Lignin is one of the main biopolymers found in plants, making up ~30% of the renewable carbon on earth. Annually, a large amount of lignin is isolated in pulp/paper and cellulosic ethanol industries with limited low-value applications. One of the proposed value-added products where lignin may be used is the implementation in carbon nanofibers for energy conversion and storage devices. For this purpose, electrospinning and carbonization of the lignin fibers were studied in the present research.
In the first part of the study, steam explosion lignin was extracted from residues of enzymatic hydrolysis in cellulosic ethanol production. Lignin content of the residue was ~62wt% which only ~40wt% of it could be extracted by N,N-dimethylformamide (DMF). Bead-free fibers were then electrospun from blends of the extracted lignin and poly(ethylene oxide) (PEO) (80/20)wt%. This study was discontinued due to the requirement of large volumes of DMF for lignin extraction.
In the next part of the study, organosolv and kraft lignin were used. PEO and lignin dissolution in 1M sodium hydroxide (NaOH) was optimized for electrospinning of submicron diameter fibers. Lignin-PEO entanglements in solution allowed reduction of the PEO ratio to below 5wt%. This decrease in PEO reduces the defects in the microstructure of carbonized fibers. Organosolv lignin/PEO fibers electrospun from DMF solutions had a Tg ≈ 100 °C and required a slow thermostabilization prior to carbonization. In contrast, electrospun fibers from NaOH(1M) solution did not exhibit Tg and were directly carbonized at 1000 °C without thermostabilization. The carbonized fibers were characterized by microscopy, nanomechanical testing, surface area analysis, Raman and X-ray diffraction spectroscopy. Effects of carbonization temperature, time and heating rate on the microstructure of the carbonized fibers were statistically analyzed by a full factorial design of experiments. The results showed that the temperature (800 vs 1100 °C) followed by the time (3 vs 10 h) and the interaction between the three parameters had the highest effects. Using NaOH(1M) solution to dissolve lignin for electrospinning has positive environmental effects by replacing DMF, reducing the PEO content, and reducing the process time and energy by omitting the thermostabilization step.
