Unveiling high specific energy supercapacitor from layer-by-layer assembled polypyrrole/graphene oxide|polypyrrole/manganese oxide electrode material

A novel layer-by-layer (LBL) based electrode material for supercapacitor consists of polypyrrole/graphene oxide and polypyrrole/manganese oxide (PPy/GO|PPy/MnO2) has prepared by electrochemical deposition. The formation of LBL assembled nanocomposite is confirmed by Fourier transform infrared spectroscopy, Raman spectroscopy and X-ray diffraction. The field emission scanning electron microscopy images clearly showed that PPy/MnO2 was uniformly coated on PPy/GO. The PPy/GO|PPy/MnO2 symmetrical supercapacitor has revealed outstanding supercapacitive performance with a high specific capacitance of 786.6 F/g, an exceptionally high specific energy of 52.3 Wh/kg at a specific power of 1392.9 W/kg and preserve a good cycling stability over 1000 cycles. It is certain that PPy/GO|PPy/MnO2 has an extraordinary perspective as an electrode for future supercapacitor developments. This finding contributes to a significant impact on the evolution of electrochemical supercapacitor.

www.nature.com/scientificreports www.nature.com/scientificreports/ close two different material in nanoscale level. Despite all these evolvements in electrode material, the energy output of SC still subtle. Yet, the bilayer film built up of PPy/GO composite and PPy/MnO 2 composite for SC, to the best of our knowledge has never been reported.
Hence, in the current investigation, we have designed and assembled an electrode consisting of PPy/GO and PPy/MnO 2 through an LBL approach. The assembled nanocomposite has various benefits. The most anticipated virtues of this LBL assembled are (1) providing an enormous number of active sides for diffusion of electrolytes in layered composite and (2) directly forming a binder-free supercapacitor electrode on a conductive substrate, in which, it would reduce the contact resistance between electrode material and substrate. Benefiting from the synergistic effect of LBL construction, this newly designed electrode material demonstrated superior specific energy, satisfactory cycling performance and enhanced specific capacitance.
Layer-by-layer assembly of PPy/GO with PPy/MnO 2 . LBL of PPy/GO with PPy/MnO 2 ( Fig. 1) was carried out by a simple and convenient electrochemical deposition method conducted using a Metrohm Autolab/ M101 potentiostat. In brief, a three-electrode system was employed for electrodeposition, wherein ITO was used as a working electrode, platinum coil served as a counter electrode and silver/silver chloride (Ag/AgCl) worked as a reference electrode. Prior to the electrodeposition, the ITO substrate (geometrical area 1 cm 2 ) was ultrasonically cleaned with ethanol, acetone and deionized water sequentially for 15 min before use. Two different solutions were prepared separately for the fabrication of LBL composite with the following procedures. 1 mg/ml GO aqueous dispersion was ultrasonicated for 60 min and was then added with 100 mM pyrrole monomer to obtain pyrrole/GO solution. Whereas, in a separate flask, 100 mM pyrrole and 0.1 M MnSO 4 .H 2 O was mixed in deionized water. For the preparation of LBL composite, first the PPy/GO film layer was electrodeposited on the cleaned ITO at a constant potential of 0.8 V for 10 minutes using chronoamperometry technique. After that, the PPy/ MnO 2 film layer was electrodeposited on the previously obtained PPy/GO layer under the same condition. For comparison, the single layer films of PPy/GO and PPy/MnO 2 were also electrodeposited using a similar method. www.nature.com/scientificreports www.nature.com/scientificreports/ Characterization. The surface morphologies of the as-prepared nanocomposites were examined by field emission scanning electron microscopy (JEOL JSM-T600F. The presence of functional groups was detected by Fourier transform infrared (FTIR, Perkin-Elmer FT-IR spectrophotometer coupled with UATR accessory) in the frequency range from 4000 to 400 cm −1 and Raman spectroscopy (WITEC Alpha 300 R) in the frequency range from 4000 to 200 cm −1 with an excitation wavelength of 532 nm. The X-ray diffraction (XRD) patterns were recorded with a Shimadzu X-ray diffractometer with Cu Kα radiation (λ = 1.54 Å).
