Synthesis of polypyrrole/nitrogen-doped porous carbon matrix composite as the electrode material for supercapacitors

Polypyrrole complex nitrogen-doped porous carbon matrix (PPy/N-PCM) was synthesized by a simple two-step method. Firstly, graphene oxide was prepared by the modified Hummers method. Secondly, Polypyrrole was compounded on the graphene oxide substrate, and the carbon matrix with a high specific surface area was obtained through high-temperature carbonization and KOH activation, and polypyrrole was used as a nitrogen source for the final nitrogen-doped composite material. The structure characterization of the carbon matrix and the final composite material shows that the carbon matrix surface has obvious porous structure, and the polypyrrole nanospheres grow uniformly on the porous carbon matrix surface. The electrochemical evaluation show that the prepared PPy/N-PCM has excellent supercapacitor performance, and its specific capacitance can reach 237.5 F g−1. When the current density reaches 10 A g−1, it has good cycle stability (the capacitance retention after 1000 charge and discharge is 88.53% of the initial capacitance value, which is better than pure PPy-60.76% and PPy/rGO-C-71.84%). The excellent capacitance performance, good-looking micro-morphology and simple synthesis method of the PPy/N-PCM provide the possibility for its commercialization.


Scientific RepoRtS
| (2020) 10:15370 | https://doi.org/10.1038/s41598-020-72392-x www.nature.com/scientificreports/ 83.8%. Chang et al. 17 used an electrochemical synthesis method to prepare graphene oxide/PPy composite membranes. The specific capacitance at a current density of 1 A g −1 was 289 F g −1 , but the cycle stability was not studied. However, the strong Vander Waals force between the graphene sheets tend to cause sheet stacking, which reduces the specific surface area of the graphene and can't support PPy well 18 . This will reduce the specific capacitance and cycle stability of the polypyrrole/graphene composite. In order to effectively improve the electrochemical performance of electrode materials, three-dimensional porous nitrogen-doped graphene-based composite materials are widely used in the study of supercapacitor electrode materials 19,20 . On the one hand, the three-dimensional porous nitrogen-doped structure can provide a larger specific surface area for the carbon substrate, promoting the diffusion and transfer of the electrolyte ions and electrons 21,22 . On the other hand, the nitrogen is doped into the carbon matrix can enhance the surface activity of the carbon matrix and produce a pseudocapacitive effect 23,24 .
In our work, we added polypyrrole to introduce nitrogen into the graphene oxide matrix, and continued to compound polypyrrole on the nitrogen-doped carbon matrix to avoid the collapse of polypyrrole morphology 25 . Firstly, polypyrrole was self-assembled on graphene oxide template to obtain the polypyrrole/graphene oxide composite material. Secondly, the polypyrrole/graphene oxide composite was then carbonized and activated to obtain the nitrogen-doped porous carbon matrix. In this process, the introduction of nitrogen atoms was from the polypyrrole, which not only solved the problem of graphene matrix aggregation, but also provided an active site for the attachment and deposition of polypyrrole. The porous structure could also increase the specific surface area. Finally, the polypyrrole nanospheres were anchored on the surface of the nitrogen-doped porous carbon matrix. With the aid of the porous structure of the carbon matrix, the morphology stability of polypyrrole was improved. Such polypyrrole/three-dimensional porous carbon matrix has high specific capacitance, good cycle stability, and good morphology, which makes it promising as one of the most popular electrode materials for supercapacitors in the future.
Preparation of PPy/GO nanocomposite. The graphene oxide (GO) was synthesized by improved Hummers method 26 . After the synthesis, a certain amount of graphene oxide was taken and dispersed in 60 ml of 1 mol L −1 HCl solution and 20 ml of ethanol solution. Pyrrole monomer was dripped in the solution and stirred for 30 min. At the same time, ammonium persulfate was dissolved in 20 ml of 1 mol L −1 HCl solution. Then the prepared two solutions were slowly mixed and stirred in an ice-water bath overnight. (The molar ratio of pyrrole monomer to APS is 1-1.5). Finally, the resulting solution was suction filtered. The filter cake was washed with HCl, distilled water and ethanol, and then dried at 50 °C overnight to obtain the product 27 . preparation of n-pcM. The PPy/GO composite prepared in the previous step was carbonized at 400 °C for 1 h under the condition of nitrogen protection in tubular furnace at a heating rate of 5 °C min −1 . After carbonization, the product was taken out and soaked in 1 mol L −1 potassium hydroxide solution and stirred, and then fully dried at 70 °C. The dried sample was placed in porcelain boat and further carbonized by gradient heating in a tube furnace (heating rate 5 °C min −1 , 450 °C 1 h, 600 °C 1 h, 700 °C 2 h). The product was washed with hydrochloric acid to remove impurities and then washed with deionized water until about neutral. The obtained product is an N-PCM (N-doped Porous Carbon Matrix). Since the amount of KOH will affect the pore size and the specific surface area of the carbon matrix, we prepared carbon matrix with different amounts of potassium hydroxide, wherein the mass ratio of KOH to carbon was 0.5:1, 1:1, 2:1, 3:1, respectively, and these products were named as N-PCM-0.5, N-PCM-1, N-PCM-2, and N-PCM-3. For comparison, rGO-C (the carbon matrix obtained by carbonizing PPy/GO composite material without potassium hydroxide treatment) was also prepared 27 .

