Electrodeposition of Polypyrrole and Reduced Graphene Oxide onto Carbon Bundle Fibre as Electrode for Supercapacitor
© The Author(s). 2017
Received: 10 February 2017
Accepted: 19 March 2017
Published: 4 April 2017
A nanocomposite comprising of polypyrrole and reduced graphene oxide was electrodeposited onto a carbon bundle fibre (CBF) through a two-step approach (CBF/PPy-rGO-2). The CBF/PPy-rGO-2 had a highly porous structure compared to a nanocomposite of polypyrrole and reduced graphene oxide that was electrodeposited onto a CBF in a one-step approach (CBF/PPy-rGO), as observed through a field emission scanning electron microscope. An X-ray photoelectron spectroscopic analysis revealed the presence of hydrogen bond between the oxide functional groups of rGO and the amine groups of PPy in PPy-rGO-2 nanocomposite. The fabricated CBF/PPy-rGO-2 nanocomposite material was used as an electrode material in a symmetrical solid-state supercapacitor, and the device yielded a specific capacitance, energy density and power density of 96.16 F g− 1, 13.35 Wh kg− 1 and of 322.85 W kg− 1, respectively. Moreover, the CBF/PPy-rGO-2 showed the capacitance retention of 71% after 500 consecutive charge/discharge cycles at a current density of 1 A g− 1. The existence of a high degree of porosity in CBF/PPy-rGO-2 significantly improved the conductivity and facilitated the ionic penetration. The CBF/PPy-rGO-2-based symmetrical solid-state supercapacitor device demonstrated outstanding pliability because the cyclic voltammetric curves remained the same upon bending at various angles.
Carbon bundle fibre modified with porous polypyrrole/reduced graphene oxide nanocomposite for flexible miniature solid-state supercapacitor.
The rapid growth of next-generation portable electronics has led to intensive efforts to develop supercapacitors with flexible, rigid, small, lightweight, eco-friendly and high storage capacity . Supercapacitors, which are also known as electrochemical capacitors, offer a promising alternative approach to energy storage devices because of their ability to store and deliver a high power density, and long life cycle with short charging time, simply by utilising the charge separation of the electrochemical interface between the electrode and electrolyte [2–4]. The conventional two-electrode system supercapacitors are planar-structured, consisting of two active electrodes kept apart by an electrolyte as an indispensable and electrically insulating separator . The supercapacitors are large, bulky and heavy, and are not suitable for portable electronic devices. Thus, to address this issue, much effort has been devoted to the development of fibre- or wire-shaped supercapacitors that are flexible, lightweight and easily shaped in portable electronic devices [6–8].
Fibre- or wire-shaped supercapacitors are commonly built on fibrous or interwoven substrates and can be directly integrated into a wearable and embedded device units in sensors, environmental monitoring, display and implanted medical devices . Metal-based fibres such as aluminium wires have previously been used as a current collector or core electrode because of its high conductivity and ease of availability. However, the performance is limited due to its heaviness and is easily oxidised under ambient conditions [6, 10]. Carbon-based fibres, like carbon microfibres and graphene fibres, have been used to replace metal-based fibres owing to its great flexibility, light weight, high mechanical strength, high conductivity and stability under ambient conditions [6, 11].
The choice of electro-active materials also plays important roles in determining the electrochemical performances of supercapacitor devices. Graphene has been studied extensively as an electro-active material for supercapacitors due to its promising properties such as large (theoretical) surface areas, high charge carrier mobility, excellent conductivity, high mechanical strength, and extremely high thermal conductivity, with the ability to store and release energy through the separation of electronic and ionic charges in the electrode and electrolyte interface [12–15]. In particular, reduced graphene oxide (rGO) is often used instead of graphene, mainly because it can be ubiquitously produced from graphene oxide (GO) through various methods such as hydrothermal reaction, laser irradiation and chemical or electrochemical reduction under mild conditions . Moreover, using GO as a starting material can provide good dispersion stability and prevent aggregation in the reaction solution .
