A green synthetic strategy to design biomassderived porous carbon electrode materials with precisely tailored structure and morphology has always been a challenging goal because these materials can fulfill the demands of next-generation supercapacitors and other electrochemical devices. Potassium hydroxide (KOH) is extensively utilized as an activator since it can produce porous carbon with high specific surface area and well-developed porous channels. The exploitation of sodium hydroxide (NaOH) as an activating agent is less referenced in the literature, although it offers some advantages over KOH in terms of low cost, less corrosiveness, and simple handling procedure, all of which are appealing particularly from an industrial viewpoint. The motivation for this present study is to fabricate porous carbon spheres in a sustainable manner via a spray drying approach followed by a carbonization process, using Kraft lignin as the carbon precursor and NaOH as an alternative activation agent instead of the high-cost and high-corrosive KOH for the first time. The structure of carbon particles can be accurately transitioned from a compact to hollow structure, and the surface textural properties can be easily tuned by altering the NaOH concentration. The obtained porous carbon spheres were applied as highly packed thin film electrode materials for supercapacitor devices. The specific capacitance value of porous carbon spheres with a highly compact structure (high packing density) is 66.5 F g −1 , which is higher than that of commercial activated carbon and other biomassderived carbon. This work provides a green processing for producing low-cost and environment-friendly porous carbon spheres from abundant Kraft lignin and important insight for selecting NaOH as an activator to tailor the morphology and structure, which represents an economical and sustainable approach for energy storage devices.

The possible explanations on how the existence of copper, CuO, or Cu2O influences the electrical conductivity and electrochemical properties of rGO.

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Graphene has been gaining tremendous attention as an active material for energy storage devices owing to its large surface area, high electrical conductivity, and high electrochemical stability. However, the restacking of graphene layers during the synthesis process has become one issue that can reduce electrode performance. In this work, the structure of electro-exfoliated graphene (EG) is regulated to improve its electrochemical properties as the supercapacitor electrode using a facile postultrasonication treatment. The ultrasonicated EG exhibited a higher exfoliation degree than the raw EG as indicated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and Brunauer–Emmett–Teller (BET) characterization results. Ultrasonicated samples were then tested using three-electrode configuration, which obtained a maximum specific capacitance of 140.5 F g–1 at 0.5 A g–1, which is higher than that of the sample prepared without ultrasonication treatment (79.0 F g–1) at the same current density. Moreover, the cycling performance of ultrasonicated EG was examined using two-electrode measurement yielding the highest capacitance retention of 92.9% after 10,000 cycles at 1 A g–1, which can improve the stability of EG than the sample without ultrasonication (82.2%). These results indicate that ultrasonication can be applied as a straightforward treatment to modify exfoliation degree and lateral size of EG sheets leading to the enhancement of supercapacitor electrode performance.

Early expansion of graphite after H2SO4/H2O2 pre-treatment and cyclic voltammograms of exfoliated graphite (EG) prepared with various volume fractions of H2O2.