Recent advances on oxygen reduction electrocatalysis: Correlating the characteristic properties of metal organic frameworks and the derived nanomaterials

This review summarizes the current advances of hetero-doped (Fe, Co, P, S, N) nanostructures derived from MOFs (metal-organic frameworks) for the oxygen reduction reaction (ORR) electrocatalysis. These nanomaterials are classified as: OD (polyhedrons, hollow and core-shell structures), 1D (nanotubes and nanorods), 2D (nanosheets), and 3D (honeycomb like frameworks) morphologies. It is thoroughly discussed the critical and very important pathway of ORR, which occurs in electrochemical devices such as fuel cells and metal-air batteries (high energy capacity, excellent conversion efficiency and low environmental impact). Emphasis is given on the mechanistic studies devoted to both the nanostructure formation from MOFs and the morphology-activity relationship of transition-metal anchored carbon nanostructures transformed from MOFs. Controlled electronic structures, extrinsic/intrinsic structures and interface (edge) properties are also discussed for the ORR performance, providing a useful preparation approach for carbon doped with heteroatoms by rationally designing MOF precursors. As a result of the ongoing flexibility of these frameworks, the doped carbon based electrocatalysts present enhanced ORR performance, which expands their applications (except for fuel cells) in other energy conversion and storage devices like supercapacitors and metal ion batteries.