The electroreduction of nitrate (NO3−) pollutants to ammonia (NH3) offers an alternative approach for both wastewater treatment and NH3 synthesis. Numerous electrocatalysts have been reported for the electroreduction of NO3− to NH3, but most of them demonstrate poor performance at ultralow NO3− concentrations. In this study, a Cu‐based catalyst for electroreduction of NO3− at ultralow concentrations is developed by encapsulating Cu nanoparticles in a porous carbon framework (Cu@C). At −0.3 V vs reversible hydrogen electrode (RHE), Cu@C achieves Faradaic efficiency for NH3 of 72.0% with 1 × 10−3 m NO3−, which is 3.6 times higher than that of Cu nanoparticles. Notably, at −0.9 V vs RHE, the yield rate of NH3 for Cu@C is 469.5 µg h−1 cm−2, which is the highest value reported for electrocatalysts with 1 × 10−3 m NO3−. An investigation of the mechanism reveals that NO3− can be concentrated owing to the enrichment effect of the porous carbon framework in Cu@C, thereby facilitating the mass transfer of NO3− for efficient electroreduction into NH3 at ultralow concentrations.

Tuning the local confinement of reaction intermediates is of pivotal significance to promote C−C coupling for enhancing the selectivity for multicarbon (C 2+ ) products toward CO 2 electroreduction. Herein, we have gained insights into the confinement effect of local CO concentration for enhanced C−C coupling over core− shell Ag@Cu catalysts by tuning the pore diameters within porous Cu shells. During CO 2 electroreduction, the core−shell Ag@Cu catalysts with an average pore diameter of 4.9 nm within the Cu shells