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

Electrochemical nitrate reduction reaction (NO 3 RR) proceeds via a well–defined tandem cascade, in which nitrate to nitrite (NO 3 – to NO 2 – ) 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 NO 3 – 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 Fe 3 O 4, and demonstrate that trace Cd–doped Fe 3 O 4 (Cd–Fe 3 O 4 ) delivers superior NO 3 RR performance over other catalysts. Systematic mechanistic investigations reveal a tandem pathway: Cd sites preferentially catalyze the NO 3 – to NO 2 – conversion, while adjacent Fe sites facilitate water dissociation to supply active hydrogen (*H) for subsequent NO 2 – hydrogenation to NH 3 . When integrated into a flow cell, Cd–Fe 3 O 4 delivers a high NH 3 production rate of 1610 mmol h –1 g –1 cat while maintaining superior Faradaic efficiency. Furthermore, the produced NH 3 is successfully recovered as high–purity struvite, demonstrating its potential in practical applications and nitrate value–added processes.