Electrochemical performance. The capacitive performance was evaluated at room temperature in a two-electrode symmetrical cell assembly, which were separated by a filter paper immersed in 1.0 M Na 2 SO 4 aqueous solution. Each sample was weighed before and after electrodeposition in order to determine the mass of deposited electroactive material on ITO. The cyclic voltammetry (CV) curves, galvanostatic charge-discharge (GCD) curves, electrochemical impedance spectroscopy (EIS) plots were obtained by using a potentiostat (Metrohm Autolab/M101). The EIS measurements were performed at open circuit potential (OCP) by using a 5 mV AC sinusoid signal in the frequency range from 100 kHz to 0.01 Hz. CV measurements were carried out at the potential window between 0 to 1 V at scan rates ranging from 25 to 200 mV/s. The GCD test was performed at different current densities (3.0 to 7.0 A/g) in the potential range of 0-1 V. The specific power, specific energy and specific capacitances were measured based on the mass of both electrodes, anode and cathode. The specific capacitance was calculated from the CV curves according to the following equation: where, C sp is the specific capacitance (F/g), and V a and V c are the integration limits of the CV (V), I is the response current (A), υ is the potential scan rate (mV/s), and m is the average mass of two electrodes (g). The specific power and specific energy were calculated from GCD curves based on the following equations: where, E is the specific energy (Wh/kg), P is the specific power (W/kg), C sp is the specific capacitance (F/g), I is the discharge current (A), and ∆V is the cell operation potential (V) and m is the average mass of two electrodes (g). www.nature.com/scientificreports www.nature.com/scientificreports/ stretching peak of PPy is located at 1550 cm −1 but this peak has shifted to 1523 cm −1 , indicating the existence of π − π and hydrogen bonding between GO and PPy.

Results and Discussion
In the spectrum of PPy/MnO 2 (Fig. 2b), the peak around 3000 cm −1 and 1684 cm −1 are assigned to O-H stretching and O-H bending, respectively. Whereas, the characteristic peak of MnO 2 is noticed at 734 cm −1 , attributed to Mn-O bond 17,18 . Despite this, all other main characteristic peaks of PPy are also seen in the spectrum which closely resembles peaks in the PPy/GO spectrum. However, a point to note here is that, after MnO 2 is incorporated with PPy in PPy/MnO 2 , the corresponding peaks of C-N and N-H are shifted to higher frequency range compared to PPy/GO spectrum 17 .
The LBL assembled composite spectrum (Fig. 2c) shows the peaks approximately similar to the combination of vibration peaks of PPy/GO and PPy/MnO 2 composites. Additionally, the bands of C-O-C and C=C-N deformation are disappeared which expected that have overlapped with both of peaks of PPy at around 1060 cm −1 and 834 cm −1 . It is noteworthy, some of the peaks in LBL assembled composite are shifted as a result of interaction between PPy/GO and PPy/MnO 2 .
Raman spectroscopy. The Raman analysis for the as-synthesized nanocomposites (Fig. 3) was carried out to further confirm the presence of active materials. The existence of D and G bands in PPy/GO spectrum (Fig. 3a) are observed at 1370 cm −1 and 1575 cm −1 , respectively. The D band is due to vibration of aromatic rings, random edges arrangement and low symmetric carbon structure 19 . While, the G band corresponds to the first-order scattering of the E 2g of sp 2 -bonded carbon atom 17,20 . It is observed that the vibration bands of PPy do not appear in the range of 1350 cm −1 to 1580 cm −1 due to low intensity and might be overlapped with GO peaks (G and D bands). Whereas, PPy/MnO 2 (Fig. 3b) spectrum shows a peak at 634 cm −1 for the Mn-O lattice vibrations. A few broad peaks at 1579 cm −1 (C=C stretching), 1390 cm −1 (C-N stretching), 1062 cm −1 and 980 cm −1 (C-H ring deformation vibration) are associated to the characteristics of PPy 19,21 . In the spectrum of LBL assembled composite (Fig. 3c), all the characteristic peaks of PPy/MnO 2 and PPy/GO clearly appear. However, there are some peaks shifted, which could be due to electrostatic interactions and hydrogen bonding 22 .

