Facile Electrodeposition of Poly(3,4-ethylenedioxythiophene) on Poly(vinyl alcohol) Nanofibers as the Positive Electrode for High-Performance Asymmetric Supercapacitor

Abstract

Poly(vinyl alcohol)/poly(3,4-ethylenedioxythiophene) (PVA/PEDOT) nanofibers were synthesized as a positive electrode for high-performance asymmetric supercapacitor (ASC). PVA/PEDOT nanofibers were prepared through electrospinning and electrodeposition meanwhile reduced graphene oxide (rGO) was obtained by electrochemical reduction. The PVA/PEDOT nanofibers demonstrated cauliflower-like morphology showing that PEDOT was uniformly coated on the smooth cross-linking structure of PVA nanofibers. In addition, the ASC showed a remarkable energy output efficiency by delivering specific energy of 21.45 Wh·kg⁻¹ at a specific power of 335.50 W·kg⁻¹ with good cyclability performance (83% capacitance retained) after 5000 CV cycles. The outstanding supercapacitive performance is contributed from the synergistic effects of both PVA/PEDOT//rGO, which gives promising materials for designing high-performance supercapacitor applications.

1. Introduction

Electrochemical energy storage devices, such as fuel cells, batteries, and supercapacitors, apply the principle of chemical-to-electrical energy conversion [1]. Supercapacitors possess significant advantages compared to other energy storages due to its fast charging and discharging, long cycle life, low cost and good environmental friendliness [2]. Supercapacitors bridging the energy storage gap between batteries and conventional capacitors and widely being used for hybrid electric vehicles, medical applications, military, and diesel locomotives [3,4]. However, high specific power, long cycle life time and portability of supercapacitors make it more demand than batteries [5]. The energy storage mechanism of a supercapacitor can be classified into the electrical double-layer capacitor (EDLC) and pseudocapacitors [6,7]. EDLCs (carbon-based electrode materials) store charges via ion accumulation at the interface of carbon-based electrodes [8] which has high specific power [9], large specific surface area and controllable pore-characteristics for high cycling stability [10,11]. Unfortunately, it suffers from low specific capacitance and specific energy which can affect the performance of supercapacitors [10]. Pseudocapacitors (transition metal oxide and conducting polymer) undergo highly reversible redox reactions taking place on the surface or near the electrodes [12] and able to deliver a higher specific capacitance and specific energy than EDLC because they have different oxidation states for rich redox reactions [13].

An asymmetric supercapacitor (ASC) consists of EDLC (power source) and pseudocapacitor (energy source) as negative and positive electrodes, respectively [10] where both electrodes have different charge storage mechanism and well separated potential windows are assembled in the same electrolyte [14]. Thus, it exhibits a wide potential window which leads to higher specific energy and specific power for asymmetric supercapacitors [14].

Reduced graphene oxide (rGO) can be prepared from graphene oxide (GO) through chemical, electrochemical, thermal, hydrothermal, and photocatalytic routes. Among these methods, electrochemical reduction of GO is easy, cheap, fast and environmentally friendly to yield the bulk amount of graphene material. Jiang et al. [15] reported that nickel foam-reduced graphene oxide//nickel-cobalt layered doubled layered hydroxide//nickel foam-reduced graphene oxide (NF-rGO/Ni-Co LDH//NF-rGO) displayed outstanding gravimetric capacitance of 1454.2 F·g⁻¹ at 1 A g⁻¹ in 1 M KOH electrolyte. In addition, asymmetric Ni-Co-O-rGO//AC possessed 1903 F·g⁻¹ of specific capacitance using KOH/redox active K₃Fe(CN)₆ as electrolyte [16]. Huang et al. [17] fabricated LDH/rGO-4//rGO composite via solvothermal approach and exhibited specific energy of 29.3 Wh·kg⁻¹ and the capacitance can be retained from 82.5 F·g⁻¹ to 21.9·Fg⁻¹ at a current density between 0.5 Ag⁻¹ to 7 Ag⁻¹, proving its good rate capability.

