Biopolymers based carbon nanofibers flexible sheet for supercapacitor application

The main objectives of this research work are to synthesize low-cost carbon nanofibers (CNFs) sheets using bio-polymer lignin to replace costly non-renewable CNFs precursors and tailor the nanofibers for various supercapacitor applications by optimizing the processing parameters. This work explores renewable precursor kraft lignin with green solvent acetic acid (according to FDA Q3C standards), which makes the whole synthesis process eco-friendly. The kraft lignin-derived polymeric free-standing nanofiber sheets were subjected to carbonization at various temperatures (600, 800, and 1000 ºC). A balanced combination of defects level, functional groups, and mesoporous surface of CNF sheet carbonized at 800 ºC leads to better supercapacitor performance. This study also explores the activation effect to improve the surface area and porosity of the CNF sheet, which provides a flexible and porous activated CNF (ACNF) sheet. An all-solid-state supercapacitor device was fabricated using an ACNF sheet, and it delivers a high energy density of 65.52 Wh/kg at the corresponding power density of 1036.27 W/kg. ACNF sheet was also explored in the composite formation with copper-metal organic framework (Cu-MOF). It is observed that ACNF surface functionality is very effective for Cu-MOF growth on its surface. The composite sheet (Cu-MOF@ACNF) revealed a higher surface area and total pore volume. This simple synthesis strategy solves the problem of MOF aggregation and improves electrochemical performance. Also, the solid-state device prepared by employing composite sheets provides an enhanced energy density of 78.71 Wh/kg at a power density of 1050.0 W/kg. The absence of agglomeration and the presence of voids between the nanofibers leads to the composite sheets' flexibility. So, this dissertation provides readers with a prospect to use the biopolymer(lignin) based CNF sheets as flexible and high-performance electrodes for supercapacitor applications by replacing traditional PAN-based CNFs. In addition, we believe that the lignin-derived green ACNFs and their composites provide versatility to prepare various well-defined nanostructures to be explored as a future flexible energy storage material.