3.1 Characterization of the composites
Figure 1a and Fig. 1b show UV-visible spectra and survey spectrum of GO and N-rGO, respectively. In the UV-visible spectrum of the N-rGO sample, the π→π* transition peak of the GO structure at 230 nm is significantly shifted to the right (~ 276 nm) and the n→ π* transitions of the C = O bonds at 300 nm was observed to be lost due to the effective removal of oxygenated functional groups in GO nanosheets. As seen in the survey spectrum, O1s peaks have high intensity due to the presence of oxygenated functional groups on the GO surface. In the survey spectrum of the N-rGO sample, because of the reduction process with urea in hydrothermal condition, the intensity of the C1s peaks increased, while the intensity of the O1s peaks decreased significantly due to effective deoxygenation and restoration of sp2 network of graphene nanosheets. When the C/O atomic ratio (13.8), in the N-rGO sample was compared to the C/O atomic ratio (2.24) in the GO sample, it was found that the oxygenated functional groups were effectively removed. Additionally, the reaction with urea under hydrothermal conditions caused the formation of N1s peaks around ~ 400 eV. With XPS analysis, it was determined that the reduced graphene oxide layers were successfully doped with N atom because of the reaction with urea under hydrothermal conditions.
Figure 1a UV-visible spectra and b Survey spectrum obtained from XPS analysis of GO and N-rGO
UV-visible spectra of N-rGO-PANI/D (0.5M DBSA, N-rGO:aniline 1–8 m/m), N-rGO-PANI/DH (0.5-1) ( DBSA: H2SO4 0.5:1 n/n, N-rGO:aniline 1–8 m/m) and N-rGO-PANI/DH (0.25-1) (DBSA: H2SO4 0.25:1 n/n, N-rGO: aniline 1–8 m/m) composites are shown in Fig. 2a. For PANI and N-rGO-PANI nanocomposite produced with different molar ratios (DBSA/H2SO4) of doping acids samples, shoulder-shaped peak between 340–360 nm wavelength caused by π→π* electron transitions of PANI, the band between 400–440 nm wavelengths associated with the protonation step of PANI chains, and the sharp peak between 780–850 nm wavelengths due to the presence of polarons, formed because of the doping process in the UV visible spectrum of each sample. In UV-visible spectra of nanocomposites, the band at approximately 280 nm due to the π→π* transition of N-rGO nanolayers is not seen expectedly, due to the N-rGO surface being completely covered with the PANI.
N-rGO-PANI (DBSA: H2SO4 0.25:1 n/n, N-rGO:aniline 1:4–10 m) prepared using PANI (DBSA: H2SO4 0.25:1 n/n) and different aniline amounts in samples are shown in the UV-visible spectrum (Fig. 2b). Similarly, it was found that all three characteristic peaks belonging to PANI were seen in the UV-visible region spectrum of the samples. However, in parallel with the increase in the amount of monomer used during the synthesis, the intensity of the characteristic bands of PANI in the UV-visible spectrum of the N-rGO-PANI samples were also increased which maybe attributed to thickening of the polymeric layer prepared on N-rGO nanosheets. As a result, PANI synthesis was successfully carried out on the surface of N-rGO nanolayers in the presence of DBSA and/or H2SO4.
Figure 2c shows, FTIR spectra of N-rGO, PANI (DBSA:H2SO4 0.25:1 n/n),N-rGO-PANI6 and N-rGO-PANI8 composites. FTIR spectra of N-rGO-PANI nanocomposites revealed all characteristic peaks belonging to the PANI structure. Additionally, the peak indicating quinone ring deformation of PANI is from 1573 cm− 1 to 1560 cm− 1, the peak due to benzoid ring deformation is from 1470 cm− 1 to 1496 cm− 1, CN tensile vibration is from 1303 cm− 1 to 1290 cm− 1, C-N+ tensile vibration from 1242 cm− 1to 1230 cm− 1, -NH+ tensile vibration from 1125 cm− 1 to 1110 cm− 1and, out-of-plane bending vibration of CH bond from 802 cm− 1to 795 cm− 1. These findings indicates the strong interaction between N-rGO and PANI, which is crucial for obtaining high electrochemical performance. Moreover, PANI synthesis proceeds on many nucleation centers of the N-rGO surface and is successfully synthesized.