Simultaneously, electrically conducting polymers such as polypyrrole (PPy) have been studied extensively as pseudocapacitor materials for supercapacitors since they offer good electrical conductivity, high charge densities, low cost and excellent pseudocapacitor behaviours [18–20]. Furthermore, the PPy also provides a greater degree of flexibility in electrochemical processing [21, 22]. Conducting polymers can improve the device by undergoing a redox reaction to store a charge in the bulk of the material and hence increase the energy stored and reduce self-discharge [23, 24]. Recently, the hybridization of carbon-based materials and conducting polymers is believed to be able to enhance the capacitance and stability of a supercapacitor performance through the favourable synergistic effect between them [25, 26].
This study focused on fabrication of flexible symmetrical solid-state supercapacitors in which two carbon bundle fibre (CBF) electrodes were assembled into a supercapacitor device by using them to sandwich polyvinyl alcohol-potassium acetate (PVA-CH3CO2K), which served as an indispensable solid-state electrolyte. The CBF served as a flexible current collector with electro-active materials, while the rGO and PPy were electrochemically deposited on it at a constant potential. The presence of the catalyst in the aqueous solution (PPy-rGO-2) during the electrodeposition was compared to those of PPy and PPy-rGO to investigate the influences of the catalyst on the surface morphology and electrochemical capacitive performance. These symmetrical solid-state supercapacitors inherited flexibility while maintaining high capacitive performances.
Graphite powder was purchased from Asbury Graphite Mills Inc. (code no. 3061). Sulfuric acid (H2SO4), phosphoric acid (H3PO4), potassium permanganate (KMnO4) and hydrogen peroxide (H2O2) were purchased from Systerm Chemicals, Malaysia. Hydrochloric acid (HCl) and iron (III) chloride (FeCl3) were purchased from Sigma-Aldrich, while potassium acetate (CH3CO2K) was purchased from BDH Reagents and Chemicals. Sodium p-toluenesulfonate (NapTs), poly(vinyl alcohol) (PVA) flakes (MW = 60,000) and glycerol purchased from Merck, followed by pyrrole (99%) purchased from Acros Organic, were stored at 0 °C and distilled before use. Hydrophilic carbon cloth (ELAT) was purchased from NuVant Systems Inc., USA.
The solid-state electrolyte was prepared using 10% (w/v) PVA in water and stirred continuously at 100 °C until complete solvation. The potassium acetate (1.96 g) was added to the solution and mixed thoroughly. This was followed by glycerine (10% w/w) which was added as a plasticiser to prevent the loss of electrolyte. Then, the as-prepared CBF/PPy-rGO-2 was used as an electrode for the fabrication of a symmetrical solid-state supercapacitor device. The two symmetrical electrodes were dipped in the solid-state electrolyte and sandwiched together side by side which served as positive and negative electrodes. Meanwhile, the solid-state electrolyte acted as both the electrolyte and ion porous separator, and the fabricated device was left to dry at room temperature.
where C cell is the specific capacitance of the cell from a charge/discharge calculation (F g − 1), V is the potential window (V) and ∆t is the discharge time (s).
Results and Discussion
The difference in the surface morphologies of the PPy-rGO and PPy-rGO-2 compared to that with PPy alone can be explained by the formation mechanism for the PPy on the rGO sheets during the potentiostatic electropolymerisation and catalyst-assisted electrodeposition process. The one-step approach of the potentiostatic electropolymerisation process for PPy-rGO resulted in a continuous layer-by-layer deposition of PPy and GO, where negatively charged GO would be attracted to the pyrrole radical cations during the electrodeposition, and a new layer of PPy would form when the existing GO was fully occupied by PPy . The reaction consequently removed oxygen from GO, thus converting GO into rGO . In contrast, the catalyst-assisted electrodeposition process for PPy-rGO-2 involved a two-step approach, in which during the stirring process, the catalyst oxidised the pyrrole monomers initially to form PPy nanoparticles on the GO sheets (PPy-GO). The subsequent electrodeposition process increased the size and amount of PPy and then bound the sheets to one another and reduced the GO to rGO, which resulted in a maximum exposed area of PPy and prevented the rGO from restacking with the neighbouring rGO . The highly porous structure of PPy-rGO-2 facilitated the electrolyte penetration and eventually increased the specific capacitance value.
