magnetic functional graphene oxide nanocomposites as novel and hybrid electrodes for highly capacitive pseudocapacitors

Accepted Manuscript Electrosynthesis, physioelectrochemical and Theoretical investigation of poly ortho aminophenol / magnetic functional graphene oxide nanocomposites as novel and hybrid electrodes for highly capacitive pseudocapacitors A. Ehsani, H. Mohammad Shiri, E. Kowsari, R. Safari, J. Shabani Shayeh, M. Barbari PII: DOI: Reference:

S0021-9797(16)30996-1 http://dx.doi.org/10.1016/j.jcis.2016.12.003 YJCIS 21837

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

8 October 2016 30 November 2016 1 December 2016

Please cite this article as: A. Ehsani, H. Mohammad Shiri, E. Kowsari, R. Safari, J. Shabani Shayeh, M. Barbari, Electrosynthesis, physioelectrochemical and Theoretical investigation of poly ortho aminophenol / magnetic functional graphene oxide nanocomposites as novel and hybrid electrodes for highly capacitive pseudocapacitors, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.12.003

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Electrosynthesis, physioelectrochemical and Theoretical investigation of poly ortho aminophenol / magnetic functional graphene oxide nanocomposites as novel and hybrid electrodes for highly capacitive pseudocapacitors

A. Ehsani*a, H. Mohammad Shirib, E. Kowsaric, R. Safaria, J. Shabani Shayehd, M. Barbaria

a

Department of Chemistry, Faculty of science, University of Qom, Qom, Iran b

Department of Chemistry, Payame Noor University, Iran

c

Department of chemistry, Amirkabir University of Technology, Tehran, Iran d

Protein Research Center, Shahid Beheshti University, GC, Tehran, Iran

*Corresponding author. E-mail address:[email protected] [email protected]

1

Abstract Magnetic functional graphene oxide (MFGO) has been synthesized in this work using FeCl4- magnetic anion paired with 1-methyl imidazolium cation. Hybrid poly ortho aminophenol

(POAP)/MFGO

films

have

then

been

prepared

via

POAP

electropolymerization in the presence of MFGO nanosheets, serving as active electrodes for electrochemical supercapacitors. The FeCl −4 functional group in MFGO plays a major part in atomic scale charge/energy transfer and consequently intramolecular electrochemical phenomena in MFGO systems, as shown by the theoretical results. POAP/MFGO composite films have been characterized by surface and electrochemical analyses. The performance of the system has been investigated by various electrochemical methods such as galvanostatic charge discharge experiments, cyclic voltammetry and electrochemical impedance spectroscopy. Novel nanocomposite compounds have been developed in this work for electrochemical redox capacitors. The advantages of these compounds include simple synthesis method, high active surface area and stability in aqueous electrolytes. Keywords: POAP, nanocomposite, supercapacitor, electrosynthesis, functionalized magnetic graphene oxide

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Introduction Electrochemical supercapacitors, which possess high power density and long cyclic life, are charge storing devices. Carbons, metal oxides and polymers are the major compounds studied as supercapacitor electrodes [1-3].

Conductive polymers are

considered the most promising materials in supercapacitors. Polyaniline (PANI) is one of the most important conducting polymers because of its unique electrical, optical, and optoelectrical properties, as well as its ease of preparation and excellent environmental stability. Aminophenols are attractive aniline derivatives. On electropolymerization in acidic media, POAP yields a surface film with interesting electrochemical and electrochromic characteristics. In aqueous and non-aqueous solutions containing protons, the film is electroactive whereas no response is observed at pHs above 7. Different conductivity data reported for POAP film [4-7] indicate that POAP electrochemical response is strongly dependent on the polymer film preparation method, dopant anions and the starting monomer purity [4-7].Carbon based nanomaterials having a high surface area and good electrical conductivity have attracted the attention of scientific community for different applications. These carbon based nanomaterials (activated carbon, carbon nanotubes, and graphene) have been used as substrates for metal oxide nanoparticles and conductive polymers for supercapacitor applications [7-11]. These conducting carbon materials provide a fast electron transfer rate during Faradaic charge transfer reactions and hence enhance the capacitance [12-17]. Additionally, these carbon nanomaterials supply the platform for the decoration of metal oxide nanoparticles to avoid their agglomeration, hence more utilization of nanoparticles. Polymer carbon nanocomposites provide the solution for the insulating nature of conducting polymers at dedoped states by

