An MEA-Relevant Operando Protocol for the Stability Study of Carbon-Based ORR Catalysts: Unraveling the Proximity-Dependent Radical Scavenging

Oxygen reduction reaction (ORR) catalysts based on M-N-C materials often exhibit promising activity in aqueous electrochemical tests but suffer from rapid degradation under practical fuel cell operation. This discrepancy can originate from different reaction environments, where conventional measurements fail to reproduce the high flux of reactive oxygen species (ROS) present at the triple-phase boundary in membrane electrode assemblies (MEAs). Here, we establish an operando protocol that quantitatively bridges this gap by determining the concentration of H2O2 and derived ROS under realistic fuel cell conditions and then reproducing such oxidative environments in aqueous media via H2O2 injection. Using CoCe-NC as model catalysts, in which Co sites act as stable ROS generators and Ce sites serve as radical scavengers, we demonstrate that atomic-scale proximity between ROS generation and scavenging sites critically governs radical mitigation efficiency and catalyst stability. By tuning inter-site distances, we show that enhanced durability arises from accelerated and localized ROS removal rather than intrinsic stabilization of the carbon matrix. This work establishes a generalizable framework for quantifying and reproducing realistic oxidative environments, enabling mechanistic investigation of ROS-driven degradation that is inaccessible by conventional methods, and providing guidance for the rational design of durable PGM-free ORR catalysts.