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Facile and scalable fabrication of graphene/ polypyrrole/MnOx/Cu(OH)2 composite for high-performance supercapacitors Hoda Nourmohammadi Miankushki, Arman Sedghi & Saeid Baghshahi

Journal of Solid State Electrochemistry Current Research and Development in Science and Technology ISSN 1432-8488 J Solid State Electrochem DOI 10.1007/s10008-018-4008-x

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Author's personal copy Journal of Solid State Electrochemistry https://doi.org/10.1007/s10008-018-4008-x

ORIGINAL PAPER

Facile and scalable fabrication of graphene/polypyrrole/MnOx/Cu(OH)2 composite for high-performance supercapacitors Hoda Nourmohammadi Miankushki 1 & Arman Sedghi 1 & Saeid Baghshahi 1 Received: 17 March 2018 / Revised: 30 April 2018 / Accepted: 29 May 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract In this study, to improve the specific capacitance of graphene-based supercapacitor, novel quadri composite of G/PPy/MnOx/ Cu(OH)2 was synthesized by using a facile and inexpensive route. First, a two-step method consisting of thermal decomposition and in situ oxidative polymerization was employed to fabricate graphene/polypyrrole/manganese oxide composites. Second, Cu(OH)2 nanowires were deposited on Cu foil. Afterwards, for the electrochemical measurements, composite powders were deposited on Cu(OH)2/Cu foil substrate as working electrodes. The synthesized samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared (FT-IR) spectroscopy, and Raman spectroscopy. The XRD analysis revealed the formation of PPy/graphene, Mn3O4/graphene, and graphene/polypyrrole/MnOx. In addition, the presence of polypyrrole and manganese oxides was confirmed using FT-IR and Raman spectroscopies. Graphene/ polypyrrole/MnOx/Cu(OH)2 electrode showed the best electrochemical performance and exhibited the largest specific capacitance of approximately 370 F/g at the scan rate of 10 mV/s in 6 M KOH electrolyte. In addition, other electrochemical measurements (charge–discharge, EIS and cyclical performance) of the G/Cu(OH)2, G/PPy/Cu(OH)2, G/Mn3O4/Cu(OH)2, and G/PPy/MnOx/Cu(OH)2 electrodes suggested that the G/PPy/MnOx/Cu(OH)2 composite electrode is promising materials for supercapacitor application. Keywords Polypyrrole . Graphene . MnOx . Cu(OH)2/Cu foil, composites, supercapacitor

Introduction Fabrication of low-cost and environmentally friendly energy storage device is one of the most important topics in recent years [1, 2]. Between different energy storage devices, supercapacitors (SCs) have been very much considered because of their high-power density, fast charge/discharge rate, and low cost and they have been widely used in various applications such as electric vehicles, industrial equipment, and energy production [3–6]. SCs can be divided into two categories: (i) electrical double-layer capacitors (EDLCs) and (ii) pseudocapacitors [7, 8] that have different charge storage mechanisms.

* Arman Sedghi [email protected] 1

Department of Materials Science and Engineering, Faculty of Engineering, Imam Khomeini International University, Qazvin 3414916818, Iran

Electrodes are one of the most important parts of the supercapacitor. Electrodes have two parts: electroactive materials and conductive substrate. In several researches, various electroactive materials have been investigated for use in SCs [9]. Up to now, different materials such as graphene and carbon nanotubes (CNTs) have been studied for EDLCs because of their surface area, low cost, and stability [10, 11]. In addition, various metal oxides [12–14], metal hydroxides [15, 16], and conductive polymers [15, 17] have been used for pseudocapacitor electrodes. Each kind of these materials has some advantages and disadvantages. Carbon materials have good cycle life and suitable mechanical properties, but their specific capacitance is low. Although transition metal oxide electrodes show high specific capacitance, their poor electrical conductivity and structural instability limit their applications; also, conducting polymers show high specific capacitance and good flexibility, but their cycle life still needs to be improved [18–20]. In order to use these advantages of electrode materials, the hybrid material is expected to be prepared by

