Asymmetric supercapacitor of functionalised electrospun carbon fibers/poly(3,4-ethylenedioxythiophene)/manganese oxide//activated carbon with superior electrochemical performance

Asymmetric supercapacitors (ASC) have shown a great potential candidate for high-performance supercapacitor due to their wide operating potential which can remarkably enhance the capacitive behaviour. In present work, a novel positive electrode derived from functionalised carbon nanofibers/poly(3,4-ethylenedioxythiophene)/manganese oxide (f-CNFs/PEDOT/MnO2) was prepared using a multi-step route and activated carbon (AC) was fabricated as a negative electrode for ASC. A uniform distribution of PEDOT and MnO2 on f-CNFs as well as porous granular of AC are well-observed in FESEM. The assembled f-CNFs/PEDOT/MnO2//AC with an operating potential of 1.6 V can achieve a maximum specific capacitance of 537 F/g at a scan rate of 5 mV/s and good cycling stability (81.06% after cycling 8000 times). Furthermore, the as-prepared ASC exhibited reasonably high specific energy of 49.4 Wh/kg and low charge transfer resistance (Rct) of 2.27 Ω, thus, confirming f-CNFs/PEDOT/MnO2//AC as a promising electrode material for the future energy storage system.

were investigated through FESEM as shown in Fig. 1. In Fig. 1(a), the cross-linking structures of the as-prepared fibers are randomly oriented with smooth surface and beads-free. After the inclusion of PEDOT and MnO 2 using the electrochemical approach, uniform growth of PEDOT and MnO 2 nanoparticles on the f-CNFs surface can be observed without any aggregation. The presence of abundance oxygenated functional groups attached on CNFs can serve as nucleation sites for the growth of PEDOT and MnO 2 23 and providing better ions diffusion process from the electrolyte onto the electrode 11 . The f-CNFs/PEDOT/MnO 2 has an average diameter of 390 ± 68 nm, which is slightly higher as compared with pure f-CNFs (354 ± 45 nm) 11 . The high-magnification FESEM image (inset of Fig. 1a) also proves that the coatings of PEDOT and MnO 2 are relatively uniform on the fibrous networks. In Fig. 1(b), the AC electrode displays a homogenous and irregular carbon spheres morphology with an average diameter of 65 ± 12 nm. The void spaces between the AC particles provide more accessible surface sites that allow more contact surface between electrode and electrolyte ions 24 . Therefore, the unique morphology for both positive and negative electrodes can provide good charge propagation behaviour in ASC. www.nature.com/scientificreports www.nature.com/scientificreports/ Raman spectroscopy. Raman spectroscopy was used to study the functional groups that exist in both f-CNFs/PEDOT/MnO 2 and AC electrodes as shown in Fig. 2. Two prominent peaks centred around 1351 and 1586 cm −1 in the AC spectrum, corresponding to the D and G bands, respectively. D band is related to the defect/ disordered carbon structure (sp 3 ) while G band originates from the crystalline graphitic layer (sp 2 ). The D band (1351 cm −1 ) and G band (1591 cm −1 ) of the f-CNFs are also observed in the Raman spectrum of f-CNFs/PEDOT/ MnO 2 . The presence of PEDOT in the spectrum is confirmed by three vibrational peaks; 986, 1087 and 1351 cm −1 which are associated with oxyethylene ring deformation, C-O-C deformation and C-C stretching vibration of PEDOT 25 , respectively. The intensity ratio (I D /I G ) of f-CNFs/PEDOT/MnO 2 is 0.85, which is slightly lower compared with AC (0.87), revealing a small number of defects in the sample 26 . In addition, a characteristic peak at 655 cm −1 is assigned to the stretching vibration of birnessite-type MnO 2 27 .

