Simultaneous reduction and covalent grafting of
polythiophene on graphene oxide sheets for
excellent capacitance retention
†
Santosh Kumar Yadav,*aRajesh Kumar,*bAshok K. Sundramoorthy,c Rajesh Kumar Singhdand Chong Min Kooef
Herein, we report room temperature reduction and covalent grafting of graphene oxide sheets by thiophene derivatives to produce pseu-docapacitive electrodes, which are capable of delivering high
capac-itance (230 F g 1 at 1 mV s 1) and, most important, 100% cycling
retention after 5000 cycles. Raman and FTIR spectroscopies
confirmed the strong interaction of PTh with rGO in the PTh-g-rGO
hybrids.
Supercapacitors are attractive energy storage devices due to their high power density and longer cycle life than batteries.1 The supercapacitors store charges based on either the accu-mulation of charges at the interface between electrode and electrolyte (electrical double layer capacitors, EDLC) or revers-ible faradic redox reactions (pseudocapacitors), or both, depending on the nature of the active materials.2Carbon based
materials are commonly used as an electrode material for the EDLC, however, due to electrostatic charge storage mechanism, their energy density is lower than batteries.1,2On contrary, metal oxides3–5 redox-active materials or conducting polymers6,7 are considered as high energy density pseudocapacitve materials which store charges via surface redox reaction. Conducting polymers are considered as a relatively low cost and a sustain-able option for the energy storage applications.8 Although, conducting polymers offer high energy density, they show poor cycling stability mainly due to low conductivity. One way to
improve their conductivity is to integrate them on conductive carbon nanostructures.7
Graphene, a two-dimensional (2D) carbon based materials have single-layer sheet of sp2 hybridized carbon atoms, has attracted tremendous attention because of its exceptional physical and chemical properties.9Graphene oxide, (GO) which consists of various oxygen containing functional groups is considered as a precursor for the synthesis of defect rich gra-phene sheets by either chemical or thermal reduction processes.10The oxygen rich surface allows GO to form stable aqueous and organic suspensions and provide a platform for the graing of various polymers.11,12The conducting polymers
like polythiophene (PTh), polyaniline (PANI) and polypyrrole are the common electrode materials for the supercapacitors appli-cations.13Among these conducting polymers, PTh is attracted
because of its good conductivity simple, relatively low cost preparation method. However, its electrochemical stability is limited because of structural conformation changes with repeated ion exchange in the electrochemical process.14
The covalent graing of the PTh on the graphene sheets may bring the synergistic impact of both materials, where former will lead to high capacitance and, later, will improve the cycling performance due to its high conductivity. Here, we have cova-lently graed the PTh on the oxygen rich graphene oxide sheets by esterication reactions and oxidative polymerization. A schematic representation for the synthesis process of the PTh-g-rGO hybrid is shown in Fig. 1, including graing of conducting polymer and simultaneous reduction of graphene oxide. The polythiophene was not only graed but also led to the reduction of graphene oxide sheets at room temperature, thus improved the conductivity of the hybrids.15When tested as a
pseudoca-pacitive electrodes, high capacitance and excellent capacitance retention of 100% was observed aer 5000 cycles at 50 mV s 1.
Graphene oxide used to gra PTh was synthesized by modied Hummer's method, as described previously.16First,
100 mg of graphene oxide (GO) was dispersed in 10 mL DMF using bath sonication up to 30 minutes to obtain a homoge-neous solution of the GO. Further, 500 mg of 3-thiophene acetic
aDepartment of Chemical and Biological Engineering, Drexel University, Philadelphia,
PA 19104, USA. E-mail: santoshkonkuk@gmail.com
bCenter for Semiconductor Components, State University of Campinas (UNICAMP),
13083-870 Campinas, Sao Paulo, Brazil. E-mail: rajeshbhu1@gmail.com
c
Department of Biological Systems Engineering, University of Wisconsin-Madison, 460 Henry Mall, Madison, Wisconsin 53706, USA
dDepartment of Physics, Indian Institute of Technology (IIT-BHU), Varanasi, India eCenter for Materials Architecturing, Korea Institute of Science and Technology (KIST),
Hwarangno 14-gil 5, Soengbuk-gu, Seoul 136-791, Republic of Korea
fNanomaterials Science and Engineering, University of Science and Technology,
Daejeon 305-350, Republic of Korea
† Electronic supplementary information (ESI) available: Fig. S1. See DOI: 10.1039/c6ra07904k
Cite this: RSC Adv., 2016, 6, 52945
