*3.1. Cellulose Aerogel Characteristics*

By altering the concentration of the MWBF dispersion from 1.5 to 2.5 wt.%, aerogels with different porosities were prepared using a freeze-drying method. Under freeze-drying conditions, a slight shrinkage was observed in these aerogels compared to their initial hydrogel dimensions. The experiments showed that the dispersion with 2.5 wt.% MWBF had very high viscosity, making it difficult to remove the air bubbles, resulting in a poor physical property. Table 1 lists the physical properties of the obtained cellulose aerogel.



As shown in Table 1, the densities of the cellulose aerogels ranged from 0.085 to 0.144 g/cm3, and all cellulose aerogels exhibited very high porosity (>90%). On the other hand, the specific surface area exhibited the low values from 8.155 to 13.419 m2/g. Actually, this is quite common for aerogels obtained through a freeze-drying process. The main point here is that the typical porosity of such system is on the macron-scale, and hence the resulting specific surface area value is modest. The similar trend was also reported by Wang et al. [39]. Figure 2 shows the microstructure of a cross-section of the cellulose aerogel at different magnifications. The cellulose aerogels possessed a highly open porous honeycomb-like structure with a pore size distribution varying over a wide range from several to tens of micrometers. In addition, a network of interconnected uniform cellulose fibers appeared on the surface of the pore wall.

**Figure 2.** Field-emission scanning electron microscopy (FE-SEM) images of cellulose aerogel 2 wt.% (**a**,**b**) and cellulose aerogel 1.5 wt.% (**c**,**d**).
