Polypyrrole

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A High Performance Supercapacitor Based on Graphene/Polypyrrole/ CuO-Cu(OH) Ternary Nanocomposite Coated on Nickel Foam 2

2

Parvin Asen, and Saeed Shahrokhian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00534 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 15, 2017

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The Journal of Physical Chemistry

A High Performance Supercapacitor Based on Graphene /Polypyrrole/Cu2O-Cu(OH)2 Ternary Nanocomposite Coated on Nickel Foam Parvin Asen1, Saeed Shahrokhian 1,2* 1 2

Institute for Nanoscience and Technology, Sharif University of Technology, Tehran, Iran Department of Chemistry, Sharif University of Technology, Azadi Avenue, Tehran 11155-9516, Iran

*Corresponding author, Tel.: +98-21-66165359; Fax: +98-21-66012983 E-mail addresses: [email protected]

Abstract: A simple and low-cost electrochemical deposition method is used to prepare reduced graphene

oxide/polypyrrole/Cu2O-Cu(OH)2

(RGO/PPy/Cu2O-Cu(OH)2)

ternary

nanocomposites as the electrode material for supercapacitor application. First, graphene oxide-polypyrrole (GO/PPy) nanocomposite is electrochemically synthesized on Ni foam by electro-oxidation of pyrrole monomer in an aqueous solution containing GO and Tiron. Subsequently, the GO/PPy film is converted to the corresponding reduced form (RGO/PPy) by an effective and eco-friendly electrochemical reduction method. Then, a thin layer of Cu2O-Cu(OH)2 is formed on RGO/PPy film by chronoamperometry. The RGO/PPy/Cu2OCu(OH)2 nanocomposite is characterized by scanning electron microscopy (SEM) , energy dispersive X-ray spectrometry (EDX), atomic force microscopy (AFM),

X-ray diffraction

(XRD) and Fourier transform infrared spectra (FT-IR). SEM images show that Cu2OCu(OH)2 nanoparticles are dispersed on the surface of RGO/PPy film with an average particle size of 50-70 nm. The electrochemical performance of the as-prepared electrode is evaluated by various electrochemical methods using cyclic voltammetry, galvanostatic chargedischarge and electrochemical impedance spectroscopy (EIS) in 0.5 M Na2SO4 solution. In three electrode system, RGO/PPy/Cu2O-Cu(OH)2 exhibits an excellent gravimetric specific capacitance of 997 F g-1 at a current density of 10 A g-1, which is far better than GO/PPy (500 F g-1), RGO/PPy (685.5 F g-1) and GO/PPy/Cu2O-Cu(OH)2 (750F g-1). The utilization of the electrical double layer capacitance (EDLC) of graphene together with the pseudocapacitive behavior of PPy and Cu2O-Cu(OH)2 leads to a maximum energy density of 20 Wh kg-1 at power density of 8000 W kg-1 , and a maximum power density of 19998.5 W kg-1 at an energy density of 5.8 Wh kg-1 for symmetric RGO/PPy/Cu2O-Cu(OH)2 supercapacitor. Furthermore, RGO/PPy/Cu2O-Cu(OH)2 nanocomposite maintains about 90 % of the initial capacitance value after 2000 cycles.

1 ACS Paragon Plus Environment


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1. Introduction To meet the ever growing demand for renewable energy devices in the modern electronic industry, tremendous research has focused on energy storage devices. Supercapacitors are promising energy storage devices because of their high power density, good cycle life, and low maintenance cost1-3. Generally, supercapacitors are classified into two types according to the different charge storage mechanisms: electrical double layer capacitors (EDLC) and pseudocapacitors. In an EDLC, the charge is stored at the limited surface of the electrode materials (typically carbon materials including activated carbon, carbon nanotubes and graphene), by means of the reversible absorption/desorption of ions to form electrical double layer4. Therefore, the electrode/electrolyte interface charging mechanism has excellent stability due to processes occurring at electrode interfaces. The mechanism of a pseudocapacitor involves surface redox reactions between the electrode material and the electrolyte. Unlike reactions in batteries, which involve bulk electrode material, surface redox reactions (pseudocapacitive reactions associated with metal oxides) happen only at the nearsurface region. Therefore, pseudocapacitors have higher energy density compared to EDLC-based electrodes. The enhancement in capacitance is related to the multiple oxidation states of transition metals 5. However, one drawback of pseudocapacitors is poor stability and high resistance during the cycling process 6. In this regard, various transition metal oxides such as ruthenium oxide (RuO2)7, manganese oxide (MnO2 )8-9, cobalt oxide (Co3O4 )10, Fe3O411 and TiO2

12

have been studied. It is found that these materials are suitable for supercapacitor

applications. Among these metal oxides, copper based materials such as cuprous oxide (Cu2O), cupric oxide (CuO) and copper hydroxide (Cu(OH)2) seem promising candidate as pseudocapacitive electrode materials due to non-toxicity, earth abundance and low cost13-15.