X-ray diffraction (XRD).
The crystallinity of f-CNFs/PEDOT/MnO 2 and AC was investigated by XRD analysis, and the corresponding patterns are shown in Fig. 3. In the XRD pattern of f-CNFs/PEDOT/MnO 2 , a broad peak at 2θ = 25° can be indexed to the (002) diffraction plane of f-CNFs 28 with an amorphous or low crystallinity carbon phase 29 . The presence of MnO 2 nanoparticles was confirmed by two diffraction peaks positioned at 37° and 65° which can be identified as the (111) and (002) crystal planes of MnO 2 (JCPDS Card. No. 24-0735) 30 . In addition, the sharp diffraction peaks at 2θ = 17° and 18° corresponded to the (200) plane of tetragonal α-MnO 2 phase (JCPDS 072-1982). Furthermore, a low-intensity peak (2θ = 24°) which is overlapped with the broad peak of f-CNFs corresponding to the diffraction peak of PEDOT (020) with high-crystallinity feature 31 . As shown in  www.nature.com/scientificreports www.nature.com/scientificreports/  32 with a high degree of graphitic crystallinity. Notably, the additional peaks (2θ = 51° and 60°) appeared in the AC spectrum can be attributed to the bare ITO glass 33 . Figure 4(a) displays the charge-discharge behavior of f-CNFs/PEDOT/ MnO 2 at different current densities to understand its capacitive performance. The GCD curves exhibit nearly symmetrical triangular shape with a small deviation, which could be raised from the pseudo-faradaic reactions of PEDOT and MnO 2 during the charging-discharging process. A noticeable IR drop at the initial portion of the discharge curves indicates the small internal resistance of the system at high charge-discharge current densities 34 . To evaluate the electrochemical performance of both f-CNFs/PEDOT/MnO 2 and AC electrodes, CV measurements www.nature.com/scientificreports www.nature.com/scientificreports/ were carried out in a standard three-electrode setup using 1 M KCl as the electrolyte as shown in Fig. 4(b). The f-CNFs/PEDOT/MnO 2 electrode was measured in the potential window of 0 to 1 V (vs. Ag/AgCl) and displays a quasi-rectangular shape, indicating the combination of both EDLC and pseudocapacitance behaviour 35 . However, a small hump observed at 0.56 V is attributed to the redox reaction of the oxygenated functional groups at the surface of the electrode 36 and pseudocapacitance of PEDOT and MnO 2 . The f-CNFs/PEDOT/MnO 2 electrode exhibits a larger enclosed CV area compared with AC, demonstrating higher charge capacity 37 and higher specific capacitance 38 . For AC, the electrode contributes the potential window from −0.6 to 0 V (vs. Ag/AgCl) with symmetrical rectangular CV curve, indicating an ideal EDLC feature 39 with good rate capability 21 . The C sp for f-CNFs/ PEDOT/MnO 2 (442.50 F/g) and AC (58.89 F/g) are calculated based on Eq. (1):

Electrochemical measurements.
where C sp (F/g) represents specific capacitance, I dV is the integrated area CV curve, v (Vs −1 ) is the potential scan rates, m (g) is the mass of sample, and dV (V) is the p, ΔV (V) is the potential window (CV) or potential drop during the discharging time (GCD) and I (A) is the applied current. The maximum potential window of the f-CNFs/PEDOT/MnO 2 //AC can be extended up to 1.6 V. Therefore, the charge of both electrodes need to be balanced in order to obtain a stable asymmetric supercapacitor via Eq. (2): where m is the mass of electrode, C is C sp of the respective electrode and ∆E is the operating voltage (0.6 V for negative and 1.0 V for positive electrode). The subscript of "+" and "−" relate to positive and negative electrodes.  Fig. 4(d). It can be seen that the ASC can work stably even at 1.6 V potential window and the quasi-rectangular of CV curves are retained, corresponding to good supercapacitive behaviour. Moreover, the specific capacitance increases from 287 F/g to 354 F/g over extended potential windows, which significantly improve the charge storage capacity of the composite 40 . Figure 4(e) represents the CV of as-obtained ASC at different scan rates, ranging from 5 to 100 mV/s. At lower scan rates, the CV profiles exhibit rectangular-like shape and as the scan rate increases from 50 to 100 mV/s, the CV curves show some distortions from an ideal rectangular shape. This is due to the limitation of the electrolyte ions to diffuse into the active material and only capable to access at the outer surface of the material, leading to the decrease of specific capacitance 41 . The calculated specific capacitance of ASC can achieve 537, 459, 354, 269, 221 and 188 F/g at a scan rate of 5, 10, 25, 50, 75 and 100 mV/s, respectively. The prominent specific capacitance of ASC could be ascribed from the synergistic effect between f-CNFs/PEDOT/MnO 2 and AC, where EDLC of f-CNFs and AC contribute to a larger ion-accessible surface area, while PEDOT and MnO 2 possess excellent electrical conductivity and high specific capacitance, respectively.
To further examine the electrochemical performance of ASCs, the GCD tests were carried out at a current density of 0.5 A/g as shown in Fig. 5(a). It can be clearly seen that f-CNFs/PEDOT/MnO 2 //AC electrode exhibits the largest charging-discharging time span, contributing to an excellent capacitive behaviour 42 . GCD measurement was also performed at various current densities; 0.3 to 8 A/g as presented in Fig. 5(b). Apparently, all GCD curves show a nearly triangular shape with a little deviation, corresponding to the signature of a redox-type storage mechanism 43 with good electrochemical reversibility 24 . The specific capacitance of 148.07 F/g is obtained at a current density of 0.3 A/g. Figure 5(c) illustrates the Ragone plot (specific energy vs specific power) of f-CNFs/ PEDOT/MnO 2 //AC asymmetric cell which exhibits maximum specific energy of 49.4 Wh/kg with a specific power of 224.02 W/kg at a current density of 0.3 A/g. Furthermore, the specific energy obtained is superior as compared to the reported values for PEDOT-and MnO 2 -based fibers for supercapacitor 4,21,39,[44][45][46] .
The electrochemical properties of different asymmetric supercapacitors were further analysed using EIS measurements and their Nyquist plots are shown in Fig. 5(d). The plot consists of two regions: high and low frequency regions. At high frequency, the semicircle arc and the intercept of the real axis (Z') indicate the charge transfer resistance (R ct ) and equivalent series resistance (ESR), respectively. The straight line in the low-frequency region represents the Warburg impedance (W) for ion diffusion at the electrolyte/electrode interface. The R ct of as-prepared f-CNFs/PEDOT/MnO 2 //AC, CNFs/PEDOT//AC and f-CNFs/MnO 2 //AC are 2.27, 1.2 and 2.74 Ω, respectively. A slightly higher R ct of f-CNFs/PEDOT/MnO 2 //AC in comparison to f-CNFs/PEDOT//AC can be ascribed to the low conductivity of MnO 2 . Furthermore, the f-CNFs/PEDOT/MnO 2 //AC electrode shows the lowest ESR value of 35.58 Ω compared to f-CNFs/PEDOT//AC (35.63 Ω) and f-CNFs/MnO 2 //AC (41.28 Ω) as the ESR is attributed to the ionic resistance of the electrolyte, the resistance of the active material and contact resistance between the current collector and active material 47 . The nearly vertical line (close to 90°) at low frequency region of the f-CNFs/PEDOT/MnO 2 //AC indicates a characteristic of an ideal capacitive behaviour 48 . In addition, f-CNFs/PEDOT/MnO 2 //AC exhibits the shortest vertical line along the imaginary axis, implying rapid ion diffusion.
The cycling stability test of f-CNFs/PEDOT/MnO 2 //AC asymmetric cell was performed over 8000 CV cycles at a potential window of 1.6 V (Fig. 5(e)). Remarkably, the ASC device displays good cycling stability and retained (2019) 9:16782 | https://doi.org/10.1038/s41598-019-53421-w www.nature.com/scientificreports www.nature.com/scientificreports/ 81.06% of its original capacitance after 8000 cycles. A good long-term cycling stability of ASC is mainly contributed from the superior mechanical strength of the f-CNFs and AC which effectively can enhance the cyclability during the charging/discharging process.

