Decoupling the Impact of Electronic Structure and Electrode Wettability of Functionalized Iron Phthalocyanine Catalysts for Electrochemical Nitrate Reduction to Ammonia

The electrochemical reduction of nitrate (NO₃⁻) to ammonia (NH₃) offers a sustainable route for nitrogen cycle remediation and decentralized NH₃ production. In this work, we systematically investigated the impact of electronic structure and wettability in regulating the catalytic performance of molecular catalysts using functionalized iron phthalocyanines (FePc-R, R = NH₂, COOH, CN, and t-Bu) supported on carbon nanotubes. The strongly hydrophilic FePc-NH₂/CNT (electron-donating functional group-containing) catalyst achieved a maximum Faradaic efficiency of 94.1% at -0.6 VRHE and a partial current density of 83.9 mA cm⁻² toward NH₃ at -0.9 VRHE. In contrast, strongly hydrophilic FePc-COOH/CNT and weakly hydrophilic FePc-CN/CNT, containing electron-withdrawing functional groups, delivered a lower performance across all potentials. Density functional theory (DFT) calculations revealed that electron-donating functional groups elevate the Fe-center HOMO level, facilitating hydrogenation of NHx intermediates and enhancing turnover frequency. In situ X-ray absorption spectroscopy (XAS) confirmed that Fe-N₄ coordination in FePc-NH₂/CNT remains stable across all tested potentials, while electron-withdrawing functional group-containing catalysts (FePc-COOH/CNT and FePc-CN/CNT) exhibited Fe-Fe cluster formation at -0.8 and -0.7 VRHE, respectively. Furthermore, coupled mass transport and reaction modeling indicated that more hydrophilic surfaces reduce the diffusion layer thickness, promoting NO₃⁻ accessibility and NH₃ formation. Together, these findings decoupled the synergistic role of electronic tuning and wettability control in governing both activity and stability, providing mechanistic design principles for molecular and heterogeneous catalysts in the reduction of electrochemical NO₃⁻ to NH₃.