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Although CuO, Cu2O and Cu(OH)2 have been investigated as electrode materials for supercapacitors, they suffer from low electrical conductivity and rapid capacity decay.16 Conducting polymers are attractive for developing supercapacitors. These polymers due to the conjugated structure of C=C bonds along the polymer bone can solve low-conductivity problems in the electrode material

17

. The main drawback of conductive polymers is poor

cycle life, because of considerable volume changes (swelling and shrinkage) during doping/dedoping processes. In recent years, conducting polymers such as polypyrrole (PPy) 12, 18

, polyaniline (PANI)19-21, and polyethylenedioxythiophene (PEDOT)22 have been

successfully used as electrode materials for supercapacitors. Among them, PPy is one of the most promising conductive polymers, because its monomer (pyrrole) is easily oxidized. Also, the monomer is soluble in water and is commercially available. In addition, PPy has advantages of high energy density, easy electrochemical processability, low cost and reversible electrochemical doping/dedoping.23 Electropolymerization is an attractive procedure for preparation of PPy-based electrodes for supercapacitor applications.24 However, loading of large amounts of the active materials using electrochemical synthesis is difficult. Therefore, increasing the loading rate and achievement of supercapacitors are

high specific capacitances at long charge–discharge cycles for highly desirable.25 The use of aromatic anionic dopants in the

electropolymerization of PPy could improve charge storage properties and cycling performance of PPy electrodes. Also, high specific capacitance and good thermal stability of PPy film are achievable using appropriate anionic dopants.26 Many researches have focused on the development of various anionic dopants for the electropolymerization of PPy.26-30 The studies have shown that with the variation of anionic dopants, the conductivity of the PPy can change by several orders of magnitude26. Also, anionic dopants can affect the stability of PPy during repeated charging and discharging processes. Liu et al. prepared PPy film on the



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functionalized partial-exfoliated graphite (FEG) substrate and dope the PPy film with β naphthalene sulfonate anion (NS). The PPy-NS/FEG electrode exhibited outstanding capacitance retention of 97.5% after 10000 cycles 31. It was found that by increasing the charge/mass ratio of the dopant molecules (sulfonate containing anionic species), conductivity of the PPy film enhances because of its significant influence on the size of particles and the electrochemical performance of PPy32-33 Recent studies generated considerable interest in the use of 4,5-dihydroxy-1,3benzenedisulfonic acid disodium salt (Tiron) as an anionic dopant for electropolymerization of PPy, because it has higher charge/mass ratio compared to that of sodium salts of ptoluenesulfonate (pTS), naphthalene sulfonate, naphthalene disulfonate, and other aromatic sulfonate anions.26, 31-32. The investigation of the electropolymerization of pyrrole on Al alloy substrate has been shown that in comparison to pTS, in the presence of Tiron as an anionic dopant, a continuous and uniform PPy film is formed on the surface of Al. Therefore, the charge transfer during the electropolymerization process increases.34 Moreover, the prepared film in the presence of Tiron shows improved conductivity and adhesion properties Similar to the most conducting polymers, the setback of PPy is low mechanical stability and short cycle life, which is due to phase changes and slow diffusion of ions within the bulk of the electrode during redox reactions. In order to improve the utilization of PPy, it can be combined with carbon-based materials

35

. Li et al. prepared PPy and PANI electrodes with

excellent cycling stabilities by deposition of a thin carbonaceous shell onto their surface. Significantly, carbonaceous shell-coated PANI and PPy electrodes maintained 95 and 85 % of their initial specific capacitance value after 10000 cycles 35. Recently, the preparation of PPy and graphene composites has attracted considerable interests36-40. Graphene has triggered a new genre for the development of supercapacitor


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electrode materials, because of its large specific surface area, extraordinarily high electrical conductivity, chemical stability and relatively low manufacturing cost.41-43 However, the conductivity of graphene film is mainly limited by restacking of graphene sheets via π-π interactions and van der Waals forces between neighboring sheets

44

. This agglomeration

decreases the surface area of graphene and thus can’t reflect the intrinsic capacitance of an individual graphene sheet.45 However, the preparation method and degree of reduction are the most important factors which can influence on the capacitance of the graphene 44-45 . Therefore, based on the above mentioned characteristics of different electrode materials, considerable attentions have been dedicated to exploring hybrids of different materials to obtain high-performance capacitive electrode materials. So the present work would be devoted to developing the hybrid materials combining advantages of double layer capacitance and pseudocapacitance. In this study, a new ternary nanocomposite of reduced graphene oxide-PPy-Cu2OCu(OH)2 (RGO/PPy/Cu2O-Cu(OH)2) has been fabricated via a facile electrochemical deposition method on the Ni foam. First, GO/PPy was deposited on Ni foam by applying a constant potential of +0.8 V. Second, electrochemical reduction method was employed to reduce GO/PPy to obtain RGO/PPy film. Finally, Cu2O-Cu(OH)2 nanoparticles are formed on RGO/PPy film

via

chronoamperometry.

It should

be

mentioned

that for the

electropolymerization of pyrrole, Tiron was used as an anionic dopant. Also, GO/PPy/Ni foam, RGO/PPy/Ni faom and GO/PPy/Cu2O-Cu(OH)2/Ni foam electrodes were prepared for comparison. Electrochemical behavior of the as-prepared electrodes was investigated by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS).



