Localized Active H* Enrichment by Cobalt Molecular Catalysts for Enhanced Electrocatalytic Nitrate Reduction

Abstract

Electrochemical nitrate reduction (NO3 - RR) offers a sustainable route for ammonia (NH3) synthesis, simultaneously enabling pollutant remediation and resource recovery. The efficiency of NO3 - RR relies on regulating the hydrogenation behaviors of active sites to drive the stepwise reduction of nitrate species. Herein, we report a molecular catalyst, cobalt tetrapyrazinoporphyrazine (CoPhz) supported on carbon nanotubes (CoPhz/CNT) that achieves outstanding NO3 - RR performance through local active H* enrichment. In neutral electrolyte, CoPhz/CNT attained a peak NH3 Faradaic efficiency (FE) of 93% and a yield rate of 8347.9 mu g h-1 cm-2, outperforming the conventional cobalt phthalocyanine (CoPc) benchmark. CoPhz/CNT exhibited exceptional stability for over 170 h at 50 mA cm-2 in a flow cell. Operando studies and theoretical calculations reveal that nitrogen atoms in the macrocycle modulate the electronic structure of the cobalt center, promoting H* generation and enrichment, as well as facilitating efficient intermediates conversion with low energy differences, leading to superior NO3 - RR efficiency. Furthermore, the practical utility of CoPhz/CNT in a Zn-NO3 - battery achieved an excellent power density of 14.5 mW cm-2. This work demonstrates that molecular engineering of macrocyclic catalysts is an effective strategy to tailor hydrogenation capability for enhanced NO3 - RR and other hydrogenation reactions.