*4.2. Preparation and Industrial Application of Nanocellulose*/*CNT Composites*

The nanocellulose component in the nanocellulose/CNT composites is used in the form of nanofibrils and nanocrystals, and carbon nanotubes usually in the form of SWCNTs or MWCNTs. Similarly as in composites containing graphene and other carbon allotropes, cellulose nanoparticles facilitate the homogeneous dispersion of CNTs in aqueous environments (Figure 3b), where the two types of nanoparticles are linked by noncovalent interactions, e.g., hydrophobic and electrostatic interactions [42,46]. The dispersion of CNTs can be further facilitated by TEMPO-mediated oxidation of cellulose nanoparticles, which endows them with abundant anionic carboxyl groups [141]. Other ways include the functionalization of CNTs with self-assembling amphiphilic glycosylated proteins [142] or the use of oil-in-water Pickering emulsions of cellulose nanoparticles [143]. From the aqueous dispersions, 2D and 3D nanocellulose/CNT composites can be formed, e.g., by vacuum filtration, centrifugal cast molding, foam forming, casting and printing [39,45,50,141]. Pickering emulsions of cellulose nanocrystals and SWCNTs or MWCNTs were used for fabricating aerogels and foams by freeze-drying [143]. Similarly as in nanocellulose/graphene composites, CNTs can be added to cultures of bacteria producing nanocellulose, and incorporated into the bacterial nanocellulose during its growth [53,57]. Composite nanocellulose/SWCNT films can be transparent or semitransparent [39, 45,141], and can transmit radiant energy [144].

Like nanocellulose/graphene composites, nanocellulose/CNT composites can be combined with various atoms, molecules and nanoparticles in order to enhance their properties for specific applications. For example, Ag nanoparticles attached to the surface of MWCNTs influenced the electrochemical properties of CNT-based films developed on a bacterial nanocellulose membrane [36]. Nanocellulose and CNTs can be also used as additives to various hybrid materials. For example, a hybrid material, created by combination of poly (3,4-ethylenedioxythiophene)-poly (styrenesulfonate) (PEDOT:PSS), silver nanoparticles (AgNPs), CNTs and a nanocellulose layer, was used for constructing a tactile sensor [103]. Incorporation of polypyrrole-coated CNTs into chemically cross-linked CNC aerogels created promising materials for flexible 3D supercapacitors [145]. A combination of cellulose acetate, chitosan and SWCNTs with Fe3O4 and TiO2 in electrospun nanofibers enables combined removal of Cr6+, As5+, methylene blue and Congo red from aqueous solutions via the adsorption and photocatalytic reduction processes [146].

Energy-related applications of nanocellulose/CNT composites include biofuel cells, varactors, supercapacitors, and electrodes for lithium batteries, thermoelectric generators for heat-to-electricity conversion or for constructing heating elements [50]. A biofuel cell comprising electrodes based on supercapacitive materials, i.e., on CNTs and a nanocellulose/polypyrrole composite, was utilized to power an oxygen biosensor. Laccase, immobilized on naphthylated MWCNTs, and fructose dehydrogenase, adsorbed on a porous polypyrrole matrix, were used as cathode and anode bioelectrocatalysts, respectively [84]. Another biofuel cell was based on a conductive MWCNT network, developed on a bacterial nanocellulose film, and functionalized with redox enzymes, including pyroquinoline quinone glucose dehydrogenase (anodic catalyst) and bilirubin oxidase (cathodic catalyst). This system generated electrical power via the oxidation of glucose and the reduction of molecular oxygen [44]. Microelectromechanical system varactors, i.e., voltage-controlled capacitors, consisted of a freestanding SWCNT film, which was employed as a movable component, and a flexible nanocellulose aerogel filling [85]. Supercapacitors with high physical flexibility, desirable electrochemical properties and excellent mechanical integrity were realized by rationally exploiting the unique properties of bacterial nanocellulose, CNTs, and ionic liquid-based polymer gel electrolytes [147]. Other flexible 3D supercapacitor devices were fabricated by incorporating polypyrrole nanofibers, polypyrrole-coated CNTs, and manganese dioxide (MnO2) nanoparticles in chemically cross-linked cellulose nanocrystal aerogels [145]. Electrospun core-shell nanofibrous membranes, containing CNTs stabilized with cellulose nanocrystals, were developed for use as high-performance flexible supercapacitor electrodes with enhanced water resistance, thermal stability and mechanical toughness [40]. Electrodes for lithium batteries were based on freestanding

LiCoO2/MWCNT/cellulose nanofibril composites, fabricated by a vacuum filtration technique [148], or on freestanding CNT-nanocrystalline cellulose composite films [41]. Thermoelectric generators for heat-to-electricity conversion were based on large-area bacterial nanocellulose films with an embedded/dispersed CNT percolation network, incorporated into the films during nanocellulose production by bacteria in culture [57]. Electrical energy can also be converted into thermal energy, as demonstrated by the composite structure of wood-derived nanocellulose, MWCNTs and pulp, designed for a heating element application [50].