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2. Experimental Section 2.1. Reagents and materials Graphite powder was purchased from Sigma Aldrich. Sulfuric acid (H2SO4, 95%–98%), potassium permanganate (KMnO4, 99.9%), hydrogen peroxide (H2O2, 30%), hydrochloric acid (HCl, 37%), sodium nitrate (NaNO3), copper sulfate (CuSO4), citric acid, sodium hydroxide, Tiron and pyrrole (99%) were purchased from Merck (Germany). The Ni foam (1cm×1cm×1.5 mm) was used as the electrode substrate. Prior to the electro-polymerization process, the monomer pyrrole was purified by a simple distillation procedure to separation the polymerized components and obtaining a colorless pure liquid. The distilled monomer stored at 0 °C. 2.2. Synthesis of graphene oxide (GO) GO nanosheets were synthesized by modified Hummers method

46

. Typically, 1 g of

graphite and 1 g of NaNO3 were mixed with 23 mL of concentrated H2SO4 and the mixture was cooled to 0 ° C using an ice bath. 4 g of KMnO4 was added gradually in portions to keep the reaction temperature below 20 ° C. The reaction was warmed to 35 °C and stirred for 1 h. Afterward, 180 mL of double-distilled water was added slowly to the solution. The temperature of the mixture increased to 95°C. In order to terminate the reaction, 10 mL of H2O2 was transferred to the suspension. The color of the mixture changed from brown to bright yellow. The obtained mixture was filtered and rinsed with 5% HCl solution and then washed several times with distilled water until a constant pH of 4–5 was obtained. The concentration of GO was 3.5 mg mL-1.


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2.3. Preparation of GO/PPy nanocomposite on Ni foam Prior to use, Ni foam (1cm × 1cm× 1.5 mm) was carefully cleaned in 1 M HCl solution in an ultrasound bath for 15 min in order to remove the surface oxide layer and then it was immersed in acetone to remove organic materials from the surface. Finally, the Ni foam was washed thoroughly with the deionized water and then dried at room temperature. A three electrode system was prepared using Ni foam, Pt foil and Ag/AgCl/KCl (3 M) as working, auxiliary and reference electrodes, respectively. The GO (1 mg mL-1) was dispersed in an aqueous solution by ultrasonication for 15 min to form a uniform GO suspension. Pyrrole (0.1 M) and Tiron (0.05 M) were added to the GO solution. The GO/PPy film was electrochemically deposited on Ni foam electrode (GO/PPy/Ni foam) at a constant potential of +0.8 V (vs. Ag/AgCl). After the electrodeposition, the film was rinsed with deionized water to remove the unreacted substances and then dried at 60 °C for 1 h.

2.4. Preparation of RGO/PPy nanocomposite on Ni foam The GO/PPy was reduced electrochemically to form a reduced GO/PPy (RGO/PPy) nanocomposite by applying a potential of -1.1 V (vs. Ag/AgCl) to GO/PPy/Ni foam in 0.5 M NaNO3 solution for 1000 s.47 After the reduction procedure, RGO/PPy/Ni foam was washed with distilled water and then dried at 60 °C for 1 h.

2.5. Preparation of GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 on Ni foam Electrochemical formation of Cu2O-Cu(OH)2 nanoparticles onto GO/PPy and RGO/PPy was carried out in a cell containing an alkaline solution of 0.05 M CuSO4 and 0.05 M citric acid as a complexing agent. The pH of the electrolyte was adjusted at 9 by adding 1 M NaOH. The electrodeposition of Cu2O-Cu(OH)2 film was performed at a constant potential of



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-1.05 V for 300 s. After the deposition, the prepared electrodes were washed with distilled water and then dried at 60 °C for 25 min. The mass loading of GO/PPy, RGO/PPy, GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 on Ni foam were between 0.95 and 1 mg.

2.6. Characterization and electrochemical measurements The RGO/PPy/Cu2O-Cu(OH)2 was characterized by X-ray diffraction analysis (GBC MMA, Instrument) with a Cu-Kα radiation. The surface morphology of RGO/PPy/Cu2OCu(OH)2/Ni foam and RGO/PPy/Ni foam were performed by scanning electron microscopy (SEM, KYKY-EM3200). The Fourier transform infrared spectra (FT-IR) were recorded using an ABB Bomem MB-100 FT-IR spectrophotometer with a construction of KBr pellets. Atomic force microscopy (AFM) was carried out in ambient conditions using DME nanotechnology GmbH. The electrochemical experiments were carried out using AUTOLAB 302N (the Netherland) electrochemical analyzer system with a standard three electrode system. Capacitive behavior and EIS of the films were investigated in 0.5 M Na2SO4 aqueous solutions. In three electrode system, cyclic voltammetry studies were performed within a potential range of -0.8 V to 0.0 V at different scan rates from 20 to 100 mV s-1. Also, galvanostatic charge-discharge tests were performed at different current densities with a potential window of -0.8 to 0.0 V. EIS measurements were carried out by applying an AC voltage with 10 mV amplitude in a frequency range from 0.01 Hz to 20 kHz at an open circuit potential condition. Also, the RGO/PPy/Cu2O-Cu(OH)2 symmetric supercapacitor was studied using a two-electrode system, which separated through a paper filtration membrane (pore size, 700 nm; Whatmann). In the two electrode system, cyclic voltammetry measurements were carried out in the potential range of 0 to 0.8 V. Moreover, galvanostatic


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charge-discharge tests were performed at various current densities with a potential window of 0 to 0.8 V.

