Compared with cellulose microfibers, nanocellulose has a finer nanostructure, superior mechanical strength, lower thermal expansion coefficient, and higher surface chemical activity. In addition, nanocellulose can be prepared into different types and materials with different properties by self-assembly, such as spinning fibers, dense thin or paper, porous aerogel, etc. These characteristics make nanocellulose more suitable for the manufacture of flexible supercapacitors for portable electronics. Flexible supercapacitor devices using nanocellulose as a key component have made extensive progress. This section will discuss the latest progress in nanocellulose-based electrode materials (including fibers, films/papers, and aerogels).

1 Fibrous electrodes for flexible supercapacitors

Nanocellulose can obtain a mechanically stable and highly oriented fibrous structure through the interaction between nanofibrils. This interaction can be achieved by extruding a nanocellulose dispersion (mixed with a specific dose of additives such as organic plasticizers, conductive polymers, metal or non-metal active materials) into a coagulation bath by wet spinning. This process can continuously prepare one-dimensional highly oriented nanocellulose long fibers [Figure 1(a)]. This new type of fiber plays an important role in the field of wearable electronics due to its advantages such as small size, light weight, good flexibility and high tensile strength. A CNF/SWCNT nonwoven microfiber mat was prepared by controlling the extrusion of a cellulose nanofibril (CNF)/SWCNT suspension in an ethanol coagulation bath [Figure 1(b)]. The average diameter of the prepared single fiber is about 50 mm [Figure 1(c)], and its rough surface has a large number of nanoscale pore structures. SWCNT can be preferentially oriented along the fiber axis, and no obvious aggregation occurs during the extrusion process [Figure 1(d)], providing abundant channels for rapid electron transmission along the axis. As a flexible supercapacitors, the CNF/SWCNT nonwoven microfiber mat exhibits an area specific capacitance of 5.99 mF/cm² and excellent electrochemical stability at a current density of 0.024mA/cm² [Figure 1(e)]. In addition, the textile integrated with this wearable supercapacitor device can be cut into specific shapes according to different usage requirements without affecting its electrochemical performance [Figure 1(f)].

Figure 1 (a) Preparation of nanocellulose fibers by extruding cellulose injection solution into a coagulation bath and drying based on wet spinning; (b)~(f) (b) physical image, (c)~(d) SEM image, (e) anti-bending electrochemical stability and (f) tailorability of CNF/SWCNT nonwoven microfiber mat; (g) preparation process of CNF-rGO/PPy fibers; (h) cross-section and surface morphology of CNF-IGO/PPy fibers: (i) preparation process of IGO/CNC hybrid fibers; (j) interaction mechanism between G0 nanosheets and CNC; (k)~(m) (k) preparation process, (i) micromorphology and (m) charge and discharge mechanism of CMC-wrapped graphene/CNT/PEDOT:PSS coaxial conductive fibers

CNF-reinforced graphene/PPy fibers (CNF-rGO/PPy) can also be prepared by similar wet spinning [Figure 1 (g)]. As an excellent scaffold material, CNF can not only inhibit the stacking of graphene nanosheets, but also provide a large number of hydrogen bonds to enhance the interlayer force, enhance the stability of its multilayer structure, and prevent its structural deformation in the dry state [Figure 1(h)]. The optimized CNF-rGO/PPy fiber can achieve a high tensile strength of 364.3 MPa and show excellent rate performance. The capacitance retention rate is as high as 98.8% when the current density increases 10 times. Studies have shown that graphite nanosheets used for spinning tend to form large liquid crystal phase aggregates, causing the spinning solution to form a high viscosity, low fluidity gel, making it difficult to obtain continuous high-strength fibers. Cher et al. proposed a strategy of non-liquid crystal spinning and chemical reduction to overcome this problem [Figure 1(i)]. As shown in Figure 1(j), in an alkaline non-liquid crystal suspension, by adding NaOH The surface of graphene oxide (GO sheet) produces a higher negative charge, resulting in stronger electrostatic repulsion to reduce the hydrogen bonding interaction between GO nanosheets and cellulose nanocrystals (CNC). This uniform non-liquid crystal GO/CNC dispersion with low viscosity ensures smooth extrusion of continuous fibers. In addition, the fibrous electrode has a high specific capacitance of 208.2F/cm³ and a strength of 199.8MPa, and shows great potential as a core material for flexible supercapacitors.

