Sawdust-based N, O and P co-doped porous biochar for high-performance supercapacitors

Abstract

In this study, a multi-heteroatom doped biochar material (NOPC-x-y) was prepared by phosphoric acid coupled urea activation and air plasma post-treatment using agricultural and forestry by-product sawdust as a carbon source. Among them, phosphoric acid is not only an activator but also a P source. Urea is both an N source and an initiator, which makes porous carbon change from a mesoporous to a microporous structure, and a large number of N, O, and P atoms are incorporated. Short-time plasma modification further increased the content of N and O and successfully increased the relative content of N-Q and N-5. In addition, density functional theory (DFT) calculations show that N, O, and P atom doping not only effectively improves the K⁺ binding energy of biochar but also significantly increases its conductivity. The prepared NOPC-1-1 electrode exhibits a high specific capacitance of 366 F g⁻¹ (at 0.5 A g⁻¹) in a three-electrode system with 6 M KOH as the electrolyte. After being assembled into a symmetrical supercapacitor (SSC) device, it has a high energy density of 18.51 W h kg⁻¹ (at 350.90 W kg⁻¹). After 10 000 cycles, the capacitance retention rate of SSC is 96.8%, and the coulombic efficiency is 100%. These excellent properties are higher than most of the biomass carbon-based capacitors reported in the literature.