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

Metal–organic frameworks (MOFs) are periodic organic–inorganic materials that have garnered considerable attention for electrocatalytic applications due to their wide tunability. Metal-hydroxide organic frameworks (MHOFs), a subset of MOFs that combine layered metal hydroxides with organic ligands of various π–π stacking energy, have shown promising catalytic functions, such as for the oxygen evolution reaction (OER). The long-term electrochemical stability of these materials for the OER is unfortunately not well understood, which is critical to design practical devices. In this study, we investigated how Ni-based MHOFs composed of two linkers with different π–π interaction strength (terephthalate; L1 and azobenzene-4,4′-dicarboxylate; L4) change as a function of cycle number and potential for the OER. All MHOFs tested showed significant increases in the number of electrochemically active Ni sites and OER activity when cycled. MHOFs constructed using the linkers with stronger π–π stacking energy (L4) were observed to remain intact in bulk with only near-surface transformations to NiOOH2–x-like phases, whereas MHOFs with linkers of weaker π–π 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, in situ characterization using Raman and UV–vis revealed that the presence of stable linkers within the MHOF structure suppresses the Ni2+/Ni(3+δ)+ redox process. We further identify NiOOH2–x as the OER active phase, while the MHOF phase serves as a precatalyst. We further propose a detailed mechanism for the phase transformation, which provides valuable insights into the future challenges for the design of both stable and catalytically active MOF-based materials for water oxidation.