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A Strategy to Enhance the Electrode Performance of Novel Three-Dimensional PEDOT/RVC Composites by Electrochemical Deposition Method Ali Aldalbahi 1, *, Mostafizur Rahaman 1 and Mohammed Almoiqli 2 1 2

*

Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia; [email protected] Nuclear Sciences Research Institute, King Abdulaziz City for Science and Technology, Riyadh 12371, Saudi Arabia; [email protected] Correspondence: [email protected]; Tel.: +966-114-697-526

Academic Editor: Do-Hoon Hwang Received: 27 March 2017; Accepted: 25 April 2017; Published: 28 April 2017

Abstract: In this article, three-dimensional (3D) microstuctured poly(3,4-ethylenedioxythiophene) (PEDOT)/reticulated vitreous carbon (RVC) composite electrodes with varying amount of PEDOT loadings were successfully prepared by electrochemical deposition method. The composites were characterized by Raman spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and cyclic voltammetry. Raman spectra suggest that there is a strong interaction between the RVC and backbone of PEDOT chain. It is revealed from the SEM images that the PEDOT amount, thickness, surface roughness, porosity, and globular structure on RVC electrode are increased with the increase in polymerization time. The capacitance of PEDOT/RVC electrode has increased by a factor of 2230 compared to a bare RVC electrode when polymerization is carried out for 120 min. Moreover, the capacitance of PEDOT was found to be very high compared with other PEDOT studies. The electrodes also show good cyclic stability. This substantial increase in capacitance of RVC electrode is due to the rough, highly porous, and honeycomb-like fine structure of PEDOT coating, which shows a flower-like morphology, consisting of numerous thin flakes with numbers of macropores and micropores. This interesting morphology has enhanced the performance of PEDOT because of increased electrode surface area, specific capacitance, and macroporous structure of RVC electrode. Keywords: PEDOT/RVC composites; electrochemical deposition; electrode performance; morphology; thermal stability; cyclic voltammetry

1. Introduction Nowadays, many researchers have focused on the development of novel carbon materials [1–9] such as activated carbon, carbon aerogel, carbon nanotubes, graphene, ordered mesoporous carbon, and their composites to make electrodes with high specific surface area, high conductivity, reasonable microstructured pores, and high electrosorption capacity. In addition, conducting polymers have been combined with such carbon materials to improve the carbon electrode properties and performance [10–12] because they have high electrical conductivity and high surface area [13,14]. All these electrodes were built as two-dimensional structure electrodes without significant thickness. However, a limited improvement in the electrosorption capacity is still far less than the theoretical value, which is mainly due to shortcomings such as the low effective surface area [15]. The conducting polymer PEDOT (poly(3,4-ethylenedioxythiophene)) is an active material for the preparation of an electrode because of its interesting properties such as high electrical conductivity,

Polymers 2017, 9, 157; doi:10.3390/polym9050157

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surface area, environmental stability [16], facile synthesis via electrochemical polymerization, etc. In addition, a novel PEDOT material can be used as a three-dimensional porous electrode with well-interconnected macropores, and numerous mesopores and micropores embedded in the reticulated vitreous carbon (RVC) walls because this improves ion diffusion by providing a decreased ion diffusion distance [15]. PEDOT has also been used in bioelectronics for coating on medical devices and in making highly performed electrochemical transistor [17,18]. The presence of macropores, serving as ion-buffering reservoirs, guarantees a shorter ion diffusion distance [19,20], which facilitates the rapid transportation of the ions into the interior of the bulk material. Moreover, it has been demonstrated that 3D porous carbon with an interconnected pore system shows an excellent performance in the field of electrochemistry [20–25]. RVC as a substrate was used for PEDOT film deposition because it has three-dimensional (3D) porous structure that provides high macroscopic surface area and other interesting characteristics such as a low density, high chemical inertness with low electrical and fluid flow resistance, and is also amenable to surface modification with conducting polymers [26–29]. This article describes and discusses the electrodeposition of various amounts of PEDOT to coat RVC electrodes. These electrodes were characterized for their morphology, Raman spectrum, thermal stability, and electrochemical properties. 2. Materials, Methods, and Experimental 2.1. Chemicals and Materials Commercial 3,4-ethylenedioxythiophene monomer (EDOT) with purity 99.9% was purchased from Sheng Chemical Ltd. (Taichung, Taiwan), and was used as received. The following chemicals, obtained from Sigma-Aldrich (Darmstadt, Germany), were also used as received: acetonitrile (ACN) (AR grade), lithium perchlorate (LiClO4 ) (AR grade), concentrated nitric acid (70%) and sodium chloride (AR grade). The reticulated vitreous carbon (RVC) (60 ppi (normal pores per linear inch)) was purchased from ERG Materials and Aerospace Engineering and used as received. Milli-Q water with a resistivity of 18.2 mΩ cm−1 was used in all preparations. 2.2. Pre-Treatment of the RVC Electrode All reticulated vitreous carbon (RVC) electrodes (length 4 cm × width 3.5 cm × thickness 0.3 cm (32.5 or 4.2 cm3 ) were cut from a block of RVC material, and soaked in 2 M HNO3 for 24 h to remove any impurities [30]. Electrodes were thoroughly washed with distilled water to remove the acid. The pH of the effluent was checked periodically until the wash became neutral. All RVC electrodes were then soaked in methanol for 2 h to remove any organic impurities [30]. The RVC electrodes were dried under a brief flow of nitrogen and kept in an oven at 110 ◦ C overnight. All RVC electrodes were weighed after drying. 2.3. Electrochemical Polymerization of PEDOT on RVC Electrode In this work, the PEDOT/RVC composite electrodes were synthesized by cyclic voltammetric and chronoamperometry. In both techniques before polymerizations, the RVC pieces were left in contact with the working electrolyte for at least 24 h to ensure their complete wetting. In order to prepare the working electrodes, electrical contact was made by a hook of Pt wire. The electropolymerization was performed in an organic electrolyte, where the reference electrode was an Ag/AgCl (3 M NaCl) electrode and the counter electrode was a Pt mesh with size 4 × 4 cm2 . The electrolyte was an acetonitrile solution containing 0.01 M monomer, EDOT and 0.1 M supporting electrolyte salt (LiClO4 ) [31,32]. The solution was thoroughly deoxygenated by nitrogen (N2 ) before the electropolymerization for 10 min prior to all electrochemical experiments at room temperature. PEDOT was deposited on the RVC working electrode by cyclic voltammetry using three-electrode systems in the voltage range between 0 and 1.3 V at 50 mV/s scan rate. In addition, PEDOT films were