X-ray diffractometry (XRD).
In order to study the crystalline structure, XRD measurements were conducted on PPy/MnO 2 , PPy/GO and LBL assembled PPy/GO|PPy/MnO 2 , as shown in Fig. 4. The existence of MnO 2 in LBL composite was further proved by comparing the LBL composite with MnO 2 that obtained via electrodeposition under the same experimental condition. In the PPy/MnO 2 spectrum (Fig. 4a), the peaks at 26.1° (002), 37.8° (100) and 64.6° (110) are ascribed to the characteristic of MnO 2 (JCPDS No. 71-0071) 23 . These diffraction peaks correspond to tetragonal MnO 2 phase. A weak and broad peak was observed in the spectra of PPy/MnO 2 (Fig. 4a) and PPy/GO (Fig. 4) at 23-35° (002) indicating the diffraction peak of PPy and further suggesting that PPy is amorphous 24 . Moreover, the peaks for MnO 2 are intense and distinguishable, showing that a highly crystalline phase of MnO 2 . Whereas, the X-ray analysis for the LBL assembled composite (Fig. 4c) shows all the characteristic peaks of PPy, GO and MnO 2 , confirming the LBL composite are made up of PPy/GO and PPy/MnO 2 .

Field Emission Scanning Electron Microscope (FESEM).
FESEM was employed to study the morphologies of PPy/GO, PPy/MnO 2 and LBL assembled composite. The image of PPy/GO displays rough and wrinkle surface (Fig. 5a) in which the PPy particles are uniformly grown together with GO sheets by hydrogen bonding and π − π interaction 16 . Whereas, Fig. 5b   www.nature.com/scientificreports www.nature.com/scientificreports/ with PPy particles. This image shows a rough surface build by granular particles of composites which could provide a large surface area that enhance the performance of SC, as well as more charges can be stored. Utilizing LBL approach to electrodeposit PPy/MnO 2 onto PPy/GO forming LBL assembled PPy/GO|PPy/MnO 2 nanocomposite (Fig. 5c) which has similar morphology as PPy/MnO 2 indicates PPy/MnO 2 is successfully deposited on the surface on PPy/GO. This structure allows more ion diffusion and migration in the electrodes, implying high energy capacity 16 . Furthermore, the existence of MnO 2 particles in the PPy matrix could increase the surface area and eventually is able to enhance the supercapacitive performance. The layered structure of LBL assembled composite can be confirmed with the cross-sectional view as shown in Fig. 5d. It is clearly seen that the film has two layers owing to PPy/GO and PPy/MnO 2 .  www.nature.com/scientificreports www.nature.com/scientificreports/ Cyclic voltammetry. In order to investigate the electrochemical behavior of the as-prepared PPy/GO| PPy/MnO 2 composite, PPy/GO and PPy/MnO 2 , cyclic voltammetric measurements were carried out using the two-electrode electrochemical system. Figure 6a shows the CV curves of LBL assembled composite, PPy/GO and PPy/MnO 2 at a scan rate of 25 mV/s in a potential range of 0 to 1 V. The CVs of LBL assembled composite and PPy/MnO 2 are quasi-rectangular, implying involvement of faradaic reaction, while the CV curves of PPy/GO is close to rectangular shape, indicating an ideal electrical double layer capacitor (EDLC) behavior 25 . It is found that C sp of LBL assembled composite (786.6 F/g) is higher than single layer composites, PPy/MnO 2 (284 F/g) and PPy/GO (78 F/g). The LBL assembled PPy/GO|PPy/MnO 2 also has a higher C sp compared to the ternary GO/ PPy/MnO 2 composite (207 F/g) 26 . This indicates that LBL assembled composite provides more active sites which enhance the electrochemical performance compared to the ternary composite. As shown in Fig. 6b, the CVs of LBL assembled composite at various scan rates (25 to 200 mV/s) exhibits quasi-rectangular shapes and the CV shape remain unchanged from the lowest scan rate (25 mV/s) to the highest scan rate (200 mV/s), implying ideal SC behaviour 27 . Moreover, the anodic and cathodic currents increase with the increasing of scan rate. However, the C sp of LBL assembled bilayer composite decreases from 25 mV/s (786.6 F/g) to 200 mV/s (206 F/g) as shown in Fig. 6c due to limited ionic diffusion in the electrode material at high scan rate 28 . Figure 7a shows the GCD curves of LBL assembled composite, PPy/GO and PPy/MnO 2 at a current density of 4.0 A/g. Based on the GCD curves, PPy/MnO 2 shows asymmetry triangular shape, whereas PPy/GO has symmetrical triangular shape. Meanwhile, the large distortion of the triangular shape of LBL assembled composite is mainly due to the domination of pseudocapacitive properties in the combination of both composites 29 . It is observed that LBL assembled composite has a longer discharging time compared to both single layer composites, indicating better capacitance behavior.