Most of the above-mentioned fiber electrode materials are assembled into devices in a parallel arrangement or spiral winding manner, which is a relatively complicated process, and the electrochemical stability of the device is largely affected by the contact area between the electrodes. More importantly, the exposed conductive layer in this structure is prone to short circuit. The preparation of composite fibers with a core-shell structure by coaxial spinning can effectively solve these problems. Cai et al. designed a graphene/CNT/PEDOT:PSS wrapped with carboxymethyl cellulose (CMC). coaxial conductive fibers [Figure 1(k)]. As shown in Figure 1(I), the ternary active material as the core layer is tightly wrapped by the CMC with ionic conductivity and electrical insulation, preventing current leakage while allowing the free penetration of ions. On the other hand, this ternary composite structure enables the double-layer capacitance of rGO and CNI and the capacitance of PEDOT:PSS to produce synergistic benefits and accelerate the electrochemical reaction kinetics [Figure 1(m)]. In addition to the typical carbon material/conductive polymer composite with nanocellulose to prepare spinning fiber supercapacitor electrode materials, nanocellulose fibers can also be directly carbonized into highly conductive carbon fibers. However, the carbonization process is often accompanied by dehydration reaction and cleavage of glycosidic bonds, resulting in structural shrinkage and poor mechanical strength of carbon fibers. Wang et al. demonstrated that the combination of nanocellulose scaffold structure and high carbon content of lignin can overcome the above defects. Therefore, they prepared carbon fibers with lignin/CNF composite fibers as precursors, which obtained a carbon retention rate of 41% and good structural retention, with a conductivity of 103 S/cm, which can be used to create fiber-shaped capacitors, microelectrodes and wearable electronic products:

2 Flexible film/paper electrodes

Nanocellulose-based film/paper is another flexible cellulose material that can be used to integrate electrochemically active materials. In the dehydrated state, high aspect ratio nanocellulose easily self-assembles through hydrogen bonding, van der Waals force interaction or electrostatic force to form flexible films/papers with numerous micropores/mesopores, which can accelerate ion diffusion and electron migration. In addition, the surface hydroxyl and carboxyl groups endow nanocellulose with excellent water holding capacity in electrolyte solutions. These advantages make nanocellulose also useful as a separator for electrochemical energy storage devices and a reinforcing material for electrolytes.

Generally, two-dimensional thin film electrodes can be prepared by depositing an electrochemically active layer on the surface of a flexible nanocellulose film, similar to the fabrication of cellulose paper electrodes mentioned in Previous article. The most common method is to immerse the nanocellulose film in a conductive polymer monomer solution and initiate in situ polymerization. The reported PPy/CNF thin film electrode exhibits a tensile strength of 71.4 MPa and provides a high area specific capacitance of 2.26 F/cm² at 2mA/cm². In addition, Ravit et al. used a rapid one-step electrochemical polymerization technique to assemble PEDOT and MnO2 nanoparticles on CNC films as flexible supercapacitors electrodes [Figure 2(a)]. The interconnected network of CNC films presents abundant nanopores, which can greatly reduce the interfacial resistance of the original PEDOT electrode and improve the charge transfer.

Figure 2(a) Schematic diagram of PEDOT/CNC/MnO electrode fabrication and two-electrode assembly; (b) CNF/CNC composite carbonized film preparation process; (c) CNF and CNC suspension; (d) TEM images of CNF and (e) CNC; (f) and (g) are macroscopic photos of hybrid aerogels and carbon aerogels; (h)~(i) Preparation of CNF paper and (j) structural diagram of printed supercapacitors; (k) Schematic diagram of the process of making printed supercapacitors using a printer; (I)~(m) Series-parallel circuits with LEDs can be easily connected to printed supercapacitors without the need for metal interconnects

