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

Abstract

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 g_cat^–1 at −1.8 V vs Ag/AgCl, corresponding to a CuCo catalytic active sites yield rate of 35.0 mol h^–1 g_CuCo^–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.