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assembling these three types of materials. Many researches have been dedicated to the synthesis of the hybrid of porous carbon, conducting polymers, and transition metal oxides, and a series of the multicomponent hybrid electrode materials with good capacitive performance have been synthesized [21–25]. Binary composite electrode materials such as carbon-conducting polymer [26, 27], carbon–metal oxide [28, 29], conducting polymer-metal oxide [30, 31], and ternary composite electrode materials such as carbon– metal oxide-conducting polymer [32–34] have been investigated and these hybrid electrode materials show superior capacitive performance [35, 36]. In recent years, the graphene-based composites with metal oxides and conducting polymers have been studied for using in supercapacitor electrodes up to 480 F g−1 due to their specific capacitance [37–39]. Polypyrrole (PPy) is one of the most attractive conductive polymers because of its high electrical conductivity, thermal and chemical stability, and low cost [40]. By polymerization of pyrrole onto graphene nanosheets, the as-prepared graphene/PPy composites achieve a maximum specific capacitances of 267 and 482 F g−1 at 0.1 and 0.5 A g−1current densities respectively [38, 41]. However, the reported specific capacitance of the graphene–PPy electrodes is still far below the theoretical value. The using of metal oxide is an effective method to enhance the properties of graphene-based electrode. During the last decades, the manganese oxides have been much considered as suitable electrode materials for SCs [42]. Among these manganese oxides, Mn3O4 has been considered as one of the promising electrodes for energy storage performance [43, 44]. Recently, different researches show that electrochemical properties of graphene nanosheet can be improved by addition of a manganese oxide [45–47]. Therefore, it is expected that the excellent properties can be achieved when the properties of PPy, graphene, and MnOx are combined together. On the other hand, current collector affects the electrochemical properties of supercapacitor electrodes. Different types of conductive substrates (e.g., Ni foam, Ti foil, stainless steel foil, copper foil, and graphite paper) are used as current collector for electrochemical supercapacitor [48–52]. Among these materials, copper is a suitable candidate for electrode substrate due to its low cost, low resistance, and good electrical conductivity despite its highly sensitive and hard to control oxidation. Fortunately, copper hydroxide is also a good conductor and can improve the stability of the copper sheet while keeping the electrical conductivity [52]. Therefore, based on the abovementioned characteristics of different electrode materials, considerable attentions have been devoted to investigating hybrids of different materials to obtain high-performance capacitive electrode materials. So, in this study, a new quadri nanocomposite of graphene/ PPy/MnOx/Cu(OH)2 has been fabricated via a simple and

inexpensive method on the Cu foil. First, a two-step method consisting of thermal decomposition and in situ oxidative polymerization was employed to fabricate graphene/polypyrrole/ manganese oxide composites. Second, Cu(OH)2 nanowires were deposited on Cu foil and for the electrochemical measurements, all composite powders were deposited on Cu(OH) 2 /Cu foil substrate as working electrodes. Electrochemical properties of electrodes were investigated by cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy (EIS).

Experimental procedure All the chemicals used were of high purity. Graphene powder was obtained from XG Sciences; manganese acetate (Mn (CH3COO)2·4H2O), pyrrole, hydrogen chloride (HCl 37%), (NH4)2S2O8, NaOH, and FeCl3 were purchased from Merck. Acetone, ethanol, and distilled water (DI) were used as a cleaning solution or agent; also, Cu foil (1 cm × 2 cm) was used as the electrode substrate.

