# Facile Fabrication of 3D Hierarchically Porous Carbon Foam as Supercapacitor Electrode Material
## 3 Results  
Compared to the smooth surface of starch-derived carbon (SC) in Figure 1a, there are plenty of macroscopic pores evenly embedded in the carbon matrix of SCF (Figure 1b). The higher magnification SEM images of SCF (Figure 1c,d) display a 3D structure with macroporesthe size of 100-500 nm are interconnected to others by smaller pores. It evidently shows that Na\({}_{2}\)CO\({}_{3}\) induces the formation of hierarchically porous texture. Figure 2 shows the synthesis steps and presumable mechanism for starch-derived carbon foam. Na\({}_{2}\)CO\({}_{3}\) can act as the template to produce macroropores due to the insolubility in ethanol system and the drastic solvothermal treatment promotes uniform mixing of sodium carbonate with starch. Moreover, the Na\({}_{2}\)CO\({}_{3}\) template decomposes and releases CO\({}_{2}\) that comes from inside the starch-derived carbon matrix serving as an activating agent to activate the carbon material producing micropores and mesopores as well as the enlarging pore sizes. This effect was named as the \"CO\({}_{2}\) inner-activation effect\" [12], which can beconfirmed by the Nitrogen adsorption-desorption results. Additionally, macroropesand large mesoporescan serve on ion-buffering reservoirs and increase the accessibility of the smaller pores [13], in terms of improving the rate performance. To further determine the porous structure of SCF, Nitrogen (77 K) adsorption-desorption is performed, showing that the BET specific surface area and pore volume of SCF reaches 1693 m\({}^{2}\cdot\)g\({}^{-1}\) and 1.106 cm\({}^{3}\cdot\)g\({}^{-1}\), respectively. The micropore volume of SCF calculated by thet-plot method is 0.255 cm\({}^{3}\cdot\)g\({}^{-1}\) and the corresponding microporosity is 23%. Figure 3a confirms that the as-prepared sample exhibits a mixture of Type I and IV isotherm with an H4-typehysteresis loop in the relative pressure range of 0.4-1.0 [14], demonstrating that not only do abundant micropores exist in this material, but ample mesopores exist as well [15]. In comparison, SC shows Type IV isotherm and the nitrogen adsorption capacity is relatively smaller (Figure S1) [16]. In Figure 3a, the strong steep adsorption uptake at a low relative pressure indicates a considerable number of micropores [17]. The adsorption plateau at a high relative pressure is not observed, implying the existence of  
Figure 1: **(a)** The scanning electron microscope (SEM) image of starch-derived carbon; **(b)**–**(d)** various magnification SEM images of starch-derived carbon foam SCF. Figure 2: The synthesis steps and presumable mechanism for starch-derived carbon foam. macroopores [9] which consist with the result of SEM images. In addition, pore size distribution calculated with non-local density functional theory NLDFT theory can be noted in Figure 3b. Plentiful peaks at tens of a nanometer testify the large mesopores. As is shown in the inset of Figure 3b, sharp peaks with pore sizes less than 2 nm and between 2 to 6 nm suggest the existence of micropores and small mesopores. Herein, the 3D structure of SCF in which macroporesare interconnected by extensive micropores and mesopores, which is favorable to the electrochemical performance [18], is verified. The XRD patterns (Figure 4) of as-prepared samples exhibit two broad peaks at 2\(\theta\) values of 22.5\({}^{\circ}\) and 43\({}^{\circ}\), which can be attributed to the amorphous characteristics of SCF and SC [19]. These broad peaks manifest that no evident graphitization happened during the formation. Obviously, the peaks of SCF are weaker and broader than what SC shows. It indicates the more disordered structure, which is further confirmed by the Raman spectra where the D band (1335 cm\({}^{-1}\)) of carbon is caused by the disorder groups and defects, while the G band at 1588 cm\({}^{-1}\) is on behalf of sp\({}^{2}\) domains and the 2D band (\(\sim\)2700 cm\({}^{-1}\)) manifests the graphitic degree [20]. It is well known that the lower the I\({}_{\rm D}\)/I\({}_{\rm G}\) ratio, the higher the graphitization degree.