Poly(3,4-ethylenedioxythiophene) (PEDOT) is used as a pseudocapacitive electrode in asymmetric supercapacitors due to its high conductivity, excellent processability and economic [18]. In addition, PEDOT is available in both an oxidized state and reduced state. Yang et al. [19] revealed that NiO/Ni(OH)₂/PEDOT electrodes could deliver 80.8 F·cm⁻³ of specific capacitance. An ASC of PEDOT//activated carbon (AC) obtained a specific capacitance of 22 F·g⁻¹ and 27 F·g⁻¹ in 1 M Et₄NBF₄/propylene carbonate and LiPF₆/ethylene carbonate/dimethyl carbonate, respectively [20].

Recently, the usage of electrospinning technique has been widely explored for preparing nanofiber membranes in ASC devices due to its facile preparation and economical. For example, manganese oxide/porous carbon nanofibers//activated carbon (MnO₂/PCNF_s/AC) was fabricated via electrospinning and subsequent chemical precipitation approach and exhibited a specific capacitance of 60.6 F·g⁻¹ [21]. High specific energy of 15.0 Wh·kg⁻¹ was achieved by the assembled ASC of cobalt sulfides/carbon//porous carbon nanofibers (CoS_x/C//PCNF_s) which was fabricated using electrospinning and hydrothermal routes [22]. Hollow carbon nanofibers (HCNF_s) coated with MnO₂ (HCNF_s/HCNF_s) which were assembled with PCNF_s in a coin cell set up producing a gravimetric capacitance of 63.9 F·g⁻¹ with an extended operating voltage up to 2.0 V [23]. Production of PVA nanofibers has attracted a lot of attention owing to its excellent mechanical properties, good adherence properties towards porous and water surfaces and environmental friendly [24,25]. PVA is one of the biodegradable polymers that has been used as a precursor to produce electrospun nanofibers. The abundance of hydrophilic functional groups (−OH) on the PVA structure makes them highly soluble in an aqueous medium. Pan et al. [26] have synthesized PVA-GO electrospun nanofibers which act as a separator via drop casting and laser-writing process for solid-state supercapacitors. The porous architecture of PVA-GO nanofibers and hygroscopic feature of GO could effectively improve the electrolyte uptake during charging/discharging process. The solid-state supercapacitor using PVA-GO nanofibers also showed high areal capacitance (9.9 mF·cm⁻²), remarkable specific energy (0.13 mW·cm⁻³) and excellent cycling stability (88% capacitance retained after 1000 cycles). However, PVA nanofibers are non-conductive materials which need to be incorporated with a conductive material such as PEDOT in order to overcome the drawback. Therefore, the combination of PVA nanofiber and PEDOT could synergistically improve the electrochemical behavior of the positive electrode. Inspired by this idea, high-performance asymmetric supercapacitor was fabricated using PVA/PEDOT nanofibers and rGO as a positive and negative electrode, respectively, with different potential windows to increase the overall voltage of asymmetric supercapacitor. The extended potential windows and synergistic effect of high capacitance, good conductivity and outstanding cycling stability of PVA/PEDOT//rGO can deliver excellent capacitive performance of ASC device, implying a promising material for future supercapacitor application.

2. Materials and Methods

2.1. Materials

Indium tin oxide (ITO) glasses were obtained from Xin Yan Technology. Polyvinyl alcohol (PVA), lithium perchlorate (LiClO₄), sodium chloride (NaCl) and 3,4-ethylenedioxythiophene (EDOT) were supplied by Sigma Aldrich. GO was obtained from Graphenea. Ethanol and potassium chloride (KCl) were purchased from HmbG Chemicals and Fisher Scientific UK, respectively. Acetone was supplied by Systerm. Acetonitrile and hydrochloric acid were obtained from J.T Baker. Deionized water (18.2 MΩ·cm) was used throughout the experiment.

2.2. Preparation of PVA Nanofiber via Electrospinning

ITO glasses (1 cm²) were cleaned with acetone followed by ethanol and deionized water for 10 min using a sonicator bath. PVA solution (10 wt%, 0.4 g) was prepared and heated at 80 °C and continuously stirred for 30 min until a clear solution was observed. The PVA solution was transferred into a 5-mL syringe and connected to the syringe pump. The collector plate was covered with aluminum foil and ITO glasses were attached on the plate surface. 15 kV of voltage was applied directly to the stainless-steel needle. The operating flow rate of solution and the distance from the syringe nozzle to the collector were controlled to 1.2 mL·h⁻¹ and 15 cm, respectively.

