High-performance flexible freestanding polypyrrole-coated CNF film electrodes for all-solid-state supercapacitors

Nowadays, supercapacitors based on cellulose nanofibril (CNF) films have attracted extensive interest due to their excellent flexibility, light weight, and unique structure. Herein, we report that highly conductive films are prepared through polypyrrole (PPy) coating on CNF films by a simple and low-cost “soak and polymerization” method. The optimized flexible film features a high electrical conductivity of 23.77 S cm−1, a superior tensile strength of 71.4 MPa, outstanding conductance stability, and a good thermal stability. Additionally, the hybrid film as a freestanding and binder-free supercapacitor electrode can provide a high areal capacitance (2.26 F cm−2 at 2 mA cm−2) and good cyclic stability (a capacitance retention of 70.5% after 5000 cycles). Remarkably, an all-solid-state flexible supercapacitor assembled by two pieces of the optimized PPy-coated CNF (PPy/CNF) film electrodes delivers an excellent areal capacitance of 1.39 F cm−2 at 0.1 mA cm−2, revealing an ultrahigh energy density of 16.95 mWh cm−3 at a power density of 1.2 mW cm−3.