was dramatically increased compared to other polymerizations in the lower AC amplitude condition (AC 50 mV: 0.240 mg → AC 100 mV: 0.501 mg). It is important that the largest aggregation structure makes the diminution of effective surface area between electrode and electrolyte. Pores in the PPy micro/nano spheres also cannot be found on the surface of electrode, resulting in a limited capacitance due to low electrolyte accessibility23. To understand these conditions, various electrochemical measurements were performed by each polymerization electrode. However, considering the discussion above, it seems the modulation of the amplitude of the AC signal using impedance spectroscopy has the same influence of PPy morphology as the time and current density in the amperometric current-time (I-t) reaction described by Fujikawa and his coworkers22. In their work, the PPy doughnuts (in our case, PPy micro/ nano structures) were


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only obtained at reaction time 40 s and current density at the order of 1.5 mA·cm-2 (peak current), and PPy clouds were formed after 100s of reaction. Comparing both works, it is clear that besides the chemicals (and the concentration), the oxidation potential is essential for obtainment of polymers with a proper morphology, considering the different techniques employed in our work and that of Fujikawa and coworkers in the PPy synthesis.

3.2. Evaluation of the AC Polymerized Polypyrrole Properties as Electrode for Supercapacitor Application In order to fully understand the influence of PPy characteristics and its viability as electrode for supercapacitor application we evaluated the electrochemical properties of cells prepared with the mostly used acid and saline electrolytes: H2SO4 and KCl.

Figure 3. (a) Cyclic voltammetric curves at a scan rate of 100 mV/s; (b) Specific capacitance per gram of active material. (c) Galvanostatic charge/ discharge profiles of the PPy-AC 50 mV at different current densities. (d) Cycling stability of PPy-AC 50 mV measured by galvanostatic


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charge/ discharge at current density of 10 A/g. (e) SEM images of PPy-AC 50 mV collected before and after testing 1000 cycles. All measurements were performed by 3-electrode system in 0.5 M H2SO4 aqueous electrolyte.

Figure 3 presents the results obtained with 0.5 M H2SO4 electrolyte. Considering the cyclic voltammetry data, it is clearly from Figure 3a that there are no redox peaks in the range of -0.1 to 0.5 V. All the curves present almost box like shape and exhibit mirror image characteristics to the E-axis in that range. This is indicative of capacitive behaviors24. The cyclic voltammograms (CV) are rectangular and symmetrical indicating high reversibility and power density of PPy electrode synthesized in the present work. From the voltammograms (Figure 3a), it could be observed that the cells prepared with Ppy obtained with different treatments shows an increased of the charge (area under the curves), and consequently an increase of the capacitance. This is a consequence of the morphological changes in Ppy observed in the different tratments as discussed in the previous section (Figure 2b to 2e). Figure 3b shows the specific capacitance (Cs) as a function of the scan rate for the cell made with PPy synthesized by the different procedures. The cell prepared with PPy obtained by ACeP present higher capacitances than the one using PPy from DCeP. The highest Cs value of 568 F/g or 271 F/cm3 (at 20 mV/s) was observed for the cell using PPy synthesized by AC at 50 mV. In fact this value is comparable with the bests ones reported in literature and is among the largest specific capacitance observed for the PPy electrode synthesized by electrochemical method without the utilization of any kind additive of preparation of composites13,25. In fact the developed methodology is among the simplest and cheapest one reported and does not require any additional step to improve the PPy characteristics/properties.


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Although the cyclic voltammetry is an effective tool to observe the capacitive behavior of the PPy electrode, the most suitable method to obtain the cell capacitance is the gavalnostatic charge/discharge (GCD) experiments (considering the discharge curve)26, 27. We perform GCD at different current densities (from 3 to 20 A/g). In Figure 3c we presented the data for the best material (based on PPy-AC 50 mV). In agreement with CV results it is evident from GDC that the procedure development by our group allowed the synthesis of a PPy (AC 50 mV) with proper characteristics for supercapacitor application. Even at a high current density as 3 A/g a discharge capacity of 543 F/g or 259 F/cm3 was obtained for this cell (Figure 3c). The capacitance values decrease with the increase of the scan rate (Figure 3b). This is typical for conducting polymer– based cells. According to the literature the inner part of the electrode material cannot contribute to the charging/discharging at high charging/discharging rate decreasing the capacitance values26, 28

.