Elemental compositions of fabricated nanocomposites
Binding energy (eV)
Atomic percentages (at.%)
Curve fitting results for core-level binding energies of fabricated nanocomposites
Binding energy (eV)
Percentages of the component (%)
Steady peak position π-π interactions
Steady peak position π-π interactions
Figure 3a demonstrates the deconvolutions of the high-resolution XPS spectrum of the S2p photoelectron line for PPy-rGO. The S2p can be resolved into three components at 165.4, 166.9 and 168.5 eV due to the contributions from the C4H4S, S–O– and SO2 species, respectively. This shows that the occurrence of these salient chemical bonding states is in good agreement with the NIST database (NIST Standard Reference Database 20, Version 4.1). The curve fitting of the high-resolution C1s photoelectron line is depicted in Fig. 3b. The deconvoluted C1s spectrum is subdivided into three fragments at three different binding energies of 282.4, 284.6 and 286.4 eV. The first two components are attributed to the presence of the carboxyl group bonding state and steady peak position π-π interactions in pyrrole rings, respectively. The third component at 286.4 eV suggests the possible occurrence of C–S/C=O/C=N/=C–NH+ bonding structures in the nanocomposites. The deconvolution of the N1s peak in the XPS spectrum offers three components with remarkably different intensities, as shown in Fig. 3c. The first peak detected at 399.7 eV is ascribed to neutral nitrogen in the pyrrole ring (–NH). The second component detected at 400.9 eV corresponds to the polaron state (–NH+–), and another peak at a relatively higher binding energy (402.1 eV) can be assigned to =NH+–, which may have originated from the presence of bipolaron charge carriers . Figure 3d shows the deconvolution of the O1s spectrum, where three segments are located at binding energy positions of 530.8, 531.6 and 532.9 eV, suggesting the presence of C=O/S=O/O=C/HO–C bonds, C=O/O–C=O bonds and O–C/C–O–C/COOH/C–OH/H2O bonds, respectively. The C–O and COOH bonds are offcuts of the oxide functional groups of graphene oxide, whereas the S–O bond originates from the NapTS used in the polymerisation process.
The as-stated three component peaks of the S2p photoelectron line for PPy-rGO-2 could be fitted to the binding energies of 167.5 eV (C4H4S), 168.5 eV (S–O–) and 169.4 eV (SO2), respectively, as depicted in Fig. 4a. The high-resolution XPS spectra of C1s and N1s are imperative because the shift in the photoelectron line position indicates a difference in the electron density of the neighbouring atoms via the existing chemical bonding states . In the PPy-rGO, the principal C1s peak at a binding energy of 284.6 eV is related to the steady peak position π-π interactions, which is consistent with an earlier report . However, for the PPy-rGO-2, the main C1s peak was found to shift slightly (0.5 eV) towards a higher binding energy, as depicted in Fig. 4b. The shift indicates a possible electronic disorder produced via new chemical bonds, which refer to the hydrogen-bridge bond between the oxygen containing the functional group of carbon and the NH-group of PPy . For this hydrogen-bridge bond, electrons transfer from the C to O atoms of the carbon functional groups and the NH-group of the PPy. As a result, the reduced electron density of the C atoms confirms a progressive transfer of the C1s XPS spectrum. However, for the N1s spectrum, the reverse phenomenon is expected because in N atoms, the electron density rises over the creation of the hydrogen bonds. This feature is clearly seen in Fig. 4c and Table 2. This negative shift of the N1s spectrum indicates the electron donation through the oxygen functionalities of carbon, which is responsible for the electron dislocations in the vicinity of the nitrogen atoms [39, 40]. A similar feature is also noticed for the O1s spectrum in Fig. 4d for the PPy-rGO-2 sample.