Received 27th March 2016 Accepted 24th May 2016 DOI: 10.1039/c6ra07904k www.rsc.org/advances
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acid (3.5 mmol) in 5 mL DMF was added to the GO solution and stirred for further 30 min. A solution of DCC (725 mg, 3.5 mmol) and DMAP (43 mg, 0.35 mmol) in 5 mL of DMF was prepared separately and added into the reaction mixture of the graphene oxide and TAA and the reaction mixture was allowed to stir for 48 h under inert atmosphere at 25C. Aer the completion of the reaction, 100 mL DMF was added to solutions and the product was obtained by vacuumltration. Further, function-alized GO was washed with plenty of DMF and methanol and dried at 40C under vacuum. Further 100 mg of TAA graed GO was sonicated in a 10 mL of dry chloroform for 25 min to obtain the homogenous solution. Later, 40 mg of ferric chloride in 10 mL of chloroform was added to the solution, and the reaction mixture was stirred at room temperature for 24 h to complete the chemical oxidative polymerization process.17 The poly-thiophene (PTh) graed GO was obtained aer vacuum ltra-tion and drying at 40C under vacuum. Surface morphologies of the samples were probed by transmission electron micros-copy (TEM, JEOL JEM-2100, Japan) with an accelerating voltage of 200 kV and scanning electron microscope (SEM, Carl Zeiss AG, Germany) operating at 3 kV. The changes in the surface chemical bonding were characterized by Fourier transform infrared (FTIR) (Thermo Nicolet Nexus) operating in trans-mission mode (8 cm 1, 32 scans per spectrum). The Raman spectrum of the as synthesized samples were recorded by Raman spectrometer Renishaw inVia spectrometer with a 632 nm laser as excitation source. Film electrodes were prepared by mixing 90 wt% active material with 5 wt% carbon black (Alfa Aesar, USA) and 5 wt% polytetrauoroethylene (PTFE) binder (Sigma-Aldrich) in ethanol. Aer evaporation of ethanol, the powder was rolled into 50mm lm. The activated carbon electrodes (150mm) were prepared by mixing 95 wt% AC and 5 wt% PTFE binder. All electrochemical tests were per-formed in a three-electrode Swagelok® cell. GO or PTh-g-rGO were used as the working electrode, activated carbonlms were used as the counter electrode, and Ag/AgCl was used as the reference electrode. A Celgard membrane served as a separator and 1 M H2SO4as the electrolyte solution. Cyclic voltammetry
(CV), galvanostatic charging/discharging, and electrochemical impedance spectroscopy (EIS) were used to study electro-chemical performance of the hybrids.
The PTh-g-rGO hybrid was synthesized via chemical oxida-tive polymerization process (Fig. 2). In this process, the mono-mer was added to the GO dispersion in DMF, and the color of the reaction mixture was monitored. The brown color disper-sion of GO was turned into dark black color, indicating the reduction of the graphene oxide and initiation of the polymer-ization process. The synthesis of PTh-g-rGO hybrids was carried out in two steps. In therst step, the monomer was graed with GO by the esterication reaction followed by oxidative poly-merization in the second step. It is worthy to point out that reduction of the GO happened at room temperature, in contrast to previous reports where high temperature for the extended period of time is needed. The proposed the reduction mecha-nism of the GO by thiophene, which could be useful for other applications. It is likely that the reduction mechanism involves donation of electrons from thiophene monomers to reduce GO to rGO during their polymerisation into the oxidised form.15
The TEM images (Fig. 3a and b) display a uniform graing of the rGO sheets by PTh without any apparent agglomeration. The overlaps and corrugation features of the hybrids occurred because of the high aspect ratio of the nanosheets. The SEM analysis (Fig. 3c) shows that PTh and rGO are interconnected with each other via covalent bonding (Fig. 2) and formed a sandwich like structure. The PTh-g-rGO hybrids display a compact layered sheet structure, which permits facile access of the electrolyte ions. Furthermore, the uniform coating of the PTh on the conductive rGO sheets permits fast electronic transport which guarantees good electrochemical behavior.
Energy-dispersive X-ray spectroscopy (EDS) elemental mapping of the bulk PTh-g-rGO was used to identify the spatial distribution of the elemental composition (Fig. S1†). The EDS mapping (Fig. 3d) of dark green (sulphur) underpins the fact
Fig. 1 Schematic presentation of the simultaneous reduction and
covalent grafting of the thiophene derivative on graphene oxide sheets.
Fig. 2 Reaction scheme of covalent grafting of the polythiophene
derivative on graphene oxide sheets.
that PTh was uniformly graed on rGO sheet. The EDS maps of sulfur (dark green) showed the presence of sulfur (3.07 at%), carbon (72.05 at%) and (d) oxygen (24.88 at%).