It is well known from the literature that conducting polymers based pseudocapacitors could present low cyclability due changes in polymer physical structure caused by doping/dedoping of ions15, 16. In fact, such devices could start to degrade even under less than one thousand cycles15. We perform charge/discharge cycles in the cells prepared using 0.5 M H2SO4 in order to evaluate the cycling effect on the electrode capacitance and polymer morphology. There are some reports in the literature that importance the use of acid solutions result in capacitors with superior capacitance and energy density26, 29. However, mostly of the works using such electrolytes just present the cycling results until 1000 cycles (or less) claiming good capacity retention26,30,31-33. From Figure 3d it is possible to see that the best cell (PPy obtained at AC 50 mV) is highly stable up to 1000 cycles losing only 10 % of its capacitance. However, above the 1500th cycle there is an abrupt fading. Figure 3e clearly shows that there is an irreversible change in the PPy


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physical structure with 1000 cycles of charge/discharge in the acid electrolyte. Before cycling test it is possible to observe very good micro/nano PPy strucutres covering the Au substrate. After cycling test, there are many cracks and damaged PPy structure looks like “burned” completely. We assumed these cracks formed as a result of structural pulverization due to the repeated volumetric shrinking and swelling by acidic electrolyte.

Figure 4. (a) Cyclic voltammetric curves at a scan rate of 100 mV/s. (b) Specific capacitance per gram of active material with different condition. (c) Cycling stability test cycling stability of PPy-AC 50 mV and 100 mV measured by galvanostatic charge/ discharge at current density of 10 A/g. (d) SEM images of PPy-AC 50 mV & 100 mV collected before and after testing 10000 cycles (x 1000 magnitude). All measurements were performed by 3-electrode system in 3 M KCl electrolyte.


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We also evaluate the PPy behavior using 3 M KCl solution as electrolyte (Figure 4). The prepared cells response followed the tendency observed previously with those prepared with acid electrolyte, however with higher capacitance values. The PPy synthesized by AC at 50 mV present Cs equal to 771 F/g or 351 F/cm3 (at 20 mV/s) and 813 F/g or 387 F/cm3 (at 3 A/g) obtained by CV and GCD, respectively (Figure 4a-b). Specific GCD curves at different current densities are shown in Figure S4. Especially, those 771 F/g and 813 F/g are outstanding values never reported before for a bare PPy electrode. This feature is consequence of the polymer characteristics obtained by the developed procedure. As previously discussed the synthesizes procedure employing AC at 50 mV provide a suitable coating of the gold current collector by a thin film of the PPy engineered with the loop-shaped morphology containing a balance between the nano/micrometer structures with necessary surface area and porosity for doping/dedoping of ions without irreversible changes of the physical structure of the matrix. This is strongly supported by the data presented in Figure 4c-d. No capacitance loss was observed bellow 3,000 cycles. With 5,000 cycles only 8 % of loss was verified and the cell maintains 77 % of its initial capacitance after 10,000 cycles with no considerable change in the original morphology (Figure 4c-d). The remarkable variation in ciclability between the cells prepared with the acid and saline electrolytes is probably related to the degree of pulverization caused by difference in the volume change due the ions size and charge and also due solvent contributions (water molecules moving into the polymer during electrochemical process)34-36. The magnitude of the volume change varies greatly from < 1 % to 120 %

33

. It seems that larger bivalent sulfate anion play an

important effect in our case.


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In order to corroborate the strong influence of the PPy physical structure and roughness/porosity (Figure S5) on the cell properties we also performed cycling test with DC and AC 100 mV assuming the difference of structure affects the electrochemical performance, including cycling stability. Figure S6 shows the GCD and Cs values at 3 A/g current densities. As an example, 100 mV and 200 mV ACeP PPy based cells present their maximum Cs at 3 A/g is 470 F/g and 448 F/g, respectively and its value is smaller than 50 mV ACeP, 813 F/g. What makes these differences? We found the reason examinee its structures. As previously said, if we apply over 50 mV amplitude, polymerization is entered to aggregation state and their size is much bigger and it leads to have smaller surface area compared with 50 mV ACeP. These differences are also affecting the cycling stability. Capacitance retention profile of AC 100 mV polymerization is keep going down and finally, its cycling stability of only 15.68 % after 10,000 cycles. As shown in Figure S7, SEM supports this reason because in the case of PPy 50 mV polymerization, maintain its structure over 10,000 cycles. However, in the case of PPy 100 mV polymerization, volumetric change was happened and its tendency is similar when using H2SO4 as shown in Figure 3e. Futhermore, some region was peel off by this affection. As previouly discussed, PPy micro/ nano structures cannot be exisited at 100 mV (and also at 200 mV) and it changed to globular structure without pores (expressed by PPy clouds). This morphological change has a considerably impact in the capacitance and cell performance. These results support our ACeP of PPy shows maximum efficiency when applying 50 mV amplitude in 3 M KCl electrolyte rather than H2SO4.


