1 Effect of graphene oxide concentration on the micromorphology of composite paper
RPC paper was prepared by integrating GO flakes with PPy/cellulose paper (5.1 Ω/sq) with the lowest sheet resistance, and then reducing the GO component using NaBH4. NaBH4 has been shown to be an effective reducing agent to obtain low-resistance RGO films. RPC composite paper supercapacitor made from GO dispersions with concentrations of 0.1 g/L, 0.5 g/L, and 2.5 g/L were labeled RPC-0.1, RPC-0.5, and RPC-2.5, respectively. The morphology of the RPC composite paper supercapacitor obtained by SEM observation is shown in Figure 1. It can be clearly seen that the papers have been modified by many flake-like rGO (white circles indicate the main distribution locations of rGO sheets) with a width of about a few microns. As shown in Figure 1(a)–(c), the proportion of rGO in the RPC paper increased significantly with the increase in the initial concentration of the GO dispersion. According to the results of thermogravimetric (TG) measurements (Figure 2, for RPC-0.1, RPC-0.5 and RPC-2.5.rGO content (mass fraction) can be roughly calculated to be 0.94%, 3.41% and 8.06%, respectively. Under low rGO content conditions, rGO is simply deposited on the surface of the PPy layer [Figure 1(a)]; with the increase of GO content, rGO flakes gradually penetrate into the porous PPy layer [Figure 1(b) and (c)], which helps to obtain stronger interactions and better interface bonding. The interaction mainly comes from the van der Waals force and π-π stacking between PPy and GO sheets. The high-magnification SEM images in Figure 1(d)~(f) show the penetration trend of rGO flakes more clearly. In addition, the introduction of rGO improves the conductivity of RPC composite paper supercapacitor. The sheet resistance of RPC-2.5 with an rGO content (mass fraction) of 8.06% is 1.7 /sq, which is 0.51 times and 0.64 times lower than the sheet resistance of RPC0.5 (3.5 Ω/sq) and RPC-0.1 (4.7 /sq) with rGO contents of 3.41% and 0.94% respectively. The results show that there is a significant negative correlation between the sheet resistance and the GO ratio, which may be due to the good contact between the rGO and PPy layers under high GO content conditions, which makes the highly conductive and large surface area rGO sheets participate in the construction of the conductive network. The increase in rGO content enhances the connection between rGO sheets and increases the number of available charge carriers.
Figure 1 [(a)~(c)] low magnification and [(d)~(f)] high magnification SEM images of RPC-0.1, RPC-0.5 and RPC-2.5
Figure 2 TG curves of cellulose paper, PPy/cellulose paper, RPC-0.1, RPC-0.5 and RPC-2.5
2 Chemical composition and specific surface area of reduced graphene oxide/polypyrrole/cellulose composite paper
Atomic force microscopy (AFM) is one of the most direct methods to characterize the degree of graphene exfoliation. Figure 3(a) shows the AFM image of the prepared GO flakes. The thickness of the GO flakes was about 1.2 nm as measured by the height profile of AFM, which is consistent with the data reported in the literature, indicating the formation of a single layer of GO. Therefore, with stirring and shaking during the impregnation process, the ultrathin GO sheets easily penetrated into the porous PPy layer. Transmission electron microscopy (TEM) observation was used to further study the microstructure of the RPC composite paper supercapacitor. As shown in Figure 3(b), the TEM image shows that the rGO sheets are covered by a large number of well-dispersed PPy nanoparticles, indicating good interfacial bonding between them. In addition, the higher magnification TEM image [Figure 3(c)] further verifies that the rGO sheets have penetrated into the porous PPy layer, which is consistent with the SEM observations. Figure 3(d) and (e) show the C 1s core level XPS spectra of GO/PPy/cellulose paper before and after GO reduction. The C 1s XPS spectrum of GO/PPy/cellulose paper showed five fitting peaks, namely 284.6eV (C-C), 285.3 eV (C-N), 286.8 eV (C-OH), 288.0 eV (C=O) and 290.0 eV (O-C=O). However, after NaBH4 reduction of GO, the C 1s XPS spectrum of the resulting product (i.e., RPC-2.5) composite paper supercapacitor showed a significant decrease in the signal intensity of oxygen-containing functional groups [Figure 3(e)], confirming the decrease in the content of oxygen-containing functional groups after reduction. In addition, the atomic ratio of C/O increased from 2.7 to 6.8 after reduction, further proving that most of the oxygen-containing groups on the GO flakes were eliminated by chemical reduction. Before and after GO reduction The Raman spectrum of GO/PPY cellulose paper is shown in Figure 3(f). For GO/PPy/cellulose paper, the relative intensity of the characteristic D band at 1339 cm-1 reflects the degree of disorder in the crystal structure, while the G band at 1567 cm-1 represents the first-order scattering E2g vibration mode, which is used to characterize the sp2 bond structure of carbon. The ID:IG ratio is a measure of the disorder of graphene. In addition, the peaks at 952 cm-1 and 1059 cm-1 belong to the characteristic peaks of PPy. After GO reduction, the ID:IG ratio increases significantly, which may be attributed to the following points: ① The content of sp2 carbon formed during the reduction process increases; ② Many structural defects are generated after removing a large number of oxygen-containing functional groups; ③ The in-situ formed rGO has a partially disordered crystal structure.
Figure 3(a) AFM image of GO nanosheets: (b) low magnification and (c) high magnification TEM images of RPC-2.5 The circled area in Figure (c) represents rGO The location of the thin film, the inset shows the magnified image of the square marked area; (d) C 1s core level XPS spectra of GO/PPy cellulose paper and (e) RPC-2.5; (f) Raman spectra of GO/PPy/cellulose paper (i) and RPC-2.5 (ii); (g) Nitrogen adsorption-desorption isotherms of cellulose paper, PPy/cellulose paper and RPC-2.5
The above results show that NaBH4 can effectively reduce the GO content in GO/PPy/cellulose paper.
Figure 3(g) shows the nitrogen adsorption-desorption isotherms of cellulose paper, PPy/cellulose paper and RPC-2.5. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, these isotherms can all be classified as type IV, and the hysteresis loop belongs to type H3, indicating the presence of slit-shaped pores. The Brunauer-Emmett-Teller (BET) surface area of RPC-2.5 (9.3 m2/g) is larger than that of cellulose paper (3.4 m2/g) and PPy/cellulose paper (6.9 m2/g). Therefore, the RPC-2.5 composite paper supercapacitor has more active sites in contact with the electrolyte, which will enhance the electrochemical performance of RPC-2.5.
