Does Turnover Number Represent a Single Value or a Distribution?

Catalyst turnover number is traditionally measured at the macroscopic scale, yielding a single numerical value under specific reaction conditions. This bulk-averaged approach assumes uniform catalyst longevity, masking intrinsic heterogeneity within a sample and limiting meaningful comparisons across different systems. Here, we present a novel method to simultaneously quantify the number of active sites and total catalytic charge of individual platinum nanoparticles during electrochemical hydrogen evolution at high current densities (>0.4 A/cm²). This enables direct calculation of single-particle turnover numbers, one at a time, which reveals orders of magnitude of variation among monodisperse particles. We further show that the observed turnover number heterogeneity cannot be primarily attributed to stochastic particle-size variation within the monodisperse population, but instead is governed by applied potential and active-site connectivity. This work redefines our understanding of catalytic turnover number, establishing a framework for assessing catalyst longevity distribution at the single-particle level. Understanding the origins of turnover number heterogeneity and the underlying molecular nuances within electrocatalysts can inform strategies to enhance durability and optimize catalytic performance.