Degradation of Fe-N-C catalyst induced by surface carbon oxidations

Abstract

Iron-nitrogen-carbon (Fe-N-C) catalyst is one of the most developed precious group metal (PGM)-free catalysts. Despite of a promising power density over 1 W∙cm-2 in PEMFC, simultaneously observed an unforeseen problem in terms of low stability hinders applications of the Fe-N-C based cathodes, which is rarely observed in alkaline environment. A few studies so far pointed out that reactive oxygen species (ROS), generated from a chemical reaction between Fe-based active site and hydrogen peroxide, could be an origin of the performance degradation based on experimental and theoretical investigations. However, specific kind of the ROS mainly responsible for the oxygen reduction reaction (ORR) deactivation and different degradation behavior according to pH conditions are still unapprehended questions. In this study, therefore, we thoroughly investigated pH-dependent H2O2-induced ORR deactivation with a model Fe-N-C catalyst exclusively comprising FeNxCy moeities. Acidic H2O2-treatment leads to severe ORR activity decrease and the degradation gradually mitigated as pH of the H2O2-treatment increases. Analyses toward surface chemistry of the H2O2-treated Fe-N-C catalysts indicated that additional oxygen functianalities were introduced to surface of the catalysts by H2O2 and extent of the H2O2-induced surface oxidation is closely related to the activity losses, inferring the chemical reaction between H2O2 and Fe centre is pH-dependent. A product of the chemical reaction which is consequently responsible for the ORR deactivation of the Fe-N-C catalyst is identified as a hydroxyl radical by electron paramagnetic resonance (EPR) spectroscopy. This ascertainment of specific ROS species offers an insight to establish a strategy to design degradation-free Fe-N-C catalysts in PEMFC operation as well as a succesful utilization of the catalysts into other types of fuel cell devices operating in alkaline condition.