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.