Tailoring surface carboxyl groups of mesoporous carbon boosts electrochemical HO production.

Oxygen-doped porous carbon materials have been shown promising performance for electrochemical two-electron oxygen reduction reaction (2e ORR), an efficient approach for the safe and continuous on-site generation of HO. The regulation and mechanism understanding of active oxygen-containing functional groups (OFGs) remain great challenges. Here, OFGs modified porous carbon were prepared by thermal oxidation (MC-12-Air), HNO oxidation (MC-12-HNO) and HO solution hydrothermal treatment (MC-12-HO), respectively. Structural characterization showed that the oxygen doping content of three catalysts reached about 20%, with the almost completely maintained specific surface area (exception of MC-12- HNO). Spectroscopic characterization further revealed that hydroxyl groups are mainly introduced into MC-12-Air, while carboxyl groups are mainly introduced into MC-12- HNO and MC-12- HO. Compared with the pristine catalyst, three oxygen-functionalized catalysts showed enhanced activity and HO selectivity in 2e ORR. Among them, MC-12-HO exhibited the highest catalytic activity and selectivity of 94 %, as well as a considerable HO accumulation of 46.2 mmol L and excellent stability in an extended test over 36 h in a H-cell. Electrochemical characterization demonstrated the promotion of OFGs on ORR kinetics and the greater contribution of carboxyl groups to the intrinsically catalytic activity. DFT calculations confirmed that the electrons are transferred from carboxyl groups to adjacent carbon and the enhanced adsorption strength toward *OOH intermediate, leading to a lower energy barrier for forming *OOH on carboxyl terminated carbon atoms.