The structure and properties of PEDOT synthesized by template-free solution method
© Zhao et al.; licensee Springer. 2014
Received: 8 August 2014
Accepted: 1 October 2014
Published: 7 October 2014
In this study, a simple one-step template-free solution method was developed for the preparation of poly(3,4-ethylenedioxythiophene) (PEDOTs) with different morphologies by adjusting various ratios of oxidant (FeCl3·6H2O) to monomer (3,4-ethylenedioxythiophene (EDOT)). The results from structural analysis showed that the structure of PEDOT was strongly affected by the oxidant/monomer ratio, and the polymerization degree, conjugation length, doping level, and crystallinity of PEDOT decreased with increasing of the oxidant/monomer ratio. The morphological analysis showed that PEDOT prepared from an oxidant/monomer ratio of 3:1 displayed a special coral-like morphology, and the branches of ‘coral’ would adjoin or grow together with increasing content of oxidant in the reaction medium; consequently, the morphology of PEDOT changed from coral to sheets (at an oxidant/monomer ratio of 9:1). The electrochemical analysis proved that the PEDOT prepared from an oxidant/monomer ratio of 3:1 had the lowest resistance and the highest specific capacitances (174 F/g) at a current density of 1 A/g with a capacity retention rate of 74% over 1,500 cycles, which indicated that the PEDOT with a coral-like morphology could be applied as a promising electrode material for supercapacitors.
With the globalization of economy, the increasing demand for portable systems, miniaturized wireless sensor networks, electric vehicles, and hybrid electric vehicles has spawned great interest in electrochemical capacitors, also known as supercapacitors[1–3]. Electrochemical supercapacitors with relatively high energy and power densities are part of the significant field of electrochemical energy storage because they bridge the gap between conventional capacitors and batteries. Besides, supercapacitors have a long lifespan, rapid charge/discharge rate, and wide operating temperature range[4–8]. Typically, a supercapacitor includes electrodes, separators, and plastic outer package[9, 10]. The electrodes are the key to a supercapacitor, which are generally made from carbon materials, transition metal oxides, and conducting polymers. In general, the conducting polymers have an advantage over carbon materials or transition metal oxides due to their good environmental stability, high conductivity, high transparency, and low oxidation potential.
Among conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) exhibits both an unusual stability and a high conductivity at an oxidized state, which has been considered as perhaps the most stable conducting polymer currently available. So far, many researchers have studied to further improve the capacitive performances of supercapacitors by increasing the specific surface area of electrode materials. As known, PEDOT exhibits an irregular amorphous shape by most methods[13, 14]. Most of the methods to prepare special nanostructures of PEDOT include template synthesis, reverse microemulsion polymerization, and interfacial polymerization in the presence of a surfactant. For example, Hu et al. synthesized a 3D flowerlike PEDOT which shows a specific capacitance of 111 F/g by using bis(2-ethylhexyl) sulfosuccinate sodium as surfactant. Lacroix et al. reported the electrodeposition of PEDOT films from an aqueous surfactant solution through a two-dimensional poly(styrene) template onto an indium tin oxide substrate. Han et al. synthesized conducting PEDOT:PSS nanospheres onto a flexible poly(ethylene terephthalate) substrate. However, it is difficult to obtain a PEDOT nanostructure with high conductivity by a simple chemical solution method without a template. Nabid et al. synthesized PEDOTs with fiber- and sphere-like morphologies by template-free route in the presence of different oxidants; however, all PEDOTs exhibited a low crystallinity and oxidation degree (or doping level), resulting in a decrease in conductivity of PEDOT.
In this paper, we report a simple one-step template-free solution method for the preparation of PEDOTs with different morphologies by adjusting various ratios of oxidant (FeCl3·6H2O) to monomer (3,4-ethylenedioxythiophene (EDOT)). The polymerization was carried out in a template-free alcoholic aqueous solution medium. The correlation between the structures and properties of the PEDOTs prepared from different contents of oxidant was discussed based on the results from Fourier transform infrared (FTIR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The possible mechanism for the formation of different morphologies of PEDOT from various ratios of oxidant to EDOT was proposed. Moreover, the galvanostatic charge/discharge measurement, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and cycle life measurement were done to evaluate the potential application of PEDOTs as a supercapacitor material.