2.3. Electrodeposition of PEDOT on PVA Nanofibers

A non-aqueous EDOT solution which consists of 0.01 M EDOT and 0.1 M LiClO₄ in acetonitrile was prepared. Chronoamperometry (CA) was used to perform electrodeposition of PEDOT on PVA nanofibers using a three-electrode system at 1.2 V for 15 min. ITO substrate coated with PVA nanofibers was used as a working electrode, silver wire coated with silver chloride (Ag/AgCl) as a pseudo-reference electrode and a platinum wire as a counter electrode.

2.4. Reduction of GO

A few drops of 1.0 mg·mL⁻¹ GO were deposited onto the ITO substrate (1 cm²) through drop-casting and dried out to form a GO-coated electrode. Electrochemical reduction of the GO-coated electrode was carried out using cyclic voltammetry (CV) in 0.5 M NaCl electrolyte between −1.5 to 0 V for 20 cycles.

2.5. Characterizations

The surface morphologies of the samples were performed using a field emission scanning electron microscope (FESEM, JEOL JSM-7600F). Fourier transform infrared spectroscopy (FTIR) was observed using a Shimadzu. The ASC spectra were analyzed in the range of 400–4000 cm⁻¹. The electrochemical properties of PVA/PEDOT nanofibers//rGO were characterized by assembling a two-electrode configuration with a piece of filter paper as separator soaked with 1.0 M KCl. CV was conducted between 0 to 1.6 V at scanning rates from 5 mV·s⁻¹ to 100 mV·s⁻¹. Galvanostatic charge-discharge (GCD) measurements were performed at different current densities of 0.4 to 0.9 Ag⁻¹. Electrochemical impedance spectroscopy (EIS) tests were carried at open circuit potentials with an amplitude of 5 mV and frequency ranging from 0.01 Hz to 100 kHz. The stability test was examined for 2000 cycles between 0 and 1.6 V. Figure 1 shows the schematic illustration of ASC design based on PVA/PEDOT nanofibers and rGO electrodes. The mass loading of PVA/PEDOT nanofibers//rGO was approximately 0.6 mg·cm⁻².

3. Results and Discussion

3.1. Structural Morphology

Figure 2 displays the FESEM images of PVA nanofibers, PVA/PEDOT nanofibers and rGO. A smooth cross-linked network structure of PVA nanofibers (Figure 2a) was observed with an average diameter of 72.65 ± 22 nm and beads-free, demonstrating better electrospinnability. The cauliflower-like morphology of PVA/PEDOT nanofibers (Figure 2b) reveals that PEDOT was uniformly coated on the cross-linking morphology of PVA nanofibers without interfering the fibril-like structure of nanofibers [27]. Figure 2c represents the FESEM image of rGO which suggests the wrinkled and crumpled morphology can facilitate rapid ion diffusion and charge transfer at the electrode material [14,28].

3.2. Fourier Transform Infrared Spectroscopy (FTIR)

Fourier transform infrared spectroscopy (FTIR) was employed to identify the functional groups and bonding configuration present [17] in PVA nanofibers, PVA/PEDOT nanofibers, GO and rGO. The FTIR spectrum of PVA nanofibers as depicted in Figure 3 shows the existence of symmetric stretching of C–H and stretching band of C-O group at absorption bands 2906 cm⁻¹ and 1087 cm⁻¹, respectively [27]. The broad peak observed at 3284 cm⁻¹ and peak at 1438 cm⁻¹ are attributed to hydroxyl (O–H) group and C–OH stretching vibration in PVA structure, respectively. From the PVA/PEDOT nanofiber spectrum, peaks of PVA nanofibers can still be seen, proving the presence of PVA nanofibers after electrodeposition of PEDOT. Three characteristic peaks of PEDOT are observed at 775, 1318 and 1602 cm⁻¹, indicating the C=S, C–C [29], C–C [30] stretching band of thiophene ring in PEDOT structure, respectively. In addition, the C–C stretching of the thiophene ring of PEDOT was observed at 1602 cm⁻¹ [30]. The FTIR spectrum of GO reveals the presence of C–O epoxide group, C–OH bond, aromatic C=C bending, C=O group in carboxy and carbonyl groups and O–H stretching vibration at 1107, 1423, 1616, 1732 and 3176 cm⁻¹, respectively. It was also observed that the FTIR spectrum of rGO shows less oxygenated functional groups after electrochemical reduction of GO, indicating rGO has been successfully prepared [31,32].