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galvanostatically deposited on the RVC working electrode with a constant voltage applied for various periods of time. The quantity of PEDOT-ClO4 coating on the RVC electrodes was determined by calculating the total charge passed in the electropolymerization. The charge values were read directly from the I to V curves by computer. 2.4. Physical Characterization The thermal stability of the PEDOT on PEDOT/RVC electrodes and the amount of PEDOT coated on the RVC were determined by thermogravimetric analysis (TGA). Experiments were performed using a Q500 (TA Instruments, New Castle, DE, USA) apparatus at a ramp rate of 5 ◦ C/min in air, with a combined gas flow of 10 mL min−1 Nitrogen (N2 ) and 90 mL min−1 air from 25 to 750 ◦ C. Moreover, the morphology and the thickness of the PEDOT deposits on RVC electrodes were analyzed by using a field emission scanning electron microscope (FESEM, ZEISS Sigma, Hamburg; Germany) at specific voltages of 0.5 KV. Furthermore, Raman spectra were measured on a Raman spectrometer equipped with a visible Raman microscope and CCD detector. The excitation wavelength was 632.81 nm and spectra were obtained over 30 s at 1.0 cm−1 resolution. 2.5. Electrochemical Characterization The capacitance and the effect of different scan rates were determined by cyclic voltammetry (CV). A PEDOT/RVC composite electrode was used as the working electrode (WE) in 1 M NaCl aqueous solution and scanned in the voltage range between −0.2 to 0.8 V using a three-electrode system; RVC electrode and Ag/AgCl (3 M NaCl) were used as counter electrode (CE) and reference electrode (RE), respectively. The scan rates range from 5 to 200 mV/s. Contacts to the WE and CE were made using Pt wire. 3. Results and Discussion 3.1. PEDOT Deposited on RVC Electrode PEDOT can be polymerized using multiple different ways, but for the purposes of this work, electrochemical polymerization has been used to synthesize PEDOT. This method is important because it requires only a small amount of monomer, short polymerization time, and can yield both electrode-supported and free-standing film. This method utilizes electrochemical oxidation of the electron-rich EDOT-based monomers by three different techniques; namely, cyclic voltammetry, chronoamperometry and chronopotentiometry. In this project, chronoamperometry method was selected to grow thick PEDOT coated RVC electrodes and cyclic voltammetry was used to select the potential of EDOT monomer oxidation to form PEDOT. 3.2. Cyclic Voltammetry EDOT monomer was prepared in acetonitrile as electrolyte because it has the advantage of a higher conductivity than that prepared in aqueous solution [33]. Figure 1 shows the cyclic voltammograms related to the EDOT. It can be seen from the figure that the EDOT oxidation started at 1.1 V for the anodic scan. Furthermore, the EDOT electropolymerization is characterized by cycles with a crossover of the reverse cathodic scan over the anodic scan, giving rise to what has been called the “nucleation loop” [34]. In particular, we observe that the current of the reverse scan is higher than that of the forward scan in the region close to the switching potential, but it drops again to the level of the current–voltage curve of the forward scan. Such kind of CV profiles have been interpreted as due to polymer nucleation effects [35] or, more recently, to homogeneous reactions between an oligomeric follow-up product and the starting monomer [36]. Moreover, an increase of both anodic and cathodic current intensities with increasing number of scans is also evident. This effect can be explained by considering the growth at the electrode surface of an electroactive polymer film, whose thickness increases regularly with the number of cycles [37]. Moreover, the RVC foam electrode was

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observed after polymerization to have a visible bluish color on the foam skeleton and this has been reported in available literature [38]. The polymerization mechanism of PEDOT has been described in Supplementary Materials. Polymers 2017, 9, 157 4 of 18

Figure 1. Cyclic voltammetry of RVC electrode in a solution containing 0.01 M EDOT and 0.1 M Figure 1. Cyclic voltammetry of RVC electrode in a solution containing 0.01 M EDOT and 0.1 M LiClO4 LiClO4 in acetonitrile, using a three-electrode system; RVC electrode and Ag/AgCl (3 M NaCl) were in acetonitrile, using a three-electrode system; RVC electrode and Ag/AgCl (3 M NaCl) were used as used as counter electrode (CE) and reference electrode (RE), respectively. counter electrode (CE) and reference electrode (RE), respectively.

3.3. Effect of Applied Constant Potential on PEDOT Electrosynthesis 3.3. Effect of Applied Constant Potential on PEDOT Electrosynthesis Chronoamperometry was used to deposit PEDOT on RVC electrodes. The aim of this Chronoamperometry to deposit PEDOT on electrodes. The aim this experiment experiment is to studywas theused effect of increasing theRVC constant potential on of electrochemical is to study the effect potential on electrochemical and to polymerization andoftoincreasing determinethe theconstant best constant potential to be used for polymerization further experiments. determine the best constant potential to be for of further experiments. These1)potentials selected These potentials were selected based on used the CV PEDOT growth (Figure obtained were previously that on showed no of over-oxidation of the polymer because polymer growth to increase with based the CV PEDOT growth (Figure 1) obtained previously thatcontinued showed no over-oxidation number of cycles. Current–time curves fortothe electrodeposition of EDOT by potential of increasing the polymer because polymer growth continued increase with increasing number of cycles. steps 1.1, 1.2curves and 1.3for V are in Figure 2. of AllEDOT of these attain to the charge consumed Current–time the shown electrodeposition bycurves potential steps 1.1,same 1.2 and 1.3 V are shown of 48 C.2. ItAll is of clear that the general of charge these curves are similar reported the in Figure these curves attain tofeatures the same consumed of 48 C.toIt those is clear that theingeneral literature [39–41]. The current starts to increase because the molecules of monomer diffuse from the to features of these curves are similar to those reported in the literature [39–41]. The current starts solution to the electrode surface and, once they are oxidized, return to the solution where increase because the molecules of monomer diffuse from the solution to the electrode surface and,the once oligomerization process occurs in the vicinity of the electrode surface. When an oligomeric high they are oxidized, return to the solution where the oligomerization process occurs in the vicinity of the density region is established, clusters are deposited onto the electrode creating the growing nuclei. electrode surface. When an oligomeric high density region is established, clusters are deposited onto After that, the current increases until it reaches a current plateau. This region is generally attributed the electrode creating the growing nuclei. After that, the current increases until it reaches a current to nucleation and growth. The phenomena of nucleation and growth of PEDOT were also observed plateau. This region is generally attributed to nucleation and growth. The phenomena of nucleation and reported in published literature [42]. Figure 2 also shows that PEDOT electrodeposition on RVC and growth of PEDOT were also observed and reported in published literature [42]. Figure 2 also at a constant voltage of 1.1 V was too slow and it took time (around 160 min) to attain a charge shows that PEDOT electrodeposition on RVC at a constant voltage of 1.1 V was too slow and it took consumed of 48 C. It can be observed that when the constant potential applied was increased from time (around 1601.3min) to attain charge consumed of 48This C. Itled can observedinthat the constant 1.1 to 1.2 and V, the currenta increased as expected. to be a decrease the when time required to potential applied was increased from 1.1 to 1.2 and 1.3 V, the current increased as expected. electrodeposit PEDOT on the RVC. For example, the same RVC electrode at a constant potential This of led1.3 toVa required decrease20 inmin the to time required to electrodeposit PEDOT onItthe RVC. Forthe example, the same attain the same charge consumed of 48 C. is clear that polymerization RVC electrode at a constant potential of 1.3 V required 20 min to attain the same charge consumed time decreased 8 times at 1.3 V compared with that at 1.1 V. In addition, when the potential of of deposition 48 C. It is is clear that the timesurface decreased 8 times at 1.3 V compared with that increased, thepolymerization porosity of PEDOT morphology is increased [43]. According to at 1.1these V. Inreasons, addition, thevoltage potential the potential porositytoofdeposit PEDOT surface the when constant of 1.3ofVdeposition was selectedisasincreased, the optimum PEDOT on RVC electrode using[43]. the potentiostatic in the work earlier. of 1.3 V was selected as morphology is increased According to mode these reasons, thepresented constant voltage the optimum potential to deposit PEDOT on RVC electrode using the potentiostatic mode in the work presented earlier.