3.

Results and discussion Schematic illustration for the synthesis of GO/PPy nanocomposite films on Ni foam

using the electrochemical polymerization process is presented in Fig. 1A. By applying the potential, pyrrole monomers were electrochemically oxidized for the beginning of polymerization. After the formation of radical cations, due to the electrostatic charge attractions, Tiron anions were attracted to the pyrrole radical cations, which caused to the formation of complex

48

. The coupling process of monomers has taken place by removing

two α-hydrogens, because the superior location for the radical in the pyrrole ring is αposition

49

. Chain growth increases by addition of freshly obtained radical cations on the

available oligomeric chain49. GO bearing a negative charge, acted as the counter-ion, attracted to the pyrrole radical cations during the electrodeposition process at a slow rate, because GO is relatively large-size anionic dopant and it has higher molecular density than Tiron. Therefore, Tiron anions are attracted to the pyrrole monomers initially.48 Moreover, the π-π interactions and hydrogen bonding between the GO layers and aromatic structure of PPy rings play a remarkable role in the formation of GO/PPy nanocomposite. After the electrodeposition, a black uniform of GO/PPy film was obtained on Ni foam. Subsequently, the GO/PPy was reduced electrochemically to form RGO/PPy nanocomposite by the chronoamperometric method at -1.1 V vs. Ag/AgCl in 0.5 M NaNO3 solution. Conductivity and capacity of the RGO/PPy film improve during this process because of the removal of some functional groups and formation of π-conjugation structure.47, 50 Cyclic voltammograms (CVs) of GO/PPy and RGO/PPy are shown in Fig. 1B. The enclosed area of the CVs for RGO/PPy is larger than those of GO/PPy composite, demonstrating the increase of capacity



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for GO/PPy film during the reduction process. The capacitance originates from double layer capacitance of graphene and pseudocapacitance of PPy. The redox peaks of GO/PPy and RGO/PPy are attributed to the dual mode behavior of doping and dedoping of cationic and anionic species from PPy. The FT-IR transmission spectra of RGO/PPy and RGO/PPy/Cu2O-Cu(OH)2 is presented in Fig. 2A. For RGO/PPy, the peak observed at 3400 cm-1 describes N–H and C–H stretching vibrations 51. Moreover, peaks appeared in 1039 and 1539 cm-1 may be assigned to C-H and C=C backbone stretching of PPy, suggesting that PPy was successfully polymerized in the presence of GO 52. Also, the peak at 1066 cm-1 is attributed to epoxy group (C-O-C) 2, 52

.The absorption band that observed around 810 cm-1 is characteristic of doped PPy chain 53.

RGO/PPy/Cu2O-Cu(OH)2 shows characteristic stretching vibrations of Cu–O bond at 600 cm1 54

.Moreover, it has been observed that the peak at 1039 cm-1 is due to the C-H in-plane

vibration of PPy ring 51. Also, absorption peaks at 1128 and 1539 cm−1 are attributed to C–N stretching and C=C stretching of PPy, which proves polymerization of PPy 17, 52. Moreover, the peak at 815 cm-1 reveals the presence of doped PPy structure.53 Fig. 2B shows the XRD pattern of RGO/PPy/Cu2O-Cu(OH)2. It should be noted that based on the XRD analysis, it is concluded that a mixed phase of Cu2O and Cu(OH)2 are present in the prepared nanocomposite. For Cu2O-Cu(OH)2, diffraction peaks at 2θ=34° and 45° can be assigned to the (111) and (200) crystal planes of Cu2O, respectively55. Moreover, the peaks located at 45°, 52°, and 77° correspond to the (111), (200), and (220) planes of Cu(OH)2, respectively56. These results confirm the presence of both Cu2O and Cu(OH)2 in the nanocomposite. It can be observed that XRD reflections shift gradually in the direction of higher 2θ values, which may be due to preparation method and type of the substrate. Also, the broad peak around 25° can be ascribed to the (002) plane of RGO or amorphous PPy2.


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SEM technique was employed to investigate the morphology of RGO/PPy and RGO/PPy/Cu2O-Cu(OH)2

nanocomposites.

The

SEM

images

of

RGO/PPy

and

RGO/PPy/Cu2O-Cu(OH)2 nanocomposite films coated on Ni foam are shown in Fig. 3. Figs. 3A and 3B show that RGO/PPy nanocomposite was firmly electrodeposited on the surface of the Ni foam. The SEM image of as-deposited RGO/PPy shows a relatively rough surface. The appearance of cracks on the RGO/PPy surface can be attributed to drying process of RGO/PPy/Ni-foam electrode. RGO/PPy nanocomposite appears in a pasty form, which is due to co-electrodeposition of PPy and GO on Ni foam substrate. In addition, agglomeration of RGO/PPy can be seen on Ni foam. The thickness of nanocomposite film can be determined by SEM. For RGO/PPy nanocomposite, the thickness obtained in the range of 40-75 nm. Figs. 3C, 3D, 3E and 3F show SEM images of Cu2O-Cu(OH)2 nanoparticles which are distributed on RGO/PPy film. The average sizes of Cu2O-Cu(OH)2 nanoparticles are in the range of 50-70 nm. It seems that the potentiostat method can be considered as a suitable method with a controlled manner to form Cu2O-Cu(OH)2 nanoparticles with a relatively uniform size on RGO/PPy film. The elemental composition of RGO/PPy/Cu2O-Cu(OH)2/Ni foam electrode was characterized by EDX spectroscopy (Fig. 4A). The EDX results confirm the presence of C, N, Cu and O elements in RGO/PPy/Cu2O-Cu(OH)2 nanocomposite. Moreover, Ni foam as the substrate appeared in the elemental analysis by EDX. Also, the presence of sulfur arises from doping of Tiron anion in the structure of PPy. Fig. 4B shows the elemental mapping of RGO/PPy/Cu2O-Cu(OH)2/Ni foam electrode, which reveals the distributions of O, C, N and Cu throughout the entire nanocomposite. AFM images provide some structural information about the as-obtained materials. Fig. 5A shows topology of the surface of RGO/PPy on Ni foil as a 2D image. It can be seen that a chain structure of RGO/PPy is formed on Ni foil. Also, it can be observed that the surface is



