New insight into electrochemical denitrification using a self-organized nanoporous VO-Co3O4/Co cathode: Plasma-assistant oxygen vacancies catalyzed efficient nitrate reduction

A novel self-organized nanoporous VO-Co3O4/Co cathode was prepared via anodization and plasma treatment and obtained a significant nitrate reduction efficiency. In the anodization, an oxide layer with the nano-sized pore structure initially grew in-situ on the Co substrate and showed a better surface area. Subsequently, He-plasma increased surface oxygen vacancies (VO) from 24 % to 57 %. Electrons in vacancies were charged into empty eg orbital of low-spin Co3+(Oh, octahedral) and firstly generated high-spin Co2+(Oh) with the configuration of t2g6eg1, accounting for 71.7 % of cobalt species. Accordingly, two original mechanisms (Vo-catalyzed and Co2+(Oh)-catalyzed) were concluded in this study. Oxygen vacancies increased the charge intensity and served as absorption sites in nitrate reduction. Meanwhile, massive Co2+(Oh) provided electrons in the eg orbital with a higher energy state and mediated the faster electron transfer through a Co2+–Co3+–Co2+ redox cycle, compared with Co2+ (Td, tetrahedral). Ultimately, a faster reaction kinetic of 0.0220 min−1 was achieved by VO-Co3O4 than other cathodes e.g., Co3O4 (0.0150 min−1). Such VO-Co3O4/Co cathode-based denitrification strategy displayed great advantages in engineering application and completely removed 90 % of TN from actual wastewater. [Display omitted] •A self-organized nanoporous Co3O4 layer grew in-situ on Co for nitrate reduction.•Oxygen vacancies increased from 24 % to 57 % after He-plasma treatment.•Empty eg orbital of low-spin Co3+(Oh) was charged by plasma to generate high-spin Co2+(Oh).•A better nitrate removal of 99 % was achieved on VO-Co3O4/Co than on Cu and Co3O4/Co.•NO3− reduction was mediated in two ways: Vo-catalyzed mechanism and Co2+(Oh)-catalyzed mechanism.