Preparation of PPy/N-PCM and PPy/rGO-C composites. The PPy/N-PCM composite image is
shown as Scheme 1. A certain amount of N-PCM was taken and dispersed in 60 ml of 1 mol L −1 HCl solution and 20 ml of ethanol solution. Pyrrole monomer was dripped in the solution and stirred for 30 min. At the same time, ammonium persulfate was dissolved in 20 ml of 1 mol L −1 HCl solution. Then the prepared two solutions were slowly mixed and stirred in an ice-water bath overnight. Finally, the resulting solution was suction filtered. The filter cake was washed with HCl, distilled water and ethanol, and then dried at 50 °C overnight to obtain the product 27  electrochemical test. The electrochemical properties of the prepared samples were investigated at room temperature with three electrode system by using 1 mol L −1 H 2 SO 4 solution as electrolyte. A mixture of electrode material, acetylene black and polytetrafluoroethylene in a mass ratio of 8:1:1 was uniformly coated with N-Methylpyrrolidone on conductive carbon paper as test sample. The cyclic voltammetry (CV), chronopotentiometry parameters, and electrochemical impedance spectroscopy (EIS) were used to study the electrochemical performance of materials.  Fig. 1a, the modified Hummers method was successfully used to prepare GO with wrinkles of multiple layers. This not only facilitates the transmission of electrons, but also provides more nucleation sites for PPy 27 . The surface of GO sheet remains smooth before PPy is compounded. PPy nanospheres are deposited on GO nanosheets by in-situ polymerization to form a flower-like structure 9 (Fig. 1b). The π-π interaction between PPy and GO makes PPy/GO an excellent precursor sample. After PPy/GO is carbonized and activated, N-PCM is obtained. The N-PCM is obtained as shown in Fig. 1c. Figure 1e further shows the microstructure of the N-PCM, the porous structure can effectively adsorb pyrrole particles by electrostatic force because of the increased specific surface area and active sites. The elemental mapping analysis of C, N, and O is performed on N-PCM. The results are shown in the figure. It can be seen that N elements are evenly distributed in N-PCM. It can be observed through SEM that the activated carbon matrix has uniform porous structure, which not only increases its specific surface area, but also promotes rapid migration of electrolyte ions. From Fig. 1d, it is found that the PPy nanospheres are uniformly distributed on the porous carbon matrix, and the porous structure of the carbon matrix is still clearly visible after the polymerization. This is consistent with the results of subsequent electrochemical performance tests. Figure 1f further illustrates the porous structure of the carbon matrix, while Fig. 1g shows that the PPy nanospheres are coated on the surface of the carbon matrix. Figure 2 presents XRD patterns of GO, rGO, rGO-C, N-PCM, PPy, PPy/GO, PPy/rGO-C and PPy/N-PCM. As shown in Fig. 2a, the diffraction peak at 2θ = 9.78° corresponds to the GO nanosheet (001) crystal plane 28 . This is because a large amount of oxygen-containing functional groups are introduced into the GO nanosheets, which causes the original graphite crystal structure destroyed and increased crystal defects. The diffraction peak at the 9.78° position indicates that the graphite is sufficiently oxidized. In addition, the rGO spectrum shows two diffraction peaks at 2θ = 26.39° and 43.19° that don't appear in GO, which means that GO has recovered 29 . The peak position of the samples of rGO-C and N-PCM in Fig. 2b are similar to that of rGO in Fig. 2a, which indicates that the carbon matrix after carbonization and doping still maintains the same crystal plane. Compared with rGO-C, the peak intensity of the diffraction peak of N-PCM at 20.78° and 25.97° is weakened. This is because nitrogen atoms are introduced into the graphene lattice, reducing the restacking of graphene sheets 30 . The diffraction peak of GO in the sample of PPy/GO disappeared significantly, demonstrating that the GO surface was completely covered by PPy. The peak positions of all composite samples are exactly the same as that of pure PPy, all of which are located at the characteristic diffraction peaks of pure PPy 2θ = 20°-30°. This indicates that PPy is completely deposited on the surface of the carbon matrix 31 .

