Functional porous carbon nanospheres from sustainable precursors for high performance supercapacitors

Abstract

Functional porous carbon nanospheres with tunable textural properties and nitrogen functionalities were synthesized from a cheap and sustainable phenolic carbon precursor (tannic acid) and nitrogen precursor (urea) using a facile one-step salt-templating method. The diverse functional carbons were obtained by calcination of mixtures of different molar ratios of urea to tannic acid (0 : 1, 5 : 1, 9 : 1, 13 : 1 and 17 : 1) with a eutectic salt (NaCl/ZnCl₂) that was used as the porogen. The physico-chemical characterization of the obtained microporous carbons demonstrated that the textural properties, morphology, surface functionalities, and conductivity were strongly influenced by the molar ratio of urea to tannic acid. The nitrogen content in the carbons increased with the molar ratio of urea, reaching a maximum of 8.83% N at the highest molar ratio while the specific surface area (S_BET) of the microporous carbons varied from 890 m² g⁻¹ to 1570 m² g⁻¹ depending on the synthesis conditions. The electrochemical performance of the carbon nanospheres in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR₁₄FSI) was also significantly influenced by the synthesis conditions due to the combined effect of textural properties, morphology, nitrogen functionalities and electrical conductivity. Supercapacitors based on the functional porous carbon synthesized with a molar ratio of urea to tannic acid of 9 : 1 exhibited the best performance, with a specific capacitance as high as 110 F g⁻¹ and a real energy density of 33 W h kg⁻¹, when charged–discharged at 3.5 V in PYR₁₄FSI.