Introducing different long-chain flexible ligands to regulate the transformation behavior of NiFe-MOF and as bifunctional catalysts for the HER/OER.

The investigation of highly efficient metal-organic framework (MOF) electrocatalysts represents a pivotal challenge in enhancing the overall efficiency of water electrolysis. In this study, we present a co-liganding method based on molecular regulation, in which succinic (s.a), adipic (a.a), and decanedioic (d.a) acids are employed to modify the original ligand HBDC-based bimetallic NiFe-MOF to obtain dual-ligand NiFe-MOF electrocatalysts with excellent OER and HER performance (NiFe-MOF s.a, NiFe-MOF a.a, and NiFe-MOF d.a). Notably, NiFe-MOF d.a performs the best, particularly in 1 M KOH solution, with an OER overpotential of 242 mV and an HER overpotential of 116 mV at a current density of 10 mA cm, and it demonstrates excellent stability under alkaline conditions, outperforming several recently reported Fe/Co/Ni-based catalysts. Bader charge and Mulliken charge calculations alongside experimental characterization data indicate that long-chain dicarboxylate ligands can influence the morphology, oxygen vacancy formation, and electronic structure of NiFe-MOFs. XPS and XRD analyses after the OER reveal that the four NiFe-MOFs undergo structural reorganization, with NiFe-MOF d.a having more unsaturated metal sites and more oxygen vacancies, promoting greater conversion of the MOF to NiOOH and FeOOH, which is beneficial for improving the OER performance. The density of states (DOS) results from DFT calculations demonstrate that the d-band center of NiFe-MOF d.a is relatively low, indicating that the adsorption energy of H* is reduced, which can promote the release of H. Furthermore, the hydrogen adsorption free energy (ΔG) values of the Fe and Ni active sites within NiFe-MOF d.a are closer to zero, thus accelerating the HER kinetics. This work paves new pathways for realizing dual-ligand MOFs.