Cellulose nanofibers as a green binder for symmetric carbon nanotubes-based supercapacitors

The demand for flexible lightweight supercapacitors (FSCs), mainly applied in flexible electronics, has spun by the ever-increasing energy storage market. To meet the market criteria, electrodes with high porosity and specific surface areas are ideal candidates as lightweight and flexible supercapacitors components. This work highlights the use of nanocellulose as a bio-derived binder for flexible supercapacitors. This binder contributes to the enhancement of electrochemical performance due to its 1D fibrous hierarchical structures. In turn, nanocellulose is a promising candidate for medical and implantable devices owing to its biocompatibility. Here, we report the fabrication of freestanding, flexible, porous, and conductive electrodes via a facile scalable fabrication process. The electrodes are comprised of multi-walled carbon nanotubes as the conductive material and cellulose nanofibers as the bio-based binder. Due to the combined synergistic effect of carbon nanotubes and cellulose nanofibers, the fabricated electrodes show a high porosity. The effect of cellulose type was evaluated at 16 wt.%, which showed no significant effect on the electrochemical performances at this percentage. However, we demonstrate that the cellulose dry content for a specific cellulose type affects the electrochemical performances, with optimized performances for a dry cellulose percentage between 4 and 6 wt.%. In these formulations, the ratio between nanocellulose and CNTs contributes to the formation of porous and conductive nanosheets. The samples' porosity was assessed regarding their specific surface area (N-2 Adsorption-desorption) while scanning electron microscopy (SEM) emphasized their morphology. Due to the high porosity as well as conductivity, the samples showed a good specific capacitance (similar to 9 F/g) and outstanding cycling stability with an increase in specific capacitance of about 10% after 3500 cycles at 0.13 A/g.