Optimization of multiscale structure and electrochemical properties of bamboo-based porous activated biochar by coordinated regulation of activation and air oxidation

Abstract

Activated biochar prepared by a single pyrolysis process often has poor pore structure and insufficient surface oxygen-containing groups, which severely inhibit its application in supercapacitors. Herein, we synthesized bamboo-based porous activated biochar (PAB) via coordinated regulation including carbonization at 450 °C, ZnCl2 activation at 400 ∼ 800 °C, and air oxidation at 200 ∼ 350 °C. The results showed that the coordinated regulation could efficiently improve the physicochemical structure and electrochemical properties of PAB. Air oxidation exhibited an obvious improving effect on the activated carbon prepared with mid-temperature activation (optimal at 600 °C), which was strengthened with increasing oxidation temperature from 200 °C to 350 °C. Air oxidation following 600 °C activation could improve the mesoporous rate by etching the pore, and simultaneously introduce more oxygen-containing groups such as carbonyl (C=O) and carboxyl (–COOH) groups, which could enhance the wettability of PAB. The optimal synergistic temperature of ZnCl2 activation and air oxidation was 600 °C and 350 °C (PAB-600-350), respectively. PAB-600-350 is a wonderful supercapacitor electrode material with a higher surface oxygen content (20.74%) and a superior mesoporous rate (35%). At 1 A/g, PAB-600-350 showed the highest capacitance of 256 F/g, which was 2-fold that of PAB-600 (128 F/g). PAB-600-350 capacitors also supplied an excellent energy density of 12.54 Wh/kg at the power density of 225 W/kg in 1 M Na2SO4 electrolyte. Furthermore, a density functional theory (DFT) analysis was performed to investigate the interaction between PAB and electrolyte ions. The results showed that oxygen-containing functional groups on the surface can increase the dipole moment of the material and enhance the adsorption of PAB for electrolyte ions (K+).