Structure and activity of metal-centered coordination sites in pyrolyzed metal-nitrogen-carbon catalysts for the electrochemical reduction of O2

Pyrolyzed metal-nitrogen-carbon (M-N-C) materials have become a mainstream research as inexpensive and sustainable catalysts for the oxygen reduction reaction (ORR) in both acid and alkaline media for low and intermediate temperature fuel cells. Tremendous advancements in the initial activity and power performance of M-N-C catalysts and cathodes have been achieved, as driven by their possible application in e.g. automotive fuel cell stacks. Based on a selected number of recent studies, this review critically discusses the advancements, but also highlights the remaining scientific questions and technical issues important in this field. The nature of the active site(s) as well as their intrinsic activity toward ORR have been clarified in particular through the preparation of model catalysts comprising only MNxCy moieties. Recently developed methods hold promise to reliably enumerate the number of electrochemically accessible active sites in such materials, which would allow deconvoluting the activity into site density and turnover frequency.