Role of sodium sulfate in electrical conductivity and structure of lignin-derived carbons

Abstract

Lignin is a promising renewable alternative to fossil fuels for producing carbon materials such as carbon fibers, activated carbons, or carbon black. Despite extensive research, lignin-derived carbon materials show limited graphitization relative to comparable petroleum-derived carbons. Further, lignin-derived carbons show high variation in graphitization and electrical conductivity depending on the source of the lignin. Herein, nine lignins, derived from various feedstocks and isolation procedures, are pyrolyzed to produce biochar at 1100∘C. These lignins have a range of chemical compositions, carbon structures, and particle sizes. As a result, the pyrolysis behavior of these lignins varies, with powdered, clumped powder, and “foam” biochar morphologies resulting from finely powdered lignin. The produced biochars vary widely in both electrical conductivity, from 0.19 to 19 S/cm, and in-plane graphitic crystallite size, from 3.4 to 41.2Å. A significant decrease in electrical conductivity is identified when Na2SO4 is removed from lignin, accompanied by an increase in graphitic crystallite size. Based on this finding, a quadratic relationship between biochar graphitic crystallite aspect ratio and electrical conductivity is proposed that builds on established quasi-percolation models for biochar electrical conductivity.