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using carbon nanomaterials as substrates to grow nanostructured polymers [7-11]. The functionalization of graphene enhances the capacitance and yields anchoring sites for decoration of metal oxide nanoparticles and conductive polymers. Metal oxide (RuO2, TiO2, and Fe3O4) nanoparticles decorated over functionalized graphene nanocomposites and conductive polymer coated functionalized graphene have also been tested for supercapacitor applications. The use of graphene as a support for metal oxide nanoparticles and conductive polymers avoids the agglomeration of nanoparticles to achieve more utilization of metal oxide characteristics and growth of nanostructured conductive polymers. This helps achieve a high value of capacitance [18-20]. Functionalization of graphene oxide (GO) renders them high solubility in polar solvents, prevents aggregation, and improves their dispersion and homogeneity in the polymer matrix of composites. Ionic liquids (ILs) are impressive media for functionalization of GO. ILs contain of a pair of organic cations and various anions. This property makes it possible to adjust the physical and/or chemical properties of ILs by altering the number of ions.

Therefore, composite materials with improved distribution of nanofillers in

polymer matrix could be expected [21], while the GO particles can hardly be dispersed in some organic solvents and aggregation problems may occur [22]. POAP/MFGO electrodes electrochemically prepared at ambient temperature have been suggested to be efficient, potential candidates for application as supercapacitors in this work. The objectives if this work were increasing the capacitance of POAP electrodes by using MFGO (Fig. 1) formation of a composite electrode and increasing the cycle ability of the electrode. The capacitive behavior of nanocomposite has been tested

4

by cyclic voltammetry, galvanostatic charge discharge and impedance spectroscopy techniques.

Figure 1. Molecular structure of magnetic functional graphene oxide.

5

2. Experimental 2.1. Reagent and materials All the chemical materials used in this work, obtained from Merck Chemical Co., were of analytical grade and used without further purification. Double distilled water was used throughout the experiments. 2.2. Characterization A potentiostat/galvanostat (Ivium V21508, Vertex) was used to perform all electrochemical experiments. To carry out POAP electropolymerization, a conventional three electrode cell with an Ag/AgCl reference electrode (Argental, 3 M KCl) was used. The counter and working electrodes were a platinum wire and a carbon paste electrode, respectively. SEM analysis was used in the investigations of the morphology of the polymeric films.

2.3. Preparation of GO and magnetic functional graphene oxide (MFGO) Firstly, GO was synthesized according to the modified Hummers method from oxidation of natural graphite [23]. Modification of GO required two steps. First, carboxyl and hydroxyl functionalities were activated by thionyl chloride (SOCl2; Merck). The synthesized GO was then refluxed in excess thionyl chloride at 75 ◦C for 3 d and the excess SOCl2 was removed by distillation. Functionalization was achieved by the formation of covalent bonds between the carbonyl group of the acyl chloride and the carbon in the alkyl halide with the nitrogen of 1-methyl imidazole. The solid product was refluxed with 1-methyl imidazole (Merck) at 90 ◦C for 24 h. The excess reagent was 6

removed by filtration and washing with dichloromethane. In the next step, MFGO was prepared. A functional GO suspension with a concentration of 1 mg/ml was obtained by dispersing the solid from the previous step in water with the aid of mild sonication for 30 min. The solution was then mixed with the proper amount of FeCl3.6H2O (Merck) (FeCl3.6H2O:FGO weight ratio of 2:1) in nitrogen atmosphere. The mixture was spun at 500 rpm for 6 h at room temperature during which ion exchange from Cl- to FeCl4occurred. MFGO was collected by centrifugation and dried at reduced pressure [24].

2.4. Synthesis of POAP and POAP/ MFGO nanocomposite POAP/MFGO composites were synthesized via electropolymerization in a solution consisting of 0.01 M monomer, 0.5 M HClO4, 0.1 M LiCO4, 5.0 ×10-3 M sodium dodecyl sulfate (SDS) and 3% of MFGO on the MFGO modified working electrode surface. The same

procedure

was

used

to

prepare

POAP

electrode

without

MFGO.

Electropolymerizations were carried out using 40 consecutive cycles at the sweep rate of 50 mV.s-1 in the potential range of -0.2-0.9V. The mass of polymer and composite films were estimated assuming a current efficiency of 100% for the electropolymerization process using Faraday’s law. Faraday’s law was also used in the estimation of the mass of

electrodeposited

films

supposing

a

electropolymerization process [25-28].

