Fine tuning dual active sites in modulating cascade electrocatalytic nitrate reduction over covalent organic framework

Abstract

Conversion of NO3− to NH3 proceeds stepwise in natural system under two different enzymes involving intermediate NO2−. Artificial electro-driven NO3− reduction also faces the obstacle of low faradaic efficiency due to insufficient utilization of this intermediate. Herein, we demonstrate a bimetallic COF-based electrocatalyst for the cascade catalysis of NO3−-to-NO2−-to-NH3 for the first time. TpBpy-Cu2Co4 exhibits a significantly improved performance, with an enhancement factor of 1.4–2 compared to monometallic TpBpy-M. The NH3 yield rate achieves 25.6 mg h−1 mgcat.−1 at −0.55 V vs RHE over TpBpy-Cu2Co4, together with excellent faradaic efficiency (93.4 %). This achievement demonstrates cascade catalysis between Co and Cu units, and their distinct roles are investigated through electrochemical experiments and theory calculations. In electrocatalytic process, Cu site facilities *NO3-to-*NO3H step, while the Co site significantly decreases the energy barrier of *NHOH-to-*NH. The present work provides a valuable inspiration in designing efficient catalysts for cascade reaction.