Biomass synchronous pyrolysis-activation-ammoniation for N-doped biochar: Relationship among biomass, biochar, and electrochemical performance

Preparing N-doped biochar from biomass pyrolysis is a highly valued utilization direction of biomass resource. Herein, porous N-doped biochar was prepared from ten types of agricultural and forestry residues via synchronous pyrolysis-activation-ammoniation of biomass, and the relationship among feedstock properties, biochar properties, and electrochemical performance was investigated. Results showed that the synergistic effect of ammonia and KOH significantly enhanced both the specific surface area (SBET) and N content of the N-doped biochar, with the SBET of biochar exceeding 799 m2/g and the N content exceeding 3.44 wt%. Specifically, the N-doped biochar from cotton stalk exhibited the highest SBET of 1470.99 m2/g and a N content of 8.61 wt%. The porous structure comprised mainly abundant micropores and narrow mesopores, which facilitated electrolyte transport effectively. Electrochemical tests showed that N-doped biochar from cotton stalk achieved a specific capacitance of 212.6 F/g in supercapacitor, maintaining an 84 % capacity retention at 10 A/g. In the oxygen reduction reaction (ORR) evaluation, N-doped biochar from cotton stalk achieved a half-wave potential (E1/2) of 0.791 V and a limiting current density (JL) of 5.77 mA/cm2, demonstrating excellent energy storage and catalytic performance. Biomass rich in lignin and low in ash yielded N-doped biochar with high SBET and N content, with abundant pyridinic-N, pyrrolic-N, quaternary-N, O-CO/-OH, and O-CO/CO functional groups; these properties of N-doped biochar exhibited excellent electrochemical performance in supercapacitor and ORR catalysis. The results could expand the research boundary for high-value utilization of biomass and the synthesis of multifunctional biochar material.