While numerous efforts are produced towards the design of sustainable and efficient nano-catalysts of hydrogen evolution reaction (HER), there is a need for the operando observation and quantification of gas nanobubbles (NBs) formation involved in this electrochemical reaction. It is achieved herein through interference reflection microscopy (IRM) coupled to electrochemistry and optical modelling. Besides analyzing the geometry and growth rate of individual NBs at single nanocatalysts, the toolbox offered by super-localization and quantitative label-free optical microscopy allows analyzing the geometry (contact angle and footprint with surface) of individual NBs and their growth rate. It turns out that after few seconds, NBs are steadily growing while they are fully covering the Pt NPs that allowed their nucleation and their pinning on the electrode surface. It then raises relevant questions related to gas evolution catalysts as for example: does the evaluation of NB growth at single nano-catalyst really reflect its electrochemical activity? 2

We describe the combination of scanning electrochemical cell microscopy (SECCM) and interference reflection microscopy (IRM) to produce a compelling technique for the study of interfacial processes and to track the...