Lignin-derived electrospun freestanding carbons as alternative electrodes for redox flow batteries

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Free-standing supercapacitors from Kraft lignin nanofibers with remarkable volumetric energy density

We have discovered a very simple method to address the challenge associated with the low volumetric energy density of free-standing carbon nanofiber electrodes for supercapacitors by electrospinning Kraft lignin in the presence of an oxidizing salt (NaNO₃) and subsequent carbonization in a reducing atmosphere. The presence of the oxidative salt decreases the diameter of the resulting carbon nanofibers doubling their packing density from 0.51 to 1.03 mg cm⁻² and hence doubling the volumetric energy density. At the same time, the oxidative NaNO₃ salt eletrospun and carbonized together with lignin dissolved in NaOH acts as a template to increase the microporosity, thus contributing to a good gravimetric energy density. By simply adjusting the process parameters (amount of oxidizing/reducing agent), the gravimetric and volumetric energy density of the resulting lignin free-standing carbon nanofiber electrodes can be carefully tailored to fit specific power to energy demands. The areal capacitance increased from 147 mF cm⁻² in the absence of NaNO₃ to 350 mF cm⁻² with NaNO₃ translating into a volumetric energy density increase from 949 μW h cm⁻³ without NaNO₃ to 2245 μW h cm⁻³ with NaNO₃. Meanwhile, the gravimetric capacitance also increased from 151 F g⁻¹ without to 192 F g⁻¹ with NaNO

Production of a pure lignin product part 2: Separation of lignin from membrane filtration permeates of black liquor

Future high value lignin-based products, such as carbon fibers, require a lignin raw material with a high purity level. Lignin with low inorganic content can be separated from kraft black liquor by the LignoBoost process. This laboratory scale study focuses on the effect of micro- and ultrafiltration of black liquor on the content of impurities in the LignoBoost lignin. Two black liquors, obtained from pulping of eucalyptus and softwood, were used in this study. The black liquors were micro- and ultrafiltered and thereafter lignin was separated from these permeates according to the LignoBoost process. It was found that micro- and ultrafiltration significantly reduced the amounts of carbohydrate residuals in the separated lignin. Eucalyptus lignin separated from permeates contained similar amounts of Al, Mn, Mg, Fe and Si, but significantly less Ca than the reference sample separated from unfiltered black liquor, resulting in 50% lower ash content. Softwood lignin separated from permeates contained lower amounts of Al, Mn, Mg, Fe, Ca and Si than the reference. In both cases, the Na and K content were unaffected by the introduction of micro- and ultrafiltration

Production of a pure lignin product part 2: Separation of lignin from membrane filtration permeates of black liquor

Future high value lignin-based products, such as carbon fibers, require a lignin raw material with a high purity level. Lignin with low inorganic content can be separated from kraft black liquor by the LignoBoost process. This laboratory scale study focuses on the effect of micro- and ultrafiltration of black liquor on the content of impurities in the LignoBoost lignin. Two black liquors, obtained from pulping of eucalyptus and softwood, were used in this study. The black liquors were micro- and ultrafiltered and thereafter lignin was separated from these permeates according to the LignoBoost process. It was found that micro- and ultrafiltration significantly reduced the amounts of carbohydrate residuals in the separated lignin. Eucalyptus lignin separated from permeates contained similar amounts of Al, Mn, Mg, Fe and Si, but significantly less Ca than the reference sample separated from unfiltered black liquor, resulting in 50% lower ash content. Softwood lignin separated from permeates contained lower amounts of Al, Mn, Mg, Fe, Ca and Si than the reference. In both cases, the Na and K content were unaffected by the introduction of micro- and ultrafiltration

Production of a pure lignin product, Part 3: Distribution and removal of inorganics from softwood lignin

Future high value lignin-based products, such as carbon fibers, require a lignin raw material essentially free from inorganic impurities. Lignin of low inorganic content can be separated from kraft black liquor by the LignoBoost process. This laboratory scale study focuses on the effect of process parameters, such as pH and retention time in re-suspension, on the content of inorganic impurities in the LignoBoost lignin. Black liquor from pulping of softwood was used in this study. Specific attention was paid to the elements Na, K, Al, Ba, Ca, Fe, Mg, Mn and Si. It was found that the softwood lignin contained significantly less Ca, resulting in lower ash content, than eucalyptus lignin separated in earlier studies made in this series. Inorganic particles, containing for example Ca-O and Na-Al-Si, were observed in the washed lignin. Prolonged retention time in the re-suspension stage reduced the content of Ca, Mn, Mg, Fe and Si in the washed lignin. A reduction of the content by 30-50% after 24 h was possible. The rate at which these elements are removed from the agglomerates was found to be dependent on the mixing conditions, which indicates that the mass transfer in the liquid film around the particles is of importance for the total mass transfer resistance