Impact of different nanostructures of a PEDOT decorated 3D multilayered graphene foam by chemical methods on supercapacitive performance

Abstract

The present research article describes chemically grown different nanostructures of poly(3,4-ethylenedioxythiophene) (PEDOT) on porous multilayered graphene foam (PEDOT/3D GF) as a supercapacitor electrode material with enhanced electrochemical performance. Different nanostructures of PEDOT on 3D GF are synthesised by a chemical method using an EDOT monomer as a precursor along with ammonium persulfate (APS) or iron chloride (FeCl₃) as an oxidizing agent. The changes in structural, morphological and electrochemical properties are examined as an impact of different oxidizing agents. Structural and morphological analysis reveals uniform growth of PEDOT nanofibers/nanoparticles over 3D GF. The morphological development of nanofibers and nanoparticles over the graphene foam is observed as an influence of the oxidizing agent. The electrochemical capacitive measurements of the PEDOT/3D GF electrode in 1 M H₂SO₄ exhibits a high specific capacitance of ∼522 F g⁻¹ and ∼88 F g⁻¹ at 2 mA cm⁻² current density for nanofibers and nanoparticle like structures, respectively. A wide porous structure, excellent conductivity and high surface area offered by multi-layered graphene framework arouses effective utilization of the deposited PEDOT with improved electrochemical charge transport and also storage capacity.

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1. Introduction

Current advancements in supercapacitors have forced researchers to address growing global power supply demands for a variety of applications such as cordless electric tools, hybrid electric vehicles, day/night storage, and industrial energy management.^1–3 The development of an energy storage device that has an energy density approaching that of a battery and the power density of a capacitor is an ultimate goal for power quality and energy storage of supercapacitors. Supercapacitors can complement or replace batteries in electrical energy storage and harvesting applications, when high power delivery or uptake is needed.^4–6 However, supercapacitors suffer from a lower energy density (normally 10 W h kg⁻¹) as compared to batteries.^7,8 To overcome this issue, modernization of accessible materials is essential in order to achieve enhanced energy and power densities for energy storage devices.

Working electrode is the key part of electrochemical devices, so electrode materials are the most important factors to determine properties of supercapacitors. In most cases, carbonaceous materials are used in the fabrication of supercapacitors due to their excellent cyclic stability and high interfacial capacitance however, their poor specific energy density limits practical application in devices.^8,9 This obstacle can be overcome by using pseudo-coactive materials, commonly, metal hydroxides/oxides and conductive polymers with multiple oxidation states, which possess a redox reaction, are used as pseudocapacitive materials.^10,11 Specifically, conducting polymers have recognized as potential electrode materials due to their high surface area, good stability and low resistance.¹² Among all the pseudo-capacitive materials (PANi, PPy, PEDOT etc.), the poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most widely used conducting polymers for supercapacitors. PEDOT polymer has multiple redox states, high doping–dedoping rate during charge–discharge and good environmental stability.^13–18 The key factors determining the performance of supercapacitors are the specific surface area (SSA) of electrode materials and the properties of electrolytes. The surface engineering research and industrial applications need low-cost processing methods for the large-scale production of materials with necessary performance and engineered surface. One such method, which is simple, low cost and applicable for large-scale production is chemical deposition method, has its own advantages such as simplicity, reproducibility, cost effectiveness, etc.^19–21 In order to improve physical and electrochemical performance of devices, manufacturing PEDOT morphology is focus of current research.

Up to now, few reports are available on the synthesis of PEDOT on different kinds of metal substrates for supercapacitor application. However, such conventional electrode system offers self-dimensional restriction during charge transport, and typical addition of a polymeric binder not only hampers the charge transport rate, but also increases the total mass of the electrodes.²² Recently, light weight (as compared to Ni foam and carbon paper) multi-layered graphene foam (GF) has been attracted intense attention of researchers due to its 3D interconnected ultrathin graphene skeleton and freestanding network capability with high surface area and good electrical conductivity.^23–25 These properties make GF a promising candidate to replace traditional current collectors such as nickel foam or carbon paper. So far, there are only few reports on chemical deposition of metal oxides/hydroxides and conducting polymers (PANi, PPy) on MGF for supercapacitor application.

