Tandem Cu–Co Sites in MOF-818 for Efficient Ammonia Electrosynthesis from Nitrate in Neutral Media

The electrocatalytic reduction of nitrate (NO3–) to ammonia (NH3) is a promising strategy for addressing environmental NO3– pollution. However, achieving a high Faradaic efficiency (FE) for NH3 production over a wide potential range in neutral electrolytes remains a major challenge for NO3– reduction reaction (NO3–RR). Herein, MOF-818(Cu)–Co was synthesized by immobilizing Co clusters within the porous framework of MOF-818(Cu). MOF-818(Cu)–Co exhibited a superior NH3 FE and the highest NH3 yield rate compared to both pristine MOF-818(Cu) and Co nanoparticles (Co NPs). Under neutral conditions, the NH3 FE exceeded 90% over a wide potential window (−1.3 to −1.8 V vs Ag/AgCl), approaching nearly 100% at −1.5 V (vs Ag/AgCl). Meanwhile, the NH3 yield rate attained 1.06 mol h–1 gcat–1 at −1.8 V vs Ag/AgCl, corresponding to a CuCo catalytic active sites yield rate of 35.0 mol h–1 gCuCo–1. In situ characterizations and theoretical calculations showed that the Cu and Co sites in MOF-818(Cu)–Co synergistically lowered the energy barrier of the rate-determining step (RDS, *NO2– → *NO) through a synergistic tandem catalytic mechanism. The Cu sites predominantly catalyzed the reduction of NO3– to NO2–, while the Co sites facilitated the subsequent conversion of NO2– to NH3. This study demonstrates that synergistic tandem catalytic systems can significantly enhance ammonia electrosynthesis in neutral media.