Cation‐Engineered Iron Porphyrin Boosting Electrocatalytic Reduction of Nitrite to Ammonia

Excessive discharge of nitrite (NO₂ ^-) into the natural environment disrupts the nitrogen cycle and poses significant health risks. Metalloporphyrins are recognized as ideal electrocatalysts owing to their well-defined M─N₄ coordination and tunable structural properties. In this study, iron 4,4',4'',4'″-(porphyrin-5,10,15,20-tetrayl)tetrakis(N, N-dimethylaniline) (FeDMA) and iron 4,4',4'',4'″-(porphyrin-5,10,15,20-tetrayl)tetrakis(N, N, N-trimethylbenzenaminium) (FeTMA) were synthesized from iron tetraphenylporphyrin (FeTPP) by introducing electron-donating dimethylammonio and electron-withdrawing trimethylammonio substituents. We systematically investigated the influence of substituent effects on the catalytic activity of iron porphyrins for the nitrite reduction reaction (NO₂RR). FeTMA achieved exceptional performance, maintaining >90% Faradaic efficiency (FE) for NH₃ over a broad potential range (-1.1 to -1.5 V versus Ag/AgCl) and a peak NH₃ yield rate of 458 ± 4 µmol h^-1 cm^-2. The NH₃ yield rate was 1.9 and 1.5 times that of FeDMA and FeTPP, respectively. Homogeneous electrochemical and spectroscopic analyses identified Fe^I as the active center for the NO₂RR. Theoretical simulations and experimental results revealed that trimethylammonio cations enhance NO₂ ^- adsorption and facilitate NH₄ ⁺ desorption through Coulombic interactions, ultimately enhancing the catalytic activity. This study demonstrates that cation-engineered metalloporphyrins are a robust strategy for NO₂RR, providing valuable guidance for ammonia synthesis.