Real-Time Investigation of Reactive Oxygen Species and Radicals Evolved from Fe-N-C Catalysts During the Oxygen Reduction Reaction

The development of platinum-group-metal-free (PGM-free) oxygen reduction catalysts is critical to improving the economic viability of various energy storage and conversion technologies such as fuel cells. However, the primary barrier to their wider commercialization is a lack of stability. Reactive oxygen species (ROS) and radicals have been proposed in many studies as a root cause for the rapid loss in activity. However, the direct detection and quantification of ROS under catalytic condutions is difficult due to the short lifetimes and dilute amounts of species such as hydroxyl radicals. To overcome this challenge, we have used an operando scanning electrochemical microscopy (SECM) technique to directly and selectively probe the production of radicals in real-time,SECM showed a monotonic production of OH radicals with increasing overpotential. This trend was distinct from that observed for hydrogen peroxide under the same conditions. Additionally, electron spin resonance (ESR) spectroscopy was used to thoroughly confirm our SECM results and confirm that the hydroxyl radical is the dominant radical species produced. Additionally, observing the production of ROS as a function of catalyst degradation also showed a decreased production as the catalytic activity decays, Finally, the structural origins of these damaging radical species was probed using a variety of model systems such as iron phthalocyanine and iron oxide nanoparticles. These results provide a comprehensive insight into the production and role of ROS on Fe-N-C catalyst systems. These experiments open the door for further mechanistic and kinetic investigations involving transient reactive oxygen species and radical intermediates on electrocatalytic systems.