Cation-Induced Coulombic Enhancement of Electrocatalytic Nitrate Reduction to Ammonia by Metalloporphyrins

The electrocatalytic conversion of environmental nitrate (NO3–) to high-value ammonia (NH3) is highly attractive, yet remains a significant challenge. Although metalloporphyrins are widely used in various electrocatalytic reactions due to their unique M-N4 structure, their application for the nitrate reduction reaction (NO3RR) has been rarely investigated. In this work, we investigated the catalytic activity of transition metal 4,4′,4″,4‴-(porphyrin-5,10,15,20-tetrayl)tetrakis(N,N,N-trimethylbenzenaminium) (MTMA) complexes, incorporating Fe, Co, Ni, or Cu centers, toward the NO3RR. The catalytic performance of the MTMAs decreased in the order: CoTMA > CuTMA > FeTMA > NiTMA. CoTMA exhibited the highest catalytic activity, achieving a maximum NH3 Faradaic efficiency (FE) of 94.7% and a peak NH3 yield rate of 366 μmol h–1 cm–2. The NH3 yield rate of CoTMA was 1.1, 1.3, and 1.9 times higher than those of CuTMA, FeTMA, and NiTMA, respectively. Theoretical calculation revealed that the superior activity of CoTMA originates primarily from its stronger interaction with the *NHO intermediate. Homogeneous control experiments further identified CoI as the active center in CoTMA. This study underscores the critical influence of the metal center’s identity and oxidation state on NO3RR activity, providing valuable insights for the rational design of efficient molecular NO3RR catalysts.