Facile synthesis of PEDOT-rGO/HKUST-1 for high performance symmetrical supercapacitor device

A novel poly(3,4-ethylenedioxythiophene)-reduced graphene oxide/copper-based metal–organic framework (PrGO/HKUST-1) has been successfully fabricated by incorporating electrochemically synthesized poly(3,4-ethylenedioxythiophene)-reduced graphene oxide (PrGO) and hydrothermally synthesized copper-based metal–organic framework (HKUST-1). The field emission scanning microscopy (FESEM) and elemental mapping analysis revealed an even distribution of poly(3,4-ethylenedioxythiophene) (PEDOT), reduced graphene oxide (rGO) and HKUST-1. The crystalline structure and vibration modes of PrGO/HKUST-1 were validated utilizing X-ray diffraction (XRD) as well as Raman spectroscopy, respectively. A remarkable specific capacitance (360.5 F/g) was obtained for PrGO/HKUST-1 compared to HKUST-1 (103.1 F/g), PrGO (98.5 F/g) and PEDOT (50.8 F/g) using KCl/PVA as a gel electrolyte. Moreover, PrGO/HKUST-1 composite with the longest charge/discharge time displayed excellent specific energy (21.0 Wh/kg), specific power (479.7 W/kg) and an outstanding cycle life (95.5%) over 4000 cycles. Thus, the PrGO/HKUST-1 can be recognized as a promising energy storage material.


Results and discussion
FESEM was performed to identify the surface morphology of the materials as depicted in Fig. 1(a). A wrinklelike morphology is observed for both GO ( Fig. 1(a)(i)) and rGO ( Fig. 1(a)(ii)). Comparatively, rGO possesses pronounce wrinkle-like sheet morphology, which is due to the reduction of GO. PEDOT ( Fig. 1(a)(iii)) reveals a homogeneous granular morphology similar to a typical polymeric structure. PrGO displays a prominent wrinkle-like sheet morphology ( Fig. 1(a)(iv)), which is contributed by the rGO. The granular PEDOT that is covered on the rGO sheet demonstrates the successful incorporation of PEDOT and rGO (inset of Fig. 1(a)(iv)). This wrinkled-like sheet morphology provides a high surface area which enables an efficient ion diffusion. The HKUST-1 ( Fig. 1(a)(v)) displays a typical octahedral shape morphology, whereas the PrGO/HKUST-1 demonstrates the presence of HKUST-1 and PrGO as both the octahedral morphology as well as wrinkle-like rGO sheet covered with PEDOT grains are observed in Fig. 1 (a)(vi).
The PrGO/HKUST-1 ( Fig. 1(b)(i)) was further analyzed via elemental mapping ( Fig. 1(b)(ii-v)) to study the elements that exist in the composite. Carbon (C), oxygen (O), sulfur (S) and copper (Cu) are evenly distributed on the PrGO/HKUST-1 surface, indicating a homogeneous formation of the composite. S and Cu elements signify the distribution of PEDOT and HKUST-1, respectively 19,20 . The presence of C and O elements are originated from PEDOT, rGO and HKUST-1.
