The role of uniformly distributed ZnO nanoparticles on cellulose nanofibers in flexible solid state symmetric supercapacitors

Abstract

Paper—a flexible, foldable, and cost-effective substrate made of one-dimensional cellulose nanofibers (1D-CNF)—is being extensively investigated as a promising aspirant for wearable as well as foldable energy storage appliances. However, the high density of flexible electrodes and structural degradation during charge and discharge remain major challenges, which limit their practical applications. Hence, efforts have been made toward the fabrication of solid-state symmetric and flexible paper-like devices for higher capacitive performance. Nanosized uniform ZnO particles were attached on the 1D-CNF surface via a facile methodology, which significantly improved supercapacitive properties. When assembled into solid-state symmetrical supercapacitors, they exhibited remarkable electrochemical performance (220 F g⁻¹, 30.2 W h kg⁻¹ at 1 A g⁻¹) and excellent cycling stability (88% after 8000 cycles). In contrast, binder/additive-free paper-like flexible solid-state symmetrical supercapacitor devices, which were carbonized at 250 °C for 30 min, exhibited superior performance. In particular, the following were observed: higher specific capacitance (140 F g⁻¹ of 1 A g⁻¹), prominent cycling performance (92% capacity retention) after 8000 cycles, and exceptional flexibility (i.e., no significant decay in the cyclic voltammetry current response/integral area after bending for 100 cycles at 90°). This work will pave the way for the application of green, lightweight, and flexible energy storage paper in wearable and foldable electronic devices.