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

ABSTRACT 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@ m UiO‐P exhibiting a more pronounced Fe‐associated higher‐shell contribution and greater post‐potential recovery. Under the investigated conditions, FeSAs@ m UiO‐P achieves a Faradaic efficiency of 99.1% at −1.05 V versus RHE and exhibits higher NH 3 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.