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Figure 5. (a) Galvanostatic charge/ discharge curves. (b) Electrode specific capacitance. (c) Ragone plot obtained at different current densities for the symmetric supercapacitor: ACeP PPy 50 mV │3 M KCl│ACeP PPy 50 mV using two-electrode configuration.

Considering the results obtained for the PPy synthesized by the procedure employing ACeP at 50 mV, we evaluated the response of a complete cell. Figure 5a presents the GCD curve for the symmetric supercapacitor: (ACeP PPy 50 mV│3 M KCl │ACeP PPy 50 mV) measured using two-electrode configuration. The curves present a triangular symmetric shape in all applied current densities, implying the reversible capacitive performance with the coulombic efficiency ranging from 94 % to 98 % (considering the different current densities). The specific capacitance take into account the mass of one electrode is 826.85 F/g or 387.41 F/cm3 (from the discarge curve) at a current density of 3 A/g. The device keeps 78 % of its capacitance with increasing


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current densities from 3 A/g to 20 A/g showing a good rate capability (Figure 5b). From the curves it is also possible to calculate the energy and power densities of the device using 3 M KCl as electrolyte to show higher efficiency as supercapacitor. Figure 5c is a Ragone plot of the corresponding specific enegy density versus power density was investigated and compared with other reported values. PPy obtained at ACeP with 50 mV deliver a maximum enegy density of 103.36 Wh/kg at power density of 0.68 kW/kg and maximum power density of 4.52 kW/kg at enegy density of 73.49 Wh/kg. These values higher than reduced graphene oxide (0.93 kW/kg and 82.5 Wh/kg)37, MnO2│FeOOH (0.45 kW/kg and 24 Wh/kg)38, NiOH2│ZnFe2O4 (0.209 kW/kg and 14 Wh/kg)39, and other PPy polymerzation case by using NiCO2O4@PPy│activated carbon asymmetric supercapacitor (AC ASC) (10.2 kW/kg and 58.8 Wh/kg)17. These data highlights the remarkable performance of the PPy film synthesized at 50 mV as both positive and negative electrodes for supercapacitor application.

4. CONCLUSION In this study we presented a simple and efficient one step procedure for the synthesis of PPy using AC impedance spectroscopy. By controlling the frequency and amplitude it was possible to obtain PPy nano/micro structures with a particular morphology of loop by forming a thin film covered with Au current collector. The properties of different electrode materials were evaluated using aqueous 0.5 M H2SO4 and 3 M KCl electrolytes. The best result was obtained with the thin film of PPy generated by applying the ACeP with 50 mV amplitude. This cell presents a specifically high capacitance and cyclability without any additional support material (retention of 77 % of the capacitance value after 10,000 charge/discharge cycles). The complet cell shows high capacitance with good rate capability and also delivery high energy (among the reported


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values) and power densities. These results are consequence of the high surface and adequate porosity generated due the balance between the nano/micro PPy loops. This allows the favoured penetration of electrolyte and high ion mobility within the polymer, and prevents the mechanical failure of the physical

structure during

volume variation

associated

to

the the

insertion/deinsertion of ions upon cycling. It demonstrates that this ACeP of PPy is one of promising electro polymerization method for supercapacitor applications.


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Supporting Information Additional SEM images of ACeP of pyrrole, AFM images of PPy micro/ nano sphere and line profile analysis, PPy film thickness by changing polymerization condition, the additional nyquist plot for DCeP and ACeP, additional Galvano charge/ discharge curve in 3 M KCl electrolyte using different polymerized PPy, specific capacitance of ACeP PPy were supplied as supporting information.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Yung Joon Jung) *E-mail: [email protected] (HeaYeon Lee) Yung Joon Jung and HeaYeon Lee contributed equally as corresponding author.

ACKNOWLEDGMENT This research was supported by the Basic Science Program of the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2014-052607). Also this research was supported by the funding from Technology Innovation Program (10050481) funded by the Ministry of Trade, Industry & Energy of Republic of Korea. R.L. Lavall thanks to the Federel University of Minas Gerais-UFMG/Brazil for the license granted for the sabbatical period at the Northeastern University.


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Table of Contents Graphic


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Nanosphere Shapes for Electrodes of

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