Results and discussion
FT-IR spectroscopy was performed on all samples to obtain detailed chemical structure information. As shown in Fig. 3, GO shows the absorption peak near 3400 cm −1 , which is due to the stretching vibration of -OH.  Figure 4 shows the Raman spectra of N-PCM, PPy and PPy/N-PCM. As shown in the spectrum of N-PCM, the peak of the D band appears at 1350 cm −1 , and the G band appears at 1580 cm −133 . The D peak is the structure caused by the defect, and the G peak is attributed to the in-plane stretching vibration of sp 2 hybridized carbon atoms on the ring and chain. The intensity ratio (I D /I G ) of the D peak to the G peak of the sample N-PCM is 0.908, which indicates that the carbonization and activation treatment can provide the sample with abundant structural defects. The PPy/N-PCM sample shows unique peaks of PPy at 980 cm −1 and 1050 cm −134 . The sample of PPy/N-PCM retains the characteristic peaks of the N-PCM carbon matrix, which confirms that PPy was successfully compounded on the surface of the carbon matrix. The results are consistent with FT-IR. Figure 5a shows that the PPy/N-PCM sample mainly contains O 1s, N 1s and C 1s signals. The surface atomic percentages of C, N and O in the sample are 72.28%, 14.6% and 13.12%, respectively. In Fig. 5c, the C 1s shows the C=C chemical state of 283.9 eV, C-C of 284.8 eV, C-N/C-O of 285.69 eV, and the chemical state of C=N/ C=O of 287.44 eV coexist. The peak near 290.58 eV is the satellite peak of C 1s, reflecting molecular structure information (It may be the pi-pi* transition of the benzene ring) 35 . This is consistent with the C 1s peak information of N-PCM and PPy in Fig. S1c and Fig. S2c, respectively. The O 1s is shown in Fig. 5d as 530.87 eV C=O and 532.28 eV C-O chemical states coexist. In Fig. 5b, The N is shown as the coexistence of C=N of 397.53 eV,  14 eV. This is consistent with the N 1s peak of PPy in Fig. S2b, indicating that PPy is successfully anchored on the surface of the N-PCM carbon matrix. In Fig. 6, the adsorption amount of all isotherms increases with the growth of relative pressure. A curve is saw in the latter section and the adsorption hysteresis loop appears in the middle section. This is a typical type IV isotherm characteristic. The pore size distribution curve (Fig. 6b) also indicates the microscopic and pore properties of the samples. The existence of pore structure is related to the release of volatile substances during   Table 1. When the mass ratio of KOH to C is 2, the obtained carbon matrix has specific surface area of 310 m 2 g −1 , and the pore size distribution curve is shown in Fig. 6b. It is apparent that the N-PCM-2 has the highest BET specific surface area and narrow pore size distribution compared to the rest of carbon matrixes. Therefore, N-PCM (N-PCM-2) is a suitable choice as composite precursor. The specific surface area of the composite covered by polypyrrole is reduced compared to carbon skeleton, which mainly due to the deposition of PPy and the filling of pores on surface of carbon substrate by PPy 37,38 . Figure 7a shows the CV curves of seven different samples at a scan rate of 20 mV s −1 and the potential window of − 0.2 to 0.8 V (with Ag/AgCl). As the picture shows, the CV curve of the carbon matrix is like a rectangle while slightly deformed. This is due to the redox reaction on the surface of carbon matrix 39 . The CV curve trend of the samples compounded with PPy is consistent with pure PPy, and both have two redox peaks, indicating that the carbon skeleton and polypyrrole are well combined and have good pseudocapacitance characteristics 40 . Compared with other samples, the CV curve surrounding area of PPy/N-PCM is significantly increased, indicating that the sample of PPy/N-PCM has higher specific capacitance. Figure 7b shows the GCD curves of the seven samples. All of the GCD curves show an approximate regular triangle, indicating that the surface of all of the fabricated electrode materials undergo redox reaction 41 . This is consistent with the CV test results. The specific capacitance value of the discharge part could be calculated according to the following formula 39 : (1) C m = It/m�V   It can be seen that N-PCM is much higher than the capacitance of GO and rGO-C. This is due to the increase of pore structure on the surface of N-PCM, which increases the effective area required for ion transfer and shortens the ion transfer path. Compared with other literatures, it is found that the specific capacitance of PPy/N-PCM is  Figure 7c reveals the effect on electrochemical performance of electrode materials with different pyrrole concentrations. As the Fig. 7c shows, when the concentration of pyrrole is 0.06 M, the area surrounded by CV curve is the largest, and a significant redox peak appears, indicating that PPy-6/N-PCM has the most excellent electrochemical performance. Combined with Fig. 7d, it is found that the 0.06 M PPy/N-PCM discharge time is longer than other samples. Calculated by the formula (1): the