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converted to a relatively uniform size after deposition of Cu2O-Cu(OH)2 on RGO/PPy surface (Fig. 5B). Fig.

6A

shows

CVs

of

GO/PPy,

RGO/PPy,

GO/PPy/Cu2O-Cu(OH)2

and

RGO/PPy/Cu2O-Cu(OH)2 on Ni foam in the potential range of -0.8 to 0.0 V at a scan rate of 20 mVs-1 in 0.5 M Na2SO4 electrolyte. CV of RGO/PPy exhibits a broader shape than GO/PPy, demonstrating the good capacitive behavior of the electrode. The conductivity of the RGO film increased with the elimination of functional groups and the restoration of sp2 hybridization of carbon.47 After the deposition of Cu2O-Cu(OH)2 on GO/PPy and RGO/PPy, CVs of electrodes show a rectangular-like shape accompanied by well-resolved pairs of cathodic and anodic peaks, indicating the contribution of both EDLC and pseudocapacitance characteristics. CVs of GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 represent a pair of redox peaks, which can be ascribed to reversible redox reactions of Cu(I)/Cu(II).23, 57 The mechanism of redox reactions occurring at GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 on Ni foam electrode may be attributed to the following reaction: Cu2+ + e-

(1)

Cu+

Displacement of redox peaks potential at GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2OCu(OH)2 can be related to the different substrates for deposition of Cu2O-Cu(OH)2. It should be noticed that the substrate has a predominant influence on potential shifts of the deposited Cu2O-Cu(OH)2 nanoparticles. The enclosed area of CV for RGO/PPy/Cu2O-Cu(OH)2 is higher than that of GO/PPy/Cu2O-Cu(OH)2, which can be attributed to higher conductivity and more effective surface area of RGO/PPy nanocomposite. Moreover, the current density of RGO/PPy/Cu2OCu(OH)2/Ni-foam electrode shows the highest among all four electrodes. These results demonstrate that RGO/PPy/Cu2O-Cu(OH)2 combine effectively two kinds of energy storage mechanisms;

double

layered

capacitance

and

pseudocapacitance.


It

seems

that

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RGO/PPy/Cu2O-Cu(OH)2/Ni foam electrode has the best performance among other electrodes for energy storage. Fig.

6B

displays the

electrochemical behavior of

RGO/PPy/Cu2O-Cu(OH)2

nanocomposite at various scan rates from 20 to 100 mV s-1 in 0.5 M Na2SO4 as the electrolyte solution. The increment of the current density can be observed by increasing the scan rate, which indicates the capacitive behavior of RGO/PPy/Cu2O-Cu(OH)2 nanocomposite. As the scan rate increases, anodic and cathodic peaks shift somewhat towards the positive and negative potentials, respectively. This arises from the internal resistance of the electrode. The capacity of RGO/PPy/Cu2O-Cu(OH)2 is negatively correlated with the scan rate. The capacity of RGO/PPy/Cu2O-Cu(OH)2 decreases with increasing the scan rate, which can be attributed to the limitation in the diffusion of electrolyte ions into the active materials. At higher scan rates, electrolyte ions can have access only to the outer surface of the active material and inner active sites cannot completely contribute in the redox processes, which caused to decreasing of the capacitance of the electrode. In order to obtain information about the prepared composites as electrode materials, galvanostatic charge-discharge measurements were carried out in Na2SO4 solution within a potential window of -0.8 and 0 V (vs. Ag/AgCl) (Fig. 6 C). The corresponding curves for GO/PPy, RGO/PPy, GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 show a slope variation of the time dependence on potential which indicates an electrical double layer capacitive behavior for the prepared electrodes. The observed deviation in linearity for GO/PPy and RGO/PPy primarily originates from pseudocapacitive behavior. In chargedischarge curves of GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 plateaus can be observed, which arise from the reversible redox reaction of Cu(I)/Cu(II) on the surface of GO/PPy and RGO/PPy films. The shape of curves is the characteristic of double layer



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capacitance and pseudocapacitance, which are in agreement with the results obtained by cyclic voltammetry. Capacitances of the constructed electrodes were obtained from charge-discharge curves according to the following equation: Cs =

