Engineering Biochar‐Derived Functional Materials for High‐Performance Supercapacitors: Design Principles, Mechanisms, and Scalable Strategies

ABSTRACT Biochar has emerged as a useful and adaptable source of carbon for supercapacitor electrodes. Its value comes from the way biomass chemistry, thermal conversion, and activation conditions shape the resulting pore network, surface groups, and degree of carbon ordering. These features govern how ions enter the structure, how charge is stored, and how the electrode behaves under cycling. Although substantial progress has been made, key limitations remain, such as variations in feedstock composition, restricted control over hierarchical porosity, modest intrinsic conductivity, and uncertainty regarding how heteroatom groups interact with the pore walls during charge storage. This review examines these issues by connecting electrochemical behavior to the underlying material structure. It describes how the proportions of cellulose, hemicellulose, lignin, and proteins influence carbon yield and aromaticity, how carbonization and activation shape micro, meso, and macropores, and how nitrogen, phosphorus, sulfur, and oxygen groups participate in fast surface redox reactions. Studies on composites, flexible electrodes, and high‐rate devices are compared with reference carbons, such as activated carbons, graphene, and carbon nanotubes, to clarify practical gains rather than isolated outcomes. Wherever possible, reported capacitance and cycling behavior are related to pore size distributions, defect densities, and the nature of surface functionalities. The review also discusses recent work on cost analysis, life‐cycle assessments, and machine learning tools that help predict yield, porosity, composition, and capacitance. Together, these findings outline the scientific and technical considerations required to improve reliability, support scalable production, and guide the development of biochar‐based electrodes for next‐generation supercapacitors.