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 (NaNO₃) 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^-2 and hence doubling the volumetric energy density. At the same time, the oxidative NaNO₃ 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^-2 in the absence of NaNO₃ to 350 mF cm^-2 with NaNO₃ translating into a volumetric energy density increase from 949 μW h cm^-3 without NaNO₃ to 2245 μW h cm^-3 with NaNO₃. Meanwhile, the gravimetric capacitance also increased from 151 F g^-1 without to 192 F g^-1 with NaNO₃.