Coordinating Mesopore Geometry and Interfacial Water Dynamics for Iron-Site Evolution in Nitrate Electroreduction.

Developing electrocatalysts with active sites that undergo structural evolution under operating conditions remains a challenge in electrochemical nitrate reduction. Here, we use single-crystalline mesoporous metal-organic frameworks (MOFs) with distinct pore geometries to investigate the potential-dependent evolution of iron active sites during electrochemical nitrate-to-ammonia conversion. Fe single atoms are anchored within hexagonal and cubic mesoporous frameworks, providing two structurally distinct environments for examining their catalytic and working-state responses. Operando FTIR, H/D isotope-exchange, and DRT analyses reveal that the 1D hexagonal mesoporous environment is associated with a distinct interfacial-water response and proton-involved reaction behavior. This response coincides with changes in the average Fe coordination environment. Operando XAS further reveals potential-dependent changes in both catalysts, with FeSAs@mUiO-P exhibiting a more pronounced Fe-associated higher-shell contribution and greater post-potential recovery. Under the investigated conditions, FeSAs@mUiO-P achieves a Faradaic efficiency of 99.1% at -1.05 V versus RHE and exhibits higher NH3 selectivity than its cubic counterpart. Collectively, these findings reveal a potential-dependent correspondence among mesopore architecture, interfacial water organization, and Fe-site evolution. Although the causal relationship between water organization and Fe-site evolution remains to be established, this correspondence provides a working framework for investigating reaction-state-dependent catalytic interfaces.