*2.4. Characterization*

The dispersion quality of aqueous CNT/CNF and GnP/CNF mixtures after sonication was examined by depositing a drop of each suspension on a glass slide for optical microscope observations. The size of the aggregates was measured and averaged over 150–200 particles. Regarding the compression test, the aerogel samples were transferred into a chamber with a temperature of 25 ± 2 ◦C and a relative humidity of 65 ± 2%. The compressive strength of the samples was measured 24 h later with a compression ratio of 1 mm min−<sup>1</sup> on a universal mechanical testing machine (CMT1000, SUST, Zhuhai, China). The morphology of CNF, CNF–CNT, CNF–GnP aerogels and CNT, GnP powder was obtained by a field emission scanning electron microscope (FE-SEM, SU-70, Hitachi, Chiyoda, Japan) after sputtering coating the samples with gold at 15 mA for 2 min under vacuum conditions (Hitachi, E-1010 Ion Sputtering System, Japan). Imaging of the samples was performed at 0.5–30 kV acceleration voltage and 1–2 nA current intensity at magnifications of 20–800,000 times. The chemical structures of CNF, CNF–CNT, CNF–GnP aerogels and CNT, GnP powder were characterized by Fourier transform infrared spectroscopy (FTIR, PerkinElmer, Spectrum 100, Waltham, MA, USA) scanning from 4000 to 400 cm−<sup>1</sup> with a resolution of 4 cm<sup>−</sup>1. Before FTIR measurements, 2 mg dry sample was ground into powder with 200 mg KBr and pressed into pellets. The specific surface area (SSA) of the samples was calculated using Equation (1) [34].

$$\text{SSA} = \frac{N\_{\text{A}} A\_{\text{MB}} (\mathbb{C}\_{\text{o}} - \mathbb{C}\_{\text{e}}) V}{M\_{\text{MB}} M\_{\text{S}}} \tag{1}$$

where *N*<sup>A</sup> is Avogadro's number (6.023 <sup>×</sup> 1023 mol−1), *A*MB is the covered area per MB molecules (typically assumed to be 1.35 nm2), *C*<sup>o</sup> and *C*<sup>e</sup> are the initial and equilibrium concentration of MB, respectively, *V* is the volume of the MB solution, *M*MB is the relative molecular mass of MB, and *M*<sup>S</sup> is the mass of the sample.