Figure 2 UV visible spectra of a N-rGO-PANI/D (0.5 M DBSA, N-rGO:aniline 1–8 m/m), N-rGO-PANI/D-H (0.5-1) (DBSA:H2SO4 0.5:1 n/n, N-rGO:aniline 1–8 m/m), N-rGO-PANI/D-H (0.25-1) (DBSA:H2SO4 0.25:1 n/n, N-rGO:aniline 1–8 m/m) b PANI (DBSA:H2SO4 0.25:1 n/n) and N-rGO-PANI (DBSA:H2SO4 0.25:1 n/n, N-rGO:aniline 1:4–10 m/m) c FTIR spectra of N-rGO, PANI/D-H (0.25-1) (DBSA:H2SO4 0.25:1 n/n, N-rGO:aniline 1–8 m/m), N-rGO-PANI6 and N-rGO-PANI8
Figure 3a shows the XRD patterns of GO and N-rGO samples. The sharp peak with a center of 10.74° in the GO diffraction pattern indicates the planar reflection (002) structure of the GO layers. At the same time, this characteristic peak of the GO structure shows that the distance (d-spacing) between the graphene layers is 0.822 nm. The entry of water molecules between the layers of graphite and the oxidation of graphite and the formation of oxygenated functional groups on the layers during the GO production process show that the distance between the layers in the GO structure increases compared to the perfect graphite structure.[23–25]. Because of the effective chemical reduction of GO by the hydrothermal method, it was observed that the (002) crystal diffraction pattern of GO disappeared completely and a broad peak was formed at 2ϴ= 24.8°. This new peak indicates that the d-spacing value for N-rGO nanolayers is 0.36 nm [26, 27]. The decrease in the distance between the graphene layers and the disappearance of the peak belonging to GO under hydrothermal conditions indicates that the oxygenated functional groups are significantly removed. Figure 3b shows, XRD patterns of the PANI, and N-rGO-PANI6 samples. Three characteristic peaks corresponding to the crystal planes (001), (020) and, (200) of the PANI (0.25-1) sample synthesized in 0.1 M H2SO4 and 0.05 M DBSA medium overlap with the diffraction pattern of the N-rGO-PANI6 nanocomposite. PANI polymeric structures with crystalline properties were successfully produced by in situ polymerization of aniline monomer on the N-rGO surface.
Figure 3a XRD patterns of GO and N-rGO b PANI (DBSA:H2SO4 0.25:1 n/n) and N-rGO-PANI6 (DBSA:H2SO4 0.25:1 n/n, N-rGO:anilin 1:6 m/m)
Figure 4 shows TGA curves of PANI (DBSA: H2SO4 0.25:1 n/n) and N-rGO-PANI6 (DBSA:H2SO4 0.25:1 n/n, N-rGO:aniline 1:6 m/m) samples. Both PANI and N-rGO-PANI (1–6) samples exhibited three main weight loss steps. The first weight loss of the samples, occurred because of the removal of moisture at a temperature of about 100°C. The second weight loss, occurred due to the degradation of H2SO4 and DBSA molecules in the temperature range of 215–430°C. Finally, since the skeletal structure of PANI started to degrade at approximately 450–550°C, considerable weight loss occurred in the samples [28, 29]. Since the analyses were carried out in a nitrogen atmosphere, the carbonization of the polymer did not occur and when the analysis temperature reached 800°C, the total weight loss percentages of PANI and N-rGO-PANI samples were 55.2% and 52.2%, respectively. This is an indication that the prepared nanocomposite structure has high thermal stability.
