(400a) Supercapacitor Electrode Configuration Based on Alkali Lignin, an Abundant Redox Biopolymer

Several efforts have been focused in the incorporation of non-conductive redox molecules to Electrically Conducting Polymers (ECPs) to improve their energy storage capacity and develop an alternative composite material to current carbon technologies for supercapacitor electrodes.  Lignin is, after cellulose, the most abundant biorenewable polymer comprising about a third of the mass of plants, yet it possesses very little value aside from its heat content. The random structure of lignin contains aromatic and aliphatic carbon subunits along with various forms of phenolic groups that can undergo redox process at a given electrochemical potential. Sodium lignosulfonate (SLS) has been used to enhance the redox capacitance in polymer electrodes, although, its availability is limited and will continue to decrease; alkali lingin (AL) accounts for 98% of lignin from paper-pulping but has limited solubility in aqueous acidic solutions thereby limiting the scope and cost advantages of lignin-polymer batteries. In this work, we describe how to overcome the solubility limitations of AL synthesized with polypyrrole (AL) to create electrodes with superior electrochemical performance; moreover we study the electrochemical behavior of SLS and AL through their incorporation in a carbon matrix.

 First, composite electrodes are prepared using PPy as the conductive element and various forms of lignin (type, MW, aromatic content) as the redox component. Compared to PPy/SLS, electrodes containing PPy/AL exhibit increased capacitance from 312 F/g to 444 F/g, and decrease in the relaxation time constant from 1.7 to 1s. Next, carbon-lignin electrodes are prepared by filtration of a Carbon/Lignin dispersion on a glass microfiber paper filters. Mesoporous carbon show to be a suitable matrix to study the chemistry behind the redox behavior of AL and SLS, differences and similarities, avoiding the possible chemical/physical interactions between lignin and ECPs, which can interfere with the real electrochemical properties of Lignin. The influence of electrolyte concentration, film deposition and preparation, lignin-carbon ratios and lignin solvent were study. Composite electrodes comprised of these materials validate the low-cost, high energy density advantages of electrodes containing abundant biorenewable polymers.