Preparation of Cu(OH)2/Cu foil A fresh Cu foil (0.05 cm) was cut into 1 cm × 2 cm sheet. It was cleaned by consecutive ultrasonication in acetone, ethanol, and distilled water for 10 min, and then was put into 1 M HCl solution to remove any surface impurities and oxide layers, which led to brightness and smoothness of the foil. The pre-cleaned Cu foil was then immersed in an aqueous solution consisting 12 mL NaOH (10 M), 6 mL (NH4)2S2O8 (1 M), and 27 mL distilled water. A few minutes later, the exposed copper foil surface turned to a faint blue color, and the initial colorless solution became increasingly blue. The reaction time of copper in the solution was 30 min. Finally, the electrode was rinsed with water and ethanol, and dried in air after extracted from the solution.

Synthesis of graphene/polypyrrole/MnOx composite electrode The synthesis of the graphene/polypyrrole/MnOx composite consists of two steps: first, 50 mg of graphene was dispersed in ethanol with ultrasonication for 1 h to make a graphene suspension. Then, 350 mg of Mn(CH3COO)2· 4H2O was added into the graphene suspension and stirred by magnetic stirring for 1 h at 90 °C and dried. Finally, the MG composite was annealed at 300 °C at a rising rate of 5 °C/min for 3 h in argon atmosphere. Second, the graphene/polypyrrole/manganese (PMG) oxide composite was prepared via in situ polymerization of pyrrole onto dispersed MG powder. The weight feeding ratio of pyrrole to GM was 10:1. The pyrrole monomer was added into a

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suspension of MG in distilled water. The suspension was ultrasonicated for 40∼60 min, then solution of FeCl3 in 0.1 M HCl was added slowly in suspension and stirred for 24 h at 0–5 °C. After filtration, the composite powder was washed with ethanol and water several times, and the product was dried at 70 °C in vacuum for 20 h. For comparison, graphene/polypyrrole (PG) composite was fabricated with same in situ polymerization method with pure graphene. Figure 1 shows the flowchart of fabrication steps of PMG composite.

Characterization and electrochemical measurements The X-ray diffraction analysis (PHILIPS, PW1730) with a Cu-Kα radiation, dispersive Raman spectrometer (Thermo Fisher Scientific Inc., Nicolet Almega XR) and the Fourier transform infrared spectra (Bruker TENSOR 27 FT-IR spectrophotometer) were introduced to analysis of phase formation and band in the synthesized composites. The surface morphology of electrodes was analyzed by field emission scanning electron microscopy (MIRA 3 TESCAN). The electrochemical experiments consisting of cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectra (EIS) were carried out using VSP-300-Biologic multichannel potentiostat/ galvanostat system with a standard three-electrode system. Capacitive behavior and EIS of the films were investigated in 6 M KOH aqueous electrolyte. For the electrochemical measurements, all the electrodes were prepared by mixing the active material with PVDF to form a slurry. Then, the slurry was cast on Cu(OH)2/Cu plate and dried under vacuum at 70 °C for 24 h. In the three-electrode system, cyclic voltammetry studies were performed within a potential range of 0 to 0.5 V at Fig. 1 Flowchart of fabrication steps of graphene/polypyrrole/ MnOx composite powder

different scan rates from 5 to 100 mV s−1. The specific capacitance was calculated based on CV measurement by using the Eq. 1 C¼

∫Idv SR  m  Δv

ð1Þ

where I is the current density (A/cm2), v is the potential (V), SR is the scan rate (V/S), and m is the deposited weight of the material on electrode (g). Moreover, the average energy density (E) and the power density (P) of the nanocomposite were evaluated from the following expressions [41]: 1 C ðΔV Þ2 ð2Þ 2 E ð3Þ P¼ t where C, ΔV, and t represent the specific capacitance (F g−1), the potential window (V), and the discharge time (s), respectively. The galvanostatic charge–discharge tests were performed at 1 A/g with a potential window of 0 to 0.5 V. EIS measurements were carried out by applying an AC voltage with 10-mV amplitude in a frequency range from 0.01 Hz to 100 kHz at an open circuit potential condition.



