Softwood Lignin as a Sustainable Feedstock for Porous Carbons as Active Material for Supercapacitors Using an Ionic Liquid Electrolyte

Abstract

We report on the facile synthesis of porous carbons based on a biopolymer lignin employing a two-step process which includes the activation by KOH in various amounts under an inert gas atmosphere. The resulting carbons are characterized with regard to their structural properties and their electrochemical performance as an active material in double-layer capacitors using for the first time an ionic liquid (EMIBF4) as the electrolyte for this type of carbon material to enhance storage ability. A capacitance of more than 200 F g–1 at 10 A g–1 is achieved for a carbon with a specific surface area of more than 1800 m2 g–1. One of the most crucial factors determining the electrochemical response of the active materials was found to be the strong surface functionalization by oxygen-containing groups. Furthermore, the sulfur content of the carbon precursor lignin does not result in a significant amount of sulfur-containing surface functionalities which might interact with the electrolyte.