The crystallinity of the composites was evaluated using XRD ( Fig. 2(a)) analysis. The peak at 2θ = 10.1° ( Fig. 2(a)(i)) represents lattice plane (001) of GO. The XRD spectrum of rGO ( Fig. 2(a)(ii)) implies a broad peak at 2θ = 25.0° (002), which confirm the successful reduction of GO to rGO 21,22 whereas Fig. 2(a)(iii) implies a diffraction peak at 2θ = 25.7° (020), verifying the presence of interchain planar ring stacking of PEDOT 23,24 . PrGO ( Fig. 2(a)(iv)) composite reveals a diffraction peak (2θ = 25.6°), representing the (020) and (002) lattice planes of PEDOT and rGO, respectively. The XRD peak of PrGO only shows one diffraction peak (2θ = 25.6°) as the peak of rGO is overlapping with PEDOT. The disorder in the rGO sheets appears when the majority of oxygenated functional groups was successfully reduced from the GO sheet during the electrodeposition 25 . The PrGO composite does not show any diffraction peak at 2θ = 10° (peak for GO), which further verifies the reduction of GO to rGO 26 . The as-synthesized HKUST-1 ( Fig. 2(a)(vi)) displays similar diffraction peaks as the simulated HKUST-1 ( Fig. 2(a)(v)) at 6.7° (200), 9.5° (220), 11.6° (222), 13.5° (400), 17.5° (400), 19.1° (600) and 29.4° (751), revealing a successful synthesis of HKUST-1 via hydrothermal 27,28 . All the XRD diffraction peaks of PEDOT, rGO and HKUST-1 are well-presented in the PrGO/HKUST-1 ( Fig. 2(a)(vii)) spectrum. The results demonstrate that the framework of HKUST-1 is retained during the synthesis process which is well supported by the FESEM images ( Fig. 1(a) The vibrational modes of different materials were examined via Raman spectroscopy ( Fig. 2(b)). GO ( Fig. 2(b) (i)) and rGO ( Fig. 2(b)(ii)) imply two intense Raman peaks at 1360 and 1598 cm −1 , resembling the D band (sp 3hybridized carbon) and G band (sp 2 -hybridized carbon), respectively. The intensity ratio of the D and G bands (I D /I G ) represents the degree of disorder in graphitic material 29 . The calculated I D /I G ratio of GO and rGO are 0.9 and 1.2, respectively. The I D /I G ratio of more than 1 indicates the presence of high sp 3 -hybridized carbon atoms compared to sp 2 -hybridized carbon 30 . The obtained I D /I G ratio of rGO (1.2) depicts the successful reduction of GO to rGO. PEDOT ( Fig. 2(b)(iii)) displays peaks at 440, 576, and 989 cm −1 , corresponding to the oxyethylene ring of EDOT monomer deformation. The C-O-C mode can be observed at 1107 cm −1 while C β -C β , C α -C α , symmetry and asymmetry C = C stretching modes of PEDOT are seen at 1260, 1366, 1427 and 1526 cm −1 , respectively 7 . The PrGO ( Fig. 2(b)(iv)) also displays two obvious peaks of rGO at 1356 (D band) and 1600 cm −1 (G band) and the calculated I D /I G ratio of PrGO (1.2) demonstrates a high degree of disorder in PrGO, revealing the majority of oxygenated functional groups in GO have been reduced successfully 31 . HKUST-1 ( Fig. 2(b)(v)) displays all vibration modes of Cu(II) species at the low frequency region (150 to 600 cm −1 ). The Raman peak at 177 cm −1 exhibits the presence of Cu-Cu dimer stretching mode, while the Cu-O vibration modes of HKUST-1 can be detected at 278 and 501 cm −1 . The C-H out-of-plane ring bending modes of trimesic acid are observed at 744 and 827 cm −1 whereas the C=C stretching mode of the trimesic acid benzene ring is spotted at 1006 cm −1 . Raman peaks of HKUST-1 ( Fig. 2(b)(v)) are observed at 1457 and 1544 cm −1 , indicating the asymmetry and symmetry C-O stretching modes, respectively. HKUST-1 incorporated with PrGO exhibits all Raman peaks of PEDOT, rGO and HKUST-1, demonstrating the successful combination of PrGO/ HKUST-1 ( Fig. 2(b) www.nature.com/scientificreports/ XPS analysis was performed in order to determine the chemical composition of the as-prepared samples. The C1s and O1s signals are observed in GO ( Fig. 3(a)) while PrGO ( Fig. 3(b)) implies the S2p, C1s and O1s signals.  