To date, there is no report on chemical synthesis of PEDOT on MGF for supercapacitor application. In this manuscript, we educed for the first time light weight chemically decorated PEDOT nanostructures on GF electrode for supercapacitor application. Moreover, we demonstrate that, effect of oxidizing agent variation on structural, morphological and supercapacitive properties of PEDOT in terms of specific capacitance, energy and power density. The results reported here will help to modify and engineer PEDOT/3D GF electrodes accordingly the energy storage applications.

2. Experimental section

A two steps procedure were carried out for fabrication of self-supported PEDOT on 3D graphene foam. In first step GF were prepared by CVD method, which includes the growth of graphene on Ni foam, followed by etching of Ni foam using etchant; the details of this process are described in ref. 25. The second step were preparation of PEDOT on 3D graphene, characteristically prepared by the mixing of EDOT, in an aqueous medium with ammonium persulfate (APS) or iron chloride hexahydrate (FeCl₃·6H₂O) via chemical oxidative polymerization. Hydrogen atoms are abstracted from the EDOT molecules during their coupling to oligomers, and polymeric structures are formed. An aqueous solution of 0.01 M EDOT monomers was prepared in 1 M KNO₃ solution with sodium dodecylbenzene sulfonate (SDBS). Then, 0.1 M ammonium persulfate (APS) or 0.1 M iron chloride hexahydrate (FeCl₃·6H₂O) in deionized (D.I.) water as an oxidizing agent is used for oxidative polymerization. Both solutions are mixed together in the same proportion at room temperature. Subsequently, a piece of 3D graphene is immersed (2 × 4 cm²) vertically in the prepared solution bath. After the reaction, a blue layer is observed on the 3D graphene sample indicating the formation of a thin layer of PEDOT growth.

2.1 Characterization

The electrode materials were structurally characterized by Raman, XPS and FESEM measurements. A Raman spectrum was recorded at ambient temperature on a WITeck CRM200 confocal microscopy Raman system with 532 nm wavelength laser. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a thermo scientific ESCALAB 250 (Thermo Fisher Scientific, UK). The morphology of the composite was examined by field-emission scanning electron microscopy (FESEM, JSM-7001F, JEOL). The supercapacitive performance was carried out by forming three electrode electrochemical system. The three electrode system consist of platinum (Pt) as counter electrode, PEDOT/GF as a working electrode and Ag/AgCl as a reference electrode in 1 M H₂SO₄ electrolyte. Cyclic Voltammetry (CV), galvanostatic charge/discharge tests were performed using ZIVE SP2 LAB analytical equipment (South Korea).

3. Result and discussions

3.1 Chemical deposition of PEDOT on GF

To decorate PEDOT on GF, the 3D porous GF is prepared by CVD method and its SEM image and Raman spectra are shown in Fig. S1 and S2 (see ESI†). The SEM image of GF electrode reveals that, the GF is exact replica of nickel foam template 3D structure with 1.5 mm thickness, along with all interconnected graphene sheets (with thickness ∼ 2.6 nm) without any breaks in the foam. The Raman spectra of 3D graphene exhibited two distinct peaks at ∼1559 cm⁻¹ (G-band) and ∼2699 cm⁻¹ (2D-band) recorded at different places on the GF. The integral ratio of the 2D and G band indicates few layered domains (multilayers) contained as-grown GF. The Raman and SEM studies confirm the formation of strongly interconnected high quality multi-layered 3D graphene foam (MGF).

Generally, there are two polymerization processes of PEDOT, by using the chemical polymerization process (Lewis acid as an oxidizing agent) and electro-polymerization process (two metal electrodes to oxidize monomer). In present work, PEDOT is prepared by chemical polymerization process where, EDOT serving as the monomer and oxidizing agent used in our case was either APS or FeCl₃. APS is a non-crosslinking oxidant in the synthesis of conducting polymers. It is well known that APS can polymerize aniline, pyrrole, EDOT, etc., via oxidative coupling and the corresponding conducting polymers in the form of a precipitate can be obtained due to both the stiffness of their backbones and strong π–π interactions among large conjugated units in their backbones. On the other hand, in case of FeCl₃ as oxidizing agent, multivalent metal ions (excessive Fe³⁺) offering an additional cross-linking force to PEDOT backbones.

