Microwave-assisted catalytic pyrolysis of biomass with biochar materials derived from spent lithium-ion batteries: Microwave absorption and pyrolysis characteristics

Biochar materials are economical and favorable microwave absorbers for microwave-assisted catalytic pyrolysis (MACP) of waste biomass into high-value biofuels (e.g., H₂-rich syngas and aromatics in bio-oil). However, the catalytic activity of biochar alone is not satisfying. Transition metals from spent lithium-ion batteries (LIBs) like Fe, Ni, Co, and Mn could improve the activity of biochar materials in MACP. Herein, a series of microwave-absorbing catalysts based on biochar materials derived from spent lithium-ion batteries were prepared. Microwave absorption of these biochar materials was studied using vector network analysis, and MACP characteristics of rice straw with these biochar materials were investigated. Results showed that the graphitization of biochar increased after oxalic acid treatment and microwave activation, resulting in increased ${\epsilon }^{\prime }$, ${\epsilon }^{\mathrm{\prime \prime }}$, and tanδ_ε. These facilitate converting microwave energy into heat and achieve higher heating rates (maximum 720 °C/min, microwave power: 800 W) to promote biomass pyrolysis. Furthermore, these biochar materials realized higher yields of H₂-rich syngas and lower yields of liquid products than the original biochar and commercial SiC in MACP of rice straw at 360 W (< 350°C), depicting enhanced catalytic activity and product selectivity. Particularly, MACP with biochar material derived from spent LiNi_xCo_yMn_1-x-yO₂ (i.e., sNCMHC-C) obtained the highest selectivity of H₂ (50.22 vol. %) in gaseous products, monocyclic aromatic hydrocarbons (MAHs, 18.86 area. %) and benzene, 1-propynyl- (17.41 area. %) in liquid products. This study suggests that MACP with biochar materials derived from spent LIBs has a high potential to promote high-value utilization of waste biomass.