7

100%

current

efficiency

for

the

3. Results and discussion In addition, to conclude our discussion, the charge and energy transfer in the GO/MFGO molecular systems (as nano-scale electrochemical systems), have been studied using theory of atoms- in- molecule (AIM). AIM is a generalization of quantum theory to proper nano-sized systems, describing atomic/molecular systems in terms of the topology of the atomic electron density, and its Laplacian, bond critical points and bond order, atomic electronic energy, and Virial/potential force [29-31]. Based on the QTAIM, the atomic electronic energy, Eelec (Ω) , is given by Eelec (Ω) = υelec (Ω) + K elec (Ω) where υelec (Ω)

(1)

and K elec (Ω) are the total atomic virial and kinetic energies,

respectively. In addition, based on the AIM theory, the electron population of the atomic basin Ω in molecule is given by nΩ = ∫ ρ ( r ) dr , where ρ (r ) the is local electron density. Ω

Therefore, the details of the atomic/local electronic transport properties of GO/ MFGO. Molecular systems are calculated, using AIM theory. The electronic wave functions corresponding to the optimized geometries were obtained at density functional level of theory (DFT-B3LYP/6-31G*). Sample results are demonstrated in Figs. 2-4. Analysis of the results show that changes can be projected as density and energy exchange between different atomic basins of the GO/MFGO molecule. Therefore, it can be predicted that thus the atomic basins of a molecular nanoelectrochemical system are grouped into donor (D) and acceptor (A) sections, and the charge/energy exchange between D/A sections due to the external electric/magnetic field

8

or external current (same as an n/p organic semiconductors). The results obtained here show that the FeCl −4 functional group, Fig.3, plays the dominant role in atomic-scale charge/energy transfer and thus intra-molecular electrochemical phenomena in GO/MFGO systems. These electronic contour maps also show that the phenyl (Ph) and Pyridine (PY) rings considered as the conjugated-bond/electron of the systems (Fig. 4), can

be reflected the parallel and perpendicular charge/energy transfer in the GO/MFGO molecular systems.

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(a)

(b)

(c)

Fig. 2: The contour maps of the local electron density (a), electronic kinetic energy (b), and virial force (c) of the graphene-oxide-structure are calculated using atoms-in-molecule (AIM) theory.

10

(a)

(b)

(c)

(d)

Fig. 3: The contour maps of the local electron density (a), Laplacian of electron density (b), electronic kinetic energy (c) and HOMO/LUMO orbital’s (d) of the molecular system FeCl −4 are calculated using DFT/AIM theory.

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(a)

(b)

(d)

(c)

Fig. 4: The contour maps of the local electron density (a), Laplacian of electron density (b), electronic kinetic energy (c) and local virial force of the Pyridine (PY) - Carbon(C)-Phenyl (Ph) plan of the graphene-oxide molecule are calculated using AIM theory.

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SEM images of MFGO and electrosynthesized POAP/MFGO have been shown in fig. 5. It can be observed in the SEM image of MFGO that the individual MFGO nanosheets have flaky and wrinkled paper-like structures which have been exfoliated through the horizontal expansion of the compact layers of graphene oxide nanosheets from the natural graphite [24]. SEM image of POAP/MFGO composite shows a porous structure and presence of the MFGO in the electrosynthesized composite film.

Figure 5. SEM image of (a)MFGO and POAP/MFGO (b) composite film

To clarify MFGO influence on POAP film properties, the electrochemical performance of composite films was investigated by CV measurements in 0.1 M HClO4. Fig. 6 shows the CVs of POAP and POAP/MFGO electrodes in 0.1 M HClO4 solution. The composite electrode capacitance is about two times greater than that of POAP electrode, showing increased electrode capacity by the application of MFGO in POAP electrode, as observed

13

in Fig. 6. The synergetic effect of the interactions of POAP and MFGO nanoparticles may influence the CV curve shapes. The CV of POAP/ MFGO electrode shows nearly rectangular shapes in potential window, indicating that the incorporation of MFGO in POAP matrix not only increase the composite capacitance due to redox transitions of MFGO between different valence states, but it also saves the ideal capacitive behavior

Figure 6. Cyclic voltammograms of POAP and POAP/ MFGO electrodes in 0.1M HClO4 .

The fractal concept was used to study the surface morphology of POAP/MFGO film. There exists a power dependence between the peak current (Ipc) in cyclic voltammograms and the corresponding potential sweep rate (ν) in diffusion controlled redox transition, 14

taking place by diffusion of an electroreactant species to a target surface, followed by heterogeneous electron transfer, as previously reported [4-7].

