The well-defined and interconnected 3D porous network of the resulting RGO hydrogel was revealed by typical SEM images of its supercritical CO2 dried purified sample as shown in Fig. 3a and Fig. SI4, from which a number of hierarchical pores with a wide size distribution were observed. Macropores (pore diameter >50 nm) with a size of ca. 1 lm are closely stacked, and solid walls of these macropores are randomly self-assembled by the inter-twisted sheet-like RGO and thus a lot of mesopores (pore diameter in the range 2–50 nm) are produced among these sheet- like structures. The existence of these mesopores was also confirmed by a hysteresis cycle in a type IV sorption isotherm curve of the dried RGO hydrogel with the BET surface area of ca. 500m2/g (see Fig. SI5). Partial coalescence of the flexible graphene sheets leads to the formation of physical cross-linking sites in the hydrogel framework. Thus the inherent flexibility of the graphene sheets is a crucial characteristic for constructing the porous 3D assembly of the RGO hydrogel.Within the assembly, the sheetswere thin enough to be transparent to the electron beam [48]. However, the AFM image as shown in Fig. 3b disclosed that typical
thickness of the sheets is 0.975 nm, consistent with the mono-layer thickness of the graphene observed by others [49,50]. More than 98.0 wt.% water encapsulated in the
hydrogel 3D network, benefiting from residual hydrophilic oxygen-containing functional groups, a direct reflection of the resulting graphene 3D assembly with the hierarchical pores.