Galvanostatic charge-discharge (GCD).
As shown in Fig. 7b, GCD curves of LBL assembled composite displays asymmetrical triangular shapes from the highest (7.0 A/g) to the lowest (3.0 A/g) current density which indicates the material has good charge-discharge reversibility 30 . However, the discharging time decreases with the increase of current density due to the incapability of the electrolyte ions to enter into the inner structure of the active material and only the outer active surface is utilized for ion diffusion at high current densities 16,31 . Figure 7c shows the Ragone plot (specific power vs. specific energy) of LBL assembled composite, PPy/MnO 2 and PPy/GO. The specific energy is inversely proportional to specific power. The LBL assembled shows the highest specific energy (52.35 Wh/kg) at a specific power of 1392.90 W/kg compared to   www.nature.com/scientificreports www.nature.com/scientificreports/ Electrochemical Impedance spectroscopy. The electrochemical performance of LBL assembled composite, PPy/GO and PPy/MnO 2 was examined by electrochemical impedance spectroscopy (EIS) which were carried out over a frequency range from 100 kHz to 0.01 Hz at open circuit potential (Fig. 8). The Nyquist plot (imaginary component (−Z″) versus the real component (Z′)) shows the frequency response in the electrode material/electrolyte system 32 . All impedance spectra exhibited a vertical line approaching 90 degrees in the low frequencies region which corresponds to an ideal capacitor and fast ion diffusion in electrode materials 27 . Based on the magnified view of the Nyquist plot (Fig. 8a), a semicircle is also observed in the high-frequency range for all impedance spectra, indicating there is a hindrance in transferring charge at the interface 33 . Technically, the diameter of semicircle is associated with the charge transfer resistance (R ct ) and the equivalent series resistance (ESR) is obtained at the intercept point of the real-axis at high-frequency region, which is related to the electrolyte resistance, the interfacial contact resistance between current collectors and active materials and the resistance of active materials 34 . Given in Fig. 8b, the equivalent circuit composes of ESR, R ct , constant phase element (CPE) for the irregular morphologies 35,36 , and the "classical" finite-length Warburg diffusion element (W) for the diffusion of electrolyte 37 is used to fit the Nyquist plot. It is clearly observed that LBL assembled composite shows a higher R ct (22.62 Ω) and ESR (35.13 Ω) compared with single layers. The high ESR value is an indication of poor contact between current collector and active materials, high intrinsic resistance of active materials and high ionic resistance of electrolytes 38 , whereas high R ct value is due to the high resistance of the movement of ions at electrolyte/ electrode interface 39 as a result of the combined resistances of both single layers.
Stability. The study on the electrochemical stability of the composites is another important parameter in evaluating the SC performance in real applications. After 1000 consecutive cycles (Fig. 9), the LBL assembled composite is able to retain exceptional cycling stability of 86.09% of its initial C sp compared with both single layer composites, PPy/GO (75.58%) and PPy/MnO 2 (75.44%). These results indicate that contribution of LBL assembled PPy/GO|PPy/MnO 2 not only on promising C sp , specific energy and specific power but also improves the stability of the composite which significantly decreases the destruction of electroactive materials 40 and significantly improves the stability during doping/dedoping process. The comparison with the previously published results (Table 1) indicates that our current work revealed outstanding performance and demonstrating that LBL assembled PPy/GO|PPy/MnO 2 is a promising electrode in enhancing the performance of supercapacitor.