Hybrid materials from organic electronic conductors and synthetic-lignin models for charge storage applications

Abstract

Homopolymers and copolymers of the monolignols syringol (S) and guaiacol (G) were prepared as well-defined lignin model compounds. Polymerisation was performed by phenol–formaldehyde condensation, also including the monomer hydroquinone (HQ) to extend the range of redox processes in these synthetic lignins (SLig). The chemical structures of the SLig samples were characterized by ¹³C and quantitative ³¹P NMR, and the molecular weight was monitored by size exclusion chromatography (SEC). Subsequently, SLig were incorporated into two different electron-conducting matrix – single-wall carbon nanotubes (SWNT) and polypyrrole (PPy), respectively. As a result, the hybrid materials, with a controlled amount of SWNT or with an unknown amount of PPy, were assembled and compared. The charge storage properties in the investigated materials are attributed to contributions from both the double-layer capacitance of the conducting matrix, and the faradaic reactions provided by quinone groups immobilized in the electrodes. The results indicate a considerable improvement of charge capacity, with the synthetic lignins incorporated in the hybrid materials. With a PPy carrying S, G and HQ, better performance is obtained than has previously been obtained with lignin derivatives, showing a maximum capacity of 94 mA h g⁻¹. Moreover, a low amount of electronic conductor (20% wt of SWNT) is adequate to perform efficient electron communication between redox active quinones and the electrode surface, providing 72 mA h g⁻¹.