One-step electrodeposition of a polypyrrole/NiO nanocomposite as a supercapacitor electrode

An electrochemical deposition technique was used to fabricate polypyrrole (Ppy)/NiO nanocomposite electrodes for supercapacitors. The nanocomposite electrodes were characterized and investigated by Fourier transform infrared spectroscopy (FTIR), X-ray Diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS). The performance of supercapacitor electrodes of Ppy/NiO nanocomposite was enhanced compared with pristine Ppy electrode. It was found that the Ppy/NiO electrode electrodeposited at 4 A/cm−2 demonstrated the highest specific capacitance of 679 Fg−1 at 1 Ag−1 with an energy density of 94.4 Wh kg−1 and power density of 500.74 W kg−1. Capacitance retention of 83.9% of its initial capacitance after 1000 cycles at 1 Ag−1 was obtained. The high electrochemical performance of Ppy/NiO was due to the synergistic effect of NiO and Ppy, where a rich pores network-like structure made the electrolyte ions more easily accessible for Faradic reactions. This work provided a simple approach for preparing organic–inorganic composite materials as high-performance electrode materials for electrochemical supercapacitors.

www.nature.com/scientificreports/ transition metal oxides (TMOs) possess the advantage of higher capacitances in practice [14][15][16] . NiO is a promising pseudocapacitive electrode due to its high capacity and stability. On the other hand, the performance of NiO electrodes is poor due to their low conductivity 17,18 . In this study, a new nanocomposite of Ppy/NiO electrodes were fabricated via one-step facile electrochemical deposition method at different currents onto the surface of graphite sheet. The electrochemical behavior of the as-prepared electrodes was investigated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS).
Characterization. The composition of the prepared Ppy 1% -DBSA 2% /NiO 97% -GS supercapacitor electrode was studied using Fourier transform infrared (FTIR) (Bruker Corporation, Ettlingen, Germany). The crystalline structure of Ppy/NiO nanocomposite and PPy were studied by using X-ray diffraction was performed using (X-ray 7000 Shimadzu-Japan) at room temperature in the range of 2 h from 10° to 100°. The X-ray source Cu target generated at 30 kV and 30 mA with a scan speed of 4° min −1 . The morphological properties of the prepared nanocomposite were investigated using scanning electron microscopy (SEM), JEOL (JSM 6360 LA, Japan) instruments. Galvanostatic electrochemical charge/discharge (GCD), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed in a three electrode cell at room temperature using a computer-controlled potentiostat (Metrohm Autolab 87070, Germany). Pt and Ag/AgCl electrodes were immersed in acetonitrile of 0.1 M LiClO 4 electrolyte solution in the three electrode cell to characterize the fabricated Ppy 1% -DBSA 2% /NiO 97% -GS SC electrodes. CV and GC/D were carried out for the fabricated SC electrodes in the potential window ranging from 0 to 1 V at sweep rates of 5, 15, 35, 50, 75, and 100 mV s −1 and current densities from 1 to 3 Ag −1 . EIS was implemented in the frequency range from 100 kHz to 0.01 Hz at 5 mV. Cyclic stability test was conducted at a current density of 1 Ag −1 for 1000 cycles. Different performance parameters of the specific capacitance (Cs), energy density (E) and power density (P) of the prepared SC electrodes were calculated by the following equations 19,20 : where I, t, ΔV, s, and m are the discharge current (A), discharge time (s), discharge potential window (V), scan rate (V s −1 ) and mass of the active material (g), respectively.