Results and discussion The XRD patterns of G and as-synthesized PG, MG, and PMG powders are shown in Fig. 2. For graphene, two diffraction peaks at 2θ = 26.3° and 2θ = 43° correspond to the graphite-like structure [53]. In PG composite, the broad peak also at 25.6° and the peak at about 43° disappeared, suggesting the PPy and GE have completely interacted [54].

dispersion of 50 mg graphene in ethanol with ultrasonication for 1 h

adding 350 mg manganese acetate to graphene suspension

stirring for 1 h at 90 °C

ultrasonication for 40-60 min

adding pyrrole monomer to dispersed GM powder

drying and annealing ( MG )

adding FeCl3 solution in GM suspension

stirring for 24 h at 0– 5 °C

washing and drying (PMG)

Author's personal copy J Solid State Electrochem Fig. 2 XRD patterns of the G, PG, GM, and PMG powders

PG

G

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a

b Fig. 3 FT-IR spectra (a) and Raman spectra (b) of G, PG, MG, and PMG samples

For MG composite in comparison with graphene spectrum, we can see different new peaks that corresponded to the Mn3O4. The positions can be indexed perfectly and matched well with the reported Mn3O4 tetragonal structure (JCPDS no. 00-001-1127). Any other impurity was not revealed in the XRD pattern which indicates the high-phase purity of the prepared samples [45, 46]. According to the XRD pattern of PMG, it is concluded that a mixed phase of Mn3O4 and MnO is present in the prepared nanocomposite. In this sample, during the polymerization process of pyrrole, Mn3O4 operated as an oxidant and redox reaction in Mn3O4 causes to form MnO particles in the PMG composite. The mechanism for Mn3O4 formation is believed to be as follows [55–57]: Mn3 O4 ðsÞ þ 6Hþ þ 2e− ⇄MnOðSÞ þ 2Mn2þ ðaqÞ þ 3h2 o

ð4Þ

Moreover, the intensity of two diffraction peaks at 2θ = 26.3° and 2θ = 43° of PMG composite decreased and this can be because of the complete coverage of the graphene nanosheets by manganese oxide.

FT-IR spectra of samples are shown in Fig. 3a. In the FT-IR spectrum of PG composite, peaks for both PPy and G are observed. The appearance of characteristic peaks of PPy at 1545 and 1450 cm−1 confirms the presence of PPy in the composite [54]. For the MG composite, the peaks at around 485 and 595 cm−1 are assigned to tetrahedral and octahedral Mn-O bands, respectively, suggesting that Mn3O4 was bounded to the surface of graphene sheets [58]. In the spectrum of PMG compared to that of the GM and PG composites, most of the bands for G, PPy, and manganese oxides exist with the difference that the intensities of two peaks at 485 and 595 cm−1 have increased and this well corresponding to the results of XRD. Figure 3b presents the Raman spectra of the G, PG, MG, and PMG samples in the wavenumber range of 500–3000 cm−1. For all G-based composites, the presence of three characteristic peaks at around 1570, 1340, and 2680 cm−1 corresponds to the G, D, and 2D bands, respectively. The D band was attributed to structural defects and disorders of graphene, whereas the G band arises from the zone center E2g mode which corresponds

Author's personal copy J Solid State Electrochem Fig. 4 FESEM images of G (a), PG (b, c), MG (d, e), and PMG (f, g) (50 and 100 kX)

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a

b

0

0

c

0

5

10

Fig. 5 EDS spectra of PG (a), MG (b), and PMG (c) electrodes

to the ordered sp2-bonded carbon. The intensity ratio (ID/IG) of D band to G band is sensitive to the disorder and crystallite size of the graphitic layer. In the Raman spectrum of PG in addition

a

to the G, D, and 2D bands, there are two new bands at around 986 and 1127 cm−1 and correspond to the ring vibration of PPy and C–H stretching respectively [59]. In MG composite, a band

b mV/s mV/s mV/s mV/s mV/s mV/s

mV/s mV/s mV/s mV/s mV/s mV/s

d

c mV/s mV/s mV/s mV/s mV/s mV/s

Fig. 6 CV curves of G (a), MG (b), PG (c), and PMG (d) electrodes

mV/s mV/s mV/s mV/s mV/s mV/s

Author's personal copy J Solid State Electrochem Table 1

Electrochemical properties of different electrodes Current collector

Specific capacitance (F/g)

