Understanding electron transfer processes and oxygen reduction electrocatalysis in nanocrystalline Cu-MOF-74

Metal-organic frameworks (MOFs) with redox properties are potentially good candidates as noble-metal free electrocatalysts due to their high specific surface area, uniform pore size and shape, and high dispersion of redox centers. Copper-based MOFs are particularly interesting as oxygen reduction electrocatalysts due to the well-known bio-affinity of this element towards oxygen in oxygen-reducing proteins. In this line, the metal–organic framework Cu-MOF-74, which has open metal sites in its 3D structure has not been studied up to date as electrocatalyst for the oxygen reduction reaction (ORR). This work reports an exhaustive electrochemical analysis of nanocrystalline Cu-MOF-74, for assessing its electrochemical behavior and electrocatalytic performance for the ORR. Nanocrystals of Cu-MOF-74 (about 40 nm in size) with intercrystal mesoporosity and high surface area (>1000 m2g−1) were synthesized by a previously reported procedure. The study of this material supported on glassy carbon (GC) rotating disk electrodes allowed to prove its good stability in neutral electrolytic medium and an efficient connectivity of the open Cu centers with redox ability. Moreover, GC/Cu-MOF-74 electrodes were capable to efficient electro-reduce dissolved oxygen to water at overpotentials higher than −0.62 V, fully preserving the framework integrity after long-time assays. It was also verified that this four-electron process is mediated by the reduction of Cu centers exposed at the inner walls of the MOF pores, involving electro-adsorbed oxygenated intermediates. The location of the active sites inside the deep structural nanopores strongly affects the mass transport properties and is detrimental for the whole performance as ORR electrocatalyst. Thus, while the results reported here provide new evidences on the redox and electrocatalytic behavior of nanocrystalline Cu-MOF-74, it is clear that its configuration must be rethought in order to attain an acceptable ORR performance.