Metal-N-C catalysts (Metal = Fe, Co primarily) are being developed and studied as alternative catalysts to platinum for catalyzing the O₂ reduction reaction (ORR), in acidic and alkaline media. The nature of their active sites is still a topic of intense discussions and advanced studies, this class of materials possibly comprising N-doped sites, Metal-N_x moieties and various crystallographic structures. The latter often result from excess metal relative to the number of defect sites in the carbon matrix that may host Metal-N_x moieties.

Improved understanding has been gained first through the combination of controlled synthesis, ex situ spectroscopic characterization and ex-situ structure / activity correlations. More recently, operando and/or in-situ spectroscopic techniques have been applied on novel Metal-NC catalysts to better understand their site formation during pyrolysis, their electronic, spin-state and coordinative environment as a function of electrochemical potential and/or time scales. Operando spectroscopy on such catalysts may also offer a way to distinguish the metal-based sites that are electrochemically accessible from those that aren’t. This is a recognized issue for pyrolysed Metal-N-C materials.

This presentation will focus on the application of X-ray absorption techniques to the operando characterization of Fe and Co in Metal-NC catalysts in various electrochemical conditions, and of what could be learnt additionally to ex situ or post mortem studies. Initial results acquired with operando ⁵⁷Fe Mössbauer spectroscopy will also be presented and discussed.