Nanocellulose is used to prepare flexible electrolyte membranes for supercapacitors

Because nanocellulose has good mechanical properties, hydrophilicity, fine nanostructure and high porosity, it is widely used as the separator/electrolyte of supercapacitors. Lee et al. reported a nanoporous cellulose separator with a bilayer nanostructure and high porosity. In this work, the top layer is a nanoporous thin pad of tripyridine functionalized CNF and the support layer is a thick macroporous pad of electrospun polyvinylpyrrolidone/polyacrylonitrile. The unique porous bilayered/asymmetric structure balances ion transport rate and leakage current. The cycling performance of the device assembled with this unique nanocellulose-based separator has been substantially improved.

Wu and colleagues reported a flexible cellulose-based hydrogel film with layered porosity and a double crosslinked structure. Inside the membrane, cellulose and polyacrylamide (PAM) are cross-linked by PDA.

By adjusting the dopamine/acrylamide ratio, it was found that the key factors affecting the mechanical properties of the hydrogel were the abundant hydrogen bond distribution in the hydrogel network and the π-π stacking of catechol groups in PDA. The optimized C4-DM-40 (cellulose4 - PDA-PAM-40) has good self-healing and mechanical properties. Fe3+ functionalized hydrogel enhances the sensitivity and conductivity of the hydrogel. It is worth noting that activated carbon material is deposited on the C4-DM-40 hydrogel film to construct an integrated micro supercapacitor. The integrated device shows a high capacity capacitor of 394.1 F cm − 3 and a surface capacitor of 275.8 mF cm − 2 at 10 mV s − 1 . Yu et al. used a scalable simple solution phase conversion method to demonstrate a renewable, transparent and flexible mesoporous cellulose membrane (mCel-membrane). The mcel membrane has a high porosity of 71.78% and a uniform ~24.7 nm mesoporosis. The mcel film with saturated KOH as the electrolyte has good mechanical robustness and flexibility, with an ionic conductivity of up to 325 mS cm − 1 and an electrolyte retention rate of up to 451.2 wt%.

Therefore, the high ionic conductivity and stability of gels or solid diaphragms/electrolytes are also essential in practical applications to improve the energy density and safety of supercapacitors. Nanocellulose has low cost, good mechanical strength and flexibility, high thermal stability and chemical stability, and can meet the key requirements of polymer membranes or electrolytes.

New strategies for adding or modifying redox active materials in nanocellulose- based electrolytes can increase the energy density of supercapacitors. Redox additives provide additional pseudocapacitance through rapid electron transfer and reversible Faraday reactions at the electrode-electrolyte interface, thereby significantly improving the specific capacitance. In addition, the construction of a crosslinking network or ionic bond in a nanocellulose electrolyte can impart additional functions to the supercapacitor such as stretchability and self-healing.

Since nanocellulose- based composite materials have the advantages of low cost, strong mechanical strength and strong porosity, they can not only be used in superconducting batteries, but also in the manufacture of other types of EES systems, especially rechargeable batteries, such as lib and lsb , sodium ion batteries (SIBs), etc.