Energy density (W h/kg)

Power density (W/kg)

Rs (Ω)

Rct (Ω)

f* (Hz)

τ = 1/2πf*(ms)

G

Cu(OH)2/Cu

PG MG PMG

Cu(OH)2/Cu Cu(OH)2/Cu Cu(OH)2/Cu

170 311 266 370

5.9 10.8 9.24 12.85

4248 6480 6652 6610

1.1 1.6 1.8 1.4

0.01 0.5 0.5 0.3

803.53 407/21 274/73 602/87

0/2 0/4 0/6 0/26

Sample

around 640 cm−1 was attributed to the Mn3O4 [45, 46]. When PPy and MnOx intercalate G to form PMG composite, the nanocomposite presents all characteristic peaks of G, PPy, and

Mn3O4 while the intensity of the band at 640 cm−1 increased and it showed that the amount of manganese oxide in ternary composite has increased and this is in good agreement with

b a

c

Fig. 7 EIS (a), the charge–discharge curves (b), and cycle performance (c) of electrodes

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XRD and FTIR results. The intensity ratios (ID/IG) of D band to G band for G, PG, MG, and PMG are 0.82, 0.84, 1.2, and 1.4, respectively. The intensity ratio increases, indicating an increase in the number of defects and porosities [60]. Moreover, the type of doping can be determined from the position shift of the G and D peaks, after hybridization with nanoparticles. As reported in the literature [61], the upshift of the G peak position and the downshift of the D peak position mean n-doping of graphene. Otherwise, the upshift of the G peak position and the upshift of the D peak were represented as the p-doping of graphene. For all samples, the position of the G and D peaks has upshift (for G peak from 1570 to 1580 cm−1 and for D peak from 1340 to 1347 cm−1). These results indicated that PPy, Mn3O4, and MnO nanoparticles play a role in the p-doping dopant of single-layer graphene film [61]. Figure 4 demonstrates the field emission scanning electron micrographs of G, PG, MG, and PMG thin films deposited on Cu(OH)2/Cu foil substrate. As clearly seen from Fig. 4a, the graphene electrode has a flake-like structure. Mixing graphene powders with PVDF causes graphene to agglomerate and form large flakes. In PG and MG nanocomposite, spherical PPy and Mn3O4 particles were deposited on graphene nanosheets (Fig. 4b, d). In Fig. 4c, e, g, the yellow arrows show the portion of the composite that EDS analysis is done. By comparing Fig. 4f with Fig. 4d, it was clear that the amount of manganese oxide particles has increased and it conformed the formation of MnO particles on the surface of graphene during the pyrrole polymerization process.

Table 2

Figure 5 shows the energy-dispersive spectroscopy (EDS) examination of prepared PG, MG, and PMG nanocomposite electrode. EDS spectrum obtained from PG electrode shows characteristic carbon, nitrogen, oxygen, and copper energy lines, which suggests the formation of PG nanocomposite on Cu/Cu(OH)2 foil. In spectrum of MG, we can see the peak signals of manganese, carbon, oxygen, and copper elements. In PMG spectra, all peaks of PG and MG composite exist with more peak intensity which is in agreement with those of the XRD, FT-IR, and Raman spectroscopy results. Electrochemical tests were characterized using a threeelectrode configuration and carried out with cyclic voltammetry, galvanostatic charge/discharge, and impedance measurements. Figure 6 displays CVs of G, PG, MG, and PMG electrodes at a scan rate of 5 to 100 mV/s. The CV curves of electrodes exhibited an irregular shape and they are different from the ideal rectangular shape of EDLCs; these profiles are very similar to those of previously reported copper substrate electrodes [52, 62–65]. This implies that charging/discharging of electrodes is dominated by a faradaic process: 2CuðOHÞ2 þ 2e− ⇄2CuOH þ 2OH− ↔Cu2 O þ H2 O þ 2OH− ð5Þ

