Supramolecular Enhancement of Electrochemical Nitrate Reduction Catalyzed by Cobalt Porphyrin Organic Cages for Ammonia Electrosynthesis in Water

The electrochemical nitrate (NO3ˉ) reduction reaction (NO3RR) to ammonia (NH3) represents a sustainable approach for denitrification to balance global nitrogen cycles and an alternative to traditional thermal Haber-Bosch processes. Inspired by bioinorganic nitrate reductase enzymes that feature a NO3ˉ transport channel to funnel substrate to the active site center and catalyze its reduction, we present a supramolecular strategy to enable capture and electrochemical conversion of NO3ˉ to NH3 in water by embedding molecular cobalt porphyrin (CoTPP) units into a porous organic cage structure. The resulting porphyrin box (CoPB-C8) promotes host-guest interactions for NO3ˉ as confirmed by 1H-NMR titrations, leading to a 13-fold increase in NH3 production rate and 200 mV decrease in overpotential relative to the parent CoTPP compound, achieving total turnover number (TON) and turnover frequency (TOF) values exceeding 200,000 (33,858 per Co) and 56 s-1 (9.4 s-1 per Co), respectively. Systematic tuning of peripheral alkyl substituents of varying lengths (C1 to C13) on the supramolecular structure reveals the importance of balancing porosity and hydrophobicity for achieving gains in NO3RR activity. These findings establish supramolecular host-guest chemistry as an effective design strategy for enhancing molecular electrocatalysis