Energy storage devices such as supercapacitors of high-performance are in great need due to the continuous expansion of digitalization and related devices for mobile electronics, autonomous sensors and vehicles of different kinds. However, the non-renewable resources and often complex preparation processes associated with electrode materials and structure pose limited scale-up in production and difficulties in versatile utilization of the devices. Here, free-standing and flexible carbon nanofiber networks derived from renewable and abundant bio-resources are demonstrated. By a simple optimization of carbonization, the carbon nanofiber networks reach a large surface area of 1670 m2 g-1 and excellent specific gravimetric capacitance of ~240 F g-1, outperforming many other nanostructured carbon, activated carbon and even those decorated with metal oxides. The remarkable electrochemical performance and flexibility of the green carbon networks enable an all-solid-state supercapacitor device, which displays a device capacitance of 60.4 F g-1 with a corresponding gravimetric energy density of 8.4 Wh kg-1 while maintaining good mechanical properties.