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Figure 2. Chronoamperometric curves at various constant potential applied to coat RVC electrodes Figure 2. Chronoamperometric curves at various constant potential applied to coat RVC electrodes by by PEDOT in a solution containing 0.01 M EDOT and 0.1 M LiClO4 in acetonitrile, using a PEDOT in a solution containing 0.01 M EDOT and 0.1 M LiClO4 in acetonitrile, using a three-electrode three-electrode system; RVC electrode and Ag/AgCl (3 M NaCl) were used as counter electrode (CE) system; RVC electrode and Ag/AgCl (3 M NaCl) were used as counter electrode (CE) and reference and reference (RE), respectively. electrode (RE),electrode respectively.

3.4. Electrodeposition of Different Amounts of PEDOT on RVC Electrodes 3.4. Electrodeposition of Different Amounts of PEDOT on RVC Electrodes The PEDOT films were electropolymerized onto (4 × 3.5 × 0.3 cm3) RVC electrodes by applying The PEDOT films were electropolymerized onto (4 × 3.5 × 0.3 cm3 ) RVC electrodes by applying a constant potential of 1.3 V vs. Ag/AgCl until polymerization times of 10, 20, 50, 70 and 120 min a constant potential of 1.3 V vs. Ag/AgCl until polymerization times of 10, 20, 50, 70 and 120 min were reached. The aim of these experiments is to increase the amount of PEDOT on the RVC. Figure were reached. The aim of these experiments is to increase the amount of PEDOT on the RVC. Figure 3 3 shows a combination of chronoamperometric curves of PEDOT deposition on RVC electrodes and shows a combination of chronoamperometric curves of PEDOT deposition on RVC electrodes and the the arrows indicate the polymerization times for each electrode. It is clear that the arrows indicate the polymerization times for each electrode. It is clear that the chronoamperogram chronoamperogram started at 0.19 A, then immediately rose sharply to 0.32 A within the first 5 min. started at 0.19 A, then immediately rose sharply to 0.32 A within the first 5 min. After that, the curve After that, the curve increased dramatically to reach 0.62 A after 35 min. The chronoamperometric increased dramatically to reach 0.62 A after 35 min. The chronoamperometric curve plateaued for curve plateaued for 18 min then started to decrease steadily to 0.46 A when finally the current 18 min then started to decrease steadily to 0.46 A when finally the current became almost stable. became almost stable. The charge consumed at each electrode was read directly from the I to V The charge consumed at each electrode was read directly from the I to V curve by computer and it curve by computer and it was 20.64, 46.04, 112.71, 185.73 and 380.98 Coulombs (C) for was 20.64, 46.04, 112.71, 185.73 and 380.98 Coulombs (C) for polymerization times 10, 20, 50, 70 and polymerization times 10, 20, 50, 70 and 120 min, respectively. Table 1 shows the polymerization 120 min, respectively. Table 1 shows the polymerization time, charge passed through the electrode, time, charge passed through the electrode, and the mass of PEDOT coating on the RVC electrode and the mass of PEDOT coating on the RVC electrode calculated using the following Equation (1) [44]: calculated using the following Equation (1) [44]:  ( Q × MEDOT ) + Q × γ × MClO4 ( × ) ( ×  × ) m= (1) (1) m= n × × F where, m is mass (g); MEDOT is molecular weight (142.16 g/mol) of EDOT; MClO4 is molecular weight is molecular weight (142.16 g/mol) of EDOT; is molecular where, m is mass (g); (99.45 g/mol) of ClO4 ; Q is charge passed (C) on the working electrode; n is the number of electrons weight (99.45 g/mol) of ClO4; Q is charge passed (C) on the working electrode; n is the number of transferred, which equals to 2 + γ, where two electrons are associated with polymerization and γ electrons transferred, which equals to 2 + γ, where two electrons are associated with polymerization electrons are associated with doping of one monomer unit in PEDOT; and F is the faraday constant and γ electrons are associated with doping of one monomer unit in PEDOT; and F is the faraday (96,485.34 C). The doping level γ can be determined from the relation Qmax = (γ/(2 + γ))Q, where constant (96,485.34 C). The doping level γ can be determined from the relation Qmax = (γ/(2 + γ))Q, Qmax is the charge density at the maximum quantity of oxidized polymer [45]. The value of Qmax is where Qmax is the charge density at the maximum quantity of oxidized polymer [45]. The value of calculated by integrating the current from its initial potential to the potential just after anodic peak. Qmax is calculated by integrating the current from its initial potential to the potential just after anodic The plot of Qmax vs. Q gives a linear line with the slope γ/(2 + γ). This results γ = 0.45, and hence peak. The plot of Qmax vs. Q gives a linear line with the slope γ/(2 + γ). This results γ = 0.45, and hence the value of n = 2.45. It is seen from the table that the PEDOT mass in each electrode increased with the value of n = 2.45. It is seen from the table that the PEDOT mass in each electrode increased with increase in polymerization time. It was 13, 29, 71, 117 and 240 mg for polymerization times 10, 20, 50, increase in polymerization time. It was 13, 29, 71, 117 and 240 mg for polymerization times 10, 20, 50, 70 and 120 min, respectively. 70 and 120 min, respectively.

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Figure 3.Figure Combination chronoamperogram V constant constant potential obtained RVC electrodes 3. Combination chronoamperogramatat1.3 1.3 V potential obtained at RVCatelectrodes 3 a solution containing containing 0.010.01 M EDOT 0.1 Mand LiClO0.1 three-electrode using a (4.2 cm3(4.2 ) incm a) insolution M and EDOT M LiClOusing 4 in aacetonitrile, 4 in acetonitrile, system; RVC electrode and Ag/AgCl (3 M NaCl) were used as counter electrode (CE) and reference three-electrode system; RVC electrode and Ag/AgCl (3 M NaCl) were used as counter electrode (CE) electrode (RE), respectively (arrows with number 1, 2, 3, 4, and 5 indicate the polymerization time for and reference electrode (RE), respectively (arrows with number 1, 2, 3, 4, and 5 indicate the each electrode). polymerization time for each electrode). Table 1. Charge consumed during polymerization, polymerization time and mass of PEDOT coating RVC electrode calculated by Equation (1). Table 1. on Charge consumed during polymerization, polymerization time and mass of PEDOT coating on RVC electrode calculated by Equation (1). Sample

Polymerization time (min)

Charge consumed (C)

Mass of PEDOT-ClO4 (mg)

