Air oxidation in surface engineering of biochar-based materials: a critical review

Abstract

Biochar always suffers from low porosity and/or poor surface functionality, which limit its performances. Among various surface engineering strategies, air oxidation favors both pore development and surface oxygenation for biochar. However, there is still a lack of systematic knowledge and critical perspective on air oxidation in surface engineering of biochar-based materials for various applications. Herein, this review analyzed the mechanisms of air oxidation, summarized the routes of air oxidation in surface engineering of biochar-based materials, investigated the impacts of controlling factors (including operation parameters and intrinsic biochar structure) on pore development and surface oxygenation during air oxidation, and discussed the performances of the resultant materials in pollution control, biomass catalytic conversion and energy storage. This review suggested that air oxidation could be conducted in oxidative torrefaction/pyrolysis, and applied as post-modification or pretreatment processes. Interestingly, air oxidation is efficient in enriching the heteroatoms in the heteroatom-doped biochar, and promoting the doping of metal species on biochar by enriching the anchor sites. This review also highlighted the future challenges concerning air oxidation in the surface engineering of biochar-based materials. Finally, this review was intended to attract broad attention and inspire new discoveries for promoting the application of air oxidation in surface engineering of biochar-based materials for various advanced applications.Graphical