The electrochemical properties of the composites were evaluated in a three-electrode configuration. From Fig. 4(a), PEDOT reveals a quasi-rectangular CV shape, suggesting the pseudocapacitance characteristic. PrGO displays a nearly rectangular CV curve, indicating EDLC characteristics. This result shows that rGO is dominant in the PrGO composite. CV curve of HKUST-1 displays a redox peak that confirms the faradic charge storage mechanism. Interestingly, the integration of PrGO with HKUST-1 (PrGO/HKUST-1) has significantly increased the redox peak currents, where the peaks are mainly contributed by the pseudocapacitance characteristic of HKUST-1. The oxidation peak demonstrates the oxidation of Cu + to Cu 2+ while, the reduction peak shows the reduction of Cu 2+ to Cu +37 . The electrochemical reactions that occur in the HKUST-1 can be explained using Eq. (1) 38 : where R is the ligand of HKUST-1. When electrolyte cation (K + ) enters the HKUST-1 network, HKUST-1 displays pseudocapacitive behavior as a reaction between copper ion (Cu 2+/+ ) and the electrolyte occurs, which mainly contributed from the K + ion insertion and deinsertion process 38 . Figure 4(b) implies the CV curves of PrGO/ HKUST-1 at different scan rates ranging from 5 to 100 mV/s. The redox current density of PrGO/HKUST-1 gradually intensifies as the scan rate increases. It can be clearly observed that PrGO/HKUST-1 is able to maintain its CV shape with well-defined redox peaks without an evident distortion at a higher scan rate (100 mV/s), signifying a good rate capability of PrGO/HKUST-1 39,40 .
The prepared materials were further analyzed via galvanostatic charge-discharge (GCD) analysis. Figure 4(c) illustrates the GCD curves of different materials at 1.8 A/g. PEDOT, HKUST-1 as well as PrGO/HKUST-1 demonstrate non-linear GCD curves 41 , indicating the good capacitive performance of materials with pseudocapacitive behavior 11,40,42 whereas PrGO exhibits a nearly linear GCD curve, indicating EDLC behavior of the electrode 18 . PrGO/HKUST-1 depicts the longest discharging time compared to other individual samples with a small and negligible voltage drop (IR drop), demonstrating an outstanding specific capacitance as well as the low internal resistance of the electroactive material 43 . Figure 4(d) presents GCD measurements of PrGO/HKUST-1 at different current densities (1.0-2.0 A/g). The GCD curves of PrGO/HKUST-1 clearly show the discharging time of the electrode reduces when the current density increases. This is because, at higher current density, the electrolyte ions movement is time limited, where only outer electroactive sites of the electrode are involved for the energy storage process. Moreover, the GCD curves retain non-linear GCD shapes at all current densities, demonstrating good electrochemical reversibility of PrGO/HKUST-1 44 .
The charge storage capacity of the as-prepared symmetrical energy storage devices was examined via a twoelectrode configuration using KCl/PVA gel as an electrolyte and separator 1 . Figure 5(a) depicts the CV curves of different materials at a potential range of 0 to 1 V. Quasi rectangular CV shapes of PEDOT and HKUST-1 prove the pseudocapacitance behavior, whereas PrGO displays a nearly rectangular CV curve, demonstrating EDLC characteristic. The PrGO/HKUST-1 depicts a quasi rectangular CV curve, suggesting a combination of EDLC and pseudocapacitance behavior 45 . The PrGO/HKUST-1 reveals the largest CV curve, signifying the highest specific capacitance (C sp ) as the area under the CV curve indicates the quantity of electrical charge stored in an electrode 13 . The C sp can be calculated using Eq. (2): where IdV indicates the integrated area of the CV curve while m, v and ∆V are mass (g) of active material, potential scan rate (V/s) and potential window (V) of the sample, respectively. The C sp obtained for PrGO/HKUST-1 www.nature.com/scientificreports/ is 360.5 F/g where it is significantly greater compared to HKUST-1 (103.1 F/g), PrGO (98.5 F/g) and PEDOT (50.8 F/g) at a scan rate of 5 mV/s. Figure 5(b) implies the CV curves of symmetrical PrGO/HKUST-1 from the scan rate of 5 to 100 mV/s. The current density and the area under the CV curves increase evidently as the scan rate is increased. The relationship between C sp and scan rate is elucidated in Fig. 5(c) and the results confirm that the C sp reduces over the increasing scan rate. At slower scan rates, the electrolyte ions are able to utilize all the electroactive sites of the material and lead to a complete redox reaction, which provides high C sp 4 . However, at faster scan rates, the movement of electrolyte ions is time limited which means that only the outer electroactive sites of the material are involved for energy storage, resulting in low C sp 46 . Interestingly, PrGO/HKUST-1 exhibits higher C sp compared to other materials, proving incorporation of HKUST-1, PEDOT and rGO can successfully boost the electrochemical performance of PrGO/HKUST-1 composite, which mainly caused by the faradic redox reaction that occurs at the surface of electroactive material 47 . The C sp achieved in this work is higher in comparison to other reported HKUST-1 based supercapacitors (Table 1).
