Atomic-Level Hydrogen Pumping Enables Near-Unity Faradaic Efficiency in Nitrate-to-Ammonia Electroreduction.

Electrochemical nitrate reduction (NO3RR) offers a sustainable route for ammonia synthesis and wastewater remediation, yet its efficiency is often constrained by the spatial mismatch between hydrogen (H) generation and consumption, leading to H accumulation, parasitic H2 evolution, and intermediate poisoning. Here we report an atomic-level hydrogen pump that directionally regulates hydrogen flux across a well-defined Cu2O@Co3O4 core-shell interface. By anchoring Pd single atoms either at the interface (Pd-in) or on the outer shell (Pd-out), we demonstrate that only the interfacial configuration enables efficient *H relay from Co3O4 hydrogen-generation sites to Cu2O nitrate-reduction centers. This design suppresses *NO2 accumulation and minimizes hydrogen evolution, achieving a near-unity Faradaic efficiency of 99.9% and an exceptional NH3 yield of 63.9 mg h-1 mgcat -1 at -0.55 V versus RHE, outperforming most reported systems. Operando spectroscopy, kinetic isotope effects (KIEs), and DFT calculations reveal that interfacial Pd lowers the *H migration barrier from 1.21 to 1.01 eV, thereby kinetically favoring hydrogenation over HER. The generality of this hydrogen-pump mechanism is further demonstrated in urea synthesis, hydrodehalogenation of 2,4,6-tribromophenol, and Zn-NO3 - battery systems. These findings establish hydrogen flux regulation via single-atom positioning as a general strategy for optimizing multi-step electrocatalytic reactions.