XPS Structural Studies of Nano=Composite Non-Platinum Electrocatalysts
The developed
methodology, described herein, combines model curve-fits and principal
component analysis (PCA), resulting in a
quantitative and unambiguous understanding of the chemical composition
and structure of complex electrocatalysts. The chemical structure of
non-platinum electrocatalysts obtained from cobalt porphyrins (CoTMPP
or CoTPP) by pyrolysis is investigated.
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High-resolution Co2p, C1s, N1s and O1s XPS spectra acquired from precursors and electrocatalysts pyrolyzed at various experimental conditions were curve-fit using experimentally obtained photopeaks from the precursor with additional peaks required for a complete curve fit. |
Principal
Component Analysis (PCA) applied to quantitative results from
the curve-fits of spectra facilitates visualization and identification
of the chemical species that are formed or destroyed, and simplifies
evaluation of critical correlations.
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The biplot of all elements shows a separation of variables and samples at 4 different temperatures into 3 groups. |
Most changes occur at
temperatures higher than 400 °C. First, the side groups of
O-CH3 are impacted and detached. Approximately 50% of the Co-N4 centers
stay intact at the completion of pyrolysis. Some of the Co that is
removed is converted to cobalt particles covered by various forms of
oxides. At increasing temperatures, the cobalt particles become
encapsulated into the graphitic network that is formed in parallel with
the Co(CO)4
species, and enhanced by the presence of oxygen from side groups. The
detached carbon-nitrogen part of the precursor centers are converted to
mixtures of pyrrolic and pyridinic nitrogens, which with an additional
increase of temperature, are converted into quaternary and oxidized
quaternary nitrogens.
Through the methodology described
herein, combining model curve-fits and PCA, it was possible to
unambiguously and quantitatively derive the structure of the
electrocatalyst and to explain structural variations resulting from
changes in processing conditions. This methodology can be universally
applied to the analysis of a variety of complex samples to assist in
identification of chemical species and tracking of
modification processes.