Understanding Pyrolysis: Operandoand in Situcharacterization of Morphology and Composition Changes during Synthesis of M-N-C Electrocatalysts

Abstract

Transition metal-nitrogen-carbon (M-N-C) materials have been continuously reported to be one of most promising platinum group metal-free (PGM-free) catalysts for oxygen reduction reaction (ORR)/cathode in low-temperature polymer electrolyte fuel cells (PEFC) and other emerging areas (such as carbon dioxide reduction reaction and nitrogen reduction reaction). They are usually obtained through pyrolysis of low molecular weight (organic) or polymetric precursors. The pyrolysis consists of heat treatment in inert (or reductive) atmosphere. Through the decades of practice, this pyrolysis process has been optimized intuitively and empirically with little-to-no knowledge of the explicit sequence of transformations. 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-Nxcatalytically 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”).Our team have investigated both stages of the pyrolytic synthesis via in situXPS, TEM, EDS, XRD and nano- and micro-XCT methods in two separate studies dedicated to (i) pyrolysis of charge-transfer organic salt (nicarbazin) when mixed with silica template (a.k.a. “sacrificial support method” of M-N-C catalyst synthesis) and (ii) re-pyrolysis (secondary, “purification” 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: pore structure evolution, initial graphitization, metal and metal carbide phase formation, secondary graphitization templated on the transition metal nanoparticles and secondary micro-pores formation. All these observations have been interpreted through cross-referencing tomography, crystallography, microscopy and spectroscopy in situdata streams.We established that the re-pyrolysis of M-N-C materials could result in the amorphization of carbon, the rearrangement of multitudinous N-containing moieties, and increased content and a more uniform distribution of Fe-Nxsites, yet without major morphology changes in carbonaceous framework. In summary, the re-pyrolysis has multiple advantages and is recommended for all M-N-C electrocatalysts, no matter what precursors and parameters are set for their synthesis.