On-Demand Atomic Hydrogen Provision by Exposing Electron-Rich Cobalt Sites in an Open-Framework Structure toward Superior Electrocatalytic Nitrate Conversion to Dinitrogen.

Electrocatalytic nitrate (NO3-) reduction to N2 via atomic hydrogen (H*) is a promising approach for advanced water treatment. However, the reduction rate and N2 selectivity are hindered by slow mass transfer and H* provision-utilization mismatch, respectively. Herein, we report an open-framework cathode bearing electron-rich Co sites with extraordinary H* provision performance, which was validated by electron spin resonance (ESR) and cyclic voltammetry (CV) tests. Benefiting from its abundant channels, NO3- has a greater opportunity to be efficiently transferred to the vicinity of the Co active sites. Owing to the enhanced mass transfer and on-demand H* provision, the nitrate removal efficiency and N2 selectivity of the proposed cathode were 100 and 97.89%, respectively, superior to those of noble metal-based electrodes. In addition, in situ differential electrochemical mass spectrometry (DEMS) indicated that ultrafast *NO2- to *NO reduction and highly selective *NO to *N2O or *N transformation played crucial roles during the NO3- reduction process. Moreover, the proposed electrochemical system can achieve remarkable N2 selectivity without the additional Cl- supply, thus avoiding the formation of chlorinated byproducts, which are usually observed in conventional electrochemical nitrate reduction processes. Environmentally, energy conservation and negligible byproduct release ensure its practicability for use in nitrate remediation.