Results and discussion
Electropolymerization of Ppy 1% -DBSA 2% /NiO 97% -GS. Figure 1 shows the potential-time curves of the chronopotentiometry processes of the synthesis of Ppy 1% -DBSA 2% /NiO 97% -GS with different current densities from 2 to 10 mA cm −2 for 600 s. The electropolymerization process usually consists of three stages, as  [21][22][23] . At the beginning of electropolymerization deposition, the voltage suddenly increases after application of different current densities due to the cathodic overpotential between Pt as a counter and GS as a working electrode. The maximum potentials recorded in the voltage-time curves of the prepared Ppy 1% -DBSA 2% /NiO 97% -GS film at 2, 4, 6, 8, and 10 mA cm −2 are 1045, 1350, 678, 680, and 717 mV, respectively. For 2 and 4 mA cm −2 the voltage increases to the supersaturation region with time due to the increases in the number of charge carriers and the critical grain size or the formation of oligomers of the Ppy 1% -DBSA 2% /NiO 97% -GS deposited on the GS electrode. This can be explained based on the first few seconds of the applied current is sufficient to form the radical cations on the pyrrole rings and start the propagation step in the stage of growth. The potential needs for the formation of radicals from the dimer or trimer is lower than that required to form the pyrrole radicals. Consequently, the potential is lower again after few seconds and attain to the plateau regions. Moreover, the temporary decays appear as a small valley in the voltage curves at 6, 8, and 10 mA cm −2 , which correspond to the diffusion limitation of the oxidation process on the Py monomer, and then small peaks appear after the valley is revealed to the nucleation end growth of the Ppy 21,22 . It is noted that at 2 mA cm −2 , the saturation potential is about 1000 mV and with increasing the current density to 4 mA cm −1 the saturation potential is raised to about 1200 mV. This indicates the formation of the DBSA doped Ppy film, which is more conductive than the GS substrate However, at 8 and 10 mA cm −2 the plateau region is declined to 700 and 750 mV, respectively and this is attributed to the degradation of the polymeric films at high current density.  The XRD pattern represents the formation of NiO/PPy composite with good crystal phase. Figure 3 presents the FTIR spectra of pure Ppy 1% -DBSA 2% /GS and Ppy 1% -DBSA 2% /NiO 97% -GS in the range of 400-4000 cm -1 . A broad band at approximately 3460 cm -1 is attributed to N-H stretching associated with the bound pyrrole ring 24 . The peaks at 1645, 1423, and 1130 cm -1 are assigned to the C-C and C-N stretching vibrations and the C-H in-plane vibrational bands of the polypyrrole ring, respectively 25 . These results indicate that Ppy has been formed. In the Ppy 1% -DBSA 2% /NiO 97% -GS sample, the observed absorption bands at 760 and 548 cm −1 are corresponded to the torsional and stretching vibration modes of the NiO bond at the octahedral and tetrahedral sites, respectively 26,27 . Most of the stretching vibrations in the Ppy 1% -DBSA 2% /NiO 97% -GS sample are the same as that of PPy, with only a slight shift of IR absorption to lower frequencies in Ppy 1% -DBSA 2% /NiO 97% -GS to 3414, 1633, 1410, and 1100 cm −1 , respectively. This suggests that an interaction between the polymer and NiO occurs. This change is due to loss in conjugation and molecular order after modification of Ppy with NiO. This result indicates a strong interaction between Ppy and NiO nanoparticles 28,29 . Surface morphology analysis. SEM micrographs of the electrochemically prepared pure Ppy 1% -DBSA 2% / GS film are illustrated in Fig. 4a. The electronic properties of polypyrrole films are linked to their morphology; the smoother and denser surface leads to a conductive film, and the more pores and ordered film facilitates charge transfer through the film. Ppy films show a cauliflower-like nodular surface morphology and microspherical grains of approximately less than 1 μm diameter. The effect of current densities and NiO on the microstructure of Ppy is indicated in Fig. 4b-e. There are a number of faceted grains elongated and rectangular blocks with different dimensions observed. These blocks result from the formation of NiO, as shown in Fig. 4b. Increasing the current density to 4 mA cm −2 for prepared in situ Ppy and NiO, as presented in Fig. 4c, drives to fusion of the cauliflower and rectangular structures and formation of pores, voids and compatible phases. The rich pores likestructure facilitates the diffusion and transfer of ions from the electrolyte to the electrode film and vice versa. On the other hand, for Ppy 1% -DBSA 2% /NiO 97% -GS@6 displayed in Fig. 4d, the increasing of the current to 6 mA results in a higher thickness of the nanocomposite and induce the phase separation between PPy and NiO. For this composite, some of the rectangular blocks are converted to long rods. For PPy 1% -DBSA 2% /NiO 97% -GS@8, the dominant phase is a cauliflower-like structure of PPy. Finally, at 10 mA for electrodeposition of the nanocomposite electrode of PPy/NiO, graded stairs layers are obtained, as illustrated in Fig. 4f. Electrochemical Properties of Ppy/NiO electrodes. The CV measurements for pure Ppy 1% -DBSA 2% / GS and Ppy 1% -DBSA 2% /NiO 97% -GS electroplated electroplated with current densities in 0.1 M LiClO 4 electrolyte solution and at different scan rates are displayed in Fig. 5. CV curves of pristine Ppy 1% -DBSA 2% /GS (shown in Fig. 5a) at different scan rates ranging from 5 to 100 mV s −1 exhibit nearly rectangular shapes symmetrical across the zero current axis, and they do not appear to be oxidation-reduction peaks 31,32 , showing the typical characteristic of electrical double layer capacitance 33,34 . It is observed that the initial or start point of CV cycle toward anodic direction is not the same the end or final point during reversed in the cathodic direction. This can be explained based on the irreversibility of oxidation reduction reaction or due to effect of high scan rate. The current density of Ppy 1% -DBSA 2% /NiO 97% -GS supercapacitor electrode is directly proportional to scan rate. After the incorporation of NiO 97% into Ppy 1% -DBSA 2% /GS at different electroplating current densities, as shown in Fig. 5b, the integrated area of the CV curves of the electroplated electrode significantly increases due to the high pseudocapacitance originating from NiO, high specific surface area, and abundant redox active sites generated by the porous NiO network 33,35 . The CV curves have a large enclosed area and good symmetrical rectangular shape, showing that the capacitive behavior of the electrode could be greatly improved by optimizing the electro- www.nature.com/scientificreports/ deposition current of the Ppy 1% -DBSA 2% /NiO 97% -GS composite. The Ppy 1% -DBSA 2% /NiO 97% -GS@4 supercapacitor electrode produces the maximum enclosed area. The CV curves of the Ppy 1% -DBSA 2% /NiO 97% -GS@4 supercapacitor electrode at various scan rates from 5 to 100 mV s −1 are displayed in Fig. 5c. A quasi-rectangular shape with no distortion appears in the CV curves. In addition, the current densities linearly increase with increasing scan rates, which may be attributed to the confirmation of the formation of efficient electrical double layers and fast charge propagation within the electrodes 36 .