I ∆t

(2)

m ∆V

Where, I(A) is the discharge current, t is the discharge time, m is the mass of the electroactive material (g), and ∆V is the potential window (V). The gravimetric specific capacitances of GO/PPy, RGO/PPy, GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 at a current density of 10 A g-1 are 500, 685, 750 and 997 F g-1, respectively. As seen, RGO/PPy/Cu2OCu(OH)/Ni-foam has the highest capacity among all four electrodes. Therefore, RGO/PPy/Cu2O-Cu(OH) nanocomposite exhibits good electrochemical performance during charge-discharge process. The constant current charge-discharge curves of RGO/PPy/Cu2O-Cu(OH)2 at different current densities are shown in Fig. 6D. The gravimetric specific capacitances for RGO/PPy/Cu2O-Cu(OH)2 at current densities of 10, 15, 20 and 25 A g-1 are 997, 900, 839 and 781, respectively. The results reveal that the capacitance decreases with increasing the current density. At lower current densities, more utilization of electroactive materials can contribute to the charge-discharge process, while at higher current densities, electrolyte ions cannot diffuse completely into the inner active sites and the capacitance of the electrode decreases. The calculated gravimetric and volumetric specific capacitances of the prepared electrodes at various current densities are shown in Fig. 6E. By increasing the current density from 10 to 25 A g-1, the gravimetric capacitance retentions of GO/PPy, RGO/PPy,


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GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 are nearly 87.0, 78.0, 87.4 and 78.3% , respectively. Also, the volumetric specific capacitance of RGO/PPy/Cu2O-Cu(OH)2 is 6.64 F cm-3 at 10 Ag-1 and decreases to 5.2 F cm-3 as the current density increases to 25 A g-1.

In addition, symmetric supercapacitor was fabricated using two same RGO/PPy/Cu2OCu(OH)2/Ni foam as positive and negative electrodes. Fig. 7A shows the CV curves of the RGO/PPy/Cu2O-Cu(OH)2 symmetric supercapacitor at different scan rates (20, 50, 75 and 100 mV s-1) between 0 and +0.8 V in 0.5 M Na2SO4 electrolyte solution. The CV curves of RGO/PPy/Cu2O-Cu(OH)2 exhibit a quasi-rectangular shape, which indicates a capacitive behavior. Fig. 7B shows the galvanostatic charge-discharge curves of RGO/PPy/Cu2OCu(OH)2 between 0.0 and +0.8 V in the two-electrode system at different values of constant current densities (10, 15, 20, 25 A g-1). The gravimetric specific capacitances of the symmetric capacitor of RGO/PPy/Cu2O-Cu(OH)2 at constant current densities of 10, 15, 20 and 25 A g-1 are 225, 146.25, 100 and 65 F g-1, respectively. Fig. 7C demonstrates the relationship between the gravimetric specific capacitances and charge-discharge current densities for the symmetric RGO/PPy/Cu2O-Cu(OH)2/Ni foam electrode. It is observed that the gravimetric specific capacitance decreases as the current density increases from 10 to 25 A g-1. Also, Fig. 7C. represents the volumetric specific capacitances of the symmetric RGO/PPy/Cu2O-Cu(OH)2 at different current densities. The volumetric specific capacitance of the symmetric RGO/PPy/Cu2O-Cu(OH)2/Ni foam electrode is 1500 mF cm-3 at 10 Ag-1 and decreases to 437 mF cm-3 as the current density increases to 25 A g-1.



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Page 16 of 40

Energy and power density are two important parameters for measuring the practical application of the as-prepared nanocomposite. The energy density and power density are calculated by charge-discharge curves at various current densities using Eqs. (3) and (4) E = 0.5

P=

Cs .( ∆ V ) 2 3.6

E t disch arge

(3)

.3600

(4)

In these equations, E and P are energy (Wh kg-1) and power density (W kg-1), respectively.

The Ragone plot (Fig. 7D) demonstrates a correlation of the power density with energy density of the symmetric cell. For the symmetric RGO/PPy/Cu2O-Cu(OH)2 supercapacitor, energy densities are 20, 13, 8.88 and 5.88 Wh kg-1 at power densities of 8000, 12000, 16000 and 19998.5 W kg-1, respectively. EIS measurements are a powerful technique to evaluate the fundamental behavior of electrode materials. Fig. 8A shows the Nyquist plots for GO/PPy, RGO/PPy, GO/PPy/Cu2OCu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 on Ni foam substrate in 0.5 M Na2SO4. Measurements were recorded at the open circuit potentials in the frequency range of 20 KHz to 10 mHz with ac amplitude of 10 mV and equilibrium time of 1 s. The Nyquist plots for GO/PPy, RGO/PPy, GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 consist of a small arc at high frequencies. The arc is related to the faradaic process, which arises from the presence of electron transfer limiting step at the interface between electrode material and electrolyte. Also, the diameter of the arc is denoted as faradaic charge transfer resistance (Rct). It is seen from the Nyquist plot that the diameter of arc goes on decreasing from GO/PPy to RGO/PPy/Cu2O-Cu(OH)2, which can be related to the enhancement of the charge transfer kinetics. The charge transfer resistance for the RGO/PPy is smaller than GO/PPy, 16 ACS Paragon Plus Environment