3.3. Electrochemical Characterizations

The electrochemical performances of both anode and cathode materials were initially evaluated using a three-electrode system in 1 M KCl electrolyte as shown in Figure 4. Figure 4a display to CV profiles of the individual PEDOT and PVA/PEDOT at a sweeping rate of 25 mV·s⁻¹ which noted that PVA/PEDOT nanofibers exhibit higher integrated CV area in comparison with PEDOT. This suggests that the formation of PVA nanofibers via electrospinning could provide more accessible sites for electron transfer [33]. The specific capacitance of the ASC is obtained from the CV curve using Equation (1) [10].

$$C=\frac{{\displaystyle \int }Idv}{vm\Delta V},$$

(1)

where C refers to the specific capacitance (F·g⁻¹), Idv is the integrated area of a CV curve, v represents the sweeping rate (mV·s⁻¹), m is the mass of the electroactive material (g) and ∆V indicates the potential (V). The calculated specific capacitances for PVA/PEDOT, PEDOT and rGO are plotted against scan rates as shown in Figure 4b. The result proves that PVA/PEDOT nanofibers still displays the highest specific capacitance of 331.49 F·g⁻¹, rather than individual PEDOT (250.02 F·g⁻¹) and rGO (85.92 F·g⁻¹) at a scan rate of 5 mV·s⁻¹. However, the rGO electrode shows good rate capability with 83% capacitance retention as compared with PVA/PEDOT (66%) and PEDOT (60%). This indicates that the presence of carbon material could provide good mechanical strength for the remarkable capacitive performance of the electrode [34]. In addition, the electrochemical performance of as-prepared GO and rGO as anode material were also investigated with an applied potential range from −1 to 0 V (Figure 4c). It is obvious that rGO possesses an ideal rectangular CV shape with a higher current response, owing to superior specific capacitance. In Figure 4d, the potential range was measured within −0.2 V to 0.6 V for PVA/PEDOT nanofibers (positive electrode) and −1.0 V to 0 V for RGO (negative electrode) at 25 mV·s⁻¹. Since both electrodes have different potential windows, the charge storage for both electrodes have to be balanced based on the Equations (2)–(4) [14]:

$$Q+=Q-$$

(2)

$$Q=C\times \Delta U\times m$$

(3)

$$\frac{m_{+}=C_{+}\times \Delta U_{+}}{m_{-}=C_{-}\times \Delta U_{-}},$$

(4)

where Q₊ and Q₋ represent charges stored in the positive electrode and negative electrodes, C₊ and C₋ indicate the capacitance for both electrodes, m is the mass loading of electroactive electrode and ΔU represents the potential. A high-performance asymmetric supercapacitor (PVA/PEDOT nanofibers//rGO) of 1.6 V can be constructed based on the sum of potential ranges of two electrodes to further examine the performance of the asymmetric device [35].

Figure 5a indicates the CV profiles of ASC at various sweeping rates (5–100 mV·s⁻¹) at potential window from 0 to 1.6 V. The ASC possesses a specific capacitance of 113.48 F·g⁻¹ at 5 mV·s⁻¹ and the rectangularity of CV curves is slightly distorted due to the combination of double-layer capacitance of rGO and pseudocapacitance of PEDOT. The specific capacitance obtained from PVA/PEDOT nanofibers//rGO is higher than those of ASC-carbon fiber composites [36,37]. Figure 5b shows that the capacitance decreased gradually when the scan rates increased due to the reduced accessibility of electrolyte ions to fully diffused the interior surface of electrodes during the redox process [10]. Figure 5c indicates the CV curves of as-prepared PVA/PEDOT nanofibers//rGO asymmetric supercapacitor at a different operating potential (1.0 to 1.6 V) at a sweeping rate of 25 mV·s⁻¹. Based on this result, the asymmetric composite obtained works stably until 1.6 V with nearly rectangular CV shape [11]. However, extending potential more than 2.0 V causes an obvious redox peak which leads to oxygen evolution [38].

