389309 Enabling High-Energy Density Supercapacitors with Conducting Polymer/Redox Biopolymer Composite Electrodes
Improvements in energy storage technologies are necessary to support the deployment of renewable energy generation systems (e.g. wind, solar), electric transportation, and power leveling systems. Future applications in these areas will demand increases in power and energy densities, as well as long term cycle-ability and scalable synthesis processes. Supercapacitors are emerging as promising devices to bridge the gap between conventional capacitors and batteries, and have already seen limited commercialization. A variety of materials can be used as supercapacitor electrodes, ranging from highly porous carbon nanomaterials to metal oxides and conducting polymers. Recent research has shown that redox polymers can be added to conducting polymers (CP) during the polymerization process to increase the Faradaic redox capacitance of the polymer film (e.g. polypyrrole, PPy). In this work, we present how redox polymer properties and process conditions can be utilized to increase the performance of mixed polymer electrodes comprising CP and redox materials for the development of high power and energy density hybrid supercapacitors.
Comprising about a third of the mass of plants and trees, lignin is one of the most abundant biorenewable polymers, 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 oxygen with phenolic groups that can undergo redox processes at a given electrochemical potential. While the sulfonated from of lignin (sodium lignosulfonate) has been used to enhance the redox capacitance in polymer electrodes, it makes up less than 1% of the available lignin. The widely available form of lignin (alkali lignin, AL) 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 and synthesize PPy-AL electrodes with superior electrochemical performance compared to the sulfonated lignin form. Mixed polymer electrodes are prepared using PPy as the conductive element and various forms of lignin (type, MW, aromatic content) as the redox component. We describe how the film processing conditions (charge, solvent, monomer content, AL weight %) affect the physical and chemical structure of the polymer electrodes to develop processing-structure-electrochemical properties relationships. Optimized electrode compositions show a 2-3 fold increase in redox capacitance when using the alkali lignin relative to the sulfonated lignin. Hybrid supercapacitors comprised of these electrode materials validate the low-cost, high energy density advantages of electrodes containing abundant biorenewable polymers.