10 20.64 13 PEDOT-ClO4 Polymerization Charge consumed Mass of 20 46.04 29 time (min) (C) (mg) 50 112.71 71 20.64 70 10 185.73 117 13 120 380.98 240 PEDOT-20 min/RVC 20 46.04 29 PEDOT-50 min/RVC 50 112.71 71 3.5. PEDOTmin/RVC Surface Properties PEDOT-70 70 185.73 117 PEDOT-120 min/RVC 120 was used to collect 380.98 Scanning electron microscopy (SEM) information about PEDOT240 formation PEDOT-10 min/RVC Sample PEDOT-20 min/RVC PEDOT-50 min/RVC PEDOT-10 PEDOT-70min/RVC min/RVC PEDOT-120 min/RVC

and its morphology along the RVC thickness. Figure 4 shows the SEM micrographs for 10 min electrodeposited PEDOT on RVC electrode. Figure 4a confirms that 10 min polymerization was enough 3.5. PEDOT Surface Properties to uniformly cover PEDOT on the RVC electrode and the average size of the pores of 60 ppi RVC is about 350 µm. It ismicroscopy clear that the(SEM) surface was of PEDOT is rough (Figure 4b)about and highly porous Scanning electron usedcoating to collect information PEDOT formation (Figure 4c). This morphology can enhance the performance of PEDOT because of high conductivity, and its morphology along the RVC thickness. Figure 4 shows the SEM micrographs for 10 min increased electrode surface area and specific capacitance, and macroporous structure of RVC electrode. electrodeposited PEDOT on RVC electrode. Figure 4a confirms that 10 min polymerization was Figure 4d shows the cross-section of a region of PEDOT/RVC composite electrode and it can be enough to uniformly cover PEDOT onofthe RVCcovering electrode average size of the pores of 60 ppi observed that the average thickness PEDOT the and RVC the is around 280 nm. RVC is aboutFigure 350 5µm. It aisSEM clear that the surface of PEDOT coating is rough (Figure 4b) and highly shows of PEDOT coated RVC electrode prepared with 120 min electropolymerization of EDOT. 4c). It is This very clear that the original pores of the electrode about µm size are not of high porous (Figure morphology can enhance theRVC performance of 350 PEDOT because significantly affected by the PEDOT coating, and the average size of the composite electrode poresstructure conductivity, increased electrode surface area and specific capacitance, and macroporous have become about 320 µm (Figure 5a). The PEDOT possesses an extraordinary fuzzy like and loose of RVC electrode. Figure 4d shows the cross-section of a region of PEDOT/RVC composite electrode structure with honeycomb-like fine structure. The fine structure of PEDOT shows a flower-like and it can be observed that the average thickness of PEDOT covering the RVC is around 280 nm. morphology, consisting of numerous thin flakes with a number of fuzziness as seen in Figure 5b,c.

Figure 5 shows a SEM of PEDOT coated RVC electrode prepared with 120 min electropolymerization of EDOT. It is very clear that the original pores of the RVC electrode about 350 µm size are not significantly affected by the PEDOT coating, and the average size of the composite electrode pores have become about 320 µm (Figure 5a). The PEDOT possesses an extraordinary fuzzy like and loose structure with honeycomb-like fine structure. The fine structure of PEDOT

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The appearance of this structure is assumed to be related to the fast kinetics and will be useful for use as a capacitive deionization (CDI) electrode. Polymers 2017, 9, 157 7 of 18 Polymers 2017, 9, 157

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Figure 4. SEM of PEDOT coated RVC electrode for 10 min electropolymerization at 1.3 V: top surface Figure SEM of of PEDOT PEDOT coated coated RVC RVC electrode electrode for for 10 10 min min electropolymerization electropolymerization at at 1.3 1.3 V: V: top top surface surface Figure 4. 4. SEM (a–c); and cross-section (d). (a–c); and cross-section (d). (a–c); and cross-section (d).

Figure 5. SEM of PEDOT coated RVC electrode for 120 min electropolymerization at 1.3 V, showing Figure 5. SEM of PEDOT coated RVC electrode for 120 min electropolymerization at 1.3 V, showing the top5.surface ofPEDOT (a) porecoated size; and fuzzy for like120 morphology. Figure SEM of RVC(b,c) electrode min electropolymerization at 1.3 V, showing the the top surface of (a) pore size; and (b,c) fuzzy like morphology. top surface of (a) pore size; and (b,c) fuzzy like morphology.

The morphology of the electrosynthesized PEDOT on RVC electrode is affected by The morphology of the electrosynthesized PEDOT on RVC electrode is affected polymerization time.ofThe PEDOT amount PEDOT increased withelectrode increased polymerization time, by as The morphology the electrosynthesized on RVC is affected by polymerization polymerization time. The PEDOT amount increased with increased polymerization time, as discussed Figure 6 shows SEM with images of the strut of the RVC time, electrode after coating by time. Theabove. PEDOT amount increased increased polymerization as discussed above. discussed above. Figure 6 shows SEM images of the strut of the RVC electrode after coating by PEDOT6 at various times.ofItthe is RVC clear electrode that PEDOT relatively smoother surface Figure shows SEMpolymerization images of the strut afterof coating by PEDOT at various PEDOT at various polymerization times. It is clear that PEDOT of relatively smoother surface covered all the strut of It RVC afterthat 10 min polymerization 6a) surface and the covered surface roughness was polymerization times. is clear PEDOT of relatively(Figure smoother all the strut of covered all the strut of RVC after 10 min polymerization (Figure 6a) and the surface roughness was increased with increased polymerization time, as seen in Figure 6c,e,g,i. After 20 min RVC after 10 min polymerization (Figure 6a) and the surface roughness was increased with increased increased with increased polymerization time, as seen in Figure 6c,e,g,i. After 20 min polymerization,time, a globular structure of 6c,e,g,i. PEDOT After started Moreover, the globular structure structure of of polymerization as seen in Figure 20growing. min polymerization, a globular polymerization, a globular structure of PEDOT started growing. Moreover, the globular structure of PEDOT started increased upon increasing the globular polymerization time and also the fuzzy likeincreasing morphology PEDOT growing. Moreover, the structure of PEDOT increased upon the PEDOT upon increasing the polymerization time and also the fuzzy like morphology increasedincreased as a time function time. The thicknesses of aPEDOT on RVC polymerization and of alsopolymerization the fuzzy like morphology increased as functiondeposited of polymerization increased as a function of polymerization time. The thicknesses of PEDOT deposited on RVC electrodes were measured from the SEM image (Figure 6b,d,f,h,j). thicknesses were time. The thicknesses of PEDOT deposited on RVC electrodes wereThe measured fromof thePEDOT SEM image electrodes were measured from the SEM image (Figure 6b,d,f,h,j). The thicknesses of PEDOT were 0.28, 0.61, 1.12, 2.53The andthicknesses 4.64 µm at of 10,PEDOT 20, 50, 70 and0.28, 120 min (Figure 6b,d,f,h,j). were 0.61,electropolymerization, 1.12, 2.53 and 4.64 µmrespectively. at 10, 20, 50, 70 0.28, 0.61, 1.12, 2.53 and 4.64 µm at 10, 20, 50, 70 and 120 min electropolymerization, respectively. and 120 min electropolymerization, respectively.

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Figure 6. The top surface and cross-section of PEDOT coated RVC strut: for 10 min polymerization Figure 6. The top surface and cross-section of PEDOT coated RVC strut: for 10 min polymerization (a,b); for 20 min polymerization (c,d); for 50 min polymerization (e,f); for 70 min polymerization (a,b); for 20 min polymerization (c,d); for 50 min polymerization (e,f); for 70 min polymerization (g,h); (g,h); and for 120 min polymerization (i,j). and for 120 min polymerization (i,j).