In this chemical oxidative polymerization via rapid mixing, the induction period becomes shorter when any solid material having a high surface area is introduced into the reaction mixture, such as graphite, carbon nanotubes or 3D graphene foam. During the chemical deposition of PEDOT, positively charged monomers (e.g. EDOT cations) were favoured to adsorb on the negatively charged 3D graphene surface by electrostatic and/or π–π stacking interactions between the PEDOT backbone and the 3D graphene surface. Therefore, favourable polymerization occurred on more and more nucleation sites facilitated by the large SSA of the 3D graphene network allowing continuous deposition of the PEDOT with good surface coverage. Therefore, PEDOT is synthesized on 3D graphene foam via in situ polymerization as shown in Fig. 1(a and b), giving rise to a good surface coverage of PEDOT and strong π–π interactions. In addition, for comparison, a commercial stainless steel (SS) substrate is used as the conventional electrode for supercapacitive evaluation with porous 3D graphene.

Fig. 1 Schematic mechanism of chemical deposition of PEDOT using (a) APS, (b) FeCl₃ as oxidizing agents.

3.2 Structural studies (Raman, XRD and XPS)

Raman spectra of 3D graphene foam, PEDOT(APS)/3D GF, and PEDOT(FeCl₃)/3D GF composite in 550–3000 cm⁻¹ shown in Fig. 2. The Raman spectrum of the PEDOT/3D GF composite shows a two common PEDOT bands at 1433.2, and 1496.8 cm⁻¹ for both samples prepared using APS and FeCl₃ (ref. 26). The common strong band at 1496.8 cm⁻¹ of PEDOT is assigned to the ring CC stretching vibration arising from neutral parts existing between localized elementary excitations such as positive polarons or bipolarons generated upon doping. However, extra small band at 448.2 cm⁻¹ is observed for PEDOT/3D GF prepared by using FeCl₃ as oxidizing agent. Besides the PEDOT peaks, the two distinct peaks at ∼1559 cm⁻¹ (G-band) and ∼2699 cm⁻¹ (2D-band) can be seen which are originated from graphene foam. These all 5 peaks originated from PEDOT and graphene foam, confirms the formation of PEDOT on the 3D graphene surface. The XRD patterns of PEDOT/3D GF electrodes are shown in http://www.sciencedirect.com/science/article/pii/S0925838810023601 Fig. S3.† The XRD patterns of PEDOT/3D GF thin films consist of small humps and no well-defined diffraction peaks other than graphene foam (http://www.sciencedirect.com/science/article/pii/S0925838810023601 Fig. S3†), indicating that amorphous or nanocrystalline nature of PEDOT film. The obtained amorphous/nanocrystalline phase is feasible for supercapacitor application, since the protons can easily permeate through the bulk of the amorphous/nanocrystalline PEDOT electrode and whole amount of electrode can be utilized.

Fig. 2 The Raman spectra of 3D GF, PEDOT(APS)/3D GF and PEDOT(FeCl₃)/3D GF electrodes.

To obtain more detailed information about elements and oxidation states of the as-prepared PEDOT(APS)/3D graphene, PEDOT(FeCl₃)/3D graphene, X-ray photoelectron spectroscopy measurements were performed and the corresponding results are presented in Fig. 3. The spectra of C (1s) shown in Fig. 3(a₁ and b₁) for the PEDOT/3D GF electrode yield three components at C=C (sp², 285.1 eV), –C–O (epoxy, 286.7 eV) and O=C–OH– (carboxyl, 289.1 eV).²⁷ The S 2p XPS peak in both PEDOT(APS) and PEDOT(FeCl₃) films exists as spin split S 2p1/2 and S 2p3/2 doublets at binding energies 164.1 and 165.2 eV, respectively (Fig. 3(a₂ and b₂)). The S 2p peak originates from S atoms bonded to the thiophene ring structure of the chains in PEDOT polymer.²⁸ Thus, XPS and Raman studies confirm the formation of PEDOT on the 3D graphene foam surface.