Trasatti method was performed to evaluate the individual capacitance contribution (EDLC and pseudocapacitance) from the total capacitance (C t ) 51 . The EDLC (surface charge) capacitance (C EDLC ) is achieved by retrieving the y-axis intercept of plot C sp vs 1/square root of scan rate (v −1/2 ) (Eq. (3)). Whereas, Eq. (4) is used to determine the C t by extracting the 1/C t value (y-axis intercept of plot 1/C sp vs square root of scan rate (v 1/2 )). The capacitance difference between C t and C EDLC is expressed as the diffusion-controlled charge (pseudocapacitance, C PC ) 52 . Figure 5(d) denotes the relationship between C sp and v −1/2 . PrGO and PrGO/HKUST-1 illustrate C EDLC of 33.2 F/g and 27.1 F/g, respectively. Meanwhile, PEDOT and HKUST-1 depict minimal C EDLC , designating the pseudocapacitive behavior of the samples. The C t values of the as-prepared samples are retrieved from Fig. 5(e). The percentage of C EDLC (%C EDLC ) and C PC (%C PC ) contribution in the samples are calculated utilizing Eqs. (5) and (6), respectively. The presented bar chart (Fig. 5(f)) illustrates a maximum %C PC of 99.9% for both PEDOT and HKUST-1, indicating the pseudocapacitive charge storage mechanism. Whereas, the %C EDLC and %C PC in PrGO (%C EDLC = 24.2% and %C PC = 75.8%) and PrGO/HKUST-1 (%C EDLC = 2.9% and %C PC = 98.1%) demonstrate hybrid supercapacitive behavior with both surface charge and diffusion-controlled charge.
(3)  The as-prepared symmetrical devices were further compared to other devices via GCD measurements at a current density of 1.8 A/g. The symmetrical PrGO/HKUST-1 (Fig. 6(a)) device exhibits a non-linear GCD curve, revealing the presence of pseudocapacitive material that is dominant in the PrGO/HKUST-1. Furthermore, the symmetrical PrGO/HKUST-1 shows the longest discharging time, suggesting a high C sp . Figure 6(b) displays GCD measurements of symmetrical PrGO/HKUST-1 device at various current densities (1.0 to 2.0 A/g). The C sp can be also obtained from GCD measurements utilizing Eq. (7), where the I, ∆t, m and ∆V refer to discharging current (A), discharging time of the device (s), average mass of two electrodes (g) and cell operating potential (V), respectively.  www.nature.com/scientificreports/ The PrGO/HKUST-1 device exhibits a C sp of 163.5 F/g at 1.0 A/g, which declines to 104.2 F/g at 2.0 A/g. The GCD results ( Fig. 6(b)) are in good agreement with the CV plots displayed in Fig. 5(b). The specific energy (E) and specific power (P) of an electrode can be measured utilizing Eqs. (8) and (9) where C sp , ∆V, I and m are the specific capacitance, potential window at discharging process, the current applied and mass of symmetrical electrode, respectively.  Table 2).
