The class of metal-nitrogen-carbon materials has emerged as a promising type of catalysts for electrochemical energy conversion at large, and for oxygen electroreduction to water in fuel cells in particular. Novel synthetic paths have allowed i) increasing their activity and selectivity and ii) reaching a better definition of the active sites that are present in such materials. Two main types of metal-based sites can be identified in the class of pyrolyzed metal-nitrogen-carbon materials, namely metallic particles surrounded by a nitrogen-doped graphite shell and atomically dispersed metal-Nx moieties. The activity for oxygen reduction and stability of these two different types of site are expected to differ in a given electrolyte, and also to change differently when switching from acid to alkaline electrolytes. With the advent of high-performance anion-exchange membrane fuel cell (AEMFC), Metal-N-C catalysts hold promise to replace platinum-group-metal catalysts in AEMFC and also in proton-exchange membrane fuel cells (PEMFC). The AEMFC and PEMFC systems have different pros and cons, the AEMFC durability being limited by the ionomer and membrane stability while, in PEMFC, the stability of Metal-N-C catalysts is the main issue. The presentation will focus on the coupling of various spectroscopic techniques (X-ray absorption and 57Fe Mössbauer spectroscopy in particular) with electrochemical investigation and other characterisation tools, in order to identify the nature of the metal-based sites in Metal-N-C catalysts at beginning-of-life, in electrochemical conditions where the oxygen reduction is catalyzed and post mortem, following accelerated stress tests. Comparison of the activity and spectroscopic behaviours of a set of well-defined metal-N-C catalysts in acid and alkaline electrolytes will be discussed, and the comparison of the stability of various types of metal-based sites (core-shell metal-NC, and Metal-Nx moieties) during electrochemical cycling, which allowed us identifying a stability-structure correlation.

Ex-situ and operando spectroscopic characterisation of inexpensive cathode catalysts for fuel cells

2019 Spring National Meeting

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