(195l) Metal-Organic Frameworks As Template Shells for Enhanced Cobalt Oxide Electrocatalyst Performance

Metal organic frameworks (MOF) are a class of materials composed of metal ions or oxo-metallic secondary building units (SBUs) coordinated with organic linkers to form a porous, open framework. The combination of rational pore design, high porosity and surface area and facile synthesis have made MOFs popular as versatile materials for an expanding portfolio of applications including gas storage, catalysis, separations etc. In the case of catalysis, MOFs can act as a multifunctional and synergistic substrate by providing a periodic, structured catalyst support in addition to facilitating size selective transport of a given species. Independently, cobalt oxide cubane material has been identified as a promising electrocatalyst for the oxygen evolution reaction (OER), an integral step in both next generation fuel production and batteries. While current methods use anodic deposition of cobalt ions to form a repeating cobalt-oxo cubane bulk structure, several studies suggest the active species is a relatively smaller domain of the whole material, i.e. edge-sharing CoO₆ clusters in the bulk material. Thus, deposition processes that favor the edge-to-bulk ratio of the cluster will improve catalytic activity of the system. Several MOF-catalyst hybrids for the OER have been explored. Typically, the framework is either assembled around existing catalyst particles or particles must be grown within the framework. Both processes require significant synthesis times and do not offer good electrical contact with the working electrode, therefore limiting catalyst performance. Here we report a novel method of electro-depositing cubane cobalt oxide on an electrode spray coated with a common MOF, UiO-66. Using cyclic voltammetry (CV), controlled potential electrolysis (CPE), and corresponding Tafel analysis we demonstrate an improvement in current density compared to bulk cobalt oxide. We further explore the morphological effects on cobalt oxide deposition and catalytic performance as they relate to the MOF by varying material ratios. Finally, we use scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to elucidate the synergistic effect between the MOF and catalyst during deposition and the OER.