3.6. Thermogravimetric Analysis 3.6. Thermogravimetric Analysis The thermal stability and the amount of PEDOT coated on RVC electrodes were determined The thermal stability and the amount of PEDOT coated on RVC electrodes were determined from the thermogravimetric analysis (TGA) curves which give the dependence of the weight loss of from the thermogravimetric analysis (TGA) curves which give the dependence of the weight loss of a a sample as a function of temperature or time. Figure 7 shows TGA curves of pure RVC electrode sample as a function of temperature or time. Figure 7 shows TGA curves of pure RVC electrode and and various PEDOT/RVC composite electrodes. It can be seen that the TGA curve of the RVC various PEDOT/RVC composite electrodes. It can be seen that the TGA curve of the RVC electrode electrode exactly matches those reported in the literature [28,46], where 10% of the initial weight loss exactly matches those reported in the literature [28,46], where 10% of the initial weight loss occurred occurred between 25 and 250 °C, then it was stable and did not show a dramatic decomposition in between 25 and 250 ◦ C, then it was stable and did not show a dramatic decomposition in the tested the tested temperature range between 250 and 550 °C followed by a major weight loss between 550 temperature range between 250 and 550 ◦ C followed by a major weight loss between 550 and 700 ◦ C. and 700 °C. The TGA curve of the PEDOT/RVC composite electrode consisted of three stages [47,48]: The TGA curve of the PEDOT/RVC composite electrode consisted of three stages [47,48]: volatilization, volatilization, decomposition of PEDOT and carbonization reaction. All PEDOT/RVC composite decomposition of PEDOT and carbonization reaction. All PEDOT/RVC composite electrodes followed electrodes followed the same behavior. The first 10% weight loss almost happened up to the the same behavior. The first 10% weight loss almost happened up to the temperature of 250 ◦ C (Stage 1). temperature of 250 °C (Stage 1). This was probably due to desorption of moisture and contaminant. This was probably due to desorption of moisture and contaminant. Furthermore, the major weight Furthermore, the major weight loss between 550 and 700 °C (Stage 3) was most likely due to loss between 550 and 700 ◦ C (Stage 3) was most likely due to carbonization by breaking chemical carbonization by breaking chemical bonds such as C–H. It is clear that all composite electrodes followed a similar decomposition with temperature in Stage 2. From 250 °C a continuous degradation occurs until major decomposition occurred in the region between 350 and 550 °C.

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Polymers 2017,such 9, 157as C–H. It is clear that all composite electrodes followed a similar decomposition with9 of 18 bonds

temperature in Stage 2. From 250 ◦ C a continuous degradation occurs until major decomposition ◦ C.the Therefore, in in Stage 2, the between amount 350 of PEDOT variousincomposite electrodes calculated occurred the region and 550 in Therefore, Stage 2, the amount ofwere PEDOT in the by loss various in weight due to decomposition, and found bein4%, 9%, 20%, and 56% forand 10,found 20, 50, 70 composite electrodes were calculated byto loss weight due to31% decomposition, to be 4%, 9%, 20%, 31% and 56% for 10, 20, 50, 70 and 120 min electropolymerization, respectively. and 120 min electropolymerization, respectively. In addition, almost no weight loss occurred after In addition, almost no weight loss occurred after 700 ◦ C. 700 °C.

Figure 7. TGA curves of of pure andvarious variousPEDOT/RVC PEDOT/RVC composite electrodes. Figure 7. TGA curves pureRVC RVCelectrode electrode and composite electrodes.

3.7. Raman Spectroscopy 3.7. Raman Spectroscopy The The Raman spectrum theRVC RVC substrate is shown in8 Figure and iswith in keeping Raman spectrum of of the substrate is shown in Figure and is in8keeping publishedwith published RVC electrodes [49,50]. It two-band has a typical two-band spectrum of disordered reportsreports for RVC for electrodes [49,50]. It has a typical spectrum of disordered polycrystalline −1 is called the D-band −1 and noncrystalline graphitic carbons. The first band at 1360 cm and the second polycrystalline and noncrystalline graphitic carbons. The first band at 1360 cm is called the D-band − 1 −1 is bandsecond at 1600band cm atis1600 calledcm the G-band which is attributed to is theattributed graphite basal and the called(graphitic) the G-band (graphitic) which to theplane. graphite figure alsofigure showsalso the shows Raman the spectra of RVC electrode afterelectrode polymerizations that afford some that basalThe plane. The Raman spectra of RVC after polymerizations information about the PEDOT structure. They confirm that the surface of RVC electrodes coatedofbyRVC afford some information about the PEDOT structure. They confirm that the was surface PEDOT. The principal assignments of the main bands have been made considering the data reported electrodes was coated by PEDOT. The principal assignments of the main bands have been made in the literature (Table 2) [34,51–53]. Raman spectra of composite electrodes have seven strong bands considering the data reported in the literature (Table 2) [34,51–53]. Raman spectra of composite that dominate the spectrum which are related to the PEDOT vibrational spectrum. The most intense electrodes seven bands that dominate the spectrum are related to the PEDOT peak is have at 1423 cm−1strong which can be assigned to the symmetric Cα =Cwhich β (–O) stretching. Asymmetric −1 −1 and −1 be vibrational spectrum. Thefrom most intense is atcm 1423 cm atwhich can assigned to the symmetric C=C stretching shifts 1509 cm−1peak to 1507 1364 cm appears a peak related to −1 to 1507 cm−1 and at − Cα=CCβα(–O) stretching. Asymmetric C=C stretching shifts from 1509 cm =Cβ stretching. The asymmetric Cα –Cα – (inter-ring) stretching band is located at 1257 cm 1 .1364 −1 cm The appears a peak related which to Cα=C β stretching. –Cα– (inter-ring) C–O–C deformation, appears at 1152, The 1120asymmetric and 1085 cm−C1αpeaks, combines asstretching one peak atband − 1 − 1 −1 −1 peaks, 1098 cm . Oxyethylene deformation peaks appear 988 andat570 cm 1120 . In addition, is located at 1257 cm . Thering C–O–C deformation, which at appears 1152, and 1085the cmother − 1 −1. Oxyethylene peaks as observed in the Raman of PEDOT coated RVC electrode at 848 at cm988(related combines one peak at 1098 cmspectrum ring deformation peaksare appear and 570tocm−1. 0 polymerization) and 685 cm−1 (symmetric C–H bending of 2,3,5-trisubstituted thiophene due to α,α In addition, the other peaks observed in the Raman spectrum of PEDOT coated RVC electrode are at C–S–C deformation). results that RVC electrodes do notdue affect structure. and −1 (related 848 cm to C–HThese bending ofindicate 2,3,5-trisubstituted thiophene to the α,α′PEDOT polymerization) 685 cm−1 (symmetric C–S–C deformation). These results indicate that RVC electrodes do not affect the PEDOT structure.

Sym Cα=Cβ(–O) str Cβ=Cβ str Cα=Cα str Cα=Cα str Polymers 2017, 9, 157

1426 1365 1252 1236

ClO4– C–H bending Sym C–S–C def Oxyethylene ring def

933 806 690 572

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Key: str: stretching; def: deformation.

