Synthesis of an ε-MnO2/metal–organic-framework composite and its electrocatalysis towards oxygen reduction reaction in an alkaline electrolyte

An ε-MnO2/metal–organic-framework (Fe) (i.e., ε-MnO2/MOF(Fe)) composite was synthesised by integrating ε-MnO2 and a MOF(Fe) support. The composite was characterised using X-ray diffraction, N2 adsorption–desorption, field emission scanning electron microscopy, transmission electron microscopy, elemental mapping, Fourier transform infrared spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The ORR activity of the composite was evaluated by cyclic and linear sweep voltammetries in an alkaline electrolyte. The results revealed that in the ε-MnO2/MOF(Fe) composite, ε-MnO2 is in the form of nanorods, each with one end protruding and the other firmly anchored on the MOF(Fe) matrix with a high porosity and a high specific surface area. This unique structure of the composite is advantageous for oxygen diffusion and contact with ε-MnO2 during reactions, resulting in much better ORR activity and stability than those of ε-MnO2 in an alkaline electrolyte. The ε-MnO2/MOF(Fe)-catalysed ORR favours an apparent 4-electron transfer pathway in which oxygen was first reduced to hydroperoxide, which was further chemically decomposed into primarily OH− and O2.