Optimization of microwave-assisted hydrothermal carbonization and potassium bicarbonate activation on the structure and electrochemical characteristics of crop straw-derived biochar

Conventional pyrolysis biochar generally has underdeveloped pore structure, low surface activity and poor electrochemical performance, which seriously limit its application in supercapacitors. Herein, biochar was fabricated through KHCO3 activation coupling microwave-assisted phosphoric acid hydrothermal carbonization (MPHTC). The results revealed that both MPHTC and KHCO3 activation can further improve the physicochemical structure and electrochemical characteristics of crop straw-derived biochar. Biochar obtained by KHCO3 activation coupling MPHTC (HBCK) exhibited the highest surface oxygen content and the most adsorbed O2/H2O and aromatic CO, as well as a larger proportion of defect structures. Moreover, CS-HBCK displayed advantageous pore structure, with the highest specific surface area (1058.36 cm2/g), the largest total pore volume (0.79 cm3/g), and a superior micropore proportion of 59.21 %. Hence, CS-HBCK exhibited the highest specific capacitance (317.01 F/g) with better rate performance and lower resistance, whose capacity retention rate could reach 87.33 % after 10,000 cycles of charge-discharge. Furthermore, the CS-HBCK symmetric capacitor could achieve an excellent energy density of 14.82 Wh/kg at the power density of 225 W/kg, whose coulombic efficiency could still be maintained at a high level (98.38 %) after 10,000 cycles of charge-discharge, indicating a great potential of recycled waste crop straw in energy storage systems.