Figure 8. Raman spectra of of PEDOT-ClO chronoamperometry V acetonitrile in acetonitrile Figure 8. Raman spectra PEDOT-ClO4 4grown grown by by chronoamperometry at at 1.31.3 V in 4 . containing 0.01 M EDOT and 0.1 M LiClO containing 0.01 M EDOT and 0.1 M LiClO4 . TableCharacterizations 2. Calculated Raman band wavenumbers and vibrational assignments of PEDOT. 3.8. Electrochemical

Cyclic voltammetry was used tocmevaluate the electrochemical properties of the −1 1 Description of the vibration Description of the vibration cm−PEDOT/RVC composite electrodes. In this section, the effect of coatingC–O–C RVC def by PEDOT on 1152, electrode capacitance, asym C= Cstr 1509 1120 and 1085 the effect ofCH increasing electron transfer and capacitance, and the 988 stability of the Oxyethylene ring def 2 Scissoring scan rate on1477 – Sym C =C (–O) str 1426 ClO 933 α β 4 electrode are considered. The specific capacitance of a PEDOT coating on RVC electrodes was Cβ =Cβ str 1365 C–H bending 806 calculated according to the following equations [54,55]:Sym C–S–C def Cα =Cα str 1252 690 Cα =Cα str

1236

Oxyethylene ring def

Cvolume = Q/(2 × Z × ΔV)

572

Key: str: stretching; def: deformation.

3.8. Electrochemical Characterizations

(2)

Cmass = Q/(2 × m × ΔV)

(3)

Carea = Q/(2 × A × ΔV)

(4)

Cyclic voltammetry was used to evaluate the electrochemical properties of the PEDOT/RVC

where Cvolume, Celectrodes. mass and Carea are capacitance of electrode in term of F/cm3, F/g, and F/cm2, respectively; Q composite In this section, the effect of coating RVC by PEDOT on electrode capacitance, the 3); A is geometric area (cm2); and V is voltage (V). is charge (C); m is mass (g); Z is on geometric (cmcapacitance, effect of increasing scan rate electron volume transfer and and the stability of the electrode are considered. The specific capacitance of a PEDOT coating on RVC electrodes was calculated according

3.9. Comparison between RVC before to the following equations [54,55]:and after PEDOT Coating Figure 9a shows the cyclic voltammograms of 1 cm3 RVC electrode and same electrode coated Cvolume = Q/(2 × Z × ∆V) (2) by PEDOT (PEDOT-120 min/RVC electrode) at the scan rate of 5 mV/s. It is clear that the CV curve of RVC compared with PEDOT coated electrode is very small. The capacitance of the RVC electrode Cmass = Q/(2 ×m × ∆V) (3) in 3 terms of geometric volume, calculated using Equation (2), is 0.002 F/cm . The current of PEDOT-120 Carea = Q/(2 × A × ∆V) (4) min/RVC composite electrode compared to a bare RVC electrode of same geometric volume has 2 whereby Cvolume , Cmass and CThis capacitance electrode in term of PEDOT F/cm3 , F/g, and F/cm area are increased a factor of 2230. is related to theoflarge surface area of compared to a ,RVC 3 2 respectively; Q isto charge (C); m is mass (g); Z is geometric ); A is geometric (cm ); electrode according the Randle–Sevcik relationship [56].volume Figure(cm 9b shows the effectarea of increasing and V is voltage (V).

3.9. Comparison between RVC before and after PEDOT Coating Figure 9a shows the cyclic voltammograms of 1 cm3 RVC electrode and same electrode coated by PEDOT (PEDOT-120 min/RVC electrode) at the scan rate of 5 mV/s. It is clear that the CV curve of RVC

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compared with PEDOT coated electrode is very small. The capacitance of the RVC electrode in terms of geometric volume, calculated using Equation (2), is 0.002 F/cm3 . The current of PEDOT-120 min/RVC Polymers 2017, 9,electrode 157 of 18 composite compared to a bare RVC electrode of same geometric volume has increased11by a factor of 2230. This is related to the large surface area of PEDOT compared to a RVC electrode PEDOT in geometric volume of RVC on specific It canthe beeffect observed that the PEDOT capacitance according to the Randle–Sevcik relationship [56].capacitance. Figure 9b shows of increasing in increased when polymerization time ofcapacitance. PEDOT increased. RVC that geometric volume capacitance geometric volume of RVC on specific It can be The observed the capacitance increased polymerization timeof of PEDOT increased. The1710 RVCatgeometric hasthe increased haswhen increased by a factor 220, 485, 1045 and PEDOT volume coated capacitance RVC, where PEDOT by a factor of 220, 1710 PEDOT coated RVC, where the PEDOTofpolymerization time polymerization time485, was1045 10, and 20, 50 andat70 min, respectively. The capacitance PEDOT coated RVC was 10, 20, and 70polymerization min, respectively. Theand capacitance of PEDOT RVC electrodesmore at various electrodes at 50 various time various scan rates coated has been discussed fully in thepolymerization next section. time and various scan rates has been discussed more fully in the next section.

3 Figure 9. (a) Cyclic voltammograms samesize sizeof ofPEDOT-120 PEDOT-120min/RVC min/RVC composite Figure 9. (a) Cyclic voltammogramsofof11cm cm3bare bareRVC RVC and and same composite electrode in in 1 1MMNaCl rate of of 55mV/s mV/s and andAg/AgCl Ag/AgCl reference referenceelectrode; electrode;and and(b)(b) effect electrode NaClusing usingaa scan scan rate effect of of increasing polymerization time of PEDOT on the specific capacitance of PEDOT/RVC electrodes. increasing polymerization time of PEDOT on the specific capacitance of PEDOT/RVC electrodes.

3.10. Effect of of Increasing 3.10. Effect IncreasingScan ScanRate Rateon onthe theElectrode Electrode Capacitance Capacitance In In this electrode was wasalso alsoselected selectedfor forthe the same thisstudy, study,the thePEDOT-120 PEDOT-120 min/RVC min/RVC composite composite electrode same previous reasons. voltammogramsofofPEDOT-120 PEDOT-120min/RVC min/RVC composite previous reasons.Figure Figure1010shows shows the the cyclic voltammograms composite electrode obtainedatatvarious variousscan scanrates rates of of 5 to 200 the CVs at at electrode obtained 200 mV/s. mV/s. It can can be be noted notedthat thatthe theshape shapeofof the CVs scan rates up to 50 mV/s were nearly semi-rectangular and highly symmetrical (Figure 10a) which scan rates up to 50 mV/s were nearly semi-rectangular highly symmetrical (Figure 10a) which indicatesan an ideal ideal behavior of double layered mechanism, fast charge/discharge process indicates behaviorcharacteristic characteristic of double layered mechanism, fast charge/discharge with insignificant ohmic resistance. The reason for its novel fast charge/discharge ability is its is process with insignificant ohmic resistance. The reason for its novel fast charge/discharge that ability granules are small and so encourage a large amount of meso-channels in the PEDOT/RVC composite, that its granules are small and so encourage a large amount of meso-channels in the PEDOT/RVC as shownas in shown Figure 5, can5, effectively reduce the diffusion length of ions (L) soofasions to reduce composite, in that Figure that can effectively reduce the diffusion length (L) sothe as to 2 /D) where D is the diffusion coefficient [57]. This diffusion time (t), which can be estimated as (L reduce the diffusion time (t), which can be estimated as (L2/D) where D is the diffusion coefficient shape helps to achieve a constant specific capacitance [58]. Furthermore, as the scan rate increased, the [57]. This shape helps to achieve a constant specific capacitance [58]. Furthermore, as the scan rate peak current also increased linearly (inset in Figure 10a). This result is similar to those published in the increased, the peak current also increased linearly (inset in Figure 10a). This result is similar to those literature [59,60]. On the other hand, when the scan rate was increased above 50 mV/s, the curves were published in the literature [59,60]. On the other hand, when the scan rate was increased above 50 characterized by non-rectangular shapes (Figure 10b). This indicated resistance-like electrochemical mV/s, the curves were characterized by non-rectangular shapes (Figure 10b). This indicated behavior because the electrode is very porous, which hinders the migration of NaCl to the pores, resistance-like electrochemical behavior because the electrode is very porous, which hinders the and this becomes pronounced at increasing scan rates. This leads to a continuous decrease in the migration of NaCl to the pores, and this becomes pronounced at increasing scan rates. This leads to a capacitance of electrodes with increasing scan rate; which is discussed in the next section. continuous decrease in the capacitance of electrodes with increasing scan rate; which is discussed in the next section.

