*3.2. Preparation and Industrial Application of Nanocellulose*/*Graphene Composites*

Similarly as in fullerenes, cellulose nanoparticles in the form of nanofibrils and nanocrystals increase the dispersion of graphene nanoparticles in water-based environments and prevent their aggregation without the need to subject them to chemical functionalization [46]. A water-based dispersion is the starting material for fabricating nanocellulose/graphene composites (Figure 2b). These composites can be created by filtration [127], filtration combined with hot pressing for fabricating films [128], or by freeze-drying [75] and freeze-casting [48] for fabricating 3D materials, such as aerogels and foams. Other methods are deposition of graphene on a nanocellulose layer [43] and incorporation of graphene into nanocellulose during its biological synthesis by bacteria [54–56].

All forms of nanocellulose and graphene have been used for constructing nanocellulose/graphene composites, i.e., CNFs, CNCs, unmodified graphene, GO and rGO. In order to modulate the properties of nanocellulose/graphene composites for specific applications, these materials can be further enriched by various substances, such as metallic or ceramic nanoparticles, oxides, carbides, sulfides, vitamin C, synthetic and natural polymers, enzymes and antibodies. For example, nanocellulose/graphene composites have high adsorption, filtration and photocatalytic ability, and they are therefore widely used for water purification, e.g., for removing antibiotics [75], dyes [43], heavy metals, such as Cu2<sup>+</sup>, Hg2<sup>+</sup>, Ni2<sup>+</sup> and Ag<sup>+</sup> [61,76], or for their bactericidal effect [56]. The water-cleaning capacity of these composites can be further enhanced by introducing additional photocatalytic agents, i.e., palladium

nanoparticles [54] or zinc oxide (ZnO) nanoparticles [73]. An optimized ultrafiltration membrane for water purification was constructed from polyvinylidene fluoride (PVDF), modified by cellulose nanocrystals functionalized with common bactericides, such as dodecyl dimethyl benzyl ammonium chloride, ZnO and GO nanosheets [129]. Another important additive is vitamin C, which reduces the GO in nanocellulose/GO composites, increases the surface area of the material and increases pore formation, and thus enhances the capacity of the composites for water purification [49]. A combination of rGO-coated cellulose nanofibers with hydrophobic and oleophilic trimethyl chlorosilane enhanced the adsorption capacity of this composite, which is necessary for effective removal of oil-based pollutants from water [74].

Another important industrial application of nanocellulose/graphene composites is in energy storage, generation and conversion. Devices for these purposes include supercapacitors [64,80, 130], hydrogen storage devices [131], electrodes for hydrogen evolution reaction [132], lithium ion batteries [133], actuators [81], solar steam generators [82] and electric heating membranes [83]. These devices can be based on pure nanocellulose/nanocarbon composites without additives [80– 83]. However, they often contain additives such as manganese oxide (MnO), which contributes to faradaic pseudocapacitance in supercapacitors [130] or polypyrrole, which acts as an insulator, but its oxidized derivatives are good electrical conductors [134]. Other additives are palladium or platinum nanoparticles for enhanced hydrogen storage [131], nitrogen-doped molybdenum carbide nanobelts in electrocatalysts for hydrogen evolution reaction [132], and silicon oxide nanoparticles in lithium ion batteries [133].

Other important industrial applications of nanocellulose/graphene composites are in the construction of fire retardants, shape memory devices, biocatalysts and materials for food packaging. Super-insulating, fire-retardant, mechanically strong anisotropic foams were produced by freeze-casting suspensions of cellulose nanofibers, GO and sepiolite nanorods, and they performed better than traditional polymer-based insulating materials [48]. Shape memory devices are based on GO/CNC thin films and nanomembranes [38,88] or on GO introduced into a nanocellulose paper made of nanofibers extracted from sisal fibers [89]. An example of a biocatalyst is a nanocellulose/polypyrrole/GO nanocomposite for immobilization of lipase, a versatile hydrolytic enzyme. This biocatalyst was employed for synthesizing ethyl acetoacetate, a fruit flavor compound [86]. Food packaging materials were constructed by filling CNCs and rGO, either separately or in the form of CNC/rGO nanohybrids, into poly (lactic acid) (PLA) matrix or in a poly (3-hydroxybutyrate*-co-*3-hydroxyvalerate) (PHBV) matrix. These composite materials exhibited better mechanical properties than the pristine polymers, and possessed antibacterial activity. In addition, the composites with CNC/rGO nanohybrids performed better than those with a single component nanofiller, i.e., either CNCs or rGO. Due to their antibacterial activity, antioxidant properties and good in vitro cytocompatibility, these composites are also promising for biomedical applications, e.g., as scaffolds for tissue engineering [67–69,87].
