Ligand-Dependent Electronic Modulation of Conjugated Metal-Organic Frameworks Enables Efficient Electrocatalytic Nitric Oxide Reduction to Ammonia.

Abstract

Electrochemical reduction of ammonia represents a highly promising approach for transforming harmful environmental pollutants into more valuable products. In this study, we successfully achieved precise control over the environment surrounding single-atom Ni sites by modifying functional groups on ligands. To validate this strategy, we constructed two similar hollow Ni-MOFs. The metal-organic frameworks (MOFs) containing the p-type tetraazanaphthotetraphene ligand (Ni-TT) demonstrated exceptional performance compared to n-type triphenylene (Ni-TP). Its overall reduction rate reached 196.9 µmol·h^-1·cm², with a Faraday efficiency as high as 90.9% at -0.4 V versus RHE. The exceptional performance of Ni-TT stems from the p-type tetranaphthalenetetracene ligand, which is nitrogen-rich and electron-deficient. The electronic properties of the ligand significantly influence the coordination environment around the single-atom Ni site, enabling precise control over proton-transfer behavior during the reaction. Theoretical calculations also indicate that hydrogen adsorption on p-type Ni-TT surfaces accelerates hydrogen transfer at Ni active sites compared to n-type Ni-TP. This significantly enhances the rate-determining step (RDS) in the NO hydrogenation process. The constructed Ni-TT-based Zn-NO battery achieved a power density of 2.1 mW cm^-2. These results provide insight into how conjugated ligand structures modulate the electronic structure and hydrogenation behavior, which may guide future studies on MOF-based electrocatalysts for nitric oxide reduction reaction (NORR).