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Figure 10. 10. Cyclic Cyclic voltammograms voltammograms of of PEDOT-120 PEDOT-120 min/RVC min/RVC electrode electrode at: at: (a) (a) 55 to to 50 50 mV/s mV/s scan Figure scan rates; rates; and (b) (b) 50 50 to to 200 200 mV/s mV/s scan and scan rates rates in in aa three-electrode three-electrode systems systems in in 11 M M NaCl NaCl solution. solution.

3.11. Capacitance 3.11. Capacitance The capacitive behavior of the PEDOT resulted mainly from electrochemical double-layer The capacitive behavior of the PEDOT resulted mainly from electrochemical double-layer charging. charging. The specific capacitances of PEDOT/RVC electrodes were calculated from the CV curves of The specific capacitances of PEDOT/RVC electrodes were calculated from the CV curves of PEDOT PEDOT coated RVC electrodes using a three-electrode system at different scan rates of 5 to 200 mV/s, coated RVC electrodes using a three-electrode system at different scan rates of 5 to 200 mV/s, as shown as shown in Figure 11a. It is observed that PEDOT has high capacitive behavior, and increasing the in Figure 11a. It is observed that PEDOT has high capacitive behavior, and increasing the amount amount of PEDOT into the RVC electrode led to a decrease in the capacitance of the composite of PEDOT into the RVC electrode led to a decrease in the capacitance of the composite electrode. electrode. The specific capacitance of the PEDOT was significantly decreased from 185.29 F/g to The specific capacitance of the PEDOT was significantly decreased from 185.29 F/g to 86.56 F/g when 86.56 F/g when the polymerization time of PEDOT in the composite electrode was increased from 10 the polymerization time of PEDOT in the composite electrode was increased from 10 to 120 min, to 120 min, determined at a low scan rate of 5 mV/s. The value of specific capacitance of PEDOT at determined at a low scan rate of 5 mV/s. The value of specific capacitance of PEDOT at PEDOT-10 PEDOT-10 min/RVC electrode was very close to the value of theoretical specific capacitance (210 min/RVC electrode was very close to the value of theoretical specific capacitance (210 F/g) [61]. F/g) [61]. Furthermore, the specific capacitances of PEDOT coated RVC electrodes, in all electrodes, Furthermore, the specific capacitances of PEDOT coated RVC electrodes, in all electrodes, were found were found to be very high compared with other PEDOT studies [61–63] at low scan rate but its to be very high compared with other PEDOT studies [61–63] at low scan rate but its capacitance capacitance markedly decreased at high scan rates. The specific capacitance of the PEDOT-10 markedly decreased at high scan rates. The specific capacitance of the PEDOT-10 min/RVC was min/RVC was significantly decreased from 185.29 to 83.57 F/g as the potential scan rate was significantly decreased from 185.29 to 83.57 F/g as the potential scan rate was increased from 5 to increased from 5 to 200 mV/s, as shown in Table 3. These results are in keeping with published 200 mV/s, as shown in Table 3. These results are in keeping with published reports for all cases of reports for all cases of PEDOT [61,64,65]. It can be seen that the specific capacitance trend of all PEDOT [61,64,65]. It can be seen that the specific capacitance trend of all PEDOT electrodes decrease, PEDOT electrodes decrease, when the scan rate was increased above 50 mV/s. It should be noted when the scan rate was increased above 50 mV/s. It should be noted that the specific capacitance that the specific capacitance values in term of mass of all electrodes were calculated using Equation values in term of mass of all electrodes were calculated using Equation (3). As mentioned earlier, the (3). As mentioned earlier, the aim of this study is to discuss the effect of increasing PEDOT amount aim of this study is to discuss the effect of increasing PEDOT amount in terms of geometric volume in terms of geometric volume and area of the electrode on the capacitance results. and area of the electrode on the capacitance results. Figure 11b shows the comparison cyclic voltammograms for the same PEDOT/RVC electrodes Figure 11b shows the comparison cyclic voltammograms for the same PEDOT/RVC electrodes using current per gram of PEDOT and current per geometric volume of electrode. It is very clear that using current per gram of PEDOT and current per geometric volume of electrode. It is very clear that current behavior in geometric volume unit was totally opposite to the current behavior in units of current behavior in geometric volume unit was totally opposite to the current behavior in units of gram. gram. Therefore, the capacitances per unit geometric volume of PEDOT/RVC electrodes were Therefore, the capacitances per unit geometric volume of PEDOT/RVC electrodes were calculated calculated using Equation (2) and are presented in the same Table 3 as F/cm3. The capacitance was3 3 . The using Equation (2) and are presented in the same Table 3 as F/cm capacitance was 0.65 F/cm 0.65 F/cm3 for the PEDOT-10 min/RVC and 6.18 F/cm3 for the PEDOT-120 min/RVC a low a scan rate for the PEDOT-10 min/RVC and 6.18 F/cm3 for the PEDOT-120 min/RVC a low a scan rate of 5 mV/s. of 5 mV/s. This indicates that the surface area of PEDOT coated in the RVC electrode was increased This indicates that the surface area of PEDOT coated in the RVC electrode was increased by increasing by increasing the amount of PEDOT, and the porous structure enhanced. Figure 11c shows that the trend of the capacitances in (F/cm3) was completely opposite to the direction of the trend for capacitance per gram.

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the amount of PEDOT, and the porous structure enhanced. Figure 11c shows that the trend of the capacitances in (F/cm3 ) was completely opposite to the direction of the trend for capacitance per gram. Furthermore, the capacitance per unit geometric area was calculated using Equation (4) and are Furthermore, the capacitance per unit geometric area was calculated using Equation (4) and 2. It is expected that the capacitance (F/cm2) behavior of reported in the same TableTable 3 as3 as F/cm 2 . It is expected that the capacitance (F/cm2 ) behavior of are reported in the same F/cm 3 electrodes followed thethe capacitance capacitance has increased a factor electrodes followed capacitance(F/cm (F/cm)3 )behavior. behavior. The The capacitance has increased by aby factor of 10 of 10 with increasing loading of PEDOT; that is, PEDOT-10 min/RVC electrode compared to PEDOT-120 with increasing loading of PEDOT; that is, PEDOT-10 min/RVC electrode compared to PEDOT-120 min/RVC electrode. min/RVC electrode.