Figure 4 TGA curves of PANI (DBSA:H2SO4 0.25:1 n/n) and N-rGO-PANI6 (DBSA:H2SO4 0.25:1 n/n, N-rGO:anilin 1:6 m/m)
SEM images of N-rGO, PANI (DBSA: H2SO4 0.25:1 n/n), N-rGO-PANI4, N-rGO-PANI6, N-rGO-PANI8, N-rGO-PANI10 are given in Fig. 5. SEM images demonstrate that the N-rGO layers with thin and curved edges (Fig. 5a) are covered by the PANI polymeric structure (Fig. 5c-f). Additionally, it is observed that the morphological structure of polyaniline synthesized by polymerization on the N-rGO surface is different from the structure of pure polyaniline synthesized directly in the solvent medium. While pure polyaniline consists of interconnected forms of rod-like structures in micron scale, polyaniline prepared by in situ polymerization in the presence of N-rGO encased the 2-D surface in thin layers. Additionally, the coating thickness of these polymeric layers on N-rGO increases with the amount of aniline monomer used. EDX spectra of PANI (DBSA: H2SO4 0.25:1 n/n) and N-rGO-PANI6 given in the inset of Fig. 5b and Fig. 5d show considerable amount of S atoms in the nanocomposite corresponding to the presence of DBSA molecules in the nanocomposite structure, which also confirm TGA results. DBSA is an effective surfactant that also undertakes the co-dopant role, responsible for preventing further growth and agglomeration of polymeric structures for the production of nanocomposite films. Moreover, DBSA increases the interaction of nanocomposite with aqueous solutions to bring processability and wettability with aqueous electrolytes. The adjustable morphological structure is very advantageous for supercapacitor applications, as the electrolyte diffusion distance can be lowered by adjusting the coating thickness controlled by the aniline/N-rGO m/m ratio. The strong interaction between N-rGO and PANI components were determined by FTIR spectrum and XRD analysis, which was also confirmed by SEM images.
Figure 5 SEM images of a N-rGO, b PANI (DBSA:H2SO4 0.25:1 n/n) (inset: EDX spectrum), c N-rGO-PANI4, d N-rGO-PANI6 (inset: EDX spectrum), e N-rGO-PANI8, f N-rGO-PANI10
TEM images of N-rGO-PANI6 sample are shown in Fig. 6. Consistent with the SEM images, it was observed that the N-rGO surface was homogeneously coated with the polymers and N-rGO-PANI ultrathin films were successfully produced.
Figure 6 TEM images of N-rGO-PANI6 sample
3.2 Electrochemical Characterization
CV curves of synthesized DBSA doped N-rGO-PANI/D, H2SO4 doped N-rGO-PANI/H, DBSA and H2SO4 co-doped N-rGO-PANI/DH and DBSA and H2SO4 co-doped PANI samples in two-electrode configuration at 20 mV/s scanning rate are given in Fig. 7a. It was observed that the characteristic two redox transitions of PANI were prominent. The redox peak at about 0.2 V indicates leucoemeraldine/emeraldine transformation. The redox couple at about 0.5 V correspond to emeraldine/pernigraniline transition. The peaks in the N-rGO-PANI/D-H (0.25-1) electrode were more prominent than those in the N-rGO-PANI/D, N-rGO-PANI/H, N-rGO-PANI/D-H (0.5-1) and PANI (DBSA: H2SO4 0.25:1 n/n) electrodes.
Having the higher CV area co-doped N-rGO-PANI nanocomposite electrodes exhibit better capacitive properties, while DBSA doped N-rGO-PANI and H2SO4 doped N-rGO-PANI samples may suffer from their significantly lower conductivity compared to co-doped samples, Fig. 7b and Fig. 7c shows the CV curves of N-rGO-PANI/DH (0.25-1) and PANI (DBSA: H2SO4 0.25:1 n/n) electrodes between 5 and 200 mV/s scan rates, respectively. As an ideal supercapacitor behavior, the response current values of the electrodes increase with increasing scanning rate in CV analysis. Also, the increase in scanning rate limits the access of electrolyte ions into the electrode material. At high scanning rates, only the surface portion of the electrode material is involved in the electrochemical process. Therefore, redox peaks that can be seen clearly at low scan rates may not be visible at high scan rates. The CV curves of the N-rGO-PANI structure (Fig. 7b) show that the composite is less affected by these restrictions compared to PANI (Fig. 7c). Micron-scale in pure PANI prevent the inner parts of the particles from participating to the electrochemical process, reduce performance and stability. PANI transition peaks, which cannot be seen at high scanning rates, indicate that the material has limited electrochemical performance. However, by coating the PANI structure on the N-rGO structure as a thin film, the ion diffusion length for ion transfer is shortened and the material is freed from structural constraints.