Structural analysis of Ppy
The GCD curves of pure Ppy 1% -DBSA 2% /GS and Ppy 1% -DBSA 2% /NiO 97% -GS supercapacitor electrodes prepared with different current densities and measured at 1 A/g are presented in Fig. 6a. It is noted that symmetric triangular curves for fabricated supercapacitor electrodes and low charge-transfer resistances during charging and discharging even at high current densities with very little ohmic drop are obtained, indicating a high rate of their performance 37 . It is also found that the discharge time of the Ppy 1% -DBSA 2% /NiO 97% -GS@4 supercapacitor electrode is the highest among the other electrodes prepared with different current densities. Consequently, the GCD curves for Ppy 1% -DBSA 2% /NiO 97% -GS@4 at 1.0, 2.0 and 3.0 A g −1 are shown in Fig. 6b, through which good linear potential-time profiles are achieved, demonstrating the good capacitance performance of this electrode. The specific capacitances are found to be 679, 333.5 and 292.7 F g −1 at 1, 2 and 3 A g −1 , respectively, showing the rate capability of the synthesized sample Ppy 1% -DBSA 2% /NiO 97% -GS@4 38 . Using Eqs. (3) and (4), the energy density of 94.4 Wh kg −1 and power density of 500.74 W kg −1 are obtained for the highest capacitance sample, Ppy1%-DBSA2%/NiO97%-GS@4 and this considers large values compared with other Ppy/NiO supercapacitors electrodes in the literatures 16,24,25,40,41 . The denser and more compact structure may have prevented cations from migrating into the electrode material 39 . For this reason, the specific capacitance of the composites first increases and then decreases with increasing Ppy/NiO film thickness 40 . Ppy 1% -DBSA 2% /NiO 97% -GS@4 (679 Fg −1 ) shows the highest performance compared with the other composites due to its high porosity, as observed in the SEM image (Fig. 4c). This provides paths to diffuse electrolyte ions into the hybrid arrays and enhances the Faradaic reactions 41 . Ppy 1% -DBSA 2% /NiO 97% -GS@10 (170 F g −1 ) exhibits the smallest value of the specific capacitance, as shown in Fig. 6a, which is in good agreement with the CV results. The specific capacitance of Ppy 1% -DBSA 2% / NiO 97% -GS@4 is larger than that of pure PPy (456 F g −1 ), and this is attributed to the synergistic effect of NiO and Ppy. The embedment of NiO as a molecular level dispersion in the Ppy matrix can reduce electron shuttling along the conjugated chains by interlinking the Ppy chains, leading to the enhancement of the overall conductivity of NiO/PPy 39 .