EDOT was obtained from Shanghai Aladdin Reagent Company (Shanghai, China) and stored in a refrigerator prior to use. FeCl3·6H2O was obtained from Tianjin Zhiyuan Chemical Reagent Co., Ltd (Tianjin, China). All other reagents were of analytical grade and used as supplied without further purification.
Preparation of PEDOT
A typical synthesis process of PEDOT was carried out in the following steps: 20 mL FeCl3·6H2O (1.0 M) aqueous solution was added in a 50-mL beaker with a magnetic stirrer. Then, 5.0 mL of 14.2 wt.% EDOT alcoholic solution was quickly added into the above solution. There was an obvious phenomenon that the orange solution of FeCl3·6H2O became black after adding the EDOT alcoholic solution. The mixing solution was under vigorous stirring for 24 h, then filtered and washed by absolute ethyl alcohol and distilled water, and at last dried at 60°C for 24 h. The obtained polymer was noted as PEDOT (3:1). In a similar manner, the molar ratio of oxidant to monomer (represented by [FeCl3·6H2O]/[EDOT]) was adjusted at 6:1 and 9:1, respectively; the resultant polymers were noted as PEDOT (6:1) and PEDOT (9:1).
The FTIR spectra of the polymers were obtained using a Bruker Equinox 55 Fourier transform infrared spectrometer (Bruker, Billerica, MA, USA) (frequency range 4,000 to 500 cm-1). The UV-vis spectra of the samples were recorded on a UV-vis spectrophotometer (UV4802, Unico, Dayton, NJ, USA). Raman spectra were recorded in a backscattering geometry with a 1,064-nm excitation wavelength using a Bruker Vertex 70 FT infrared spectrometer (equipped with RamIIFT Raman Module). XRD patterns have been obtained using a Bruker AXS D8 diffractometer with a monochromatic Cu-Kα radiation source (λ = 0.15418 nm); the scan range (2θ) was 10° to 80°. Morphology and microstructure of the samples were investigated by FESEM (Hitachi S-4800, Hitachi Ltd., Chiyoda-ku, Japan). TEM measurements were performed on a TEM instrument (JEOL model 2100, JEOL Ltd., Tokyo, Japan).
The electrodes were prepared by mixing 85 wt.% active materials (3 mg), 10 wt.% carbon black, and 5 wt.% polytetrafluoroethylene (PTFE) to form a slurry. The slurry was pressed on a graphite current collector (area, 1 cm2) and then dried at 60°C for 24 h. All electrochemical experiments were carried out using a three-electrode system, in which the sample was used as the working electrode, platinum as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode, and 1 M H2SO4 was used as the electrolyte. The CV, galvanostatic charge/discharge tests were done in the potential window ranging from -0.2 to 0.8 V by using CHI 660C electrochemical workstation (CH Instruments Inc., Shanghai, China). EIS measurements were performed by using CHI 660C electrochemical workstation in the frequency range of 0.01 Hz to 100 kHz. The cycle life measurement of polymers was recorded by sequential CV cycling (over 1,500 cycles) at a scan rate of 10 mV/s.
Results and discussion
Figure 1b shows the UV-vis absorption spectra of PEDOTs. As seen from Figure 1b, the different molar ratios of oxidant to monomer bring different absorption spectra. All PEDOTs show two characteristic peaks at approximately 400 to 700 nm and approximately 1,000 to 1,060 nm with a free tail extending into the near-infrared region. The peaks at approximately 400 to 700 can be ascribed to the π-π* transitions of thiophene ring, whereas the peaks at approximately 1,000 to 1,060 nm can be attributed to polaron and/or bipolaron bands, which are characteristic of oxidized PEDOT with high conjugation length[25–28]. On comparison, decreasing of the [FeCl3·6H2O]/[EDOT] ratio causes a redshift in the π-π* absorption peak from 507 to 541 nm. This implies that the oxidation degree and conjugation length of PEDOT increase with decreasing of the [FeCl3·6H2O]/[EDOT] ratio. Moreover, the intensity ratio of the polaron and/or bipolaron band to the π-π* transition band (I1,000-1,060/I400-700) of the PEDOT (3:1) is higher than that of others, suggesting that the highest doping level occurs in the case of PEDOT (3:1)[25–28].