The FTIR spectrum of GO (Fig. 3e) displays broad and intense peak centered at 3365 cm 1 attributed to the –OH stretching vibration. The peaks at around 1225 and 1045 cm 1 can be attributed to stretching vibration of epoxy and alkoxy groups. The sharp peak at 1711 and broad peak at 1370 cm 1
are the C]O stretching vibration adsorption peaks of carboxyl and carbonyl groups. The sharp and narrow peak at 1620 cm 1 is ascribed to the stretching vibration of aromatic C]C bonds. The other peak at 769 cm 1is due to the C–H bonds.18These
peaks evidence shows the existence of different abundance functional groups such as epoxy, hydroxyl, carboxyl and carbonyl on the GO surfaces which has been utilized as active sites to graing the PTh.19
Moreover, comparison of the spectra of GO and PTh-g-rGO hybrids shows a dramatic decrease in intensity of the absorp-tion peaks of oxygen-containing funcabsorp-tional groups (1711, 1045 cm 1), which indicates GO reduction in rGO in PTh-g-rGO hybrids. The one new peak at lower wavenumber side at 695 cm 1corresponds to C–S stretching in the thiophene ring
which is due to PTh in PTh-g-rGO hybrid.20,21The overall FTIR
observation conrms that GO converted into rGO and also successfully covalently incorporated in the PTh matrix in the PTh-g-rGO hybrids.
Raman spectroscopy of GO and PTh-g-rGO hybrids materials was carried out to investigate the possible interactions of PTh with rGO. Raman spectra have been commonly used to
characterize the structural information of sp2carbon materials and reveal the quality of synthesized sample in terms of gra-phene layers, degree of defects and various doping level.22The Raman spectra of GO and PTh-g-rGO hybrids are shown in Fig. 3f. The Raman spectrum of the GO shows two characteristic peaks at 1341 and 1599 cm 1that correspond to the D and G
bands, respectively.19,22 The Raman spectra of PTh-g-rGO
hybrids show several sharp peaks at 697, 1043, 1334, 1451, 1587 and 2676 cm 1. Among the above peaks, the two peaks as 1334 and 1587 shows the D band and G band, respectively in PTh-g-rGO hybrids due to PTh-g-rGO. It can be seen that the both (G and D band) bands are shied towards lower wave number side, which shows the red shiing in PTh-g-rGO hybrids. This shiing in G and D bands is mainly due to the charge-transfer interaction and p–p interaction between thiophene rings of PTh with rGO.23Furthermore, the ID/IGdecreased in PTh-g-rGO hybrids
from 0.91 (GO) to 0.72, which is due to the reduction of GO to rGO in PTh-g-rGO hybrids. The other peak at 697, 1043 and 1451 cm 1corresponds to the C
a–S–Cadeformation, Cb–H bending
and Ca]Cb ring stretching, respectively due to presence of thiophene in PTh-g-rGO hybrids.24,25The graphene oxide is an
insulator, however, the PTh-g-rGO hybrid showed the conduc-tivity of approximately 1.0 S m 1via four probe measurement, which is also supporting the reduction as well as graing.
Electrochemical measurement like CV of PTh-g-rGO hybrids was performed in potential window between 0.2 and 0.8 V at different scan rates of 5, 10, 20, 30, 40, and 50 mV s 1(Fig. 4a).
At lower scan rates, the shapes of the CV curves indicate that their capacitive characteristics are similar to that of electrical double layer capacitor (EDLC) for which the CV curve resembles to a nearly rectangular shape which is typical behavior for graphene-based supercapacitors. The peak current increases progressively with increasing scan rate. But at higher scan rates we observe that the signal has both characteristics as EDLC and faradaic processes associated with a pseudocapacitor. The specic capacitances of the PTh-g-rGO hybrids electrodes were calculated similar to reported work.26
Fig. 4b shows the specic capacitance of PTh-g-rGO hybrids at different scan rates. The specic capacitances evaluated from the CV curves for PTh-g-rGO hybrids were found to be 230, 195, 165, 152, 135, 120 and 115 F g 1at scan rates of 1, 2, 5, 10, 20, 50 and 100 mV s 1, respectively. The decrease in specic capaci-tance with increasing scan rates can be attributed to the fact that the electrolyte ions are not fully accessible to the interior surfaces of the active materials for charge-storage because of the reduced diffusion time at a high scan rate. It can be seen that the electrode materials, PTh-g-rGO hybrids showed the maximum specic capacitance of 230 F g 1at a scan rate of 1
mV s 1, whereas pure GO, exhibit very low specic capacitance (5 F g 1at 1 mV s 1), at the same scan rate. Fig. 4b shows that
the specic capacitance PTh-g-rGO hybrids which are 46 times higher than the pure GO. In situ reduction of GO by thiophene not only enhance the surface area of rGO but also improved the conductivity aer removal the functional groups such as epoxy, hydroxyl, carboxyl and carbonyl from GO surfaces. This enhanced surface area and conductivity of rGO in PTh-g-rGO hybrids are the most likely reasons for the high specic
Fig. 3 TEM images (a, b), SEM images (c) and elemental mapping (d) of
PTh-g-rGO hybrids, showing uniform coating of the polythiophene on the rGO sheets, (e) FTIR spectrum of the GO (below) and PTh-g-rGO (f) corresponding Raman spectrum of GO and PTh-g-rGO (below) shows grafting of the polythiophene on the GO sheets.
capacitance. The uniformly dispersed conducting polymer in rGO not only effectively inhibit the stacking and agglomeration of GO, resulting in high capacitance but also provides a highly conductive network for electron transport during the CV processes.
