Solid-state yet flexible supercapacitors made by inkjet-printing hybrid ink of carbon quantum dots/graphene oxide platelets on paper

Paper-based flexible supercapacitors (SCs) show advantages due to the improved adhesion between paper and active materials, the simplified printing process and the lower cost, compared to other substrates such as plastics. Here we report the fabrication of solid-state yet flexible SCs by inkjet-printing a hybrid ink consisting of carbon quantum dots (CQDs) and graphene oxide (GO) platelets, followed by casting of polyvinyl alcohol (PVA)/sulfuric acid (H2SO4) gel electrolyte. The SC obtained from 100-time-printing of the hybrid ink shows a specific capacitance of similar to 1.0 mF cm(-2) at a scan rate of 100 mV s(-1), which is enhanced by nearly 150%; the whole device including paper substrate, gel electrolyte and active material demonstrates an energy density of 0.078 mW h cm(-3) at a power density of 0.28 mW cm(-3). In addition, the excellent mechanical strength of GO platelets ensures the good flexibility and mechanical robustness of the printed SCs, which show a retention of 98% in capacitance after being bended for 1,000 cycles at a bending radius of 7.6 mm. This study demonstrates a promising strategy for the large-scale preparation of low-cost, lightweight, and flexible/wearable energy storage devices based on carbon-based ink and paper substrate.