Cadmium Sites Accelerate Nitrate-to-Nitrite Kinetics in Cd-Fe₃O₄ 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.