Design challenges in bimetallic molecular catalysts for electrochemical nitrate reduction to ammonia revealed by FePc-CuPc as a case study

Electrochemical reduction of nitrate (NO3 −) to ammonia (NH3) is a promising approach for sustainable nitrogen-cycle remediation and green NH3 synthesis. Bimetallic molecular catalysts combining two active sites offer a potential route to enhanced activity and selectivity, yet their rational design remains challenging. Here, we report a series of iron phthalocyanine (FePc) and copper phthalocyanine (CuPc) catalysts supported on carbon nanotubes (CNTs) and identify fundamental limitations in their cooperative function. The 2FePc:1CuPc/CNT achieved a Faradaic efficiency of 91.1% and a maximum NH3 partial current density of 66.3 mA cm−2. Density functional theory calculations show that NO3 − reduction is energetically more favorable on Fe than on Cu sites. In situ X-ray absorption spectroscopy reveals that Fe sites retain their molecular coordination under operating conditions, whereas Cu sites undergo irreversible reduction to metallic clusters. These findings expose two critical design challenges in bimetallic molecular catalysts: uncontrolled spatial distribution of active sites and instability of the Cu component under reductive conditions.