GCD curves of PANI (DBSA: H2SO4 0.25:1 n/n), N-rGO-PANI/D, N-rGO-PANI/H, N-rGO-PANI/DH (0.5-1) and N-rGO-PANI/DH (0.25-1) electrodes at 1 Ag− 1 current density are given in Fig. 7d. The capacitances of PANI, N-rGO-PANI/D, N-rGO-PANI/H, N-rGO-PANI/DH (0.5-1) and N-rGO-PANI/DH (0.25-1) samples are 282.1 Fg− 1, 134.5 Fg− 1, 283.13 Fg− 1, 264.33 Fg− 1, and 342.7 Fg− 1, respectively. Because of the analyses performed at a current density of 1 Ag− 1, the electrode with the best performance was determined as N-rGO-PANI/DH (0.25-1). Similarly with CV curves GCD analysis also displays, co-doped N-rGO-PANI/DH nanocomposite samples exhibit better capacitive properties compared to pristine PANI and DBSA doped N-rGO-PANI/D electrodes. The enhanced specific capacitance of co-doped nanocomposites is because of their excellent surface properties and maintaining their high conductivity. The co-doping process provides formation of ultrathin stable nanocomposite 2-D networks having high electroactive surface area, low ion transfer distance, excellent wettability with electrolyte ions and high conductivity. However, the increase in DBSA/H2SO4 molar ratio caused a decrease in the specific capacitance, which may be attributed to the presence of increased number of DBSA molecules having high interaction with the nanocomposite structure, consequently reducing electronic conductivity and ion transfer.
Figure 7a CV curves of PANI (DBSA: H2SO4 0.25:1 n/n), N-rGO-PANI/D, N-rGO-PANI/DH (0.5-1), N-rGO-PANI/DH (0.25-1) and N-rGO-PANI/H electrodes at 20 mVs− 1, b CV curves of N-rGO-PANI/DH (0.25-1) electrodes at 5-200 mVs− 1, c CV curves of PANI (DBSA: H2SO4 0.25:1 n/n) electrodes at 5-200 mVs− 1, d GCD curves of PANI (DBSA: H2SO4 0.25:1 n/n), N-rGO-PANI/D, N-rGO-PANI/DH (0.5-1), N-rGO-PANI/DH (0.25-1) and N-rGO-PANI/H electrodes at 1 Ag− 1
Subsequently, syntheses containing different ratios of N-rGO:aniline (1:4–10 m/m) were carried out to determine the effect of PANI coating the N-rGO nanosheets on the electrochemical performance of N-rGO-PANI samples. Figure 8a shows CV curves of N-rGO-PANI4, N-rGO-PANI6, N-rGO-PANI8, and N-rGO-PANI10 at 20 mV/s scan rate in -0.2-0.8 V potential range. Each nanocomposite display the redox transitions of PANI, while N-rGO-PANI6 and N-rGO-PANI8 electrodes display higher CV area, revealing their higher specific capacitance. CV curves of N-rGO-PANI6 and N-rGO-PANI8 samples with different scan rates are given in Fig. 8b and Fig. 8c. In both samples, an increase was observed in current density values with the increase in the scan rate, and no significant change was observed in the shapes of the CV curves, the redox transitions of PANI are still prominent at the highest applied scan rate, because of their high electrochemical stability and ion transfer properties.