The excellent interfacial contact between PTh and rGO can signicantly improve the accessibility of PTh-g-rGO hybrids to the electrolyte ions and shorten the ion diffusion and migration pathways. Meanwhile, the ion transfer paths are largely short-ened because PTh-g-rGO hybrids have thick sheet like structure. All these factors led to improved capacitance.
Galvanostatic charge/discharge behavior of the PTh-g-rGO hybrids (Fig. 4c) electrode at different current densities of 3, 5, 7, 10, 12 and 15 A g 1showed an increase in discharge time at low current densities due to better percolations of the ions and vice versa. Inset of the Fig. 4c shows the charge/discharge prole of PTh-g-rGO hybrids measured at a current density of 10 A g 1 which exhibited almost symmetric triangular shape with no obvious ohmic drop, indicating the negligible internal resis-tance and good columbic efficiency.27Nyquist plot of PTh-g-rGO
hybrids (Fig. 4d) shows a straight line in the low-frequency region and a very small arc in the high frequency region. The intersection of the real axis in high-frequency region represents
the solution resistance and is related to the charge transfer between the electrode materials and electrolyte.28–31 It is seen that the PTh-g-rGO hybrids have a very low solution resistance of 0.14 U, suggesting good electronic conductivity of the hybrids. Furthermore, negligible semicircle indicate minimal charge-transfer resistance (Rct) which ensures better charge
percolation during charging/discharging. The low charge-transfer resistance and negligible semicircle further indicate good integration of PTh onto rGO sheets which exhibited low Warburg diffusion resistance. The interlinked graing structure of PTh-g-rGO hybrids facilitates the fast electrolyte ion transport during the electrochemical processes and provides a contin-uous conductive network. Ragone plot (Fig. 4e) shows that the PTh-g-rGO hybrids exhibited high energy density of 8 W h kg 1 in aqueous 1 M H2SO4electrolyte.
It is well-known that long-term cyclic stability is a signicant consideration for the practical application of a material in a supercapacitor electrode. The cycling stability of the hybrids was measured using CV technique at a scan rate of 50 mV s 1up
to 5000 cycles (Fig. 4f). The nearly 100% capacitance retention was observed which again conrmed the usefulness of the covalent graing and is a future guideline for the synthesis of supercapacitor hybrids. The achieved long term stability and retention properties of as synthesized PTh-g-rGO hybrids shows better performance than the works reported by others on carbon materials with different polymers based super-capacitors.32–39 It shows that our hybrids have outperformed reports in cycling stability due to covalent network and synergic contribution of the polythiophene and reduced graphene oxide sheets.
Conclusions
In conclusion, we have shown simultaneous reduction and covalent grating of thiophene derivatives on GO sheets to synthesize PTh-g-rGO hybrid by a simple and inexpensive in situ chemical oxidative polymerization method. The SEM images showed uniform coating of the two-dimensional rGO sheets. The Raman and FTIR spectroscopies conrmed the strong interaction of PTh with rGO in the PTh-g-rGO hybrids. The formation of such a special type of morphology enhances the electrochemical properties of the PTh-g-rGO hybrids. Such two-dimensional morphology provide facile ionic and electronic transport for better electrochemical performance. The maximum specic capacitance of 230 F g 1 (1 mV s 1) was
obtained for the rGO hybrids. Most important, the PTh-g-rGO hybrid showed the excellent stability without distorted and degradation nature of CV even aer 5000 cycles and capacitance retention ration of 100%, which indicating excellent stability. Based on the results, the PTh-g-rGO hybrids can be used as electrode material for high-performance supercapacitor electrodes.
Acknowledgements
We would like to gratefully acknowledge anonymous referees for useful comments and constructive suggestions.
Fig. 4 (a) Cyclic voltammograms of PTh-g-rGO at various scan rates
between 2 and 50 mV s 1, (b) rate performance of GO and PTh-g-rGO
at 2–100 mV s 1, (c) corresponding GCPL plots at various current
densities. Inset shows the symmetrical GCPL curves at 10 A g 1
yielding high coloumbic efficiency, (d) Nyqiust plots of PTh-g-rGO
showing negligible semicircle, (e) Ragone plot of the PTh-g-rGO, (f) cyclic stability up to 5000 cycles of PTh-g-rGO showed negligible capacitance loss due to covalent interaction between polymer and rGO sheets.
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