The specific capacitance (Csp) was calculated from the CV curves by integrating the area under the CV curve using Eq. (1). As shown in Fig. 7a, Ppy 1% -DBSA 2% /NiO 97% -GS@4 has larger Csp values than Ppy 1% -DBSA 2% /GS at different scan rates from 5 to 100 mV s −1 . The Csp value of Ppy 1% -DBSA 2% /NiO 97% -GS@4 at 5 mV s −1 is calculated to be 605 F g −1 and is higher than the value of 364 F g −1 for Ppy 1% -DBSA 2% /GS at the same scan rate. The dependence of Csp on scan rate exhibits a decay of 35% Csp of Ppy 1% -DBSA 2% /NiO 97% GS@4 with increasing scan rate from 5 to 100 mV s −1 . The area under the CV curves increases. It is noted that the shape of CVs at different scan rates is the same indicating the excellent rate capability and reversibility of the SC electrodes. At low scan rate, the electrolyte ions diffuse and migrate into active Ppy and high specific capacitances are produced. On the    Csp values can be derived from GCD curves by using Eq. (2). The Ppy 1%-DBSA 2% /NiO 97% -GS@4 sample shows much larger Csp values at all current densities than Ppy 1% -DBSA 2% /GS with current densities increasing from 1 to 3 A g −1 , which is in agreement with the results obtained from CV tests. The Csp of Ppy 1%-DBSA 2% /NiO 97% -GS@4 at 1 A g −1 is calculated to be 679 F g −1 , which is much larger than the value of 456 F g −1 for Ppy 1% -DBSA 2% /GS. Depending on these results calculated above, the capacitance utilization of Ppy 1%-DBSA 2% /NiO 97% -GS@4 is higher than that of Ppy 1% -DBSA 2% /GS, indicating that a homogeneous distribution of PPy and NiO particles is beneficial for the transport of ions in full-gapped nanoparticle systems and for the increase of the PPy/electrolyte interfacial area. The decline of the specific capacitance at elevating current density is due to the inaccessibility of electroactive sites by the electrolyte ions. Figure 8 shows the Nyquist plots for the Ppy/NiO nanocomposite electrodes synthesized electrochemically with different currents at the frequency range from 0.01 to100 kHz with amplitude of 5 mV. The long tails in the low-frequency region or the diffusion region are nearly vertical to the real axis. The intercept of the highfrequency curve in the real part reflects the equivalent series resistance (Rs) between the electrodes and electrolyte and equal to the summation of the Ohmic resistance of the electrolyte, the contact resistance, and the internal resistance of the material. From the inset of Fig. 8, the Ppy1%-DBSA2%/NiO97%-GS@4 electrode possesses the smallest Rs (8.1 Ω) compared to pure Ppy1%-DBSA2%/GS (10 Ω), and Ppy1%-DBSA2%/NiO97%-GS@6 exhibits the largest Rs (11.8 Ω). However, no distinct semicircles are observed in the plots of all electrochemically prepared samples, indicating a small charge transfer resistance between the electrode and electrolyte and consequently the low effect of capacitive double layer. This resulted from the Ppy effect as a conducting polymer which proposed to have a redox behavior. Ppy1%-DBSA2%/NiO97%-GS@4 and Ppy1%-DBSA2%/NiO97%-GS@6 have more vertical slopes demonstrating that they possess low diffusion resistances and contact resistances  The cycle stability as an important parameter for supercapacitors for Ppy 1% -DBSA 2% /NiO 97% -GS@4 and pure Ppy 1% -DBSA 2% /GS electrodes can be valued by their consecutive GCD at 1 Ag −1 for 1000 cycles. As shown in Fig. 9, the specific capacitance retentions are 83.9% and 59.6% of the initial value after 1000 cycles for Ppy1%-DBSA2%/NiO97%-GS@4 and pure Ppy1%-DBSA2%/GS, respectively. The Cs reduction resulted from a degradation of the PPy chains due to the excessive swelling and shrinking of the PPy polymer during the charge/discharge process. The clearly excellent long-term cycling stability of the Ppy1%-DBSA2%/NiO97%-GS@4 composite may be attributed to the porous network gapped structure and good conductivity, which were favorable for charge transportation and electrolyte diffusion 39 . The presence of NiO nanoparticles not only enhances the capacitance value but also improves the cycling stability 44 .

Conclusion
The electrochemical results were improved for the chronopotentiometry deposited Ppy/NiO electrode at different currents onto the graphite sheet compared with the pristine Ppy electrode. It was found that the Ppy 1% -DBSA 2% / NiO 97% -GS@4 electrode demonstrated a high specific capacitance of 679 Fg −1 at a current density of 1 Ag −1 and capacitance retention of 83.9% of its initial capacitance after 1000 cycles at 1 Ag −1 . The high electrochemical performance of Ppy 1% -DBSA 2% /NiO 97% -GS@4 was due to the synergistic effect of NiO and Ppy, where a uniform porous network-like structure made the electrolyte ions more easily accessible for Faradic reactions. Figure 9. Cycling stability measurement of the pure Ppy 1% -DBSA 2% /GS and Ppy 1% -DBSA 2% /NiO 97% -GS@4 electrodes at 1 A g −1 for 1000 cycles.