Figure 8a CV curves of N-rGO-PANI (DBSA:H2SO4 0.25:1 n/n, N-rGO:anilin 1:4–10) samples at 20 mVs− 1, b CV curves of N-rGO-PANI6 at 5-200 mVs− 1, c CV curves of N-rGO-PANI8 at 5-200 mVs− 1
GCD analysis was also carried out to examine the specific capacitance values of N-rGO-PANI electrodes produced with various aniline/N-rGO ratios. Figure 9a shows the GCD curves of N-rGO-PANI (DBSA:H2SO4 0.25:1 n/n, N-rGO:aniline 1:4–10) samples at 1 Ag− 1 current density between − 0.2–0.8 V. The capacitance values of N-rGO-PANI4, N-rGO-PANI6, N-rGO-PANI8, and N-rGO-PANI10 electrodes at 1 Ag− 1 current density were calculated as 310 Fg− 1, 346.3 Fg− 1, 342.7 Fg− 1, 325.6 Fg− 1, respectively. Compared to pristine PANI (282.1 Fg− 1, Fig. 7d), N-rGO-PANI nanocomposite samples exhibited higher specific capacitance values, because of uniform coating of PANI polymeric films on heteroatom doped, highly conductive N-rGO nanosheets. Additionally, the increase in aniline/N-rGO ratio between 1:4–1:8 resulted in enhencament of the specific capacitance but further increase in the aniline amount (N-rGO-PANI10) led to a decrease in the capacitance because of the thickening of the PANI layer on the nanocomposite film, which is also responsible for the increase in the ion transfer distance. GCD curves of N-rGO-PANI6 and N-rGO-PANI8 electrodes between 1–10 Ag− 1 are shown in Fig. 9b and 9c. The capacitance values of N-rGO-PANI6 and N-rGO-PANI8 electrodes at 1 Ag− 1 to 10 Ag− 1 were determined as 346.3 Fg− 1 to 345.9 Fg− 1 (Fig. 9b), and 342.7 Fg− 1 to 342.2 Fg− 1 (Fig. 9c) with displaying excellent rate capability (99.9% and 99.8%, respectively) thank to its high conductivity, strong interaction with electrolyte ions and low ion transfer distance because of its ultrathin 2-D film structure.
Figure 9a GCD curves of N-rGO-PANI (DBSA:H2SO4 0.25:1 n/n, N-rGO:aniline 1:4–10) samples at 1 Ag− 1, b GCD curves of N-rGO-PANI6 between 1–10 Ag− 1, c GCD curves of N-rGO-PANI8 between 1–10 Ag− 1
The Nyquist plots of N-rGO-PANI4 N-rGO-PANI6, N-rGO-PANI8 and N-rGO-PANI10, are given in Fig. 10. The Nyquist plots consist of a semicircle in the high frequency region giving charge transfer resistance (Rct), and a straight line at the low frequency region related to the capacitive behaviour of the electrodes. The Rct values of N-rGO-PANI4 N-rGO-PANI6, N-rGO-PANI8 and N-rGO-PANI10 were determined as 0.96 Ω, 0.6 Ω, 0.56 Ω and 0.19 Ω, respectively. Because of their excellent surface properties and high conductivity, N-rGO-PANI nanocomposites exhibited very low Rct values. Additionally, the slope of the curve in the Nyquist plot of N-rGO-PANI electrodes in the low frequency region is an indication that their properties are close to the ideal capacitor.
Figure 10 Nyquist plots of PANI (DBSA:H2SO4 0.25:1 n/n), N-rGO-PANI (1–6) (DBSA:H2SO4 0.25:1 n/n, N-rGO:aniline 1:6) and samples N-rGO-PANI (1–8) (DBSA:H2SO4 0.25:1 n/n, N-rGO:aniline 1:8) samples.
Another important parameter for the applicability of the electrode material as a supercapacitor is the cycle stability. Among the N-rGO-PANI samples, the samples showing the highest electrochemical performance were determined as N-rGO-PANI6. Accordingly, the cycle stability of N-rGO-PANI6 was investigated, and the change in the capacitance of N-rGO-PANI6 electrode with the number of cycles are shown in Fig. 11a. N-rGO-PANI6 sample exhibited a specific capacitance of 346.1 Fg− 1 at a current density of 5 Ag− 1 and the sample exhibited 81.3% specific capacitance retention after 5000 cycles. Because of the excellent mechanical strength of N-rGO nanosheets and strong interaction between PANI film with N-rGO the nanocomposite electrodes exhibited high cyclic stability. This interaction limits continuous swelling-shirinking of PANI structures during the cycling process. Figure 11b shows the change in capacitanece of N-rGO-PANI6 electrode between 1–10 Ag− 1 after the 5000 cycles. As shown in the Fig. 11.c N-rGO-PANI6 sample exhibited a specific capacitance of 290 Fg− 1 at 1 Ag− 1 and 251.9 Fg− 1 at 10 Ag− 1. It was determined that the electrode still exhibited high stability (81.3%) even after 5000 cycles.