specific capacitance of 0.02 M, 0.04 M, 0.06 M and 0.08 M PPy/N-PCM are 114.375 F g −1 , 118.875 F g −1 , 237.5 F g −1 , and 130.5 F g −1 , respectively. The result is consistent with Fig. 7c. This is because the specific surface area of the porous carbon matrix is increased, so that more polypyrrole nanospheres are embedded on the surface of the carbon matrix. At the same time, the pyrrole monomer concentration of 0.06 M allows the polypyrrole to be uniformly anchored on the surface of the carbon matrix without stacking. Figure 7e shows the cyclic voltammetry curves for PPy/N-PCM at different scan rates. The area of the CV curve of PPy/N-PCM increases as the scan rate increases. It indicates that the prepared composite sample has good rate performance, and an increase in the scanning rate causes a slight change in shape due to the resistance of the electrode. The symmetrical GCD curves at different current densities indicate that the PPy/N-PCM has good capacitance characteristics (Fig. 7f). When the current density is increased to 10 A g −1 , the GCD curve of PPy/N-PCM remains unchanged, further confirming that it has high rate performance. Such excellent specific capacitance of the electrode is due to the functionalization of carbon matrix by nitrogen doping. In addition, the presence of nitrogen groups increases the tantalum capacitance response, which in turn contributes to the increased energy storage [44][45][46] . Figure 8 shows the Ragone plots of the samples of PPy, PPy/rGO-C, and PPy/N-PCM. It is worth noting that the PPy/N-PCM has an energy density of 76.04 Wh kg −1 at a power density of 0.4 kW kg −1 , which is greater than PPy and PPy/rGO-C. This shows that the carbon support can effectively control the volume change of PPy, thereby improving its electrochemical performance. Comparing PPy/N-PCM and PPy/rGO-C, it was found that the nitrogen-doped carbon matrix was subjected to alkali-activated pore-forming and then compounded with polypyrrole, the sample had higher energy density. This is because the specific surface area of the carbon matrix after pore formation becomes larger than rGO-C so that PPy can be uniformly compounded on the carbon www.nature.com/scientificreports/ matrix. In addition, it can be found from the figure that the rate performance of PPy/N-PCM is not as good as that of pure PPy. This may be due to the support of the N-PCM matrix, PPy nanospheres can only grow along the template. This will reduce the specific surface area of the sample to a certain extent, thereby affecting the ion transport and transfer of the PPy/N-PCM sample in the electrolyte. When the high current density is 10 A g −1 , the power density is 4 kW kg −1 , and the energy density is 30 Wh kg −1 , which indicates that the PPy/N-PCM composite has objective application prospects as an electrode material for supercapacitors. The EIS analysis of the PPy/N-PCM, PPy/rGO-C and PPy were used to study the conductivity and charge transport behavior. The result is shown in Fig. 9a. It is observed that the radius of the high frequency region of PPy/N-PCM and PPy/rGO-C are smaller than the radius of PPy, indicating that the charge transfer resistance is small 47 . The slope of the oblique line presented in the low frequency region is large, indicating that the diffusion rate of ions in electrolyte is very fast 48 . The cycling stability of PPy, PPy/rGO-C and PPy/N-PCM electrodes was tested at a current density of 10 A g −1 . As can be seen from Fig. 9b, the PPy/N-PCM has excellent cycle stability and still maintains an initial specific capacitance of 88.53% after 1000 cycles of testing. This is attributed to the synergy between PPy nanospheres and N-PCM which inhibits the expansion and contraction of polypyrrole during charge and discharge. The PPy/rGO-C and PPy electrodes only retained 71.84% and 60.76% of the initial capacitance, demonstrating good stability of the PPy/N-PCM. Therefore, PPy/N-PCM composite has better cycle stability than pure PPy nanospheres. The PPy/N-PCM will be an ideal for high-performance energy storage devices.

conclusions
In summary, we have continuously tried to develop a new method to synthesize conductive composites with 3D structure. The synthesized PPy/N-PCM was proved to have an excellent three-dimensional structure by SEM, TEM, FT-IR, Raman, XPS and XRD characterization. The surface of the porous carbon matrix provides active sites, which is beneficial to the successful anchoring of PPy and improves utilization of PPy, so that the PPy nanospheres morphology is well controlled. In the electrochemical test, the specific capacitance of PPy/N-PCM reached 237.5 F g −1 due to the synergistic effect of PPy and N-PCM. When the current is expanded to 10 A g −1 in cycle stability test, the capacitance retention rate can reach 88.53%. The sample of PPy/N-PCM not only solves the morphology problem of polypyrrole, but also increases specific capacitance of carbon matrix. This may be another major breakthrough in the field of conductive polymers.