Experimental and theoretical modeling of Fe-, Co-, Cu-, Mn-based electrocatalysts for oxygen reduction

Experience gained during efforts towards optimization of noble-metal-free electrocatalysts for oxygen reduction is simultaneously used to understand the chemical and morphological necessities for inducing efficient multi-electron transfer catalysis. The analysis of many preparative experimental steps between the moderately performing metal porphyrines and the highly efficient transition metal- and sulfur-containing pyrolized catalyst material contributes to the following model of the catalyst: The metals function enclosed in nitrogen or graphitic environment where they are shielded against irreversible oxidation. The metals can be exchanged but are not identical in their efficiency. Higher efficiency is achieved, when the function of a binary reaction center is warranted. The carbonization of the environment is critical and provides intercalated metal centers and attached metal complexes in graphite environment for interaction with the nitrogen-chelated partner center in the simultaneously obtained graphene layers. Three alternatives for the binary catalytic center are presented and their relevance discussed on the basis of EXAFS, RAMAN, EPR, Mössbauer and X-ray spectroscopy. A parallel is drawn with the cytochrome oxidase oxygen reduction catalysis, which is proposed to proceed according to roughly the same mechanism.