## 3 Results and Discussion  
3 DG was prepared by annealing of freeze-dried RGO hydrogel, formed firstly, via hydrothermal reduction of GO. As observed by SEM, 3 DG after thermal treatment still exhibited porous structure with pore sizes distributed from several hundred nanometers to several micrometers (Figure 1a), demonstrating that further annealing of freeze-dried RGO hydrogel has less impact on its pore structure [31]. High magnification of SEM image (Figure 1b) clearly shows the wrinkle structure of graphene sheets stacking of 3 DG, which is also confirmed via low magnification TEM (Figure 1c). Obvious graphene crystalline strips are also presented for 3 DG, as imaged from high-resolution TEM (Figure 1d). Raman spectrum of 3 DG (Figure 2a) shows the two characteristic peaks at 1335 and 1591 cm\({}^{-1}\), attributed to D and G bands of graphene materials, respectively. The intensity ratio of D and G bands (I\({}_{\text{D}}\)/I\({}_{\text{G}}\)) reflects the defect extent of materials. The I\({}_{\text{D}}\)/I\({}_{\text{G}}\) value of prepared 3 DG was measured to be 1.28, obviously higher than that of its RGO hydrogel precursor with a value of 1.04 [30], indicating sp\({}^{2}\) domains and edge defects were increased after thermal reduction [32]. C 1s XPS spectra of 3 DG (Figure 2b) reveal typical peaks at ca. 286 and 289 eV ascribed to C-O and COOH groups, respectively. But the intensity for these peaks related to oxygenated groups is obviously weaker than the corresponding peaks from RGO hydrogel. Lower oxygen content for 3 DG was also demonstrated by the ratio of C/O measured by XPS. 3 DG had a C/O ratio of 7.1:1, higher than that of RGO hydrogel of 6.2:1 [30]. The oxygenated groups were further removed from the 3 DG via thermal reduction, improving its electrical conductivity. Residual oxygenated groups of a small amount at 3 DG surface benefit its contact with the electrolyte solution. Figure 1: SEM (**a**,**b**) and TEM (**c**,**d**) images with different magnifications for 3 DG. Low magnification (**a**,**c**), and high magnification (**b**,**d**). Figure 2: Raman spectrum (**a**) and C 1s XPS spectra (**b**) of 3 DG. Cyclic voltammetry (CV) curves of the 3 DG at various potential scanning rates are shown in Figure 3a. They always remain in a typical rectangular shape when the scanning rate changes from 10 to 100 mV-s\({}^{-1}\). While the CV curves for the RGO hydrogel distorted rapidly when increasing the scanning rate [30]. Therefore, the obtained 3 DG via thermal treatment possesses better rate performance for supercapacitors, and no obvious redox peaks relevant to oxygenated groups are seen, supporting the electrical double-layer capacitance for 3 DG. Figure 3b displays the GCD curve of 3 DG at a current density of 1 A-g\({}^{-1}\). According to this figure, the specific capacitance of 3 DG at 1 A-g\({}^{-1}\) was calculated to be 115 F-g\({}^{-1}\), lower than RGO hydrogel (198 F-g\({}^{-1}\) at ca. 1 A-g\({}^{-1}\)) [30]. Lower capacitance for 3 DG was induced by the disappearance of pseudo-capacitance due to the removal of oxygenated groups via thermal reduction [33]. Furthermore, the elimination of oxygenated groups increased the hydrophobicity of 3 DG, not benefiting its contact with electrolyte solution [29]. This value is comparable to commercial activated carbon with a typical specific capacitance of 80 to 120 F-g\({}^{-1}\)[34; 35]. The discharge curves at different current densities are shown in Figure 3c. According to these curves, specific capacitances were calculated and plotted versus current densities, as shown in Figure 3d. When current density increases up to 40 A-g\({}^{-1}\), the specific capacitance for 3 DG still remains 81 F-g\({}^{-1}\). A 70% capacitance retention ratio for 3 DG is presented when current density increasing 40 fold, obviously better than most reported results for 3 DG, while the corresponding RGO hydrogel only showed a 16% capacitance retention when current density increased to ca. 20 A-g\({}^{-1}\).