Biowaste Enabled Porous Carbon towards Ultrahigh Capacitance for Energy Storage

Lignin is the second most abundant biopolymer and the most abundant aromatic-containing polymer in nature. Typically, lignin accounts for 15–30% of plants and serves as an adhesive by forming a lignin-carbohydrate complex to bind cellulose and hemicellulose together. Lignin is a waste product in the production of paper or chemicals (e.g. ethanol) from biomass. If not removed, the lignin could cause the paper-derived wood pulp to weaken and discolor rapidly.
Valorization of useless lignin to high value-added materials for advanced applications would help address the environmentally detrimental biowaste and satisfy the societal need. To date, lignin has been used as a functional component in polymer synthesis. For example, polyhydroxyalkanoate (PHA)/lignin shows better microbial resistance than PHA alone due to the formation of strong hydrogen bonds. Lignin blended polylactic acid (PLA) exhibits a great non-flammable capability. Furthermore, monomers in lignin have been used to synthesize polymers, such as polyester and polyurethanes. However, owing to the chemically recalcitrant nature, it is very challenging and costly to convert lignin into value added biopolymers.
Recently, the focus on using lignin and its derivatives has been shifted to fabricating energy storage devices, including lithium-ion batteries and supercapacitors. Regarding batteries, lignin has been used by either combining it with additives and binders as electrodes for lead-acid batteries or by fabricating it into three-dimensional (3D) hierarchical porous carbon as electrodes for lithium-ion batteries. Without a doubt, the lignin-based electrode is cheaper and greener, which is a great alternative to existing electrodes for energy storage.
In this presentation, a thick freestanding electrode coupled by lignin carbon and sodium without any binder and additives was fabricated demonstrating a specific area capacitance of 19.7 F cm^-2 at a current density of 1 mA cm^-2, which is the highest among to date reported freestanding lignin carbon electrodes with similar thickness. This excellent electrochemical performance originates from high electro-positivity and oxygen content promoted by the sodium. Our brings a new strategy towards lignin utilization and energy storage through coupling lignin carbon and alkali metals.