Structural changes of a NiFe-based metal-organic framework during the oxygen-evolution reaction under alkaline conditions

The design of new supramolecular complexes and metal-organic frameworks (MOFs) as water-oxidizing catalysts requires the stability studies because many metal-organic compounds are decomposed under the harsh conditions of water oxidation. Metal-organic frameworks have been extensively reported as catalysts for water splitting toward hydrogen production. Recently, a NiFe MOF has been claimed as an excellent catalyst for oxygen-evolution reaction (OER) under alkaline conditions (KOH (1.0 M)). Herein, using electrochemical methods, X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, transmission electron microscopy, and Raman spectroscopy, the stability of this NiFe MOF was investigated during OER. In the Raman spectrum, the peaks related to C–C and C–O in 1100–1550 cm−1 become weak after OER, indicating the decrease of the carboxylate functional group. Comparing energy-dispersive spectra for the MOF before and after OER shows a decrease in carbon, but an increase in oxygen contents. Similar to the results of linear sweep voltammetry, energy-dispersive spectroscopy spectra suggest that the surface of the MOF converts into an oxide-based structure after OER. Transmission electron microscopy also shows some new crystalline areas after OER. The crystalline areas indicate a crystal lattice spacing of 0.21–0.23 nm, corresponding to (012) plane of the NiFe layered double hydroxide. Taken together, these experiments show that during OER the MOF converts to NiFe oxide with significant defects and imperfections in the regular geometrical arrangement of the ions, and the NiFe oxide is a candidate for catalyzing OER. This finding could be a roadmap for progress in the field of sustainable catalysis.