Rapid, selective surface oxygenation of activated biochar via microwave-induced air oxidation shock toward organic pollutant adsorption and electrochemical energy storage

Oxidation is a commonly used strategy to enhance the surface functionality of biochar. Disappointingly, oxidation always enriches high polarity poor-capacitance O-C=O groups (i.e., carboxyl/lactone) on the carbon material surface, which act as a double-edged sword for biochar's application in organic pollutant adsorption and electrochemical energy storage. Herein, microwave-induced air oxidation shock (MW-AOS) is proposed as a rapid and simple strategy to selectively oxygenate the surface of activated biochar (AB). Characterization results show that the surface of the pristine AB is remarkably oxygenated from 3.8 % to 11.8 % after MW-AOS at an output power of 800 W for 15 s. Interestingly, surface oxygenation is achieved while reducing the high-polarity poor-pseudocapacitance O-C=O groups (i.e., carboxyl/lactone) is selectively achieved at a shorter irradiation time (15 s). Batch adsorption results indicate that MW-AOS remarkably increases the adsorption of various organic pollutants (dyes and antibiotics) by AB. Electrochemical analysis reveals that the specific capacitance of the AB is improved by 250 % (from 60 to 208 F/g at 1 A/g after AOS at 800 W for 15 s), attributed to reduced electrical resistance and enhanced ion transport. Finally, this study could pave a new route for the surface engineering of AB for these applications.