I pc = σ Fν α

(2)

Where α is the fractal parameter and σ F is a proportionality factor. Thus, the fractal parameter can be obtained easily by plotting the peak current against the sweep rate in log-log scale. On the other hand, the fractal parameter is related to the fractal dimension (Df) of the electrode surface as [4]:

α=

Df −1 2

(3)

Eq. (2) is applicable to electrochemical methods, as it has been successfully used for calculating the fractal dimension of electrode surface. Based on this information, Fig. 7 represents cyclic voltammograms of POAP/MFGO films recorded in different potential sweep rates in the range of 10-400 mVs-1. Fractal dimension has been obtained from the relationship between the anodic peak current and potential sweep rate in log-log scale. The α value of the composite film is given by the slope of the line. The fractal dimension of 2.59 is obtained for electrosynthesized film by substituting the value in Eq. (3). The porous structure of the electrosynthesized composite film is verified by the calculated values for fractal dimension.

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Figure 7. Cyclic voltammograms of POAP/ MFGO films recorded in different potential sweep rates in the range of 10-400 mVs-.

The capacitance of POAP/MFGO composite electrodes has been focused on by galvanostatic charge/discharge method. The charge/discharge behavior of POAP and POAP/MFGO electrodes in the range of 0.0-0.6 V is shown in Fig. 8. A triangular shape is located between these potential ranges, indicating good columbic efficiency and ideal POAP/MFGO supercapacitor electrode capacitive behavior, as observed. The following equation is applied to find the specific capacitance of the nanocomposite film by charge/discharge curves [4]:

16

(4) Where I is the current loaded, m is reactive material mass, V is the potential change during discharge process and t is the discharge time. The mass of deposited polymer on the electrode was calculated from the charge (Q) passed during characteristic CV of electrode based on Faraday’s law: Q = znF, where Q is the difference of charge passed through polymer electrodes, z is the number of exchanged electrons and n is the number of moles. By substituting the obtained values in equation 5, the SC of POAP/ MFGO electrode was found to be 333 F.g-1. While the POAP electrode loses its stability fast, the composite electrode maintains its stability and saves more that 93.5% of its capacitance of the first cycle under consecutive cycles after 1000 cycles.

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Figure 8. Galvanostatic charge and discharge measurements of (a) POAP and POAP/MFGO electrode in 0.1M HClO4 solution at 0.005mA,(b) different current for composite, (c) during 10 consecutive charge-discharge and (d) during 100 consecutive charge-discharge of composite film.

One of the best methods for the analysis of the properties of conducting polymer electrodes, which has been widely discussed in the literature via different theoretical models, is electrochemical impedance spectroscopy (EIS) [32-41]. EIS gives information about the processes, occurring in the polymer matrix when doped, including kinetic 18

values of the doping process and parameters of ion diffusion into the polymers, in electrochemical systems. EIS analysis has been used for POAP and POAP/FGO films in HClO4 acidic solution. Fig. 9 shows the Nyquist diagrams of electrodes in OCP. The plot in Fig. 9 illustrates a single, semi-circle in the high frequency region and a straight line in the low frequency region for two spectra. The high frequency arc is the overall contact impedance generated from the electrical connection between POAP/FGO composites, the backing plate and the charge transfer at the contact interface between the electrode and the electrolyte solution. In spite of the similar shape of the impedance spectra, there is an obvious difference between the diameters of the two semi-circles. The diameters of the semicircles decline in the presence of MFGO in POAP. In other words, the bulk film transport of electrons and the charge transfer resistance (Rct) of POAP/ FGO films are lower than those of the pure POAP films. This signifies the faster electron transport in the bulk film and charge transfer in the parallel POAP film/solution and FGOs/solution interfaces of nanocomposites, compared to that in the originally single POAP film/solution interface. This may suggest that MFGO has an obvious improvement effect, making the composites have more active sites for faradic reactions and a larger specific capacitance than pure POAP.

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Figure 9. Nyquist plots with an ac amplitude of 5 mV for POAP (1) and POAP/MFGO (2) electrode.

4. Conclusions Magnetic graphene oxide has been dispersed within conducting polymer/MFGO composite

films

using

a

simple

and

general

strategy,

namely

in

situ

electropolymerization. POAP/MFGO composite electrode has been shown to improve the specific capacitance and power characteristic of electrochemical capacitance. MFGO has an obvious improvement effect, which makes the composites have more active sites for faradic reaction and larger specific capacitance than pure POAP. In addition, this results in enhanced electric conductivity, lowers the resistance, and facilitates the charge transfer of the composites. Based on our main objective to produce high active composite 20

materials for supercapacitor application, this work introduces new nanocomposite materials for electrochemical redox capacitors with such advantages as the ease of synthesis, high active surface area and stability in an aqueous electrolyte.

Acknowledgements The authors would like to express their deep gratitude to the Iranian Nano Council for supporting this work.

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Graphical abstract

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