The performance of the assembled ASC is further evaluated by GCD analysis at current densities from 0.4 to 0.8 A·g⁻¹ at 1.6 V operating potential as shown in Figure 6a. PVA/PEDOT nanofibers//rGO shows a nearly symmetric triangular shape, showing an ideal capacitive behavior of PVA/PEDOT nanofibers//rGO [33]. A slight IR drop is observed at the beginning of these discharging curve, illustrating the influence of the internal resistance of electrode [39]. The ASC device displays equivalent series resistance (ESR) of about 33.33 Ω at a current density of 0.4 A·g⁻¹. However, the initial voltage drops are small at higher current densities, implying small internal resistance of the asymmetric device [10]. As a result, the PVA/PEDOT nanofibers//rGO composite can achieve a total potential of 1.6 V at high current density, in agreement with a potential window of CV. The specific energy and specific power of ASC are obtained based on these equations [8]:

$$E=\frac{1}{2}C\times \Delta V^2$$

(5)

$$P=\frac{E}{\Delta t},$$

(6)

where E represents the specific energy (Wh·kg⁻¹), P indicates the specific power (W·kg⁻¹), C refers to the specific capacitance (F·g⁻¹), ∆t is the discharge time (h) and ∆V is the potential (V). Ragone plot (Figure 6b) of ASC PVA/PEDOT//rGO can deliver reasonably high specific energy of 21.25 Wh·kg⁻¹ at a specific power of 335.51 W·kg⁻¹. Interestingly, the superior specific energy of ASC surpasses other PEDOT-based electrodes [4,27,40,41,42]. This remarkable supercapacitor behavior is due to the high specific capacitance obtained from the discharged time and extended potential window [43].

EIS measurements were performed to study the charge-transfer resistance (R_ct) and ESR of PVA/PEDOT nanofibers//rGO ASC (0.01 Hz to 100 kHz) as shown in Figure 7. From the Nyquist plot, two major parts can be identified. At the high-frequency region, the semicircle arc corresponds to the R_ct, where transport of ions at the electrode/electrolyte interface take place [33]. ESR comprises of intrinsic resistance of the electrode, ionic resistance of electrolyte and contact resistance at electrode/current collector [14]. The ESR is obtained from the x-intercept on the real axis (Z’) at the high-frequency region. It is observed that the values for R_ct and ESR of ASC are 13.10 Ω and 37.81 Ω, respectively. Furthermore, the presence of a nearly vertical line along the imaginary axis in the low-frequency region suggests the good capacitive behaviors of the electrode [44]. The equivalent circuit fitting based on the Nyquist plot of PVA/PEDOT nanofibers//rGO ASC is shown in the inset of Figure 7. R_s represents the solution resistance, R_ct is charge transfer resistance and W is the Warburg impedance that related to the ionic diffusion. Constant phase element (CPE) was used to replace double layer capacitance due to the inhomogeneity and porous structure of the electrode surface [45].

The cycling stability of the asymmetric PVA/PEDOT nanofibers//rGO supercapacitor was investigated at 100 mV·s⁻¹ for 5000 cycles. From Figure 8, the ASC presents excellent cycling stability by maintaining 83% of the initial capacitance after 5000 cycles. It is noted that a small increase of specific capacitance at cycle 600 is due to the improved wettability of PVA/PEDOT nanofibers//rGO [46] and high favorable of ions diffusion into the electrode surface [47]. As the cycle number continued, the capacitance decayed slightly with considerable retention, representing good cycling durability. The rGO as a negative electrode primarily provides high mechanical strength for the excellent cycling stability of the supercapacitor [48].

4. Conclusions

In conclusion, an ASC of PVA/PEDOT//rGO was successfully prepared using facile electrospinning and electrodeposition techniques. The PVA nanofibers provide a large surface area, enhancing more ion migration of electrolyte onto the electrode surface, resulting to a better specific capacitance at the maximum potential window of 1.6 V. A wrinkled morphology of rGO contributes good mechanical strength of ASC for high cycling stability over 5000 cycles. Interestingly, the synergistic effects of both electrodes illustrate remarkable specific energy (21.25 Wh·kg⁻¹) at 0.4 A·g⁻¹. Therefore, these favorable capacitive performances of PVA/PEDOT//rGO can be served as a highly potential ASC electrode for future development supercapacitor application.