Moreover, the charge transfer resistance (Rct) can also be calculated from the diameter of the semicircle formed by the Nyquist plot, which relates to the interfacial processes of the counter-ions through the electrode/electrolyte interface . The Rct values of the PPy, PPy-rGO and PPy-rGO-2 modified electrodes were 33.38, 22.57 and 4.85 Ω, respectively, revealing that among the investigated electrodes, the PPy-rGO-2 modified electrode had the lowest interfacial resistance with a good charge propagation behaviour. Moreover, CBF/PPy-rGO-2 had a highly porous structure, which enabled easier access for the electrolyte ions, resulting in less resistance in the electrode . Thus, the high conductivity and good diffusion of the electrolyte contributed to a higher specific capacitance value.
Galvanostatic charge/discharge (GCD) test for the supercapacitor devices was performed at a constant current density of 1 A g− 1. The CBF/PPy supercapacitor device failed to charge up to the highest applied potential (1.0 V), as depicted in Fig. 5c. This phenomenon could be attributed to the gap between the deposited layer and the current collector, as shown in Fig. 1b, which eventually disrupted the penetration of the electrolyte and the faradaic charging/discharging between the electrolyte and electrode. In contrast, the CBF/PPy-rGO and CBF/PPy-rGO-2 had asymmetrical charge and discharge curves, which imply pseudo-capacitance behaviours . In addition, the IR drops were due to the presence of internal resistance in the electrode associated with the electrical connection resistance, bulk solution resistance and resistance of ion migration in the electrode, which contributed to the non-linear discharge curves . As seen in Fig. 5c, CBF/PPy-rGO-2 had a lower IR drop than CBF/PPy-rGO at the beginning of the discharge process, indicating better charge efficiency . The specific capacitance of the CBF/PPy-rGO-2 supercapacitor device was 96.16 F g− 1, which was 1.51-fold higher than that of CBF/PPy-rGO (63.27 F g− 1). The CBF/PPy-rGO-2 supercapacitor device had an energy density and power density of 13.35 Wh kg− 1 and 322.85 W kg− 1, respectively.
In order to evaluate the cycling stability of the CBF/PPy-rGO-2 supercapacitor device, GCD studies were performed at a relatively high current density of 1 A g− 1, and the capacity retention as a function of the number of GCD cycles is presented in Fig. 5d. During the first 25 cycles of GCD, the capacity retention decreased by about 21%, presumably related to the low electrochemical stability of PPy, degradation of the polymer chain and deterioration of the electro-active materials after an abrupt and excessive swelling and shrinking process during the GCD cycles [27, 45]. Encouragingly, the GCD cycles stabilised thereafter, and after 500 cycles, the CBF/PPy-rGO-2 had a capacity retention of 71% at a current density of 1 A g− 1.
A flexible and bendable supercapacitor device was fabricated by using a simple and low-cost two-step electrochemical deposition of PPy and rGO on the surface of carbon bundle fibre and assembled into supercapacitor devices by using them to sandwich a solid-state electrolyte. The formation of a hydrogen bond in CBF/PPy-rGO-2, as depicted in the XPS results, transferred electrons efficiently between the rGO and PPy components, leading to an excellent electrochemical performance for the symmetrical solid-state carbon bundle fibre supercapacitor. The formation of a high porosity structure in the PPy-rGO-2 efficiently increased the ionic penetration, which remarkably enhanced the specific capacitance value of 96.16 A g− 1 at a current density of 1 A g− 1. The CBF/PPy-rGO-2 supercapacitor device has a capacity retention of 71% after 500 GCD cycles, and it showed outstanding stability when subjected to bending at various angles. Therefore, these results demonstrate the feasibility of fabricating flexible supercapacitors for portable electronic devices using the simple electrochemical deposition of active materials on a carbon bundle fibre.
This research work was financially supported by Putra Grant IPB (GP-IPB/2014/9440701) and IRU-MRUN (9399901) in the design of the study and collection, analysis and interpretation of the data and writing of the manuscript.
HAAB and HNL conceived and designed the experiments. HAAB performed the experiments and wrote the manuscript together with HNL. HAAB, HNL, MMR, MA, ZTJ and PA analysed the data. HNL, NMH and ZTJ contributed the reagents, materials and analysis instruments. HNL, SK, SAR, RY, NMH, CYY, MMR, MA, ZTJ and PA gave the idea and advice on every problem that occurred while doing this research. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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