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because of the presence of RGO in nanocomposite film.50 Also, the diameter of arcs for GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 is smaller than GO/PPy and RGO/PPy, which indicates the fast redox reactions of Cu(I)/Cu(II) at the electrode surfaces. The observed arc with the least diameter for RGO/PPy/Cu2O-Cu(OH)2 can be attributed to the incorporation of Cu2O-Cu(OH)2 nanoparticles and RGO in nanocomposite which improves the kinetics of the charge transfer process at the electrode surface. The line with 45° slope at the intermediate frequency region demonstrates the Warburg resistance (W) of ionic diffusion/transport from the electrolyte to the electrode surface. Moreover, it is observed that in comparison to GO/PPy and RGO/PPy, straight lines with slopes close to 90° are resulted for GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 in low frequencies, which is due to their capacitive behavior. It is a result of the fast diffusion and quick adsorption of electrolyte ions from the solution onto the electrode surface. From impedance spectra, it is obvious that the RGO/PPy/Cu2O-Cu(OH)2 shows a lower charge transfer resistance and a better capacitive behavior when compared to the GO/PPy, RGO/PPy and GO/PPy/Cu2OCu(OH)2. An equivalent circuit based on Nyquist plots is shown in Fig. 8 A (inset). In this model, Rs represents the resistance of the electrolyte solution. W and Rct are joined in series which can be attributed to the faradaic impedance. In this model, the double layer capacitor (Cdl) is replaced by a constant phase element (CPE) because of the surface roughness and distribution of the reactions on the surface.58 CPE and Rct are in parallel with an equivalent circuit, which can be related to the charge transfer limiting processes. Rct can be determined from Nyquist plots

as

semicircle

arc

diameter.

The

calculated

Rct

values

for

GO/PPy,

RGO/PPy,GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 are 200, 118, 56 and 46 Ω, respectively. It can be concluded that in comparison to other active materials,



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RGO/PPy/Cu2O-Cu(OH)2 has the lowest Rct value. Therefore, this nanocomposite indicates a fast electron transfer rate between the electrode material and the electrolyte. The cycling stability and the capacitance retention of GO/PPy, RGO/PPy, GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 were investigated in Na2SO4 solution using galvanostatic charge-discharge tests between -0.8 and 0 V at a current density of 10 A g-1 for 2000 cycles (Fig. 8B). The results reveal that GO/PPy and RGO/PPy retain 79 and 84 % of its original capacitance after 2000 cycles, respectively. More capacity retention of RGO/PPy relative to GO/PPy can be attributed to the incorporation of RGO in nanocomposite film. In fact, removal of the functional groups by electrochemical reduction results in relatively stable RGO film. Also, after electrodeposition of Cu2O-Cu(OH)2 on GO/PPy and RGO/PPy , the capacity of GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2OCu(OH)2 reaches 88 and 90% of initial value after 2000 cycles. The presence of Cu2OCu(OH)2 nanoparticles not only enhances the capacitance value, but also improves the cycling stability. The cycling instability of the polymer-based electrodes is due to large volumetric swelling and shrinking during the cycling process. Doping and dedoping of ions into the polymer structure during repeated charge-discharge processes lead to degradation of the polymer chain. Therefore, reduction in the capacitance may be attributed to the structural breakdown of the polymer and deterioration of the active materials during charge-discharge tests. Nevertheless, the RGO/PPy/Cu2O-Cu(OH)2 nanocomposite is able to retain a significant amount of its specific capacitance, which demonstrates that only a minor degradation occurred during the charge-discharge process. Table 1 shows a comparison between the specific capacitance of different copper oxide/hydroxide composites prepared on different substrates (such as stainless steel (SS), Ni


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foam, Cu foil, Cu foam) in various electrolytes. It is shown that the RGO/PPy/Cu2O-Cu(OH)2 have a suitable capacitive behavior in Na2SO4 as the electrolyte solution. Conclusions RGO/PPy/Cu2O-Cu(OH)2

ternary

nanocomposite

was

constructed

by

the

electrodeposition of GO/PPy on Ni foam, electrochemical reduction of GO/PPy (RGO/PPy) and then electrodeposition of Cu2O-Cu(OH)2 nanoparticles onto RGO/PPy film. The XRD pattern of nanocomposite confirms the formation of Cu2O-Cu(OH)2 loaded on the RGO/PPy film. SEM images of RGO/PPy/Cu2O-Cu(OH)2

reveal the presence of Cu2O-Cu(OH)2

nanoparticles in the range of 50-70 nm on RGO/PPy surface. Electrochemical performance of RGO/PPy/Cu2O-Cu(OH)2

nanocomposite

was investigated by cyclic

voltammetry,

galvanostatic charge-discharge and EIS. In a three electrode system, the prepared RGO/PPy/Cu2O-Cu(OH)2 nanocomposite exhibits an excellent gravimetric specific capacitance of 997 F g-1 at 10 A g-1, which can be ascribed to double layer capacitance of graphene and pseudocapacitance behavior of PPy and

Cu2O-Cu(OH)2. The symmetric

RGO/PPy/Cu2O-Cu(OH)2 supercapacitor represents a gravimetric specific capacitance of 225 F g-1 at a current density of 10 A g-1. Also, the prepared symmetric electrode showed very good electrochemical performances with an energy density of 20 Wh kg-1 at a power density of 8000 W kg-1. The as-prepared RGO/PPy/Cu2O-Cu(OH)2 nanocomposite maintains 90 % of its initial specific capacitance value after 2000 cycles. Therefore, the enhanced electrochemical performance of RGO/PPy/Cu2O-Cu(OH)2 suggests that it can be considered as a suitable active material for supercapacitor applications.

