Dozens-fold improvement of biochar redox properties by KOH activation

Recently redox-active biochar has been widely used as an electron shuttle in many redox systems to accelerate diverse reactions. However, low value of electron exchange capacity (EEC) and a lack of effective modification methods prevent it from further applications. Thus, how to efficiently enhance biochar's redox properties becomes a hot challenge. In this study, we developed a two-step KOH activation method to modify pine wood biochar and obtained a significant improvement in biochar's EEC, with a maximum increase of 50-fold in electron donor capacity (EDC) to 1.07 mmol e-/g biochar, and 30-fold in electron acceptance capacity (EAC) to 5.23 mmol e-/g biochar, respectively. Considering the gradually decreasing yield with increasing KOH dosage, six was the optimal KOH-to-biochar mass ratio for preparing high EEC char in large quantities, with a yield of 46.36% and a maximum EEC value of 2.80 mmol e-/g raw biochar. Results showed that KOH activation could increase O-containing functional groups, convert them to unstable forms, and also promote biochar's aromatization, which contributed to the EEC increase. In addition, a significant linear correlation (p < 0.05) between biochar's EDC and specific surface area (SSA) was observed, which provided a reference value for exploring modification methods targeted to biochar's redox properties and also made it a clue how to refine a low-quality biochar into a value-added product with high redox properties.

Automated summary

The study develops a two-step KOH activation method to modify pine wood biochar, significantly enhancing its EEC values. The research provides insights into the redox properties of biochar, offering new avenues for its applications and modifications.