Oxygen reduction in the nanocage of metal-organic frameworks with an electron transfer mediator

A highly porous metal–organic framework (MOF) containing copper metal centres and nanocages was modified onto a glassy carbon electrode as a noble-metal-free electrocatalyst for oxygen reduction reaction. The nanocages in the metal–organic framework were fully activated by the solvent-exchange method. Although both the as-prepared MOF and activated MOF samples showed electrochemical activity of Cu2+/Cu+ redox pairs by cyclic voltammetric studies, only the activated MOF samples could catalyze an oxygen reduction reaction. In order to avoid detachment of the activated sample from the glassy carbon electrodes surface owing to a low-effective electron-transfer pathway during electrochemical scanning in aqueous solution, reduced graphene oxide (RGO) was immobilized onto a glassy carbon electrode surface as a binder and electron transfer mediator under MOF active layer. The MOF layer on RGO immobilized glassy carbon electrode can catalyze the oxygen reduction reaction through a 2–4 electrons reduction pathway. Furthermore, the occurrence potential of ORR versus Ag/AgCl by MOF catalyst shifted to the positive near 100 mV in comparison with other MOF catalysts.