Acknowledgments



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The authors gratefully acknowledge the support of this work by the Research Council and the Center of Excellence for Nanostructures of the Sharif University of Technology, Tehran. They are grateful to Institute of National Science Foundation (INSF, 94/S/44025, Iran) for financial supports of this work.

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Electrodes of Electrochemical Supercapacitors. Nanoscale Res. Lett. 2010, 5, 518. 59.

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of Lotus-Like CuO/Cu(OH)2 Hybrid Material Electrodes. J. Electroanal. Chem. 2012, 673, 43-47. (60)

Li, Y.; Chang, S.; Liu, X.; Huang, J.; Yin, J.; Wang, G.; Cao, D. Nanostructured CuO

Directly Grown on Copper Foam and Their Supercapacitance Performance. Electrochim. Acta 2012, 85, 393-398. (61) (OH)

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Figures Captions Fig. 1. (A) Schemata illustration of the preparation procedure for RGO/PPy nanocomposite (B) cyclic voltammograms of RGO/PPy and GO/PPy Fig. 2. (A) FT-IR spectrum of RGO/PPy and RGO/PPy/Cu2O-Cu(OH)2 nanocomposites , (B) X-ray diffraction pattern of RGO/PPy/Cu2O-Cu(OH)2 Fig. 3. SEM images of (A and B) RGO/PPy, (C, D, E and F) RGO/PPy/Cu2O-Cu(OH)2 at different magnification Fig. 4. (A) EDX spectrum of RGO/PPy/Cu2O-Cu(OH)2 /Ni foam electrode ,(B) elemental mapping images of Cu, C, N and O. Fig. 5. 2D AFM images of (A) RGO/PPy and (B) RGO/PPy/Cu2O-Cu(OH)2 on Ni foil Fig. 6. (A) Cyclic voltammograms of GO/PPy, RGO/PPy, GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 in 0.5 M Na2SO4 at scan rate of 20 mV s-1, (B) Cyclic voltammograms of RGO/PPy/Cu2O-Cu(OH)2 in 0.5 M Na2SO4 at various scan rates, (C) Galvanostatic charge-discharge curves of GO/PPy, RGO/PPy,GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 at current density of 10 A g-1 in 0.5 M Na2SO4, (D) Galvanostatic charge-discharge curves of RGO/PPy/Cu2O-Cu(OH)2 in in 0.5 M Na2SO4 at different current densities from 10 to 25 A g-1, (E) Gravimetric and volumetric specific capacitances of GO/PPy, RGO/PPy,GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 versus different current densities from 10 to 25 A g-1 Fig. 7. (A) Cyclic voltammograms of symmetric RGO/PPy/Cu2O-Cu(OH)2 supercapacitor in 0.5 M Na2SO4 at various scan rates, (B) Charge-Discharge Curves of the Symmetric RGO/PPy/Cu2O-Cu(OH)2 Supercapacitor at various current densities from 10 to 25 A g-1, (C) Gravimetric and volumetric specific capacitances of symmetric RGO/PPy/Cu2O-Cu(OH)2 supercapacitor versus different current densities from 10 to 25 A g-1, (D) The Ragone plot of the symmetric RGO/PPy/Cu2O-Cu(OH)2 supercapacitor



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Fig. 8. (A) Nyquist plots of GO/PPy, RGO/PPy, GO/PPy/Cu2O-Cu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 at the open circuit potential (equivalent circuit in inset), (B) Variation of the remained specific capacitance of GO/PPy, RGO/PPy, GO/PPy/Cu2OCu(OH)2 and RGO/PPy/Cu2O-Cu(OH)2 as a function of cycle number measured at 10 A g-1


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Fig. 1



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Fig. 2


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Fig. 3



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Fig. 4


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Fig. 5



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Fig. 6


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Fig. 7



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Fig. 8


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Table 1. A comparison among specific capacitances of different copper oxides/hydroxides electrodes for supercapacitor applications

Specific Current density or Sample

Electrolyte

capacitance scan rate (F g-1) 57

Copper oxide/ SS

1 M Na2SO4

36

20 mV s-1

CuO/Cu(OH)2/Cu Foil

5 M NaOH

278

2 mA cm-1

CuO/Cu foam

6 mol dm -3 KOH

212

0.4 mA mg-1

CuO/Cu(OH)2 /SS

2 M KOH

459

5 mV s-1

Cu2O/CuO/RGO /SS

6 M KOH

173

1 A g-1

Cu(OH)2@RGO/Ni foam

6 M KOH

602

2 A g-1

CuO/Cu2O/Cu/Ni foam

6 mol dm -3 KOH

878

1.67 A g-1

63

CuO/Cu foil

1 M KOH

284.5

0.5 mA cm-1

64

RGO/Cu2O / Cu foil

1 M KOH

98.5

1 A g-1

55

0.5 M Na2SO4

997

10 A g-1

This work

RGO/PPY/Cu2OCu(OH)2/Ni foam


59

60

61

62

56


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"TOC Graphic"


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