Carbon-based catalysts for oxygen reduction reaction: A review on degradation mechanisms

Abstract

This review focuses on the historic and recent progress of the stability of the carbon-based non-platinum-group metal (PGM) and metal-free catalysts for oxygen reduction reaction (ORR). It highlights the reported degradation mechanisms which are responsible for the instability issue. These mechanisms are (a) oxidative attack of the ORR intermediates, (b) demetalation of the metal-active site, (c) protonation followed by anionic adsorption, and (d) micropore flooding. For the metal-based catalysts, mechanisms (a) and (b) are the most commonly observed. From the review, it is known that the oxidative attack of ORR intermediates leads to the carbon oxidation, which can also result in the demetalation of the metal-active site. Additionally, the demetalation of the iron-based catalysts can also indirectly oxidize the carbon-support via Fenton reaction. The mechanisms (c) and (d) are sharply criticized. Finally, for the metal-free carbon-based catalysts, there is only a limited research available on the stability of these catalysts. Among the reported study, the mechanisms (a) and (c) can be responsible for the degradation of these catalysts. Therefore, it is recommended that more study should be focused on investigating the degradation parameters and mechanisms for metal-free catalysts.