(Invited) In Situ Pyrolysis: Direct Observations of Transformations during Synthesis of M-N-C Electrocatalysts

Abstract

Transition metal-nitrogen-carbon (M-N-C) materials have been considered to be one of most promising platinum group metal-free (PGM-free) catalysts for polymer electrolyte fuel cells 1,2. They are usually obtained through pyrolysis of low molecular weight or polymetric precursors: heat treatment in inert atmosphere. Morphological, structural, interfacial and chemical changes during pyrolysis enable the emergence of the new material with atomically dispersed transition metal ions. These ions are incorporated in the graphene-like carbonaceous matrix, doped with nitrogen and forming various Fe-Nx catalytically active sites. Often, a secondary pyrolysis process is being carries out, after the completion of the initial one, resulting in better defined, and in many cases, more active and stable catalysts (a.k.a. “re-pyrolysis”). Understanding the process leading to formation of the active sites helps advancing their development addressing activity and durability conundrum and paves to way for their introduction in main-stream fuel cell technology.We have reported our findings on dramatic transformations during pyrolysis of M-N-C derived by the Sacrificial Support Method: a hard templating with amorphous silica, removed after the pyrolysis by reactive etching (HF).3 This talk will cover the observations made in situ for both stages of the pyrolytic synthesis via a set of synchrotron and laboratory techniques including (i) pyrolysis of charge-transfer organic salt (nicarbazin) when mixed with silica template and (ii) re-pyrolysis of the M-N-C material as a path to obtain atomically dispersed PGM-free catalyst. During the first pyrolysis we have observed for the first time the explicit melting of the precursor (at exact thermal signature), wetting of the hard template and followed major transformation in morphology. We established that the re-pyrolysis of M-N-C materials results in the rearrangement of multitudinous N-containing moieties, and increased content and a more uniform distribution of Fe-Nx sites.A new set of data will be introduced corresponding to structural changes in “self-templated” pyrolysis of metal-organic framework (MOF) compounds to yield M-N-C with defines morphology. The role of different transition metals and “morphology memory” in retaining MOF structure after pyrolysis will be discussed. Asset and P. Atanassov, Joule, 4 (2020) 33-44Specchia, P. Atanassov and J. Zagal, Current Opinion in Electrochemistry, (2021) 100687Huang, et al., Materials Today, (2021) DOI: 10.1016/j.mattod.2021.02.006