The conductivity as well as the ion mobility at the interface of electrode/electrolyte were evaluated via EIS analysis by retrieving the information of the internal resistance along with the interface resistance amidst an electrode and electrolyte 57 . The Nyquist plots (Fig. 7(a)) consist of equivalent series resistance (ESR) as well as the resistance of charge transfer (R ct ) at the high frequency region, while the vertical line (Warburg line) at low frequency region. The ESR is the intersection point that appears at the real axis whereas R ct is the semicircle diameter. The PrGO/HKUST-1 exhibits the lowest ESR (35.0 Ω) and R ct (1.16 Ω) compared to PEDOT (ESR = 40.1 Ω, R ct = 3.7 kΩ), PrGO (ESR = 39.3 Ω, R ct = 2.72 Ω) and HKUST-1 (ESR = 36.2 Ω, R ct = 2.56 Ω). The lowest ESR of PrGO/HKUST-1 reveals a good contact between the current collector and electrode material while the small R ct shows a low resistance at the electrode/electrolyte interface, demonstrating high conductivity of PrGO/HKUST-1 57 . Moreover, PrGO/HKUST-1 illustrates the shortest vertical line at the low-frequency region, signifying a rapid ion diffusion rate within the electrode/electrolyte interface 58 .
Inset Fig. 7(c) displays an equivalent circuit that represents the electrochemical system of the PrGO/HKUST-1 composite. The equivalent circuit consists of ESR, R ct , Warburg (W) and the constant phase element (CPE). The double layer capacitor (C dl ) is replaced by CPE due to the electrode surface inhomogeneity 7 . Chi-square (χ 2 ) is the sum of the square differences between theoretical and experimental results 61 . From the fit and simulation analysis, the value of χ 2 obtained is 7.2 × 10 -3 , proving that the equivalent circuit is suitable for the electrochemical system of PrGO/HKUST-1.
The cycling stabilities of HKUST-1 and PrGO/HKUST-1 were evaluated over 4000 CV cycles at 100 mV/s. From Fig. 7(d), the capacitance retention of PrGO/HKUST-1 is 95.5% compared to HKUST-1 (85.4%), confirming excellent long-term stability of the PrGO/HKUST-1 device. A slight increment in capacitance retention (first 300 cycles) can be noticed in both HKUST-1 (121.3%) and PrGO/HKUST-1 (103.2%), indicating a self-activation process where electrolyte ions continuously penetrate all the active sites of the composite 62 . During the long-term cycling stability, HKUST-1 depicts an obvious decrease in specific capacitance compared to PrGO/HKUST-1 due to the swelling and shrinking properties of HKUST-1 during the redox reaction 63 . The inset of Fig. 7(d) demonstrates that the shape and the size of the CV curve at the 4000 th cycle are almost similar to the 1st cycle, proving that the specific capacitance of PrGO/HKUST-1 only drops slightly. The high cycling stability of PrGO/ HKUST-1 is due to the presence of rGO, where it is able to provide high mechanical strength to the composite 64 . After 4000 cycles, the EIS measurement of the PrGO/HKUST-1 device implies an ESR and R ct of 45.6 Ω and 1.19

Conclusions
A novel PrGO/HKUST-1 composite was successfully synthesized as an outstanding supercapacitor device. The octahedral HKUST-1 on wrinkled-like sheet PrGO exhibited a unique morphology which boosts the electrochemical performance of the electrode by demonstrating a superior specific capacitance (360.5 F/g), remarkable specific energy (21 Wh/kg) at a specific power of 479.7 W/kg and excellent cyclability (95.5% energy retention over 4000 cycles). Thus, the combination of PrGO and HKUST-1 with enhanced electrochemical performance is a promising energy storage material.
1 mM EDOT monomer was then added into the 1 mg/ml GO and the solution was shaken and left overnight. The PrGO was electrodeposited on ITO glass (current collector) at a fixed potential of 1.2 V for 10 min 7 . For comparison, PEDOT was also prepared via a similar deposition technique using 10 mM EDOT and 0.1 M LiClO 4 in DI water. The electrodepositions of PEDOT and PrGO were carried out using a potentiostat (Autolab