Linker-Dependent Stability of Metal-Hydroxide Organic Frameworks for Oxygen Evolution

Metal-organic frameworks (MOFs) are periodic organic-inorganicmaterials that have garnered considerable attention for electrocatalyticapplications due to their wide tunability. Metal-hydroxide organicframeworks (MHOFs), a subset of MOFs that combine layered metal hydroxideswith organic ligands of various & pi;-& pi; stacking energy,have shown promising catalytic functions, such as for the oxygen evolutionreaction (OER). The long-term electrochemical stability of these materialsfor the OER is unfortunately not well understood, which is criticalto design practical devices. In this study, we investigated how Ni-basedMHOFs composed of two linkers with different & pi;-& pi;interaction strength (terephthalate; L1 and azobenzene-4,4 & PRIME;-dicarboxylate;L4) change as a function of cycle number and potential for the OER.All MHOFs tested showed significant increases in the number of electrochemicallyactive Ni sites and OER activity when cycled. MHOFs constructed usingthe linkers with stronger & pi;-& pi; stacking energy (L4)were observed to remain intact in bulk with only near-surface transformationsto NiOOH2-x -like phases, whereasMHOFs with linkers of weaker & pi;-& pi; stacking energy(L1) showed complete reconstruction to NiOOH2-x -like phases. This was confirmed using X-ray diffraction,X-ray absorption spectroscopy, and electron microscopy. Further, insitu characterization using Raman and UV-vis revealed thatthe presence of stable linkers within the MHOF structure suppressesthe Ni2+/Ni(3+& delta;)+ redox process. We furtheridentify NiOOH2-x as the OER activephase, while the MHOF phase serves as a precatalyst. We further proposea detailed mechanism for the phase transformation, which providesvaluable insights into the future challenges for the design of bothstable and catalytically active MOF-based materials for water oxidation.