conclusions
In summary, a promising positive electrode derived from f-CNFs/PEDOT/MnO 2 was successfully fabricated for asymmetric supercapacitor. By combining with AC as a negative electrode, the ASC device induces a strong synergistic effect which greatly enhanced its electrochemical performance. A well-adhered of PEDOT and MnO 2 on the surface of the f-CNFs as well the highly porous morphology of AC was beneficial in providing ease ion-diffusion pathways at the electrolyte/electrode interface, thus can increase the specific capacitance. The www.nature.com/scientificreports www.nature.com/scientificreports/ assembled f-CNFs/PEDOT/MnO//AC could operate reversibly at a maximum voltage of 1.6 V, and displayed specific capacitance as high as 537 F/g, with good cycling stability (81.06%) after 8000 cycles. It also offers high specific energy of 49.4 Wh/kg at a specific power of 224.02 W/kg and low R ct , implying its good rate performance and enhanced conductivity. These outstanding results prove that f-CNFs/PEDOT/MnO//AC ASC can hold great potential for achieving high-performance supercapacitors.

experimental Section
Materials. Indium

Characterisations.
The morphologies of f-CNFs/PEDOT/MnO 2 and AC were examined using field emission scanning electron microscopy (FESEM, JEOL JSM-7600F). The structure and elemental composition of both positive and negative electrodes were identified using Raman spectroscopy (Alpha 300R, 532 nm Ar-ion laser) and X-ray diffraction (XRD, Shimadzu with Cu K∝ radiation (λ = 1.54 Å) Electrochemical test. Electrochemical measurements of the individual f-CNFs/PEDOT/MnO 2 (positive) and AC (negative) were performed in a three-electrode configuration in 1.0 M KCl electrolyte. An Ag/AgCl and Pt wire were employed as a reference electrode and the auxiliary electrode, respectively. The CV of positive and negative electrodes was recorded at the voltage of 0-1.0 V and −0.6 to 0 V, respectively at a scan rate of 25 mV/s, respectively. The ASC two-electrode configuration was assembled by sandwiching both positive and negative electrodes together with a filter paper soaked in 1.0 M KCl as a separator. A series of electrochemical measurements, including CV, galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) was performed for ASCs. The CV analysis was performed from 0 to 1.6 V potential window at various scan rates (5-100 mV/s). GCD analysis was tested at a current density from 0.3-8 A/g. The C sp , specific energy (E) and specific power (P) of the ASC obtained from GCD curves were calculated according to Eqs (3-5): where C sp (F/g) represents specific capacitance, Δt is discharge time (h), m (g) is the mass of active material (kg), ΔV (V) is the potential drop during the discharging time and I (A) is the applied current. EIS measurements were carried out in a frequency ranging from 0.01 Hz to 100 kHz at AC amplitude of 5 mV.