(Invited) Spectroscopy Techniques Applied to Operando Characterization of PGM-Free Catalysts for Fuel Cells

Abstract

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-Nx 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-Nx 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.