Fig. 3 The core level XPS spectra of (a₁ and b₁) C 1s and (a₂ and b₂) S 2p for PEDOT deposited on 3D GF using APS and FeCl₃ as oxidizing agents (black line – original data, red line – fitted data).

3.3 Morphological analysis

The surface morphologies of the PEDOT/3D graphene foam were investigated by scanning electron microscopy (SEM) and images are shown in Fig. 4(a and b). A wide porous graphene foam skeleton is well decorated by PEDOT, seen at a low magnification SEM image as shown in Fig. 4(a₁ and b₁). As synthesizing 3D graphene by CVD method, functional groups such as –COOH, –OH, –C=O were introduced on the 3D graphene layer, so that EDOT monomer can be absorbed to the surface by the π–π* binding, and hydrogen bonding with the carboxyl groups.²⁹ The SEM micrographs of the thin PEDOT/3D GF film deposited on graphene foam at different magnifications are shown in Fig. 4(a_1–4 and b_1–4). From the SEM images at different magnifications, it is clearly seen that, the changing oxidant has major influence in the emerging form of PEDOT morphology. Fig. 4(a₃ and a₄) shows the SEM images of PEDOT prepared over 3D GF using APS as oxidant and it reveals that, the formation of microspheres composed of nanoparticles. On the other hand, nanofibrous like structure for PEDOT on 3D GF is observed for FeCl₃ oxidant (shown in Fig. 4(b₃ and b₄)).^30,31

Fig. 4 (a) FESEM images of PEDOT/3D GF electrodes prepared using (a₁–a₄) APS, (b₁–b₄) FeCl₃ as oxidizing agents. (b) Schematic of nanostructures formation mechanism on 3D GF surface.

To account, formation of different nanostructures of PEDOT on GF and their robust adhesion, we present the schematic of growth mechanism in Fig. 4(b). The nanostructured PEDOT formation process of the precursor can be classified in two steps, as heterogeneous nucleation and subsequent crystal growth. Initially, EDOT monomer surrounded by SDBS get adsorbed on the GF surface. Considering the hydrophobic property of EDOT, SDBS was dissolved in de-ionized water in order to ensure the suggested micelles in other conducting polymers, which act as a soft templates for the formation of nanostructures. Through the polymerization of EDOT, nucleation's seed can form on GF surface for further growth of nanostructures, through the reaction. If only crystal seeds are produced on the heterogeneous substrate, the subsequent growth of a porous film is feasible. However, different oxidant used for polymerization plays vital role to alter the morphology of PEDOT over the GF surface. When the oxidant is FeCl₃, the growth of PEDOT is tempered and tends to accord with these fiber-like micelles because of the weak oxidation/reduction potential, and so finally PEDOT can be found in nanofiber-like structure. On the other hand, when the oxidant is APS, the growth of PEDOT is very aggressive and does not accord with the micelles (fiber-like) well because of the powerful oxidant competence of APS, and finally aggregated particle-like PEDOT can be found. Thus, the different nanostructures (nanofibrous and nanoparticle) may formed on the GF surface, due to different oxidant competence of FeCl₃ and APS. Morphological analysis confirms that, different oxidizing agents leads to the formation of different nanostructures. Different nanostructures of PEDOTs over 3D porous GF may contribute to improve the supercapacitive performance with ease of ion transport, dead volume reduction by a short diffusion distance from the porous multi-layered graphene.