Nanocellulose films can also be converted into conductive carbon materials through a simple pyrolysis process, which can be used as good anode materials for flexible supercapacitors. Li et al. combined the structural advantages of good compatibility of CNF and CNC to design a hierarchical porous structure for flexible nanocellulose carbon film [Figure 2 (b) ~ (g)]. In this carbonized nanomembrane, long CNF constitutes a macroporous framework, while short CNC assembles around the CNF framework and produces micropores and mesopores, eventually forming a highly conductive, highly graphitized nanocellulose fiber network with a specific surface area of up to 1244m²/g. As supercapacitor electrode materials, these films exhibit better electrochemical performance than commercial activated carbon in a high current range. Torvinen et al. prepared CNF paper with a nanoporous network structure for flexible supercapacitors by printing technology [Figure 2 (h) and (i)]. As shown in Figure 2 (j), graphene and CNT active layers are printed on both sides of the CNF paper as high surface area active electrodes and current collectors, while the CNF paper serves as a dual-function separator substrate in supercapacitors. Later, Choi et al. used a conventional desktop inkjet printer to directly print on the CNF paper. A novel printed supercapacitor was fabricated on CNF paper, featuring integral integration with the paper substrate and high-resolution printed images. As shown in Figure 2(k), the nanoporous CNF matrix, interdigitated electrode layer (SWCNT/activated carbon/silver nanowires), and solid electrolyte layer were deposited stepwise by conventional inkjet printing process, followed by UV-induced crosslinking reaction. The inkjet printing technique brings the aesthetic appeal advantage of versatility to supercapacitors. In addition, without the help of metal interconnects, series and parallel circuits can be easily connected on the inkjet-printed supercapacitors [Figure 2(i) and (m)], reflecting that the battery voltage and capacitance can be flexibly controlled according to the usage. Given the simplicity and scalability of the inkjet printing process and the design versatility of the device, it is foreseeable that the application of inkjet-printed nanocellulose devices will expand to a wider range of fields.

In addition to the above-mentioned methods for fabricating nanocellulose membrane electrodes, conductive polymers and two-dimensional graphite can also be mixed into the suspension of nanocellulose to assemble composite membranes with good flexibility, high mechanical strength and conductivity. To date, researchers have developed various nanocellulose/conductive polymer composite nanofilms or papers by simple methods, such as: CNF/PANI, CNF/PPy, CNF/PEDOT, etc. More importantly, the introduction of a specific proportion of GO as a reinforcing and structure-directed additive can give the composite membranes better flexibility and higher surface area. Hou et al. reported a highly flexible CNF-based film containing PPy and rGO nanosheets made by a regulated filtration method [Figure 3(a)]. Since the hydrophilic CNF provides more ion diffusion paths within the rGO layer and the sandwich structure reduces the charge transfer resistance [Figure 3(b) and (c)], this sandwich PPy/rGO/CNF membrane (S-PRC) shows better performance than the sandwich PPy/hrGO membrane (S-PR) and the randomly mixed PPy/rGO/CNF film (M-PRC) has a higher specific capacitance [Figure 3(d)]. In addition, the introduction of lignin sulfonate into the CNF-PEDOT:PSS system, the all-organic flexible supercapacitors paper electrode composed of wood derivatives and conductive polymers [Figure 3(e)~(g)] also exhibits a high specific capacitance ranging from 110 F/g to 230 F/g.

In recent years, two-dimensional metal carbides and nitrides called MXene have emerged. They can be combined with one-dimensional nanocellulose to construct various new functional films, and exhibit excellent metallic conductivity, good hydrophilicity and high redox activity. Tian et al. prepared mechanically strong composite films by filtering stable dispersed colloids of MXene (Ti₃C₂T_x/CNF) and CNF [Figure 3 (h) and (i)]. They proposed that the stable mixing of the two colloids can be attributed to the electrostatic force between CNF and Ti₃C₂T_x flakes, and their adhesion benefits from the functional groups present on the surface of the outer transition metal layer of Ti₃C₂T_x [Figure 3 (j) and (k)]. This special interaction gives the composite film 341 MPa and a high capacitance of 298F/g. They also demonstrated that this MXene/CNF hybrid dispersion can be used as ink to print flexible supercapacitors with precise dimensions [Figure 3(I) and (m)].

Figure 3 (a) Fabrication process of flexible sandwich PPy/rGO/CNF film; (b)~(d) PPy/rGO/CNF film (b) physical image, (c) cross-sectional SEM image and (d) electrochemical performance test; (e) Preparation process of all-organic component paper electrode and (f)~(g) foldability; (h)~(i) Preparation process of Ti₃C₂T_x/CNF nanofilm; (j) Zeta potential of Ti₃C₂T_x/CNF dispersions with different CNF contents at different times; (k) Ti3C2Tx flakes and Functional relationship between ionic strength and adhesion between CNF interfaces; (I) Preparation process of Ti₃C₂T_x//CNF-based micro-supercapacitors; (m) CV curves of Ti₃C₂T_x/CNF-based micro-supercapacitors under different states