Free-standing supercapacitors from Kraft lignin nanofibers with remarkable volumetric energy density.

We have discovered a very simple method to address the challenge associated with the low volumetric energy density of free-standing carbon nanofiber electrodes for supercapacitors by electrospinning Kraft lignin in the presence of an oxidizing salt (NaNO) and subsequent carbonization in a reducing atmosphere. The presence of the oxidative salt decreases the diameter of the resulting carbon nanofibers doubling their packing density from 0.51 to 1.03 mg cm and hence doubling the volumetric energy density. At the same time, the oxidative NaNO salt eletrospun and carbonized together with lignin dissolved in NaOH acts as a template to increase the microporosity, thus contributing to a good gravimetric energy density. By simply adjusting the process parameters (amount of oxidizing/reducing agent), the gravimetric and volumetric energy density of the resulting lignin free-standing carbon nanofiber electrodes can be carefully tailored to fit specific power to energy demands. The areal capacitance increased from 147 mF cm in the absence of NaNO to 350 mF cm with NaNO translating into a volumetric energy density increase from 949 μW h cm without NaNO to 2245 μW h cm with NaNO. Meanwhile, the gravimetric capacitance also increased from 151 F g without to 192 F g with NaNO.