Abstract
Three different types of lignin (kraft, organosolv and phosphoric acid lignin) were characterized and tested as precursors of electrospun nanofibers. Polyethylene oxide (PEO) was added as a plasticizer and dimethyl formamide (DMF) employed as a solvent. It was found that the molecular weight of lignin was the key parameter to understand the differences of the mechanical stability of the resultant fiber mats. In the case of kraft lignin (KL), the influence of some changes in the synthetic process was also tested: applied voltage, pretreatment in air or not, and the addition of a small amount of Ketjen black. After pyrolysis in nitrogen flow, the obtained carbon nanofibers (CNFs) were characterized by different techniques to analyze their differences in morphology and surface chemistry. Vanadium electrochemistry in 3M sulfuric acid was used to evaluate the different CNFs. All fibers allowed electrochemical reactions, but we observed that the oxidation of V(II) to V(III) was very sensitive to the nature of the raw material. Materials prepared from kraft and phosphorus lignin showed the best performances. Nevertheless, when 1 wt.% of Ketjen black was added to KL during the electrospinning, the electrochemical performance of the sample was significantly improved and all targeted reactions for an all-vanadium redox flow battery were observed. Therefore, in this work, we demonstrated that CNFs obtained by the electrospinning of lignin can be employed as electrodes for vanadium electrochemistry, and their properties can be tuned to improve their electrochemical properties.
Highlights
Lignin is produced as a waste product from the cellulose pulping process, which yields more than 60 megatons per year of this byproduct, which is one of the most abundant polymers on Earth [1]
While most lignin is employed as a fuel, other applications have been explored in the literature to obtain value-added products such as chemicals [2,3]
Over the last decades, research efforts have focused on exploring other ways to valorize the lignin, and one of the most important is the synthesis of carbon materials such as activated carbons [4,5], hierarchical porous carbons [6,7], and carbon nanofibers [8,9,10,11,12]
Summary
Lignin is produced as a waste product from the cellulose pulping process, which yields more than 60 megatons per year of this byproduct, which is one of the most abundant polymers on Earth [1]. The diameter of the fiber depends on the so-called Taylor’s cone formation, and so it is related to different experimental variables such as the nature of the solvent, viscosity of the suspension, flow of injection, needle diameter, needle-to-collector distance, and applied voltage [1,14,18]. This fact increases the versatility of the process, allowing fibers of very different sizes to be obtained and, as long as these variables are strictly controlled, the process is reproducible. Their performances as promoters of vanadium oxidation and reduction reactions were related to the CNF surface chemistry and morphology
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