Figure 6b shows the galvanostatic charge/discharge curves of PEDOTs performed at a current density of 3 mA cm-2 in the three-electrode system between -0.2 and 0.8 V; the supporting electrolyte is 1 M H2SO4. The specific capacitance (SC) of the electrode materials is calculated by means of SC = (I × Δt)/(ΔV × m), where I is the charge/discharge current, Δt is the discharge time, ΔV is the electrochemical window (1 V), and m is the mass of active materials within the electrode (3 mg). The SC of PEDOT (3:1), PEDOT (6:1), and PEDOT (9:1) is 174, 132, and 124 F/g, respectively. This result is consistent with the CV analysis. It should be noted that the SC of PEDOT (3:1) is higher when compared with other reports[38–40]. Furthermore, PEDOT (3:1) also has both longer charge and discharge time and has a high charge/discharge efficiency (η) of 98.8%, while the charge/discharge efficiency of PEDOT (6:1) and PEDOT (9:1) is 81.9% and 83.0%, respectively. The results confirm that PEDOT (3:1) has a better capacitance behavior. This enhanced SC of the PEDOT (3:1) electrode can be attributed to the fact that the coral-like morphology of the polymer offers a higher specific surface area which is convenient for ions accessing into the polymer matrix and inducing higher charge to keep stable. And, the special coral structure provides broader room and shorter diffusion distances for ion transport between the electrolyte and PEDOT molecules.
All electrochemical testing suggests that PEDOT (3:1) has the promising electrocapacitive property among PEDOTs. This enhanced SC of the PEDOT (3:1) electrode can be attributed to the following reasons: (1) the highest polymerization degree, doping level, conjugation length, and crystallinity occurring in the case of PEDOT (3:1) among PEDOTs, which can enhance the conductivity of polymer chains, and (2) the coral-like morphology of PEDOT (3:1), which offers a high specific surface area for convenient ions accessing into the polymer matrix and inducing higher charge to keep stable. And, the special coral structure provides broader room and shorter diffusion distances for ion transport between the electrolyte and PEDOT molecules.
In this study, a simple one-step template-free solution method was developed for the preparation of PEDOTs with different morphologies by adjusting various ratios of oxidant (FeCl3·6H2O) to monomer (EDOT). The polymerization was carried out in a template-free alcoholic aqueous solution medium. The results showed that the alcoholic aqueous solution medium was beneficial for the dispersion of EDOT and the uniform contact between EDOT and FeCl3·6H2O, and this would increase the possibility of formation of PEDOT with high polymerization degree, conjugation length, doping level, and crystallinity. The results also showed that the coral-like morphology of PEDOT occurring in the case of the molar ratio of [FeCl3·6H2O]/[EDOT] at 3:1 mainly resulted from nanofibers growing together. And, with increasing content of FeCl3·6H2O, the branches of ‘coral’ would adjoin or grow together and consequently lead to a morphology of mixing up tentacles with sheets ([FeCl3·6H2O]/[EDOT] at 6:1) and a sheet-like morphology ([FeCl3·6H2O]/[EDOT] at 9:1). With the highest polymerization degree, conjugation length, and crystallinity, PEDOT prepared from [FeCl3·6H2O]/[EDOT] at 3:1 displayed a higher conductivity, a lower internal resistance, and a stable cycling performance. This enhanced behavior which could be attributed to the special coral structure will provide broader room and shorter diffusion distances for ion transport between the electrolyte and PEDOT molecules, which demonstrated that PEDOT with coral-like morphology displayed an excellent electrochemical capacitive behavior and could be used as an electrode material for supercapacitors.
We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21264014, 21464014).
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