3.4 Supercapacitive performance

For detailed characterization analysis of different capacities, cyclic voltammogram were used to analyse electrochemical capacitive properties. Comparative supercapacitor performance of the PEDOT on the stainless steel (SS) and 3D graphene foam was evaluated using a three-electrode electrochemical cell in which PEDOT/SS or PEDOT/3D GF electrode as working electrode, platinum as counter electrode, and Ag/AgCl as reference electrode in 1 M H₂SO₄ electrolyte. Typical cyclic voltammograms of PEDOT/SS and PEDOT/3D GF electrodes for different oxidizing agents in aqueous 1 M H₂SO₄ electrolyte at sweep rate of 50 mV s⁻¹ within optimized potential range of −200 to +500 mV/Ag/AgCl are shown in Fig. 5(a). The figure shows that, the current under curve of CV is higher for PEDOT(APS)/3D GF electrode than PEDOT(FeCl₃)/3D GF, PEDOT(APS)/SS and PEDOT(FeCl₃)/SS electrodes. This result confirms the PEDOT prepared on MGF electrode by using APS oxidizing agent shows more storage capacity than FeCl₃ oxidizing agent. An ideal capacitor displays a rectangular shape with no resistance, but the prominent peaks that occur within voltage ranges are usually evidence of pseudocapacitive behaviour.³² The intercalation and de-intercalation reaction of ions involved in a basic electrolyte is the characteristic source of pseudocapacitance.

Fig. 5 Electrochemical studies of PEDOT/3D GF and PEDOT/SS electrodes, (a) CV curves of PEDOT prepared using APS and FeCl₃ oxidizing agent on 3D GF and SS substrates within optimized potential window of −0.2 to 0.6 V in aqueous 1 M H₂SO₄ at 50 mV s⁻¹ scan rate. (b and c) Scan rate dependent (20–200 mV s⁻¹) CV curves of PEDOT(APS)/3D GF and PEDOT(FeCl₃)/3D GF electrodes in 1 M H₂SO₄ electrolyte.

The Fig. 5(b and c) shows the CV curves at different scan rates ranging from 20 to 200 mV s⁻¹ of PEDOT(APS)/3D GF and PEDOT(FeCl₃)/3D GF electrodes, respectively. The CV curves of PEDOT(APS)/SS and PEDOT(FeCl₃)/SS electrodes at different scan rate shown in the ESI Fig. S4(a and b).† The current under curve increasing with scan rate; concludes that the voltammetric current is directly proportional to the scan rates of CV and this is the indication of the pseudocapacitive behaviour of electrode.³³ The shapes of CV curves do not change with scan rate, suggesting that the mechanism of electrochemical activity is the same throughout the high frequency too. Thus, the capacitance of PEDOT/3D graphene foam is pseudocapacitive, mainly arising from faradaic reactions of PEDOT on the graphene surface, while the capacitance of the graphene framework is overwhelmed by the electric double-layer capacitance.

For the evaluation of PEDOT(APS)/3D GF or SS, PEDOT(FeCl₃)/3D GF or SS electrodes capacity, comparative galvanostatic charge–discharge evaluation at a current density of 2 mA cm⁻² has been performed. As shown in Fig. 6(a and b), the shape of the charge–discharge curves do not show the characteristic of pure double-layer capacitor (linear charge–discharge), but mainly pseudocapacitance (non-linear charge–discharge). These curves show that the resistive drop (iR) for PEDOT(APS or FeCl₃)/SS is more significant than that of the PEDOT(APS or FeCl₃)/3D GF electrodes. Thereby, PEDOT/3D GF confirms more proficiency as an electrode material for supercapacitors. Thus, clearly 3D graphene framework offers much better conduction pathways for charge transportation. The calculated supercapacitance values from the charge discharge curves are shown in Fig. 6(c). The calculated specific capacitances are ∼527.22, 88.23, 97.30 and 9.11 F g⁻¹ for PEDOT(APS)/3D GF, PEDOT(APS)/SS, PEDOT(FeCl₃)/3D GF and PEDOT(FeCl₃)/SS electrodes, respectively. Both PEDOT nanofiber and nanoparticle structures on 3D GF show enhancement in specific capacitance than on conventional SS substrates. Nanoparticle like structure of PEDOT shows significant performance on 3D GF electrode. A Ragone plot is shown in Fig. 7 with the corresponding energy and power densities being calculated from the galvanostatic charge–discharge measurements with 2 mA cm⁻² current densities.

Fig. 6 (a and b) Galvanostatic charge–discharge (GCD) plots of PEDOT(APS) and PEDOT(FeCl₃) on 3D GF and SS substrates, within potential window of −0.2 to 0.6 V at constant charging current 2 mA cm⁻². (c) Graph of charge–discharge curve dependent specific capacitance of PEDOT electrodes.