Figure 11. (a) specific capacitance electrodes various rates; (b) cyclic Figure 11.The (a) The specific capacitance(F/g) (F/g)ofofPEDOT/RVC PEDOT/RVC electrodes at at various scanscan rates; (b) cyclic voltammograms of PEDOT/RVC electrodes pergram gramof of PEDOT current voltammograms of PEDOT/RVC electrodesusing using current current per PEDOT andand current per per geometric volume of electrode mV/sscan scan rate; rate; and specific capacitance of PEDOT/RVC geometric volume of electrode atat5 5mV/s and(c) (c)the the specific capacitance of PEDOT/RVC 3 electrodes in terms of F/g and F/cm mV/sscan scan rate. rate. Electrolyte: NaCl solution. Potential 3 atat electrodes in terms of F/g and F/cm 5 5mV/s Electrolyte:1 M 1M NaCl solution. Potential range: between −0.2 and 0.8 V vs. Ag/AgCl using a three-electrode system. range: between −0.2 and 0.8 V vs. Ag/AgCl using a three-electrode system.

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Table 3. Specific capacitance of PEDOT/RVC electrodes measured in various units at various scan rates. Composite electrode

PEDOT-10 min/RVC

PEDOT-20 min/RVC

Scan rate (mV/s)

PEDOT-50 min/RVC

PEDOT-70 min/RVC

PEDOT-120 min/RVC

Capacitance F/g

5 10 20 50 100 200

185.29 160.12 140.15 125.66 108.23 83.57

171.04 147.41 130.90 111.32 100.36 72.95

155.36 137.16 118.63 102.04 91.52 65.81

142.53 126.51 107.59 94.78 81.47 57.96

86.56 78.62 67.77 62.41 54.34 39.45

Composite electrode

PEDOT-10 min/RVC

PEDOT-20 min/RVC

PEDOT-50 min/RVC

PEDOT-70 min/RVC

PEDOT-120 min/RVC

Capacitance F/cm2

Scan rate (mV/s) 5 10 20 50 100 200

0.08 0.07 0.06 0.06 0.05 0.04

0.19 0.17 0.15 0.12 0.11 0.08

0.41 0.36 0.31 0.27 0.24 0.17

0.66 0.59 0.50 0.44 0.38 0.27

0.80 0.73 0.63 0.58 0.50 0.36

Composite electrode

PEDOT-10 min/RVC

PEDOT-20 min/RVC

PEDOT-50 min/RVC

PEDOT-70 min/RVC

PEDOT-120 min/RVC

5.14 4.57 3.89 3.42 2.94 2.09

6.18 5.62 4.84 4.46 3.88 2.82

Capacitance F/cm3

Scan rate (mV/s) 5 10 20 50 100 200

0.65 0.56 0.49 0.44 0.38 0.29

1.48 1.28 1.14 0.97 0.87 0.63

3.18 2.81 2.43 2.09 1.87 1.35

3.12. Cycling Stability of PEDOT/RVC Electrodes Stability is one of the most critical characteristics and electrodes with poor mechanical and electrochemical stabilities may lose their function. The electrochemical cycling performance of the PEDOT/RVC electrodes was investigated for 200 cycles at 5 mV/s, as shown in Figure 12. It can be observed that the CVs’ shape do not change much and are almost identical at the beginning and end of the stability test, as shown in other studies [66]. The PEDOT-120 min/RVC electrode shows quite good cycling stability and retains approximately 96% of its current density after 200 cycles. The loss may be due to the irreversible reactions of the PEDOT backbone, which represents a deterioration of the electrochemical reversibility [67]. The 3D mesoporous network structure can enable PEDOT to avoid shrinkage during continuous quick charging/discharging [68].

of the stability test, as shown in other studies [66]. The PEDOT-120 min/RVC electrode shows quite good cycling stability and retains approximately 96% of its current density after 200 cycles. The loss may be due to the irreversible reactions of the PEDOT backbone, which represents a deterioration of the electrochemical reversibility [67]. The 3D mesoporous network structure can enable PEDOT to Polymers 2017, 9, 157 15 of 18 avoid shrinkage during continuous quick charging/discharging [68].

Figure 12. Figure The electrochemical cycling stability of PEDOT-120 min/RVC electrode in 1 M NaCl 12. The electrochemical cycling stability of PEDOT-120 min/RVC electrode in 1 M NaCl solution recorded in in the rangerange between −0.2 and 1.0 V vs. Ag/AgCl a three-electrode system at solution recorded thepotential potential between −0.2 and 1.0 V using vs. Ag/AgCl using a three-electrode 5 mV/s scan rate for 200 cycles. system at 5 mV/s scan rate for 200 cycles. 4. Conclusions In this study, PEDOT has been successfully deposited by electropolymerization on RVC. The polymerization time decreases with the increase in electrode potential. SEM images confirm that 10 min polymerization was enough to uniformly cover PEDOT on the RVC electrode. The morphology of the PEDOT on RVC electrode is affected by polymerization time. The average pore size of RVC electrode is reduced marginally with the increase in polymerization time. The PEDOT amount, thickness, surface roughness, porosity, and globular structure on RVC electrode are increased with the increase in polymerization time. The surface of PEDOT coating was rough and highly porous, and loose with honeycomb-like fine structure, which shows a flower-like morphology, consisting of numerous thin flakes with numbers of macropores and micropores. This observed morphology can enhance the performance of PEDOT because of high conductivity, increased electrode surface area and specific capacitance, and macroporous structure of RVC electrode. TGA results show that the weight loss value of PEDOT increases with polymerization time. Raman spectroscopy and cyclic voltammetry suggest a strong interaction between the RVC and the backbone of the PEDOT chain. This improves the surface area and conductivity of the PEDOT/RVC composite electrode. It has been shown that the capacitance of PEDOT-120 min/RVC electrode has increased by a factor of 2230 compared to a bare RVC electrode. The capacitance of PEDOT was found to be very high compared with other PEDOT studies. The value of specific capacitances decreases with the increase in scan rate irrespective of polymerization time. The specific capacitance per unit mass is decreasing but the specific capacitance per unit volume and unit area is increasing with the increase in polymerization time irrespective of scan rate. The electrodes show good cyclic stability. Hence, the performance of RVC electrode has substantially increased by the electrodeposition of PEDOT on it. The results show that the PEDOT/RVC composites can be used as a capacitive deionization electrode. Supplementary Materials: The following are available online at www.mdpi.com/2073-4360/9/5/157/s1, Figure S1: Scheme of oxidative polymerization mechanism of PEDOT. Acknowledgments: The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group No. (RG-1436-005).

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Author Contributions: Mostafizur Rahaman and Mohammed Almoiqli have designed and performed the experiments, and analyzed the data. Ali Aldalbahi also has analyzed the data and written the manuscript. All the authors have read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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