Precise fine-tuning of the defect degree in FeNi-TDC nanoarrays via fluorinated acetic acid for rapid urea-assisted hydrogen production in seawater

Precisely regulating the defect degree in metal-organic framework (MOF) crystals and exploring the structure-activity relationship is of great significance for urea-assisted seawater electrolysis to produce green hydrogen. Herein, a series of X-FeNi-TDC nanoarrays with various defects are successfully constructed through the competitive coordination of ligand 2,5-thiophenedicarboxylic acid (H₂TDC) and fluorinated acetic acid modulators with different pK_a and coordination ability of -COO. The 20D-FeNi-TDC nanosheets containing more unsaturated sites and moderate electron delocalization exhibit multifunctional activity with an overpotential of 194 mV in OER, 116 mV in HER and an applied voltage of 1.326 V in UOR at 10 mA cm^-2. In addition, small angle X-ray scattering (SAXS), small angle neutron scattering (SANS), X-ray absorption near-edge structure (XANES) and density functional theory (DFT) calculation further elucidate alterations in pore structure and electronic arrangement of metal sites. This study provides insight into the mechanism by which modulators regulate defects in MOF-based electrocatalyst.