Ligand engineering via the hard–soft-acid–base theory: enabling the design of MOFs for efficient electrocatalytic oxygen evolution reaction

Abstract

Metal–organic frameworks (MOFs) exhibit excellent catalytic activity in the electrochemical oxygen evolution reaction (OER), but their poor stability in alkaline solutions hinders their wider application in anion exchange membrane (AEM) water electrolysis. In this work, we design and synthesize a layered MOF Co-Tdp-Bpy based on the Hard–Soft-Acid–Base (HSAB) theory, which features a novel phosphorus-containing ligand 2,5-thiophenediphosphinic acid (Tdp) and 4,4′-bipyridine (Bpy) for synergistic coordination with Co. Co-Tdp-Bpy/NF exhibits excellent OER catalytic performance (211 mV @ 10 mA cm⁻² and 240 mV @ 100 mA cm⁻²) and stability (>330 h @ 10 mA cm⁻² and 100 mA cm⁻² in sequence, >120 h @ 1 A cm⁻²). Single crystal XRD analysis indicates that Co-Tdp-Bpy enhances the activity of oxygen atoms through shorter Co–O bonds, synergistically improves the structural stability by sulfur/phosphorus atoms, and optimizes the electronic structure by Co–N bonds, which are the fundamental sources of its excellent catalytic performance. Meanwhile, density functional theory calculations show that the introduction of the Tdp ligand facilitates electron transfer and reduces the free energy of adsorbed intermediates during the electrocatalytic OER process. This work demonstrates that the design of ligands and MOFs based on the HSAB theory is an effective way to enhance the stability of MOFs, broadening the design scope of MOFs and their application as efficient OER electrocatalysts.