Revisiting Metal-Organic Frameworks for Oxygen Evolution: A Case Study.

Abstract

Water splitting is a promising reaction for storing sustainable but intermittent energies. In water splitting, water oxidation is a bottleneck, and thus different catalysts have been synthesized for water oxidation. Metal-organic frameworks (MOFs) are among the highly efficient catalysts for water oxidation, and so far, MOF-based catalysts have been divided into two categories: MOF-derived catalysts and direct MOF catalysts. In particular, a nickel/cobalt MOF is reported to be one of the best direct catalysts for water oxidation. For the first-row transition MOF structures in general, a hypothesis is that the harsh conditions of OER could cause the decomposition of organic ligands and the formation of water-oxidizing oxide-based structures. By electrochemical methods, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and X-ray absorption spectroscopy, a nickel/cobalt MOF known to be a highly efficient catalyst for water oxidation is shown to form Ni/Co oxide, making it a candidate catalyst for oxygen evolution. MOFs are interesting precatalysts for metal oxide water-oxidizing catalysts, but control experiments are necessary for determining whether a certain MOF or other MOFs are true catalysts for OER. Thus, finding a true and direct MOF electrocatalyst for OER is a challenge.