Tuning lignin-derived carbon for high-performance supercapacitors

Abstract: Lignin accounts for up to 30% of organic carbon in the biosphere making it a major source of renewable carbon. However, the vast majority of lignin produced in industry is burned as low-value fuel, contributing to carbon emissions at accelerated timescales despite being classified as biogenic carbon. As an alternative to combustion, carbon materials offer an effective pathway for long-term carbon sequestration. Beyond simple carbon sinks, porous carbons can be used in advanced technology to address some of the world’s most pertinent challenges, such as energy storage, water treatment, and CO2 capture. Owing to its high carbon content, aromatic motifs, and natural tendency to form nanoparticles, lignin is one of the most promising precursors of tunable carbon materials. Carbons represent a diverse class of materials, highly tunable with respect to their carbon architecture, pore structures and surface chemistries. Often high surface areas with accessible pore structure are targeted for efficient adsorption and mass transport. Chemical activation approaches are effective, but the resulting carbons often have disordered pore structures, and the use of activation chemicals limit scalability. Contemporary approaches focus on producing well-defined pore structures, in this context, lignin nanoparticles offer tunable carbon morphologies, with high surface to volume ratios and predictable packing densities. Supercapacitors are high power density energy storage devices that store charge at the electrode-electrolyte interface through electrostatic mechanisms. Since the capacitance is proportional to the accessible surface area, carbon materials with high surface areas make excellent supercapacitor electrodes. This work investigates the formation of porous carbon from kraft lignins as electrodes for high-performance supercapacitors. Traditional chemical activation is compared with emerging approaches using lignin nanoparticles. The approaches are evaluated for their impact on the resulting carbon properties and supercapacitor performance. Insights are gained into the advantages of different processing methods for different applications of lignin-based carbon.