Controlling the graphite-like microcrystalline structure of lignin-based ultrafine carbon fibers via the design of condensed structures.

Obtaining lignin-based graphite-like microcrystallites at a relatively low carbonization temperature is still very challenging. In this work, we report a new method based on condensed structures, for regulating graphite-like microcrystalline structures via the incorporation of 4,4'-diphenylmethane diisocyanate (MDI) into the main structure of lignin. The effects of MDI on the thermal properties of lignin and the graphite-like microcrystalline structure of lignin-based ultrafine carbon fibers were extensively studied and investigated. The incorporation of MDI decreased the thermal stability of lignin, increased the carbon yield and enhanced the formation of graphite-like microcrystallites, which are beneficial for reducing energy consumption during the preparation of lignin-based carbon fibers. The modified lignin-based ultrafine carbon fibers (M-LCFs) demonstrated satisfactory electrochemical performance, including high specific capacitance, low charge transfer resistance, and good cycle performance. The M-LCFs-3/2 electrode had a specific capacitance of 241.3 F g-1 at a current density of 0.5 A g-1, and a residual ratio of 90.2 % after 2000 charge and discharge cycles. This study provides a new approach to control the graphite-like microcrystalline structure and electrochemical performance while also optimizing the temperature.

Hardwood Kraft lignin-derived carbon microfibers with enhanced electrochemical performance.

It is of great challenge to prepare lignin-derived carbon microfibers with suitable graphite crystallites due to the volatilization of incorporated polymers. In this work, we proposed a simple method for the construction of graphite crystallites based on the regulation of the hydrogen-bonding interaction between hardwood Kraft lignin (HKL) and poly(m-phenylene isophthalamide) (PMIA). The strong hydrogen-bonding interaction demonstrated by the results of TG, FTIR, XPS, Raman and XRD increased the graphite crystal size and perfected the crystal structure of HKL-based carbon microfibers, which further enhanced the electrochemical performance of HKL/PMIA-based carbon microfibers electrodes, especially for the increase of capacitance and cycle performance and the decrease of charge transfer resistance. The specific capacitance, energy density and power density of P2H2-based (HKL/PMIA = 1:1) carbon microfibers electrode were up to 190.8 F g-1, 34.4 Wh kg-1 and 540 W kg-1 at a current density of 0.5 A g-1, respectively, which were comparable to or even higher than those of lignin composites-based carbon fibers electrodes. This work reveals the relationship between hydrogen-bonding interaction and crystalline structure, which can be further considered in the preparation of lignin-based carbon fibers electrodes.

Effect of chemical structure and molecular weight on the properties of lignin-based ultrafine carbon fibers.

Unlocking the effects of chemical structure and molecular weight of lignin on the properties of carbonized fiber can accelerate the development of lignin-based carbon fiber which was mainly limited by its complex structure. Hardwood kraft lignins (HKLs) with different structures and molecular weights prepared via heat treatment and fractionation processes were spun into ultrafine fibers using electrospinning technique at the assistance of 1 wt% polyoxyethylene (PEO), which was further removed during the carbonization process to eliminate the potential impacts. The structure and molecular weight of HKLs together with their influences on the thermal behavior, fiber morphology, crystal structure and mechanical performance of HKLs ultrafine fibers or carbonized ultrafine fibers were systemically investigated to provide an elaborate knowledge on the relationship between physico-chemical structure and properties of HKLs ultrafine fibers. Results suggest that a high molecular weight of HKL is beneficial to the formation of graphite-like crystallite, and the formed graphite-like crystallite and condensed structure of HKLs are crucial for the improvement of the mechanical performance of carbonized ultrafine fibers.