Transient Imaging of the Oxygen Reduction Reaction at Single Nanocatalysts

The initiation of Faradaic reactions at the single-particle level is crucial for uncovering fundamental kinetic principles. However, resolving this early stage is highly challenging because of its ultrafast dynamics and dominant interference from capacitive currents. Here, we report a transient electrogenerated chemiluminescence microscopy (ECLM) that enables real-time visualization of the initiation and evolution of the oxygen reduction reaction (ORR) at individual single-atom catalysts. By integrating a potentiostatic double-pulse-relaxation strategy, sparsely generated H 2 O 2 at the onset of ORR can be sensitively detected and transduced into photon emission by the ECL reaction. Monitoring these transient product yields allows ECL imaging to resolve mechanistic features inaccessible to conventional electrochemical methods, including interfacial potential buildup, dynamic adsorption and conversion of reactants, and the establishment of kinetic equilibrium at catalytic sites. From these dynamics, we define the characteristic time for ORR establishment as a new kinetic parameter that quantitatively describes the transition from a prereactive state to dynamic equilibrium. This parameter provides fundamental mechanistic insight into transient electrocatalysis and establishes a benchmark for evaluating the catalyst performance under varying reaction conditions. Importantly, in this framework, the temporal resolution of ECL imaging is no longer limited by camera exposure time but is instead dictated by the pulse window, which can be tuned with microsecond precision. By coupling microsecond temporal control with single-photon sensitivity, transient ECLM not only deepens the mechanistic understanding of ORR but also enables a new generation of high-resolution methodologies for probing interfacial electrochemistry.