  MnOx ° C β þ δCþ þ δe‐ ⇄MnOx‐β ° C βþδ

ð6Þ

PPy þ nA‐ ⇄PPynþ ðA‐ Þn þ ne‐

ð7Þ

In Fig. 6a–c, the anodic peaks are not distinct, while cathodic peaks exist. A cathodic peak in the curve of graphene electrode

Comparison of properties of graphene composite electrodes

Activated material

Current collector

Electrolyte

Scan rate or current density

Capacitance (F/g)

Ref.

G/PPy

Glassy carbon

G/PPy G/PPy Mn3O4/GO Mn3O4/G Mn3O4/G Mn3O4/C Mn3O4/G Mn3O4/G

Graphite – Nickel foam Nickel foam Stainless steel Nickel foam Nickel foam Nickel foam

2 M H2SO4 1 M H2SO4 1 M KCl 1 M Na2SO4 6 M KOH 1 M Na2SO4 6 M KOH 6 M KOH 1 M Na2SO4

– – 10 mV/s 1 A/g 5 mV/s 5 mV/s 1 mV/s 0.1 A/g 0.1 A/g

267 270 277 194.8 245 344 266 270 171

[41] [59] [71] [45] [60] [72] [73] [74] [75]

Mn3O4/G RGO/MnO2/PPy CNT/MnO2/PPy G/PPy/MnOx G/PPy/MnO2 G/PPy G/Mn3O4 G/PPy/MnOx

Nickel foam Nickel foam – Nickel foam Stainless steel Cu(OH)2/Cu Cu(OH)2/Cu Cu(OH)2/Cu

1 1 – 1 1 6 6 6

1 A/g 5 mV/s 10 mV/s 5 mV/s 1 A/g 10 mV/s 10 mV/s 10 mV/s

115 589 272 45 258 311 266 370

[44] [76] [77] [66] [68] This work This work This work

M Na2SO4 M Na2SO4 M Na2SO4 M Na2SO4 M KOH M KOH M KOH

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is attributed to the reduction reaction in Cu(OH)2 substrate according to Reaction 5. Composite electrodes (PG and PM) have broader cathodic peaks than graphene electrode which is attributed to the synergistic effects between graphene, Cu(OH)2, PPy, and Mn3O4 particles (Reactions (5–7)). In these curves, the anodic peaks did not exist for some reason: (1) water splitting is negligible under 0.5 V cutoff, (2) typical behavior of water splitting is observed when the potential cutoff was higher than 0.5 V, and (3) high conductivity of electroactive materials causes ions to move fast on the surface of electrodes [63]. Results showed that the specific capacitance of G, PG, MG, and PMG electrodes was 170, 311, 266, and 370 F g−1 at 10 mV s−1 scan rate, respectively. The improvement in electrochemical performance of the PG, MG, and PMG nanocomposite, as compared to G, was probably due to the following reasons: (I) The oxidation or deoxidation of α–C or β–C atoms of PPy rings has been facilitated by the presence of graphene nanosheets (GNS). (II) Attachment of PPy on the surface of GNS plays a part in reducing the diffusion and migration length. (III) Synergistic effect between PPy and GNS [41] (IV) Synergistic effect between manganese oxides and GNS [45, 46] The energy density and power density of G, PG, MG, and PMG electrodes were calculated and are shown in Table 1. Figure 7a illustrates the electrochemical impedance spectra (Nyquist plots) of the pure graphene and their composites performed in the frequency range from 0.01 Hz to 100 kHz by applying an AC voltage. They are typical supercapacitor AC impedance curves with three parts involving high-, intermediate-, and low-frequency areas [31, 36]. In the highfrequency region, we can get some messages: the intercept on the real axis is related to the combined resistance (Rs) containing ionic resistance of electrolyte, intrinsic resistance of electrode materials, and contact resistance between current collector and electrode; the diameter of the semicircle corresponds to the charge transfer resistance (Rct), and it shows the charge transfer rate at the electrode and electrolyte interface, which further incurs the Faradaic redox process [66]. The bigger the diameter of the semicircle, the larger the Rct. The values of Rs and Rct of samples are shown in Table 1. Rs of graphene is smaller than other electrodes, and it is due to the excellent electrical conductivity of graphene. Rs of PMG is lower than that of PG and MG electrodes; it shows that the conductivity of materials gets an increasing after mixing polypyrrole with manganese oxide [67]. In addition, G exhibited a negligible semicircle in the high-frequency region indicating low Rct and good charge transfer behavior than other samples. Moreover, the almost-linear vertical line across the x-axis of

