## 13 Nanocellulose Aerogels for Electromagnetic Interference Shielding  
Cellulose is one of the most abundant polysaccharides on the planet and is a key component present in the cell wall of plants. Basically, it is a linear biopolymer composed of 1,4-anhydro-D-glucopyranoside units and is considered as a replenishable feedstock which has the potential for meeting the increasing demand for green and sustainable products. By virtue, cellulose has a fibrillar structure and coexists with hemicellulose and lignin in the plant cell walls. Inside individual cellulose fibrils, there exists regions that are highly ordered (crystalline) and disordered (amorphous), with strong inter/intra-molecular hydrogen bonding between the hydroxyl groups present in the cellulose backbone. At the nanoscale level, each macro-cellulose bundle is composed of several elementary cellulose nanofibers (5-20 nm diameter) that can be extracted and utilized for many technological applications, such as water purification membranes, substrates for flexible electronics, electrode materials for batteries and biomedical scaffolds [154]. These applications are made possible because of the excellent physicochemical properties of nanocellulose, such as high stiffness, high crystallinity, tunable surface chemistry and impeccable film-forming ability. However, for designing lightweight and robust solutions to the current challenges that the industries are facing, cellulose nanofiber (CNF)-based aerogels have gained much attention. CNF aerogels are highly porous solids with high porosity, ultra-low density and high specific surface area, and are foreseen as a feasible solution for advanced applications ranging from oil absorbents to biomedical scaffolds. In this view, [PERSON] et al. [155] developed a series of CNF aerogels decorated with silver nanowires (AgNWs) with minimal density and flexibility for high-performance EMI shielding. Herein, three different aerogel morphologies were fabricated using bidirectional, unidirectional and randomly directional freeze-casting methods using facile ice-templated freezing. Figure 15 depicts some of the key findings, such as the hierarchical structure of CNF, the Tyndall effect confirming the dispersion stability of CNF in water, different freezing approaches employed for making anisotropic and honeycomb-like aerogels, interactions of CNF and AgNPs, different aerogel morphologies and the bendability/twistability of CNF/AgNW aerogels. These aerogels demonstrated a maximum EMI SE value of 70 dB, with the highest specific EMI SE value of 178,235 dB-cm\({}^{2}\).g\({}^{-1}\), which far exceeds the values reported for biobased aerogels [155]. Upon addition of magnetic nanoparticles, one can enhance the overall permeability values of the nanocomposites, which can lead to enhanced microwave absorption. This approach was used by [PERSON] and coworkers for fabricating cellulose/rGO/Fe\({}_{3}\)O\({}_{4}\) aerogels by decorating Fe\({}_{3}\)O\({}_{4}\) nanoparticles within the porous network of these aerogels. The steps involved in the fabrication of these aerogels are shown in Figure 16. Cellulose was dissolved in a sodium hydroxide/urea/H\({}_{2}\)O mixture along with calculated amounts of graphene oxide (GO). Vitamin C solution (30 g/L) was used to reduce GO to form cellulose/rGO hydrogel, which was later freeze-dried for 48 h to form cellulose/GO aerogels. These hydrogels were also simultaneously immersed in an aqueous FeCl\({}_{3}\) and FeCl\({}_{2}\) solution to form Fe\({}_{3}\)O\({}_{4}\)-decorated hydrogel and lyophilized in a freeze drier to form cellulose/rGO/Fe\({}_{3}\)O\({}_{4}\) aerogels. In order to elucidate the effect of porosity, film samples of these compositions were also fabricated. A maximum EMI SE value of 52.4 dB was achieved at a 2.0 mm aerogel thickness at 10 GHz [156]. With the increasing magnetic content and thickness of these aerogels and films, shielding by absorption was also found to increase.