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

Philipp Schlee, Servann Herou, Rhodri Jervis, Paul R. Shearing, Dan J. L. Brett, Darren Baker, Omid Hossein, Per Tomanid, M. Mangir Murshed, Yaomin Lif, María José Mostazo-López, Diego Cazorla-Amorós, Ana Belen Jorge Sobrido and Maria-Magdalena Titirici

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 cm2 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 cm2 in the absence of NaNO3 to 350 mF cm2 with NaNO3 translating into a volumetric energy density increase from 949 mW h cm3 without NaNO3 to 2245 mW h cm3 with NaNO3. Meanwhile, the gravimetric capacitance also increased from 151 F g1 without to 192 F g1 with NaNO3.