Fig. 7 A comparative Ragone plot of energy density and power density. Inset shows, schematic of charge transport mechanism of PEDOT/SS and PEDOT/3D GF electrodes.

The maximum electrical parameters such as specific power density (SP) and specific energy density (SE) for PEDOT(APS)/3D GF (half test cell) at 2 mA cm⁻² are found to be ∼6 kW kg⁻¹ and ∼61 W h kg⁻¹, respectively. By forming two electrode full test cell (symmetric cell of PEDOT(APS)/3D GF//PEDOT(APS)/3D GF), the maximum specific capacitance value is found to be 47.5 F g⁻¹ at 2 mA cm⁻², with 15 W h kg⁻¹ and 2.8 kW kg⁻¹, specific energy and specific power density, respectively. A rough comparison of the obtained energy and power density for half and full test cells in a typical Ragone plot, with some recent reports, is shown in Fig. 7. Fig. 7 shows that, the power density at moderate energy density obtained in the present work, laid in previous literature values reported for aqueous electrolyte based PEDOT supercapacitors.^34–36 The stability (shown in Fig. S5†) of PEDOT(APS)/3D graphene electrode in 1 M H₂SO₄ was tested by galvanostatic charge–discharge curve. Fig. S5† showed the specific capacitance values from 1^st to 1000^th cycles. The specific capacitance is decreased by ∼15% up to 1000 cycles. Hence, the stability of electrode retained to 85% after 1000^th cycles. The small decrement (15%) in the specific and interfacial capacitance values with higher number of cycles of PEDOT (APS)/3D graphene electrode was observed due to the loss of active material. The steerages like charging–discharging at slow current densities, and “conductive wrapping” of active electrode can improve the stability of electrode materials.³⁷

The maximum specific capacitance of chemically deposited PEDOT(APS)/3D GF electrode ratify its potential features and applicability in supercapacitor devices. The nanoparticle like PEDOT structure on graphene shortens ion diffusion paths and facilitates migration of electrolyte ions at a large current density, also, the 3D GF provides a highly conductive network to enhance the charge transport to the PEDOT, (schematic of charge transport mechanism shown in inset of Fig. 7). The direct growth of PEDOT by chemical deposition method on 3D GF avoids the addition of binder and compared to lightweight GF a metal-based current collector which not only hamper the charge transport rate, but also increases the total mass of the electrode and results in to reduce in specific capacitance. Thus, the facile chemical synthetic approach may provide a convenient route for the preparation of light weight PEDOT/3D GF as efficient electrode in energy storage application.

4. Conclusion

In summary, a chemical deposition method was successfully employed for the deposition of PEDOT different nanostructures (nanoparticle and nanofibers) onto 3D GF surface by using different oxidizing agents (APS and FeCl₃). The PEDOT(APS)/3D GF electrode reveals maximum specific capacitance up to 522 F g⁻¹. The improved supercapacitive performance of PEDOT(APS)/3D GF emanates from the synergistic cooperation between graphene and PEDOT nanoparticles, leading to a high specific power and energy densities of 6 kW kg⁻¹ and 60 W h kg⁻¹, respectively. Thus, the simple PEDOT synthetic approach using APS oxidizing agent may provide a convenient route for the preparation of light weight PEDOT/3D GF as efficient electrode in high-energy storage application. This enhanced PEDOT/3D GF electrode performance open up further scope in asymmetric, non-aqueous and solid state supercapacitive devices as per requirement of application (high energy or power).

Acknowledgements

This work was partially supported by the Priority Research Centers Program (2009-0093823), Basic Science Research Program (2013R1A1A2A10013147), and the Korean Government (MSIP) (No. 2015R1A5A1037668) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST), and the Korea Railroad Research Institute (2015-11-0165) .This research was supported by the Korea Research Fellowship Program funded by the Ministry of Science, ICT and Future Planning through the National Research Foundation of Korea (2015-11-1063).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra21851a

‡ These authors contributed equally.

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