Easy preparation of nitrogen-enriched carbon materials from peptides of silk fibroins and their use to produce a high volumetric energy density in supercapacitors

A novel nitrogen-rich carbon material, silk carbon, was prepared from Bombyx mori silk fibroins by simple heat-treatment under inert atmosphere. The nitrogen atoms were originated from copious peptide bonds contained in the silk fibroins. The adequate heat-treatment temperature turned the nitrogen atoms of the peptide bonds to the silk carbons in the form of nitrogen-containing functional groups. Furthermore, the activation of the silk carbons with steam and potassium hydroxide provided nitrogen-containing activated carbons (ACs) with different pore-size distribution. In the application to electric double layer capacitors, the silk carbon-based ACs showed a higher capacitance and an excellent electrochemical stability in the high voltage region, compared with a typical AC prepared from phenolic resin. More significantly, the steam-activated silk carbon showed the energy density that was comparable to that of the phenolic resin-based AC prepared by chemical activation. This indicates the superiority of steam-activated silk carbons to conventional KOH-activated carbon materials in supercapacitor application, due to the low production cost by the facilities of steam activation, and the relevant capacitance comparable with that of chemical activation.