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

• Published in: Chemical science (Cambridge) 2019-03, Vol.1 (1), p.298-2988
• Main Authors: Schlee, Philipp, Herou, Servann, Jervis, Rhodri, Shearing, Paul R, Brett, Dan J. L, Baker, Darren, Hosseinaei, Omid, Tomani, Per, Murshed, M. Mangir, Li, Yaomin, Mostazo-López, María José, Cazorla-Amorós, Diego, Jorge Sobrido, Ana Belen, Titirici, Maria-Magdalena
• Format: Article
• Language: English
• Subjects: Capacitance; Carbon fibers; Carbonization; Chemistry; Electrodes; Energy; Flux density; Gravimetry; kraft lignin; Lignin; Microporosity; Nanofibers; nanofibre; Oxidation; Packing density; Process parameters; Reducing agents; Sodium hydroxide; supercapacitor; Supercapacitors; volumetric energy density
• ISSN: 2041-6520; 2041-6539; 2041-6539
• DOI: 10.1039/c8sc04936j

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 3 ) 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 NaNO 3 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 NaNO 3 to 350 mF cm −2 with NaNO 3 translating into a volumetric energy density increase from 949 μW h cm −3 without NaNO 3 to 2245 μW h cm −3 with NaNO 3 . Meanwhile, the gravimetric capacitance also increased from 151 F g −1 without to 192 F g −1 with NaNO 3 . A very simple method to enhance the low volumetric energy density of free-standing carbon nanofiber electrodes.