Atomically Dispersed Iron Sites in Nanometer-Thick-Coordination Polymer Films Based on 2,5-Dimercapto-1,3,4-thiadiazole for Oxygen Reduction Electrocatalysis

Homogeneous distribution of single-atomic redox-active sites on the electrode surface through the coordination network offers advantages in multiproton-coupled electron transfer catalytic reactions. Herein, we report that iron ions are coordinated with 2,5-dimercapto-1,3,4-thiadiazole (DMcT), forming cross-linked coordination structures. Spectroscopic analyses, including XPS, XAS, Raman, and FT-IR, confirm the presence of Fe–N and Fe–S coordination linkages, which contribute to structural robustness and uniform distribution of active sites for photoelectrocatalytic reactions. Powder XRD and selected area electron diffraction (SAED) patterns suggest the formation of an amorphous phase of the material. Repeatable n-type photocurrent responses were observed in I^–/I₃^– redox couple, in neutral (0.5 M Na₂SO₄) and protic electrolyte (0.1 M H₂SO₄). Unlike oxides and sulfides of iron, this photoelectrode displays a stable n-type photocurrent response in protic media. This iron coordination system exhibits good oxygen and nitrate reduction activities. The onset oxygen reduction potential is observed at +0.6 V vs RHE with a limiting current density of ∼3 mA cm^–2 in 0.1 M H₂SO₄. The tautomeric shuttle between neighboring S and N units of DMcT enables it to act as a possible proton relay to the iron sites, enhancing the proton-coupled electron transfer activity. The charge delocalization through DMcT cross-linkage in the poly-Fe-DMcT metallopolymer enables faster electron/proton transfer, contributing to the improved ORR catalytic activity.