the higher resistivity region implies a nearly ideal capacitive behavior for the nanocomposite, and the steeper sloped line indicates the fast diffusion of the electrolyte ions [66, 68]. In the EIS plots of PMG composite at low frequency, the straight line leans more towards the vertical Z″ axis, indicating that these electrodes have a better capacitive performance and lower ion diffusion resistance in contrast to PG and MG. In fact, the more vertical the curve, the more closely the supercapacitor behaves as an ideal capacitor [69]. From the frequency (f*) corresponding to the maximum of the semicircle, the time constant is calculated using the expression: τ ¼ 1=2πf *

ð8Þ

The values of time constant (τ) obtained from the data demonstrated in Fig. 7a are low (of the order of ms), which are preferred for electrochemical supercapacitors to ensure fast charge–discharge characteristics [70]. The charge–discharges of electrodes were investigated at current densities of 1 A/g, as shown in Fig. 7b. The non-linear discharge curves with a slight curvature, indicating their pseudocapacitance behaviors. All the curves have an IR drop, which indicates the presence of internal resistance [66]. Compared with G, the composite electrodes exhibited longer discharge times at the same current density that attribute to the higher capacitance of these electrodes. Cycling stability is very important for the practical application of supercapacitors. Figure 7c provides the cycling curves of G, PG, MG, and PMG at 1 A/g in KOH electrolyte. It can be seen that the capacitance of all samples decreases gradually with increasing cycling number. Capacitance of PG is more stable than that of other samples after 1000 charge/ discharge cycles. Table 2 shows the comparison between the performances of our electrodes in this work and those reported before. The comparison shows that G/PPy, G/Mn3O4, and G/PPy/MnOx electrodes with Cu(OH)2/Cu as current collector can compete with electrodes with other current collectors specially nickel foam. So, with that achievement, a new and inexpensive electrode with good properties was fabricated for electrochemical applications.

Conclusions 1. Graphene/polypyrrole, graphene/Mn3O4, and graphene/ polypyrrole/manganese oxide were prepared successfully by a two-step method. The structure and morphology of these composites and pure graphene were fully characterized with different techniques including FT-IR, XRD, Raman spectroscopy, and FESEM. 2. Graphene/polypyrrole, graphene/Mn3O4, and graphene/ polypyrrole/manganese oxide were deposited on

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Cu(OH)2/Cu foil as current collector and electrochemical properties of electrodes were investigated. 3. Cyclic voltammetry and other electrochemical measurements showed that synergistic effect between polypyrrole, graphene and manganese oxide, and decreasing charge transfer resistance improve electrochemical properties of PMG electrodes. 4. High specific capacitance and good cycling stability were achieved for PMG with the highest specific capacitance of 370 F g−1 and energy density of 12.85 W h/kg.

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