Atomistic Insights into the Degradation of Metal Phthalocyanine Catalysts during Oxygen Reduction Reaction

Oxygen reduction catalysts frequently suffer from degradation
under
harsh operating conditions, and a limited understanding of the underlying
mechanisms hampers the development of effective mitigation strategies.
In this study, we integrate first-principles calculations with a time-dependent
microkinetic model to investigate the deactivation pathways of six
highly active metal phthalocyanines (MPc, M = Cr, Mn, Fe, Ru, Rh,
and Ir) during the oxygen reduction reaction (ORR). We quantitatively
assess the ORR processes, hydrogen peroxide generation, radical generation,
and three primary degradation mechanisms, namely, carbon oxidation,
nitrogen protonation, and demetallation, through a reaction network
involving 40 chemical species and 75 elementary reactions. Our findings
reveal that the dominant degradation mechanism varies significantly
across the MPcs. Under typical alkaline conditions, the primary byproducts
arise from carbon oxidation, driven by •OH radical
attack and structural reorganization of surface adsorbates, and from
protonation at either the metal center or nitrogen sites. In the kinetics-controlled
region, the ORR activity follows the order of RhPc > IrPc >
FePc >
MnPc > RuPc > CrPc. Notably, RhPc and IrPc demonstrate both
higher
ORR activity and greater stability than the widely studied FePc under
elevated potentials.