Driving biochar applications via intrinsic redox superiority: electron transfer mechanisms, quantification, aging effects, and design strategies

Abstract Biochar, a carbon-rich product of biomass pyrolysis, has attracted attention for its applications in pollution control, soil amendment, and carbon sequestration. However, its large-scale application is hindered by its inherently lower surface area and conductivity compared to activated carbon or graphene, and by the added costs/pollution of post-modifications. Notably, the intrinsic advantages of biochar, particularly redox properties (i.e., electron exchange capacities, EEC) arising from redox-active moieties (RAMs), enable it to outperform other materials in facilitating electron transfer for pollutant degradation and energy recovery, thereby enhancing its competitive edge. Herein, we systematically review (i) the types and microspatial distribution of RAMs governing redox availability and spatial accessibility, which dominate the contribution of EEC in electron transfer; (ii) chemical, electrochemical, and microbiological techniques for quantifying EEC, highlighting methodological strengths, limitations, and interferences; (iii) the multifactorial impact of environmental aging on EEC, relating to long-term electron transfer performance; (iv) targeted strategies to enhance EEC, with precise tuning and trade-offs between performance and economic/environmental costs being recognized as current challenges. Future research perspectives are proposed to unveil electron transfer mechanisms controlled by redox potential and spatial accessibility behind different scenarios, refine the identification and visualization techniques of RAMs to assist mechanism interpretation and structure tuning, standardize EEC quantification protocols to eliminate interferences, monitor long-term performance changes, and regulate the internal elements in feedstocks through co-pyrolysis integrated with intelligent multi-objective optimization for targeted performance enhancement. By prioritizing the inherent redox properties of biochar, this work aims to guide sustainable, cost-effective strategies for maximizing its environmental utility. Highlights The redox properties of biochar, outperforming other materials in electron transfer, enable large-scale application. Redox potential and spatial accessibility of RAMs probably affect electron transfer performance and quantitation. Targeted enhancement of EEC and long-term performance of biochar in electron transfer during aging needs investigation. Graphical Abstract