In this review, the formation mechanism of biochar in the pyrolysis of a range of biomass precursors is discussed in detail. Because biochar directly obtained from the pyrolysis of biomass has a limited number of surface functional groups and porosity, a functionalization process is required before it can be used as a routine functional material. Because of its easily tuned surface functionality and porosity, biochar is recognized as a promising platform material for the synthesis of a broad range of functional materials. By tuning the surface functionality through oxidation, amination, sulfonation, and recombination, abundant functional groups (e.g., C═O, phenolic OH, COOH, NH₂, and SO₃H), metal nanoparticles, and inorganic nanostructures can all be introduced onto the biochar surface. This is crucial for rendering biochar useful as a functional material. In addition to surface functionality, the biochar porosity is also important. Through in situ tuning and postactivation with various chemical or physical methods, porosity with a high surface area and controllable pore size distribution can be imparted, thus expanding the range of potential applications.

These include catalysis, energy storage, and environmental protection. For catalytic applications, sulfonated biochar has shown favorable activity for various acid-catalyzed chemical reactions, including esterification of organic acids in aqueous media, acylation of alcohols and amines, alkylation of aromatics, and biomass hydrolysis. Biochar-supported metal nanoparticles exhibit high performance for the catalysis of many organic reactions, including ORRs in fuel cells, hydrogenation and dehydrogenation, and thermal decomposition of biomass in pyrolysis or gasification. For energy storage applications, when used as electrode materials for supercapacitors, biochar-based porous carbon materials exhibit high performance, including large specific capacitance and excellent cycle stability. When used for H₂ storage, biochar-based materials also show high adsorption capacities. For environmental protection applications, their highly abundant surface functional groups and easily tuned porosities make biochar-based materials promising for CO₂ capture and pollutant adsorption. Thus, overall biochar provides a cost-effective and sustainable platform for the development of a new generation of functional materials.