Hierarchical structural carbon with properly modulated compositions and porosity is essential for energy storage capacity. Here, N-doped porous carbon was synthesized using abundant rice straw under the sequential hydrothermal treatment and calcination by KHCO 3 in the presence of melamine. The activation with KHCO 3 resulted in about a 50% increase in the yields of porous carbons and performance comparable to that of KOH. The extra additional melamine not only introduces the N-containing functional groups but also enhances the mesoporosity and specific surface area (2786.5 m 2 g −1 ). Meanwhile, wettability and conductivity are improved. The obtained N-doped porous carbon exhibits outstanding capacitance of 317 F g −1 at 1 A g −1 . The fabricated symmetric supercapacitor displays a stable cycling performance (99.4% retention after 5000 cycles), a reasonable rate performance, and the maximum specific energy of 18.4 W h kg −1 . Our research provides a promising method for effectively converting biowaste into energy storage materials through green synthetic strategy.

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Hydrothermal carbonization (HTC) has been demonstrated as an effective method for preparing hydrochar to realize efficient utilization of biomass resources. In this study, both trifluoroacetic acid (TFA) and phosphorous acid (PA) under low temperature were used as swelling solvents to disrupt the cellulose crystalline structure within 1 h with distinct physiochemical properties. After TFA and PA swelling pretreatment, by means of hydrothermal carbonization, two varieties of swollen cellulose-derived hydrochars were produced. The results showed that the TFA and PA swelling pretreatment significantly enhanced the dehydration and deoxidation of cellulose through an HTC process. The H/C value and O/C value of swelled cellulose are lower than nonswelled by more than 4% at 220 °C, which demonstrates that the dehydrogenation reaction is enhanced during the hydrothermal process of swollen cellulose. Particle size as well as microscopic arrangement of hydrochars were distinctly different due to swelling cellulose. The particle size of hydrothermal carbon microspheres increases by a swelling process at the same hydrothermal carbonazition temperature. TFA-HTC have an estimate diameter of 0.2 μm at 220 °C, which is equivalent to the size of CEL-HTC-280. This study illustrates the mechanism for hydrothermal carbonization of TFA and PA swollen cellulose and provides a novel route for directionally regulating hydrothermal carbon microspheres and developing carbon-based catalysts.

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