Phosphorus functionalization for the rapid preparation of highly nanoporous submicron-diameter carbon fibers by electrospinning of lignin solutions

Abstract

This work presents a fast and versatile method to prepare carbon fibers from lignin. It involves the production of submicron-sized phosphorus-functionalized lignin fibers in only one step by electrospinning of lignin/H₃PO₄ solutions. The phosphorus functionalities enable shortening of the conventional stabilization process from more than 90 h to only 2 h thus avoiding fiber fusion or even stabilizing the lignin fibers in an inert atmosphere. The incorporation of H₃PO₄ into the initial lignin solution produces more oxidized spun lignin fibers, due to the reaction of phosphoric acid with the dissolved lignin, generating phosphate (and/or polyphosphate) esters throughout the structure of lignin fibers. These phosphate groups seem to be responsible for the production of cross-linking reactions during the stabilization step that are, in this case, very active and effective in increasing the glass transition temperature of the lignin fibers, reducing the time needed for the stabilization step and improving this process. Moreover, they promote the chemical activation of lignin fibers and greatly increase their oxidation resistance, avoiding their complete combustion during carbonization under a low concentration of O₂ at temperatures as high as 900 °C. The resulting carbon fibers gather different interesting properties, such as sub-micron diameters (≤1 μm), large surface area (≈2000 m² g⁻¹), relatively high performance in relation to their mechanical properties for functional applications and a rich variety of uniformly distributed O and P surface functionalities, which make them very attractive for heterogeneous catalysis, adsorption and electrochemical applications.