Figure 11a Cycle stability of N-rGO-PANI6, b GCD curves of N-rGO-PANI6 between 1–10 Ag− 1 after 5000 cycle, c Capacitance of N-rGO-PANI6 with different current densities after 5000 cycle
The values of Rct, the specific capacitances, the energy density and the power density of PANI, N-rGO-PANI4, N-rGO-PANI6, N-rGO-PANI8 and N-rGO-PANI10 are given in Table 1. The prepared composites showed good energy density compared to the aqueous electrolyte with limited operating voltage range. Since the potential range is determined as 1 V in all electrochemical analyses, the energy density varies directly with the capacitance. Also, the samples retained their energy density at high power density values. This is due to the high velocity stability of the composites containing N-rGO.
Table 1 represents a comparison of the electrochemical performance of co-doped N-rGO-PANI nanocomposite with reported studies in the literature containing organic acids as dopants for PANI. The two-electrode configuration reflects more accurate results compared to the three-electrode configuration because of the similar configuration and charge transfer mechanism similar to practical supercapacitors. The electrochemical measurements carried out in symmetrical two-electrode configuration exhibit N-rGO-PANI nanocomposites have high specific capacitance, high cyclic stability, and excellent rate capability. The in situ polymerization of aniline on highly conductive N-rGO nanosheets, which have high surface area and plenty of active centres for polymerization, provided effective coating of N-rGO with polymeric structure. Moreover, co-doping of PANI with DBSA and H2SO4 ensured the formation of ultrafine polymeric films on N-rGO nanosheets, having high stability and wettability with electrolyte while maintaining its high conductivity.
Table 1
Comparison for the electrochemical property of the N-rGO-PANI and the reported Graphene-PANI based electrodes
Study | Electrode | Doping Agent | Configuration | Specific Capacitance (1 Ag− 1) (Fg− 1) | Specific Capacitance (10 Ag− 1) (Fg− 1) | Capacitance Retention | Cyclic Stability |
[30] | PANI | Camphorsulfonic acid (CSA) | Three-electrode | 600.7 | 419.2 | 70% | 74% 1000 |
[31] | PANI-Au | Dodecylbenzene sulfonic acid (DBSA) | Three-electrode | 292.2 (0.5 Ag− 1) | 173 | 60% | 86% 3000 |
[32] | GO/Pt/DBSA/PANI | Dodecylbenzene sulfonic acid (DBSA) | Three-electrode | 227 | Not reported | Not reported | 96% 1500 |
[33] | PANI | Dodecylbenzene sulfonic acid (DBSA), Sulfuric Acid | Three-electrode | 516 (0.5 Ag− 1) | 266 (25 Ag− 1) | 51.5% | 57% 1000 |
[34] | NCF-PANI | P-TSA (p-toluene sulfonic acid) | Three-electrode | 139 | 97.3 | 70% | 89% 5000 |
[35] | PANI | HCl-Phytic Acid | Three-electrode | 350 | 315 | 90% | 99% 500 |
[36] | SPANI/SrGO | Naphthalene Sulfonic Acid | Two-electrode | 345.7 | 235.8 (5 Ag− 1) | 68% | 89% 2500 |
[37] | PANI | hyperbranched polyesters (HBPE)- Dodecyl hydrogen sulfate-Sulfiric Acid | Two-electrode | 300 (0.2 Ag− 1) | Not reported | Not reported | 75% 1500 |
This Work | N-rGO/PANI | Dodecylbenzene sulfonic acid (DBSA)-Sulfiric Acid | Two-electrode | 346.3 | 345.9 | 99.9% | 81.3% 5000 |