Cadmium Sites Accelerate Nitrate-to-Nitrite Kinetics in Cd–Fe ₃ O ₄ Tandem Electrocatalysts for Ammonia Electrosynthesis

Electrochemical nitrate reduction reaction (NO3RR) proceeds via a well–defined tandem cascade, in which nitrate to nitrite (NO3– to NO2–) conversion is universally identified as the kinetically rate–determining step (RDS). Copper (Cu) is widely recognized as the active site for this critical RDS, yet systematic investigations into the intrinsic NO3– activation activity of metals adjacent to Cu and its diagonal elements in the periodic table remain largely unexplored. Herein, we systematically screen a series of metals (Cu, Ni, Sn, Cd) as dopants in Fe3O4, and demonstrate that trace Cd–doped Fe3O4 (Cd–Fe3O4) delivers superior NO3RR performance over other catalysts. Systematic mechanistic investigations reveal a tandem pathway: Cd sites preferentially catalyze the NO3– to NO2– conversion, while adjacent Fe sites facilitate water dissociation to supply active hydrogen (*H) for subsequent NO2– hydrogenation to NH3. When integrated into a flow cell, Cd–Fe3O4 delivers a high NH3 production rate of 1610 mmol h–1 g–1 cat while maintaining superior Faradaic efficiency. Furthermore, the produced NH3 is successfully recovered as high–purity struvite, demonstrating its potential in practical applications and nitrate value–added processes.