Dual Action of Lignin: Electrode and Electrolyte for Sustainable Supercapacitor Application

From a sustainability and economic point of view, lignin is a potential biopolymer that can be used as an electrode and electrolyte source for renewable energy applications. In this work, an electrode, i.e., microporous carbon, is prepared from the kraft lignin using a ball-milling method followed by vacuum-assisted carbonization. The milling time of kraft lignin has not only a significant effect on developing large surface area carbon (∼606 m2 g–1) without activation, but at the same time, vacuum-assisted carbonization also minimizes the oxygen content up to ∼1%. The microporous carbon (referred to as KL-8) developed from kraft lignin by optimizing milling time of 8 h in a symmetric supercapacitor displays a specific capacitance of 117.9 F g–1 at a current density of 0.5 A g–1 in lignin-containing 6 M KOH electrolyte (denoted as Lig/6 M KOH) compared to pure 6 M KOH solution (88 F g–1). The enhanced capacitance of a symmetric supercapacitor in Lig/6 M KOH electrolyte is attributed to the electronic carrier transported between the carbon electrode and electrolyte interface by virtue of oxygenated groups of lignin dissolved in the 6 M KOH electrolyte. The symmetric supercapacitor shows ultra-stable performance up to 75,000 repetitive charging–discharging cycles at 5 A g–1 with 98.2% capacitance retention. Furthermore, a lignin-derived gel electrolyte membrane is made and used in a solid-state supercapacitor, which exhibits improved electrochemical performance compared to the state-of-the-art polyvinyl alcohol/KOH-based gel electrolyte. Thus, the formulation of the active electrode and electrolyte from a sustainable source of lignin for supercapacitor applications represents a milestone in functionality.